Using Dynamic Covalent Chemistry in Crosslinked Glassy Polymers to Achieve Stress Relaxation and Improve Material Performance
Covalent adaptable network (CAN) has attracted significant interest in the soft matter community, as it enables stress relaxation in a crosslinked polymer network. However, all CAN systems have developed thus far are in the rubbery state. This thesis is focused on the development of mechanically strong, high glass transition temperature (Tg) polymer networks that are capable of dynamic bond exchange at ambient temperature while maintaining superior mechanical properties. This goal was achieved by introducing moieties capable of dynamic covalent chemistry (DCC) in the resin matrix and at the resin-filler interface of thermosetting polymers to relax internal and applied stresses at the glassy state. The first part of this thesis covers the implementation of reversible addition fragmentation transfer (RAFT) into the resin phase of glassy copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) polymer networks. Correspondingly, various azide and alkyne monomers containing structurally variable allyl sulfide (AS) or trithiocarbonate (TTC) moieties were designed and synthesized following a scalable synthetic strategy. While CuAAC reactions as a highly efficient and orthogonal “click” chemistry yields glassy polymers composed of rigid triazole linkages with enhanced stiffness and toughness, the RAFT moieties independently undergo bond exchange leading to glassy state stress relaxation upon light exposure. This capability of glassy state stress relaxation enables significant enhancement in toughness and ductility, light-activated shape reconfiguration and photoreversal of physical aging.
The second part of this thesis covers the development of adaptive interface (AI) platform by introducing moieties capable of RAFT and thiol-thioester exchange (TTE) at the resin-filler interface of a highly crosslinked, glassy thiol-ene polymer network to promote interfacial stress relaxation in the glassy state and explore the resulting evolution of composite performance. Employing this active, bond-exchanging interface overcomes the limitation of interfacial stress concentration in polymer composites that incorporate filler materials of different modulus relative to the resin phase and imparts significant benefits to the composite performance including improvements in toughness and polymerization shrinkage stress. This improvement in mechanical performance that results from the successful relaxation of interfacial stresses is a platform technology that will broadly impact the field of polymeric composites.
Full pdf - https://www.colorado.edu/mse/node/513/attachment