WHISKERS .4. COPOLYESTERS OF 4-HYDROXYBENZOIC ACID AND 2-(4'-HYDROXYPHENYL)BENZOXAZOLE-5-CARBOXYLIC ACID

Acetylated 2-(4'-hydroxyphenyl)benzoxazole-5-carboxylic acid and 4-acetoxybenzoic acid were polycondensed in high concentration (0.5 mol/L) at 350-degrees-C in a liquid reaction medium. The resulting copolyesters isolated in yields above 90% possess a composition which parallels the feed ratio. Further polycondensations were conducted at 400-degrees-C and low monomer concentrations to yield whisker-like crystals. Yet whiskers were only obtained at a composition of 1:9 in favor of 4-hydroxybenzoic acid (4-HBA). Polycondensation of the silylated monomers yielded thick whiskers also at a composition of 2:8. The WAXS powder patterns indicate that all copolyesters are highly crystalline, but only the whisker-forming copolyesters adopt an orthorhombic crystal lattice similar to that of pure poly(4-HBA). At a composition of 3:7 a hexagonal chain packing prevails. The DSC measurements show that incorporation of the benzoxazole unit into poly-(4-HBA) lowers the temperature of all phase transitions.