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In-Plane Birefringence of Polyimide Films Prepared on Substrates Having Thermal Expansion Anisotropy

Polyimide films whose refractive indices in the film plane show anisotropy (in-plane birefringence, Δn) were prepared by a novel method based on the anisotropy of thermal expansion coefficients (Δα) of single crystal substrates. Polyimide molecular chains were oriented along the crystal axis having a larger thermal expansion coefficient. The Δn of polyimide films increases as the Δα and the larger thermal expansion coefficient of the substrates increase.