BROWNIAN DYNAMICS SIMULATION OF REVERSIBLE POLYMERIC NETWORKS

In this paper we describe the construction of a Brownian Dynamics simu lation of reversibly cross-linked networks. The simulation differs fro m existing analytical approaches and computer simulations of networks in the sense that we also take the topology of the network into accoun t. The motion of the junction points between different molecules is no t prescribed but is calculated from a force balance. This makes it pos sible to measure the effect of network reorganisations on the stress r elaxation. The response of networks to shear flow is measured and anal ysed in terms of transient network theory and within the framework of linear viscoelasticity. It is shown that the average motion of the jun ction is affine but that there is a long time diffusive process around the affine path. It was found that, even in systems with Gaussian cha ins and fixed association and disociation rates, a shear thickening of the viscosity and primary normal stress coefficient can occur. The re ason was found to be that dangling segments are recaptured by the netw ork before they had the opportunity to fully relax to the equilibrium state where the probability of reattachment to the network increases l inearly with the length of a segment. Due to this mechanism the fracti on of long segments present in the network is increased. This explanat ion of shear thickening seems to be consistent with experimental findi ngs.