Access Type

Open Access Dissertation

Date of Award

1-1-2010

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemical Engineering and Materials Science

First Advisor

Rangaramanujam M. Kannan

Second Advisor

Esin Gulari

Abstract

Effective dispersion of the fillers in a polymer matrix and improvement of polymer-clay interactions are two key challenges in the field of nanocomposites. 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 was explored.

Significant dispersion was achieved using the scCO2 process with Cloisite 10A without the presence of an organic phase as evident by the absence of the diffraction peak in WAXD and the presence of individual tactoids that lost their parallel registry. The expanded flexible structure of the scCO2 processed clay 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.

Enhanced nano-clay dispersion and improved properties were achieved in the scCO2 processed polystyrene/clay nanocomposites compared to melt compounding and solution blended benchmarks. There was a three-fold reduction in the number of tactoids in the scCO2 samples compared to solution blended benchmark that was independent of the clay modifier used leading to an increase in the surface area available for polymer-clay interactions. Significant dispersion, without strong polymer-clay interactions, was not sufficient for significant nanocomposites property improvement. Also, favorable polymer-clay interaction without clay dispersion was also not enough to obtain the maximum property enhancement. Improvements as high as 3 orders of magnitude in the low frequency storage modulus were observed in nanocomposites processed with 5wt% 10A and 15A scCO2 that had favorable polymer-clay interactions. Supercritical carbon dioxide processed nanocomposite showed a reduction of 45% (5wt% 10A) and 60% (5wt% 15A) in oxygen permeability compared to PS. Moreover, the scCO2 processed nanocomposites reduced the oxygen permeability by 20 to 50% compared to solution blended benchmark samples depending on the clay used. Replacing "as received" clay with pre-dispersed clay increased 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 showed solid-like behavior at loadings as low at 2 wt%, and elastic modulus improvements as high as 3.5 orders of magnitude in the 10 wt% nanocomposites over the pure polymer.

Comparison of scCO2-processed and water-processed Cloisite Na+ /PVME nanocomposites with "weak polymer-filler interactions" suggested that high level clay dispersion resulted in non-terminal while intercalation resulted in a filler effect with the relaxation behavior of the bulk polymer virtually unaltered by the presence of the nano-clay. In contrast, for intercalated systems with "stronger" polymer-clay interactions (PVME/I.30P), there was more than two orders of magnitude increase in low-frequency storage moduli. Similarly to what was observed in the PS, in PVME systems, when `strong' polymer-clay interactions are present coupled with significant nano-clay dispersion produced significant property improvements.

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