Broadband Dielectric Study of Dynamics of Polymer and Solvent in Poly(vinyl pyrrolidone)/ Normal Alcohol Mixtures N. Shinyashiki,* D. Imoto, and S. Yagihara Department of Physics, Tokai UniVersity, Hiratsuka, Kanagawa 259-1292, Japan ReceiVed: August 22, 2006; In Final Form: NoVember 7, 2006 Broadband dielectric measurements of poly(vinyl pyrrolidone) (PVP)-monohydroxyl alcohol mixtures of various normal alcohols with the number of carbon atoms per molecule ranging from 1 to 9 were made in the frequency range of 20 Hz to 20 GHz at 25 °C. Two relaxation processes due to the reorientation of dipoles on the PVP and alcohol molecules were observed. The relaxation process at frequencies higher than 100 MHz is the primary process of alcohols, and that at frequencies lower than 10 MHz is attributed to the local chain motion of PVP. For mixtures of alcohol molecules that are smaller than propanol, the relaxation time of the alcohol increases with increasing PVP concentration, whereas for mixtures of alcohol molecules larger than butanol, the relaxation time of the alcohol decreases with increasing PVP concentration. The increase in the density of hydrogen-bonding sites upon the addition of PVP reduces the relaxation time of alcohol in the mixture, and vice versa. The relaxation time of the local chain motion of PVP increases with PVP concentration and solvent viscosity. Different time scales of the molecular motions of polymer and solvent coexist in homogeneous mixtures with hydrogen-bonded polar solvent and polymer. Introduction The dynamics of solvent molecules and the micro-Brownian motion of polymer chains govern various physical and chemical properties of polymer solutions. In addition, the dynamics of polymer solutions can be effectively treated as a simple and fundamental model system for molecular motions directly related to the functions of biopolymers. Poly(vinyl pyrrolidone) (PVP) is a semicrystalline and nontoxic synthetic polymer that exists as a randomly coiled and highly flexible chain in polar solvents. Molecular interactions between PVP and solvent molecules change the dynamics of neat components. The dynamics of polymer solutions has been investigated by dielectric relaxation spectroscopy. Dielectric studies of PVP solutions have provided various important pieces of information on molecular dynamics, the degree of intermo- lecular interaction, and cooperativity between guest and host molecules. Furthermore, the solvent molecular structure de- pendences of the dielectric properties of PVP solutions show various characteristic properties. The dielectric behavior of PVP solutions has been extensively studied in water, 1-12 alcohol, 13 and ethylene glycol oligomer (EGO). 14,15 Despite the fact that the dielectric relaxation process of polymers in solutions of a nonpolar solvent has been studied extensively, only a few results have been reported on the observation of the dynamics of polymers in solutions of a polar solvent in a solvent-rich region. 11,12,14,15 The difficulty in the observation is caused by the existence of the large contributions of conductivity and electrode polarization. Even if the local chain motion of a polymer existed at frequencies lower than those of the polar solvent, the dc conductivity and electrode polarization would mask the relaxation process of the chain motion. In this case, the local chain motion of the polymer should be detected with a large error 11,12 or not detected at all. 16 In addition, the observation of the dielectric relaxation processes of both solvent and solute polymers requires an extremely wide frequency range. Therefore, the systematic experimental study of a polymer chain in a polar solvent has not yet been carried out. Alcohols are well-known hydrogen-bonding molecular liq- uids. Various types of alcohol are available, from which we can obtain information on the alcohol structure dependences of physical and chemical properties systematically. The hydroxyl groups of alcohol can form hydrogen bonds, which cause various unique properties. The dielectric properties of alcohols and their water mixtures have been studied extensively. 17-31 The large dipole moment of the hydroxyl group results in a large primary process and small secondary processes. 17-23 The large primary relaxation process is considered to be due to the cooperative reorientational motion of alcohol molecules accompanying the formation and deformation of intermolecular hydrogen bonds. On the other hand, the secondary processes are thought to be due to a local reorientation of the hydroxyl groups. We reported the dielectric relaxation process of PVP in water mixtures with various PVP concentrations at 25 °C. 2 However, the relaxation processes observed in the frequency range from 300 kHz to 100 MHz reported in our previous papers 1,2,13 were incorrect owing to the wrong choice of time window for the time domain reflectometry (TDR) measurements. The limited time window produced a truncation error in the dielectric constant and loss in the frequency range corresponding to the reciprocal of the time window. The truncation error resembles a relaxation process. In addition, to subtract the contribution of the dc conductivity of the PVP-water mixtures, we used NaCl aqueous solution as a reference sample for the time domain measurements. The difference in the high-frequency tail of the electrode polarization between the reference sample and the PVP-water mixture affected the dielectric spectrum observed below 100 MHz. Therefore, experimental results below 100 MHz in the previous papers 1,2,13 were not reliable. * Address correspondence to this author. 2181 J. Phys. Chem. B 2007, 111, 2181-2187 10.1021/jp065414e CCC: $37.00 © 2007 American Chemical Society Published on Web 02/09/2007