Composition and molecular weight dependence of the interdiffusion coefficient and critical behavior in a binary polymer blend as measured by quasi-elastic light scattering

Abstract

Quasi-elastic light scattering experiments by means of photon correlation spectroscopy have been employed for the study of diffusional dynamics in the compatibble polymer blend poly(dimenthylsiloxane)PDMS/poly(ethylmethylsiloxane)PEMS at various temperatures (−20 to 120°C) and over the entire composition (θ) 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 D up to 30 000 D below the entanglement value. In addition the static structure factor, S(q), was measured. It was found to be indepents of the waveve ctor, q, at temperatures far above the spinodal. The Onsager transport coefficient Ω = D ̃ S(q = 0 ) was found to be independent from chain length implying ‘Rouse’-like behavior. Further, the composition dependence of D ̃ in the present blend, which possesses a virtually θ-independent glass transition temperature, T 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 0 and the θ-weighted tracer diffusivities if one assumes θ-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 tempperature, T c. Up to very close T c, we observe a mean field type behavior.