A Molecular View of the Dynamics and Mechanics of Model Polymer Networks

Cross-linked polymer networks are stress supporting structures that represent an important class of soft materials with a broad range of applications in adhesives, coatings, membranes and natural rubber. Recent advances in synthetic end-linked polymer gels have received growing attention in the biomedical field for their structural and mechanical similarities to tissues. Synthetic efforts that utilize linear chains end-linked with multi-functional cross-linkers have produced nearly homogeneous poly(ethylene glycol) gels capable of reproducing some of the remarkable mechanical properties found in natural materials. The elastic properties of gels, as imparted by the network architecture, are critically important to the performance of these soft materials under external stress. Characterizing the architecture of end-linked networks and quantification of topological defects, such as loops where both ends of a polymer bind to the same cross-linker, and understanding their role in the elastic and failure properties of polymer networks will provide the basis for the development of a real elastic network theory. Previous studies of glass-forming materials (another class of disordered solids) have shown that local packing and rearrangements dictate the mechanical response and site of failure. As many synthetic and natural soft materials are inherently inhomogeneous, an important step is to elucidate how structural heterogeneities, such as network defects and phase boundaries, affect the response of the network to deformation. We use coarse-grained molecular dynamics to investigate the response of swollen polymer networks to deformation and the influence of network heterogeneities on the response. We have developed a scheme to “synthesize” realistic polymer networks during a molecular simulation, and we examine the influence of network structures on soft materials’ bulk mechanical properties using tensile deformation to study both reversible and irreversible responses in gels as a function of polymer concentration, strand length, and network defects. We find that the network formation process dictates local network structure, which is highly correlated to the dynamic material response under deformation. Finally, we turn to the highly complex environment of synthetic and natural gels with phase boundaries. We explore how interfaces separating phases with different material properties influence the local mobility and mechanical responses of microphase separated polymer networks