NC Gels Research Articles

Mechanism of Forming Organic/Inorganic Network Structures during In-situ Free-Radical Polymerization in PNIPA−Clay Nanocomposite Hydrogels

The process of forming the unique organic/inorganic network structure of nanocomposite hydrogels (NC gels) was studied through changes in viscosity, optical transparency, X-ray diffraction, and mechanical properties. It was concluded that, during the preparation of the initial reaction solutions, a specific solution structure was formed from monomer (NIPA) and clay, where NIPA prevents gel formation of clay itself, and initiator (KPS) is located near the clay surface through ionic interactions. In subsequent in-situ free-radical polymerization, it was observed that the viscosity increased markedly during NC gel syntheses and in a manner similar to that in OR gel syntheses. Also, NC gels with different polymer contents exhibit characteristic two-step changes in the stress−strain curves, which correspond to the primary network formation and subsequent increase of cross-link density. These are because the polymerization proceeds on the clay particles which are relatively immobile, and clay platelets act as e...

Compositional Effects on Mechanical Properties of Nanocomposite Hydrogels Composed of Poly(N,N-dimethylacrylamide) and Clay

Nanocomposite type hydrogels (DMAA-NC gels) consisting of organic (polymer)/inorganic (clay) networks were prepared by in-situ free-radical polymerization of N,N-dimethylacrylamide (DMAA) in the presence of inorganic clay in aqueous solution. The composition of the NC gels could be controlled directly by altering the composition of the initial reaction mixture. The resulting DMAA-NC gels were mostly uniform and transparent, irrespective of their clay and polymer contents. From DSC, X-ray, TEM, and tensile mechanical measurements, the network structure was established. Contrary to conventional chemically cross-linked hydrogels (DMAA-OR gels) prepared by chemical cross-linking with a difunctional monomer, DMAA-NC gels exhibit superb mechanical properties with astonishingly large elongations at break, near to or greater than 1500%. The effects of the composition, such as the amounts of clay, polymer, and water content in DMAA-NC gels, on the tensile mechanical properties were investigated in detail. With inc...

Effects of Clay Content on the Properties of Nanocomposite Hydrogels Composed of Poly(N-isopropylacrylamide) and Clay

For two different types of poly(N-isopropylacrylamide) (PNIPA) hydrogels, i.e., nanocomposite type PNIPA hydrogels (NC gel) and conventional chemically cross-linked PNIPA hydrogels (OR gel), the effects of cross-linker contents on various physical properties were investigated. In NC gels composed of a unique organic (PNIPA)/inorganic (clay) network, the inorganic clay acts as a multifunctional cross-linker in place of an organic cross-linker (BIS) as used in OR gels. In NC gels, which generally exhibit extraordinary mechanical toughness, the tensile moduli and tensile strengths are almost proportional to the clay content (Cclay), while the elongation at break tends to decrease slightly with increasing Cclay. On the other hand, in OR gels, which always exhibit weak and brittle natures, there was no detectable change in properties on altering the concentration of BIS (CBIS). The deswelling rate was affected markedly by the cross-linker content in both gels though in opposite directions. On increasing cross-linker contents NC gels exhibit decreases and OR gels exhibit increases in rates of deswelling. In NC gels, high deswelling rates and high structural homogeneities (transparencies) were achieved simultaneously. Also, volume changes related to the phase transition of PNIPA at LCST were also inclined to decrease on increasing cross-linker contents in both gels, although swelling ratios at temperatures below LCST were generally larger in NC gels than those in OR gels. As for transparency changes at LCST, in OR gels changes in transparency decrease on increasing CBIS, because below the LCST the transmittances themselves decrease steeply with increasing CBIS. On the contrary, NC gels exhibit large transparency changes regardless of Cclay and show a tendency to increase their transmittances above LCST in the high Cclay region. All results obtained were consistent with the proposed model structure for NC gels. On the basis of the theory of rubber elasticity and using tensile mechanical data, the number of effective cross-links and the molecular weight between cross-linking points were evaluated for all NC gels.

Nanocomposite Hydrogels: A Unique Organic–Inorganic Network Structure with Extraordinary Mechanical, Optical, and Swelling/De-swelling Properties

Novel nanocomposite hydrogels (NC gels) with a unique organic-inorganic (clay) network structure (see Figure) have been synthesized by in-situ free radical polymerization. The resulting NC gels exhibit high structural homogeneity, superior elongation with near-complete recovery, good swellability, and rapid deswelling in response to temperature changes.