Shrinkage stress evolution during photopolymerization: Theory and experiments

Abstract

Photopolymerized materials are widely used for a broad range of industrial applications including coatings, three-dimensional (3D) printing, micro/nano-scale fabrications, restorative dentistry, etc. However, the service quality and life of these materials are dramatically impaired by polymerization shrinkage stress evolved during the photopolymerization process in practices. A theoretical model for mechanism exploration and accurate prediction of the shrinkage stress evolution is thus-far inaccessible and strategies for alleviating the stress based on existing studies are quite controversial. Herein, we develop a comprehensive theoretical model that can accurately capture the evolution of the shrinkage stress under various working conditions. The model is established by a time-incremental method based on 3D viscoelasticity theory coupling the evolutions of reaction kinetics and material properties during photopolymerization. Theoretical predictions of the shrinkage stress are then validated by groups of experimental measurements using a series of photopolymers and their composites under various testing conditions. We show that our model is able to correctly predict both the individual and combined effects of instrumental compliance, specimen geometry, material composition, and photocuring protocol on the shrinkage stress evolution, results of which well clarify the diverse conflicts existed in the literature. The magnitudes of the predicted stresses based on our model agree quantitatively with those of the experimental ones, far exceeding the prediction accuracy of all existing models. In addition, our model also successfully predicts the results measured in the literature using various testing devices, indicating that our model could be applied to existing setups for shrinkage stress measurement of bulk materials. Our theory-experiment-combined study not only provides a theoretical approach to accurately predict and thoroughly uncover the shrinkage stress evolution during photopolymerization, it will also guide further explorations on shrinkage stress reduction and material property optimization for photopolymers with improved service life. Additionally, the presented model on the shrinkage stress evolution paves a potentially new way for the derivation of material mechanical property evolutions and thus revealing the mechanical-chemical-coupled mechanisms during photopolymerization.