TRACER DIFFUSION OF LINEAR POLYSTYRENES IN POLY(VINYL METHYL-ETHER) GELS

The tracer diffusivity, D(t), has been determined for linear polystyrenes (PS) in poly(vinyl methyl ether) (PVME) gels, by dynamic light scattering (DLS). The tracer molecular weight, M, ranged from 3.2 x 10(4) to 1.05 x 10(6). Gel concentrations, c, of 0.200 and 0.235 g/mL in toluene, and monomer-to-cross-linker ratios of 100:1, 50:1, and 25:1, were employed. The gels were synthesized cationically in toluene with the PS tracers incorporated in the reaction mixture and with the gel concentration identical to that used during the DLS measurements. Each gel was dried to constant weight and cut into three pieces. The central piece was reswollen with toluene for the DLS measurements, while the outer two pieces were swollen to equilibrium, to provide an estimate of the molecular weight between cross-links, M(x). The diffusivity scaled approximately as D(t) is similar to M(x), consistent with the reptation model incorporating a Rouse-like friction for the subchains. A series of measurements for gels prepared at c = 0.235 g/mL, and subsequently swollen or deswollen, yielded D(t) is similar to c-0.8 +/- 0.2, consistent with an affine deformation of the cross-links. Comparison with extensive data for PS in PVME solutions indicated that Dt in the gels was consistently less than or equal to D, in solutions with the same c values, with equality found for the solution data acquired with a high molecular weight matrix polymer. The most important observation, however, was that in the gels D(t) is similar to M-2.7 +/- 0.1 and M-2.8 +/- 0.1 for c = 0.200 and 0.235 g/mL, respectively, in contrast to the reptation model prediction. Possible reasons for this discrepancy are identified, but the results as a whole are apparently not consistent with any currently available theory.