Extrusion of CNT-modified Polymers with Low Viscosity - Influence of Crystallization and CNT Orientation on the Electrical Properties

Several polymers were modified with multiwalled carbon nanotubes (CNT) to study the influences of the crystallization in the polymeric matrix and of the CNT orientation during extrusion on the electrical conductivity. Experiments were carried out with common semi-crystalline polymers (polypropylene, polyethylene, polyamide 6) and compared to an amorphous polymer (ethylene vinyl acetate). All polymers were grades with low viscosity, so that the CNT could be oriented well during extrusion. For all materials, the percolation threshold was determined, and the lowest value of 3% was found in polypropylene. The percolation threshold was correlated to the degree of crystallinity of the matrix polymers, so that crystallites could be seen as an excluded volume for CNT. The crystallization itself was analyzed by differential scanning calorimetry (DSC), whereby nucleation effects and changes in the crystallization temperature were found. The shear rate during extrusion had a large influence on the electrical conductivity. This effect was analyzed by transmission electron microscopy (TEM), with which the orientation of CNT in the direction of extrusion was visualized and differences between the polymer matrices were explained.