Stress Relaxation and Underlying Structure Evolution in Tough and Self-Healing Hydrogels

Abstract

The tough and self-healing hydrogels composed of polyampholytes (PA gels) are drawing great attention due to their multiscale structures and the resultant multiple mechanical properties. This work studies the stress relaxation behavior of PA gels and reveals the underlying multiscale structure evolutions by combining birefringence and small-angle X-ray scattering measurements. The PA gels show a fast and strong stress relaxation that obeys the stress-optical rule, which could be associated with relaxation of chain segment orientation by the breaking of ionic bonds. A slow and weak relaxation of phase structure (∼100 nm) is also observed, which tells that the stress redistributes and local strain amplification gradually builds in the phase network at long relaxation times as a result of synergetic breaking of multiple ionic bonds. This work gives insight into exploring the formation of the crack precursor that is important in the fracture and fatigue of self-healing hydrogels.

Conductive Hydrogels with Dynamic Reversible Networks for Biomedical Applications

“…DCHs, especially physically crosslinked hydrogels, are viscoelastic and exhibit stress relaxation, which means that in response to applied strain, the stress decreases, also known as creep behavior (i.e., the tendency to undergo permanent deformation in response to applied stress). [34,37,42] The DCH stress relaxation can result from numerous dissipative events, such as polymer disentanglement, weak physical interactions, and rearrangement of the reversible network. Additionally, the degradation of the polymer can also affect the network viscoelasticity and cause stress relaxation.…”

Solution-Processable Conductive Composite Hydrogels with Multiple Synergetic Networks toward Wearable Pressure/Strain Sensors

“…Each CCH coin (3 mm in thickness and 16 mm in diameter) served as a sensor unit/pixel. Different weights (5,10,20,50, and 100 g) were put on different electrodes and the resistance changes were recorded by the digital multimeter.…”

“…The circuit was encapsulated with insulating PVC and connected to a Single Chip Micyoco (SCM, Arduino MEGA 2560) for the signal processing and a computer for outputting the signal. The resistance changes induced by different weights (10,20, and 50 g) on the same site or different sites were read out by the SCM and graphically shown on the computer that prewrote programs.…”

Aggregated structures and their functionalities in hydrogels

“…[104] PA hydrogels have a multiscale structure: reversible ionic bonds between opposite charges at ∼0.1 nm scale, transient polymer network at ∼1 nm scale, permanent polymer network at ∼10 nm scale, and bicontinuous hard/soft phase networks at ∼100 nm scale. [6,24,[105][106][107][108] Such a multiscale structure allows the gel a multiple mechanical performance: high stiffness (Young's modulus: 0.1-1 MPa), high toughness (fracture energy: 1000-4000 J/m 2 ), 100% self-recovery, and high fatigue resistance. During deformation, the bicontinuous phase network shows affine deformation up to a large strain and then an extensive nonaffine deformation.…”