Synergistic effect between carbon black nanoparticles and polyimide on refractive indices of polyimide/carbon black nanocomposites

High refractive index polyimide (PI)-chloride modified carbon black hybrid materials were prepared. The introduction of chloride modified CBs endowed the PI/CB nanocomposites with high refractive index, and the effect of the interaction between the five-membered imide rings and aromatic rings of CBs on the high refractive indices and transparency of the resulting PI/CB nanocomposites was discussed. The refractive indices of the prepared hybrid films at 633 nm were in the range of 1.711-1.833, which increased with the CB content from 0 to 10 wt%. According to the comparison of the theoretical and experimental values, the higher refractive index is mainly due to the effective enhancement of intra-and intermolecular charge transfer (CT) interactions between the five-membered imide rings and chlorine atoms attached directly to the CBs and the adjacent benzene rings of CBs. A theoretical equation based on the modified Lorentz-Lorenz theory provided reasonably close estimation of the refractive indices to the experimentally observed values. Transmission electron microscopy results indicated that the incorporation of CBs into polymer matrices on the nanoscale would assure the optical transparency.