Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
pubs.acs.org/Macromolecules
Chain-End Functionalization with a Saccharide for 10 nm Microphase Separation: “Classical” PS‑b‑PMMA versus PS‑b‑PMMASaccharide Kohei Yoshida,† Shunma Tanaka,† Takuya Yamamoto,† Kenji Tajima,† Redouane Borsali,‡ Takuya Isono,*,† and Toshifumi Satoh*,† †
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Graduate School of Chemical Sciences and Engineering and Faculty of Engineering, Hokkaido University, Hokkaido 080-8628, Japan ‡ Univ. Grenoble Alpes, CNRS, CERMAV, 38000 Grenoble, France S Supporting Information *
ABSTRACT: Microphase-separated structures of block copolymers (BCPs) have attracted much attention as template materials for bottom-up nanofabrication. At the same time, chain-end modification has become a leading facile and efficient technique for fine-tuning the morphologies of microphaseseparated structures generated from BCPs. Herein, we describe the preparation of well-defined polystyrene-block-poly(methyl methacrylate)s (PS-b-PMMAs) bearing highly hydrophilic mono/oligosaccharide moieties at their PMMA chain ends (SM-mono/oligosaccharides) as well as the impact of the mono/oligosaccharide on microphase separation behavior. PS-b-PMMAs were terminal-selectively transesterified using the titanium alkoxide of 6-azido-1-hexanol to introduce azido groups into the side chains of the terminal MMA units. The azidofunctionalized PS-b-PMMAs were subsequently click reacted with ethynyl-functionalized mono/oligosaccharides to yield SMmono/oligosaccharides. Small-angle X-ray scattering and microscopy experiments reveal that PS-b-PMMAs bearing maltotrioses at their chain ends (SM-MTs), and with total molecular weights of ∼10 kg mol−1, successfully form microphase-separated structures, although the unmodified PS-b-PMMAs exist in miscible states. Interestingly, the SM-MT with equivalent PS- and PMMA-MT-block volume fractions microphase separated to form a hexagonally close-packed cylinder with a domain spacing of 11.5 nm, rather than a lamellar structure, implying that the phase diagram for microphase separation is significantly affected by strong maltotriose aggregation. Hence, the results presented herein demonstrate that the incorporation of oligosaccharide moieties at chain ends is an efficient means of fine-tuning the size features as well as the morphologies of BCP microphaseseparated structures.
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N BCPs. BCPs with high χ values usually consist of highly hydrophilic or hydrophobic segments; however, such BCPs require difficult to access monomers as well as elaborate syntheses. Therefore, to expand the applications of BCP-based nanofabrication, the development of a novel methodology for the production of sub-10 nm-scale microphase-separated structures from BCPs composed of classical monomers is of great interest. The partial structures of BCPs, such as the junction points between blocks and polymer-chain ends, have been found to significantly affect self-assembly behavior.21−30 Recently, Park demonstrated a simple way of tuning the microphase-separated structures generated from polystyrene-block-poly(ethylene oxide) (PS-b-PEO) through modification of the PEO chainend structure.30 For example, PS-b-PEO bearing sulfonic acid
INTRODUCTION Block copolymers (BCPs) spontaneously self-assemble to form periodic structures termed “microphase-separated structures”, with various microdomains on the ∼10−100 nm length scale over large areas in the bulk and thin-film states. These nanostructures have been studied for a broad range of applications, including high-density magnetic data storage,1−3 photovoltaics,4−6 microelectronics,7−9 and membranes.10 The morphologies of the nanoscale periodic structures generated from BCPs are varied and depend on the volume fractions of the constituent polymers, block incompatibilities, degrees of polymerization, and molecular architectures.11,12 Because the domain spacing (d) of a microphase-separated structure is mainly governed by the pairwise interaction parameter (χ) and the overall degree of polymerization (N), polymer designs that realize both high χ and low N are normally required to fabricate small features. For example, Hawker et al.,13 Hillmyer et al.,14,15 and other groups16−20 demonstrated the fabrication of extremely small structures through the design of high-χ/low© XXXX American Chemical Society
Received: September 25, 2018 Revised: October 15, 2018
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DOI: 10.1021/acs.macromol.8b02069 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Scheme 1. Synthesis of PS-b-PMMAs Bearing Mono/Oligosaccharides at Their PMMA Chain Ends (SM-Mono/ Oligosaccharides) by Terminal-Selective Transesterification and Click Chemistry
were found to microphase separate with a d of ca. 10 nm in both bulk and thin-film states, even though their unmodified PS-b-PMMAs exist in miscible states because of their low χ and N values. The results of this study reveal that the installation of an oligosaccharide segment at the PMMA chain end of a PS-bPMMA induces microphase separation. Hence, we successfully produced extremely small patternings from classical BCPs, which intrinsically exist in completely disordered states, by installing an oligosaccharide as a hydrophilic component.
groups at the PEO chain ends formed a lamellar nanostructure, while the unmodified PS-b-PEO with terminal hydroxyl groups was less ordered, demonstrating that block incompatibility can be controlled by the end groups. In addition, the morphology was clearly observed to change from the lamellar to cylindrical phase as the end group was changed from sulfonic acid to lithium sulfonate. This pioneering study suggested that suitable adjustments of chain end structure can lead to enhancements in χ and the fine-tuning of the BCP morphology. According to this principle, we expected that the introduction of a highly hydrophilic component at the BCP chain end would be an efficient way of inducing microphase separation of classical low-χ BCPs, leading to sub-10 nm size features. However, the knowledge required to generate microphase separation from a classical BCP, which is intrinsically disordered, by simple chain-end modification is still lacking. Polystyrene-block-poly(methyl methacrylate)s (PS-bPMMAs), which are classical BCPs, are widely employed in the fabrication of nanostructures for a broad range of applications because of practical advantages that include commercial availability and ease of synthesis. However, PS-bPMMA suffers from insufficient segregation strength between its blocks, making it difficult to microphase separate with a d of
