Effective dispersion of the fillers in a polymer matrix and improvement of polymer-clay interactions are two key challenges in the field of nanocomposites. We explore a novel processing method that utilizes the unique properties of supercritical carbon dioxide (scCO₂) to disperse nano-clay and prepare a series of polymer/clay nanocomposites with enhanced properties. Significant dispersion was achieved using the scCO₂ process with both Cloisite 93A and Cloisite 10A without the presence of an organic phase as evident by the absence of d₀₀₁ diffraction peak in WAXD. SEM revels that the individual tactoids in the scCO₂ samples have delaminated from each other and lost their parallel registry. The expanded flexible structure of the scCO₂ processed particles/tactoids exposes more of the available surface area allowing for more contact between the polymer and clay surface, and the platelets should be easier to disperse into a polymer matrix than the “as received” clay. Furthermore, multiple scCO₂ processing of the nano-clays leads to an increase level of dispersion which opens up a larger surface area.

We have also prepared a series of polystyrene/clay nanocomposites using the scCO₂ processing method to investigate the effects of scCO₂ depressurization rate, solution blending, clay weight fraction, and pre-dispersion on clay dispersion, nanocomposites morphology and polymer-clay interactions. High molecular weight polystyrene is not processable at low enough temperatures that would prevent clay modifier degradation and a co-solvent is used to improve polymer mobility during processing. Although significant dispersion of the nano-clay is achieved when processes without polymer present, the WAXD of the high molecular weight polystyrene/clay nanocomposites processed in the presence of a co-solvent reveals a strong intercalation peak that is similar in all the nanocomposites regardless of the processing conditions. However, despite the lack of differentiation in WAXD between all the nanocomposites studied, TEM images reveal that there is a larger fraction of dispersed platelets present in the scCO₂ sample compared to solution blended control sample which displays much larger tactoids and a lack of individual clay sheets. There is a three fold reduction in the number of tactoids in the scCO₂ samples leading to an increase in the surface area available for polymer-clay interactions. Processing polymer and clay in scCO₂ followed by quick depressurization leads to significant reinforcement of the polymer matrix compared to solution blending analog alone, implying that clay platelets are effectively being dispersed by the dramatic CO₂ expansion. Replacing “as received” clay with pre-dispersed clay increases the surface area available for polymer-clay interactions, resulting in a significant doubling of G' at low frequencies over the standard scCO₂ processed sample. The scCO₂ samples show solid-like behavior at loadings as low at 2 wt%, and elastic modulus improvements as high as 3 orders of magnitude in the 10 wt% nanocomposites over the pure polymer.

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