Dark Polymerization Following Partial Radical Photocuring: Effect of Light Intensity and Exposure Duration

Free-radical photopolymerization enables the spatial and temporal customization of the viscoelastic properties of polymers for multifunctional designs. Adjusting light intensity and exposure time can be used to control the kinetics of photopolymerization and, therefore, the nonuniformity in the viscoelastic properties of the polymerized material. Additionally, polymerization that occurs in the dark after partial radical photopolymerization must be taken into account to accurately assess the state of polymerization and maximize photopolymerization efficiency when complete curing is required. In this study, we present a systematic methodology for examining the combined effects of light intensity and exposure time on dark polymerization following partial radical photopolymerization of two acrylate-based photocuring adhesives applicable for bonding dissimilar materials. The findings indicate that light power intensity and material composition are interconnected factors influencing polymerization in the absence of light. Additionally, the results show that, for each material, the stage of photopolymerization─whether in the autoacceleration or autodeceleration phase─significantly affects both the extent and the kinetics of dark polymerization. The similarity of trends in dark radical polymerization, regardless of process and material conditions, led to the proposal of a two-parameter power-law function to empirically describe the trend and rate of dark polymerization.