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

• Thermal expansion for rigid and planar polyimides depends on film thickness.
• Charge transfer complex is formed more favorably at thinner films.
• Variable thermal expansion originates from complex morphological effects.
• Variable thermal expansion provides optimum film thickness for display substrates.

Polyimide films with thicknesses ranging from 6 μm to 80 μm were prepared with a solvent casting method to explore film thickness effects on the in-plane thermal expansion coefficient (CTE). In the case of polyimide films composed of bulky and flexible molecular units, CTE is consistent regardless of 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, which then plateau for thicker films. It is apparent that the film thickness dependent thermal expansion originates from complex effects of molecular orientation, charge transfer complex formation, and crystal formation as a function of film thicknesses, through characterization on UV–Vis absorption, crystalline structure, glass transition behavior, and optical retardation. These results provide insight into the design of polymer structures for flexible display substrates that require appropriate CTE values.

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