Bi-modal polymer networks: Viscoelasticity and mechanics from molecular dynamics simulation

Abstract

The high strain-rate rheological and mechanical properties of bi-modal epoxy polymer networks were characterized using molecular dynamics simulation. The complex Young's modulus was found by applying a cyclic sinusoidal strain over a wide range of temperatures spanning the glass transition. The non-linear stress response was studied in the glass transition region using uni-axial deformation. We discuss special considerations in computing viscoelastic properties at the high strain-rates available to molecular dynamics. As in experimental studies, the complex modulus is shown to be a function of the network composition and strain rate. However, the high strain-rate simulations performed here predict the existence of broad peaks in the temperature-dependent loss modulus and slow relaxation of the storage modulus. In general, it is observed that network compositions with larger amounts of short, stiff 4,4’-methylenebis(cyclohexylamine) (MCA) cross-linkers lead to an increase in the mechanical glass transition temperature as well as the breadth of the glass transition compared to longer, more flexible poly(oxypropylene) diamine (POP) cross-linkers. When the networks of any composition were deformed beyond the linear region, the stress response displayed a plateau that was associated with the extension of network chains.