Symmetric, Isotopic Blends of Poly(dimethylsiloxane)

Neutron scattering experiments were performed on three molecular weight pairs of symmetric, isotopic blends of poly(dimethylsiloxane) (PDMS) of near-critical composition. Scattering data covering close to 3 decades in size were globally fit using the random phase approximation (RPA) and the Debye function for Gaussian polymer coils using the interaction parameter, ø, and statistical segment length, b, as free parameters. These wide q range fits differ from the standard, narrow q range RPA fits in that the power-law scaling regime and exponential decay regimes, related to b, are accounted for. Values for ø showed a well-behaved linear dependence on inverse temperature. Critical temperatures were estimated from these data. Direct observations of the miscibility limit, through neutron cloud points, were made in several cases which agree to some extent with the extrapolated critical points. Monotonic dependencies in temperature of the coil expansion factor, R, as calculated from the statistical segment length, were observed. Under the assumption that the thermal dependence of R 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 ø is observed to vanish in the high-molecular-weight limit in keeping with a Flory-Huggins/Hildebrand description of ø as B/T. The molecular weight dependence of ø supports the view that, after accounting for coil expansion, end-group effects are the sole source of noncombinatorial entropy in this model system.