UV reactive polymers for refractive index modulation based on the photo-Fries rearrangement

In this study we report on the synthesis and characterization of photoreactive polymers which display large changes in their refractive index upon UV irradiation. These polymers contain aromatic ester groups which undergo a photo-Fries rearrangement from aryl esters to hydroxyketones. The polymers under investigation were poly(4-acetoxystyrene) and two derivatives of polynorbornene, poly(bicyclo[2.2.1]hept-5-ene-2-carboxylic acid, phenyl ester) and poly(bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenyl ester). The kinetics of the photoreaction was studied by FTIR spectroscopy. Ellipsometric measurements of thin films showed that the photo-Fries rearrangement causes a large increase of the refractive indices in these polymers (between +0.03 and +0.05). The modulation of the refractive index is of interest for applications in the field of optics and data storage. The photoreaction also leads to an increase in surface energy as evidenced by contact angle measurements with water and diiodomethane as test liquids.