Carbon nanofiber (CNF) and carbon nanotube (CNT) composites have interesting mechanical and electrical properties that make these composites interesting for reinforcing applications. These applications require good dispersion of CNF within a polymeric matrix. Presently high shear methods, such as twin screw extrusion, are used to make well dispersed CNF composites but these methods reduce the physical properties due to a reduction in the aspect ratio of the CNF.
Low shear methods to functionalize CNT and CNF have been used to obtain good dispersion while maintaining the high aspect ratio. In this research three ways of making CNF/polymer composites by low shear methods were explored. The first reaction used bisphenol A cyclic carbonate oligomer as a low molecular weight precursor. The oligomers were polymerized to disperse the CNF within the matrix. These composites were characterized by electrical resistivity, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravametric analysis (TGA) and gel permeation chromatography (GPC). The composites had a percolation threshold at 6 wt % CNF decreasing the resistivity to 10 4ohm•cm. The second way used heterocoagulation where a cationic polystyrene latex was combined with anionically charged oxidized CNF. The composites were melt pressed and characterized using electrical resistivity, SEM, and TGA. The percolation threshold was 2 wt % and the resitivity dropped to 10 6ohm•cm.
Finally, it was found that synthesizing a hyperbranched polyol was possible by chemically modifying oxidized CNF with glycidol and BF 3OEt 2. The resulting polyol CNF were characterized by TGA, Fourier transform infrared spectroscopy (FTIR), TEM, and X-ray photoelectron spectroscopy (XPS). The OH groups were reacted with heptafluorobutyryl chloride to determine the amount of OH in the sample. The resulting fluorinated composite was characterized by FTIR and elemental analysis. The amount of OH for the polyol CNF increased by 550% compared with oxidized CNF.
It was also found that purification of CNF was possible using methylene chloride and an increase in the thermal stability of CNF was observed. Purified CNFs were also reacted with the glycidol and characterized by FTIR, TGA, TEM, and XPS. The heptafluorobutyryl chloride reaction was also done with the purified CNF and purified polyol CNF and characterized by the same methods mentioned for the previous sample. The amount of OH increased by 440% for the purifed polyol CNF compared with the purified CNF.
The polyol functionalized CNF should aid dispersion into hydrophilic polymers and the oxidized CNF, oxidized polyol CNF, purified CNF and purified polyol CNF were combined with an epoxy matrix. The resulting composites were characterized using electrical resistivity, TGA, SEM, and TEM. There was no percolation threshold observed for any of the composites except or the purified polyol CNF which had a percolation threshold at 5 wt % to 10 6ohm•cm which is significantly higher than most CNT/epoxy composites reported in the literature.