Preparation and Characterization of Thermally Stable Polybenzoxazole Copolymer Films Fabricated from Their Precursors

The polyhydroxyamide copolymers (F-PHAs) were synthesized by low-temperature solution polycondensation of 2,2-bis(3-amino-4-hydroxyphenyl) hexafluoropropane (BAHHFP) and/or 3,3 '-dihydroxybenzidine (DHB) with terephthaloyl chloride (TPC) in N,N-dimethylacetamide (DMAc) with the aid of lithium chloride (LiCl). A series of polybenzoxazole copolymer (F-PBO) films were fabricated via an efficient solution-casting and thermal-treatment technique, using their precursors (F-PHA films) with different BAHHFP fractions (25-100 mol%). The solubility results show that F-PHAs with higher BAHHFP content (>= 75 mol%) are readily soluble in anhydrous DMAc and N-methyl-2-pyrrolidinone (NMP) without LiCl at room temperature. Differential scanning calorimetry (DSC) indicates that the thermal cyclization temperature of the F-PHA films decreases with increasing BAHHFP content. Thermogravimetric analysis (TGA) reveals that the F-PHA and F-PBO films are well prepared and completely converted. The UV-visible spectra indicate that the F-75-PBO film (BAHHFP content: 75 mol%) has reasonable transparency, with 84.9 % transmittance at 600 nm and 397-nm UV cut-off wavelength. It also has lower coefficient of thermal expansion (CTE) (48 ppm/degrees C) and water absorption (WA) values (0.14%). The BAHHFP as diamine component provides the final F-PBO film with desirable properties, e.g., flexibility, thermal stability, transparency, and WA values.