Study of Strain-Induced Crystallization and Enzymatic Degradation of Drawn Poly( l ...

Study of Strain-Induced Crystallization and Enzymatic Degradationof Drawn Poly(L‑lactic acid) (PLLA) FilmsDeepika Rangari and Nadarajah Vasanthan*

Department of Chemistry, Long Island University, One University Plaza, Brooklyn, New York 11201, United States

ABSTRACT: Poly(lactic acid) (PLLA) melt-pressed films with low crystal-linity were crystallized by stretching at a constant strain rate. The strain-inducedcrystallization and enzymatic degradation of drawn PLLA films in the presenceof proteinase K at 37 °C was investigated using weight loss measurements,differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR)spectroscopy. The results show that drawing has a significant effect on thecrystallinity, molecular orientation, and enzymatic degradation. The absorbanceratio of the bands at 921 and 956 cm−1 (A921/A956) was chosen to determinethe structural changes during strain-induced crystallization and hydrolysis. TheDSC crystallinity and A921/A956 showed an increase with the draw ratio. Sincewe were unable to obtain the transition moment angle for the bands at 921 and956 cm−1, the dichroic ratios were compared. It was found that the crystallineorientation develops rapidly at lower draw ratios whereas the amorphousorientation develops much more slowly. The enzymatic degradation of annealed PLLA films was reported, and surprisingly, quitedifferent results were observed for the enzymatic degradation of oriented PLLA films. The extent of degradation was lower forthe drawn PLLA film than for the undrawn melt-pressed PLLA film. The DSC crystallinity and A921/A956 of drawn PLLA filmsincreased with the degradation time, suggesting an increase in the crystalline phase with degradation. This reveals thatdegradation occurs in both the free and the restricted amorphous region in the case of drawn PLLA films, whereas it occurs onlyin the free amorphous region in annealed unoriented PLLA films.

■ INTRODUCTIONPoly(L-lactic acid) (PLLA) is a biodegradable and biocompat-ible semicrystalline polymer. It has been widely studied as analternative to conventional commercially available polymersbecause it is biodegradable, compostable, and nontoxic.1−7

Studies have shown that the mechanical properties of PLLA arecomparable to those of other polymers such as polyethyleneand polystyrene, and therefore, it can be used for manufacturingpackaging films and fibers.1−7 PLLA can be processed intotransparent films or fibers, or it can be injection-molded intobottles in the same way as poly(ethylene terephthalate)(PET).8 Poly(lactic acid) has two stereoisomeric forms: D-lactide and L-lactide. It can also exist as a racemic equimolarmixture of D- and L-lactides, designated as DL-lactide.9,10

PLLA has a glass transition temperature (Tg) of ∼60 °C,which is relatively higher than that of other thermoplastics, anda crystalline melting temperature (Tm) ranging from 130 to 180°C, which is relatively lower than that of other thermo-plastics.11,12 PLLA can be crystallized either by thermallyinduced crystallization or by strain-induced crystallization.13−15

The mechanical and physical properties of semicrystallinePLLA have been widely reported to depend on theirmorphology and crystal structure.16−20 In this light, thedevelopment of the structure of PLLA during thermallyinduced crystallization and strain-induced crystallization hasbeen extensively studied.13−15,21−25 PLLA has been found tocrystallize into one of three crystal forms, α, β, or γ, dependingon the crystallization conditions. The α form is formed either

by melt or by cold crystallization; the β form, by high-speedspinning and drawing to high draw ratios at high drawingtemperature; and the γ form, by epitaxial crystallization.Amorphous PLLA has lower strength and dimensional stability,and therefore, stretching or annealing above the glass transitiontemperature is required to increase the molecular orientationand crystallinity for commercial applications. The developmentof the structure of PLLA has been studied by X-raydiffraction26,27 and differential scanning calorimetry,28,29 andit has been characterized by IR spectroscopy as a comple-mentary technique.30−32

Many studies have investigated the degradability ofPLLA.33−49 It has been found that the degradability can bemodified significantly by changing the microstructure of thepolymer. Many studies have also investigated the effect ofstructural parameters on the hydrolysis of PLLA in differentmedia such as phosphate buffered solution (pH = 7.4), alkalinemedia, acidic media, and enzymes.33−49 Our group studied thehydrolytic degradation of PLLA films in the presence of 0.1 MNaOH and proteinase K.48,49 It has been found that thedegradation of PLLA films in the presence of a base increasedwith increasing crystallinity. On the other hand, degradation inthe presence of an enzyme decreased with increasingcrystallinity.

Received: July 17, 2012Revised: August 24, 2012Published: September 4, 2012

Article

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© 2012 American Chemical Society 7397 dx.doi.org/10.1021/ma301482j | Macromolecules 2012, 45, 7397−7403

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Although the degradation of PLLA has been widely studied,the influence of the microstructure on degradation has not yetbeen clarified. This study aims to investigate the effect of strain-induced crystallization on the enzymatic degradation of PLLAusing weight loss measurements, DSC, and FTIR spectroscopy.The microstructural changes in PLLA after degradation are alsostudied. Furthermore, PLLA subjected to strain-inducedcrystallization and thermally induced crystallization is com-pared. This study aims to provide some new insights into howmicrostructural properties such as the crystallinity andmolecular orientation affect enzymatic degradation.

■ EXPERIMENTAL SECTIONMaterials. Poly(L-lactic acid) (PLLA) with a molecular weight of

300 000 was obtained from Polysciences, Inc. The enzyme proteinaseK from Tritirachium album in the form of lyophilized powder, 1 MTris-HCl buffer solution with pH 8, and sodium lactate 60% (w/w)with a density of 1.3 g/mL were obtained from Sigma-AldrichChemical Co. The pH of the buffer solution was adjusted to 8.5, andthe solution was stored in a refrigerator until further use.Preparation of PLLA Films. PLLA films were prepared by

solvent-casting followed by melt-pressing using a Carver press. PLLApellets (0.5 g) were dissolved in a 25 mL 50:50 mixture ofdichloromethane and chloroform. The resulting solution was pouredinto a glass Petri dish, and it was left to stand overnight at roomtemperature for evaporation. Solvent-cast PLLA films were placedbetween the preheated platens at 200 °C for 5 min, and a pressure of10 000 lb was applied. The films were then removed and quicklyquenched in ice-cold water to prevent further crystallization. Thesefilms were essentially had low crystallinity and had a thickness of 50−60 μm. Rectangular films with dimensions of 4 × 3 cm2 were cut fromthe undrawn films, and they were uniaxially stretched to different drawratios of 1 to 3 at a constant strain rate of 8.333 × 10−3 s−1. Thevarious films obtained were labeled as PLLA1, PLLA1.5, PLLA2,PLLA2.5, and PLLA3, with the numeral in the label indicating thedraw ratio.Enzymatic Degradation. PLLA films with dimensions of 2 × 1.5

in.2 were immersed in a 100 mL beaker containing 2.0 mL of theenzyme solution and 18.0 mL of the Tris-HCl buffer solution. Theenzyme solution was prepared by dissolving 1 mg of proteinase Kenzyme in 1 mL of e-pure water. The 0.05 M Tris-HCl buffer solutionwith pH 8.5 was prepared by taking 50 mL of 1 M Tris-HCl solutionand adjusting the pH to 8.5 using 1 M NaOH. The solution was thentransferred to a 1 L volumetric flask, and its volume was made up to 1L with e-pure water. Enzyme solutions with films were maintained at37 °C in a constant-temperature bath. The degradation data weremeasured every day for 10 days. To do so, every day, the film wasremoved, washed with ice-cold water, and dried in a vacuum oven. Theweights of the films were measured before and after degradation usingan analytical balance. The degradation studies were carried out for atleast two sets of films. The degraded film solutions were collected andstored in the refrigerator for further use.Weight Loss Measurements. The percentage weight loss of the

degraded film was calculated using the mass of PLLA films before andafter degradation by the equation

= × −W W W W(%) 100 ( )/loss before after before

where Wloss (%) is the percentage weight loss of the degraded PLLAfilm; Wbefore, the weight of the dried PLLA film before degradation;and Wafter, the weight of the dried PLLA film after degradation.Differential Scanning Calorimetry (DSC). A Perkin-Elmer DSC

7 differential scanning calorimeter was used to investigate the thermalproperties of PLLA films before and after degradation. Samplesweighing ∼4−6 mg were used for the DSC experiments. Theinstrument was calibrated for the temperature and heat of fusion usingan indium standard (Tm = 156.6 °C and ΔH = 28.5 J/g). Theinstrument was heated from 30 to 200 °C at a constant heating rate of10 °C/min. Thermal properties such as Tg, Tm, and ΔH of the

stretched films before and after degradation were obtained. Allexperiments were carried out under a constant nitrogen flow rate of 20mL/min. The degree of crystallinity, χc, was calculated from the heat offusion and obtained from the DSC scans by the equation

χ =Δ − Δ

Δ×

H HH

% 100cm c

100%

where ΔHm is the enthalpy of melting; ΔHc, the enthalpy ofcrystallization; and ΔH100%, the heat of fusion of 100% crystallinePLLA. The crystallinity of PLLA was determined from the heat offusion of 100% crystalline PLLA for all samples (93 J/g).

Fourier Transform Infrared Spectroscopy (FTIR). The infraredspectra of the melt-pressed, stretched, and degraded films wereobtained using a Nicolet Magna 760 spectrometer. The spectra werecollected in the mid-IR region from 4000 to 500 cm−1 in thetransmission mode with a resolution of 2 cm−1. The absorbance ofvarious infrared bands was determined using Omnic software. Twodifferent transmission spectra were collected for each sample fororientation studies by using an incident beam parallel andperpendicular to the fiber axis using a polarizer. The infrared dichroicratio was determined using the following equation: D = A∥/A⊥, whereA∥ is the absorbance parallel to the draw direction and A⊥ thatperpendicular to the stretching direction.

■ RESULTS AND DISCUSSIONStrain-Induced Crystallization Studies. The melt-

pressed PLLA films (PLLA1) were stretched to differentdraw ratios to obtain PLLA films with varying crystallinity andmolecular orientation. Figure 1 shows DSC scans of PLLA1,

PLLA2, and PLLA3. The DSC scan of the unoriented melt-pressed film shows three transitions: melting temperature (Tm),glass transition temperature (Tg), and cold crystallizationtemperature (Tc). DSC scans of the PLLA film drawn todifferent draw ratios suggest a significant difference from thethermal behavior of the undrawn film, and they only show themelting temperature (Tm). Tm increases with the draw ratio,and this increase is often accompanied by increasing lamellarthickness. For example, PLLA1, PLLA2, and PLLA3 melt at179, 182, and 184 °C, respectively. Tc and Tg disappearcompletely for films that are stretched to different draw ratios,suggesting that all of the stretched films have a significantamount of crystallinity. As the crystallinity develops, themobility of polymer chains in the noncrystalline region isexpected to be significantly reduced, resulting in a slower rate ofcrystallization. The thermal properties of PLLA films drawn at astrain rate of 0.17 s−1 have been reported previously,15 and ithas been shown that Tc appears in DSC scans for PLLA films

Figure 1. DSC scans of melt pressed and PLLA films stretched todifferent draw ratios.

Macromolecules Article

dx.doi.org/10.1021/ma301482j | Macromolecules 2012, 45, 7397−74037398

drawn up to draw ratio of 5. This is different from ourobservation in which Tc was not observed at all for drawn PLLAfilms. It should be noted that the PLLA films used in this studywere drawn at a much lower strain rate (0.00833 s−1) comparedto that in the previous study, and therefore, the differentthermal behavior may be attributed to the different strain rateused for stretching.The crystallinity of the PLLA films was measured using DSC.

Figure 2 shows the development of the crystallinity with the

draw ratio. The crystallinity increases with the draw ratio, whichis consistent with the previously reported data for othersemicrystalline polyesters such as PET50,51 and poly-(trimethylene terephthalate) (PTT)52,53 drawn at temperaturesabove Tg. The crystallinity increases rapidly from theunstretched PLLA film to the PLLA film stretched to a drawratio of 2, and the crystallinity increases more gradually for adraw ratio above 2. For example, the crystallinity of anunoriented PLLA film was ∼20%; it increased to 40% for thefilm drawn to a draw ratio of 2 and 43% for one drawn to adraw ratio of 3.Figure 3 shows the FTIR spectra of PLLA1, PLLA2, and

PLLA3 in the region from 1000 to 650 cm−1. An obviousspectral difference can be seen between the FTIR spectra ofdrawn and undrawn PLLA films. As mentioned above, PLLA

crystallizes into one of three possible forms: α, β, or γ. The αform, which is most common, is obtained either by melt orsolution crystallization, whereas the β form is obtained bydrawing to high draw ratios at high drawing temperatures.Infrared band assignments were reported for both forms. Thebands at 697, 739, 921, and 1293 cm−1 were attributed to the αcrystalline phase and those at 710, 757, 895, 956, and 1302cm−1 to the amorphous phase. It was found that the absorbanceof crystalline bands increases relative to that of amorphousbands with increasing draw ratios. This observation furtherconfirms that crystallinity increases with the draw ratio. It hasbeen reported that bands at 921 and 908 cm−1 can be used toidentify the α and β crystalline phases, respectively. Figure 3shows no band at 908 cm−1, suggesting that there is no β crystalform present in both drawn and undrawn PLLA films.Vasanthan and Ly used FTIR spectroscopy to investigate the

structural changes in PLLA films after thermally inducedcrystallization and hydrolytic degradation.48,49 The bands at921 and 956 cm−1 were chosen to determine the structuralchanges that occur during crystallization at different annealingtemperatures and after hydrolytic degradation because these arerelatively isolated from other bands. These bands wereattributed to the coupling of C−C stretching with CH3 rockingvibration. In this study, these bands were used to examine thestructural changes in PLLA during stretching as well asenzymatic degradation. Figure 4 shows the infrared bands at

921 and 956 cm−1 for all drawn PLLA films. It is seen that theabsorbance of the band at 921 cm−1 increases relative to that ofthe band at 956 cm−1. To measure the changes in the crystallinephase quantitatively, the spectra of unoriented films arerequired. We have measured the spectra of uniaxially orientedfilms under parallel and perpendicular polarization. Figure 5shows the FTIR spectra of a PLLA film drawn to a draw ratio of3 measured under parallel and perpendicular polarization. Thepolymer chain usually orients parallel to the drawing direction.The absorption of infrared radiation depends on the transitionmoment vector and the applied electric vector. In the case ofsolids and crystals, each molecule has a fixed direction.Therefore, the absorbance of a vibrational mode depends onthe transition moment vector of the normal vibration (M) andthe applied electric vector (E). The quantity M·E determinesthe molecular transition moment. Maximum absorption willoccur when M and E are parallel and minimum absorptionwhen they are perpendicular. Parallel and perpendicular bands

Figure 2. DSC crystallinity as a function of draw ratio for all PLLAfilms drawn to different draw ratios.

Figure 3. FTIR spectra of PLLA films stretched to different drawratios in the region of 1000−650 cm−1: (a) DR 1, (b) DR 2, and (c)DR 3.

Figure 4. IR spectra of melt pressed and PLLA films stretched todifferent draw ratios in the region of 1000−900 cm−1 .



respectively become stronger and weaker in absorbance whenthe polarized IR light is parallel to the draw direction. On theother hand, perpendicular bands become stronger when the IRlight is perpendicular to the draw direction. Figure 5 suggeststhat the bands at 1384, 1360, 1214, 921, 870, and 751 cm−1 areperpendicular, whereas those bands at 1370, 1188, 956, 739,and 710 cm−1 are parallel. The unoriented absorbance of an IRband was then calculated using the spectra obtained underparallel and perpendicular polarization by the formula

= + ⊥A A A( 2 )/3un

where Aun is the absorbance of the unoriented film; A∥, theabsorbance under parallel polarization; and A⊥, the absorbanceunder perpendicular polarization. Figure 6 shows a plot of the

unoriented absorbance ratios of 921 and 956 cm−1 against thedraw ratio. It is apparent that the absorbance ratio (A921/A956)increases with the draw ratio. The absorbance ratio increasesrapidly up to a draw ratio of 2, and it increases more graduallyfor a draw ratio of 2−3; this is in strong agreement with ourDSC results.Polarized infrared studies were used to obtain information

about the molecular orientation of the crystalline and theamorphous phase of PLLA. One of the practical difficulties is to

identify infrared bands that have an absorbance lower than ∼1in order to satisfy the Beer−Lambert law. On the basis ofprevious studies, we have identified the bands at 921 and 956cm−1 as being suitable for investigating the crystalline andamorphous orientation of PLLA because their absorbance isless than 1. The transition moment angles of these vibrationsare not known, and therefore, the dichroic ratio is used tocompare the molecular orientation. The dichroic ratio (D) of aperfectly oriented polymer parallel to the draw axis is given byD0 = 2 cot2 α, where α is the transition moment angle of aparticular vibration. The dichroic ratio of a partially orientedpolymer is given by

= ⊥D A A/

where A∥ is the absorbance parallel to the draw direction andA⊥ that perpendicular to the stretching direction. Figure 7

shows the changes in the dichroic ratio of the bands at 921 and956 cm−1 as a function of the draw ratio of the PLLA films. Thedichroic ratio of the respective bands decreases and increases asthe draw ratio increases, indicating that both the crystalline andthe amorphous orientation increases with the draw ratio. Theorientation of the crystalline phase appears to reach a maximumat a draw ratio of 2, whereas that of the amorphous phasecontinues to increase until a draw ratio of 3. The rapiddevelopment of the crystalline orientation to an almost full fiberaxis compared with the slower development of the amorphousorientation is commonly observed in semicrystalline polymers.

Enzymatic Degradation and Morphological Changesafter Degradation. The undrawn and drawn melt-pressedPLLA films were subjected to enzymatic degradation usingproteinase K enzyme for 10 days. The percentage weight loss ofthe oriented and the unoriented PLLA films was calculatedfrom the mass of dried PLLA films before and after degradationby the equation

= × −W W W W(%) 100 ( )/loss before after before

where Wbefore is the initial weight of dried PLLA films beforedegradation and Wafter the weight of dried PLLA films afterdegradation.Figure 8 shows the percentage weight loss as a function of

degradation time for PLLA1, PLLA1.5, PLLA2, and PLLA3,which have different crystallinity and molecular orientation.The percentage weight loss increases with the degradation timefor both undrawn and drawn films. It is not easy to completely

Figure 5. FTIR spectra of PLLA film drawn to draw ratio of 3measured under parallel and perpendicular polarization in the regionof 1000−650 cm−1: (a) perpendicular polarization; (b) parallelpolarization.

Figure 6. Absorbance ratio (A921/A956) changes as a function of drawratio for drawn PLLA films.

Figure 7. Dichrotic ratio of bands at 921 and 956 cm−1 as a function ofdraw ratios: (●) 956 cm−1 and (○) 921 cm−1.



analyze how the microstructure affects the enzymaticdegradation of PLLA films because the crystallinity as well asthe molecular orientation changes with the draw ratio. Toinvestigate the effect of PLLA crystallinity on enzymaticdegradation, an undrawn PLLA film and one drawn to adraw ratio of 2 were compared. The highest percentage weightloss was observed in the former. Furthermore, the crystallinityof the latter (40%) was ∼2 times larger than that of the former(20%). It is well-known that the degradation of semicrystallinepolymers first occurs in the amorphous phase and then in thecrystalline region, and therefore, our observation should beexpected. This is also consistent with results reported by otherswho investigated the effect of the crystallinity of an undrawnPLLA film on enzymatic degradation. The degradation rate foran unoriented PLLA film is found to be much higher comparedto that for a PLLA film stretched to draw ratios of 2 and 3. Itshould be noted that the molecular orientation, which is specificto the drawn PLLA film, might affect enzymatic degradation.The molecular orientation includes the crystalline and theamorphous orientation. These can respectively be measuredusing the IR bands at 921 and 958 cm−1. The differencebetween the crystallinity of PLLA2 and PLLA3 is very small(40% and 43%, respectively), and therefore, PLLA films drawnto draw ratios of 2 and 3 are compared to determine the effectof amorphous orientation. The degradation rate and extent ofdegradation are similar for both PLLA2 and PLLA3, suggestingthat the amorphous orientation plays a minimal role in thedegradation. This in turn suggests that the crystallinity plays adominant role in determining the extent of degradation relativeto the molecular orientation.Figure 9 shows the DSC scans of undrawn and drawn PLLA

films before and after enzymatic degradation. The onset valueswere taken as Tg, Tc, and Tm (Table 1). Tg, Tc, and Tm clearlyshow a significant increase with degradation for the undrawnPLLA film. For example, Tg of neat PLLA is ∼54 °C, but itincreases to 62 °C after 10 days of degradation. Similarly, Tc ofneat PLLA is ∼89 °C, but it increases to 96 °C. The Tg and Tcdepend primarily on chain flexibility, molecular weight, andcross-linking, suggesting that segmental interaction increased asa function of degradation time for the undrawn PLLA film. Tgand Tc could not be detected for PLLA films drawn to differentdraw ratios before and after degradation. Table 1 shows Tm of

undrawn and drawn films as a function of degradation time. Tmwas compared for all PLLA films before and after degradation.Tm increased with the draw ratio; however, Tm for all PLLAfilms showed a small increase and stayed relatively constantduring degradation.Figure 10 shows the crystallinity of PLLA1, PLLA 1.5,

PLLA2, and PLLA3 as a function of degradation time. Thecrystallinity values of both drawn and undrawn PLLA filmsincrease with the degradation time. It has been shownpreviously that the crystallinity of annealed PLLA film increasedwith degradation time in the phosphate buffer; this wasattributed to the selective removal of PLLA chains from the freeand restricted amorphous region.43,44 Recently, we studied theeffect of crystallinity on the enzymatic degradation of annealedPLLA film and found that there is no significant change incrystallinity with degradation time; this was attributed to theselective removal of PLLA chains from the free amorphousregion.49 Furthermore, we observed an increase in thecrystallinity of a drawn PLLA film with degradation time; thiswas attributed to the selective removal of PLLA chains from thefree and restricted amorphous region.The FTIR spectra of degraded PLLA films were obtained and

compared with those of the original film. The crystallinitychanges as a function of degradation time were determinedusing FTIR spectroscopy. FTIR spectra taken for degraded

Figure 8. Percentage weight loss as a function of degradation time(days) for melt pressed and drawn PLLA films: (●) DR1, (○) DR1.5,(▲) DR2, and (△) DR3.

Figure 9. DSC scans of PLLA melt pressed PLLA films before andafter enzymatic degradation.

Table 1. Thermal Properties and Crystallinity Obtained byDSC of Drawn PLLA Films before and after EnzymaticDegradation at Various Times

sampledegradation time

(days)Tg(°C)

Tc(°C)

Tm(°C)

crystallinity(x)

PLLA1 0 54 89 178 19.74 58 90 178 20.07 59 95 179 20.610 62 96 179 23.2

PLLA1.5 0 57 92 180 32.04 58 93 181 33.27 60 95 183 35.510 62 97 183 40.1

PLLA2 0 182 40.94 183 41.07 183 43.010 184 48.0

PLLA2 0 183 42.64 183 43.67 187 45.610 185 52.6



PLLA films showed that the band at 956 cm−1 decreased andthat at 921 cm−1 increased with increasing degradation time,suggesting an increase in crystallinity with degradation; this is inagreement with our DSC observation. Furthermore, Figure 11

shows a plot of absorbance ratio, A921/A956, against thedegradation time. The plot shows an increase in absorbanceratio with the degradation time, suggesting an increase incrystallinity with time. The dichroic ratio of the bands at 921and 956 cm−1 was calculated for all degraded samples using thespectra obtained under parallel and perpendicular polarization.No significant change was observed in the dichroic ratio withtime for both drawn and undrawn PLLA films.

■ CONCLUSIONSThe effect of strain-induced crystallization on enzymaticdegradation was studied using weight loss measurements,DSC, and FTIR spectroscopy, and it was compared with theenzymatic degradation of annealed PLLA films. Undrawn PLLAfilms showed three transitions, Tg, Tc, and Tm, whereas drawnfilms showed only Tm. Tm showed a slight increase with thedraw ratio. The crystallinity showed a rapid increase from thecase of an undrawn film to a draw ratio of 2 and a more gradualincrease from a draw ratio of 2 to 3. The absorbance of thebands at 697, 739, 921, and 1293 cm−1attributed to thecrystalline phaseincreased and that of the bands at 710, 757,895, 956, and 1302 cm−1attributed to the amorphous

phasedecreased with increasing draw ratio. The bands at921 and 956 cm−1 were chosen to determine the structuralchanges during strain-induced crystallization. The IR band ratio(921/956) increased with the draw ratio, which was inagreement with the DSC results. The dichroic ratio at 921and 956 cm−1 was determined using polarized FTIR spectros-copy. The dichroic ratio at 921 cm−1 decreased and that at 956cm−1 increased with increasing draw ratio. The extent ofdegradation was lower for the drawn PLLA film than for theundrawn melt-pressed PLLA film. The DSC crystallinity as wellas A921/A956 increased with degradation time, suggesting anincrease in the crystalline phase with degradation. This revealsthat degradation occurs in both the free and the restrictedamorphous region, whereas it occurs only in the freeamorphous region in annealed unoriented PLLA films.

■ AUTHOR INFORMATION

Corresponding Author*E-mail [email protected].

NotesThe authors declare no competing financial interest.

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Figure 10. DSC crystallinity of undrawn and drawn PLLA films as afunction of degradation time: (●) DR1, (○) DR1.5, (▲) DR2, and(△) DR3.

Figure 11. IR band ratio (A921/A956) of undrawn and drawn PLLAfilms as a function of degradation time: (●) DR1, (○) DR1.5, (▲)DR2, and (△) DR3.



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Study of Strain-Induced Crystallization and Enzymatic Degradation of Drawn Poly( l ...

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