Coefficients of molecular packing and intrinsic birefringence of aromatic polyimides estimated using refractive indices and molecular polarizabilities

Molecular structure dependences of the coefficients of molecular packing (K_p) and the intrinsic birefringence (Δn⁰) were investigated for aromatic polyimides (PIs) using a method based on the modified Lorentz–Lorenz equation and the Vuks equation. This method needs only average refractive indices and polarizability tensors ( ) for PI repeating units. αˆ was calculated using the density functional theory at the B3LYP/6-31+G(d) theory level that was confirmed as optimal for PI repeating units. Because bent or kinked structures in main chain hinder the ordering of polymer chains, K_p decreases in cases that ether linkages are introduced in the main chain and that kinked-structured dianhydrides are used. Introduction of side groups, trifluoromethyl ( CF₃) groups in particular, also causes decreases in K_p. On the other hand, the estimated values of Δn⁰ are proportional to the calculated anisotropy in αˆ (Δα/V_vdw), thus Δn⁰ can be well predicted by computation. The values of Δα/V_vdw are decreased not only by bent structures in the main chain (e.g., ether linkage), presence of side groups, but also by large fractions of imide rings in the polymer structures. The estimated values of Δn⁰ range from 0.338 to 0.470, which are much larger than the reported ones for optically transparent conventional polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2354–2366, 2004