*3.5. Self-Healing Performance of the Hydrogels*

The composite hydrogel possessed a dynamic self-healing PVA-borate network, and the TOCNF-CNT@PANI composite fibers network provided an additional platform to strengthen the structure [33]. In Figure 5a, the *G* and *G*" of TOCNF-CNT@PANI/PVA-2 were 8.1 and 4.2 kPa at γ = 1%, respectively. The value of *G* was larger than that of *G*", indicating that the elastic character of hydrogel became the dominant factor. When the strain increased to γ = 80%, the corresponding *G* and *G*" of TOCNF-CNT@PANI/PVA-2 were 0.2 and 0.6 kPa, respectively. The value of *G* was less than that of *G*", indicating hydrogel turned to the quasi-liquid state. Interestingly, when the strain dropped to 1% again, the corresponding *G* and *G*" immediately restored the original values, indicating that the hydrogel recovered the quasi-solid state. The rapid and repeatable phase transition between the quasi-solid state and quasi-liquid state demonstrated the intrinsic preeminent self-healing capability of the hydrogel. To visualize the self-healing property of the hydrogel, two blocks of TOCNF-CNT@PANI/PVA-2 were pushed together for 20 s, and their contact surfaces would fuse. In addition, after the self-healed hydrogel was stretched to 300% strain, there was no crack at the healing interface (Figure 5b).

**Figure 5.** (**a**) The storage modulus (*G* ) and loss modulus (*G"*) dependence of time in continuous step strain measurements. (**b**) Illustration of self-healing property for TOCNF-CNT@PANI/PVA-2 hydrogel. (**c**) Hydrogel stretching curve before and after 20 s self-healing. The solid lines represent the original hydrogels, and the dashed lines represent the self-healed hydrogels. (**d**) Histogram of tensile stress of the original hydrogels and the self-healed hydrogels after healing in the air for 20 s.

The stress-strain curves of hydrogels are shown in Figure 5c. The maximum break strain values of original TOCNF-CNT/PVA, TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-2, and TOCNF-CNT@PANI/PVA-3 were 450.5 ± 23.2%, 387.8 ± 19.0%, 345.1 ± 15.1%, and 326.7 ± 12.5%, respectively. The highest tensile stress values of original TOCNF-CNT/PVA, TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-2, and TOCNF-CNT@PANI/PVA-3 were 57.9 ± 2.1, 63.5 ± 2.5, 95.3 ± 3.2, and 72.9 ± 2.9 kPa, respectively. The TOCNF-CNT@PANI/PVA-2 composite hydrogel possessed the highest tensile stress, which was 1.6 times that of TOCNF-CNT/PVA hydrogel without PANI, indicating that the proper incorporation of PANI could effectively improve the tensile stress of composite hydrogels. This was consistent with previous measurements of mechanical strength and dynamic viscoelasticity.

To calculate the self-healing efficiency of hydrogels, these self-healed hydrogels after 20 s healing in the air were measured by a tensile test. The tensile curves of the self-healed hydrogels were basically the same as that of the original hydrogels. The maximum break strain values of healed TOCNF-CNT/PVA, TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-2, and TOCNF-CNT@PANI/PVA-3 were 443.8 ± 22.1%, 381.1 ± 18.0%, 338.5 ± 13.3%, and 321.0 ± 10.2%, respectively. The corresponding η<sup>k</sup> values were 98.5%, 98.3%, 98.1%, and 98.2%, respectively. The maximum tensile stress of healed TOCNF-CNT/PVA, TOCNF-CNT@PANI/PVA-1, TOCNF-CNT@PANI/PVA-2, and TOCNF-CNT@PANI/PVA-3 were 56.3 ± 2.0, 61.9 ± 2.4, 92.3 ± 3.7, and 69.7 ± 2.8 kPa, respectively. The corresponding η<sup>F</sup> values were 97.2%, 97.5%, 96.8%, and 96.7%, respectively. These data of

dynamic viscoelasticity properties are summarized in Table 3, which demonstrated the outstanding self-healing property of as-prepared hydrogels with the dynamic borate-assisted cross-linking network. The complexation between borate and hydroxyl was extremely fast (0.33 s), and TOCNFs and PVA chains contained large number of hydroxyl groups so that the hydrogels could recover quickly [43].


**Table 3.** Tensile strength and tensile rate of hydrogels before and after a 20 s self-healing.

Note: (1) *F*original means the break stress of initial hydrogel; (2) *K*original means the break strain of initial hydrogel; (3) *F*healed means the break stress of healed hydrogel; (4) *K*healed means the break strain of healed hydrogel; (5) η<sup>F</sup> means the healing efficiencies of break stress; (6) η<sup>K</sup> means the healing efficiencies of break strain.
