Bismaleimide/preceramic polymer blends for hybrid material transition regions

The problem of phase separation in polycarbosilane (Starfire resin)–Matrimid thermoset blend structures, designed for thermal protection applications, was addressed in this study. The structural design of the compatibilizers synthesized via a hydrosilylation–deprotection sequence was primarily based on a polymer structure with a nonpolar segment consisting of a carbosilane unit and a polar segment with a bisphenol-type moiety. Besides the homopolymer, block copolymers with varying contents of the polycarbosilane unit and the bisphenol A unit were synthesized and characterized. Curing experiments on the blends of Starfire resin–Matrimid incorporating 1–5% of the compatibilizers were performed followed by morphological characterization of the cured resin plaques. The effectiveness of the compatibilizers was assessed by the StarFire domain size and the distribution of cured specimens was examined in the scanning electron micrographs. Results indicated that the compatibilizers that were more effective in minimizing phase separation were relatively more polar and comprised a larger fraction of the phenolic functional units. In fact, the blend composition incorporating 5% of the homopolymer compatibilizer with the largest fraction of the polar bisphenol A unit was highly effective in providing a homogeneous distribution of very small StarFire domains, that is, fairly uniform domain sizes of the order of 5 µm or less.