Surface Modification of Montmorillonite and Application to the Preparation of Pmma/montmorillonite Nanocomposites in Supercritical Carbon Dioxide

Partially exfoliated poly (methyl methacrylate) (PMMA) / montmorillonite clay nanocomposite was synthesized via the free radical polymerization of MMA in the presence of alkyl ammonium substituted polysilsesquioxane surfactant-modified clay and AIBN initiator in supercritical CO2. MMT clay was initially cation exchanged with the surfactant to enhance its hydrophobicity and to expand the interlamellar spaces of silicate platelets. The 3-D structured surfactant reduces surface energy of the clay dramatically which promotes miscibility of polymer/clay nanocomposite. The intercalation and dispersion of clay were quantified by both X-ray diffraction and transmission electron microscopy. The reaction was also conducted in aqueous ethanol solvents in order to compare the morphologies of nanocomposites obtained from different solvents. INTRODUCTION Layered silicate based nanocomposites have received considerable attention in recent years as an effective method of enhancing polymer properties, e.g. increased stiffness and strength, improved gas barrier resistance, superior thermal and fire stabilities. Montmorillonite, the most preferred and widely used smectic clay is of particular interest because it offers a high aspect ratio (10-1000) and a high surface area. Due to an isomorphic substitution within the layers, the clay layer is negatively charged, which is counterbalanced by cations in the interlayer space. The natural montmorillonite is hydrophilic and is generally considered unsuitable for hosting nonpolar organic molecules without prior treatment. Interlayer cations can be exchanged with organic cations to obtain organophilic montmorillonite producing an ‘organoclay’, which has an expanded interlayer spacing and more readily produces a polymer–clay nanocomposite. Employing suitable surfactants with simple ion-exchange reactions could achieve different natures of morphologies: intercalated, intercalated and flocculated and exfoliated. Although the exfoliated structures are usually claimed to have the most significant property improvements, there are in reality very few unequivocal