Synthesis, characterization, and reinforcement effect of graphene oxide on rheological behaviour of gum...

Hydrogels derived from natural polysaccharides often lack the mechanical and thermal robustness required for advanced soft-material applications. In this study, we report the synthesis and systematic characterization of gum acacia-grafted poly(N,N-dimethylacrylamide) (GA-g-P(DMA), GADMA) hydrogels and elucidate the influence of graphene oxide (GO) nanosheets on their morphology, thermal behaviour, and multimodal rheological properties. GA-g-P(DMA) was synthesized via free-radical graft polymerization using ammonium persulfate as the initiator and N,N'-methylenebisacrylamide as the crosslinker. GO was incorporated at varying loadings (GO-0 to GO-5) to obtain a series of nanocomposite hydrogels. Scanning electron microscopy revealed a transition from porous gum acacia lamellae to densely crosslinked microstructures upon GO incorporation. Thermogravimetric (TGA/DTG) analysis demonstrated enhanced thermal stability in GO-reinforced hydrogels, evidenced by delayed degradation, increased T₅%, T₅₀%, and T₇₅%, and higher char residues compared with neat gum acacia and the pristine hydrogel. Comprehensive rheological evaluation including steady-shear flow, dynamic frequency sweep, strain sweep, and temperature-cycling tests revealed a non-monotonic, GO loading-dependent mechanical response. Low GO contents disrupted the hydrogel network, resulting in reduced viscosity and viscoelastic strength, whereas optimized or higher loadings, particularly GO-5, produced significant reinforcement characterized by increased storage modulus (G'), complex viscosity (η*), yield behaviour, improved strain tolerance, and superior thermo-mechanical recoverability. Power-Law and Herschel-Bulkley models effectively captured the transition from nanofiller-induced plasticization at suboptimal GO levels to pronounced reinforcement at well-dispersed concentrations. To the best of our knowledge, this work provides the first integrated rheological framework linking GO dispersion, nanosheet-polymer interactions, and macroscopic thermo-mechanical performance in GA-g-P(DMA) hydrogels. The demonstrated dual role of GO as a network disruptor at low contents and a multifunctional reinforcing agent at optimized loadings offers a tunable and scalable strategy for designing thermally stable, mechanically resilient, and injectable hydrogels for biomedical, environmental, and soft-device applications.