CALCULATION OF REFRACTIVE-INDEXES OF POLYIMIDES AND THEIR MOLECULAR PACKING

Refractive indices at 589.3 nm of 19 polyimides were measured using an Abbe refractometer and calculated from their Van der Waals volumes and molecular polarizabilities. The calculated refractive index (n(calc)) when the packing coefficient was assumed to be 0.681 shows a linear relationship with the measured index (nBAR) 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 (K(p)) of polyimides are estimated by comparing nBAR with calculated parameter phi0. The molecular chains of the polyimides with high nBAR are densely packed, and a planar structure of pyromellitic dianhydrides or ether linkages lead to high K(p). In contrast, the molecular chains of polyimides with low nBAR are loosely packed, and trifluoromethyl groups cause a decrease of interchain interaction and a intra-chain steric hindrance that inhibits molecular packing.