Dynamical coupling between stress and concentration fluctuations in a dynamically asymmetric polymer mixture, investigated by time-resolved small-angle neutron scattering combined with linear mechanical measurements

Abstract

By time-resolved small-angle neutron scattering (SANS) monitoring of two kinetic phenomena of (i) an early stage of spinodal decomposition and (ii) relaxation of shear-induced phase separation after shear cessation, we quantitatively investigated a growth or relaxation rate R(q) of the concentration fluctuations in a polymer mixture of polystyrene (PS)/poly (vinyl methyl ether) (PVME), where q is the magnitude of scattering vector. R(q) thus determined is largely influenced according to a scheme of dynamical coupling between stress and diffusion. The coupling effect gives rich varieties depending on two crossovers of (i) from viscous to gel-like limits and (ii) from dynamic symmetry to asymmetry, which are tuned by temperature change in the intermediate temperature region between largely different glass transition temperatures, T_g, for each constituent. With respect to q-behaviors of R(q), we observed as follows; (i) in a conventional (dynamically symmetric) viscous limit (T ≥ 110 °C), R(q) shows q², (ii) in a dynamically asymmetric viscous limit (T = 100–80 °C) R(q) becomes q-independent, (iii) shallow in a dynamically asymmetric gel-limit (T = 80–60 °C), R(q) shows q² for low q and q-independent for high q, and (iv) deep in a gel-like dynamically asymmetric limit (50–40 °C), R(q) splits into fast and slow modes (showing q² and q-independent, respectively). In order to explain the rich variety found in R(q), we employed a time-dependent Gintzburg-Landau equation (TDGL) modified to consider the stress and diffusion coupling effect. Two mechanical relaxation times of terminal reptation and Rouse modes were determined by linear viscoelasticity measurements to successfully reproduce the rich variety in R(q) according to the modified TDGL.