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Supercritical CO2 Exfoliated Polymer Nanocomposites

S.E. Horsch, R.M. Kannan and E. Gulari
Wayne State University, US

Keywords:
nanocomposites, nano-scale fillers, supercritical CO2 processing

Abstract:
Achieving a high degree of dispersion of nano-scale fillers in a polymer matrix produces significantly improved mechanical, barrier, thermal, and flame retarding properties. Most commonly used nano-scale fillers, such as clays, graphite, carbon nanotubes, and other platy minerals, aggregate due to a variety of particle-particle interactions. Therefore, a major technical challenge is to significantly exfoliate and disperse these naturally layered fillers in a host polymer matrix. To address this challenge, we employ several strategies based on supercritical carbon dioxide (scCO2) processing.
 
The first strategy involves subjecting the nano-scale filler, clay or graphite, to scCO2 for a period of time and then rapidly depressurizing. High diffusivity and low viscosity of scCO2 enables its diffusion between the layers. During depressurization, it expands and pushes the layers apart resulting in significant delaminated structures. A second strategy involves introducing a low molecular weight organic compound soluble in CO2, a coating agent, in the scCO2 processing step. Again, favorable transport properties of the solution facilitate diffusion of the organic compound along with CO2 between the layers. During depressurization, the coating agent precipitates and coats layers while CO2 expands and pushes the layers apart. A third strategy exploits scCO2’s ability to swell and plasticize a polymer and facilitate its mixing with the fillers. The polymer and the processed filler are mixed thoroughly and processed in scCO2 at a temperature and pressure suitable for the given polymer/filler system followed by depressurization to remove CO2. The degree of filler dispersion is a function of the processing temperature, pressure, and depressurization rate.
 
We have investigated polydimethylsiloxane(PDMS)/clay, polystyrene(PS)/clay, and polyvinylmethylether(PVME)/clay nanocomposites. The microstructure of the processed clays and nanocomposites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and rheology. Some benefits of scCO2 processing include 1) use of natural clay instead of chemically modified organoclays, 2) lower processing temperatures avoiding thermal degradation, 3) elimination of organic solvents in processing. Our studies indicate that scCO2 processing produces significant dispersion of nano-scale fillers resulting in improved properties of the polymer nanocomposites.

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