Crossover from gelation to precipitation

(Received 19 July 1991, accepted 17 October 1991)

Abstract
Gelation and precipitation are two possible outcomes for the polymerization of monomers dissolved in a solvent. Gelation is obtained when the polymers grow with a fractal dimension lower than 3 ; precipitation when equals 3, i.e. when they are dense. In sol-gel processes, a continuous range of final products may be obtained, with fractal, dimensions ranging from (gelation of branched polymers) to (precipitation of dense particles). We show that the selection of the growth process (and of the final state) may result from the relative reactivities of monomers with different chemical environments. If reacted monomers are much less reactive for further reactions, the system starts with pairs and then oligomers; the recombination of polymers through the tips of branches leads to gelation. If monomers are much more reactive when they have already made some bonds then the densest clusters scavenge everything else in the reaction bath : this is precipitation. Systems used in actual sol-gel processes have a weaker hierarchy of chemical reactivities ; then the reaction bath may cross over during reaction from recombination of bushy polymers to precipitation of the densest one. This allows control of gel structure through manipulation of chemical preferences.