Refractive index tuning of highly transparent bismuth containing polymer composites

A new method for the preparation of transparent bismuth containing composites is reported. With the organic ligands 2,3-Dihydroxypropylmethacrylate (HMA) and 2-(Methacryloyloxy)-ethylacetoacetate (AcMA), bifunctional monomers with a functional group for metal complexation on the one hand and a reactive group for polymerization on the other hand were found. By integrating up to 20 wt-% bismuth in the monomer mixtures, an increase of the refractive index of 4.0% (∆n = 0.058) is determined. The metal complexation was observed via infrared spectroscopy (AcMA) and NMR spectroscopy (HMA). Subsequent addition of trimethylolpropane triacrylate (TMPTA) as cross-linking agent and 2,4,6-trimethyl-benzoyldiphenylphosphine oxide (TPO) as photoinitiator to the bismuth containing monomers Bi-HMA and Bi-AcMA lead to a UV-curable liquid. Photoinduced polymerization was used to generate transparent bismuth containing hybrid materials with increased refractive index. The polymers were characterized by UV/VIS spectroscopy, ellipsometry and DTA/TG. The materials show a high transmission (>80% in 2 mm thick plates) in the visible spectral range, and an increased refractive index with increasing bismuth content.