The gelation mechanism of poly(N-isopropylacrylamide)-clay nanocomposite gels (NC gel) was investigated by dynamic light scattering (DLS) and contrast variation small-angle neutron scattering (SANS). It was found that the gelation mechanism of NC gels is similar to that of conventional gels made with organic cross-linker (OR gels). Namely, time-resolved DLS measurements captured all of the characteristic features of gelation at the threshold. This indicates that the gelation of NC gels is also classified to an ergode-nonergode transition. However, the size of the clusters at the gelation threshold is much larger than that of OR gels. This results in a significant depression of optical transmittance exclusively at the gelation threshold for NC gels. Partial scattering functions, i.e., two self-terms S PP (q) and S CC (q) and the corresponding cross-term S CP (q), were obtained by contrast-variation SANS, where P and C denote polymer and clay, respectively, and q is the magnitude of the scattering vector. The detailed analysis of S PP (q), S CC (q), and S CP (q) indicates that (i) each clay platelet is surrounded by polymer layers, (ii) the volume fraction of the polymer layer per clay platelet is independent of the concentrations, and (iii) the correlation length of the network polymer decreases with increasing clay concentration. These results confirm that the screening length of the system is influenced by the concentrations of clay platelets as well as of polymer chains, and the local structures of polymers near clay platelets are similar between in a sol state near the gelation threshold and in bulk NC gels.

The nanocomposite (NC) gels made of poly(N-isopropylacrylamide) (PNIPA) and synthetic clay show extraordinarily high mechanical properties, e.g., high extensibility and high strength at break. The structure and deformation mechanism of the NC gels have been investigated by small-angle neutron scattering (SANS). Two-dimensional SANS intensity patterns were obtained for deformed NC4 gels with the stretching ratio up to 6 times, where 4 denotes the clay concentration being 0.04 mol/L (≈3.05 wt %). A so-called abnormal-butterfly pattern was observed in the low q region, i.e., q < 0.02 Å -1 , while a normalbutterfly pattern was obtained in the larger q (g0.02 Å -1 ). Here q is the magnitude of the scattering vector. However, contrast-matched SANS experiments disclosed that the abnormal-butterfly pattern is not originated from cross-link inhomogeneities observed in conventional gels but from oriented-clay platelets by deformation. These results indicate that clay platelets, 300 Å in diameter and 10 Å thick embedded in the PNIPA network, are highly aligned with their surface normal parallel to the stretching direction, and the PNIPA chains are elongated parallel to the stretching direction. It is concluded that the high mechanical properties of NC gels are ascribed to "plane cross-linking" with long PNIPA chains between platelets compared with those of conventional chemical gels having "point cross-linking". Experimental SectionSamples. N-Isopropylacrylamide (NIPA) monomer (Kohjin Co., Ltd., Japan

The structure of poly(N-isopropylacrylamide)−clay nanocomposite gels (NC gel) is investigated. The sophisticated contrast variation small-angle neutron scattering technique leads to obtain the partial scattering functions of each component quantitatively. The extracted partial scattering functions of clay, polymer, and the polymer−clay cross-term are analyzed in detail, and the selective interaction between clay and polymer is revealed. On the basis of these structural observations, it is confirmed that the clay nanoparticles serve as 2-dimensional cross-linkers.