Polymer-clay nanocomposite research is largely focused on improving material properties by maximizing polymer-clay interactions and increasing particle surface area through delamination of clay platelets. A novel method utilizing supercritical CO₂ (scCO₂) has recently been shown to be effective in separating clay platelets with or without polymer present, and can be used to disperse them in a polymer matrix.¹ By investigating the rheological changes that occur upon scCO₂ processing, we can gain a better understanding of the physical process of delamination. High molecular weight polystyrene and montmorillonite with varied organic modifiers were processed with a co-solvent under multiple processing conditions. Use of a co-solvent is a convenient way to reduce processing temperatures, allowing us to avoid thermal degradation of clay modifier, and enabling direct comparison of our nanocomposite properties with solution blended analogs.

Results indicate that scCO₂ processing produces a dramatic rheological improvement at low frequencies, over both the neat polymer and solution blended benchmark samples, even at 2 wt% nano-clay. As much as a 3 order of magnitude improvement in storage modulus is observed despite a lack of change in the x-ray diffraction peak of the clays, suggesting that a bi-modal distribution of dispersed platelets and small tactoids exist in the scCO₂ processed materials. The supercritical fluid process appears to produce dispersion that is also dependent on the chemical species on the nano-clay organic modifier. Finally, the rate at which the system is depressurized as well as the number of times the clay is processed in CO₂ have a strong effect on rheological enhancement with better results produced by faster depressurization rates and a larger number of CO₂ depressurizations. These results are analyzed in the scope of interpreting the clay structural changes that occur due to the catastrophic depressurization of CO₂ in our processing method.

[1] Horsch, S.; Serhatkulu, G.; Gulari, E.; Kannan, R. M. Polymer 2006, 47, (21), 7485-7496.