Pressure induced variations in refractive index of aromatic polyimide film analyzed by brillouin scattering

Abstract

Pressure-induced variations in refractive indices and acoustic velocities of a spin-coated aromatic polyimide (PI) film, poly(4,4’-oxidiphenylene pyromellitimide), were analyzed between atmospheric pressure (0.1 MPa) and 2 GPa by Brillouin scattering measurements with the symmetric platelet and the back scattering geometries for the film surface and the cross-sectional surface. At high pressures, anisotropies in acoustic velocity and refractive indices were clearly observed between the in-plane and out-of-plane directions. Additionally, the average refractive index of the PI film was estimated to increase from 1.682 to 1.80 by applying pressure up to 2 GPa due to the densification of aggregation structures of the PI film.