Ternary Stereocomplex and Hetero-Stereocomplex Crystallizability of

Article pubs.acs.org/crystal

Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Ternary Stereocomplex and Hetero-Stereocomplex Crystallizability of Substituted and Unsubstituted Poly(lactic acid)s Hideto Tsuji,* Noriaki Masaki, Yuki Arakawa, Kazumasa Iguchi, and Tadashi Sobue Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan S Supporting Information *

ABSTRACT: The ternary stereocomplex (TSC) crystallizability of ternary substituted and unsubstituted poly(lactic acid) blends composed of poly(D-2-hydroxy-3-methylbutanoic acid) [P(D-2H3MB)], poly(L-2-hydroxy-3-methylbutanoic acid) [P(L2H3MB)], and poly(L-2-hydroxybutanoic acid) [P(L-2HB)] or poly(L-lactic acid) (PLLA), together with heterostereocomplex (HTSC) crystallizability of binary blends composed of P(D-2H3MB) and PLLA, were investigated for solvent evaporated and precipitated samples. For the solvent evaporated P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) (mol/mol/mol) blend, formation of TSC crystallites with a very small amount of P(D-2H3MB) and/or P(L-2H3MB) homocrystallites was observed, whereas in the precipitated P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) blend, P(D-2H3MB)/P(L-2HB) HTSC crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and P(L-2HB) homocrystallites were formed without formation of TSC crystallites. This is the first report for TSC crystallization of all substituted PLAs with linear and branched side chains. In contrast, in both solvent evaporated and precipitated P(D-2H3MB)/P(L-2H3MB)/PLLA (50/25/25) (mol/mol/ mol) blends, P(D-2H3MB)/P(L-2H3MB) homostereocomplex crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and PLLA homocrystallites were formed without crystallization of TSC crystallites. It was confirmed that HTSC between P(D-2H3MB) and PLLA is not formed in both solvent evaporated and precipitated P(D-2H3MB)/PLLA (50/50) (mol/mol) blends. Based on reported and present results, we proposed the rule for TSC and HTSC crystallization of, respectively, binary and ternary substituted and unsubstituted poly(lactic acid)s, wherein all the optically active polymer components are included in the same SC crystalline lattice. The difference in carbon numbers of side chains between the two polymers with different chemical structures and opposite configurations is one. ing,20 crystallization temperature range between the melting temperatures of SC- and homocrystallites,21 stepwise assembly by alternate soaking in PLLA and PDLA solutions,22 utilization of supercritical fluid,23 repeated casting,24 layer-by-layer deposition by inkjet printing,25 and solvent evaporation with solvent and nonsolvent mixture.26 Recently, Zhang et al. illustrated that through the interplay of polarization terahertz (THz) spectroscopy and solid-state density functional theory, the chiral symmetry is not conserved in a PLA SC system.27 Very recently, Tashiro et al. confirmed that PDLA and PLLA

1. INTRODUCTION Poly(L-lactide), i.e., poly(L-lactic acid) (PLLA), is a biobased sustainable polymer derived from corn and sugar cane and is widely used in various applications including commodity, environmental, and pharmaceutical applications due to its relatively high mechanical performance, biodegradability, and affinity to the living body.1−10 Stereocomplex (SC) crystallization of PLLA with its enantiomer poly(D-lactide), i.e., poly(D-lactic acid) (PDLA) is versatile for the enhancement of mechanical properties, thermal/hydrolysis-resistance required for commodity applications.10−18 A wide variety of methods and parameters enhancing SC crystallization have been extensively investigated.12,17−26 The methods or factors for enhancement of SC crystallization include shearing,19 draw© XXXX American Chemical Society

Received: November 7, 2017 Revised: December 6, 2017 Published: December 6, 2017 A

DOI: 10.1021/acs.cgd.7b01559 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 1. Schematic representation of ternary blends of P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) (red line) and P(D-2H3MB)/P(L2H3MB)/PLLA (50/25/25) (purple line), and binary blend of P(D-2H3MB)/PLLA (50/50) (green line).

during bulk crystallization from the melt.42 HTSC, TSC, and QSC stated above are composed of two to four polymers with two different chemical structures. However, we recently found SC crystallization from three polymers with three different chemical structures, i.e., one L-configured P(L-2HB) and two Dconfigured PDLA and P(D-2H3MB).43 This result also indicates that an optically active polymer (L-configured or Dconfigured polymer) like unsubstituted or substituted optically active PLAs can act as “a configurational or helical molecular glue” for two oppositely configured optically active polymers (two D-configured polymers or two L-configured polymers) to allow their cocrystallization.43 On the other hand, SC crystallization occurs in oppositely configured random copolymers, L- and D-configured poly(2-hydroxybutanoic acid-co-lactic acid)s during solvent-evaporation of solution and during bulk crystallization from the melt, and interestingly the copolymer SC lattice contains four types of monomer units L- and D-2hydroxybutanoic acids and L- and D-lactic acids.44 In the present study, we investigated the TSC crystallizability of ternary blends composed of D-configured P(D-2H3MB) and L-configured [P(L-2H3MB)] with L-configured P(L-2HB)] or PLLA, together with HTSC crystallizability of binary blends composed of D-configured P(D-2H3MB) and L-configured PLLA (Figure 1), by the use of typical crystallization methods of solvent-evaporation and precipitation. These polymer combinations were selected to investigate the effects of the difference in carbon numbers of side chains between the two polymers with different chemical structures and opposite configurations on SC crystallization. Based on the wide-angle X-ray diffractometry (WAXD) data, we report for the first time TSC crystallization from all substituted ternary PLAs with linear and branched side chains, one D-configured P(D2H3MB) and two L-configured P(L-2H3MB) and P(L-2HB). Based on the reported and present results, we proposed the rule for TSC and HTSC crystallizability of substituted and unsubstituted PLAs.

chains coexist in the SC crystal lattice at the various ratios in the wide range of PLLA/PDLA from 70/30 to 30/70 by crystalline infrared spectorscopy28 and proposed a new model for the crystal structure of PLLA/PDLA SC on the basis of the X-ray diffraction data analysis, which is suitable for the wide range of PLLA/PDLA.29 For the crystallization of SCs, various crystallization techniques such as solvent-evaporation, precipitation, and bulk crystallization from the melt have been utilized. Homostereocomplexes (HMSCs), which are defined as SCs formed from polymers with identical chemical structures and opposite configuration, are formed in enantiomeric unsubstituted poly(lactide)s, i.e., poly(lactic acid)s (PLAs), PLLA, and PDLA,17 enantiomeric substituted PLAs such as enantiomeric poly(2-hydroxybutyrate)s [i.e., poly(2-hydroxybutanoic acid)s, P(2HB)s] with linear side chains (ethyl groups), poly(L2-hydroxybutyrate) [i.e., poly(L-2-hydroxybutanoic acid) P(L2HB)] and poly(D-2-hydroxybutyrate) [i.e., poly(D-2-hydroxybutanoic acid), P(D-2HB)],30−32 and enantiomeric poly(2hydroxy-3-methylbutyrate)s [i.e., poly(2-hydroxy-3-methylbutanoic acid)s, P(2H3MB)s] with branched side chains (isopropyl groups), poly(L-2-hydroxy-3-methylbutyrate) [i.e., poly(L-2-hydroxy-3-methylbutanoic acid) [P(L-2H3MB)] and poly(D-2-hydroxy-3-methylbutyrate) [i.e., poly(D-2-hydroxy-3methylbutanoic acid), P(D-2H3MB)].33,34 On the other hand, heterostereocomplexes (HTSCs) are formed from unsubstituted and substituted PLAs with the different chemical structures and opposite configurations, which include polymer pairs of PLA and P(2HB) 35−37 and of P(2HB) and P(2H3MB).38,39 Ternary stereocomplex (TSC) is formed from one Lconfigured P(L-2HB) and two D-configured P(D-2HB) and PDLA during solvent-evaporation of solution and during bulk crystallization from the melt.40,41 Moreover, quaternary stereocomplex (QSC) is formed from two L-configured P(L-2HB) and P(L-2H3MB) and two D-configured P(D-2HB) and P(D2H3MB) during solvent-evaporation of solution as well as B



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2. EXPERIMENTAL SECTION

Table 2. Polymer Compositions of Blends

2.1. Materials. P(D-2H3MB), P(L-2H3MB), and P(L-2HB) were synthesized by polycondensation of 2.0 g of D-2-hydroxy-3methylbutanoic acid [(R)-2-hydroxy-3-methylbutyric acid or D-αhydroxyisovaleric acid] (≥98.0%, Sigma-Aldrich Co.), L-2-hydroxy-3methylbutanoic acid [(S)-2-hydroxy-3-methylbutyric acid or L-αhydroxyisovaleric acid] (99%, Sigma-Aldrich Co.), and L-2-hydroxybutanoic acid [(S)-2-hydroxybutyric acid] (≥97.0%, Sigma-Aldrich Co., Tokyo, Japan), respectively, using 5 wt % p-toluenesulfonic acid (monohydrate, Nacalai special grade GR, Nacalai Tesque inc., Kyoto, Japan) as the catalyst, as reported previously under a constant nitrogen gas flow at 130 °C under an atmospheric pressure for 5 h and then at a reduced pressure of 5.0 kPa for 13 h for the synthesis of P(D-2H3MB) and P(L-2H3MB), and of 2.0 kPa for 24 h for the synthesis of P(L2HB).32,34,37,42 For the synthesis of P(D-2H3MB) and P(L-2H3MB), 0.5 mL of distillated water (HPLC grade, Nacalai Tesque Inc.) was added before polycondensation. PLLA was synthesized by ringopening polymerization of L-lactide (PURASORB L, Purac Biomaterials, Gorinchem, The Netherlands) in bulk at 140 °C initiated with 0.03 wt % of tin(II) 2-ethylhexanoate (practical grade CP, Nacalai Tesque, Inc.) in the presence of 0.6 wt % of 1-propanol (Tokyo Kasei special grade GR, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) as a co-initiator.36,40,45 Before use, tin(II) 2-ethylhexanoate was purified by distillation under reduced pressure and L-lactide was purified by repeated recrystallization using ethyl acetate (Nacalai special grade GR, Nacalai Tesque inc.) as the solvent. Other chemicals were used as received. The synthesized P(D-2H3MB), P(L-2H3MB), P(L-2HB), and PLLA were purified by reprecipitation using chloroform (Nacalai special grade GR, Nacalai Tesque inc.) and methanol (Nacalai special grade GR, Nacalai Tesque inc.) as the solvent and nonsolvent, respectively, and then dried under reduced pressure for at least 6 days.41 The molecular characteristics of the polymers used in the present study are listed in Table 1.

code A B C D E a

Mwa (g mol−1)

Mw/Mna

[α]25589b (deg dm−1 g−1 cm3)

P(D-2H3MB) P(L-2H3MB) P(L-2HB) PLLA

× × × ×

2.44 3.01 1.60 1.27

73.8 −74.5 −116.2 −141.2

2.93 3.43 2.29 1.67

3

10 103 104 104

P(D2H3MB)a (mol %)

P(L2H3MB)a (mol %)

P(D-2H3MB)/P(L2H3MB) (50/50) P(D-2H3MB)/P(L2HB) (50/50) P(D-2H3MB)/ PLLA (50/50) P(D-2H3MB)/P(L2H3MB)/P(L2HB) (50/25/25) P(D-2H3MB)/P(L2H3MB)/PLLA (50/25/25)

50

50

0

0

50

0

50

0

50

0

0

50

50

25

25

0

50

25

0

25

P(L2HB)a PLLAa (mol %) (mol %)

Mol% in monomer unit.

polymers were evaluated in chloroform at 40 °C with a Tosoh gel permeation chromatography (GPC) system (refractive index monitor: RI-8020) with two TSK Gel columns (GMHXL) using polystyrene standards.46 The glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) and enthalpies of cold crystallization and melting (ΔHcc and ΔHm, respectively) of the blends were determined by a Shimadzu (Kyoto, Japan) DSC-60 differential scanning calorimeter.46 The samples were heated from 0 to 230 °C at a rate of 10 °C min−1 under a nitrogen gas flow of 50 mL min−1 for DSC measurements.40 The crystalline species and crystallinity (Xc) values of blend samples were estimated by WAXD measurements. The WAXD measurements were performed at 25 °C using a Rigaku (Tokyo, Japan) RINT-2500 equipped with a Cu Kα source [wavelength (λ) = 1.5418 Å], which was operated at 40 kV and 200 mA.47 In a 2θ range of 5−30°, the crystalline diffraction peak areas of respective crystalline species relative to the total area between a diffraction profile and a baseline were used to estimate the Xc values.34,38,41

Table 1. Molecular Characteristics of Polymers Used in the Present Study polymer

componentsa (compositions in mol %)

3. RESULTS AND DISCUSSION 3.1. Wide-Angle X-ray Diffractometry. For the estimation of formed crystalline species and their Xc values, interplanar distance (d) values of the solvent evaporated and precipitated blends, WAXD measurements were performed. Figure 2 shows the WAXD profiles of the solvent evaporated and precipitated blends. The Xc values of respective crystalline species were evaluated from the WAXD profiles in Figure 2 and are summarized in Table 3. In previous papers, solvent evaporation and melt-crystallization were utilized for crystallization of P(D-2H3MB)/P(L-2H3MB) (50/50) blend,33,34 the precipitation method was applied for the first time for P(D2H3MB)/P(L-2H3MB) (50/50) blend (A) in the present study. For the solvent evaporated P(D-2H3MB)/P(L-2H3MB) (50/50) blend (A), only P(D-2H3MB)/P(L-2H3MB) HMSC crystalline diffraction peaks were seen at 2θ values around 9.7°, 16.9°, 17.8°, and 19.5°, whereas surprisingly, for the precipitated P(D-2H3MB)/P(L-2H3MB) (50/50) blend (A), only P(L-2H3MB) and P(D-2H3MB) homocrystalline diffraction peaks were observed at 2θ values around 13.8°, 16.9°, 18.8°, and 21.2°,34 indicating that solvent evaporation and precipitation correspondingly facilitated the P(D-2H3MB)/ P(L-2H3MB) HMSC crystallization and P(L-2H3MB) and P(D-2H3MB) homocrystallization, respectively. The completely separated homocrystallization of P(L-2H3MB) and P(D-2H3MB) from their blend solution is reported for the first time in the present study. The crystalline species of solvent evaporated P(D-2H3MB)/ P(L-2H3MB) blend (A) is consistent with the previously

a

Mw and Mn are weight- and number-average molecular weights, respectively, estimated by GPC. bMeasured in chloroform. Solvent evaporated blends were prepared by the procedure stated in previous papers.35,38,40,43 Briefly, each solution of two or three polymers was prepared separately to have a polymer concentration of 1.0 g dL−1 and then admixed with each other under stirring.40 Dichloromethane was used as the solvent40 and the compositions of the prepared blends are tabulated in Table 2. The solutions were cast onto Petri-dishes, followed by solvent evaporation at 25 °C for approximately 1 day and then drying in vacuo for at least 6 days. The precipitated blends were obtained by dissolving solvent evaporated blends using dichloromethane/1,1,1,3,3,3-hexafluoro-2-propanol (abbreviated as HFIP, HPLC grade, Nacalai Tesque Inc.) (v/v = 95/5) as the solvent to have a polymer concentration of 10 g dL−1 (40 mg/0.4 mL) and reprecipitation with stirred methanol as the nonsolvent.43,45 The volume ratio of blend solution and methanol 0.4/30 (mL/mL). The precipitated blends were separated by centrifugation, rinsed with fresh methanol twice, and dried under reduced pressure for at least 6 days. In the present study, in addition to P(D-2H3MB)/P(L2H3MB)/P(L-2HB) (50/25/25) (D), P(D-2H3MB)/P(L-2H3MB)/ PLLA (50/25/25) (E), and P(D-2H3MB)/PLLA (50/50) (C) blends, P(D-2H3MB)/P(L-2H3MB) (50/50) (A) and P(D-2H3MB)/P(L2HB) (50/50) (B) blends were prepared for reference. 2.2. Physical Measurements. The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the C



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Figure 2. WAXD profiles of solvent evaporated (a) and precipitated (b) P(D-2H3MB)/P(L-2H3MB) (50/50) blends (A), P(D-2H3MB)/P(L2HB) (50/50) blends (B), P(D-2H3MB)/PLLA (50/50) blends (C), P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) blends (D), and P(D2H3MB)/P(L-2H3MB)/PLLA (50/25/25) blends (E). Dotted lines, broken lines, and alternate long and short dashed lines are crystalline diffraction peak positions of P(L-2H3MB)/P(D-2H3MB) HMSC crystallites, P(L-2HB)/P(D-2H3MB) HTSC crystallites, and P(L-2H3MB) and/ or P(D-2H3MB) homocrystallites, respectively.

Table 3. Crystallinity of Blend Samples crystallization method solvent evaporation

code A B C D E

precipitation

A B C D E

a

components (compositions in mol %) P(D-2H3MB)/P(L-2H3MB) (50/50) P(D-2H3MB)/P(L-2HB) (50/50) P(D-2H3MB)/PLLA (50/50) P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) P(D-2H3MB)/P(L-2H3MB)/PLLA (50/25/25) P(D-2H3MB)/P(L-2H3MB) (50/50) P(D-2H3MB)/P(L-2HB) (50/50) P(D-2H3MB)/PLLA (50/50) P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) P(D-2H3MB)/P(L-2H3MB)/PLLA (50/25/25)

type of SC

Xc(SC) (%)

Xc[P(2H3MB)] (%)

Xc[P(2HB)] (%)

Xc(PLA) (%)

Xc(tot)a (%)

HMSC HTSC NFb TSC

70.0 68.1 0.0 73.2

0.0 6.0 35.4 2.7

0.0 0.0 0.0 0.0

0.0 0.0 32.7 0.0

70.0 74.1 68.1 75.9

HMSC

36.6

12.9

0.0

16.0

65.5

NFb HTSC NFb HTSC

0.0 51.9 0.0 20.9

65.7 6.7 4.9 38.4

0.0 6.3 0.0 5.6

0.0 0.0 23.8 0.0

65.7 64.9 28.7 64.9

HMSC

7.6

22.6

0.0

22.3

52.5

Xc(tot) = Xc(SC) + Xc[P(2H3MB)] + Xc[P(2HB)] + Xc(PLA). bSC was not formed.

reported result33,34 and can be ascribed to the fact that the critical concentration below which crystallization occur during solvent evaporation is lower for P(D-2H3MB)/P(L-2H3MB) HMSC crystallites than for P(D-2H3MB) and P(L-2H3MB) homocrystallites, as reported for PLLA/PDLA blend solutions.12,24 This will result in the crystallization of P(D2H3MB)/P(L-2H3MB) HMSC crystallites prior to that of P(D-2H3MB) and P(L-2H3MB) homocrystallites during solvent evaporation, causing exclusive formation of P(D2H3MB)/P(L-2H3MB) HMSC crystallites. It is interesting to note that even such low-molecular-weight P(L-2H3MB) and P(D-2H3MB) with Mw around 3 × 103 g mol−1 formed homocrystallites in the precipitated (D-2H3MB)/P(L2H3MB) blend (A). The critical molecular weight below which only homocrystallites are formed decreased with increasing carbon number of side chains of poly(2-hydroxyalkanoic acid)s in the following order, PLLA/PDLA blend (Mn in the order of 105) > P(L-2HB)/P(D-2HB) blend (Mn in the order of 104) > P(L-2H3MB)/P(D-2H3MB) blend (Mn in the order of 103).12,32,34 On the other hand, the fact that the

crystallization rates of P(L-2H3MB) and P(D-2H3MB) homocrystallites from the melt are higher than that of P(D2H3MB)/P(L-2H3MB) HMSC crystallites,34 which is in marked contrast with the results for PDLA/PLLA and P(D2HB)/P(L-2HB) blends,31,48 can explain the crystalline species of precipitated P(D-2H3MB)/P(L-2H3MB) blend (A). During a short time of precipitation, only P(L-2H3MB) and P(D2H3MB) homocrystallites with the high crystallization rates could have been formed. In contrast, for the solvent evaporated and precipitated P(D2H3MB)/P(L-2HB) (50/50) blends (B), main crystalline species were P(D-2H3MB)/P(L-2HB) HTSC crystallites with crystalline peaks at 2θ values around 10.2°, 17.5°, 18.6°, and 20.8°. In addition to P(D-2H3MB)/P(L-2HB) HTSC crystallites, the solvent evaporated P(D-2H3MB)/P(L-2HB) blend (B) contained P(D-2H3MB) homocrystallites with main crystalline peaks at 12.9°, whereas the precipitated P(D2H3MB)/P(L-2HB) blend (B) had P(D-2H3MB) homocrystallites with the main crystalline peak at 13.8° and P(L-2HB) homocrystallites with the main crystalline peak at 14.8°. It D



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Table 4. Interplanar Distance (d) Values of SCs of Solvent Evaporated Blends d (Å) [2θ (deg)]

components code

(compositions in mol %)

type of SC

lowest 2θ

second lowest 2θ

third lowest 2θ

fourth lowest 2θ

A B D

P(D-2H3MB)/P(L-2H3MB) (50/50) P(D-2H3MB)/P(L-2HB) (50/50) P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25)

HMSC HTSC TSC

9.09 [9.73] 8.71 [10.16] 8.96 [9.87]

5.25 [16.89] 5.07 [17.50] 5.18 [17.11]

4.98 [17.82] 4.77 [18.59] 4.91 [18.06]

4.55 [19.52] 4.26 [20.84] 4.50 [19.73]

Figure 3. DSC thermograms of solvent evaporated (a) and precipitated (b) P(D-2H3MB)/P(L-2H3MB) (50/50) blends (A), P(D-2H3MB)/P(L2HB) (50/50) blends (B), P(D-2H3MB)/PLLA (50/50) blends (C), P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) blends (D), and P(D2H3MB)/P(L-2H3MB)/PLLA (50/25/25) blends (E).

should be noted that the P(L-2H3MB) or P(D-2H3MB) homocrystallites have two different crystal structures formed during solvent evaporation and melt-crystallization.34 Considering the main diffraction angle of P(D-2H3MB) homocrystallites, it seems that the homocrystalline structure of P(D2H3MB) crystallites formed during precipitation is the same as that formed during melt-crystallization.34 The solvent evaporated and precipitated P(D-2H3MB)/ PLLA blends (C) contained P(D-2H3MB) and PLLA homocrystallites without formation of P(D-2H3MB)/PLLA HTSC crystallites, irrespective of crystallization procedure. This result is consistent with the result reported by Andersson et al. for a solvent evaporated sample.33 However, the present study revealed that even the precipitation method with stirred nonsolvent, which is known to facilitate SC crystallization of even high-molecular-weight PLLA/PDLA blends which are prone to crystallize in homocrystallites in normal crystallization methods, cannot induce HTSC crystallization between P(D2H3MB) and PLLA. Furthermore, the Xc values of P(D2H3MB) and PLLA homocrystallites (35% and 33%, respectively) are similar to each other for the solvent evaporated P(D-2H3MB)/PLLA blend (C), whereas for the precipitated P(D-2H3MB)/PLLA blend (C) the Xc value of P(D-2H3MB) homocrystallites (5%) is much lower than that of PLLA homocrystallites (24%), indicating suppressed P(D2H3MB) homocrystallization during a short time of precipitation, probably due to lower crystallization rate of P(D2H3MB) homocrystallites, which originated from bulky side chains (isopropyl groups) compared to small side chains (methyl groups) of PLLA. The WAXD data for these three types of binary blends (A−C) are reference data to elucidate crystallization behavior of ternary P(D-2H3MB)/P(L-

2H3MB)/P(L-2HB) (50/25/25) blends (D) and P(D2H3MB)/P(L-2H3MB)/PLLA (50/25/25) blends (E). The solvent evaporated ternary P(D-2H3MB)/P(L2H3MB)/P(L-2HB) (50/25/25) blend (D) showed a SC type diffraction pattern. As seen in Figure 2a (magnified Figure 2(a) is shown in Figure S1), the SC crystalline peaks were located between those of HMSC crystallites of solvent evaporated P(D-2H3MB)/P(L-2H3MB) blend (A) and HTSC crystallites of solvent evaporated and precipitated P(D-2H3MB)/P(L-2HB) blends (B). The d values of SC crystallites estimated from solvent evaporated blends (A, B, and D) were evaluated from Figure 2a and are tabulated in Table 4. The respective d values of solvent evaporated P(D-2H3MB)/ P(L-2H3MB)/P(L-2HB) (50/25/25) blend (D) are between the d values of HMSC crystallites of the solvent evaporated P(D-2H3MB)/P(L-2H3MB) blend (A) and HTSC crystallites of the solvent evaporated P(D-2H3MB)/P(L-2HB) blends (B). This result indicates the incorporation of P(L-2HB) in P(D2H3MB)/P(L-2H3MB) HMSC crystalline lattice or TSC crystallization in the solvent evaporated P(D-2H3MB)/P(L2H3MB)/P(L-2HB) (50/25/25) blend (D). This is the first report for TSC crystallization from all substituted PLAs with linear and branched side chains. In contrast, the precipitated P(D-2H3MB)/P(L-2H3MB)/ P(L-2HB) (50/25/25) blend (D) had P(D-2H3MB)/P(L2HB) HTSC crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and P(L-2HB) homocrystallites, without TSC crystallites. Interestingly, this result suggests that during precipitation the crystallization rates of P(D-2H3MB)/P(L2HB) HTSC crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and P(L-2HB) homocrystallites were higher than that of P(D-2H3MB)/P(L-2H3MB) HMSC crystallites. E



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Table 5. Thermal Properties of Blend Samples crystallization method

code

type of SC

Solvent evaporation

A B C D E A B C D E

HMSC HTSC NFc TSC HMSC NFc HTSC NFc HTSC HMSC

Precipitation

Tga (°C) 30.8 50.8 50.4 39.5 40.0 56.2 50.0 40.2

Tcca (°C)

145.4 106.9 108.6, 140.7

Tma (°C) 192.3 145.5, 162.6 137.8, 149.1, 199.3, 105.4, 163.5 179.2, 163.6,

159.0, 206.1 147.0, 195.4 162.8, 182.8, 193.3 205.9 180.1, 205.3, 208.1 197.1 198.5

ΔHccb (J g−1)

ΔHmb (J g−1)

ΔH (tot)b (J g−1)

0.0 0.0 0.0 0.0 0.0 −2.0 0.0 −8.9 0.0 −5.8

38.1 40.3 36.5 38.7 27.7 27.1 39.0 35.6 38.9 25.2

38.1 40.3 36.5 38.7 27.7 25.1 39.0 26.7 38.9 19.4

a Tg, Tcc, and Tm are glass transition, cold crystallization, and melting temperatures, respectively. bΔHcc and ΔHm are cold crystallization and melting enthalpies, respectively. cSC was not formed.

crystallites become dominant at higher crystallization temperatures (Tc’s) above 130 °C when crystallized from the melt,34 transition from P(D-2H3MB) and P(L-2H3MB) homocrystallites to P(D-2H3MB)/P(L-2H3MB) HMSC crystallites might have occurred during DSC heating. Such a transition from homocrystallites to HMSC crystallites during heating is reported for relatively high molecular weight PLLA/PDLA blend49 and should have caused the similar melting behavior of the solvent evaporated and precipitated P(D-2H3MB)/P(L2H3MB) (50/50) blends (A). For the solvent evaporated P(D-2H3MB)/P(L-2HB) (50/ 50) blend (B), the large sharp melting peak of P(D-2H3MB)/ P(L-2HB) HTSC crystallites and the small broad melting peak of P(D-2H3MB) homocrystallites were seen at 206 and 159 °C, respectively, of which relative areas are consistent with the relative Xc values of HTSC and homocrystallites (Table 3). Similar to the solvent evaporated blend, the precipitated P(D2H3MB)/P(L-2HB) (50/50) blend (B) had a large melting peak of P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) HTSC crystallites and the small melting peak of P(D-2H3MB) homocrystallites at 208 and 180 °C, correspondingly. In addition to these melting peaks, the melting peak of P(L-2HB) homocrystallites was seen at 105 °C in the precipitated P(D2H3MB)/P(L-2HB) (50/50) blend (B). The crystalline species traced by their melting peaks of the solvent evaporated and precipitated P(D-2H3MB)/P(L-2HB) (50/50) blends (B) are in agreement with those monitored by WAXD measurements (Table 3). In the solvent evaporated and precipitated P(D-2H3MB)/ PLLA (50/50) blends (C), P(D-2H3MB) and PLLA homocrystallites were formed (Table 3). Despite the presence of two types of homocrystallites [P(D-2H3MB) and PLLA], only one melting peak was seen at 163 and 164 °C for the solvent evaporated and precipitated blends, respectively. Probably due to the rapid crystallization of PLLA homocrystallites, P(D-2H3MB) homocrystallites should have crystallized in the space confined by PLLA homocrystalline lamellae and thereby the growth of P(D-2H3MB) homocrystallites was disturbed. As a result, small size P(D-2H3MB) homocrystallites, whose Tm was lower than the value observed for the precipitated P(D-2H3MB)/P(L-2H3MB) blend (A) (199 and 206 °C) and similar to that of PLLA homocrystallites, should have been formed. The solvent evaporated P(D-2H3MB)/P(L-2H3MB)/P(L2HB) (50/25/25) blend (D), which had a large amount of P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) TSC crystallites and a very small amount of P(D-2H3MB) and/or P(L-2H3MB)

The lower crystallization rate of P(D-2H3MB)/P(L-2H3MB) HMSC crystallites compared to those of P(D-2H3MB) and/or P(L-2H3MB) homocrystallites during precipitation agrees with the outcome of precipitated P(D-2H3MB)/P(L-2H3MB) blend (Blend A), resulting in the formation of P(D-2H3MB) and P(L-2H3MB) homocrystallites and no formation of P(D2H3MB)/P(L-2H3MB) HMSC crystallites. High crystallizability of P(D-2H3MB)/P(L-2HB) HTSC crystallites in the precipitated P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/ 25) blend (D) is consistent with the result of the precipitated P(D-2H3MB)/P(L-2HB) (50/50) blend (B). The solvent evaporated and precipitated P(D-2H3MB)/P(L2H3MB)/PLLA (50/25/25) blends (E) contained P(D2H3MB)/P(L-2H3MB) HMSC crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and PLLA homocrystallites, without TSC crystallites. Similar to the results of solvent evaporated and precipitated P(D-2H3MB)/P(L2H3MB) blends (A), the Xc(SC) of P(D-2H3MB)/P(L2H3MB) HMSC crystallites was much higher for solvent evaporated P(D-2H3MB)/P(L-2H3MB)/PLLA blend (E) than for the precipitated P(D-2H3MB)/P(L-2H3MB)/PLLA blend (E). It is interesting to note that a significant amount of P(D2H3MB)/P(L-2H3MB) HMSC crystallites was formed in the precipitated P(D-2H3MB)/P(L-2H3MB)/PLLA blend (E), in marked contrast with no formation of P(D-2H3MB)/P(L2H3MB) HMSC crystallites in precipitated P(D-2H3MB)/ P(L-2H3MB) blend (A) and P(D-2H3MB)/P(L-2H3MB)/ P(L-2HB) (50/25/25) blend (D). Comparison of the components of blends (A, D, and E) suggests that PLLA homocrystallites with the high crystallization rate should have acted as a nucleating agent for P(D-2H3MB)/P(L-2H3MB) HMSC crystallites. 3.2. Differential Scanning Calorimetry. To investigate the thermal properties of the blends, DSC measurements were carried out for the blends and their thermograms are shown in Figure 3. The thermal properties were estimated from the DSC thermograms in Figure 3 and summarized in Table 5. Although crystalline species were different in the solvent evaporated and precipitated P(D-2H3MB)/P(L-2H3MB) (50/50) blends (A), i.e., P(D-2H3MB)/P(L-2H3MB) HMSC crystallites and P(D2H3MB) and P(L-2H3MB) homocrystallites, respectively (Table 3), these blends show the similar melting peaks at around 200 °C. Considering the facts that the reported Tm range of P(D-2H3MB) or P(L-2H3MB) homocrystallites (160−190 °C) was wider to a lower temperature compared to that of P(D-2H3MB)/P(L-2H3MB) HMSC crystallites (ca. 190 °C) and that P(D-2H3MB)/P(L-2H3MB) HMSC F



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Table 6. SC Crystallizability of Substituted and Unsubstituted PLAs Reported So Far and Studied in the Present Study type of polymer chain 1

2

3

4 a

type of SC

type of chemical structure

PLLA/PDLA P(L-2HB)/P(D-2HB) P(L-2H3MB)/P(D-2H3MB) PLLA/P(D-2HB) or PDLA/P(L-2HB) PLLA/P(D-2H3MB) or PDLA/P(L-2H3MB)

HMSC HMSC HMSC HTSC NFa

1 1 1 2 2

1 2 3 1, 2 1, 3

P(L-2HB)/P(D-2H3MB) or P(D-2HB)/P(L-2H3MB) PLLA/P(L-2HB)/P(D-2HB) or PDLA/P(L-2HB)/P(D-2HB) PLLA/P(L-2H3MB)/P(D-2H3MB) or PDLA/P(L-2H3MB)/P(D2H3MB) P(L-2HB)/P(L-2H3MB)/P(D-2H3MB) or P(D-2HB)/P(L2H3MB)/P(D-2H3MB) PLLA/P(D-2HB)/P(L-2H3MB) or PDLA/P(L-2HB)/P(D2H3MB) PLLA/PDLA/P(L-2HB)/P(D-2HB) P(L-2HB)/P(D-2HB)/P(L-2H3MB)/P(D-2H3MB)

HTSC TSC HMSC

2 2 2

2, 3 1, 2 1, 3

50 30 33, 34 35 33, present study 38 40 Present study

TSC

2

2, 3

Present study

TSC

3

1, 2, 3

43

HSC QSC

2 2

1, 2 2, 3

44 42

components

carbon number of side chains

refs

SC was not formed.

carbon number of side chains up to three. Two combinations of binary substituted and unsubstituted PLAs with different chemical structures and opposite configurations are known to form HTSC [PLLA/P(D-2HB) or PDLA/P(L-2HB)35 and P(L-2HB)/P(D-2H3MB) or P(D-2HB)/P(L-2H3MB)38], wherein the difference in carbon numbers of side chains between the two polymers is one (Figure 4). However, as

homocrystallites (Table 3), showed a melting peak at around 195 °C. Small melting peaks of P(D-2H3MB) and/or P(L2H3MB) homocrystallites should have been included in the large melting peak of P(D-2H3MB)/P(L-2H3MB)/P(L-2HB) TSC crystallites. On the other hand, the precipitated P(D2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) blend (D), which contained large amounts of P(D-2H3MB)/P(L-2HB) HTSC crystallites and P(D-2H3MB) and/or P(L-2H3MB) homocrystallites and a small amount of P(L-2HB) homocrystallites (Table 3), showed a double melting peak at 179 and 197 °C. Due to a similar Tm values of P(D-2H3MB)/P(L-2HB) HTSC crystallites and P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, it is difficult to ascribe the double melting peak. Also, due to a very small amount of P(L-2HB) homocrystallites, its melting peak should not have been observed explicitly. The solvent evaporated and precipitated P(D-2H3MB)/P(L2H3MB)/PLLA (50/25/25) blends (E), which had P(D2H3MB), P(L-2H3MB), and PLLA homocrystallites (Table 3), showed melting peaks at 163 or 164 °C and 193 or 199 °C. The former and latter melting peaks are attributed to those of PLLA homocrystallites and P(D-2H3MB) and/or P(L2H3MB) homocrystallites, respectively. Due to imperfect separation of most melting peaks of respective crystalline species, we do not provide detailed discussion on the enthalpies of crystalline species. 3.3. QSC, TSC, HTSC, and HMSC Crystallizability. The present study revealed that TSC can be formed in P(D2H3MB)/P(L-2H3MB)/P(L-2HB) blends [and probably in P(D-2H3MB)/P(L-2H3MB)/P(D-2HB) blends] but not in P(D-2H3MB)/P(L-2H3MB)/PLLA blends [and probably not in P(D-2H3MB)/P(L-2H3MB)/PDLA blends]. Moreover, the present study exhibited that HTSC cannot be formed in P(D2H3MB)/PLLA blends [and probably not in P(D-2H3MB)/ PLLA blends]. The TSC and HTSC crystallizability of substituted and unsubstituted PLAs as well as their QSC and HMSC crystallizability reported so far and studies in the present study are summarized in Table 6. HMSC crystallization has been reported for three combinations of binary substituted or unsubstituted PLAs with identical chemical structures and opposite configurations [PLLA/PDLA,50 P(L-2HB)/P(D2HB),30 and P(L-2H3MB)/P(D-2H3MB)33,34] with the

Figure 4. Schematic representation of HTSC crystallizable substituted and unsubstituted PLA blends.

confirmed in the present study, the combination of PLLA/P(D2H3MB) [or PDLA/P(L-2H3MB)], whose difference in carbon numbers of side chains between the two polymers is two, cannot form HTSC, strongly suggesting that the difference in carbon numbers of side chains between the two components should be as low as one for HTSC crystallization. Similar to the results for HTSC crystallizability, TSC crystallization is reported for two combinations of ternary substituted and unsubstituted PLAs [PLLA/P(L-2HB)/P(D2HB) or PDLA/P(L-2HB)/P(D-2HB)40 and P(L-2HB)/P(L2H3MB)/P(D-2H3MB) or P(D-2HB)/P(L-2H3MB)/P(D2H3MB) (present study)], wherein the difference in carbon numbers of side chains between the two polymers with G



Article

2HB) (50/25/25) blend, P(D-2H3MB)/P(L-2HB) HTSC crystallites, P(D-2H3MB) and/or P(L-2H3MB) homocrystallites, and P(L-2HB) homocrystallites were formed without formation of TSC crystallites. This is the first report for TSC crystallization of all substituted PLAs with linear and branched side chains. In contrast, in both solvent evaporated and precipitated P(D-2H3MB)/P(L-2H3MB)/PLLA (50/25/25) blends, P(D-2H3MB)/P(L-2H3MB) HMSC crystallites, P(D2H3MB) and/or P(L-2H3MB) homocrystallites, and PLLA homocrystallites were formed without formation of TSC crystallites. It was confirmed that HTSC between P(D2H3MB) and PLLA is not formed in both solvent evaporated and precipitated P(D-2H3MB)/PLLA (50/50) blends. For the P(D-2H3MB)/P(L-2H3MB) (50/50) blends, solvent evaporation and precipitation correspondingly facilitated the HMSC crystallization and P(L-2H3MB) and P(D-2H3MB) homocrystallization, respectively. For the solvent evaporated and precipitated P(D-2H3MB)/P(L-2HB) (50/50) blends, main crystalline species were P(D-2H3MB)/P(L-2HB) HTSC crystallites. Based on the reported and present results, we proposed the rule for TSC and HTSC crystallization of respectively binary and ternary substituted and unsubstituted PLAs, wherein all the optically active polymer components are included in the same SC crystalline lattice: The difference in carbon numbers of side chains between the two polymers with different chemical structures and opposite configurations is one.

different chemical structures and opposite configurations is one (Figure 5). However, the polymer combination of PLLA/P(L-

Figure 5. Schematic representation of TSC crystallizability substituted and unsubstituted PLA blends.

2H3MB)/P(D-2H3MB) or PDLA/P(L-2H3MB)/P(D2H3MB) (present study), wherein the difference in carbon numbers of side chains between the two polymers with different chemical structures and opposite configurations is two, resulted in P(L-2H3MB)/P(D-2H3MB) HMSC crystallization and homocrystallization of PDLA, P(L-2H3MB), and P(D2H3MB). Furthermore, PLLA/P(D-2HB)/P(L-2H3MB) or PDLA/P(L-2HB)/P(D-2H3MB) blend, wherein the difference in carbon numbers of side chains between the two polymers with different chemical structures and opposite configurations is one, forms TSC.43 The aforementioned rule can be applied to the QSC crystallizability of P(L-2HB)/P(D-2HB)/P(L-2H3MB)/P(D2H3MB) blends, wherein the difference in carbon numbers of side chains between the two polymers with different chemical structures and opposite configurations is one.42 However, this rule cannot be applied to the quaternary PLLA/PDLA/P(L2HB)/P(D-2HB) blends, wherein the difference in carbon numbers of side chains between the two polymers with opposite configurations is one but PLLA/PDLA and P(L2HB)/P(D-2HB) HMSC crystallites were separately formed in the blends.44 Considering the reported and present results, TSC and HTSC from correspondingly ternary and binary substituted and unsubstituted PLAs, wherein all the polymer components are included in the same SC crystalline lattice, are formed when the difference in carbon numbers of side chains between the two polymers with opposite configurations is one.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01559. Magnified WAXD profile (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hideto Tsuji: 0000-0001-9986-5933 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This research was supported by JSPS KAKENHI Grant Number 16K05912. REFERENCES

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4. CONCLUSIONS The TSC crystallizability of ternary substituted and unsubstituted PLA blends composed of P(D-2H3MB), P(L2H3MB), and P(L-2HB) or PLLA, together with HTSC crystallizability of binary blends composed of P(D-2H3MB) and PLLA, was investigated for solvent evaporated and precipitated samples. For the solvent evaporated P(D2H3MB)/P(L-2H3MB)/P(L-2HB) (50/25/25) blend, formation of TSC crystallites with a very small amount of P(D2H3MB) and/or P(L-2H3MB) homocrystallites was observed, whereas in the precipitated P(D-2H3MB)/P(L-2H3MB)/P(LH



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Ternary Stereocomplex and Hetero-Stereocomplex Crystallizability of

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