EUROPEAN

European Polymer Journal 43 (2007) 4431–4439

www.elsevier.com/locate/europolj

POLYMERJOURNAL

DSC analysis of isothermal melt-crystallization, glasstransition and melting behavior of poly(L-lactide)

with different molecular weights

Yong He a, Zhongyong Fan a,*, Yanfei Hu a, Tong Wu a, Jia Wei a, Suming Li a,b,*

a Department of Materials Science, Fudan University, Shanghai 200433, Chinab Research Center on Artificial Biopolymers, Max Mousseron Institute on Biomolecules, UMR 5247, Faculty of Pharmacy,

University Montpellier I, 34060 Montpellier, France

Received 17 April 2007; received in revised form 29 June 2007; accepted 4 July 2007Available online 20 July 2007

Abstract

Isothermal melt-crystallization, glass transition and melting behavior of poly(L-lactide) (PLLA) with different molecularweights were investigated by using differential scanning calorimetry. Analysis by Avrami equation showed that crystalli-zation was initiated by heterogeneous nucleation, followed by 3-dimensional growth. The maximum reciprocal half-timeof crystallization (1/t1/2) was detected at 105 �C. Double endothermic peaks were observed around the glass transition forPLLA with intermediate crystallinities, indicating the coexistence of bulk-like and confined amorphous regions. Double-melting behavior was analyzed and combined with the equilibrium melting temperature evaluation by non-linearHoffman–Weeks extrapolation, from which a value of 207.6 �C was deduced for PLLA of infinite molecular weight.Lauritzen–Hoffman theory was employed to analyze the crystallization kinetics. Regime II-III transition was found tooccur at 120 �C for PLLA of lower molecular weight. The crystal morphology was also examined by scanning electronmicroscopy through chemical etching method.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Poly(L-lactide); Crystallization kinetics; Glass transition; Melting behavior; Equilibrium melting temperature

1. Introduction

In the family of aliphatic polyesters, poly(L-lac-tide) (PLLA) has been extensively studied because

0014-3057/$ - see front matter � 2007 Elsevier Ltd. All rights reserved

doi:10.1016/j.eurpolymj.2007.07.007

* Corresponding authors. Address: Research Center on Artifi-cial Biopolymers, Max Mousseron Institute on Biomolecules,UMR 5247, Faculty of Pharmacy, University Montpellier I,34060 Montpellier, France (S. Li).

E-mail addresses: zyfan@fudan.edu.cn (Z. Fan), lisuming@univ-montp1.fr (S. Li).

of its degradability and biocompatibility. PLLAcan be used for various biomedical and pharmaco-logical applications, in particular for the fabricationof osteosynthetic devices due to its good mechanicalproperties which are comparable to those of poly-ethylene and polystyrene [1–4]. Attention is alsopaid to PLLA as environmentally degradable plasticmaterials [4,5]. For all applications, it is of majorimportance to understand the degradation charac-teristics of PLLA, which strongly depend on itscrystallization process.

.

4432 Y. He et al. / European Polymer Journal 43 (2007) 4431–4439

The crystallization behavior of PLLA has beenlargely investigated. The overall crystallizationkinetics of PLLA have been analyzed by usingAvrami equation and Lauritzen–Hoffman theory[6–10]. Different values of Avrami exponent (n) werereported: n = 3 by Kolstad and Iannace [6,7], andn = 4 by Miyata [8]. Iannace reported that the tran-sition from Regime III to Regime II occurs at115 �C [7], while Di Lorenzo and Abe observedthe transition at 120 �C [9,10]. The equilibriummelting temperature of PLLA ðT 0

m) was estimatedto be 207 �C by Vasanthakumari and Pennings[11] and in the range of 198 �C to 212 �C by Tsujiet al. with different procedures [12]. The glass tran-sition of PLLA after crystallization has also beeninvestigated [13–15]. Mano reported double endo-thermic behavior by using non-isothermal crystalli-zation procedure [15].

The molecular weight (MW) is an important fac-tor influencing the crystallization behavior of poly-mers. Vasanthakumari and Pennings studied themorphology and crystal growth as a function ofundercooling and MW mainly by using opticalmicroscopy, regime I and regime II crystallizationbeing applied for the evaluation of substrate lengths[11]. Later on, Miyata examined the crystallizationcharacteristics of PLLA films of different MWs,and observed that the growth rate of spherulitesincreased with decreasing MW [8]. However, DSCanalysis of PLLA crystallization behavior has notbeen investigated in detail in terms of MW and iso-thermal conditions, especially the glass transitiondynamics and double-melting behaviors.

Recently, we reported the basic crystallizationbehavior and crystal morphology of PLLA [16,17].Circumferential and hexagonal cracks were detectedin PLLA spherulites which were formed duringmelt-crystallization at 135 �C and quenching inliquid nitrogen. Rhythmic growth and thermalshrinkage were suggested to be the two main factorsaccounting for the formation of periodic cracks.The crystallization temperature range (120–155 �C)belongs to Regime II. In this work, we investigatedthe isothermal melt-crystallization, melting behav-ior and glass transition of three PLLA polymersof different MWs obtained by ring opening poly-merization of L-lactide using zinc lactate as catalyst.Avrami equation was applied to analyze the overcrystallization kinetics. T 0

m was reevaluated by usingboth linear and non-linear Hoffman–Weeks extrap-olations and by using Flory’s equation. Hoffmannsecondary nucleation theory was also employed to

estimate the kinetic parameters. Finally, etchingexperiments were conducted to allow a direct exam-ination of crystal morphologies.

2. Experimental

2.1. Synthesis

Poly(L-lactide) was synthesized by ring openingpolymerization of L-lactide using zinc lactate as cat-alyst and ethylene glycol as co-initiator. L-lactide(14.4 g), ethylene glycol (25, 45 or 85 mg) and zinclactate (10 mg) were introduced into a round-bot-tomed flask. After degassing, the flask was sealedunder vacuum and polymerization was allowed toproceed at 140 �C for 7 days [18]. The obtainedpolymers were purified by the dissolution/precipita-tion method using dichloromethane as solvent andethanol as non-solvent to remove low MW speciesand residual monomer. The three PLLA sampleswere labeled as PLLA1, PLLA2 and PLLA3, withthe number-average molecular weight (Mn) of7.45 · 104, 5.56 · 104, and 2.57 · 104 g/mol, respec-tively, and the same polydispersity index (Mw/Mn)of 1.4. The specific rotation value ½a�D20 measuredin chloroform solution was �153�, which confirmsthe optical purity of PLLA polymers.

2.2. Methods

GPC measurements were performed with aWaters apparatus equipped with a refractive indexdetector. Chloroform was used as the mobile phaseat a flow rate of 1.0 mL/min. 20 lL samples of 0.5%(w/v) solution were injected for each analysis. Thecolumns were calibrated with polystyrene standards(Polysciences).

DSC experiments were conducted by using aDuPont DSC 910 instrument calibrated with indiumstandard. Isothermal melt-crystallization was per-formed after melting at 200 �C for 3 min and rapidcooling to the crystallization temperature at ca.60 �C/min. Each sample of 5(±0.5) mg was scannedat a heating rate of 10 �C/min or 20 �C/min underN2 atmosphere after various thermal treatments.For glass transition studies, the samples wereallowed to crystallize for different isothermal periodsat 90 �C or 115 �C, quenched down to 45 �C andannealed for 48 h in an isothermal furnace beforescanning at a heating rate of 20 �C/min. Tm valuescorresponding to the lower-temperature endother-mic peak were taken after 10 min isothermal crystal-

Y. He et al. / European Polymer Journal 43 (2007) 4431–4439 4433

lization for the determination of equilibrium meltingtemperature ðT 0

mÞ values.The 1:2 water–methanol mixture containing 0.025

mol/L of sodium hydroxide was used for etchingexperiments. PLLA3 was melted on a glass plate,followed by isothermal crystallization at 110 �C or130 �C for 45 min, respectively. The crystallizedsamples were then subjected to etching in the mixedsolution of sodium hydroxide at 60 �C for 6 h. APhilips XL30 scanning electron microscope (SEM)was used to observe the crystal morphologies afteretching.

3. Results and discussion

3.1. Isothermal melt-crystallization

Isothermal melt-crystallization of PLLA wasinvestigated in a temperature range from 85 �C to

Fig. 1. Heat flow as a function of isothermal crystallization timeof PLLA1 at different temperatures (j), fitting results ofdifferential expression of Avrami equation shown in solid lines.

Table 1Overall crystallization kinetic data based on Avrami equation

Tc (�C) PLLA1 PLLA2

na ka

(1/min)n

t1/2 (min) DHc

(J/g)na ka

(1/min

85 3.3 0.00008 15.6 18.6 3.1 0.000590 3.3 0.00085 7.7 26.4 3.0 0.004495 3.2 0.01225 3.6 32.4 2.7 0.0890

100 3.0 0.06215 2.2 37.8 3.0 0.1112105 2.9 0.15715 1.7 40.2 2.9 0.5136110 2.9 0.10904 1.9 42.6 3.0 0.3022115 2.8 0.03311 3.0 42.0 3.0 0.0338120 2.7 0.01573 4.0 39.0 3.0 0.0150125 2.7 0.00378 7.1 40.2 2.8 0.0055130 2.6 0.00095 12.9 40.8 2.6 0.0040

130 �C. Typical DSC scanning of PLLA1 is pre-sented in Fig. 1 (shown in symbols). The bell-shapeof the peaks suggests a primary crystallization withno obvious secondary crystallization which usuallyoccurs at later stages of the crystallization process,as reported in literature [7,19]. It should be notedthat the beginning of crystallization is hardly distin-guishable from the heat flow curves.

The relative crystallinity (Xc) can be definedaccording to the following equation:

X c ¼R t

0ðoH c=otÞdtR1

0ðoH c=otÞdt

ð1Þ

where (dHc/dt) is DSC heat flow, and can be ana-lyzed by using Avrami equation [20–22]:

X c ¼ 1� expð�katnaÞ ð2Þ

where ka is the Avrami crystallization rate constantand na is the Avrami exponent.

Differential expression corresponding to Avramiequation was introduced to analyze the isothermalexothermic behavior, in order to avoid deviationresulting from the assignation of the beginning ofcrystallization. In fact, a small variation in the start-ing time of crystallization exotherms can inducelarge errors in Avrami analysis. The fitting linesare shown in Fig. 1, in good agreement with the ori-ginal data. Table 1 summarizes the results derivedfrom Avrami analysis.

The values of na are in the range of 3.3–2.6 forPLLA1, 3.1–2.6 for PLLA2, and 3.2–2.8 for PLLA3,suggesting a bulk crystallization with heteroge-neous nucleation followed by 3-dimensional growth[16,23]. Fractional na values have been reported inliterature and ascribed to the occurrence of randomnucleation or 2-dimensional growth [7,8]. In fact,

PLLA3

)n

t1/2 (min) DHc

(J/g)na ka

(1/min)n

t1/2 (min) DHc

(J/g)

7 10.1 25.8 3.2 0.00091 7.8 28.25 5.5 31.8 3.2 0.00904 4.0 34.21 2.1 34.2 2.9 0.10201 1.9 36.6

1.8 37.2 2.9 0.29852 1.3 51.69 1.1 39.6 3.0 0.83324 0.9 43.28 1.3 43.2 2.9 0.43958 1.2 45.64 2.7 43.2 2.8 0.05259 2.5 42.66 3.7 46.2 2.9 0.01917 3.5 34.84 5.8 48.0 2.9 0.00593 5.3 35.47 8.1 50.4 2.8 0.00402 6.2 29.4

4434 Y. He et al. / European Polymer Journal 43 (2007) 4431–4439

separate spherulites were hardly detectable becauseof the high nucleation density resulting from thehomogeneous nucleation when the isothermal tem-perature was below 110 �C [9,11]. In contrast, attemperatures above 120 �C, spherulites could reach100 lm in diameter with 2-dimensional growth, inthe case of thin PLLA films formed in DSC cansafter melting [16,17,24].

The half time of crystallization t1/2 can be calcu-lated from Ka, as shown in the following expression:

t1=2 ¼ln 2

ka

� �1=n

ð3Þ

Reciprocal half-time of crystallization (1/t1/2) can beconsidered approximately proportional to the crys-tal growth rate (G) [7]. As shown in Fig. 2, the tem-perature dependence of 1/t1/2 shows a bell-shapewith the maximum at about 105 �C, which is consis-tent with literature data [7,8]. PLLA3 exhibited thehighest 1/t1/2 value and PLLA1 the lowest one. Inother words, crystallization rate increases withdecreasing MW of PLLA in the investigated tem-perature range, as previously reported [8,10,11].Okui et al. suggested that (1/t1/2)max can be definedas a characteristic value for the crystal growthof polymers [25]. A power law is expressed as(1/t1/2)max /M a. The exponent a is deduced to be�0.5 from the slope in the insert of Fig. 2, in agree-ment with the value reported in literature [25].

The values of the crystallization enthalpy (DHc)are also listed in Table 1. For PLLA1, DHc increasesfrom 18.6 to 42.6 J/g with increasing crystallization

Fig. 2. Reciprocal of half-time of crystallization (1/t1/2) as afunction of melt-crystallization temperature for PLLA1 (m),PLLA2 (j), PLLA3 (.). A plot of common logarithm of themaximum 1/t1/2 against common logarithm of MW (Mn) isshown as the inset.

temperature (Tc) from 85 �C to 110 �C. Beyond, lit-tle changes are detected. A constant DHc increase isobserved for PLLA2, from 25.8 J/g at 85 �C to50.4 J/g at 130 �C. In the case of PLLA3, DHc ini-tially increases with increasing Tc, from 28.2 J/g at85 �C to 51.6 J/g at 100 �C. Afterwards, DHc stea-dily decreases to reach 29.4 J/g at 130 �C. Therefore,DHc depends on both the molecular weight and thecrystallization temperature which strongly influencethe chain mobility of the polymers. The appearanceof a DHc maximum at 100 �C for PLLA3 can beassigned to the combined effects of chain diffusion,chain folding and nucleation.

3.2. Glass transition dynamics

Fig. 3 presents the DSC endotherms around theglass transition for PLLA1 after crystallization at

Fig. 3. DSC scans (20 �C/min) for PLLA1 samples annealed at45 �C for 48 h after melt-crystallization at 90 �C and 115 �C fordifferent periods.

Fig. 4. DSC scans (10 �C/min) for PLLA1 (a), PLLA2 (b) andPLLA3 (c) after isothermal crystallization at varioustemperatures.

Y. He et al. / European Polymer Journal 43 (2007) 4431–4439 4435

90 �C or 115 �C for various time periods, followedby aging at 45 �C for 48 h. The aging processsignificantly enhanced molecular arrangementsinvolved in the glass transition, the segments relax-ing towards equilibrium. Enthalpy recovery arisesdepending on the mobility of PLLA chains duringheating. An endothermic peak is observed at 67 �Cwhen the PLLA1 sample is nearly amorphous(2.5 min at 90 �C and 2 min at 115 �C). A singlepeak is also detected at 75 �C for highly crystal-line samples after long-time crystallization (30 minat 90 �C and 20 min at 115 �C). On the other hand,the highly crystalline samples exhibited lowerenthalpy recovery during glass transition as com-pared to the nearly amorphous ones. Therefore,crystallinity is an important factor to the glass tran-sition and enthalpy recovery behaviors of PLLA.

It is of interest to note the presence of doubleendothermic peaks when the crystallization timeperiod was close to t1/2 (7.7 min at 90 �C and3.0 min at 115 �C according to the Avrami analysis).Mano et al. previously reported the double endo-thermic behavior for PLLA by using some coolingprocedures from the melt [15].The low-temperaturepeak was ascribed to bulk-like glass transition andthe high-temperature one to restricted motions ofthe amorphous phase confined by the crystallizedfraction. It is also noted that the high-temperaturepeak is around 75 �C in our work, higher than thevalue of 70 �C reported by Mano. This deviationcould result from the difference between the isother-mal and non-isothermal crystallization processes.

The peak positions appeared independent of thecrystallinity and the isothermal temperatures. How-ever, the high temperature peak appears less sharperat 115 �C as compared with that obtained at 90 �C(curves of 20 min in Fig. 3). This result might beattributed to more perfect lamellae growth and bet-ter organized spherulites in which amorphous por-tion is much more confined at 115 �C. From aboveanalysis, it could be concluded that both crystallinityand isothermal crystallization temperature contrib-ute to the glass transition dynamics of PLLA.

3.3. PLLA melting behavior after isothermal

crystallization

Fig. 4 presents the melting endotherms of thethree PLLA samples after appearance of the wholeexothermic peak during isothermal crystallizationat various Tc ranging from 85 to 135 �C. Either sin-gle or double endotherms were observed. The low-

and high-temperature melting endotherms werelabeled as peak I and peak II, respectively. Peak Igradually shifted to higher temperature withincreasing crystallization temperature. In contrast,peak II showed much less dependence on the iso-thermal temperature.

Table 2Estimated equilibrium melting temperatures and thickeningratios for PLLA according to linear and non-linear Hoffman–Week extrapolations

Samples Linear Non-linear

T 0m (�C) c T 0

m (�C) c a dl (nm)

PLLA1 180.3 5.6 201.3 1.0 3.86 4.2PLLA2 175.4 6.3 192.2 1.0 4.90 5.4PLLA3 171.4 7.1 183.5 1.0 6.48 7.1

4436 Y. He et al. / European Polymer Journal 43 (2007) 4431–4439

The double-melting behavior of isothermallycrystallized PLLA has been reported in literature,and assigned to melt–recrystallization by Yasuniwa[26], or to the possible degradation of PLLA melt byWang and Mano [27]. In our previous work, similarresults were obtained, confirming the melt-recrystal-lization mechanism [17]. Peak I is attributed to theprimary crystallites formed at Tc, with peak IIreflecting the relatively perfect lamella stacksresulted from recrystallization during the heatingscan.

In order to determine the T 0m of PLLA, Tm values

corresponding to the lower-temperature endother-mic peak were taken after 10 min isothermal crystal-lization to prevent data deviation caused bysecondary crystallization, lamellar thickening, andcrystallization perfection. The resulting Tm and Tc

data are plotted in Fig. 5. T 0m is generally estimated

by linear extrapolation of the Tm versus Tc plot tothe line Tm = Tc, as expressed by linear Hoffman–Weeks (HW) extrapolation

T m ¼T c

cþ T 0

m 1� 1

c

� �ð4Þ

where c is the thickening ratio [28].As shown in Table 2, linear extrapolation gives

T 0m values of 180.3, 175.4, 171.4 �C for PLLA1,

PLLA2, and PLLA3, respectively, which are muchlower than the value of 207 �C reported in literature[11,29]. The thickening ratio c also exhibits largedeviations from commonly obtained coefficients

Fig. 5. Non-linear Hoffmann–Weeks extrapolation of Tm � Tc

plots for PLLA1 (j), PLLA2 (m) and PLLA3 (.), and a plot of1/Tm versus 1/Pn for PLLA with different MWs is shown as theinset.

between 2 and 3 by linear HW analysis, and thelinear HW extrapolation is no longer applicable inT 0

m determination of low MW PLLA.Actually, the obtained Tm data do not exhibit a

strict linear relationship with Tc, as previouslyreported [30]. A modified non-linear Hoffmann–Weeks extrapolation has been proposed as follow-ing [31,32]:

T 0m

T 0m � T m

¼ cT 0

m

T 0m

� T c þ a

� �ð5Þ

where a is a constant, c the thickening ratio. Thenon-linear extrapolations are also shown in Fig. 5.T 0

m values of 201.3, 192.2, 183.5 �C are obtainedfor PLLA1, PLLA2 and PLLA3, respectively (Table2). The value of c derived from the fitting process isclose to 1, which means that crystals correspondingto peak I do not thicken isothermally or upon heat-ing. This result confirms the invalidity of linear HWextrapolation which is based on the assumption thedifference between crystallization and observedmelting temperatures is solely due to the thickeningof lamellae formed at the crystallization tempera-ture [31]. On the other hand, it could imply thatpeak II is derived from thickened lamellae stacksand the melt–recrystallization occurs during theheating process.

According to the value of c close to 1, the averageinitial lamellar thickness (l*) observed at Tc andpeak I can be expressed according to the LH theory,given by

l� ¼ 2re � T 0m

DH � ðT 0m � T cÞ

þ dl ¼ 2re � T 0m

DH � ðT 0m � T mÞ

ð6Þ

where dl is the thickness increment above the mini-mum lamellar thickness, fold surface energy re is6.09 · 10�2 J/m2, heat of fusion DH is 1.11 · 108 J/m3 [11].

By combining (5) and (6), the value of dl can beobtained, as shown below:

dl ¼ a � 2re

DHð7Þ

Fig. 6. Plots of ln(1/t1/2) + U(*)/ R(Tc � T1) versus (1/TcDTf)for three PLLA samples.

Table 3Kinetics results of PLLA obtained by using Lauritzen–Hoffmannequation

Samples Kg · 10�5 rre · 106 (J2/m4) r · 103 (J/m2) Regime

PLLA1 7.60 1187 19.5 IIIPLLA2 5.94 945 15.5 IIIPLLA3 5.27 855 14.0 III

2.61 847 13.9 II

Y. He et al. / European Polymer Journal 43 (2007) 4431–4439 4437

The results are included in Table 2. The value of dl

is comparable to that found for polyethylene (ca.4.3 nm) and has an essential apportion to lamellarthickness. However, this term is neglected in the lin-ear HW extrapolation and causes its invalidity. Thevalue of dl increases with PLLA of lower MW,which implicates that higher thickness incrementabove the minimum lamellar thickness, is neededfor lower MW PLLA to enable the secondary sur-face nucleation and growth at the fastest rate atTc. Chain ends seem to have significance on PLLAcrystallization kinetics.

Using the following equation proposed by Flory[33],

1

T m

� 1

T 0m

¼ k � 1

P n

ð8Þ

where Pn is the number of repeat units of polymerchains, k is a constant. The T 0

m value of infiniteMW was evaluated to be 207.6 �C (Fig. 5), whichis in good agreement with the value reported in lit-erature [11,29].

The various T om and 1/t1/2 values of PLLA with

different MW are analyzed based on Lauritzen–Hoffman theory [34], as expressed as

1

t1=2

� �¼ 1

t1=2

� �0

exp�U �

R � ðT c�T1Þ

� �exp

�Kg

T c �DT � f

� �

ð9Þ

where Kg is the nucleation constant, DT ¼ðT 0

m � T cÞ is the undercooling, f is expressed as2T c=ðT 0

m þ T cÞ, U(*) is the activation energy with avalue of 1890 cal/mol [17], T1 is (Tg � 30 �C) withTg = 59.5 �C [17], and (1/t1/2)0 is the front factor.

ln(1/t1/2) + U(*)/R (Tc � T1) versus (1/TcdTf) isplotted in Fig. 6. Two Kg values are obtained forPLLA3 (Table 3), corresponding to regime III andregime II, respectively. The ratio of the two Kg val-ues is equal to 2, i.e. the same as the value predictedby Lauritzen–Hoffman theory. The transition fromregime III to regime II is estimated to occur at120 �C. Such a regime transition was not observedfor PLLA1 and PLLA2 of higher MW. Iannacehas reported regime transition at 115 �C, but theMW of PLLA employed is unknown [7]. On theother hand, Pennings reported regime I to regimeII transition at 163 �C for PLLA with Mv =150 K, PLLA with higher MW exhibiting regimeII crystallization only [11].

A new crystal modification named the a 0 formhas been recently proposed for the crystal structure

of PLLA samples annealed at temperatures below120 �C. Such crystal modification with loose 103

helical chain packing is thermally less stable thanthe standard a form of PLLA [35]. The deviationof the nucleation constant Kg around 120 �C mightalso be attributed to the formation of different crys-tal modifications [35,36]. The surface free energy(rre) and lateral surface energy (r) can be deducedfrom the following expression:

Kg ¼nbrreT 0

m

kB � DHð10Þ

where n equals 4 in regime III and 2 in regime II, kB

is the Boltzmann constant, b is the layer thickness(5.17 · 10�10 m) [11]. The rre value is in the rangeof 800–1200 · 10�6 J2/m4 while that of r is in therange of 14–20 · 10�3 J/m2 (Table 3), both decreas-ing with decreasing MW. This finding may be attrib-uted to the presence of more defects related to endgroups in the lateral surface for PLLA of lowerMW.

3.4. Morphological examination after etching

Etching experiments were carried out on PLLA3after crystallization at 110 �C or 130 �C, respec-tively, in order to examine the crystallization mor-

Fig. 7. SEM images of PLLA3 etched by the mixed solution ofsodium hydroxide at 60 �C for 6 h after crystallization at 130 �C(a) or 110 �C (b).

4438 Y. He et al. / European Polymer Journal 43 (2007) 4431–4439

phology in the two regimes. Fig. 7 shows the SEMmicrographs of the crystals after etching. Finespherulite structures were observed for PLLA3allowed to crystallize at 130 �C (Fig. 7a), with thelath-like lamellae arranged in radial directions,while the amorphous parts were removed due to dis-ordered chain organization. The central part of thespherulites is also etched, possibly because of theless organized structure of the nucleus. Similarresults were reported by Fisher [37]. In contrast,spherulite structures were hardly distinguishable forPLLA3 allowed to crystallize at 110 �C (Fig. 7b),the lamellae appearing irregularly arranged. WhenPLLA3 crystallizes in regime III at 110 �C, the crys-tal growth rate is controlled by the diffusion terminstead of the nucleation term. Nucleation densityis significantly high to decrease the size of spheru-lites by an order of magnitude. Besides, a large num-ber of nucleation sites can be formed in the samegrowth layer of a lamella, which contributes to theformation of defects in the crystal. As a conse-

quence, spherulites appeared more hydrolyzed ascompared to those formed at 130 �C.

4. Conclusions

Avrami equation was applied in the isothermalmelt-crystallization of PLLA with different MWs.Differential equation was introduced to determinethe kinetics parameters through a fitting process.The crystallization was generally initiated by heter-ogeneous nucleation, followed by 3-dimensionalgrowth. The crystal growth rate of PLLA increasedwith decreasing MW, the maximum growth rateappearing at 105 �C.

Double endothermic peaks were detected aroundthe glass transition of PLLA with intermediate crys-tallinities. The low-temperature peak was assignedto the bulk-like amorphous PLLA, and the highone to the confined amorphous region. Both crystal-linity and isothermal crystallization temperaturecontribute to the glass transition dynamics of PLLA.

Double-melting behavior was observed for all thePLLA samples. The values of T 0

m were estimatedbased on non-linear HW extrapolation, exhibitingMW dependence. Through non-linear HW extrapo-lation, the origins of peak I and peak II were distin-guished. A T 0

m value of 207.6 �C was deduced forinfinite MW PLLA. Using the specific values of T 0

m

for PLLA with different MWs, the Lauritzen–Hoff-man theory was applied. The rre value is in therange of 800–1200 · 10�6 J2/m4 while that of r isin the range of 14–20 · 10�3 J/m2, both decreasingwith decreasing MW. Regime II-III transition wasfound to occur at 120 �C for PLLA3 only. PLLA3crystal morphologies in both regimes were observedby SEM through chemical etching method.

Acknowledgements

The authors are indebted to the Shanghai-Unilever Research and Development Fund (No.05SU07097) and the Pujiang Talents Project (No.06PJ14010) of the Shanghai Science and Technol-ogy Committee for financial support.

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