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 (η0) 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.

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SUMMARY: The miscibility of random copolymers (COP), consisting of dimethylsiloxane and methylphenylsiloxane units, with poly(dimethylsiloxane)s (PDMS) was studied in the absence and in the presence of shear experimentally as well as theoretically. Blends of COP 0.86 28 with PDMS 33 (subscripts: volume fraction of DMS in the copolymer, numbers after the abbreviations: weight average molar masses in kg/mol) were investigated far from critical conditions on the PDMS side of the phase diagram. According to these experiments the two phase regime increases by shear without exception and the maximum effects grow from 3 to 12 K as the PDMS concentration increases. Theoretical calculations were performed under the premise that shear destroys clusters of like segments formed under equilibrium conditions. The effects calculated in this manner are of the correct order of magnitude, but their concentration dependence contradicts the measurements. Blends of COP 0.71 7 with PDMS 27, PDMS 33, or PDMS 38 exhibit critical concentrations at approx. 23 wt.-% PDMS. For sufficiently low PDMS contents shear reduces the miscibility again according to experiment and theory. However, measurements demonstrate that the susceptibility of the blends towards shear decreases as the concentration of PDMS increases until the effect changes sign and the homogeneous region expands as the systems flow, in contrast to the calculations which yield a monotonous increase of shear effects. Possible reasons for the observed discrepancies are discussed.

Phase equilibria were calculated by means of a new method (direct minimization of the Gibbs energy of mixing) for polymer blends consisting of monodisperse polymer A and polydisperse polymer B. The results obtained for a Schulz-Flory distribution of B (molecular nonuniformity U = (M,/M,) -1 = 1 and 100 components of model B) agree quantitatively with that of computations on the basis of continuous thermodynamics. The influence of V , on the miscibility of A and B in 1 :1 mixtures was studied for constant M, of B, quantifying the incompatibility of the polymers by the length of the tie lines. The outcome of these calculations demonstrates that the typical effect of an augmentation of U, (keeping M, and the overall composition constant) consists in an enlargement of the mutual solubility of A and B. However, for an almost compatible pair of polymers (i.e., interaction parameters g are only slightly larger than the critical values for U, = 0), this statement remains true only in the case of sufficiently small U,. In order to gain some understanding of these findings, calculations were also performed for ternary systems (A and two species B). They demonstrate that it is the distance of the overall composition in the Gibbs phase triangle to the critical line (connecting the critical points for different U,) which governs the changes in compatibility. Normally the critical point comes closer to the overall composition as U, is raised, except for low g values where the critical pointafter an initial approach -drifts apart as U, becomes larger.

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