Residual Strain Development in Rapid Frontally Curing Polymers

Frontal polymerization (FP) has emerged as a rapid and energy-efficient process for fabricating thermoset polymers and composites. In this process, a self-propagating reaction front cures the polymer rapidly by the exothermic heat of polymerization reaction instead of an external heat source. Design for FP-based manufacturing in commercial applications requires more comprehensive characterization and prediction of material evolution and residual deformation throughout the process. Here, we report experimental and numerical studies in response to this need. The experimental study focuses on measuring the temperature and cure-dependent properties of mono/poly dicyclopentadiene to capture the strain evolution during the frontal polymerization process. The experimentally measured elastic moduli, Poisson's ratios, and coefficients of thermal expansion and chemical shrinkage show strong dependence on the degree of cure. Based on the experimental output, a coupled thermo-chemo-mechanical model has been developed to capture the measured residual strains. The chemical shrinkage is closely related to the curing rate, leading to strong localization of residual strains in accelerated reaction regions, especially where two fronts merge. Preheating of the monomer (or gel) at the fronts merging area is suggested as an effective method to mitigate residual deformations.