M. Kapnistos, A. Hinrichs, D. Vlassopoulos, S. H. Anastasiadis, A. Stammer and
B. A. Wolf

"Rheology of a LCST binary polymer blend in the homogeneous, phase separated
and transitional regimes"

*Macromolecules* **1996**, *29*, 7155-7163.

**Abstract:** Small amplitude oscillatory shear rheology is employed in order
to investigate the linear viscoelastic behavior of the lower critical solution
temperature blend polystyrene/poly(vinyl methyl ether), PS/PVME, as a function
of temperature and composition. At low temperatures, where the mixture is homogeneous,
the dependence of the zero shear viscosity (eta(0)) on concentration is measured
and is well-described by means of a new mixing rule, based on surface fractions
instead of volume fractions. Shift factors from time-temperature superposition
(TTS) exhibit a Williams-Landel-Ferry (WLF) behavior. As the macrophase separation
temperature is approached (the phase diagram being established by turbidity
measurements), the blend exhibits a thermorheologically complex behavior. A
failure of TTS is observed at low frequencies, both in the homogeneous pretransitional
and in the two-phase regimes. Its origin is attributed to the enhanced concentration
fluctuations, which exhibit a critical slowing down near the phase boundary
in the homogeneous regime, and in the two-phase morphology inside the phase-separated
regime. The anomalous pretransitional behavior can be quantified using a recent
mean field theory, yielding the spinodal temperature. Furthermore, in the two-phase
region an intermediate region of enhanced moduli at low frequencies is observed,
followed by flow at even lower frequencies, which is attributed to the two-phase
structure. The linear viscoelastic properties of the phase-separated blends
are, to a first approximation, adequately described by a simple incompressible
emulsion model considering a suspension of droplets of one coexisting phase
in the matrix of the other phase.

preprint number: 171