Critical dynamics of binary polymer mixtures.

Mode coupling corrections to the thermal decay rate of binary polymer mixtures near a critical point are investigated. It is found that the classical (Van Hove) theory due to Binder breaks down in several limiting cases. In the regime qR 1 and qxi >> 1, where q is the scattering wavevector, R is the radius of gyration of a chain, and xi is the correlation length, we find that the classical theory fails when qR N(e)/N(3/2). Here N is the number of segments on a chain and N(e) is the number of segments between entanglements. In the regime qR 1 and qxi 1, we find a breakdown in the classical theory when xi >> RN(3/2)/N(e). The thermal decay rate is predicted to vary like q(3) and q(2/xi in these two new regimes, respectively. Particularly in the case of polymer mixtures below their entanglement molecular weight we expect to see the mode coupling corrections in light scattering experiments.