Thermal Transitions and Barrier Properties of Olefinic Nanocomposites

Differential scanning calorimetry (DSC) was used to study the thermal transitions of smectite organoclays and their dispersions in olefinic systems, which included paraffinic waxes and polyethylene. The organoclays, with treatment on both the basal and edge surfaces, produced nanocscale dispersions without the aid of external coupling agents or compatibilizers. In addition to DSC measurements, the nanocomposites were further characterized using X-ray diffraction and oxygen transmission. The DSC measurements indicated that a clay/wax nanocomposite phase was generated with melt/freeze transition temperatures that were different from those of the individual components, while X-ray data indicated that the nanocomposite phase was in equilibrium with an intercalate phase. Barrier improvement of over 300× was observed and ascribed to a tortuosity effect resulting from dispersed clay platelets having a high aspect ratio and strong cohesion between the wax and the organoclay surface. Available data indicate that the barrier enhancement decreases as the difference between the freezing points of the organoclay and the wax widens. The cause of poor barrier performance in polyolefin systems is traced to the large difference in recrystallization temperatures, such that when the polymer begins to crystallize the surface of the organoclay is still in a liquid state, which leads to phase separation.