Effects of Non-covalent Functionalization and Initial Mixing Methods on SWNT/PP and SWNT/EVOH Composites

We report that a simple, low-cost type of spray-freeze drying (SFD) significantly improves the dispersion of single-walled carbon nanotubes (SWNTs) in thermoplastic polymers. Conventional SFD requires costly specialized equipment and large amounts of material, both of which are impediments to laboratory research on nanomaterial composites. Our method uses more readily available equipment and can be adapted to use milligrams to grams of material. A household spray bottle containing an aqueous nanomaterial dispersion is used to spray the dispersion into a dish of liquid nitrogen. The resulting material is then lyophilized in a standard laboratory freeze dryer. The usefulness of this simplified method was explored by comparing the properties of polypropylene (PP) composites produced by this method to those produced by a previously reported rotary evaporation method in which the dispersion is vacuum-dried onto the polymer. The role of the initial dispersion state was explored by using pristine SWNTs as well as SWNTs stabilized by two common SWNT stabilizers: polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate. Based on rheological, thermal, and morphological characterization, the porous friable structures produced by SFD resulted in better SWNT dispersion compared to composites produced by a previously reported rotary evaporation method. However, the PP/PVP-SWNT nanocomposites produced by both methods contained large aggregates. To verify that this aggregation behavior was the result of thermodynamic incompatibility between PP and PVP, ethylene vinyl alcohol (EVOH) nanocomposites containing PVP-SWNT were also produced using the SFD method. The results of this research show how a low-cost alternative to SFD along with careful consideration of compatibility is a promising approach to produce nanocomposites.