Optical Properties of Polyimide Thin Films. Effect of Chemical Structure and Morphology

The optical properties of polyimide (PI) thin films with various backbone structures were investigated with prism coupler. PI films were prepared from two types of dianhydrides: pyromellitic dianhydride (PMDA) and 4,4’-hexafluoroisopropylidene bis(phthalic anhydride) (6FDA), and two type of diamines: 4,4’-oxydiphenylene diamine (ODA) and p-phenylene diamine (PDA). To relate the free volume change with refractive indices in PI thin films, the fractional free volume and the average refractive index were estimated for PI thin films. The fractional free volume of PI thin films varied from 0.2168 to 0.3865, and was in the following increasing order: PMDA-PDA < PMDA-ODA < 6FDA-ODA < 6FDA-PDA. The average refractive indices varied from 1.5778 to 1.7427, and were in the following increasing order: 6FDA-PDA < 6FDA-ODA < PMDA-ODA < PMDA-PDA. Morphological structure of the films was characterized with wide-angle X-ray diffraction (WAXD) analysis. The birefringence for different PIs was measured from in-plane and out-of-plane refractive indices using the prism coupler technique. The birefringence of PI film was in the following order: PMDA-PDA > PMDA-ODA > 6FDA-PDA ≍ 6FDA-ODA. Finally the effect of film thickness on birefringence was determined by preparing PI films having various backbone structures with different thickness. Thus effect of morphological structure, including their in-plane orientation and molecular packing on optical properties of PI thin films were studied.