Effects of Composition, Orientation and Temperature on the O2 Permeability of Model Polymer/Clay Nanocomposites

Studies of the barrier properties of polymer/layered silicate nanocomposites have traditionally focused on systems with low organoclay contents, with very little work comparing theory and experiment over broad composition ranges. As a result, in spite of many reports of promising enhancements in barrier properties, the envelope of applicability nanocomposite barrier models is generally unknown, making predictive modeling and materials design difficult if not impossible. In this work, model polystyrene (PS)/dimethyl ditallow modified montmorillonite (DMDT-MMT) nanocomposites are produced via a novel spray casting technique capable of creating homogeneous, free-standing nanocomposite films. This approach provides a single experimental methodology for producing films of pure polymer, pure organoclay, or any intermediate composition, with consistently high levels of layer orientation in all cases. The results of oxygen permeation analysis (OPA) performed on these model materials (0−100 vol % organoclay in 10% increments) are compared to the results from all models commonly used for nanocomposite barrier properties modeling, both before and after the addition of a correction factor for actual layer orientation as measured by 2D wide-angle X-ray diffraction (WAXD), and with fitting parameters limited to physically meaningful values. We report substantial improvements in barrier properties in spite of the absence of exfoliation, with the model fits implying that the permeating species remain sensitive to the aspect ratio of individual platelets at all organoclay contents. While all models match our experimental data at low organoclay contents, significant differentiation occurs as organoclay content is increased. Finally, we confirm that the permeability of these materials follows an Arrhenius relation vs temperature, albeit scaled to lower values as a function of inorganic content.