Solvent and Probe Diffusion in Aroclor Solutions of Polystyrene, Polybutadiene, and Polyisoprene

Pulsed-field-gradient NMR has been used to measure the Aroclor 1248 solvent diffusion in solutions of polystyrene (PS), polybutadiene (PB), and polyisoprene (PI), at 28.5 and 63.5 °C. Six samples of PS (M = 2.0 × 10³, 5.5 × 10³, 9.0 × 10³, 2.04 × 10⁴, 3.2 × 10⁴, and 1.05 × 10⁶), one of PB (M = 1.44 × 10⁵), and one of PI (M = 1.35 X 10⁵) were examined; in all cases the polydispersities were less than 1.1. Additional measurements of azobenzene diffusion in PS (M = 1.79 × 10⁵), PB, and PI/Aroclor 1248 solutions were made by forced Rayleigh scattering, at 25.0 °C. Polymer volume fractions, ϕ, ranged up to 0.2. Reduced diffusion coefficients, D/Do, where D₀corresponds to the diffusion coefficient in neat solvent, were equal for the two experiments within the data uncertainty. For PS solutions, D/D₀ was also independent of molecular weight. For PS solutions at 28.5 °C, the solvent diffusion decreased with increasing ϕ more rapidly than the decrease observed for a variety of other polymer/solvent systems. For PB solutions at the same temperature, D/D₀ increased with increasing ϕ, while for PI solutions, the mobility was essentially independent of ϕ. At the higher temperature the differences in D/D₀behavior among the Aroclor solutions of all three polymers, and relative to data for other chemical systems, were substantially reduced. The composition dependence of the solution glass transition temperature was also determined for each of the three systems. The diffusion results correlate with the changing solution free volume, as both PB and PI have lower glass transition temperatures than the neat solvent. However, it is doubtful whether the data for all three systems can be described concurrently by using a free volume equation based on the model developed by Fujita. The results for solvent diffusion in PB and PI solutions do correspond closely to previously reported anomalous viscosity, viscoelasticity, and oscillatory flow birefringence properties for these systems. If the composition dependence of D/D₀ is viewed as an effective local friction, then chain dynamics measurements on these systems may require some reinterpretation. In particular, the postulate that anomalous high-frequency viscoelastic and flow birefringence properties, reported for these and other systems, reflects primarily the modification of bulk solvent properties by the presence of polymer chains is supported by the diffusion results reported here.