Re-examination of the relationship between packing coefficient and thermal expansion coefficient for aromatic polyimides

The existence of a possible relationship between molecular packing coefficient and thermal expansion coefficient for various aromatic polyimides was investigated. Rod-like low-thermal-expansion polyimides without side groups were seen to have very high packing coefficients, pointing to free volume as a factor in lowering their thermal expansion coefficients. But the small packing coefficients for low-thermal-expansion polyimides with side groups indicated that this was not so. Also, even if the large packing coefficients tended to increase the Young's moduli for these polyimides without side groups, the rod-like polyimides with side groups have small packing coefficients and large Young's moduli. The polyimides with low packing coefficients were found to have very small diffusion coefficients for water vapour.