Structure–Property Relationship of Co-PI Film Made by Incorporating 1,2,4,5-Benzenetetracarboxylic Anhydride into 3,3',4,4'-Biphenyltetracarboxylic Dianhydride/P-Phenylenediamine

The aim was to prepare copolyimide (BPDA/PMDA-PDA) films of low thermal expansion coefficient by incorporating 1,2,4,5-benzenetetracarboxylic anhydride (PMDA) into polymer backbones 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine (PDA). FTIR and WAXD were used to explore the chemical and physical microstructure of the PIs films. DMA, TMA and TGA were used to characterize the mechanical and thermal performance of the PIs film respectively. The results show that the smectic structure of the film was destroyed by introducing rigid PMDA groups. The free volume fraction of polymers reached its maximum value when 40 wt % PMDA was added. The glass transition temperature of the polymers increased from 330 to 430°C and the thermal decomposition temperature declined slightly with the addition of PMDA. The inflection point in thermal expansion coefficient (CTE) of the BPDA-PDA polymers in TMA test reached as high as 470°C. The glass transition temperature of the polymers increased but the service temperature range narrowed with the addition of PMDA. The optimized PMDA concentration to obtain a best CTE was 5% as was revealed by the experimental results. The insights obtained from the experiments indicated that excessive introduction of PMDA into the polymers could increase the glass transition temperature significantly. However, the operating temperature range may be reduced. This research could offer practical and valuable references to the application for co-PI films.