Tuning the structure and mechanical properties of double-network elastomer: Molecular dynamics simulation

Abstract

Double-network polymers constructed by physical interaction network and covalent network, have aroused many concerns for their improved mechanical properties. Even though many experiments have revealed how to combine the physical interaction network with the covalent network, we still need a understanding of the structure-property relationship in the double-network polymers. Through coarse-grained molecular dynamics (MD) simulation, we have successfully designed the double-network system consisting of repairable physical interaction network and covalent cross-linking network. Firstly, we examined the kinetic of the covalent cross-linking reaction under high temperature ( T *>2.0 k B T / e ), which is consistence with the experiment. With the model above, we try to tune the mechanical properties by adjusting the covalent cross-linking density and the strength of non-bonding physical interaction. By systematically analyzing the radial distribution function, the virial coefficient of the beads constructing the physical interaction network under equilibration state and the stress-strain curves of all systems under unixial deformation, we found that the strength of non- bonding interaction finally dominate the contribution from physical network on the mechanical properties. Typically, only when the strength of non-bonding interaction of the physical network is higher than 5.0 e can the physical network finally form via the aggregation of physical cross-linking sites and then enhance the mechanical properties of the system. Meanwhile, the covalent network with high cross-linking density evidently perform an improvement of the tensile stress, which also better promote the contribution from physical network on the total non-bonding interaction energy. Moreover, the stress relaxation curves of double-network systems with different covalent cross-linking density also validate that the coarse-grained cross-linking process is reasonable. Finally, to investigate the self-healing property of the physical interaction network, we defined the efficiency of self-healing and study the effect of temperature. The physical interaction network could be fully reversible at high temperature ( T *>1.0 k B T / e ), though it cannot fully recover to its initial state after long enough self-healing process at the normal temperature T *>1.0 k B T / e . In general, we found the threshold of the physical interaction network where the physical network can effectively strength the mechanical properties. Constructing double-network with proper strength of physical interaction (above 12.5 kJ mol-1) may achieve the formation and good dispersion of physical cross-linking sites. Recently, the double-network polymer consisting of the physical and chemical cross-linking network has become the new trend, which may combine good mechanical properties and self-healing properties. Our work above may provide some guidances for the design of high-performance elastomer by studying the structure double-network, and it also meets the demand of tuning the structure and property of the double-network polymer.