LIGHT-SCATTERING FROM DILUTE POLYSTYRENE IN MIXTURES OF SEMIDILUTE POLY(DIMETHYLSILOXANE) AND TETRAHYDROFURAN

Ternary solutions of tetrahydrofuran (THF), polystyrene (PS), and poly(dimethylsiloxane) (PDMS) were studied using total low-angle light scattering and dynamic light scattering. The PS concentration, c2, was varied but was always kept very dilute. The PDMS concentration was held constant at the semidilute concentration of 15 vol %, corresponding to 3c3, where c3 is the entanglement concentration. The range of PS molecular weights investigated was such that RH,2/ζ3, where RH,2 is the hydrodynamic radius of PS in pure THF and ζ3 is the dynamic correlation length of the semidilute PDMS matrix, fell in the “crossover” regime between 0.7 and 3.0. The tracer diffusion coefficient of PS at infinite dilution, D22(0), was determined from the “measured” mutual diffusion coefficient, λ2(c2), according to λ2(c2) = D22(0)(1 + kDc2). D22(0) in the examined system scales with molecular weight according to D22(0) ~ Mw,2-0.91. Similar to a binary mixture, we show that kD defined above may be represented by two terms, one arising from thermodynamic interactions and the other from hydrodynamic interactions. The dependence of kD on molecular weight is found to be kD ~ -Mw,20.57 and is dominated here by the thermodynamic term. © 1990, American Chemical Society. All rights reserved.