SUPERCRITICAL CARBON DIOXIDE (scCO2) PROCESSING OF DISPERSED POLYMER/CLAY NANOCOMPOSITES: STRUCTURAL AND BARRIER PROPERTIES
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Improved polymeric packaging materials have significantly economic and environmental impact on food, beverage and other industries. In this dissertation, we developed polymer/clay nanocomposites, by incorporating impermeable layered silicates into polymeric matrix to enhance barrier properties. We utilized the unique properties of super critical carbon dioxide (scCO2) to exfoliate clay layers and improve polymer-clay interactions, which are two key challenges in the field of nanocomposites. Firstly, the scCO2 processing was applied to pre-disperse various commercial Cloisite® clays, the effects of scCO2 processing and chemical and physical properties of clay particles on clay pre-dispersion were investigated and the extent of clay pre-dispersion was assessed by SEM, WAXD and TGA. We found that the scCO2 processing results in pre-dispersion of organic modified clays regardless what kind of modifier on them. The degree of dispersion of different kind of clays actually is a competitive result between carbon dioxide philicity and modifiers interaction. In addition, TGA data confirm that the scCO2 processing does not remove surface modifiers from clays, which maintain good solubility of clay in organic solvent and compatibility with organic phase. Secondly, the pre-dispersed clay particles were solution blending with polystyrene (PS) to form nanocomposites, whose structural physical properties were characterized and evaluated by WAXD and TEM. The effects of scCO2 processing, clay dispersion, modification and fraction on improvements of physical properties such as rheology and barrier of nanocomposites were investigated. In addition, structure-barrier properties relationship was assessed based on several phenomenological models. In particular, we identified that 10A is the best reinforcement for improving PS barrier properties. Microstructural and barrier properties characterizations revealed that scCO2 processed clays prevent reorganization of platelets and lead to more homogenously clay dispersion with improved interfaces. As the increase of clay fraction and dispersion, gas permeation decreased continuously, in this study ~83% reduction of permeation had been achieved with 3.1 vol% of scCO2 processed clays with a calculated effective aspect ratio of 109. Finally, the concept of scCO2 processing reinforced polymer/clay nanocomposites based on PS matrix was translated to more complex melting extrusion of engineering polymers, such as polyethylene terephthalate (PET) and high density polyethylene (HDPE). The effects of scCO2 processing, clay dispersion, modification and fraction and temperature on improvements of structural and physical properties of matrix were investigated and discussed. 30B and 20A were identified as the best candidate for improving barrier properties of PET and HDPE respectively. Similarly, we found that scCO2 processed clays lead to more homogenously clay dispersion with improved interfaces. Nevertheless, unlike previous research, permeation of PET did not decrease continuously with increase of clay fraction or dispersion. In this study, with only 1wt% of clay, pre-dispersed clay lead to a more significant reduction of oxygen permeation (~33%) than as-received clay did (~8%) and a maximum reduction (44%) of oxygen permeation was achieved by adding 3wt% of pre-dispersed clay. In addition, we confirmed that effect of temperature on the permeation of PET/clay nanocomposites depended both on the Arrhenius behavior of the organic phases and tortuous path effect, where improved clay dispersion resulted in a higher effective activation energy. Moreover, both the transparency and mechanical properties have been improved by using scCO2 processed clay.