Advanced composite materials processable by cost-effective manufacturing play an important role in developing lightweight structures for future space and planetary exploration missions. With the growing demand for improved performance in the aerospace sector, advances in polymer systems with extreme thermomechanical properties are critical in providing excellent retention of performance in high-temperature environments, and high resistance to microcracking at cryogenic temperatures.
Extensive research and development, and material characterization, was performed on nonflammable, high-strength, lightweight aromatic thermosetting copolyester (ATSP) composites, which found that they are suitable for making reliable and low-cost cryogenic tanks and high-temperature structures for spaceflight. ATSP composites are resistant to microcracking due to their ability to locally match coefficient of thermal expansion (CTE) at ordered interfaces, and are capable of withstanding hundreds of extreme hot and cold temperature cycles. Additionally, they are mechanically durable and can maintain properties at high temperatures (Tg up to 307 °C). ATSP has a further unique feature in that it is self-bondable in the solid state by interchain transesterification reactions (ITR) — fracture toughness experiments conducted at the bondline between plies joined via ITR indicate high resistance to delamination even at elevated and cryogenic temperatures. This indicates that ATSP has utility towards rapid joining schemes previously not viable for thermoset resins.
The most widely used resins — epoxies — are stable only up to 100 to 120 °C for long-term usage. Other polymeric resin systems that have been designed for high-temperature stability are phenyl-based epoxies, polyimides, bismaleimides, PEEK, and Vectran. However, these polymeric systems generally possess either limited thermal properties or have extremely high melt viscosities, which preclude fabrication of low-porosity, high-quality composites.
ATSP forms a strong bond with the reinforcing carbon fibers and this was of benefit when shear strength and modulus, and fracture toughness properties were measured by testing the samples under tension. The results were comparable or better than polyimide and epoxy/C composites. Composites made from liquid crystalline oligomers (C2A2/C) yield tougher matrices compared to non-liquid crystalline oligomers (C1A1/C). Thus toughened matrices could be achieved without incorporating any additives and instead by merely changing the morphology of the final polymer.