Solid-State Fabrication and Characterization of Polymer-Graphite Nanocomposites

Paul J. Hubert and Katsuyuki Wakabayashi. Department of Chemical Engineering, Bucknell University, Lewisburg, PA 17837

Polymer nanocomposites are a class of advanced materials that contain small amounts of nanoscale fillers dispersed in a polymer matrix. These “nano”fillers can greatly enhance the physical properties of the resulting composites, such as mechanical stiffness and toughness, thermal stability, electrical conductivity, chemical resistance, and reduced gas permeability. Numerous potential property improvements in polymer nanocomposites lead to a wide range of high-performance applications, from packaging to automotive parts and sporting goods.

There are several widely-used techniques to produce polymer nanocomposites, all of which are very different from one another. Each has notable advantages and disadvantages. Solution mixing and in situ polymerization are methods which employ large quantities of solvent or monomer during nanocomposite fabrication. While both techniques have shown to produce well-dispersed nanocomposites on the laboratory scale, the required volumes of external chemicals make the processes impractical for scale-up to industrial production.

Melt-state processing of polymer nanocomposites has been performed on a batch scale as well as with a continuous extruder. Extrusion processes give melt-state processing the potential to be applied to mass production of nanocomposites. While both melt-based methods have led to the production of nanocomposites containing relatively good dispersion, the low shear involved in these processes limit nanocomposites from achieving the highest nanofiller exfoliation and distribution. Additionally, the high-temperature processes can lead to practical issues like polymer degradation and filler re-agglomeration.

Recently, solid-state processing of polymer nanocomposites has been gaining popularity in the research community. This is a novel method which utilizes high amounts of shear forces carried out below the melt and/or glass transition temperature of the polymer. Cryogenic milling is a batch process in which a blend of polymer and nanofiller is ground within a cylinder at cryogenic temperatures. Solid-state shear pulverization (SSSP) is a continuous analog of the cryogenic milling technique. In SSSP, polymer/nanofiller blends are pulverized within a modified twin-screw extruder which operates at sub-ambient temperatures [1].

In this paper, polymer-graphite nanocomposites were fabricated using two different solid-state processing techniques, namely batch-scale cryogenic compounding and continuous SSSP. 1 wt% of pure, unmodified graphite was incorporated in four different polymer matrices: polypropylene, polystyrene, poly(methyl-methacrylate), and styrene-butadiene-styrene block copolymer. Solid-state processed nanocomposites were compared with equivalent samples made via conventional melt-mixing processes, batch-scale melt-mixing and continuous single-screw extrusion. Thermal, electrical, mechanical, and morphological characterization was conducted to determine superior processing methods for the polymer-graphite nanocomposites.

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