| Abstract: | 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. These composites were exposed to the LEO environment, for 50 h, as part of the “Effect of Oxygen Interaction with Materials” (EOIM-III) experiment aboard the space shuttle STS-46. XPS analysis showed primarily silicon oxides on the LEO-exposed surfaces and evidence of a thermally 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 found, surprisingly, that these materials erode slower when far-UV radiation is combined with the atomic oxygen. XPS analysis of PISX exposed only to far-UV allowed a partial mechanism to be proposed for the effect of far-UV radiation on the PISX copolymers. Polyhedral oligosilsequioxane (POSS)–siloxane copolymers were evaluated in a simulated LEO environment and results indicate that the POSS–siloxane copolymers are even 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. © 1996 John Wiley Sons, Inc. |
