No abstract

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 crosslinker 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 (C clay), 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 (C BIS). 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 C BIS, 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 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 increasing clay content from C clay = 1 to 7 (C clay is proportional to the weight of clay per unit volume of water, and a value of 1 corresponds to 0.762 g clay per 100 mL of water), the modulus and the ultimate tensile strength increased almost proportionally to the clay content, but the elongation at break decreased slightly. By altering the polymer content over 2 orders of magnitude, it was observed that the nature of the mechanical properties of DMAA-NC gels changed markedly. Above a lower critical polymer content (C p ≈ 0.13), the elongation at break increased rapidly from near zero to more than 1000%. Thereafter, the hydrogels became very tough NC gels. With further increases in polymer content, the strength and the elongation at break showed maxima at certain polymer contents, whereas the modulus increased monotonically. The mechanical properties were also changed by altering the water content. The cross-link density was estimated for DMAA-NC gels with different clay contents. The effects of clay and polymer contents on the mechanical properties, as described above, were discussed on the basis of the network structure model for NC gel.

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 effective multifunctional cross-linking agents (plane cross-link). Then, it was proposed that clay−brush particles, consisting of exfoliated clay platelets with numbers of polymer chains grafted to their surfaces, were formed in the very early stage of polymerization, at around 7% of monomer conversion. Novel decreases in transparency were observed corresponding to the formation of clay−brush particles, but transparency recovered on further polymerization. Clay−brush particle formation was confirmed by XRD measurements on dried NC gels prepared using small amounts of monomer. Thus, a mechanism for forming the unique organic/inorganic network structure, including the formation of clay−brush particles in the synthetic pathway, is proposed. Furthermore, it was found that NC gels with excellent mechanical properties and structural homogeneity could not be prepared using other methods such as mixing clay and polymer solutions or by in-situ polymerization in the presence of the other inorganic nanoparticles instead of clay. These results indicate that the formation of organic/inorganic network structures in NC gels is highly specific and only realized by in-situ free-radical polymerization in the presence of clay.

The mechanical properties and structures of nanocomposite gels (NC gels), consisting of poly-(N-isopropylacrylamide) (PNIPA) and inorganic clay (hectorite), prepared using a wide range of clay concentration (∼25 mol % against water) were investigated. All NC gels were uniform and transparent, almost independent of the clay content, C clay . The tensile modulus (E) and the strength (σ) were controlled without sacrificing extensibility by changing C clay . The E, σ, and fracture energy observed for as-prepared NC gels attained 1.1 MPa, 453 kPa, and 3300 times that of a conventional chemically cross-linked gel, respectively, and σ increased to 3.0 MPa for a once-elongated NC25 gel. From the tensile and compression properties, in addition to optical transparency, it was concluded that a unique organic/inorganic network structure was retained regardless of C clay . The effects of C clay on the tensile mechanical properties on the first and second cycles, the time-dependent recovery from the first large elongation and the optical anisotropy of NC gels, and also the disappearance of the glass transition and the formation of clay-polymer intercalation in the dried NC gel were revealed. Thus, it became clear that the properties and the structure changed dramatically for an NC gel with a critical clay content (C clay c ≈ NC10) or above. The structural models for NC gels with low and high C clay , exhibiting different clay orientation and residual strain, were depicted.