THE EFFECT OF SURFACE MODIFICATION ON THE STABILITY CHARACTERISTICS OF POLY(N-ISOPROPYLACRYLAMIDE) LATICES UNDER BROWNIAN AND FLOW CONDITIONS

The stability of thermoreversible microgel particles of poly(N-isoprop ylacrylamide) having carboxylate surface charge groups has been studie d in the presence of electrolyte and non-adsorbing polymer. Methylatio n of the surface charge groups leads to a decrease in the electrophore tic mobility of the particles and also the interparticle electrostatic repulsive potential, resulting in the material becoming more suscepti ble to flocculation. The Hamaker constant of the microgel particles in creases with the decrease in the hydrodynamic size of the particles fo llowing heating. This brings about an increase in the van der Waals at tractive energy which results in the particles aggregating in the pres ence of sufficient electrolyte. Under conditions of flow through membr anes, where shearing forces are operative, the flocculation observed f ollowing the heating of the dispersions results in the blockage of por es.