Bulk magnetization and nuclear magnetic resonance of magnetically purified layered silicates and their polymer-based nanocomposites

Abstract

The bulk magnetization and the H1 and Si29 nuclear-magnetic-resonance (NMR) spectra of two layered silicates, montmorillonite (MMT) and hectorite (HCT), purified by high-gradient magnetic separation, and of HCT-polymer nanocomposites have been measured. At 300K, the magnetization of MMT as received shows a behavior typical of paramagnets and does not change significantly even after ∼100h of magnetic separation. The magnetization of HCT as received is typical of a weak ferromagnet but it changes drastically after magnetic separation. The extracted particles have sizes varying from a few to about 150μm and show ferromagneticlike properties at 300K. While the magnetization/magnetic-field ratio, M∕H, of HCT is reduced 50-fold after ∼15min of magnetic separation and shows predominantly diamagnetic properties at 300K after ∼30h, we have also found that HCT contains ∼0.2mass% of paramagnetic Fe ions, with an effective magnetic moment of ∼5.2μB per Fe ion, as a regular element of its lattice. This is much smaller than the ∼3.8mass% in MMT. Spinning sidebands in H1 NMR spectra of HCT as received are reduced by magnetic separation. This indicates that dipolar interactions between nuclei and paramagnetic ions even in separated phases can affect the NMR spectra. Magnetically purified hectorite was used for preparing polymer-HCT nanocomposites where the NMR spectra show increased signal-to-noise ratios.