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.

The temperature‐dependent ballistic performance and the ductile‐to‐brittle transition in polymer networks

“…Alternative methods of improving the temperature‐dependent ballistic impact performance include the use of toughening additives, as well as self‐segregating materials . These are generally engineering approaches to improved ductility, with the filler materials acting to alter fracture behavior and effectively improve toughness .…”

Influence of nano-scale morphology on impact toughness of epoxy blends

“…A previous investigation revealed a maximum in the high rate impact toughness at an amine volume fraction of 0.5, which was hypothesized to arise from nanoscale phase-separated soft domains in a rigid matrix [4]. The soft domains appear to preferentially deform [23,24], and the fine dispersion of these domains may be what gives rise to the improved ballistic performance [25]. This data is reproduced here in Figure 1 1 including two additional mixtures with exactly 50 volume percent amines for both the D2000 and the D4000 blends.…”

Integration of Atomistic Simulation with Experiment Using Time−Temperature Superposition for a Cross‐Linked Epoxy Network

“…We believe that the TIA dynamics is a characteristic of highly cross‐linked polymers, where non‐Gaussian molecular units of disparate lengths and flexibilities are covalently bound to form a network. In the future, it would be interesting to explore this phenomenon in the context of ongoing research38,65,66 on the use of mixed networks for ballistics applications.…”