PHASE-SEPARATION OF BLENDS OF POLYDISPERSE POLYMERS - COMPARISON BETWEEN EXPERIMENT AND THEORY FOR THE SYSTEM POLY(DIMETHYLSILOXANE) POLY(ETHYLMETHYLSILOXANE)/

Cloud point curves were measured turbidimetrically for blends (upper c ritical solution temperatures; UCSTs) of poly(ethylmethylsiloxane) (PE MS) and four different samples of poly(dimethylsiloxane) (PDMS); the w eight average molar masses in kg/mol are 31.2 for PEMS and 10.4, 15.5, 18.1 and 24.0 for PDMS; all components have polydispersity indices D (= (M) over bar(w)/(M) over bar(n)) within the range from 1.82 to 2.67 . Corresponding calculations on the basis of the Flory-Huggins theory (three adjustable parameters) account for the polydispersity; molecula r weight distributions are represented by the generalized Schulz-Flory equation. Calculated cloud point curves agree very well with those me asured, if individual sets of parameters are admitted for each of the different mixtures. Even for an identical set of parameters the maximu m deviations remain moderate. Realistic model calculations concerning the influences of D on cloud points and on critical points were perfor med with this set of parameters. For the present (only slightly endoth ermal) system, the precipitation threshold increases from 318 to 558.6 K as D-PEMS is raised from 1.87 to 4.00, keeping the number average d egree of polymerization constant.