Toughening Hydrogels Through Force-triggered Chemical Reactions that Lengthen Polymer Strands

Abstract

<p>The utility and lifetime of materials made from polymer networks, including hydrogels, depend on their capacity to stretch and resist tearing. In gels and elastomers, those mechanical properties are often limited by the covalent chemical structure of the polymer strands between cross-links, which is typically fixed during the material synthesis. Here, we report polymer networks in which the constituent strands 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. </p>

Mechanochemically accessing a challenging-to-synthesize depolymerizable polymer

“…The mechanochemical activation enabled depolymerization demonstrated here can enrich the growing toolbox for force-induced small molecule release. [37][38][39][40] The utility of cyclobutane in controlling depolymerization of polymers again captures the versatility of this mechanophore, as it has been used to toughen hydrogels, 41 access conjugated polymers, 32,33 regulate reactivity, 25 and control polymer degradation. [9][10][11][12] In previous works on mechanically active degradable polymers, cyclobutane was used to lock the degradable functionalities; [9][10][11][12] here, the function of cyclobutane is distinct in that the two alkenes generated from the cycloreversion of cyclobutane are used for ole n metathesis.…”

Chain breaking in the statistical mechanical constitutive theory of polymer networks

“…Polymer networks are often synthesized to contain sacrificial bonds that are designed to break or activate before the rest of the bonds in the network (Ducrot et al, 2014;Clough et al, 2016;Wang et al, 2021). We can adjust our previous relations to accommodate these cases -we consider the same uFJC model, but now with N b breakable links and N # b unbreakable links.…”

“…The sacrificial network is often embrittled as a swollen gel (Gong et al, 2003), but could also be pre-stretched using the secondary networks (Ducrot and Creton, 2016) or even designed without need for pre-stretching (Nakajima et al, 2019). The breaking in the sacrificial network may involve some additional functionality, such as mechanoluminescence (Ducrot et al, 2014) and recently, chain-lengthening (Wang et al, 2021). Secondly, elastomers may utilize a range of reversible bonds in order to allow chains to reform after they have been broken.…”

Ultra-tough self-healing hydrogel via hierarchical energy associative dissipation

“…reach satisfactory strength and elasticity 1,2 . Methods such as employing tensile-resistant groups 3,4 and introducing structural heterogeneity 5 ,6 have been developed to fabricate tough hydrogels. However, those techniques significantly increased the complexity and cost of material preparation, and only had limited applicability.…”

“…To address the issue, a number of approaches have been proposed. One popular idea is to incorporate tensile-resistant units into hydrogel, such as micellar crosslinkers 3,13 , force responsive groups 4 , peptide crosslinkers 14 , covalent organic frameworks 15 , and ionic crosslinking points that are usually found in double network hydrogels 16,17 . During structural deformation, the tensileresistant units are subject to quick chemical changes, which absorbs energy and converts into a more extended state.…”