PICOSECOND MICROSECOND STRUCTURAL RELAXATION DYNAMICS IN POLYPROPYLENE GLYCOL - IMPULSIVE STIMULATED LIGHT-SCATTERING EXPERIMENTS

Picosecond time-resolved impulsive stimulated light-scattering experiments are conducted on polypropylene glycol at temperatures above the glass transition temperature T(g). Through the use of a wide range of scattering angles, longitudinal acoustic waves are characterized in the 20 MHz-5 GHz frequency range. In addition, time-dependent thermal expansion is observed on nanosecond-microsecond time scales. The results are consistent with an empirical description of structural relaxation dynamics in terms of a stretched exponential relaxation function with exponent-beta = 0.4 and with the average relaxation time given by the Vogel-Tamman-Fulcher form. Comparisons to Brillouin scattering and photon correlation spectroscopy results indicate that this description holds for a dynamic range of over 10 orders of magnitude. Comparison to dielectric relaxation measurements which probe mainly orientational motions of polymer segments indicates that density fluctuations involve different segmental motions with significantly faster dynamics for T > T(g).