Improving dimensional stability at high temperature and toughness of polyimide films via adjustable entanglement density

Abstract Polyimide (PI) films with ultralow coefficient of thermal expansion (CTE) and high glass transition temperature (Tg) are in great demand for applications as substrates in flexible displays. Furthermore, trade-off between CTE and toughness is highly concerned due to strategies for reducing CTE always result in weakness of toughness. In this work, we developed a facile method to improve dimension stability and toughness of PI containing benzimidazole units via regulating entanglement density based on controlling molecular weight and solid content of precursor polyamic acid (PAA) solution. As a result, the Tg of PI films increased from 394.3 to 441.4 °C and the in-plane CTE (40–350 °C) decreased from 65.7 to 3.9 ppm/K with entanglement density increasing from 253.3 to 2868.6 mol/m3. Moreover, mechanical properties especially toughness of PI films were enhanced significantly (from 7% to 18%) as entanglement density increased. In short, we delineated the effects of entanglement on the properties of PI films and found that the decrease of CTE mainly arises from the increase of entanglement density. Our studies provide new insights into the future design of flexible PI substrates with low CTE.