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.