UC San Diego

Vitrimers have recently emerged as a class of polymers combining great processability, self-healing capability and high-temperature mechanical properties. Most of those salient features of vitrimers originate from the existence of dynamic covalent bonds in the polymer network. Although intensive research has been conducted for understanding the constitutive properties of these new materials, their fracture behaviors have been largely unexplored. Moreover, improving the creep resistance and fracture toughness of vitrimers are critical for the commercial applications, which need to be addressed. In this dissertation, we first experimentally show that, if the network with only regular covalent bonds and the network with dynamic covalent bonds are immiscible with each other, hybrid network can be synthesized with a low molar ratio of dynamic covalent bonds (less than 20 mol%), which still maintains its reprocessing and self-healing capabilities. Our discoveries will enable much greater tunability of the thermo-mechanical properties of vitrimers such as stress relaxation, creep resistance and fracture toughness, which can be important in many of their applications. Additionally, we investigate the rate-dependent fracture of vitrimers, and we, for the first time, obtain the intrinsic fracture energy and bulk dissipation of vitrimers during crack extension. Then, we find that the transient nature of a vitrimer network yields peculiar fracture characteristics that cannot be understood from existing fracture theories. Crack propagation is a non-equilibrium process whose velocity depends on the interplay between external load, bond dynamics and network damage. To explain the transient life of a crack in the vitrimer, we extend the linear elastic fracture theory to a dynamic model that predicts the time-dependent evolution of a crack during loading. Finally, we propose that a combination of crack tilting and crack bridging determines the effective fracture toughness of the fiber-reinforced composite with the plywood structure. Based on our quantitative analysis, it is found that the effective fracture toughness of the composite can be maximized for a certain pitch angle of the oscillated/twisted plywood structure, which agrees well with experiments.