Study of the factors influencing the exfoliation of an organically modified montmorillonite in methyl methacrylate/poly(methyl methacrylate) mixtures

The factors that affect the dispersion of exfoliated organically modified montmorillonite in a solution of poly(methyl methacrylate) in methyl methacrylate are explored. Exfoliation of montmorillonite in the solution is achieved with the assistance of ultrasound, and rheological measurements indicate a very significant increase in the viscosity, a dramatic shear thinning behavior, and a finite yield stress, all of which are direct consequences of the exfoliated state of the clay platelets. A number of factors, including the sonication power, clay loading, use of a swelling agent, and moisture content of the modified montmorillonite, are found to influence the exfoliation process. The effect of addition of a range of titanate coupling agents (LICA-01, 12, 38, 44, and 97) on the viscosity of the nanoclay dispersions was investigated. It was found that LICA-44 had the effect of reducing the viscosity of the exfoliated montmorillonite dispersion without apparently influencing the extent of the exfoliation. Molecular modeling, UV-visible and Fourier transform infrared spectrometry were used to investigate the possible reasons for efficacy of this LICA. The LICA appears to act through a combination of steric effects and the presence of certain charges on the organic molecule. The magnitude of the negative charges on elements of the LICA appears to influence its ability to bind to the clay and also its ability to reduce the viscosity of the nanoclay. This article indicates how the apparently conflicting requirements of achieving a highly exfoliated state and also maintaining a viscosity low enough for processing can be effectively addressed. (c) 2005 Wiley Periodicals, Inc.