Microscopic Structure of Solvated Poly(benzyl methacrylate) in an Imidazolium-Based Ionic Liquid: High-Energy X‑ray Total Scattering and All-Atom MD Simulation Study Kenta Fujii,* ,† Takeshi Ueki,* ,‡ Kei Hashimoto, § Yumi Kobayashi, § Yuzo Kitazawa, § Kazu Hirosawa, ∥ Masaru Matsugami, ⊥ Koji Ohara, # Masayoshi Watanabe, § and Mitsuhiro Shibayama ∥ † Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 1-16-2 Tokiwadai, Ube, Yamaguchi 755-8611, Japan ‡ National Institute for Materials Science, 1-1 Namiki, Tsukuba-city, Ibaraki 305-0044, Japan § Department of Chemistry & Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Kanagawa 240-8501, Japan ∥ Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan ⊥ Faculty of Liberal Studies, National Institute of Technology, Kumamoto College, 2659-2 Suya, Koshi, Kumamoto 861-1102, Japan # Japan Synchrotron Radiation Institute (JASRI), Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan *S Supporting Information ABSTRACT: We report a new approach for investigating polymer structures in solution systems, including polymer− solvent interactions at the molecular level. The solvation structure of poly(benzyl methacrylate) (PBnMA) in an imidazolium-based ionic liquid (IL) has been investigated at the molecular level using high-energy X-ray total scattering (HEXTS) with the aid of all-atom molecular dynamics (MD) simulations. The X-ray radial distribution functions derived from both experimental HEXTS and theoretical MD (G exp (r) and G MD (r), respectively) were in good agreement in the present PBnMA/IL system. The G(r) functions were successfully separated into two components for the inter- and intramolecular contributions. Here, the former corre- sponds to polymer solvation (or polymer−solvent interactions) and the latter to polymer structure, such as conformation and interactions between side chains (benzyl groups) in PBnMA. The intermolecular G MD inter (r) revealed that the side chains are preferentially solvated by imidazolium cations rather than anions. On the other hand, the intramolecular G MD intra (r) suggested that PBnMA is also stabilized by interactions among the aromatic side chains (π−π stacking). Thus, polymer (benzyl group)− cation interactions and benzyl group stacking within a PBnMA chain coexist in the PBnMA/IL system to give a more ordered solution structure. This behavior might be ascribed to negative mixing entropy in the solution state, which is key to the lower critical solution temperature (LCST)-type phase behavior in the PBnMA/IL solutions. ■ INTRODUCTION Stimuli-responsive polymers undergo changes in solubility in response to external stimuli, such as temperature, pressure, and solution pH. 1−3 Thermoresponsive polymers show conforma- tional changes in solution that are ascribed to changes in the solvation environment around the polymer at different temperatures. Poly(N-isopropylacrylamide) (PNIPAm), which is a well-known thermoresponsive polymer, exhibits lower critical solution temperature (LCST)-type phase separation in aqueous solution, i.e., a coil-to-globule transition at a critical temperature (T c ) near body temperature. 4 Structurally, hydration of a polymer, particularly around hydrophobic side chains, plays a key role in LCST phase behavior. In aqueous PNIPAm solutions, PNIPAm exists as a coil structure below T c that is stabilized by hydrophobic hydration, i.e., hydrogen- bonding networks with “water−water interactions” around the hydrophobic moieties within the side chain. 5−9 Above T c , dehydration collapses the polymer into a globular structure, 10 resulting in an increase in “polymer−polymer interactions”. The phase behavior in ionic liquids (ILs) is quite different from that in aqueous solution, 11,12 e.g., PNIPAm in a typical aprotic IL, such as 1-ethyl-3-methylimidazolium bis(trifluoromethane- sulfonyl)amide ([C 2 mIm][TFSA]), exhibits upper critical solution temperature (UCST)-type phase separation, which is Received: April 24, 2017 Revised: June 8, 2017 Article pubs.acs.org/Macromolecules © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.7b00840 Macromolecules XXXX, XXX, XXX−XXX