Mechanical Properties of Tetrapolyethylene and Tetrapoly(ethylene oxide) Diamond Networks via Molecular Dynamics Simulations

The tensile response to uniaxial deformation of polyethylene-based (Tetra-PE) and poly(ethylene oxide) based (Tetra-PEO) networks of various strand lengths with idealized diamond connectivity have been studied via atomistic molecular dynamics simulations. Tetra-PE and Tetra-PEO diamond networks with the same strand length show comparable maximum extensibility but the Young's moduli and tensile strength of the former are significantly lower than those of the latter, consistent with stronger intersegmental attractions in the amorphous Tetra-PEO networks. The stress strain curves show that the stress in short-stranded networks increased rapidly and monotonically with strain while for long-stranded networks it increased very little at small strain and then very sharply as the limit of extensibility was approached. The soft response observed at small strains and the strain-induced crystallization that ensues at large strains rendered the deformations largely nonelastic (irreversible). Spontaneous partial crystallization of a long-stranded Tetra-PE diamond network under supercooling was demonstrated, and the resulting system was used to (1) estimate its melting point as the temperature where any crystalline material disappeared abruptly and (2) show that the presence of crystalline material in the undeformed state leads to higher stress responses upon deformation compared to amorphous samples, a result consistent with experimental observations. The spontaneous crystallization of Tetra-PEO networks at large supercooling was unsuccessful due to the slow motions of the network beads and the prohibitively long crystal nucleation times entailed.