Highly Tough and Highly Transparent Soluble Polybenzoxazoles (II): Effect of Sulfone Group

Abstract

Novel semi-cycloaliphatic (semi-alicyclic) colorless polybenzoxazoles (PBOs) were polycondensed from 3,3'-diamino-4,4'-dihydroxydiphenylsulfone (ABPS) with several cyclohexanedicarboxylic acid (CHDCA) isomers in poly(phosphoric acid) (PPA) at an elevated temperature. The PBO system derived from ABPS with 1,4-CHDCA (trans/cis mixture) possessed excellent combined film properties, i.e., a very high Tg at 294 °C, high transparency (transmittance = ca. 80% at 400 nm for 20 μm thick film), high thermal stability (5% weight loss temperature, T5d > 400 °C in an air atmosphere), sufficient film toughness (elongation at break, Eb > 20%), and good solubility in a variety of common organic solvents (good solution-processability) except for a drawback, a relatively high water absorption ( WA = ca. 2%). The influence of synthetic route on the film properties was also investigated in a comparison with the same chemical composition of PBO film prepared via the two-step method, namely, thermal cyclodehydration of its precursor (polyhydroxyamide). The results revealed that the PBO obtained from the one-pot PPA method is much more profitable from the viewpoints of higher film transparency than that from the two-step way. This result was discussed on the basis of the difference of the polymerization mechanisms. The properties of the colorless PBOs were also compared with those of the corresponding sulfone-containing semi-cycloaliphatic polyimide and poly(ether sulfone) as a typical high temperature engineering plastic. Copolymerization using minor fractions of 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (BAHP6F) into the ABPS-based PBO system enabled us to reduce significantly the water absorption ( WA = 0.83%) without sacrificing other target properties. The colorless PBO developed in this work is a promising candidate as a new type of high-temperature plastic substrate materials for realizing flexible LCDs.