Synthesis and Characterization of Boron Trifluoride Doped High Performance Polyaniline

We report simple synthesis of boron trifluoride (BF3) doped defect free high performance polyaniline (HPPANI) in two step method. Firstly, HPPANI was prepared via self-stabilization dispersion polymerization method in a heterogeneous reaction medium. Second step involves doping of emeraldine base form of HPPANI with boron trifluoride under reduced vacuum. The resultants BF3 doped HPPANI have been well characterized by using UVVisible spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and thermogravimetry. The spectroscopic data indicated that the interaction between HPPANI and BF3.Thermogravimetry studies revealed that the BF3 doping improved the thermal stability of defects free PANI.


Introduction
Conducting polymers have attracted increasing attention because they offers the possibility of generation of novel materials with diverse applications for electromagnetic interference (EMI) shielding, rechargeable battery, chemical sensor, organic light emitting devices, corrosion devices, and microwave absorption [1][2][3][4][5] .Among conducting polymers, polyaniline (PANI) is the promising electrical conducting polymer due to its a broad range of tunable properties derived from its structural flexibility, good environmental stability, easy preparation in aqueous solution, and organic solvents, optical, electrical properties and unique redox chemistry [6][7] .Moreover, PANI exhibits a large spin density so interesting electrical and magnetic properties arise that are highly dependent on the doping level and the structure of the polymer.However, PANI suffers from poor processability because it is infusible and insoluble in common solvents.PANI synthesized in standard Mac Diarmid method 8 , the macroscopic precipitation polymerization of aniline occurs at the interface of growing particles and the aqueous reaction medium and also inside the swollen particles.The resulting PANI has defects due to a randomly branched backbone, some cross linking, ortho-coupling, and Michael reductive addition of aniline 9 .To prevent the undesired side reaction and macroscopic precipitation, several methods have been developed and the most effective method is self-stabilized dispersion polymerization in a heterogeneous medium.A defect free PANI was prepared by self-stabilized dispersion polymerization method in heterogeneous medium 10 .The rationale behind this method is that the organic phase acts to separate the insoluble aniline oligomers and grow PANIchains from the reactive ends of the chains in the aqueous phase.Thus prevent the macroscopic precipitation and undesired side reactions.
In this study, we reoprt synthesized defect free PANI via self-stabilized dispersion polymerization method in heterogeneous reaction medium.The emeraldine base form of defect free PANI was doped with boron trifluoride (BF 3 ) by using BF 3 -etharate complex under control atmosphere.

Experimental
Aniline, Boron trifluoride-Etherate (BF 3 -Etherate), Ammonium persulphate [APS, (NH 4 ) 2 S 2 O 8 ], Chloroform, Methanol, Sodium hydroxide (NaOH) were obtained from Merck Chemicals, India and used as received.Aniline was double distilled under reduced vacuum and stored at 0-5 o C before use.Double distilled water was used throughout all the synthetic processes.All other reagents were analytical grade and used without further purification.

Synthesis of High performance Polyaniline (HPPANI)
In typical synthesis, aniline solution in 1M HCl was added to a chloroform / water mixer (1:2 vol./vol.)and stirred at -16 0 C until the solution became colloidal dispersion.A precooled acidic solution of oxidant, ammonium persulfate, (APS) was added drop wise to the colloidal dispersion, with rigorous stirring.The polymerization reaction was preceded for another 12 hours at -16 0 C. The colloidal dispersion slowly tuned to the dark green colour characteristic of PANI.The reaction mixture was filtered, washed with water and methanol periodically to remove unreacted reagents.Finally, dedoped with 0.5 M NaOH solution and dried under dynamic vacuum at room temperature.

Preparation of Boron Trifluoride (BF 3 ) Doped High Performance Polyaniline
EB form of HPPANI powder was doped with boron trifluoride (BF 3 ) using boron trifluorideetharate complex under anhydrous condition because high susceptibility of BF 3 towards hydrolysis, doping was carried out strictly anhydrous conditions.BF 3 -Etherate, 1:1 complex of BF 3 and diethyl ether was used for doping.In order to reduce the exposure to atmosphere, BF 3 -etherate was distilled in vacuum and distillate was collected directly over the emeraldine base HPPANI powder.The reactivity of BF 3 in BF 3 -etherate complex is remarkably reduced, makes it a lot easier to handle.In order to achieve maximum doping, excess dopant was added and reaction mixture was left to equilibrate for 24 hours and after which un-reacted BF 3 -etherate was removed under dynamic vacuum at room temperature.Pumping for longer period of time, leads to partial dedoping.

Characterization
The UV-Visible absorption spectra of the samples were recorded on a Perkin-Elmer double beam LS-50 spectrophotometer.The samples were dissolved in dry dimethylsulphoxide (DMSO) and centrifuged to remove any undissolved polymer.The clear solutions were taken in quartz cuvettes.The infrared spectra were recorded over the range 4000 -400 cm -1 in a Perkin-Elmer Model SPECTRUM 1000 FTIR spectrometer.The powdered samples were mixed thoroughly with KBr and pressed into thin pellets.Morphology of BF 3 doped HPPANI was examined by scanning electron microscopy (SEM).

Results and Discussion
PANI can be considered as Lewis base due to a lone electron pair on Nitrogen atoms of PANI.Recently, it was evidenced that the PANI can be doped with Lewis acids and forms acid-base complexes 11 .Like other Lewis acids, BF 3 can also form a complex with PANI.The general structure of PANI doped with Lewis acid (LA) is presented in figure1.Similarly to the case of protanation, one molecule of dopant is coordinated to nitrogen atom of PANI, but contrary to the protation, both types of nitrogen atoms of PANI( amine as well as imine ones) are coordinated by LA.In this case of the formation of a covalent or mixed ionic covalent bond was proposed.The structures of Lewis acid doped PANI is differ significantly from those of protanated PANI.Lewis acid-doped PANI systems are expected to be different from the conventional protonated PANI owing to a qualitatively different chemical interaction between the dopant and the polymer; for instance, the absence of any counter ion in these systems may have different influence on the properties.Thus spectroscopic properties of BF 3 doped PANI systems differ from those of protonated PANI.Molecular structure of BF 3 doped HPPANI was investigated by UV-Visible spectroscopy and Fourier-transform infrared (FTIR) spectroscopy.UV-Visible absorption spectra for both undoped HPPANI and BF 3 doped HPANI shown in Figure 2. Undoped HPANI gives two electronic absorption bands at 330 nm and 630 nm approximately.The broad absorption feature at 630 nm has been assigned to quinoid formation in the backbone of the PANI, while the band at 330 nm is assigned to the π →π * electronics transition of benzenoid rings in the HPPANI.In case of BF 3 doped HPANI, new absorption bands appears at 450 nm and 830 nm.These features confirmed the BF 3 doping to HPPANI.It is important to note that the same changes have also been observed in the case of proton doped PANI [12][13] .Figure 3 shows FTIR spectrum for BF 3 doped high performance polyaniline, which well agreed with the previous literature [14][15][16][17][18] .The FTIR spectrum gives main characteristic peaks at 3180, 1595, 1496, 1408, 1313, and 827 cm -1 .Peaks at 3180, 1595, and 1496 cm -1 due to N-H stretching vibration, C=C stretching of quinoid phenyl and benzenoid phenyl rings, respectively.The peak at 1408 cm -1 is due to stretching frequency of B-N=Q moiety (B refers to benzenoid and Q refers to quinoid ring).The presence of this peak confirms that the HPPANI is doped with BF 3 .The peak at 1313 cm -1 is assigned to C-N stretching vibrations of the 1, 4-disubstituted benzene ring of HPPANI.The peak at 827 cm -1 corresponds to C-H out of plane bending of 1, 4-disubstituted benzene rings of HPPANI.

Conclusion
We have demonstrated a simple, reproducible, and facile method of synthesis of BF 3 doped HPPANI via self-stabilization dispersion oxidative chemical polymerization technique and doping with BF 3 -etherate under controlled atmosphere.The spectroscopic results indicated that successful synthesis of BF 3 doped HPPANI.TG data indicated that the thermal stability of BF 3 doped HPPANI have higher than PANI prepared by conventional method.TG reveals that higher thermal stability is due to the doping HPPANI with BF 3.

Figure 5
Figure 5 shows a TG -DTG curve of BF 3 doped HPANI prepared via self-Stabilized dispersion polymerization of aniline under N 2 flow at a heating rate of 20 0 C per minute from 50-850 0 C. The first region at lower temperatures (<350 0 C) is due to removal water and other volatiles in BF 3 doped HPPANI.The weight loss in temperature range of 350-550 0 C can be ascribed to the removal of BF 3 and decomposition of HPPANI chain.The third weight loss region of 550-840 0 C is due to decomposition of HPPANI chain.Thermal studies indicated that thermal decomposition of HPPANI chain occurs at higher temperature for BF 3 doped HPANI as compared to PANI synthesized by standard method.

Figure 5 .
Figure 5. TG-DTG of BF 3 doped HPPANI under N 2 atmosphere flow at a heating rate of 20 0 C min -1 .