Thermal expansion behavior of polyimide films containing benzoxazole unit

Thermal expansion behavior of polyimide (PI) films based on benzoxazole containing diamine, 5,4-diamino-2-phenyl benzoxazole (DAPBO), was investigated systematically. Effects of preparation parameters on the values of in-plane coefficient of thermal expansion (in-plane CTE) were studied from the viewpoint of film thicknesses, curing rates, and substrates. It was found that the in-plane CTE values increased as a function of film thicknesses and curing rates, this pattern was also applicable to PI based on DAPBO and pyromellitic dianhydride (PMDA), which showed negative in-plane CTE value. The influence of preparation parameters on in-plane CTEs was interpreted by the effect of residual solvent. Thermogravimetric and dynamic mechanical analyses were used to study the curing process of poly(amic) acid films with various amounts of residual solvent and further to illustrate the effect of residual solvent on in-plane CTE values. In-plane and out-of-plane CTE of PI film based on DAPBO and 3,3,4,4-oxydiphthalic dianhydride (ODPA) were directly measured using thermomechanical analysis instrument, and the values were 22.5 and 62.3 ppm K-1, respectively, indicating the orientation character of PI chains along the film plane direction.