Dispersion and rheological aspects of SWNTs in ultrahigh molecular weight polyethylene

A new method is developed to homogeneously disperse single-walled carbon nanotubes bundles (SWNTs) in an intractable polymer, for example, ultrahigh molecular weight polyethylene (M-w > 3 x 10(6) g/mol) (UHMWPE). The dispersion is obtained by spraying an aqueous solution of SWNTs onto a fine UHMWPE powder directly obtained from synthesis. The SWNTs are adsorbed on the surface of the polymer powder. A composite film is prepared from the solution of the polymer powder dissolved in xylene. The high viscosity of UHMWPE in solution prevents coagulation of the adsorbed SWNTs. Scanning electron microscopy (SEM) of the films reveals that SWNT bundles are randomly dispersed in the UHMWPE matrix. The observed "shish-kebab" morphology in the SEM pictures of the film shows that the polymer chains tend to crystallize from solution as chain-folded crystals (kebab). The nanotube surface can act as a nucleating site (shish). The orientation of the dispersed SWNTs in UHMWPE matrix is achieved on solid-state drawing the solution crystallized films. Crystallization of the UHMWPE melt followed by rheometry shows that the presence of SWNTs enhances the overall crystallization rate. The observed rheological behavior of the UHMWPE/SWNT nanocomposites is rather unusual. Varying the content of SWNTs, the dynamic viscosity/storage modulus shows a minimum. The decrease in viscosity is attributed to the selective adsorption of the high molar mass fraction onto the nanotubes surface. The increase in viscosity upon further increasing the nanotube content is attributed to the formation of an elastic nanotube-polymer network. The formed nanotube-polymer network is conductive at percolation threshold of 0.6 wt % SWNTs.