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International Journal of Biological Macromolecules 61 (2013) 121– 126

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules

jo ur nal homep age: www.elsev ier .com/ locate / i jb iomac

ynthesis of lignocellulosic polymer with improved chemicalesistance through free radical polymerization

ijay Kumar Thakura,∗, Manju Kumari Thakurb, Raju Kumar Guptac

Department of Materials Science and Engineering, Iowa State University of Science and Technology, Ames, IA 50011, USADivision of Chemistry, Govt. Degree College Sarkaghat, Himachal Pradesh University, Shimla 171005, IndiaDepartment of Chemical Engineering, Indian Institute of Technology, Kanpur 208016, India

r t i c l e i n f o

rticle history:eceived 4 April 2013eceived in revised form 12 June 2013ccepted 26 June 2013vailable online 2 July 2013

a b s t r a c t

Rising environmental awareness has resulted in a renewed interest in biological macromoleculesobtained from renewable resources. So in view of technological significance of natural lignocellulosicpolymers in numerous applications, the present study is an attempt to synthesize lignocellulosic polymersbased graft copolymers using free radical polymerization. Different reaction conditions have been studiedto synthesize the lignocellulosic graft copolymers. The graft copolymers have been characterized with

eywords:ignocellulosic polymersraft copolymerizationhermalorphological and physico-chemical

haracterization

scanning electron micrography (SEM), Fourier transform infrared spectroscopy (FTIR) and thermogravi-metric analysis (TGA). The grafted samples have also been screened against different physico-chemicalconditions to assess their applicability in different applications.

Published by Elsevier B.V.

. Introduction

Recently, rising environmental awareness has resulted in aenewed interest in green materials obtained from renewableesources [1–3]. Commercial interest in manufacturing these prod-cts is driven by the derivation of the polymers from renewableources as well as by their specific properties including biodegrad-bility [4–8]. Indeed there has been rapid growth in the area ofellulosic polymers, facilitated by novel technologies, which hasesulted in a more urgent focus on developing potential greenaterials more effectively and efficiently [9–11]. Natural cellulosic

olymers e.g. natural fibers offer a number of advantages whichnclude biodegradability, easy availability, eco-friendliness, toxi-ologically harmless, enhanced energy recovery, low cost etc. asompared to the traditional synthetic materials such as glass fibers,arbon fibers, aramid fibers [12–15]. Last few years have seen aesurgence of natural cellulosic materials as potential reinforce-ent in green composites as well as in toxic metal ion removal,

ackaging and biomedical applications [16–20]. In conventional

ynthetic polymer composites none of the component belongso natural polymers and thus affects the environment directly orndirectly. On the other hand composite materials which contain

∗ Corresponding author. Tel.: +1 0512 259 6972; fax: +1 0512 259 0104.E-mail addresses: vijayisu@hotmail.com (V.K. Thakur), guptark@iitk.ac.in

R.K. Gupta).

141-8130/$ – see front matter. Published by Elsevier B.V.ttp://dx.doi.org/10.1016/j.ijbiomac.2013.06.045

natural cellulosic fibers as one of the component normally exhibitenhanced eco-friendly properties because of the partial replace-ment of the synthetic filler materials with low cost economic greenreinforcing material [21–24]. Natural fibers polymer compositeshave been the subject of intense research efforts all around theglobe since last few years. A number of practical applications ofnatural fibers reinforced composites are now emerging in differ-ent field such as in interior decorating, automotive componentsand biomedical applications [24–27]. However the natural fibersreinforced composites suffers from few drawbacks such as theseare inferior to harsh environmental conditions as the cellulosefibers are of hydrophilic nature. In order to improve the proper-ties of natural cellulosic polymers a number of surface modificationtechniques have been used such as mercerization, benzoylation,acetylation and graft copolymerization [27–31]. Among these graftcopolymerization is most facile to induce desired functionalitiesinto the cellulosic polymers. The aim of this study is to improvethe existence properties such as poor physico-chemical resistance,hydrophilic nature and thermal stability of natural lignocellu-losic polymer: Eulaliopsis binata through graft copolymerization ofethyl acrylate monomer. These grass fibers species are traditionalin a number of countries and so far have been the unexploredmaterials in spite of their wide potential. The present communi-

cation for the first time describes in detail the effect of differentreaction conditions on the percentage of grafting onto these lig-nocellulosic polymers. The grafted lignocellulosic polymers weresubjected to different physico-chemical studies and have been

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ound to exhibit better properties after graft copolymerization isccomplished.

. Experimental

.1. Materials

Lignocellulosic Eulaliopsis binata fibers were collected from localesources of Himalayan region. Prior to their use in graft copo-ymerization synthesis, lignocellulosic Eulaliopsis binata fibers wereoaked in a detergent solution for 120 min, followed by exten-ive washing with tap water until free from any detergent. Thelean lignocellulosic fibers were then washed with distilled water,queezed, and allowed to dry in an air oven at 60 ◦C, and finallytored in a vacuum desiccator ready for use. Ethyl acrylate (EA),errous ammonium sulphate (FAS), potassium per sulphate (KPS),odium hydroxide (NaOH), alcohol (methanol &isobutanol), chlo-oform and acetone were kindly supplied by Sigma–Aldrich.

.2. Graft copolymerization procedure

Free radical induced graft copolymerization synthesis onto lig-ocellulosic Eulaliopsis binata fibers was carried out by immersinghem in a precise amount of distilled water for 24 h. Subsequentlyhe appropriate amount of redox initiator (FAS-KPS) was added in

preheated round bottomed flask and stirred in a water solution toreate free radical sites on the surface of the lignocellulosic fibersollowed by the addition of monomer into the reaction flask to ini-iate the graft copolymerization reaction. When the grafting timeas over, the grafted lignocellulosic fibers were thoroughly washedith distilled water, and then extracted with acetone in a Soxhlet

xtraction apparatus for 50 h in order to dissolve any homopoly-er which may have adhered to the surface of the lignocellulosic

bers. The grafted lignocellulosic fiber samples were then allowedo dry in an air oven at 50 ◦C, until constant weight. The percentagerafting (Pg) onto the lignocellulosic fibers was determined as pertandard method [25–27] in the following manner.

Percent grafting

g = Wg − W

W× 100

here W is the weight of raw Eulaliopsis binata fibers; Wg is theeight of grafted Eulaliopsis binata fibers.

.3. Characterization of the lignocellulosic samples

The raw and grafted lignocellulosic samples were character-zed using Fourier transform infra red spectroscopy (FTIR) withhe help of KBr pellets on PERKIN ELMER RXI Spectrophotome-er in the range of 400–4000 cm−1. The surface morphology of rawnd grafted lignocellulosic samples was investigated using scan-ing electron microscopy (LEO 435 VP). The excitation energy useduring the analysis was 5 keV. The fiber samples were gold coatedo improve its conductivity to obtain good images. Thermal behav-or was explored using thermo gravimetric analyses in nitrogentmosphere on a thermal analyzer (Perkin Elmer) at a heating ratef 10 ◦C/min.

.4. Physico-chemical characterization of the lignocellulosicamples

The raw and grafted lignocellulosic samples were screenedgainst different physico-chemical conditions to assess their appli-ability in different applications. A brief overview of the methodssed to determine the behavior is given below:

gical Macromolecules 61 (2013) 121– 126

2.4.1. Swelling behaviorSwelling studies of the raw and grafted lignocellulosic samples

was carried out in different solvents such as dimethyl formamide,water, methanol, and isobutyl alcohol from the increase in initialweight in the following manner:

Percent swelling

(PS) = Wf − Wi

Wi× 100

2.4.2. Chemical resistance behaviorThe chemical resistance behavior of the raw and grafted ligno-

cellulosic samples was studied in acid and base environments byplacing a known weight (Tw) in fixed volume of 1 N HCl and 1 NNaOH. After certain intervals, the samples were taken out fromthe solution, wiped up with paper and dried at room temperatureuntil constant weight is obtained. Finally, the weight of the sampleswere noted down and the chemical resistance of grafted as well asungrafted fibers to acid and base in terms of percentage weight losswas studied as in the following manner:

Percent chemical resistance

Pcr = Tw − Waci

Tw× 100

where Tw = total weight of raw and grafted lignocellulosic samples;Waci = weight after certain interval of raw and grafted lignocellulosicsamples.

3. Results and discussion

The mechanism of graft copolymerization of vinyl monomersonto cellulose is related to the concept of “accessibility” of hydroxylgroups onto the backbone polymer. The accessibility refers to theproficiency of OH groups present in the lignocellulosic back-bone to react under precise reaction conditions. In the presentwork, a mechanism for graft copolymerization onto lignocellulosicEulaliopsis binata fibers containing glucan chains with repeating(1→4) ß-glucopyranose units with several OH groups is described[25–27]. In lignocellulosic Eulaliopsis binata fibers; C2, C3 andC6 OH groups along with C H sites are the active centers for graftcopolymerization of ethyl acrylate monomer chains. The initia-tor (FAS:KPS) results in the formation of lots of free radicals ontothe lignocellulosic backbone as well as the monomer, and theseradicals could be thought to be capable of initiating graft copolym-erization on the lignocellulosic Eulaliopsis binata fibers. Scheme 1depicts the detailed mechanism of graft copolymerization syn-thesis. Different reaction parameters have been optimized to getthe optimum percentage of grafting. Fig. 1(a–e) shows differentvalues of percentage of grafting during optimization of these reac-tion parameters. The optimum reaction conditions for maximumpercentage grafting (56.9%) were: solvent-105 ml; Time-120 min;Temperature −50 ◦C, FAS: KPS- 1:1 and monomer- (0.75 mol/L).Fig. 1a shows the effect of the amount of solvent (water) dur-ing graft copolymerization synthesis of EA monomer. It has beenobserved from the figure that percentage of grafting of EA ontolignocellulosic Eulaliopsis binata fibers shows an initial jump in per-centage of grafting with increasing amount of solvent, from 35 mLto 170 mL giving maximum percentage grafting in 105 mL. Beyondthis amount of solvent, the percentage grafting decreases and thenbecomes almost constant [25–27]. The effect of reaction time onpercentage grafting is presented in Fig. 1b. With increase in thereaction time from 30 to 120 min, an increase in percentage graft-

ing is observed, after this it slightly decreases and then a levelingoff are observed. This behavior can be explained on the basis thatthe amount of graft polymerization increases with time account-ing for the initial rise in percentage grafting. However with further

V.K. Thakur et al. / International Journal of Biological Macromolecules 61 (2013) 121– 126 123

Fig. 1. (a) Optimization of solvent amount for graft copolymerization onto lignocellulosic Eulaliopsis binata. (b) Optimization of reaction time for graft copolymerization ontolignocellulosic Eulaliopsis binata. (c) Optimization of reaction temperature for graft copolymerization onto lignocellulosic Eulaliopsis binata. (d) Optimization of initiator ratiofor graft copolymerization onto lignocellulosic Eulaliopsis binata. (e) Optimization of monomer for graft copolymerization onto lignocellulosic Eulaliopsis binata.

124 V.K. Thakur et al. / International Journal of Biological Macromolecules 61 (2013) 121– 126

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cheme 1. Mechanism for graft copolymerization onto lignocellulosic Eulaliopsisinata.

ncrease in time, the rate is expected to slow as the monomer issed up and the initial radicals generated on the lignocellulosiculaliopsis binata fibers are depleted by grafting. Fig. 1c presentshe effect of temperature onto percentage grafting. The percentagerafting was found to increase with increase in temperature proba-ly due to increased decomposition of the initiator, greater mobilityf monomer molecules, and enhancement of rates of the reactions.eyond optimum temperature, the maximum percentage graftingtarts slightly decreasing, and then levels off. This behavior cane attributed to more homopolymerization at high temperaturer to the decomposition of graft copolymers. The effect of varia-ion of initiator amount onto the percentage grafting is shown inhe Fig. 1d. The results demonstrate that grafting increases withncrease in the initiator concentration. The optimum molar ratioor the optimum percentage grafting was found to be 1:1, beyondhich it decreases. This trend can be attributed to the formation

f more Fe3+ ions at higher molar ratio, resulting in the termina-ion of growing polymer chains. Fig. 1e shows the effect of amountf monomer on the percentage grafting. The grafting percentageas been found to increase with the monomer concentration upo a maximum monomer concentration of 0.75 mol/L. At a higher

onomer concentration, the grafting percentage was observed toall and then leveled off. The initial rise in grafting is expected ashere is a higher amount of the monomer available for copolym-rization, but beyond this, the excess monomer concentration didot result in graft copolymerization product. This behavior can bettributed to the fact that under a given initiator concentration, thevailable number of active sites for graft copolymerization synthe-is is constant and the monomer molecules compete for the limitedites for grafting available onto the lignocellulosic Eulaliopsis binatabers. Hence at high concentration of the monomer there is super-uous of the monomer molecules relative to the available sites

or graft copolymerization formation, resulting in the formation ofomopolymer.

.1. Physico-chemical characterization of the lignocellulosicamples

Both grafted and ungrafted lignocellulosic Eulaliopsis binatabers were subjected to swelling and chemical resistance studies asescribed in the succeeding paragraph. Raw and poly ethylacrylarterafted lignocellulosic Eulaliopsis binata fibers have been found to

Fig. 2. Swelling behavior of raw and grafted lignocellulosic Eulaliopsis binata indifferent solvents.

exhibit different swelling behavior in diverse solvents (Fig. 2) andfollows the trend as H2O >CH3OH > iso-BuOH > DMF due to the pres-ence of hydrophilic OH groups at C2, C3 and C6 of the glucose unitand their superior affinity with water. However reversal in swellingbehavior has been observed as a function of percentage of grafting(Pg) and follow the trend; DMF > CH3OH > H2O > iso-BuOH, whichcan be attributed to the obstruction of reactive sites on polymericsubstrate by poly ethylacrylarte (PEA) chains causing changes inthe sorption of the different solvents [21–25].

The resistance of the Eulaliopsis binata fibers toward acid andbases has been studied in terms of percent weight loss in 1 N HCland 1 N NaOH respectively (Fig. 3a, b). The resistance toward chem-icals has been found to increase with the increase in the percentagegrafting and can be attributed to the blockage of active sites vulner-able to the chemical attack by poly ethyl acrylate (PEA) chains onthe lignocellulosic backbone resulting in more resistance towardthe chemicals.

3.2. Structural characterization of the lignocellulosic samples

The presence of poly (ethyl acrylate) chains on the sampleswas verified by the FTIR spectra of the lignocellulosic Eulaliopsisbinata fibers and PEA-grafted- Eulaliopsis binata fibers, as shownin Fig. 4 (a, b). Both spectra of the Eulaliopsis binata fibers showa characteristic broad absorption band of the hydroxyl grouparound 3400–3100 cm−1 due to hydrogen bonded ( OH) groups.The presence of the peaks near 1732 cm−1 could be attributed tothe stretching vibration. However, the comparative FTIR spectrumof PEA-grafted- Eulaliopsis binata fibers shows significant changesin the spectrum confirming the successful graft copolymerizationonto Eulaliopsis binata fibers (Fig. 4b). The grafting of poly (EA)chains onto the lignocellulosic fibers is inveterated by the addi-tional characteristics absorption band at 1735 cm−1 with increasedintensity along with the changes in the intensity of the some of thebasic peaks of Eulaliopsis binata fibers.

SEM micrographs of the ungrafted Eulaliopsis binata fibers andPEA-grafted- Eulaliopsis binata fibers are shown in Fig. 5a and brespectively. After the graft copolymerization process, the surface

of the lignocellulosic Eulaliopsis binata fibers were homogeneouslycovered by the poly (ethyl acrylate) copolymer and is stronglyattached onto the fibers surface as it is chemically bonded.

V.K. Thakur et al. / International Journal of Biological Macromolecules 61 (2013) 121– 126 125

Fig. 3. (a) Acid resistance behavior of raw and grafted lignocellulosic Eulaliopsisbg

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Fig. 4. FTIR spectrum of (a–b) raw and grafted raw and grafted lignocellulosicEulaliopsis binata.

inata in 1 N HCl at different time intervals. (b) Base resistance behavior of raw andrafted lignocellulosic Eulaliopsis binata in 1 N NaOH at different time intervals.

Thermal stability of raw Eulaliopsis binata fibers and polyethyl acrylate) grafted fibers was studied as a function ofercentage weight loss with the increase in temperature forssessing the applicability of these fibers in practical appli-ations. In case of raw Eulaliopsis binata fibers, degradationccurs due to dehydration, glycogen formation and depolymer-zation. The initial decomposition (IDT) temperature for rawulaliopsis binata fibers has been found to be 215 ◦C with aercent weight loss of 14% and the final decomposition tem-erature (FDT) to be 481 ◦C with a percent weight loss of0%. While the poly (ethyl acrylate) grafted Eulaliopsis binatabers showed enhanced thermal stability due to the incorpora-ion of hydrophobic poly (ethyl acrylate) chains. Thermal stability

ncreases with an initial decomposition temperature (IDT) of25 ◦C with percentage weight loss of 11% and the final decom-osition temperature of 497 ◦C with percentage weight loss of4%.

Fig. 5. Scanning electron micrographs of (a–b) raw and grafted lignocellulosicEulaliopsis binata.

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. Conclusions

Lignocellulosic Eulaliopsis binata fibers were grafted in aque-us solution using a redox initiated free radical polymerizationith ethyl acrylate monomer. The method was both efficient and

elective as graft copolymerization was dominant over homopoly-erization due to optimization of different parameters. This kind

f modification may offer a simple way to improve the functionalroperties of lignocellulosic polymers with synthetic polymers,ffering new potential cellulosic materials for numerous appli-ations. These results from this work suggest that lignocellulosiculaliopsis binata fibers are a good substrate for copolymerization.ellulose based graft copolymers have been proved to be usefulaterials in several applications, and value added products could

airly easily be obtained from lignocellulosic Eulaliopsis binata fibershat otherwise is waste material.

cknowledgements

Authors wish to thanks their parental institutes for providinghe necessary facilities to accomplish the present research work.

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