(267b) Characterization of Biopolymer Composites Created Using Solid-State Shear Pulverization

The incorporation of an additional material into a polymer matrix is a conceptually simple and affordable method for achieving improved properties. The selection of a filler is based upon many criteria including cost, size, and desired effect. For this work, solid-state shear pulverization (SSSP) was used to create polymer composites with clay or starch. This process utilizes a modified twin-screw extruder which is cooled to keep the polymer below the glass or melt transition temperature. Consequently, all processing occurs in the solid state. This eliminates any thermodynamic limitations associated with blend formation, providing the opportunity for a well dispersed system. In addition, SSSP has the potential to eliminate the need for solvents, organic modifiers, and plasticizers necessary in conventional processes.

By using a nanofiller such as clay, the thermal, barrier, and mechanical properties of the polymer can be altered. The overall effect of the clay is dependent upon many factors, including dispersion. The ideal platelet dispersion, exfoliation, is often difficult to achieve using conventional polymer processing techniques. The unique mixing capabilities of SSSP have been shown to improve the chances of achieving exfoliation. Nanocomposites were created from thermoplastic starch, poly(lactic acid), and poly(caprolactone) with clay. Small angle x-ray scattering demonstrated increasing levels of exfoliation in the thermoplastic starch nanocomposites processed using different experimental conditions. The incorporation of the clay was shown to improve the thermal stability of the thermoplastic starch when held at a constant temperature near the polymer degradation temperature. Both poly(lactic acid) and poly(caprolactone) exhibited essentially no scattering peaks in small angle x-ray scattering, suggesting significant levels of exfoliation. For poly(caprolactone), processing via SSSP produced changes in the crystallization behavior of the neat polymer. This was demonstrated by differential scanning calorimetry and oxygen permeation. By incorporating small quantities of clay, the oxygen permeability coefficient of the poly(caprolactone) was further reduced.

Granular starch has also been added to several polymer matrices as an inexpensive, abundant filler. Blends were created using polyethylene and poly(lactic acid). Pulverization was found to damage the starch granules in the polyethylene systems, i.e. increase starch surface area and crack the outer starch casing. This breakdown of the granular structure altered the water adsorption behavior of the blend. When poly(lactic acid) was the matrix, very few starch granules were damaged. The impact of the damage of starch via SSSP processing on blend properties is under investigation.