Ultrafast nonexponential dynamics in a polymer glass forming liquid

Abstract

The orientational dynamics of phenyl side groups in poly(methylphenylsiloxane) (PMPS) melts are examined over a broad range of viscosity/temperature (η/T) using subpicosecond transient grating optical Kerr effect (TGOKE) measurements. Measurements on poly(dimethylsiloxane) are also reported. Following ultrafast (hundreds) of fs librational dynamics, the phenyl side group orientational dynamics occur over a range of times from 2 ps to a few hundred ps. The experiments were performed from 25 to 143 °C, resulting in η/T changing by a factor of 40. In spite of the large change in η/T, the side group dynamics remain unchanged throughout the entire temperature range. Comparison of the dynamics of PMPS in the melt and PMPS in dilute CCl4 solution shows that chain–chain interactions influence the phenyl side group dynamics in the melt. The dynamics are described as local orientational relaxation of phenyl groups in the microenvironments defined by the backbone geometry and side group steric interactions rather than rotational diffusion. The dynamics exhibit power-law behavior, t−α, over two decades of signal decay. Two possible physical processes that can give rise to a power-law decay are discussed. The relationship of the observed dynamics to the β and α relaxations of glass forming liquids is also discussed.