Preferential Stereocomplex Crystallization in Enantiomeric Blends of

Sep 9, 2015 - Although stereocomplex (sc) crystallization is highly effective for improving the thermal resistance of poly(lactic acid) (PLA), it is m...
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Preferential Stereocomplex Crystallization in Enantiomeric Blends of Cellulose Acetate‑g‑Poly(lactic acid)s with Comblike Topology Jianna Bao, Lili Han, Guorong Shan, Yongzhong Bao, and Pengju Pan* State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China

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S Supporting Information *

ABSTRACT: Although stereocomplex (sc) crystallization is highly effective for improving the thermal resistance of poly(lactic acid) (PLA), it is much less predominant than homocrystallization in high-molecular-weight (HMW) poly(L-lactic acid)/ poly(D-lactic acid) (PLLA/PDLA) racemic blends. In this contribution, the sc crystallization of HMW PLLA/ PDLA racemic blends was facilitated by using comblike PLAs with cellulose acetate as the backbone. Competing crystallization kinetics, polymorphic crystalline structure, and structural transition of comblike PLLA/PDLA blends with a wide range of MWs were investigated and compared with the corresponding linear/comblike and linear blends. The HMW comblike blend is preferentially crystallized in sc polymorphs and exhibits a faster crystallization rate than does the corresponding linear blend. The sc crystallites are predominantly formed in nonisothermal cold crystallization and isothermal crystallization at temperatures above 120 °C for the comblike blends. Except for the facilitated sc formation in primary crystallization, synchrotron radiation WAXD analysis indicates that the presence of a comblike component also facilitates the transition or recrystallization from homocrystallite (hc) to sc crystallite upon heating. Preferential sc formation of comblike blends is probably attributable to the favorable interdigitation between enantiomeric branches and the increased mobility of polymer segments. After crystallization under the same temperature, the comblike blends, which mainly contain sc crystallites, show smaller long periods and thinner crystalline lamellae than do the corresponding PLLA with homocrystalline structures.

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INTRODUCTION Poly(lactic acid) (PLA) is a well-known biobased and biodegradable thermoplastic and has many advantages such as versatile processing ability and high mechanical properties. Except for biomedical applications, PLA has been considered as a candidate for the substitution of the conventional petroleumderived thermoplastics.1 Because lactic acid is a chiral molecule, PLA has two isotactic stereoisomers, i.e., poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). As semicrystalline polymers, PLLA and PDLA can crystallize in different polymorphs under different conditions.2 One of the most interesting polymorphs is the stereocomplex (sc) formed between PLLA and PDLA.3 The melting point (Tm) of sc crystallites (210−230 °C) is 50 °C higher than that of homocrystallites (hc). Therefore, sc crystallization has been regarded as one of the most efficient methods for the improvement of the thermal resistance and long-lasting durability of PLA material.4,5 Because of the presence of intermolecular hydrogen bonds between enantiomers,6−8 sc crystallites possess denser chain packing and slower molecular relaxation than their hc analogs.8 Such unique structure renders sc-type PLA with highly improved physical properties such as superior (thermo)mechanical strength and moduli9,10 and better resistances to thermal and hydrolytic degradation.11−13 © 2015 American Chemical Society

Because sc and hc crystallizations share similar crystallization temperature windows, the polymorphic crystalline structure of PLLA/PDLA blends is quite complicated. Because PLLA and PDLA enantiomers must arrange in a combined manner during sc crystallization, sc formation has a longer diffusion path and suffers from a larger kinetic barrier than homocrystallization, despite the fact that the former is more thermodynamically favorable. Therefore, the sc crystallization of PLLA/PDLA racemic blends is usually accompanied by the homocrystallization of the individual enantiomer. It has been well-recognized that molecular weight (MW) is a critical factor for the competing crystallization of sc and hc polymorphs in PLLA/ PDLA blends. The sc formation is just favorable for the racemic blends containing one or two low-MW components (MW 300 k) with different MWs were synthesized via ring-opening polymerization (ROP) using cellulose acetate (CA) as the backbone.42 The crystallization kinetics, polymorphic crystalline structure, crystalline transition, and lamellae structure of comblike PLLA/PDLA racemic blends were systematically investigated and compared with those of HMW linear/ comblike and linear blends. It was found that sc formation is highly favored in the crystallization of HMW comblike blends. Mechanisms for the topology-dependent crystallization kinetics and polymorphic structure of HMW racemic blends were discussed and proposed.

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EXPERIMENTAL SECTION Materials. The L- and D-lactide (>99%) were purchased from Purac Co. (Gorinchem) and purified by recrystallization from ethyl acetate. Cellulose acetate (CA, Mn = 42.7 kg/mol, Mw/Mn = 2.26) with an acetyl content of 39.8 wt %, a degree of acetyl substitution of 2.2 and an average degree of polymerization (DP) of 176 was purchased from Sigma-Aldrich. Tin(II) 2-ethylhexanoate [Sn(Oct)2, > 98%, Sigma-Aldrich] was distilled before use. Linear PLLA (l-PLLA, Mw = 143 kg/mol, Mw/Mn = 1.71) was obtained from the Shimadzu Co. (Kyoto, Japan). Linear PDLA (l-PDLA, Mw = 191 kg/mol, Mw/Mn = 1.44) was prepared by bulk ROP of D-lactide at 130 °C using Sn(Oct)2 as the catalyst and dodecanol as the initiator. Synthesis of Comblike PLLA and PDLA with CA Backbone. Comblike PLLA and PDLA were synthesized via the ROP of lactide using CA as the macroinitiator and Sn(Oct)2 as the catalyst, according to a published method.42 MW and PLA graft length of comblike polymers were controlled by changing the lactide/CA molar ratio. A typical procedure to synthesize comblike PLLA with an expected Mn of 12690

DOI: 10.1021/acs.jpcb.5b05398 J. Phys. Chem. B 2015, 119, 12689−12698

Article

The Journal of Physical Chemistry B Small-angle X-ray scattering (SAXS) and in situ WAXD were measured on the beamline BL16B1 of the Shanghai Synchrotron Radiation Facility (SSRF) under an X-ray wavelength of 0.124 nm. Scattering patterns were collected by a Rayonix SX-165 CCD detector (Rayonix, Illinois, USA) with a resolution of 2048 × 2048 pixels and a pixel size of 80 × 80 μm2. In SAXS analysis, the film samples (thickness of ∼0.3 mm) isothermally crystallized at different Tcs were prepared on a Linkam THMS600 hot stage using the same thermal program as that used in the DSC isothermal crystallization. For in situ WAXD analysis, an amorphous film sample with a thickness of ∼0.2 mm was prepared by quenching the melted (250 °C, 3 min) sample into liquid nitrogen. The sample was placed between two pieces of polyimide films and heated from 50 to 260 °C at 10 °C/min on a Linkam THMS600 hot stage. The WAXD patterns were collected by a time interval of 30 s during heating. Sample-to-detector distances were 0.12 and 2.5 m in WAXD and SAXS analyses, respectively. Acquisition times in WAXD and SAXS measurements were 15 and 90 s, respectively. The two-dimensional (2D) data were converted into a one-dimensional profile by circularly averaging with a Fit2D software.

The resonance peaks for methyl proton of acetyl group in CA appears at 1.8−2.1 ppm (peak c).42 The GPC curve of comblike PLLA and PDLA shows a single peak (Figure S1). On basis of the NMR data, the degree of lactyl substitution (DS) in the glucose unit and the average degree of polymerization (DP) of PLA grafts (DPPLA) of comblike PLAs were calculated DS = 2.2 × 3 × Ab ′/Ac

DPPLA = (Ab + Ab ′)/Ab ′

where A is the area of the corresponding NMR peak, and 2.2 is the degree of acetyl substitution in CA. As shown in Table 1, the lactyl DS of comblike PLA increases when increasing the lactide/CA feed ratio, and it is close to 0.8 when the molar feed ratio of the lactide to the glucose unit is larger than 26, suggesting that almost all hydroxyl groups of CA initiate the ROP of lactide, and that the synthesized comblike PLAs have similar grafting densities. As calculated from the Mn (42.7 kg/mol) and the degree of acetyl substitution (2.2) of CA, a CA macroinitiator contains approximately 140 hydroxyl groups. Therefore, it is estimated that the averaged graft number in the comblike PLA is 104−140. The averaged DP of the PLA branches (DPPLA) ranges 38−103, corresponding to the MW for each branch of 2.7−7.4 kg/mol. MW and DPPLA increase with the lactide/CA feed ratio, indicating that the MW and the graft length of comblike PLAs can be well tailored from the feed ratio. The DPPLA calculated from NMR agrees with that calculated from the feed molar ratio of lactide to glucose (or unsubstituted hydroxyl group in CA). These results can demonstrate the successful synthesis of comblike PLLA and PDLA. Crystallization Kinetics. The crystallization kinetics of comblike PLLA/PDLA racemic blends with different MWs were studied via DSC and also compared with those of the linear/comblike and linear blends. Based on the DSC heating curves of melt-quenched samples (Figure 2), glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm), and enthalpy (ΔHm) of hc and sc crystallites were calculated (Table 2). As shown in Figure 2 and Table 2, no distinct change in Tg is observed for the blends with different MWs and topologies. Interestingly, except for the cold crystallization at 90−120 °C, all comblike blends (i.e., c-PLLA/ c-PDLA blends) exhibit a single melting region at high temperatures (210−230 °C), suggesting the predominant formation of high-Tm sc crystallites in nonisothermal cold crystallization. The Tcc of comblike blend decreases and its ΔHm,sc increases with increasing MW, suggesting that the comblike blends with longer PLA branches have faster crystallization rates and higher abilities to stereocomplex.

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RESULTS AND DISCUSSION Synthesis of Comblike PLLA and PDLA with CA Backbone. The chemical structure and MW of comblike polymers were characterized by 1H NMR and GPC. As shown in Figure 1, resonance peaks corresponding to the methyl,

Figure 1. 1H NMR spectrum of comblike PLLA with a CA backbone.

methine, and terminal methine protons of PLA are observed at 1.5 (peak a), 5.1 (peak b), and 4.3 ppm (peak b′), respectively.43

Table 1. Feed Ratio and Chemical Characteristics of Comblike PLLA and PDLAs with CA Backbone samplea

LA/glucoseb (mol/mol)

Mn,GPCc (kg/mol)

Mw,GPCc (kg/mol)

PDIc

degree of lactyl substitution

DPPLAd

c-PLLA-341k c-PDLA-365k c-PLLA-501k c-PDLA-467k c-PLLA-684k c-PDLA-611k

15.1:1 15.1:1 26.3:1 26.3:1 43.1:1 43.1:1

217.8 231.5 330.9 312.2 440.6 423.5

340.7 364.6 500.5 467.0 684.0 610.7

1.56 1.57 1.51 1.50 1.55 1.44

0.65 0.59 0.75 0.8 0.8 0.8

42 38 68 51 103 91

a

Numerals in sample code denote Mw (kg/mol) of comblike PLLAs and PDLAs measured by GPC. bFeed molar ratio of lactide (LA) to glucose unit in CA, which is 0.8 of the feed molar ratio of LA to unsubstituted hydroxyl group in CA. cMn, Mw, and polydispersity index (PDI) measured by GPC. d Averaged DP for PLA branches with respect to the lactic acid unit. 12691

DOI: 10.1021/acs.jpcb.5b05398 J. Phys. Chem. B 2015, 119, 12689−12698

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The Journal of Physical Chemistry B

Because the crystallization was controlled by nucleation and chain diffusion at low and high supercooling, respectively, the t1/2s values of all racemic blends exhibit minimums (i.e., maximums of crystallization rate), and their k values show maximums under the medium Tc (100−140 °C). The t1/2 increases and k decreases as Tc is increased toward Tm or decreased toward Tg (Figure 3). Normal spherulites with typical Maltese-cross patterns are observed at different Tc values for all PLLA/PDLA blends in the measurement of polarized optical microscopy (POM). No discernible difference is seen between the spherulitic morphologies of hc and sc crystallites (Figure S3). The polymorphic crystallization kinetics of PLLA/PDLA blends are strongly influenced by the macromolecular topologies and graft length. As shown in Figure 3, under the same Tc, the t1/2 of comblike blend reduces, and its k increases remarkably with increases of the MW and graft length. POM analysis also indicates that the spherulitic growth rate of comblike blend increases with the MW and graft length (Figure S4). All of these results suggest that a smaller graft length decrease the crystallization rate. Branching and grafting topologies usually delay and disturb the crystallization of polymers due to the lower degree of molecular symmetry.45 For the comblike polymers, each graft chain is tethered on the backbone, and its diffusion ability is retarded due to the branching and steric effects. In the case of sc crystallization, the diffusion of polymer chains is a more critical factor because sc crystallization only proceeds when both enantiomers are diffused to the crystalline growth faces. The branching effect would become more severe for the comblike polymers with shorter grafts because of the elevated number of branching points per unit mass. Besides this, the change of chain direction and incorporation of initiator moiety in comblike polymers can also decrease the crystallization rate.46 Even though the branching topology of PLA decreases its homocrystallization rate,43,45 the comblike blends

Figure 2. DSC curves recorded during the heating of melt-quenched PLLA/PDLA racemic blends with different topologies.

However, for the linear/comblike and linear blends (i.e., l-PLLA/c-PDLA and l-PLLA/l-PDLA blends), two melting peaks corresponding to the hc and sc crystallites are present at 150−180 and 210−230 °C, respectively (Figure 2). ΔHm,hcs of linear/comblike and linear blends are much larger than their ΔHm,scs (Table 2), indicating the predominant formation of hc. Notably, the linear/comblike blend shows smaller ΔHc,hc and larger ΔHc,sc values than does its linear analog, demonstrating that the presence of comblike component facilitates sc crystallization of PLLA and PDLA. The isothermal crystallization kinetics of PLLA/PDLA racemic blends were further investigated in a wide temperature range (Tc = 80−180 °C). On the basis of the DSC curves collected in isothermal melt crystallization (Figure S2), the kinetic parameters (including the crystallization half-time (t1/2) and overall crystallization rate constant (k)) of PLLA/PDLA blends crystallized at different Tcs were analyzed by Avrami equation44 and are shown in parts a and b of Figure 3, respectively.

Table 2. Thermal Properties of PLLA/PDLA Racemic Blends with Different Topologies Collected in Nonisothermal Cold Crystallization of Melt-Quenched Samples

a

blend

Tg (°C)

Tcc (°C)

Tm,hc (°C)

ΔHm,hc (J/g)

Tm,sc (°C)

ΔHm,sc (J/g)

l-PLLA/l-PDLA l-PLLA/c-PDLA-611k c-PLLA-341k/c-PDLA-365k c-PLLA-501k/c-PDLA-467k c-PLLA-684k/c-PDLA-611k

61.0 59.4 59.4 61.6 60.5

111.7 107.9 115.0 109.0 102.3

178.4 166.8 NPa NP NP

40.8 34.8 0 0 0

221.4 219.1 222.2 210.5 213.0

19.9 24.9 21.3 22.6 30.6

No peak is observed.

Figure 3. Kinetic parameters of PLLA/PDLA racemic blends with various topologies crystallized at different Tcs: (a) crystallization half-time and (b) overall crystallization rate constant. 12692

DOI: 10.1021/acs.jpcb.5b05398 J. Phys. Chem. B 2015, 119, 12689−12698

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The Journal of Physical Chemistry B exhibit faster crystallization (i.e., smaller t1/2 and larger k) than the linear and linear/comblike blends with the same comblike component (Figure 3). This can be attributed to the effect of crystalline polymorph, as illustrated in the following parts. Polymorphic Crystalline Structure. Because the hc and sc crystallites have distinct Tms and WAXD patterns, polymorphic crystalline structures of PLLA/PDLA racemic blends crystallized at different Tcs (100−180 °C) were analyzed by DSC and WAXD (Figures 4 and 5). The macromolecular topology

Figure 5. (a) WAXD patterns of PLLA/PDLA racemic blends meltcrystallized at Tc = 160 °C: (A) l-PLLA/l-PDLA; (B) l-PLLA/c-PDLA611k; (C) c-PLLA-341k/c-PDLA- 365k; (D) c-PLLA-501k/c-PDLA467k; and (E) c-PLLA-684k/c-PDLA-611k. (b,c) WAXD patterns of (b) c-PLLA-684k/c-PDLA-611k and (c) l-PLLA/c-PDLA-611k blends after melt-crystallization at different Tcs. The wavelength of the X-ray is 0.154 nm.

Figure 4. DSC heating curves of PLLA/PDLA racemic blends: (a) racemic blends with different topologies after isothermal melt crystallization at Tc = 160 °C; (b) c-PLLA-684k/c-PDLA-611k, and (c) l-PLLA/c-PDLA-611k blends after isothermal melt-crystallization at different Tcs.

and Tc influence the crystalline structure, and sc and hc contents remarkably. As shown in Figures 4a and b and 5a, comblike blends merely exhibit the melting peaks and characteristic diffractions of sc crystallites (2θ = 12.0, 20.8, and 24.0°),47 while the linear/comblike and linear blends show the melting peaks and characteristic diffractions of both hc (2θ = 16.8 and 19.0°)48 and sc crystallites after crystallization at Tc = 160 °C. On the basis of the WAXD patterns, the relative fraction of sc crystallites ( fsc) in the blends was estimated by comparing the diffraction peak area of the sc crystallites with the total areas of both the sc and the hc diffractions, i.e., fsc = Isc/(Isc + Ihc), where Isc and Ihc are the total peak areas of sc and hc diffractions, respectively.22,35 We have tried to calculate the absolute crystallinities of sc and hc crystallites from WAXD, but the broad halo peak of PLA amorphous phase is not obvious in the WAXD patterns of PLLA and PLLA/PDLA blends,19,48 leading to the conclusion that the crystallinity estimated from WAXD is much larger than that estimated from DSC. Therefore, fsc was only estimated here. As shown in Figures 5b,c and 6, sc diffraction intensity and sc fraction steadily increase with Tc for all the PLLA/PDLA blends because sc crystallites are more thermally stable and favored at high Tc values. The fsc of the

Figure 6. Fraction of sc crystallites (fsc) for PLLA/PDLA racemic blends melt-crystallized at different temperatures: (A) l-PLLA/lPDLA; (B) l-PLLA/c-PDLA-611k; (C) c-PLLA-341k/c-PDLA-365k; (D) c-PLLA-501k/c-PDLA- 467k; (E) c-PLLA-684k/c-PDLA-611k.

c-PLLA-684k/c-PDLA-611k blend is about 40% at Tc = 100 °C, and it increases to 100% at Tc = 140 °C. All of the racemic blends crystallize in sc polymorph at Tc = 180 °C because this Tc has exceeded the Tm of hc. After crystallization at the same Tc (100−160 °C), the comblike blends show less obvious melting of hc (Figures 4b and c), more predominant sc diffraction (Figures 5b and c), and a larger sc fraction (Figure 6) than do the corresponding linear/comblike and linear blends. Moreover, the linear/ comblike blend has a higher sc fraction than its linear analog after crystallization at Tc = 100−160 °C (Figure 6). Crystalline structures of solution-crystallized comblike blends are consistent with those in nonisothermal and isothermal crystallizations. 12693

DOI: 10.1021/acs.jpcb.5b05398 J. Phys. Chem. B 2015, 119, 12689−12698

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The Journal of Physical Chemistry B The melting peaks and WAXD diffractions of sc crystallites are mainly observed, while those of hc are much less obvious (Figure S5). Therefore, it is rational to conclude that the comblike topology of PLLA and PDLA promotes their sc formation in racemic blends. The presence of one or two comblike enantiomers facilitates the sc crystallization of HMW PLLA/PDLA blends. Interestingly, the degree of sc crystallization in comblike blends also depends on MW and graft length. After crystallization at Tc = 100 and 120 °C, the sc fraction of the comblike blend first increases and then decreases with increasing MW and graft length (Figure 6). First, the branching effect, which impedes chain diffusion45 and rearrangement in sc crystallization, is more predominant in the comblike polymer with shorter graft chains. Second, it has been well-documented that the competing formations of hc and sc crystallites in PLLA/ PDLA blends are MW dependent, and that the sc crystallization is facilitated in low-MW blends. As the MW and graft length of comblike polymer increase, the viscosity and entanglement between branches enhance, impeding the polymer segment interdiffusion and sc formation. As reported by Sakamoto and Tsuji, when the Mn of each branch in two- and four-armed PLLA/PDLA blends exceeds 4.5 kg/mol, the homocrystallization takes place in addition to sc crystallization.49 The Mn of each branch in our comblike PLLA and PDLA ranged between 2.7 and 7.4 kg/mol, leading to the occurrence of homocrystallization with increasing graft length. Due to the synergetic effects of branching, MW, and graft length, the comblike blend with moderate graft length (i.e., c-PLLA-501k/c-PDLA-467k) shows the largest sc fraction and the best ability to stereocomplex (Figure 6). By comparing the results shown in Figures 4b and 5b, one can see that the polymorphic structural information derived from DSC and WAXD is contradictory. It is notable that the DSC and WAXD samples have the same thermal history in melt crystallization, both of which are quenched directly to Tc after melting. In the case of c-PLLA-684k/c-PDLA-611k blend, the melting of sc crystallites is solely observed at different Tc values (100−180 °C). However, hc diffractions are clearly detected for this sample at Tc = 100−120 °C, indicating the existence of both hc and sc crystallites. The difference between the DSC and WAXD results, which has also been observed in the linear PLLA/PDLA racemic blend,50 is ascribed to the heating-induced hc-to-sc crystalline transition. It has been reported that hc reorganizes into its sc counterpart upon heating or annealing at elevated temperatures.38−41 This hc-tosc transition is also influenced by the initial state of hc. The hc, which is formed at low Tcs (e.g., 80 °C) and possesses a less ordered structure, can reorganize or melt-recrystallize into sc more easily than those formed at high Tc values (e.g., >120 °C).41 As illustrated in the following in situ WAXD results, the endothermic melting of hc and exothermic recrystallization of sc take place simultaneously upon heating, especially for the racemic blends crystallized at low Tc values. Offset of the hc melting and sc recrystallization would lead to the disappearance of the melting peak of hc in the temperature range of 140−170 °C. Polymorphic Crystallization and Crystalline Transition Investigated by In Situ WAXD. Crystallization and structural reorganization of amorphous PLLA/PDLA racemic blends upon heating were investigated by synchrotron radiation WAXD (Figures 7, S6). On the basis of the temperature-dependent WAXD profiles, intensity changes for sc (110) diffraction and

Figure 7. Temperature-dependent WAXD patterns for melt-quenched PLLA/PDLA racemic blends collected upon heating: (a) c-PLLA501k/c-PDLA-467k and (b) l-PLLA/ c-PDLA-467k. The wavelength of the X-ray is 0.124 nm.

Figure 8. Temperature-dependent peak areas of sc (110) diffraction and hc (110)/(200) diffraction collected during the heating of meltquenched PLLA/PDLA racemic blends: (a) c-PLLA-501k/c-PDLA467k; (b) l-PLLA/c-PDLA-467k; and (c) l-PLLA/l-PDLA. The data was obtained from in situ WAXD patterns shown in Figures 7 and S6.

hc (110)/(200) diffraction were evaluated and plotted as a function of temperature in Figure 8. No discernible diffraction is observed in all of the melt-quenched PLLA/PDLA blends before heating, confirming their amorphous structures. For the comblike blends, sc starts to form upon heating to 100 °C, and its diffraction intensity continuously increases with heating to 190 °C, indicating the gradual sc crystallization (Figures 7a and 8a). Upon further heating to above 190 °C, sc diffraction intensities decrease due to the sc melting. It is notable that hc diffraction is much less obvious throughout the heating process. A tiny hc (110)/(200) diffraction appears at 120 °C and disappears completely upon further heating to 170 °C. In the case of linear/comblike blends, the variation trend of sc diffraction intensity is similar to that observed for the comblike blend, but the hc diffraction is obviously shown upon heating to 110 °C (Figures 7b and 8b). This demonstrates that 12694

DOI: 10.1021/acs.jpcb.5b05398 J. Phys. Chem. B 2015, 119, 12689−12698

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The Journal of Physical Chemistry B

between enantiomeric branches, favoring the nucleation and crystallization of sc crystallites. However, the HMW linear PLLA/PDLA blend lacks the geometrical advantage for sc crystallization. Furthermore, it has been demonstrated that the sc crystallites generated in initial state of crystallization can act as the physical cross-links and lead to form the network structure,51,52 which would significantly restrain the diffusion of bridged and surrounded chains. However, the existing sc crystallites, which are formed prior to homocrystallization, can promote the heterogeneous nucleation of homocrystallization.52 Therefore, sc crystallization is significantly depressed compared to homocrystallization in the HMW linear blends. Because of the unique grafting topology and short PLA segments, the formation of network structures may be less predominant in comblike blends; this would decrease the kinetic barrier of sc crystallization. Notably, the investigated comblike blends with CA as the backbone show different polymorphic crystalline structures than do the reported racemic blends of poly(norbornene)-gPLAs.53,54 It has been found that the sc crystallization in the poly(norbornene)-g-PLLA/poly(norbornene)-g-PDLA blend is restricted in comparison to the comblike stereoblock PLAs54 or the linear/brush enantiomeric blends.53 Even though the exact reasons for the different stereocomplexation ability between our comblike blends and the poly(norbornene)-g-PLA racemic blends are still unclear, it is conjectured that this difference may originate from the effects of graft density, uniformity of graft length, and the backbone and graft compatibility (or miscibility). In the studies of Grubbs et al.53 and Satoh et al.,54 the poly(norbornene)-g-PLAs were prepared by the ring-opening metathesis polymerization of norbornene end-functionalized PLAs. Both the prepared norbornene-functionalized PLA macromonomer and poly(norbornene)-g-PLAs have the very narrow MW distributions, with PDI 120 °C). The crystallization rate of the comblike blend increases with increasing graft length due to the decreased branching effect. Furthermore, the presence of a comblike component in the HMW PLLA/PDLA blend also facilitates the hc-to-sc crystalline transition or recrystallization of sc crystallites upon heating. The mechanism for the facilitated sc crystallization of comblike blends is proposed and might be attributable to the favored interdigitation between enantiomeric branches and the increased mobility of the polymer segment. Due to the large fraction of sc crystallites, the comblike blend exhibits smaller long periods and thinner crystalline lamellae compared to those of the corresponding PLLA with homocrystalline structure. This study has provided an effective method for facilitating the sc crystallization of HMW PLLA/PDLA blends and has also shed light on the polymorphic crystallization mechanism and crystalline transition of enantiomeric polymer blends with complicated topologies.

Figure 9. SAXS data of the c-PLLA-684k/c-PDLA-611k blend after isothermal melt-crystallization at different Tcs: (a) Lorentz-corrected SAXS and (b) correlation function. The inset of panel b shows the evaluation of the lamellae parameter from the correlation function.

where I(q) or I is the scattering intensity, q is the scattering vector (q = 4π sin θ/λ with 2θ as the scattering angle), and r(z) is the electron-density correlation length. Of note is the fact that the correlation function is valid for the two-phase structure. The correlation function was used to estimate the average values of lamellar repeat distance (or long period, dac) and crystalline (dc) and amorphous (da) layer thicknesses (Figure 9b), even though the comblike blends crystallized at low Tc values (≤120 °C) contain both hc and sc crystallites. Lorenz-corrected SAXS profiles of comblike blend exhibit a single scattering peak (Figure 9a). Long spacing (LP) was calculated from the Bragg equation, LP = 2π/qmax, where qmax corresponds to the q value of scattering peak top in Lorenz-corrected SAXS profiles. The scattering peak shifts toward low q with increasing Tc, indicating the formation of more perfect crystallites with larger LP values (Table 3). Table 3. Lattice Parameters of the Comblike PLLA/PDLA Racemic Blend and the Comblike PLLA Melt-Crystallized at Various Temperatures c-PLLA-684k/c-PDLA-611k

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c-PLLA-684k

Tc (°C)

LP (nm)

dac (nm)

dc (nm)

da (nm)

LP (nm)

dac (nm)

dc (nm)

da (nm)

100 120 140 160 180

15.3 16.3 17.4 19.3 23.9

14.4 15.5 16.1 18.9 22.8

6.3 6.7 7.0 8.1 9.0

8.0 8.8 9.1 10.8 13.8

28.2 23.2 28.5 30.6 −

25.5 22.7 26.1 26.5 −

11.0 9.7 11.2 11.4 −

14.5 13.0 14.8 15.0 −

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b05398. Additional details on the measurement of polarized optical microscopy, GPC and isothermal crystallization DSC, polarized optical microscopy micrographs, spherulitic growth rate of PLLA/PDLA racemic blends, DSC and WAXD data of solvent-cast blends, and in situ WAXD patterns of linear blends. (PDF)

As shown in Table 3, the LP and dac calculated from the Lorenz-corrected SAXS profiles and correlation function are similar, and both of them increase with Tc. The LP and dac of comblike blends (14−24 nm) are larger than those reported for sc crystallites formed in linear racemic blends19,59 and stereoblock copolymers,60 which are generally in the range of 11−17 nm. Larger LP and dac values of comblike blends may be

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-571-87951334; e-mail [email protected]. 12696

DOI: 10.1021/acs.jpcb.5b05398 J. Phys. Chem. B 2015, 119, 12689−12698

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The Journal of Physical Chemistry B Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Natural Science Foundation of China (21274128 and 21422406) and the Fundamental Research Funds for the Central Universities (2015XZZX004-08). SAXS and in situ WAXD were measured on beamline BL16B1 of SSRF, China.

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DOI: 10.1021/acs.jpcb.5b05398 J. Phys. Chem. B 2015, 119, 12689−12698