Structure and mechanics of bundled semiflexible polymer networks

Fibrous networks of semiflexible polymers are prevalent in biological materials such as connective tissues, the cytoskeleton, and biofilm extracellular matrices. While the mechanical behavior of permanently cross-linked fibrous networks has been studied in detail, much less is understood about how bundles and physical entanglements influence the response of semiflexible polymer networks. Here we use coarse-grained molecular dynamics simulations to ascertain how fiber bending rigidity, interfiber cohesion, and network architecture influence the tensile mechanical behavior of these systems. We find that the features of self-assembled networks, namely mean bundle thicknesses, link lengths, and number of links and nodes, depend strongly on fiber properties as well as assembly conditions. Tensile tests show that high strength networks have high densities arising from high cohesive energy and/or low persistence lengths. Adjusting the cohesive potential has the greatest impact on mechanical response, influencing bundle thickness and therefore both network cohesive and bending energies. Changes in network architecture such as fiber alignment, bundle thickening, and link extension also contribute to stiffening. When stretched to fracture, networks show a power law scaling for ultimate tensile strength with both fiber persistence length and cohesive potential. Furthermore, similar exponential scaling of the stress−strain relationship is seen during stiffening of networks. These results shed light on the competing influences of fiber stiffness, cohesion, architecture, and structural evolution in entangled semiflexible polymer networks, particularly underlining the significance of bundle formation and thickening as an advantageous mechanism for generating mechanically robust networks.