Large amplitude oscillatory shear behavior of thermoresponsive hydrogels: Single versus double network

Abstract

Double network (DN) hydrogels have been recognized as new tough materials for several industries due to their precise structural platforms and significant properties. However, a comprehensive understanding of microstructural changes of DN hydrogels under large deformations is required to extend their applications. In this work, we use the large amplitude oscillatory shear (LAOS) technique to study the nonlinear response of a thermoresponsive κ-carrageenan/polyacrylamide DN system and its nanocomposite containing graphene oxide (GO) in comparison to its single network components. The results show a combination of strain stiffening and shear thickening nonlinear responses. The elastic intracycle strain stiffening was mainly attributed to the shear-induced increase in the elasticity of network chains and non-Gaussian stretching of individual chains. In addition, the orientation of the κ-carrageenan double helix segments and their enhancing effect on molecular orientation could be proposed as another possible mechanism of strain stiffening. The viscous intracycle shear thickening is also interpreted by two mechanisms of shear-induced temporary structure formation and reformation of dissociated physical interactions. It is also found that the GO nanosheets could contribute to the viscoelastic response by increasing the molecular interactions and, thus, amplification of energy dissipation. Furthermore, temperature dependency of the DN hydrogel owing to the conformational changes of the κ-carrageenan network at sufficiently high temperatures is used to investigate the effect of temperature on nonlinear behaviors. Increasing the temperature is found to have a significant decreasing effect on viscous nonlinearity, while its effect on the elastic nonlinearity was strongly dependent on the strain amplitude. This study provides a better understanding of the correlation between the microstructure and viscoelastic properties for designing tough hydrogels.

All-Natural, Robust, and pH-Responsive Glycyrrhizic Acid-Based Double Network Hydrogels for Controlled Nutrient Release

“…It is worth noting that for the GN-ALG-DN hydrogel, the changing trend of the S factor with strain amplitudes is similar to that of the GN-SN hydrogel, which implies that the nonlinear elasticity in the form of strain stiffening is most likely attributed to the increasing contacts between the semiflexible GN nanofibrils within the gel network, thus inducing the transition from a bending-dominated to a stretching-dominated deformation. Similar behaviors of intracycle strain stiffening have been reported in these hydrogels with fibrin or filamentous protein networks. ,, In addition, at 50% strain, the S factor values for the GN-ALG-DN and GN-SN hydrogels are significantly lower than those of the ALG-SN hydrogel, suggesting a lower degree of nonlinearity, which further confirms the greater ability of the GN fibrillar network to resist large deformations. This is in good agreement with the results of Lissajous plots and linear SAOS rheology (Figures A,B, and D–I).…”

“…Similar behaviors of intracycle strain stiffening have been reported in these hydrogels with fibrin or filamentous protein networks. 41,44,46 In addition, at 50% strain, the S factor values for the GN-ALG-DN and GN-SN hydrogels are significantly lower than those of the ALG-SN hydrogel, suggesting a lower degree of nonlinearity, which further confirms the greater ability of the GN fibrillar network to resist large deformations. This is in good agreement with the results of Lissajous plots and linear SAOS rheology (Figures 3A,B, and 4D−I).…”

“…Compared to the SAOS, the LAOS test, in which the strain amplitude is sufficiently high to evoke the nonlinear viscoelastic behavior, can provide more information about the mechanical response of materials during practical processing. ,− In particular, the elastic and viscous Lissajous–Bowditch curves decomposed from the shear stress waveform in each domain of the viscoelastic LAOS test, which shows the instantaneous intracycle stress of the material as a function of the applied strain or shear rate, can together provide useful information about their structural transitions and viscoelastic response under large deformation. It is known that the strain amplitude has no impact on the elastic and viscous moduli of materials within the LVR, in which the oscillatory stress response is sinusoidal and the Lissajous curve shows an elliptical shape.…”

“…Further, at relatively low amplitude (0.5−10%), the positive T factor values of all hydrogels reveal a nonlinear intracycle shear thickening of viscous response (Figure S6B, Supporting Information), which is due to the formation of a temporary structure within hydrogels under shear-induced alignment by enhancing the molecular interactions. 41,47,48 However, at the large strain of 50%, the intensity of intracycle shear thickening is reduced and even transformed into a shear thinning behavior, especially for the ALG-SN hydrogel (Figure S6B, Supporting Information). This implies that the gel network is broken and becomes more fluid by a large shearing at higher strains, which is related to the fact that the reformation of physical interactions is postponed due to the insufficient time for relaxation under high deformation amplitudes (i.e., 50% strain), further supporting the greater ability of the GN-ALG-DN hydrogel to respond to large shear deformations.…”

Comparative Large Amplitude Oscillatory Shear (LAOS) Study of Ionically and Physically Crosslinked Hydrogels

“…In order to obtain such quantitative parameters for multicomponent hydrogels, researchers propose mechanisms of shear-induced temporary structure formation and reformation based on increased or decreased molecular interactions. For example, the amplification of energy dissipation [ 16 ], the shifting of the nonlinear behavior onset and increase in yield stress [ 17 ], and the abrupt yielding of the gel structure as a function of the protein fractionation [ 18 ] from the LAOS analysis have all been reported.…”