Composition and Molecular Weight Dependence of the Interdiffusion Coefficient and Critical Behaviour in a Binary Polymer Blend as Measured by Quasi-Elastic Light Scattering

Quasi-elastic light scattering experiments by means of photon correlation spectroscopy have been employed for the study of diffusional dynamics in the compatible polymer blend poly(dimethylsiloxane)PDMS/poly(ethylmethylsiloxane)PEMS at various temperatures (--20-120 deg C) and over the entire composition ( phi ) range. From the relaxation time of the concentration fluctuations the mutual diffusion coefficient, D, could be determined for a series of molecular weights ranging from 5000-30 000 D below the entanglement value. In addition the static structure factor, S(q), was measured. It was found to be independent of the wavevector, q, at temperatures far above the spinodal. The Onsager transport coefficient Omega = DS(q = 0) was found to be independent from chain length implying "Rouse"-like behaviour. Further, the composition dependence of D in the polymer blend, which possesses a virtually phi -independent glass transition temperature, T sub g , in order to distinguish between the Binder--Brochard--de Gennes and the Kramer--Sillescu approaches which connect the mutual diffusion D with the tracer diffusivities differently, was studied. The experiments support the Kramer--Sillescu approach, which means that there is a linear dependence between W exp O and the phi -weighed tracer diffusivities if one assumes phi -independent monomeric friction coefficients for the mixture components. Further, it was possible to measure that the interdiffusion coefficient, D, undergoes a critical slowing down for the critical composition as the temperature is lowered towards the critical temperature, T sub c . Up to very close to T sub c , a mean field type behaviour is observed. Graphs. 25 ref.--AA