Synergy in Melting of Initially Disentangled Polymers: Linear versus Ring

Extensive molecular dynamics simulations of the Kremer–Grest model have been conducted to explore kinetic features and molecular mechanisms of the melting process of the initially disentangled linear and ring polymers with varying chain lengths (up to 800) in bulk, with an emphasis on illuminating the specific role of chain topology therein. In the meantime, some interesting issues concerning chain explosions have also been addressed in this study. It is revealed from our computational study that the melting of initially disentangled polymers, both linear ones and rings, is a three-stage process, where there is a synergy of such nonequilibrium processes as globule–coil transitions of polymer chains, polymer interpenetration, and thus formation of nontrivial topological states (i.e., entanglements or threadings) of polymers. We found that the melting of initially disentangled linear polymers seems to be accomplished through chain explosions, while such a picture is obscure for the ring case. In the linear case, it is through sufficient “releasing” of the two chain ends that the disentangled linear chains become able to explode for arriving at their well-equilibrated coil conformations, while chain expansion of ring polymers during the melting occurs with the aid of the formation of more wrinkled conformations due to the absence of chain ends. Moreover, it is concluded that a concomitant development of polymer interpenetrations essentially acts as a requisite for the occurrence of chain expansion during the melting, and a cooperation of chain expansion and interpenetrations leads to the emergence of entanglements in the linear systems and threadings in the rings.