Authors J.-U. Sommer and S. Lay
Title Topological Structure and Non-Affine Swelling of Bimodal Polymer Networks
Date 14.11.2002
Abstract Using the bond-fluctuation model, we study bimodal networks obtained from end-linking long (100 repeat units) and short (5 repeat units) chains in the melt state. The short chains are dilute among the long chains but provide the majority of chain ends. We discuss the necessary conditions under under which the end-linking process results in the formation of clusters of short chains within the network structure. In our simulations, the onset of this behavior can be observed. On the other hand, the tendency of short chains to form loops is much increased. In the melt state, these loops are likely to form trapped-entanglement long chains, thus yielding a very particular topological structure. We swell the cross-linked systems in an athermal solvent close to equilibrium. The swelling process can be subdivided into two steps. A fast intrusion of solvent into the network provides a large part of the swelling ratio. This is followed by a slow relaxation process, which allows for cooperative reorganization of the network. Our simulations show that swelling of individual strands is highly subaffine. Unfolding of the network structure is necessary in order to provide the global swelling ratio. We systematically analyze the dependency of nonaffine deformation processes as a function of the size of connected substructures. As a result, we conclude that clusters comprising hundreds of strands are necessary to provide affine behavior. These findings can be related to the topological structure formed by the cross-linking process. In particular, we define a folding length which corresponds to the minimal topological distance along the network structure which connect two neighboring monomers in space. The distribution of folding lengths shows a broad peak in the range of the observed nonaffine swelling behavior. Our results show that entanglement constraints do not suppress unfolding of the network structure and challenge simplified theoretical models.
Journal Macromolecules 35 (2002) 9832-9843

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