Exfoliation of Kaolinite Nanolayers in Poly(methylmethacrylate) Using Redox Initiator System Involving Intercalating Component

Abstract

Urea was intercalated into kaolinite (K) and the resulting modified clay (K-U) was characterized by X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and fourier transform infrared (FTIR). The K-U was added to promote the formation of Poly(methylmethacrylate)/kaolinite nanocomposites via in-situ intercalative emulsion polymerization of methylmethacrylate (MMA) in the temperature range 50–70°C using a redox initiation system based on urea as intercalatable component while potassium persulphate (PPS) was dissolved in the aqueous medium. Additionally, potassium persulphate/sodium bisulfite (PPS/SBS) as a powerful redox initiation system was employed in absence of any kaolinite form for comparison. The polymerization rate was found to increases slowly with the temperature but still lower if compared to the polymerization in absence of kaolinite, whereas the activation energy (Ea) decreased from 15.6 to 9.55 × 104 J/mol. On replacing the K with K-U, the rate of polymerization was faintly increased with the reaction temperature, also Ea was markedly influenced and reduced by factor of approximately 5 if a comparison was set relative to the polymerization in absence of kaolinite which highlights the role of urea as an effective intercalant that can constitute a redox initiation system in combination with PPS. The produced nanocomposites were of the exfoliated type as confirmed by XDR and Transmission electron microscope (TEM). In most cases, the nanocomposites exhibited improved thermal stability as compared with the pure PMMA at the temperature range 250–550°C, while the onset of degradation was not influenced as revealed by TGA. The DSC thermograms of PMMA/K-U nanocomposites did not show any transition and no clear T g was observed which contradicts with the pure PMMA which showed a small endothermic peak at 100°C related to the glass transition temperature, which implies the restricted molecular motion of PMMA chains in case of strong interaction forces between the dispersed silicate layers and the surrounding polymer phase where exfoliation exists.