Temperature-dependent structural properties of poly (vinylpyrrolidone)/alcohols using time-domain reflectometry (TDR)

Abstract

The complex dielectric permittivity of Polyvinylpyrrolidone (PVP K-30) [C₆H₉ON]_n in four different alcohols (methanol, ethanol, propanol, and butanol) with no. of carbon atoms ranging from one to four has been determined in the frequency range of 10 MHz to 50 GHz over the temperature range of 278.15–298.15 K at various concentrations of PVP (C) using time-domain reflectometry technique. The dielectric permittivity spectra were described using the Cole-Davidson model. Two modes of relaxation processes were observed, namely low-frequency (secondary) and high-frequency (primary) relaxation. The low-frequency relaxation is due to the interaction between solute–solvent molecules, whereas the reorientation of solvent molecules is responsible for the high-frequency dispersion of relaxation. By using the Harviliak-Negami equation, the frequency-dependent complex dielectric permittivity has been studied. Dielectric parameters such as static dielectric constant (ε_j), relaxation time (τ_j), dipole moment (û), Kirkwood correlation factor (g₁), number of solvent molecules irrotationally bound to solute molecules (Z_ib), and effective volume of rotation (V_eff) were determined. In addition to this, thermodynamic properties such as the free energy of activation (ΔF_j), enthalpy of activation (ΔH_j), and entropy of activation (ΔS_j) were also calculated. It was observed that the low-frequency static dielectric constant (ε_l) increases with the PVP concentration in all cases except in methanol. ε_l values strongly depend on the temperature, and they increase as the temperature of the system decreases, whereas the high-frequency dielectric constant (ε_h) increases towards higher concentrations of solute molecule as well as towards lower temperatures.