Preparation and Characterization of Transparent Polyimide–Silica Composite Films Using Polyimide with Carboxylic Acid Groups
Polyimide (PI) composite films with thicknesses of approximately 100 &micro;m were prepared via a sol&ndash;gel reaction of 3-aminopropyltrimethoxysilane (APTMS) with poly(amic acid) (PAA) composite solutions using a thermal imidization process. PAA was synthesized by a conventional condensation reaction of two diamines, 3,5-diaminobenzoic acid (DABA), which has a carboxylic acid side group, and 2,2&prime;-bis(trifluoromethyl)benzidine (TFMB), with 4,4&prime;-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in N,N-dimethylacetamide (DMAc). The PAA&ndash;silica composite solutions were prepared by mixing PAA with carboxylic acid side groups and various amounts of APTMS in a sol&ndash;gel process in DMAc using hydrochloric acid as a catalyst. The obtained PI&ndash;silica composite films showed relatively good thermal stability, and the thermal stability increased with increasing APTMS content. The optical properties and in-plane coefficient of thermal expansion (CTE) values of the PI&ndash;silica composite films were investigated. The CTE of the PI&ndash;silica composite films changed from 52.0 to 42.1 ppm/&deg;C as the initial content of APTMS varied. The haze values and yellowness indices of the composite films increased as a function of the APTMS content.