Conductivity And Mechanism Of Bromine-doped Carbon Nanotube-Conjugated Polymer Composites

Since the discovery of conductive polymers in 1970s,their excellent electrical,electrochemical and optical properties make conductive polymers have the very big application prospect.With the development of conductive polymers study in batteries,hydrogen storage materials,supercapacitors,electromagnetic shielding materials and sensors,many works found that the relative low conductivity limited the improvement of other properties.Thus it is necessary to further improve the conductivity of conductive polymers.In this paper,in allusion to the current widespread study of polyaniline(PANI),polypyrrole(PPy),poly(aniline-co-pyrrole)(P(An-Py))and polythiophene(PTh),we performed the following studies:1.Effect of pre-polymerization on the conductivity of P(An-Py)A highly conductive copolymer P(An-Py)was fabricated by properly controlling pre-polymerization time.The electrical conductivity of P(An-Py)by proper pre-polymerization achieved the same order of magnitude(10-1 S/cm)with their homopolymers,or even slightly higher,which was irrelated to the molar ratio of two monomers.Only by 15 min pre-polymerization,the electrical conductivity of P(1 An-3Py)and P(lAn-lPy)improved more than 3 orders of magnitude and 4 times,respectively.Electron microscopic analysis showed there was no obvious change in the morphology of the copolymers with pre-polymerization.Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)revealed that the copolymers backbone contained many small aniline chains after pre-polymerization.The number of quasi-hydrogen bonding between N of pyrrole and adjacent free H+ of protonated aniline was greatly reduced.As a result,the number of free H+ of protonated aniline chain in copolymer was significantly increased,thus the carrier concentration and electrical conductivity increased sharply.2.Effect of bromine doping methods on the conductivity of CNTs-PANIWe prepared polyaniline wrapped carbon nanotubes composites(CNTs-PANI)by in-situ polymerization,and used three different treated methods of bromine water,bromine vapor and liquid bromine treating to fabricate bromine-doped compounds.Only with the method of liquid bromine processing could we highly improve the conductivity of composites.When treating by liquid bromine,the conductivity of 10%CNTs-PANI significantly improved near 3×106 times,which was already near that of pure CNTs we used.Compared with pure PANI doped by bromine,we found that there was a synergistic effect in CNTs-PANI composites between bromine and CNTs.Based on Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS)and heat treatment experiment,we proposed that the interaction between bromine and CNTs-PANI had both the doped effect acted as a promoter and the destructive effect on the electrical conductivity.3.Effect of bromine doping and dosage on the conductivity of CNTs-conjugated polymer composites.Highly conductive bromine-doped carbon nanotubes-conjugate polymer composites were prepared by in-situ polymerization and liquid bromine doping.Compared to undoped samples,the conductivity of CNTs-PPy and CNTs-P(l An-3 Py)barely improved.However,that of CNTs-PANI and CNTs-PTh could increased by 7 and 5 orders of magnitude,respectively.Notably,the conductivity of 50Br/10%CNTs-PANI almost achieved the same with that of pure CNTs we use in our study.Based on Hall-effect,XRD,UV-Vis,FTIR and XPS spectra,both the improvement of the carrier concentration and mobility resulted in superior electrical properties.The conductive mechanism was varied with the heteroatom in the composites.When the heteroatom is nitrogen,the synergistic effect between bromine and CNTs helps to form a stronger p-? conjugated system.The mobility and carrier concentration of N+,Br3-and Br-increased simultaneously,promoting the conductivity of composites apparently.When the heteroatom is sulfur,under the influence of the charge-transfer complex and ?-? conjugated interaction,the carrier concentration and mobility of composites were boosted,as well as the conductivity.Our work supplied a method of pre-polymerization before copolymerization to simple and effective improve the conductivity of P(An-Py),which could be applied to other copolymers of aniline with other monomers.The present work suggested liquid bromine was an excellent dopant to highly improve the electrical conductivity of some carbon nanotube-conjugate polymer composites.