Stereocomplex Crystallization between L- and D-Configured

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

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Stereocomplex Crystallization between L- and D‑Configured Staggered Asymmetric Random Copolymers Based on 2‑Hydroxyalkanoic Acids Hideto Tsuji,* Katsuya Osanai, and Yuki Arakawa Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan

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ABSTRACT: Stereocomplex (SC) crystallization in staggered asymmetric 2-hydroxyalkanoic acid-based random copolymer blends of L-configured poly(L-2-hydroxybutanoic acid-co-L-lactic acid) [P(L-2HB-co-LLA)] (50/50) and Dconfigured poly(D-2-hydroxybutanoic acid-co-D-2-hydroxy-3butanoic acid) [P(D-2HB-co-D-2H3MB)] (50/50) composed of monomers with three different chemical structures was reported for the first time. The wide-angle X-ray diffractometry profiles and interplanar distance values of SC crystallites of the staggered asymmetric random copolymer blends were similar to those of poly(L-2-hydroxybutanoic acid)/poly(D-2-hydroxybutanoic acid) SC crystallites. The melting temperature values of SC of copolymer blend samples were much higher than those of P(L-2HB-co-LLA) and P(D2HB-co-D-2H3MB) individual crystallites of the nonblended P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) samples (90.5 and 96.2−126.0 °C, respectively). Moreover, this article reports for the first time that the 2-hydroxyalkanoic acid-based random copolymer P(D-2HB-co-D-2H3MB) with 2-hydroxybutanoic acid unit content at approximately 50 mol % is crystallizable, strongly suggesting that the random copolymers of 2-hydroxybutanoic acid and 2-hydroxy-3-methylbutanoic acid with the identical configurations should be crystallizable at all 2-hydroxybutanoic acid unit contents. The crystallizability of the nonblended random copolymer P(D-2HB-co-D-2H3MB) was higher than that of the nonblended random copolymer P(L-2HBco-LLA). The present and reported results exhibit the high cocrystallizability of 2-hydroxyalkanoic acid monomer units with other types of 2-hydroxyalkanoic acid monomer units in their L- and D-configured homopolymer blends, nonblended L- or Dconfigured random copolymers, and L- and D-configured random copolymer blends.

1. INTRODUCTION Optically active isotactic poly(L-lactide) or poly(L-lactic acid) (PLLA) composed of one of 2-hydroxyalkanoic acids, L-lactic acid, has been attracting much attention, because of its producibility from renewable plant resources such as corn, sugar cane, tubers, and roots and the mechanical performance appropriate for a wide range of commodity articles.1−10 To widen its application range, stereocomplex (SC) crystallization of PLLA with its enantiomer, i.e., optically active isotactic poly(D-lactide) or poly(D-lactic acid) (PDLA) (Figure 1), has been extensively studied.11−20 SC crystallization between PLLA and PDLA enhances mechanical performance, and resistance to hydrolytic/thermal degradation.11−20 The effects of stereoblock copolymerization,12,13,17,21−27 thermal treatment conditions,12,17,28,29 shearing force,12,17,30−33 compression,17,34 layer-by-layer deposition,17,35,36 repeated casting,17,37 casting with solvent/nonsolvent mixtures,17,38 incorporated third polymers,17,39−47 and so on have been widely studied to enhance the SC crystallization. Recently, the interplay of polarization terahertz spectroscopy and solid-state density functional theory revealed that the © XXXX American Chemical Society

symmetry between L- and D-type helices of poly(lactide) or poly(lactic acid) (PLA) is not conserved in a SC system.48 Crystalline infrared spectroscopy indicated that PDLA and PLLA chains can coexist in the SC crystal lattice in the wide range of PLLA/PDLA from 70/30 to 30/70,49 and a new crystal model applicable to PLLA/PDLA SC for the wide range of PLLA/PDLA is proposed.50 Very recently, elastic modulus in the direction parallel to the chain axis was found to be higher for the PLLA/PDLA SC crystalline region (20 GPa) than for the homocrystalline regions of neat PLLA and PDLA (14 GPa).51 α-Hydroxy acid (or 2-hydroxyalkanoic acid) and α-amino acid-derived enantiomeric poly(lactic acid-coalanine)s were synthesized, and their SC crystallization and homocrystallization were investigated.52 However, blending alternating stereocopolymer poly(L-lactic acid-alt-D-lactic acid), i.e., syndiotactic PLA with isotactic PLA, PLLA, did not induce SC crystallization or their cocrystallization, in Received: June 6, 2018 Revised: August 7, 2018 Published: August 10, 2018 A

DOI: 10.1021/acs.cgd.8b00863 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

SC crystallization is not restricted to enantiomeric homopolymer blends or stereoblock copolymers of unsubstituted and substituted PLAs. SC crystallization occurs in enantiomeric symmetric 2-hydroxyalkanoic acid-based random copolymer blends of L-2-hydroxybutanoic acid and L-lactic acid [P(L-2HB-co-LLA)] and D-2-hydroxybutanoic acid and Dlactic acid [P(D-2HB-co-DLA)] composed of monomers with two different chemical structures, i.e., lactic acid (LA) and 2hydroxybutanoic acid (2HB) (Figure 2).17,61 The unsubsti-

Figure 1. Molecular structures of enantiomeric unsubstituted and substituted PLA homopolymers.

marked contrast with isotactic and syndiotactic poly(methyl methacrylate) blends.53 SC crystallization has been reported for enantiomeric, optically active isotactic substituted PLAs, or 2-hydroxyalkanoic acid-based homopolymers such as enantiomeric poly(2hydroxybutyrate)s or poly(2-hydroxybutanoic acid)s [P(2HB)s], i.e., poly(L-2-hydroxybutanoic acid) [P(L-2HB)] and poly(D-2-hydroxybutanoic acid) [P(D-2HB)]17,54 and enantiomeric poly(2-hydroxy-3-methylbutyrate)s or poly(2hydroxy-3-methylbutanoic acid)s [P(2H3MB)s], i.e., poly(L-2hydroxy-3-methylbutanoic acid) [P(L-2H3MB)] and poly(D2-hydroxy-3-methylbutanoic acid) [P(D-2H3MB)]17,55,56 (Figure 1). In addition to PLA SC, these SCs from substituted PLAs are called homo-stereocomplexes (HMSCs) because the enantiomeric polymers are composed of monomers with the identical chemical structures although their configurations are opposite. Hetero-stereocomplex (HTSC) is formed between optically active and isotactic P(2HB) and PLA17,57 or P(2H3MB)17,58 with the different chemical structures and opposite configurations. Ternary stereocomplex (TSC) crystallization occurs in the blends of L- and D-configured P(2HB)s [i.e., P(L-2HB) and P(D-2HB)] and L- or D-configured PLA (i.e., PLLA or PDLA),17,59 of L- and D-configured P(2H3MB)s [i.e., P(L-2H3MB) and P(D-2H3MB)] and L- or D-configured P(2HB),19 and of D-configured PDLA, L-configured P(L-2HB), and D-configured P(D-2H3MB).20 Moreover, PLA quaternary stereocomplex (QSC) is formed in the blends of L- and Dconfigured P(2HB)s and L- and D-configured P(2H3MB)s.17,60 However, in the blends of L- and D-configured PLAs and L- and D-configured P(2HB)s, not QSC crystallization but dual HMSC crystallization occurred.17,61 That is, HMSCs of Land D-configured PLAs and of L- and D-configured P(2HB)s were separately formed.

Figure 2. Molecular structures of random copolymer P(L-2HB-coLLA), its enantiomeric symmetric random copolymer P(D-2HB-coDLA), and staggered asymmetric random copolymer P(D-2HB-co-D2H3MB).

tuted and substituted PLAs-based SCs are summarized in Table 1.17,19,20,54−62 However, as far as we are aware, SC crystallization of L- and D-configured staggered asymmetric random copolymer blends composed of monomers with three different chemical structures, i.e., LA, 2HB, and 2-hydroxy-3methylbutanoic acid (2H3MB), has not been reported so far. In the cases of nonblended 2-hydroxyalkanoic acid-based random copolymers, they normally lose crystallizability when the monomer unit ratio approaches 50/50 (mol/mol), as in the case of poly(L-lactic acid-co-glycolic acid).63−67 Marubayashi et al. reported that L-lactic acid (LLA) units and some relatively small-sized 2-hydroxyalkanoic acid comonomer units of LLA-rich copolymers cocrystallize in the same lattice.66 Interestingly, some of 2-hydroxyalkanoic acid-based random copolymers, P(L-2HB-co-LLA)68 and P(D-2HB-co-DLA)61,69 are crystallizable for all monomer unit contents [0/100−100/0 (mol/mol)] and two types of monomer units cocrystallize in the same lattice.61,68,69 However, another example of a 2hydroxyalkanoic acid-based random copolymer which is crystallizable at a monomer unit ratio of approximately 50/ 50 and both monomer units cocrystallize in the same lattice has not been reported so far. We have been pursuing the limitation of polymer molecular structure and combination for individual and SC crystallizability of 2-hydroxyalkanoic acid-based optically active polymers. In the present study, we synthesized L- and Dconfigured staggered asymmetric 2-hydroxyalkanoic acid-based B



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Table 1. SC Crystallizability of Substituted and Unsubstituted PLAs or 2-Hydroxyalkanoic Acid-Based Polymers Reported so Far and Studied in the Present Studyb type of polymer chain 1

2

3

4

components

type of SC

type of chemical structure

carbon number of side chains

refs

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

HMSC HMSC HMSC HTSC HTSC RCSCa RCSCa

1 1 1 2 2 2 3

1 2 3 1, 2 2, 3 1,2 1, 2, 3

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

TSC TSC

2 2

1, 2 2, 3

62 54 55, 56 57 58 61 present study 59 19

TSC

3

1, 2, 3

20

HMSC × 2 QSC

2 2

1,2 2, 3

61 60

a

Random copolymer SC. bThis table is based on refs 19, 20, 54−62.

random copolymers, poly(L-2-hydroxybutanoic acid-co-L-lactic acid) [P(L-2HB-co-LLA)] (50/50) and poly(D-2-hydroxybutanoic acid-co-D-2-hydroxy-3-methylbutanoic acid) [P(D-2HBco-D-2H3MB)] (50/50) with a common monomer unit of 2HB and different monomer units of LA and 2H3MB (Figure 2). We report for the first time the example of SC crystallization between L- and D-configured staggered asymmetric 2-hydroxyalkanoic acid-based random copolymer blends of P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) composed of monomer units of three different chemical structures and an example of individual crystallization of 2hydroxyalkanoic acid-based random copolymer, P(D-2HB-coD-2H3MB) with the monomer unit ratio at approximately 50/ 50, using wide-angle X-ray diffractometry (WAXD) and differential scanning calorimetry (DSC). In the SC crystallites of the L- and D-configured staggered asymmetric random copolymers and the individual crystallites of the random copolymer, all types of monomer units were found to cocrystallize in the same crystalline lattice.

(0.0045 mol) of D-2-hydroxy-3-methylbutanoic acid were used for the synthesis of P(D-2HB-co-D-2H3MB). The synthesized P(L-2HB-coLLA) was purified by reprecipitation using chloroform and methanol (both guaranteed grade, Nacalai Tesque Inc.) as the solvent and nonsolvent, respectively, whereas the synthesized P(D-2HB-co-D2H3MB) was purified by extraction with methanol. The purified polymers were dried under reduced pressure for at least 6 days. The molecular characteristics of the polymers used in the present study are summarized in Table 2.

2. EXPERIMENTAL SECTION

The crystallization of unblended and blended polymers was carried out by the following three methods: solvent evaporation (casting), precipitation, and melt-crystallization. Equimolar binary polymer blend was prepared by the procedure stated in the previous papers.54,57,58 Briefly, each solution of the two enantiomeric polymers was prepared separately to have a polymer concentration of 0.5 g dL−1 and then admixed equimolarly based on the average molecular weights of monomer units of the random copolymers under vigorous stirring. Dichloromethane (guaranteed grade, Nacalai Tesque Inc.) was used as the solvent. The mixed solution was cast onto a Petri-dish, followed by solvent evaporation at 25 °C for approximately 1 day. The obtained polymer blend was further dried under reduced pressure at least 6 days. The precipitated samples were prepared by dissolving solvent evaporated samples at a concentration of 10 g dL−1 (25 mg/0.25 mL) and reprecipitation with stirred methanol (15 mL) as the nonsolvent.20,68 The melt-crystallization of the solvent evaporated samples sealed in test tubes under reduced pressure was performed at a crystallization temperature of 70 °C for 1 h after melting at 200 °C for 3 min.61 2.2. Physical Measurements and Observation. The weightand number-average molecular weights (Mw and Mn, respectively) of the polymers were evaluated in chloroform at 40 °C using a Tosoh (Tokyo, Japan) gel permeation chromatography (GPC) system with two TSK gel columns (GMHXL) and polystyrene standards. Therefore, the Mw and Mn values are given relative to polystyrene.61

Table 2. Characteristics of Polymers polymer P(L-2HB-coLLA) P(D-2HB-coD-2H3MB)

Mn × 10−3a Mw/Mna

2HB contentb (mol %)

[α]25589c (deg dm−1 g−1 cm3)

6.59

3.42

49.5

−134.4

7.16

1.60

49.9

89.3

a

Estimated by GPC. b2-Hydroxybutanoic acid (2HB) content estimated by 1H NMR. cMeasured in chloroform.

2.1. Materials. 2-Hydroxyalkanoic acid-based random copolymers, P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB), were synthesized by polycondensation of L-2-hydroxybutanoic acid [(S)-2hydroxybutyric acid] (≥97.0%, Sigma-Aldrich Co., Tokyo, Japan) and L-lactic acid and of D-2-hydroxybutanoic acid [(R)-2-hydroxybutyric acid] (≥98.0%, Sigma-Aldrich Co.) and D-2-hydroxy-3methylbutanoic acid [(R)-2-hydroxy-3-methylbutyric acid or D-αhydroxyisovaleric acid] (≥98.0%, Sigma-Aldrich Co.), respectively, using 5 wt % p-toluenesulfonic acid (monohydrate, guaranteed grade, Nacalai Tesque Inc., Kyoto, Japan) as the catalyst, as reported previously.70,71 The polycondensation reaction was performed at 130 °C under a nitrogen atmosphere and atmospheric pressure for 6 h for the synthesis of both polymers and then under reduced pressure of 2.0 kPa for 18 h for the synthesis of P(L-2HB-co-LLA) and of 1.0 kPa for 17 h for the synthesis of P(D-2HB-co-D-2H3MB). L-Lactic acid aqueous solution used for the synthesis of P(L-2HB-co-LLA) was prepared by hydrolytic degradation of L-lactide (1 g, PURASORB L, Purac Biomaterials, Gorinchem, The Netherlands) in the presence of 0.3 mL of water (HPLC grade, Nacalai Tesque Inc., Kyoto, Japan) at 98 °C for 0.5 h. A total of 0.536 g (0.0051 mol) of L-2hydroxybutanoic acid and 0.450 g (0.0060 mol) of L-lactic acid aqueous solution were used for the synthesis of P(L-2HB-co-LLA), and 0.468 g (0.0045 mol) of D-2-hydroxybutanoic acid and 0.530 g C



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Figure 3. 1H NMR spectra of P(L-2HB-co-LLA) (a) and P(D-2HB-co-D-2H3MB) (b). The specific optical rotation ([α]25589) of the polymers was measured in chloroform at a concentration of 1 g dL−1 and 25 °C using a JASCO (Tokyo, Japan) P-2100 polarimeter at a wavelength of 589 nm.61 The glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) and the enthalpies of cold crystallization and melting (ΔHcc and ΔHm, respectively) were determined with a Shimadzu (Kyoto, Japan) DSC-60 differential scanning calorimeter under a nitrogen gas flow at a rate of 50 mL min−1.61 The samples (ca. 3 mg) were heated from 0 to 185 or 250 °C at a rate of 10 °C min−1).60 WAXD was carried out at 25 °C using a RINT-2500 (Rigaku Co., Tokyo, Japan) equipped with a Cu−Kα source [wavelength (λ) = 1.5418 Å].61 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.19

3. RESULTS AND DISCUSSION 3.1. Monomer Compositions and Sequences of Copolymers. 1H and 13C NMR measurements were carried out to estimate the monomer compositions and sequences of the copolymers, respectively. Figure 3 shows the 1H NMR spectra of P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB). For P(L-2HB-co-LLA), the 1H NMR peaks at approximately 5.05, 1.99, and 1.03 ppm are attributed to methine (c), methylene (b), and methyl (a) protons of L-2-hydroxybutanoic acid (L-2HB) units, whereas those at approximately 5.17 and 1.59 ppm are ascribed to methine (e) and methyl (d) protons of LLA units, consistent with the reported values.68 For P(D2HB-co-D-2H3MB), the 1H NMR peaks at approximately 5.07, 1.99, and 1.04 ppm are attributed to methine (c), methylene (b), and methyl (a) protons of D-2-hydroxybutanoic acid (D-2HB) units, whereas those at approximately 4.98, 2.35, and 1.04 ppm are ascribed to methine (f), methylene (d), and methyl (e) protons of D-2-hydroxy-3-methylbutanoic acid (D2H3MB) units. It should be noted that the methyl (a) proton of D-2HB units and methyl (e) proton of D-2H3MB units appear at similar chemical shift values. Using the ascriptions of protons shown in Figure 3, the monomer unit contents of the copolymers were estimated and shown in Table 2. Figure 4 shows the 13C NMR spectra of P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) for carbonyl carbons. On the basis of the previous report,68 the 13C NMR peaks observed at approximately 169.6 and 169.1 ppm for P(L-2HB-co-LLA) are attributed to the carbonyl carbons of the two monomer unit sequences of same monomer types, LLA-LLA and L-2HB-L2HB sequences, respectively. In the expressions for monomer

Figure 4. 13C NMR spectra of P(L-2HB-co-LLA) and P(D-2HB-co-D2H3MB).

sequences in Figure 4 and the main text, two monomers are connected with ester groups with a direction of “-COO-” not “−OCO-”. On the other hand, based on the previous report,68 the 13C NMR peaks at approximately 169.8 and 168.9 ppm are assigned to the two monomer unit sequences of different monomer types, LLA-L-2HB and L-2HB-LLA sequences, respectively. If P(L-2HB-co-LLA) has a blocky or alternating monomer distribution, only one sharp peak should be observed for the LLA or L-2HB units. Therefore, the presence of two peaks for the LLA and L-2HB units and the similar peak areas of LLA-LLA and LLA-L-2HB sequences and of L-2HB-L-2HB and L-2HB-LLA indicate that P(L-2HB-co-LLA) had random distributions of two types of monomer units. On the other hand, for P(D-2HB-co-D-2H3MB), only two broad 13C NMR peaks were observed at approximately 169.2 and 168.7 ppm for carbonyl carbons. These two peaks are attributed to the carbonyl carbons of D-2HB and D-2H3MB units, respectively. Considering the peak widths of these broad peaks, it is expected that peaks of two sequential monomer units with the identical and the different chemical structures overlapped and appeared as seemingly broad peaks. Actually, the peak of D-2HB-D-2HB at approximately 169.1 ppm seems to be contained in the broad peak at approximately 169.2 ppm. To verify this, we performed the subtraction of the peak of LD



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Figure 5. WAXD profiles of solvent evaporated, precipitated, and melt-crystallized P(L-2HB-co-LLA) (a), P(D-2HB-co-D-2H3MB) (b), and their blends (c). The main diffraction peak angle of each crystalline species is shown with a dotted line.

2HB-L-2HB, which is an equivalent of the peak of D-2HB-D2HB, from the spectrum of P(D-2HB-co-D-2H3MB). Figure S1 shows the subtraction operation. As seen in this figure, the original D-2HB unit peak appears at 169.19 ppm, and the subtraction of the peak of L-2HB-L-2HB (D-2HB-D-2HB) sequences at 169.15 ppm in the range of 169.30−169.05 ppm multiplied by 0.6 leaves the peak at 169.23 ppm. The remaining peak at 169.23 ppm should be ascribed to D-2HBD-2H3MB sequences. If P(D-2HB-co-D-2H3MB) has a blocky or alternating monomer distribution, only one sharp peak should be observed for the D-2HB units. Therefore, the presence of two peaks (b) and (c) in Figure S1 for the D-2HB units and the similar areas of the peaks (b) and (c) strongly suggest the random monomer distribution of D-2HB units in P(D-2HB-co-D-2H3MB) and also that of D-2H3MB units. Considering this result, it is expected that the 13C NMR peak of D-2H3MB units at approximately 168.7 ppm should consist of two peaks of D-2H3MB-D-2H3MB and D-2H3MB-D-2HB sequences with higher and lower chemical shift values, respectively. 3.2. Wide-Angle X-ray Diffractometry. To estimate the crystalline species, interplanar distance (d), crystallinity of the samples, and WAXD measurements were performed. As seen in Figure 5a, P(L-2HB-co-LLA) was crystallizable only for a precipitated P(L-2HB-co-LLA) sample. The main crystalline diffraction peak angles (15.6° and 18.0°) of the precipitated P(L-2HB-co-LLA) sample were consistent with those reported for a precipitated P(L-2HB-co-LLA) with 2-hydroxybutanoic acid (2HB) unit content of 44 mol % (15.7° and 18.1°)68 and between the main crystalline diffraction peak angles reported for P(L-2HB) homopolymer (14.7° and 17.2°)54 and PLLA homopolymer (16.7° and 19.0°).29,72,73 These results confirm the previously reported result that P(L-2HB-co-LLA) with the 2HB unit content at approximately 50 mol % is crystallizable and L-2HB and LLA units cocrystallized in the same lattice.68 The noncrystallizability during melt-crystallization in the present study can be attributed to the slightly lower Tm (91 °C) compared to that reported for P(L-2HB-co-LLA) with 2HB content of 44 mol % (95 °C),68 which should have reduced the degree of supercooling. In contrast, all P(D-2HB-co-D-2H3MB) samples with 2HB content at approximately 50 mol % had crystalline diffraction peaks, indicating that P(D-2HB-co-D-2H3MB) samples were

crystallizable, irrespective of crystallization method (solvent evaporation, precipitation, and melt-crystallization) [Figure 5b]. The highest crystalline diffraction peak angles of the P(D2HB-co-D-2H3MB) samples (14.1°) were between those reported for the homopolymers of P(L-2HB) (14.7°)54 and P(L-2H3MB) (13.7° and 13.8°)56,74 and, therefore, those of their enantiomers P(D-2HB) and P(D-2H3MB). On the other hand, the second highest crystalline diffraction peak angles of the solvent evaporated and precipitated P(D-2HB-co-D2H3MB) samples (16.8°) were lower than that reported for P(L-2H3MB) homopolymer (17.0° and 17.1°)56,74 but the second highest crystalline diffraction peak angle of the meltcrystallized P(D-2HB-co-D-2H3MB) sample (17.2°) was higher than that reported for the P(L-2HB) homopolymer (17.0°).54 The former finding for the highest crystalline peak anlges indicates that D-2HB and D-2H3MB units cocrystallized in the same lattice, whereas the latter finding for the second highest crystalline peak angles reflects that the crystalline modification of the P(D-2HB-co-D-2H3MB) samples are slightly different from those of P(L-2HB) [and, therefore, its enantiomer P(D-2HB)] and P(L-2H3MB), and, therefore, its enantiomer P(D-2H3MB)]. This is the first report that optically active 2HB and 2-hydroxy-3-methylbutanoica acid (2H3MB) units cocrystallize in the random P(D2HB-co-D-2H3MB) copolymer. All staggered asymmetric random copolymer blend samples had crystalline diffraction peaks at approximately 11°, 19°, and 21° [Figure 5c], whose patterns are similar to those reported for unsubstituted and substituted PLA SCs,12,17 revealing the SC crystallization between the staggered asymmetric random copolymers of P(L-2HB-co-LLA) and P(D-2HB-co-D2H3MB). The highest SC diffraction peak angles of the blend samples and the third highest SC diffraction peak angles of the blend samples (10.6° and 21.4°, respectively) are similar to those of P(L-2HB)/P(D-2HB) SC crystallites (10.7° and 21.5°, respectively)54 and, as expected, are lower than those of PLLA/PDLA SC crystallites (11.9° and 24.0°, respectively)75,76 and higher than those of P(L-2H3MB)/P(D2H3MB) SC crystallites (9.7° and 19.4°, respectively).56 The former result is indicative of the fact that the SC crystalline lattice sizes of staggered asymmetric random copolymer blend samples are similar to those of P(L-2HB)/P(D-2HB) SC crystallites. In addition to SC crystalline peaks, the precipitated E



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Table 3. Interplanar Distance (d) Values of Samples d (Å) [2θ (deg)] sample P(L-2HB-co-LLA)

P(D-2HB-co-D-2H3MB)

P(L-2HB-co-LLA)/P(D-2HB-co-D-2H3MB) blend

PLLAd P(L-2HB)f P(L-2H3MB)g PLLA/PDLA SCd P(L-2HB)/P(D-2HB) SCf P(L-2H3MB)/P(D-2H3MB) SCg

crystallization solvent evaporation precipitation melt solvent evaporation precipitation melt solvent evaporation precipitation melt melt (120 °C)e melt (70 °C)e melt (100 °C)e melt (120 °C)e melt (130 °C)e melt (130 °C)e

2θ ≈10.7°a

2θ ≈14.1°b

2θ ≈15.7°c

2θ ≈16.8 or 17.3°b

5.66 [15.65] 5.26 [16.86] 5.28 [16.79] 5.14 [17.24]

8.25 [10.72]

6.30 [14.05] 6.28 [14.10] 6.31 [14.03]

8.31 [10.65] 8.35 [10.59]

6.30 [14.07] 6.28 [14.10]

4.14 [21.46] 5.69 [15.57]

5.26 [16.86]

5.32 [16.68]

7.41 [11.94] 8.30 [10.66] 9.14 [9.68]

2θ ≈ 21.4°a

4.92 [18.02]

6.23 [14.21]

6.04 [14.66] 6.45 [13.72]

2θ ≈ 18.0°c

4.14 [21.46] 4.17 [21.32] 4.66 [19.06]

5.16 [17.18] 5.22 [16.98] 3.71 [23.98] 4.14 [21.46] 4.57 [19.42]

a

Peaks of SC crystallites. bPeaks of P(D-2HB-co-D-2H3MB), P(L-2HB), or P(L-2H3MB) individual crystallites. cPeaks of P(L-2HB-co-LLA) or PLLA individual crystallites. eThe values in the parentheses are crystallization temperatures. dRef 75. fRef 54. gRef 56.

different from those of P(D-2HB) and P(D-2H3MB) homocrystallites, regardless of the crystallization method, and the crystal modification of the solvent evaporated and precipitated P(D-2HB-co-D-2H3MB) samples is different from that of the melt-crystallized P(D-2HB-co-D-2H3MB) sample. The d values of the highest SC crystalline peaks (8.25, 8.31, and 8.35 Å) and the SC crystalline peaks approximately 21° (4.14, 4.14, and 4.17 Å) of precipitated, solvent evaporated, and melt-crystallized blend samples are similar to those of P(L2HB)/P(D-2HB) SC crystallites (8.30 and 4.14 Å, respectively),54 higher than those of PLLA/PDLA SC crystallites (7.41 and 3.71 Å, respectively)75 and lower than those of P(L2H3MB)/P(D-2H3MB) SC crystallites (9.14 and 4.57 Å, respectively).56 These results are indicative of the fact that 2H3MB, 2HB, and LA units cocrystallized in P(L-2HB-coLLA)/P(D-2HB-co-D-2H3MB) SC crystallites and that P(L2HB-co-LLA)/P(D-2HB-co-D-2H3MB) SC crystallites had the crystalline lattice sizes similar to those of P(L-2HB)/P(D2HB) SC crystallites, despite the fact that the blend samples were composed of the random copolymers of P(L-2HB-coLLA) and P(D-2HB-co-D-2H3MB), not homopolymers of P(L-2HB) and P(D-2HB). Considering the similar lattice sizes between P(L-2HB-co-LLA)/P(D-2HB-co-D-2H3MB) SC crystallites and P(L-2HB)/P(D-2HB) SC crystallites, it is expected that in the P(L-2HB-co-LLA)/P(D-2HB-co-D-2H3MB) SC crystallites, the D-2H3MB units with a large-sized isopropyl side chain in the D-configured polymer neighbor the LLA units with a small-sized methyl side chain in the L-configured polymer and vice versa to avoid the steric hindrance of the large-sized isopropyl side chain in the D-configured polymer. As a result, the D-2HB units with an ethyl side chain in the Dconfigured polymer adjoin the L-2HB units with the samesized ethyl side chain in the L-configured polymer. The Xc values of P(L-2HB-co-LLA), P(D-2HB-co-D2H3MB) individual crystallites, and SC crystallites [Xc(L), Xc(D), and Xc(SC), respectively] of the samples evaluated from the WAXD profiles in Figure 5 are summarized in Table

blend sample had crystalline diffraction peaks of P(D-2HB-coD-2H3MB) individual crystallites at 14.0° and 16.8° and of P(L-2HB-co-LLA) individual crystallites at 15.7°, whereas the solvent evaporated and melt-crystallized blend samples had a smaller crystalline diffraction peak of P(D-2HB-co-D-2H3MB) individual crystallites at 14.0° but no crystalline diffraction peak of P(L-2HB-co-LLA) individual crystallites. We do not discuss the SC crystalline peak at approximately 19° because the peak here was double or very broad depending on the crystallization method, which should have been caused by a disordered lattice structure and/or small crystalline regions. The interplanar distance (d) values of the samples were estimated from the WAXD profiles in Figure 5 and are tabulated in Table 3. The d values of the highest and the second highest peak of the precipitated P(L-2HB-co-LLA) sample (5.66 and 4.92 Å, respectively) were very similar to those reported for precipitated P(L-2HB-co-LLA) sample with a 2HB content of 44 mol % (5.65 and 4.93 Å, respectively), wherein cocrystallization of two monomer units occurred.68 This confirms the cocrystallization of two monomer units in the P(L-2HB-co-LLA) copolymer samples in the present study. Although the d values of the highest peak of the precipitated, solvent evaporated, and melt-crystallized P(D-2HB-co-D2H3MB) samples (6.23, 6.30, and 6.28 Å) were higher than that reported for P(L-2HB) (6.04 Å), which is the enantiomer of P(D-2HB)54 and lower than that reported for P(L-2H3MB) (6.45 Å), which is the enantiomer of P(D-2H3MB),56 supporting the cocrystallization of two monomer units in the P(D-2HB-co-D-2H3MB) samples. The d values of the second highest peak of the solvent evaporated and precipitated P(D2HB-co-D-2H3MB) sample (5.28 and 5.26 Å, respectively) were higher than that reported for P(L-2H3MB) (5.22 Å),56 which is the enantiomer of P(D-2H3MB), whereas the d value of the second highest peak of the melt-crystallized P(D-2HBco-D-2H3MB) sample (5.14 Å) was lower than that of P(L2HB) (5.16 Å),54 which is the enantiomer of P(D-2HB). Interestingly, these findings affirm that the crystalline modifications of P(D-2HB-co-D-2H3MB) samples are slightly F



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Figure 6. Crystallinity (Xc) of solvent evaporated, precipitated, and melt-crystallized P(L-2HB-co-LLA) (a), P(D-2HB-co-D-2H3MB) (b), and their blends (c). Xc(L), Xc(D), and Xc(SC) are Xc values of P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) individual crystallites and SC crystallites, respectively. Xc(tot) = Xc(L) + Xc(D) + Xc(SC).

Figure 7. DSC thermograms of solvent evaporated, precipitated, and melt-crystallized P(L-2HB-co-LLA) (a), P(D-2HB-co-D-2H3MB) (b), and their blends (c).

respectively) [Figure 6b] much higher than those of P(L2HB-co-LLA) samples (0, 25, and 0% for solvent evaporation, precipitation, and melt-crystallization, respectively) [Figure 6a]. The latter finding indicates the higher crystallizability of P(D-2HB-co-D-2H3MB) than that of P(L-2HB-co-LLA). The Xc(D) values of the precipitated and melt-crystallized P(D2HB-co-D-2H3MB) samples were much higher than that of

S1 and are plotted in Figure 6. Again, as seen in Figure 6a, P(L-2HB-co-LLA) was crystallizable only during precipitation and had a positive Xc(L) value of 25%. In contrast, P(D-2HBco-D-2H3MB) was crystallizable, irrespective of crystallization method (solvent evaporation, precipitation, and melt-crystallization), and had positive Xc(D) values (11, 48, and 57% for solvent evaporation, precipitation, and melt-crystallization, G



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Table 4. Thermal Properties of Samples sample

crystallization

P(L-2HB-co-LLA)

solvent evaporation precipitation melt solvent evaporation precipitation

P(D-2HB-co-D2H3MB)

blend

melt solvent evaporation precipitation melt

Tga (°C)

Tcca (°C)

Tm(L)a (°C)

Tm(D)a (°C)

Tm(SC)a (°C)

ΔHccb (J g−1)

ΔHm(L)b (J g−1)

ΔHm(D)b (J g−1)

ΔHm(SC)b (J g−1)

ΔH(tot)c (J g−1)

46.0 42.4 38.1 37.4

90.5 67.9

30.2

102.4

21.1 31.7

83.8

d 30.1

88.3

89.2

13.2 118.1, 124.4 96.2, 125.6 126.0 127.0 124.1 126.8

13.2

−2.5

10.1

7.6

−0.1

20.1

20.0

183.7

−0.9

23.9 4.3

26.5

23.9 29.9

181.0 181.5

−1.4

3.7 4.0

35.1 30.5

40.3 33.1

1.5

a

Tg, Tcc, Tm(L), Tm(D), and Tm(S) are glass transition and cold crystallization temperature, and melting temperatures of P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) individual crystallites and SC crystallites, respectively. bΔHcc, ΔHm(L), ΔHm(D), and ΔHm(SC) are enthalpies of cold crystallization and melting of P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) individual crystallites and SC crystallites, respectively. cΔH(tot) = ΔHcc + ΔHm(L) + ΔHm(D) + ΔHm(SC). dThe peak was too broad to estimate Tg.

co-LLA) sample with 2HB content of 44 mol % (95 °C).68 For all P(D-2HB-co-D-2H3MB) samples, melting peaks were observed [Tm(D) = 118−126 °C] [Figure 7b and Table 4]. The Tm(D) values of random P(D-2HB-co-D-2H3MB) copolymer are reported for the first time in the present study, and these Tm(D) values were between the Tm values reported for the homopolymers of P(D-2HB) (77−111 °C)77 and P(D-2H3MB) (147−178 °C).56 The lower temperature melting subpeak observed at around 100 °C for the precipitated P(D-2HB-co-D-2H3MB) sample can be due to the imperfect crystallites. In addition to Tm(L) and Tm(D) values of some blend samples, all P(L-2HB-co-LLA)/P(D2HB-co-D-2H3MB) blend samples had Tm(SC) values (181− 184 °C), which are much higher than Tm(L) and Tm(D) values of the P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) samples (91 and 96−126 °C, respectively) [Figure 7c and Table 4]. This finding supports the SC crystallization in the blend samples. The basic trend for the dependence of ΔHm values on sample type and crystallization methods are similar to that observed for the Xc values in Figure 6 and Table S1. 3.4. SC Crystallization and Individual Crystallization of 2-Hydroxyalkanoic Acid-Based Random Copolymers. In the previous paper, SC crystallization not only between random copolymers, P(L-2HB-co-LLA) and poly(D-2-hydrolxybutyrate-co-D-lactic acid) P(D-2HB-co-DLLA), but also between a homopolymer P(L-2HB) or PLLA and the random copolymer P(D-2HB-co-DLA) was reported.61 In these blends, quaternary monomer units (L-2HB, D-2HB, LLA and DLA) or ternary monomer units (L-2HB or LLA, D-2HB and DLA) cocrystallized in SC crystallites, but these monomer units had only two different chemical structures (2HB and LA). The present article reports for the first time SC crystallization of staggered asymmetric random copolymers of L-configured P(L2HB-co-LLA) and D-configured P(D-2HB-co-D-2H3MB). In the SC crystallites of the staggered asymmetric random copolymers with opposite configurations, four types of monomers, i.e., D-2H3MB, L-2HB, D-2HB, and LLA, were contained. These monomer units have three different chemical structures (2H3MB, 2HB, and LA). Regarding the SC crystallites of unsubstituted and substituted PLA homopolymers, three homopolymers, P(D-2H3MB), P(L-2HB), and PDLA, form TSC crystallites, wherein monomer units with three different chemical structures (2H3MB, 2HB, and LA) are

solvent-evaporated P(D-2HB-co-D-2H3MB) sample. This result confirms the result previously reported for P(L-2HBco-LLA) that the crystallizability of copolymers were much higher for precipitation and melt-crystallization than for solvent evaporation.68 As seen in Figure 6c, all staggered asymmetric random copolymer blend samples had the positive Xc(SC) values, and Xc(SC) was higher for the melt-crystallized blend sample (36%) than for the solvent evaporated and precipitated blend samples (25 and 24%, respectively). In the case of solvent evaporation, the P(L-2HB-co-LLA) sample did not crystallize individually [Xc(L) = 0%] and the P(D-2HB-co-D-2H3MB) sample crystallized individually to have a low Xc(D) of 11%, whereas due to the high crystallizability of SC crystallites, the Xc(SC) and Xc(tot) [= Xc(L) + Xc(D) + Xc(SC)] values of the blend sample were as high as 24 and 29%, respectively. In the case of precipitation, in agreement with the precipitated P(L2HB-co-LLA) sample, only the precipitated blend sample had a low X c (L) value (3%), affirming the low individual crystallizability of P(L-2HB-co-LLA). The lower Xc(L) and Xc(D) values of the precipitated blend sample (3 and 20%, respectively) compared to those in the precipitated P(L-2HBco-LLA) and P(D-2HB-co-D-2H3MB) samples (25 and 48%) are attributed to the incorporation of most P(L-2HB-co-LLA) and P(D-2HB-co-D-2H3MB) segments into SC crystallites, leaving a small amount of P(L-2HB-co-LLA) and P(D-2HB-coD-2H3MB) segments for the formation of their individual crystallites. In the case of melt-crystallization, the Xc(D) value of the blend sample (2%) was much lower than that in the P(D-2HB-co-D-2H3MB) sample (57%), which can be ascribed to the same reason for the low Xc(L) and Xc(D) values of the precipitated blend sample. 3.3. Differential Scanning Calorimetry. For estimation of thermal properties of the samples, DSC measurements were performed (Figure 7). The obtained thermal properties, Tg, Tcc, and Tm of P(L-2HB-co-LLA), P(D-2HB-co-D-2H3MB), and SC [Tm(L), Tm(D), and Tm(SC), respectively], ΔHcc, and ΔHm of P(L-2HB-co-LLA), P(D-2HB-co-D-2H3MB), and SC [ΔH m (L), ΔHm (D), and ΔH m (SC), respectively] are summarized in Table 4. Consistent with the WAXD result, for P(L-2HB-co-LLA) samples, only the precipitated sample had a melting peak at Tm(L) = 91 °C [Figure 7a and Table 4], which is similar to that reported for the precipitated P(L-2HBH



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contained.20 Therefore, the present article reports the first example of SC crystallites composed of monomer units with three different chemical structures from two staggered asymmetric random copolymers not from three different homopolymers. It is probable that only L-2HB unit sequences in P(L-2HBco-LLA) and D-2HB unit sequences in P(D-2HB-co-D2H3MB) form P(L-2HB)/P(D-2HB) SC crystallites. Considering the reported result that the minimum monomer unit sequence length of 7 or more is required for the formation of PLLA/PDLA SC crystallites,78 we assumed the same minimum monomer unit sequence length for P(L-2HB)/P(D-2HB) SC crystallites. On the basis of this assumption and the L-2HB unit fraction in P(L-2HB-co-LLA) or the D-2HB unit fraction in P(D-2HB-co-D-2H3MB) (approximately 0.5), the fraction of segments that can form P(L-2HB)/P(D-2HB) SC crystallites equals 0.57 ≈ 0.0078. This fraction is far below the Xc(SC) values (24−36%) in the present study, strongly supporting the incorporation of LLA and D-2H3MB units in SC crystallites in addition to L-2HB and D-2HB units. The previous paper reported that the nonblended random copolymers of chemically synthesized P(L-2HB-co-LLA)68 or biosynthesized P(D-2HB-co-DLA)69 were crystallizable for all 2HB unit contents (0−100%). In other words, 2HB and LA units of the nonblended random copolymers can cocrystallize in the same lattice at an arbitrary 2HB unit content. In the present article, the nonblended random P(D-2HB-co-D2H3MB) copolymer at approximately 2HB unit content of 50 mol % was crystallizable, regardless of crystallization method (solvent evaporation, precipitation, and melt-crystallization), which is reported for the first time. The crystallizabilty and crystallinity of the nonblended random P(D-2HB-co-D-2H3MB) copolymer were higher than those of the nonblended random P(L-2HB-co-LLA) copolymer which was crystallized only by precipitation to have the low crystallinity value. Although the effects of 2HB unit content on the crystallizability of nonblended random copolymers were not investigated in the present study, the crystallizability of nonblended P(D-2HB-co-D-2H3MB) with the 2HB unit content at approximately 50 mol % strongly suggests that the random copolymers of 2HB and 2H3MB with the identical configurations should be crystallizable at all 2HB unit contents (0−100%). Unsubstituted and substituted LAs (LA, 2HB, and 2H3MB) are 2-hydroxyalkanoic acids. HTSC crystallization between optically active isotactic 2-hydroxyalkanoic acid-based homopolymers with the opposite configurations (L- and Dconfigurations) and different chemical structures such as between PLA and P(2HB)19,57 or between P(2HB) and P(2H3MB)19,58 as well as TSC19,59 and QSC60 crystallization of PLA, P(2HB), and P(2H3MB) indicate the high cocrystallizability of L- and D-configured 2-hydroxyalkanoic acid-based homopolymers. Furthermore, at the monomer unit ratio of approximately 50/50, the cocrystallization of monomer units takes place in nonblended 2-hydroxyalkanoic acid-based random copolymers of P(L-2HB-co-LLA)68 and P(D-2HB-coDLA), 61,69 and P(L-2HB-co-LLA)/P(D-2HB-co-DLA) blends61 in the previous study and in the nonblended random copolymer of P(D-2HB-co-D-2H3MB) and P(L-2HB-coLLA)/P(D-2HB-co-D-2H3MB) blends in the present study. The present and reported results indicate that the exceptionally high cocrystallizability of 2-hydroxyalkanoic acid monomer units with other types of 2-hydroxyalkanoic acid monomer

units in their L- and D-configured homopolymer blends, nonblended L- or D-configured random copolymers, and L- and D-configured random copolymer blends.

4. CONCLUSIONS By the use of WAXD and DSC, this article reports for the first time SC crystallization in staggered asymmetric 2-hydroxyalkanoic acid-based random copolymer blends of L-configured P(L-2HB-co-LLA) and D-configured P(D-2HB-co-D-2H3MB) with monomer units of three different chemical structures. The WAXD profiles and d values of SC crystallites of the staggered asymmetric random copolymer blend samples were similar to those of P(L-2HB)/P(D-2HB) SC crystallites. The Tm(SC) values (181.0−183.7 °C) of the copolymer blend samples were much higher than Tm(L) and Tm(D) values of the P(L-2HBco-LLA) and P(D-2HB-co-D-2H3MB) samples (90.5 and 96.2−126.0 °C, respectively). Additionally, this article reports for the first time that the 2-hydroxyalkanoic acid-based random copolymer P(D-2HB-co-D-2H3MB) with the 2HB unit content at approximately 50 mol % is crystallizable, strongly suggesting that the random copolymers of 2HB and 2H3MB with the identical configurations should be crystallizable at all 2HB unit contents. The crystallizability of the nonblended random copolymer P(D-2HB-co-D-2H3MB) was higher than that of the nonblended random copolymer P(L-2HB-co-LLA). The present and reported results exhibit that the high cocrystallizability of 2-hydroxyalkanoic acid monomer units with other types of 2-hydroxyalkanoic acid monomer units in their L- and D-configured homopolymer blends, nonblended Lor D-configured random copolymers, and L- and D-configured random copolymer blends.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00863. Peak analysis of 13C NMR spectrum, crystallinity (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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Stereocomplex Crystallization between L- and D-Configured

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