Opto-thermal characteristics of amorphous polyimides for optical applications

Polymeric components are desirable in optical and photonic applications because of their light weight, high impact resistance, and/or ability to be formed into sophisticated shapes or gradient index (GRIN) optics. However, relatively large thermal effects in polymers can limit applications. We studied a selected series of amorphous polyimides in order to evaluate their potential application in components of optical devices. Several of these polyimides have thermo-optic coefficients (dn/dT) and volume coefficients of thermal expansion (VCTE) about 50% smaller than those of standard optical polymers, such as PMMA and polycarbonate. Surprisingly, these low dn/dT and VCTE values were found in amorphous polyimides which have sterically hindered, kinked linkages between phenyl rings. This suggests a different structural dependence than that found in previous studies, which showed that low thermal expansion in crystalline polyimides is correlated with a rigid linear backbone molecular structure. Thus, amorphous polyimides with favorable backbone structures represent a class of materials with improved thermo-optical stability for polymeric optical devices.