Polarization-dependent refractive indices were utilized to probe the free volume and anisotropic chain orientation of several types of microporous polyimides (PIs)/copolyimides (co-PIs) based on Tr?ger''''''''''''''''''''''''''''''''s base (TB). The properties related to the free volume and chain orientation, such as dielectric constants and linear thermal expansion behaviors, as well as the out-of-plane thermal diffusivity (D_⊥), were quantitatively investigated. The free volume fractions (V_f) of 0.183–0.228 were estimated for the membranes based on the Lorentz-Lorenz equation and molecular polarizability calculations. The TB-based PIs/co-PIs exhibited relatively low average refractive indices (1.5208–1.6061), low-to-moderate birefringence (0.0181–0.0505) at 1310 nm, and ultra-low D_⊥ values ranging from 6.2 to 9.6 × 10^?8 m²

https://www.sciencedirect.com/science/article/pii/S0032386117306596