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The Production and Properties of Exfoliated Polyethylene-Clay Nanocomposites

Cynthia Pierre, Materials Science & Engineering, Northwestern University, 2220 Campus Drive, Cook Hall Room 2036, Evanston, IL 60208-3108 and John M. Torkelson, Chemical and Biological Engineering and Materials Science & Engineering, Northwestern University, 2145 Sheridan Road, Room E136, Evanston, IL 60208.

Polymer-clay nanocomposites are an important class of materials with major potential for technological application. There have been many studies of polymer-clay hybrids since the Toyota Research group's success with nylon-6 in the early 1990s. However, only a small fraction of studies that have focused on melt mixing of polymer and clay have led to well-exfoliated clay and thus true nanocomposite formation. The primary route of investigation has been the modification of polymer or clay to improve thermodynamic compatibility, a method that has met with limited success in achieving exfoliation. As a result of the poor clay dispersion, the performance of many polymer-clay hybrids falls below expectations.

Here we demonstrate that a continuous, industrially applicable process called solid state shear pulverization (SSSP) can produce well-exfoliated polymer-clay nanocomposites. The SSSP process employs a cooled, modified twin-screw extruder that maintains the polymer in the solid state. During SSSP, the material is exposed to high compressive and shear forces, undergoing fragmentation and fusion cycles. This results in exfoliation of clay in polymer that is kinetically stable to long-term, high-temperature melt processing. The structure, thermal stability, mechanical strength, and barrier properties of the well-exfoliated nanocomposites are investigated using x-ray diffraction, electron microscopy, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, and oxygen permeation. Because SSSP can tune the level of exfoliation, a fundamental understanding of how clay dispersion affects macroscopic properties of nanocomposites will be gained, in addition to insight on the behavior of polymer in a confined state.