High Refractive Index Polymers

Rodlike polymers with repeat units composed of groups with high π-electron density directly on the backbone, such as linearly conjugated double bonds and aromatic heterocyclic rings, exhibit high isotropic refractive indexes and high birefringence values when oriented. Typical polyamide birefringence values for oriented films far exceed 0.5 and several are as high as 0.85; the corresponding parallel and perpendicular indexes for these polymers are in the 2.2-2.4 and 1.5-1.7 range, respectively, depending on the molecular structure and degree of orientation. The isotropic index values may be as high as 1.8-2.0.

In order to make optical devices utilizing these highly birefringent films, it is frequently necessary to bond them to films or glass, which necessitates the use of index-matching, optical-quality bonding agents. We devised two synthetic strategies to obtain polymers that exhibit these properties. In one approach, we synthesized a series of colorless, nonbirefringent, polyacrylates (and methacry-lates) containing halogenated carbazole rings; these results will be reported in a forthcoming paper that is currently being prepared. We have also modified rodlike polyamides by substituting index-enhancing groups directly on the amide nitrogen; the refractive index values of the resulting materials are in the 1.60-1.67 range. The most important finding of the latter research was the realization that rodlike polyamides not only exhibit unusually high birefringence, but also greatly enhanced isotropic refractive indexes, which is directly related to their rodlike conformation.