SANS study on the solvated structure and molecular interactions of a thermo-responsive polymer in a room temperature ionic liquid

Abstract

We have utilized small-angle neutron scattering (SANS) to quantitatively characterize the LCST-type phase behavior of the poly(benzyl methacrylate) (PBnMA) derivative poly(2-phenylethyl methacrylate) (PPhEtMA) in the deuterated ionic liquid (IL) d₈-1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (d₈-[C₂mIm⁺][TFSA⁻]). The SANS curves showed a discontinuous change from those characteristics of dispersed polymer chains to those of large aggregates of PPhEtMA chains suspended in the IL solution, indicating that phase separation occurs discontinuously at T_c. Furthermore, we evaluated the enthalpic and entropic contributions to the effective interaction parameter χ_eff of PPhEtMA in [C₂mIm⁺][TFSA⁻] and compared them with those of PBnMA. The absolute value of the enthalpic contribution observed for PPhEtMA was smaller than that for PBnMA. This difference in the enthalpic term can be attributed to the unfavorable interaction between the IL and the alkyl group in the side chain of PPhEtMA. In addition, the temperature dependence of χ_eff was smaller than the previously reported value for a thermo-responsive polymer in an aqueous system. It was found out that the strong dependence of T_c on the chemical structure in IL systems originated from the relatively smaller temperature dependence of χ_eff.

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Introduction

Ionic liquids (ILs) consist of only ion species. Thus, they exhibit unique solvent properties such as high ion conductivity, electrochemical stability, negligible volatility and nonflammability.³ More importantly, the solvent properties of ILs can be tuned by selecting appropriate combinations of ions, and they can dissolve various solutes including metal ions,^4,5 organic molecules^6,7 and biopolymers.^8–11 In polymer science, the combination of polymer materials with appropriate ILs permits the fabrication of functional soft materials by exploiting the unique properties of ILs.^12–17 Recently, various systems of different types of stimuli-responsive polymers in ILs have been developed for applications to sensors, actuators and switching devices possessing the intrinsic properties of the ILs.^1,18–27 As it is well-known, poly(N-isopropylacrylamide) (PNIPAm) shows a lower critical solution temperature (LCST)-type phase separation in aqueous solutions. In contrast, PNIPAm exhibits an upper critical solution temperature (UCST)-type phase separation in a typical hydrophobic IL, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C₂mIm⁺][TFSA⁻], as shown in Scheme 1).¹⁸ Interestingly, it was found for the first time that poly(benzyl methacrylate) (PBnMA, shown in Scheme 1) and its derivatives show an LCST-type phase separation in [C₂mIm⁺][TFSA⁻].^1,19 Furthermore, a cross-linked PBnMA gel in [C₂mIm⁺][TFSA⁻] shows discontinuous LCST-type volume phase transition in response to the temperature change.¹⁹ One of the characteristics of stimuli-responsive polymers in IL systems is that their phase transition temperature (T_c) strongly depends on both the polymer chemical structures and the structures of the ILs. For example, the LCST-type phase transition temperature of poly(2-phenylethyl methacrylate) (PPhEtMA, shown in Scheme 1) is 63 K lower than that of PBnMA although the chemical structures of PPhEtMA and PBnMA differ only by one methylene spacer inserted in the side chains.¹ This strong dependence of T_c on the chemical structure provides a mechanism by which one can control the phase behavior of polymers in IL systems by designing the chemical structure of polymers or ILs appropriately.

Scheme 1 Schematic illustration for the chemical structure of (a) PPhEtMA, (b) PBnMA and (c) [C₂mIm⁺][TFSA⁻], together with the reported phase separation temperature, T_c's of PPhEtMA and PBnMA in [C₂mIm⁺][TFSA⁻].¹

In a previous study, we studied PBnMA in the [C₂mIm⁺][TFSA⁻] system using small-angle neutron scattering (SANS). In this system, it was found that the PBnMA chains exist as independent Gaussian chains in [C₂mIm⁺][TFSA⁻] below T_c but suddenly form aggregates at T_c.² Furthermore, our investigations of the microscopic solvation structure of PBnMA in [C₂mIm⁺][TFSA⁻] by high-energy X-ray total scattering (HEXTS) modeled with the aid of MD simulations demonstrated that the C₂mIm⁺ cations are localized above and below the benzyl groups of PBnMA.²⁸ This partially ordered microscopic solvation structure of PBnMA in [C₂mIm⁺][TFSA⁻] solution decreases the mixing entropy, resulting in the observed LCST-type phase behavior. From a thermodynamic aspect, it was reported that the mixing enthalpy and entropy values of PBnMA in [C₂mIm⁺][TFSA⁻] are significantly smaller than those of PNIPAm in H₂O, indicating that T_c of the system can be easily modified by a slight change in the free energy of mixing.²⁹ Although several research groups have studied the structural aspects of thermo-responsive polymers in IL systems using infrared spectroscopy,³⁰ SANS,^31–33 and theoretical investigations of the phase diagram,³⁴ the correlation between the microscopic or the mesoscopic solvation structure and macroscopic phase behavior is still unclear. For example, the physical origin of the above-mentioned differences in the T_c's of PBnMA and PPhEtMA is completely unknown.

In this study, we chose PPhEtMA in [C₂mIm⁺][TFSA⁻] solution as a model system for investigating the key factors that determine the phase behavior of polymers in IL systems. We performed SANS measurements on this system and carried out quantitative data analysis to determine the thermodynamic parameters such as the second virial coefficient, A₂, the effective interaction parameter, χ_eff, and the Θ temperature. The enthalpic and entropic contributions to χ_eff were estimated from temperature dependence of χ_eff and compared with those obtained for PBnMA.

This study is an attempt to understand the phase behavior of polymers in IL systems from a viewpoint of microscopic molecular interactions. Such physicochemical characterization of the phase behavior will contribute enormously to facilitating a systematic design of novel systems of polymers in ILs.

Experimental

Materials

PPhEtMA was prepared by atom transfer radical polymerization (ATRP) following a previously-reported procedure.³⁵ CuBr 5.8 mg and CuBr₂ 0.9 mg were dried at 60 °C in a 50 mL Schlenk flask under vacuum and then stored in a nitrogen atmosphere. Anisole 10 mL and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) 10.3 μL were added to the flask and stirred until the mixture became transparent. 2-Phenylethyl methacrylate (PhEtMA) 10 mL and the initiator (2-bromoisobutyric acid ethyl ester) 23.9 μL were added and the system was degassed twice using freeze–pump–thaw cycles. The polymerization reaction of PhEtMA was conducted under a nitrogen atmosphere at 40 °C for 6 h, before termination by quenching with dry ice and methanol. The resulting product was purified by reprecipitation three times using ethyl acetate as the good solvent and methanol as the poor solvent and dried for 24 h at room temperature under vacuum. The structure of the polymer was characterized using ¹H-NMR spectroscopy and size exclusion chromatography (SEC). SEC was conducted using N,N-dimethylformamide (DMF) containing 0.01 mol L⁻¹ LiBr as the eluent. The number average molecular weight, M_n, was determined to be M_n = 31.5 k by ¹H-NMR and the polydispersity index (M_w/M_n) was measured to be M_w/M_n = 1.30 using Tosoh columns calibrated with PMMA standards, where M_w is the weight average molecular weight.

The ionic liquid [C₂mIm⁺][TFSA⁻] and the partially deuterated ionic liquid d₈-[C₂mIm⁺][TFSA⁻] whose H atoms are substituted by D for the ethyl group and the imidazolium-ring of the C₂mIm cation but not for the methyl group were synthesized according to the procedures reported before.^36,37 The deuteration ratio of d₈-[C₂mim⁺][TFSA⁻] was estimated to be higher than 93% based on ¹H-NMR spectroscopy measurements. All of the sample solutions were prepared using the cosolvent evaporation method with THF as the good solvent.^38,39

Viscosity measurements

Viscosity measurements were carried out on PPhEtMA in [C₂mIm⁺][TFSA⁻] solutions using a rheometer (MCR501, Anton Paar, Austria) at 298 K with a cone-plate geometry and a shear rate of 100 s⁻¹.

Dynamic light scattering (DLS) measurements

DLS measurements were carried out using a SLS/DLS 5022F-PCC-MS compact goniometer (ALV, Langen) coupled with an ALV photon correlator. A 22 mW He–Ne laser (wavelength, λ = 632.8 nm) was used as the incident beam and the scattering angle θ was 90°. Each measurement required 30 s. The solvent IL [C₂mIm⁺][TFSA⁻] was passed through a PTFE filter (pore size: 0.5 μm) prior to use. The temperature was changed at a rate of approximately 0.5 K min⁻¹, and DLS measurements were performed at each temperature after allowing the system to stabilize for at least 5 min.

The distribution function of the hydrodynamic radius R_h was obtained by applying the inverse Laplace transformation to the time correlation function of scattering intensity using the well-established CONTIN program.⁴⁰ The temperature dependence of the viscosity of [C₂mIm⁺][TFSA⁻] was determined using the Vogel–Tammann–Fulcher (VTF) equation as previously reported.⁴¹ The refractive index of [C₂mIm⁺][TFSA⁻] at each temperature was obtained from a previous report.⁴²

Cloud point observation

PPhEtMA in [C₂mIm⁺][TFSA⁻] and d₈-[C₂mIm⁺][TFSA⁻] solutions was poured into test tubes immersed in a water bath. Temperature was elevated at a rate slower than 0.2 K min⁻¹, and the cloud point was determined by visual observation. We measured the highest temperature at which sample solution looked completely transparent (T₁) and the lowest one at which it looked completely opaque (T₂). We defined the cloud point (T_c) as the middle between T₁ and T₂, i.e., T_c = (T₁ + T₂)/2, and defined the uncertainty of T_c (ΔT_c) as ΔT_c = (T₂ − T₁)/2.

SANS measurements

PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻] solutions of polymer concentration, c = 8–29 mg mL⁻¹ were injected into 2 mm thick spectroscopic grade quartz “banjo” cells, each containing 700 μl of the sample. The temperature was changed from 288 K to 310 K at a rate of 0.5 K min⁻¹; the SANS measurements were performed at each temperature after the solution had stabilized for at least 20 min. The SANS measurements were performed at the CG-2 General Purpose SANS instrument in the High Flux Isotope Reactor (HFIR) facility in Oak Ridge National Laboratory (ORNL, USA). Two instrument configurations were used to yield a q-range from 0.0028 Å⁻¹–0.46 Å⁻¹ where q is the magnitude of the scattering vector defined as q = (4π/λ)sinθ and 2θ is the scattering angle. The configurations were (i) source-to-sample distance (SSD) = 17.3 m, sample-to-detector distance (SDD) = 19.3 m and (ii) SSD = 5.1 m and SDD = 2.8 m. Both configurations used a wavelength of λ = 4.72 Å.

The measured data were corrected for background scattering (empty sample holder), transmission and detector efficiency according to the standard protocol using the Igor Pro based software provided by ORNL. The corrected scattering profiles were scaled to absolute units using a calibrated attenuated direct-beam measurement. The coherent scattering intensity from the sample solutions was obtained by the subtraction of the incoherent scattering calculated from chemical composition. We subtracted the coherent scattering intensity of the neat d₈-[C₂mIm⁺][TFSA⁻] from those of sample solutions to obtain the coherent scattering intensity of PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻], referred to hereafter as I(q).

The mass densities of [C₂mIm⁺][TFSA⁻], d₈-[C₂mIm⁺][TFSA⁻] and PhEtMA (monomer) were measured at 298 K using a digital density meter (DMA 4500, Anton Paar, Austria) to provide the values used for analysis; the results of density measurements are summarized in Table S1 (ESI†).

Results and discussion

Cloud points of PPhEtMA in [C₂mIm⁺][TFSA⁻] and d₈-[C₂mIm⁺][TFSA⁻]

We first determined the cloud points T_c of PPhEtMA in [C₂mIm⁺][TFSA⁻] and d₈-[C₂mIm⁺][TFSA⁻] solutions by visual observation. Fig. 1 shows the concentration dependence of T_c. As Fig. 1 indicates, the T_c of PPhEtMA was in the range of 306 K–310 K in d₈-[C₂mIm⁺][TFSA⁻] which is significantly lower than in protonated [C₂mIm⁺][TFSA⁻] (315–320 K). Such a difference of T_c in deuterated and protonated solvents has been previously reported for PNIPAm in aqueous solutions,^43–45 poly(styrene) in cyclohexane solutions,⁴⁶ polymers in IL solutions,³¹ and for polymer blends.^47,48 For example, the T_c of PNIPAm in D₂O was reported to be 0.6–0.8 K higher than in H₂O, due to the more stable hydrogen-bonding interactions in D₂O.

Fig. 1 The cloud points of PPhEtMA in [C₂mIm⁺][TFSA⁻] (black squares) and d₈-[C₂mIm⁺][TFSA⁻] (red circles) solution at various concentration as obtained by visual observation. The broken lines are guides to eye. The error bars represent the uncertainty of T_c, ΔT_c.

The effect of deuteration on T_c originates mainly from differences in molecular interactions in deuterated and protonated solvents.^31,44,45 It is thus expected that the observed differences in T_c of PPhEtMA in protonated and deuterated ILs similarly originate from the isotope effect on the strength of molecular interactions. The molar volume of neat d₈-[C₂mIm⁺][TFSA⁻] (2.5730 × 10⁻⁴ m³ mol⁻¹) calculated from its mass density (see Table S1, ESI†) was slightly smaller than that of the protonated [C₂mIm⁺][TFSA⁻] (2.5777 × 10⁻⁴ m³ mol⁻¹). This result indicates that cohesive interactions between IL ions are slightly enhanced by deuterium substitution. As mentioned in the Introduction, it has been reported previously that the enthalpy and entropy of mixing of PBnMA in [C₂mIm⁺][TFSA⁻] are quite small compared to those of PNIPAm in the H₂O system.²⁹ Hence, even a slight difference in the interactions can affect the T_c of PPhEtMA, one of the PBnMA derivatives in the IL. We conclude that the lower T_c of PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻] and the larger difference in T_c's (≈10 K) can be ascribed to the slightly stronger cohesive interactions between deuterated IL ions compared to those between protonated ones.

However, it should be noted that T_c decreases for PPhEtMA in an IL system while it increases for PNIPAm in water by deuterium substitution on solvents. This opposite tendency suggests that the solvation mechanism is different between ILs and aqueous systems. Here, the former is ascribed to the side-chain “philic” interaction between the phenyl group of PPhEtMA and the C₂mIm cation,²⁸ while the latter in the side-chain “phobic” interaction is stabilized by the solvent-solvent interaction.^44,45,49 When the cohesive interaction between solvent molecules is enhanced by deuterium substitution,⁴⁶ the side-chain “philic” interaction should become unfavorable whereas the side-chain “phobic” interaction favorable. We thus conjecture that the opposite deuteration effects on T_c of polymers in ILs and in water solutions reflect the difference in their solvation, i.e., the side-chain “philic” interaction of PPhEtMA in the [C₂mIm⁺][TFSA⁻] system and the side-chain “phobic” interaction of PNIPAm in the water system.

Viscosity measurements

Fig. 2 shows the reduced viscosity η_red of PPhEtMA in [C₂mIm⁺][TFSA⁻] solutions as a function of PPhEtMA concentration c, where η_red is defined by the following equation:Here, η and η₀ are the viscosities of the solution and the solvent, respectively. The chain-overlapping concentration (c*) was determined by the following equation;where Φ is the universal constant. We employed the value of Φ = 2.1 × 10²³ mol⁻¹ in our analysis.^50,51 The intrinsic viscosities, [η], were obtained by extrapolating η_red to c → 0, as shown by the solid line in Fig. 2. The c* value was estimated to be approximately 130 mg mL⁻¹; we performed the DLS and SANS measurements for PPhEtMA in [C₂mIm⁺][TFSA⁻] solution systems at concentrations lower than the estimated c*.

Fig. 2 Concentration dependence of the reduced viscosities (η_red) of PPhEtMA in [C₂mIm⁺][TFSA⁻] solutions. The intrinsic viscosities ([η]) were obtained by extrapolating η_red to c = 0 (solid line).

DLS measurements

Fig. 3(a) and (b) show the time-correlation function g⁽²⁾(τ) − 1 and the corresponding R_h distribution functions G(R_h) observed for 15 mg mL⁻¹ PPhEtMA solution at various temperatures, respectively. The DLS measurements were carried out below the phase-separating temperature as the solution became opaque and strong multiple scattering prevented DLS measurements above the phase-separating temperature. The G(R_h)s data at T < 303 K showed a single intense peak at around R_h ∼ 45–50 Å with the peak positions independent of temperature. Above 303 K, a small peak (slow-mode peak) appeared at R_h > 1000 Å while the peak at R_h ∼ 45–50 Å remained intense. This slow-mode peak was also observed in the previous study of PBnMA in the [C₂mIm⁺][TFSA⁻] system; it indicates the appearance of local micro-phase separation below the bulk phase separation temperature.² This result suggests that the majority of the PPhEtMA chains are homogeneously dispersed but that a small amount of aggregates coexists at the temperature near the critical temperature.

Fig. 3 (a) The time correlation functions of scattering intensity, g⁽²⁾(τ) −1 and (b) the distribution functions of the hydrodynamic radius, G(R_h) obtained for PPhEtMA in [C₂mIm⁺][TFSA⁻] solution of c = 15 mg mL⁻¹ at various temperatures. The correlation functions in (a) are vertically shifted for visibility. The solid lines in (b) are guide to eye.

Fig. 4 shows the temperature dependence of R_h of PPhEtMA chains corresponding to the fast mode. It is clear that R_h was almost independent of temperature, indicating that molecular conformation of solvated PPhEtMA chains remained essentially unchanged across the examined temperature range.

Fig. 4 The temperature dependence of the R_h values obtained for the PPhEtMA in [C₂mIm⁺][TFSA⁻] solution at c = 15 mg mL⁻¹.

SANS measurements

We performed SANS measurements for the present system using deuterated IL, d₈-[C₂mIm⁺][TFSA⁻], as the solvent to investigate the chain conformation and evaluate thermodynamic parameters for PPhEtMA in the [C₂mIm⁺][TFSA⁻] system. The SANS curves observed for PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻] solution (c = 16 mg mL⁻¹) at various temperatures are shown in Fig. 5. Here, c of the examined sample solution was substantially lower than c* (∼130 mg mL⁻¹) and the system was in the dilute region. As Fig. 5 indicates, the scattering intensity slightly increased with increasing temperature, but then showed a steep upturn in the data at temperatures higher than T_c (∼309 K, see Fig. 1). Here, the upturn at the low-q region (q < 0.006 Å⁻¹) and the slope in the middle-q region (0.03 Å⁻¹ < q < 0.13 Å⁻¹) are described by I(q) ∼ q⁻⁴ and I(q) ∼ q⁻², respectively. We modeled the data by fitting to the following scattering function based on Zimm-approximation;^52–54where, φ, χ_eff, V₁ and V₂ are the volume fraction of PPhEtMA, the effective interaction parameter, and the molar volumes of the solvent and solute components, respectively. Here, N_A is the Avogadro number, R_g is the radius of gyration, and Δρ is the difference in the scattering length density between the solute and the solvent. This function is well-established as the scattering function describing Gaussian chains in a dilute solution where a single-contact intermolecular interference is taken into account. The solid lines in Fig. 5 are the results of fitting the data using the functions described in eqn (3). As shown here, the scattering profiles below the critical temperature are well reproduced by eqn (3) except for a deviation from the theoretical curve in the low-q region of q < 0.006 Å⁻¹ and in the high-q region of q > 0.2 Å⁻¹. Here, R_g and χ_eff of dispersed PPhEtMA chains were obtained from curve fitting. The obtained R_g was almost constant below T_c (Fig. S3, ESI†). This result indicates that the conformation of PPhMA chains does not change below T_c. We will discuss χ_eff in detail in the later section. The slight upturn in the low-q region is attributed to the existence of small amounts of cluster observed in DLS as the slow mode in DLS (see Fig. 3) and is often observed for polymer solutions.^55,56 We can neglect this upturn in the curve fitting because the fraction of such a cluster in the system should be small and considering the fact that scattering intensity from them was very weak. In the high-q region, the absolute value of the exponent of I(q) against q was smaller than 2, indicating the deviation from the Gaussian conformation on the microscopic scale. In previous studies on aqueous solution of hydrophobic polyelectrolytes,^57,58 it is reported that the collapsed region (“pearl”) and the elongated region (“necklace”) coexist in a single chain. This structure is called the “pearl-necklace structure”. In the present study, as mentioned above, the exponent of I(q) vs. q showed a deviation from −2 at around q* = 0.2 Å. We conjecture that such a deviation reflects the existence of locally rigid structures consisting of PPhEtMA chains and their aggregates, i.e., the “pearl-necklace structure”. We roughly estimated the characteristic length scale, R* of “pearl” from q*, as R* ∼ 10 Å. Here, we used the relationship of R* = 1.91q*⁻¹ which is often used to estimate the persistence length of these wormlike chains.⁵⁹ We will not dwell further on this point as our primary concern in this study is determining the mesoscopic structure of the system and the phase separation process.

Fig. 5 SANS profiles observed for the PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻] solution (c = 16 mg mL⁻¹) at various temperatures shown in the form of the log–log plot. The solid lines are the fits with eqn (3) or eqn (3) + Aq⁻⁴ functions. The SANS profiles are also shown in the form of the semi-log plot in the inset.

For the scattering profiles at T > 309 K, we performed curve fitting by using a sum of eqn (3) and Debye–Bueche functions,

where Ξ is the characteristic size of inhomogeneities in the phase separated structure. In this study, we employed I(q) = eqn (3) + Aq⁻⁴ as a first approximation because of the absence of experimental data in the low-q region. Eventually, the scattering profiles at T = 310 K were successfully reproduced by eqn (3) with the Debye–Bueche term. It was impossible to estimate the size of aggregates only from SANS profiles because of the lack of data in the low-q region. In the DLS results (see Fig. 3), a slow-mode peak was observed near the critical temperature which corresponds to the size of a small amount of aggregates, R_h > 1000 Å. Based on these results, we presume that the size of the aggregates formed above the critical temperature is at least 1000 Å. These results show that PPhEtMA chains are homogeneously dispersed as the Gaussian chains before the phase separation, and that they suddenly form large aggregates at the critical temperature.

Most previous studies in the literature were performed using semi-dilute solution of phase-separating polymers in water,^43,60 organic solvents^55,61 and in ILs.^2,31–33 The small-angle scattering profiles of these showed behavior typical of spinodal decomposition: the divergence of scattering intensity correlation length as the critical point is approached. In contrast, in a recent study of PNIPAm in dilute aqueous solution, it was reported that the SANS profiles show a discontinuous change at the critical point where the steepness of the change decreased with the increasing fraction of the meso-diad in the PNIPAm chain, i.e., decreasing cooperativity of hydration.^62–64 Here, cooperative solvation means that the desorption of a solvent molecule from a site induces the desorption of another solvent molecule from its neighboring site. From this, we concluded that the sharpness of the phase separation results from the cooperative dehydration of PNIPAm at the critical point.

It should be noted that the present study shows a discontinuous change in the scattering from that of dispersed polymer chains to that of large aggregates in dilute solution. Based on the previous study on the PNIPAm dilute aqueous solution, we conjecture that the discontinuous change in SANS profiles observed for PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻] also originates from the cooperative dissociation of IL ions at T_c.

Earlier in this section, we demonstrated that the phase separation behavior of PPhEtMA in the [C₂mIm⁺][TFSA⁻] system in SANS was well described by the typical functions for the Gaussian chains in dilute solution with a phase separated structure. To investigate the system more quantitatively, we evaluated the thermodynamic parameters as determined from the Zimm plot. At the limit of low-c and low-q, a scattering intensity from a polymer solution I(q) can be approximated as follows:

where K = Δρ²/N_Ad_polymer² (for the present case, K = 0.0015 mol cm² g⁻²) is the contrast factor. Here, Δρ is the difference of scattering length densities between PPhEtMA and d₈-[C₂mIm⁺][TFSA⁻] (i.e., 3.044 × 10¹⁰ cm⁻²) and d_polymer, N_A, R_g,0 and A₂ are the mass density of PPhEtMA, the Avogadro number, the radius of gyration as c → 0, and the second virial coefficient, respectively. We used the mass density of the PhEtMA monomer (see Table S1, ESI†) instead of the corresponding value of PPhEtMA in the analysis, because the mass density of PPhEtMA in solution is unknown.

Fig. 6 shows the Zimm plot at various temperatures (288, 293, 298, and 303 K) below T_c. We successfully obtained reasonable values of M_w for PPhEtMA (the obtained M_w values are in the range of 32–34 kDa) by fitting the data to the function described in eqn (5), confirming the validity of the experiment and the analysis. Fig. 7 shows the temperature dependence of (a) A₂ and (b) R_g,₀ obtained from the Zimm plot. As Fig. 7(a) illustrates, A₂ decreased with increasing temperature and then changed its sign at T = 301 K, i.e., the Θ temperature. This result indicates that the compatibility of PPhEtMA decreases gradually with increasing temperature. As Fig. 7(b) demonstrates, R_g,0 was independent of temperature within its error, similar to the results of the DLS experiments (see Fig. 4). The stability of R_h and R_g,0 even near the critical point indicates that the desolvation of IL ions from the solvated PPhEtMA occurs suddenly at the critical temperature. This interpretation is supported by the sudden and discontinuous changes observed in SANS profiles at T_c (see Fig. 5).

Fig. 6 Zimm plots for the PPhEtMA in d₈-[C₂mIm⁺][TFSA⁻] solutions at (a) T = 288 K, (b) T = 293 K, (c) T = 298 K and (d) T = 303 K.

Fig. 7 The temperature dependence of (a) the second virial coefficient, A₂ and (b) the radius of gyration at c → 0, R_g,0 obtained from the Zimm plots shown in Fig. 6.

Comparison between PPhEtMA and PBnMA in [C₂mIm⁺][TFSA⁻] solutions

In the former sections, we evaluated the structural parameters for PPhEtMA in the [C₂mIm⁺][TFSA⁻] system. Here, we compare the structural parameters of PPhEtMA with those of PBnMA in [C₂mIm⁺][TFSA⁻] to probe the physical origin of the difference between the measured T_c 's (T_c of PPhEtMA in [C₂mIm⁺][TFSA⁻] is approximately 60 K lower than that of PBnMA).¹ We have previously reported a SANS study on PBnMA in the [C₂mIm⁺][TFSA⁻] system in which we estimated the effective interaction parameter χ_eff in the dilute region where the chain overlapping concentration was also determined by viscosity measurements (Fig. S1, ESI†).²Fig. 8 shows the comparison of the temperature dependence of χ_eff obtained for PPhEtMA and PBnMA using the same analytical procedure of fitting the curve using the function given in eqn (3). The value of χ_eff thus obtained for PPhEtMA solution at c = 16 mg mL⁻¹ was very close to the value converted from A₂ obtained using the Zimm plot shown in Fig. S2 (ESI†), confirming the validity of the curve fitting.

Fig. 8 The temperature dependence of the effective interaction parameter χ_eff of the PPhEtMA in [C₂mIm⁺][TFSA⁻] solution obtained from the curve fitting shown in Fig. 5, together with the corresponding values for PBnMA in the [C₂mIm⁺][TFSA⁻] solution system.² The solid lines are linear fits to χ_eff = A + B/T; the fitted values for A and B are shown in the graph.

In the vicinity of the critical point, χ_eff can be approximated as a function of temperature as follows:

where A and B are the entropic and enthalpic contributions to χ_eff, respectively.^33,65 As can be seen, the value of |B| obtained for PPhEtMA (=56 K⁻¹) was smaller than the corresponding value obtained for PBnMA (=84 K⁻¹) whereas there was a small difference in the values of |A| obtained for PPhEtMA (=0.69) and for PBnMA (=0.73).

The observed differences between the obtained |A| and |B| for PPhEtMA and PBnMA imply that the chemical modification of PBnMA by the insertion of a methylene group into the side chains lowers the absolute value of the enthalpy of mixing, resulting in the decrease of T_c. In our previous study, we pointed out that the dominant microscopic interaction between PBnMA and [C₂mIm⁺][TFSA⁻] is the cation-π interaction between C₂mIm and PBnMA whereby C₂mIm cations preferentially solvate above and below the benzyl groups in the PBnMA side chains.²⁸ Considering this fact, alkyl groups in side chains of PBnMA or PPhEtMA do not contribute to the interactions between C₂mIm cations and the side chains. Hence, we conclude that the lower |B| value of PPhEtMA than PBnMA originates from an increase of unfavorable interactions between PPhEtMA and the IL caused by the insertion of the methylene group.

Furthermore, we compared the values of A and B between the polymer in IL systems and PNIPAm in aqueous solution systems. The obtained values of |A| and |B| for PPhEtMA and PBnMA in [C₂mIm⁺][TFSA⁻] were smaller than those estimated for PNIPAm in aqueous solution using previously reported data (|A| = 1.15, |B| = 203 K⁻¹).⁶⁴ This reduced temperature dependence of χ_eff in IL systems indicates that the phase equilibrium of the IL system can be sensitively affected by a slight change in A or B, i.e., chemical modification to the polymer or IL. Hence, it was determined that the strong dependence of T_c on the chemical structure in IL systems originates from the relatively smaller temperature dependence of χ_eff in these systems.

Conclusions

The solvated structure and phase behavior of the thermo-responsive polymer PPhEtMA in the [C₂mIm⁺][TFSA⁻] solution system were investigated quantitatively by SANS experiments. The following facts were disclosed: (1) the obtained SANS profiles showed a discontinuous change from that of dispersed polymer chains to that of large aggregates for PPhEtMA in the IL solution, indicating that the phase separation occurs discontinuously at T_c. The scattering profiles were analyzed by curve fitting with a single-contact Debye function and by the Zimm plot. The second virial coefficient A₂ was successfully evaluated, and the results showed that the compatibility of PPhEtMA with the IL gradually decreases with increasing temperature. (2) The absolute value of the enthalpic term |B| in the effective interaction parameter χ_eff obtained for PPhEtMA was smaller than that for poly(benzyl methacrylate) (PBnMA). This difference in the enthalpic term can be attributed to an increase of the unfavorable interaction between IL and PPhEtMA caused by the insertion of the methylene group. (3) The obtained |A| and |B| values of the polymer in IL systems were significantly smaller than those of the PNIPAm in aqueous solution systems. It was determined that the strong dependence of T_c on the chemical structure in IL systems originated from the relatively-smaller temperature dependence of χ_eff.

Acknowledgements

This work has been financially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 22245018 to M. S.). The experiment (IPTS12408.1) using the General-Purpose SANS (CG-2) at Oak Ridge National Laboratory was supported by the US-Japan Cooperative Program on Neutron Scattering. Travel expenses for the experiment were also supported by General User Program for Neutron Scattering Experiments, Institute for Solid State Physics, The University of Tokyo (proposal no. 14906), at JRR-3, Japan Atomic Energy Agency, Tokai, Japan. The High Flux Isotope Reactor and beamline CG2 of ORNL was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. K. H. was supported by Japan Society for the Promotion of Science through Program for Leading Graduate Schools (MERIT).

Notes and references

1. K. Kodama, H. Nanashima, T. Ueki, H. Kokubo and M. Watanabe, Langmuir, 2009, 25, 3820.
2. K. Fujii, T. Ueki, K. Niitsuma, T. Matsunaga, M. Watanabe and M. Shibayama, Polymer, 2011, 52, 1589–1595.
3. T. Welton, Chem. Rev., 1999, 99, 2071–2083.
4. S. Seki, Y. Kobayashi, H. Miyashiro, Y. Ohno, A. Usami, Y. Mita, N. Kihira, M. Watanabe and S. Terada, J. Phys. Chem. B, 2006, 110, 10228–10230.
5. K. Fujii, H. Hamano, H. Doi, X. Song, S. Tsuzuki, H. Hayamizu, S. Seki, Y. Kameda, K. Dokko, M. Watanabe and Y. Umebayashi, J. Phys. Chem. C, 2013, 117, 19314–19324.
6. J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser and R. D. Rogers, Chem. Commun., 1998, 1765–1766.
7. C. Chiappe and D. Pieraccini, J. Phys. Org. Chem., 2005, 18, 275–297.
8. R. Madeira Lau, F. Van Rantwijk, K. R. Seddon and R. A. Sheldon, Org. Lett., 2000, 2, 4189–4191.
9. R. P. Swatloski, S. K. Spear, J. D. Holbrey and R. D. Rogers, J. Am. Chem. Soc., 2002, 124, 4974–4975.
10. F. Rantwijk and R. A. Sheldon, Chem. Rev., 2007, 107, 2757–2785.
11. M. Moniruzzaman, N. Kamiya, K. Nakashima and M. Goto, Green Chem., 2008, 10, 497–500.
12. R. T. Carlin and J. Fuller, Chem. Commun., 1997, 1345.
13. P. Scovazzo, J. Kieft, D. A. Finan, C. Koval, D. DuBois and R. Noble, J. Membr. Sci., 2004, 238, 57–63.
14. M. Galinski, A. Lewandowski and I. Stepniak, Electrochim. Acta, 2006, 51, 5567–5580.
15. N. Winterton, J. Mater. Chem., 2006, 16, 4281.
16. J. M. Lu, F. Yan and J. Texter, Prog. Polym. Sci., 2009, 34, 431–448.
17. T. Ueki and M. Watanabe, Bull. Chem. Soc. Jpn., 2012, 85, 35–50.
18. T. Ueki and M. Watanabe, Chem. Lett., 2006, 35, 964–965.
19. T. Ueki and M. Watanabe, Langmuir, 2007, 23, 988–990.
20. R. Tsuda, K. Kodama, T. Ueki, H. Kokubo, S. Imabayashi and M. Watanabe, Chem. Commun., 2008, 4939.
21. S. Tamura, T. Ueki, K. Ueno, K. Kodama and M. Watanabe, Macromolecules, 2009, 42, 1315.
22. T. Ueki, A. Yamaguchi, N. Ito, K. Kodama, J. Sakamoto, K. Ueno, H. Kokubo and M. Watanabe, Langmuir, 2009, 25, 8845.
23. T. Ueki, M. Watanabe and T. P. Lodge, Macromolecules, 2009, 42, 1315–1320.
24. H.-N. Lee and T. P. Lodge, J. Phys. Chem. Lett., 2010, 1, 1962–1966.
25. H. N. Lee and T. P. Lodge, J. Phys. Chem. B, 2011, 115, 1971–1977.
26. S. Imaizumi, H. Kokubo and M. Watanabe, Macromolecules, 2012, 45, 401–409.
27. Y. Kitazawa, T. Ueki, K. Niitsuma, S. Imaizumi, T. P. Lodge and M. Watanabe, Soft Matter, 2012, 8, 8067–8074.
28. M. Matsugami, K. Fujii, T. Ueki, Y. Kitazawa, Y. Umebayashi, M. Watanabe and M. Shibayama, Anal. Sci., 2013, 29, 311–314.
29. T. Ueki, A. A. Arai, K. Kodama, S. Kaino, N. Takada, T. Morita, K. Nishikawa and M. Watanabe, Pure Appl. Chem., 2009, 81, 1829–1841.
30. W. Li and P. Wu, Soft Matter, 2013, 9, 11585–11597.
31. H. N. Lee, N. Newell, Z. Bai and T. P. Lodge, Macromolecules, 2012, 45, 3627–3633.
32. H. Asai, K. Fujii, T. Ueki, S. Sawamura, Y. Nakamura, Y. Kitazawa, M. Watanabe, Y.-S. Han, T.-H. Kim and M. Shibayama, Macromolecules, 2013, 46, 1101–1106.
33. M. L. Hoarfrost, Y. He and T. P. Lodge, Macromolecules, 2013, 46, 9464–9472.
34. R. P. White and J. E. G. Lipson, Macromolecules, 2013, 46, 5714–5723.
35. Y. Kitazawa, T. Ueki, S. Imaizumi, T. P. Lodge and M. Watanabe, Chem. Lett., 2014, 43, 204–206.
36. K. Fujii, R. Kanzaki, T. Takamuku, Y. Kameda, S. Kohara, M. Kanakubo, M. Shibayama, S. Ishiguro and Y. Umebayashi, J. Chem. Phys., 2011, 135, 244502.
37. K. Fujii, H. Asai, T. Ueki, T. Sakai, S. Imaizumi, U. Chung, M. Watanabe and M. Shibayama, Soft Matter, 2012, 8, 1756–1759.
38. P. Snedden, A. I. Cooper, K. Scott and N. Winterton, Macromolecules, 2003, 36, 4549–4556.
39. Y. He, Z. Li, P. Simone and T. P. Lodge, J. Am. Chem. Soc., 2006, 128, 2745–2750.
40. S. W. Provencher, Comput. Phys. Commun., 1982, 27, 213–227.
41. H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan and M. Watanabe, J. Phys. Chem. B, 2005, 109, 6103.
42. T. Ueki, T. Karino, Y. Kobayashi, M. Shibayama and M. Watanabe, J. Phys. Chem. B, 2007, 111, 4750–4754.
43. M. Shibayama, T. Tanaka and C. C. Han, J. Chem. Phys., 1992, 97, 6829–6841.
44. H. Shirota and K. Horie, Chem. Phys., 1999, 242, 115.
45. H. Shirota, N. Kuwabara, K. Ohkawa and K. Horie, J. Phys. Chem. B, 1999, 103, 10400.
46. C. Strazielle and H. Benoit, Macromolecules, 1975, 8, 203–205.
47. H. Yang, M. Shibayama, R. S. Stein, N. Shimizu and T. Hashimoto, Macromolecules, 1986, 19, 1667–1674.
48. E. L. Atkin, L. A. Kleintjens, R. Koningsveld and L. J. Fetters, Makromol. Chem., 1984, 185, 377–387.
49. M. Shibayama, S. Mizutani and S. Nomura, Macromolecules, 1996, 29, 2019–2024.
50. H. Fujita, Polymer Solution, Elsevier, Amsterdam, 1990.
51. H. Asai, K. Fujii, T. Ueki, T. Sakai, U. Chung, M. Watanabe, Y. S. Han, T. H. Kim and M. Shibayama, Macromolecules, 2012, 45, 3902–3909.
52. H. Benoit and M. Benmouna, Polymer, 1984, 25, 1059–1067.
53. H. Benoît, C. Picot and M. Benmouna, J. Polym. Sci., Part B: Polym. Phys., 1984, 22, 1545–1548.
54. J. S. Higgins and H. C. Benoit, Polymers and Neutron Scattering, Clarendon Press, Oxford, 1994.
55. Y. Xie, K. F. Ludwig Jr, R. Bansil, P. D. Gallagher, X. Cao and G. Morales, Physica A, 1996, 232, 94–108.
56. B. Hammouda, D. Ho and S. Kline, Macrmolecules, 2002, 35, 8578–8585.
57. Q. Liao, A. V. Dobrynin and M. Rubinstein, Macromolecules, 2006, 39, 1920–1938.
58. M. N. Spiteri, C. E. Williams and F. Boué, Macromolecules, 2007, 40, 6679–6691.
59. A. K. Gupta, J. P. Cotton, E. Marchal, W. Burchard and H. Benoit, Polymer, 1976, 17, 363–366.
60. I. R. Nasimova, T. Karino, S. Okabe, M. Nagao and M. Shibayama, Macromolecules, 2004, 37, 8721–8729.
61. Y. B. Melnichenko, G. D. Wignall, A. W. Van Hook, J. Szydlowski, H. Wilczura and L. P. Rebelo, Macromolecules, 1998, 31, 8436–8438.
62. B. H. Zimm and J. K. Bragg, J. Chem. Phys., 1959, 31, 526–535.
63. Y. Okada and F. Tanaka, Macrmolecules, 2005, 38, 4465–4471.
64. K. Nishi, T. Hiroi, K. Hashimoto, K. Fujii, Y.-S. Hang, T.-H. Kim, Y. Katsumoto and M. Shibayama, Macromolecules, 2013, 46, 6225–6232.
65. A. R. Shultz and P. J. Flory, J. Am. Chem. Soc., 1952, 74, 4760–4767.

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Footnote

† Electronic supplementary information (ESI) available: Mass density of the material, viscosity of PBnMA in [C₂mIm⁺][TFSA⁻] solutions and χ_eff calculated from A₂. See DOI: 10.1039/c6cp02254e

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