Molecular dynamics simulation of thermo-mechanical behaviour of elastomer cross-linked via multifunctional zwitterions

Coarse-grained (CG) molecular dynamics simulations have been employed to study the thermo-mechanical response of a physically cross-linked network composed of zwitterionic moieties and fully flexible elastomeric polymer chains. In this work, we used the effective Lennard-Jones interactions for our bead-spring model of zwitterionic cross-linkers (ZCLs). The effects of ZCL functionality, its chain length and the attractive interaction strength (epsilon) of end-groups on the thermo-mechanical properties are explored. We developed one particular system with versatile functionality of ZCLs at constant molecular weight with variable functional sites, observing that the stress-strain curve is increased with the functionality, evidenced by the bond orientation through the second-order Legendre polynomials. In the case of the length of the tri-functional ZCL, we found that a shorter length leads to a higher modulus and a relatively greater value of glass-transition temperature (T-g). In addition, we found that with the increase of the attractive interaction strength (epsilon) on the tri-functional ZCL, the stress-strain behaviour and the chain orientation are reinforced, accompanied by an enhancement of T-g. Lastly, we examined the effect of the cross-linker functionality on the tension-recovery process, as well as the fracture behaviour, by performing the typical tri-axial deformation. The results of tension-recovery and tri-axial deformation correspondingly proved the presence of physical linkages of end-groups, and strong attractive forces between end-linking groups were revealed through visual molecular dynamics (VMD) analysis. Generally, we anticipate that our work could provide some guidelines for the rational design and preparation of high performance zwitterionic cross-linked elastomeric materials.