Time-resolved light scattering studies on kinetics of phase separation and phase dissolution of polymer blends. 4. Kinetics of phase dissolution of a binary mixture of polystyrene and poly(vinyl methyl ether)

Abstract

The dynamics of liquid-liquid phase separation of a polymer blend of polystyrene and poly(viny1 methyl ether) was studied by time-resolved elastic light scattering techniques in both the nucleation-growth (NG) and spinodal-decomposition (SD) regimes. It was found that in the early stage of SD the scattered intensity at a given momentum transfer q = (4r/X) sin (8/2) increases exponentially with time after the initiation of the isothermal phase separation involved by a temperature jump from the temperature well below the binodal point. The relaxation rate 2R(q) of the intensity increase is a function of q such that R(q)/q2 linearly decreases with q2, in accord with the linear theories of SD originally proposed by Cahn for small molecules and extended by de Gennes for polymers. The spinodal temperature was obtained from the dynamics measured as a function of temperature in the linear SD regime. In the later stage of SD, the intensity increase with time starts to deviate from exponential behavior and the scattering maximum shifts to smaller q, corresponding to the onset of the coarsening process. The higher the superheating, the earlier the stage where the coarsening starts. In the NG regime the intensity increases nonexponentially with time. I. Introduction Our objective in this series of studies is to clarify the structure-property relationships of polymer blends in terms of basic molecular parameters such as degrees of polymerization NA and NE of polymers A and B, Flory- Huggins interaction parameter x, Kuhn's statistical seg- ment lengths UA and uB, etc. The role of the molecular parameters on the structure and properties is dual, af- fecting both equilibrium and kinetic aspects of phase separation. That is, they affect mechanisms of the phase separation (nucleation growth (NG) vs. spinodal decom- position (SD)) and the dynamics of the phase separation in the regimes of NG and SD.l Although the equilibrium structure obtained after the phase separation of the mixture is independent of the mechanism of the phase separation but dependent only upon temperature and composition, the intermediate structures developed during the course of phase separation and hence the resulting properties are significantly dif- ferent for different mechanisms.'P2 In this regard it is crucial to understand the molecular parameters affecting the mechanism of the phase separation, i.e., the phase boundaries (binodal and spinodal lines1p3) for the molecular design of the structure and consequently of the properties. In order to obtain the intermediate structures one must quench the phase-separated structures at various stages of the phase separation below glass transition tempera- tures, at a rate much faster than the relaxation rate for the growth of the phase separation. One must therefore know the molecular parameters affecting the dynamics of phase separation. In the event when the processing re- quires molecular mixing at the processing temperatures, one must know the molecular parameters affecting the binodal temperature and those affecting the dynamics of the phase di~solution.~ The time required to dissolve the phase-separated structure is a function of the size of the phase-separated domain and the relaxation rate (or the