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Nanostructured Materials via the Pendant Self-Assembly of Amphiphilic Crystalline Random Copolymers Goki Hattori, Mikihito Takenaka, Mitsuo Sawamoto, and Takaya Terashima J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03838 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018
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mol%, Figure 2b).19 This importantly means that the heat of fusion normalized by the ODA content is almost identical for each of the copolymers. Thus, the octadecyl pendants are efficiently crystallized in the presence of amorphous and hydrophilic PEG chains. !0#
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of the domain spacing much smaller than that generally obtained with common block copolymers. The size and morphology are controlled by composition, independent of chain length. Thus, random copolymers easily prepared by free radical polymerization also afford sub-10 nm selfassembly. Additionally, owing to hydrophilic PEG and hydrophobic and crystalline octadecyl units, those copolymers gave crystalline core, thermoresponsive micelles and vesicles in water and reverse micelles in hexane. Thus, the self-assembly systems with the common copolymers would bring innovation in nanotechnologies, material sciences, and related research fields. Amphiphilic random copolymers bearing hydrophilic PEG and hydrophobic octadecyl pendants (P2-P7) and homopolymers carrying PEG or octadecyl pendants (P1, P8) were synthesized by ruthenium-catalyzed living radical (co)polymerization of PEG methyl ether acrylate (PEGA) and octadecyl acrylate (ODA). The ODA content was changed from 0 to 100 mol%. PEGA and ODA were simultaneously consumed with a ruthenium catalyst [RuCp*Cl(PPh3)2/n-Bu3N] and a bromide initiator (benzyl 2-bromo-2-methylpropanoate) in toluene at 80 oC, giving well-controlled random copolymers (P2-P7: Mn = 1670021000, Mw/Mn = ~1.1 by size-exclusion chromatography (SEC) in THF calibrated against PMMA standards) (Figure S1, Table S1). Synchronized consumption of the two monomers, independent of monomer feed ratio, supports the random sequence distribution of both monomer units into polymers. The degree of polymerization (DP) and monomer unit number (m/n: PEGA/ODA) estimated by 1H NMR (Figure S2) were consistent with the values calculated from monomer feed ratio and conversion (DP = 67 - 82). Absolute weight-average molecular weight (Mw) of P1-P8 was determined by SEC coupled with multi-angle laser light scattering (MALLS) in THF: Mw = 24600 – 36500. A PEGA/ODA (50 mol%) random copolymer with different molecular weight (P9: Mn = 24700, Mw/Mn = 1.4) was also prepared by free radical copolymerization (FRP) of PEGA and ODA with 2,2’-azobis(isobutyronitrile). Crystallinity of P1-P9 was evaluated by differential scanning calorimetry (DSC). Upon heating from -80 oC to 150 oC, PEGA and ODA homopolymers (P1, P8) clearly showed melting temperatures (Tm) of PEG and octadecyl pendants at 1.6 oC and 47.8 oC, respectively (Figure 2a and S3). This indicates that, at room temperature around 25 oC, PEG chains are amorphous and octadecyl units are crystallized. PEGA/ODA copolymers showed crystallinity dependent on composition. Copolymers with over 40 mol% ODA (P3-P7) exhibited Tm derived from the octadecyl pendants although P2 comprising 25 mol% ODA did not show such a melting peak. The melting temperature slightly increased from 38 oC to 43 oC with increasing ODA content from 40 to 80 mol%. Enthalpy of the melting peaks was proportional to the weight fraction of ODA (over 40 wt%: 50
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Figure 2. Crystallization and micro-phase separation of PEGA/ODA random copolymers. (a) DSC thermograms recorded during heating for the copolymers (P2: black, P3: blue, P4: red, P5: green) and a ODA homopolymer (P8: dash line) and (b) melting enthalpy for the crystalline ODA units of P3-P8 (ODA = 25 – 100 mol%): heating = 10 o C/min. (c) Small angle X-ray scattering and (d) X-ray diffractograms of the copolymers (P2: black, P3: blue, P4: red, P5: green, P9: red dash), a ODA homopolymer (P8: dash line), and a methacrylate copolymer with PEG and dodecyl pendants (50 mol%, gray).
To investigate the crystal structure of the octadecyl pendants, P1-P8 were analyzed by X-ray diffraction (XRD) at 25 oC (Figure 2d and S4). The solid polymer samples were obtained from the evaporation of the CH2Cl2 solutions of their polymers, followed by drying under vacuum. P1 and P2 with 25 mol% ODA only showed halo peaks originating from amorphous units (Figure 2d, S4), whereas P8 exhibited a sharp peak at about 22o of 2! (Figure 2d); this peak represents hexagonally packed structure of the octadecyl groups with lattice spacing (d) of ~4 Å.20 Copolymers with over 40 mol% ODA (P3-P7) showed superimposed peaks of the crystalline octadecyl groups at 22o and amorphous halo; the crystalline peaks increased with increasing ODA content. This is consistent with crystallinity characterized by DSC. These diffractograms further exhibited another peak at 3o (d = ~30 Å), suggesting microphase separation of the copolymers. Thus, P2 - P7, P9 were analyzed by small angle X-ray scattering (SAXS) at 25 oC (Figure 2c). All the copolymers induced microphase separation in solid state. The copolymers with 25 or 40 mol% ODA (P2, P3) formed spherical or BCC (domain space = 7.27 nm) phase
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separation, whereas the copolymers with over 50 mol% ODA (P4-P7) showed integer order peaks (1:2:3) to have lamellar structure with small domain space (d) about 5 - 6 nm (Figure 2c, S4,S5). The d-spacing of lamellar structure decreased with increasing ODA content (P4: 1.07o; d = 5.98 nm, P5; 1.12o: d = 5.63 nm, P6; 1.28o: d = 4.90 nm, P7; 1.35o: d = 4.65 nm). Importantly, the lamellar structure just depends on copolymer composition; it is independent of chain length and molecular weight distribution. In the case of 50 mol% ODA, P9 (Mn = 24,700) had the same d-space lamellar structure as P4 (Mn = 17,700). The d-space was smaller than that of lamellar structure generally obtained with polyethylene (PE)-b-poly(ethylene oxide) (PEO) block copolymers (e.g., E29EO20: d = ~12 nm).21 In contrast to the acrylate copolymers, methacrylate copolymers with PEG and octadecyl pendants (octadecyl methacrylate: ODMA = 50 mol%) didn’t induce lamellar-type microphase separation owing to low crystallinity of the octadecyl units (Tm = 27 oC). These results indicate that PEGA/ODA random copolymers with 50 - 80 mol% ODA effectively form sub-10 nm lamellar structure, where the amorphous layer of hydrophilic PEG plus methacrylate backbones and the hydrophobic crystalline layer of the octadecyl pendants are repeated. Such efficient microphase separation is attributed to amorphous PEG chains and flexible acrylate backbone. Importantly, sub-10 nm lamellar structure can be obtained with PEGA/ODA random copolymers prepared by FRP, ).+ !A 3 '(
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