Preparation and properties of ionic crosslinked nanocomposite high strength hydrogel

Traditional chemical crosslinked hydrogels have poor mechanical properties due to their uneven network structure. The low elongation at break and poor tensile strength limit the application of hydrogels in various stress scenarios. In this paper, a kind of ion crosslinked nanocomposite hydrogel with good comprehensive properties was prepared by combining free radical polymerization with salt solution immersion.

Short-chain chitosan (CS) with low molecular weight could be directly dissolved in water without the need for additional acid solutions, significantly simplifying subsequent experimental procedures. Initially, water-soluble short-chain CS was utilized to modify halloysite nanotubes (HNTs). A specified amount of acrylamide (AM), acrylic acid (AA), N,N-methylenebisacrylamide, and ammonium persulfate were then added to the HNTs @CS dispersion. Following ultrasonic defoaming, the reaction was conducted at 60℃ for 18 hours to obtain the initial nanocomposite hydrogel matrix (original-gel). The resulting gel was immersed in 0.2 mol/L Fe(NO) solution for 3 hours, followed by immersion in 1 mol/L NaSO solution for 8 hours to yield the HNTs @CS/Poly(AM-co-AA) ionically crosslinked nanocomposite double network hydrogel (DN-gel). The initially entering Fe ions can bind to the —COO groups of the polymer chains, forming strong tridentate coordination. Meanwhile, SO ions interact with the —NH groups on the CS, thereby establishing ion crosslinking points with HNTs @CS as the crosslinking centers by binding to the entangled CS on HNTs or directly connecting adjacent free CS chains as crosslinking points. This ionic crosslinked double network effectively utilizes the active groups on the macromolecular chain, optimizing the three-dimensional network structure of the hydrogel and resulting in a dense crosslinked network.

After standing, it is observed that the HNTs @CS exhibit superior dispersion stability in water compared to unmodified HNTs. FTIR and TEM results confirm the successful modification of HNTs by CS. SEM images of the composite hydrogel reveal a more compact structure and a significant reduction in pore size following ion immersion. The elongated cylindrical DN-gel demonstrates the ability to be knotted and stretched like a rope, displaying no wrinkles, cracks, or other surface imperfections, indicating excellent flexibility. The spherical DN-gel, when dropped from a height of 20 cm, contacts the bottom plate and rebounds to a height of 13 cm, showcasing good elasticity. The sheet-like DN-gel is capable of lifting a reaction kettle weighing up to 3 kg without rupture, demonstrating high strength. As the AA content increases from 6 mol% to 15 mol%, the modulus exhibits a significant increase from 0.235 MPa to 5.765 MPa, nearly a 24-fold rise. The breaking strength of the hydrogel also increases significantly with higher AA content, although the rate of increase gradually diminishes. The breaking elongation initially increases and then decreases with increasing AA content. However, both tensile strength and elongation at break exhibit a trend of first increasing and then decreasing with increased HNTs content. The optimal mechanical properties are observed at an AA content of 12 mol% and HNTs content of 3.5 wt%, where the fracture strength reaches a maximum of 3.81 MPa, and the breaking elongation is 494%. After immersion in Fe and SO ion solutions, the tensile strength and elongation at break of the original gel improve to 3.96 MPa and 553%, respectively. The compressive strength at 85% strain measures 13.4 MPa, indicating good stability and self-healing properties. Following a 48-hour immersion in deionized water, the mass of the DN gel remains nearly unchanged, demonstrating anti-swelling properties, and the tensile strength increases to 5.64 MPa, with a modulus of up to 15 MPa. Additionally, the DN gel, when frozen at -20℃ and bent with tweezers, quickly rebounds, maintaining its excellent flexibility.Conclusions: By combining free radical polymerization with salt solution immersion, a type of ionically crosslinked nanocomposite hydrogel (DN-gel) with excellent overall performance is obtained when the CS content is 2 wt%, AM is 88 mol%, AA is 12 mol%, and HNTs is 3.5 wt%, followed by immersion in Fe and SO ion solutions. The DN gel exhibits high tensile strength (3.96 MPa), significant elongation at break (553%), high compressive strength (13.4 MPa at 85% strain), and a modulus of 4 MPa, along with good self-recovery capabilities, while maintaining a flexible shape. After soaking in deionized water for 48 hours, the rigidity of the DN gel further increases, achieving a tensile strength of 5.64 MPa and a modulus of up to 15 MPa, while the mass remains almost unchanged, demonstrating unique anti-swelling properties. The low moisture content and high ion concentration endow the DN gel with frost resistance, allowing it to maintain good flexibility at -20℃ and supporting normal use under lower temperature conditions.