We report results on the linear viscoelasticity (oscillatory shear in the t
emperature range T-g(glass-transition temperature) less than or equal to T
less than or equal to T-g+ 90 K) of miscible blends of polystyrene (PS) and
poly(vinyl methyl ether) (PVME) and segmental relaxations, measured by die
lectric spectroscopy. The Flory-Huggins interaction parameter of this blend
is weakly negative, and the glass transitions of the pure components are q
uite disparate (Delta T-g = 125 K). PS/PVME blends have been found to be co
nsistently thermorheologically complex at both the segmental and terminal l
evels: 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 distrib
ution of segmental relaxation times of PVME, in both the pure and blended s
tates. The slowest relaxing segments thus control the longer-time relaxatio
n processes of the chains. Miscible blends with weak interactions and large
Delta T-g have concentration fluctuations that broaden the distribution of
segmental relaxation times. This distribution narrows as the temperature i
s raised in the blend, leading to the failure of tTS for terminal dynamics.