Mechanically tough and highly stretchable poly(acrylic acid) hydrogel cross-linked by 2D graphene oxide

Abstract

The mechanical performances of hydrogels are greatly influenced by the functionality of cross-linkers and their covalent and non-covalent interactions with the polymer chains. Conventional chemical cross-linkers fail to meet the demand of large toughness and high extensibility for their immediate applications as artificial tissues like ligaments, blood vessels, and cardiac muscles in human or animal bodies. Herein, we synthesized a new graphene oxide-based two-dimensional (2D) cross-linker (GOBC) and exploited the functionality of the cross-linker for the enhancement of toughness and stretchability of a poly(acrylic acid) (PAA) hydrogel. The 2D nanosheets of GO were modified in such a way that they could provide multifunctional sites for both physical and chemical bonding with the polymer chains. Carboxylic acid groups at the surfaces of the GO sheets were coupled with the acrylate functional groups for covalent cross-linking, while the other oxygen-containing functional groups are responsible for physical cross-linking with polymers. The GOBC had been successfully incorporated into the PAA hydrogel and the mechanical properties of the GOBC cross-linked PAA hydrogel (PAA-GOBC) were investigated at various compositions of cross-linker. Seven times enhancement in both toughness and elongation at break has been achieved without compromising on the modulus for the as-synthesized PAA-GOBC compared to the conventional N,N'-methylenebis(acrylamide) (MBA) cross-linked PAA hydrogel. This facile and efficient way of GO modification is expected to lead the development of a high-performance nanocomposite for cutting-edge applications in biomedical engineering.

Fig. 1. Schematic diagram of the preparation of GOBC cross-linked PAA composite hydrogel.

Fig. 2. UV-Vis spectra (a), and FT-IR spectra (b) of GO (red), CGO (blue), and GOBC (green).

Fig. 3. TGA (solid red lines) and DSC (dashed blue lines) curves of GO (a), CGO (b), and GOBC (c).

Fig. 4. FESEM images of GO (a), CGO (b), and GOBC (c).

Fig. 5. FTIR of freeze-dried PAA-GOBC-0.05 hydrogel (a), XRD patterns of neat PAA and PAA-GOBC-0.05 hydrogels (b), FESEM images of freeze-dried PAA-GOBC-0.05 hydrogel after swelling in DI water for 24 hours with 100× (c) and 500× (d) magnifications.

Fig. 6. Photographs of high stretchability PAA-GOBC hydrogels to withstand 19 folds stretching (a and b), and 15 folds stretching from a knotted state (c and d), a schematic illustration of cross-linked polymer chains forming randomly entangled conformation, that dissipates deformation stress by chain disentanglement and desorption (e).

Fig. 7. Stress–strain curves of PAA composite hydrogels prepared with different cross-linkers (a) and PAA-GOBC hydrogel with different compositions of GOBC. (b) Comparison of Young's modulus and tensile strength (c), elongation at break and toughness (d) of the hydrogels.

Fig. 8. Swelling ratio with respect to time of PAA composite hydrogels.