Highly Tough and Highly Transparent Soluble Polybenzoxazoles

Abstract

Semi-cycloaliphatic (semi-alicyclic) polybenzoxazoles (PBOs) were investigated as candidates for flexible plastic substrates for liquid crystal displays (LCDs). PBOs were prepared by the one-pot polycondensation in polyphosphoric acid (PPA) from 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (BAHP6F) with 1,3- and/or 1,4-cyclohexanedicarboxylic acid (CHDCA). The BAHP6F-based PBOs were soluble in various common organic solvents. In addition, the PBO films showed a relatively a high Tg exceeding 250° C, excellent transparency, and a relatively low value of water absorption ( < 0.5%), simultaneously. The BAHP6F-based PBO films were flexible, but the elongation at break ( Eb ) in the stretching test did not exceed 20%. On the other hand, an ether-containing bis(o-aminophenol), 3,3' -diamino-4,4' -dihydroxydiphenylether (ADPE) exhibited much higher polymerization reactivity with CHDCA than BAHP6F, and as a result led to higher molecular weight PBOs. The ADPE-derived PBO films were relatively tough and showed a very high Eb , which exceeded 50%. However, the values of Tg were lower than those of the BAHP6F-derived PBOs, owing to the presence of the flexible ether linkages in the structure. Copolymerization using BAHP6F and ADPE led to excellent combined properties, namely, high Tg , high transparency, a high degree of toughness, and high solution-processability (solubility). The influence of the PBO preparation routes (one-pot technique or two-step method via thermal cyclization of PBO precursors) on the film transparency and other properties are also discussed. Some of the semi-cycloaliphatic PBOs examined in this work practically attained the target properties required for flexible substrates in LCDs..