POLY(DIMETHYLSILOXANE) CHAINS CROSS-LINKED AT RANDOM - PERCOLATION AND NMR APPROACHES

The formation of network structures associated with randomly cross-lin ked polydimethyl-siloxame chains is analyzed as a function of time. A cross-linking reaction between the methyl groups and the vinyl groups of vinylmethylsiloxane comonomeric units was induced by heating (150 d egrees C) in the presence of a peroxide; transient states of gelation were observed by quenching samples in ice. The gel fraction, the swell ing ratio, and an NMR structural parameter were the physical quantitie s used to investigate the network structures. The fraction of all viny l groups that react in a time t is found to be proportional to t(1/2) in agreement with a mean field percolation treatment of systems of cro ss-linked chains. The maximum swelling effect was interpreted in terms of the packing of swollen Gaussian domains. The presence of silica ag gregates did not perturb the network structures, as observed from thei r swelling properties.