HETEROGENEITY IN POLYMER LATICES - DETECTION BY ZONAL CENTRIFUGATION

Mono- and paucidisperse latex particles are often assumed to have unif orm surface properties and chemical compositions from particle to part icle, but experimental evidence proving this assumption is scarce. Rec ent work from this laboratory showed that zonal centrifugation in dens ity gradients coupled to the scanning of the centrifugation tubes to d etermine light scattering by their contents can give useful informatio n on the chemical heterogeneity among latex particles and on latex agg regation behavior. In this work, we describe experiments done using tw o latices: one from polystyrene and the other from poly[styrene-co-(ac rylic acid)]. The shapes of the isopycnic bands of both are more compl ex than expected for samples made of particles of uniform density. PS latex sedimentation behavior depends on the presence of NaCl (10(-4)-1 0(-2)M) within the sucrose density gradients; even in the lowest salt concentrations there is aggregation of a fraction of the latex, showin g the presence of components easier to aggregate than others. On the o ther hand, PSAA latex is less sensitive to salt concentration, which i s assigned to its higher particle surface charge. Centrifugation profi les obtained at 10(-3)-10(-1)M are essentially the same as without sal t, but there is partial resolution of the isopycnic bands, showing tha t density changes may be induced by salt and that different fractions respond differently to salt. Extensively dialyzed,non-Newtonian PS lat ex particles sediment slowly, leaving behind a cloud extending to the top of the solution column and confirming the correlation between late x viscosity and latex particle friction coefficient dependence on salt , already presented in the literature. Zonal centrifugation of mixed P S and PSAA latices shows that their particles migrate independently an d reach the isopycnic equilibrium expected in the absence of particle interaction.