Nanotechnology and its applications in lignocellulosic ... · Nanotechnology and its applications...

1. Introduction

1.1. Nanotechnology

Technology is the major driving factor for growthat every level of an economy. At the 1 nanometer(nm) scales and below, quantum mechanics rules,and at dimension above 100 nm classical quantummechanics, physics, and chemistry dictate proper-ties of matter. Between 1 and 100 nm, a hybridexists, and interesting things can happen such as

mechanical, optical, electrical, magnetic, and avariety of other properties can behave quite differ-ently [1]. A nanometer is a billionth of a meter, or80 000 times thinner than human hair. So, nanome-ter domain covers sizes bigger than several atomsbut smaller than the wavelength of visible light.Nanotechnology (based on the Greek word fordwarf) is defined as the manipulation of materialsmeasuring 100 nm or less in at least one dimension.

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*Corresponding author, e-mail: [email protected]© BME-PT and GTE

Nanotechnology and its applications in lignocellulosic composites, a mini review

S. Kamel*

University of Malakand, Pharmacy Department, Chakdara Dir, N.W.F.P., PakistanPermanent address; National Research Center, Cellulose & Paper Department, El-Tahrir St. Dokki, Cairo, P.O. 12622,Egypt

Received 1 March 2007; accepted in revised form 14 June 2007

Abstract. Nanotechnology has applications across most economic sectors and allows the development of new enabling sci-ence. The ability to see materials down to nanoscale dimensions and to control how materials are constructed at thenanoscale is providing the opportunity to develop new materials and products in previously unimagined ways.This review covers the academic and industrial aspects of the preparation, characterization, material properties, crystalliza-tion behavior; melt rheology, and processing of polymer/cellulose or cellulose/cellulose nanocomposites. Cellulosic mate-rials have a great potential as nanomaterials because they are abundant, renewable, have a nanofibrillar structure, can bemade multifunctional, and self-assemble into well-defined architectures. The fibrillation of pulp fiber to obtain nano-order-unit web-like network structure, called microfibrillated cellulose, is obtained through a mechanical treatment of pulp fibers,consisting of refining and high pressure homogenizing processes. Also, nano-whisker can be used as novel reinforcementin nanocomposites; it can be obtained by acid hydrolysis from various sources such as wood, tunicin, ramie, cotton, wheatstraw, bacterial cellulose, and sugar beet. The properties of nanocomposite materials depend not only on the properties oftheir individual parents, but also on their morphology and interfacial characteristics. Compared with plant cellulose, bacte-rial cellulose has found many applications in the biomedical field as tissue engineering materials due to their good biocom-patibility, mechanical properties similar to those of hard and soft tissue and easy fabrication into a variety of shapes withadjustable interconnected porosity. One of the drawbacks of cellulose whiskers with polar surfaces is poor dispersibil-ity/compatibility with nonpolar solvents or resins. Thus, their incorporation as reinforcing materials for nanocomposites hasso far been largely limited to aqueous or polar systems. To overcome this problem and broaden the type of possible poly-mer matrices, efforts of surface modification have been made. These attempts include surfactant coating or graft copoly-merization.

Keywords: nanomaterials, nanocomposites, reinforcements, rheology and mehanical properties

eXPRESS Polymer Letters Vol.1, No.9 (2007) 546–575Available online at www.expresspolymlett.comDOI: 10.3144/expresspolymlett.2007.78

Where the physical, chemical, or biological proper-ties are fundamentally different from those of thebulk material. By expanding our understanding andcontrol of matter at such levels, new avenues inproduct development can be opened [2].Nanotechnology can be defined as the science andengineering involved in the design, synthesis, char-acterization, and application of materials anddevices whose smallest functional organization inat least one dimension is on the nanometer scale orone billionth of a meter [3]. Classified by nanofillerdimensionality, there are a number of types ofnanocompomposites. Zero-dimensional (nanoparti-cle), one-dimensional (nanofiber), two-dimensional(nanolayer), and three-dimensional (interpenetrat-ing network) systems can all be imagined [4]. Also,lamellar nanocomposites can be divided into twodistinct classes, intercalated and exfoliated. Inintercalated nanocomposites, the polymer chainsalternate with the inorganic layers in a fixed com-positional ratio and have defined number(s) ofpolymer layers in the intralamellar space. In exfoli-ated nanocomposites, the number of polymerchains between the layers is almost continuouslyvariable and the layers stand >100 Å apart. Deter-mining and altering how materials and their inter-faces are constructed at nano- and atomic scaleswill provide the opportunity to develop new materi-als and products. Because of this ability, nanotech-nology represents a major opportunity for woodand wood-based materials to improve their per-formance and functionality develop new genera-tions of products, and open new market segments inthe coming decades [5]. Now it is possible to ask,why nanotechnology is important.

The answer is [6]:a) Less space, faster, less material, and less

energy.b) Novel properties and phenomena.c) Most efficient length scale for manufacturing.d) Intersection of living/non-living.

Polymer nanocomposites are produced by incorpo-rating materials that have one or more dimensionson the nanometer scale (<100 nm) into a polymermatrix. These nanomaterials are in the literaturereferred to as for example nanofillers, nanoparti-cles, nanoscale building blocks or nanoreinforce-ments. Nanocomposites have improved stiffness,strength, toughness, thermal stability, barrier prop-erties and flame retardancy compared to the pure

polymer matrix. Nanoreinforcements are alsounique in that they will not affect the clarity of thepolymer matrix. Only a few percentages of thesenanomaterials are normally incorporated (1–5%)into the polymer and the improvement is vast due totheir large degree of surface area [7]. Because ofthe nanometric size effect, these composites havesome unique outstanding properties with respect totheir conventional microcomposite counterparts.Since the pioneering work by the Toyota group[8–10] polymer nanocomposites have attracted anincreasing amount of attention.The properties of nanocomposite materials dependnot only on the properties of their individual par-ents, but also on their morphology and interfacialcharacteristics. In the particular case of polymerreinforced with rigid nanofillers, various parame-ters seem to be of importance in characterizing thefillers: geometrical factors such as the shape, thesize, and the aspect ratio; intrinsic mechanical char-acteristics such as the modulus or the flexibility;surface properties such as specific surface area andsurface treatment [11]. The type of polymer matrixused and the possible effects of nanofillers on itsmicrostructure and its intrinsic properties are alsoessential parameters determining the compositeproperties. In addition, the processing conditionscan affect on composite properties. To study thisfactor high aspect ratio nanofibers/poly (styrene co-butyl acrylate) composites were prepared with twodifferent processing conditions. It was found that,in case of evaporated cellulose filled composites;the highest mechanical reinforcement (with amechanical percolation phenomenon) coupled to anincrease in composites thermo-mechanical stabil-ity. While in case of freeze-dried cellulose filledcomposites; the freeze-drying process prevents thecreation of strong contacts between nanofibrils, alower mechanical reinforcement is measured [12].Whereas the general class of inorganic nanocom-posites has enjoyed much discussion and is still afast-growing area of research, exciting newresearch on bio-based nanocomposites have agreater potential because the bio-resource can beboth sustainable and genetically manipulated.Wood cellulose nanofibrils have about 25% of thestrength of carbon nanotubes, which are expectedto be the strongest fibers that can be produced.Their potential cost, however, might be 10 to 100times less, giving cellulose nanofibrils a uniqueeconomic advantage [5].

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Kamel – eXPRESS Polymer Letters Vol.1, No.9 (2007) 546–575

1.2. Nanocomposite materials

The definition of nanocomposite materials has sig-nificantly broadened in the last few years. Thisterm now encompasses a large variety of systemscombining one-, two-, and three dimensional mate-rials with amorphous materials mixed at the nano-meter scale. Natural fibers are pervasive throughoutthe world in plants such as grasses, reeds, stalks,and woody vegetation. They are also referred to ascellulosic fibers, related to the main chemical com-ponent cellulose, or as lignocellulosic fibers, sincethe fibers usually often contain a natural polyphe-nolic polymer, lignin, in their structure.The use of lignocellulosic fibers derived from annu-ally renewable resources as a reinforcing phase inpolymeric matrix composites provides positiveenvironmental benefits with respect to ultimate dis-posability and raw material use [13]. By comparingwith inorganic fillers, the main advantages of ligno-cellulosics are;– Renewable nature– Wide variety of fillers available throughout the

world– Nonfood agricultural based economy– Low energy consumption– Low cost– Low density– High specific strength and modulus– High sound attenuation of lignocellulosic based

composites– Relatively reactive surface, which can be used

for grafting specific groups.

– The recycling by combustion of lignocellulosicfilled composites is easier in comparison withinorganic fillers systems.

Therefore, the possibility of using lignocellulosicfibers as a reinforcing phase has received consider-able interest. In addition, it is necessary to discusesthe intrinsic nanoscale properties of wood and sim-ilar lignocellulosic materials for developingadvanced nanomaterials and use nanoprocesses tomodify lignocellulosic materials.

1.2.1. Structure of wood fiber

The structure of wood spans many length scales:meters for describing the whole tree, centimetersfor describing structures within the tree cross sec-tion (pith, heartwood, sapwood, and bark), millime-ters for describing growth rings (early wood,latewood), tens of micrometers for describing thecellular anatomy, micrometers for describing thelayer structure within cell walls, tens of nanometersfor describing the configuration of cellulose fibrilsin a matrix of hemicellulose and lignin, andnanometers for describing the molecular structuresof cellulose, hemicellulose, and lignin and theirchemical interactions.Wood fibers, the most abundant biomass resourceon earth, are hollow tubes made up of celluloseembedded in a matrix of hemicellulose and lignin.Most of the cell-wall materials are located in thesecond layer, which consists of a helically woundframework of microfibrils (Figure 1) [14]. Themost important attribute of wood is its mechanicalproperties, in particular its unusual ability to pro-vide high mechanical strength and high strength-to-

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Figure 1. Microstructure of wood fiber cell wall: S1, S2, and S3 are the inner, middle and outer layers of the secondarywall, respectively. Reproduced from; Jupiter.phys.ttu.edu/corner/1999/dec99.pdf

weight ratio while allowing for flexibility tocounter large dimensional changes due to swellingand shrinking. These unique properties of wood area direct result of its hierarchical internal structure.Microfibrils, nano-order-unit (10 nm×3.5 nm) [15]fibers that compose most fiber cell walls, consist ofmonocrystalline cellulose domains with cellulosechains parallel to the microfibril axis. As they aredevoid of chain foldings and contain only a smallnumber of defects, each microfibril can be consid-ered a string of polymer whiskers having a modulusclose to that of the perfect crystal of native cellu-lose, which is estimated to be 150 GPa, and pos-sessing a strength of about 10 GPa [16]. Celluloseis a polydisperse linear polymer of poly-β(1,4)-D-glucose. The monomers are linked together by con-densation such that glycosidic oxygen bridges jointhe sugar rings. In nature, cellulose chains have adegree of polymerization of approximately 10 000glucopyranose units in wood cellulose and about15 000 in native cellulose cotton. These cellulosechains are biosynthesized by enzymes, deposited ina continuous fashion and aggregate to formmicrofibrils, long threadlike bundles of moleculesstabilized laterally by hydrogen bonds betweenhydroxyl groups and oxygen of adjacent molecules.Depending on their origin, the microfibril diame-ters range from about 2 to 20 nm for lengths thatcan reach several tens of microns. These microfib-rils highly ordered (crystalline) regions alternatewith less ordered (amorphous) regions [17]. Prop-erties of cellulose crystallites from different earlierreports are concluded and shown in Table 1 [13,17–22].Beside cellulose I, cellulose II, III and IV are pres-ent with the possibility of conversion from oneform to another [23]. Regenerated cellulose II isformed whenever the lattice of cellulose I isdestroyed for example on swelling with strongalkali or on dissolution of cellulose. Since thestrongly hydrogen bonded cellulose II is thermo-chemically more stable than cellulose I, so cellu-lose I can be converted into cellulose II but cellu-lose II can not be converted into cellulose I [17].

1.2.2. Cellulosic whiskers

Native cellulose that reinforces most plant cellwalls is a typical example of a material that can bedescribed as whisker-like. Since amorphousregions act as structural defects, it is responsible forthe transverse cleavage of the microfibrils intoshort monocrystals under acid hydrolysis [24, 25].This procedure can be used to prepare highly crys-talline particles called microcrystalline cellulose[26]. Microcrystalline cellulose consists generallyof stiff rod-like particles called whiskers. Whiskersare obtained from natural fibers such as wood [27,28], sisal [29], ramie [30], cotton stalks [31] wheatstraw [22], bacterial cellulose [32, 33], sugar beet[34], chitin [35, 36], potato pulp [37, 38] as well astunicin [39, 40].The sea animals have a mantle consisting of cellu-lose microfibrils or tunicin embedded in a proteinmatrix. After deproteinization and acid hydrolysis,tunicin breaks down in the form of whiskers havingseveral microns in length [41, 42].Starting from the raw material and after successivechemical processing steps and, ultimately, con-trolled acid hydrolysis, the cellulose whiskermicrocrystals are suspended in aqueous media.Geometrical characteristics of cellulose whiskersdepend on the origin of cellulose microfibrils, aswell as on acid hydrolysis process conditions suchas time, temperature, and purity of material. DeSouza et al. [43] studied two rodlike systems inaqueous suspensions, cotton and tunicate whiskersthe average size whisker dimensions are L = 255 nmand d = 15 nm for the cotton (ratio L/d = 17) whileL = 1160 nm and d = 6 nm (ratio L/d = 72.5) for thetunicate whiskers. The whiskers are relatively easyto prepare as they are dispersible in water, in a vari-ety of sizes, and can be used in composite liquids.The main characteristics of the whiskers are theirhigh aspect ratio and their nanoscopic size. For thisreason, the interface area offered by the whiskerssurface is high. This might lead to the formation ofan interphase in which mechanical properties of thematrix are modified like the nanocomposite of plas-ticized poly (vinyl chloride) matrix reinforced bycellulose whiskers [44]. In addition, Dong et al.[45] also studied the effect of preparation condi-tions (time, temperature, and ultrasound treatment)on the resulting cellulose microcrystals structurefrom sulfuric acid hydrolysis of cotton fiber. Theyreported a decrease in microcrystalline cellulose

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Table 1. Properties of cellulose crystallites

Property Cellulose crystallitesLength [nm] 300–2000Diameter [nm] 5–20Aspect ratio, L/d 20–60Tensile strength [MPa] 10 000E-modulus [GPa] 150

length and an increase in their surface charge withprolonged hydrolysis time. On the other hand,using of sulfuric acid for cellulose whiskers prepa-ration leads to more stable whiskers aqueous sus-pension than that prepared using hydrochloric acid[29]. Indeed, the H2SO4-prepared whiskers presenta negatively charged surface, whereas the HCl pre-pared whiskers are not charged. Another way toachieve charged whiskers consists of the oxidationof the whiskers surface [46, 47] or the post-sulfa-tion of HCl-prepared microcrystalline cellulose[48]. So, whiskers can be defined as the fibers thathave been grown under controlled conditions thatlead to the formation of high-purity single crystals[49–51]. Acid hydrolysis of native cellulose leadsto aqueous rod-like suspensions of elongatedmicrocrystals with high aspect ratio. Depending ontheir origin, their lateral dimensions range from 2 to50 nm, with length up to several micrometers.When suspended in water, the cellulose whiskersdo not flocculate as they are stabilized by electro-static repulsion arising from ionic species graftedduring the acidic treatment.Cellulose whiskers have a mechanical strengthsequivalent to the binding forces of adjacent atomsleading to highly ordered structure, which producesnot only unusually high strength but also signifi-cant changes in electrical, optical, magnetic, ferro-magnetic, dielectric, conductive, and even super-conductive properties. So, it can be used as a rein-forcement of polymer matrix [52]. The reinforcingability of the cellulose whiskers lies in their highsurface area and good mechanical properties [53].However, to obtain a significant increase in mate-rial properties the whiskers should be well sepa-rated and evenly distributed in the matrix material.One of the drawbacks in using polar surface cellu-lose whiskers is that they cannot be uniformly dis-persed in non-polar media such as organic solventsor monomers. Thus, their incorporation as rein-forcement material for nanocomposite processingor their use as complex fluids has so far beenmainly limited to aqueous or polar environment. Inorder to obtain non-flocculated dispersion of cellu-lose in non-polar solvents such as alkanes, one canenvisage two routes, namely (a) coat the surface ofthe whiskers with surfactants having polar headsand long hydrophobic tails, and (b) graft hydropho-bic chains at the surface of the cellulose whiskers.By the first route, Bonini et al. [54] dispersed thesurfactant coated whisker in toluene by mixing of

surfactants with cellulose whisker in aqueous sus-pensions. After freeze-drying of these suspensions,the surfactant coated whisker could be dispersed incyclohexane. By surface acetylation stable suspen-sion of cellulose whiskers with degree of substitu-tions of 0.75 could be obtained in acetone, but notin solvents of lower polarity [55]. Another recentapproach has described suspensions of cellulosewhisker in toluene using phosphoric ester of poly(ethylene oxide) as surfactant (Figure 2), theseaqueous cellulose whisker suspensions are usuallystabilized by steric repulsion between poly (ethyl-ene glycol) chains grafted on the surface.Using surface silylation of the cellulose whiskerswith different silylating agents such as; isopropyldimethyl chlorosilane, n-butyldimethyl chlorosi-lane, n-octyldimethyl chlorosilane or n-dodecyl-dimethyl chlorosilane (Figure 3), the whiskerscould be homogeneously dispersed without aggre-gation in various organic solvents of medium polar-ity, such as acetone or tetrahydrofurane but not insolvents of very low polarity such as toluene orhexane [56].As mentioned before, wood cellulose nanofibrilshave about 25% of the strength of carbon nan-otubes, by comparing the mechanical and electricalbehavior of poly (styrene-co-butyl acrylate)nanocomposites [57, 58] reinforced by cellulosenanofibrils (obtained from sugar beet pulp) andmulti-walled carbon nanotubes (synthesized fromthe catalytic decomposition of acetylene at 720°Con supported cobalt/iron catalyst) [59], a high rein-forcement effect is achieved for cellulose filledmaterials, suggesting the presence of a rigid cellu-lose nanofibril network, linked by strong hydrogenbonds, within the material. With carbon nanotubesas fillers, no strong interactions are possiblebetween multi-walled carbon nanotubes. The soft

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Figure 2. Chemical structure of the surfactant phosphoricester of poly(ethylene oxide)

Figure 3. Preparation of silylated cellulose whiskers R = i-C3H7, n-C4H9, n-C8H17, n-C12H25

entangled nanotube network was found to have aninfluence on the composite tensile behavior only athigh temperatures. Such a soft network is efficientfor mechanical reinforcement when the polymermatrix is highly viscous. Opuntia ficus-indicacladode cells were individualized under alkalineconditions and homogenized under a shear actionto produce cellulose microfibril suspensions. Indi-vidualized microfibrils consist of flexible andhairy, high aspect ratio fibers almost 5 nm in width.The resulting suspension was used to processnanocomposite materials with a high level of dis-persion using latex of poly (styrene-co-butyl acry-late) as a matrix. Cellulose microfibrils bring agreat reinforcing effect at high temperature (T > Tg

of the matrix) and improve the thermal stability ofthe composite materials, even at very low fillerloading. The swelling behavior of the polymericmatrix was found to strongly decrease even at only1 wt% of cellulose microfibrils and was almostindependent of the filler content [60].Three types of surface characteristics cellulosewhiskers were compared; aggregated whiskerswithout surface modification, aggregated whiskersgrafted with maleated polypropylene, and novelsurfactant-modified whiskers. The whiskers wereincorporated as nanometric fillers in, polypropy-lene, by solvent casting from toluene followed byfilm pressing. The crystallization behavior of thefilms, as evaluated by X-ray diffraction, displayedtwo crystalline forms (α and β) in the nanocompos-ites containing aggregated whiskers without sur-face modification and novel surfactant-modifiedwhiskers, whereas the neat matrix and the materialreinforced with aggregated whiskers grafted withmaleated polypropylene only crystallized in the α-form. Differential scanning calorimetry experi-ments also indicated that the aggregated surfactant-modified whiskers acted as nucleating agents forthe polypropylene. The α-phase crystallites repre-sent the predominant part of the neat polypropy-lene, whereas the appearance of the other twophases (β or γ) may eventually be favored by thepresence of fillers, under high pressure or by ther-mal annealing. The mechanical properties of thenanocomposite films were evaluated by dynamicmechanical analysis, and were found to be signifi-cantly enhanced by the incorporation of the cellu-lose whiskers. Particularly, the materials with novelsurfactant-modified whiskers and aggregatedwhiskers without surface modification displayed

increased moduli as compared to the neat matrixand the aggregated whiskers grafted with maleatedpolypropylene composite. The presence of the β-phase may have an important influence on themechanical properties of the resulting composites,since its toughness is higher than that of the α-phase [61]. An environmental friendly chemicalmodification route to confer high hydrophobicity tocrystalline cellulose was developed. With lowreagent consumption and simple treatment proce-dures, highly hydrophobic whiskers can be obtained.The acylated whiskers, using iso-octadecenyl succ-nic anhydride and n-tetradecenyl succinic anhy-dride as acylating agent, could disperse in medium-to low-polarity solvents, i. e., dimethyl sulfoxide to1,4-dioxane. By controlling the heating time,whiskers with different dispersibility could beobtained. Based on its organic-solvent dispersibil-ity, the acylated whiskers are expected to be usefulin direct mixing with synthetic resins to formnanocomposites with improved dispersion andadhesion with matrices [55].

1.2.3. Bacterial cellulose

Besides being the cell-wall component of plants,cellulose is also secreted extracellularly as synthe-sized cellulose fibers by some bacterial species.Bacterial cellulose is produced by Acetobacterspecies cultivated in a culture medium containingcarbon and nitrogen sources. It presents uniqueproperties such as high mechanical strength and anextremely fine and pure fiber network structure.This network structure is in the form of a pelliclemade up of a random assembly of ribbon shapedfibrils, less than 100 nm wide, which are composedof a bundle of much finer microfibrils, 2 to 4 nm indiameter. Bacterial cellulose microfibrils have adensity of 1600 kg/m–3 [62–65]. In addition, it hassufficient porosity, 3-dimensional (3-D) networkstructure, water holding capability, and biocompat-ibility [66].Instead of being obtained by fibrillation of fibers,bacterial cellulose is produced by bacteria in areverse way, synthesizing cellulose and building upbundles of microfibrils. These bundles are some-what straight, continuous, and dimensionally uni-form. In addition, compared with animal-derivedpolymers, bacterial cellulose is free of any occur-rence of cross infection likely associated with col-lagen. Current applications for bacterial cellulose

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include use as a dietary food, as medical pads forskin burns, as reinforcement in high-strengthpapers, as binding or thickening agents, and asdiaphragms of lectoracoustic transducers [67–70].For the last application, Nakagaito et al. [71]reported a markedly high dynamic Young’s modu-lus, close to 30 GPa, for sheets obtained from bac-terial cellulose pellicles when adequately processed.Due to this remarkable modulus, bacterial cellulosesheets seemed to be an ideal candidate as raw mate-rial to further enhance the Young’s modulus ofhigh-strength composite. When bacterial cellulosepellicles compressed into sheets and impregnatedwith phenolic resin to produce high-strength com-posites. The Young’s modulus of the compositeswas significantly higher when compared to that ofmicrofibrial cellulose-based composites, 28 GPaagainst 19 GPa, respectively. The higher modulusof bacterial cellulose composites was credited tothe extremely fine, pure, and dimensionally uni-form ribbon-like cellulose microfibril bundles,arranged in a network of relatively straight and con-tinuous alignment, and also to the planar orienta-

tion of these elements obtained through the com-pression of the bacterial cellulose pellicles intosheets.

1.2.4. Starch

There are numerous examples where animals orplants synthesize extracellular high-performanceskeletal biocomposites consisting of a matrix rein-forced by fibrous biopolymers. Cellulose and chitinare classical examples of these reinforcing ele-ments, which occur as whisker-like microfibrils thatare biosynthesized and deposited in a continuousfashion. Starch is another example of natural semi-crystalline polymer that is produced by many plantsand occurs as microscopic granules. It acts as astorage polymer in cereals and tubers. These abun-dant and natural polymers can be used to createhigh performance nanocomposites presenting out-standing properties. Starch granules become swollenand gelatinized when water is added or when theyare heated, and water is often used as a plasticizerto obtain desirable product properties and during

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Figure 4. Starch gelation

this swelling amylose leaches out the water butamylopectin forms gel (Figure 4) [72]. Gelatiniza-tion of starch was found to lead to the destruction ordiminution of hydrogen bonding in granules and adecrease in crystallinity of starch [73].Aqueous suspensions of crystallites can be pre-pared by acid hydrolysis of the purified substrates.The object of this treatment is to dissolve awayregions of low lateral order so that the water-insol-uble, highly crystalline residue may be convertedinto a stable suspension by subsequent vigorousmechanical shearing action. For cellulose andchitin, these monocrystals appear as rod-like nano-particles, dimensions of which depend on the bio-logical source of the substrate. In the case of starchthey consist of platelet-like nanoparticles. Highreinforcing capability was reported resulting fromthe intrinsic chemical nature of these polymers andfrom their hierarchical structure [74]. On the otherhand, amylose content in starch may affect theproperties of the prepared composites. Ke et al.[75] studied the effect of amylose content instarches on the mechanical properties. Four drycorn starches with different amylose content wereblended with poly lactic acid at various starch/polylactic acid ratios and characterized for morphology,mechanical properties and water absorption. Ten-sile strength and elongation of the blends decreasedas starch content increased, but no significant dif-ference was observed among the four starches atthe same ratio of starch/poly lactic acid. The rateand extent of water absorption of starch/poly lacticacid blends increased with increasing starch.Blends made with high-amylose starches had lowerwater absorption than the blends with normal andwaxy corn starches.Starch can be used as a reinforcing or a matrix.Nanocomposite materials were obtained usingglycerol plasticized starch as the matrix and a col-loidal suspension of cellulose whiskers as the rein-forcing phase. After mixing the raw materials andgelatinization of starch, the resulting suspensionwas cast and evaporated under vacuum. The com-posites were conditioned at various moisture con-tents in order to evaluate the effect of this parame-ter on the composite structure. The specific behav-ior of amylopectin chains located near the interfacein the presence of cellulose probably led to a tran-scrystallization phenomenon of amylopectin oncellulose whiskers surface [76]. The reinforcingeffect of whiskers strongly depended on the ability

of cellulose filler to form a rigid network, resultingfrom strong interactions between whiskers such ashydrogen bonds, and therefore on the moisture con-tent. It was shown that increasing water contentinduced the crystallization of amylopectin chainsand the accumulation of plasticizer in the cellu-lose/amylopectin interfacial zone [77]. Anotherapproach is using latex of poly (hydroxyoctanoate)as a matrix and using a colloidal suspension ofhydrolyzed starch or cellulose whiskers as naturaland biodegradable filler. High-performance materi-als were obtained from these systems, preservingthe natural character of poly (hydroxyoctanoate).Specific polymer-filler interactions and geometri-cal constraint due to the particle size of the latexhave to be considered to account for the mechanicalreinforcement effect of cellulose whiskers [78].

2. Preparation

2.1. Nanofibrill

A variety of techniques have been used to makenanostructures.

2.1.1. By mechanical fibrillation

The fibrillation of pulp fiber to obtain nano-order-unit web-like network structure, called microfibril-lated cellulose, is obtained through a mechanicaltreatment of pulp fibers, consisting of refining andhigh pressure homogenizing processes. The refin-ing process used is common in the paper industry,and is accomplished via a piece of equipment calleda refiner. In a disk refiner, the dilute fiber suspen-sion to be treated is forced through a gap betweenthe rotor and stator disks, which have surfaces fit-ted with bars and grooves, against which the fibersare subjected to repeated cyclic stresses. Thismechanical treatment brings about irreversiblechanges in the fibers, increasing their bondingpotential by modification of their morphology andsize. In the homogenization process, dilute slurriesof cellulose fibers previously treated by refining arepumped at high pressure and fed through a springhigh pressure loaded valve assembly. As this valveopens and closes in rapid succession, the fibers aresubjected to a large pressure drop with shearing andimpact forces. This combination of forces promotesa high degree of microfibrillation of the cellulosefibers, resulting in microfibrillated cellulose [79].

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The refining process is carried out prior to homoge-nization due to the fact that refining produces exter-nal fibrillation of fibers by gradually peeling off theexternal cell wall layers (P and S1 layers) and expos-ing the S2 layer and also causes internal fibrillationthat loosens the fiber wall, preparing the pulp fibersfor subsequent homogenization treatment [80].Nakagaito et al. [81] studied how the degree of fib-rillation of pulp fibers affects the mechanical prop-erties of high strength cellulose composites. It wasfound that fibrillation solely of the surface of thefibers is not effective in improving compositestrength, though there is a distinct point in the fib-rillation stage at which an abrupt increase in themechanical properties of composites occurs. In therange between 16 and 30 passes through refinertreatments, pulp fibers underwent a degree of fibril-lation that resulted in a stepwise increment ofmechanical properties, most strikingly in bendingstrength. This increase was attributed to the com-plete fibrillation of the bulk of the fibers. For addi-tional high pressure homogenization-treated pulps,composite strength increased linearly against waterretention values, which characterize the cellulose’sexposed surface area, and reached maximum valueat 14 passes through the homogenizer (Figure 5.).

2.1.2. By electrospinning of polymer

Electrospinning derived from electrostatic spin-ning. Electrospinning has been recognized as an

efficient technique for the fabrication of polymernanofibers. Various polymers have been success-fully electrospun into ultrafine fibers e.g. celluloseacetate. There are basically three components tofulfill the process: a high voltage supplier, a capil-lary tube with a pipette or needle of small diameter,and a metal collecting screen. In the electrospin-ning process a high voltage is used to create anelectrically charged jet of polymer solution or meltout of the pipette. Before reaching the collectingscreen, the solution jet evaporates, and is collectedas an interconnected web of small fibers [82, 83].One electrode is placed into the spinning solu-tion/melt, the other is attached to the collector. Theelectric field is subjected to the end of the capillarytube that contains the solution fluid held by its sur-face tension. This induces a charge on the surfaceof the liquid. The potential difference depended onthe properties of the spinning solution, such aspolymer molecular weight and viscosity. When thedistance between the spinneret and the collectingdevice was short, spun fibers tended to stick to thecollecting device as well as to each other, due toincomplete solvent evaporation. Mutual chargerepulsion and the contraction of the surface chargesto the counter electrode cause a force directly oppo-site to the surface tension [84]. As the intensity ofthe electric field is increased, the hemisphericalsurface of the fluid at the tip of the capillary tubeelongates to form a conical shape known as theTaylor cone [85]. By further increasing in the elec-tric field, a critical value is attained with which therepulsive electrostatic force overcomes the surfacetension and the charged jet of the fluid is ejectedfrom the tip of the Taylor cone. The dischargedpolymer solution jet undergoes an instability andelongation process, which allows the jet to becomevery long and thin. Meanwhile, the solvent evapo-rates, leaving behind a charged polymer fiber. Inthe case of the melt the discharged jet solidifieswhen it travels in the air.Most of the polymers were dissolved in some sol-vents before electrospinning. When the solid poly-mer or polymer pellet is completely dissolved in aproper amount of solvent that is held, for example,in a glass container, it becomes a fluid form calledpolymer solution. The polymer fluid is then intro-duced into the capillary tube for electrospinning.Both the dissolution and the electrospinning areessentially conducted at room temperature withatmosphere condition.

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Figure 5. Homogenizer (Microfluidizer) used for the pro-duction of microfibrillated cellulose

Polymers, molten in high temperature, can also bemade into nanofibers through electrospinning.Instead of a solution, the polymer melt is intro-duced into the capillary tube. However, differentfrom the case of polymer solution, the electrospin-ning process for a polymer melt has to be per-formed in a vacuum condition [86–88]. Namely,the capillary tube, the traveling of the charged meltfluid jet and the metal collecting screen must beencapsulated within a vacuum. A schematic dia-gram to interpret electrospinning of polymernanofibers is shown in Figure 6.A polymer solution, such as cellulose acetate dis-solved in 2:1 acetone: dimethyl acetamide wasintroduced into the electric field. The polymer fila-ments were formed, from the solution, between twoelectrodes bearing electrical charges of oppositepolarity. One of the electrodes was placed into thesolution and the other onto a collector. Onceejected out of metal spinnerets with a small hole,the charged solution jets evaporated to becomefibers which were collected on the collector [89].

2.1.3. From sea animals

Colloidal suspensions of cellulose whiskers inwater were prepared as following; the shells of thetunicates cut into small fragments and bleach bythree successive treatments with sodium hypochlo-rite in dilute acetic acid. Heat the mixture to70–80°C and keep at this temperature for 1 hour.After the third cycle, the tunicate mantles isolatevia decanting, wash with ice water, and disintegratein blender into an aqueous suspension (tunicatecontent ~3% w/w). The disintegrated mantles sub-sequently hydrolyze by adding concentrated sulfu-ric acid, heating the mixture to 80°C, and rigorousstirring at this temperature for 20 minute to yield asuspension of cellulose whiskers. After washingwith water until the pH is neutral, adding water so

that the whisker concentration, suspension of cellu-lose whiskers will be obtained [90].

2.1.4. From microcrystalline cellulose in organicsolvent

The microcrystalline cellulose was swelled andpartly separated to whiskers by chemical and ultrasonification treatments. Dimethyl acetamide with0.5 wt% LiCl solution was used as swelling agent.The microcrystalline cellulose in LiCl/dimethylacetamide was 10 wt% which was agitated using amagnetic stirrer for 12 hour at 70°C to swell themicrocrystalline cellulose particles. The slightlyswelled particles were then sonicated in an ultra-sonic bath for 3 hours over a period of 5 days withlong intervals between each sonication treatment,to separate cellulose nano whiskers [91].

2.1.5. By acid hydrolysis

Suspensions of nanocrystalline cellulose were pre-pared. Hydrolysis was carried out with sulfuric acidwith constant stirring. Immediately following theacid hydrolysis, the suspension dilute 10-fold withdeionized water to quench the reaction. The sus-pension centrifuges at 6000 rpm for 10 min to con-centrate the cellulose and to remove excessaqueous acid. The resultant precipitate should berinsed, recentrifuged, and dialyzed against waterfor 5 days until constant neutral pH [92].

2.1.6. From bacterial cellulose

Cellulose can be synthesized by some bacteria [31,33]. For example, the cellulose was produced bystatic cultivation of Acetobacter xylinum, subspecies BPR2001, in a fructose/CSL medium at30°C [93]. The bacteria were grown in 400 mlErlenmeyer flasks containing 100 ml of media. Inorder to remove the bacteria and to exchangeremaining media, the produced cellulose pellicleswere boiled in 1 M NaOH at 80°C for 1 hour fol-lowed by repetitive boiling in deionised water. Toprevent drying and to avoid contamination, thewashed cellulose was stored in diluted ethanol in arefrigerator.The advantage in using bacterial cellulose as amodel for plant cellulose lies in its high purity, finefibrils (high surface area) [94], high tensile strengthand water-holding capacity. So, bacterial cellulose

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Figure 6. Schematic diagram to show polymer nanofibersby electrospinning.

has been used as a reinforcing in nanocomposites[95].

2.2. Preparation of nanocomposite films

2.2.1. From regenerated cellulose

Microcrystalline cellulose powder is produced byacid hydrolysis of amorphous domains, whichresults in high crystallinity. Microcrystalline cellu-lose was activated for 6 hours in distilled H2O atroom temperature. Subsequently, the cellulose wasdehydrated in ethanol, acetone, and N,N-dimethylacetamide for 4 hours each. After decanting N,N-dimethyl acetamide from the dehydrated cellulose,LiCl/N,N-dimethyl acetamide solution was pouredonto cellulose sample and stirred for 5 minutes.The solutions were then poured into Petri dish, andleft at ambient atmosphere for 12 hour. After thistime a 5–8 mm thick transparent gel had formedwhich was washed in distilled water and dehy-drated between gently compressed sheets of paper.The final nanocomposite films were optically trans-parent and had a thickness between 0.2 and 0.5 mm[96].

2.2.2. By solution casting

For preparing solid polymer nanocomposite film,combine appropriate amounts of the nanoreinforce-ment’s solution and dissolved polymer matrix.Two processing conditions can be used to preparethe composites film from this mixture.The mixture cast in a Petri dish and put in a dryingoven under vacuum. The chosen temperatureallows the solvent evaporation and the film forma-tion (i. e. polymer particles coalescence). A so-called evaporated film is obtained and materialscompression in mold under heating and pressure[92, 97].The second route used to elaborate composite film,the mixture is first freeze-dried to allow water sub-limation, and a compact soft powder is obtained.This powder then press under heating and pressure[62].

2.2.3. By extrusion

For mixing dry material with suspension solution;the composite materials will be compounded usinga co-rotating twin-screw extruder with a gravimet-ric feeding system for dry materials and a peristaltic

pump for the cellulose whiskers suspension. Fig-ure 7, shows a schematic picture of the compound-ing process.For preparing nanocomposite film of poly (lacticacid)-malic anhydride-poly (ethylene glycol) rein-forced with cellulose whiskers suspended in LiCl/dimethyl acetamide, poly (lactic acid) was fed inzone 1 and the cellulose whiskers suspension waspumped into the melted polymer at zone 4. The liq-uid phase was removed by atmospheric venting inzones 7 and 8 and by vacuum venting in zone 10.Both poly (lactic acid)-malic anhydride and poly(ethylene glycol) were premixed and fed in zone 1.The LiCl/dimethyl acetamide was removed by theventing system during the extrusion. The extrudedmaterials were compression moulded to ~1.3 mmthickness.The color on the samples was changed from trans-parent yellow to light brown which indicates ther-mal degradation. The color change and decreasedmechanical properties indicates that the LiCl/dimethyl acetamide is not suitable as swelling-sep-aration agent for cellulose whiskers if high temper-ature processing is used [91].

2.2.4. By electrostatic layer-by-layer

Electrostatic layer-by-layer self-assembled filmshave been exploited for the fabrication of sophisti-cated nanocomposite incorporated the linear poly-mer cellulose sulfate. In this method, a chargedsolid substrate is exposed to a solution of oppo-sitely charged polyelectrolyte, followed by rinsing.The polymeric material adhering to the surface hasmore than the stoichiometric number of chargesrequired for charge neutralization, thereby revers-ing the surface charge. This allows for easy adsorp-tion of the next oppositely charged polyelectrolyte,also resulting in charge reversal. The amount ofadsorbed polymer is self-limiting as a result of rins-ing and allows for stepwise film growth [98, 99].

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Figure 7. Schematic picture of the compounding process:feeding of poly (lactic acid) in zone 1, pumpingof dispersed of cellulose whiskers in zone 4 andremoval of the liquid using atmospheric andvacuum venting in zones 7, 8 and 10

Resultant films and coatings show long-life stabil-ity as well as self-healing characteristics.Structured layer-by-layer films have potentialapplications as antireflective coatings [100], wave-guides, bio/optical sensors [101], separation tech-nologies and drug delivery systems [102]. Conven-tionally, layer-by-layer assembly has employedsolution-dipping (or dip-coating) in beakers of var-ious sizes containing dilute aqueous polymer solu-tions. This inexpensive method works for most sub-strates independent of shape but has not alwaysresulted in adequately homogeneous films. Alter-natively, spin-coating is the most widely used tech-nique for obtaining uniform films in lithographyand other micromachining applications. The spin-coating process involves the acceleration of a liquidsolution on a rotating substrate and is characterizedby a balance of centrifugal forces (spin speed) andviscous forces (solution viscosity). Films createdby this way have been found to be consistent andreproducible in thickness [103]. Nanocrystallinecellulose is amenable to sequential film growth bylayer-by-layer assembly, as presented schemati-cally in Figure 8.Thin multilayered films incorporating polyelec-trolyte layers such as poly (allylamine hydrochlo-ride) and nanocrystalline cellulose layers wereprepared by the electrostatic layer-by-layer method-ology, as well as by a spin coating variant. Bothtechniques gave rise to smooth and stable thinfilms, as confirmed by atomic force microscopysurface morphology measurements as well as scan-ning electron microscopy investigations. Films pre-pared by spin-coating were substantially thickerthan solution-dipped films. Thus both techniquesare viable for producing structured nanocompos-ites, where the large aspect ratio cellulose unitsmay serve to strengthen the elastic polymer matrix[93].

3. Characterization

Characterization of nanofibers and nanocompositescan be performed using different techniques suchas transmission electron microscopy (TEM), X-rayand neutron diffraction, dynamic infrared spec-troscopy, atomic force microscopy (AFM), differ-ential scanning calorimetry (DSC), small angleneutron scattering (SANS), etc.

3.1. Nanoindentation techniques

Mechanical properties of materials have been com-monly characterized using indentation techniques.Properties that are measured by indentationdescribe the deformation of the volume of materialbeneath the indenter (interaction volume). Defor-mation can be by several modes: elasticity, vis-coealasticity, plasticity, creep, and fracture. Thesedeformation modes are described by the followingproperties, respectively: elastic modulus, relaxationmodulus, hardness, creep rate, and fracture tough-ness [2].

3.2. Microscopy characterization

Scanning electron microscopy (SEM) as well asatomic force microscopy (AFM) can be used forstructure and morphologies determination of cellu-lose whiskers and their nanocomposites. From con-ventional bright-field transmission electronmicroscopy (TEM) it was possible to identify indi-vidual whiskers, which enabled determination oftheir sizes and shape. AFM overestimated the widthof the whiskers due to the tip-broadening effect.Field emission SEM allowed for a quick examina-tion giving an overview of the sample; however,the resolution was considered insufficient fordetailed information.By comparing TEM, Field emission scanning elec-tron microscopy (FESEM) and AFM analysis ofcellulose whiskers as in Figure 9, it is shown that;a) the whiskers did not differ significantly in con-trast from the carbon film, b) it was difficult toclearly discern individual whiskers from agglomer-ated structures, and therefore an estimate of thewidth of the whiskers was not obtained while in, c)the structures differed from the needlelike shape asobserved in TEM. The whiskers appeared signifi-cantly broader having a rounded shape. AFM couldtherefore be a powerful alternative to conventional

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Figure 8. Schematic representation of the build-up of elec-trostatically adsorbed multilayered films. Thepolyelectrolyte, cationic poly(allylaminehydrochloride), is shown by the curved line andcolloidal cellulose nanocrystals are representedby straight rods; counterions have been omittedfor clarity

bright-field transmission electron microscopy insuch composite materials [104].

3.2.1. Small angle neutron scattering

Due to the sensitivity of this technique, this methodwas used for instance to investigate polymer layersgrafted onto spherical particles with high specificarea such as whisker, silica, latex particles or sur-factants adsorbed onto ferrofluid particles [54, 105,106].

3.3. X-ray diffractometry

The structural properties of the nanocompositefilms can be characterized using X-ray diffractionlike, the size of the cellulose crystallites [107] andcrystallinity index [108].

3.4. Tensile tests

The nonlinear mechanical behavior of compositesand unfilled matrix was analyzed using testingmachine in tensile mode. The true strain ε wasdetermined by the Equation (1):

(1)

where L and L0 are the length of the specimen at thetime of the test and the length at zero time, respec-tively.The true stress σ was calculated by the Equation (2):

(2)

where F is the applied load and S is the cross-sec-tional area at the time of the test.From the stress-strain curves, the Young modulus(E) was determined from the slope of the low-strainregion [21].

3.5. Electrical conductivity

For measuring ionic conductivities of ion-conduct-ing solid polymer electrolytes or composites, thesample must be coated at their end with a silverpaint to ensure a good electrical contact. Electricalconductivity measurements are performed at ambi-ent temperature using several frequencies. Thecomplex admittance is Y* recorded versus time.From this admittance, the conductivity σ*c can bededuced by the Equation (3):

(3)

where W, L and T are the width, the length and thethickness of the sample during the test, and V0 (thesample initial volume) = L0·T0·W0 [58].

3.6. Transparency measurements

Transparency can be carried out using UV-visualspectrometry which gives the amount of light beingtransmitted trough the nanocomposite films at dif-ferent wavelengthes [97].

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0

2***

V

LY

WT

LYc ==σ

0

lnL

L=ε

S

F=σ

Figure 9. TEM, FESEM and AFM images of cellulose whiskers. a) Bright-field TEM image of stained cellulose whiskerson a porous carbon film, b) FESEM backscatter overview image of stained cellulose, c) AFM phase image ofcellulose whiskers

3.7. Thermomechanical analysis

Thermomechanical measurements are performusing a thermomechanical analyzer. This analysisshows the temperature dependencies of a dynamicstorage modulus [92].

3.8. Rheological behavior of suspensions

Rheological data could be collected with a rotatingrheometer.

4. Applications

4.1. Cellulose–cellulose nanocomposites

Composite materials, typically glass fibers or car-bon fibers embedded into epoxy resin or unsatu-rated polyester, show excellent mechanical andthermal properties; thus, they are widely used invarious applications ranging from aerospace tovehicles to sports utensils [109]. However, theseadvantages cause environmental problems whendisposing by incineration. Consequently, there aregrowing demands for environmentally friendlycomposites. Cellulose is the most abundant bio-mass resource and possesses excellent mechanicaland thermal properties as mention before. Naturalcellulose (cellulose I) also boasts an elastic modu-lus, El, of 138 GPa for the crystalline regions in thedirection parallel to the chain axis. This is compara-ble with the El values of high performance syntheticfibers such as poly (phenylene terephthalamide)130 GPa. In addition, the maximum macroscopicYoung’s modulus of natural plant cellulose (up to128 GPa) is higher than those of aluminum(70 GPa) and glass fibers (76 GPa). The ultimatetensile strength of cellulose is estimated to be17.8 GPa. This is 7 times higher than that of steel.Intrinsically, the very high elastic modulus and ten-sile strength (not specific modulus and specificstrength) imply that cellulose possesses the poten-tial to replace glass fiber, and it shows promise as areinforcement fiber for composites where the den-sity is not a concern. Current trends toward envi-ronmentally friendly composites focus on the useof cellulose fibers [110]. The interface between thefiber and the matrix often brings serious problemssuch as poor adhesion and water uptake by thecomposites. On the other hand, when the fiber andthe matrix are composed of the same material,

some benefits relevant to recyclability, and a betteradhesion at interface, can be expected [111].Cellulose is well known not to melt, but showsthermal degradation at high temperature. There-fore, to process cellulose, a wet process should beemployed. Consequently, cellulose/cellulose com-posite was manufactured by the wet process usingLiCl/N,N-dimethyl acetamide as cellulose solventand by controlling the solubility of cellulosethrough pretreatment conditions. This composite istotally composed of sustainable cellulosicresources, so it can be biodegradable after service.Cellulose self-reinforced composite, possessedexcellent mechanical and thermal properties duringuse as well as transparent to visible light. This com-posite can be used as an alternative of the glass-fiber-reinforced composite. By choosing the pre-treatment condition to the fiber, the transversemechanical properties of the composite can be alsoenhanced through the molecular diffusion acrossthe interface between the fiber and the matrix [92,111].The ratio of cellulose I and II affects the propertiesof the resulting nanocomposites. So, cellulose-based nanocomposite films with different ratios ofcellulose I and II were produced by partial dissolu-tion of microcrystalline cellulose powder inLiCl/N,N-dimethyl acetamide and subsequent filmcasting. The films are isotropic, transparent to visi-ble light, highly crystalline, and contain differentamounts of un-dissolved cellulose I crystallites as afiller. By varying the cellulose I and II ratio, themechanical performance of the nanocompositescan be tuned depending on the composition. Also,the nanocomposites clearly surpass the mechanicalproperties of most comparable cellulosic materials,their greatest advantage being the fact that they arefully biobased and biodegradable, but also of rela-tively high strength [92].

4.2. Nanocomposites from cellulose derivatives

The chemical modification of dissolving-gradewood pulp fibers with a variety of acids and anhy-drides represents longstanding industrial practice.Cellulose ethers and cellulose esters are used for awide variety of products in the food, householdproducts, health care, textile, and many other indus-tries. Esters with short alkyl chains (acetate or pro-pionate) form solvent-soluble, spinnable fibers;

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esters with bulky substituents (butyrate, hexanoate,and higher) are progressively more thermallydeformable under pressure. Such cellulose esterpowders in the presence of different plasticizersand additives are extruded to produce variousgrades of commercial cellulose plastics in pal-letized form [112, 113]. Among biopolymericmaterials, cellulose and cellulose derivatives enjoywidespread use and remain the single largestbiopolymer. In multiphase polymer materials, cel-lulose may find application in both polymericblends and as fibers in reinforced polymeric com-posites. Recently, composite materials based oncellulose derivatives have been the subject ofintense research.Polymer nanocomposites are one of the importantapplication areas for nanotechnology, as well asnaturally derived organic nanophase materials areof special interest in the case of polymer nanocom-posites. Nanocomposites have been successfullyfabricated from cellulose acetate, triethyl citrateplasticizer, and organically modified clay Cloisite30B with and without maleic anhydride grafted cel-lulose acetate butyrate as a compatibilizer. The cel-lulosic plastic with cellulose acetate/triethyl citrate(80/20 wt%) was used as the polymer matrix fornanocomposite fabrication. Cellulosic plastic-based nanocomposites obtained using increasedpre-plasticizing times showed better exfoliatedstructures. In the system containing compatibilizer,the minimum retention time required for obtainingalmost completely exfoliated hybrid nanocompos-ites was shorter than in the system without compat-ibilizer [114–118].Cellulose diacetate films incorporated withsmall amount of montmorillonite nanoclay(Al4Si8O20(OH)4·nH2O) were prepared from meth-ylene chloride/ethanol (9:1 wt/wt) casting solu-tions. The various nanoclays were incorporated intothe cellulose structure in order to enhance themechanical properties as well as thermal stabilityof cellulose. The plasticizers used were: dibutylphthalate, diethyl phthalate, poly (ethylene glycol).The films were completely transparent in the com-position range of 10 to 30 w/w plasticizers and 1 to7 w/w montmorillonite nanoclay. The strength offilms decreased with the increase in the plasticizercontent. All the films gave a single glass transitiontemperature, Tg, which decreased sharply from180°C of the original cellulose diacetate to approx-

imately 95°C according to the content and kind ofplasticizer [119, 120]. When the plasticizer wasadded into the cellulose diacetate film up to 30 wt%,the Young's modulus of film was decreased from1930 MPa to 1131 MPa but was increased from1731 MPa to 2272 MPa when the montmorillonitenanofiller was added into the film up to 7 wt%. Themechanical properties of cellulose diacetate filmswere decreased by addition of plasticizer butstrengthened by the incorporation of montmoril-lonite nanofiller. Also, nanocomposites were syn-thesized using cellulose acetate bioplastic, citratebased plasticizer and organically modified claynanofillers. Transmission electron microscopyrevealed the existence of intercalated clay dis-persed throughout the cellulose acetate matrix. Theintercalated reinforcements resulted in enhance-ments of the composite tensile strength, tensilemodulus, and coefficient of thermal expansion. Thecomposite tensile strength of cellulose acetateincreased approximately 38% after incorporating5 wt% clay. The tensile modulus was also enhancedas much as 33% [121].Biodegradable cellulose acetate/layered silicategrafted poly(ε-caprolactone) nanocomposites wereprepared by in situ polymerization of ε-caprolac-tone in the presence of cellulose acetate and organ-ically modified layered silicate [122].Nearly monodisperse nanoparticles have been syn-thesized based on a naturally occurring polymer ofhydroxypropyl cellulose. The hydroxypropyl cellu-lose nanoparticle assembly in water has been fur-ther stabilized by covalently bonding neighboringparticles to form a three-dimensional network.This network contains a large amount of water sim-ilar to a conventional bulk gel [123]. From mor-phology analysis of hydroxypropyl cellulose fibrilreinforced nanocomposites using nanoindentation.It was observed that hydroxypropyl cellulose com-posite with a fibril showed lighter and darker struc-tures that can be explained by a contrast of crys-talline fibril areas and the amorphous polymermatrix. The direct use of aqueous fibril suspensionsfor the compounding with hydroxypropyl cellulosewas found to be an effective method of dispersingthe fibrils within the polymer matrix. The resultsshow a homogeneous distribution of cellulose fib-rils at higher magnifications [124].Another less studied form of cellulose, microcrys-talline cellulose, which is used in the production ofpharmaceutical tablets as binding material and

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compatibility enhancer. Borges et al. [125] pro-duced and characterized composites based on a cel-lulose derivative, 2-hydroxypropylcellulose used asmatrix and reinforced with microcrystalline cellu-lose fibers. As both the matrix and Avicel fibershave free hydroxyl groups in the anhydroglucloseunits, they proposed a new way of elaborating thesenatural composites through the direct couplingbetween the fibers and the matrix. Different diiso-cyanates were used as coupling agents and theobtained composites presented improved mechani-cal properties as the result of a better fiber-matrixadhesion. In addition, the fibers concentration inthe composite material used up to 30 w/w%. Thismeans that, the inclusion of fibers in a hydrox-ypropylcellulose matrix produces composites withsuperior mechanical properties.Carboxymethyl cellulose is a polyelectrolytederived from natural materials. It has been exten-sively studied as a hydrogel polymer. The effect ofnano-sized fillers on the properties of carboxy-methyl cellulose-based composites is of interest inthe development of novel or improved applicationsfor hydrogel polymers in general and carboxy-methyl cellulose in particular. The composite mate-rial composed of carboxymethyl cellulose,microcrystalline cellulose or cellulose nanocrys-tals, with glycerol as a plasticizer was prepared.Cellulose nanocrystals improved the strength andstiffness of the resulting composite compared tomicrocrystalline cellulose. In addition, a simpleheat treatment was found to render the nanocom-posite water resistant [126].Ethyl cellulose/montmorilloniote nanocompositefilm plasticized with environmental-friendly plasti-cizer epoxidized soybean oil was prepared by meltprocess using Haake mixer. The addition of 10 wt%epoxidized soybean oil causes a decrease of Tg

from 81 to 61°C. When the plasticizer was addedinto the ethyl cellulose films, the mechanical prop-erties of ethyl cellulose films was decreased, how-ever the addition of montmorillonite into the ethylcellulose films or the ring opening reaction ofepoxidized soybean oil plasticizer cause enhance-ment of mechanical properties [127].The reaction of dissolving-grade wood pulp fiberswith a mixed p-toluenesulfonic/hexanoic acidanhydride system and a titanium (IV) isopropoxidecatalyzed system under non-swelling reaction con-ditions produces fibers that represent biphasic cel-lulose derivatives. Transparent or semitransparent

composite sheets can be formed in which the ther-moplastic cellulose hexanoate phase is consoli-dated into a continuous matrix reinforced with dis-continuous cellulose I domains. The cellulose Icomponent also varies with the extent of modifica-tion, and it provides for biodegradability and rein-forcement (i. e., high modulus). Although no pre-cise dimensions can be determined for the respec-tive phase domains, the low solubility of theproducts and the decrease in the crystal size of cel-lulose I suggest that they are on the nanometerscale. There are differences in some propertiesbased on the difference in the distribution of subsis-tent between the materials generated by toluenesul-fonic/hexanoic acid anhydride and the titanium(IV) isopropoxide catalyzed systems [107].

4.3. Cellulose–inorganic nanocomposites

The integration of polymers and inorganic materi-als is an attractive field in materials science. Unfor-tunately, because of the differences in their individ-ual intermolecular interaction forces, the interfacialincompatibility between inorganic and organicpolymers often causes failures in the preparation ofthese composites. The lack of affinity and hydropho-bic polymers make it difficult for a homogeneousmixture to be achieved. Tourmaline, from sing-halese tourmaline, a mixed stone, is a naturallycomplex group of hydrous silicate minerals con-taining Li, Al, B, and Si and various quantities ofalkalis (K and Na) and metals (Fe, Mg, and Mn). Itis the principle boron-containing mineral in thecrust and has its genesis in both igneous (princi-pally pegmatites) and metamorphic rocks. Its struc-tural formula is Na(Li,Al)3Al6(BO3)3Si6O18(OHF)4.Tourmaline forms hexagonal and prismatic crystalsand possesses as antibacterial activity. On the otherhand, the NaOH/thiourea aqueous system ishydrophilic, so it can be expected to dispersehydrophilic tourmaline nanoparticles. By usingNaOH/thiourea aqueous solutions as cosolvents ofcellulose and nanoparticles provides a simple,cheap, and pollution-free way of preparing suchcomposite materials. By this way Dong et al. [128]prepared cellulose/tourmaline nanocompositesfilms via a casting method through coagulationwith CaCl2 and HCl aqueous solutions. Resultsindicated that adhesion between regenerated cellu-lose and filled nanocrystals can occur, and the dis-persion of nanocrystals in cellulose is homoge-

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neous but the induction of tourmaline breaks thepartial intermolecular hydrogen bonds of cellulose,and this result in a reduction of the thermal stabilityof the composite films but composite films withtourmaline exhibit satisfying antibacterial actionagainst Staphylococcus aureus. So, using NaOH/thiourea aqueous solutions as cosolvents of cellu-lose and nanoparticles provides a simple, cheap,and pollution-free way of preparing compositematerials.Suber et al. [129] study the synthesis and the com-parative structural and morphological study of ironoxide nanoparticles in polystyrene-based ion-exchange resins and cellulose. The results showthat the matrix influences the iron oxide particlesize; the average size is about 7 nm in the resinsand 25 nm in the cellulose. In the resins, particlesare present inside the pores and as aggregates onthe surface of the resin beads, whereas in the cellu-lose they are present on the surface and in theswollen network of the microfibers constituting thesingle fibers.

4.4. Starch nanocomposites

A number of researchers have presented work in thefield of starch nanocomposites. Park et al. [131]reported on the preparation and properties of gelati-nized starch/montmorillonite clay nanocompositesusing both naturally occurring sodium montmoril-lonite and a number of alkyl ammonium modifiedclays. X-ray diffraction and transmission electronmicroscopy showed an intercalated structure for thestarch/sodium montmorillonite, however the modi-fied clays appeared either unchanged or agglomer-ated in structure [130]. Also they found that theunmodified sodium montmorillonite/starch nano-composite also exhibited the greatest increase inmodulus of all the clays used. Park et al. [132] alsoinvestigated the formation of montmorillonite-rein-forced glycerol/plasticized thermoplastic starch.Scanning electron microscopy showed well-dis-persed montmorillonite platelets. The Fourier-Transformed Infrared spectra indicated cooperationexisted between montmorillonite and starch mole-cules and hydrogen bonds that formed between thereactive hydroxyl groups of montmorillonite and thehydroxyl groups of starch molecules. The mechani-cal and thermal properties of the starch nanocom-posites formed showed significant improvements.Wilhelm et al. [133] have also investigated the for-

mation of starch/clay nanocomposites, using a Ca2+

hectorite (Mg6Si8O20(OH)4·nH2O). Solution caststarch/clay nanocomposites showed no X-ray dif-fraction due to the first basal spacing, indicatingalmost total exfoliation in the starch matrix.Although this was a good result, starch does requiresome kind of plasticizer to reduce the brittleness ofthe starches. When glycerol was added by itself tothe clay, the interplanar distance increased. Fischeret al. [134, 135] also investigated starch/claynanocomposites and a number of experimentalpathways were investigated, including the disper-sion of Na+ montmorillonite clay in water, followedby blending in an extruder at a temperature of85–105°C with a premixed powder of potato starch,glycerol and water. The resulting material appearedto be fully exfoliated and exhibited a reduction inhydrophilicity, and improved stiffness, strength andtoughness.The natural smectite clays, montmorillonite andhectorite, readily formed nanocomposites withthermoplastic starch which prepared by melt-pro-cessing of starch and glycerol [136]. In all cases,clay increased the elastic modulus of thermoplasticstarch. The moduli of treated-hectorite and kaolin-ite composites were very similar at similar clayloadings and were lower than the nanocomposites,for nanocomposites, montmorillonite generallyprovided a slightly greater improvement in themodulus than untreated hectorite [137]. Anothergroup of starch-based nanocomposites is those thatare blended with biodegradable polyesters.McGlashan and Halley studied the dispersion ofnanoclays in a number of different biodegradablestarch/polyester blend formulations. The crystal-lization temperature of the nanocomposite blendswas found to be significantly lower than the baseblend, probably due to the clay platelets inhibitingorder, and hence crystallization, of the starch andpolyester. The best dispersions were found in the30 wt% starch blends [138]. Kalambur and Rizvi[139] also investigated starch nanocompositesblends and successfully made starch/polylactic acidblends in the presence of montmorillonite nanoclay(Al4Si8O20(OH)4·nH2O). By study the thermal sta-bility of nanocomposites of starch/clay it is foundthat there is no significant effect of clay on the ther-mal degradation of starch, whereas a significantincrease in thermal stability was observed whennanocomposites of thermoplastic starch and unmod-

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ified montmorillonite was prepared by melt interca-lation method even at 5% filler content. Thus, thepreparation method might have an effect on theproperties. Composites have been prepared by solu-tion method after drying of starch and clay at110°C. All composites show highest weight loss at296°C. It was assumed that thermal degradationwas influenced by hydroxyl group exposure, claydispersion and reassociation of starch chains whereclay dispersion was more important than others[140]. However, hydrophobic poly (lactic acid) andhydrophilic starch are thermodynamically immisci-ble, leading to poor adhesion between the two com-ponents, and hence poor and irreproducible per-formance. Various compatibilizers and additiveshave been investigated to improve the interfacialinteractions of these blends. Wang et al. [141, 142]used methylenediphenyl diisocyanate to improvethe interface and studied a blend of 55/45 w/w mix-ture of poly (lactic acid) and dried wheat starch inan intensive mixer with or without a low level ofmethylenediphenyl diisocyanate. Blends with meth-ylenediphenyl diisocyanate had enhanced mechani-cal properties that could be explained by the in situformation of a block copolymer acting as a compat-ibilizer. Scanning electron microscopy showedreduced interfacial tension between the two phases.The presence of methylenediphenyl diisocyanatealso enhanced the mechanical properties of theblend at temperatures above Tg. Water uptake bythe poly (lactic acid)/ starch blends with and with-out methylenediphenyl diisocyanate did not differ.Wang et al. [143] also studied the effect of starchmoisture content on the interfacial interaction of anequal-weight blend of wheat starch and poly (lacticacid) containing 0.5% methylenediphenyl diiso-cyanate by weight. Starch moisture (10–20%) had anegative effect on the interfacial bonding betweenstarch and poly (lactic acid). The tensile strengthand elongation of the blend both decreased asstarch moisture content increased. In blends of poly(lactic acid) /starch using dioctyl maleate as a com-patibilizer markedly improved the tensile strengthof the blend, even at low concentrations (below5%). When dioctyl maleate functioned as a plasti-cizer at concentrations over 5%, significantenhancement in elongation was observed. Compat-ibilization and plasticization took place simultane-ously according to the blends [144]. With dioctylmaleate as a polymeric plasticizer, thermal loss inthe blends was not significant. Water absorption of

poly (lactic acid)/starch blends increased withdioctyl maleate concentration. Other compatibiliz-ers were also studied for the starch/poly (lacticacid) blends, such as poly (vinyl alcohol) [145] andpoly(hydroxyester ether) [146] it was added to astarch and poly (lactic acid) blend (50/50, w/w) toenhance compatibility and improve mechanicalproperties.

4.5. Reinforcing agent for polymer electrolytes for lithium batteries application

Ion-conducting solid polymer electrolytes haveattracted considerable interest, because of theirpotential application in rechargeable batteries, fuelcells, light-emitting electrochemical cells, elec-trochromics, and many other electrochemicaldevices [147–149]. Cellulose crystallites in theform of microcrystalline cellulose are currently uti-lized widely industrially. In the nanocompositefield, cellulose whiskers can be used as mechanicalreinforcing agents of low-thickness polymer elec-trolytes for lithium batteries application but, thefiller content is generally relatively low, below10 wt%, avoiding significant decrease of the ionicconductivity. Nanocomposite polymer electrolytesbased on high-molecular weight poly (oxy ethyl-ene) were prepared from high aspect ratio cellulosenanocrystalline whiskers and lithium trifluo-romethyl sulfonyl imide. The main effect ofwhisker is thermal stabilization of the modulus ofcomposites above the melting point of the poly(oxy ethylene)/lithium trifluoro methyl sulfonylimide complexes. The filler provides a high rein-forcing effect, while a high level of ionic conduc-tivity is retained with respect to unfilled polymerelectrolytes. So the ionic conductivity was quiteconsistent with the specifications of lithium batter-ies [150, 151].To study the effect of cellulose whiskers onmechanical properties of nanocomposite an aque-ous suspension of high aspect ratio rod-like cellu-losic particles composed of tunicin whiskers and apoly (oxy ethylene) aqueous solution casted in aPetri dish. After water evaporation a solid compos-ite film was obtained, the mechanical behavior oftunicin whiskers/poly (oxy ethylene) nanocompos-ites was evaluated in the linear range over a broadtemperature range from dynamic mechanical analy-sis. The main effect of the filler was a thermal sta-

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bilization of the storage modulus for the compos-ites above the melting temperature of the poly (oxyethylene) matrix. It was shown that the formationof the cellulosic network through inter-whiskershydrogen bonds, assumed to be responsible for thehigh mechanical properties of the composites, wasnot affected by the matrix crystallization processand filler/poly (oxy ethylene) interactions [152].The incorporation of liquid compounds like plasti-cizers into the polymer electrolytes improves theionic conduction. It generally results from thedecrease of glass transition temperature of the com-plex, the reduction of crystallinity, the increase ofsalt dissociation capability and the rise of chargecarrier diffusions. However, a decrease in mechani-cal strength of the resulting polymer electrolytes ispredictable. Different plasticizers were used toenhance the conductivity. Plasticizers generallyconsist of low molecular weight organic moleculeslike propylene carbonate, ethylene carbonate,dimethyl carbonate, dioctyl phthalate, etc. An effi-cient plasticizer must display several properties:miscibility with poly (oxy ethylene), low viscosity,low volatility, electrochemical stability and highcapability to solvate lithium salts. Low molecularweight poly (ethylene glycols) have chemical struc-ture similar to that of poly (oxy ethylene), are ther-mally stable and can solvate lithium salts. To obtainnew nanocomposite polymer electrolytes with bothimproved mechanical properties and improved con-ductivities, Azizi et al. [153] studied, poly (oxy eth-ylene)-lithium trifluoromethyl sulfonyl imide-based polymer electrolyte with tetra(ethylene)glycol dimethyl ether as plasticizer and cellulosewhiskers as nanometric filler. The plasticizerinduces both a loss of mechanical stiffness in therubbery state of poly (oxy ethylene) and an increaseof the ionic conductivity of the electrolyte. For salt-free systems and polymer electrolytes based onpoly (oxy ethylene), a high reinforcing effect wasobserved above Tg of poly (oxy ethylene) whenadding a low amount of tunicin whiskers. In addi-tion, the filler provides a thermal stabilizationeffect of the material above the melting of poly(oxy ethylene). Both phenomena were ascribed tothe formation of a rigid cellulosic network withinthe matrix as well as the conduction performanceswere similar for unplasticized and unfilled elec-trolytes on the one hand and plasticized filled sys-tems on the other hand. Therefore, the later allows

conciliating good ionic conductivities and highmechanical performances.The processing of a composite polymer electrolytefrom an aqueous suspension of cellulose whiskersis not easy to consider since water can react withthe negative electrode and reduce the battery cyclelife. On the other hand, cellulose whiskers are verydifficult to disperse in a polymeric matrix as theyhave a large surface area and possess large hydro-gen forces among themselves. It can lead to the for-mation of strongly bound aggregates. A surfactantcan be used to disperse cellulose whiskers in a non-polar solvent [39, 54] like toluene. However, thelarge amount of surfactant necessary to maintainthe stability of the suspension, due to the high spe-cific area of the filler, prevents the use of this tech-nique for composites processing in organic solvents.Another way is the surface chemical modificationof cellulose whiskers to disperse cellulose whiskersin organic solvents [56] but, the mechanical per-formances of the resulting composites stronglydecrease after chemical modification. Azizi et al.[21] prepared nanocomposite film reinforced withtunicin whiskers from a N,N-dimethylformamide asan organic solvent without a surfactant addition ora chemical surface modification. Both the highvalue of the dielectric constant of dimetyl for-mamide and the medium wettability of tunicinwhiskers were supposed to control the stability ofthe suspension. The nanocomposite materials wereprepared by UV cross-linking; with thermally sta-ble photoinitiator, 4-(2-hydroxyethoxy)-phenyl-(2-hydroxy-2-propyl) ketone, using an unsaturatedpolyether as matrix.Cross-linking is one of the most common methodsused to disrupt polymer crystallinity and to ensuremechanical properties. It is classically performed toprovide both low-temperature conductivity andhigh-temperature mechanical stability. In compar-ing the behavior of weakly cross-linked poly etherα,ω-dihydroxyoligo(oxyethylene) filled with tunicinwhiskers suspended in N,N-dimethylformamideand the one of unfilled materials exhibiting differ-ent cross-linking density. The cellulosic nanofillerprovided a much higher reinforcing effect at hightemperature than the cross-linking process, a pho-toinitiator, 4-(2-hydroxyethoxy) phenyl-(2-hydroxy-2-propyl) ketone used as cross-linking agent. Inaddition, nanocomposite electrolytes display ahigher ionic conductivity on the whole temperaturerange due to the high crosslinking density that

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should be used for unfilled electrolytes in order toensure satisfactory mechanical properties. It wasalso shown that tunicin whiskers seem to have noeffect on the conduction mechanism of the polymerelectrolyte. Therefore, the used cross-linked nano-composite polymer electrolytes allow conciliatingboth higher ionic conductivities and higher mechani-cal performances [154]. Lithium perchlorate-dopednanocomposites of ethylene oxide-epichlorohydrincopolymers and cellulose whiskers can readily beproduced by solution casting tetrahydrofuran/watermixtures comprising the components and subse-quent compression molding of the resulting nano-composites. Films of these materials display sub-stantially improved mechanical properties, whencompared to the not reinforced lithium perchlo-rate/ethylene oxide-epichlorohydrin (Figure 10),and their electrical conductivities experience com-parably small reductions [90].High performance solid lithium-conducting nano-composite polymer electrolytes based on poly (oxyethylene) were prepared from high aspect ratio cel-lulosic whiskers and lithium trifluoro methane sul-fonyl imide. The filler provided a high reinforcingeffect while a high level of ionic conductivity wasretained with respect to the unfilled polymer elec-trolytes [155, 156].

4.6. In biomedical

From a biological viewpoint, almost all of thehuman tissues and organs are deposited in nanofi-brous forms or structures. Examples include: bone,dentin, collagen, cartilage, and skin. All of them arecharacterized by well organized hierarchicalfibrous structures. In biomedical applications, forsoft tissue replacement a developed material thatwill not only display similar mechanical propertiesas the tissue it is replacing, but also showsimproved life span, biocompatibility, nonthrombo-genic, and low degree of calcification needed.Hydrophilic bacterial cellulose fibers of an averagediameter of 50 nm are produced by the bacteriumAcetobacter xylinum, using a fermentation process.

They can be used in combination with poly (vinylalcohol) to form biocompatible nanocomposites.Millon et al. [157] prepared poly (vinyl alcohol)/bacterial cellulose nanocomposites and they foundthat, the resulting nanocomposites possess a broadrange of mechanical properties and can be madewith mechanical properties similar to that of car-diovascular tissues, such as aorta and heart valveleaflets.Silk fibroin-microcrystalline cellulose (cellulosewhisker) composite films with varied compositionswere prepared by casting mixed aqueous solution/suspensions of the two components. Silk fibroinwas dissolved in lithium thiocyanate followed bydialysis; a cellulose whisker suspension was pre-pared by sulfuric acid hydrolysis of tunicate cellu-lose. Composite films showed improved mechani-cal strength at 20–30 wt% fibroin content, withbreaking strength and ultimate strain about fivetimes those of the constituent materials. From theobserved shift in the infrared absorption bands ofamide I and amide II of fibroin, the anomaly in themechanical strength is considered to arise from thecontact of fibroin with the highly ordered surface ofcellulose whiskers. This phenomenon is not practi-cable for producing bulk materials because of thelengthy procedure of solubilization and dialysisinvolved, but may be useful in biomedical applica-tions such as for cell culture media and implantmaterials, since both components are chemicallyinert and known to be compatible with living tis-sues [158]. Hydroxyapatite (Ca10(PO4)6(OH)2)-bacterial cellulose as a novel class of nanocompos-ites were prepared by Wan et al. [159, 160]. Thestructure characterizing reveals that the crystallitesizes of the hydroxyapatite crystals are nano-sizedand their crystallinities are low. The Fourier-Trans-formed Infrared spectroscopy results show thathydroxyapatite crystals are formed when the phos-phorylated and CaCl2-treated bacterial cellulosefibers are soaked in a 1.5 simulated body fluid thehydroxyapatite crystals are partially substitutedwith carbonate, resembling natural bones. Thenanocomposites containing hydroxyapatite withstructural features close to those of biologicalapatites are attractive for applications as artificialbones. From the scheme, it is believed that the non-ionic hydroxyl groups on the unphosphorylatedbacterial cellulose may firstly bind the calcium ions

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Figure 10. Chemical structure of ethylene oxide-epichlorohydrin coploymer

through ionic-dipolar interaction, and then hydrox-yapatite crystals grow around these trapped ions.The process is schematically displayed in Fig-ure 11a. However, a different process is involvedfor the phosphorylated bacterial cellulose. Asshown in Figure 11b, esterification takes place dur-ing phosphorylation and thus anionic phosphategroups are bonded to the cellulose chain throughstrong covalent bonds. The negatively chargedphosphate groups are capable of trapping calciumions, forming calcium phosphate complexes thatact as nuclei of hydroxyapatite and hydroxyapatitegrows by further complexation with phosphate ionsin SBF as shown in Figure 11c. Note that the bond-ing between the calcium ions and bacterial cellu-lose chain is via strong ionic bonds for thephosphorylated bacterial cellulose while the cal-cium ions complexation with nonionic hydroxylgroups proceeds via ionic-dipolar interaction forthe unphosphorylated bacterial cellulose (Fig-ure 11a). An organic-mineral composite of hydrox-yapatite (Ca10(PO4)6(OH)2) nanoparticles and

carboxymethyl cellulose is synthesized via copre-cipitation from a solution containing CaCl2, aque-ous ammonia, (NH4)2HPO4, and carboxymethylcellulose. The hydroxyapatite nanoparticles areshown to form agglomerates about 200 nm in size.The interaction between the nanoparticles and car-boxymethyl cellulose macromolecules leads to theformation of a pore structure potentially attractivefor biomedical applications [161].

4.7. In papers industry

The pulp and paper industry is a materials industry.It provides materials for use in communications,packaging, consumer products, and other products.The intersection of wood fiber-based materials withnano-materials provides nearly unlimited opportu-nities for the pulp and paper industry to developnew products with enhanced functionality andgreater value. Paper in itself provides an excellentplatform for developing nano-material fiber com-posites for use in higher value printing, barrierpackaging, and intelligent communications media.TiO2/cellulose nanocomposites were preparedthrough the titanyl sulphate hydrolysis in acidicmedium in the presence of cellulosic fibers in spe-cific experimental conditions, the cellulose fiberspromote the nucleation and growth of TiO2 parti-cles, yielding hybrid materials containing up to46% TiO2. Two series of paper handsheets havingdistinct TiO2 content have been prepared, one froma selected hybrid composition and the other frommixtures of commercial TiO2 and cellulose fibers.Comparative optical studies performed on thepaper handsheets revealed a much higher opacityfor the synthetic sample [162].The pore structure of the cell wall of the never-dried pulp fibers has been identified as a generalmicropackaging or encapsulation system for abroad range of both organic and inorganic chemi-cals [163–166] these substances are entrapped inthe cellulosic fiber matrix during the collapse of thecell wall pores as the pulp is dried [167]. Fahmyet al. [168] used sucrose as the nanoadditive to themercerized non-dried cotton linter fibers. Relativeto the sucrose-free paper, the sucrose-containingcounterparts exhibit greater breaking length andremarkably high water uptake up to a sucrose con-tent of 8–15% w/w.

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Figure 11. Schematic diagrams of various interactionprocesses. a) Hydroxyapatite growth onunphosphorylated bacterial cellulose, b) Phos-phorylation of bacterial cellulose, c) Hydroxya-patite growth on phosphorylated bacterialcellulose (HPA = Hydroxyapatite)

4.8. In packing

In cellulose-based base material (e.g., copier paperor packing); a cellulose particles in nano-size rangecan be obtained from starting product containingcellulose (e.g., wood) by mixing with water; themixture is heated, so that the product containingcellulose is hydrolyzed; the mixture then undergoesa shearing process giving base material and thebase material is added to another solid- or liquidmaterial-containing fiber substances [169]. On theother hand, biodegradable polymers such as starch[170, 171] poly (lactide) [172, 173] and poly (ε-caprolactone) [174–176] have attracted consider-able attention in the packaging industry. In additionpoly (lactic acid) is biodegradable thermoplasticpolyester produced from L-lactic acid, which isderived from the fermentation of corn starch. Poly(lactic acid) is becoming increasingly popular as abiodegradable engineering plastic owing to its highmechanical strength and easy processability com-pared to other biopolymers [177]. It has gainedenormous attention as a replacement for conven-tional synthetic packaging materials in the lastdecade [178] but the problem is the amount oftransmitter light through the poly (lactic acid)sheet, for reducing this transmitter light, bentonite alayered silicate and microcrystalline cellulose cho-sen as nanoreinforcement for poly (lactic acid).By comparing the mechanical, thermal and barrierproperties of two different types of biopolymerbased nanocomposites such as, bentonite a layeredsilicate and microcrystalline cellulose. The poly-mer matrix used was poly (lactic acid), a commonavailable biopolymer. The bentonite nanocompos-ite showed great improvements in both tensilemodulus and yield strength, while the microcrys-talline cellulose nanocomposite only showed ten-dencies to improve the yield strength. There are afew factors that will help to explain these differ-ences. First of all, the bentonite added to the poly(lactic acid)/bentonite material has theoreticallytwice the surface area of the added swelled micro-crystalline cellulose. A larger surface area willallow the nanoreinforcement to interact with alarger amount of polymer chains and thereby hav-ing a larger effect on the mechanical properties.Secondly, the bentonite clay is organically modi-fied to be compatible with polymers like poly (lac-tic acid) and will therefore have better interactionwith the poly (lactic acid) matrix. Good interaction

between the reinforcing phase and the matrix in acomposite will allow for good stress transfer to takeplace in the composite. This gives rise to largeimprovements in the mechanical properties of theweaker matrix.Also these two nanoreinforcements would affectpoly (lactic acid) as a packaging material. Theresults showed a reduction in the oxygen perme-ability for the bentonite nanocomposite, but not forthe microcrystalline cellulose nanocomposite. Theamount of light being transmitted through thenanocomposites was reduced compared to purepoly (lactic acid) indicating that both nanorein-forcements were not fully exfoliated. [97].

4.9. Nanotechnology and wood as a buildingmaterials

Half of the wood products now used in constructionare engineered wood composites. Nanotechnologywill result in a unique next generation of wood-based products that have hyper-performance andsuperior serviceability when used in severe envi-ronments. They will have strength properties nowonly seen with carbon-based composites materials,and they will be both durable while in service andbiodegradable after their useable service-life. Nan-otechnology will also promote the development ofintelligent wood- and biocomposite products withan array of nanosensors built in. Building function-ality onto lignocellulosic surfaces at the nanoscalecould open new opportunities for such things asself- sterilizing surfaces, internal self-repair andelectronic lignocellulosic devices. The high strengthof nanofibrillar cellulose together with its potentialeconomic advantages will offer the opportunity tomake lighter weight, strong materials with greaterdurability [5]. However, as in all markets, technol-ogy and shifting demographics give rise to hangingmarket demands. Materials and products used inhousing construction are not immune to suchchanges. Because a home or a commercial buildingis typically the largest purchase a family will makeand one of the larger investments a corporation willmake, consumers want structures that maintaintheir value over time and are safe and secure,healthy, comfortable, long-lasting (durable), lowmaintenance, affordable (lower in cost and provid-ing more value for the dollar), easily adaptable tonew and modified architectural designs, and allowfor personalized customization, have smart system

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capabilities, and reduce costs for heating and airconditioning.Wood-based construction materials functionextremely well under a variety of end-use condi-tions. Under wet conditions, however, they can beprone to decay, mold, mildew, and insect attack.Wood can be protected from biodeterioration bytreatments using toxic chemicals or by maintaininglow moisture content in wood. Achieving control ofmoisture is a major opportunity for nanotechnologyto aid in preventing biodeterioration of wood andwood-based materials. New non- or low-toxicitynanomaterials such as nanodimensional zinc oxide,silver, titanium dioxide, and even possibly claysmight be used as either preservative treatments ormoisture barriers. In addition, resistance to firemight be enhanced by use of nanodimensionalmaterials like titanium dioxide and clays.Composites allow an array of disparate materialswith greatly differing properties to be engineeredinto products matched to end-user needs and per-formance requirements. For example, future nano-composite construction materials may use combi-nations of wood, wood fiber, plastics, steel, andconcrete. To achieve this, it will be necessary to beable to make hydrophilic materials compatible withhydrophobic materials such as wood and plastics[179].

5. Conclusions

Nanotechnology presents a tool to extend structuralperformance and serviceability by orders of magni-tude. Nanotechnology will allow engineers and sci-entists to manipulate and systematically eliminatethe formation of random defects that now dictatethe properties, performance, and serviceability ofbiocomposites as known today. This new ability tominimize and eliminate naturally occurring andhuman-made internal defects will allow realizingthe true potential of biomaterials. Nanotechnologywill help the ability of manipulate and controlfiber-to-fiber bonding at a microscopic level, and itwill also offer an opportunity to control nanofibril-lar bonding at the nanoscale. Nanocomposites willbe the new frontier. So, understanding the synthe-sis-structure-property relationship of nanocompos-ites is vital for the development of advanced poly-mer nanocomposites with enhanced mechanicalstrength, stiffness and toughness for structural engi-neering applications.

In the nanocomposite field, cellulose whiskers canbe used as mechanical reinforcing agents of low-thickness polymer electrolytes for lithium batteriesapplication but, the filler content is generally rela-tively low, below 10 wt%, avoiding significantdecrease of the ionic conductivity. But using aque-ous suspension of cellulose whiskers is not easy toconsider since water can react with the negativeelectrode and reduce the battery cycle life. On theother hand, a surfactant can be used to disperse cel-lulose whiskers in a nonpolar solvent like toluene.However, the large amount of surfactant necessaryto maintain the stability of the suspension, due tothe high specific area of the filler, prevents the useof this technique for composites processing inorganic solvents. Another way is the surface chem-ical modification of cellulose whiskers to dispersecellulose whiskers in organic solvents but, themechanical performances of the resulting compos-ites strongly decrease after chemical modification.Nanocomposites reinforced with cellulose whiskersuspended in organic solvent without surfactantaddition or surface modification lead to high ionicconductivities and high mechanical performances.Wood can be protected from biodeterioration bytreatments using toxic chemicals or by maintaininglow moisture content in wood. Achieving control ofmoisture is a major opportunity for nanotechnologyto aid in preventing biodeterioration of wood andwoodbased materials. New non- or low-toxicitynanomaterials such as nanodimensional zinc oxide,silver, titanium dioxide, and even possibly claysmight be used as either preservative treatments ormoisture barriers. In addition, resistance to firemight be enhanced by use of nanodimensionalmaterials like titanium dioxide and clays.Future nanocomposite construction materials mayuse combinations of wood, wood fiber, plastics,steel, and concrete. To achieve this, it will be nec-essary to be able to make hydrophilic materialscompatible with hydrophobic materials such aswood and plastics.

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