STABILITY OF FILLED POLY(DIMETHYLSILOXANE) AND POLY(DIPHENYLSILOXANE-CO-DIMETHYLSILOXANE) ELASTOMERS TO CYCLIC STRESS AT ELEVATED-TEMPERATURE

The response of aluminum oxide-filled poly(dimethyl siloxane) and poly (diphenylsiloxane) and poly(dimethylsiloxane-co-dimethylsiloxane) elas tomers, containing 3-24 mol % diphenylsiloxane, to cyclic stress at el evated temperatures (dynamic creep) was evaluated. The materials could be divided into two classes, based on their response to the applicati on of cyclic stress: no or low-diphenylsiloxane content elastomers in which substantial creep and a decrease in crosslink density were obser ved, and high diphenylsiloxane content (16-24 mol %) elastomers that s howed decreased creep with increasing diphenylsiloxane content and an increase in crosslink density. It was suggested that the phenyl groups stabilize the siloxane bond in the polymer backbone, decreasing the r ate of chain scission reactions as the diphenylsiloxane content increa ses and stabilizing the elastomer against creep. The balance of chain scission, chemical crosslinking, and cyclic formation reactions varies depending on diphenylsiloxane content, giving rise to the differences in dynamic creep behavior. An activation energy of 12.9 kcal/mol was measured for dynamic creep of poly(16 % diphenylsiloxane/84 % dimethyl siloxane), suggesting that a catalyzed degradation mechanism was resp onsible. The primary catalysts of the degradation reactions are postul ated to be the filler particles. (C) 1996 John Wiley & Sons, Inc.