High-Refractive-Index Halogenated Urethane Acrylates in Polyurethane for 2‑Stage Material Applications

High-refractive-index (HRI) monomers swollen in a rubbery host matrix to form 2-stage composite materials play a crucial role in modern optics applications and are constantly being redesigned to produce enhanced material properties. To aid in this effort, we report the synthesis of two HRI halogenated acrylate monomers and characterize their affinity for high-spatial-frequency photopatterning when swollen in a polyurethane matrix. Through refractive index measurements and high-resolution photopatterning, we show that these 2-stage films support submicrometer spatial frequencies with refractive index modulation n ₁ on the order of 0.01 and change in index upon polymerization Δn of 0.0023. Photopolymerization kinetics from FTIR fit a three-species kinetic model, where the dependence of the polymerization rate on incident light intensity reveals that the dominant termination kinetics in these 2-stage films is unimolecular termination, as opposed to the standard bimolecular. Critically, we show that photopatterning does not induce undesirable optical or mechanical properties such as haze, discoloration, stiffening of the films, or increased dispersion, making these materials suitable for photopatterning applications. High-fidelity holographic diffraction gratings and 2D photo-patterns with micron-scale features are presented to show proof of concept.