Homo- and Stereocomplex Crystallization of Star-Shaped Four-Armed

Article pubs.acs.org/JPCB

Homo- and Stereocomplex Crystallization of Star-Shaped FourArmed Stereo Diblock Copolymers of Crystalline and Amorphous Poly(lactide)s: Effects of Incorporation and Position of Amorphous Blocks Hideto Tsuji,* Michiaki Ogawa, and Yuki Arakawa Department of Environmental and Life Sciences, Graduate School of Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan ABSTRACT: Star-shaped four-armed stereo diblock copolymers with an amorphous poly(DL-lactide) (PDLLA) core and a crystalline poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA) shell (4-DL-L and 4-DL-D) and with a crystalline PLLA or PDLA core and amorphous PDLLA shell (4-L-DL and 4-DDL) copolymers were synthesized. The effects of incorporation and position of amorphous PDLLA blocks on homoand stereocomplex (SC) crystallization behavior of unblended copolymers and their blends (4-DL-L/4-DL-D and 4-L-DL/4D-DL) were investigated during isothermal crystallization from the melt and DSC heating. The incorporated amorphous PDLLA blocks disturbed the SC crystallization and orientation of SC lamellae in the copolymer blends as well as homocrystallization and orientation of homocrystalline lamellae in the unblended copolymers compared to those reported for four-armed PLLA/four-armed PDLA homopolymer blends or four-armed PLLA homopolymers, irrespective of the position of amorphous PDLLA blocks. The disturbance effect was stronger for the amorphous PDLLA shell than for the amorphous PDLLA core. The crystallizability was higher for SC crystallites in the copolymer blends than for homocrystallites in the unblended copolymers, irrespective of the position of amorphous PDLLA blocks. SC crystallization in the copolymer blends disclaimed the positional effects of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks on spherulite growth rate and crystallinity. 105 g mol−1 2−7 and the crystallization rate was higher for stereo block PLA polymers having longer L-lactyl and D-lactyl unit sequences monitored by spherulite growth rate and overall crystallization rate.25,26 However, even the stereo diblock PLA having sufficiently long L-lactyl and D-lactyl unit sequences had the lower crystallization rate compared to that of the one-armed PLLA/PDLA (1-L/1-D) blend, for PLA polymers with Mw values around 1 × 104 g mol−1.22,26 The lower crystallization rate was observed for one-armed stereo diblock copolymer PLLA-b-PDLA (1-L-D) compared to two-armed PLLA/PDLA (2-L/2-D) blends, for PLA polymers with Mw values around 1 × 104 g mol−1.31 These results strongly suggest that the intermolecular SC crystallization rate in the enantiomeric polymer blends is higher than the intramolecular SC crystallization rate in the stereo diblock copolymers or stereo block architecture delays SC crystallization.22,26,31 The crystallization behavior of star-shaped multiarmed PLLA/PDLA blends has been studied. Branching architecture disturbs or decelerates SC crystallization, as estimated by

1. INTRODUCTION Poly(L-lactide) (PLLA) [i.e., poly(L-lactic acid) (PLLA)] is a biobased polymer and can be used in various applications such as commodity, industrial, biomedical, pharmaceutical, and environmental applications.1 As advanced poly(lactide) [i.e., poly(lactic acid) (PLA)]-based materials, the stereocomplexed materials prepared from enantiomeric PLLA and poly(Dlactide) [i.e., poly(D-lactic acid) (PDLA)] have been attracting considerable attention because of their superior mechanical performance and hydrolytic/thermal degradation resistance of stereocomplexed PLA materials compared to conventional PLA-based materials.2−7 The molecular architectures such as molecular weight, tacticity, or optical purity of PLA polymers affect stereocomplex (SC) crystallization. High molecular weight and low optical purity are known to disturb SC crystallization.2−7 Recently, a variety of stereo block8−32 and star-shaped33−45 PLAs were synthesized, and the effects of stereo block and star-shaped or branching architectures on crystallization were intensively studied and found to have crucial effects on SC crystallization behavior. Stereo block architecture can suppress homocrystallization, resulting in predominant SC crystallization for PLA polymers with weight-average molecular weight (Mw) values above 1 × © XXXX American Chemical Society

Received: August 8, 2016 Revised: October 3, 2016

A

DOI: 10.1021/acs.jpcb.6b07987 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 1. Molecular and schematic structures of 4-DL-L, 4-DL-D, 4-L-DL, and 4-D-DL copolymers.

spherulitic growth rate and final crystallinity,42 as can be expected from the results for homocrystallization of unblended star-shaped multiarmed PLLA polymers.46,47 In contrast, incorporation of star, comb, and hyper-branched PDLA to 1L dramatically decreased the half time of crystallization or increased the overall crystallization rate due to a higher number of nuclei per unit mass.44 The melting temperature (Tm) or crystalline thickness of both one or multiarmed PDLA/PLLA [1-L/1-D, 2-L/2-D, and four-armed PLLA/four-armed PDLA (4-L/4-D) blends] was determined by the molecular weight per one arm, not by the total molecular weight, irrespective of the arm number.42 The branching architecture of PLLA and PDLA was reported to facilitate the recrystallization of SC crystallites of the multiarmed PLLA/PDLA blends during cooling from the melt compared to that of linear one-armed 1-L/1-D blends.33 For 1-L/4-D or six-armed PDLA (6-D) blends38 and threearmed PLLA (3-L)/three-armed PDLA (3-D) or 1-L blends,40 the Tm and enthalpy of melting (ΔHm) of SC crystallites depended on the blending ratio and the arm-length or molecular weight of PDLA. The phase separation molecular weight decreased gradually in the following order: one-armed/ one-armed > one-armed/three-armed > three-armed/threearmed.40

Furthermore, the crystallization behavior of one-, three-, and six-armed PLLA-b-PDLA stereo diblock copolymers (1-L-D, 3L-D, and 6-L-D, respectively) was investigated.48 The increasing arm number of stereo diblock copolymers decreased the crystallization temperature upon cooling, melting temperature, degree of crystallinity, spherulitic growth rate, crystalline size, long period, and crystalline layer thickness.48 Recently, we investigated the crystallization behavior of four-armed PLLA-bPDLA (4-L-D) with different L-lactyl unit contents and molecular weights and found that the branching architecture rather than diblock architecture disturbs SC crystallization and simultaneous crystallization of SC- and homocrystallites does not occur at intermediate L-lactyl unit content between 0 and 50% or 50 and 100%.49,50 For the crystallization of 4-L-D/1-L and 4-L-D/1-D blends during solvent evaporation, SC crystallites were the main crystalline species, although the crystallinity was lower than those of 1-L/1-D blends and 1-L or 1-D/1-L-D blends.51 Besides, the incorporated 4-L-D can act as a crystallization accelerating or nucleating agent for homocrystallization of high molecular weight 1-L or 1-D, and such an accelerating effect was higher for 4-L-D added to 1-L than for 4L-D added to 1-D.53 The different accelerating effect should be caused by the configurational disagreement and agreement of B


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Table 1. Molecular Characteristics of Four-Armed Homopolymers and Four-Armed Stereo Dibock Copolymers Synthesized in the Present Study polymer type homopolymer

copolymer

a

code

Mn(GPC)a (g mol−1)

Mn(NMR)a (g mol−1)

Mw(GPC)/Mn(GPC)a

[α]25589b (deg dm−1 g−1 cm3)

4-DL 4-L 4-D 4-DL-L 4-DL-D 4-L-DL 4-D-DL

× × × × × × ×

× × × × × × ×

1.09 1.06 1.06 1.20 1.27 1.17 1.15

0.2 −147.2 147.4 −74.1 76.1 −76.0 79.1

1.38 1.23 1.26 2.33 2.69 2.62 2.25

4

10 104 104 104 104 104 104

1.00 8.36 8.78 1.82 1.95 1.57 1.55

4

10 103 103 104 104 104 104

L-lactyl

unit content (%) 50.0 100.0 0.0 75.2 24.2 75.8 23.2

Mn and Mw are number- and weight-average molecular weights, respectively. bSpecific optical rotation measured in chloroform.

the shell of 4-L-D with 1-L and 1-D, respectively. The former combination will enhance the formation of SC crystallites as nucleating agents. Also, blending 4-L-D having a PDLA shell with four-armed PDLA-b-PLLA (4-D-L) having a PLLA shell accelerated the overall crystallization compared to that of unblended 4-L-D or 4-D-L, because of the elevated number of SC crystalline nuclei of the blend.52 On the other hand, effects of incorporated amorphous onearmed poly(DL-lactide) (1-DL) on homocrystallization of 1-L (i.e., in 1-L/1-DL blends)54−56 or SC crystallization of 1-L and 1-D (i.e., in 1-L/1-D/1-DL blends)57,58 were studied. In the case of 1-L/1-DL blends, the homocrystallization rate of 1-L decreased and the induction period increased when the similar molecular weight 1-L and 1-DL were blended and 1-DL chains were excluded from the amorphous region between 1-L lamellae upon homocrystallization,54,56 whereas the incorporated low molecular weight 1-DL accelerated homocrystallization of 1-L due to the plasticizing effect.55 In the case of 1-L/1D/1-DL blends, incorporated low molecular weight 1-DL delayed the SC crystallization of 1-L and 1-D but disturbed the homocrystallization of either 1-L or 1-D.58 The mechanical properties and shape memory behavior of 1-DL were improved by addition of 1-L and 1-D up to 10 wt %, due to SC crystallization.59 It is expected that an amorphous PDLLA chain linked to a crystalline PLLA or PDLA chain decreases the homocrystallizability and crystallization rate of unblended PLLA or PDLA and SC crystallizability and crystallization rate of the PLLA/PDLA blend. However, as far as we are aware, neither homocrystallization in stereo diblock copolymers of PLLA or PDLA and PDLLA nor SC crystallization in their enantiomeric blends has been reported so far, except for the investigation for the twoarmed PLLA-b-PDLLA/two-armed PDLA-PDLLA blend.60 In this report,60 the narrowly dispersed SC nanocrystals for allPLA nanocomposites were successfully prepared by predominant removal of PDLLA segments via hydrolytic degradation but the focus was not on the crystallization behavior of the copolymer blends. In the present study, we synthesized fourarmed diblock PDLLA-b-PLLA (4-DL-L) and PDLLA-b-PDLA (4-DL-D) copolymers having a crystalline PLLA or PDLA shell and an amorphous PDLLA core and four-armed diblock PLLAb-PDLLA (4-L-DL) and PDLA-b-PDLLA (4-D-DL) copolymers having an amorphous PDLLA shell and a crystalline PLLA or PDLA core (Figure 1), and investigated the homocrystallization behavior of unblended four-armed stereo diblock 4-DL-L, 4-DL-D, 4-L-DL, and 4-DL-D copolymers and SC crystallization of the 4-DL-L/4-DL-D (50/50) blend having crystalline PLLA and PDLA shells and amorphous PDLLA cores and the 4-D-DL/4-D-DL (50/50) blend having amorphous PDLLA shells and crystalline PLLA and PDLA

cores. Their crystallization behavior was studied by the use of wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). On the basis of the obtained results, the effects of incorporation of amorphous PDLLA blocks and the positions of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks on crystallization behavior are discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. The precursors for four-armed diblock copolymers, i.e., four-armed PDLLA (4-DL), 4-L, and 4-D, were synthesized by bulk ring-opening polymerization of DLlactide (3,6-dimethyl-1,4-dioxane-2,5-dione) (Sigma-Aldrich Japan, K.K., Tokyo), L- and D-lactides (2 g) (PURASORB L and D, Purac Biochem BV, Gorinchem, The Netherlands) initiated with 0.03 wt % of tin(II) 2-ethylhexanoate (Practical grade, Nacalai Tesque, Inc., Kyoto, Japan) in the presence of pentaerythritol (Sigma-Aldrich Japan, K.K., Tokyo, Japan, 27.6 mg) as the co-initiator at 140 °C for 10 h.42,47,49,52 Before polymerization, DL-, L-, and D-lactides were purified by repeated recrystallization using ethyl acetate (JIS special grade, Nacalai Tesque Inc.) as the solvent. Also, tin(II) 2-ethylhexanoate was purified by distillation under reduced pressure and pentaerythritol was used as received. Synthesized polymers were purified by reprecipitation using acetone (JIS special grade, Nacalai Tesque Inc.) as the solvent for 4-DL or chloroform (JIS special grade, Nacalai Tesque Inc.) as the solvent for 4-L and 4-D and methanol (JIS special grade, Nacalai Tesque Inc.) as the nonsolvent for all polymers and then dried in vacuo for at least 6 days. Four-armed stereo diblock copolymers with a crystallizable shell, 4-DL-L and 4-DL-D copolymers, were synthesized by ring-opening polymerization of L- or D-lactide (0.4 g), in toluene (Nacalai special grade, H2O < 30 ppm, Nacalai Tesque Inc.) (3.2 mL) initiated with 0.3 wt % of tin(II) 2-ethylhexanoate in the presence of 4-DL (0.4 g) as the coinitiator at 120 °C for 36 h.42,47,49,52 Four-armed stereo diblock copolymers with an amorphous shell, 4-L-DL and 4-D-DL copolymers, were synthesized using DL-lactide (0.4 g) and 4-L or 4-D (0.4 g) by the same procedure with 4-DL-L and 4-DL-D copolymers. The synthesized copolymers were purified by precipitation using chloroform and methanol as the solvent and nonsolvent, respectively, and then dried in vacuo for at least 6 days. The samples of unblended polymers and their 1:1 blends (4DL-L/4-DL-D and 4-L-DL/4-D-DL) for crystallization experiments were prepared by the procedure stated in the previous papers.31,42,52 Briefly, for the preparation of blend samples, each solution of the two polymers was prepared separately to have a polymer concentration of 1.0 g dL−1 and then admixed with each other under vigorous stirring. Dichloromethane (JIS C


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nitrogen gas flow at a rate of 50 mL min−1. About 3 mg of sample was heated at a rate of 10 °C min−1 from ambient temperature to 250 °C. The transition temperatures and enthalpies were calibrated using tin, indium, and benzophenone as standards. The crystalline species and crystallinity (Xc) values of crystallized samples were estimated by the use of WAXD. The WAXD measurements were performed at 25 °C using a Rigaku (Tokyo, Japan) RINT-2500 equipped with a Cu Kα source (λ = 1.5418 Å), which was operated at 40 kV and 200 mA. The isothermal spherulite growth of 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, UK) 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 3 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 with a higher sensitivity than that of LM-332 which we normally use, due to the low crystallinity of samples.

special grade, Nacali Tesque Inc.) was used as the solvent, and the mixing ratio of two copolymers was fixed at 1/1 (w/w). The mixed solution was cast onto a Petri-dish, followed by solvent evaporation at 25 °C for approximately 2 days and further drying under reduced pressure for at least 6 days. The unblended polymer samples were prepared with the same procedure as that of the blend samples without mixing solutions. The melt-crystallization of the solution-crystallized samples sealed in test tubes under reduced pressure was performed at the crystallization temperature (Tc) for 3 h after melting at 220 °C for 3 min. The samples after meltcrystallization were quenched at 0 °C to stop further crystallization. 2.2. 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) GPC system with two TSK gel columns (GMHXL) using polystyrene standards. Therefore, the molecular weights are those relative to polystyrene. The estimated molecular characteristics of the synthesized polymers are summarized in Table 1. 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. 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 (I2), observed at around 5.2 and 4.4 ppm, respectively,42,49 M n(NMR) = 136.2 + (144.1/2) × 4[(I1 + I2)/I2]

3. RESULTS AND DISCUSSION 3.1. Crystallizability and Crystallinity. To determine the crystalline species and crystallizability of isothermally crystallized samples from the melt, WAXD measurements were performed. Figure 2 shows the WAXD profiles of unblended copolymers and their blends crystallized for 3 h from the melt. As seen in Figure 2a−d, unblended copolymers with a crystalline PLLA or PDLA shell, 4-DL-L and 4-DL-D copolymers, formed α- or δ-form homocrystallites of either PLLA or PDLA with diffraction peaks at 17 and 19° (refs 61−63) in the Tc ranges of 80−100 and 90−100 °C, respectively, whereas unblended copolymers with an amorphous PDLLA shell, 4-L-DL and 4-D-DL copolymers, were noncrystallizable in all Tc. The homocrystalline peak intensities of copolymers were much lower than those reported for 4-L homopolymers.47 These results indicate that the incorporated amorphous PDLLA blocks disturbed the homocrystallization of unblended copolymers, irrespective of the position of amorphous PDLLA blocks, and the disturbance was stronger for the amorphous PDLLA shell than for the amorphous PDLLA core. In contrast, as seen in Figure 2e and f, two types of copolymer blends, 4-DL-L/4-DL-D blend with crystalline PLLA and PDLA shells and 4-L-DL/4-D-DL blend with amorphous PDLLA shells, formed SC crystallites with diffraction peaks at 12, 21, and 24° 2−7 in the wide Tc ranges of 80−160 and 80−140 °C, respectively, without formation of PLLA or PDLA homocrystallites. The SC crystalline peak intensities of copolymer blends were lower than those reported for 4-L/4-D homopolymer blends42 but higher than the homocrystalline peak intensities of unblended copolymers (Figure 2a−d). Similar to the homocrystallization in the unblended copolymers, these results are indicative of the fact that the incorporated amorphous PDLLA blocks disturbed SC crystallization in the copolymer blends but crystallizability was higher for SC crystallites in the copolymer blends than for

(1)

where 136.2 g mol−1 is the molecular weight of the co-initiator (pentaerythritol), 144.1 g mol−1 is the molecular weight of DL-, L-, and D-lactides, and 4 is the arm number. The Mn(GPC) or Mn(NMR) values of four-armed diblock polymers were twice those of precursors and Mw(GPC)/Mn(GPC) values were as low as about 1.2, indicating the successful synthesis of fourarmed diblock copolymers. The specific optical rotation ([α]25589) values of polymers were 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. The L-lactyl unit contents of the polymers were estimated by the following equation L‐lactyl unit content

(%)

25

= 100 × {[α]

589

− [α]25589 (4‐D)}/{[α]25589 (4‐L)

− [α]25589 (4‐D)}

(2)

where [α]25589(4-L) and [α]25589(4-D) are those of 4-L (−147.2 deg dm−1 g−1 cm3) and 4-D (147.4 deg dm−1 g−1 cm3), respectively, measured in chloroform. Table 1 summarizes the molecular characteristics of polymers synthesized in the present study. The L-lactyl unit contents evaluated for four-armed diblock polymers are around 75 and 25%, indicating that synthesized polymers contain similar lengths of crystalline PLLA or PDLA blocks and amorphous PDLLA blocks. The glass transition, cold crystallization, and melting temperatures (Tg, Tcc, and Tm, respectively) and the enthalpies of cold crystallization and melting (ΔH cc and ΔH m , respectively) were determined with a Shimadzu (Kyoto, Japan) DSC-50 differential scanning calorimeter under a D


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Figure 3. Crystallinity (Xc) values of unblended 4-DL-L, 4-DL-D, 4-LDL, and 4-D-DL copolymers (a) and 4-DL-L/4-DL-D and 4-L-DL/4D-DL blends (b) estimated by WAXD measurements, as a function of Tc.

higher disturbance effect of amorphous PDLLA blocks at the shell compared to that at the core, or strong shielding effects of the amorphous PDLLA shell to interaction between the crystalline PLLA or PDLA cores in 4-L-DL or 4-D-DL copolymer. The lower Xc values of 4-DL-D copolymer compared to those of 4-DL-L copolymer should be ascribed to the slightly higher molecular weight of the former, which should have decreased the chain mobility and crystallization rate. However, the Xc values of blend from the copolymers with crystalline PLLA and PDLA shells (4-DL-L/4-DL-D blend) are similar to those of blend from the copolymers with amorphous PDLLA shells (4-L-DL/4-D-DL blend) for Tc below 120 °C but became higher for Tc above 140 °C. The maximum Xc values of 4-DL-L/4-DL-D and 4-L-DL/4-D-DL copolymer blends (31 and 30%, respectively) were lower than those of 4L/4-D homopolymer blends with averaged Mn(NMR) values of 5.9 × 103 and 9.8 × 104 g mol−1 (67 and 67%, respectively),42 supporting the disturbance effect of incorporated amorphous PDLLA blocks on SC crystallization, irrespective of the position of the amorphous PDLLA block. The maximum Xc values were higher for the copolymer blends (30 and 31%) than for the unblended copolymers (0−7%), affirming the higher crystallizability of SC crystallites in the blends than of homocrystallites in the unblended copolymers. The crystallizable Tc range of the 4-DL-L/4-DL-D blend was wider than that of the 4-L-DL/4-DDL blend, which is due to the higher Tm of the former than that of the latter (see section 3.2). 3.2. Thermal Properties. To investigate the thermal properties of isothermally crystallized samples from the melt, DSC measurements were carried out. Figure 4 show the DSC thermograms of unblended polymers and their blends crystallized for 3 h or quenched (Tc = 0 °C) from the melt. Thermal properties obtained from DSC thermograms in Figure 4 are tabulated in Table 2. The absence of a cold crystallization peak for all the melt-quenched unblended copolymers (Figure 4a−d) indicates their noncrystallizability during DSC heating, in contrast with the homocrystallizability reported for 4-L homopolymers with Mn(NMR) = 9.5 × 103 and 1.8 × 104 g mol−1.47 On the other hand, unblended 4-DL-L and 4-DL-D copolymers crystallized at Tc = 80−100 and 90−100 °C, respectively, had melting peaks with ΔHm = 2−6 J g−1 at 121.7−133.4 °C, whereas unblended 4-L-DL and 4-D-LD copolymers crystallized at Tc = 80−100 °C and 4-DL-D polymer crystallized at Tc = 80 °C had no melting peak. These results exhibit the no or very low homocrystallizability of

Figure 2. WAXD profiles of unblended 4-DL-L (a), 4-DL-D (b), 4-LDL (c), and 4-D-DL (d) copolymers and 4-DL-L/4-DL-D (e) and 4L-DL/4-D-DL (f) blends crystallized at the different crystallization Tc values from the melt. The broken lines show the 2θ values for SC crystallites.

homocrystallites in the unblended copolymers, irrespective of the positions of crystalline and amorphous blocks. Xc values estimated from the WAXD profiles in Figure 2 are plotted in Figure 3 as a function of Tc. It should be noted that the Xc range for panel b (0−40%) is wider than that of panel a (0−10%). The maximum Xc values of unblended copolymers (0−7%) were much lower than those of 4-L homopolymers with Mn(NMR) values of 6.1 × 103 to 5.7 × 104 g mol−1 (67− 73%),47 confirming the strong disturbance effect of incorporated amorphous PDLLA blocks on homocrystallization, regardless of the position of amorphous PDLLA blocks. The lower maximum Xc values of 4-L-DL and 4-D-DL copolymers having the amorphous shell and crystalline core (0%) compared to those of 4-DL-L and 4-DL-D copolymers having the crystalline shell and amorphous core (3 and 7%) reflects the E


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weight per one PLLA arm (ca. 2.4 × 103 g mol−1) is similar to those of crystalline PLLA or PDLA blocks of 4-DL-L and 4-DLD copolymers,47 suggesting the disturbed growth of homocrystallites in 4-DL-L and 4-DL-D copolymers, even when amorphous PDLLA blocks were located at the core. Considering their Tg of 52 and 47 °C, such low Tm values of unblended 4-DL-L and 4-DL-D copolymers should have extremely limited the crystallizable Tc range. In marked contrast to the results of melt-quenched unblended copolymers, the presence of a cold crystallization peak of both copolymer blends at around 136−137 °C (Figure 4e and f) reflects their SC crystallizability during DSC heating, in agreement with SC crystallizability of 4-L/4-D homopolymer blends with averaged Mn(NMR) values from 5.9 × 103 to 2.9 × 104 g mol−1 during DSC heating.42 However, the observed cold crystallization peaks were broader for the copolymer blends than for 4-L/4-D homopolymer blends,42 reflecting the disturbance effects of amorphous PDLLA blocks in the copolymer blends on SC crystallization. The large area of cold crystallization of melt-quenched 4-DL-L/4-DL-D blend (ΔHcc = 15 J g−1) with crystalline PLLA and PDLA shells compared to that of melt-quenched 4-L-DL/4-D-LD blend (ΔHcc = 5 J g−1) with amorphous PDLLA shells means the higher SC crystallizability of the former during DSC heating and the disturbance effect of amorphous PDLLA blocks at the shells on SC crystallization was stronger than that of amorphous PDLLA blocks at the cores. Also, the cold crystallization peak was observed for the 4-L-DL/4-D-DL blend crystallized at Tc = 160 °C, confirming no isothermal crystallizability at this Tc, which was monitored by WAXD, and crystallizability during DSC heating. Isothermally crystallized copolymer blends had cold crystallization and melting peaks at around 106−138 and 135−186 °C, respectively, and ΔH(tot) = ΔHcc + ΔHm had nonzero values in the range 3−32 J g−1 (Table 2), supporting the crystallizability during isothermal crystallization and DSC heating. The Tm values of 135−186 and 137−176 °C for the SC crystallites of 4-DL-L/4-DL-D and 4-L-DL/4-D-DL copolymer blends with crystalline PLLA and PDLA blocks having Mn values around 2.5 × 103 g mol−1 (Table 2) are lower than the Tm value reported for the 4-L/4-D homopolymer blend with averaged Mn(NMR) = 9.8 × 103 g mol−1 (198 °C), whose molecular weights per one PLLA or PDLA arm (ca. 2.5 × 103 g mol−1) are similar to those of crystalline PLLA or PDLA blocks of 4-DL-L, 4-DL-D, 4-L-DL, and 4-D-DL copolymers,42 suggesting that the SC crystalline growth was interrupted by the presence of amorphous PDLLA blocks. The higher maximum Tm values of the 4-DL-L/4-DL-D blend (186 °C) compared to that of the 4-L-DL/4-D-DL blend (176 °C) should have given the higher Xc values and radial growth rate values of spherulites (section 3.3) of the former at high Tc values and confirm that the disturbance effects of amorphous PDLLA blocks at the shells on SC crystalline growth was stronger than that at the cores. 3.3. Polarized Optical Microscopy. To investigate the spherulitic morphology and growth behavior of samples, POM observation was performed. Figure 5 show the polarized optical photomicrographs of unblended copolymers and their blends crystallized from the melt. As seen in Figure 5a−d, spherulites without Maltese cross or with a very disordered Maltese cross were observed for unblended 4-DL-L and 4-DL-D copolymers having the crystalline shell, whereas no spherulite was formed in unblended 4-L-DL and 4-D-DL copolymers having the amorphous shell (photos not shown). The spherulites of 4-L

Figure 4. DSC thermograms of unblended 4-DL-L (a), 4-DL-D (b), 4L-DL (c), and 4-D-DL (d) copolymers and 4-DL-L/4-DL-D (e) and 4-L-DL/4-D-DL (f) blends, crystallized at the different crystallization Tc values from the melt. The arrows in panels a and b indicate melting peaks of homocrystallites, whereas the arrows in panels e and f show cold crystallization peaks.

unblended copolymers during isothermal crystallization or DSC heating. However, the presence of a melting peak for 4-DL-L and 4-DL-D copolymers confirms the WAXD result that isothermal homocrystallizability was higher for the unblended copolymers with the crystalline PLLA or PDLA shell than for those with the amorphous PDLLA shell. The Tm values of 122−133 °C observed for the homocrystallites of 4-DL-L and 4-DL-D copolymers with crystalline PLLA or PDLA blocks having Mn values around 2.5 × 103 g mol−1 are lower than the Tm value reported for 4-L homopolymer with averaged Mn(NMR) = 9.5 × 103 g mol−1 (139 °C), whose molecular F


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The Journal of Physical Chemistry B Table 2. Thermal Properties of Unblended Copolymers and Blends Crystallized from the Melt sample type unblended copolymer

code 4-DL-L

4-DL-D

4-L-DL

4-D-DL

blend

4-DL-L/4-DL-D

4-L-DL/4-D-LD

Tca (°C)

Tgb (°C)

0 80 90 100 0 80 90 100 0 80 90 100 0 80 90 100 0 80 100 120 140 160 0 80 100 120 140 160

52.0 51.3 48.0 52.3 47.3 51.8 53.0 51.8 50.4 48.6 49.8 49.6 49.6 49.2 51.7 50.6 52.5 51.3 47.8 52.4 48.7 51.6 51.0 48.3 51.5 51.8 53.3 52.0

Tccb (°C)

137.2 114.2

136.4 125.4 106.1

137.7

Tmb (°C)

ΔHccc (J g−1)

ΔHmc (J g−1)

121.7, 133.4 124.8, 131.6 126.6

1.5 5.3 5.5

125.4 126.8

1.7 2.8

185.9 183.3 183.1 135.0, 152.4, 187.1 157.8, 171.7, 173.9 137.1, 176.0 171.2

−15.1 −5.2 184.2 181.6 174.3 174.4

−5.0 −6.2 −1.1

173.9 −17.3

14.4 24.2 26.9 31.9 29.0 20.5 5.2 25.3 33.2 30.6 27.4 19.9

ΔH(tot)d (J g−1) 1.5 5.3 5.5

−0.7 19.0 26.9 31.9 29.0 20.5 0.2 19.1 32.1 30.6 27.4 2.6

Tc is the crystallization temperature. Tc = 0 °C means that the sample was melt-quenched. bTg, Tcc, and Tm are the glass transition, cold crystallization, and melting temperatures, respectively. cΔHcc and ΔHm are the enthalpies of cold crystallization and melting, respectively. dΔH(tot) = ΔHcc + ΔHm. a

noted the G values of unblended 4-L-DL and 4-D-DL copolymers with the amorphous PDLLA shell could not be measured because no spherulite formation was observed. The G values of unblended 4-DL-L and 4-DL-D copolymers were in the ranges of 0.03−0.15 and 0.05−0.08 μm min−1, respectively, which are 1 order lower than those reported for 4-L homopolymers [0.8−6.7 μm min−1 for 4-L with Mn(NMR) = 1.8 × 104 g mol−1, whose molecular weight is similar to those of 4-DL-L and 4-DL-D copolymers, and 0.5−3.3 μm min−1 for 4L with Mn(NMR) = 9.5 × 103 g mol−1, whose molecular weight per one PLLA arm (2.4 × 103 g mol−1) is similar to those of crystallizable PLLA or PDLA blocks of 4-DL-L, 4-DL-D, 4-LDL, and 4-D-DL copolymers].47 These results reflect the fact that the amorphous blocks disturbed homocrystallization and decreased G values, irrespective of their positions, and the disturbance effect was stronger when amorphous blocks were located at the shell. On the other hand, the G values of 4-DL-L/4-DL-D and 4-LDL/4-D-DL blends were in the ranges of 0.10−0.72 and 0.05− 0.70 μm min−1, respectively, whose maxima were higher than those of unblended 4-DL-L and 4-DL-D copolymers but lower than those reported for 4-L/4-D blends [2.8−35.0 μm min−1 for the 4-L/4-D blend with averaged Mn(NMR) = 1.8 × 104 g mol−1 (unpublished results), whose molecular weight is similar to those of 4-DL-L and 4-DL-D copolymers and 2.4−18.0 μm min−1 for the 4-L/4-D blend with averaged Mn(NMR) = 9.8 × 103 g mol−1, whose molecular weights per one PLLA or PDLA arm (2.5 × 103 g mol−1) are similar to those of crystallizable

homopolymers having no amorphous PDLLA block showed a well-defined Maltese cross if Tc was carefully selected.47 These results indicate that the incorporated amorphous PDLLA blocks at the shell prohibited the formation of homocrystallite spherulites and those at the core disturbed the orientation of homocrystallite lamellae along the radius direction. On the other hand, as seen in Figure 5e−h, the spherulites with a welldefined Maltese cross were seen for the 4-DL-L/4-DL-D blend whose constituent copolymers have crystalline PLLA and PDLA shells, whereas the spherulites with a disordered Maltese cross (130 °C) or without a Maltese cross (150 °C) were observed for the 4-L-DL/4-D-DL blend whose constituent copolymers have amorphous PDLLA shells. In contrast, spherulites with a well-defined Maltese cross are formed in 4L/4-D homopolymer blends having no amorphous PDLLA block.42 These results exhibit that the amorphous PDLLA shells of the 4-L-DL/4-D-DL blend disturbed the orientation of SC lamellae along the radial direction, although the incorporated amorphous PDLLA blocks did not prohibit the formation of SC spheurulites, regardless of their positions. The spherulite number per unit area was higher for the 4-DL-L/4-DL-D blend than for the 4-L-DL/4-D-DL blend, indicating the nucleation of SC spherulites was disturbed by the amorphous PDLLA blocks at the shells. The radial growth rate of spherulites (G) was estimated from the photos obtained at different crystallization times, and the thus obtained G values are plotted for unblended copolymers and their blends in Figure 6a and b, respectively. It should be G


Article


reflect the strong disturbance effects of the incorporated amorphous PDLLA blocks, irrespective of the position of amorphous PDLLA blocks in the copolymers of blends. More interestingly, the G values of the 4-DL-L/4-DL-D blend with crystalline PLLA and PDLA shells were similar to those of the 4-L-DL/4-D-DL blend with amorphous PDLLA shells in the Tc range below 125 °C but higher than those of the 4-L-DL/4-DDL blend only in the Tc range above 130 °C. The former result indicates that the SC crystallization disclaimed the positional effects of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks, whereas the latter result can be attributed to the higher Tm of the 4-DL-L/4-DL-D blend than that of the 4-LDL/4-D-DL blend. The nucleation constant (Kg) and the front constant (G0) of the samples were estimated using the nucleation theory established by Hoffman et al.,65,66 in which G can be expressed by the following equation G = G0 exp[−U * /R(Tc − T∞)] exp[−K g /(TcΔTf )]

(3)

where ΔT is supercooling Tm − Tc when Tm is equilibrium Tm, f is the factor expressed by 2Tc/(Tm0 + Tc) which accounts for the change 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 7 as a function 0

0

Figure 5. Polarized optical photomicrographs of 4-DL-L (a, b) and 4DL-D (c, d) copolymers and 4-DL-L/4-DL-D (e, f) and 4-L-DL/4-DDL (g, h) blends crystallized at the shown Tc and crystallization time from the melt. Figure 7. ln G + 1500/R(Tc − T∞) of unblended 4-DL-L and 4-DL-D copolymers (a) and 4-DL-L/4-DL-D and 4-L-DL/4-D-DL blends (b).

of 1/(TcΔTf), using Tm0 = 212 °C of homocrystallites67 for unblended 4-DL-L and 4-DL-D copolymers and Tm0 = 279 °C of SC crystallites68 for 4-DL-L/4-DL-D and 4-L-DL/4-D-DL blends, U* = 1500 cal mol−1, and T∞ = Tg − 30 K. Also, as Tg values, those of melt-quenched samples were used (52, 47, 53, and 51 °C for unblended 4-DL-L and 4-DL-D copolymers, 4DL-L/4-DL-D and 4-L-DL/4-D-DL blends, respectively) (Table 2). The plot in Figure 7 gives Kg as a slope and the intercept ln G0 and thus estimated Kg and G0 values are shown in Table 3. All samples had only one Kg value, indicating that the crystalline growth mechanism was not varied by Tc,

Figure 6. Radial growth rate of spherulites (G) of unblended 4-DL-L and 4-DL-D copolymers (a) and 4-DL-L/4-DL-D and 4-L-DL/4-DDL blends (b) as a function of Tc.

Table 3. Front Constant (G0) and Nucleation Constant (Kg) of Unblended Copolymers and Blends

PLLA or PDLA blocks of 4-DL-L and 4-DL-D copolymers].42 It is interesting to note that, because of the higher rate of SC crystallization compared to that of homocrystallization,64 SC spherulites in the blends had much higher G values compared to those of homocrystallite spherulites in the unblended copolymers even in the presence of amorphous PDLLA blocks. The very lower G values of the blends compared to those of 4L/4-D homopolymer blends with no amorphous PDLLA block

sample type unblended copolymer blend

H

code

G0 (μm min−1)

4-DL-L 4-DL-D 4-DL-L/4-DL-D 4-L-DL/4-D-DL

× × × ×

1.50 2.48 6.13 6.33

9

10 107 1013 1016

Kg (K2) 4.85 3.69 1.27 1.64

× × × ×

105 105 106 106


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increase in Tc. These results reflect the rapid overall crystallization of the 4-DL-L/4-DL-D blend having crystalline PLLA and PDLA shells than that of the 4-L-DL/4-D-DL blend with amorphous PDLLA shells, which should have been caused by the higher number of SC spherulites per unit area (Figure 5). Isothermal crystallization kinetics traced by light intensity measurements was analyzed with the Avrami theory,71−73 which is expressed by the following equation

regardless of the position of crystalline PLLA and PDLA blocks and amorphous PDLA blocks. In the case of unblended 4-DL-L and 4-DL-D copolymers, unchanged Kg values can be ascribed to too narrow Tc range for regime change observation. Only one crystalline growth mechanism observed for unblended 4DL-L and 4-DL-D copolymers is in contrast with two (regimes II and I) or three (regimes III, II, and I) crystalline growth mechanisms reported for 4-L homopolymers,47 whereas one crystalline growth mechanism observed for 4-DL-L/4-DL-D and 4-L-DL/4-D-DL blends agrees with the results for 4-L/4-D homopolymer blends.42 The Kg values of unblended 4-DL-L and 4-DL-D copolymers were 4.85 and 3.69 × 105 K2, respectively, which are comparable with, higher than, and comparable with 5.28, 1.87, and 4.03 × 10 5 K 2 for correspondingly regime III, II, and I kinetics of 4-L having a Mn(NMR) value of 1.8 × 104 g mol−1,47 and are comparable with and lower than 3.60 and 6.87 × 105 K2 for correspondingly regime II and I kinetics of 4-L having a Mn(NMR) value of 9.5 × 103 g mol−1.47 On the other hand, the Kg values of 4-DL-L/4DL-D and 4-L-DL/4-D-DL blends were 1.27 and 1.64 × 106 K2, respectively, which are lower than 8.22 × 105 K2 reported for regime III kinetics of the 4-L/4-D blend having an averaged Mn(NMR) value of 9.5 × 103 g mol−1. However, without the presence of another Kg value or regime kinetics, mere comparison between the present and reported Kg values could not determine the type of regime kinetics of unblended 4-DL-L and 4-DL-D copolymers and 4-DL-L/4-DL-D and 4-LDL/4-D-DL blends in the present study. 3.4. Overall Crystallization Behavior. The overall isothermal crystallization behaviors of the samples at different Tc values were 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)26,49,69,70 X r (%) = 100(It − I0)/(I∞ − I0)

1 − X r (%)/100 = exp(−ktc n)

(5)

where k is the crystallization rate constant and n is the Avrami exponent. Equation 5 can be transformed to eq 6: log[− ln(1 − X r /100)] = log k + n log tc

(6)

To avoid deviation from the theoretical curves, as suggested by Mandelkern et al. and Lorenzo et al.,74,75 we used Xr in the range 5−20% for estimating n and k. The plots with eq 6 are shown in Figure 9. The plots with eq 6 give n as slope and

Figure 9. log[−ln(1 − Xr/100)] of blends crystallized at different Tc values from the melt, as a function of crystallization time (log tc).

intercept log k. Thus, obtained n and k values are listed in Table 4. The n values of the 4-DL-L/4-DL-D blend were in the range 2.2−3.6, whereas those of the 4-L-DL/4-D-DL blend were in the range 2.0−3.5. Assuming the thermal nucleation,76 the n values from 2 to 4 mean the growth morphology changed from one-dimensional or linear to three-dimensional or spherical, depending on Tc, and the position of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks did not affect the growth morphology. In addition, crystallization half time [tc(1/2)] was calculated using the following equation:

(4)

where It and I0 are the I values at crystallization time (tc) = t and 0, respectively, and I∞ is the I value when it leveled off. The thus obtained Xr values are plotted in Figure 8 as a function of

tc(1/2)(Cal.) = [(ln 2)/k]1/ n

(7)

Figure 8. Relative crystallinity (Xr) of blends crystallized at different Tc values from the melt, as a function of crystallization time (tc).

The values of tc(1/2)(Cal.) thus obtained are summarized in Table 4. The tc(1/2)(Cal.) values were very similar to the experimental tc(1/2) [tc(1/2)(Exp.)] values. The tc(1/2)(Exp.) values of the 4-DL-L/4-DL-D blend with crystalline shells were lower than those of the 4-L-DL/4-D-DL blend with amorphous shells, confirming the rapid overall crystallization of the former.

tc. Here, due to the very small change of I, the data for all the unblended polymers at all Tc values and 4-DL-L/4-DL-D blend at Tc = 100 °C could not be taken. As seen in Figure 8, the Xr values of the 4-DL-L/4-DL-D blend for all Tc values increased similarly and leveled off within 80 min, whereas the Xr values of the 4-L-DL/4-D-DL blend increased and leveled off within 100 and 200 min for Tc = 110 and 120 °C and Tc = 130 °C, respectively, and the crystallization rate became lower with an

4. CONCLUSIONS The homo- and SC crystallization behavior of star-shaped branched diblock copolymers with crystalline PLLA or PDLA and amorphous PDLLA blocks and their blends were investigated, and the effects of incorporated amorphous PDLLA blocks and the positional effects of PLLA and PDLA crystalline and amorphous PDLLA blocks on crystallization behavior were elucidated. During isothermal crystallization I


Article

The Journal of Physical Chemistry B Table 4. Avrami Exponent (n), Crystallization Rate Constant (k), and Crystallization Half Time [tc(1/2)] of Blends code 4-DL-L/4-DL-D

4-L-DL/4-D-DL

a

Tc (°C) 100 110 120 130 110 120 130

k (min−n)

n 2.19 3.62 3.20 2.78 3.43 3.50 2.01

5.20 8.10 3.62 2.13 5.52 2.01 4.31

× × × × × × ×

tc(1/2)(Exp.)a (min)

tc(1/2)(Cal.)a (min)

23.5 23.3 22.2 18.9 31.3 41.7 103.0

26.9 23.1 21.7 18.4 27.6 38.1 122.6

−4

10 10−6 10−5 10−4 10−6 10−6 10−5

tc(1/2)(Exp.) and tc(1/2)(Cal.) are experimental and calculated crystallization half times.

Notes

from the melt, the unblended copolymers having a crystalline shell (4-DL-L, 4-L-DL) were homocrystallizable, whereas those having an amorphous shell (4-L-DL, 4-D-DL) were noncrystallizable. All the unblended copolymers were noncrystallizable during DSC heating. The incorporated amorphous PDLLA blocks disturbed the isothermal homocrystallization and orientation of homocrystalline lamellae of unblended copolymers compared to those reported for 4-L homopolymers, irrespective of the position of amorphous PDLLA blocks, and the disturbance effect was stronger for the amorphous PDLLA shell than for the amorphous PDLLA core. In contrast, all of the blends, 4-DL-L/4-DL-D blend with crystalline PLLA and PDLA shells and 4-L-DL/4-D-DL blend with amorphous PDLLA shells, were SC crystallizable during isothermal crystallization or DSC heating, regardless of the position of amorphous PDLLA blocks. Similar to homocrystallization and orientation of homocrystalline lamellae in the unblended copolymers, the incorporated amorphous PDLLA blocks disturbed the SC crystallization and orientation of SC crystalline lamellae in the copolymer blends compared to those reported for 4-L/4-D homopolymer blends, but crystallizability was higher for SC crystallites in copolymer blends than for homocrystallites in unblended copolymers, irrespective of the position of amorphous PDLLA blocks. Interestingly, for the blends, the G and Xc values of the 4-DLL/4-DL-D blend with crystalline PLLA and PDLA shells were similar to those of the 4-L-DL/4-D-DL blend with amorphous PDLLA shells in the Tc ranges below 125 and 120 °C, respectively, but higher than those of the 4-L-DL/4-D-DL blend in the Tc ranges above 130 and 140 °C, respectively. The former result indicates that SC crystallization disclaimed the positional effects of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks for the blends, whereas the latter result is due to the higher Tm of the 4-DL-L/4-DL-D blend. The higher number of SC spherulites per unit area of the 4-DLL/4-DL-D blend compared to that of the 4-L-DL/4-D-DL blend increased the overall crystallization rate of the former. The unblended copolymers (4-DL-L and 4-L-DL) and two types of blends showed only one regime, exhibiting no change in crystalline growth mechanism by Tc, regardless of the positions of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks. However, the SC crystalline growth morphology of the 4-DL-L/4-DL-D and 4-L-DL/4-D-DL blends changed from one-dimensional or linear to three-dimensional or spherical depending on Tc, irrespective of the positions of crystalline PLLA and PDLA blocks and amorphous PDLLA blocks.

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The authors declare no competing financial interest.

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

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