Highly Transparent and Refractive Polyimides with Controlled Molecular Structure by Chlorine Side Groups

Transparent polyimides (PIs) that have a high refractive index and low birefringence as well as good thermal and mechanical stability were synthesized by the judicious introduction of di- and tetrachlorinated aromatic diamines with aromatic/aliphatic dianhydrides. All the PI films, except for the PI derived from aromatic dianhydride and dichlorinated diamine (BPDA/2DCDB and BPDA/3DCDB), exhibited high optical transparency over the entire visible region. In particular, tetrachlorinated, fully aromatic polyimide (BPDA/TCDB) with a highly distorted conformation of the main chain exhibited excellent optical transparency in the visible range that was comparable to highly fluorinated aromatic PIs. This was attributed to the effective suppression of intra- and intermolecular charge transfer (CT) interactions by steric hindrance between the five-membered imide rings and chlorine atoms attached directly to the adjacent benzene rings as well as between chlorine atoms in the diamine molecules. A high transmittance of 82% at 400 nm with an average transmittance > 90% in the visible region and strong fluorescent emission were achieved in a fully aromatic PI (BPDA/TCDB). In addition, the chlorinated aromatic Pis exhibited high average refractive indices (1.702-1.732), suitable for applications as optical materials, such as microlens, and low birefringences (0.028-0.091) at 633 nm, due to the high atomic polarizability of chlorine and the bulky molecular structure of chlorinated PIs. In particular, a mean refractive index of 1.7018 was observed for BPDA/TCDB PI, which is extraordinarily high for a colorless PI.