Dynamics of the α relaxation of a glass-forming polymeric system: Dielectric, mechanical, nuclear-magnetic-resonance, and neutron-scattering studies

Abstract

The dynamics of the α relaxation in a glass-forming polymeric system, poly(vinyl methyl ether) (PVME) has been studied by means of dielectric and mechanical spectroscopies and nuclear magnetic resonance, as well as by means of quasielastic neutron scattering. By using these techniques we have covered a wide time scale ranging from mesoscopic to macroscopic times (${10}^{\mathrm{-}10–}$ ${10}^1$ sec). The dielectric and mechanical data have been interpreted in terms of a Havriliak-Negami relaxation function, ${\mathrm{\Phi }}_{\mathrm{HN}}$. Nuclear-magnetic-resonance data were interpreted by means of a spectral density function J(ω) based on ${\mathrm{\Phi }}_{\mathrm{HN}}$. Neutron-scattering data were described in terms of a scattering law S(Q,ω) which was also built starting from ${\mathrm{\Phi }}_{\mathrm{HN}}$. The results obtained from different experimental techniques indicate that the dynamics of the α relaxation in PVME can be well described by means of (i) a common temperature-independent spectral shape and (ii) a common temperature behavior of the relaxation times. We deduce for the spectral shape very similar parameters by fitting the Havriliak-Negami relaxation function to the different experimental data. These shape parameters are found to be not very sensitive to changes of temperature. The resulting characteristic relaxation times follow a Vogel-Fulcher-like temperature behavior in the temperature range ${\mathit{T}}_{\mathit{g}}$-5 KT${\mathit{T}}_{\mathit{g}}$+150 K. Therefore, this implies a self-consistent description of the dynamics of the α relaxation obtained by very different probes in PVME.