Interplay of Luminophores and Photoinitiators During Synthesis of Bulk and Patterned Luminescent Photopolymer Blends

Abstract

Four-dimensional printing with embedded photoluminescence is emerging as an exciting area in additive manufacturing. Slim polymer films patterned with three-dimensional lattices of multimode, cylindrical waveguides (waveguide-encoded lattices, WELs) with enhanced fields of view can be fabricated by localizing light as self-trapped beams within a photopolymerizable formulation. Luminescent WELs have potential applications as solar cell coatings and smart, planar optical components. However, as luminophore-photoinitiator interactions are expected to change photopolymerization kinetics, the design of robust luminescent photopolymer sols is non-trivial. Here, we use model photopolymer systems based on methacrylate-siloxane and epoxide homopolymers, and their blends to investigate the influence of the luminophore Lumogen® Violet (LV) on the photolysis kinetics of the Omnirad™ 784 photoinitiator through UV/Vis absorbance spectroscopy. Initial rate analysis with different bulk polymers reveals differences in the first-order rate constants in the absence and presence of LV, with a notable increase (40%) in the photolysis rate for the 1:1 blend. Fluorescence quenching studies, coupled with density functional theory calculations, establish that these differences arise due to electron transfer from photoexcited LV to the ground state photoinitiator molecules. We also demonstrate an in-situ UV/Vis absorbance technique that enables real-time monitoring of both waveguide formation and photoinitiator consumption during the fabrication of WELs. The in-situ photolysis kinetics confirm that LV-photoinitiator interactions also influence the photopolymerization process during WEL formation. Our findings show that luminophores play a non-innocent role in photopolymerization and highlight the necessity for both careful consideration of the photopolymer formulation and a real-time monitoring approach to enable the fabrication of high-quality micropatterned luminescent polymeric films.