SYMMETRICAL, ISOTOPIC BLENDS OF POLY(DIMETHYLSILOXANE)

Neutron scattering experiments were performed on three molecular weigh t pairs of symmetric, isotopic blends of poly(dimethylsiloxane) (PDMS) of near-critical composition. Scattering data covering close to 3 dec ades in size were globally fit using the random phase approximation (R PA) and the Debye function for Gaussian polymer coils using the intera ction parameter, chi, and statistical segment length, b, as free param eters. These wide q range fits differ from the standard, narrow q rang e RPA fits in that the power-law scaling regime and exponential decay regimes, related to b, are accounted for. Values for chi showed a well -behaved linear dependence on inverse temperature. Critical temperatur es were estimated from these data. Direct observations of the miscibil ity limit, through neutron cloud points, were made in several cases wh ich agree to some extent with the extrapolated critical points. Monoto nic dependencies in temperature of the coil expansion factor, alpha, a s calculated from the statistical segment length, were observed. Under the assumption that the thermal dependence of alpha can be described in a Flory-Krigbaum form, this offers a second measure of the critical point in these blends. If coil expansion is accounted for in this way , the noncombinatorial entropic component of chi is observed to vanish in the high-molecular-weight limit in keeping with a Flory-Huggins/Hi ldebrand description of chi as B/T. The molecular weight dependence of chi supports the view that, after accounting for coil expansion, end- group effects are the sole source of noncombinatorial entropy in this model system.