Microscopic Theory of Chain Pullout in Amorphous Polymers

Abstract

The statistical-mechanical problem of chain pullout from an amorphous polymer under the influence of a constant force has been investigated. A simple microscopic model is proposed to describe the pullout process in both polymer glasses and elastomeric materials from a single point of view. A mean-field approximation for the interchain potential is used to account for the entanglements with the polymer network of the chain being pulled out. The chain mobility and the pullout rate were calculated as a function of pullout force, length of the chain, and other model parameters. In glassy polymers the pullout force was found to be almost rate independent at low pullout rates and linear in rate at high pullout rates. For polymer glasses the model also predicts the existence of some characteristic degree of polymerization Ne of the chain being pulled out, such that the pullout force scales as N for N > Ne. In elastomeric materials the pullout force was found to have a nonlinear dependence on both chain length and rate of pullout. ~J~~~~-~~ One can attempt to separate these models into two major groups. The first group of models has its origin in work devoted to polymer adhesion and interfacial toughne~s.~-l~ To this end the authors usually consider the process of failure of the diblock-reinforced interface between two immiscible amorphous polymers. The failure of such an interface by crack propagation causes either scission or pullout of diblock copolymer chains entangled in the bulk of polymeric material. Although models of this process differ in their approach to the micromechanical problem of crack propagation, the descriptions of the individual chain pullout process have a number of features in common. For example, the pullout process is dominated by viscous forces, and so the pullout rate is linear in the pullout force. However, it is assumed to be different from a purely viscous process in the sense that the pullout force tends to a nonzero constant as the pullout rate approaches zero. pullout .2-21