Filling Aggregation-Induced Extinction Mechanism in Near-Infrared Photopolymerization for Gradient and Highly Filled Bulk Materials

The efficiency and curing depth of filled photopolymerization considerably depend on light penetration that is affected by filler extinction behaviors. In this study, the underlying mechanism of filling aggregation-induced extinction (FAiE) utilizing low-absorption and high-dissipation (scattering and refraction) fillers for improved light penetration in upconversion materials-assisted near-infrared photopolymerization (UCAP) is reported. Results revealed that the extinction mechanism depends on the relationship between filler characteristic size (Sc) and incident-light wavelength (λi = 980 nm). When Sc < λi, the filler extinction performance was gradually strengthened with the increase in the filler content, and high filling further induced spontaneous aggregation of filler, resulting in the growing Sc. The highest extinction was observed at Sc = λi and then tended to be stable (Sc > λi), which was mainly caused by a variable scattering intensity; this was lower than the extinction in the ideal dispersion state and was beneficial for highly filled photopolymerization. The extinction mechanism was theoretically evaluated by a light-attenuation gradient model and experimentally confirmed by different Sc particles coupled with an electron microscope as well as polymerization kinetics test. Moreover, the extra "promotion effect" for functional-group conversion and mechanical properties of materials via the dissipation light of the fillers was demonstrated, and the exploitation of such characteristics realized the preparation of multidimensional gradient materials. The established model integrated with FAiE in UCAP provides a theoretical foundation to successfully fabricate centimeter-level objects with 92% filling in a few minutes, highlighting the potential applications of UCAP technology in ultrahighly filled material manufacturing.