Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Interplay between Stereocomplexation and Microphase Separation in PS‑b‑PLLA‑b‑PDLA Triblock Copolymers Huan Ge,†,‡ Fajun Zhang,§ Haiying Huang,*,†,‡ and Tianbai He*,†,‡ †
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State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany S Supporting Information *
ABSTRACT: Polymers with different tacticities or chiralities can selfassemble into stereocomplex structure (sc-structure) which may further self-assemble into sc-crystals. This stereocomplexation behavior has a great potential in enhancing the crystallization and mechanical properties of polymers. However, less is known about its interplay with microphase separation (MS) in controlling the microscopic structures of block copolymers. In this work, we have designed and synthesized two series of PS-b-PDLA-b-PLLA (PS-b-PLLA-b-PDLA) triblock copolymers. By introducing a PDLA block into a strong segregation PS-b-PLLA system, we were able to explore the interplay between stereocomplexation, crystallization, and MS. The first series of triblock polymers have a fixed total molecular weight and a PLA weight fraction about 0.3, corresponding to a cylinder MS structure. By increasing the relative PDLA:PLLA composition approaching to unity within the PLA block, we found that the long period, L0, of the MS structure decreases due to the formation of sc-structure. However, crystallization was completely suppressed under the strong confinement as no characteristics of sc-crystal could be detected by DSC and WAXS. In the second series, triblock copolymers with the same PS-b-PLLA block but a varying PDLA block were synthesized. With increasing the PDLA length, the MS structure undergoes a transition from cylinder to lamellae. Interestingly, because of the sc-structure formation, the long period of the lamellar phase only increases slightly with increasing PDLA length. Despite the confinement of MS, exclusive sc-crystals could be formed within the lamellae. Our results demonstrate that the confinement of MS structure has a strong impact on sc-crystallization and sterocomplexation within the PLA domains, which provide a new strategy for further adjusting the microstructure as well as various properties of PLA-based materials.
1. INTRODUCTION Microphase separation (MS) of block copolymers (BCP) provides a robust way to facilitate order structures in nanometer scales, which has a wide range of applications in water filtration, electronic circuits, template, drug delivery, and so forth.1−14 In light of academic research and industry applications, it is crucial to control the long-period (L0) of microphase-separated structure of BCPs. In theory, L0 is proportional to χN (L ∼ χ1/6N2/3) in the strong segregation system, where χ is the Flory−Huggins interaction parameter and N is the total degree of polymerization.15−17 Thus, the simplest way of controlling L0 is to vary the molecular weight (N) or blend BCPs with various homopolymers or diblock copolymers.16,18−23 One of the limits of blending is that macroscopic phase separation may occur and/or the original nanostructures of BCPs may change when the blending ratio reaches a critical level.24 Recently, a new strategy of controlling MS morphologies of BCPs by modifying the junctions between the blocks has been applied in several systems.25−27 Woo et al. © XXXX American Chemical Society
introduced a short yet phase-separation-promoting hydrophilic middle block poly(methacrylic acid) at the junction of poly(styrene-b-methyl methacrylate). The resulting BCP has a higher order−disorder transition temperature (ODT) without altering the lateral concentration fluctuations.25 Luo et al. demonstrated that the ionic junction can be used as an efficient control parameter to improve the segregation strength and the ODT of BCPs.26 Furthermore, Wen manifested the competition between the π−π interactions and MS on the selfassembly of BCPs by introducing aromatic junctions into a polystyrene-b-poly(L-lactide) (PS-b-PLLA) system.27 These studies highlighted the high efficiency of chemical modification of the block junctions and paved the way for tailoring the MS of BCPs. As one of the most promising biodegradable materials, polylactide or poly(lactic acid) (PLA) has drawn a great Received: December 11, 2018
A
DOI: 10.1021/acs.macromol.8b02627 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthesis of PS-b-PDLA-b-PLLA via a Sequential ROP Method Following the Procedure Reported in the Literature54−56
attention to meeting the requirements of “green chemistry” in recent years.28,29 Depending on the chirality of monomers, traditional PLA materials are divided into semicrystalline poly(L-lactide) (PLLA), poly(D-lactide) (PDLA), and amorphous poly(DL-lactide) (PDLLA).30−32 In addition to these, a new type of material with a stereocomplexed PLA (sc-PLA) was reported by Ikada when blending PLLA and its enantiomer PDLA. The stereocomplexation was attributed to the intermolecular interactions including the hydrogen bonding or dipolar interactions as reported in many chirality polymers.33−35 The sc-PLA with a compact side-by-side helical structure of sc-crystal (31 helical conformation) shows outstanding properties when compared with the pure PLLA (107 helical conformation), such as a higher melting temperature (Tm ∼ 220 °C), better crystallization rates, and higher thermal-resistant temperatures.36 The stereocomplexation effects were further studied on the crystallization and nucleation of PLLA,37−40 higher thermal resistance,41,42 unique morphologies,43,44 and functional materials.45−47 So far, most of investigations focus on various blend systems, in which the intermolecular interactions drive the stereocomplexation.48,49 Unfortunately, because of the weak segregation force of the block copolymers, breakout crystallization prevails in the blending samples.50−52 Thus, little is known about the interplay between the stereocomplexation and the MS in these systems. Recently, Wen et al.53 introduced PLA chains into a strong segregation system PS-b-PLLA, and they found that the chirality effect can be used to tune the microphase separation structure. In this work, we propose to introduce a PDLA block into a PS-b-PLLA system to study the stereocomplexation effects on the MS and confined crystallization. The advantage of this strategy is twofold: on the one hand, the strong segregation between PS and PLA will ensure the MS; on the other hand, both intra- and intermolecular stereocomplexation may occur for the PLLA-b-PDLA stereoblocks. In this context, we have synthesized two series of PS-b-PLLA-b-PDLA samples using a sequential ring-opening polymerization (ROP) method: one with a fixed total PLA length and varying the PLLA and PDLA length and the other with a fixed PS−PLLA diblock and varying the PDLA block. For the first series, the low molecular weight and volume fraction of PLA block render a convenient study of the effect of stereocomplexation on the MS of BCPs,
whereas for the second series, the focus is on the structure change after MS and the confined sc-crystallization.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Diblock and Triblock Copolymers. Chloroform (CHCl3) and stannous 2-ethylhexanoate (tin octoate, Sn(Oct)2) were purchased from Sigma-Aldrich and used directly without further purification. Toluene (from Beijing Chemical Works) was dried over 4 Å molecular sieves, refluxed over a fresh sodium−benzophenone complex, and then distilled prior to use. Methanol was supplied by Beijing Chemical Works. Lactide (L- and D-form) was purchased from Ji’nan Daigang Co. Ltd. (China). Hydroxy-terminated polystyrene (PS-OH, Mn = 20.4 kg/mol, PDI = 1.06) was kindly supplied by Prof. Ji from CIAC, Changchun, China, which was synthesized according to the literature.54 Di- and triblock copolymers were synthesized following the routes shown in Scheme 1.54−56 For diblory 50 mL two-neck flask was charged with PS-OH (1.0 g, 0.05 mmol) and lactide (0.1−0.7 g, 0.69−4.86 mmol). The monomer was further dried in flask at 50 °C for 6 h under vacuum. After that, 20.0 mL of toluene was added in the flask. At last, Sn(Oct)2 solution was quickly injected into the flask (5.0 mg/mL in toluene, 0.2−1.4 mL, ∼0.01 equiv of lactide). The mixture was stirred at 120 °C for 3 h and precipitated in excess methanol. The white solid powder was further dried under vacuum for 24 h at room temperature. The synthesis route for triblock copolymers PS-b-PLLAb-PDLA or PS-b-PDLA-b-PLLA (Scheme 1) was similar to that for diblock copolymers, except for using the corresponding diblock macroinitiator instead of PS-OH. Detailed molecular structure information about the resulting copolymers are presented in the Results section. The synthesized products were characterized by 1H NMR and GPC. The 1H NMR was measured on a Bruker (500 MHz) with CDCl3 as the solvent at room temperature. The chemical shift was calibrated from the solvent signal. GPC analysis was performed using a GPC-515 (Waters) either in CH2Cl2 for series I polymers or in CHCl3 for series II polymers at 30 °C. One-step thermal degradation analysis (TGA) was performed on a Q50 (TA Instruments) from room temperature to 600 °C with a scan rate of 10 °C/min. Differential scanning calorimetry (DSC) analysis was performed on TA Q100 equipped with a refrigerated cooling system. Typically, about 3 mg of sample was sealed into an aluminum pan, and each sample was taken through two thermal cycles; i.e., samples were first equilibrated at 20 °C, followed by a heating scan to 200 or 240 °C for di- and triblock copolymers, respectively, stay for 2 min, and a cooling scan down to 0 °C. Then, a second heating scan was performed to 200 or 240 °C for di- and triblock copolymers, respectively. The scan rate was 10 °C/min. For isothermal crystallization, the samples were B
DOI: 10.1021/acs.macromol.8b02627 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Sample Characteristics of SL, SLD, and SDL BCPs Mn (kg/mol) sample code series I
series II
SL-1 SDL-1 SDL-2 SDL-3 SLD-1 SLD-2 SLD-3 SL-2 SL2D-1 SL2D-2 SL2D-3
PSa
PDLAb
PLLAb
total Mnb,c
PDIc
SAXS qc (nm−1)
DSC Tm (°C)
ΔHmd (J/g)
Xcd (%)
20.4 20.4 20.4 20.4 20.4 20.4 20.4 20.4 20.4 20.4 20.4
9.7 9.3 7.9 4.1 9.1 7.6 4.7 10.7 10.7 10.7 10.7
30.1/31.9 30.2/31.0 30.2/27.8 29.3/27.2 29.8/27.0 30.2/31.5 29.7/29.2 31.1/35.7 33.1/35.0 39.7/37.3 50.3/50.0
1.09 1.07 1.12 1.10 1.08 1.08 1.08 1.10 1.11 1.13 1.08
0.224
0.5 1.9 4.8 0.3 2.2 4.6 0 2.0 8.6 19.2
157.6 161.3 186.7 192.2 160.8 192.7 193.8 158.6/167.3 197.3 204.5 230.5
21.7 15.4 13.8 20.8 20.0 8.4 21.2 21.7 13.4 32.0 11.6
53.5 38.0 38.3 31.5 45.6 19.2 48.4 46.7 24.0 45.1 19.8
0.241 0.283 0.240 0.246 0.258 0.208 0.177 0.254 0.230
Determined by GPC (eluent: THF 1 mL/min at 40 °C, standard: PS). bDetermined by 1H NMR analysis. cDetermined by GPC (eluent: CH2Cl2 and CHCl3, standard: PS). dDetermined by DSC after crystallized at 120 °C for 48 h. a
quenched quickly from 240 °C to the crystallization temperatures (Tc = 80−180 °C) and kept for 6 h, followed by a heating scan to 240 °C. 2.2. Wide- and Small-Angle X-ray Scattering (WAXS/SAXS). WAXS and SAXS were conducted on the beamline 1W2A in Beijing Synchrotron Radiation Facility (BSRF) equipped with a Linkam hot stage (TST 350) under a constant nitrogen atmosphere. The wavelength of the X-ray was 0.154 nm, and the sample-to-detector distances were 289 and 4744 mm for WAXS and SAXS, respectively. For in situ WAXS analysis, the sample was first heated to 245 °C and then cooled to room temperature at 10 °C/min, and the acquisition time was 100 s at each temperature. One-dimensional (1-D) linear profiles were converted from the 2D patterns using the Fit-2D software. Additional WAXS measurements for powder samples were performed with a Rigaku smartLab from 10° to 30° with a rate of 0.5°/min. Additional SAXS measurements were conducted on a Bruker Nanostar instrument equipped with a Linkam hot stage. The wavelength of the X-ray was 0.154 nm, the exposure time was 1800 s, and the sample-to-detector distance was 1122 mm. In situ SAXS experiments with thermal annealing samples were annealed at 180 °C for 6 h after erasing the thermal history under vacuum. 2.3. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). SEM images were obtained using a ZEISS Gemini Merlin FE-SEM and a Hitachi S-4800 operated with an acceleration voltage of 1 kV. Tapping mode AFM images were obtained by using a commercial instrument SPA-300HV/SPI3800N (Seiko). Etched Si tips (Olympus) with a resonance frequency of ∼75 kHz and a spring constant about 1.7 N/m were used. The scan rate was 1.0 Hz. The residual solvent of thin films was further removed under vacuum for 24 h at room temperature. 2.4. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR measurements were performed using a Nicholet 6700 spectrometer (Thermo Fisher) equipped with a Linkam hot stage and a TM94 cooling system. Polymer thin films were obtained by casting the solution (10 mg/mL) on a KBr plate, and the solvent was further removed under vacuum. The samples were first heated at 10 °C/min above their melting points and kept for 2 min to erase the thermal history, followed by a cooling to 40 °C and a second heating scan at 10 °C/min. FTIR spectra were collected by 16 scans at a 4 cm−1 resolution in the range from 400 to 4000 cm−1.
mol) and a PS weight fraction (∼0.7) but different in PDLA/ PLLA lengths, and in the second series, copolymers have the same PS and PLLA blocks but a different PDLA block. In addition, we have two types of triblock copolymers in series I with a similar chemical composition but different block sequences, i.e., SDL and SLD, which rendered a comparison of chain structure possible. The chain structure was characterized by GPC and 1H NMR, and the results are summarized in Table 1. GPC measurements (Figure 1a,b)
Figure 1. GPC traces (a) for PS-OH and series I polymers as indicated in the figure with the eluent of CH2Cl2, and (b) for series II polymers with the eluent of CHCl3. Representative 1H NMR spectra (c) of PS-OH, SL-1, and SDL-3 and (d) decoupling spectra. Characteristic chemical shifts for the aromatic protons of PS (region a′) and the methine protons of PLA (region b′) were identified.
show a narrow single peak (PDI below 1.15) for all copolymers, indicating a good control of polymerization and no residual unreacted macroinitiators. The 1H NMR spectra of PS-OH and di- and tri-BCPs (Figure 1c) were consistent with the previously reported results,55 and the characteristic resonance peaks of PS (aromatic protons, region a′) and PLA (methine proton, region b′) were observed at about 6.3− 7.2 and 5.2 ppm, respectively. Furthermore, Mn of the PLA block was obtained by calculating characteristic proton signal of styrene and lactide, which is consistent with the GPC results
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Di- and Triblock Copolymers. Following the synthetic route shown in Scheme 1, di- and triblock copolymers were synthesized using the hydroxyl-terminated PS or the diblock copolymers as macroinitiators, respectively. For a systematic comparison, two series of well-defined BCPs were synthesized: in the first series, copolymers have a similar total molecular weight (∼30.0 kg/ C
DOI: 10.1021/acs.macromol.8b02627 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules (both are shown in Table 1). The isotactic structure was demonstrated by the homonuclear decoupling (of the methyl resonance) spectra on the methine resonance (Figure 1d). The single peak indicates the well-defined block sequence which is distinctly different from poly(meso-lactide) and poly(raclactide).57−59 The isotactic block sequence was further supported by the 13C NMR spectra of methine resonances and carbonyl resonances (Figure S1), though a small racemi sequence was observed in 169.4 ppm (integral ratio
