Primary and secondary relaxation processes in poly(propylene glycol) monobutyl ether: a broadband dielectric spectroscopy investigation

摘要

Several relaxation modes present in poly(propylene glycol) monobutyl ether [PPGMBE] were explored over a wide temperature range (120-310 K) and frequency band range (10 mHz-1.0 GHz) using wideband dielectric spectroscopy. Two resolvable secondary relaxations (beta- and gamma-processes) were observed below T-g, along with a structural relaxation mode (alpha-) above T-g. The beta-relaxation process was well fitted with the Cole-Cole equation and was identified as true Johari Goldstein relaxation based on an aging experiment and a coupling model prediction. The spectra for the gamma-process showed asymmetric behavior below T-g, hence it was well described by the Havriliak-Negami equation and did not show any of the peculiar behavior seen with many PPG families. The T-dependences of several dielectric properties, e.g., tau(alpha), tau(beta), tau(gamma) , tau(sigma), and sigma(dc), were thoroughly investigated. It was noticed that the T-variation of the relaxation time for the alpha-mode was not satisfactorily described with only one VFT equation, and the derivative plot indicated a crossover temperature of similar to 240 K. Our experimental findings demonstrated that a logarithmic plot of the sigma(dc) versus tau(alpha) data showed nonlinear dependency and thus were expressed by the fractional Stokes-Einstein-Debye law, with a change in the exponent from zeta = -0.70 to -0.85 for high-T (>240 K) and low-T behavior (<240 K), respectively.