Structural and compositional effects on thermal expansion behavior in polyimide substrates of varying thicknesses

Abstract: Polyimide films with thickness range from 6 μm to 80 μm are prepared with a solvent casting method to explore film thickness effects on the in-plane thermal expansion coefficient (CTE). In case of polyimide films composed of bulky and flexible molecular units, CTE is consistent regardless of the change in film thickness. In contrast, films with rigid and planar molecular structure show CTE increase according to the increase of film thickness up to 40 ？ 50 μm, and then plateau at thicker films. It is justifiably apparent that the unforeseen thermal expansion originates from the difference between the polymer morphology at the film top surface and in the bulk of the films, through characterization on UV-Vis absorption, crystalline structure, glass transition behavior, and optical retardation in various film thicknesses. These results provide insight into the design of polymer structure for flexible display substrates.

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