(670f) Studies On the Tensile Properties of Polymer Networks with Heterogeneous Microstructure: Bimodal and Idealized Regular Networks

Through both coarse-grained molecular modeling and experimental measurements, our investigation aims to gain a deeper knowledge on the underlying relationships between the microstructure and macroscopic mechanical properties in polymer networks. We are currently focused on how tensile properties strongly depend upon the microstructure in two different types of systems of polymer networks: first, networks made of chains with bimodal distributions of molar masses; and second, model networks made of semi-flexible chains, with completely regular topologies. Bimodal polymer networks exhibit pronounced increases in the energy of rupture (toughness), compared to equivalent unimodal networks, when the chain lengths are widely separated. The impact of the network composition (mol% of shorter chains) on the network topology, chain-segment orientation, and mechanical enhancement is studied through molecular simulations in addition to ²H-NMR and SANS measurements on experimental polydimethylsiloxane networks. Good agreement is found between experimental and simulation results. On the other hand, idealized polymer networks with chains exhibiting regular connectivity are also being studied further via simulations. This type of networks has been shown to display a peculiar step-wise elastic response and extension mechanism (as seen in super-tough natural materials, such as, organic adhesives inside the abalone shells, and the muscle protein ?titin?) under iso-stress deformations. We investigate the impact of network topology, chain stiffness, and length on the shape of the stress-strain curve. For some regular networks, we observe a dual tensile response: a liquid-like response at small (iso-strain) deformations, and a saw-tooth shaped stress-strain curve at moderate to large deformations. Additionally, stiffer regular networks exhibit a noticeable irreversibility in the deformation mechanism, and a pronounced hysteresis over loading-unloading cycles. This ?system memory? upon deformations may be deleted by heating or performing deformations along different axes. Furthermore, chain stiffness and length seem to play an important role in determining the shape of the stress-strain curve.