LOW-EARTH-ORBIT RESISTANT SILOXANE COPOLYMERS

Two classes of siloxane copolymers were evaluated for their resistance to the low earth orbit (LEO) environment. Poly(imide-siloxane) (PISX) copolymers were used as the resin for PISX-carbon fiber composites. T hese composites were exposed to the LEO environment, for 50 h, as part of the ''Effect of Oxygen Interaction with Materials'' (EOIM-III) exp eriment aboard the space shuttle STS-46. XPS analysis showed primarily silicon oxides on the LEO-exposed surfaces and evidence of a thermall y accelerated oxidation. The results of simulated LEO exposure of the PISX composites show that they are one to two orders of magnitude more resistant than are homopolyimide-based composites. Furthermore, we fo und, surprisingly, that these materials erode slower when far-UV radia tion is combined with the atomic oxygen. XPS analysis of PISX exposed only to far-UV allowed a partial mechanism to be proposed for the effe ct of far-UV radiation on the PISX copolymers. Polyhedral oligosilsequ ioxane (POSS)-siloxane copolymers were evaluated in a simulated LEO en vironment and results indicate that the POSS-siloxane copolymers are e ven more resistant to the simulated LEO environment than are the PISX copolymers; POSS-siloxanes actually gained weight during the exposure and healed the microcracks present. (C) 1996 John Wiley & Sons, Inc.