TRANSIENT GELATION BY SPINODAL DECOMPOSITION IN COLLOID-POLYMER MIXTURES

We have investigated with small angle light scattering and optical mic roscopy transient gelation phenomena which occur in phase-separating c olloid-polymer mixtures. The scattering intensity distribution shows a peak at non-zero wave vector and satisfies the asymptotic q(-4) Pored behaviour. Consistent with these observations, optical micrographs sh ow an alternating pattern of dark and bright domains. These findings s uggest that the polymer-induced depletion forces lead to the formation of a bicontinuous network of colloid-rich and colloid-poor domains, v ia a spinodal decomposition process. This bicontinuous network rapidly attains a gellike character as indicated by the arrest of speckle flu ctuations. The occurrence of the gel is ascribed to polymer-induced ag gregation between the colloids in the colloid-rich phase. Due to the r eversible nature of the aggregation the network restructures and event ually the gel collapses, as is manifested by the rapid separation of t he colloid-rich phase from the colloid-poor phase.