Published: December 15, 2011 r2011 American Chemical Society 775 dx.doi.org/10.1021/jp210376u | J. Phys. Chem. B 2012, 116, 775–781 ARTICLE pubs.acs.org/JPCB Molecular Dynamics of Poly(N-isopropylacrylamide) in Protic and Aprotic Solvents Studied by Dielectric Relaxation Spectroscopy Shinya Nakano, † Yasuhiro Sato, † Rio Kita,* ,† Naoki Shinyashiki, † Shin Yagihara, † Seiichi Sudo, ‡ and Masaru Yoneyama § † Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan ‡ Department of Physics, General Education Center, Tokyo City University, Tamazutsumi, Setagaya, Tokyo 158-8557, Japan § Department of Chemistry and Chemical Biology, Gunma University, Kiryu, Gunma 376-8515, Japan ’ INTRODUCTION Dielectric relaxation spectroscopy has been utilized for study- ing the molecular dynamics of polymer solutions. 1À6 In the case of polymer solutions composed of polar solvents in a solvent-rich region, relaxation processes due to the reorientation of dipoles of solvents and polymer chains are observed separately at higher and lower frequencies, respectively. 3À5,7,8 Typically, the relaxa- tion process observed at frequencies on the order of 10 GHz is associated with the molecular motion of solvent molecules, while the relaxation process observed at kHz - MHz frequencies is attributed to the relaxation modes of polymer chains. The relaxation process owing to solvent molecules is affected by the addition of polymers. 3,4,9À11 This implies that the dynamical structures of solvent molecules are related to the polymers through interactions at the molecular level. The relaxation process that arises from the polymer chains should also be affected by the solvent molecules. This interdependence of polymer chains and solvent molecules can be analyzed by investigating the dielectric relaxation spectrum as a function of concentration and/or temperature. Dielectric relaxation spectra can be described by, for example, the relaxation time, the relaxation strength, and the shape parameter characterizing the distribution of the relaxation process. Therefore, the relaxation parameters obtained by the variation of polymer concentration or temperature, as well as the solvent species, can provide important information leading to greater understanding of molecular interactions. Recently, the relaxation processes of polymer chains and solvent molecules have been studied system- atically for the poly(vinylpyrrolidone) [PVP] system in various polar and nonpolar solvents in broad temperature and frequency ranges. 3À5 It has been revealed that the cooperation between segmental motion and the reorientation of solvent molecules provides intrinsic information about the molecular dynamics of polymer solutions. In this study, we report the experimental results of dielectric relaxation behavior for the systems of poly(N-isopropylacryl- amide) (PNiPAM) in protic and aprotic solvents as a function of PNiPAM concentration studied by dielectric relaxation spectroscopy. An aqueous solution of PNiPAM has a Θ-temperature of 30.6 °C and undergoes a coil Àglobule transition upon heating. 12À15 The transition of PNiPAM chains in water is also observed upon the addition of a second water-miscible solvent, such as methanol, Received: October 28, 2011 Revised: December 13, 2011 ABSTRACT: We report the experimental results of dielectric relaxation spectroscopy for the systems of poly(N-isopropyl- acrylamide) [PNiPAM] in various solvents in the frequency range of 40 kHz to 20 GHz at the solution temperature of 25.0 °C. The solvents used in this study were protic solvents (water, methanol, ethanol, and 1-propanol) and aprotic solvents (acetone, methyl ethyl ketone, and dimethyl sulfoxide). Two relaxation processes were observed at frequencies of approximately 1 MHz and 10 GHz in all the solutions. The origins of the two relaxation processes are considered to be the reorientation of dipoles of the PNiPAM chains at middle frequencies (m-process) and that of solvent molecules at higher frequencies (h-process). For the PNiPAM solutions composed of protic solvents except for 1-propanol, the relaxation time of the h-process increased with increasing PNiPAM concentration, whereas that of the h-process for the 1-propanol decreased with increasing PNiPAM concentration. In contrast, the relaxation times of the h-process for the aprotic solvents were independent of the density of hydrogen-bonding sites. For the m-process, which is attributed to the local chain motion of PNiPAM, the extrapolated relaxation time to zero polymer concentration τ m0 was scaled by the solvent viscosity for all the protic solvents, whereas for the aprotic solvents τ m0 showed no correlation with the solvent viscosity. The dynamics of polymer chains and solvent molecules in their solution state are clarified in terms of cooperative motion, which is associated with the interactions through hydrogen bonding at the molecular level.