Sisal cellulose whiskers reinforced polyvinyl acetate nanocomposites

Nancy Lis Garcia de Rodriguez, Wim Thielemans and Alain Dufresne*Ecole Francaise de la Papeterie et des Industries Graphiques (EFPG-INPG), BP 65, 38402 Saint-Martind’Heres, France; *Author for correspondence (e-mail: Alain.Dufresne@efpg.inpg.fr; phone: +33-4-76-82-69-95; fax: +33-476-82-69-33)

Received 30 September 2005; accepted in revised form 7 December 2005

Key words: Cellulose whiskers, Nanocomposites, Polyvinyl acetate, Sisal, Thermal behaviour, Waterabsorption

Abstract

Sisal nanowhiskers were used as novel reinforcement to obtain nanocomposites with polyvinyl acetate(PVAc) as matrix phase. They are seen as attractive materials due to the widespread availability and lowcost of the sisal source material. Statistical analysis of the sisal whisker length and diameter resulted inaverage values of 250 nm and 4 nm, respectively, resulting in an average aspect ratio in the upper range ofreported cellulose nanowhisker values. The high aspect ratio ensures percolation, with resulting mechanicalimprovements and thermal stability, at lower fiber loads. Water uptake and thermal behaviour of the sisalwhisker –PAVc composites were studied. Whisker addition was found to stabilize the nanocomposites withno benefit seen when increasing the whisker content beyond the percolation threshold: For all whiskercontents studied above percolation, the water uptake stays constant, and the Tg does not vary with whiskercontent at a given relative humidity. The water diffusion rate however increases due to water accumulationat the whisker –PVAc interface. Below whisker percolation, stabilization is only noticed at low relativehumidity, whereas high humidity results in disruption of whisker –PVAc interactions. This work shows thepotential of cellulose nanowhiskers to stabilize polar polymers even at high humidity conditions withminimal reinforcement addition.

Introduction

Composite materials are being used in an enormousamount of applications due to their versatility andwide applicability. While their properties andbehaviour can be easily adjusted by changes incompositionof the reinforcing andmatrixphases, aswell as the fibre load, many commonly used com-posite materials pose problems after their intendedlife and are derived from petroleum. The fast-pacedconsumption of petroleum, roughly 100,000 timesfaster than nature can replenish, and the generaldisposal possibilities, incineration and land filling,contribute to the unsustainability of the current sit-uation (Netravali and Chabba 2003). General solu-

tions to this problem can focus either on the supplyside, the life-end side, or on both at the same time. Avast amount of publications are available withworkfocussing on the development of polymers fromrenewablematerials andonbiodegradablepolymersand composites (Wool and Sun 2005).

Sisal is a commonly occurring plant with anannual worldwide fibre production reported toattain 4.5 million tons (Chand et al. 1988). Cellu-lose content of sisal varies between 49.6 wt% and61 wt%, depending on the age of the plant (Chandand Hashmi 1993). Sisal fibre length is found to bebetween 1 and 1.5 m, with average diameters of200 lm (Li et al. 2000). The main utilisation ofsisal fibres is found as a rope source for marine

Cellulose (2006) 13:261 –270 � Springer 2006

DOI 10.1007/s10570-005-9039-7

and agricultural applications (Mukherjee andSatyanarayana 1984). The most importantadvantages of sisal fibres are their low cost (0.16US$/pound vs. 1.48US$/pound for glass fibres),light weight (1.5 g/cm3 vs. 2.5 g/cm3 for glassfibres), and one of the highest tensile strengths ofall natural fibres (511 –635MPa) (Li et al. 2000;Eichhorn et al. 2001).

Sisal was used in this work as a novel source ofcellulose whiskers. These cellulose nanowhiskersare obtained from native cellulose sources by acidhydrolysis of native cellulose, which removes theamorphous regions. The remaining particles areindividualized nanosize monocrystalline particles.It has been well established that the size of cellu-lose nanowhiskers depends on the source materialwith aspect ratios varying between 10 and 65 forcotton and tunicate, respectively (Azizi Samiret al. 2005). The use of sisal as a novel sourcematerial will allow us to obtain new nanoparticledimensions, widen the range of potential applica-tions and increase the supply of cellulosic materialsusable for nanocomposite materials. Polyvinylacetate (PVAc) was chosen as matrix material fortwo important reasons. PVAc has been shown tobiodegrade under certain conditions, adding to thegreen character of the final composite material(Trejo 1988; Crowly et al. 2005). The specificconditions for biodegradation also guaranteesbiodegradation to occur only after the intendedlife of the product. In addition, PVAc is also arelatively polar polymer with limited hygroscop-icity. Its hydrophilicity can be placed in betweenthe hydrophilic plasticized starch and naturalrubber, two commonly used matrices for cellulosenanocomposites studies. It is anticipated thatstrong interactions may form between the hydro-xyl-rich surface of the cellulose whiskers and theester linkages abundantly protruding from thePVAc backbone. The resulting hydrogen bondsare expected to strengthen the interface signifi-cantly with a positive impact on the mechanicalproperties of the composite material.

Experimental

Sisal hydrolysis

Native sisal (Mahatma Gandi University, Kotta-yam, Kerala, India) was cut up with a 300 Wmixeruntil a fine particulate substance was obtained. The

sisal pieces were washed four times in boiling 2 wt%aqueous NaOH for 4 h under mechanical stirring.The fibers were filtered and rinsed with distilledwater in between each treatment step. The NaOHsolution did not discolor significantly upon the lasttreatment. A subsequent bleaching treatment at80 �C for 6 h was used to render the sisal fiberswhite. The bleaching solution contained equal partsof aqueous chlorite (1.7 wt%NaClO2 in water) andan acetate buffer (27 g NaOH and 75 ml glacialacetic acid, diluted to 1 L using distilled water). Thesisal content was approximately 5 wt% and thebleaching step was repeated two times. The fibersagain were filtered and rinsed with distilled waterbetween each treatment step. The fibers were sub-sequently dried for 24 h at 40 �C in a convectionoven. The dried treated fibers were ground a secondtime to a fine powder using a 300 W mixer anddispersed in 65 wt% sulphuric acid in water (4 wt%sisal). This suspension was held at 60 �C undermechanical stirring for 15 min to allow sisalhydrolysis. The suspension was subsequently di-lutedwith an equal part of coldwater andwashedbysuccessive centrifugation at 10,000 rpm and 10 �Cuntil a turbid supernatant became visible (�3times). Dialysis against distilled water was per-formed to remove free acid in the dispersion. Thiswas verified by neutrality of the dialysis effluent.Complete dispersion of the nanowhiskers was ob-tained by a sonication step using a Branson sonifier.The dispersions were stored in the refrigerator afterfiltration over a No. 2 fritted glass filter to removeresidual aggregates and addition of several drops ofchloroform. Determination of the whisker contentwas donebyweighing aliquots of the solution beforeand after drying. The nanowhisker yield wasapproximately 30% of the original sisal weight.

Transmission Electron Microscopy

Transmission electron micrographs of cellulosewhiskers were taken with a Philips CM200 trans-mission electron microscope with an accelerationvoltage of 80 kV. Nanowhiskers were depositedfrom an aqueous dispersion on a microgrid (200mesh, Electron Microscopy Sciences, Hatfield, PA,USA) covered with a thin carbon film (�200 nm).The deposited nanowhiskers were subsequentlystained with a 2% uranyl acetate solution to en-hance the microscopic resolution.

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Film processing

An aqueous dispersion of polyvinyl acetate wasobtained by emulsion polymerization of vinylacetate. At 60 �C, 100 g vinyl acetate was poly-merized in a mixture containing 1 kg water, 10 gsodium dioctyl sulfosuccinate and 0.84 g ammo-nium persulfate. After 10 min of reaction, anextra 211 g vinyl acetate was added to theemulsion. The particle diameter in the finalpolymer dispersion was 77 nm, as measured bydynamic light scattering, and contained 19.3%solids. Specific amounts of this dispersion wasmixed with a specific amount of the aqueousdispersion of sisal nanowhiskers and mixed at10,000 rpm for 15 min using a Dispermat CA20-C (VMA-Getzmann GMBH, Reichshof, Ger-many). The resulting mixture was poured in aTeflon mould and placed in a 45 �C oven underconstant air stream to evaporate water. The ob-tained films of 300 –400 lm thickness were de-molded after approximately 24 h. Sisalnanowhisker loads in the final composites were 0,1.0, 2.5, 5.0 and 10.0 wt%.

Film conditioning

Plasticizing water may have an important effect onthe behavior of the composite films. They weretherefore conditioned at six relative humidities(RH) of 0, 35, 43, 58, 75 and 98% at room tem-peratures using saturated salt solutions. The saltswere P2O5 (0% RH), CaCl2 Æ 6H2O (35% RH),K2CO3 Æ 2H2O (43% RH), NaBr Æ 2H2O (58%RH), NaCl (75% RH), and CuSO4 Æ 5H2O (98%RH). Films were kept at the respective RH for aminimum of 2 weeks to ensure equilibrium con-ditions. The weight of the films was also moni-tored to confirm stabilization of the film weight.

Water content of the films was measured byrecording the weight gain of the films using amicrobalance. Several films were also tested usinga SETARAM TGA 92 thermogravimetric ana-lyzer (Caluire, France) to confirm the obtainedresults. Several micrograms of sample were placesin the crucible, which was heated to 150 �C at arate of 5 �C/min under inert nitrogen atmosphere.Results from both methods were found to bewithin experimental error.

Water uptake experiments

The kinetics of water absorption was measuredusing thin films with dimensions 20�5�0.3 mm ata RH level of 98% and room temperature. Thethickness of the film (2 L) allows the assumptionof one-dimensional water diffusion. Samples werefirst dried at 100 �C under vacuum for 2 days.After weighing to determine the initial weight(Mo), they were placed in a container conditionedto 98% RH with a saturated copper sulfate solu-tion. At specific time intervals (t), the sampleweight (M) was determined until an equilibriumvalue (M¥) was reached. The water diffusioncoefficient was subsequently determined from theinitial slope of (M)Mo)/m¥ vs. (t/L2)0.5, where m¥denotes the equilibrium water sorption and 2 L isthe film thickness, as described elsewhere in detail(Angles and Dufresne 2000). The sample thicknesswas measured using an Adamel Lhomargy M120micrometer (0.1 lm accuracy, Roissy en Brie,France). Three samples were tested for eachwhisker content.

Differential Scanning Calorimetry

Differential Scanning Calorimetry (DSC) wasperformed on a DSC Q100 (TA Instruments, NewCastle, DE, USA) fitted with a manual liquidnitrogen cooling system. Conditioned film sectionswere placed in hermetically closed DSC crucibles.Samples were tested in the range of )100 to 200 �Cat a heating rate of 10� C/min under a nitrogenatmosphere. Sample weight was between 2 and5 mg.

Results and discussion

Nanowhisker characterisation

Figure 1 shows an electron micrograph of the sisalnanowhiskers.

The average diameter and length were calcu-lated using digital image analysis (Axone). Aminimum of 800 measurements for both thediameter and length were fitted using a three-parameter Weibull model. This model was foundto give the best fit to measurements for all thestatistical models attempted. Both the obtained

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histograms and model fits are shown in Figure 2.The geometric average diameter and length of thesisal whiskers are approximately 4 ± 1 and250 ± 100 nm, while the three-parameter Weibullmodel averages are 3.6 ± 1 and 211 ± 106 nm,respectively, giving rise to an aspect ratio around60. The presented errors are the standard devia-tions of the distributions. The aspect ratio of sisalwhiskers is quite high as it corresponds to some ofthe highest cellulose nanowhisker aspect ratiosreported in the literature (Beck-Candanedo et al.2005). Tunicate whiskers, with a similar aspectratio, however pose problems for large-scale use.The relatively easy harvest of sisal makes thesenanowhiskers thus even more attractive. While theWeibull slope, b, was 2.05, it was found that a log-normal distribution (distribution similar to Wei-bull with b� 2) did not capture the distributionaccurately. No relation was found between diam-eter and length, such that whiskers with smalldiameters did not always display short lengths.

The measured average sisal whisker dimensionsresult in a percolation threshold value, vRC, of 1.11vol% or 1.40 wt% using:

vRC ¼0:7

L=dð1Þ

with L and d the average length and diameter ofthe nanowhiskers, respectively. The conversionbetween vol% and wt% was carried out by usingthe values of 1.19 and 1.50 g/cm3 for PVAc(Lindemann 1989) and sisal whisker (Gindl andKeckes 2004), respectively.

Nanocomposite properties

The equilibrium water content of conditionedPVAc/sisal films as function of the water activity,aw, and sisal whisker content is shown inFigure 3(a). At low water activity (0<aw<0.5),very little water uptake is noticed. The watercontent in these films does not surpass 3 wt% forall whisker contents and whisker content appearsto exhibit only a small effect on the water uptakeof the films. A similar effect is seen for interme-diate activity levels (0.5<aw<0.75), where theequilibrium water content is seen to increase butstill stays below 4 w%. Significant changes areonly seen for the high water activity between 0.75and 1, where higher whisker contents result in amore pronounced uptake of water during condi-tioning. It is important to note that for the 10 wt%sample, an average of 30% water uptake increase

Figure 1. Transmission electron micrograph of cellulose nano-

whiskers obtained from sisal.

Figure 2. Length and diameter histograms of sisal nanowhis-

kers. The solid line denotes the three parameter Weibull model

fit to the experimental data.

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compared to uncharged PVAc is measured,regardless of the water activity. This similar rela-tive water uptake with varying water activity isnoticed for all whisker contents. Figure 3(b) showsthe water uptake equilibrium values for PVAc/sisalwhisker films conditioned at 98% RH as functionof whisker content. A significant increase in watercontent with increasing whisker content can benoticed as well an apparent plateau reachedaround 12% for whisker loads of 2.5 wt% and up.

Absorbed water molecules can accumulate in threeregions within the composite: the PVAc matrix,the cellulose whiskers and the polymer –whiskerinterface. PVAc water uptake is determinedexperimentally. Crystalline cellulose does not ab-sorb significant amounts of water (Stromme et al.2003), leaving only the whisker –polymer interfaceto accommodate the additional amount of ab-sorbed water molecules at equilibrium. This effectcan be quite substantial since a water monolayer

Figure 3. Equilibrium water uptake of PVAc/sisal whisker films (a) as function of water activity for various whisker contents: 0 (h),

1 ()), 2.5 (D), 5 (�), and 10 wt% (s), and (b) as function of whisker content for 98% RH.

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(3.1 A) along the whisker surface will increase thewater uptake by 0.25% for 1 wt% whiskers in thecomposite. This water accumulation effect can beattributed to the higher hydrophilicity of the sisalwhisker surface compared to the PVAc polymer.The contact angle of water on PVAc was measuredto be 75� using digital analysis of recorded imagesof a 10 ll water droplet on a solution cast PVAcfilm. Pure cellulose –water contact angles havebeen reported in the range of 33� –24� (Erikssonet al. 2005). This large difference in water contactangle clearly shows the higher hydrophilicity ofcellulose. Nevertheless, the increasing whiskercontent should cause a linear increase in equilib-rium water uptake as the whisker –polymer inter-facial area increases, in contradiction with theplateau value above the percolation thresholdfound in this work. It has been previously shownthat the cellulose nanowhiskers form a rigid three-dimensional network above whisker percolationdue to hydrogen bond formation between theindividual whiskers (Azizi Samir et al. 2005). It isprobable that this three-dimensional networkformed by hydrogen bonds between the sisalnanowhiskers significantly inhibits swelling of thenanocomposites, similarly to the formation ofchemical cross-links within a polymer phase. Thismight explain the plateau attained for theequilibrium water uptake above the whisker per-

colation threshold where increased interfacial area,where water might accumulate, is negated by areduction in water uptake by the polymer. Thekinetics of water diffusion through the PVAc/sisalfilms was measured at 98% RH by following theweight gain of the films with time (Figure 4).Water absorption by the film is rapid at shorttimes (<24 h), after which weight gain of the filmdue to water uptake slows down to reach a plateauvalue at longer times. Experiments were left up to30 days to confirm equilibration. The water dif-fusion coefficient as function of whisker contentare shown in Figure 5 and Table 1. They werecalculated from the initial slope of the waterabsorption data as described in the experimentalsection.

The water diffusivity coefficient in PVAc is inagreement with other reported values (2.5 �10)8cm2/s at 22 �C) (Kishimoto et al. 1960). Thehigh activation energy makes that a sharp decreasewith temperature is expected ()20% for 3 �C). It isseen that, initially, sisal whisker addition does nothave a significant effect on water diffusion, withany variation falling within experimental error.Upon crossing the percolation threshold however,the water diffusivity coefficient decreases mark-edly. Water diffusion is subsequently seen to occurmore actively with as the sisal whisker content isincreased beyond the percolation whisker content.

Figure 4. Water uptake of PVAc/sisal whisker films with time at 98% RH and room temperature for different whisker contents (0 (h),

1 ()), 2.5 (D), 5 (�), and 10 wt% (s)).

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Below the percolation threshold, it can be expectedthat the reinforcement will have very limited effecton water diffusion, largely taking place in thepolymeric matrix. However once a three-dimen-sional network, spanning the entirety of the com-posite, is formed, it is expected to inhibit swellingof the polymeric matrix, thereby hindering andthus slowing down, water diffusion. This networkthus ties the matrix together in a similar manner ascross links within the matrix. While this apparentlydid not result in a decrease in equilibrium wateruptake, it does seem to affect the kinetics of watersorption quite significantly. The surprising in-crease in the water diffusivity coefficient withfurther increase in whisker content, is ascribed tothe more pronounced hydrophilicity of the cellu-lose nanowhisker surface compared to PVAc.While the whisker network effectively reduces thediffusivity through the polymeric matrix, the watermolecules will not stay contained within thisphase, and might migrate to and accumulate at the

whisker –polymer interface. This water accumula-tion during transient water sorption depletes thepolymer matrix phase, therefore speeding up dif-fusion, until equilibrium saturation is reached atthe cellulose –polymer interface and in the polymermatrix phase. The latter accumulation wasapparent from equilibrium water uptake data.This reduces the polymer water content during thetransient sorption process at any given time,compared to the uncharged polymer, until equi-librium is established. Water diffusion along thecellulose whisker surface throughout the perco-lated network might also occur. It can thus beanticipated that increasing the whisker content,will also increase the availability and reach of thesediffusion pathways for water molecules through-out the nanocomposites, thereby increasing thewater sorption kinetics. Similar effects have seen tooccur in starch/tunicate whisker nanocomposites,where the hydrophilicity of matrix and reinforce-ment were more closely matched (Angles andDufresne 2000; Mathew and Dufresne 2002).

PVAc is a completely amorphous polymer so itsDSC trace only shows a glass transition tempera-ture. Table 2 combines the measured glass transi-tion temperatures as function of relative humidityat which the samples were conditioned and thesisal whisker content. A graphic representation isshown in Figure 6.

Increased humidity clearly decreases the Tg ofall nanocomposites due to the plasticizing effect of

Figure 5. Water diffusion coefficient through a PVAc/sisal nanocomposite as function of sisal whisker content.

Table 1. Water diffusivity coefficient in PVAc/sisal whisker

composites measured at 98% RH and room temperature.

Whisker content (wt%) Diffusivity coefficient (10)8 cm2/s)

0 1.34

1.0 1.35

2.5 0.50

5.0 0.70

10.0 1.54

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the water molecules. The sharpest decrease is seenfor uncharged PVAc where the Tg drops from 39.4to )6.84 �C as the relative humidity increases from0% RH to 98% RH. This drop is still visible butless marked for whisker contents of 2.5% and up,where no significant difference in Tg is seen withincreasing reinforcement loadings. The 1 wt% sisalnanocomposite displays behaviour between thesetwo extreme cases: Up to a RH of 75%, its Tgfollows the behaviour of the nanocomposites withhigher whisker charges, where at 98% RH, a steepdrop in Tg is seen to a value in close agreement

with the Tg of uncharged PVAc. Figure 6(b) de-picts the Tg as function of the sisal whisker contentfor different relative humidities and clarifies thispeculiar behaviour. Above the percolationthreshold (1.4 wt%), the sisal whisker content doesnot appear to affect the variation of the glasstransition temperature with relative humidity. Thisis in agreement with the equilibrium water uptakeplateau at 98% RH for values above this thresh-old. As the water uptake by the matrix is limited toa constant fraction by the percolated whiskernetwork, the plasticizing effect of this constant

Figure 6. Glass transition temperature of PVAc/sisal whisker composites as a function of (a) whisker content (0 (h), 1 ()), 2.5 (D),5 (�), and 10 wt% (s)) and (b) relative humidity (0 (h), 35 (D), 43 (�), 58 (d), 75 (s), and 98% RH (+)).

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water content on the matrix will also stay constant.Surprisingly however, the 1 wt% composite sam-ple is less plasticized than uncharged PVAc at arelative humidity of 75% RH and below, eventhough the absence of a percolated whisker net-work should allow the matrix phase to swell freelyand accumulate water as it does in the unchargedstate. The water content of the 1 wt% compositesample is also higher than the uncharged PVAcsample for these relative humidities (Figure 3(a)).Despite this higher water content, PVAc polymerchains are thus found to be more restricted in thecomposite than they are in the pure polymersample, even without a fully percolated whiskernetwork, except at high water accumulation in thecomposite sample as is apparent from the 98% RHdata. As the polymer matrix is not restricted in itswater uptake, with its inherent detrimental plasti-cizing effect, Tg increases can only be due torestrictions in chain movement by interactionswith the cellulose nanowhiskers. As the watercontent increases, as is the case at 98% RH, wateraccumulation at the cellulose –PVAc interface, theonly possible cause for the increased water uptakeat equilibrium, appears to break these interactionsdown sufficiently with complete matrix plasticiza-tion and a drop in Tg as resulting effects.

Conclusions

Sisal has been used as a novel source for cellulosenanowhiskers. Analysis of the resulting whiskersrevealed rod-like particles with an average aspectratio in the higher end of the range of reportedvalues for other cellulose nanowhiskers. Statisticalaverages for length and diameter were around 250and 4 nm, respectively. This high aspect ratio,when compared to other polysaccharide nano-whiskers, and coupled with the low cost and wide-

spread availability of the sisal source material,make these particular nanowhiskers very attractiveas reinforcement for polymeric materials. Biode-gradable sisal whisker reinforced nanocompositeswere formed with PVAc as the polymer matrix,which showed the similar behaviour of these novelsisal reinforcements compared to other cellulosenanowhiskers. Water diffusion into the nanocom-posites was found to be hindered by a percolatedthree-dimensional nanowhisker network above thepercolation threshold. Water uptake was found toincrease with increasing reinforcement contentwith a plateau value established once whiskerpercolation occurs. However, the higher hydro-philicity of the cellulose nanowhisker surface,compared to the polymer matrix, resulted in apreviously unreported increase in diffusion ratewith increasing whisker content above the perco-lation threshold, believed to be due to wateraccumulation at the cellulose –polymer interface.Water uptake was not found to plasticize thecomposite significantly above the whisker perco-lation threshold. Below this threshold value, thecomposite was plasticized significantly at highwater uptake (98% RH), whereas at low valuesplasticization was not found to occur, presumabledue to strong whisker –PVAc interactions. Watersorption and DSC experiments point towards theopposing effects of the formation of a restrictivethree-dimensional cellulose network above thewhisker percolation threshold and water accumu-lation at the polymer –reinforcement interface.Further experiments are currently in progress todetermine the consequences of these effects on themechanical behaviour of these nanocompositesand to elucidate the structure of the polymer –reinforcement interface. These results show theenormous potential of cellulose nanowhiskers inpolar matrices and its stabilizing effects at rela-tively low whisker contents. This stabilization will

Table 2. Glass transition temperature of PVAc/sisal whisker composites as function of whisker content and relative humidity.

Whisker content (wt%) Relative humidity (%)

0 35 43 58 75 98

0 39.4 32.4 24.2 18.6 7.9 )6.81.0 40.2 33.5 31.9 29.7 25.3 )5.12.5 43.3 37.6 31.3 29.6 25.8 14.0

5.0 40.6 40.6 32.0 29.5 27.1 14.4

10.0 42.5 37.0 36.2 32.0 26.5 13.0

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enhance the behaviour of polar polymers underhumid conditions and widens their applicability.

Acknowledgments

The authors would like to acknowledge professorSabu Thomas from Mahatma Gandhi University(Kottayam, Kerala, India) for supply of sisal andDr. Elodie Bourgeat-Lami from the Laboratoirede Chimie et Procedes de Polymerisation (CNRS,Villeurbanne, France) for the polyvinyl acetatelatex. Financial support for this work was pro-vided by ADEME (Agence Francaise de l’Envi-ronnement et de la Maıtrise de l’Energie).

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