Heterogeneity in Crosslinked Polymer Networks: Molecular Dynamics Simulations

Abstract

Crosslinked network formation occurs when multifunctional precursors react to form a three-dimensional polymer network. In attempting to link network topology to physical properties, materials are typically characterized by the functionality of the crosslink nodes, the typical chain length between crosslinks and the concentration of the crosslink junctions. Statistical models that are in common usage calculate average values of these quantities and do not determine the possible variation in these properties. Molecular dynamics studies show not only how networks form and whether the average values conform to the predictions of these statistical theories, but also how they vary locally. An important result from the simulations is that there are significant regions where there are few bonds connecting neighboring chains. In addition, very long loops and dangling chains occur. None of this should be surprising in a system where the reactions and spatial arrangements are influenced by random chance and there are such a huge number of precursor molecules and possibilities that a number of nonideal configurations are possible. The simulations permit visualizations of these imperfections and show how large they are. Many of the end use properties of polymer networks depend on the continuity of the network, so this heterogeneity must have an impact on the physical properties. Here, the simulations use coarse-grained techniques that are not specific to any reactive chemistry, but such research may indicate how networks can be assembled that are less prone to large heterogeneities and are thus more robust.