Effect of Bidispersity on Structure and Entanglement of Confined Polymer Films

Using Monte Carlo simulations combined with a geometric primitive path analysis method (Z1 algorithm), we investigate the effect of bidispersity on the structure and entanglement of polymer films which consist of short (the molecular length is below the characteristic entanglement molecular length) and long (the molecular length is above the characteristic entanglement molecular length) chains between two neutral walls. Our results demonstrate the length-based migrations of chains in bidisperse films (the longer chains reside away from the walls and the shorter chains are close to the walls), which becomes more obvious with the decrease in the weight fraction of long chains. With decreasing the weight fraction of long chains, the number of short-long entanglements exhibits a dramatic increase, whereas the number of long-long entanglements exhibits a slight decrease, which indicates that short chains can significantly affect the local situations of entanglements of bidisperse polymer films. On the basis of the constraint release mechanism, our simulations imply that for the lower weight fraction of long chains, the local degree of confinement instead of the long-long entanglements has a marked effect on the relaxation of long chains, due to the fast relaxation of short chains dilating the tube diameter of long chains. However, for the higher weight fraction of long chains, after the relaxation of short chains, the long-long entanglements are in sufficient quantities to restrict long chains within a tube, which implies that the relaxation of long chains is hardly affected by the number of short-long entanglements. Our work can be helpful for understanding the microscopic structure and entanglement of bidisperse polymer films, which can provide computational support for their various technological applications.