Dynamics of Pregel Solutions and Gels in a ϑ-Solvent near a Spinodal

We examine the influence of the diverging concentration fluctuations near a spinodal on the dynamics of branched polymer solutions in the vicinity of a sol−gel transition. Dynamic light scattering (DLS) measurements on pregel solutions and gels of methyl methacrylate (MMA) cross-linked with varying amounts of ethylene dimethacrylate (EDMA) in a ϑ-solvent (4-heptanone) reveal a fast exponentially decaying cooperative mode and a slow, long-time tail arising from the dynamics of the partially penetrating clusters. Just as in the case of a sol−gel transition in a good solvent, the slow part is a stretched exponential below the gel point and a power law at the gel point. The cooperative diffusion constant D c ∼ ( T − T s ) ν‘ and the decay rate characterizing the dynamics of the interpenetrating clusters in the pregel solutions Γ s ∼ ( T − T s ) ν‘‘ as the temperature approaches the spinodal temperature, T s . While the vanishing of D c is expected due to the divergence of the concentration correlation length near a spinodal, the vanishing of the stretched exponential relaxation rate with the exponent v‘‘ = 1.20 ± 0.05 is a new observation. The exponent ν‘ varies from 0.5 to 0.85 as the degree of cross-linking approaches the gel point. This implies that the effective viscosity diverges more strongly near T s than observed in regular fluids. For the sample at the gel point, the decay rate of the power law tail also slows down in the vicinity of the spinodal. We suggest that the unusual temperature dependences arise from a coupling of the cluster diffusion dynamics to the dynamics of the concentration fluctuations in the polymer matrix.