Article pubs.acs.org/Macromolecules
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 (Gexp(r) and GMD(r), respectively) were in good agreement in the present PBnMA/IL system. The G(r) functions were successfully separated into two components for the interand intramolecular contributions. Here, the former corresponds 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 GMDinter(r) revealed that the side chains are preferentially solvated by imidazolium cations rather than anions. On the other hand, the intramolecular GMDintra(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.
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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 conformational 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 (Tc) 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 Tc © XXXX American Chemical Society
that is stabilized by hydrophobic hydration, i.e., hydrogenbonding networks with “water−water interactions” around the hydrophobic moieties within the side chain.5−9 Above Tc, 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(trifluoromethanesulfonyl)amide ([C2mIm][TFSA]), exhibits upper critical solution temperature (UCST)-type phase separation, which is Received: April 24, 2017 Revised: June 8, 2017
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DOI: 10.1021/acs.macromol.7b00840 Macromolecules XXXX, XXX, XXX−XXX
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completely opposite to the behavior in aqueous solution systems.13 In such polymer/IL systems, an understanding of polymer solvation (interactions between polymers and IL ions) is essential for controlling the phase behavior. We have previously reported the relation between phase behavior and solvation of polymers in IL solutions, including neutral polymers with aromatic groups in their side chains, such as poly(benzyl methacrylate) (PBnMA) derivatives in imidazolium-based IL solutions.14−16 PBnMA in [C2mIm][TFSA] shows LCST-type phase separation at Tc of ∼100 °C. The Tc values of such systems strongly depend on the combination of polymer and IL species and their chemical structures. In general, from a thermodynamic viewpoint, LCST-type phase behavior is characterized by negative changes in both enthalpy and entropy upon mixing (ΔHm < 0 and ΔSm < 0).17,18 We pointed out that a partially ordered solvation structure is formed for PBnMA in imidazolium-based ILs; i.e., specific “solvent−solute” interactions are formed owing to cation−π interactions between imidazolium cations and the aromatic side chains, leading to negative ΔSm values.16,19−21 Thus, the origin of structural ordering resulting in negative ΔSm values is essentially different between IL and aqueous PNIPAm systems, that is, direct solute−solvent interactions in the former case and solvent−solvent interactions in the latter case. Notably, water molecules do not directly interact with the polymer side chain. In structural studies on soft matter systems, small-angle Xray/neutron scattering (SAXS/SANS) and dynamic/static light scattering (DLS/SLS) are powerful methods for investigating polymer structures, such as chain conformation, size, and aggregation of polymers in solution. In theoretical studies, molecular dynamic (MD) simulations with a coarse-grained model are often applied to polymer or macromolecular solution systems because this technique allows for larger-scale and longer-time simulations compared with all-atom MD simulations.22−24 However, such experimental and theoretical investigations have a serious drawback, as it is difficult to evaluate the local polymer solvation (polymer−solvent interactions) at the atomistic level. Hence, solvation effects or polymer−solvent interactions are discussed using only an interaction parameter, χ; i.e., in the range of 0 < χ < 0.5, as it approaches to 0.5, an interaction between polymer and solvent becomes weaker, whereas a smaller χ indicates a stronger polymer−solvent interaction. However, it is clear that the χ parameter is just an indirect indicator of polymer−solvent interactions that is not based on the microscopic structure. To understand LCST-type phase behavior in polymer solutions, direct experimental and theoretical evidence for polymer solvation is needed, which has been limited at the present stage. In this work, we propose a new approach for investigating polymer structures in solution systems that provides information about polymer−solvent interactions at the molecular level. Thus, we report herein a structural study that combines high-energy X-ray total scattering (HEXTS) experiments and all-atom MD simulations to reveal the microscopic structure of PBnMA in a typical imidazolium-based ionic liquid, [C2mIm][TFSA]. Such combined X-ray scattering and MD simulation study is useful to elucidate local structure at the molecular or atomistic level and thus has been applied to neat ILs and IL solutions containing solutes (with low molecular weight) such as metal ion, water and organic molecules, and so on.25−29
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EXPERIMENTAL SECTION
Materials. The IL solvent, [C2mIm][TFSA], was synthesized according to a previously reported procedure.30 The water content in the IL was lower than 100 ppm, as determined by Karl Fisher titration. PBnMA homopolymers having different molecular weights were synthesized by atom transfer radical polymerization (ATRP), which is a well-established living radical polymerization method, and were characterized by 1H NMR and gel permeation chromatography. The number-average molecular weight (Mn) was determined to be Mn = 4 kDa. The polydispersity index (Mw/Mn, where Mw is weight-average molecular weight) for polymers were confirmed to be 1.1−1.2. The number-average degree of polymerization for PBnMA (4 kDa) corresponds to n = 22.8. The schematic structure of PBnMA is shown in Figure 1. All polymer samples were dissolved in [C2mIm][TFSA] using the cosolvent evaporation method using tetrahydrofuran as a cosolvent.14,31
Figure 1. Schematic structures of PBnMA, C2mIm cation, and TFSA anion. HEXTS. HEXTS measurements for PBnMA (4 kDa)/IL solutions were performed using a high-energy X-ray diffraction apparatus (BL04B2 beamline at SPring-8, Japan Synchrotron Radiation Research Institute, JASRI, Japan).32,33 All measurements were performed at room temperature (298 K). Monochromatized X-ray radiation of 61.4 keV was obtained using a Si(220) monochromator. The observed Xray scattering intensities were corrected for absorption, polarization, and incoherent scattering to obtain coherent scattering intensities (Icoh(q)).34−36 The experimental X-ray structure factor (Sexp(q)) per stoichiometric volume was obtained by taking into account the atomic scattering factors as follows:
S exp(q) =
Icoh(q) − ∑ nifi (q)2 { ∑ nifi (q)}2
+1 (1)
where ni and f i(q) correspond to the number and atomic scattering factor of atom i, respectively. The radial distribution function (Gexp(r)) was obtained by the inverse Fourier transform of Sexp(q) as follows: G exp(r ) − 1 =
1 2π 2rρ0
∫0
qmax
q{S exp(q) − 1}sin(qr ) exp(−Bq2) dq
(2) where ρ0 and B correspond to the number density and damping factor, respectively. MD Simulations. MD simulations were carried out using the GROMACS 4.5.5 program.37 The system in a cubic cell under NTP ensemble condition controlled by a Nosé−Hoover thermostat38,39 and a Parrinello−Rahman barostat.40 The former was a target to be 298 K and the latter to be 1 atm during the simulation. The composition (number of ion pairs and solutes) of a given system and the resulting box size at the equilibrium state are listed in Table S1. The long-range interactions were evaluated using the smooth particle mesh Ewald (PME) method41 with real-space cutoff distance of 12 Å. The simulation time was set to 100 ns, which is long enough for the polymer solution system to reach an equilibrium state. The system was equilibrated for the first 20 ns with an interval of 2.0 fs, and the data collected at 10 ps intervals during the last 50 ns were analyzed to obtain the X-ray weighted structure factors and radial distribution functions (SMD(q) and GMD(r), respectively). CLaP and OPLS-AA force fields, including intermolecular Lennard-Jones and Coulombic interactions and intramolecular interactions with bond stretching, angle bending, and torsion of dihedral angles, were used for IL ions B
DOI: 10.1021/acs.macromol.7b00840 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 2. (a) Sexp(q)s and (b) r2[Gexp(r) − 1]s obtained by HEXTS measurements for PBnMA/IL solution (solid line) and neat IL (dashed line). and solutes (PBnMA with n = 2 and 23), respectively.42−46 The detailed procedure for the MD simulations is described elsewhere.
good agreement with the corresponding experimental values, which are also listed in Table S1. The SMD(q) functions were calculated by using the trajectory obtained from the simulations as follows:
30,47
The density values obtained by the present MD simulations were in
⎧ ∑ ∑ {ni(nj − 1)f (q)f (q)/N (N − 1)} r sin qr i j ⎪ i j 4πr 2ρ0 (gij MD(r ) − 1) d r + 1 ( i = j) 2 ⎪ qr 0 {∑k (nk fk (q)/N )} ⎪ S MD(q) = ⎨ ⎪ ∑i ∑j {2ninjfi (q)f j (q)/N 2} r sin qr ⎪ 4πr 2ρ0 (g ijMD(r ) − 1) d r + 1 ( i ≠ j) 2 ⎪ qr 0 {∑k (nk fk (q)/N )} ⎩
∫
∫
where N is the total number of atoms in the simulation box given by N = ∑knk and gijMD(r) is the atom−atom pair correlation function between atoms i and j. The GMD(r) functions were obtained by Fourier transformation of SMD(q).
(3)
closest cation−anion interactions and ion−ion interactions between similarly charged ions).48,51 We observed changes in the two broad peaks following the addition of PBnMA into the IL, which are due to the solvation of PBnMA; in particular, changes in the second peak (at ∼10 Å) were significant. Here, it should be noted that the intermolecular interactions at r = 3−8 Å overlap considerably with the intramolecular interactions of the IL (C2mIm or TFSA ions) and PBnMA.21,51−53 PBnMA is significantly larger than the C2mIm and TFSA ions, and thus, such overlap in a wide r range makes it difficult to discuss the solvation structure of PBnMA using the experimental G(r) functions. Therefore, to evaluate the intermolecular interactions between PBnMA and IL ions quantitatively, we examined the possibility of deconvoluting the observed total Gexp(r) functions into their intra- and intermolecular components by using allatom MD simulations. MD Simulations. Figure 3 shows the total GMD(r) and its intra- and intermolecular components, G MD intra (r) and GMDinter(r), for 20 wt % PBnMA/IL solution. The corresponding Gexp(r) obtained by HEXTS is also shown in Figure 3 (open circles). We found that the total GMD(r) reproduces the experimental Gexp(r) well at r-values up to 25 Å. With regard to intermolecular interactions, GMDinter(r) exhibited small peaks at 3.5, 4.5, 5.9, and 6.5 Å and broad peaks at approximately 9, 16, and 25 Å. The GMDinter(r) is composed of six intermolecular components: solute−cation, solute−anion, solute−solute, cation−anion, cation−cation, and anion−anion. In this work, we focused on the solvation of PBnMA in [C2mIm][TFSA]. Thus, we deconvoluted the total GMDinter(r) into its six components and then extracted the PBnMA−C 2 mIm, PBnMA−TFSA, and PBnMA−PBnMA interactions (partial GMDsol−cat(r), GMDsol−an(r), and GMDsol−sol(r), respectively), which will be discussed later. With intramolecular interactions,
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RESULTS AND DISCUSSION HEXTS Experiments. Figure 2a shows structure factors Sexp(q) in the q-range of 0−6 Å−1 obtained from HEXTS measurements of 20 wt % PBnMA/[C2mIm][TFSA] solution and neat [C2mIm][TFSA].30,48 The corresponding Sexp(q) functions over the whole q-range (0−23.0 Å−1) are shown in Figure S1. In general, peaks observed at lower q values (
