th

ga, Paelvc

Hydrolytic degradationCellulose nanowhiskers

parad stico by

ing usty. Theasily ading com

nanocomposite both, the polymer matrix and the nanoreinforce-ment have to be derived from renewable resources.

In this way, highly crystalline rod-like nanostructures obtainedfrom cellulose, called cellulose nanowhiskers (CNW) or cellulosenanocrystals (CNC), have attracted signicant attention during the

between them [12]. Several attempts have been made to achieveagooddispersionof thenanowhiskers indifferentpolymermatrices,including the use of surfactants [13,14] and the chemical surfacemodication of the whiskers [15e17].

Among biodegradable polymers, the family of polylactides hasrecently received a great deal of investigation [18e21]. Lactic acid iseasily obtained by biotechnological processes (usually from lacto-bacillus) in an economically feasible manner. To produce polylac-tide, corn or sugar sources are processed to produce D-glucose,

* Corresponding author. Tel.: 33 31 3409 5753; fax: 33 31 3409 5700.

Contents lists availab

Polymer Degradat

ev

Polymer Degradation and Stability 96 (2011) 1631e1638E-mail address: fabianovargas@yahoo.com (F.V. Pereira).matrix [1,2].Interest has recently increased in the use of biopolymers as

matrices for nanocomposites because of the environmental impactcaused by conventional plastics derived from petroleum. However,themajorityof thesebiopolymershavepoormechanical and thermalproperties that make them unsuitable for some applications. In thisway, inorganic nanoparticles [3e5] or natural organicllers [6,7] canbe added to different biopolymers to prepare different bio-basednanocomposites with improved physicalemechanical properties.Additionally, to produce a fully renewable and biodegradable

amorphous regions and release crystalline cellulose nanoparticlesor cellulose nanowhiskers [10]. These particles consist of highlycrystalline rod-shaped nanomaterials (the length and lateraldimensions of which depend on the source of cellulose), presentinga high modulus (around 100 GPa) [11].

One of the major challenges in the preparation of nano-compositesusingCNWnanollers is theability to achieve acceptablelevels of dispersion of the cellulose nanocrystals within a polymericmatrix. The elementary crystallites commonly aggregate laterallydue to their high specic area and the strong hydrogen bondsPoly(D,L-Lactide)Bionanocomposite

1. Introduction

Polymeric nanocomposites are beapplications because of their versatiliof a nanocomposite system can be ecompositionof thenanoscale reinforc0141-3910/$ e see front matter 2011 Elsevier Ltd. Adoi:10.1016/j.polymdegradstab.2011.06.006incorporation of cellulose nanocrystals in the PDLLA also made the biopolymer more thermally stable,increasing the initial temperatureofmass loss even after the degradation inphosphatemedium. The resultspresented here show the possibility of controlling the biodegradability and prolonging the service life ofa polylactide through the incorporation of a small quantity of nanollers obtained from renewablematerials.

2011 Elsevier Ltd. All rights reserved.

ed in a wide variety ofproperties andbehaviorjusted by changing theponent or thepolymer

last decade as potential nanoreinforcements for different polymers[8e10]. Cellulose is abundant in nature and is found in both plantsand bacteria. Cellulose molecules are stabilized laterally in the cellwalls by hydrogen bonding between hydroxyl groups, resulting inthe formation of bundles of microbrils. Upon controlled acidhydrolysis, the microbrils undergo transverse cleavage along theKeywords:1%. This effectwas related to thephysical barrier createdby thehighlycrystalline CNWs that inhibitedwaterabsorption and hence retarded the hydrolytic degradation of the bionanocomposites. In addition, theAvailable online 7 July 2011 delay in the hydrolytic degradation of the PDLLA, evenwhen the concentration of the nanollers was onlyInuence of cellulose nanowhiskers onof poly(D,L-lactide)

Everton Luiz de Paula a, Valdir Mano b, Fabiano VaraDepartment of Chemistry, Federal University of Minas Gerais, Av. Antnio Carlos, 6627bDepartment of Natural Sciences, Federal University of So Joo Del-Rei, Praa Dom H

a r t i c l e i n f o

Article history:Received 18 March 2011Received in revised form8 June 2011Accepted 10 June 2011

a b s t r a c t

This paper reports the prewhiskers (PDLLA/CNWs) anthe polylactide. The hydrolysample weight loss and als

journal homepage: www.elsll rights reserved.e hydrolytic degradation behavior

s Pereira a,*

mpulha, CEP 31270-901 Belo Horizonte, MG, Brazilio, 74 Fbricas, CEP 36301-160 So Joo Del Rei, MG, Brazil

tion of bionanocomposites based on poly(D,L-lactide) and cellulose nano-tudies the inuence of the CNWs on the hydrolytic degradation behavior ofdegradationprocesswas studied in a phosphate buffermedium through theFTIR, DSC and TGAmeasurements. The presence of CNWs induced a strong

le at ScienceDirect

ion and Stability

ier .com/locate /polydegstab

datiowhich is then fermented to produce lactic acid [22]. Using anappropriate catalyst and heat, lactic acid is converted to the cycliclactide dimmer. A suitable catalyst can induce ring opening poly-merization of the lactide dimer, producing poly(lactic acid). Theresulting plastic has mechanical properties that are intermediatebetween the polyolens and other biopolymers [2], and it can beused in a variety of applications, including biomedical uses [23]such as sutures and medical implants, food products and pack-aging materials [2]. In addition, this biopolymer can be easilyprocessed by conventional processing techniques. The proportionof the L and D isomeric forms will determine the properties of thepolymer, i.e., if the material is amorphous or semi-crystalline. Aracemic mixture of isomers L and D is usually called D,L-lactide. Themonomers L and D,L-lactide are used exclusively for the productionof polylactides and are called, poly(L-lactide) e PLLA, and poly(D, L-lactide) e PDLLA, respectively [24]. The random distribution ofD and L units in the poly(D, L-lactide) hinders the orientation of thechains, producing a polymer with mechanical properties that areinferior to those of PLLA. In addition, because the PDLLA is anamorphous polymer, it only presents a glass transition temperature(ranging from 50 to 60 C).

The polylactides have also been studied in the preparation ofblends, copolymers and nanocomposites. Wan and colleagues [25]produced polymeric PDLLA/chitosan membranes. Xiong andcolleagues [26] prepared the copolymer PDLLAPVPPDLLA to ach-ieve a higher biodegradability compared to pure PDLLA, while Luoand colleagues [27] prepared an amphiphilic poly(N-vinyl-pyrrolidone)-block-poly(D,L-lactide) (PVP-b-PDLLA) diblock copol-ymer. Recently, PDLLA has been studied for use in controlled drugrelease systems. Zhang and colleagues [28] showed the productionof a porous tissue based on PDLLA porous biphasic calcium andphosphate (BCP) that has applications in controlled drug releasesystems.

The degradation of a synthetic polymer is caused by severalfactors and results in the loss of physical properties [18,29]. Thisprocess is complex and can proceed via hydrolysis (most oftencatalyzed by enzymes) and/or oxidation (UV- or thermo-induced).In the degradation mechanism, polymer chain scission and thebreakdown of the structure in the crystal lattice generally occurs.Factors that can cause the degradation of a polymer can be relatedto the processing conditions and also the environmental effects ofthe matrix. Certain characteristics of polymers can inuence thedegradation process. In addition to the chemical structures ofpolymers, other factors also inuence the rate of degradation suchas crystallinity due to the different chain packing arrangements ofeach polymer. The amorphous structure of the PDLLA allows morewater to penetrate this structure, resulting in a faster degradationthan PLLA [30]. Some authors have studied the hydrolytic degra-dation of PDDLA and composites of this polymer [30e35]. Forexample, Fu and co-workers [32] prepared a multi-arm star poly-mer by ring-opening polymerization of D,L-lactide using multi-functional epoxidized soybean oil as an initiator and concluded thatthe degradation rate of the star-shaped polymer was slower thanthat of the linear PDLLA. One explanation for this phenomenonwasthat soybean oil can minimize the amount of trapped water,therefore slowing the permeation of water into the polymer. Inanother work, Chen et al. [35] observed greater hydrolytic degra-dation for the composites based on PDLLA/chitosan/tricalciumphosphate than neat PDLLA. This behavior was explained in termsof the hydrophilicity of the composites using contact angle andwater absorption measurements.

Investigations on polymer nanocomposites with differentbiopolymers and using different nanollers, such as clay, carbonnanotubes or CNWs, usually focus on improvements in the

E. Luiz de Paula et al. / Polymer Degra1632mechanical or barrier properties. Thus, the possible inuence of thenanoller addition on the polymer degradation matrix remains tobe addressed, mainly if the nanoller is also a biodegradablematerial.

The goal of the present work was to study the effect of theaddition of organic nanollers (cellulose nanowhiskers) on thehydrolytic degradation behavior of a biodegradable matrix. Toachieve this aim, we studied the controlled hydrolytic degradationbehavior of a PDLLA/CNW nanocomposite in phosphate buffermedium. The degradation process of the nanocomposites wasmonitored using FTIR spectroscopy, differential scanning calorim-etry (DSC) and thermogravimetric analysis (TGA). To the best of ourknowledge, this is the rst time that the inuence of CNWs on thedegradation behavior of a biodegradable matrix has beendetermined.

2. Experimental

2.1. Materials

Eucalyptus kraft wood pulp with high a-cellulose content(96e98%) was generously supplied by Bahia Pulp Company(Brazil). Sulfuric acid, sodium hydroxide, hydrogen peroxide andsodium chlorite were purchased from Aldrich, So Paulo, Brazil.Acetic acid 99% and hydrochloric acid were purchased from Synth,So Paulo, Brazil. All reagents were used without furtherpurication.

2.1.1. Cellulose Nanowhisker PreparationSulfuric acid hydrolysis of eucalyptus wood pulp was performed

as described in the literature with minor modications [36,37].Briey, the received wood pulp was treated with NaOH solution,and then a bleaching treatment was performed using acetate bufferand aqueous chlorite (1.7 wt% in water). Then, the wood pulp wasground using a Willey mill until a ne particulate was obtained.Subsequently, 10.0 g of cellulose was added to 160 mL of 65 wt%sulfuric acid under strong mechanical stirring. Hydrolysis wasperformed at 50 C for approximately 50 min. After hydrolysis, thedispersion was diluted 2-fold, and the suspensions were thenwashed using three cycles of centrifugation and resuspension. Thelast washing was conducted through dialysis with deionized wateruntil the dispersion reached pHw6. Afterward, the dispersionswere sonicated (Unique Sonicator, 40 kHz) for approximately 5 minand were nally ltered using a lter paper with a 20 mm pore size.The nal concentration of the CNW dispersions was approximately1 wt %.

2.1.2. Preparation of nanocompositesThe nanocomposites lms were prepared via a solution casting

method. Firstly, freeze-dried CNWs were dispersed in DMF (0.5%),using sonication to achieve full dispersion in the solvent. Thedispersion of the cellulose nanocrystals in DMF was checked byobserving the ow birefringence between crossed polarizers (notshown here). The appropriate amount of CNW/DMF dispersionwasadded to a PDLLA solution with the same solvent to prepare nalcomposites with the desired whiskers concentrations: 0%, 1%, and5%. The samples were named PDLLA, PDLLA-1%, and PDLLA-5% for0%, 1%, and 5% whisker content, respectively. After mixing for 12 h,the mixtures were poured into a Teon plate and left for 18 h toallow slow evaporation of the solvent. The lms were further driedunder vacuum for 8 h.

2.2. Hydrolytic degradation

The hydrolytic degradation of each of the samples (30 mg of dry

n and Stability 96 (2011) 1631e1638weight) was performed in 10 mL of phosphate-buffered solution

the wet lm at each time point and the weight of the original dry

Thermogravimetric analyses (TGA) were performed using a Hi-Res TGA 2950 thermogravimetric analyzer from TA instruments

images were obtained from a diluted suspension (0.01%) and showindividual nanocrystals and some laterally-aggregated elementary

3.2. Weight loss of the nanocomposites PDLLA/CNWs

Fig. 1. TEM image of the eucalyptus CNWs. The scale bar is 200 nm.

dation and Stability 96 (2011) 1631e1638 1633crystallites. The size of these agglomerated nanocrystals dependson the sample preparation (for example, the use of ultrasoundwaves to disperse the CNWs). Nevertheless, the presence of theseaggregates is highly expected even in suspension because of thehigh specic area and strong hydrogen bonds established betweenthe whiskers. However, when the dispersing medium is removed,as in the case of TEM sample preparation, bundles of whiskers canbe even more numerous than individualized rods [12,36]. Fromseveral TEM images, the mean values of the length (L) and diameter(D) of the isolated eucalyptus CNW were determined to be145 25 nm and 6.01.5 nm, respectively, giving an aspect ratiowith a heating ramp of 20 Cmin1 under air ow (74 cm3min1)from room temperature to 600 C.

3. Results and discussion

3.1. Cellulose nanowhisker characterization

Fig. 1 shows a typical TEM image of the eucalyptus CNWs. Theweight of the lm divided by the weight of the original dry weightof the lm.

2.4. Instrumental analysis

Transmission electron microscopy (TEM) images of CNWs weretaken using a FEI Tecnai G2-Spirit with 120 kV acceleration voltage.The nanocrystals were deposited from aqueous dispersions ona carbon- Formvar-coated copper (300-mesh) electron microscopygrid. The samples were subsequently stainedwith 2% uranyl acetatesolution to enhance the microscopy resolution.

Fourier transform infrared spectroscopy (FTIR) was recordedusing a Perkin Elmer FTIR spectrometer (GX). The infrared spectrawere obtained in the 4000e400 cm1 region in lm samples on theZnSe window.

DSC experiments were conducted on a PerkinElmer DSC Dia-mond in a temperature range of 30200 C using 10 mg of samplespecimen, according to the following temperature program: from30 to 200 C at 10 Cmin1; cooling from 200 to 30 C at20 Cmin1; and heating from 30 to 200 C at 10 Cmin1.(pH 7.4). The phosphate-buffered solutionwas replaced by a freshsolution every ve days. The asks were immersed in a water bathat 37 C. At predetermined times (one week, two weeks, onemonth, two and a half months and three months), samples wereremoved from the buffered solution and rinsed several times withdistilledwater. Subsequently, the samples were dried to remove thebuffer solution adhered on the surface of the lms and weighed todetermine the residual mass. After the hydrolytic degradation, thespecimens were rinsed, dried and placed in a desiccator forcharacterization.

2.3. Water uptake

To verify whether the cellulose nanocrystals can hinder theadsorption of water by the PDLLA polymer, water uptake experi-ments were conducted using distilled water at 37 C. The waterabsorption was calculated as the difference between the weight of

E. Luiz de Paula et al. / Polymer Degra(L/D) of approximately 24.A phosphate buffer solution (pH 7.4) was used for the hydro-lytic degradation of PDLLA and its nanocomposites because this isthe most common buffer in studies for biomedical applications dueto the similarity with body uids, including pH. The hydrolyticdegradation in buffered medium was monitored for 12 weeks, andthe weight loss of the samples during this period is shown in Fig. 2.

Fig. 2 clearly shows the inuence of the CNWs on the degra-dation time of the PDLLA polymer. The mass remaining of the neatbiopolymer decreases linearly up to eight weeks, with only 20% ofthe polymer mass remaining after 12 weeks. For the nano-composite with only 1% CNWs, we observed a signicant differencein degradation behavior compared with the neat PDLLA. ThePDLLA-1% exhibited only a small weight loss during the rst twoweeks, after which the nanocomposite was observed to havea constant mass. No weight loss was observed for the nano-composite with 5% CNWs, even after 12 weeks. This behaviorindicates that the presence of the CNWs in the matrix has a cleardelaying effect on the hydrolytic degradation of PDLLA.Fig. 2. Residual mass of the neat PDLLA, PDLLA-1%, and PDLLA-5% as a function ofdegradation time.

absorption and hence retard the degradation, modifying thekinetics of the hydrolytic process in PDLLA polymers.

These results are in agreement with those of the residual massin the hydrolytic degradation experiment (Fig. 2), in which weobserved a much smaller weight loss for the sample with only 1%CNWs and no degradation during the experiment in the PDLLA-5%sample. Previous results demonstrated that for chitosan/CNWs andalso for starch/CNWs systems, the cellulose nanocrystals improvedthe water resistance of these matrix, leading to a decrease in the

Fig. 3. FTIR spectra of PDLLA and nanocomposites before (d) and after () degrada-tion in phosphate buffer for three months: PDLLA (a), PDLLA-1% (b), and PDLLA-5% (c).Scales in the left Y-axis are the absorbance values for the samples before degradationand in the right Y-axis are the absorbance values obtained after degradation.

datioIt is important to mention that we also performed tensilemechanical tests on the nanocomposites (results not shown here),to characterize the inuence of the CNWs on the mechanicalproperties of the PDLLA polymer. The results showed that themaximum tensile strength of the PDLLA/CNWs nanocomposite(with 1% CNW)was approximately 25% greater than that of the neatPDLLA.

3.3. Infrared spectroscopy (FTIR)

Degradation of poly(D,L-lactide) occurs via electrophilic attackby water on the carbonyl ester groups present in the polymer. Theamorphous structure of PDLLA facilitates the diffusion of water inthe polymer matrix, accelerating the degradation process. Thus,long polymer chains are converted into smaller chains and intooligomers and monomers [38]. In this way, the degradation causesan increase in the number of chains and consequently in thenumber of terminal carboxyl groups. This degradation mechanismof PDLLA can be conrmed by FTIR analysis. Fig. 2 shows the FTIRspectra of the PDLLA and the nanocomposites before and afterdegradation (after three months) in phosphate buffer.

The increase in the number of terminal carboxyl groups in thePDLLA polymer after degradation is demonstrated by the appear-ance or increase in the absorbance of the band at 3508 cm1, whichcorresponds to the OH stretching vibration, and by the appearanceof the 1602 cm1 band representing C]O bond stretching incarboxylic acids. For the neat PDLLA polymer (Fig. 3a), the bands at3508 cm1 and at 1602 cm1 were clearly observed after thedegradation process. For the PDLLA-1% composite (Fig. 3b), we alsodetected the 1602 cm1 band after degradation, but the absorbanceintensity of the OeH stretching decreased compared to the neatPDLLA sample after degradation. For the PDLLA-5% (Fig. 3c), thestretching band related to carboxylic acids (1602 cm1) was notdetected, and the band at 3508 cm1 remained unchanged afterthree months in phosphate buffer.

The above data indicate that the presence of CNWs delayed theoverall degradation process of the PDLLA. The amount of waterabsorbed by the PDLLA polymer strongly inuences the rate ofchain scission [33]. In this way, one possible explanation for theincrease in stability under hydrolytic degradation conditions forPDLLA is that the presence of cellulose nanocrystals in the matrixacts as a physical barrier, preventing the absorption and/or thediffusion of water within the polymer matrix.

3.4. Water uptake

The PDLLA polymer is well know to be able to absorb a consid-erable amount of water [30,31] during the rst few days ofhydrolysis due to its amorphous nature that allows water mole-cules to penetrate more easily than a semi-crystalline polymer,such as PLLA. To verify whether the inuence of the CNWs on thedegradation behavior occurred by increasing the water resistanceof the biopolymer, we measured the water absorption. Because thedegradation occurs faster in phosphate buffer than in distilledwater, we used this last medium to detect the absorption of water,avoiding the interference of the weight loss that occurs during thedegradation (as could be the case using phosphate buffer). Fig. 4shows the water absorption for the neat PDLLA and the nano-composites as a function of time. The neat PDLLA absorbs approx-imately 9% of its mass in water in one week and reaches 15% waterabsorption in four weeks. For the nanocomposite with only 1%CNWs, the water absorption values were much smaller, approxi-mately 2% after two weeks, whereas the PDLLA-5% did notdemonstrate any water uptake after four weeks. These results show

E. Luiz de Paula et al. / Polymer Degra1634that a small quantity of cellulose nanowhiskers inhibit watern and Stability 96 (2011) 1631e1638swelling in water [39e41].

dation and Stability 96 (2011) 1631e1638 1635Additionally, comparable results, where the presence of nano-llers reduced the biodegradation rate of polylactides, have beendescribed [42,43]. Defeng et al. [42] reported the preparation ofPLA/carbon nanotube (CNT) nanocomposites and observed that theaddition of the nanollers retarded the biodegradation of the PLAin soil. This result was related to the physical barrier promoted bythe CNT that can block the entry of water into the polymer matrixto some extent. In another study, Fukushima et al. [43] studied thedegradation in compost of PLA and PLA nanocomposites based onlayered hydrated magnesium silicates (sepiolites) and found thatthe presence of the nanoparticles seemed to partially delay thedegradation of the PLA matrix. This result was also explained interms of barrier effects, in which the nanollers acted as cross-linking entanglements leading to a lower water absorption in thenanocomposite as compared to neat PLA. However, in some cases,the presence of nanollers can enhance the degradation, asdescribed in several studies in which the authors studied thehydrolytic [44] or compost degradation [18,45] in polylactide/nanoclay nanocomposites. In these studies, the authors observedthat nanoclays exert a catalytic role on the degradation of thepolylactides, and these results were attributed to the high relativehydrophilicity of the clays, facilitating the permeation of water intothe polymer matrix and thus accelerating the degradation process.

On the other hand, in this work the presence of the CNCs in theamorphousPDLLApolymercouldprovidephysicalbarriers,decreasingthe water absorption through the creation of a tortuous path for the

Fig. 4. Water absorption of the neat PDLLA, PDLLA-1%, and PDLLA-5% as a function oftime at 37 C in distilled water.E. Luiz de Paula et al. / Polymer Degrapermeating of water. This behavior can be explained in terms of thehigh degree of crystallinity of the nanocrystals which play an impor-tant role tohinder theabsorptionofwaterby thelms.Webelieve thatthe high level of crystallinity of the CNCs (C.I. 87%, calculated fromXRD analysis), and also their rigid hydrogen-bonded network gov-erned by a percolation mechanism were the responsible for theimprovement of barrier properties of the amorphous PDLLA polymer.

The high crystallinity of the nanowhiskers was also used toexplain the decrease in water vapor transmission rate formembranes based on PVA/CNCs [46] and xylan/CNCs [47].

Interestingly, the results of the present work showed that thebarrier properties of PDLLA can be enhanced through the incor-poration of cellulose nanoparticles in the matrix. This observationis particularly important because one of the limitations of the use ofPDLLA is its low barrier properties.

3.5. Thermogravimetric analysis

Fig. 5 shows the thermograms of PDLLA (Fig. 5a) and thenanocomposites (Fig. 5b and c) before and after degradation (also

Fig. 5. TGA curves of PDLLA and the nanocomposites before (d) and after (.)degradation in phosphate buffer (during three months) with a heating rate of20 Cmin1 under air ow: PDLLA (a), PDLLA-1% (b), and PDLLA-5% (c).

cyclic oligomers. Also, CO, CO2, methylketene, acetaldehyde and

Before the degradation process, the Tg values were observed todepend on the addition of CNWs (Table 2). The increase in thesevalues with concentration of whiskers in the nanocomposites canbe explained in terms of the restriction of the chain motionsimposed by the presence of the nanocrystals [49], which act asmicrodomains preventing the movement of polymer chains.

DSC measurements were also used to investigate the degrada-tion of the neat PDLLA and the nanocomposites, and the results arepresented in Fig. 6.

PDLLA is an amorphous polymer and does not show a crystalli-zation peak. However, during the degradation process of the lac-tides, crystalline domains appear in the degraded polymer matrix.Some authors have suggested that this phenomenon is due to therearrangement of small chains that are generated during thedegradation [50e52]. The formation of crystalline domains in thestructure of PDLLAwas conrmed by DSC analysis only for the neatPDLLA, inwhich an endothermic peak appeared at 91 C (Fig. 6). Forthe nanocomposites, we did not observe a signal related to thecrystalline domains, even when the CNW content was only 1 wt%.

Table 1TGA data (heating rate of 20 Cmin1) of PDLLA and PDLLA/CNW nanocompositesbefore and after degradation.

% CNWs Tbegin1/C Tend1/C Tbegin2/C Tend2/C

0 290 410 260 3541 302 400 299 4005 316 402 307 378

E. Luiz de Paula et al. / Polymer Degradation and Stability 96 (2011) 1631e16381636possible H2O from fragmentation reactions can be identied asa result of primary pyrolysis. In addition, polylactides samplescontaminatedwith residual Sn from the polymerization process canshow a preceding selective depolymerization step (Tmxw 300 C)which produces lactide exclusively [48].

Aside from the improvements in mechanical and barrier proper-ties, adding nanollers to a polymer matrix could also lead to anincrease in the degradation temperature. The results obtained in thiswork reveal that the incorporation of cellulose nanocrystals in PDLLAimproves the thermal stability of this biopolymer, increasing itsinitial temperature ofmass loss by approximately 12 C and 26 C for1% and 5% CNW content, respectively. A more interesting nd is theeffect of the CNW addition on the thermal properties after degra-dation in phosphate medium. With the addition of only 1% of CNWsin the matrix, the initial temperature of mass loss increases byapproximately 39 C (from 260 to 299 C), and this effect is morepronouncedwith theadditionof5%ofCNWs(Tbegin2 307 C). Theseresults demonstrate that the addition of CNWs to PDLLA inducesa considerable change in the thermal properties of the polymer. Inaddition, whereas the thermal properties of the neat PDLLA changefrom Tbegin1290 C (before degradation) to Tbegin2 260 C (afterdegradation), the Tbegin of the nanocomposites undergo slight vari-ations during the degradation process (comparing the Tbegin1 andTbegin2 values of the nanocomposites). The same behavior wasobserved for Tend: the signicant difference observed between Tendbefore andafter thedegradation for theneat PDLLAwasnotobservedfor the nanocomposites.

The analysis of the residual mass after degradation process up to600 C resulted in more than 98% of thermal degradation for all thesamples (less than 2% solid residue).

3.6. DSC results

To evaluate the thermal behavior of the PDLLA with the incor-poration of the CNWs, we also conducted DSC measurements withthe nanocomposites before and after degradation. The glass tran-sition temperatures (Tg) obtained from these curves are summa-measured after three months) in phosphate buffer. Table 1summarizes the data obtained from the thermogravimetriccurves for the stages of mass loss.

In a thermal decomposition of a polylactide, the dominantreaction pathway [48] (which Tmxw360 C) is an intramoleculartransesterication of the polylactide, giving rise to the formation of

Tbegin e temperature of the beginning of mass loss; Tend e temperature of the end ofmass loss; 1 and 2 refer to before and after degradation, respectively.rized in Table 2. The values were obtained from the second heatingrun.

Table 2DSC data obtained from the second heating run (at 10 Cmin1) on PDLLA andPDLLA/CNWs before and after degradation.

Sample Tg/C

Before degradation After degradation

PDLLA 53 57PDLLA-1% 55 54PDLLA-5% 56 55Moreover, we observed that the Tg value was increased by thedegradation process for the neat PDLLA, and for this sample, theregion of the Tg in the DSC curve broadened after degradation (notshown here). The crystalline domains that originated during thedegradation process act as a type of cross-linking, hindering themotion of polymer chains and consequently increasing the Tg.However, for the nanocomposites PDLLA-1% and PDLLA-5%, noincrease in the Tg values was observed after the degradationprocess (Table 2). In this case, because the presence of CNWsstrongly delays the degradation process of the polymer, the Tgvalues were not affected. These DSC results are in agreement withthe results obtained from weight loss measurements and FTIR,showing that the cellulose nanocrystals delay the degradation ofthe PDLLA matrix.

Although PDLLA is an amorphous polymer, we observed that itsdegradation induces a partial crystallization. Li and Vert [53]emphasized that the degradation process is more complex thanjust the relationship between the amorphous and crystalline parts.Using different PDLLA lms, these authors showed that thedegradation process is heterogeneous, occurring more rapidly inthe center than at the surface when the lm is in contact withaqueous media. At the beginning of the process, the degradationlikely occurs mainly on the surface due to the gradient of waterabsorption, but as the concentration of carbonyl groups increases inthe center, these groups serve as catalysts for the process. Lam and

1Fig. 6. DSC curves (second heating, at 10 Cmin ) of the samples after degradationfor twelve weeks in phosphate buffer.

dation and Stability 96 (2011) 1631e1638 1637colleagues [54] conrmed this self-catalysis characteristic, showingthat non-porous membranes are degraded faster than porousmembranes because the latter facilitated the spread of the degra-dation products dissolved in the aqueous environment, thushindering self-catalysis. In our case, we concluded that the cellulosenanowhiskers act in the rst stage of the degradation, hinderingthe degradation by decreasing water absorption. Because of thismechanism, only a small concentration of CNWs can havea pronounced effect on the degradation behavior of the polymer.

As mentioned, to prepare the composite studied in this work,we rstly achieved a good dispersion of the CNWs in the polarorganic solvent DMF. We then used the solvent casting method toprepared nanocomposites with PDDLA without using any type ofcompatibilizer, surfactant or chemical modication of the CNWs.This strategy of using DMF as a dispersion medium for freeze-driedCNWs, which has been previously used by other authors [55,56],has been described as a method to prepare CNWs nanocompositesusing organic solvents.

Finally, because we observed a large change in the degradationrate of the PDLLA polymer with the addition of only 1% CNWs, webelieve that this work can represent a rst step toward the use ofnanollers obtained from renewable resources to control thebiodegradability of a polylactide and could be extended to otherbiodegradable polymers.

4. Conclusions

The inuence of the cellulose nanowhiskers on the hydrolyticdegradation behavior of a bionanocomposite based on PDLLA wasstudied in a phosphate buffer medium. The nanocomposites wereprepared by a simple solvent casting method using DMF asa solvent to disperse the freeze-dried CNWs before the preparationof the nanocomposites.

The CNWs induced a strong delay in the PDLLA degradation,even when the concentration of the nanollers was only 1%. Thecellulose nanocrystals delay the degradation because they act asa physical barrier, hindering the absorption and/or the diffusion ofwater among the polymer chains and consequently modifying thekinetics of the hydrolytic process. This behavior was explained interms of the high degree of crystallinity of the nanocrystals whichimprove the barrier properties of the lms, through the creation ofa tortuous path for the permeation of water.

The incorporation of cellulose nanocrystals in the PDLLA alsomakes the biopolymer more thermally stable, increasing its initialtemperature of mass loss even after degradation in phosphatemedium. In addition, Tg values increased with whiskers concen-tration due to the restriction of the chain mobility resulting fromthe presence of the nanocrystals.

Finally, the results presented here showed the possibility toprolong the service life and to control the degradation rate of thePDLLA biopolymer, through the preparation of a nanocomposite,introducing a small quantity of nanollers obtained from renew-able materials: the cellulose nanocrystals.

Further work will study the biodegradability of PDLLA/CNWsnanocomposites by compost and investigate nanocomposites withdifferent low concentrations of CNWs to achieve control over thebiodegradability of these materials.

Acknowledgments

The authors thank CAPES (Nanobiotec e EDT N 04/2008),FAPEMIG (PPP- EDT N 021/2008) and Pr-reitoria de Pesquisa-UFMG for their nancial support. Centro de Microscopia-UFMG is

E. Luiz de Paula et al. / Polymer Degragratefully acknowledged for the TEM images.References

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E. Luiz de Paula et al. / Polymer Degradation and Stability 96 (2011) 1631e16381638

Influence of cellulose nanowhiskers on the hydrolytic degradation behavior of poly(d,l-lactide)1 Introduction2 Experimental2.1 Materials2.1.1 Cellulose Nanowhisker Preparation2.1.2 Preparation of nanocomposites

2.2 Hydrolytic degradation2.3 Water uptake2.4 Instrumental analysis

3 Results and discussion3.1 Cellulose nanowhisker characterization3.2 Weight loss of the nanocomposites PDLLA/CNWs3.3 Infrared spectroscopy (FTIR)3.4 Water uptake3.5 Thermogravimetric analysis3.6 DSC results

4 Conclusions Acknowledgments References