poly(d -lactic acid) blends

Polymer 53 (2012) 747e754

Contents lists available at SciVerse ScienceDirect

Polymer journal homepage: www.elsevier.com/locate/polymer

Synchronous and separate homo-crystallization of enantiomeric poly (L-lactic acid)/poly(D-lactic acid) blends Hideto Tsuji a, *, Kohji Tashiro b, Leevameng Bouapao a, Makoto Hanesaka b a b

Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan Department of Future Industry-Oriented Basic Science and Materials, Graduate School of Engineering, Toyota Technological Institute, Hisakata, Tempaku, Nagoya 468-8577, Japan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2011 Received in revised form 12 December 2011 Accepted 15 December 2011 Available online 21 December 2011

High-molecular-weight poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) are blended at different ratios and their crystallization behavior was investigated. Solely homo-crystallites mixtures of PLLA and PDLA were synchronously and separately formed during isothermal crystallization in the temperature (Tc) range of 90e130
C, irrespective of blending ratio, whereas in addition to homo-crystallites, stereocomplex crystallites were formed in the equimolar blends at Tc above 150 and 160
C. Interestingly, in isothermal crystallization at Tc ¼ 130
C, the spherulite morphology of blends became disordered, the periodical extinction (periodical twisting of lamellae) in spherulites disappeared, and the radial growth rate of spherulite (G) of the blends was reduced by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. However, the interplane distance (d), the crystallinity (Xc), the transition crystallization temperature (Tc) from a0 -form to a-form, the alternately stacked structure of the crystalline and amorphous layers, and the nucleation mechanism were not altered by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. The unchanged d, Xc, transition Tc, long period of stacked lamellae, and nucleation mechanism strongly suggest that the chiral selection of PLLA or PDLA segments on the growth sites of PLLA or PDLA homo-crystallites to some extent was performed during solvent evaporation and this effect remained even after melting. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Crystallization Spherulites growth Polylactide

1. Introduction Due to the existence of L- and D-form lactic acids, there are two typical types of optically pure crystalline polymers, poly(L-lactic acid) [i.e., poly(L-lactide) (PLLA)] and poly(D-lactic acid) [i.e., poly(Dlactide) (PDLA)]. Blending PLLA with its enantiomaric PDLA leads to the formation of stereocomplex, the crystalline model of which is shown in Fig. 1(a). The stereocomplexation is known to increase the mechanical performance, thermal/hydrolytic degradationresistance, and gas barrier properties [1e6]. However, for crystallization in bulk from the melt, the exclusive stereocomplex formation is limited to the PLLA and PDLA pair at least either of which has the molecular weight below 1  104 g mol1, whereas solely homo-crystallites are formed in the equimolar PLLA/PDLA blends where both polymers have the molecular weights higher than 1  105 g mol1 [3,7], which is shematically illustrated in Fig. 1(c). Similar to high-molecular-weight PLLA/PDLA blends,

* Corresponding author. E-mail address: [email protected] (H. Tsuji). 0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2011.12.023

homo-crystallization, not stereocomplexation, is reported to occur in poly[(S)-propylene sulfide]/poly[(R)-propylene sulfide] blend [8], poly[(S)-3-hydroxybutyrate]/poly[(R)-3-hydroxybutyrate] blend [9] and poly[(S)-3-hydroxypentanoic acid]/poly[(R)-3hydroxypentanoic acid] blend [10], although the effect of molecular weight on crystalline species was not investigated. Because of aforementioned peculiar and superior properties of stereocomplex-type PLLA/PDLA blends, most of the reported articles focus on the stereocomplex crystallization [1e6]. On the other hand, optically pure neat PLLA is known to crystallize in a0 - and a-form in the absence of orientation (shearing force) or specific substrates, in b-form in the presence of orientation, and in g-form in the presence of specific substrates upon which PLLA can epitaxially crystallize [11e14]. The transition from a0 - to a-form is reported to occur at 110e120
C [15e19]. On the other hand, nucleation mechanism changed from regime II to regime III at around 120
C [20e22]. As stated earlier, PLLA/PDLA blends both with high-molecular-weights above 1  105 g mol1 will cause homo-crystallization of PLLA and PDLA. However, to the best of authors’ knowledge, the detailed homo-crystallization mechanism and behavior of L-polymer/D-polymer blends have not

748

H. Tsuji et al. / Polymer 53 (2012) 747e754

D L D L

L D L D

D L D L

L D L D

D L D D

L D L D

a L L D L

L D L D

b

L L L L

L L L L

D D D D

L L L L

D D D D D D D D

c

Fig. 1. Crystallites composed of equimolar L- and D-polymers; (a) stereocomplex (racemic) crystallite where L- and D-polymers are packed side by side, (b) crystallite in which L- and D-polymer chains are packed randomly, (c) mixture of homo-crystallites of L- and D-polymers. This figure referred to the structural models suggested in Ref. [8].

been reported so far. In the present study, as a model case of homocrystallization of L-polymer/D-polymer blends, high-molecularweight PLLA and PDLA were synthesized and the homocrystallization of their blends with different PDLA contents was monitored by wide-angle X-ray scattering (WAXS), small-angle Xray scattering (SAXS), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). 2. Experimental section 2.1. Materials The PLLA [weight-average molecular weight (Mw) ¼ 8.4  105 g mol1, Mw/number-average molecular weight (Mn) ¼ 2.3] and PDLA (Mw ¼ 7.0  105 g mol1, Mw/Mn ¼ 1.8) were synthesized by ring-opening polymerization of L- and D-lactide, respectively, in the presence of stannous octoate as the initiator. The synthesized polymers were purified by reprecipitation using dichloromethane as the solvent and methanol as the nonsolvent. The films of neat PLLA, PDLA, and their blends (thicknesses of 20 and 500 mm for POM observation and physical measurements, respectively) with different PDLA contents [XD ¼ WPDLA/ (WPLLA þ WPDLA), where WPLLA and WPDLA are weights of PLLA and PDLA, respectively, in the blend] were prepared by casting 1 g dL1

solutions of the polymers using dichloromethane as the solvent, and subsequent solvent evaporation at room temperature (25
C) for approximately two days, followed by drying under vacuum for 7 days [23]. For preparation of blends, the separately prepared solutions of PLLA and PDLA were mixed vigorously before solution casting. For preparation of crystallized films, neat PLLA, PDLA, and their blend films were sealed in test-tubes under reduced pressure, melted at 250
C for 3 min, crystallized at an arbitrary Tc in the range of 90e160
C for 10 h, and quenched at 0
C for 5 min. 2.2. Physical measurements and observation The Mw and Mn of the polymers were evaluated in chloroform at 40
C by a Tosoh (Tokyo, Japan) GPC system (refractive index monitor: RI-8020) with two TSK Gel columns (GMHXL) using polystyrene standards. The fractions of stereocomplex and homocrystalline species in isothermally crystallized films were estimated by the use of WAXS. The WAXS measurements were performed at 25
C using a Rigaku (Tokyo, Japan) RINT-2500 equipped with a Cu-Ka source (l ¼ 0.1542 nm). The degree of crystallinity (Xc) of films was determined from WAXS profiles according to the procedure reported previously [24]. The SAXS profiles were measured using a Rigaku Nanoviewer with a Cu-Ka line as an incident X-ray beam, where an imaging plate was used as a detector. The spherulite growth in the films of the neat PLLA, PDLA, and their blends was observed by an Olympus (Tokyo, Japan) polarized optical microscope (BX50) equipped with a heating-cooling stage and temperature controller (LK-600PM, Linkam Scientific Instruments, Surrey, UK) under a constant nitrogen gas flow. The neat PLLA, PDLA, and blends were first heated from room temperature to 250
C at 100
C min1, held at the same temperature for 3 min to erase thermal history, cooled to an arbitrary Tc in the range of 90e160
C at 100
C min1, and then held at the same Tc, where the spherulite growth was observed. 3. Results and discussion 3.1. Wide-angle X-ray scattering To investigate the effect of Tc and XD on crystallization behavior, crystalline species, and crystallinity, the WAXS measurements were carried out. Fig. 2 shows the WAXS profiles of the neat PLLA, PDLA, and their equimolar blend films (XD ¼ 0.5) crystallized at different Tc values from 90 to 160
C. The comparison of three figures shows that the diffraction profiles are not altered by the equimolar blending of PLLA and PDLA, regardless of Tc values. The crystallization peaks at around 15, 17, and 19
, which are respectively due to

Fig. 2. WAXS profiles of neat PLLA, PDLA, and their blend (XD ¼ 0.5) films crystallized at different crystallization temperatures (Tcs) for 10 h from the melt. Arrows show the diffraction peaks from stereocomplex crystallites.

H. Tsuji et al. / Polymer 53 (2012) 747e754

a 2.76

c

b 2.44 d ( 110/200)

749

80

X

d ( 203)

X =0

2.74

2.42

0.5

α

α'

1

(3.7)

c

D

(0)

60

(1.6) (0)

2.72

X (%)

(0)

40

2.70

2.38

2.68

2.36

20

α

α'

2.66 80

100

120 140 T (°C)

160

(0)

c

d (Å)

d (Å)

2.40

2.34 80

100

c

120 140 T (°C)

160

c

0 80

α

α'

100

120 140 T (°C)

160

c

Fig. 3. d for (110/200) (a) and (203) (b), and Xc (c) of neat PLLA, PDLA, and their blend (XD ¼ 0.5) films crystallized for 10 h from the melt as a function of crystallization temperature (Tc). The values in the parentheses are the Xc values of SC crystallites for XD ¼ 0.5.

the diffraction from the (010), (110/200), and (203) planes of a0 - or a-form of PLLA or PDLA homo-crystallites [14e19], are observed for all the films, whereas no stereocomplex crystalline peaks were observed at around 12, 21, and 24
for the PLLA/PDLA blend films (XD ¼ 0.5), excluding the blend films crystallized at Tc ¼ 150 and 160
C [1,25]. This finding indicates that the blend films contained only a0 - or a-form homo-crystallites. In other words, PLLA and PDLA in the blend films are separately crystallized into the a0 - or a-form PLLA and PDLA homo-crystallites for the Tc range of 90e130
C. The interplane distances (d) of (110/200) and (203) planes were estimated from the 2q values at around 17 and 19
, respectively, whereas the Xc values were evaluated from the area of all crystalline diffractions relative to total diffraction area for 2q range from 10 to 25
. The d and Xc values thus obtained are plotted in Fig. 3 as a function of Tc. The d values of (110/200) and (203) dramatically decreased when Tc was elevated from 100 to 110
C. This dramatic change is attributable to the transition from a0 - to a-form of PLLA or PDLA homo-crystallites in the previous papers [15e19], but not due to the normal slow decrease of d values with an increase in Tc. Moreover, this transition is confirmed by the fact that the crystalline diffraction at 22.5
from (015), which is specific to a-form of PLLA or PDLA homo-crystallites [15] appeared in all the films at Tc ¼ 110
C (Fig. 3). The transition from a0 - to a-form of PLLA is reported to occur at 110e120
C [15e19]. The Xc values of all the films showed the similar Tc dependence and increased monotonically with Tc, although the small amounts of stereocomplex crystallites were contained in the equimolar blend films at Tc ¼ 150 and 160
C. It is interesting to note that the transition Tc from a0 - to a-form and the Xc values were not altered by the equimolar blending of PLLA and PDLA. Fig. 4 shows the WAXS profiles of the blend films having different XD values and crystallized at Tc ¼ 130
C, together with those of the neat PLLA and PDLA films. All the films showed the very similar diffraction profiles; that is, all the films crystallized in the aform of PLLA and PDLA homo-crystallites based on the result of Fig. 2. The d values of (110/200) and (203) planes and Xc values were estimated from Fig. 4 and are plotted in Fig. 5 as a function of XD. The d values of (110/200) and (203) planes and Xc values were independent of XD within the experimental errors.

values, where the scattering vector q ¼ (4p/l)sin(q) (l: incident Xray wavelength and 2q: scattering angle). In this figure, the neat PLLA, PDLA, and their blend films with different XD values crystallized at 130
C for 10 h were used for measuremts. The q giving the maximal Iq2 (qmax) was estimated from Fig. 6(a). The long periods [LP(qmax)] were calculated from thus obtained qmax values using Equation (1) and are tabulated in Table 1:

LPðqmax Þ ¼ 2p=qmax

(1)

The obtained LP(qmax) values of the blend films with XD ¼ 0.1e0.9 were in the range of 196e217 Å, which are practically constant, irrespective of XD, and are similar to 209 Å of the neat PLLA or PDLA. Fig. 7 illustrates the typical electron density correlation function [K(z)] for the lamellar system. Under the assumption of the two-phases model consisting of the alternately stacked structure of the crystalline and amorphous layers, the K(z) is defined by the following equation and can be calculated from SAXS data [26,27]:

3.2. Small-angle X-ray scattering To investigate the effect of XD on the stacked lamellar structure, SAXS measurements were performed. Fig. 6(a) shows the Iq2

Fig. 4. WAXS profiles of neat PLLA, PDLA, and their blend films with different XD values crystallized at 130
C for 10 h from the melt.

750

H. Tsuji et al. / Polymer 53 (2012) 747e754

Fig. 5. d for (110/200) (a) and (203) (b), and Xc (c) of neat PLLA, PDLA, and their blend films crystallized at 130
C for 10 h from the melt as a function of XD.

between the neat PLLA or PDLA film and the blend films indicate that the alternately stacked structure of the crystalline and amorphous layers was not varied by synchronous and separate crystallization of PLLA and PDLA. The LP values obtained from K(z) plot were smaller than but similar to the LP(qmax) values. If either PLLA or PDLA crystallizes first and then another polymer crystallizes in the blends, the second polymer should crystallize in the restricted space between already formed homocrystallites to form the crystallites smaller than that of the first polymer. In this case, there should be two peaks in SAXS curves depending on the thicknesses of lamellae. However, single peak diffraction curves were observed [Fig. 6(a)]. Therefore, two homocrystallites of PLLA and PDLA should have crystallized synchronously.


      KðzÞf h z!  < h> h z þ z!   < h> f2p1

ZN q2 IðqÞcosðqzÞdq

(2)

0

where < > means the statistical average and h(z) and are the electron density variation along the lamellar normal and the mean electron density, respectively. I(q) is the experimental SAXS intensity at a given q, and z is the correlation distance. Fig. 7 shows the two cases with different Xc ranges. Model (a) is the case with the Xc lower than 0.5. In this case the correlation function gives the averaged lamellar thickness (), thickness of the amorphous phase (), and long period (LP) as shown in this figure. Model (b) shows the case with Xc higher than 0.5. In the corresponding correlation curve, the and are given in the opposite manner when compared with the case (a). From the correlation curves calculated in Fig. 6(b), the degree of crystallinity can be evaluated as Xc ¼ /LP. The result is shown in Table 1. The thus obtained Xc values correspond well to those shown in Fig. 5(c) which were obtained from the WAXS data analysis. The LP (184e189 Å), (109e114 Å), and (75e77 Å) values of the blend films were in complete agreement with the corresponding values of the neat PLLA and PDLA films, 187 and 188 Å, 111 and 112 Å, 76 and 76 Å, respectively. Similarity of these structural values

3.3. Polarized optical microscopy To investigate the effect of XD on the macroscopic morphology and spherulite growth behavior, polarized optical microscopy was carried out. Fig. 8 shows the polarized optical photomicrographs of the blend films with different XD values crystallized at Tc ¼ 130
C for 10 h. The well-defined spherulites, wherein periodical extinction along the radial direction was seen, were observed for the neat PLLA and PDLA films. The periodical extinction indicates the periodical twisting of lamellae along the radial direction. The well-

b

a Iq

2

X =0

0.7

0.1 0.3 0.5

0.9 1

D

1.5

K(z) 1.0

Iq

2

K(z)

0.5

0.0

-0.5

-1.0 0.02

0.03

0.04 -1

q (Å )

0.05

0

50

100

150 200 z/Å

250

300

Fig. 6. SAXS profiles (a) and correlation function K(z) (b) of neat PLLA, PDLA, and their blend films with different XD values crystallized at 130
C for 10 h from the melt.

H. Tsuji et al. / Polymer 53 (2012) 747e754 Table 1 Structural parameters of PLLA/PDLA blends crystallized at Tc of 130
C for 10 h estimated by SAXS measurements. XDa

1 0.9 0.7 0.5 0.3 0.1 0

qmaxb

LP(qmax)c

LPd

e

f

Xcg

(Å1)

(Å)

(Å)

(Å)

(Å)

(%)

0.030 0.029 0.032 0.030 0.030 0.031 0.030

209 217 196 209 209 203 209

187 188 184 189 186 185 188

111 111 109 114 111 110 112

76 77 75 75 75 75 76

59.4 59.0 59.2 60.3 59.7 59.5 59.6

a XD ¼ WPDLA/(WPLLAþWPDLA), where WPLLA and WPDLA are weights of PLLA and PDLA, respectively, in the blend. b The q giving maximum Iq2. c Long period estimated by qmax: LP(qmax) ¼ 2p/qmax. d Long period estimated from K(z) plot. e Mean lamellar thickness. f Amorphous region thickness: da ¼ LP  . g Degree of crystallinity: Xc ¼ < dc>/LP.

defined spherulites were observed for the blend films, with the exception of the blend with XD ¼ 0.7, but the periodical extinction was not seen. At XD ¼ 0.7, the assemblies of crystallites, not spherulites, were observed. Interestingly, this finding indicates that the lamella orientation is disturbed by the synchronous and separate crystallization of PLLA and PDLA, although the incorporation of PLLA in PDLA and vice versa did not change the Xc values, transition Tc from a0 - to a-form, and alternately stacked structure of the crystalline and amorphous layers. The disturbance was most dramatic for the blend with XD ¼ 0.7. However, we cannot give an appropriate reason why the lamellar orientation was most disturbed at XD ¼ 0.7. Furthermore, it is interesting to note that the periodical extinction observed for neat PLLA and PDLA films disappear in the

751

blend films, despite the fact only homo-crystallites are formed and Xc is constant for all the blend film. This indicates that the periodical twisting of lamellae can be attained when solely PLLA or PDLA homo-crystallites are formed in the spherulites but the presence of the polymer having the opposite configuration and the synchronous and separate crystallization of PLLA and PDLA homocrystallites disturbed the periodical twisting of lamellae in the spherulites. Such periodical extinction or banded spherulites were observed not only for low-molecular-weight PLLA [28e31] and crystallizable poly(L-lactide-co-D-lactide) (83/17) [32] but also for PLLA in the presence of low-molecular-weight polymers such as atactic poly(DL-lactic acid) [33,34], atactic poly(3-hydroxybutyrate) [34e36], and poly(ethylene oxide) [37e39]. With ultrathin films, Maillard and Prud’homme observed S- and Z- shaped lamellae for S-configured PLLA and R-configured PDLA, and found that the direction of curvature of the lamellae can be linked with the sense of twisting of lamellae in banded spherulites [39,40]. The periodical rotation of PLLA or PDLA lamellae in neat PLLA and PDLA films may be disturbed by the coexistence of PLLA and PDLA in the blend films. The radial growth rate of the spherulites (G) was estimated from polarized optical photographs taken at different crystallization times. The G values thus obtained are plotted in Fig. 9(a) as a function of Tc. The G values averaged for a0 - and a-form (regimes III and II) are plotted in Fig. 9(b) as a function of XD. The plot in Fig. 9(b) are limited for Tc ¼ 90e140
C to avoid the effect of stereocomplex crystallization. The G values of the blend films averaged for a-form (regimes III and II), except for that at XD ¼ 0.5 in regime III, were lower than those of neat PLLA and PDLA. On the other hand, the G values of the blend films averaged for a0 -form, except for that at XD ¼ 0.3, are similar to those expected from neat PLLA and PDLA assuming the linear dependence of G on XD. Interestingly, this indicates that the effect of synchronous and separate formation

Fig. 7. Stacked lamellar structures and corresponding correlation functions calculated for the two types of models with different degrees of crystallinity (Xc), where the lamellae of various thicknesses are assumed to be stacked with random spacing. The models (a) and (b) show respectively the cases with Xc lower and higher than 0.5, in which Xc is defined as Xc ¼ /LP with the averaged lamellar thickness and long period (LP). is the thickness of the amorphous phase, and LP ¼ þ . It should be noted here that the relation of and < da> in the correlation function is opposite in these two cases.

752

H. Tsuji et al. / Polymer 53 (2012) 747e754

Fig. 8. POM photographs of neat PLLA, PDLA, and their blend films with different XD values crystallized at 130
C for 10 h from the melt.

3.5 X =0

G against T

3.0

c

D

2.5

)

)

11

2.0

G ( μm min

1.5 1.0

III

1.5 1.0

II

α'-form α-form (regime III) α-form (regime II)

0.5

0.5

α'

α

100

110

120

130

140

T (°C )

150

0

0.2

0.4

0.6 D

a0 -

X =0 D

0.5 1

0.8

9 8

α' α

7

0.0

X

c

ln G + 1500/R (Tc-T∞)

10

-1

-1

G ( μm min

2.0

12

Averaged G against X

0.1 0.3 0.5 0.7 0.9 1

2.5

0.0

c

b3.0

D

lnG + 1500/R (Tc-T∞)

a

1

6 2.6

III 2.8

II

3.0 3.2 3.4 5 -1 -2 10 ( T ΔTf ) (K )

3.6

3.8

c

Fig. 9. G as a function of Tc (a), G averaged for and a-form (regimes III and II) as a function of XD (b), and ln G þ 1500/R(Tc  TN) for a-form as a function of 1/(TcDTf) (c) for neat PLLA, PDLA, and their blend films crystallized from the melt.

H. Tsuji et al. / Polymer 53 (2012) 747e754

of PLLA and PDLA homo-crystallites on G values depends on the crystalline form and XD. It seems that the slow growth of spherulites, wherein a0 -form homo-crystallites were formed, suppressed the effect of synchronous and separate homo-crystallization on G. However, constantly low G values at XD ¼ 0.3 and 0.7, irrespective of the crystalline form, and the disordered spherulite morphology or lamella orientation at XD ¼ 0.7 may be ascribed to the imperfect chiral selection around homo-crystallites during solvent evaporation, which disturbs the growth and lamella orientation of the spherulites. We estimated the nucleation constant (Kg) and the front constant (G0) for the neat PLLA, PDLA, and their blend films by the use of the nucleation theory established by Hoffman et al. [41,42], in which G can be expressed by the following equation:

h i   G ¼ G0 exp  U * =RðTc  TN Þ exp  Kg =ðTc DTf Þ

(3)

where U* is the activation energy for transportation of segments to the crystallization site, R is the gas constant, TN is the hypothetical temperature where all the motions associated with viscous flow ceases, and f is the factor expressed by 2Tc/(Tm þ Tc) that accounts for the change in heat of fusion as the temperature is decreased 0 . Fig. 9(c) illustrates the typical ln G þ 1500/R(T  T ) below Tm c N plots of the neat PLLA, PDLA, and their equimolar blend films as 0 value of PLLA (or PDLA) crysa function of 1/(TcDTf), using the Tm
tallites (212 C) [43] and the Tg values of the melt-quenched polymers in the present study. Also, we used the universal values of U* ¼ 1500 cal mol1 and TN ¼ Tg  30 K. To avoid the effect of crystalline form transition of a0 to a-form at around Tc ¼ 105
C, we analyzed the data only for a-form, i.e., for Tc above 105
C. We did not analyze the data for a0 -form, i.e., for Tc below 105
C because of the small number of data. Normally, the plot as in Fig. 8(c) gives Kg as a slope and the intercept ln G0. The thus obtained values for the neat PLLA, PDLA and their blends are summarized in Table 2. Two Kg values observed for the neat PLLA, PDLA, and their blend films, indicating the presence of two different nucleation mechanisms, depending on Tc. The larger Kg values of the blend films (5.0e5.9  105 K2) are about twice the smaller Kg values of the blend films (2.5e2.9  105 K2) strongly suggest that the larger and smaller Kg values are those for regimes III and II kinetics, respectively [20e22]. The Kg values of the blend films for regimes III and II were similar to those of neat PLLA and PDLA (5.7 and 5.4  105 K2, 2.5 and 2.7  105 K2). This indicates that although the G values and the orientation and periodical rotation of lamellae in the spherulites of the blends are altered by the coexistence of PLLA and PDLA, the nucleating mechanism was not varied even in the blends. Such relatively small effects may be attributed to the separate crystallization of PLLA and PDLA during solvent evaporation to some extent and its memory effect during melt-crystallization process. Table 2 Estimated front constants (G0) and nucleation constant (Kg) of PLLA/PDLA blends crystallized in a-from from the melt. XDa

1 0.9 0.7 0.5 0.3 0.1 0 a

Tc (II-III)b

G0 (III)c

G0 (II)c

(
C)

(mm min1)

(mm min1)

120 120 120 120 120 120 120

1.31 7.88 3.68 5.94 8.81 5.43 3.53

      

1011 1010 1010 1010 1010 1011 1011

3.20 3.57 4.79 1.65 9.92 5.63 1.62

      

Kg (III)c

107 107 107 107 106 107 107

Kg (II)c

(K2) 5.42 5.24 5.04 5.23 5.45 5.88 5.73

(K2)       

105 105 105 105 105 105 105

2.71 2.73 2.85 2.53 2.49 2.87 2.49

      

105 105 105 105 105 105 105

XD ¼ WPDLA/(WPLLAþWPDLA), where WPLLA and WPDLA are weights of PLLA and PDLA, respectively, in the blend. b Tc at which transition from regime II to III occurs. c (III) and (II) mean regimes III and II, respectively.

753

4. Conclusions High-molecular-weight PLLA and PDLA crystallized synchronously and separately into a0 - or a-form homo-crystallites but not into stereocomplex crystallites in the Tc range of 90e130
C. Interestingly, the spherulite morphology was disturbed, the periodical extinction (periodical rotation or twisting of lamellae) as observed in the PLLA or PDLA spherulites was upset, and the G values of the blends were decreased by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo- crystallites. However, the transition Tc from a0 - to a-form homo-crystallites, Xc, d, the alternately stacked structure of the crystalline and amorphous layers and the nucleation mechanism were not altered by the synchronous and separate crystallization of high-molecular-weight PLLA and PDLA. The unchanged d, Xc, transition Tc, long period of stacked lamellae, and nucleation mechanism may be attributed to the fact that the chiral selection of PLLA or PDLA segments on the growth sites of PLLA or PDLA homo-chiral crystallites was performed during solvent evaporation to some extent and this effect remained even after melting. Acknowledgment This research was supported by a Grand-in-Aid for Scientific Research, Category "C", No. 19500404, from Japan Society for the Promotion of Science (JSPS). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

Ikada Y, Jamshidi K, Tsuji H, Hyon SH. Macromolecules 1987;20:904e6. Slager J, Domb AJ. Adv Drug Deliv Rev 2003;55:549e83. Tsuji H. Macromol Biosci 2005;5:569e97. Fukushima K, Kimura Y. Polym Int 2006;55:626e42. Kakuta M, Hirata M, Kimura Y. Polym Rev 2009;49:107e40. Tsuji H, Tsuruno T. Macromol Mater Eng 2010;295:709e15. Tsuji H, Ikada Y. Macromolecules 1993;26:6918e26. Sakakihara H, Takahashi Y, Tadokoro H, Sigwalt P, Spassky N. Macromolecules 1969;2:515e20. Yokouchi M, Chatani Y, Tadokoro H, Teranishi K, Tani H. Polymer 1973;14: 267e72. Yokouchi M, Chatani Y, Tadokoro H, Tani H. Polym J 1974;6:248e55. Tsuji H. In: Doi Y, Steinbuchel A, editors. Biopolymer, vol. 4 (Polyester 3). Weinheim: Wiley-VCH, Germany; 2002. p. 129e77. Auras R, Harte B, Selke S. Macromol Biosci 2004;4:835e64. Jiang X, Luo Y, Tian X, Huang D, Reddy N, Yang Y. In: Auras R, Lin L-T, Selke SEM, Tsuji H, editors. Poly(lactic acid): synthesis, structures, properties, processing, and applications (Wiley Series on Polymer engineering and technology). NJ: John Wiley & Sons, Inc; 2010. p. 69e82. Wasanasuk K, Tashiro K, Hanesaka M, Ohhara T, Kurihara K, Kuroki R, et al. Macromolecules 2011;36:6441e52. Pan P, Kai W, Zhu B, Dong T, Inoue Y. Macromolecules 2007;40:6898e905. Pan P, Zhu B, Kai W, Dong T, Inoue Y. J Appl Polym Sci 2008;107:54e62. Kawai T, Rahma N, Matsuba G, Nishida K, Kanaya T, Nakano M, et al. Macromolecules 2007;40:9463e9. Zhang J, Tashiro K, Tsuji H, Domb AJ. Macromolecules 2007;40:1049e54. Bouapao L, Tsuji H, Tashiro K, Zhang J, Hanesaka M. Polymer 2009;50: 4007e17. Vasantahkumari R, Pennings AJ. Polymer 1983;24:175e8. Abe H, Kikkawa Y, Inoue Y, Doi Y. Biomacromolecules 2001;2:1007e14. Tsuji H, Tezuka Y, Saha S-K, Suzuki M, Itsuno S. Polymer 2005;46:4917e27. Tsuji H, Ikada Y. Polymer 1999;40:6699e708. Tsuji H, Nakano M, Hashimoto M, Takashima K, Katsura S, Mizuno A. Biomacromolecules 2006;7:3316e20. Okihara T, Tsuji M, Kawaguchi A, Katayama K, Tsuji H, Hyon S-H, et al. J Macromol Sci Phys 1991;B30:119e40. Strobl GR, Schneider M. J Polym Sci Polym Phys Ed 1980;18:1343e59. Strobl GR, Schneider M. J Polym Sci Polym Phys Ed 1980;18:1361e81. Yasuniwa M, Tsubakihara S, Iura K, Ono Y, Dan Y, Takahashi K. Polymer 2006; 47:7554e63. Bouapao L, Tsuji H. Macromol Chem Phys 2009;210:993e1002. Tsuji H, Sugiura Y, Sakamoto Y, Bouapao L, Itsuno S. Polymer 2008;49: 1385e97. Zhang F, Xie X, Yuang J, Yuan W, Yan Q. Acta Polym Sinica; 2011:488e93. Tsuji H, Ikada Y. Macromol Chem Phys 1996;197:3483e99.

754

H. Tsuji et al. / Polymer 53 (2012) 747e754

[33] Tsuji H, Ikada Y. Polymer 1996;37:595e602. [34] Xu J, Guo B-H, Zhou J-J, Li L, Wu J, Kowalczuk M. Polymer 2005;46:9176e85. [35] Focarete ML, Ceccorulli G, Scandola M, Kowalczuk M. Macromolecules 1998; 31:8485e92. [36] Ohkoshi I, Abe H, Doi Y. Polymer 2000;41:5985e92. [37] Nakafuku C. Polym J 1996;28:568e75. [38] Kim KS, Chin IJ, Yoon JS, Choi HJ, Lee DC, Lee KH. J Appl Polym Sci 2001;82: 3618e26.

[39] Maillard D, Prud’homme RE. Macromolecules 2008;41:1705e12. [40] Maillard D, Prud’homme RE. Macromolecules 2006;39:4272e5. [41] Hoffman JD, Davis GT, Lauritzen JI. In: Hannay NB, editor. Treatise on solid state chemistry, vol. 3 (Crystalline and nonscrystalline solids). New York: Plenum Press; 1976 [Chapter 7]. [42] Hoffman JD, Davis GT, Lauritzen JI. J Res Nat Bur Std-A Phys Chem 1975;79A: 671e99. [43] Tsuji H, Ikada Y. Polymer 1995;36:2709e16.