1994 Volume 51 Issue 4 Pages 251-257
Refractive indices at 589.3nm of 19 polyimides were measured using an Abbe refractometer and calculated from their Van der Waals volumes and molecular polarizabilities. The calculated refractive index (ncalc) when the packing coefficient was assumed to be 0.681 shows a linear relationship with the measured index (n) with a square correlation coefficient of 0.900. However, the slope considerably deviates from 1.0, which indicates that the molecular packing of polyimides changes according to their molecular structure. Packing coefficients (Kp) of polyimides are estimated by comparing fi with calculated parameter φ0. The molecular chains of the polyimides with high n are densely packed, and a planar structure of pyromellitic dianhydrides or ether linkages lead to high Kp. In contrast, the molecular chains of polyimides with low n are loosely packed, and trifluoromethyl groups cause a decrease of interchain interaction and a intra-chain steric hindrance that inhibits molecular packing.
