Poly(ethylene terephthalate) (PET) is widely used for various applications ranging from food packaging to bottles to automotive parts. There have been many studies aimed at optimizing the physical properties of PET through the addition of nanofillers. However, due to the high melt temperature of PET, only a small fraction of studies have led to well-exfoliated fillers and thus, true nanocomposite formation. As a result of the poor nanofiller dispersion, the performance of many PET hybrids falls below expectations. Here we demonstrate the ability to enhance the physical properties of PET via two routes, by simply processing PET using solid-state shear pulverization and through the addition of either clay or graphite to produce well-exfoliated PET nanocomposites.

Solid-state shear pulverization (SSSP) is a continuous, industrially applicable process that employs a cooled, modified twin-screw extruder that maintains the polymer in the solid-state during processing. We demonstrate the ability to very strongly tune the physical and mechanical properties of PET by changing the processing conditions of neat PET during solid-state shear pulverization without addition of any fillers or nucleating agents. Using differential scanning calorimetry, we observe a roughly factor of 3 increase in crystallinity of PET that has been pulverized and subsequently melted relative to the unprocessed PET. We also observe a dramatic increase in the rate of crystallization of the pulverized samples. Rheological characterization has demonstrated an increase in viscosity of the pulverized material, which can be ascribed to chain branching in the pulverized product. We also observe significant reductions in the oxygen permeability of the PET with pulverization as well as enhancements in mechanical properties that are commensurate with the modified crystallization properties of the pulverized PET.

Via a combination of methods, including x-ray diffraction and transmission electron microscopy, we demonstrate that the well-exfoliated state can be achieved via SSSP processing of PET-clay and PET-graphite nanocomposites. Using x-ray diffraction of powder and melt-state samples, we show that SSSP has yielded well-exfoliated states in all of the systems studied. Even more important is the fact that the well-exfoliated state is stable to long-term annealing in the melt state; this means that the exfoliation will be maintained during any subsequent melt processing. Among the properties that have been characterized in these PET nanocomposites include mechanical, barrier and thermal properties. Comparisons will be drawn between the property enhancements achieved through simple processing of the PET and the two fillers used.