Refractive indices of aromatic polyimides

Experimentally measured refractive indices of polyimides of varying chemistry are compared to predicted values calculated from the methods of van Krevelen (1) and Bicerano (2). The refractive index reducing effects of fluorinated substituents, such as the 6F and SF5 groups, are of particular interest. The prediction techniques successfully estimate the decrease in refractive index with increasing fluorine content that has been widely observed experimentally. In addition, while measured refractive indices of non-fluorinated polyimides generally deviate above predicted values, measured refractive indices of polyimides with high fluorine content are very close to predicted values. Disparities between measured and predicted refractive indices are explained in terms of crystallinity and inter-chain interaction, both of which are expected to be more prevalent in the non-fluorinated materials.