Preparation Of Noncovalently Functionalized Multiwalled Carbon Nanotubes With Sodium Lignosulfonate And Investigation Of Its Pickering Emulsifying Properties

Carbon nanotubes(CNTs) have excellent optical, electrical, magnetic, thermal, chemical and mechanical properties. However, they are easy to form cluster structures and cannot be dispersed well in both water and organic solvents due to the strong van der waals force between tubes, which greatly limits their application. Functionalization of CNTs is an effective method to improve their dispersion. There are usually two methods, namely, covalent and noncovalent functionalization. In contrast to covalent functionalization, noncovalent functionalization methods are more preferable since they do not destroy the one-dimensional electronic structure of CNTs and thus the excellent properties of the CNTs can be better preserved.Industrial sodium lignosulfonate(SLS), existing in the waste liquor from the pulping and paper-making industry, was extracted as dispersing agent to prepare noncovalent functionalized CNTs and CNTs-based conductive composites. It not only improves the dispersion of CNTs, but also broadens the value-added utilization of industrial SLS, which has both theoretical and practical significance. The main results are shown as follws:(1) Noncovalently functionalized MWCNTs/SLS systems were prepared, and the effect of molecular weight of SLS on the dispersion property of MWCNTs/SLS systems in water was intensively discussed. Ultrafiltration membranes of molecular weight cutoff of 5000, 10000, and 50000 were used to obtain SLS with different molecular weights. Different SLS fractions were then characterized by UV-vis spectrophotometer, Fourier transform infrared spectroscopy, gel permeation chromatography and elemental analysis. Then, SLS frations were applied to disperse MWCNTs, MWCNTs/SLS dispersions were prepared under different conditions. Functionalized MWCNTs with different SLS fractions were thus characterized by Fourier transform infrared spectroscopy, elemental analysis, resistance rate testing, atomic force microscopy, HRTEM, and Raman spectroscopy. The results showed that MWCNTs could be well dispersed by SLS and achieve its noncovalent functionalization. Among different fractions of SLS, F3 fraction with the highest molecular weight showed the best dispersing performance. The particle size of dispersed MWCNTs was the smallest, and the prepared MWCNTs/SLS had the minimum resistivity and best conductivity. Between SLS with the highest molecular weight and MWCNTs, there were stronger π-π interaction and bigger adsorption capacity. Among the MWCNTs adsorping SLS with the highest molecular weight, the electrostatic repulsion and the steric hindrance were larger and thus showed better dispersion stability.(2) Noncovalent functionalized MWCNTs /SLS particles were applied as the stabilizers to form Pickering emulsions. The emulsifying performance of MWCNTs /SLS particles under different solution conditions was systematically studied. The particle size of functionalized MWCNTs wth fraction F3 was between 1-10μm. Functionalized MWCNTs became amphiphilic due to the adsoption of SLS, thus could be used as Pickering emulsion stabilizers. Properties of Pickering emulsion were systematically studied by changing the oil/water volume ratio, the initial concentration of stabilizers, the oil phase type, and the addition of HCl and other conditions, and characterized by polarized light microscopy. The results showed that, the stabilizer of F3-MWCNTs had better emulsification properties for different oil phases, including different polymer monomers. Formed Pickering emulsion exhibited good stability and pH-responsibility.(3) Multi-wall carbon nanotubes/polyaniline(MWCNT/PANI) composites were prepared by Pickering emulsion polymerization. On the basis of emulsifying property researches, using F3-MWCNTs as stabilizer and aniline(ANI) monomer as oil phase, F3-MWCNTs/PANI composites were successfully synthesized by Pickering emulsion polymerization method. UVvisible spectra and scanning electron microscope image showed that ANI was polymerized on the wall of F3-MWCNTs, and there was strong interaction between PANI and MWCNTs. F3-MWCNTs were successful doped into PANI chains. Conductivity results showed that the prepared carbon nanotubes/polyaniline composites had better conductivity than that prepared by normal method.