DYNAMIC LIGHT-SCATTERING FROM TERNARY MIXTURES OF 2 HOMOPOLYMERS IN SOLUTION

The dynamical behavior of a ternary mixture of poly(methylmethacrylate)/poly(dimethylsiloxane)/tetrahydrofuran (PMMA/PDMS/THF), where the THF is isorefractive for the PDMS, has been investigated by quasi-elastic light scattering (QELS). Two distinct modes have been observed in the autocorrelation function of the scattered light. In the semi-dilute range, at one total polymer concentration C(T) > C* (where C* is the overlap concentration) and far from the 'cloud point' C(T) < C(sp) (where C(sp) is the spinodal concentration), the variation of the two relaxation processes characterized by their frequencies, GAMMA-C, GAMMA-I, and their amplitudes, a(c), a(I), has been examined as function of the relative composition of the 'visible' polymer (e.g. PMMA) x = C(PMMA)/C(T) and the scattering vector q. The experimental results show a good agreement with recent theoretical calculations based on the random phase approximation (RPA) developed by Benmouna and Benoit. Indeed, the first mode GAMMA-C, which is fast, is independent of the composition and characterizes the total concentration fluctuations. Its value is identical to that measured in the corresponding binary system (e.g. PMMA/THF); it is the cooperative diffusion coefficient mode. The second mode GAMMA-I, which is slow, is sensitive to the composition. It describes the motion of one species with respect to the other and is called the interdiffusion mode. As regards the amplitudes, a(C) and a(I), of these two processes, the experimental results show a sensitive variation with the relative composition, x, according to the theoretical predictions.