Cryogenic Milling of Classical Polymer Nanocomposites: Polypropylene-Clay and Polypropylene-Carbon Nanotube Hybrids

Edwin B. Gienger IV, Austin K. Murata, Paul J. Hubert, and Katsuyuki Wakabayashi. Department of Chemical Engineering, Bucknell University, Lewisburg, PA 17837

Polymer nanocomposites are of great scientific and technological interest due to the numerous potential physical property improvements compared to the neat polymer or the conventional macrocomposites. Due to the very small loading of the nanofillers, these polymer hybrids have a wide range of high-performance applications at relatively low cost.

The two widely studied systems are polymer-clay and polymer-carbon nanotube nanocomposites [1,2]. Clay, commonly known as layered silicate, is an inorganic material composed of closely-packed silica and alumina layers that are approximately 1 nm apart. A typical filler clay type is montmorillonite and its organically modified analog. Carbon nanotubes, on the other hand, are bundles of rolled graphene sheets with diameters on the order of nanometers. Carbon nanotubes are available in varying purity levels and morphologies such as single-walled and multi-walled structures.

Many of the previously studied polymer nanocomposites were made by melt-state processing, in situ polymerization, or solution blending. Recently, solid-state processing of polymer nanocomposites has been gaining popularity in the research community. This method imparts high shear and/or impact forces to the materials below the melt and/or glass transition temperature of the matrix and the filler, thereby achieving separation (exfoliation) and dispersion of the filler.

This paper investigates the feasibility of cryogenic milling as a polymer nanocomposite fabrication technique. Cryogenic milling is a batch process in which a tungsten carbide bar repeatedly impacts the materials inside a cylinder at the cryogenic temperature (~77K). The low-temperature mechanochemical process can be considered as the batch-scale analog of the solid-state shear pulverization, which has recently succeeded in continuously fabricating different polymer nanocomposites [3,4].

Two model systems under investigation are polypropylene-based hybrids that have commercial applicability. The first system incorporates pristine montmorillonite without any modifications or compatibilizers, while the second system uses industrial grade, multi-walled carbon nanotubes suitable for eventual scale-up for mass-production. The processing parameters, such as milling time, material phase, and cryogenic soaking time, were varied for evaluation of their effects upon the final structure and properties of the resulting hybrids.

Micro- and nanostructure of the processed samples were probed using X-ray diffraction and electron microscopy methods. Thermal properties were measured by differential scanning calorimetry. Mechanical characterization was conducted using a uniaxial tensile test.

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[2] J. H. Koo. Polymer Nanocomposites. McGraw-Hill, New York (2006).

[3] K. Wakabayashi, C. Pierre, D.A. Dikin, R.S. Ruoff, T. Ramanathan, L.C. Brinson, J. M. Torkelson. Macromolecules 41, 1905 (2008).

[4] J. Masuda, J.M. Torkelson. Macromolecules 41, 5974 (2008).