POLYMERIZATION-INDUCED PHASE-SEPARATION - A MAXIMUM IN THE INTENSITY OF SCATTERED-LIGHT ASSOCIATED WITH A NUCLEATION-GROWTH MECHANISM

The appearance of a maximum in the intensity of scattered light at a n onzero wave vector for systems undergoing a polymerization-induced pha se separation (PIPS) has been considered in the past as conclusive evi dence of the presence of a spinodal demixing mechanism. However, recen t results from Light scattering studies of colloidal aggregation and p hase separation in aqueous biopolymers systems and polymer blends prov e that the maximum may also be generated by a nucleation-growth proces s (NG). The origin of this scattering behavior is the presence of a la yer surrounding dispersed-phase particles that contains less solute co ncentration than the bulk (depletion layer). We apply this concept to a system undergoing PIPS through an NG mechanism. The analysis is cons trained to the generation of a diluted dispersion of spherical particl es where concentration profiles around particles may be analytically d erived. Both Rayleigh-Gans and Mie scattering theories are used to des cribe the patterns of scattered light. It is shown that in a diffusion -controlled growth process a maximum will appear in the scattered ligh t pattern at a nonzero wave vector. This maximum increases in intensit y and shifts to lower values of the wave vector as the population of p articles grows. For particular cases where the continuation of nucleat ion leads to a decrease in the average size of the particles, the maxi mum may shift to higher values of the wave vector, as recent experimen tal evidence has demonstrated. For the diluted dispersion, situations where the usual patterns ascribed to NG are obtained, i.e., scattered intensity decaying from the zero wave vector, are (a) the absence of d iffusion control in the growth process, (b) starting solutions that ar e very diluted in the component that will be phase separated, and (c) generation of a very broad distribution of particle sizes.