Stereocomplex Crystallization of Star-Shaped Four-Armed Stereo

Article Cite This: J. Phys. Chem. B XXXX, XXX, XXX-XXX

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Stereocomplex Crystallization of Star-Shaped Four-Armed Stereo Diblock Poly(lactide) from the Melt: Effects of Incorporated Linear One-Armed Poly(L‑lactide) or Poly(D‑lactide) Hideto Tsuji,* Ryota Ozawa, and Yuki Arakawa 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: Star-shaped four-armed stereo diblock poly(lactide) (4-LD) and linear one-armed PLLA or PDLA (1-L or 1-D) having a molecular weight similar to that of 4-LD [higher molecular weight 1-L(H) or 1-D(H)] and that of one block of 4-LD [lower molecular weight 1-L(L) or 1-D(L)] were synthesized, and the effects of incorporated 1-L or 1-D on the isothermal and nonisothermal crystallization of 4-LD blends from the melt were investigated. Solely stereocomplex crystallites were formed in unblended 4-LD and 4-LD blends incorporated with 1-L or 1-D during isothermal and nonisothermal crystallization. Incorporated 1-L or 1-D increased normalized stereocomplex crystallinity and accelerated cold nonisothermal crystallization and isothermal crystallization. The accelerating effect became higher with decreasing the molecular weight of 1-L or 1-D. The crystalline growth mechanism was not altered by the incorporation of 1-L and 1-D, whereas the crystalline growth geometry changed from line to sphere or circle, depending on the type of sample and Tc. The difference in crystallization half time and cold crystallization temperature between 4-LD/1-L(H) and 4-LD/ 1-D(H) blends or 4-LD/1-L(L) and 4-LD/1-D(L) blends was explained by the difference in radial growth rate and spherulite density, which was further discussed considering the non-interpenetrating and interpenetrating models.

1. INTRODUCTION The improvement of mechanical properties and thermal/ hydrolytic degradation-resistance of biobased biodegradable poly(L-lactide), i.e., poly(L-lactic acid) (PLLA), is a crucial issue for actualization of its widespread use as an alternative material to petro-derived polymers. 1−5 A promising means for improving mechanical properties and thermal/hydrolytic degradation-resistance of PLLA-based biodegradable materials is stereocomplex (SC) formation with poly(D-lactide), i.e., poly(D-lactic acid) (PDLA), which is an enantiomer of PLLA.6−13 SC crystallization occurs in enantiomeric polymer blends of PLLA and PDLA and in stereoblock poly(lactide) [i.e., poly(L-lactic acid) (PLA)] copolymers.6−13 Besides, binary, ternary, and quaternary SC crystallization of optically active unsubstituted and/or substituted PLAs, and their copolymers having opposite configurations and identical or two dif ferent chemical structures, has been reported and extensively studied.14−29 Recently, ternary SC crystallization of PDLA, poly(L-2-hydroxybutanoic acid), and poly(D-2hydroxy-3-methylbutanoic acid) having three dif ferent chemical structures was reported.30 The results reported here strongly suggest that an optically active polymer (L-configured or Dconfigured polymer) like unsubstituted or substituted optically active poly(lactic acid)s can act as “a configurational or helical molecular glue” for two oppositely configured optically active © XXXX American Chemical Society

polymers (two D-configured polymers or two L-configured polymers) to allow their cocrystallization.30 The various parameters such as molecular weight, optical purity, branching, stereo block architecture, and their mixed effects affect SC crystallization behavior.6−13 It was found that low molecular weight, high optical purity, and stereo diblock copolymerization facilitate SC crystallization,6−13 whereas the branching architecture disturbs SC crystallization.31 Although stereoblock architecture facilitates SC crystallization,6−13,32 SC crystallization rates of relatively low molecular weight stereo diblock copolymers are lower than those of PLLA/PDLA blends.33,34 Melting temperature (Tm) or crystalline thickness of branched PLLA/PDLA blends is determined by the molecular weight of one arm, not by the total molecular weight.31 Moreover, 4-L and 4-D having amorphous poly(DLlactide) [i.e., poly(DL-lactic acid) (PDLLA)] blocks at the core and shell were synthesized and the effects of the position of amorphous PDLA blocks were investigated.35 The PDLLA blocks at the shell strongly disturbed the homocrystallization of unblended block copolymers compared to those at the core, whereas PDLLA blocks had very weak disturbing effects on SC Received: July 27, 2017 Revised: September 21, 2017 Published: September 21, 2017 A

DOI: 10.1021/acs.jpcb.7b07420 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

higher number-average molecular weight (Mn) values of about 1.4 × 104 mol−1] and that of one block of 4-LD [abbreviated as 1-L(L) or 1-D(L) with lower number-average molecular weight (Mn) values of about 2 × 103 mol−1] were synthesized and the effects of incorporation of 1-L or 1-D, molecular weight of incorporated 1-L or 1-D, and the disagreement and agreement of configuration of shell of 4-LD with 1-L or 1-D on the isothermal and nonisothermal crystallization of 4-LD from the melt were investigated using wide-angle X-ray diffractometry (WAXD), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). A blending ratio of 4LD and 1-D or 1-L (w/w = 2/1) was selected assuming that all 1-L and 1-D molecules form SC crystallites correspondingly with the PDLA shell and the PLLA core of 4-LD. On the basis of the obtained results, the effects of incorporation of 1-L or 1D, molecular weight and configuration of 1-L or 1-D, and crystallization temperature (Tc) on the crystallization behavior are discussed.

crystallization of enantiomeric block copolymer blends, irrespective of the position of amorphous PDLLA blocks. The multiplicate effects of branching architecture and block copolymerization have been studied.32,36−41 In the case of equimolar one-, three, and six-armed stereo diblock PLLA-bPDLA copolymers (1-LD, 3-LD, and 6-LD, respectively), the higher arm number disturbed the SC crystallization of stereo diblock copolymers during cooling, when compared at the similar total molecular weights.32 For equimolar four-armed stereo diblock PLLA-b-PDLA (4-LD) copolymers, both branching and diblock architectures disturbed the SC crystallization and spherulite growth of equimolar 4-LD copolymers and the disturbance effect was higher for branching architecture than for diblock architecture.36 Also, simultaneous SC crystallization and homocrystallization was highly hindered in nonequimolar 4-LD copolymers with L-lactyl unit contents of about 30 and 70% for isothermal crystallization directly from the melt, slow cooling from the melt, and precipitation.37,38 Interestingly, blending of equimolar 4-LD (four-armed stereo diblock copolymers of PLLA and PDLA having PDLA at the shell) and equimolar 4-DL (four-armed stereo diblock copolymers of PDLA and PLLA having PLLA at the shell) increased spherulite nuclei and accelerated the overall SC crystallization.39 Recently, click chemistry was applied for the synthesis of PLA stereoblock copolymers,40 which is expected to facilitate the synthesis of various types of PLA stereoblock copolymers. Normally, for preparation of stereocomplexationable materials, blending between enantiomeric polymers, PLLA and PDLA homopolymers, or synthesis of PLA stereoblock copolymers has been performed.6−13 However, there are few reports on the combination of PLLA or PDLA homopolymer and PLA stereoblock copolymer. Exceptions are summarized as follows.41−43 For the crystallization of equimolar 4-LD/linear one-armed PLLA (1-L) and 4-LD/linear one-armed PDLA (1D) blends (w/w = 50/50) during solvent evaporation, SC crystallites were formed as the main crystalline species, although the crystallinity was lower than those of 1-L/1-D blends, and 1-L or 1-D/1-LD blends.41 Moreover, homocrystallites were formed in equimolar 4-LD/higher molecular weight one-armed 1-D, in contrast with equimolar 4-LD/higher molecular weight one-armed 1-L, wherein no homocrystallites were formed.41 Also, for high molecular weight linear onearmed 1-L or 1-D/equimolar 4-LD blends (w/w = 95/5), the incorporated 4-LD can act as a crystallization accelerating agent for homocrystallization of high molecular weight 1-L or 1-D and such an effect was higher for 4-LD added to 1-L than for 4LD added to 1-D, which difference should be caused by the configurational disagreement and agreement of the shell of 4LD (i.e., PDLA block) with 1-L and 1-D, respectively.42 Recently, SC crystallization of solution-cast and melt-crystallized multiwalled carbon nanotube (CNT) grafted poly(Llactide)-block-poly(D-lactide) copolymer (CNT-L-D), 1-L/ CNT-L-D, and 1-D/CNT-L-D blends was investigated.43 It was found that, for 1-D/CNT-L-D blends, SC crystallization mainly occurs between PLLA blocks of CNT-L-D and free 1-D molecules, and for 1-L/CNT-L-D blends, SC crystallization mainly occurs between PLLA blocks and PDLA blocks in CNT-L-D.43 In the present study, star-shaped four-armed stereo diblock 4-LD and higher and lower molecular weight linear one-armed 1-L and 1-D were synthesized to have a molecular weight similar to that of 4-LD [abbreviated as 1-L(H) or 1-D(H) with

2. EXPERIMENTAL SECTION 2.1. Materials. 1-L and 1-D with Mn values of about 1.4 × 104 and 2 × 103 g mol−1 were synthesized by bulk ring-opening polymerization of L- and D-lactides (2 g) (PURASORB L and D, Purac Biochem BV, Gorinchem, The Netherlands) initiated with 0.03 wt % tin(II) 2-ethylhexanoate (Nacalai Tesque, Inc., Kyoto, Japan) in the presence of different amounts of 1propanol (7.5 or 60 mg, Tokyo Chemical Industry, Co., Ltd., Tokyo, Japan) as the co-initiator at 140 °C for 10 h.31 Tin(II) 2-ethylhexanoate was used after purification by distillation under reduced pressure. The precursor of 4-LD [i.e., fourarmed PLLA (4-L)] was synthesized according to the same procedure with 1-L using 34 mg of pentaerythritol as coinitiator, instead of 1-propanol.31,44 Synthesized 1-L, 1-D, and 4-L polymers were purified by reprecipitation using chloroform and methanol as the solvent and nonsolvent, respectively, and then dried in vacuo for at least 6 days. 4-LD was synthesized by ring-opening polymerization of D-lactide (500 mg) in toluene (4 mL) initiated with 0.3 wt % tin(II) 2-ethylhexanoate in the presence of the purified and dried 4-L (500 mg) as the coinitiator at 120 °C for 36 h.36−39 After removal of toluene, synthesized 4-LD polymers were purified by precipitation using a mixed solvent of chloroform (Guaranteed grade, Nacalai Tesque Inc.)/1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (HPLC grade, Nacalai Tesque Inc.) (vol/vol = 95/5) and methanol as the solvent and nonsolvent, respectively, and then dried in vacuo for at least 6 days. HFIP was added to chloroform to increase the solubility of SC crystallites. The molecular characteristics of polymers synthesized in the present study are summarized in Table 1, and the 1H NMR spectrum of 4-LD is shown in Figure S1. Figure S1 indicates the successful incorporation of pentaerythritol units in 4-LD or synthesis of star-shaped 4-LD. For preparation of blend samples (4-LD and 1-L or 1-D) (w/ w = 2/1), separately prepared solutions of two polymers were mixed vigorously, followed by solvent evaporation. The mixed solvent of chloroform/HFIP (vol/vol = 95/5) was utilized as a solvent. For preparation of isothermally crystallized samples, each sample (3 mg) packed in a DSC aluminum pan was sealed in a test tube under reduced pressure, melted at 220 °C for 150 s, crystallized at different Tc values of 100−180 °C for 3 h, and quenched at 0 °C for at least 5 min to stop further crystallization. For preparation of melt-quenched samples, each sample packed in a DSC aluminum pan was sealed in a B


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The Journal of Physical Chemistry B Table 1. Molecular Characteristics of Synthesized Polymers code 4-L 4-LD 1-L(H) 1-D(H) 1-L(L) 1-D(L)

Mn(NMR) (g/mol) 7.4 1.5 1.3 1.4 2.0 1.8

× × × × × ×

a

103 104 104 104 103 103

Mn(GPC) (g/mol) 1.3 2.2 2.5 2.6 2.6 2.7

× × × × × ×

M n(NMR) = M(coinitiator) + (144.1/2)

a

104 104 104 104 103 103

× f [(I1 + I2)/I2]

Mw(GPC)/Mn(GPC)a 1.10 1.17 1.33 1.35 1.32 1.34

(1)

where M(co-initiator) is the molecular weight of initial coinitiator (60.1 and 136.2 g mol−1 for 1-propanol (1-L and 1-D) and pentaerythritol (4-L and 4-LD, respectively), 144.1 g mol−1 is the molecular weight of L- and D-lactides, and f is the arm number (1 and 4 for 1-L and 1-D and for 4-L and 4-LD, respectively). An increase was observed for the Mn(NMR) value of 4-LD after the second-step polymerization, in agreement with the result for the Mn(GPC) value (Table 1). The specific optical rotation ([α]25589) values of 4-LD and 4L polymers were measured in the mixed solvent of chloroform/ HFIP (vol/vol = 95/5), 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. The L-lactyl unit contents of 4-LD polymers were estimated by the following equation36−39

a Mn and Mw are number- and weight-average molecular weights, respectively.

test tube under reduced pressure, melted at 220 °C for 150 s, and quenched at 0 °C for at least 5 min. 2.2. Physical Measurements and Observation. The weight- and number-average molecular weight [Mw(GPC) and Mn(GPC), respectively] values of the synthesized polymers were evaluated in chloroform at 40 °C by a Tosoh (Tokyo, Japan) gel permeation chromatography (GPC) system with two TSK gel columns (GMHXL) using polystyrene standards. Therefore, the molecular weights are those relative to polystyrene. For GPC sample preparation, the mixed solvent of chloroform/HFIP (vol/vol = 95/5) was used for 4-LD, whereas chloroform was used for 1-L, 1-D, and 4-L. The estimated molecular characteristics of the synthesized polymers are summarized in Table 1. The Mn(GPC) values of 4-LD were higher than those of precursor 4-L, indicating the successful synthesis of four-armed stereo diblock copolymers. Also, the Mn(NMR) values of the synthesized polymers were determined from the 400 MHz 1H NMR spectra obtained in deuterated chloroform (50 mg mL−1) by a Bruker BioSpin (Kanagawa, Japan) AVANCE III 400 using tetramethylsilane as the internal standard. Due to the overlapping of peaks for the methine protons of the lactyl unit at the chain terminal and HFIP, we used only deuterated chloroform as the solvent, even for 4-LD. The Mn(NMR) values were estimated according to the following equation using the peak intensities for methine protons of lactyl units inside the chain (I1) and at the hydroxyl terminal (I 2 ), observed at around 5.2 and 4.4 ppm, respectively31,36−39

L‐lactyl unit content

(%)

= 100 × {[α]25589 (PLLA) + [α]25589 } /{2 × [α]25589 (PLLA)}

(2)

where [α]25589(PLLA) is that of 4-L (−162.4 deg dm−1 g−1 cm3). Using eq 2 and [α]25589 for 4-LD (−9.0 deg dm−1 g−1 cm3), the L-lactyl unit content of 4-LD was evaluated to be 52.8%. The glass transition, cold crystallization, melting temperatures (Tg, Tcc, and Tm, respectively), and enthalpies of cold crystallization and the melting (ΔHcc and ΔHm, respectively) of samples for heating were determined with a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter under a nitrogen gas flow at a rate of 50 mL min−1. About 3 mg of melt-quenched samples were heated at a rate of 10 °C min−1 from ambient temperature to 230 °C. The Tg, Tcc, Tm, ΔHcc, and ΔHm values were calibrated using tin, indium, and benzophenone as standards. By definition, ΔHcc and ΔHm are negative and positive, respectively. The crystalline species and crystallinity (Xc) values of the samples were estimated by the use of WAXD. The WAXD measurements were performed at 25 °C using a RINT-2500 (Rigaku Co., Tokyo, Japan)

Figure 1. WAXD profiles of samples crystallized at Tc = 140 °C (a) and 100 °C (b) from the melt. C


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Figure 2. Non-normalized (a) and normalized (b) crystallinity of SC crystallites [Xc(S)] of samples crystallized from the melt as a function of crystallization temperature (Tc).

Figure 3. DSC thermograms of samples crystallized at Tc = 140 °C (a) and quenched (Tc = 0 °C) from the melt (b).

fractions of L- and D-lactyl units of the blends, it is surprising that all samples had the crystalline diffractions only at 2θ = 12, 21, and 24° specific to SC crystallites,6−13,45 except for unblended 4-LD at Tc = 120 °C and 4-LD/1-D(L) at Tc = 160 °C, wherein a very small main peak of homocrystallites was observed at 2θ = 17°.45−47 This result indicates that only SC crystallites were formed in almost all of the samples crystallized from the melt, irrespective of the presence or absence of 1-L or 1-D, molecular weight and configuration of 1-L or 1-D, and Tc. The non-normalized crystallinities of SC crystallites and homocrystallites [Xc (S) and X c (H), respectively] were estimated from the WAXD profiles in Figure 1 and are summarized in Table S1. Non-normalized Xc(S) and Xc(H) values of the blend samples were normalized by the maximum fraction of SC crystallites, according to the following equations, assuming that 4-LD contains equimolar L- and D-lactyl units, PLLA and PDLA molecules, and segments form SC crystallites as much as possible, and remaining excess PLLA or PDLA molecules or segments form homocrystallites:

equipped with a Cu Kα source (λ = 1.5418 Å), which was operated at 40 kV and 200 mA. The isothermal spherulite growth of the samples was observed using an Olympus (Tokyo, Japan) polarized optical microscope (BX50) equipped with a heating−cooling stage and a temperature controller (LK-600PM, Linkam Scientific Instruments, Surrey, U.K.) under a constant nitrogen gas flow. The samples were heated from room temperature to 220 °C at 100 °C min−1, held at this temperature for 2 min, cooled at 100 °C min−1 to an arbitrary Tc, and then held at the same temperature (spherulite growth was observed here). With the same heating and cooling program utilized for the observation of isothermal spherulite growth, the overall crystallization behavior of samples during isothermal crystallization was monitored by measuring the light intensity transmitted through the samples using BX-50 equipped with LK-600PM and an As One (Osaka, Japan) LM-331 photometer.

3. RESULTS 3.1. Wide-Angle X-ray Diffractometry. To investigate the crystalline species and Xc of the isothermally crystallized samples, WAXD measurements were performed. Figure 1 shows typical WAXD profiles of samples crystallized at Tc = 140 and 100 °C for 3 h from the melt. Despite nonequimolar D

Normalized Xc(S) = 3/2 × Non‐normalized Xc(S)

(3)

Normalized Xc(H) = 3 × Non‐normalized Xc(H)

(4)


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Figure 4. Polarized photomicrographs of unblended 4-LD (a), 4-LD/1-L(H) (b), 4-LD/1-D(L) (c), 4-LD/1-L(L) (d), and 4-LD/1-D (e) blends crystallized at 140 °C for 5 min from the melt.

D(L) blends at Tc = 140 °C estimated by WAXD measurements, very small cold crystallization and melting peaks at around 80 and 130 °C, respectively, are ascribed to those of homocrystallites. Actually very similar absolute values are noticed for ΔHcc and ΔHm of homocrystallites (Table S2), reflecting that homocrystallites were formed during DSC heating scan, not during isothermal crystallization. For melt-quenched samples (Figure 3b), glass transition, cold crystallization, and melting were observed at 44−54, 81−100, and 182−188 °C, respectively. Considering the relatively high Tm values (182−188 °C), the melting peaks are attributed to SC crystallites, not homocrystallites, exhibiting only SC crystallites were formed in melt-quenched samples during DSC heating. The Tcc values became lower when linear onearmed 1-L or 1-D was incorporated, and the lowering effect was higher for the lower molecular weight 1-L(L) or 1-D(L) than for the higher molecular weight 1-L(H) or 1-D(H). These results are indicative of the fact that linear one-armed 1-L or 1D accelerated cold SC crystallization and such an accelerating effect was higher for the lower molecular weight 1-L(L) or 1D(L) with higher segmental mobility, which should have accelerated cold crystallization and thereby should have given the lower Tcc values. When compared between the 4-LD blends with similar molecular weight 1-L and 1-D [i.e., between 4-LD/ 1-D(L) blend and 4-LD/1-L(L) blend or between 4-LD/1D(H) blend and 4-LD/1-L(H) blend], Tcc was lower for 4-LD/ 1-D(L) blend than for 4-LD/1-L(L) blend but was lower for 4LD/1-L(H) blend than for 4-LD/1-D(H) blend. 3.3. Polarized Optical Microscopy. To inquire about the crystallization behavior of spherulites, POM observation was performed. Figure 4 shows the polarized optical photomicrographs of the samples crystallized at 140 °C for 5 min from the melt. Compared to unblended 4-DL, 4-LD blends with 1-L or 1-D had larger sized spherulites and the shperulite size was larger for 4-LD blends with lower molecular weight 1-L(L) or 1-D(L) than for 4-LD blends with the higher molecular weight of 1-L(H) or 1-D(H). All samples contained spherulites with Maltese crosses, indicating the lamella orientation along the radial direction in the spherulites. The radial growth rate of

The thus obtained normalized Xc(S) and Xc(H) values are tabulated in Table S1 and non-normalized and normalized Xc(S) values are plotted as a function of Tc in Figure 2. As seen in Table S1, Xc(H) values were nil, except for very small positive values of unblended 4-LD at Tc = 120 °C and 4-LD/1D(L) blend at Tc = 160 °C. Also, as seen in Figure 2, nonnormalized Xc(S) values were similar for all of the samples, regardless of the presence or absence of 1-L or 1-D, molecular weight and configuration of 1-L or 1-D, and Tc, whereas normalized Xc(S) values of blends [4-LD/1-L(L), 4-LD/1D(L), 4-LD/1-L(H), and 4-LD/1-D(H) blends] were higher than those of unblended 4-LD, indicating that linear one-armed 1-L or 1-D formed SC crystallites with 4-LD copolymer and the incorporation of 1-L or 1-D facilitated SC crystallization compared to that in unblended 4-LD. The molecular weight and configurational effects of 1-L and 1-D were insignificant on non-normalized and normalized Xc(S) values. 3.2. Differential Scanning Calorimetry. To explore the thermal properties of samples isothermally crystallized from the melt and to monitor nonisothermal crystallization behavior of melt-quenched samples during heating, DSC measurements were carried out. Parts a and b of Figure 3 correspondingly show the typical DSC thermograms of the samples isothermally crystallized at Tc = 140 °C and quenched samples from the melt. Thermal properties estimated from DSC thermograms are summarized in Table S2. As seen in Figure 3a, only large melting peaks of SC crystallites were observed in the range 145−191 °C for almost all of the samples, except for 4-LD/1L(L) and 4-LD/1-D(L) blends with very small cold crystallization peaks at around 80 °C and very small melting peaks of homocrystallites at around 130 °C (the peaks with an arrow). Large peak areas of melting of SC crystallites and no or negligibly very small peak areas of cold crystallization and melting of homocrystallites at Tc = 100−160 °C support the results by WAXD measurements that SC crystallites were predominantly formed in the samples crystallized from the melt, regardless of the presence or absence of 1-L or 1-D, molecular weight and configuration of 1-L or 1-D, and Tc. Considering nil Xc(H) values of 4-LD/1-L(L) and 4-LD/1E


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Figure 5. Radial growth rate of spherulites (G) (a) and ln G + 1500/R(Tc − T∞) (b) of samples as functions of Tc and (TcΔTf)−1, respectively.

Table 2. Maximum G [G(max)], Tc Which Gives G(max) [Tc(max)], Front Constant (G0), and Nucleation Constant (Kg) of Samples Crystallized from the Melt

spherulites (G) was estimated from the POM photomicrographs taken at different crystallization time (tc) values, and the obtained G values are plotted in Figure 5a as a function of Tc. It is seen that the G values of unblended 4-LD were the lowest among the samples. Incorporation of linear one-armed 1-L or 1-D increased G values of 4-LD blends compared to those of unblended 4-LD, and the accelerating effect was higher for the lower molecular weight 1-L(L) or 1-D(L) than for the higher molecular weight 1-L(H) or 1-D(H), in agreement with the sizes of spherulites shown in Figure 4. The higher acceleration of SC spherulite growth by lower molecular weight 1-L(L) or 1-D(L) should be due to their higher segmental mobility compared to that of higher molecular weight 1-L(H) or 1D(H). When compared between the 4-LD blends with similar molecular weight 1-L and 1-D, G values were higher for 4-LD/ 1-D(L) blend than for 4-LD/1-L(L) blend, whereas G values of 4-LD/1-L(H) blend were similar to those of 4-LD/1-D(H) blend at Tc ≥ 140 °C but slightly higher than those of 4-LD/1D(H) blend at Tc ≤ 130 °C. The nucleation constant (Kg) and the front constant (G0) of the samples were estimated using the nucleation theory established by Hoffman et al.,48,49 in which G can be expressed by the following equation G = G0 exp[−U */R(Tc − T∞)] exp[−K g /(TcΔTf )]

code 4-LD 4-LD/1-L(H) 4-LD/1-D(H) 4-LD/1-L(L) 4-LD/1-D(L)

Tc(max) (°C) 120 130 130 130 130

G(max) (μm min−1)

G0 (μm min−1)

4.2 6.6 5.8 14.9 18.4

× × × × ×

1.08 5.57 1.75 9.23 3.75

15

10 1012 1013 1012 1013

Kg (K2) 1.34 1.05 1.10 1.05 1.11

× × × × ×

106 106 106 106 106

Also, the spherulite number per unit area (spherulite density) was evaluated from the photomicrographs at 1, 2, 4, 4, 10, 12, and 20 min at 120, 130, 140, 150, 160, 170, and 180 °C, respectively, for the area 311 × 233 μm2 (larger than the area shown in Figure 4). The evaluated values are tabulated in Table 3. Compared to unblended 4-LD, 4-LD blend with higher molecular weight 1-L(H) or 1-D(H) had higher spherulite density values at Tc = 140−170 °C, except for 4-LD/1-L(H) blend at Tc = 170 °C, whereas 4-LD blends with lower molecular weight 1-L(L) or 1-D(L) had lower spherulite density values at Tc = 120−170 °C. This result reflects that the spherulite density depended on the molecular weight of 1-L or 1-D. When compared between the 4-LD blends with similar molecular weight 1-L and 1-D, spherulite density values were higher for 4-LD/1-D(H) blend than for 4-LD/1-L(H) blend, but those of 4-LD/1-L(L) and 4-LD/1-D(L) blends did not show the clear trend or were similar to each other. However, for the latter result, it is expected that a higher number of spherulites at lower Tc will give the result with higher reliability. Considering this expectation and spherulite density values at lower Tc = 120 and 130 °C, 4-LD/1-L(L) blend should have had higher spherulite density than that of 4-LD/1-D(L) blend. 3.4. Overall Crystallization Behavior. The overall isothermal crystallization behavior of the samples at different Tc values was estimated by the time change of light intensity transmitted through a sample (I) using a polarized optical microscope. I increases with an increase in crystallinity and finally levels off when crystallization completes. We used the I defined by the following equation as an index of relative crystallinity (Xr)33,51,52

(5)

where ΔT is supercooling Tm0 − Tc when Tm0 is equilibrium Tm, f is the factor expressed by 2Tc/(Tm0 + Tc) which accounts for the changes in the heat of fusion as the temperature is decreased below Tm0, U* is the activation energy for the transportation of segments to the crystallization site, R is the gas constant, and T∞ is the hypothetical temperature where all motion associated with viscous flow ceases. The ln G + 1500/ R(Tc − T∞) of the samples are plotted in Figure 5b as a function of 1/(TcΔTf), using Tm0 = 279 °C50 of SC crystallites, U* = 1500 cal mol−1, and T∞ = Tg − 30 K.48 Also, as Tg values, those of melt-quenched samples were used [51, 51, 54, 46, and 44 °C for unblended 4-LD, 4-LD/1-L(H), 4-LD/1-D(H), 4LD/1-L(L), and 4-LD/1-D(L) blends, respectively]. The plot in Figure 5b gives Kg as a slope and ln G0 as an intercept, and thus estimated Kg and G0 values are tabulated in Table 2. All samples had only one Kg value and the Kg values of 4-DL blends with linear one-armed 1-L or 1-D (1.1 × 106 K2) were similar to those of unblended 4-DL (1.3 × 106 K2), indicating that the crystalline growth mechanism was not altered by the incorporation of 1-L and 1-D, molecular weight and configuration of 1-L and 1-D, and Tc.

X r (%) = 100(It − I0)/(I∞ − I0)

(6)

where It and I0 are the I values at tc = t and 0, respectively, and I∞ is the I value when it levels off. The typical Xr values at Tc = 140 °C are plotted in Figure 6a as a function of tc. Similar to the F


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The Journal of Physical Chemistry B Table 3. Spherulite Density of Samples Crystallized from the Melt spherulite density (number mm−2) Tc (°C) 120 130 140 150 160 170 180 a

4-LD 4.3 2.4 5.7 3.8 2.8 1.1

× × × × × ×

103 103 102 102 102 102

4-LD/1-L(H) >3.5 >1.0 1.1 1.2 2.5 1.8 3.3

× × × × × × ×

4-LD/1-D(H)

103 a 103 a 103 102 102 102 102

>5.5 >1.7 >1.2 >1.2 >1.2 >9.0

× × × × × ×

4-LD/1-L(L)

103 a 103 a 103 a 103 a 103 a 102 a

1.6 6.4 2.8 1.7 1.2 3 6

× × × × × × ×

4-LD/1-D(L)

103 102 102 102 102 101 101

7.1 3.7 2.9 2.2 4 4 1

× × × × × × ×

102 102 102 102 101 101 101

Uncountable unified spherulites are observed. Due to the reason, actual spherulite density should be higher.

Figure 6. Relative crystallinity (Xr) (a) and Avrami plot (b) of samples crystallized at Tc = 140 °C from the melt.

results of Tcc and G in Figures 3b and 5a, respectively, overall crystallization rate was the lowest for unblended 4-LD among the samples and increased by the incorporation of linear onearmed 1-L or 1-D. The overall crystallization rate was higher for 4-LD blends with lower molecular weight 1-L(L) or 1-D(L) than for 4-LD blends with the higher molecular weight 1-L(H) or 1-D(H), due to the same reason stated for the comparison of Tcc and G values. Isothermal crystallization kinetics traced by light intensity measurements was analyzed with the Avrami theory,53−55 which is expressed by the following equation 1 − X r (%)/100 = exp(−ktc n)

Table 4. Avrami Exponent (n), Crystallization Rate Constant (k), and Crystallization Half Time [tc(1/2)] of Samples Crystallized from the Melt code 4-LD

4-LD/1-L(H)

(7)

where k is the crystallization rate constant. Equation 7 can be transformed to eq 8: log[− ln(1 − X r /100)] = log k + n log tc

4-LD/1-D(H)

(8)

In addition, crystallization half time [tc(1/2)] was calculated using the following equation: tc(1/2) = [(ln 2)/k]1/ n

4-LD/1-L(L)

(9)

To avoid deviation from the theoretical curves, as suggested by Mandelkern et al. and Lorenzo et al.,56,57 we used Xr in the range 3−20% for estimating n and k. Typical Avrami plots at Tc = 140 °C with eq 8 are shown in Figure 6b. The plots with eq 8 give n as a slope and log k as an intercept. The thus obtained n and k values and the tc(1/2) values evaluated using eq 9 are listed in Table 4, and tc(1/2) values are plotted in Figure 7 as a function of Tc. The n values of unblended 4-LD are in the range of 2.3−3.5, whereas those of 4-LD blends with linear 1-L or 1D were 2.1−3.8, 1.8−2.9, 1.8−3.4, and 2.3−3.0 for 4-LD/1L(H), and 4-LD/1-D(H), 4-LD/1-L(L), and 4-LD/1-D(L) blends, respectively. Assuming the thermal nucleation,58 the n values of 2−3 or 2−4 of the samples mean the change from line

4-LD/1-D(L)

Tc (°C)

n

120 130 140 150 160 120 130 140 150 160 120 130 140 150 160 120 130 140 150 160 120 130 140 150 160

3.46 2.32 3.34 2.53 2.56 2.92 3.75 2.74 2.12 2.11 1.79 2.87 2.77 2.85 2.53 1.76 1.78 2.06 3.36 1.79 2.28 2.78 3.04 2.68 2.75

k (min−n) 2.23 7.44 1.06 2.47 7.02 7.60 1.31 1.74 5.21 5.83 1.03 3.41 2.98 1.82 5.35 3.95 1.07 1.01 1.15 5.99 1.14 3.89 6.80 4.97 8.94

× × × × × × × × × × × × × × × × × × × ×

10−4 10−3 10−3 10−4 10−6 10−2 10−3 10−2 10−4 10−4 10−1 10−2 10−2 10−3 10−4 10−1 10−1 10−1 10−2 10−3

× × × ×

10−2 10−2 10−3 10−4

tc(1/2) (min) 10.2 7.06 6.97 23.1 89.3 2.13 5.32 3.84 29.8 28.7 2.91 2.86 3.11 8.05 17.0 1.38 2.86 2.55 3.39 14.2 0.804 2.82 2.15 6.31 11.2

growth geometry to circular growth geometry or from line growth geometry to spherical growth geometry, depending on the type of sample and Tc. G


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melt-quenched samples, as reported for neat PLLA,59 should have suppressed the difference in the spherulite density values between 4-LD/1-L(H) and 4-LD/1-D(H) blends. In the case of 4-LD blends with lower molecular weight 1-L(L) or 1-D(L), despite the higher G values of 4-LD/1-D(L) blend compared to those of 4-LD/1-L(L) blend, the similar tc(1/2) values of 4LD/1-L(L) and 4-LD/1-D(L) blends are attributable to the higher spherulite density values of 4-LD/1-L(L) compared to those of 4-LD/1-D(L). On the other hand, the lower Tcc values of 4-LD/1-D(L) blend compared to those of 4-LD/1-L(L) blend can explain by the higher G values of the former compared to those of the latter. If the initial state before nucleation or crystallization is assumed that 1-L or 1-D and 4-LD exist separately without interpenetrating (non-interpenetrating model), it is expected that linear one-armed 1-L (not 1-D) can readily diffuse into and interact with the PDLA shell of 4-LD, resulting in faster SC nucleation or crystallization. This can explain the formation of homocrystallites in addition to that of SC crystallites reported for 4-LD/higher molecular weight 1-D during solvent evaporation.41 In the present study, the higher G values of 4LD/1-L(H) than for 4-LD/1-D(H) at Tc below 130 °C and the higher SC spherulite density values for 4-LD/1-L(L) than for 4LD/1-D(L) may be explained by the facile molecular diffusion of 1-L(L) and its interaction with the PDLA shell of 4-LD in the non-interpenetrating model. However, this model can be applied only for the solution-casting during solvent evaporation, although it is probable that such an effect on crystallization during solvent evaporation remains and affects the meltcrystallization behavior. After solution casting as in the present study, SC crystallization should have completed. Therefore, the initial state before nucleation or crystallization from the melt is assumed that 1-L or 1-D and 4-LD interpenetrate or overlap each other (interpenetrating model), and the memory effect of crystalline regions after melting determines the nucleation or crystallization rate. Considering the shorter distance from the branching or cross-linking point in the co-initiator moiety of the PLLA core of 4-LD copolymer compared to that of the PDLA shell, the former SC crystallites with 1-D located near the branching or cross-linking points, which restrict the segmental or molecular motion, should have a higher memory effect of SC crystallites after melting, resulting in rapid nucleation or crystallization compared to that of the SC crystallites with 1-L located apart from the branching or crosslinking points. If the memory effect in the interpenetrating model is strong, rapid nucleation or crystallization will occur. This can

Figure 7. Crystallization half time [tc(1/2)] of samples crystallized from the melt.

The tc(1/2) values of 4-LD blends with linear one-armed 1-L or 1-D were shorter than those of unblended 4-LD when compared at the same Tc, except for 4-LD/1-L(H) at Tc = 150 °C, and the tc(1/2) values were shorter for 4-LD blends with lower molecular weight 1-L(L) or 1-D(L) than for 4-LD blends with higher molecular weight 1-L(H) or 1-D(H), in agreement with the results of Tcc and G in Figures 3b and 5a. When compared between the 4-LD blends with similar molecular weight 1-L and 1-D, the lowering effect was higher for 1-D(H) than for 1-L(H) but was similar for 1-L(L) and 1-D(L). 3.5. Comparison between the Effects of 1-L and 1-D on SC Crystallization Behavior. The comparison of G, spherulite density, tc(1/2), and Tcc values of 4-LD/1-L(L) or 1D(L) and 4-LD/1-L(H) or 1-D(H) blends are summarized in Table 5. Basically, G and spherulite density determine overall crystallization rate and thereby Tcc and tc(1/2). High G and spherulite density increase the overall crystallization rate and thereby decrease Tcc and tc(1/2). In the case of 4-LD blends with higher molecular weight 1-L(H) or 1-D(H), despite the fact that the G values of 4-LD/1-D(H) blend were similar to or lower than those of 4-LD/1-L(H) blend, shorter tc(1/2) values of 4-LD/1-D(H) blend compared to those of 4-LD/1-L(H) blend, except for that at the low Tc = 120 °C, can be ascribed to higher spherulite density values of 4-LD/1-D(H) blend. On the other hand, the lower Tcc values of 4-LD/1-L(H) blend compared to those of 4-LD/1-D(H) blend are attributable to the higher G values at lower Tc of 4-LD/1-L(H) blend, which can be expected from the higher G values below 130 °C. It is probable that the very high spherulite density values of the

Table 5. Orders of Properties of 4-LD/1-L(H) or 1-D(H) and 4-LD/1-L(L) or 1-D(L) Blends property

ordera

4-LD/1-L(H) or 1-D(H)

G

4-LD/1-L(L) or 1-D(L)

spherulite density tc(1/2) Tcc G spherulite density tc(1/2) Tcc

4-LD/1-L(H) ≈ 4-LD/1-D(H) (Tc ≥ 140 °C) 4-LD/1-L(H) > 4-LD/1-D(H) (Tc ≤ 130 °C) 4-LD/1-D(H) > 4-LD/1-L(H) 4-LD/1-D(H) < 4-LD/1-L(H) (except for Tc = 120 °C) 4-LD/1-L(H) < 4-LD/1-D(H) 4-LD/1-D(L) ≥ 4-LD/1-L(L) 4-LD/1-L(L) > 4-LD/1-D(L) 4-LD/1-L(L) ≈ 4-LD/1-D(L) 4-LD/1-D(L) < 4-LD/1-L(L)

sample type

a

The samples on the left have crystallization rates higher than or similar to those on the right, except for spherulite density. In the case of spherulite density, the samples on the left have values higher than or similar to those on the right. H


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compared to those of 4-LD/1-D(L) blend. On the other hand, the lower Tcc values of 4-LD/1-D(L) blend compared to those of 4-LD/1-L(L) blend can be explained by the higher G values of the former compared to those of the latter. The different effects of incorporated 1-L and 1-D on SC nucleation or spherulite growth were further discussed considering the diffusion effect in the non-interpenetrating model and the memory effect in the interpenetrating model.

explain the SC crystallization between PLLA blocks of CNT-LD and free 1-D molecules for 1-D/CNT-L-D blends and that between PLLA blocks and PDLA blocks in CNT-L-D for 1-L/ CNT-L-D blends.43 In the present study, The higher SC spherulite density values of 4-LD/1-D(H) blend compared to those of 4-LD/1-L(H) blend and the higher G values of 4-LD/ 1-D(L) blend compared to those of 4-LD/1-L(L) blend can be explained by the difference in the memory effect in the interpenetrating model. However, the higher G values of 4-LD/1L(H) than for 4-LD/1-D(H) at Tc below 130 °C and the higher SC spherulite density for 4-LD/1-L(L) than for 4-LD/1D(L) cannot be explained by the interpenetrating model. The notable result observed in the present study is that all the orders of G, spherulite density, tc(1/2), and Tcc could not be explained by only the memory effect in the interpenetrating model or the diffusion effect of the non-interpenetrating model. It is probable that the very high crystallizability of SC crystallites35 reduced the difference in the interaction between the PDLA shell of 4-LD with 1-L and between the PLLA core of 4-LD with 1-D and thereby lowered the difference in SC nucleation or crystallization rates between 4-LD/1-L and 4-LD/1-D blends. Further detailed experiments and accumulation of information are required to elucidate what parameter induces the inconsistent orders of G, spherulite density, tc(1/2), and Tcc.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.7b07420. Figure showing the 1H NMR spectrum of 4-LD and tables showing crystallinity (Xc) values and thermal properties (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-532-44-6922. ORCID

Hideto Tsuji: 0000-0001-9986-5933 Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS Star-shaped four-armed 4-LD and linear one-armed 1-L or 1-D having a molecular weight similar to that of 4-LD [1-L(H) or 1D(H)] and that of one block of 4-LD [1-L(L) or 1-D(L)] were synthesized, and the effects of incorporated 1-L or 1-D on the isothermal and nonisothermal crystallization of 4-LD from the melt were investigated. Solely SC crystallites were formed in 4LD blends incorporated with linear one-armed 1-L or 1-D as well as unblended 4-LD during isothermal and nonisothermal crystallization, irrespective of the presence or absence of 1-L or 1-D, molecular weight and configuration of 1-L or 1-D, and Tc, with the exception of a trace amount of homocrystallites in two samples at the certain Tc values. Incorporated linear one-armed 1-L or 1-D increased normalized Xc(S) and accelerated cold nonisothermal crystallization and isothermal crystallization monitored by Tcc, G, and Xr. The accelerating effect was higher for lower molecular weight 1-L(L) or 1-D(L) compared to that of higher molecular weight 1-L(H) or 1-D(H), due to the higher segmental mobility of the former compared to that of the latter. The crystalline growth mechanism was not altered by the incorporation of 1-L and 1-D, molecular weight and configuration of 1-L and 1-D, and Tc. However, the crystalline growth geometry changed from line to circle or from line to sphere, depending on the type of sample and Tc. The difference in crystallization half time and cold crystallization temperature between 4-LD/1-L(H) and 4-LD/1-D(H) blends or 4-LD/1L(L) and 4-LD/1-D(L) blends was explained by the difference in G and spherulite density as follows. The shorter tc(1/2) values of 4-LD/1-D(H) blend compared to those of 4-LD/1L(H) blend can be ascribed to higher spherulite density values of 4-LD/1-D(H) blend, whereas the lower Tcc values of 4-LD/ 1-L(H) blend compared to those of 4-LD/1-D(H) blend are attributable to the higher G values of 4-LD/1-L(H) blend. Despite the higher G values of 4-LD/1-D(L) blend compared to those of 4-LD/1-L(L) blend, the similar tc(1/2) values of 4LD/1-L(L) and 4-LD/1-D(L) blends can be attributed to the higher spherulite density values of 4-LD/1-L(L) blend

ACKNOWLEDGMENTS This research was supported by JSPS KAKENHI Grant Number 16K05912 and MEXT KAKENHI Grant Number 24108005

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Stereocomplex Crystallization of Star-Shaped Four-Armed Stereo

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