HETEROGENEITY IN POLYMER LATICES - DETECTION BY ZONAL CENTRIFUGATION

Mono- and paucidisperse latex particles are often assumed to have uniform surface properties and chemical compositions from particle to particle, but experimental evidence proving this assumption is scarce. Recent work from this laboratory showed that zonal centrifugation in density gradients coupled to the scanning of the centrifugation tubes to determine light scattering by their contents can give useful information on the chemical heterogeneity among latex particles and on latex aggregation behavior. In this work, we describe experiments done using two latices: one from polystyrene and the other from poly[styrene-co-(acrylic acid)]. The shapes of the isopycnic bands of both are more complex than expected for samples made of particles of uniform density. PS latex sedimentation behavior depends on the presence of NaCl (10(-4)-10(-2)M) within the sucrose density gradients; even in the lowest salt concentrations there is aggregation of a fraction of the latex, showing the presence of components easier to aggregate than others. On the other hand, PSAA latex is less sensitive to salt concentration, which is assigned to its higher particle surface charge. Centrifugation profiles obtained at 10(-3)-10(-1)M are essentially the same as without salt, but there is partial resolution of the isopycnic bands, showing that density changes may be induced by salt and that different fractions respond differently to salt. Extensively dialyzed,non-Newtonian PS latex particles sediment slowly, leaving behind a cloud extending to the top of the solution column and confirming the correlation between latex viscosity and latex particle friction coefficient dependence on salt, already presented in the literature. Zonal centrifugation of mixed PS and PSAA latices shows that their particles migrate independently and reach the isopycnic equilibrium expected in the absence of particle interaction.1072095209