Synthesis and properties of transparent random and multi-block polyamide-imide films with high modulus and low CTE

Two new series of transparent polyamide-imides (PAIs) with high modulus and low coefficient of thermal expansion (CTE) were prepared by random or block copolymerization of 4,4′-biphthalic dianhydride (BPDA) and p -phthaloyl chloride (TPC) with 2,2′-bis(trifluoromethyl)benzidine (TFDB). These series of PAIs displayed low CTE of 16–42 ppm/K, good mechanical properties with high tensile modulus ranging from 4.9 to 6.6 GPa, tensile strengths of 112–170 MPa and elongations at break of 2.0–5.4%, as well as high transparency with transmittances at 500 nm (T 500nm ) of 84–87%, which was greatly affected by the feed ratio of monomer BPDA & TPC, and the block length caused by block polymerization. The block copolymer PAI-3-B1 presented the best comprehensive performance with high transparency (T 500nm ) of 87%, low CTE of 16 ppm/K and high tensile modulus of 6.1 GPa.