ABSTRACT:Ultrahigh molecular weight polyethylene (UHMWPE) and multi-wall carbon nanotube (MWNT) composites were prepared using either decalin or paraffin as solvent. Electrical conductivity measurements were performed for the original and heat-treated composites. The drastic increase in conductivity occurred at low MWNT content for the composite prepared in paraffin, while the conductivity of the composite prepared in decalin increased slightly up to 10 wt % MWNT content. Scanning electron microscopy observations revealed that the MWNTs within the composite prepared in decalin were covered by UHMWPE, and their average diameters were much greater than those of the original MWNTs, while the average diameter of the MWNTs within the composite prepared in paraffin was similar to the diameter of the original MWNTs. Such different morphology was found to be due to the different crystallization. The composites prepared in decalin had high drawability. Interestingly, the conductivities of the drawn (up to 50 times) and undrawn UHMWPE-MWNT composites were almost equal, independent of the MWNT content. As for the UHMWPE-MWNT composite (drawn up to 50 times) with 15 wt % MWNT content, the conductivity reached up to 10 À1 S/cm, and the modulus was higher than 50 GPa at ca. 25 C, indicating that a polymeric material with high modulus and relative high conductivity was successfully prepared.

Multiwalled carbon nanotubes (MWNTs) and ultrahigh molecular weight polyethylene (UHMWPE) composites are prepared using paraffin as solvent. The resulting electrical properties show lower electrical percolation threshold, which is 1.4 vol %, than that of UHMWPE-MWNT composites prepared by gelation/crystallization from decalin solution. Good reproducibility and higher maximum of electrical conductivity are obtained. The extremely low electrical percolation threshold indicates that the dispersion of MWNTs is segregated, which can be proved by the morphology of the film observed by optical microscopy. The complex planes of electric modulus are useful to analyze the dispersion of MWNTs and the quality of interparticle contacts. In addition, the result of thermogravimetry shows that the retardation of onset of UHMWPE decomposition in inert gas is observed in the mixture with MWNTs.

Gelation mechanisms of ultra-high-molecular-weight polyethylene (UHMWPE) in dispersion solutions containing multiwall carbon nanotubes (MWNTs) were investigated in terms of liquid-liquid phase separation mechanisms. When an incident beam of He-Ne gas laser was directed to the dispersion solution quenched to a desired temperature, the logarithm of scattered intensity increased linearly with elapsing time and tended to deviate from this linear relationship. The two different increasing behaviors of the scattered intensity were termed as initial and latter stages, respectively, of the phase separation. The linear increase in logarithmic light-scattered intensity was analyzed within the framework of the linear theory of spinodal decomposition in terms of the viewpoint that the quenched solution becomes thermodynamically unstable and tends to incur phase separation to resolve the unstable state. The growth rate of the concentration fluctuation of neat UHMWPE solution (without MWNTs) calculated from the slope of linear increase was faster than that of the dispersion solution. In spite of the rapid growth rate, however, the gelation time of the dispersion solution was much shorter than that of the neat UHMWPE solution and the gelation and quasispinodal temperatures were slightly higher. These phenomena were in contradiction with the result of the concentration fluctuation. This contradiction was thought to be due to the fact that diffused UHMWPE chains in the dispersion solution were gelled easily on the MWNT surface rather than the gelation by self-coagulation.

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