One-dimensional Core-shell Structured Multi-walled Carbon Nanotubes/polyaniline Nanocomposites

Multi-walled Carbon nanotubes (MWNTs) have attracted the fancy of many researchers due to their unique electrical and mechanical characteristics, as well as an extremely high aspect ratio with low density. The combination of MWNTs and conducting polymers into composite materials has been receiving interest as one-dimensional nanostructured materials, due to a large enhancement in the physical characteristics of electrical, thermal and structural properties as a result of interaction between the polymer and MWNTs. However, the MWNTs aggregate easily and are difficult to uniformly disperse in the polymeric matrix due to the strong van der Waals forces among the tubes, thus the MWNTs were generally treated with strong acidic solutions via a reflux process to eliminate impurities including metallic catalysts prior to the preparation. Unfortunately, such treatment always resulted in the damage of carbon nanotubes and formation of functional groups at defect sites of MWNTs.In this work, we describe a simple approach to purify the MWNTs by refluxing them in concentrated hydrochloric acid under low-power ultrasonic irradiation. It is found that the impurities including metallic catalysts absorbed on MWNTs can be efficiently removed prior to the preparation, and the characteristic of extremely high aspect ratio of MWNTs can be remained. During this refluxing process, carbon nanotubes with more structural defects would be broken into pieces, whereas outer layer with less structural defects was easy to peel off the MWNTs surface until the perfect surface appeared. The MWNTs treated for 15h at 70℃with less defects and more perfect structure were obtained.Then the nanocomposites with the aforementioned MWNTs and polyaniline were prepared by the in situ chemical polymerization of aniline on the carbon nanotubes using hydrochloric acid as an oxidant. The in situ polymerization of aniline in the presence of these well-dispersed nanotubes gave rise to a new tubular composite of carbon nanotubes having a uniform encapsulation of doped polyaniline. It was found that the mixing sequence of the initiator, the monomer and MWNTs strongly influenced the microstructure of the nanocomposites. The coated polyaniline layers in above-mentioned one-dimensional nanocomposites could be controlled easily by adjusting a feed ratio of aniline to MWNTs. It also was found the polyaniline on MWNTs surfaces gained obvious improvement on the structural order, molecular packing and delocalization of the charges in the material. For the ratio of MWNTs versus aniline monomer of 2:1, the resulting PANI could be completely assembled onto the surface of MWNTs with the thickness of the PANI shell layer of about 15-20nm, and the nanocomposites exhibited good thermal stability and electrical conductivity up to 28.46 (Ω·cm)^-1.The Doping effects on the morphology, electronic conductivity, and thermal stability of the nanocomposites also were studied. It was demonstrated that strong n-n conjugation interaction between carbon nanotubes and polyaniline (PANI) caused by the doping effects led to PANI in emeraldine salt (ES) form closely wrapping onto the MWNT surfaces. However, the weakπ-π^* conjugation interaction appeared due to the change of conformation of PANI in the subsequent de-doping process and the removal of aniline oligomer in emeraldine base (EB) form from the MWNT surfaces, and thus the loose core-shell structure formed for EB-MWNT. The improvement in the thermal stability of the nanocomposites was observed by replacing hydrochloric acid with sulphonic acid as the dopant. More attention should be paid to the unique influence of MWNTs on the delocalization of theπelectrons of PANI.