SILICA-FILLED POLYDIMETHYLSILOXANE - CRYSTALLIZATION PROCESS OF THE ADSORBED LAYER

Mechanical mixtures of silica particles and molten polydimethylsiloxane give rise to an adsorbed layer. Crystallization properties of this polymeric layer were investigated as a function of the residual amount of polymer Qr left bound to silica after removing all free chains. The partial crystallinity ρ{variant}c was determined from the measured enthalpy of crystallization divided by the enthalpy of crystallization per monomer unit. No crystallization effect was observed from very short chains ( M ̄n<6×103 g mol-1); accordingly, it was considered that loops formed from the adsorption effect are too short to participate in the crystallization. The partial crystallinity ρ{variant}c associated with longer adsorbed chains was found to vary as 1-ρ{variant}c∝Q-1r. It is shown that eight monomeric units, on average, are locked in space by the formation of one H bond between a silanol group and a skeleton oxygen atom. The sol-gel transition resulting from the connection of particles by polymer chains is clearly perceived from the state of crystallinity of the adsorbed layer: when all particles and polymer chains form a gel, the resulting adsorbed layer has a constant partial crystallinity ρ{variant}c=0.7, whatever the silica concentration and the amount of adsorbed polymer. © 1990.