DIFFUSION OF LINEAR POLYMER-CHAINS IN METHYL-METHACRYLATE GELS

We have used dynamic light scattering (DLS) to investigate the probe diffusion of linear polymer chains of varying molecular weight (M(p)) in swollen methyl methacrylate (MMA) gels. Two different systems were studied: (i) with polystyrene (PS) as the probe and toluene as solvent and (ii) with poly(methyl methacrylate) (PMMA) as the probe and dioxane as solvent. For system i we found Stokes-Einstein type scaling of the diffusion constant D(t) vs M(p) at low molecular weights, and a crossover to reptation behavior above a critical molecular weight M(c). The crossover occurs when the radius of gyration of the probe becomes comparable to the correlation length of the gel. For comparison, we also measured probe diffusion of PS in linear PMMA solutions polymerized under conditions identical to that for the gel. A comparison of the effective viscosity of the matrix and D(t) for the same probe suggests that the permanent cross-links of the gel help stabilize the transient entanglements in this system. Laplace inversion analysis of the correlation functions shows a broadening of the probe diffusion peak with increasing M(p). Although the average value of D(t) is approximately M(p)-1.8+/-0.2, the width implies that faster and slower diffusional mechanisms also occur, with the D(t) vs M(p) exponent ranging from -0.5 to -2.7. Our measurements in system ii establish that DLS can be used to study probe diffusion in gels even without using an isorefractive solvent. Laplace inversion analysis of composite correlation functions covering 4 decades shows that it is possible to separate the fast collective diffusion of the gel from the slower diffusion of the probe polymer. The average value of this broad mode scales with molecular weight according to the reptation prediction.