Aggregation-Induced Emission to Visualize and Monitor Progression of Photopolymerization

Light-induced polymerization is widely used in high-resolution fabrication such as additive manufacturing and photolithography as well as low-cost industrial processes such as UV curable coating and adhesives. Polymer parts made from acrylate-based resins exhibit a large variation in materials properties due to differences and inhomogeneities in the degree of conversion within each part and thus suffer from quality control challenges for high-precision applications. As new photochemistries are being actively developed, it is increasingly important to examine the uniformity of polymerization with high spatial resolution and monitor its progression kinetics with detailed temporal information. In this work, we introduced a class of fluorophores based on aggression-induced emission (AIE) as an additive in the photo-resin formulation; their fluorescence intensity increases as the local rigidity of the cross-linked polymer increases. We conducted systematic ex-situ photo-rheology and FTIR measurements to establish the calibrated relation between fluorescence intensity and polymerization conversion for representative resin formulations. We used fluorescence microscopy and confocal microscopy to demonstrate high resolution grayscale mapping of polymerization conversion in 3D printed parts made by various photopolymerization methods including Digital Light Processing (DLP), Two-Photon Polymerization (2PP), and tomographic Volumetric Additive Manufacturing (VAM). With this unique visualization method, we observed the periodic variation of higher and lower conversion across each printed layer in DLP-printed dog-bone samples, which led to large differences in tensile strength as a function of the printing orientation with respect to the loading direction. Finally, we implemented a fluorescence monitoring setup to a VAM system that actively tracks the conversion of the 3D printed parts as tomographic printing progresses, which offers vast opportunities for real-time in-process metrology and feedback control to improve printing quality.