A comprehensive study on the effect of molecular chain flexibility on the low-temperature curing ability of polyimides

Abstract

As stringent demands for PI materials with low-temperature curable properties have increased in the high-frequency communication era, the introduction of flexible structures has gained prominence for enhancing molecular chain mobility. However, systematic studies on the effect of flexible structures on low-temperature curing ability remain limited. In this work, we designed a new dianhydride monomer with isopropylidene and ester groups named TABPP. Low-temperature curable polyimides with different molecular chain flexibility were prepared by choosing different diamines and anhydrides. Surprisingly, the degree of imidization was not straightforwardly aligned with molecular chain flexibility. An exemplar instance was PI-9-200, exhibiting the greatest flexibility among the samples, yet possessing the lowest degree of imidization (ID) of 68.59%. Based on the analysis of experimental results and front-line orbital energy levels, it could be seen that the mismatch between the ID and the flexibility may originate from the influence of electronic effects of the monomers. Notably, when enough flexible structures were introduced into the polyimide backbone, the effect of increasing the free volume appeared to outweigh the influence of incomplete imidization, thereby favoring the preparation of low-temperature curable PI films with outstanding dielectric properties. In particular, the dielectric constant of the prepared low-temperature curable PI films was as low as 2.50, which is the best performance among the low-temperature curable PI films. This work throws new light on the correlation between flexibility and low-temperature curing ability and offers fresh perspectives on the preparation of low-temperature curable PIs with excellent dielectric properties.