(161d) Supercritical CO2-Processing of Polymer-Clay Nanocomposites with Improved Mechanical and Barrier Properties
Improvements in clay dispersion and polymer-cloay interactions are keys to producing superior polymer-clay nanocomposites. Supercritical CO2 has been shown to be effective at addressing both these needs.1,2 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 . 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 . 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. Recent studies have focused on the improvement in gas barrier properties of PS-clay nanocomposite films. More than a 50% reduction in oxygen permeability could be achieved in scCO2-processed PS-clay films.
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