Accurate assessment of the three-dimensional shrinkage stress evolution during photopolymerization: mechano-chemo-thermo-coupled finite element modeling and experimental validation

Abstract

Accurate evaluation of the stress evolution caused by constrained polymerization shrinkage under complex three-dimensional (3D) boundary conditions is critical for the practical applications of photopolymerized materials. However, the lack of direct experimental method for measuring 3D stress and the infeasibility of theoretical prediction due to the intricate mechano-chemo-thermo-coupled photopolymerization pose significant challenges to accurately assessing the 3D shrinkage stress. In this study, we develop, for the first time, an accurate mechano-chemo-thermo-coupled finite element method (FEM) capable of accurately capturing the 3D shrinkage stress evolved during photopolymerization. The real-time evolutions of the multi-field-coupled material properties during photopolymerization are first derived based on the measured development of the degree of conversion and temperature change. The coupled FEM is then established by accounting for the evolution of the material properties and the thermal expansion/contraction. Mechanical, chemical, and thermal experiments are utilized to parameterize the coupled FEM. Simulation models based on this coupled FEM under three photocuring protocols and various boundary conditions are subsequently built according to the typical applications of photopolymerized materials. The accuracy of the FEM-predicted shrinkage stress is finally validated by three different groups of experiments: uniaxial shrinkage stress measurement, full-field optical measurement, and acoustic emission measurement using typical dimethacrylate photopolymerized materials. It is shown that the predicted shrinkage stresses based on our mechano-chemo-thermo-coupled FEM agree quantitatively with the experimental results, far exceeding the accuracy of all existing methods. In addition, our coupled FEM could accurately predict the spatial debonding locations of the photopolymerized material based on the distribution of the predicted shrinkage stress, which is otherwise inaccessible by any existing methods. Our simulation-experiment-combined study not only provides an effective method for accurately assessing the internal 3D shrinkage stress under complex boundary conditions, but also realizes the quantitative prediction of the spatial and temporal evolutions of the shrinkage stress that could ultimately guide the material design for extended service life and reliability.

Keywords: photopolymerization, 3D shrinkage stress, mechano-chemo-thermo-coupled finite element method, experimental validation