*3.4. Dynamic Viscoelastic Performance of Hydrogels*

Figure 4a shows the *G* curves of hydrogel samples based on strain at ω = 1 Hz. Within the LVR, the *G* and *G* of the hydrogel were independent of strain, as determined by dynamic strain scanning tests. The critical strain (γc) of hydrogel was a strain point, where the *G* value decreased from the platform value by 5%, indicating deviation from LVR [38]. The *G* value corresponding to the strain higher than γ<sup>c</sup> would gradually decrease, indicating that the quasi-solid hydrogel had changed to a quasi-liquid state. The γ<sup>c</sup> values of TOCNF-CNT/PVA, TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-2, and TOCNF-CNT@PANI/PVA-2 were 2.5%, 2.1%, 1.2%, and 1.5%, respectively. Therefore, in the following dynamic oscillation measurement, the γ<sup>c</sup> value was selected as γ = 1%, which could ensure that deformations of the hydrogel samples were within the LVR. For all the hydrogel samples in the LVR, the *G* values were independent of strain, and the corresponding *G* max was 2.8, 4.1, 7.5, and 5.1 kPa, respectively (Figure 4a). The *G* max of TOCNF-CNT@PANI/PVA-1 was 1.5 times that of TOCNF-CNT/PVA. TOCNF-CNT@PANI/PVA-2 possessed the largest *G* max (7.5 kPa), which was nearly 1.8-fold larger than TOCNF-CNT@PANI/PVA-1 (4.1 kPa) and 1.5-fold greater than TOCNF-CNT@PANI/PVA-3 (5.1 kPa). Incorporation of an appropriate amount of PANI could improve remarkably the stiffness of hydrogel. The shorter the LVR, the closer the sample was to the solid-state. Compared with TOCNF-CNT@PANI/PVA-1 and TOCNF-CNT@PANI/PVA-3, it could be known that the TOCNF-CNT@PANI/PVA-2 possessed a higher *G* max and shorter LVR, indicating that TOCNF-CNT@PANI/PVA-2 was the strongest hydrogel. The result was consistent with the mechanical strength test.

**Figure 4.** Dynamic viscoelastic properties of hydrogels at 25 ◦C. (**a**) storage modulus (*G* ) curves based on strain (γ) at angular frequency (ω) = 1 Hz; (**b**) *G* and loss modulus (*G*) curves based on ω at γ = 1%; (**c**) complex modulus (*G*\*) curves based on ω at γ = 1%; (**d**) stretching demonstration of TOCNF-CNT@PANI/PVA-2 hydrogel.

In order to study the effect of TOCNF-CNT@PANI composite fiber on the viscoelasticity of hydrogels, the *G* (elasticity) and *G* (viscosity) of hydrogels versus ω at γ = 1% in the LVR are shown in Figure 4b. As shown, the *G* and *G*" curves of all the hydrogels followed similar trends. With the increase of ω, the *G* increased monotonically and arrived at plateau value (*G* <sup>∞</sup>), indicating the formation of neighboring polymer chains entanglements; the *G*" increased preliminarily to reach the maximum value (*G*"max), then decreased gradually. For all the hydrogels, the *G* values were always higher than the *G*" values throughout the ω range, suggesting hydrogels showed typical solid-like characteristics, indicating that a dynamic cross-linked network was established inside hydrogel [6,42]. The *G* <sup>∞</sup> and *G*"max values of TOCNF-CNT/PVA were 4.3 and 2.6 kPa, respectively. After the introduction of PANI, the *G* <sup>∞</sup> (6.1 kPa) and *G*"max (3.1 kPa) values of TOCNF-CNT@PANI/PVA-1 were 1.4 and 1.2 times those of TOCNF-CNT/PVA, respectively. It was shown that the combination of PANI and TOCNF-CNT to form a TOCNF-CNT@PANI composite fiber with a "core-shell" structure could significantly improve the viscoelasticity of hydrogel. Comparing between TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-2, and TOCNF-CNT@PANI/PVA-3, the TOCNF-CNT@PANI/PVA-2 showed the highest *G* <sup>∞</sup> (18.2 kPa) and *G*"max (7.6 kPa). These data of dynamic viscoelastic properties are summarized in Table 2. It showed that an appropriate proportion of PANI could develop a hierarchical network structure and increase viscoelasticity together with TOCNF-CNT. However, excessive PANI would form aggregation due to insufficient TOCNF-CNT to disperse and load. A large amount of PANI blocked the cross-linking between PVA and borax, reducing the dynamic viscoelasticity of the hydrogel [33]. Figure 4c shows the curves of complex modulus (*G*\*) versus ω, which provided a clear contrast of viscoelasticity. The trend was TOCNF-CNT@PANI/PVA-2 > TOCNF-CNT@PANI/PVA-3 > TOCNF-CNT@PANI/PVA-1 > TOCNF-CNT/PVA within the entire range of ω. The TOCNF-CNT@PANI/PVA-2 showed the highest *G*\*, further proving that TOCNF-CNT@PANI/PVA-2 was the most quasi-solid hydrogel among these hydrogels. In Figure 4d, a piece of rubbery TOCNF-CNT@PANI/PVA-2 hydrogel could be stretched to 400% strain without damage, exhibiting excellent flexibility, viscoelasticity, and efficient energy dissipation capability. Inside hydrogel, flexible PVA chains and long TOCNF-CNT@PANI composite fiber were physically entangled or hydrogen-bonded to build a 3D network, which could unravel and reconstruct the energy dissipating ability of the hydrogel.


**Table 2.** Rheological characteristics from viscoelasticity curves.

Note: (1) γ<sup>c</sup> means critical strain; (2) G max means the maximum value of storage modulus based on strain; (3) G ∞ means the plateau value of storage modulus based on angular frequency; (4) G"max means the maximum value of loss modulus based on angular frequency.
