MOLECULAR-WEIGHT DEPENDENCE OF THE CLOUD-POINT CURVE OF POLY(DIMETHYLSILOXANE) POLY(METHYLETHYLSILOXANE) MIXTURES

Poly(dimethylsiloxane)s (PDMS) of molecular weight, M(w), = 8 x 10(4)-1.15 x 10(6) were found to be partially miscible with poly(methylethylsiloxane)s (PMES) of M(w) = 4 x 10(3)-9 x 10(3), exhibiting the upper critical solution temperature. The extremum temperature, T(cl,ex), of the cloud-point curve ranged from room temperature to 100-degrees-C, and the T(cl,ex) of ternary blends consisting of PDMS and PMES having a bimodal molecular weight distribution could be controlled by changing the composition of the low and high molecular weight fractions in PMES. The temperature dependence of the interaction parameter chi between PDMS and PMES could be reasonably determined from the molecular weight dependence of T(cl,ex). The result is expressed as chi (per PMES monomer unit) = 4.07/T(K)-2.00 x 10(-3), showing that the enthalpy term in chi is dominant. For a particular mixture with very narrow molecular weight distributions, the critical point was determined by measuring the volume ratio of two demixed phases and was found to shift from the extremum of the cloud point curve, indicating that the location of the critical point is very sensitive to the polydispersity.