The degradation of polymeric spacecraft materials by far-UV radiation and atomic oxygen

In the Low Earth Orbital (LEO) environment, spacecraft experience damaging solar far-UV radiation and high velocity atomic oxygen (AO). While significant research has been conducted in the area of AO degradation of polymeric spacecraft materials, little is known about their response to far-UV photons. The detailed study of polymer photodegradation has been largely restricted to wavelengths $\u3e$250nm, due in part to the fact that far-UV ($\u3c$250nm) radiation forms a negligible component of terrestrial sunlight. Above the protective ozone layer, spacecraft will be subjected to the unfiltered solar spectrum, including significant intensity below 200nm.^ This work focuses on the degradative nature of far-UV radiation and AO on polymeric materials used for spaceflight applications. Of primary concern are the far-UV photodegradation effects since little is known in this area. The materials studied were polycarbonate (PC), fluorinated ethylene propylene copolymer (FEP), and a polyimide-siloxane block copolymer (PISX). Far-UV irradiation of solid PC produced significant mass loss by volatilization (photoablation). The remaining polymer surface was modified (deoxygenated) to a depth of only tens of nanometers, leaving the bulk unaffected. Far-UV/AO exposure of solid PC resulted in large mass loss. The polymer was cleaved by AO into small volatile pieces. Far-UV/AO irradiation of FEP thermal control film resulted in large mass loss. Defluorination and chain scission from AO impingement eroded the polymer surface quickly. FEP thermal control film exposed on the NASA LDEF mission was dependent upon the sample location as to the type and degree of modification observed. Samples exposed in the ram direction (direction of flight) showed enormous mass loss and destruction of the optical properties. Far-UV/AO irradiation of PISX films resulted in appreciable mass loss. The advanced hybrid structure, designed to protect the polymer from AO attack by forming a protective Si-O layer from PDMS blocks at the surface, performed marginally well. While attempts have been made to protect and stabilize polymeric materials from the aggressive LEO environment, little is known about the detailed chemical response.