Investigation of Clay Modifier Effects on the Structure and Rheology of Supercritical Carbon Dioxide-Processed Polymer Nanocomposites

Superior property enhancements in polymer-clay nanocomposites can be achieved if one can significantly enhance the nanoclay dispersion and polymer-clay interactions. Recent studies have shown that nanoclays can be dispersed in polymers using supercritical carbon dioxide (scCO(2)). However, there is need for a better understanding of how changing the clay modifier affects the clay dispersability by scCO(2) and the resultant nanocomposite rheology. To address this, the polystyrene (PS)/clay nanocomposites with "weak" interaction (Cloisite 93A clay) and "strong" interaction (Cloisite 15A clay) have been prepared using the supercritical CO2 method in the presence of a co-solvent. Transmission electron microscopy images and small-angle X-ray diffraction illustrate that composites using 15A and 93A clays show similar magnitude of reduction in the average tactoid size, and dispersion upon processing with scCO(2). When PS and the clays are co-processed in scCO(2), the "dispersion" of clays appears to be independent of modifier or polymer-clay interaction. However, the low-frequency storage modulus in the scCO(2)-processed 15A nanocomposites is two orders of magnitude higher than that of 93A nanocomposites. It is postulated that below percolation (solution blended composites), the strength of polymer-clay interaction is not a significant contributor to rheological enhancement. In the scCO(2)-processed nanocomposites the enhanced dispersion passes the percolation threshold and the interactions dictate the reinforcement potential of the clay-polymer-clay network. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 823-831, 2010