Negative in-plane CTE of benzimidazole-based polyimide film and its thermal expansion behavior

Non-stretched polyimide films based on 5,4′-diamino-2-phenyl benzimidazole (DAPBI) show curious thermal expansion properties: the in-plane CTE value of PI film is negative when cured at 350 °C (contract upon heating). However, the value of CTE turns positive when cured at 400 °C. In-plane and out-of-plane CTE of PI films annealed at various temperatures have been measured to study the annealing effect on thermal expansion feature. The in-plane CTE value increases from negative to positive as the raise of the annealing temperatures (T_anneal), while the out-of-plane CTE decreases as a function of T_anneal. Morphologies of PI films change from amorphous to semi-crystalline accompanied with the change of in-plane CTE from negative to positive. Mechanism of the thermal expansion behavior of DAPBI-based PI films is proposed: negative in-plane CTE is generated under the combination of the more preferential thermal expansion in the out-of-plane direction and the amorphous structure when the films are cured at lower temperatures; while thermal expansion in the in-plane and out-of-plane directions are both available for semi-crystallized PI films, affording positive in-plane CTE values.