Pressure Response of a Thermoresponsive Polymer in an Ionic Liquid

Oct 21, 2016 - The cloud point temperature of the IL solution increased monotonically with pressure, which was quite different from that of convention...
5 downloads 12 Views 1MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Pressure Response of a Thermoresponsive Polymer in an Ionic Liquid Kazu Hirosawa,† Kenta Fujii,*,‡ Takeshi Ueki,§ Yuzo Kitazawa,∥ Masayoshi Watanabe,∥ and Mitsuhiro Shibayama*,† †

Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Graduate School of Sciences and Technology for Innovation, Yamaguchi University, 2-16-1 Tokiwadai, Ube, Yamaguchi 755-8611, Japan § Polymer Materials Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044 Japan ∥ Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡

S Supporting Information *

ABSTRACT: We investigated pressure effects on the lower critical solution temperature (LCST)-type phase behavior of a thermoresponsive polymer in an ionic liquid (IL) solution. The cloud point temperature of the IL solution increased monotonically with pressure, which was quite different from that of conventional aqueous polymer solutions reported in previous work, which exhibited a convex-upward-type pressure dependence. We compared the IL and aqueous systems and concluded that the difference results from their solvation mechanisms. Dynamic light scattering (DLS) measurements showed an appearance of a slow mode (corresponding to aggregation) in addition to the fast mode (corresponding to molecular dispersion) at the cloud point pressure, indicating an onset of pressure-induced phase separation. This work contributes to the fundamental understanding of the phase behavior of polymers in IL systems under high pressure.

■

INTRODUCTION One of the most well-known thermoresponsive polymers, poly(N-isopropylacrylamide) (PNIPAm), undergoes a lower critical solution temperature (LCST)-type phase separation in water in increasing temperature.1 From a thermodynamic viewpoint, such LCST-type phase behavior is characterized by negative change in both enthalpy and entropy upon mixing: ΔHm < 0 and ΔSm < 0, respectively. In aqueous solutions with hydrophobic solutes, including PNIPAm, the formation of aqueous hydrogen-bonding networks around hydrophobic groups is the origin of negative ΔSm.2−5 Here, the partial structure formation of solvent molecules (i.e., “solvent− solvent” interactions around hydrophobic molecules) plays a key role in the phase behavior. Ionic liquids (ILs) are liquids composed of only ion species. Their solvent properties, such as hydrophobicity and hydrophilicity, can be controlled by the combination of cations and anions. We recently reported that neutral polymers with aromatic groups in their side chains, such as poly(benzyl methacrylate) (PBnMA) derivatives, show LCST-type phase separation in imidazolium-type ILs.6,7 We showed that in the polymer/IL systems the negative ΔSm values originate from structural ordering due to “solvent−solute” interactions: cation−π interactions between the imidazolium cation and the aromatic side chains of the polymers.8,9 The structural ordering in solutions of hydrophobic polymers is essentially different between aqueous and IL systems. In aqueous PNIPAm solutions, the polymer is solubilized by the formation © XXXX American Chemical Society

of networks of water molecules around the polymer (“solvent− solvent” interactions); water molecules do not directly interact with the polymer. On the other hand, in the case of PBnMA derivatives in IL solutions, IL ions interact with the polymer and form a partially ordered structure based on direct “solvent− solute” interactions. To understand this difference at the molecular level and to elucidate solvation effects on LCST phase behavior, we focused on pressure as an experimental variable. It has been reported that pressure affects the miscibility of synthetic polymers10−12 and proteins5,13,14 in aqueous solutions by affecting molecular volumes and intermolecular interactions in solution.15 For this study, we investigated pressure dependence of the miscibility and solvated structure of a hydrophobic polymer in IL solution and discuss the relation between LCST phase behavior and polymer solvation. The combination of poly(2-phenylethyl methacrylate) (PPhEtMA, see Chart 1) and the IL 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA], see Chart 1) was selected as a typical thermoresponsive system. The pressure response of this system was compared to that of an aqueous PNIPAm solution.11 Received: September 9, 2016 Revised: October 15, 2016

A

DOI: 10.1021/acs.macromol.6b01987 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Chart 1. Chemical Structure of (a) Poly(2-phenylethyl methacrylate) (PPhEtMA) and (b) 1-Ethyl-3methylimidazolium Bis(trifluoromethanesulfonyl)amide ([C2mIm][TFSA])

■

EXPERIMENTAL SECTION

Materials. [C2mIm][TFSA] was synthesized according to previously reported procedures.16 PPhEtMA was synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization following the scheme in a previous work (detailed information is shown in the Supporting Information).17 The synthesized PPhEtMA was characterized using 1H NMR spectroscopy and size exclusion chromatography (SEC). The number-average molecular weight (Mn) was calculated from the monomer conversion in reaction solution determined by 1H NMR to be Mn = 29 kDa. The polydispersity D (Mw/Mn, where Mw is the weight-average molecular weight) was determined to be D = 1.49 by SEC calibrated with PMMA standards using tetrahydrofuran (THF) as the eluent. The synthesized PPhEtMA was dissolved in [C2mIm][TFSA] at concentration of 3 wt % using the cosolvent evaporation method with THF as the good solvent.18 The prepared solution was used as sample solution. The cloud point temperature of the sample solution at atmospheric pressure (Tc0) was measured to be Tc0 = 43 °C by visual observation. Cloud Point Measurement and Dynamic Light Scattering Experiment. Pressure-dependent dynamic light scattering (DLS) experiment and cloud point measurement were carried out with an inner-cell-type pressure cell with a set of sapphire optical windows, PCI-400, Syn. Co. Ltd., Kyoto, Japan. The basic concept of the pressure cell is described elsewhere.19 A He−Ne laser with a power of 22 mW (wavelength, λ = 632.8 nm) was used as the incident beam in the measurements. DLS experiment was conducted with a static/ dynamic compact goniometer (SLS/DLS-5000), ALV, Langen, Germany. The scattering angle was fixed to 90°. Each measurement required 30 s. Sample solution was passed through a PTFE filter (pore size: 0.2 μm) prior to use. The intensity of transmitted beam was monitored by power and energy analyzer (FieldMaster GS, Coherent Inc., USA) for the estimation of transmittance. After temperature and pressure stabilization, the hydrostatic pressure was decreased stepwise under isothermal condition, and cloud point measurement and DLS experiment were performed at various pressures, P. Sample solution was stabilized for 3 min after changing pressure at each P before the measurements.

Figure 1. Pressure dependence of the transmittance of 3 wt % PPhEtMA/[C2mIm][TFSA] solution at various temperatures.

examined range of T and P (T: 44−77 °C; P: 0.1−364 MPa). Taking Pc as the pressure at which the transmittance value becomes lower than 0.5, we constructed a P−T phase diagram which shows the pressure dependence of the cloud point temperature (Tc). Figure 2 shows the P−T phase diagram obtained for the 3 wt % PPhEtMA/[C2mIm][TFSA] solution together with that of

Figure 2. P−T phase diagram of 3 wt % PPhEtMA/[C2mIm][TFSA] solution (red circles) and of aqueous 8 wt % PNIPAm solution (black squares).11

■

RESULTS AND DISCUSSION Cloud Point Measurements and a P−T Phase Diagram. Figure 1 shows the P dependence of the transmittance of 3 wt % PPhEtMA/[C2mIm][TFSA] solution at various temperatures of T > Tc0. Here, transmittance is defined as Itr/I0, where Itr and I0 denote the transmitted light intensities from the sample solution and from an empty cell, respectively. The transmittance decreased dramatically at certain cloud point pressures (Pc) at all the temperatures examined. These results indicate that PPhEtMA is soluble at P > Pc but undergoes phase separation at Pc. The observed Pinduced phase transition was confirmed to be reversible by transmittance measurement in a repressurizing process (see Figure S1). The Pc gradually increased with increasing T, suggesting that Pc is a monotonically increasing function with T. Here, we also measured trasmittances at higher P (140 MPa < P < 364 MPa), but we did not observe any significant change in transmittances (see Figure S2). It suggests that the PPhEtMA/[C2mIm][TFSA] system has only one Pc in the

the aqueous PNIPAm solution reported elsewhere.11 The pressure dependence of Tc of the PPhEtMA/[C2mIm][TFSA] solution was much larger than that of the aqueous PNIPAm solution. The Tc of the PPhEtMA/[C2mIm][TFSA] system increased monotonically with increasing P, which differs from the aqueous PNIPAm system.11 The system of PNIPAm in aqueous solution exhibits a convex-upward P dependence, which is commonly observed for hydrophobic solutes in aqueous solutions.12,20−22 The difference between the IL and aqueous systems might be ascribed to the solvation environments around polymeric solutes, and the pressure response of the LCST phase behavior strongly depends on the solvent species. The phase boundary line in the P−T phase diagram is given by the following Clapeyron-type equation:15 T ΔVm(P) dT = dP ΔHm(P) B

(1) DOI: 10.1021/acs.macromol.6b01987 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules where ΔHm(P) and ΔVm(P) denote the changes in the enthalpy and volume by mixing as a function of pressure, respectively. Here, the enthalpy of mixing of the PPhEtMA/ [C2mIm][TFSA] system at ambient pressure is reported to be smaller than that of the aqueous PNIPAm system,23,24 which is confirmed by the experimental results in this work. The larger |dT/dP| obtained in the IL system compared to the aqueous PNIPAm system (Figure 2) can be ascribed to a smaller |ΔHm(P)| in the IL system, based on eq 1. The physical origins of the different behavior in the P−T diagram between the IL and aqueous polymer solutions are discussed below. As observed from eq 1, the sign of dT/dP is determined by that of ΔVm(P). ΔHm(P) must be negative for the system to exhibit LCST phase behavior, so a negative ΔVm(P) results in a positive dT/dP and vice versa.15 Here, ΔVm(P) might be separated into two contributions: the release of free volume by mixing (ΔVm,free(P)) and the formation of a solvation shell around polymer side chains (ΔVm,solv(P)), as illustrated in Figure 3a. The sign of ΔVm,free(P) is negative in

Schematic models showing the solvation structure before and after pressurization for the aqueous PNIPAm solution and the PPhEtMA/[C2mIm][TFSA] solution are shown in Figure 3b. In aqueous polymer solutions, the polymer is solubilized by a hydration shell formed by enhanced hydrogen bonding between water molecules around hydrophobic side chains. The formation of this hydration shell causes expansion of the system upon mixing, leading to a positive ΔVm,solv(P) in the aqueous system. Because |ΔVm,free(P)| is a decreasing function of P,20,22 a positive contribution from the ΔVm,solv(P) overwhelms a negative contribution from the ΔVm,free(P) in the high-P region, resulting in the change of the sign of ΔVm(P) (= ΔVm,solv(P) + ΔVm,free(P)) from negative to positive. Hence, dT/dP changes from positive to negative with pressure at a certain P. This phenomenon is the origin of the convex-upward P−T phase diagram in the aqueous polymer system.20,25 On the other hand, in the PPhEtMA/[C2mim][TFSA] system, the polymer is mainly solubilized by cation−π interactions in which the IL cations directly interact with the polymer. In this situation, there is no significant contribution of solvation to the volume change. That is, |ΔVm,solv(P)| is negligible compared to |ΔVm,free(P)|. Thus, ΔVm,solv(P) does not cause a significant positive contribution to ΔVm(P) even in the high-P region. In the observed results, the ΔVm(P) remained negative and the dT/dP did not change its sign in the IL system, even under the highest examined value of P. DLS Experiments. To obtain more insight into the pressure effects of polymer solvation in ILs, we performed light scattering measurements at various P on PPhEtMA in [C2mIm][TFSA]. Figure 4 shows (a) pressure dependence of the time correlation function of scattering intensity, g(2)(τ) − 1, and (b) the distribution function, G(Γ−1) of decay time (Γ−1) of 3 wt % PPhEtMA/[C2mIm][TFSA] solution at T = 44 °C. Here, G(Γ−1) was obtained by applying the inverse Laplace transformation to g(2)(τ) − 1 using the well-established CONTIN program.26 The Pc at T = 44 °C was 3 MPa (Figure 1). As observed from Figure 4, a single translational diffusion mode was observed at P > 7 MPa, indicating the presence of homogeneously dispersed polymer chains. In the vicinity of Pc (P ≤ 5 MPa), an additional slow mode was observed; its intensity was enhanced with decreasing pressure. In addition, as shown in Figure 4c, the time-averaged scattering intensity ⟨I⟩T diverged in the vicinity of the cloud point pressure. These results indicate that aggregates of PPhEtMA are formed at pressures near Pc , resulting in pressure-induced phase separation. This behavior is also observed in the temperatureinduced phase separation of this system at ambient pressure.24 In conclusion, we observed a specific pressure effect on the solubility of PPhEtMA in [C2mIm][TFSA] solution showing an LCST-type phase transition. Pressurization resulted in a monotonic increase of the cloud point temperature in the IL system, which was a quite different behavior from that of conventional aqueous systems. The observed differences in the pressure response reflected the difference in the characteristics of solvation in each system, i.e., whether the polymer is solubilized by direct “solvent−polymer” interaction (IL systems, this work) or enhanced hydrogen bonding between solvent molecules around the polymer (aqueous systems). Furthermore, the pressure-induced phase transition of the PPhEtMA/[C2mIm][TFSA] solution was also investigated by dynamic light scattering experiments. The appearance of a slow mode in the time-correlation function of scattering intensity, as well as the divergence of time-averaged scattering intensity, was

Figure 3. (a) Illustration of the volume change in the mixing process of polymer and solvent. (b) Schematic models showing the solvation structure of aqueous PNIPAm solution (above) and PPhEtMA/ [C2mIm][TFSA] solution (below) before and after pressurization.

polymer solution systems because a large volume of polymer is released upon mixing.20,22 Hence, the behavior of dT/dP is mainly determined by the P dependence of the ΔVm,solv(P) term. Based on the above discussion, the observed difference between the pressure response of the PPhEtMA/[C2mIm][TFSA] solution and that of the aqueous system is characterized by the magnitude of |ΔVm,solv(P)| in each system. C

DOI: 10.1021/acs.macromol.6b01987 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

mittance of the PPhEtMA sample solution at T = 77 °C and P = 0.1−364 MPa (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (K.F.). *E-mail [email protected] (M.S.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work has been financially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 16H02277 and 25248027 to M.S.). K.H. was supported by the Japan Society for the Promotion of Science through the Program for Leading Graduate Schools (MERIT).

■

Figure 4. (a) Time correlation functions of scattering intensity, g(2)(τ) − 1, measured at various pressures. The temperature is fixed at T = 44 °C. (b) Distribution function, G(Γ−1), of decay time (Γ−1) obtained from the inversed Laplace transformation procedure for g(2)(τ) − 1. (c) Time-averaged scattering intensity of light, ⟨I⟩T, at various pressures.

observed in the vicinity of the cloud point pressure, indicating that aggregates were suddenly formed at the cloud point.

■

REFERENCES

(1) Schild, H. G. Poly(N-isopropylacrylamide): Experiment, Theory and Application. Prog. Polym. Sci. 1992, 17, 163−249. (2) Shibayama, M.; Mizutani, S.; Nomura, S. Thermal Properties of Copolymer Gels Containing N-isopropylacrylamide. Macromolecules 1996, 29, 2019−2024. (3) Ono, Y.; Shikata, T. Hydration and Dynamic Behavior of Poly(Nisopropylacrylamide)s in Aqueous Solutions: A Sharp Phase Transition at the Lower Critical Solution Temperature. J. Am. Chem. Soc. 2006, 128, 10030−10031. (4) Frank, H. S.; Evans, M. W. Free Volume and Entropy in Condensed Systems. Entropy in Binary Liquid Mixtures; Partial Molar Entropy in Dilute Solutions; Structure and Thermodynamics in Aqueous Electrolytes. J. Chem. Phys. 1945, 13, 507−532. (5) Kauzmann, W. Thermodynamics of unfolding. Nature 1987, 325, 763−764. (6) Ueki, T.; Watanabe, M. Lower Critical Solution Temperature Behavior of Linear Polymers in Ionic Liquids and the Corresponding Volume Phase Transition of Polymer Gels. Langmuir 2007, 23, 988− 990. (7) Kodama, K.; Nanashima, H.; Ueki, T.; Kokubo, H.; Watanabe, M. Lower Critical Solution Temperature Phase Behavior of Linear Polymers in Imidazolium-Based Ionic Liquids: Effects of Structural Modifications. Langmuir 2009, 25, 3820−3824. (8) Matsugami, M.; Fujii, K.; Ueki, T.; Kitazawa, Y.; Umebayashi, Y.; Watanabe, M.; Shibayama, M. Specific Solvation of Benzyl Methacrylate in 1-Ethyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquid. Anal. Sci. 2013, 29, 311−314. (9) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.; Hardacre, C.; Rogers, R. D. Liquid Clathrate Formation in Ionic Liquid-Aromatic Mixtures. Chem. Commun. 2003, 4, 476−477. (10) Otake, K.; Karaki, R.; Ebina, T.; Yokoyama, C.; Takahashi, S. Pressure Effects on the Aggregation of Poly(N-isopropylacrylamide) and Poly(N-isopropylacrylamide-co-acrylic acid) in Aqueous Solutions. Macromolecules 1993, 26 (9), 2194−2197. (11) Shibayama, M.; Isono, K.; Okabe, S.; Karino, T.; Nagao, M. SANS Study on Pressure-Induced Phase Separation of Poly(Nisoproptylacylamide) Aqueous Solutions and Gels. Macromolecules 2004, 37, 2909−2918. (12) Osaka, N.; Shibayama, M. Pressure-Induced Phase Transitions of Hydrophobically Solvated Block Copolymer Solutions. Phys. Rev. Lett. 2006, 96 (4), 048303. (13) Smeller, L. Pressure-temperature Phase Diagram of Biomolecules. Biochim. Biophys. Acta, Protein Struct. Mol. Enzymol. 2002, 1595, 11−29. (14) Osaka, N.; Takata, S. I.; Suzuki, T.; Endo, H.; Shibayama, M. Comparison of Heat- and Pressure-induced Gelation of Beta-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01987. Synthetic procedures of PPhEtMA, confirmation of reversibility of P-induced phase transition, and transD

DOI: 10.1021/acs.macromol.6b01987 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules lactoglobulin Aqueous Solutions Studied by Small-angle Neutron and Dynamic Light Scattering. Polymer 2008, 49 (12), 2957−2963. (15) Rebelo, L. P. N. A Simple gE-model for Generating All Basic Types of Binary Liquid-liquid Equilibria and Their Pressure Dependence. Thermodynamic Constrains at Critical Loci. Phys. Chem. Chem. Phys. 1999, 1, 4277−4286. (16) Fujii, K.; Kanzaki, R.; Takamuku, T.; Kameda, Y.; Kohara, S.; Kanakubo, M.; Shibayama, M.; Ishiguro, S.; Umebayashi, Y. Experimental Evidences for Molecular Origin of Low-Q peak in Neutron/X-ray Scattering of 1-Alkyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)amide Ionic Liquids. J. Chem. Phys. 2011, 135, 244502. (17) Kitazawa, Y.; Ueki, T.; McIntosh, L. D.; Tamura, S.; Niitsuma, K.; Imaizumi, S.; Lodge, T. P.; Watanabe, M. Hierarchical Sol−Gel Transition Induced by Thermosensitive Self-Assembly of an ABC Triblock Polymer in an Ionic Liquid. Macromolecules 2016, 49 (4), 1414−1423. (18) He, Y.; Li, Z.; Simone, P.; Lodge, T. P. Self-Assembly of Block Copolymer Micelles in an Ionic Liquid. J. Am. Chem. Soc. 2006, 128 (8), 2745−2750. (19) Matsumoto, M.; Murakoshi, K.; Wada, Y.; Yanagida, S. High Pressure Effects on Fluorescence-Quenching of Coumarin 343 Adsorbed on TiO2 Nanocrystallites in Methanol and in N,NDimethylformamide. Chem. Lett. 2000, 29, 938−939. (20) Sawamura, S.; Kitamura, K.; Taniguchi, Y. Effect of Pressure on the solubilities of Benzene and Alkylbenzens in water. J. Phys. Chem. 1989, 93, 4931−4935. (21) Rebelo, L. P. N.; Visak, Z. P.; de Sousa, H. C.; Szydlowski, J.; Gomes de Azevedo, R.; Ramos, A. M.; Najdanovic-Visak, V.; Nunes da Ponte, M.; Klein, J. Double Critical Phenomena in (Water + Polyacrylamides) Solutions. Macromolecules 2002, 35, 1887−1895. (22) Sawamura, S.; Nagaoka, K.; Machikawa, T. Effects of Pressure and Temperature on the Solubility of Alkylbenzenes in Water: Volumetric Property of Hydrophobic Hydration. J. Phys. Chem. B 2001, 105 (12), 2429−2436. (23) Ueki, T.; Arai, A. A.; Kodama, K.; Kaino, S.; Takada, N.; Morita, T.; Nishikawa, K.; Watanabe, M. Thermodynamic Study on Phase Transitions of Poly(benzyl methacrylate) in Ionic Liquid Solvents. Pure Appl. Chem. 2009, 81, 1829−1841. (24) Hirosawa, K.; Fujii, K.; Ueki, T.; Kitazawa, Y.; Littrell, K. C.; Watanabe, M.; Shibayama, M. SANS Study on the Solvated Structure and Molecular Interactions of a Thermo-responsive Polymer in a Room Temperature Ionic Liquid. Phys. Chem. Chem. Phys. 2016, 18 (27), 17881−17889. (25) Nasimova, I. R.; Karino, T.; Okabe, S.; Nagao, M.; Shibayama, M. Small-angle Neutron Scattering Investigation of Pressure Influence on the Structure of Weakly Charged Poly(N-isopropyl acrylamide) Solutions and Gels. Macromolecules 2004, 37 (23), 8721−8729. (26) Provencher, S. W. A Constrained Regularization Method for Investing Data Represented by Linear Algebraic or Integral Equations. Comput. Phys. Commun. 1982, 27, 213−227.

E

DOI: 10.1021/acs.macromol.6b01987 Macromolecules XXXX, XXX, XXX−XXX