Bridging Molecular Mechanochemistry and Network Fracture Mechanics

Ph.D. Defense Stephen Craig, Ph.D, Advisor Abstract. The fracture of polymer networks is usually perceived macroscopically and is usually considered to be a mechanical engineering problem. However, to advance a crack in a polymer network, lots of polymer strands that bridge the crack need to be broken, thus network fracture is molecular as well. In the past 80 years, scientists have been trying to build up quantitative connections between network fracture mechanics and the molecular details of the networks, but to date, there is still no well-accepted quantitative molecular model for network fracture. This is due to the lack of understanding of the molecular details (i.e., strand scission reaction, network topology, etc.) in polymer networks. Developments in polymer mechanochemistry and polymer physics open the door to understanding network fracture from the molecular level. With the concepts of polymer mechanochemistry and polymer physic, this dissertation investigates the correlations between bond/strand scission reaction and the network fracture mechanics theoretically and experimentally. We presented a conceptual framework for adding force-coupled polymer strand scission and network connectivity to the Lake-Thomas model of tearing energy in fracture of rubbery polymer networks. Mechanophores were designed, characterized, and incorporated into end-linked and sidechain-crosslinker networks to investigate network fracture. We showed that the tearing energy of end-linked networks is directly correlated to the force-coupled reactivities of network strands. The tearing energies of end-linked networks with coexisting strand scission reactivities suggest the crack propagation is controlled by the reactivity of stands that are percolated, which is different from the classical point of view on network fracture. Instead of weakening the polymer network, we showed that the incorporation of mechanochemically weak covalent crosslinkers into sidechain-crosslinked networks can significantly toughen the polymer network when the primary chains in the network are long and mechanochemically strong.