INTERDIFFUSION IN A HOMOGENEOUS POLYMER BLEND FAR ABOVE ITS GLASS-TRANSITION TEMPERATURE

We report measurements of the mutual diffusion coefficient D of the compatible polymer mixture poly(dimethylsiloxane)/poly(ethylmethylsiloxane), PDMS/PEMS, with degrees of polymerization N(A) = 80 and N(B) = 90, respectively, at various compositions and temperatures. We have employed photon correlation spectroscopy PCS to obtain both D and the static structure factor S (q=0). Additionally, we have measured the self-diffusion coefficient D(s) by pulsed-field gradient NMR. The friction coefficients derived from the latter experiments have been compared with those from measurements of the dynamic shear modulus. For this model system far above the glass transition and macrophase separation we found a consistency between the dynamic quantities D(s), the viscosity, and the kinetic factor D0 relevant for polymer interdiffusion. The resulting composition dependence for D0 is given by the reciprocal friction coefficient zeta0-1 extracted from the mechanical measurement, a result only in harmony with the so-called ''fast mode'' approach and a more recent random phase approximation.23