Load-Induced Shear Band Formation in Microscale Epoxy Materials

Thermosetting polymer thin films exhibit distinct mechanical behaviors at the microscopic scale compared to bulk materials. Experimental results reveal significant necking and unexpected shear band formation under tensile load. This study investigates the mechanisms underlying shear band formation in epoxy resin systems composed of bisphenol-diglycidyl-ether and diamines. Mechanical testing methods, including creep, relaxation, and cyclic testing, as well as ex situ and in situ high-resolution infrared (IR) spectroscopy, are used synergistically with quantum mechanical calculations to elucidate the underlying molecular mechanisms. Additionally, molecular dynamics (MD) simulations on a nanoscale model explored the (visco-)plastic behavior and network strain in epoxies. Our findings reveal a strong correlation between shear band formation and shifts in IR spectra, specifically the redshift of para-phenylene and the blueshift of out-of-plane vibrations of aromatic moieties. These shifts are attributed to load-induced aromatic stretching in the polymer backbone. The robust agreement between experimental data and simulation results supports these observations at both the atomic and nanoscale. These insights enhance the understanding of epoxy resin mechanics, potentially informing the design of advanced composite materials.