Synthesis, Characterization, and Solid-State NMR Investigation of Organically Modified Bentonites and Their Composites with LDPE

Polymer/clay nanocomposites show remarkably improved properties (mech. properties, as well as decreased gas permeability and flammability, etc.) with respect to their microscale counterparts and pristine polymers.  Due to the substantially apolar character of most of the org. polymers, natural occurring hydrophilic clays are modified into organophilic clays with consequent increase of the polymer/clay compatibility.  Different strategies have been developed for the prepn. of nanocomposites with improved properties, esp. aimed at achieving the best dispersion of clay platelets in the polymer matrix.  In this paper we present the prepn. and characterization of polymer/clay nanocomposites composed of low-d. polyethylene (LDPE) and natural clay, montmorillonite-contg. bentonite.  Two different forms of the clay have been considered: the first, a com. organophilic bentonite (Nanofil 15), obtained by exchanging the natural cations with dimethyldioctadecylammonium (2C18) cations, and the second, obtained by performing a grafting reaction of an alkoxysilane contg. a polymerizable group, 3-(trimethoxysilyl)propyl methacrylate (TSPM), onto Nanofil 15.  Both the clays and LDPE/clay nanocomposites were characterized by thermal, FT-IR, and x-ray diffraction techniques.  The samples were also investigated by means of 29Si, 13C, and 1H solid-state NMR, obtaining information on the structural properties of the modified clays.  Moreover, by exploiting the effect of bentonite paramagnetic (Fe3+) ions on proton spin-lattice relaxation times (T1's), useful information about the extent of the polymer-clay dispersion and their interfacial interactions could be obtained.