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 lambda. 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 lambda (lambda <= 2.0). The correlation length, xi, obtained by the OZ functions was a decreasing function of C-clay, i.e., xi similar to C-clay(-1/3), and was in proportion to the inter-platelet distance (similar to 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 similar to 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.