Phase separation in dynamically asymmetric unentangled polymer blend: An experimental study

We examined a phase separation dynamics of a blend of unentangled polyisoprene (PI) and poly(4-ethylstyrene) (PC2St), which exhibits the upper critical solution temperature. Since there is a significant difference in glass transition temperatures of these polymers, these samples are dynamically asymmetric. While PC2St is dielectrically inert, PI has a type-A dipole along the chain backbone and its dynamics can be dielectrically detected. Thus, by combining the dielectric and rheological measurements, we can detect the dynamics of each component in blends. We used this combination to estimate the composition-dependence of the mobility, which is required to describe the phase separation dynamics. For this purpose, we determined the chain friction coefficient (ζ) of each component in a uniform state sufficiently above the phase separation temperature (T_s). The temperature dependence of ζ was reasonably expressed by the Williams–Landel–Ferry (WLF) equation. From the extrapolation of this WLF-type dependence obtained for blends of various compositions to the test temperature T^∗ below T_s, ζ at T^∗ (ζ^∗) can be estimated as a function of the composition. This ζ^∗ was then used to determine the mobility Λ defined for the material fluxes at T^∗. The time-dependent Ginzburg-Landau equation with this Λ reasonably described the experimental phase-separated structure observed with the optical microscope.