Effect of microstructure on hydrolytic degradation studies of poly (L-lactic acid)by FTIR spectroscopy and differential scanning calorimetry

Nadarajah Vasanthan*, Onah LyDepartment of Chemistry, Long Island University, One University Plaza, Brooklyn, NY 11201, USA

a r t i c l e i n f o

Article history:Received 9 February 2009Received in revised form14 May 2009Accepted 19 May 2009Available online 28 May 2009

Keywords:Hydrolytic degradationFTIR spectroscopyCrystallinityCrystallization

a b s t r a c t

Structural changes during thermally induced crystallization and alkaline hydrolysis of Poly(L-lactic acid)(PLLA) films were investigated using differential scanning calorimetry (DSC), FTIR spectroscopy, weightloss, HPLC and optical microscopy. It was shown that crystallinity (cc), glass transition temperature (Tg)and melting temperature (Tm) were found to be strongly annealing temperature (Ta) dependent. The FTIRstudy of PLLA films suggested that the bands at 921 and 956 cm�1 could be used to monitor the structuralchanges of PLLA. An independent infrared spectroscopic method was developed for the first time todetermine crystallinity of PLLA before degradation and it showed good qualitative correlation with DSCcrystallinity. The higher crystallinity values determined by FTIR were attributed to the intermediatephase included in the IR crystallinity. Both the weight loss data and the percentage of lactic acid obtainedby HPLC showed that the alkaline hydrolysis of PLLA films increased with increasing crystallinity. TheDSC observation showed an increase in Tg and no significant change in Tm and heat of fusion, while IRshowed an increase in IR crystallinity with increasing hydrolysis time. The increase in IR crystallinity andTg with hydrolysis time suggested that degradation progressed from the edges of the crystalline lamellaswithout decreasing lamellar thickness, but increased the intermediate phase and the short-range order.

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1. Introduction

Polylactic acid (PLA) is a biodegradable polymer and it is widelystudied and used in the area of industrial packaging and biomedicalapplications such as resorbable sutures, surgical implants, scaffoldsfor tissue engineering and controlled drug-delivery devices [1–5].However, it has been demonstrated that these systems are notsuitable for hard tissue regeneration due to its weak mechanicalproperties [6–10]. PLA can exist as two stereoisomers, designated asD and L, or as a racemic mixture, designated as DL. The D and L formsare optically active while the DL form is optically inactive. Poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) are semi-crystallinewhile poly(DL-lactic acid) (PDLLA) is amorphous [2,11,12].

PLA has a glass transition temperature (Tg) of about 60 �C andcrystallinemelting temperature (Tm) in the range from 130 to 180 �C.[6] Tg and Tm depend on optical composition, thermal history andmolecular weight [13–15]. Pure PDLA or PLLA has an equilibriumcrystallinemelting temperature (Tm

o) of 207 �C [16–19]. Baratian et al.carried out the isothermal crystallization of a series of poly(L-lactide-co-D-lactide) [20]. Kolstad [21] studied the crystallization

kinetics of poly(L-co-meso-lactide) and found that the crystallizationhalf time increased approximately 40% for every 1wt% increase in themeso-lactide. PLLA crystallizes into three different crystal modifica-tions, a, b and g form, depending on the crystallization conditions.However, the a and b forms are the common ones [22]. The a form isformed either bymelt or cold crystallization of PLLAwhile the b formis produced during either drawing of PLLA films at a high draw ratioand high drawing temperature or spinning fibers at a high spinningspeed. The g form is produced by epitaxial crystallization [22]. DeSantis andKovacs showed that thea formof PLLA fromsolution-spunfibers has a pseudo-orthorhombic unit cell containing two left-handed 103 polymeric helices which are arranged in an antiparallelfashion. The unit cell dimensions of a crystal form are a ¼ 1.07 nm,b ¼ 0.645 nm and c ¼ 2.78 nm. Hoogsteen et al. showed that theb form of PLLA adopts an orthorhombic unit cell containing sixleft-handed 31 polymeric helices. The unit cell dimensions of b formare a ¼ 1.031 nm, b ¼ 1.821 nm and c ¼ 0.900 nm [22,23].

Polymer degradation means the changes in physical propertiesdue to the chemical bond scission reactions in macromolecules[24,25]. Polymer degradation can be induced by thermal activation,hydrolysis, biological activity (i.e. enzymes), oxidation or photol-ysis. Depending on the mode of initiation, polymer degradationcan be classified as thermal, mechanical, photochemical, biologicalor chemical degradation [25]. In addition to the environmental

* Corresponding author. Tel.: þ1 718 246 6328; fax: þ1 718 488 1465.E-mail address: nadarajah.vasanthan@liu.edu (N. Vasanthan).

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Polymer Degradation and Stability

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Polymer Degradation and Stability 94 (2009) 1364–1372

conditions such as moisture, temperature and microorganisms,polymer degradation also depends on the structural propertiesof the polymer. Chain orientation, stereochemical composition,degree of crystallinity, molecular weight, molecular weight distri-bution and degree of crosslinking of polymers are among theimportant ones [26–28].

Hydrolysis belongs to the class of chemical degradation, whichoccurs by scission of chemical bond in the main chain by reactionwith water. The mechanism associated with hydrolysis of esterlinkage in neutral or acidic media is different from the one inalkaline media [29]. In neutral or acidic media (Scheme 1), thehydrolysis is initiated by protonation [29] and is followed by theaddition of water and the cleavage of the ester linkage. On the otherhand, in alkalinemedia (Scheme 2) hydroxyl ions are attached to thecarbonyl carbons and followed by the breaking of the ester linkages.Hydrolytic degradation of PLLA was reported to occur either bysurface erosion or bulk erosion mechanism. Surface erosion mech-anism of polymer degradation occurs only at the polymer–waterinterface. Bulk erosion mechanism is the uniform degradationthroughout the polymer [26,30]. The mechanism associated withalkaline hydrolysis is still not well understood and therefore in thepresent study, the effect of crystallinity and morphology on hydro-lytic degradation and microstructure changes with degradationwasinvestigated using weight loss, HPLC, DSC, optical microscopy andFTIR spectroscopy.

2. Experimental

2.1. Materials

Poly(L-lactic acid) (PLLA) with 6%D content was provided byProfessor Dennis Smith from Clemson University. TheMn andMw ofthis PLLA are 92,000 and 225,000 Da, respectively. Its polydispersityindex (PDI) is 2.44. NaOH solution of 0.1Mwas obtained from FisherScientific. Sodium lactate solution of 60% (w/w) with a density of1.3 g/mL (C3H5O3Na) was purchased from Sigma–Aldrich. All ofthese materials were used without further purification.

2.2. Preparation of PLLA film

PLLA films were prepared by melt-pressing technique witha Carver press. The press was preheated to 200 �C and the PLLApellets were placed between the two heated platens and allowed toheat to 200 �C for 5 min. Pressure of 20,000 pounds was applied onthe sandwich for about 4 min. The film was then removed andquenched quickly in ice cold water to prepare fully amorphous film(thickness is about 40–50 m). The amorphous PLLA filmwas dried atroom temperature and kept in a desiccator for further studies.These amorphous PLLA films were then annealed at differenttemperatures from 80 �C to 120 �C with the increment of 10 �C for30 min for hydrolysis studies.

2.3. Hydrolysis

Alkaline hydrolysis was performed on both annealed andas-prepared PLLA films. The PLLA films were annealed at 80 �C and110 �C in the oven for 30 min. The alkaline hydrolysis of each PLLAfilm (1 in� 1 in)was performed in a 100mLbeaker containing 50mLof 0.1MNaOH solution at room temperature for 1, 2, 4, 6, 8,10 and 12days. After hydrolysis, the PLLA films were removed and washedwith ice cold water to remove any traces of NaOH solution tostop further hydrolysis. Then the PLLA films were dried at roomtemperature and kept in a desiccator at constantweight until furtheruse. These PLLA films were used to study hydrolysis behavior. Thehydrolyzed solutions were used for HPLC studies.

2.4. Weight loss measurements

The percentage weight loss of hydrolyzed films was calculatedfrom theweights of the dried PLLA films before and after hydrolysisaccording to the following equation:

Wlossð%Þ ¼ 100��Wbefore �Wafter

�.Wbefore

where Wloss(%) is the percentage weight loss of hydrolyzed PLLAfilm.

Wbefore is the weight of the dried PLLA film before hydrolysis.Wafter is the weight of the dried PLLA film after hydrolysis.

2.5. High-performance liquid chromatography (HPLC) study

The chromatograms of all standard lactic acid and hydrolyzedsamples were obtained using the Hewlett–Packard 1050 HPLCsystem consisting of diode array detector (DAD) and 250 � 4.6 mmPhenomenex and Maxsil 5 RP 18 column. The 0.012 M HCl mobilephase was run with the flow rate of 1.0 mL/min; the injectionvolume was 20 mL. The hydrochloric acid solution with concentra-tion of 0.012 M was used as the mobile phase for HPLC. This solu-tionwas prepared by diluting the 12.1 M concentrated hydrochloricacid solutionwith E-pure water with the ratio of 1:1000 by volume.

Preparation of standard lactic acid solution and hydrolyzedsolutions were carried out by the following procedures. Lactic acidsolution of 13.9 mM stock was prepared by diluting 200 mL of6.96 M sodium lactate with E-pure water, adjusted to pH 2.10 using1 M hydrochloric acid (HCl) solution, and brought to the mark ofa 100 mL volumetric flask with E-pure water. A series of fivestandard lactic acid solutions was prepared by diluting 13.9 mMstock lactic acid solution with E-pure water with the ratio of 1:2,1:5, 1:10, 1:20 and 1:50 by volume for obtaining the concentrationof 6.950, 2.780, 1.390, 0.695 and 0.278 mM, respectively. Standardsolutions were placed in capped auto injector vials for HPLC

O

C O H C

OH

O C

OH

O

C

OH

O + H2O

+

C

OH

O HO+ H +

Scheme 1. Hydrolysis reactions of ester linkage in neutral or acidic media.

C

O

OOH

C

O

O

OH

C

O

O

OH

C OH

O

+ O

C

O

O + HO

Scheme 2. Hydrolysis reactions of ester linkage in alkaline media.

N. Vasanthan, O. Ly / Polymer Degradation and Stability 94 (2009) 1364–1372 1365

analysis. Each of the hydrolyzed samples was prepared and placedin capped auto injector vials for HPLC analysis. One-day and two-day samples were prepared by dilution with 0.1 M HCl solutionswith a ratio of 1:5 by volume. All the other samples (4d, 6d, 8d, 10d,and 12d) were prepared by dilution with 0.1 M HCl solutions witha ratio of 1:20 by volume. 1d, 2d, 4d, 6d, 8d, 10d and 12d samplesrepresent the hydrolyzed solutions after removing the PLLA film,which was immersed in 0.1 M NaOH solution for 1, 2, 4, 6, 8, 10 and12 days, respectively.

2.6. Differential scanning calorimetry (DSC)

Tg, Tm and DH of annealed PLLA films before and after hydrolysiswere determined using the Perkin Elmer DSC 7 differential scanningcalorimeter. The instrumentwas calibrated for temperature andheatof fusion using standard indium (Tm ¼ 156.6 �C and DH ¼ 28.5 J/g).All experiments were performed under nitrogen atmosphere withthe flow rate of 20mL/min. The samples were prepared by sealing inaluminum panwith the weight of 4–6 mg. DSCmeasurements wereperformed using the following procedure: The samples were heldfor 5 min at 25 �C and subsequently heated from 25 �C to 200 �C at10 �C/min. Heat of fusion is directly proportional to crystallinity (cc),and it was calculated according to the following equation, cc ¼ DHs/DHo, where DHo is the theoretical heat of fusion for 100% crystallinePLLA and it was taken as 93 J/g [30].

2.7. Fourier transform infrared spectroscopy (FTIR)

Infrared spectra of the annealed and hydrolyzed PLLA filmswerecollected using a NicoletMagna 760 spectrometer. The spectrawerecollected using transmission mode with a resolution of 4 cm�1

and 64 scans per sample in the mid-IR region of 4000–500 cm�1.Grams software was used for peak fitting. The linear baseline andLorentzian shape were used for all peak fitting.

3. Results and discussion

3.1. Crystallization study

Fig. 1 shows DSC scans of melt-pressed PLLA film and PLLA filmsannealed at 80 �C and 110 �C. All DSC scans show two transitions,which are glass transition, Tg and melting temperature, Tm. It can beseen that both Tg and Tm increase substantially with increasing Ta.Double-melting endotherms named as low and high melting peakswere observed for all annealed PLLA films and they can be attributedto either recrystallization of metastable crystals upon annealing orcrystallization from the amorphous region or a combination of both. Itshould be noted from Fig. 1 that a strong endothermic transitionsimilar to melting transition occurred for Tg . Similar observation hasbeen made by number of groups and it was attributed to enthalpy ofstress relaxation [31]. Fig. 2 shows Tg and Tm of PLLA as a function ofannealing temperatures. Both Tg and Tm observed for annealed PLLAfilms increase with increasing Ta. Tg of melt-crystallized PLLA wasstudied recently by Fitz et al. [32] and shown that a significantreduction inTg occurs during crystallization of PLLAunder constrainedconditions. Our PLLA samples were annealed under unconstrainedconditions and thus exhibit the conventional trend of increasing Tgwith increasing Ta. It shouldbepointedout that our PLLA samples havelowermelting points thanpreviously reported values [33,34], thismaybe attributable to the few D-lactic acid units incorporated in our PLLAsample. Fig. 3 shows the variation in crystallinity of PLLA films asa function of Ta. The crystallinity values show an increase withincreasing Ta, which is consistentwith previously reported data [35]. Itcan be seen that the crystallinity increases slowly up to Ta¼ 80 �C andincreases rapidly for Ta from 80 �C to 110 �C. It is also apparent that

crystallinity increase becomes smaller when Ta exceeds 110 �C. Thismay be attributable to the slower crystallization rate at highertemperatures that reduces the amount of final crystallinity due toinsufficient crystallization within a given period of time.

In this study, crystallization of PLLA during annealing wasfollowed by FTIR spectroscopy. Fig. 4 shows the FTIR spectra of PLLAfilms annealed at different Ta from 80 �C to 120 �C in the regions of1320–1280and 1000–600 cm�1. The structural changes wereinvestigated by comparing the spectral differences between semi-crystalline and amorphous PLLA. It is obvious that the IR spectrum ofsemicrystalline and IR spectrum of amorphous PLLA have distinctdifferences. Amorphous spectra showa band at 1302 cm�1 that splitsinto two bands at 1302 and 1293 cm�1 for the films annealed at Tafrom 100 to 120 �C. It appears that the band ratio of 1293 cm�1/1302 cm�1 increases with increasing Ta. It should be noted that theband at 956 cm�1 decreases in intensity while the band at 921 cm�1

increases in intensity with increasing Ta. The band at 921 cm�1 wasattributed previously to the combination of C–C backbone and CH3

rocking mode of PLLA a crystals [36,37]. The bands at 895 and871 cm�1 become weaker as Ta increases. The bands at 757 and710 cm�1 appear as a single band in the IR spectrum ofmelt-pressedfilm and both bands split into two bands as Ta exceeds 100 �C. Itis also apparent that the bands at 739 and 697 cm�1 increase in

Fig. 1. DSC scans of PLLA at annealing temperature (Ta) of RT, 80 �C and 110 �C.

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120 140T

a (°C)

Tg/T

m (°C

)

TgTmLinear (Tm)Linear (Tg)

Fig. 2. Tg and Tm of PLLA films as a function of annealing temperature.

N. Vasanthan, O. Ly / Polymer Degradation and Stability 94 (2009) 1364–13721366

intensity with increasing Ta. In summary, increases in absorbance ofbands at 697, 739, 921 and 1293 cm�1 are observed while decreasesin absorbance of bands at 710, 757, 895, 956 and 1302 cm�1 areobserved with increasing Ta. It appeared that there is no significantchange in the absorbance of the C–H stretching region. It is wellknown that annealing of PLLA films produce a significant amount ofcrystallinity and therefore the bands showing an increase in absor-bance can be attributed to the crystalline phase of PLLA while thebands showing a decrease in absorbance can be attributed to theamorphous phase of PLLA.

In order to confirm the reference band, curve fitting was carriedout to separate different components in the C–H stretching regionfrom 3060 to2840 cm�1. Five Lorentzian peaks were found to fit theregion, shown in 5a. Five peaks at 2996, 2961, 2945, 2901 and2880 cm�1 were obtained. Fig. 5b shows the peak areas of threebands (2996, 2961 and 2945 cm�1) of PLLA films as a function of Ta.No significant changes in peak area for all PLLA films as a function ofTa were observed and therefore in the present study, absorbance ofthe bands at 2996, 2961 and 2945 cm�1 were used as referencebands tomonitor structural changes of PLLA during annealing. If we

assume PLLA satisfies a two-phase model, these bands can be usedto measure the crystallinity by calibration against another methodsuch as DSC measurement. Since we know the bands are associatedwith the crystalline and amorphous phases, respectively, the band

Annealing Temperature (°C)

20 40 60 80 100 120 140

Cry

sta

llin

ity

(%

)

5

10

15

20

25

30

35

Fig. 3. Crystallinity of PLLA films obtained by DSC as a function of annealingtemperature.

Fig. 4. FTIR spectra of PLLA films at various annealing temperatures in the region of1000–600 cm�1: (a) 80 �C, (b) 90 �C, (c) 100 �C, (d) 110 �C and (e) 120 �C.

Temperature (°C)

70 80 90 100 110 120 130 140

Ban

d A

rea

10

20

30

40

50

a

b

Fig. 5. a. Curve fitting in the C–H stretching region. b. Peak areas of three bands at2996, 2961 and 2945 cm�1versus annealing temperature.

DSC Crystallinity (%)

0 10 20 30 40 50

Ban

d A

rea R

atio

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Fig. 6. Plot of absorbance ratio (peak area) of crystalline and amorphous bands (A921/A956) versus DSC crystallinity (%) of the same PLLA film.

N. Vasanthan, O. Ly / Polymer Degradation and Stability 94 (2009) 1364–1372 1367

ratios of 921 and 956 cm�1 can be correlated with the crystallinityobtained from DSC in Fig. 6. It is apparent from Fig. 6 that (A921/A956) increases with increasing DSC crystallinity. Crystallinity of anunknown sample may then be obtained from this calibration plot.This plot will be used to estimate the crystallinity change withhydrolytic degradation; this will be discussed later.

It would be very advantageous to develop a new method ofdetermining crystallinity using FTIR that does not involve anycalibration. FTIR spectroscopywas used for the first time to quantifythe amount of crystalline and amorphous fraction of PLLA withincreasing Ta and it was correlated with the crystalline fractionvalues obtained by DSC measurement. The bands at 921 and956 cm�1 are used to represent the crystalline and amorphousphase, respectively. If we assume PLLA satisfies a two-phase mod-el, these bands can be used to measure the distribution of thecrystalline and amorphous components. We assume a two-phasecrystalline and amorphous model for PLLA as follows:

P1�A921=ARef

�þ P2�A956=ARef

� ¼ 1

or ARef=A956 ¼ P1ðA921=A956Þ þ P2 (1)

where P1 and P2 are constants associated with crystalline and amor-phousbands, respectively. Thebandratioof referenceandamorphousarea (ARef/A956) versus band ratio of crystalline and amorphous area(A921/A956) of PLLA films at different Ta is plotted in Fig. 7 and showsa good linear fit. P1 and P2 were found to be 272.9 and 217.5, respec-tively. Crystalline fraction (P1(A921/ARef)) and amorphous fraction(P2(A956/ARef)) were obtained for PLLA films at different Ta; the resultsare shown in Table 1. Table 1 also includes the sum of crystallinefraction and amorphous fraction for all PLLA films annealed at

different Ta and DSC crystallinity. The results show the sum ofcrystalline and amorphous fraction obtained by IR to be 1.00 � 0.05for all PLLA films at different Ta. Crystallinity obtained by FTIR versuscrystallinity obtained by DSC of PLLA films at different Ta is plotted inFig. 8; the extrapolation of the plot to zero IR crystallinity yielded theintercept at 6.5% instead of 0% due to the differences of crystallinityobtained from FTIR and DSC studies. It is not surprising to see thecrystalline fraction obtainedby FTIR is higher than crystalline fractionobtained by DSC. It is known for sometime that FTIR always provideshigher crystallinity values than DSC because DSC measures themeltable portion of the crystals while FTIR measures the short-rangeorder that includes crystalline phase as well as the intermediatephase.

3.2. Hydrolytic degradation

The percentage of weight loss of PLLA films annealed at differentTa was calculated from dried PLLA films before and after thehydrolysis as described in the Experimental section. A PLLA filmwas placed in distilledwater for aweek and no significant change inthe mass of PLLA film was shown. Annealing of PLLA films werecarried out to obtain films with varying crystallinity. All PLLA filmswere placed in water for a week and no change in crystallinity was

A921

/A956

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

AR

ef/A

956

200

240

280

320

360

400

440

Fig. 7. Plot of ARef/A956 versus A921/A956 of PLLA films at different annealing temper-atures where ARef is the absorbance of reference band.

Table 1Crystalline fraction, amorphous fraction, sum of crystalline fraction and amorphousfraction of PLLA films obtained by FTIR along with crystalline fraction obtained byDSC at different Ta. Standard deviation is þ/� 0.05 based on at least fourmeasurements.

Annealingtemperature(�C)

Crystallinefractionby DSC

Crystallinefractionby FTIR

Amorphousfractionby FTIR

Sum of crystallineand amorphousfraction

80 0.12 0.07 0.88 0.9590 0.16 0.15 0.91 1.06100 0.20 0.31 0.70 1.01110 0.32 0.40 0.56 0.96120 0.33 0.47 0.55 1.03

DSC Crystallinity (%)

0 10 20 30 400

10

20

30

40

50

FT

IR

crystallin

ity (%

)

Fig. 8. FTIR crystallinity (%) versus DSC crystallinity (%).

Hydrolysis Time (days)

0 2 4 6 8 10 12

We

ig

ht L

os

s (%

)

0

20

40

60

80

100

Fig. 9. Percentage weight loss as a function of hydrolysis time for PLLA films annealedat different temperatures (C) PLLA film as-prepared; (B) PLLA film annealed at 80 �C;(:) PLLA film annealed at 110 �C.

N. Vasanthan, O. Ly / Polymer Degradation and Stability 94 (2009) 1364–13721368

observed. This confirms that there is no additional crystallizationoccurred from the free amorphous region during hydrolysis. Thepercentage of weight loss as a function of hydrolysis time for allPLLA films with different crystallinity is shown in Fig. 9. It appearsthat the percentage of weight loss increases with hydrolysis timefor all PLLA films studied and increases with initial crystallinity ofthe PLLA film. Fig. 9 shows that weight loss of PLLA films withcrystallinity of 9.8%, increased up to 6 days and then stayed pretty

much constant while theweight loss of PLLA filmswith crystallinityof 12.4% and 31.5% increased up to 10 days during hydrolyticdegradation. Alkaline and enzymatic hydrolysis of PLA copolymerswas reported recently30 and it has been shown that alkalinehydrolysis of copolymers depends on initial crystallinity of the filmand not the amount of the co-monomer unit. On the other hand,enzymatic degradation depends on both initial crystallinity and theco-monomer unit. It has also been shown that both alkaline and

Concentration (mM)

0 2 4 6 8

Peak A

rea

0

100

200

300

400

500

600

a

b

Fig. 10. a. Chromatogram of 2.780 mM standard lactic acid solution, Fig. 10b.Calibration curve of standard lactic acid solutions.

Time (days)

0 2 4 6 8 10 12 14

We

ig

ht %

L

ac

tic

A

cid

0

10

20

30

40

50

60

70

80

Fig. 11. Weight percentage of lactic acid as a function of hydrolysis time for PLLA filmsannealed at different temperatures (C) PLLA film as-prepared; (B) PLLA film annealedat 80 �C; (:) PLLA film annealed at 110 �C.

Fig. 12. a. DSC scans of PLLA films as prepared before and after hydrolysis, Fig. 12b. DSCscans of PLLA films annealed at 80 �C before and after hydrolysis, Fig. 12c. DSC scans ofPLLA films annealed at 110 �C before and after hydrolysis.

N. Vasanthan, O. Ly / Polymer Degradation and Stability 94 (2009) 1364–1372 1369

enzymatic hydrolyzibility of the PLA film decreased with increasingcrystallinity, which is the opposite of what we have observedduring alkaline hydrolysis. The crystallinity range of the PLAcopolymer films investigated was very small (26–22%) while thecrystallinity of PLA films used in our investigation was in the rangeof 9–30%. That the maximum weight loss appeared to increasewith increasing crystallinity in alkaline hydrolysis suggests that themechanism associatedwith alkaline hydrolysis is different from theenzymatic degradation reported previously.

In order to determine the amount of lactic acid in the hydro-lyzed solutions, the calibration curve of standard lactic acidsolutions was constructed based on the result obtained from thechromatograms of a series of five standard lactic acid solutionswith different concentrations. Chromatogram observed fora standard lactic acid solution with a concentration of 2.780 mMshows only one peak with a retention time of 4.403 min, shownin Fig. 10a. Therefore the retention time of 4.403 min wasattributed to lactic acid. Fig. 10 shows the calibration curve ofstandard lactic acid solutions obtained by integrating the areaunder the lactic acid peak of all standard lactic acid solutions.Retention time for all standard lactic acid solutions was about4.4 min. The calibration curve of the standard lactic acid solutionsin Fig. 10b revealed that the peak area of lactic acid linearlyincreases with lactic acid concentration. The peak area of lacticacid was obtained by integrating its area under the curve for allhydrolyzed samples of PLLA annealed at RT, 80 �C and 110 �C.Lactic acid concentrations in hydrolyzed samples were calculatedusing the calibration curve of standard lactic acid solutions andthe peak areas of lactic acid peaks in hydrolyzed samples. Thenthe percentages of lactic acid in hydrolyzed samples were calcu-lated by dividing the weight of the lactic acid contained in thehydrolyzed sample to the weight of PLLA film before hydrolysisand multiplied by 100. The percentage of lactic acid in hydrolyzedsamples obtained by HPLC as a function of hydrolysis time for thePLLA samples with varying crystallinity is plotted in Fig. 11. It canbe seen that the percentage of lactic acid increases as hydrolysistime increases. The percentage of lactic acid in the hydrolyzedsamples was found to decrease as the following for most of thetime: PLLA-110 �C (Xc ¼ 31.5%) > PLLA-80 �C (Xc ¼ 12.4%) > PLLA-RT (Xc ¼ 9.8%). These results were in agreement with the resultsobtained in our weight loss study.

3.3. Microstructure changes after degradation

Fig.12a–c show the DSC scans of PLLA films annealed at RT, 80 �Cand 110 �C, respectively, before and after hydrolysis. The values ofDH, Tg and Tm for PLLA films annealed at RT, 80 �C and 110 �C beforeand after hydrolysis are tabulated in Table 2. It is clear from Table 2that there was no significant change in heat of fusion of PLLA filmsbefore and after hydrolysis. Table 2 also shows Tg of PLLA films

before and after hydrolysis with respect to hydrolysis time. It isapparent that Tg increases with increasing hydrolysis time for allPLLA films annealed at RT, 80 �C and 110 �C. It should also be notedthat there is no significant change in Tm as a function of hydrolysistime. Tm was found to be 140–141 �C for PLLA films at RT,144–145 �C for PLLA films annealed at 80 �C and 149–151 �C forPLLA films annealed at 110 �C. The lack of significant change in heatof fusion as well as melting temperature suggests that the crys-talline region is not affected by alkaline hydrolysis. The increase inTg may be attributed to increase in intermediate phase of PLLA filmwith degradation time that reduces the mobility of PLLA chainwhich is supported by FTIR observation (it will be discussed later)but not by DSC observation.

The optical micrographs of PLLA films annealed at 110 �C beforeand after alkaline hydrolysis are given in Fig. 13. PLLA film beforehydrolysis (Fig. 13a) shows spherulitic morphology. Fig. 13a illus-trates typical regions in the PLLA film and shows larger spherulitessurrounded by smaller spherulites. Three regions are apparent inthe optical micrographs: 1. crystalline region; 2. amorphous regionwithin the spherulite; and 3. free amorphous region betweenthe spherulites. Rotation of sample results in extinction patternssuggests the presence of birefringence. Fig. 13b shows themorphology of PLLA film degraded for six days and, in Fig. 13b,spherulitic boundaries are clearly visible. Close observation indi-cates some of the materials may have been etched away. Fig. 13cillustrates the morphology of PLLA film degraded for 10 days. It isclear from Fig. 13c that spherulites are not impinging with oneanother. Loss of spherulitic boundaries suggests that materialbetween spherulites is etched away. Therefore it was concludedthat alkaline hydrolysis progressed through the edge of thelamellas without changing lamellar thickness; the lack of change inTm as a function of degradation time supports this conclusion.

FTIR spectra of PLLA films before and after degradation weretaken and they were very similar to the semicrystalline spectrumshown in Fig. 4. FTIR spectra suggest that PLLA films before andafter degradation consists of a a crystalline phase and amorphousphase. In order to see how crystallinity changes as a function ofhydrolysis for all PLLA films degraded in NaOH solution, the plotof the band ratio of crystalline and amorphous area (A921/A956)versus hydrolysis time was constructed. The band ratio of crys-talline and amorphous bands was obtained; the results of A921/A956 for all PLLA films are tabulated in Table 3. Table 3 representsA921/A956 of PLLA films before and after hydrolysis with respect tohydrolysis time at RT, 80 �C and 110 �C. It is apparent that A921/A956 increased as hydrolysis time increased. However, there wasa small increase in A921/A956 as hydrolysis time increased for allPLLA films at RT, 80 �C and 110 �C. This suggests that FTIRcrystallinity increased with increasing hydrolysis time of all PLLAfilms annealed at RT, 80 �C and 110 �C. Crystallinity change withdegradation was estimated using the Fig. 6 and it was about 9%

Table 2The values of DH, Tg and Tm of PLLA films before and after hydrolysis for the sample annealed at RT, 80 �C and 110 �C.

Hydrolysis time (day) RTa 80 �C 110 �C

DH DH b (J/g) Tgc (�C) Tm

d (�C) DH (J/g) Tg (�C) Tm (�C) DH (J/g) Tg (�C) Tm (�C)

0 9.1 59.1 140.0 11.6 61.6 144.6 29.3 62.3 150.92 7.2 60.5 141.0 9.2 63.4 145.1 28.6 63.2 150.16 9.6 61.2 140.9 9.2 63.7 144.9 26.6 64.6 149.410 10.3 61.2 140.8 9.9 65.3 145.0 27.2 65.2 149.1

Confidence limits for temperatures (þ/� 1 �C) and DH (þ/�0.5 J/g) based on three replicates.a Room temperature about 25 �C.b Heat of fusion.c Glass transition temperature.d Melting temperature.

N. Vasanthan, O. Ly / Polymer Degradation and Stability 94 (2009) 1364–13721370

increase for PLLA as prepared, 6% for PLLA annealed at 80 �C and3% for PLLA annealed at 110 �C after 10 days of degradation. Sincethere was no change in DSC crystallinity, it is reasonable toassume there is an increase in intermediate phase with hydro-lytic degradation. The increased intermediate phase is probablydue to hydrolysis of restricted amorphous region that providemore intermediate phase.

4. Conclusions

Thermally induced crystallization of PLLA films was studiedusing FTIR spectroscopy and DSC. Tg and Tm showed an increasewith increasing Ta. The crystallinity also increased as a function ofTa, as expected. An increase in absorbance of bands at 697, 739, 921and 1293 cm�1 and a decrease in absorbance of bands at 710, 757,956 and 1302 cm�1 were observed with increasing Ta. Infraredbands showing an increase in absorbance were attributed to thecrystalline phase while the bands showing a decrease in absor-bance were assigned to the amorphous phase. It was shownthat absorbance of the bands at 2996, 2961 and 2945 cm�1 do notchange significantly with increasing Ta and therefore these bandswere used as the thickness band for normalization. In the presentstudy the bands at 921and 956 cm�1 were used to characterize thecrystalline and amorphous phases, respectively. The crystallinity ofPLLA films at different Ta was obtained by an independent IRmethod and it was in good agreement with the crystallinityobtained by DSC. The degradation of PLLA films in alkaline solutionobtained by weight loss study showed an increase with increasingcrystallinity, which was in agreement with the percentage of lacticacid in hydrolyzed samples obtained by HPLC study. The DSCshowed an increase in Tg and no significant change in Tm and heat offusion while IR showed an increase in IR crystallinity with hydro-lysis. The increase in IR crystallinity and Tg with revealed thatdegradation progressed from the edges of the crystalline lamellaswithout decreasing lamellar thickness, but increases the interme-diate phase and the short-range order.

Acknowledgement

The authors thank Professor Glen Lawrence of Long IslandUniversity for his technical help of HPLC analysis. This work hasbeen partially supported by Department of Commerce and NationalTextile Center. The authors also thank reviewers for their helpfulcomments to improve the quality of this paper.

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