The Effect of Clay Content on PMMA-Clay Nanocomposite Foams

In this study the CO₂ sorption at 45 °C in PMMA nanocomposite films containing 2 wt.% of nanoclay has been measured using an in-situ gravimetric technique. The films examined were prepared by compression moulding material obtained by dry-blend and solvent co-precipitation techniques. The CO₂ diffusion coefficients were found to be higher for the dry-blended nanocomposite due to the larger agglomerations of the organoclay agglomerations, which prevented the polymer chains from fully wetting and intercalating the clay particles. The T_g-p profile for PMMA nanocomposite containing 2 wt.% nanoclay in the presence of CO₂ was also measured using high-pressure DSC. The glass transition phase envelope was shifted vertically by approximately 10 °C when compared to the value reported in the literature for neat PMMA. This result suggests that the nanoclay affects the plasticization behaviour of PMMA under high-pressure CO₂ conditions. The cellular morphologies obtained for these PMMA nanocomposite foams produced by batch processing with subcritical CO₂ are strongly dependent upon the clay content and the dispersion of the nanoclay in the material. In the case of intercalated nanocomposites, most clay particles exist as agglomerated stacks of silicate sheets. On foaming the cells tend to form around the clay particles causing either irregular-shaped cells or layers to be produced. As a result, the cell density increases and the mean cell size decreases in the foamed nanocomposite on increasing the nanoclay content. Accordingly, the resulting cell structures are highly non-uniform and show large variations in cellular morphologies throughout the foam.