Clay Concentration Dependence of Microstructure in Deformed Poly(N-isopropylacrylamide)−Clay Nanocomposite Gels

The microscopic structure of poly( N -isopropylacrylamide)−clay nanocomposite gels (NC gels) having various clay concentrations ( C clay ) was investigated by mechanical measurement and small-angle neutron scattering (SANS) as a function of stretching ratio λ. The mechanical measurements indicated that the number of poly( N -isopropylacrylamide) (PNIPA) chains bridging clay platelets, N chain, per unit volume increased with C clay 4/3 . Two-dimensional SANS intensity patterns exhibited the so-called “abnormal-butterfly” pattern. However, both sector-averaged scattering intensities parallel and perpendicular to the stretching direction were well represented by Ornstein−Zernike (OZ) scattering functions, suggesting that the NC gels have very little cross-linking inhomogeneities regardless of C clay and of λ (λ ≤ 2.0). The correlation length, ξ, obtained by the OZ functions was a decreasing function of C clay, i.e., ξ ∼ C clay -1/3, and was in proportion to the inter-platelet distance (∼ C clay -1/3 ). This suggests that the concentration fluctuations are effectively screened by clay platelets. The additional C clay dependence in N chain, i.e., N chain /C clay ∼ C clay 1/3, is due to localization of cross-links to the two-dimensional space on clay platelets. This is one of the direct evidence of “plane” cross-linking, which contributes to the advanced mechanical properties of NC gels.