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 Troger'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-perpendicular to), 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 x 10(-8) m(2)/s, depending on the chemical structures and the preparation conditions. Their excellent overall performance suggests their potential applications, for example, in protective sunshades, heat insulation tape, and thermal barrier coatings. (C) 2017 Elsevier Ltd. All rights reserved.