Improvements in the thermal stability and mechanical robustness of holograms recorded in acrylamide/polyvinyl alcohol-based photopolymers through the use of D-Sorbitol as a plasticizer

In this work, a photopolymer formulation is presented that demonstrates significant improvements in layer hardness and temperature stability, while retaining the excellent holographic recording characteristics and ease of preparation associated with acrylamide (AA)/polyvinyl alcohol (PVA)-based photopolymers. This is achieved through the use of N-phenyl glycine as an electron donor and, for the first time, D-Sorbitol, a naturally occurring sugar-alcohol, as a plasticizer. The plasticizing effect of D-Sorbitol is demonstrated through the minimal loss of diffraction efficiency (η) observed upon UV bleaching (fixing) of 500 lines/mm gratings, which is evidence for effective mass transport during holographic recording. This solves a key challenge for holographic optical elements: balancing the need for the permeability that ensures diffusion during holographic recording, with the need for a permanent refractive index modulation in a hard, robust layer/coating. Gratings made in previous photopolymer formulations using NPG demonstrated improved hardness but were partially erased during the UV bleaching step. Plasticizers such as TEA and glycerol helped, but also caused a severe reduction in hardness. Here, we report holographic gratings made with 21 wt. % D-Sorbitol that demonstrates stability after prolonged exposure to a temperature of 95 °C and has a permanent RIM of 0.002. An increased indentation hardness was also observed in samples with D-Sorbitol compared to the TEA or glycerol, at the optimum concentration of 21 wt. % D-Sorbitol layers were measured to have more than three times higher hardness than the samples with glycerol or TEA. A similar trend was observed for the elastic modulus (E), which was found to be approximately 4 GPa for layers with 21 wt. % D-Sorbitol; one order of magnitude higher than the elastic modulus of samples prepared with glycerol or TEA.