All-Aromatic Liquid Crystalline Thermosets with High Glass Transition Temperatures

TLDR: In this paper, a new family of low melting all-aromatic ester-based liquid crystal oligomers end-capped with reactive phenylethynyl end groups was synthesized and characterized.

Abstract: We have synthesized and characterized a new family of low melting all-aromatic ester-based liquid crystal oligomers end-capped with reactive phenylethynyl end groups. In a consecutive, high-temperature step, the reactive end groups were thermally activated and polymerization was initiated. This reactive oligomer approach allows us to synthesize liquid crystal thermosets with outstanding mechanical and thermal properties, which are superior to well-known high-performance polymers such as PPS and PEEK. We have modified an intractable LC formulation based on hydroquinone and terephthalic acid, with M n = 1000, 5000, and 9000 g mol -1 , and varied the backbone composition using isophthalic acid, resorcinol, 4-hydroxy-benzoic acid, 6-hydroxy-2-naphthoic acid, and chlorohydroquinone. All fully cured polymers showed glass transition temperatures in the range of 164-275 °C, and high storage moduli at room temperature (∼ 5 GPa) and elevated temperature (∼ 2 GPa at 200 °C). All oligomers display nematic mesophases and in most cases, the nematic order is maintained after cure. Rheology experiments showed that the phenylethynyl end group undergoes predominantly chain extension below 340 °C and crosslinking above this temperature. Highly aligned fibers could be spun from the nematic melt, and we found that the order parameter was not affected by the chain extension and crosslink chemistry.