A Molecular Dynamics Study on Hysteresis of Amorphous Polymers

Abstract

For a new insight on the mechanism of hysteresis of polymers, several molecular dynamics simulations are conducted on an amorphous polyethylene under cyclic deformation. Each role of the bond stretch, bending, torsion and van der Waals is investigated in detail, revealing that the bond stretch and van der Waals dominate the hysteresis in the cyclic deformation. In the 1st cycle, or the “pre-stretching”, the trans nodes increase to orient the molecular chains in loading direction. However, the trans⇒gauche transition takes place during the unloading, resulting in the “curled” chain structure which has more gauche nodes than the initial configuration. In the 2nd cycle up to the maximum strain of the 1st cycle, the gauche⇔ trans change occurs in the “pseudo reversible” manner among the gauche nodes introduced in the 1st cycle. When the polyethylene is elongated in the 2nd cycle beyond the maximum strain, the dihedral nodes with high torsional energy, which are not in the trans nor gauche angles, decrease to lead the increase of trans nodes. This suggests that the overloading orients the entangled chains in loading axis, resulting in the dissociation of the chain entanglements.