Study on the chain orientation and interdiffusion of polyimide affected by drying and curing processes

Abstract

Chain orientation in polyimide (PI) film and its interdiffusional behavior influenced by the process conditions were investigated. The amount of residual solvent and the degree of imidization were proved to be the key factors that determine the final chain orientation and interdiffusion width of PIs. To observe the effect of film thickness, the films made by poly(amic acids) or PAA, the precursor of polyimide, having different thicknesses were prepared. During the drying and curing processes, the depth-wise distribution of residual solvent and imidization rate were observed. For thick PI, it showed a lower degree of in-plane chain orientation, particularly near the substrate. This non-uniform distribution of chain orientation was similar to that of a dried PAA. Fast imidization with higher solvent content for thick PI retarded the formation of a well-ordered structure and resulted in a lower degree of in-plane orientation. Chain orientation of a PI can also be changed by thermal histories. PAA began to imidize quickly with retaining more solvent as the initial drying temperature increased. The fully imidized PI showed the lowest degree of in-plane chain orientation when it was processed by the protocol of the highest drying temperature. It influenced the in-plane thermal expansion coefficient most significantly, while no significant change in crystallinity or glass transition temperature was observed. In multi-layer PI film, controlling the interdiffusion and chain orientation is essential to achieve a good adhesion performance. In the present study, bilayer PI films were made by two different drying methods, direct multi-layer (DML) and layer-by-layer (LBL). The extent of interdiffusion between two PAAs was affected by both the amount of solvent and contact time. As a result, the interdiffusion width of the cured PI made by DML was larger than that made by LBL. The chain orientation of the bottom layer, which consists of more rigid chain structure, was observed to be varied more significantly according to the drying conditions. In the bilayer PI film made by LBL method, the degree of in-plane orientation of the bottom layer became reduced compared to the single-layer when it was dried at lower temperature. The result originates from the fast diffusion of solvent in the top layer into the bottom layer.