(50f) Morphological and Rheological Effects of Supercritical Carbon Dioxide Processing of Polystyrene-Clay Nanocomposites

Recent research shows that polymer-clay nanocomposites have significant potential to improve a variety of mechanical, thermal and mass transfer properties and therefore there is much interest in studying methods for producing the best possible enhancement.   The state of clay platelet dispersion and extent of polymer-clay interactions are 2 of the most important variables dictating how much of the “potential” property enhancement is actually converted to true measured property enhancement.  In this study we investigate the morphological and rheological differences in polystyrene-clay nanocomposites created with a wide range of commercial organically modified clays and two different processing methods: solution blending and a novel, solvent-assisted, supercritical carbon dioxide (scCO₂) processing method ^[1]. 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 ^[2].

Nanocomposites with 4 different organically modified clays at 5wt% in high molecular weight polystyrene were solution blended in toluene and split into two samples: one that received only the solution blending protocol and another that received additional soaking time with scCO₂ followed by rapid depressurization.  After driving off all of the solvent, SAXD, TEM, and rheological characterization was preformed on each sample.  It was found that scCO₂ produces similar reductions in average tactoid size in all of the clay’s investigated despite them displaying significantly different interaction strength with the polymer as evidenced by rheology.  It has been anticipated and subsequently shown in literature that clays exhibiting strong interaction with the polymer matrix should be the most difficult to disperse, however it is these clays that researchers are generally most interested in dispersing.  Our results show that using scCO₂ processing allows us to disperse nano-clays similarly regardless of interaction. 

As much as a 3 order of magnitude improvement in storage modulus was observed despite a lack of change in the x-ray diffraction peak of any of the clays, suggesting that a bi-modal distribution of dispersed platelets and small tactoids exist in the scCO₂ processed materials. 

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

2. Manitiu, M.; Bellair, R.; Horsch, S; Gulari, E.; Kannan, R. M. Macromolecules 2008, 41, (21), 8038-8046