COEXISTENCE CURVES OF POLYSTYRENE POLY(DIMETHYLSILOXANE) BLENDS

The coexistence curves of blends of polystyrene (PS, M(w) = 600 and 2. 2 x 10(3) g mol(-1)) and poly(dimethylsiloxane) (PDMS, M(w) = 460, 140 0, 3700 and 2.3 x 10(5) g mol(-1)) were determined using a specially d esigned refractometer. The coexistence curve of the oligomer blend PS (M(w) = 600)/PDMS (M(w) = 460) was of the upper critical solution temp erature type, and sensitively reflected molecular-weight polydispersit y of the PS, although the polydispersity index M(w)/M(n) was less than 1.10. The coexistence curve at the critical composition could be fitt ed to phi(+/-) = a(+/-)epsilon(beta) + b(+/-) epsilon(beta+Delta), whe re phi(+) and phi(-) are the volume fractions of PDMS of the coexistin g phases (phi(+)> phi(-)), the exponent Delta is 0.5 and E is the redu ced temperature. The critical exponent beta was determined to be 0.33( 6), which was that for non-renormalized Ising behaviour, showing no po lydispersity effect. The Flory-Huggins theory could reasonably describ e the coexistence curves of blends with different molecular weights, a lthough it showed a slight discrepancy for the effects of molecular-we ight asymmetry on the coexistence curve. By fitting the calculated coe xistence curves to the experiments, the interaction parameter chi was evaluated as a function of temperature. The obtained chi parameter inv olved a large enthalpy term, dominating the entropy term, and indicate d that PS/PDMS was an extremely immiscible blend compared with other r eported blends. Critical fluctuation effects on the coexistence curve were also discussed.