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Progress in Polymer Science 38 (2013) 1653– 1689

Contents lists available at ScienceDirect

Progress in Polymer Science

journa l h om epa ge: www.elsev ier .com/ locate /ppolysc i

iobased plastics and bionanocomposites: Current statusnd future opportunities

urali M. Reddya, Singaravelu Vivekanandhana,b, Manjusri Misraa,b,ujata K. Bhatiac, Amar K. Mohantya,b,∗

Bioproducts Discovery & Development Centre (BDDC) , Department of Plant Agriculture, Crop Science Building, University of Guelph,uelph, N1G 2W1, ON, CanadaSchool of Engineering, Thornbrough Building, University of Guelph, Guelph, N1G 2W1, ON, CanadaSchool of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA

a r t i c l e i n f o

rticle history:eceived 2 May 2012eceived in revised form 23 April 2013ccepted 1 May 2013vailable online 9 May 2013

eywords:iobased plasticsionanocompositesanofillersiodegradable

a b s t r a c t

This paper presents a broad review on the recent advances in the research and develop-ment of biobased plastics and bionanocomposites that are used in various applicationssuch as packaging, durable goods, electronics and biomedical uses. The development ofbiobased materials is driven by renewability, low carbon footprint and in certain casesbiodegradability (compostability) issues and helped them in moving from niche marketsto high-volume applications. The inherent drawbacks of some biobased plastics such as thenarrow processing window, low heat deflection temperatures, hydrophilicity, poor barrier,and conductivity and inferior biocompatibility can be overcome by bionanocomposites.The first part of the paper reviews the recent advances in the development of biobasedand biodegradable materials from renewable resources and their advantages and disad-

From the Progress in Bionanocomposites: from green plastics to biomedical applications Special Issue

ompostableelt intercalation

olvent castingurface treatmentackagingiomedical

vantages. In the second part, various types of bionanocomposites based on four types offillers i.e. nanocellulose, carbon nanotubes, nanoclays, and other functional nanofillers arediscussed. This review also presents up-to-date progress in this area in terms of processingtechnologies, product development and applications.

© 2013 Elsevier Ltd. All rights reserved.

lectronics

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.1. Bioplastics: classification and current status . . . . . . . . . . . . . . .

Abbreviations: PLA, poly(lactic acid); PHA, polyhydroxyalkanoates; PHB, polyholycaprolactone; PBAT, poly(butylene adipate-co-terephthalate); PTT, poly (trimne; PP, polypropylene; PVOH, poly(vinyl alcohol); PET, poly(ethylene terephtholy(propylene carbonate); PBSA, poly (butylene succinate-co- adipate); PEA, poplastic starch; CA, cellulose acetate; CAP, cellulose acetate propionate; CAB, c

arbon nanotube; SWCNT, single-walled carbon nanotube; EMI, electromagnetic

red silicates; HAp, hydroxyapatite; LDH, layered double hydroxides; POSS, polyhater vapor transmission rate; CTE, coefficient of thermal expansion; OLED, orga∗ Corresponding author at: Bioproducts Discovery & Development Centre (BDDuelph, Guelph, N1G 2W1, ON, Canada. Tel.: +1 519 824 4120x56664; fax: +1 519

E-mail address: [email protected] (A.K. Mohanty).

079-6700/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.progpolymsci.2013.05.006

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1654 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1654

ydroxybutyrate; PHBV, poly(hydroxybutyrate-co-hydroxyvalerate); PCL,ethylene terephthalate); PBS, poly(butylene succinate); PE, polyethyl-

alate); PEO, poly(ethylene oxide); CNW, cellulose nanowhiskers; PPC,olyesteramide; PDO, 1,3-propanediol; TPA, terephthalic acid; TPS, ther-ellulose acetate butyrate; CNT, carbon nanotube; MWCNT, multi-walledinterference; CEC, cation exchange capacity; OMLS, organomodified lay-edral oligomeric silsequioxanes; MCC, microcrystalline cellulose; WVTR,nic light emitting diode; ESD, electrostatic discharge.C), Department of Plant Agriculture, Crop Science Building, University of

821 8660.

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1654 M.M. Reddy et al. / Progress in Polymer Science 38 (2013) 1653– 1689

1.1.1. Poly(lactic acid) (PLA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16561.1.2. Polyhydroxyalkanoates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16561.1.3. New trend in plastics from biological sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16571.1.4. Agro-polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1659

2. Nanofillers for bionanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16612.1. Cellulose based nanofillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16612.2. Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16622.3. Nanoclays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16642.4. Functional nanofillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1666

3. Nanocomposites from renewable resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16673.1. Cellulose nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16673.2. CNT Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16683.3. Clay nanocomposites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16713.4. Functional nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1673

4. Processing aspects of bionanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16744.1. Processing of cellulose based bionanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16744.2. Processing of polymer-carbon nanotubes nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16744.3. Processing of polymer-clay nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16754.4. Processing of functional nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1675

5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16775.1. Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16775.2. Electronics, sensor and energy applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16795.3. Bionanocomposites for medical applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1680

5.3.1. Bionanocomposites for tissue engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16815.3.2. Bionanocomposites for drug delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16815.3.3. Bionanocomposites for gene therapy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1682

6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1682Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683

. . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction

Worldwide annual plastics production is estimatedto surpass 300 million tons by 2015 [3] representingtrillions of dollars in terms of global economic returns.Petroleum resources are extensively used in producingthese polymers, which leads to concerns in terms ofboth economic and environmental sustainability. Overde-pendence on petroleum resources can be alleviated bybioplastics development using biological resources or moreprecisely annually renewable resources. Bioplastics repre-sent a wide spectrum of thermoplastics that are obtainedfrom biological resources and fossil resources, or com-binations of both. Currently, the bioplastics market iscoming out of its infancy and capturing the plastics mar-ket at a growth rate of 30% annually [4]. Many researchersare working to derive new compounds from biologicalresources either by industrial biotechnology or by chem-ical methods. Efforts are also being made to producebiopolymers or polymer building blocks in transgenicplants or micro-organisms. PHA synthesis in plants hasadvanced considerably, recently it has been demonstratedthat switchgrass is capable of producing PHB as nearly3.7% of its weight, but for commercial viability at least5% of weight as PHB should be produced [5]. A bioplasticmay be biobased and/or biodegradable — that is, it maybe a biopolymer derived from nature and/or a polymer
that can return to nature. The terms biobased plastic andbiodegradable plastic are sometimes used interchange-ably, but this is not correct. Bioplastics can be madeof 100%-renewable material, biodegradable fossil-based
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683

polymers, or a combination of renewable and fossil mate-rials.

1.1. Bioplastics: classification and current status

The current classification of bioplastics based on theirproduction method is shown in Fig. 1 and the followingsection describes this classification and reviews some ofthe important polymers in these segments.

(i) Renewable-resource-based bioplastics: These bioplas-tics are either synthesized naturally from plants& animals, or entirely synthesized from renew-able resources. This class includes starch, cellulose,proteins, lignin, chitosan, poly lactic acid (PLA)and polyhydroxyalkanoates/polyhydroxybutyrates. Arecent breakthrough in this class of bioplastics is thedevelopment of technology to synthesize polymerslike polyethylene, polypropylene and nylon from bio-logical resources.

(ii) Petroleum-based bioplastics: These polymers are syn-thesized from petroleum resources but are biodegrad-able at the end of their functionality. Polycaprolactone(PCL) and poly(butylene adipate-co-terephthalate)(PBAT) are included in this category.

(iii) Bioplastics from mixed sources: These are made fromcombinations of biobased and petroleum monomers;
they include polymers like poly (trimethylene tereph-thalate) (PTT), bio-thermosets and biobased blends.PTT is manufactured using petroleum derived tereph-thalic acid and biologically derived 1,3-propanediol.

M.M. Reddy et al. / Progress in Polymer Science 38 (2013) 1653– 1689 1655

Petroleum bas ed Biodegradable Polym ers

Poly (vinyl alcohol) (PVOH)

Aliph a�c-Aroma�c polyesters

Aliph a�c polyesters

Ex: Poly(bu tylene adipate –co-terephthala te) (PBAT)

Ex: Polycaprolactone (PCL), Poly(bu tylene succ inate) (PBS)*

Starch Plas�cs

Cell ulosics

Ex: Cellulose Esters

Renewable Resource bas ed Polymers

Poly (lac�c acid) (PLA)

Prote ineous Plas�cs

Ex: Wheat/Potato/Corn-based Pla s�cs

Ex: Plant and animal proteins based plas�cs

Polyhydroxyalkanoates (PHAs)Ex: Polyhydroxybu tyrate (PHB),

Poly(hydroxybu tyrate-co-hydroxyvalerate) (PHBV)

Ex: Poly(D-lac�de) (PDLA), Poly(DL-la c�de) (PDLL A)

Polymers from Mixed Sources (Bio-/Petro-)

Thermoset s

Polyesters

Ex: Poly (trimethylene terephthala te) (PTT )

Ex: Biobased Epoxy, Biobased Polyurethane

* PBS: can be renewable resourcebase d with renewable conte nt > 50%

Recent Renewable Resource-based Pl as�cs: Bio-Polyethylene, Bio-Polypropylene & B io-nylon**** Bio-nylon can be bo th 100 % renewab le resou rce ba sed and pa r�al renewab le resou rce ba sed. For example Nylon 6,10 and Nylon 11 are pa r�ally renewab le , wh ile Nylon 10 ,10 is 100% renewab le resou rce ba sed.

Fig. 1. Classification of bioplastics based on their production routes. Redrawn after Ref. [303].

Fig. 2. Biobased plastics polymers and their monomers produced by microbial fermentations combined with chemical synthese [9]. Copyright 2012Redrawn with permission from ref. the American Chemical Society.

Polymer
1656 M.M. Reddy et al. / Progress in
The biodegradability of the plastics mentioned in thisclassification depends on the chemical nature not onthe source. For e.g. 100% bio-derived PE and partiallybio-derived PTT are not biodegradable where as 100%petro-derived PBAT is biodegradable. Also, poly(vinyl alco-hol) is not highly biodegradable in short term tests [6].Isotactic poly(vinyl alcohol) (PVOH) biodegrades fasterthan commercial PVOH in presence of Pseudomonas strain[7]. All of the biobased plastics are synthesized either byproduction within bacteria or by chemical polymerization[8,9]. Fig. 2 shows the chemical structures and differentsynthesis routes of bioplastics that are available in thecommercial quantities in the market. This review focuseson bioplastics that belong to category 1, i.e. Renewable-resource-based bioplastics or biobased plastics. The followingsection presents the current status of production and uti-lization of the renewable-resource-based bioplastics, morespecifically PLA, PHAs, starch, proteins, cellulose and newtrends in obtaining biobased polymers.

1.1.1. Poly(lactic acid) (PLA)PLA is renewable, biocompatible and also biodegradable

polymer and is one of the most widely used bioplastics. PLAis obtained either by ring opening polymerization (ROP) oflactide or by direct polycondensation of lactic acid [10].The lactide used in ROP is a cyclic dimer of lactic acid.[10]. The monomer lactic acid is a chiral molecule andexists as D- and L-lactic acid and this can be obtainedeither biologically or chemically. The molecular weightand yield of poly (lactic acid) depends on purity of themonomer used. Therefore, purification of lactic acid dur-ing its production is very important for the production ofPLA with consistent properties. Another important issuewith PLA production via polycondensation reaction is thedifficulty to remove water; residual water in the poly-mer can lead to reduction in molecular weights of thefinal product. Hence, the commercial production of PLAis mostly carried out by ROP of lactides. PLA’s stereoreg-ularity makes it a highly crystalline polymer [11]. It ispossible to control this stereoisomerism by using differentcatalysts. The chiral nature of lactic acid results in dis-tinct forms of polylactide, namely, poly(L-lactide) (PLLA),poly(D-lactide) (PDLA) and poly(DL-lactide) (PDLLA) whichare synthesized from the L-, D- and DL-(racemic mixture ofL- and D-) lactic acid monomers, respectively, or from thecorresponding L,L-lactide, D,D-lactide and D,L-lactide [12].Highly crystalline PLA can be obtained with low D content(<2%), while fully amorphous PLA can be obtained withhigh D content (>20%) [11,13–15]. Semi-crystalline PLAare obtained with 2 to 20% of D content. Stereochemistrycontrols the physico-chemical and mechanical propertiesof PLA. Isotactic PLA i.e. PLLA is a semicrystalline poly-mer and melts around 180 ◦C, the melting point can beincreased to 230 ◦C by mixing equivalent proportions ofL-PLA and D-PLA which will yield crystalline stereocom-plex of PLA. Semi-crystalline PLA displays both melting(Tm) and glass transition temperature (Tg). The Tg of PLA
can range from 50 to 65 ◦C depending on its molecularweight, physical gaining, polymer architecture and degreeof crystallinity. The Tm can range anywhere between 170and 180 ◦C.
Science 38 (2013) 1653– 1689

The properties of PLA such as thermal stability andimpact resistance are inferior to those of conventionalpolymers used for thermoplastic applications. Therefore,PLA is not ideally suited to compete against the con-ventional polymers [16]. The applications of PLA can bewidened by improving its properties. To achieve this,copolymers of lactic acid and other monomers such asderivatives of styrene, acrylate, and poly (ethylene oxide)(PEO) have been developed. PLA has also been formulatedand associated with nanosized fillers. Modification of PLA,copolymerization with other monomers, and PLA compos-ites are some approaches that are being used to improve thestiffness, permeability, crystallinity, and thermal stabilityof PLA [17–20]. Considerable research is being carried outto develop and study modified PLA, PLA-based copolymers,and PLA-based composites. The manufacturing cost of PLAhas dwindled due to the advances in obtaining glucose fromcorn using bacterial fermentation. Today it is easily avail-able and cost-competitive with most of the commoditypolymers. PLA-target markets include packaging, textilesand biomedical applications [21]. Natureworks from USAis the largest PLA producer from corn, while Mitsui Chem-icals, Mitsubishi, Shimadzu, Toyota and Dainippon InkChemicals from Japan, Futerro from Belgium, Biomer fromGermany and Purac from Netherlands also produce PLA.

1.1.2. PolyhydroxyalkanoatesPolyhydroxyalkanoates (PHA) are the family of

biopolyesters which are totally synthesized by microor-ganisms from various substrates as carbon sources.Recent demonstration of PHA accretion in transgenicplants such as Arabidopsis and their expression to bac-terial PHA biosynthetic genes has led to creating costcompetitive PHA production. Also, research efforts areproceeding towards developing PHAs in transgenicplants. This group of polymers have diverse structuresand display properties accordingly. Over 150 differ-ent types of PHAs, i.e. homopolymers, copolymers,can be synthesized by employing different bacterialspecies and growth conditions. Polyhydroxybutyrate(PHB) and poly(hydroxybutyrate-cohydroxyvalerate)(PHBV) are the most well-known polymers of thepolyhydroxyalkanoates family. They are producedwhen bacteria are exposed to carbon source whileall other necessary nutrient becomes limited [22].Poly(3-hydroxybutyrate-co-3-hydroxyvalerate),poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), poly(3-hydroxybutyrate-co-3-hydroxypropionate), and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) are fourimportant copolymers produced by several bacterialstrains. These strains also use various carbon substrates inthe synthesis of the copolymers PHA synthesis in plantshas advanced considerably, recent research has identifiedthat switchgrass is capable of producing PHB as seen inFig. 3 [5].

PHAs are renewable, biodegradable and biocompati-ble; also the properties of the PHBV co-polymer can be
easily tailored by varying the valerate content. PHAs arevery sensitive to processing conditions and exhibit a verynarrow processing window. Under higher shears, they dis-play rapid reduction in the molecular weight due to chain

M.M. Reddy et al. / Progress in Polymer

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leavage, and pose problems during most of the polymerrocessing operation. They exhibit thixotropic behaviornd are very sensitive to temperature and shear. Addi-ives, blends, and composites are the most obvious wayso overcome these problems. Impact strength increasesith increasing hydroxyvalerate (HV) content, while Tm,

g, crystallinity, water permeability and tensile strengthecreases in the co-polymers [23–25]. The biodegradabilityf PHAs depends mainly on crystallinity and the polymerype; copolymers degrade faster than the homopolymers.

The diversity of PHA’s properties makes it suitable foride range applications including packaging, fibers, and

iomedical uses [22]. Also, the PHA monomers can be usedor biofuels, drugs or chiral intermediates [26]. Researchersre working on improve PHA’s mechanical, biodegradation,nd morphological properties to broaden its applicabilityn various industries. PCL [27], PLA [28], PBAT [29] andtarch [30] have been blended with PHBV for this purpose.lso, in another approach various natural fibers such asood fiber [31], bamboo fiber [32], jute [33], wheat straw

34], and coir fiber [35] have used to fabricate lightweightnd affordable composites. With the continuous develop-ent of new PHBV-based blends and composites and new

rocessing technologies, an even broader range of applica-ions are anticipated for biobased and biodegradable PHBV.lobally more than 20 companies have been established toommercialize these developments [36].

.1.3. New trend in plastics from biological sourcesA new trend is emerging in the synthesis of tra-

itional polymers by utilization of biological sources.
dvances in biotechnologies have helped in commer-ializing many polymers that are otherwise obtainedrom petroleum feedstocks. Among many polymers pos-ible, Table 1 gives a clear picture of new biobased
Science 38 (2013) 1653– 1689 1657

polymers and the leading manufacturers around theworld. This section gives an insight about the advancesin obtaining biobased poly (butylene succinate) (PBS),poly(trimethylene terephthalate) (PTT), polyethylene (PE),polypropylene (PP), polyethylene terephthalate (PET) andbiobased polyamides.

Biobased Poly(butylene succinate) (Bio-PBS): Poly(butylenesuccinate) (PBS) is generally obtained by direct polymer-ization of succinic acid and 1,4-butanediol as it is a simpleprocess that produces high molecular weight polymer[9]. Currently there are efforts to obtain succinic acid byutilizing biological feedstocks such as corn starch, cornsteep liquor, whey, cane molasses, glycerol, lignocellu-loses, cereals, and straw hydrolysates [37–39]. Bacteria’ssuch as Actinobacillus succinogenes, Mannheimia suc-ciniciproducens and Anaerobiospirillum succiniciproducensare used in the biological production of succinic acid[38]. Researchers are trying to improve the downstreamprocessing economics and also to improve the microbialconversion process in the production of succinic acid. Var-ious manufacturers are working to produce succinic acidincluding BioAmber, DSM-Roquette, BASF, Myriant Tech-nologies, and Mitsubishi chemicals. BioAmber is workingwith NatureWorks and Mitsubishi chemicals to commer-cialize biobased PBS.Biobased Poly(trimethylene terephthalate) (PTT):Poly(trimethylene terephthalate) is a non-biodegradablearomatic polyester obtained by polycondensation reac-tion between 1,3-propanediol (PDO) with terephthalicacid (TPA). Advances in obtaining cost effective PDO frombiological resources have attracted companies to producePTT via bio-routes. PDO has been successfully obtainedin aerobic bioprocessing with glucose by DuPont, Tate& Lyle and Genencor. Glucose from starch was used inthese processes. Biobased PTT has similar properties tothat of its petrochemical counterpart and displays verygood strength, stiffness, toughness and heat resistance.It is reported to have very high surface appearance andgloss [40]. It finds applications in carpets, textiles, films,packaging, and automotive and high performance appli-cations [4]. Currently, DuPont is producing biobased PTTin the trade name known as Sorona®.Biobased Polyethylene (Bio-PE) and Biobased Polypropylene(Bio-PP): Polyethylene is one of the largely used polymersin the world which is manufactured by polymerization ofethylene obtained from petroleum feedstock. However,now efforts are being made to derive this polymer frombiological resources. The conventional process of obtain-ing biobased ethylene from dehydration is being adoptedas the route to synthesize bio-PE. Bioethanol can beproduced using different biological feedstocks includingstarchy crops, sugar crops and lignocellulosic materials.The polymerization of biobased ethylene is identical to theprocess for polymerizing petrochemical ethylene, whichhas been applied at a very large scale for decades. TheBrazilian company Braskem is the first company to offer
biobased polyethylene at a commercial scale. Biobased PPcan also be obtained in similar fashion as that of PE. Itinvolves production of biobutanol and its dehydration tobutylenes and other intermediates step- to convert it to

1658M

.M.

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et al.

/ Progress

in Polym

er Science

38 (2013) 1653– 1689

Table 1New biobased polymers and the leading manufacturers.

Biobased polymer Synthesis route Manufacturer

Bio-Poly (butylenesuccinate) (PBS)

Glu cos eFermentation−−−−−−−−→Succinic acid

Succinic acidHydrogenation−−−−−−−−→1, 4 Butanediol

Succinic acid +1, 4 Bu tan ediol

Melt Polymerization−−−−−−−−−−→Poly butylene succinate [9]

1. Bio-Amber2. Mitsubishi Chemicals3. DSM- Roquette4. Showa Denko K.K (SDK)5. Myriant Technologies

Bio-Poly(trimethyleneterephthalate) (PTT)

Corn SyrupFermentation (E.coli)−−−−−−−−−−−→ 1, 3 − Propanediol

Terephthalic acid + 1, 3 −

Propanediol

CondensationPolymerization−−−−−−−−−−−−→Poly(trimethylene terephthalate)

1. DuPont

Bio-Polyethylene (PE) Glu cos eFermentation (Yeast)−−−−−−−−−−−→Etholol

Dehydration−−−−−−−−→EthylenePolymerization−−−−−−−−→ Polyethylene 1. Braskem

2. Dow Chemical – Mitsui JV(commercial production from2015) [309]

Bio-Polypropylene (PP) Glu cos eFermentation (E.Coli)−−−−−−−−−−−→isobu tan ol

Dehydration−−−−−−−−→ButylenesIntermediateSteps−−−−−−−−−→Propylene

Polymerization−−−−−−−−→Polypropylene 1. Braskem

Bio-PolyethyleneTerephthalate (PET)

(Partial biobased-PET)

Glu cos eFermentation (Yeast)−−−−−−−−−−−→Etholol

Dehydration−−−−−−−−→EthyleneOxidation−−−−−→Ethylene glycol

Terephthalic acid + Ethylene glycolPolymerizatoin−−−−−−−−→Polyethylene Terephthalate

1. Toyota Tsusho Corporation2. Futura Polyesters [310]

(100% Biobased -PET)

Glu cos eFermentation (Yeast)−−−−−−−−−−−→Etholol

Dehydration−−−−−−−−→EthyleneOxidation−−−−−→Ethylene glycolSugar

Catalytic Conversion−−−−−−−−−−→Bio paraxyleneCatalytic Conversion−−−−−−−−−−→Bio −

Terephthalic acid

Bio − Terephthalic acid + Bio − Ethylene glycolPolymerization−−−−−−−−→Bio − Polyethylene Terephthalate

1. Coca-Cola – Gevo Venture2. PepsiCo- Virent Venture [311](Not in Commercial scale yet)


right 20

11ipapeaheasdtf

Fig. 4. Thermoplastics starch formation [305]. Copy

propylene. This makes the process of obtaining bio-PP dif-ficult compared to bio-PE. Braskem has announced its planto produce bio-PP commercially by 2013 [9].Biobased Polyethylene terephthalate (PET): PET is one ofthe widely used polyesters for one trip/time packag-ing applications and is obtained by polyesterification ofterephthalic acid (TPA) with ethylene glycol. This reactioncan be easily carried out using bio derived ethylene gly-col. Biobased ethylene glycol is produced using biobasedethylene; bio-ethylene is oxidized to ethylene oxide, fol-lowed by its hydrolysis [41,42]. The biobased content willbe partial and will be according to the stoichiometry ofthe reaction. Coca-cola and other beverage companies areusing biobased PET for bottles for their products [43]. Toy-ota Tsusho Corporation, Japan and Futura Polyesters, Indiaare the producers of bio-PET.Biobased Polyamides: Polyamides are also known as nylonshave recurring amide groups [–CONH–] as an integral partof the main polymer chain. They are widely used as engi-neering thermoplastic in automotive, flexible electronics,packaging and electrical applications. Nylons are gener-ally synthesized from diamines and dibasic acids. Castoroil is used in the synthesis of biobased nylons. Nylon 6,10, Nylon 11 are partially biobased nylons whereas Nylon10,10 is 100% biobased nylon. Arkema, DSM, BASF, DuPontare the major producers of biobased nylons.

.1.4. Agro-polymers

.1.4.1. Starch. Starch is a widely used bioplastic whichs actually a storage polysaccharide in plants. It is com-osed of both linear and branched polysaccharides knowns amylose and amylopectin respectively. The ratio of theseolysaccharides varies with their botanical origin and gen-rally, native starches contain around 85–70% amylopectinnd 15–30% amylose. Starch softening temperature isigher than its degradation temperature due to the pres-nce of many intermolecular hydrogen bonds [44], whichffects its processing. Plasticizers like water, glycerol and
orbitol will help in increasing the free volume and therebyecreasing the glass transition and softening tempera-ures [45]. Thermoplastic-like material can be obtainedrom starch by disrupting its molecular interactions by
12. Reproduced with permission from Elsevier Ltd.

using plasticizers under specific conditions. Heating ofstarch granules will lead to non-irreversible transition andswelling of amorphous regions in the presence of plasti-cizer, thereby disrupting starch molecular structure [46].This process of disrupting starch molecular structure isknown an as gelatinization and the resultant materialis known as thermoplastic starch (TPS). The schematicshowing the process of obtaining TPS is shown in Fig. 4.Traditional extrusion, injection molding and compressionmolding can be used to process thermoplastic starch.The melt processing technique of obtaining thermoplasticstarch is a complex operation that involves plasticization,devolatilization, melt-melt mixing and morphology con-trol. The final morphology of TPS depends on composition,mixing time, temperature, shear and elongation rate ofthe operation. Although it is possible to make useful prod-ucts from TPS alone, extreme moisture sensitivity of starchleads to limited practical application. Therefore, the real-ity in commercialization of starch-based plastics involvesblending of TPS with other polymers and additives. Earlywork in this area was mostly symptomatic of developmentof biodegradable plastics involving starch with polyethyl-ene and other polymers. Halley and coworkers [47] havereviewed thermoplastic starch polymers and highlightedthe research in this area. It is noteworthy there are manyefforts in obtaining completely biodegradable materialsfrom starch and biodegradable polyesters including blendsof TPS with PLA, PHB, PCL, PBS and PBAT. NovamontItaly, Rodenberg Biopolymers, Netherlands, Biotec GmbHGermany, Earthshell, USA and Plantic Technologies Ltd.,Australia are the major producers of starch-based plasticsin the world. Novamont is the largest producer of starchplastics in the world.

1.1.4.2. Cellulose. Cellulose is an abundant and ubiquitousnatural polymer. It is the major structural component ofplant cells and is found throughout nature. It is widelyused in industrial applications in different forms. Cellu-
lose is mostly obtained from wood and cotton at presentfor many applications; on the other hand, cellulose pulp isalso being extracted from agricultural byproducts such asbagasse, stalks and crop straws.

Polymer Science 38 (2013) 1653– 1689

Chitin

Chitosan

O

H

H

NHCOCH3

CH2OH

H

OH

O

O

OH

H

H

NHCOCH3

CH2OH

HH

O

n

O

H

H

NH2

CH2OH

H

OH

O

O

OH

H

H

NH2

CH2OH

HH

OH

n


Naturally occurring cellulose is a polydisperse lin-ear homogeneous polysaccharide based on �-l,4-D-glucopyranose repeat units, with an average degree ofpolymerization of 3000 to 1500 depending on the source[48]. Currently, cellulose based materials are used in twoforms on an industrial scale [23].

1. Regenerated cellulose used for fiber and film productionand cannot be melt processed.

2. Cellulose esters used in broad array of applicationsincluding coatings, biomedical uses and other usual plas-tic applications.

Cellulose esters are obtained from esterification of cel-lulose; cellulose acetate (CA) is product of esterificationreaction between cellulose and acetic anhydride. Celluloseacetate butyrate (CAB) and cellulose acetate propionate(CAP) are obtained with esterification reaction with appro-priate acids and anhydrides.

Non-plant resources can also be used to producecellulose, especially bacteria and tunicates. There is aconsiderable interest in obtaining cellulose from bacte-ria, popularly known as bacterial cellulose. Acetobacterxylinum produces this cellulose under unique culturingconditions to form a fibrous network [49]. Acetobac-ter xylinum produces cellulose with good mechanicalstrength and biodegradability. ‘Nanocellulose’ comprisesof fibrous or crystalline units of cellulose between 5 and500 nm in diameter with a length in several micrometers.Nanocellulose is available in two forms i.e. microfibrils andnanowhiskers [50]. Detailed discussion on nanocellulose isgiven in Section 2.1.

1.1.4.3. Chitin and Chitosan. Interest in these polymersis driven by their unique properties such as renewable,biocompatible, biodegradable and non-toxic with excel-lent adsorption properties [46]. Chitin is an abundantlyavailable natural polysaccharide and is the supportingmaterial in many invertebrate animals such as insects andcrustaceans. The monomers in chitin are 2-acetamido-2-deoxy-�-D-glucoses which are attached t � (1→4) linkagesand this polymer degrades by chitinase. The deacetylatedchitin is known as chitosan, this deacetylation is more than50% [51]. The chemical structures of these polymers areshown in Fig. 5. Chitosan is a semi-crystalline polymer andits crystallinity depends on the extent of deacetylation [52].Chitosan is receiving more attention as a possible polysac-charide resource for biomedical applications [52].

The process of obtaining chitin from the shells of crab orshrimp starts with the extraction of proteins followed bytreatment with calcium carbonate for dissolution of shells.The chitin obtained from this process is then deacetylatedwith 40% sodium hydroxide for 1–3 h at 120 ◦C. This yieldsa 70% deacetylated chitosan [46]. The molecular weightof chitosan depends on the source and it varies from 100to 1100 kDa [53]. Commercial chitosan have around 50 to90% deacetylation degree [52]. Chitosan has been exten-
sively explored for films and fibers [51]. Fibers from thesepolymers are very useful as wound dressing materials andabsorbable sutures [53,54]. They have generated interestin biomedical applications [52]. Lysozyme studies have led
Fig. 5. Structure of chitin and chitosan.

to interest in chitosan as it is present in human body fluids[55]. Porous structures that can be used in the tissue regen-eration and cell transplantation can be easily obtainedfrom chitosan. Freezing and lyophilizing chitosan-aceticacid solution in suitable molds leads to porous structures[56]. More details are discussed in Section 5.3.

1.1.4.4. Proteins. A protein is random copolymer of dif-ferent amino acids. Based on the origin proteins can beclassified as plant proteins (e.g. soy, pea, canola, wheatprotein) and animal proteins (e.g. gelatin, whey, casein andkeratin). Generally, proteineous biomaterial can be definedas a stable 3D polymeric network which is strengthenedby hydrophobic interactions and hydrogen bonding.[57].Unfolding and realigning of the proteins is necessaryto produce useful biomaterials. Hydrophilic compoundsand lipid compounds are used for plasticization of proteins.Water, glycerols, fatty acids and oils are commonly usedplasticizers for proteins. Plasticizers reduce interactionsbetween functional units and improve the polymeric chainmobility and intermolecular spacing. This also results inreduction in glass transition temperature of the proteins.Wet processing and dry processing are used to obtainbiomaterials from proteins [58]. Wet processing involvessolubilization and dispersion of proteins in solvent. Thedissolved protein is casted to obtain films by drying thesolvent. Dry processing is the more conventional methodin which proteins are mixed with suitable additives fol-lowed by thermo-mechanical processing by conventional
techniques such as extrusion and/or molding. Althoughboth animal and plant based proteins are used in designingmany non-food applications, these proteins plastics have

Polymer Science 38 (2013) 1653– 1689 1661

na

cabtibepaMc

2

bbono

1234

2

aatommnmcsdtcibaarrt[liurr

da

Fig. 6. Schematics of (a) single cellulose chain repeat unit, showing thedirectionality of the 1–4 linkage and intrachain hydrogen bonding (dot-

M.M. Reddy et al. / Progress in

ot progressed significantly towards commercialization at large scale.

Gelatin is water soluble protein and is obtained fromollagen; its ability to form transparent gels has gener-ted lot of interest in researchers [59–61]. This protein haseen commonly used in food and pharmaceutical applica-ions. [62]. Apart from these applications, due to its almostdentical composition as that of natural collagen [63], it iseing explored as a scaffolding material for many tissuengineering applications. Also, the denatured state of thisrotein can circumvent the concerns of immunogenicitynd pathogen transmission associated with collagen [63].ore details on gelatin based bionanocomposites are dis-

ussed in Section 5.3.

. Nanofillers for bionanocomposites

Bionanocomposites which are obtained from 100%iobased materials, in which the fillers and the matrixoth are obtained from renewable resources, are the focusf this review paper. This section presents three differentanofillers which are extensively used for the preparationf bionanocomposites:

. Cellulose based nanofillers

. Carbon nanotubes

. Nanoclays

. Functional nanofillers

.1. Cellulose based nanofillers

As discussed in earlier sections, cellulose is a widelyvailable and low cost material which is both renew-ble and biodegradable. These attributes combined withheir environmentally benign nature makes the nanofibersbtained from cellulose very attractive for use as reinforce-ents in the preparation of bionanocomposites. Celluloseicrofibrils and nanocrystalline celluloses or cellulose

anowhiskers (CNW) are the two types of nanoreinforce-ents obtained from cellulose [50]. Cellulose microfibrils

onsist of bundles of molecules that are elongated andtabilized through hydrogen bonding [50,64]. The typicalimensions of these nanofibrils are 2–20 nm in diame-ers, while lengths in micrometer range. Also, these fibrilsonsist of both amorphous and crystalline regions. Thiss shown in Fig. 6. The crystalline regions can be isolatedy various techniques and resultant material is knowns whiskers. These whiskers are also known as nanorodsnd nanocrystals. The length of these whiskers typicallyange from 500 nm to 1–2l m in length and diameter in theange of 8–20 nm [65]. Also, it was found cellulose crys-als have a modulus around 150 GPa and strength is 10 GPa66]. This is very interesting data as it suggests that cellu-ose can replace single-walled carbon nanotubes (SWCNTs)n many applications. Acid hydrolysis is the most widelysed method for extracting cellulose nanowhiskers, whichemoves the amorphous regions while crystalline regions
emain intact [67].
The properties of the cellulose based nanocompositesepend on the dimensions and consequent aspect ratioss well as mechanical and percolation effects [68,69].

ted line), (b) idealized cellulose microfibril showing one of the suggestedconfigurations of the crystalline and amorphous regions, and (c) cellulosenanocrystals after acid hydrolysis dissolved the disordered regions.

Research has indicated that the tensile properties andtransparency of the nanocomposites increases with theaspect ratio of the cellulose nanowhiskers [70,71]. Also,Kvien and Oksman [72] showed that the tensile propertiesalso depend on the orientation of the nano fibers. However,Jiang et al. [71] showed that filler orientation and distri-bution plays a important role in the realizing the meanaspect ratio. Hence, the actual mechanical properties ofthe filled systems can differ by large extent when fillers donot follow a symmetric distribution and that those follow.The maximum improvement in properties of the compos-ites occurs when there are just enough filler in the matrix,so that they can form a continuous structure. This is alsoknown as percolation threshold [66,73], the enhancementis influenced by the proper dispersion of filler within thematrix. In other words, improvement occurs when eachfiber is in contact with two or more fibers[38]. It wasobserved by Chakraborty et al. [73] that a 2.5-fold increasein PVOH modulus with 5% fibers, but after this any addi-tion of fibers has not improved the modulus. The resultwas due to high aspect ratio was observed at 5% leading toimprovement in the properties in the final compositions.The hydrophilic surface of the cellulose based nanore-inforcements leads to poor interaction between matrixand the filler [74]. Furthermore the chemical compati-bility is very important in controlling the dispersion and
the adhesion among them. Therefore, it is common to seeweak filler–matrix interactions when hydrophilic whiskerswere added to hydrophobic matrices [68]. Similarly, thehigh polarity of cellulose surface leads to certain problems


tion patn from

Fig. 7. (a) TEM images (height) and (b) their corresponding WAXS diffracand annealing process [306]. Copyright 2012. Reproduced with permissio

when added to non-polar polymer matrices. These prob-lems include weak interfacial compatibility, poor moisturebarrier properties and aggregation of fiber by hydrogenbonding [75]. The extremely hydrophilic surface of cel-lulose fibrils leads to poor moisture resistance propertieswhich is not desired in many potential applications [68].Fig. 7 shows the report the TEM images of CNWs before andafter silanization. Bondeson and Oksman [74] investigatedthe potential use of PVOH for improving the dispersibilityof cellulose nanowhiskers in PLA matrix. However, it wasrevealed that two immiscible phases were formed whenPVOH and PLA were blended in presence of whiskers.PVOH was discontinuous phase where most of the whiskerswere dispersed while PLA was continuous phase. Hence,both thermal and mechanical properties did not showany improvements compared to their matrix. The misci-
bility of cellulose nanofillers with hydrophobic matricescan be improved by various surface modifications. Ester-ification and acylation of the cellulosic fibers will lead tothe improvement in interfacial compatibility with polymer
terns of methacrylic-based CNWs ([MPS]0 = 100 mM) after freeze-dryingElsevier Ltd.

matrix. This results in the enhancement of both mechanicaland thermal properties. Also, these surface modificationshave lead to the reduction in water uptake capacity [75,76].Cellulose whiskers with surfactant coating has lead tothe improvement in interactions with matrix and therebyenhancing percent elongation [77]. In a study by Grunertand Winter [78] it was revealed that the weight of modifi-cation groups have a negative impact on reinforcing effectsof nanowhiskers. When trimethylsilylated whiskers werecompared with unmodified whiskers, the unmodified cel-lulose whiskers showed a higher reinforcing effect. Thereduction in reinforcing effect was attributed to higherweight of whisker arising from silyl groups which has ledto the restricted filler/filler interactions.

2.2. Carbon nanotubes

Carbon nanostructures including fullerene (bucky-balls), carbon nanotubes (single wall and multi wall),carbon nanofibers, carbon nanoparticles and graphene

Polymer

neefshnnacmScgaamonpbvcimc(ramssvnfrbmpFsmet

paeppttracteAfrp

(


anosheets have been widely investigated due to theirxcellent physicochemical, mechanical and electrical prop-rties [79–81]. The allotropic behavior of carbon arisesrom different bonding states representing sp3, sp2, andp hybridization. In general, the degree of carbon bondybridization-n (spn) determines the structure of carbonanomaterials and their functional properties [79]. Carbonanotubes have been synthesized using many methods,mong them (i) arc-discharge, (ii) ablation using laser (iii)hemical vapor deposition, (iv) high pressure of carbononoxide are found to be most popular [82]. In 1991,

umio Iijima [83] first demonstrated the arc discharge pro-ess for the fabrication of carbon nanotubes using tworaphite rods with different potentials, which is kept inn argon filled enclosure. The optimum distance betweennode and cathode caused the formation of arc. After ainute, carbon nanotubes can be collected on the cath-

de. Laser ablation was first used to grow high qualityanotubes by Guo and co workers in 1995 [84]. In thisrocess, a high intense laser pulse is used to ablate a car-on target, which is kept in argon filled tube-furnace atery high temperature of 1200 ◦C. Carbon nanotubes can beollected as deposition on the furnace wall [84]. In chem-cal vapor deposition, a substrate coated with metal or

etal oxide catalysis is heated up to 700 ◦C. Growth ofarbon nanotubes occurs when the gaseous hydrocarbonacetylene or methane) passes through along with the car-ier gas (nitrogen or hydrogen or argon) [85]. An addeddvantage of chemical vapor deposition is the control ofulti/single walled architectures in nanotubes through the

uitable pretreatment of the substrate surface [86]. Con-tructive developments have been made on the chemicalapor deposition process for the development of carbonanotubes, which promoted this process as a common one

or industrial purposes. Again in 1999 Nikolaev and hisesearch team [87] developed a new method for the car-on nanotubes synthesis by using high pressure carbononoxide as a carbon source and named it as HiPco (High

ressure carbon monoxide method). In this process he usede(CO)5 as a catalyst, and was able to obtain high qualityingle-walled carbon nanotubes. Among all of the aboveentioned techniques, chemical vapor deposition has been

stablished for industrial scale synthesis of carbon nano-ubes with desired structures.

Carbon nanotubes find many applications that includeolymer nanocomposites, electrochemical energy stor-ge/conversion, catalysis, hydrogen storage, health andnvironmental hygiene products and electronics [88]. Inolymer nanocomposites they have been utilized as higherformance functional fillers, which not only improvedhermal/mechanical performance but also provided addi-ional functionalities such as fire retardant, moistureesistance, electromagnetic shielding and barrier perform-nces [89]. The field of polymer nanocomposites witharbon nonmaterial has highly diversified; carbon nano-ubes (CNTs) based polymer composites have been widelyxplored in many aspects as described elsewhere [90].
n exponential growth on polymer nanocomposites rein-
orced with carbon nanotubes occurred after the firstesearch work published by Ajayan et al. [91]. This is hap-ening not only because of their well studied properties

Science 38 (2013) 1653– 1689 1663

but also due to the advancement in recent years on theirfabrication techniques at industrial scale [92]. Especiallyfor polymeric composites, carbon nanotubes impart severaladvantages:

(i) versatility as reinforcement in both thermoplastic andthermoset regime [90]

(ii) extremely high theoretical/experimental tensilestrength (150–180 GPa) and modulus (640 GPa to1 TPa) [90]

iii) one dimensional electronic structures, which enablesignificantly non scattering electron transport [93]

(iv) their compatibility with other chemical compounds,metal/metal oxides/chalcogenides nanoparticles andpolymeric materials [89]

Researchers have effectively utilized these advantagesof carbon nanotubes for the development of various com-posites systems from renewable resource based polymericmaterials. Cao et al. reported the fabrication of plas-ticized starch/multiwalled carbon nanotubes (MWCNTs)composites with improved thermal, mechanical and waterabsorption properties [94]. Improved performance of PLAbioplastic was reported by many researchers by reinforcingit with single wall and multiwall carbon nanotubes [95,96].Carbon nanotubes have also been utilized for the enhance-ment of polyurethane foams. For example, Liang et al.[97] reported ∼25–30% property enhancement of soybasedpolyurethane foam reinforced with 0.1–1% carbon nano-tubes. A recent study on the carbon nanotube/epoxy soyoil composites was performed by Thielemans et al. [98]in order to evaluate the ability of MWCNTs dispersionwith their increasing content. Their research investigationfound that composites displayed significant improvementsin mechanical properties even at very low amounts ofreinforcement of 0.28 wt%. They also indentified that theaddition of a higher weight fraction of carbon nanotubes toepoxy systems caused the agglomeration of fiber bundles.

Reinforcement of carbon nanotubes into various poly-mer systems not only provides improvement in mechanicaland thermal properties but also creates additional func-tional properties. Hapuarachchi et al. [99] demonstratedmultiwall carbon nanotubes as a successful flame retar-dant agent for PLA and their natural fiber reinforcedbiocomposites. They found a 58% reduction in heat releas-ing rate (HRR) due to the addition of multiwall carbonnanotubes compared to virgin PLA. The development ofhigh aspect ratio carbon nanotubes with better conduc-tive properties created a new application for their polymercomposites in electromagnet interference shielding andnumerous reports were published on this aspect in dif-ferent thermoplastic and thermoset platforms. Thomassinet al. [100] demonstrated the exceptional electromagneticinterference (EMI) shielding properties of MWCNT rein-forced poly(�-caprolactone) nanocomposites. However,such research on renewable resource based biopoly-mer/CNT composites has not been extensively explored.

One of the critical issues in the fabrication ofCNT/polymer nanocomposites is their dispersion in apolymer matrix, as CNTs have the tendency to formagglomeration and remains in bundles because of their

1664 M.M. Reddy et al. / Progress in Polymer

Fig. 8. SEM images of fracture surfaces indicates the (a) Nonuniform dis-persion of pristine SWNT and (b) Uniform dispersion of functionalized

coordinated atoms amalgamated to edge-shared octahe-

SWNT in epoxy matrix [105]. Copyright 2003. Reproduced with permis-sion from the American Chemical Society.

high aspect ratio and strong van der Waals interac-tions [101]. In order to enhance their solubility, variousattempts have been made: (i) oxidation of their surface byacid treatment [102] (this will generate the carboxyl andhydroxyl groups), (ii) creation of long alkyl/polymer chainsor biomolecules onto carbon nanotubes [101] and (iii)physical treatments of sonochemical oxidation and plasmamodification [103,104]. An effective improvement on thesurface functionalities of carbon nanotubes and their dis-persion (Fig. 8) through fluorination was reported by Zhu
[105]. Eitan et al. [106] demonstrated the functionalizationof carbon nanotubes with epoxide-based functional groupsfor the effective reinforcement of the polymer matrix.
Fig. 9. HRTEM images of the Ag decorated CNTs and the corresponding EDX speLtd.

Science 38 (2013) 1653– 1689

Quantitative titrations were performed in order to con-firm the attachment of epoxide-based functional groupson carbon nanotubes. Tseng et al. [103] effectively demon-strated the plasma modification of carbon nanotubes forthe fabrication of covalent-integrated epoxy composites.They also reported the improved mechanical and con-ductivity properties through the reinforcement of plasmamodified carbon nanotubes [103]. These functionalitieswill enable the compatibility of carbon nanotubes with var-ious polymeric matrixes and enhance the effectiveness ofload transfer between polymer matrix and carbon nano-tubes.

Recent interest on carbon nanotubes has turned tocombinations with various types of inorganic nanomate-rials (metal/metaloxides/chalcogenides) and the resultantpolymeric composites show additional properties, whichenable diversified applications [107,108]. Zhao et al. [109]reported the microwave absorbing property of epoxycomposites reinforced with functional carbon nanotubescombined with Ni and Ag. Their extensive research foundthat the microwave absorption of metal coated CNTs/epoxycomposites was contributed by both the dielectric andmagnetic losses of composite material. Ma et al. [108] foundthat the incorporation of carbon nanotubes filled with sil-ver nanoparticles into polymer matrices enhances theirelectrical conductivity significantly. Such high conductingpolymer composites have potential applications in elec-tronic industries. A study by Liu et al. [110], provides theinformation on the functionalization of carbon nanotubeswith NiFe2O4 as shown in Fig. 9 and their enhancementof electrical conductivity. Such innovative developmentswill provide the opportunity for carbon nanotubes ashigh performance multi-functional fillers for new biobasedcomposite systems.

2.3. Nanoclays

Layered silicates also known as nanoclays are mostcommonly utilized nanofillers in the synthesis of poly-mer layered silicate nanocomposites. Among these layeredsilicates, phylloscilicates (2:1) are extensively used inpreparing clay based nanocomposites. The crystal arrange-ment in the silicate layers is made up of two tetrahedrally

dral sheets. These sheets are made up either magnesiumor aluminum hydroxide as seen in Fig. 10.These layershave thickness of 1 nm and their tangential dimensions

ctrum [108]. Copyright 2008. Reproduced with permission from Elsevier


Table 2Structural characteristics of principal 2:1 layered silicatesa [114]. Copyright 2003. Reproduced with permission from Elsevier Ltd.

2:1 Phyllosilicates General formula CEC (meq/100 g) Aspect ratio

Montmorillonite Mx(Al4−xMgx)Si8O20(OH)4 110 100–150Hectorite Mx(Mg6−xLix)Si8O20(OH)4

Saponite MxMg6(Si8−xAlx)O20(OH)4

a M: monovalent cation; x: degree of isomorphous substitution (between 0.5 a

Fd2

rdpaTrgptcacabce

alkylammonium chain lengths showed that higher the

ig. 10. 2:1 layered silicate structure (T, tetrahedral sheet; O, octahe-ral sheet; C, intercalated cations; d, interlayer distance) [114]. Copyright008. Reproduced with permission from Elsevier Ltd.

ange from 300 A to a few microns. The variation in theimensions depends on clay source, particulate silicate andreparation technique. Hence, these layers have a very highspect ratio (length/thickness) and surface area [111–114].he van der Waals gap between these layers is due to theegular stacking of the layers; this gap is known as interallery spacing. The negative charges generated by isomor-hic substitution (for e.g. Mg2+ replaced by Li1+) withinhe layers are countered by inter-gallery alkali and alkalineations. Generally, a moderate surface charge is found bessociated with these layered silicates which is known asation exchange capacity (CEC). CEC is commonly denoted
s mequiv/100 g [109]. Also, CEC is not a constant value,ut varies with layers and generally average values areonsidered. The key structural properties of major 2:1 lay-red silicates are given in Table 2. The end properties of
Fig. 11. Schematic picture of an ion-exchange reaction [307]. Copy

120 200–30086.6 50–60

nd 1.3).

nanocomposites are influenced by the dispersibility of sil-icates into their individual layers in the matrix.

The dispersibility of layered silicates into individuallayers is governed by its own ability for surface modifica-tion via ion exchange reactions that can replace interlayerinorganic ions with organic cations. Renewable polyestersare mostly organophilic compounds, while the pristinesilicate layers are miscible only with hydrophilic poly-mers. The silicate layers can be made miscible withhydrophobic polymer by introducing/exchanging inter-layer cations galleries (Na+, Ca2+, etc.) of layered silicateswith organic compounds. Generally, phosphonium orammonium cations with at least one long alkyl chain areused for this purpose as shown in Fig. 11. Ion exchangesilicates with organic cations results in an increased inter-layer spacing due to bulkiness of alkyammoniums. Thesealkyammoniums cations can even provide certain func-tional groups that can initiate polymerization in presenceof monomers and also they can compatibilize polymermatrix [115]. The conducive chemical environment in theinterlayer helps in the intercalation of polymer chains[116]. Conversely, the organic modification improves bothcompatibilization between hydrophilic clay and hydropho-bic polymer matrix and also increases interlayer spacing[117].

Moreover, organic cations can be used as silicate-attached initiators or mediators for polymerization, thusproviding a mechanism for improving interfacial adhesionbetween matrix and the silicate and a route for effec-tive stress transfer [118]. The separation between claylayers increases with the surfactant chain and chargedensity of the clay. A study on organoclays modified with

chain alkyamine chain length greater is the improvementin the interlayer spacing [119]. This study also foundthat when same nanoclay was organically modified with

right 2003. Reproduced with permission from Elsevier Ltd.


Table 3Commercial (O) MMT and their characteristics [21]. Copyright 2009. Reproduced with permission from Elsevier Ltd.

Commercial clays Clay type Organomodifier type Modifier concentration (meq/100 g) w (%)a d-spacing (Å)

Supplier/trade name/designationSouthern Clay Products (USA)Cloisite®Na CNa MMT – – 7 11.7Cloisite®15A C15A MMT N+ (Me)2(tallow)2 125 43 31.5Cloisite®20A C20A MMT N+ (Me)2(tallow)2 95 38 24.2Cloisite®25A C25A MMT N+ (Me)2(C8)(tallow) 95 34 18.6Cloisite®93A C93A MMT NH+(Me)(tallow)2 90 37.5 23.6Cloisite®30B C30B MMT N+ (Me)(EtOH)2(tallow) 90 30 18.5

Süd-Chemie (Germany)Nanofil®804 N804 MMT N+ (Me)(EtOH)2(tallow) 21 18

Laviosa Chimica Mineraria (Italy)Dellite® LVF LVF MMT – 105 4–6 9.8Dellite® 43B D43B MMT N+ (Me)2(CH2-Ø;)(tallow) 95 32–35 18.6

CBC Co. (Japan)Somasif MEE SFM N+ (Me)(EtOH)2(coco alkyl) 120 28

hydroxide (LDH) is generally referred to as a naturallyoccurring mineral, and chemically described as [MII xMIII1 − x(OH)2] intra [Am-x/m .nH2O] inter, where MII and

MAE SFM N+ (Me)2(tallow)2

Tallow: ∼65% C18; ∼30% C16; ∼5% C14.a %Weight loss on ignition.

alkyammoniums chain with 12, 16, 18 and 20 carbonatoms, the increase in the interlayer spacing increasedwere found to be 1.36, 1.79, 1.85 and 2.47 nm, respectively.These clays that are modified with organic ions are alsoknown as organomodified layered silicates (OMLS). Thisway, it is feasible to compatibilize both polymer and thesilicates and thereby improving the end properties ofthe nanocomposites. Table 3 presents the characteristicfeatures of commercially available nanoclays.

2.4. Functional nanofillers

Nowadays, medical implants are very common inpractice that utilize a wide range of biocompatiblematerials such as metal, alloys, ceramics, polymers andcomposites [120]. Among them, bionanocomposites thatare fabricated using the combination of biopolymers andvarious nanostructured inorganic/organic functional fillersreceive extensive attention due to their diversified biomed-ical as well as biotechnological application [121]. Nano-structured fillers play an important role in biocompositefabrication, since they bring various desired function-alities to the composites [121]. Functional nano fillerssuch as cellulose nanofibers, hydroxyapatite (HAp), layereddouble hydroxides (LDH), silica nanoparticles, polyhedraloligomeric silsequioxanes (POSS) are most investigated forthis proposes [121,122]. Recently, hydroxyapatite and lay-ered double hydroxides have received more attention dueto their versatility in the fabrication of various nanocom-posites for biomedical application [121].

Hydroxyapatite is a well known bioactive and bio-compatible ceramic found in bones and teeth, whichexhibits other advantages of osteoconductivity, and non-toxic/non-inflammatory/non-immunogenic behaviors[123,124]. Hydroxyapatite is chemically represented as
Ca10(PO4)6(OH)2 with hexagonal crystal structure and it isschematically shown in Fig. 12 [125]. Functionality of HA isdetermined by various properties such as crystal structure(shape, size, crystalinity), morphology, thermal stability,
120 41

and solubility. Synthesis route influences all these proper-ties [123]. Thus various processes such as hydrothermal,sonochemical, solgel and emulsion have been investigatedand reported for the synthesis of hydroxyapatite withcontrolled morphology [123,126–128]. Layered double

Fig. 12. Schematic representation of hydroxyapatite and layered doublehydroxide chemical structure [129]. Copyright 2001. Reproduced withpermission from the American Chemical Society.

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III- metal cations; A – anion; intra and inter- intralayeromain and the interlayer space [129]. The positivelyharged layers of LDHs are counterbalanced by exchange-ble interlayer anionic species. A schematic representationf LDH structure is shown in Fig. 12 [129]. LDH exhibitsositively charged host lattices and this unique featurenables an interesting layer and interlayer chemistry,hich can be tuned to various applications [130]. The chal-

enging issue in the fabrication of LDH nanocompositess tailoring the interfacial interactions [130]. A new classf intercalation compounds is constituted from variousypes of charged polysaccharides with various potentialpplications including biomedical and electrochemicalensors [131].

. Nanocomposites from renewable resources

Expansion of nanotechnology in recent years has influ-nced the scientific, technical and economical competitive-ess of renewable resource-based polymers in developing

range of high performance engineering/consumer prod-cts [132]. Recently scientists/engineers from academiand industries are investigating the use of nanostruc-ures (cellulose nanostructures, carbon nanotubes andano clays) as reinforcements in order to produce aew class of bionanocomposites [133,134]. The inher-nt properties of these nanoparticles in enhancing thehermal, mechanical, dimensional stability and otherypes of functional properties (electrical/electromagnetichielding/barrier/fire retardant/triggered biodegradabil-ty/solvent resistance) of the composite materials with thedded advantages of eco friendliness were effectively uti-ized to create new class of materials. They also providedditional advantages like ease to process, transparent,ow density and recyclable [50,132]. These nano enabledunctional properties are achieved by those biobased nano-tructures due to their larger surface to volume ratio,mproved surface reactivity at nano regime and molecu-ar level distribution in the polymeric matrices [50]. Manyf these properties are precisely controlled by the sizend the morphology of reinforced nanomaterials, whichill enable tuning of desired properties for high per-

ormance applications including biomedical, sensor, andransducer, with relatively low concentration [131]. Inddition to materials development, deeper research shoulde focused on diversified application fields with prospectsor commercialization of these novel bionanocomposites.ost reduction and awareness of toxicological and envi-onmental impacts of these nanocomposites should alsoe addressed [132].

In this review bionanocomposites are classified in to fol-owing major categories depends on the polymer matrixuch as (i) nanocomposites based on starch plastics[135],ii) nanocomposites using polylactic acid (PLA), whichs synthesized using biomass-derived monomers [136],iii) nanocomposites fabricated using polymers produced

icro-organisms (polyhydroxyalkanoates (PHA)) [137]
nd (iv) nanocomposites formulated using petroleumased biodegradable plastics. Further it can also be clas-ified in to various classes depending of the reinforcedanomaterials [21,138]. This section describes a few
Science 38 (2013) 1653– 1689 1667

important categories of bionanocomposites based on thereinforcements that are fabricated from various biobasedplastics.

3.1. Cellulose nanocomposites

The importance of cellulose nanocomposites arises fromthe unique properties of cellulose nanomaterials such asstructural stability against various processing windows,excellent mechanical properties in terms of Young’s mod-ulus (about 138 GPa) compared to other lignocellulosicnatural fibers (35–45 GPa for flax fiber) and very lowcoefficient of thermal expansion in longitudinal direction(10-7 K-1) [139,140]. Structures of cellulose nanofibersare stabilized by hydrogen bonds with high levels ofcrystallinity, which makes cellulose nanofibers an idealreinforcing material in polymeric materials. In general,cellulose nanocomposites made using renewable resourcebased biopolymers exhibit superior thermal, mechani-cal and barrier properties with minimum reinforcement(∼5 wt%) compared to macro reinforcements with theadded advantages of recyclability and biodegradability[141].

Biopolymers of polysaccharides and proteins have beenwidely investigated due to their advantages of eco friendli-ness and biodegradability [142]. The major challenge fortheir sustainability is their poor moisture resistance athigher humidity conditions. Incorporation of nanostruc-tured materials with high aspect ratio as reinforcementsreduced water-permeability compared to virgin matrix[143]. In contrast, use of cellulose nanostructures in tovast ranges of polysaccharides and protein biopolymersresult in enhanced moisture resistance without com-promising their immense advantages of biodegradability[142,143]. Processing of starch-based materials with nanostructured cellulose by traditional melt processing isalways critical as the agglomeration of fiber materialshas to be presented/controlled [144]. Very few researchreports are available on conventional melt processing ofstarch/cellulose fiber nanocomposites. Teixeira et al. [143]reported the incorporation of nanofibers derived from cas-sava bagasse into cassava based starch thermoplastic. Theywere successful in employing a torque rheometer to studythe effective dispersion of nanofibers in starch matrices.They also investigated the melt processing of thermoplas-tic corn starch with cotton derived cellulose nanofibersusing a twin-screw extruder [144]. Their studies indicatedthat the existence of residual sugars and the plasticizationof starch had heavily influenced the performance of thenanocomposites. Solution casting of thermoplastic starchwith cellulose nano fibrils derived from various renewableresources like wheat straw and flax was also explored forthe fabrication of novel green nanocomposites [145,146].

Cellulose based nanostructures has been utilized forimproving PLA’s. Microcrystalline cellulose (MCC) is thebest source for cellulose nanowhiskers; however, disper-sion of MCC into individual fibers is difficult especially
in melt processing. Mathew et al. reported that the rein-forcement of MCC in PLA processed through twin screwextrusion resulted in the retention of MCC as bundlesand reduced in the mechanical properties [147]. However,

Polymer Science 38 (2013) 1653– 1689
Nakagaito et al. [148] experimented with the reinforce-ment of microfibrillated cellulose (nano to submicron widefibers) in PLA through a solvent casting process, and con-cluded that cellulose was a promising reinforcing material.In order to address the challenges in melt compound-ing of cellulose nanofiber/nanowiskers, Oksman et al.[149] developed a novel technique for the industrial scaleprocessing of cellulose nanocomposite. They were usingN,N-dimethylacetamide (DMAc) and LiCl as separationagent. It was found that the mechanical properties werenot increased compared to virgin PLA, which is due tothe presence of additives and unsuitable processing tem-peratures. Reinforcement of cellulose nanostructure intoPLA creates a diversified impact that includes improve-ment in barrier properties, nucleation effects and foamformation. Sanchez-Garcia et al. [150] reported the bar-rier properties of PLA/cellulose nanowhisker composites.Their research ensured that the addition of 3 wt% cellulosenanowhiskers into PLA was able to reduce the water andoxygen permeability by 82% and 90%, respectively. Cellu-lose nanowhiskers acted as shield in PLA and caused thecrystallinity development, which resulted in high barrierproperties [150]. Effects of microfibrillated cellulose-reinforcement in PLA on crystallization were studied bySuryanegara et al. [151]. They found that the microfibrilacted as nucleating agent and altered the crystallizationbehavior of PLA, which resulted in the enhancement ofstorage modulus up to 1 GPa [151]. Boissard et al. demon-strated the water functionalized microfibril cellulose as afoaming agent, and the formation of foamy PLA biocom-posites with the lowest density of 0.49 g/cm3 [152]. Watermolecules absorbed in the microfibril cellulose were con-verted into vapor during the melt processing, serving as ablowing agent and causing the foam formation [152]. Cel-lulose nanofibers that are produced by bacteria have beeneffectively used to tailor the surface of natural fibers forthe development of hierarchical reinforcement as shownin Fig. 13. Pommet et al. [153] developed a novel methodto modify the surface of natural fibers with nanosized bac-terial cellulose though the high number of hydrogen bonds[153]. Furthermore, the reinforcement of modified natu-ral fibers into PLLA created the new class of hierarchicalcomposite with the enhanced properties [153].

The technical challenges for PHA are its narrowprocessing window and high brittleness. This has beenaddressed by forming the copolymer of with valerateresulting in PHBV that exhibit lower crystallinity [154]. Thelower crystallinity of PHBV can be addressed by reinforcingit with nanostructured materials that can act as nucle-ating agents and result in enhanced crystallinity. Amongthem, cellulose nanostructures play a remarkable role inenhancing the performance of PHBV. Nanocomposites ofPHBV/cellulose nanostructures were first reported by Jianget al. using cellulose nanowhiskers as reinforcing material[32]. In this study, a solution casting process was used tofabricate PHBV/CNW composites and the obtained resultswere compared with the extrusion/injection molded sam-
ples. The extrusion process caused CNW agglomerationand resulted in reduced properties compared to the com-posite samples prepared using solution casting [32]. Tenet al. [155] reported the solution casting processing of
Fig. 13. (a) Photograph of sisal fiber modified with bacterial cellulose and(b) SEM micrographs of sisal fiber with bacterial cellulose [153]. Copyright2008. Reproduced with permission from the American Chemical Society.

PHBV/CNW nanocomposites using polyethylene glycol(PEG) as a compatibilizer between fiber and matrix. Theyconfirmed the nucleating effect of cellulose nanowhiskerand their positive impact in increasing their mechani-cal properties [155]. Effects of CNW on other propertiesof PHBV such as dielectric and rheology were recentlyreported by Ten et al. [156]. The agglomeration of cellu-lose nanowhiskers in PHBV/CNW reflects in the reductionof real permittivity. In addition the rheological analysis ofPHBV/CNW composites exhibits a lower transition point(1.2 wt%) due to the possibility of PHBV-CNW network for-mation without geometrical overlapping [156].

3.2. CNT Nanocomposites

In general carbon nanostructures (including carbonnanotubes, nanofibers and graphene sheets) exhibit arange of properties, such as higher electrical conductivity,enhanced mechanical strength and better thermal behav-ior, which make them desirable reinforcing materialsfor polymer nanocomposites [157]. Especially carbonnanotubes receive more attention due their uniquenessin efficient load transfer, which arises from the enormoussurface area and high aspect ratio. Mechanical behaviorof carbon nanotubes composites is influenced by carbon
nanotubes type, geometry of the carbon nanotubes andpolymer nature (amorphous/crystalline/semicrystalline),and the processing method (in situ polymerization/solventcasting/melt processing) [158]. In addition, other key

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actors in improving the properties of CNT based compos-tes are the adhesion between carbon nanotubes and theolymer matrix, which can be tuned by imparting differ-nt functional groups on the walls of carbon nanotubes159–161]. The homogenous dispersion of nanotubes inhe polymer matrix also helps in improving the proper-ies [159–161]. In bioplastic based composites, carbonanotubes play a vital role in enhancing their thermal,echanical, electrical and crystallization properties alongith their biodegradation [162].

In starch based plastic materials utilization of carbonanotubes as nano reinforcement is limited by theirffective distribution. One of the successful methods ofispersing CNTs into a starch matrix is functionalization.amá et al. investigated the fabrication of starch/multi-alled carbon nanotubes composites and reported their

mproved mechanical properties. They were successful inispersing MWCNTs into a starch matrix by adopting newtrategies to wrap the carbon nanotubes surfaces with atarch–iodine complex. This also creased a strong adhesionetween nanotubes and starch matrix, which caused anffective load transfer that improved their mechanicalroperties [163]. They were able to achieve a 70% incre-ent in the stiffness with the MWCNTs reinforcement of

nly 0.055 wt%. Another investigation of Famá et al. ontarch based nanocomposites using MWCNT modified withtarch–iodine complex resulted in lower water perme-bility with high storage modulus [164]. These enhancedroperties were obtained due to the uniform dispersionf MWCNT caused by the coating of same material thatas been used for matrix. Liu et al. reported the effectivesage of glycerol for the dispersion of MWCNTs in waterolution, which lead to the formation of homogeneoushermo plastic starch/MWCNTs nanocomposites by aolution casting method [165]. Fig. 14 shows the clearvidence of glycerol’s role in effective dispersion of carbonanotubes in water. A significant improvement in pastingiscosity as well as thermal stability was observed on
he thermo plastic starch by the effective reinforcementf MWCNTs [165]. Improved electrical conductivity oflycerol plasticized starch polymer by the incorporationf MWCNT was reported by Ma and co workers [166]. A
ig. 14. Comparison of TEM micrograph of MWCNTs dispersed in water (a) and grom Elsevier Ltd.

Science 38 (2013) 1653– 1689 1669

minimum reinforcement of MWCNT of about 4.75 wt%enhanced their electrical conductivity to 10 S/cm, whichindicates that the starch/MWCNT nanocomposites wouldbe a promising alternative to novel electroactive polymericmaterials. This will also create a potential opportunityfor such novel materials in various applications includingbiosensor, artificial muscles, and electronic shielding[166].

As global research is moving towards the diversifica-tion of PLA based products, it is necessary to enhancetheir crystallization behavior in order to achieve requiredthermal and mechanical properties as well as industrialscale processability [96]. It is well understood that thecrystallization kinetics of PLA obeys different mechanismsgoverned by various chemical and physical environments.In addition it also depends on the nucleating agent andits interface with the PLA matrix. Wu et al. studied bothmelt and cold crystallization of PLA/CNT composites andcompared the obtained results with their biodegradation[96]. They found that the addition of CNT to PLA playsdual roles as nucleating agent and physical barrier. Theirresults indicate that the addition of carbon nanotube to PLAaccelerates the melt crystallization and hinders the coldcrystallization. In addition, they identified that the com-posite samples with cold crystallization history exhibitedhigher degradation compared to the composite with meltcrystallization history. This suggested that the crystalliza-tion behavior significantly influences the biodegradation[96]. Xu et al. [167] reported the improved nucleation effectof acid oxidized carbon nanotubes in PLA and investigatedthe effect of different aspect ratios of carbon nanotubes.Their result indicates that the addition of acid oxidizedMWCNT increases the nucleation density of PLA. Compar-ing the effect of MWCNT with two different lengths, thelowest (0.5–2 �m) increase the nucleation density of PLAsignificantly since they offer more nucleating sites thanMWCNT with 30 �m; this is shown in Fig. 15. Recent inter-est in the fabrication of porous structure in plastics, which
contributes in altering their physical properties signifi-cantly along with material and cost savings, motivated thefabrication of porous PLA/CNT nanocomposites. Rizvi et al.[168] developed the porous polylactide-multiwall carbon
lycerol solution (b) [165]. Copyright 2011. Reproduced with permission


rmal crT (with 0

Fig. 15. Polarized optical micrographs of spherulite growth during isothe(with 30 �m length)/PLA composite, and (c) 0.5 wt% acid oxidized MWCNpermission from the American Chemical Society.

nanotube composites using two-stage batch foaming. Theywere able to obtain porous PLA-MWCNT composites withhighly expanded cores and denser skin. Increasing MWCNTcontent caused the rise in degree of expansion comparedto neat PLA. Addition of nanomaterials to PLA significantlyimproves the flame retardancy [169]. Bourbigot and theirresearch team investigated the reactive extrusion of PLAand its CNT nanocomposites using L,L-lactide precursor andcatalysis in a co-rotating twin screw extruder and inves-tigated their flame retardancy [170]. They found slightimprovements in their flame retardancy. PLA fiber productsare in large demand in packing, suture, and biomedi-cal industries [171]. In general PLA fibers were preparedby melt spinning and solution spinning, which result inmicron scale fibers. As the demand grows for nanosizedfibers in many fields, the electrospinning process becomesa simple and versatile tool for the fabrication of many poly-mer fibers including PLA [171]. Yang et al. [171] reported
the successful fabrication of polylactide/CNT compositefiber using electrospinning technique. His study indicatedthat the morphology of PLA/CNT composite fiber stronglydepends on the solution concentration as well as solvent.
ystallization at 130 ◦C for (a) neat PLA, (b) 0.5 wt% acid oxidized MWCNT.5–2 �m length)/PLA composite [167]. Copyright 2011. Reproduced with

The major issue in expanding commercial opportunitiesfor PHB and related polymer materials is their crystal-lization behavior [172]. Reviewing the work of PHB basedblends, there is not much work has been done on the crys-tallization of PHB/CNT nanocomposites. Carbon nanotubeshave been widely used as an excellent nucleating agentin PHB polymers and enhanced their crystallization prop-erties [162,172–174]. Xu et al. prepared PHB/MWCNTsnanocomposites and investigated the effect of variousMWCNTs loadings on their thermal behavior, morphologyand also kinetics of isothermal crystallization [172]. Theyidentified the heterogeneous nucleation effect of MWCNT,which improved the nonisothermal melt crystallizationof PHB. Yun et al. reported the fabrication of PHB/CNTcomposite film for biomedical applications [173]. Theyalso studied their mechanical properties and found thatthe hardness and Young’s modulus increased significantlywith the addition of SWCNTs and resulted in the formation
of film with more brittleness than the neat PHB films.This result confirms the nucleating effect of SWCNT onPHB polymer. Sanchez-Garcia et al. studied the solutioncasting synthesis of PHBV/CNT nanocomposites films and

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nvestigated their conductivity, thermal, mechanicalnd gas barrier properties. It was found that barrier,onductivity and mechanical properties of PHBV/CNTanocomposites increased with 1 wt% carbon nanotubes162]. Shan et al. [174] employed melt mixing processor the fabrication of PHBV/MWCNT nanocompositesnd studied their crystallization process. They observedhat the addition of CNT with PHBV increased the nucle-tion density of crystals and caused formation of smallerpherulites. Vidhate et al. [175] reported the improvedechanical and electrical properties of PHBV/MWCNT

anocomposites fabricated by melt mixing. Their reportndicated that the addition of MWCNT increases the recrys-allization temperature significantly, which is about 70%ompared to virgin PHBV, and it varied substantially withhe increase in MWCNT content in the composite [175].

.3. Clay nanocomposites

Bioplastic–clay nanocomposites have been receiv-ng extensive attention due to their improved thermal,

echanical and barrier properties as well as reducedammability compared to their respective virgin polymers114]. Other positive aspects of nanoclay reinforcements it does not hampers the biodegradation of biodegrad-ble polymers [176]. Nanoclays has been discussed inetail in earlier section. These clay materials were clas-ified into many types based on their chemical nature,tructure and their unique properties of swelling as wells exfoliation [177]. In composite fabrication montmoril-onite, hectorite and saponite are the three most commonlysed nanoclay minerals, which belong to the smectitesamily [176]. Montmorillonite, saponite and hectorite arehe three most commonly used nanoclays in the synthe-is of polymer nanocomposites, these nanoclays belong tomectites family [178]. Enhancement of polymer proper-
ies by individual clay layers can be obtained through theirigh aspect ratio and interfacial interactions with polymeretworks. Also it requires very minimum reinforcement ofbout 3–6 wt% in order to achieve significant enhancement
ig. 16. Photograph of hot pressed (a) starch film (b) starch/pristine clay nanocoopyright 2007. Reproduced with permission from Elsevier Ltd.

Science 38 (2013) 1653– 1689 1671

in their tensile and other physical properties [179]. Versa-tility of layered silicates (clays) in composite fabrication isone of the most important composite fabrications not onlydue to their availability, low cost, and significant enhance-ment of properties but also their simple processability aswell as adoptive nature with both thermoset and thermo-plastic systems [114]. Polymer–clay nanocomposites werecommonly prepared by three different techniques, whichare discussed in Section 4.3 in detail.

Starch based biodegradable materials have attractedtremendous interest for many applications, because oftheir cost effectiveness, easy availability, and their renew-able origin [180]. However, their applications have beenlimited due to their poor processability, weak materialproperties, lower moisture barrier and higher humiditysensitivity [180]. Numerous research efforts address theseissues/drawbacks to achieve better performance of starch-based biomaterials. The first starch–clay composite wasmade by Carvalho et al. [181] using melt intercalation tech-niques. They showed that the starch-clay composite filledwith 50 phr kaolin (clay) increased the tensile strengthfrom 5 to 7.5 MPa and also reduced the water uptake. There-after several research works have explored starch/claynanocomposites. Gao et al. [180] investigated starch–claynanocomposites using various clay materials with differenthydrophilicities by the film blowing method. The proper-ties of starch-nanoclay films were greatly influenced by thehydrophilicity of the clay, this study found that mediumrange hydrophilicity favored the nanocomposite forma-tion. Zhang et al. [182] investigated the thermal behavior,morphology and crystallinity of starch/clay nanocompos-ites and compared the properties of composites made withpristine and organically modified clays. Fig. 16 shows thephotograph of hot pressed transparent starch film and theirpristine as well as organically modified clay nanocompos-ites. From this study, it was identified that the pristine clay
enhanced the onset and maximum degradation temper-atures of starch plastic, however modified clay enhancedthe onset degradation temperature alone [182]. A changein crystalline structure from B-type in to Va-type was
mposite and (c) starch/organically modified clay nanocomposites [182].


l burial

duced w
Fig. 17. Photographs of Starch/Cloisite (clay) nanocomposites; before soidays (e), for 90 days (f), and for 120 days (g) [183]. Copyright 2009. Repro
observed after melt extrusion. Over all they found thatthe compatibility is essential in addition to d- spacing ofnanoclay for better intercalation. Magalhães et al. [183]demonstrated the extrusion of glycerol-plasticized cornstarch plastic with two types of clay with various for-mulations and investigated their processability as well asbiodegradation. Their soil burial tests indicate that theaddition of the clay enhanced their biodegradation ratecompared to virgin starch plastic, which is shown in Fig. 17.Their detailed investigation revealed that the biodegrada-tion of nanocomposites was influenced by their relativecrystallinity as well as hydrophilicity [183]. The chal-lenge in establishing the food packaging technology withstarch–clay nanocomposites is to understand the effectof food-nanocomposite contact. Avella et al. [184] fabri-cated the biodegradable starch/clay nanocomposite filmsand extended their research for food packaging applica-tions. This research revealed that optimum intercalation ofpolymeric chains into clay has enhanced the modulus aswell as tensile strength of the nanocomposites [184]. Theyalso performed the migration and food contact testing andconfirmed that their samples are compatible with the Euro-pean regulations and directives on biodegradable materials[184].

PLA is becoming an attractive alternative to con-ventional polymers in many applications, however itsprocessing window and barrier properties needs to beenhanced to widen its application window [185]. Among

(a); after soil burial for 15 days (b), for 30 days (c), for 45 days (d), for 60ith permission from Elsevier Ltd.

most of the nanofillers, nanoclays are found improvethe material properties and processing windows of thepolymer even at a very low filler loading at 1 wt%[185]. Rhim et al. [186] processed PLA/clay nanocompos-ite films and investigated their tensile, moisture barrierand antimicrobial properties. They reported the increasedmoisture barrier properties suitable for food and bever-age packaging. Their conclusion indicated that in PLA/claynanocomposites the concentration of clay plays an impor-tant role in enhancing their properties.

Wu et al. [187] investigated melt rheology and thermalbehavior of PLA/clay nanocomposites. Their study indi-cated the solid-like behavior of composite material whenthe clay loading reaches 4 wt% or high, which indicatingthe percolation threshold of about 4 wt% [187]. It is wellknown that PLA exhibits less flexibility and breaks downeven at low deformation. Reinforcing clay with PLA furtherincreases their stiffness, which is undesired for somefeatured applications. This can be overcome by its plas-ticization; Pluta et al. [188] performed the plasticizationof polylactide and fabricated their clay nanocomposites,and they were successful in reducing the brittleness ofPLA. Tanoue et al. [189] used poly(ethylene glycol) asplasticizing agent for PLA and fabricated the organoclay
nanocomposites by melt processing. They investigatedtwo different types of poly(ethylene glycol) with differentmolecular weights (2000 and 300,000–500,000). Theyfound that the poly(ethylene glycol) with the molecular

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eight of 2000 is a good plasticizer for PLA in improvingechanical properties.Commercialization of PHB/HV copolymer and their

omposites can be achieved by improving their crystal-ization/processing behaviors, cost reduction and propertynhancement. One possible approach is the reinforcementf layered clay nanostructures in to polymer matrix. Theajor advantages of nano reinforcement are the possibil-

ty of significant enhancement of mechanical propertiesith a small amount of clay, along with complementary

arrier properties, which is not possible using conven-ional glass fiber reinforcement [190]. Choi et al. fabricatedhe PHB/HV–organoclay nanocomposites employing meltntercalation methods [190]. They were successful inbtaining an intercalated structure, which is driven by thetrong hydrogen bonding between polymer matrix and theanoclay. Nanoclay played the role of nucleating agentnd caused the enhancement of crystallization rate. Chennd his research team [191] investigated the crystalliza-ion kinetics of PHBV/clay nanocomposites and reportedhat the addition of nanoclay enhanced the crystalliza-ion rates. However, it was found that the crystallizationate reduced with increase in clay content. Melt interca-ation was employed by Bordes et al. for the fabricationf PHA/clay nanocomposites and the nucleating effectf clay and their influence in crystal size of PHA waseported [192]. Fig. 18 shows the decreasing crystal sizeith increasing clay content in composite material. Threeifferent techniques namely solvent casting, extrusion and
elt mixing were compared for the fabrication of PHBV
nd clay nanocomposites by Cabedo et al. [193]. Solventasting or melt mixing method of composite fabricationnabled the formation of intercalated morphology, where

ig. 18. POM photographs of (a) neat PHB, (b) PHB/Clay 1 wt.%, (c) PHB/Clay 2.2ermission from John Wiley & Sons.

Science 38 (2013) 1653– 1689 1673

as melt blending result in sub-micron aggregates. PHBVdegraded significantly during melt intercalation and thiswas influenced by filler concentration and residence time[193]. The ultimate solution to control this degradation isusing nitrogen gas environment during melt processing.It is well known that the compatibility between nanoclayand the polymer plays an important role in achieving theimprovement in the nanocomposite properties. In orderto enhance the compatibility between clay nanostruc-ture and PHB matrix, Parulekar et al. [194] experimentedwith the titanate-modification of montmorillonite clayand obtained improved mechanical properties. They foundthat the addition of 5 wt% titanate-modified clay loadingresults in about 400% improvement in impact propertiesin compared with virgin PHB. In addition the toughenedand compatibilized PHB exhibits poor biodegradation com-pared to virgin PHB, which was enhanced several-foldby incorporating titanate-modified clay [194]. Delamina-tion of clay layers into individual layers and uniformdispersion in the matrix are the major challenges innanocomposite fabrication [195]. Organic cations such asdistearyl dimethyl ammonium chloride and quaternaryammonium-modified starches, etc., are applied to improvethe dispersion of nanoclay [196]. Achieving miscibility andnanoclay dispersion are challenges till date.

3.4. Functional nanocomposites

Biobased polymers with functional fillers such as
HAp and LDH found a wide range of applicationsspecially towards tissue engineering, drug deliver andgene therapy due to their compatibility and also non-cytotoxic and non-inflammatory towards with biological
wt.%, and (d) PHB/Clay 5 wt.% [192]. Copyright 2008. Reproduced with

Polymer
system [197,198]. Yamaguchi et al. reported fabricationof chitosan/hydroxyapatite nanocomposites using co-precipitation method. They reported the formation of flex-ible composites with homogenous dispersion of hydroxy-apatite. Heat treatment of this composite caused theformation of hydrogen bonds between chitosan moleculesand result in the enhanced mechanical properties [199].Chen et al. [200] also demonstrated the precipitationtechnique for the fabrication of chitosan/hydroxyapatitenanocomposites for biomedical applications. Similarly,wide range of literature available for the fabricationand characterization of hydroxyapatite reinforced colla-gen nanocomposites for biomedical applications. Chan[201] et al. reported the simultaneous titration methodfor the fabrication of hydroxyapatite/collagen nanocom-posites with glutaraldehyde cross linking. Rhee et al.[202]reported the precipitation method for the fabricationof hydroxyapatite/collagen/chondroitin sulfate nanocom-posites with the possible shaping and consolidation withmechanical pressing. Dagnon et al. [203] reported thesolution casting fabrication of PLA/LDH nanocompositesand reported the increment of overall crystallinity. Chianget al. [204] investigated the fabrication of biodegradablePLA/LDH nanocomposites by solution mixing process andreported the significant increment of mechanical proper-ties with 1.2% LDH reinforcement. More discussion on thefabrication and their biomedical application of HAp andLDH reinforced bionanocomposites will be found in Section5.3.

4. Processing aspects of bionanocomposites

4.1. Processing of cellulose based bionanocomposites

The polarity of cellulose whiskers leads to the difficultyin dispersing them in non-polar medium leading to theirapplication in polar or aqueous environments [149]. Thisbehavior has led to difficulty in processing cellulose basedbionanocomposites. In aqueous medium, a higher degreeof filler dispersion in the polymer matrix is expected.Generally, two main techniques are used to prepare cel-lulose based bionanocomposites [205], namely, solventcasting and melt processing. Solvent casting is currentlymost commonly used for three main types of polymers:(i) water soluble polymers, (ii) polymer emulsions, and(iii) non-hydrosoluble polymers. For polymer emulsionsand non-hydrosoluble polymers, researchers have envis-aged two different routes to obtain uniform dispersion ofcellulose nanofillers in an appropriated organic medium[206,207]:

a. Coating of the surface of the cellulose nanocrystals withsurfactants having polar heads and long hydrophobictails.

b. Grafting of hydrophobic chains at the surface of cellulosenanocrystals.

Melt extrusion is being explored for the purpose ofobtaining cellulose based nanobiocomposites as it is theone most industrially prevalent techniques used for poly-mer processing [74,149]. However, the bigger problem in

Science 38 (2013) 1653– 1689

using this technique is usage of dried cellulose nanofillers.These cellulose nanoparticles establish strong hydrogenbonds between amorphous parts and form aggregateswhen they are dried. To overcome these limitations,researchers have tried to pump the cellulose nanocrys-tals during the melt extrusion of cellulose nanowhiskerreinforced PLA nanocomposites [149]. Also, Bondesonand Oksman [74] have investigated the preparation ofpolylactic acid/cellulose whisker nanocomposites by meltextrusion by using two different feeding methods i.e.dry-mixing and pumping the suspension. Their TEM inves-tigation concluded the better dispersion with liquid feedingcompared to dry mixing.

4.2. Processing of polymer-carbon nanotubesnanocomposites

Orientation and dispersion extent determines theextent of reinforcement in filled polymer systems. Dispers-ing both SWNTs and MWCNTs into a polymer matrix is oneof the biggest even today. Van der Waals between thesetubes leads to their aggregation of these nanotubes in thepolymer matrix.

It is important to obtain uniform dispersion for tworeasons; for effective stress transfer from matrix to thefiller and also for the obtain network structure that canhelp in thermal and electrical conductivities. Accordingto Thostenson et al. [208], uniform dispersion [209–211],wetting and orientation in the matrix [91,211], andfunctionalization to improve the compatibility are theimportant issues that influence the processing of CNT poly-mer nanocomposites.

Also, due to difference in size followed by larger sur-face found in SWNTs leads to their aggregation comparedto the MWCNTs in the polymer matrix. Among variousapproaches used in dispersion carbon nanotubes in poly-mer matrices include surface modification of nanotubes[106,212–215], polymer coating on the surface of car-bon nanotubes [216], in situ polymerization technique toobtain CNT nanocomposite [217,218], dispersion of CNTins polymer solutions using ultrasonic [219,220], meltprocessing [221–225], surfactant chemistry [210,226],electrospinning [227], electrode chemistry [228], and crys-tallization [229]. Also plasma treatment and/or chemicaloxidation to attach functional group.

In melt extrusion, first a preblend is obtained by mix-ing the nanotubes powder with polymer powder or pellets.This preblend is fed into extruder to melt the polymer anddisperse the CNT into the matrix by the application of shearand temperature in predetermined residence time. Thenanocomposites is extruded as desired shape into strands,films or fiber [230]. Uniform dispersion always improvesthe property of the matrix, while orientation of nanotubescan have further improvements. It is has been found thatmelt compounding followed by melt drawing has led to sig-nificant improvements in the tensile properties [225]. Gooddispersion and orientation in melt compounded materials
has been found using transmission and scanning elec-tron [224,230]. Also, most of these improvements can beseen upto a optimum loading level after which no furtherimprovements were observed [225,231].


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Fig. 19. Different Routes of Preparing of Polyme

.3. Processing of polymer-clay nanocomposites

Intercalation of polymers in the layered silicates isroved to be the best approach in the preparation of theolymer clay nanocomposites. The nanocomposites prepa-ation methods are classified according to the processingechniques into three main categories (Fig. 19).

(i) Polymer solution intercalation: The addition of polymersolutions to the dispersions of complete delami-nated interlayers leads to the strong interactionsbetween macromolecules and individual layers. Orthe macromolecules intercalate into the interlayerof clay and displace the solvent within that space.When the solvent in removed, these intercalated struc-tures remains, resulting in the formation of polymernanocomposites. This intercalation of polymer chainsinto clay layers in the process is governed by theentropy gained by the removal of solvent [232]. Thisgained entropy compensates decreased entropy ofintercalated and confined polymer chains [232]. Thismethod is suitable for producing hybrids from very lowpolar or non-polar polymers. This method is touted asnot non-environmental benign process due to the largeamounts of the solvent required.

(ii) In situ polymerization: This process involves the poly-merization of monomers in presence of the layeredsilicates. In this process, first nanoclay is swollen inliquid monomer or monomer solution which is fol-lowed by its polymerization. During polymerization,the polymer is formed between the layered silicatesleading to either intercalated or exfoliated structures.Initiators, heat, radiation and catalyst are used to ini-tiate the polymerization in this process. Also, cationexchange can be applied to fix the catalyst inside the
interlayer of the silicates before the engorgement stepwhen required [114].
iii) Melt intercalation method: This method is most widelyresearched due to its prevalence in the polymer

lay composites. Redrawn after reference [114].

industry. In this process, polymer matrix and nan-oclay are annealed above the melting temperature ofthe chosen matrix. In case of amorphous polymer, thisis carried out above its glass transition temperature.If these conditions are conducive or silicate surfaceare compatible, the polymer chains can crawl into theinterlayers of the silicates forming either intercalatedor exfoliated structures. Among many advantages ofthis technique, it is solvent free and making it econom-ically viable and green method. Using this technique,a wide range of nanocomposites with different mor-phologies can be obtained. However, this depends onextent of polymer chain intercalation which is in turninfluenced by the surface functionalization and theirinteractions with the polymer matrix. The mechanismof melt intercalation under high shearing is shownin Fig. 20.The nanocomposite formation depends ontwo things (a) an optimal interlayer structure, relatingto the size of surfactant chains and number per unitarea, (b) the polar interactions between the clay layersand the polymer melt adequate for the intercala-tion of macromolecular chains. Vaia et al. [232,233]studied the thermodynamics associated with thenanocomposites formation by applying a mean-fieldstatistical lattice model and comparing the resultswith the experimental results. These studies of Vaiaet al. [232,233] can be used to construct the productmaps and general guidelines for selecting potentiallycompatible polymer/layered silicate systems.

4.4. Processing of functional nanocomposites

HAp is an inorganic part of the naturally occurring bone.HAp based nanocomposites are developed for biomedi-cal application especially for bone tissue engineering. HAp
nanocomposites can be prepared by both conventionalprocessing technologies and physico chemical methods[234]. In physico-chemical method, mineral crystals areeither precipitated or bioceramic particles are dispersed


mer ma illustrat

Fig. 20. Proposed mechanism of how nanoclay platelets disperse into polybetween chemistry and process conditions in the extruder. (b) Schematic2001. Reproduced with permission from Elsevier Ltd.

in the polymer solution followed by consolidation suchas solvent casting. The adequate dispersion and uniformdistribution of these biocermaic particles in this processdepends on solvent and concentration of the polymersolution. Also, processing variables such as gelation rate,stirring time and mixing mode also influence the process.The low reactivity of HAp particles arises due to the lackof adhesive active hydroxyl groups leads to the poor adhe-sion in composite fabrication [235]. This can be overcomeby modifying the HAp surface [234]. Surface treatment of
HAp particles helps in both uniform distribution of the par-ticles and in delaying the debonding process in the polymermatrix. It is important to note that these surface modifiersused in HAp particles must be non-toxic, biocompatible and
trix during melt intercalation (a) shows three cases involving the interplayion of how platelets peel apart under the action of shear [308]. Copyright

should not change any properties of the fillers. The surfacemodification of HAp is reviewed elsewhere [234].

As discussed previously in the section, the posi-tive surface charge of LDH layers is counterbalancedby anions located in the domains between adjacentlayers, [236]. Therefore the usual approach of interca-lation of large anionic species by a delamination andentrapping-restacking process for synthesizing nanoclay-based nanocomposites is not feasible for producing LDHbased nanocomposites. LDH nanocomposites are prepared
by (a) monomer exchange and in situ polymerization,(b) co-precipitation method or polymer exchange pro-cess and (c) rearranging of the exfoliated layers over thepolymer [129]. A schematic showing different routes of


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ig. 21. Various methods of fabricating LDH nanocomposites [129]. Copy

reparing these nanocomposites is given in Fig. 21. Theirect ion exchange is widely used for several polymer-LDHanocomposites fabrication, its kinetics is limited by thelow diffusion of the polymer into LDH gallery [237]. Co-recipitation is proving to be very useful for the purposef intercalating the anionic polymers of high moleculareight within the layered double hydroxides [237]. This
ethod consists of the “co-organized assembly” synthe-
is of the LDH in the presence of the polymer, which wille incorporated between the LDH sheets [237]. Detailedeviews on LDH based nanocomposites are given elsewhere

01. Reproduced with permission from the American Chemical Society.

[129,131,237]. LDH based systems for biomedical applica-tions are reviewed in Section 5.3.

5. Applications

5.1. Packaging

Bioplastics suffer from three main disadvantageswhich are performance, processing, and cost comparedto the petro-plastics. Narrow processing window, poorgas and water barrier properties, unbalanced mechanical

1678 M.M. Reddy et al. / Progress in Polymer

modified montmorillonite (Cloisite 30BTM). Surprisingly,

Fig. 22. Proposed model for the tortuous zigzag diffusion path in a poly-mer clay nanocomposite when used as a gas barrier.

properties, low softening temperature and weak resistivityof the plastics has limited their use in wide range of appli-cations. As discussed previous sections, nanotechnologyhelps in overcoming these problems. Nanofillers help inimproving the above discussed properties of the bioplas-tics. Bionanocomposites exhibit remarkable improvementcompared to the neat matrix and conventional compositesdue to nanoreinforcements.

Permeability properties apart from tensile propertiesare considered as the most important parameters in select-ing materials for food packaging applications [238]. Highbarrier to gases and vapors are key attribute found in glassand metal based packaging, the polymers are expected toperform on par with these materials. Polymers provideexcellent balance in properties including mechanical, ther-mal and barrier properties. It is well recognized that theincorporation of nano-fillers especially nanoclay into thepolymeric matrix can lead to significant enhancement inthe barrier properties [114]. This improved barrier prop-erties in nanocomposites is explained on the basis ofincreased path length due to the presence of nanofillersthat the same molecules needs to traverse while diffusingthrough the matrix. In nanoclay based nanocomposites, theclay layers create a more tortuous diffusive path as shownin Fig. 22 and delays the transfer. Also the barrier proper-ties of bionanocomposites depend on the orientation andthe nanofillers state of dispersion in the polymer matrix[239]. Nanoclays are more effective nanofillers comparedto fibrillar nanocellulose for improving the barrier proper-ties. Nanoclay based bionanocomposites have gained moreimportance by the packaging industry, due to the ease ofavailability, processing, and low costs compared to othernanocellulose, carbon nanotubes [240]. PLA, PHB and starchbased nanocomposites have been attractive for packagingapplications.

PLA fulfills the requirements for direct food contactwith aqueous, acidic and fatty acids [241]. Cups, cutleryand food containers are being manufactured using PLAby many companies [242]. PLA can be laminated to paperand paperboard by extrusion coating for further use
as packaging material [238]. The insolubility of PLA inwater limits its applications for use in paper products andindustrial coatings. PLA is considered as a poor oxygen
Science 38 (2013) 1653– 1689

barrier; however its barrier property is high compared toall other bioplastics. Nanoclay has been used overcomethis property of PLA. The effects of different organicallymodified nanoclays on the oxygen gas permeability of PLAhas been studied by Chang et al. [243]. Melt intercalatednanocomposites were prepared and it was found that thepermeability decreased for all the nanocomposites. In astudy conducted by Plackett et al. [244] PLA nanoclaybased nanocomposites were studied for their suitability incheese packaging. This study also showed the both waterand oxygen properties have decreased significantly bythe incorporation of nanoclay. Furthermore researchers[245] have studied the effect of shear and feed rate on thepermeability of PLA nanocomposite based films. It wasfound that films showed an improvement over PLA matrix.Oxygen barrier properties improved by a 15–48% forPLA nanocomposites compared PLA. Also, both shear andfeed rate had no influence on nanocomposites i.e. barrierproperties were independent of these two factors. For neatPLA, the properties were dependent on the processingvariables leading to conclusion that processing param-eter needs to be optimized for PLA processing. Similartrend was observed for water vapor transmission rate,PLA nanocomposite films displayed a reduction of about40–50% in transmission rate. The observed improvementwas again independent of processing variables [245].

In case of PHAs their water resistant arises from theirhighly hydrophobic nature [246]. It is interesting to notethat PHAs are suitable for coating and film application. Also,the water vapor barrier property of PHAs is very close tothat of the polyethylene, this makes them very attractivematerials for the food packaging. However, the poor gasbarrier properties and narrow processing window limitstheir applications. Sanchez-Garcia et al. [247] investigatedthe relationship between morphology and barrier proper-ties in PHB/clay nanocomposites. In this study, the nanoclayshowed a very high dispersion in the PHB matrix due to theorgano-modification of the clay layers. This high dispersedstate has led to improvement in oxygen, D-limonene, andwater barrier proprieties of PHB/clay nanocomposites.

Similarly, thermoplastic starch is also highly sensitiveto water due to its high hydrophilicity. This is hindering itsapplication in packaging which can be overcome by usingnanocomposites technology. Park et al. [248,249] focusedtheir attention on water permeability. It appears that therelative water vapor transmission rate (WVTR) of the TPSnanocomposites was reduced by nearly a half compared tothe neat TPS at only 5 wt% of montmorillonite. This resultleads to exploration of TPS formulations for food packagingapplications and protective coatings. The significant reduc-tion in WVTR is attributed to the tortuous pathway createdby nanoclay in the matrix. This pathway increases the timerequired for molecules to travel along the diffusion path,thus decreasing the WVTR.

Park et al. [248] also compared the effect of claymodification on WVTR and tensile properties of TPS byusing unmodified montmorillonite (Cloisite Na+TM) and

this study revealed that the unmodified montmorilloniteTPS showed improvement over modified montmorilloniteTPS composites. The reason for observed improvement in

Polymer Science 38 (2013) 1653– 1689 1679

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nmodified montmorillonite TPS nanocomposites is theigher hydrophilicity of unmodified montmorillonite com-ared to modified montmorillonite. TPS used in the work

s highly hydrophilic and has led to the intercalation alongith better dispersion with hydrophilic unmodified mont-orillonite than organophilic modified montmorillonite.

his study leads to conclusion that the degree of clay dis-ersion is controls the water permeability rates in theanocomposites.

Cellulose nanofibers were found have the same reduc-ng effect on barrier properties of TPS, this effect isttributed to the increase of the tortuosity induced by theresence of the nanofibers [250]. However, the celluloseanofibers are not as effective as that of nanoclay probablyue to their shape which limits the increment in tortuousath.

It is important to note that the decrease in permeabilityf nanocomposites, there are some contradictory observa-ions in these properties for different solvents and gases. Its well known that the saturation is related to agglomera-ion phenomena of the nanofillers in the polymer matrix.

ost of the times, different morphologies co-exist withinhe nanocomposites leading to different permeabilites thatause complex transport phenomena. Also, it is well knownhat the semi-crystalline polymers itself have two regionsrystalline and amorphous leading to different permeabil-ties as crystalline regions are impermeable to penetrant

olecules. It is well known that the decrease in the perme-bility is attributed to tortuous path created by nanofillershich increases the effective path length for vapors and

as molecules [251]. However, these fillers also have anffect on the crystallinity and chain mobility of the poly-er matrix leading to the reduction in permeation. Hence,

oth crystallinity changes and tortuous path has to be takennto consideration for analyzing the effect of nanofillers onhe permeability of nanocomposites [252].

.2. Electronics, sensor and energy applications

Polymers reinforced with engineered/functional nano-tructures provide additional electrical, optical, electro-agnetic shielding and magnetic properties and lead to

he development of various advanced devices includingight emitting, diodes, sensors, solar cells, display panelsnd other medical devices [253,254]. As the global demandor flexible electronic devices increases polymer nanocom-osites, receive extensive attention in developing variousevices [255]. Besides, ever-increasing uses of electricalnd electronic equipment creates environmental issues athe end of their life span and generates enormous wasteroducts (e-waste) [256]. As a result, applications of bio-lastics and their composites in electronic products are

ncreasing due to their biodegradability and renewabil-ty with less environmental impact/carbon footprint [257].his section summarizes various device applications of bio-anocomposites.

(i) Advanced electronics: The advantage of cellulosenanofibers in fabricating transparent and flexiblecomposites found wide applications not only for elec-tronic devices such as displays, solar cells and organic

Fig. 23. Demonstration of organic light-emitting diode (OLED) depositedon cellulose nanocomposite [257]. Copyright 2008. Reproduced with per-mission from John Wiley & Sons.

light emitting diodes but also for roll-to-roll fab-rication techniques [257]. In roll-to-roll technique,continuous deposition of various functional compo-nents is facilitated leading to fabrication of electronicdevices. The functional components used in this fab-rication include metal wiring, active/gas barrier films.This technology has been widely used for the devel-opment of the flexible electronic devices. However,the application of this technology for plastic materi-als (while using as a substrate for active components)suffers due to a high coefficient of thermal expan-sion (CTE) [258,259]. Nogi et al. [257] reported thatthe addition of bacterial cellulose nano fibers intopolymeric resin caused the reduction in CTE. Theyalso suggested that such new composite materialscan be used as the potential substrate for roll-to-rollfabrication. A wide range of research has been per-formed in order to enhance the thermal and tensileproperties of petro/bio based polymeric materials forthe purpose of roll-to-roll fabrication. Fig. 23 showsthe demonstration of organic light emitting diode(OLED) deposited on cellulose nanocomposites. As anexample, Petersson et al. [260] reported fabrication ofpoly(lactic acid)/cellulose whiskers nanocompositesand found that the reinforcement of nanostructu-red cellulose whiskers enhanced their thermal andmechanical properties. They found that the bothcellulose whiskers and their nanocomposites werethermally stable up to 220 ◦C, which indicating theirsuitability for various device applications [260].

(ii) Sensors: Polymeric nanocomposites have beenidentified as suitable candidates to develop flexi-ble sensors due to their advantages such as goodelectrical performance, simplicity in handling,light-weight, economic benefits, biocompatibil-ity and eco friendliness while using biopolymers[261]. In this perspective, a wide range of poly-
meric composites were fabricated and investigatedwith the reinforcement of various electric, mag-netic, bio, optical and mechanical sensitive/activefunctional fillers for the development of various

Polymer
temperature, chemical, bio and humidity sensors[261]. Cellulose based composite films (which arescientifically known as active paper) with active nano-materials such as gold, silver and carbon nanotubeshave been used to construct strain, chemical and biosensors. Similarly PLA and their carbon nanotube-reinforced composite films were effectively used asthe liquid sensing material. Yun et al. [262] reportedthe fabrication MWNTs–cellulose composites assmart paper and demonstrated their applicationas chemical vapor sensors. Kobashi et al. [263,264]have investigated the liquid sensing properties ofmelt-processed PLA/MWCNTs composite films bymeans of the electrical resistance variance on liquidcontact. The sensing performance was optimizedthrough the resistances of the composite due to thesolvent transport in the structure.

(iii) Electromagnetic shielding: Electromagnetic shieldingis the action to block the electromagnetic field byusing suitable barrier materials with efficient con-ductivity and/or magnetic properties [265]. In recentyears a number of polymer composites reinforcedwith high conductive fillers have been reported as EMIshielding materials [266,267]. Such materials foundapplications in high performance shielded connectors,scientific/medical/consumer electronic devises andmilitary/security products. Based on the conductivityrange, materials can find applications in either elec-trostatic discharge (ESD) prevention (105 to 109 S/cm)or EMI shielding (greater than 10 S/cm) [168,268].Profound research work has been reported for EMIshielding application using various types of petro-based polymer composites reinforced with manyconducting fillers. Application of renewable resourcebased biocomposites for this purpose is just budding.Recently, Rizvi et al. reported the fabrication of solidand porous PLA-MWCNT composites and investigatedtheir electrical conductivity. They have obtained lowfrequency complex conductivity of PLA-MWCNT com-posites reinforced with 5% MWCNT that exhibitedsuitable conductivity for electrostatic discharge (ESD)prevention application [168]. Duan et al. fabricatedthe polylactide based hybrid nanocomposites usinggraphite nanosheets and MWCNTs as reinforcementand demonstrated the enhancement of their electricalproperties. Such enhancement of electrical propertiesindicates possible applications for EMI shielding [268].

(iv) Thermoelectric systems: Thermoelectric systems area very promising technology in harvesting electricityfrom any kind of heat sources, where the temperaturegradient plays major role. Also, they offer additionaladvantages such as (i) reduced operational noise,(ii) long-term, maintenance-free operation and (iii)simplified structure with the absence of movingparts compared to traditional electricity generatingsystems [269]. Thermoelectric semiconductors havebeen widely used for energy conversion; however
they face disadvantages due to their expensive syn-thetic protocol [270]. This motivated researchersto move towards polymer nanocomposites, sincethey are light and generally require relatively simple
Science 38 (2013) 1653– 1689

manufacturing processes relative to traditional semi-conductor based thermoelectric materials [271]. Ingeneral biobased polymers are electrical and thermalinsulating materials, which could be altered to behighly conductive through the effective reinforcementof conductive carbon fillers. These nanocompositesmaterials with high electrical and thermal conduc-tivity can be an excellent material for thermoelectricconversion. Antar et al. reported the thermoelectricbehavior of melt processed carbon nanotube/graphiteco reinforced poly(lactic acid) composites and foundthat their thermal and electrical conductivity hasincreased compared to virgin plastic [272].

(v) Solar cell: Another application of polymer nanocom-posites reinforced with conducting carbon nanotubesis flexible photovoltaic cells [273]. In polymer-basedflexible photovoltaic cells, the challenging issueis to improve the conductivity of polymer, whichis directly influenced by the conduction of photo-generated carriers to the electrodes. Reinforcinghigh-conducting carbon nanotubes enhances thecollecting performance of photogenerated electronsand improves the overall performance of flexiblephotovoltaic cells [274]. For example, Valentin et al.made a novel approach to develop carbon nanotubebased polymer nanocomposites and studied theirelectrical performance [275]. They concluded that thepolymer/SWNTs composite can perform as organicsemiconducting material, which can be used tofabricate photovoltaic cells. Landi et al. also char-acterized the photovoltaic cell based on single-wallcarbon nanotube– poly(3-octylthiophene)-(P3OT)[276]. Strange et al. have successfully fabricatedPLLA–nanoclay composites films as bio degradablesubstrates for the solar cell application [277]. How-ever, extensive research on biobased polymer/carbonnanotubes composites for high performancephotovoltaic applications has not been conducted.

5.3. Bionanocomposites for medical applications

The versatility and adaptability of bionanocompositesenable these materials to be utilized for biomedical appli-cations. An essential characteristic of medical biomaterialsis biocompatibility, the ability to function appropriately inthe human body to produce the desired clinical outcome,without causing adverse effects [197]. Bionanocompositesare an intuitive choice for medical applications, given thatsuch materials are constructed from bio-derived polymers,and such materials possess tunable mechanical proper-ties. Biobased polymers are increasingly being recognizedas biocompatible materials for clinical use. For exam-ple, plastics and films made from corn-derived PDO havebeen shown to be non-cytotoxic and non-inflammatory toclinically relevant cell lines [198]. Moreover, soy-derivedpolymers have been demonstrated to be useful as bonefillers [278]. Bionanocomposites that combine the tissue
compatibility of natural polymers and bio-derived poly-mers along with the physical and chemical properties ofnanoreinforcements will find widespread use in clinicalmedicine. In particular, three emerging areas of medical

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pplications for bionanocomposites are tissue engineering,rug delivery, and gene therapy.

.3.1. Bionanocomposites for tissue engineeringBionanocomposites are well-suited for regeneration of

ative tissue structures, as nanocomposite materials mimiche natural morphology of the extracellular matrix thaturrounds cells. Namely, the extracellular matrix exhibitshree key characteristics [279]:

1) The extracellular matrix is composed of a combinationof macromolecules, such as proteins and polysaccha-rides, and inorganic matter;

2) Macromolecules in the extracellular matrix are typ-ically present in a fiber form, and possess an axial(length/diameter) ratio greater than 100;

3) Macromolecules in the extracellular matrix typicallyhave nanoscale diameters of less than 500 nm.

Within the overall area of tissue engineering, the mostidely explored application of bionanocomposites has

een that of bone tissue engineering. Native human bones itself a nanocomposite, made up of collagen proteinbers, hydroxyapatite mineral nanocrystals, and proteo-lycans, hierarchically arranged at the nanometer scale280]. Collagen provides bone a structured matrix anduperior tensile strength along with flexibility. The stiff-ess and compressive strength to the bone are providedy the hydroxyapatite nanocrystals. Collagen is a triple-elical protein with a fiber length of 500 nm, these fibersre the result of self-assembly of 10 nm long individualelical chains of collagen. Toward re-creating the naturaltructure of bone, nanocomposites comprised of poly-er scaffolds and hydroxyapatites have been synthesized

121]. For bone regeneration in clinical orthopedics, it isssential that bionanocomposites are biodegradable andioresorbable structures, which induce and promote newone formation at the site of implantation. Moreover, bio-anocomposites for bone tissue engineering must haveufficient macroporosity to facilitate new tissue growth. Aydroxyapatite- collagen composite material has been fab-icated at 37 ◦C and pH 7. These physiological conditionsave led to the simultaneous assembly of collagen fibrilsnd crystallation of hydroxyapatite nanoparticles result-ng in a porous scaffold [281]. Such scaffolds exhibit highlasticity, and have been successfully implanted for repairnd reconstruction of bony defects in animal models [282].he three-dimensional pore structure of hydroxyapatite-ollagen nanocomposites can be precisely controlled viace crystal growth and subsequent freeze-drying, to give

material with high flexibility and shape-recovery prop-rties [283]. Chitosan-hydroxyapatite composites with aomogeneous nanostructure have been created by a co-recipitation method [199]. These composites have been

mplanted in the bone marrow with no inflammation,nd new bone formation has been observed surround-ng the implanted chitosan-hydroxyapatite composites.
elatin-hydroxyapatite nanocomposite fibers have beenrepared via a process that involves precipitation followedy electrospinning [284]. Bone-derived osteoblast cellsttach, grow, and proliferate on the gelatin-hydroxyapatite
Science 38 (2013) 1653– 1689 1681

nanocomposite fibrous mesh. These nanocomposites addi-tionally improve the differentiation and functional activityof osteoblast cells [285]. Hydroxyapatite/chitosan-gelatinnetwork composites have been created by phase separa-tion methods [286]. Osteoblasts attach to the network, andsynthesize extracellular matrices that include collagen Iand proteoglycan-like substrate; the scaffolds induce bone-like tissue formation and biomineralization. Moreover, anintercalated nanocomposite of gelatin/montmorillonite-chitosan has been fabricated via a solution intercalationprocess [287]. Rat bone marrow-derived stromal stem cellsattach and proliferate on gelatin/montmorillonite-chitosannanocomposite membranes.

The natural polysaccharide alginate has also beencombined with hydroxyapatite for the purpose of boneregeneration. Alginate provides a polymeric sponge struc-ture for tissue engineered scaffolds. Apatite crystals havebeen directly nucleated on self-assembling alginate chainsto create a biomimetic composite; such materials sup-port osteoblast growth and functionality [288]. In addition,nanocomposites of silk fibroin and hydroxyapatite havebeen synthesized by co-precipitation [289]. Further, athree-dimensional scaffold comprised of porous nonwovensilk fibroin net/nano-hydroxyapatite composite has beenprepared [290]; the silk-hydroxyapatite scaffold is not onlycytocompatible with osteoblasts, it also improves the via-bility of osteoblasts. Cell multilayers wrapped around thescaffold frame, and cells deposited extracellular matrixwith fibrillar components on the silk-hydroxyapatite net-work. Hydroxyapatite has successfully been deposited onsilk sericin films to create bionanocomposites as well [290].Another natural biopolymer, bovine serum albumin, hasbeen assembled to nanocrystalline hydroxyapatite [291].The milk protein, sodium caseinate, has additionally beenincorporated into the structure of hydroxyapatite, withmicropores formed by means of emulsion templating [292].

Nanocomposites based on poly-L-lactic acid (PLLA)have been evaluated as bone replacement materials. Themechanical properties of PLLA alone are inadequate forload-bearing applications; nanoreinforcements such as ori-ented fibers, hydroxyapatite, or clays can impart enhancedproperties to PLLA-based bionanocomposites. For exam-ple, nanoreinforcements can increase the flexural modulusand strength of PLLA-based nanocomposites, to give valuesconsistent with bone replacement implants [293]. Silicananoparticles and organically modified montmorillonitehave been incorporated into PLLA matrices, resulting innanocomposites with improved mechanical and thermalproperties [294]. Finally, “self-reinforced” bionanocom-posites have been fabricated by incorporating orientedPLLA fibers into a matrix of the same material [295];self-reinforced poly(lactic acid) rods have demonstratedsuccess in bone fixation procedures in animal models.

5.3.2. Bionanocomposites for drug deliveryBionanocomposites designed for implantation and tis-

sue replacement/regeneration can simultaneously serve as
reservoirs for drug delivery. A hydroxyapatite/collagen-alginate bionanocomposite has been developed as a bonefiller and drug delivery vehicle; the composite is loadedwith bone morphogenetic protein, a growth factor that

Polymer
stimulates bone formation [296]. Porous hydroxyapa-tite/collagen scaffolds have additionally been designedas carriers for fibroblast growth factor-2; these drug-releasing scaffolds are efficacious in stimulating bothbone regeneration and cartilage regeneration in animalmodels of large osteochondral defects [297]. Hydroxyap-atite/chitosan nanocomposites have also been prepared asdrug-loaded matrices, and the controlled release of vita-mins from such matrices has been achieved [298].

Nanocomposites based on layered double hydroxides(LDH) represent another attractive strategy for controlledrelease of pharmaceuticals. LDHs can serve as hosts forintercalation of biopolymers, to produce bionanocom-posite structures. Specifically, anionic polysaccharidesincluding alginate, carrageenan, and pectin have beenassembled to Zn-LDH matrices [237]. Layered solids can actas a “molecular container” for pharmaceutical agents [299].Poly(lactic acid) nanocomposites with LDHs have beenfunctionalized with the anti-inflammatory drug ibuprofen[203]. The natural biopolymer xyloglucan, extracted fromBrazilian jatobá seeds, has also been combined with LDHsto create a vehicle for slow release of the anti-hypertensiveagent enalaprilate [300].

5.3.3. Bionanocomposites for gene therapyNatural nucleic acid biopolymers can be intercalated

into layered double hydroxides; such composite materialscan be utilized for gene therapy. For instance, an ion-exchange method has been used for intercalation of DNAbetween the interlayers of LDH, leading to the formationof bionanocomposites [301]. In such matrices, the DNAchains are arranged in a double-helix conformation, ori-ented parallel to the basal plane of the LDH [301]. Thesematrices can transfer genetic material to the intracellularspace, as the inorganic matrix shields the DNA’s negativecharge, enabling the bionanocomposite system to be trans-ported through cell membranes. Once the matrix entersthe cell, the slightly acidic pH of the lysosome dissolvesthe LDH, allowing the release of DNA that is then trans-ferred to the cell nucleus. Nanohybrid particles comprisedof DNA intercalated in LDH can successfully enter leukemiacell lines and release oligonucleotides to inhibit cancerouscell growth [302]. These bionanocomposites can serve asnonviral vectors for gene therapy of cancers and other dis-eases. Bionanocomposites therefore represent a promisingmethod for creating functional, bio-inspired materials withwide-ranging applications in clinical medicine [131].

6. Concluding remarks

This review presents the current status of biobasedplastics and their nanocomposites. The usage of renew-able resource based plastics can lead to the reduction ofenergy consumption and greenhouse gas emissions in cer-tain applications. Biobased plastics are still at an earlystage of commercialization, only starch-based bioplasticsand PLA are available considerably for packaging and other
industrial applications. A renaissance in producing tradi-tional plastics by the use of biobased resources has led tonew interest in polymers like PTT, PE, PP, PBS, PET andnylons. Advancements in microbial technology have led
Science 38 (2013) 1653– 1689

to the creation of new metabolic pathways to producemonomers/polymers in a cost-effective way. The price ofPLA has reduced and currently competes with some petro-based polymers in the market place. It has found usesin various applications due to its renewable nature withadded advantages of compostability. Also, many companieshave been working on producing traditional polymers viabiochemical pathways using renewable resources ratherthan petro-based resources. The inherent weaknesses ofbiopolymers are being overcome by different strategies.

Bionanocomposites lead to dramatic improvements inthe properties of the bioplastics. Addition of nanofillers(nanoclay, nanocellulose, carbon nanotubes and functionalnano fillers) helps in tuning the properties of biopoly-mers as desired. Bioplastics are thermally sensitive andhence solvent-based processing techniques, especially sol-vent intercalation, have been widely updated for thepreparation of bionanocomposites. Also, the irreversibleagglomeration of nanocellulose when dried hinders it frombeing used in melt processing techniques. However, themelt intercalation technique is more industrially preva-lent and helps in fine and uniform dispersion of nanofillersin the matrix. Also, to overcome the compatibility issues,often the nanofillers are organically modified using ammo-nium or phosphonium cations bearing at least one longalkyl chain.

Extensive research has been explored for the fabrica-tion of nanocomposites using renewable resource basedbiopolymers, which exhibit a remarkable increase in theirthermal, mechanical and other functional properties (gasbarrier, fire retardant, EMI shielding). Also, it is found thatthe addition of nanostructured fillers with biopolymersenhances their degradation rate significantly. In gen-eral, biopolymers experience various issues such as poorenvironmental stability, lower barrier properties, poorcrystallization kinetics and narrow processing window,which inhibits their commercial opportunities. In starchbased plastics the challenging issue is their poor stabilityagainst moisture, and it has been improved by reinforcingnanostructured fillers such as cellulose nanofibers, carbonnanotubes and nanoclay. It is well known that polylac-tide has shown much promise towards nanocompositesfabrication due to its cost effectiveness and comparableproperties with conventional plastics. Several reports havebeen made on PLA nanocomposites aiming to addresstheir crystallization related issues, to enhance the pro-cessability in various processing lines including injectionmolding, fiber formation and film fabrication to createwide commercial opportunity. In polymer composites,nanostructure fillers act as an excellent nucleating agentand dominate in altering the crystallization behavior. Inaddition to PLA, PHBV also receives benefits from nano-structured materials as nucleating agents, which improvesvarious properties.

Bionanocomposites find great potential in manyapplications, remarkably in packaging, electronics andbiomedical sectors. Currently food packaging films with
high barrier properties are being explored. Even the appli-cations of bionanocomposites in various electronic systemsare budding, and it is expected to be a vibrant area ofresearch in future. Significant research is in progress to

Polymer

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xtend the application of these nanocomposites for variousonsumer electronic systems, aiming at the reduction oflobal e-wastes. Polymer nanocomposites reinforced witharbon nanotubes show improved electrical conductivity,nd extend their application as EMI shielding materials.ionanocomposites with HAp and layered double hydrox-

des (LDH) have been explored for various biomedicalpplications. Given these functionalities, nanofillers inioplastics have excellent potential to revolutionize thetilization of nanocomposites and expand their use in highalue applications.

cknowledgements

The authors are thankful to the Ontario Ministry ofgriculture, Food, and Rural Affairs’ (OMAFRA) New Direc-

ions and Alternative Renewable Fuels “Plus” Researchrograms; to the OMAFRA-Univ. of Guelph Bioeconomy-ndustrial Uses Program; to the Ontario Research FundORF) Research Excellence (RE) Round-4 from the Ontario

inistry of Economic Development and Innovation (MEDI),he Natural Sciences and Engineering Research Council ofanada (NSERC) Discovery grant individuals to M.M. &.K.M.; AUTO21-NCE (Canada’s automotive R&D program);

he Hannam Soybean Utilization Fund (HSUF) and the Grainarmers of Ontario (GFO) fund for supporting research onarious aspects of biobased materials, biofuel co-productsnd bionanotechnology at the Univ. of Guelph’s Bioprod-cts Discovery and Development Centre. S.B. is thankful tohe students and faculty of the Harvard School of Engineer-ng and Applied Sciences.

Bioplastic: A plastic that is made from biologi-al/renewable resources or degrades by the action oficro-organisms/biological activity or both [1].Biobased plastic: A plastic that is obtained totally or

artially from biological resources, i.e. all the monomers orny of the monomer used in the synthesis is derived fromiological resources [2].

Degradable plastic: A plastic, which undergoes majortructural changes under prescribed environmental condi-ions.

Biodegradable plastic: A plastic which degrades by thection of micro-organisms or undergoes lowering of itsolecular weight by biological activity[2]Compostable plastic: A plastic that undergoes

iodegradation in composting environment to yieldarbon dioxide, water, inorganic compounds and biomasst a rate equivalent to standard compostable materials andeaves no toxic materials.

Biocomposite: Composite that consists of either fillerr polymer matrix derived from biological resources.

Nanocomposite: Polymer composites that have fillersith atleast one dimension in nano meters.

Biobased nanocomposite: Nanocomposite in whichither fillers or polymer matrix has been obtained fromiological resources.

Bionanocomposite: Nanocomposite in which both
llers and polymer matrix obtained from biologicalesources.
Biodegradable nanocomposite: Nanocomposites thategrades by the action of micro-organisms.

Science 38 (2013) 1653– 1689 1683

Biocompatible: Refers to the ability of a material tocarry out its intended function without any adverse reac-tions or unintended responses [2].

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http://www.icis.com/Articles/2012/07/02/9573828/bioplastics-surge-towards-commercialization.html

http://www.icis.com/Articles/2012/07/02/9573828/bioplastics-surge-towards-commercialization.html

http://www.bseindia.com/bseplus/annualreport/500720/5007200611.pdf

http://www.bseindia.com/bseplus/annualreport/500720/5007200611.pdf

http://plasticsengineeringblog.com/2012/08/13/the-race-to-100-bio-pet/

http://plasticsengineeringblog.com/2012/08/13/the-race-to-100-bio-pet/

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