Engineering Mechanics of Polymers and Gels through Molecular Design

Abstract

Covalent polymer mechanochemistry has attracted significant attention in the last decade due to its broad potential in stress-responsive materials. Force-sensitive chemical moieties (mechanophores) embedded into polymer backbones are triggered by mechanical loads, with demonstrated responses that include mechanochromism, small-molecule release, mechano-catalysis, enhancements in material toughness and even electrical conductivity. While many qualitative studies have demonstrated the structure-activity correlation between mechanophore designs and the force-responsive properties of bulk materials, quantitative investigations remain rare. Gaining quantitative perspectives is advantageous for the rational, molecular design of polymer materials with tailored mechanical properties, which ultimately could lead to cost-effective and less wasteful materials. Here, we probe quantitative structure-activity relationships on three fronts: 1) stereochemical effects in the mechanochemical reactivity of endo- vs. exo- furan-maleimide Diels-Alder adducts; 2) the mechanochemical reaction pathways of dichlorocyclopropane diester ring opening reaction; 3) enhanced mechanics of hydrogels through stress-responsive covalent extension of single strands within the polymer network. Chapter 2 presents a systematic study of a structure-activity correlation in polymer mechanochemistry, specifically investigating how stereoisomerism affects the mechanochemical scission of furan-maleimide Diels-Alder adducts. In this study, we evaluated the internal competition between the mechanically triggered retro-DA reaction and the mechanochemical ring opening of gem-dichlorocyclopropane (gDCC) mechanophores in the pulsed sonication of polymer solutions. The relative extent of the two mechanochemical reactions in the same polymer shows that the endo DA isomer exhibits greater mechanical lability than its exo isomer. This result contrasts with recent measurements of the relative rates of scission in a similar system and points to potential enhanced sensitivity obtained through the use of internal competition as opposed to absolute rates in assessing mechanical reactivity in sonication studies. Chapter 3 investigates how the mechanically accelerated ring opening of gDCC mechanophores is influenced by the use of ester, rather than alkyl, handles. We find that when ester groups are used to link the mechanophore to the attached polymer chains, both cis- and trans- gDCC pulling lead to reaction rates and outcomes that are consistent with the symmetry-allowed disrotatory pathways at forces relevant to either single-molecule force spectroscopy and sonication experiments. Finally, Chapter 4 presents a quantitative study of how the properties of a bulk materials can be enhanced by embedding mechanophores capable of covalent reactive strand extension (RSE) into the constituent strands of a polymer network. RSE allows constituent strands to lengthen through force-coupled reactions that are triggered as the strands reach their nominal breaking point. Reactive strand extensions of up to 40% lead to hydrogels that stretch 40-50% further than, and exhibit tear energies twice that of, networks made from analogous control strands. The enhancements are synergistic with those provided by double network architectures, and complement other existing toughening strategies.