Epoxy resin thermo-mechanics and failure modes: Effects of cure and cross-linker length

The effects of molecular weight (MW) of cross-linker and degree of cure on the structure and thermo-mechanical properties of the Bisphenol A diglycidyl ether epoxy resin have been studied using MD simulations with reactive force field ReaxFF and non-reactive General AMBER Force Field (GAFF). Cross-linked structures are created from stoichiometric mixtures of Epon and Jeffamine (R) using a multi-step cross-linking algorithm. The glass transition temperature (T-g) is determined by annealing where the cross-linked epoxy is cooled from the rubbery state to below room temperature. Deformation mechanisms of the cross-linked epoxy including bond breakage are studied under tensile and shear loadings. The effects of cross-linkers of increasing MW (Jeffamine (R) D-230, Jeffamine (R) D-400 and Jeffamine (R) D-600) are studied for highly cured (98.5% degree of cure) systems. MD predicted T-g is in good agreement with experiments after cooling rate correction using the WLF relationship. The highest T-g is obtained for the lower MW cross-linker that exhibits a denser network structure. In addition, the effects of varying degrees of cure on properties are studied for Epoxy/Jeffamine (R) D-230. In this case, the MD results shows that T-g increases linearly with degree of cure and that the DiBenedetto relationship can be applied using the MD fitted parameters. Lower MW cross-linker yields higher modulus and yield stress and reduced strain to failure and energy absorption than the higher MW cross-linkers. Results from GAFF, which is about 100 times more computationally efficient, agree well with ReaxFF predictions up to the strain limit at which bond breakage becomes significant.