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

Four-dimensional printing with embedded photoluminescence is emerging as an exciting area in additive manufacturing. Micropatterned polymer films containing embedded waveguide channels (or waveguide-encoded lattices, WELs) can be fabricated by localizing incoherent light within a photocurable formulation, with potential applications in solar cell coatings, holographic data storage and optical components. However, as luminophore-photoinitiator interactions are expected to change the 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 by UV/Vis absorption spectroscopy. Initial rate analysis on bulk polymers reveals differences in the first-order rate constants for all species 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. We then demonstrate an in-situ UV/Vis absorption technique that enables real time monitoring of both waveguide channel formation and photoinitiator consumption during the fabrication of micropatterned WEL films. 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 fabrication of high-quality micropatterned luminescent polymeric films.