Self‐recovery and fatigue of double‐network gels with permanent and reversible bonds

Abstract

Double‐network (DN) gels subjected to cyclic deformation (stretching up to a fixed strain followed by retraction down to the zero stress) demonstrate a monotonic decrease in strain with time (self‐recovery). Observations show that the duration of total recovery varies in a wide interval (from a few minutes to several days depending on composition of the gel), and this time is strongly affected by deformation history. A model is developed for the kinetics of self‐recovery. Its ability to describe stress–strain diagrams in cyclic tests with various periods of recovery is confirmed by comparison with observations on several DN gels. Numerical simulation reveals pronounced enhancement of fatigue resistance in multi‐cycle tests with stress‐ and strain‐controlled programs when subsequent cycles of deformation are interrupted by intervals of recovery. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 438–453

Supramolecular Dimerization in a Polymer Melt from Small-Angle X-ray Scattering and Rheology: A Miscible Model System

“…No diffusion over longer distances will be allowed, and the same partners are found. This contrasts strongly with the hydro-or organo-gel systems where the mobility is high due to the presence of solvent molecules [50,51]. Material is transported over longer distances and a strong partner exchange is likely to occur.…”

Bond Reformation, Self-Recovery, and Toughness in Hydrogen-Bonded Hydrogels

“…Yet, the physical bonds can reform after dissociation to re-establish newly formed cross-linking points in the network and maintain networks’ integrity. As such, hydrogels with physical cross-linking such as ionic and hydrogel bonding can undergo network recovery after dissociation of their physical bonds if enough time is allowed for physical bond reformation. − In this study, to theoretically model the contribution of network reformation to mechanical properties of hydrogels with physical cross-linking, an exponential correlation was proposed for the reformation of physical bonds as a function of time. The correlation between physical bond reformation and time was then used to determine how fast physical bonds could form in a constantly deforming network (e.g., fracture phenomenon).…”

“…In more recent versions of double network hydrogels, the covalent cross-linking in the tightly cross-linked network was replaced by physical bonds (e.g., ionic), which resulted in a time-dependent load recovery behavior. The dynamics and kinetics of the transient bonds in such hybrid hydrogels were explained using relaxation and uniaxial load-unload tensile cyclic tests. , The full recovery in such topologies may take some time from a few minutes to days depending on the composition of the gels and the rate of bond reformation in the network. 13, Nevertheless, incorporating a dynamic physical cross-linking into the polymer networks has become an effective strategy to increase toughness and improve load recovery.…”

Structure–property relations in linear viscoelasticity of supramolecular hydrogels

“…[1][2][3] Owing to the transient nature of physical bonds between chains (that associate and dissociate being driven by external stimuli), these gels demonstrate high strength, 4 exceptional stretchability, 5 high fracture toughness 6 and fatigue resistance, 7 and rapid self-recovery. 8 Among other important features of supramolecular gels, it is worth mentioning (i) their self-healing ability, 9,10 (ii) shape memory property, 11,12 (iii) self-assembling (the ability to undergo sol-gel-sol transitions driven by mechanical and chemical triggers), 13,14 and (iv) the ability to secure strong and robust adhesion to a variety of surfaces. [15][16][17] As the mechanical behavior of supramolecular gels is strongly affected by rearrangement (dissociation and re-association) of temporary bonds between chains, a number of studies analyzed the kinetics of this process by means of the standard rheological tests (shear oscillations in the frequency sweep mode).…”

“… 1–3 Owing to the transient nature of physical bonds between chains (that associate and dissociate being driven by external stimuli), these gels demonstrate high strength, 4 exceptional stretchability, 5 high fracture toughness 6 and fatigue resistance, 7 and rapid self-recovery. 8 Among other important features of supramolecular gels, it is worth mentioning (i) their self-healing ability, 9,10 (ii) shape memory property, 11,12 (iii) self-assembling (the ability to undergo sol–gel–sol transitions driven by mechanical and chemical triggers), 13,14 and (iv) the ability to secure strong and robust adhesion to a variety of surfaces. 15–17 …”