New insights into mechanism of negative in-plane CTE based on bio-based adenine-containing polyimide film

Abstract Non-stretched adenine-containing polyimide films based on pyromellitic dianhydride (PMDA) and adenine-containing diamine (p-APA) were prepared and annealed at various temperatures. IR studies indicated the existence of transformation of hydrogen bonding types from N-H⋯N (in purine) to N-H⋯O (in carbonyl). The PMDA API films showed negative in-plane coefficient of thermal expansion (CTE) even annealed much higher than Tg. Morphology studies showed that the groups in PMDA API were inclined to self –arranged along the in-plane direction when annealed higher than Tg. The free volume fraction (FFV) of PMDA API films decreased with increasing annealing temperature. An orientation & hinge model was proposed to discuss the negative in-plane CTE behavior.