The mechanical properties of polymer composite hydrogels have been extensively studied owing to their potential applications in various fields. Under optimum conditions, incorporating inorganic nanoparticles into polymeric hydrogels improves the mechanical performance of polymer hydrogels. This has been mainly attributed to dispersing the nanoparticles in hydrogels as well as suitable interactions between the nanoparticles and the polymer. Additionally, the mechanical performance is also influenced by the molecular weight of the polymer.
Among the available materials for reinforcing polymer hydrogels, disk-shaped clay particles have attracted research attention. They have positively charged edges and negatively charged surfaces and serve as multifunctional crosslinkers in polymer hydrogels. Thus, clay particles have been deemed appropriate for fabricating composite hydrogels with remarkable mechanical properties. Unfortunately, incorporating clay particles into polymer gels causes the aggregation of clay nanoparticles, resulting in a serious reduction in mechanical performance.
Two clay/polymer composite hydrogels production methods have been recommended to address this problem: using in-situ free-radical polymer polymerization in the presence of clay particles and using clay nanoparticles dispersing agent to mix the polymer and clay platelets. Although composite hydrogels prepared via these methods have several advantages, their mechanical performance often decreases at higher clay content attributed to structural inhomogeneity in the gel.
Recently, Silica (SiO2) nanoparticles have been identified as a potential solution owing to their excellent characteristics, including high surface-volume ratio, low production cost and high dispersibility in water. However, producing SiO2/polymer hydrogels with remarkable mechanical properties requires surface modifications to overcome the weak interactions between the silica and the polymer. Since SiO2 and clay nanoparticles exhibit different binding and dispersibility behaviors, fabricating composite gels using silica or clay as reinforcing agents might differently affect the mechanical performance of the resulting nanocomposite hydrogels.
Herein, Professor Hiroyuki Takeno, Mr. Y. Aoki and Mr. K. Kimura from Gunma University in Japan investigated the effects of clay and silica nanoparticles on the mechanical performance of polymer hydrogels. In particular, poly (vinyl alcohol) (PVA) nanocomposite hydrogels containing borax were utilized as polymer gels. Additionally, clay (laponite) and SiO2 nanoparticles with smaller sizes were used to improve the mechanical properties of the composite hydrogels. Besides clarifying the underlying mechanism behind the mechanical performance of the hydrogels, the authors also discussed the synergistic effects of adding silica and clay nanoparticles on the mechanical properties of PVA/borax hydrogels. The new research work is currently published in the journal, Colloids and Surfaces A: Physicochemical and Engineering Aspects.
The researchers showed that clay/PVA/borax hydrogels exhibited higher tensile strength at low clay concertation, but their mechanical performance reduced drastically at higher clay concentrations. In contrast, an increase in the concentration of silica enhanced the tensile strength of silica/PVA/borax hydrogels under moderate to high elongation. The differences in the mechanical behaviors of these composite hydrogels were attributed to the differences in the interactions between PVA and the respective fillers. Despite the direct adsorption of PVA chains on clay nanoparticles, they were not adsorbed on silica nanoparticles but instead crosslinked with them through borax. Furthermore, adding clay and silica nanoparticles eliminated the inhomogeneities in the composite gel, allowing the production of highly stretchable composite hydrogels.
In summary, the effects of incorporating two fillers nanoparticles (clay and silica) on the mechanical performance of PAV/borax nanocomposite hydrogels were investigated. The results confirmed that the tough polymer network produced by the multiple crosslinking, like the interactions and complexities between PVA and clay and silica and PVA, provided an effective tool for fabricating composite hydrogels with remarkable mechanical properties. Additionally, the synergistic effects of silica and clay on the mechanical properties of PVA/borax hydrogels were examined, and underlying mechanics clarified. In a statement to Advances in Engineering, Professor Takeno explained the resulting composite hydrogel are promising soft functional materials with potential applications in different fields like tissue engineering.
Takeno, H., Aoki, Y., & Kimura, K. (2021). Effects of silica and clay nanoparticles on the mechanical properties of poly(vinyl alcohol) nanocomposite hydrogels. Colloids And Surfaces A: Physicochemical and Engineering Aspects, 630, 127592.