This thesis is situated at the critical intersection between the physico-chemical understanding of degradation mechanisms in polymers and polymer composites and the development of robust constitutive models capable of predicting their physico-chemical and chemo-mechanical behavior across scales in aggressive space environments. Environmental degradation, specifically UV photo-oxidation and erosion due atomic oxygen (AO) in Low Earth Orbit (LEO), fundamentally alters the material’s physico-chemical state, which in turn governs its mechanical performance. Consequently, accurate lifetime prediction requires a unified framework that explicitly couples these distinct degradation mechanisms. Following an introductory overview highlighting the industrial strategic relevance of these materials, the environmental degradation challenges, and the limitations of current empirical modeling approaches, the dissertation is organized into two distinct parts, each addressing a specific class of materials and degradation mechanisms.