Brillouin scattering studies of structural relaxations in poly(propylene glycol)

Abstract

Structural relaxation processes in poly(propylene glycol) (PPG), molecular weight 10 000, have been examined at the high frequencies of Brillouin scattering. By combining the high-frequency results with reported photon correlation data, the relaxation time τ is established over a wide timescale of 1–10 −11s. Over the whole range, the relaxation times can be described by a single equation, τ=τ 0 exp[ B (T−T 0) ] , where τ 0=3.5 × 10 −14 s, T 0=170 K and B=1169 K. At high temperatures ( 1 T →0 ), τ extrapolates to the characteristic bending and stretching times of the polymer chain (10 −14 s). At the other extreme, the value of T 0, the so-called ‘ideal glass’ transition temperature, is in accordance with viscosity, photon correlation and dielectric relaxation results. Single relaxation time theory does not fit the experimental results; the width is characterized by β=0.4 in a Williams-Watts distribution of relaxation times. The observed behaviour is explained in terms of restricted local intrachain motions.