MELT MIXED PCL - MWCNT NANOCOMPOSITES PREPARED AT DIFFERENT MIXING SPEEDS

摘要

Composites of poly(caprolactone) (PCL) and multiwalled carbon nanotubes (MWCNTs) were produced by melt-mixing in a small scale compounder by varying the screw speed between 25 and 400 rpm at a constant mixing time of two minutes. By that, different levels of dispersions, as assessed by quantitative analysis of area ratio of remaining primary agglomerates from light microscopy, were achieved. With increasing screw speed the state of dispersion increases and levels off starting at about 100 rpm.Melt Theological properties were measured in frequency sweeps. Interestingly, distinct differences in the complex viscosity η~* and the storage modulus G' were found in dependence on the agglomerate area ratio, whereas the loss modulus G" was not much influenced. The storage modulus at 0.1 rad/s initially increased with decreasing area ratios, showing that especially the storage modulus is very sensitive to the nanotubes dispersion state. It increased up to a mixing speed of about 75 rpm illustrating improved dispersion followed by a decrease when further increasing the speed. As GPC investigation showed no significant differences in the degradation of the PCL matrix depending on the rotation speed, the effect of decreasing rheological parameter was assigned to nanotube shortening. Both effects, improved dispersion and nanotube shortening, are also reflected in the electrical resistivity values of compression molded samples. Here, up to 75 rpm a decrease in resistivity due to the better dispersion was observed, whereas above 75 rpm, where dispersion had leveled off, again an increase was found reflecting the reduction in nanotube aspect ratio.Thus, it could be shown that rheological measurements are suitable to detect differences in the dispersion state in composites with a fixed type of CNTs and concentration but also the effect of nanotube shortening reflected in lower aspect ratios.