Competitive Stereocomplexation, Homocrystallization, and

May 4, 2015 - Competitive crystallization kinetics, polymorphic crystalline structure, and transition of poly(l-lactic acid)/poly(d-lactic acid) (PLLA...
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Competitive Stereocomplexation, Homocrystallization, and Polymorphic Crystalline Transition in Poly(L‑lactic acid)/Poly(D‑lactic acid) Racemic Blends: Molecular Weight Effects Pengju Pan,* Lili Han, Jianna Bao, Qing Xie, Guorong Shan, and Yongzhong Bao State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China S Supporting Information *

ABSTRACT: Competitive crystallization kinetics, polymorphic crystalline structure, and transition of poly(L-lactic acid)/ poly(D-lactic acid) (PLLA/PDLA) racemic blends with a wide range of molecular weights (MWs) were symmetrically investigated. Stereocomplex (sc) crystallites are exclusively formed in the low-MW racemic blends. However, stereocomplexation is remarkably depressed, and homocrystallization becomes prevailing with increasing MWs of PLLA and PDLA. Suppressed stereocomplexation in high-MW (HMW) racemic blends is proposed to be due to the low chain diffusion ability and restricted intermolecular crystal nucleation/growth. Equilibrium melting point of sc crystallites first increases and then decreases as MW increases. Crystallinity and relative fraction of sc crystallites in racemic blends enhance with crystallization temperature (Tc), and the sc crystallites are merely formed at Tc > 170 °C because of their higher thermodynamic stability. In situ wide-angle X-ray diffraction (WAXD) analysis reveals that the stereocomplexation and homocrystallization are successive rather than completely simultaneous, and the stereocomplexation is preceding homocrystallization in isothermal crystallization of HMW racemic blends. Both initial crystalline structure of homocrystallites (hc) and MW influence the heating-induced hc-to-sc transition of HMW racemic blend drastically; the hc-to-sc transition becomes easier with decreasing Tc and MW. After crystallization at the same temperature, sc crystallites show smaller long period than their hc counterparts.

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°C), high mechanical strength and modulus,11 efficient nucleation ability to homocrystallization,12−15 rheological modification ability to PLLA,16 superior thermomechanical properties,17 better chemical resistance, and lower thermal and hydrolytic degradation rates.18−20 However, not all of the PLLA/PDLA racemic blends can achieve full stereocomplexation, i.e., without any homocrystallization of PLLA or PDLA. Stereocomplexation and homocrystallization are in competition upon cooling the racemic blends. Because both enantiomers of PLLA and PDLA must combine in stereocomplexation, they have larger diffusion path than regular homocrystallization. Therefore, stereocomplex formation suffers from a larger kinetic barrier than homocrystallization, even though it is more thermodynamic favorable. Molecular weights (MWs) of PLLA and PDLA have profound influences on the competing formation of sc and hc crystallites in their enantiomeric blends. Stereocomplexation is favorable for the PLLA/PDLA blend which contains at least one low-MW (LMW) enantiomer.6,21−25 It has been found that the critical MW of PLLA and PDLA to achieve exclusive stereocomplexation is around 6K in melt crystallization.6 This

INTRODUCTION Poly(lactic acid) (PLA) is one of the most promising biobased and biodegradable thermoplastics with good processing ability, high mechanical properties, and relatively low-cost production from annually renewable resources. PLA has been widely used in the biomedical and commodity applications for substitution of conventional oil-based thermoplastics.1,2 Although the melting temperature of conventional PLA, i.e., poly(L-lactic acid) (PLLA), is around 170 °C and its glass transition temperature (Tg) is ∼60 °C, its thermal resistance is limited by a relatively low heat deflection temperature due to the slow crystallization and low crystallizability in practical processing such as injection molding. In this regard, stereocomplexation of PLLA and poly(D-lactic acid) (PDLA) is considered as an effective method to modify the thermal resistance and longlasting durability of PLA materials.3,4 Since the first report of Ikada et al.,5 it has been known that equal-mass PLLA and PDLA can form specific stereocomplex (sc) crystallites in the solution, melt, and cold crystallizations of their racemic blends3,6 and stereoblock copolymers.7 It has been identified that sc crystallites have denser chain packing and slower molecular relaxation than homocrystallites (hc)8 due to the intermolecular H-bond interactions between enantiomers.9,10 This specific structure offers sc-type PLA many unique properties such as high melting point (220−240 © 2015 American Chemical Society

Received: April 13, 2015 Revised: April 26, 2015 Published: May 4, 2015 6462

DOI: 10.1021/acs.jpcb.5b03546 J. Phys. Chem. B 2015, 119, 6462−6470

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The Journal of Physical Chemistry B critical value is much lower than the MW of commercial PLA. As MWs of PLLA and PDLA increase, the degree of stereocomplexation decreases significantly, and homocrystallization becomes predominant in conventional heating and cooling processes.26−34 Both stereocomplexation and homocrystallization can take place in medium-MW (MMW) and high-MW (HMW) PLLA/PDLA blends. However, it is still unclear whether these two types of polymorphs grow simultaneously or successive in PLLA/PDLA racemic blends. Therefore, investigation on the competing, synergetic formation of sc and hc crystallites as well as its dependence on MW would be crucial to tailor the polymorphic structure and physical performance of PLLA/PDLA blends. Because of the higher thermal stability of sc crystallites than their hc analogues, homocrystallites can transform into their sc counterparts upon heating and annealing at elevated temperatures.35−38 Diffusion, refolding of polymer chains, and initial crystalline state of homocrystallites are key factors for the hc-tosc crystalline transition.36,37 Since MWs of PLLA and PDLA influence their chain mobility, crystallizability, and crystalline transition of homocrystallites significantly,39,40 we envision that MW will be a crucial factor for the hc-to-sc crystalline transition. However, the relationships between MW and hcto-sc crystalline transition of PLLA/PDLA blends have not been explored. In this work, crystallization kinetics and polymorphic crystalline structure of PLLA/PDLA racemic blends with a wide range of MWs (Mw = 18K−192K) and high stereoregularity under different conditions were investigated. Structural evaluation during isothermal crystallization and heating-induced hc-to-sc crystalline transition of racemic blends were studied by in situ wide-angle X-ray diffraction (WAXD). The mechanism for the MW-dependent stereocomplexation, homocrystallization, and hc-to-sc crystalline transition of PLLA/PDLA racemic blends was discussed.

Table 1. Molecular Weights of PLLA and PDLA sample

code

Mn (kg/mol)

Mw (kg/mol)

Mw/Mn

PLLA

L18 L41 L58 L82 L192 D22 D40 D58 D91 D191

16.9 35.8 47.4 58.5 133.4 18.3 32.6 46.9 68.2 132.5

18.1 41.1 57.7 81.9 192.2 21.9 39.5 57.6 91.3 191.2

1.07 1.15 1.22 1.40 1.44 1.19 1.21 1.23 1.34 1.44

PDLA

sample was cooled from 260 to 0 °C and then reheated to 260 °C again. Both the heating and cooling rates were 10 °C/min. On basis of the DSC curves, the temperature and enthalpy of melt crystallization upon cooling (Tmc, ΔHmc), temperature and enthalpy of cold crystallization upon subsequent heating (Tcc, ΔHcc), and melting temperature and enthalpy of hc (Tm,hc, ΔHm,hc) and sc crystallites (Tm,sc, ΔHm,sc) were calculated. For investigating isothermal melt crystallization, the sample was quickly cooled from 260 °C to the desired temperature at 100 °C/min and then held at this temperature for crystallization. It was then reheated to 260 °C to examine the melting behavior. ΔHm,hc and ΔHm,sc were evaluated from the DSC melting curves. Degrees of crystallinity for hc (Xc,hc) and sc crystallites (Xc,sc) were estimated by comparing ΔHm to the corresponding value of an infinitely large crystal (ΔH0m), taken as ΔH0m = 93 and 142 J/g41,42 for hc and sc crystallites, respectively. The relative fraction of sc crystallites in crystalline phase of PLLA/ PDLA blends was calculated by fsc,DSC = Xc,sc/(Xc,sc + Xc,hc). WAXD and Small-Angle X-ray Scattering (SAXS). WAXD analysis of melt-crystallized PLLA/PDLA blends was performed on a Rigaku RU-200 instrument (Rigaku Co., Tokyo, Japan) using the Ni-filtered Cu Kα radiation (λ = 0.154 nm), operated at 40 kV and 200 mA. The sample was stepscanned from 7° to 40° at a 2θ scanning rate of 2°/min. The sample was prepared by the same thermal procedure as that used in DSC. On the basis of WAXD results, the relative fraction of sc crystallites ( fsc,WAXD) in crystalline phase of PLLA/PDLA racemic blends was estimated by comparing the diffraction peak area of sc crystallites with the total peak areas of both sc and hc diffractions; that is, fsc,WAXD = Isc/(Isc + Ihc), in which Isc and Ihc are the peak areas of sc and hc diffractions, respectively.33,43 In situ WAXD and SAXS were measured on the beamline BL16B1 of Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of radiation source is 0.124 nm. The temperature of the sample was controlled by a Linkam THMS600 hot stage (Linkam Scientific Instrument Ltd., Surrey, UK). Scattering data were acquired by a Rayonix SX165 CCD detector (Rayonix, Evanston, IL), which had a resolution of 2048 × 2048 pixels and a pixel size of 80 × 80 μm2. Sample-to-detector distances were 0.13 and 2.0 m for WAXD and SAXS, respectively. Acquisition times in WAXD and SAXS measurements were 30 and 90 s, respectively. For analyzing the structural evaluation in isothermal crystallization, the sample was sandwiched by two pieces of polyimide films and was melted at 260 °C for 2 min in hot stage to erase the thermal history. It was then quickly cooled to 160 °C for crystallization. To investigate hc-to-sc crystalline transition, the racemic blends crystallized at 80 and 160 °C were sandwiched

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EXPERIMENTAL SECTION Materials. Both L- and D-lactide (optical purities >99.9%) were purchased from Purac Co. (Gorinchem, The Netherlands) and further purified by recrystallization from ethyl acetate. Tin(II) 2-ethylhexanoate [Sn(Oct)2] and lauryl alcohol were purchased from Sigma-Aldrich Co. (St. Louis, MO). PLLA and PDLA with various MWs were synthesized by bulk ring-opening polymerization at 130 °C using lauryl alcohol as the initiator and Sn(Oct)2 as the catalyst. MW was controlled by changing the molar ratio of lactide to lauryl alcohol. MW and polydispersity index (PDI) of PLLA and PDLA are listed in Table 1. In this work, we denote the L18/D22 sample as LMW blend, L41/L40 and L58/D58 as MMW blends, and L82/D91 and L192/D191 as HMW blends. Measurements. Gel Permeation Chromatography (GPC). MWs of PLLA and PDLA were measured on a Waters GPC (Waters Co., Milford, MA) equipped with a 2414 refractive index (RI) detector, a 1525 binary HPLC pump, a 717 autosampler, and a PLgel 10 μm MIXED-BLS column at 30 °C. THF was used as the mobile phase, and polystyrene was used as the standard. Differential Scanning Calorimetry (DSC). DSC measurements were performed on a NETZSCH DSC 214 Polyma instrument (NETZSCH, Germany) equipped with an IC70 intracooler. Temperature and heat flow were calibrated by an indium standard. The sample sealed in an aluminum pan was first heated to 260 °C and held for 2 min to erase thermal history. To investigate nonisothermal melt crystallization, the 6463

DOI: 10.1021/acs.jpcb.5b03546 J. Phys. Chem. B 2015, 119, 6462−6470

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The Journal of Physical Chemistry B by two pieces of polyimide films and then heated from 140 to 270 °C at a heating rate of 5 °C/min. The WAXD pattern was collected each 1 min during the crystallization or heating process. All the data were corrected from background and air scattering. Two-dimensional data was converted into onedimensional data by circularly averaging with Fit2D software.

As seen in Figure 1b, the LMW racemic blend (e.g., L18/ D22) shows melting region at a high temperature of 210−250 °C, corresponding to the melt of sc crystallites. However, MMW and HMW racemic blends exhibit two melting regions at 160−180 and 210−250 °C upon heating. The melting endotherms at low and high temperatures correspond to hc and sc crystallites, respectively. These results imply that the sc crystallites are exclusively formed in LMW racemic blends, but the mixed hc and sc crystallites are generated in MMW and HMW racemic blends. Notably, ΔHm,sc decreases steadily while ΔHm,hc first increases and then decreases as the MWs of PLLA and PDLA increase (Table 2). Therefore, it is concluded that stereocomplex formation of PLLA and PDLA is favorable in LMW racemic blend but drastically suppressed in HMW racemic blend. Isothermal crystallization and melting behavior of PLLA/ PDLA racemic blends with various MWs were further studied. To obtain the kinetic parameters, DSC curves collected in isothermal crystallization (Figure S1) were analyzed by Avrami model.46 Crystallization half-time (t0.5) and overall crystallization rate constant (k) were obtained from the Avrami analysis. Because the crystallization of L18/D22 blend was very fast at low Tc, their kinetic parameters at high crystallization temperature (i.e., Tc ≥ 160 °C) were just analyzed. t0.5 and k of PLLA/PDLA racemic blends with different MWs are plotted as a function of Tc in panels a and b of Figure 2, respectively. As

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RESULTS AND DISCUSSION Crystallization Kinetics. Crystallization kinetics and melting behavior of PLLA/PDLA racemic blends with different MWs were first investigated by DSC. Figure 1 shows the DSC

Figure 1. DSC curves of PLLA/PDLA racemic blends collected upon cooling and subsequent heating. Both the heating and cooling rates are 10 °C/min.

thermograms recorded upon cooling and subsequent heating. Thermal parameters derived from the DSC curves are shown in Table 2. As seen in Figure 1, crystallization kinetics and melting behavior of PLLA/PDLA blends depend strongly on MW. When the MWs of PLLA and PDLA are less than ∼40K, the racemic blend exhibits fast crystallization, and it can complete crystallization upon cooling at 10 °C/min. However, as the MWs of PLLA and PDLA are increased to above 50K, a cold crystallization peak is detected in the subsequent heating. Upon cooling, both Tmc and ΔHmc decrease with increasing MW (Table 2). These results suggest that the crystallization rate of PLLA/PDLA racemic blend is decreased with increasing MW, similar as the MW-dependent crystallization kinetics of PLLA.39 It is notable that the synthesized PLLA and PDLA bear relatively long alkyl (i.e., lauryl) ends. Tsuji et al. have reported that the presence of long alkyl terminal in LMW PLLA or PDLA (typically MW < 10 kg/mol) will increase the rates of homo- and stereocomplex crystallizations.44,45 Because PLLA and PDLA used here have relatively high MW and all bear the same initiator terminal, the effects of lauryl terminal on homoand stereocomplex crystallizations of PLLA/PDLA blends can be negligible.

Figure 2. Plots of (a) crystallization half-time and (b) overall crystallization rate constant as a function of crystallization temperature for PLLA/PDLA racemic blends with different MWs.

shown in this figure, t0.5 shows a minimum and k shows a maximum at around Tc = 100 °C, meaning that the racemic blends crystallize fastest around this Tc. Crystallization rate of racemic blend decreases as Tc is approaching Tg or melting temperature. At the same Tc, t0.5 increases and k decreases with increasing MW, meaning the decrease of crystallization rate. Melting Behavior and Polymorphic Crystalline Structure. Because sc and hc crystallites have distinct melting temperatures, polymorphic structure of racemic blends crystal-

Table 2. Thermal Parameters of PLLA/PDLA Racemic Blends Collected upon Nonisothermal Melt Crystallization and Subsequent Melting cooling

subsequent heating

sample

Tmc (°C)

ΔHmc (°C)

Tcc (°C)

ΔHcc (°C)

Tm,hc (°C)

ΔHm,hc (°C)

Tm,sc (°C)

ΔHm,sc (°C)

T0m,sc (°C)

L18/D22 L41/D40 L58/D58 L82/D91 L192/D191

159.0 136.5 105.7 104.0 100.3

−89.4 −53.5 −13.7 −13.4 −5.2

− 86.6 103.7 104.9 109.8

0 −1.0 −28.4 −27.3 −30.2

− 171.0 175.9 177.5 180.0

0 37.4 54.1 50.0 42.9

215.0, 238.2 227.1, 247.4 228.2, 247.7 225.7 221.3

91.2 51.6 33.1 13.7 8.23

256.5 275.4 262.4 258.5 251.8

6464

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The Journal of Physical Chemistry B lized at different Tcs can be evaluated from the melting behavior. Figure 3 shows the DSC melting curves of racemic

Figure 4. WAXD patterns of PLLA/PDLA racemic blends after melt crystallization at different temperatures: (a) L18/D22, (b) L82/D91, and (c) L192/D191. (d) Relative fraction of sc crystallites derived from WAXD data. Wavelength of used X-ray is 0.154 nm.

because the crystallites formed at high Tc are sufficiently perfect and do not undergo recrystallization upon heating. For the MMW and HMW racemic blends (L41/D40, L82/ D91, and L192/D191), two melting regions corresponding to hc and sc crystallites are observed at Tc < 170 °C, indicating the formation of mixed hc and sc crystallites. This is consistent with the WAXD data, in which both the characteristic diffractions of sc and hc crystallites are observed (Figure 4b,c). Because of the high melting point of sc crystallites, its crystallization temperature region is wider than that of homocrystallites. When Tc is increased to higher than the melting point of homocrystallites (170−180 °C), sc crystallites are exclusively formed in MMW and HMW blends, showing the melting peak at high temperature and characteristic diffractions of sc crystallites (Figures 3b−d and 4b,c). Of note is that the sc crystallization rate at Tc > 170−180 °C is very slow due to the small undercooling (Figure 2 and Figure S1). Sc crystallites formed in HMW blends (L82/D91, L192/D191) at Tc ≥ 160 °C also undergo melt recrystallization upon heating, in which double melting endotherms are observed above 200 °C. Melting behavior and crystalline structure of homocrystallites in MMW and HMW racemic blends also show the Tc dependence. Multiple melting behavior of homocrystallites has been ascribed to the phase transition and melt recrystallization mechanism.39 With increasing Tc from 80 to 160 °C, two dominant diffraction peaks (110/200 and 203) of homocrystallites shift to larger angle due to the transition from disordered α′ to ordered α homocrystallites.39,49,50 On basis of the DSC data, Xc,hc, Xc,sc, and fsc,DSC of racemic blends were calculated, as shown in Figure 5. Because higher Tc facilitates the formation of more perfect and ordered crystallites, Xc,hc increases with Tc at Tc < 140 °C (Figure 5a), consistent with the results of PLLA.50 Because sc

Figure 3. DSC melting curves of PLLA/PDLA racemic blends after isothermal melt crystallization at different temperatures: (a) L18/D22, (b) L41/D40, (c) L82/D91, and (d) L192/D191.

blends melt-crystallized at different Tcs. As shown in this figure, melting behavior of racemic blends depends strongly on MW and Tc. The LMW blend (e.g., L18/D22) merely shows melting region at high Tc (210−250 °C) under all Tcs investigated, indicative of the exclusive formation of sc crystallites. This is consistent with the WAXD data (Figure 4a), in which only the (110), (300)/(030), and (220) reflections characteristic of sc crystallites are observed at 2θ = 12.0°, 20.8°, and 24.1°, respectively.47,48 It is notable that sc crystallites of LMW blend show multiple melting peaks upon heating, which may be attributed to the melt recrystallization mechanism. At low Tc (e.g., 120 °C), an exotherm is observed prior to the melting peak of the L18/D22 blend. This exotherm gradually disappears, and a melting endotherm appears at lower temperature (P1) prior to the endotherm at higher temperature (P2), with increasing Tc from 120 to 180 °C. Endotherm P1 can be attributed to the melting of primary crystals developed in isothermal crystallization, and peak P2 is ascribed to the melting of crystallites formed in recrystallization upon heating. As Tc increases, endotherm P1 gradually shifts to higher temperature; peak P1 increases and peak P2 decreases in magnitude steadily. This is attributed that more perfect crystallites with high melting temperature are formed, and the fraction of crystallites undergoing recrystallization decreases with increasing Tc. A similar phenomenon is observed for HMW blends crystallized at Tc > 160 °C (Figure 3c,d). With further increasing Tc to 190 °C, the double melting peaks (P1, P2) merge into one peak 6465

DOI: 10.1021/acs.jpcb.5b03546 J. Phys. Chem. B 2015, 119, 6462−6470

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The Journal of Physical Chemistry B

L192/D191) is less than 15% at all Tcs investigated, which is much smaller than the Xhc of racemic blends crystallized at Tc < 140 °C. Tsuji et al. have reported that the racemic blends with ultrahigh MWs (e.g., MW > 700 kDa) hardly form sc crystallites but solely crystallize in homocrystallites at Tc ≤ 130 °C.31 It is proposed that the suppressed stereocomplexation in HMW racemic blends is ascribed to two aspects, i.e., low chain diffusion ability and restricted intermolecular crystal nucleation/growth. First, chain diffusion is a crucial factor for stereocomplexation compared to homocrystallization due to a high level of mixing required between PLLA and PDLA chains in stereocomplexation. HMW PLLA and PDLA have higher viscosity and lower diffusion ability compared to the LMW polymers. As shown in the following section, stereocomplexation is preceding homocrystallization upon cooling the HMW racemic blend. Stereocomplexation between multiple chains will lead to the formation of three-dimensional networks through physical cross-links.14,15,51 This can restrain the diffusion and mobility of bridged and surrounded chains. Besides, it has been well recognized that sc crystallites can promote the heterogeneous nucleation of homocrystallization.12−14 Therefore, stereocomplexation suffers from a larger kinetic barrier than homocrystallization, even though it is more thermodynamically favorable. Second, there are two extreme paths in polymer crystal nucleation, i.e., intramolecular and intermolecular nucleations. The former corresponds to folded-chain nucleation with most of crystalline stems truncated with adjacent chain-folding at the bundle-end surface, and the latter represents fringed-micelle nucleation with all the crystalline stems stretching out of the bundle-end surface.52 Because of the lower free energy barrier, polymer may prefer to choosing the path of intramolecular nucleation rather than intermolecular nucleation.52 Stereocomplexation between enantiomeric chains is a typical example of intermolecular crystal nucleation and growth. In HMW PLLA/PDLA blends, the difficulty of intermolecular growth than intramolecular growth may depress stereocomplexation. However, for LMW PLLA/PDLA blend, the short chain length, high mobility, and intermolecular H-bond interactions between enantiomers may favor the intermolecular crystal nucleation and growth, resulting in the enhanced stereocomplexation. Based on the DSC melting curves of racemic blends crystallized at Tc ≥ 170 °C, equilibrium melting point (T0m) of sc crystallites with different MWs were evaluated by the Hoffman−Weeks plot. The melting temperature of sc crystallites at lower temperature, which corresponds to the crystallites formed in isothermal crystallization, is used to determine T0m. As shown in Table 1, the obtained T0ms of sc crystallites vary from 252 to 275 °C, which agrees with that of racemic blend with high optical purity (263 °C) reported by Tsuji and Ikada,53 but larger than that of the asymmetric PLLA/PDLA blend reported by Saeidlou et al.54 It has been found that T0m of homocrystallites increases with MW.39 However, less perfect sc crystallites would be formed in racemic blends with increasing MW, due to the suppressed stereocomplexation. Therefore, T0m of sc crystallites shows a unique MW dependence. It first increases and then decreases with increasing MW. Sc crystallites of L41/D40 blend show the largest T0m of 275 °C. Structural Evaluation in Isothermal Crystallization. Competing formation of sc and hc crystallites of PLLA/PDLA racemic blends in isothermal crystallization was analyzed via

Figure 5. Degree of crystallinity of (a) hc, (b) sc crystallites, and (c) relative fraction of sc crystallites in PLLA/PDLA racemic blends after melt crystallization at different temperatures.

crystallites are more thermally stable than their hc counterparts, high Tc is favorable to form sc crystallites.29,33 With increasing Tc from 80 to 160 °C, diffraction intensities of sc crystallites and fsc,WAXD steadily increase for the MMW and HMW racemic blends (Figure 4b−d). fsc,WAXD of MMW and HMW blends is less than 5% at Tc = 80 °C, but it increases to 20−60% with increasing Tc to 160 °C (Figure 4d). With increasing Tc from 160 to 180 °C, Xc,hc of MMW and HMW blends gradually decreases and their Xc,sc, fsc,DSC, and fsc,WAXD increase (Figures 4d and 5b,c), suggesting the favorable formation of sc crystallites. At Tc > 180 °C, homocrystallites completely disappear and sc crystallites are exclusively formed. However, Xsc and fsc,DSC derived from DSC contradict the corresponding data obtained from WAXD. As seen in Figures 5b and 5c, Xsc and fsc,DSC derived from DSC are nearly independent of Tc at Tc ≤ 160 °C. fsc,DSC strongly depends on MW and ranges from 10 to 50% at Tc = 80−100 °C, which is much larger than fsc,WAXD calculated from WAXD (Figure 4d). This difference is ascribed to the heating-induced hc-to-sc crystalline transition. Several authors have reported that the homocrystallites transform into sc crystallites in PLLA/PDLA racemic blends upon heating.35−38 Na et al. have found that hcto-sc transition is easier for the homocrystallites formed at low Tc (e.g., 80 °C) than those formed at high Tc (e.g., 120 °C).37 Therefore, ΔHm,sc, Xc,sc, and fsc,DSC obtained from DSC include not only the contributions of sc crystallites formed in isothermal crystallization but also those formed upon heatinginduced hc-to-sc transition. This will result in the overestimation of ΔHm,sc, Xc,sc, and fsc,DSC in DSC analysis, which is more pronounced for the blends crystallized at low Tc due to the facilitated crystalline transition. Polymorphic behavior of racemic blend shows an interesting MW dependence in isothermal crystallization. At the same Tc, Xsc and fsc decrease significantly with increasing MW (Figures 4d and 5b,c), demonstrating that the HMW PLLA and PDLA are unfavorable for stereocomplexation. The L18/D22 blend has a large Xsc of 60−70%. Xsc of HMW blends (L82/D91 and 6466

DOI: 10.1021/acs.jpcb.5b03546 J. Phys. Chem. B 2015, 119, 6462−6470

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The Journal of Physical Chemistry B synchrotron radiation WAXD. Figure 6a shows the timeresolved WAXD patterns of L82/D91 blend collected in melt

Figure 6. (a) WAXD patterns of L82/D91 racemic blend collected in melt crystallization at 160 °C for different times. (b) Plots of diffraction peak area and sc crystallite fraction as a function of crystallization time. Peak area was normalized by the maximum value of (110)/(200) hc diffraction. Wavelength of used X-ray is 0.124 nm.

Figure 7. WAXD patterns collected upon heating of PLLA/PDLA racemic blends melt-crystallized at different temperatures: (a) L82/ D91, Tc = 80 °C; (b) L192/D191, Tc = 80 °C; (c) L82/D91, Tc = 160 °C. Wavelength of used X-ray is 0.124 nm.

crystallization at 160 °C. The intensities of (110) sc diffraction and (110)/(200) hc diffraction and fsc are plotted as a function of crystallization time in Figure 6b. The intensities were normalized by the maximum value of (110)/(200) hc diffraction. As seen in Figure 6, the sc crystallites appear immediately after cooling to 160 °C from melt state, while homocrystallites start to form after 3 min. After 3 min, diffraction intensities of sc crystallites increase little while those of homocrystallites enhance significantly with the crystallization proceeding. This imply that stereocomplex formation is preferential in the early stage of crystallization in HMW racemic blends. Because of the heterogeneous nucleation of precedingly formed sc crystallites to homocrystallization, homocrystallization becomes predominant and stereocomplexation is suppressed dramatically with the crystallization progressing. Therefore, it is rational to conclude that the formations of sc and hc crystallites are not completely simultaneous but successive in the crystallization of HMW racemic blend. Because of the successive formation of sc and hc crystallites, two exotherms are detected in the DSC thermograms of MMW and HMW racemic blends crystallized at certain Tcs; e.g., L82/D91 crystallized at Tc = 170 °C (Figure S1). The exotherms at early and later crystallization stages are ascribed to the formations of sc and hc crystallites, respectively. Heating-Induced hc-to-sc Crystalline Transition. Heating-induced crystalline transition of PLLA/PDLA racemic blends was investigated via in situ WAXD. Figure 7 shows the WAXD patterns of racemic blends crystallized at Tc = 80 and 160 °C collected upon heating. Intensity changes of (110) sc diffraction and (110)/(200) hc diffraction were evaluated and plotted as a function of temperature in Figure 8. For comparison, the intensities were normalized by the maximum value of (110)/(200) hc diffraction. As seen in Figure 7a,b, the

Figure 8. Temperature-dependent diffraction peak areas upon heating of PLLA/PDLA racemic blends melt-crystallized at different temperatures: (a) L82/D91, Tc = 80 °C; (b) L192/D191, Tc = 80 °C; (c) L82/D91, Tc = 160 °C. Peak area was normalized by the maximum value of (110)/(200) hc diffraction.

mixed α′ homocrystallites and sc crystallites were formed after crystallization at a low Tc of 80 °C. The disordered α′ homocrystallites are characteristic of (110)/(200) and (203) diffractions at lower angles and the appearance of diffraction peak at 2θ = 19.6° (indicated by an asterisk in Figure 7a,b).39,49,50 Upon heating to 170 °C, (110)/(200) and (203) diffraction peaks of homocrystallites shift to higher angle, and characteristic (106) and (206) diffractions of α homocrystallites 6467

DOI: 10.1021/acs.jpcb.5b03546 J. Phys. Chem. B 2015, 119, 6462−6470

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The Journal of Physical Chemistry B are present at around 2θ = 19.6° (indicated by arrows in Figure 7). This indicates that the disordered α′ homocrystallites transform into their ordered α analogues upon heating, which is accompanied by the contraction of crystal lattice. Meanwhile, the diffraction intensities of homocrystallites decrease and those of sc crystallites increase upon heating from 160 to 200 °C, indicative of the hc-to-sc transition. Notably, the increase in sc diffraction intensity is less obvious as compared to the decreased hc diffraction intensity in hc-to-sc transition (Figure 7a,b). Therefore, it is concluded that majority of homocrystallites melt directly and just minority of homocrystallites undergo hc-to-sc transition upon heating the HMW racemic blend. At a high Tc of 160 °C, the mixtures of α homocrystallites and sc crystallites are formed after isothermal crystallization (Figure 7c). Upon the following heating, diffraction intensities of sc crystallites do not increase but slightly decrease in the melting region of homocrystallites (160−190 °C), meaning that the hc-to-sc transition is negligible upon heating the racemic blend initially contained ordered α crystallites. Therefore, initial crystalline structure of homocrystallites influences strongly on the heating-induced hc-to-sc transition. Compared to the α crystallites with ordered structure, the α′ homocrystallites with disordered conformation and loose chain packing are preferential to recrystallize or transform into sc crystallites. This may due to the higher chain mobility and smaller size of α′ homocrystallites. Except for the crystalline transition, secondary crystallization of chains in amorphous region may be another origin for stereocomplex formation.38 Because of the smaller crystallinity of α′ homocrystallites formed at low Tc than that of α homocrystallites formed at high Tc,55 the high portion of amorphous zone in α′ homocrystallites can lead to further stereocomplexation at elevated temperatures. Interestingly, MW is also a factor for the heatinginduced hc-to-sc crystalline transition. The hc-to-sc transition becomes more difficult with increasing MW. By comparing Figures 8a and 8b, it can be clearly seen that the intensity increase in sc diffraction of L82/D91 blend is more pronounced than that of L192/D191 blend upon heating in the melting region of homocrystallites. Crystalline Lamellae Structure Analyzed by SAXS. The crystalline lamellae structure of PLLA/PDLA racemic blends was investigated via SAXS. Figure 9 shows the Lorentzcorrected SAXS profiles of racemic blends crystallized at various Tcs. Long spacing (L) was calculated by Bragg equation (L = 2π/qmax), where qmax corresponds to the peak position in Lorentz-corrected SAXS profiles. The LMW blend (e.g., L18/ D22) exhibits single scattering peak in SAXS profiles, corresponding to the scattering of sc crystallites. This scattering peak shifts to low q with increasing Tc, meaning that perfect sc crystallites with larger lamellae thickness are formed at high Tc (Table 3). For the HMW blends (e.g., L82/D91 and L192/D191), no discernible scattering peak corresponding to the long period is observed at Tc ≤ 100 °C. Generally, it is hard to observe the long period peak of homocrystallites in SAXS, except for that annealed at high temperature.37,56 This may be due to the less regular crystalline laminar structure and wide distribution of long period for homocrystallites formed at low Tc. At Tc = 120−160 °C, a scattering peak, marked as Phc, appears at q < 0.4 nm−1, and it disappears as Tc is increased to 180 °C. However, a new broad scattering peak, marked as Psc, is observed at higher angle (q > 0.4 nm−1) with increasing Tc to

Figure 9. Lorentz-corrected SAXS profiles of (a) L18/D22, (b) L82/ D91, and (c) L192/D191 racemic blends after melt crystallization at different temperatures. The profiles were shifted vertically for clarity. I is the scattering intensity, and q is the scattering vector.

Table 3. Long Spacing (nm) of Hc and Sc Crystallites in PLLA/PDLA Racemic Blends Melt-Rrystallized at Different Temperatures L18/D22

L82/D91

L192/D191

Tmc (°C)

hc

sc

hc

sc

hc

sc

120 140 160 170 180

− − − − −

11.3 11.7 12.8 13.9 14.1

16.8 17.0 18.1 21.2 −

− − 10.1 12.7 15.4

22.0 23.6 25.8 27.2 −

− − − 13.8 16.8

≥160 °C. Because the sc crystallites are exclusively formed at Tc = 180 °C, Phc and Psc can be assigned to the scattering of hc and sc crystallites, respectively. Both Phc and Psc shift to low q with increasing Tc, meaning the enhancement of long period of hc and sc crystallites (Table 3). Long periods of sc crystallites range 11−17 nm, which agree with the results reported by Li et al.57 and Woo et al.58 It is notable that, at the same Tc, the long period of sc crystallites is much smaller than that of homocrystallites. Long diffusion path and large kinetic barrier of stereocomplexation may restrict the formation of thick crystalline lamellae.

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CONCLUSIONS In conclusion, MW is a crucial factor influencing the competing stereocomplexation and homocrystallization, polymorphic crystalline structure, and crystalline transition of PLLA/PDLA racemic blends. Sc crystallites are exclusively formed in both nonisothermal and isothermal crystallizations of LMW PLLA/ PDLA blend. Sc crystallization is dramatically suppressed, and homocrystallization becomes predominant with increasing MWs of PLLA and PDLA. It is proposed that the suppressed stereocomplexation in HMW racemic blends is ascribed to the low chain diffusion ability and restricted intermolecular crystal nucleation/growth. Crystallization rate of racemic blends 6468

DOI: 10.1021/acs.jpcb.5b03546 J. Phys. Chem. B 2015, 119, 6462−6470

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The Journal of Physical Chemistry B decreases, while T0m of sc crystallites first increases and then decreases with increasing MW. Crystallinity and relative fraction of sc crystallites increase with Tc, and the sc crystallites are merely formed at Tc > 170 °C. Formations of sc and hc crystallites are successive rather than completely simultaneous in isothermal crystallization of HMW racemic blends. The formation of sc crystallites is preceding that of homocrystallites. The heating-induced hc-to-sc crystalline transition of HMW racemic blend depends on both Tc and MW, which becomes easier as Tc and MW decrease. Under the same Tc, sc crystallites show smaller long period than their hc counterparts. This study has illustrated the effects of MW and crystallization conditions on polymorphic crystalline structure and transition of PLLA/PDLA racemic blends. This would provide potential approaches to control the microstructure and physical performance of PLLA/PDLA blends as well as the assembled structures and properties of copolymer systems containing PLLA and PDLA segments.

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ASSOCIATED CONTENT

S Supporting Information *

DSC curves of isothermal crystallization. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b03546.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-571-87951334, e-mail [email protected] (P.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Prof. Wenbing Hu of Nanjing University for valuable discussions. This work was financially supported by the Natural Science Foundation of China (21274128, 21422406) and the Fundamental Research Funds for the Central Universities (2015XZZX004-08). SAXS and in situ WAXD experiments were performed at the BL16B beamline of SSRF, China.

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