We report results on the linear viscoelasticity (oscillatory shear in the temperature range
T
g (glass-transition temperature) ≤ T ≤ T
g + 90 K) of miscible blends of polystyrene (PS) and poly(vinyl
methyl ether) (PVME) and segmental relaxations, measured by dielectric spectroscopy. The Flory−Huggins
interaction parameter of this blend is weakly negative, and the glass transitions of the pure components
are quite disparate (ΔT
g = 125 K). PS/PVME blends have been found to be consistently thermorheologically
complex at both the segmental and
terminal levels: the empirical time−temperature superposition (tTS)
principle applies to neither their oscillatory shear response nor their dielectric response. Using the tube
model, we quantitatively compare dielectric and mechanical results. At low temperatures, the effective
time scale for motion of a Kuhn segment (the shortest Rouse mode) is near the long-time end of the
distribution of segmental relaxation times of PVME, in both the pure and blended states. The slowest
relaxing segments thus control the longer-time relaxation processes of the chains. Miscible blends with
weak interactions and large ΔT
g have concentration fluctuations that broaden the distribution of segmental
relaxation times. This distribution narrows as the temperature is raised in the blend, leading to the
failure of tTS for terminal dynamics.
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