Abstract:  Microtubules are extremely stiff biopolymers that play important roles in maintaining the shape, strength and organization of cells and serve as an excellent model system for the study of rigid rod polymer networks. To determine the structure-mechanics relationships in dense microtubule networks, we measure the time- and force-dependent viscoelastic responses of entangled and crosslinked networks subjected to precise microscale manipulation. We use confocal microscopy to determine the morphology as a function of polymer concentration and crosslinking, and a suite of custom-built magnetic tweezers devices to apply calibrated forces as a function of time. We find that in this rigid system, bond breakage kinetics and network geometry dominate the mechanical responses, while thermal fluctuations and hydrodynamics play a secondary role. Our results are important to understanding the origins of nonlinearity and stress dissipation in rigid polymer networks, and provide insight into how cytoskeletal biopolymers regulate cargo transport and