Theoretical Prediction and Experimental Measurement of the Evolution of Polymerization Shrinkage Stress Under Different Photocuring Protocols

Background The service quality and life of photopolymerized materials are dramatically impaired by shrinkage stress generated during the polymerization process. Several soft-start photocuring protocols including two-step, ramp, and pulse delay have been proposed to reduce the shrinkage stress. However, the mechanism for the shrinkage stress reduction by soft-start photocuring remains largely elusive. Objective This study aims to explore the mechanism for shrinkage stress reduction in soft-start photocuring protocols and then propose a universal strategy to maximize the stress reduction. Method A theory-experiment-combined method was developed to investigate the effect of soft-start photocuring protocols on the shrinkage stress evolution. Shrinkage stresses under different protocols were measured by a standardized cantilever beam-based instrument. An improved theoretical model incorporating the evolutions of the reaction kinetics and material properties was developed to predict the shrinkage stress evolution under different curing protocols. Results Compared to the standard protocol with a constant photo-irradiation, all the soft-start photocuring protocols could effectively reduce the shrinkage stress and the two-step protocol achieved a maximum reduction of 25% among all experimental conditions. The elastic modulus of photopolymers coincided under the same radiant exposure and irradiation intensity. Unlike previous studies focusing on the mechanical properties of the photopolymers, we found that the shrinkage stress reduction by soft-start photocuring protocols could be attributed to a delayed gelation and a reduction in the peak temperature change after gelation. Based on these mechanisms, adding a delay time before the gelation was proposed as an effective strategy to reduce the shrinkage stress, leading to a reduction of up to more than 40% according to the theoretical predictions. Additionally, the timing for introducing the delay and its duration can be effectively and conveniently determined by monitoring the real-time evolution of shrinkage stress in the standard photocuring protocol. Conclusions This theory-experiment-combined study not only uncovers that the shrinkage stress reduction by soft-start photocuring protocol is attributed to the delay in the gelation and the reduction of the peak temperature change after the gelation but also proposes an effective approach to mitigate shrinkage stress by adding a delay time before the gelation. Such a strategy for maximizing the shrinkage stress reduction while maintaining the mechanical and curing properties is to guide the practical applications of photopolymers.