A new approach to solubility and processablity of polyaniline by poly ( aniline-coo-anisidine ) conducting copolymers

Homopolymers of aniline, o-anisidine and their copolymers were synthesized by chemical oxidative polymerization using different ratios of monomers in the feed of H2SO4 medium. The synthesized polymers are characterized by employing Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) techniques for understanding the details of the structure of the synthesized polymers. Morphological, thermal and electrical conductivity of the as synthesized polymers were also studied by employing scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and dc electrical conductivity respectively. Rod and spherical shaped nanoparticles were observed for PANI and for copolymers respectively. A three step thermal degradation was observed for all the polymers. The electrical conductivities of the copolymers are less compared with PANI, and at higher temperature the conductivities of all the polymers are more or less same. The copolymers show better solubility but lower conductivity than PANI.

synthesis, good environmental stability, and ability to dope with protonic acids. 1,2However conductive form of polyaniline is difficult to process, since it is insoluble in common organic solvents and unstable at melt processing temperatures, has restricted its applications.To overcome this, several substituted polyaniline homopolymers (single type of monomers involved) soluble in organic solvents have been prepared such as alkyl 3 and alkoxy 4 as well as alkyl-N-substituted polyanilines. 5Another approach followed is copolymerization (more than one type of monomers involved) of aniline with suitable substituted aniline.Using the later method, Chen and Hwang 6 have synthesized the first water soluble self-acid doped polyaniline, poly(aniline-co-Npropanesulphonicacidaniline).Heeger et al. 7 have prepared soluble polyaniline in its conducting form by doping it with functionalized surfactants such as camphorsulphonic acid and dodecylbenzenesulphonic acid.Among substituted polyanilines, polytoluidines polyanisidines have attracted considerable attention since they exhibit better solubility 8 in many organic solvents and better processability 9 than polyaniline with moderate to good conductivity.However polyanisidines show less conductivity than polyaniline,which may be due to steric constraints imposed by methoxy group that could induce additional deformation along the polymer backbone as well as increase in the inter-chain distance.Both factors reduce the mobility of the carriers and as a result lower conductivity is exhibited.In order to combine the high conductivity of polyaniline with the good solubility of the polyaniline der ivatives, copolymerization received greater attention as it helps to tailor-make a material with specifically desired proper ties, like excellent electrical, optical and mechanical properties.
Although there are repor ts on the copolymers of aniline with anisidine [10][11][12] , however, these studies were only partially characterized, and not focused on the morphology.Moreover homopolymers and copolymers reported to be soluble in solvents like dimethyl sulphoxide (DMSO), N-methylpyrrolidone (NMP) N,Ndimethylformamide (DMF), and tetrahydrofuran (THF) which are hazardous, costly and comparatively viscous.As our main aim of the study was to obtain copolymer salts with improved solubility and better processability compared to PANI and early reported PANI derivatives.We synthesized copolymers which are soluble in most common, economical, non-viscous, comparatively safe, non-hazardous solvent like ethyl alcohol, which helps in making films.In the present research program homopolymers of aniline, anisidine and their copolymers [poly (aniline-coo-anisidine)] of different compositions have been synthesized by chemical polymerization in acidic (H 2 SO 4 ) medium.Resulted homopolymers and copolymers were characterized by Fourier transform infrared (FTIR) spectroscopy and Xray diffraction (XRD) techniques.We have evaluated their conductivity (dc electr ical conductivity), thermal (thermogravimetric analysis (TGA)) and morphological (scanning electron microscopy (SEM)) properties.We also made an effort to understand the effect of having electron donating methoxy group in the polymer chain and noted the differences between the homopolymers and copolymers.

Materials
Aniline and o-ansidine (Merck) were distilled twice and all other chemicals (analytical grade) were used as procured.Double distilled water was used for the preparation of required solutions.

Synthesis of homopolymers (polyaniline (PANI))
In a typical experiment, aqueous solution of 0.1 M oxidizing agent, ammonium persulfate was added dropwise into 1.0 M H 2 SO 4 solution containing 0.1 M aniline at a temperature of 0-5 0 C. The oxidation of aniline is highly exothermic and therefore, the rate of addition of the oxidant was adjusted to prevent any increase in the temperature of the reaction mixture.After the addition of oxidant, the reaction mixture was left stirring at constant temperature for 4 h.The precipitated polyaniline was filtered and then washed with distilled water until the washing liquid was colourless.In order to remove oligomers and other organic byproducts, the precipitate was washed with acetone until the solution was colourless.Finally, the resulting polymer salt was dried at 100 0 C till a constant mass.(Polyaniline base was prepared by dedoping polyaniline-sulfate salt (1 g), with constant stirring at ambient temperature in 100 mL sodium hydroxide solution (1 M) for 12 h.The resultant solid was filtered and washed with water, followed by acetone and finally dried at 100 0 C till a constant mass.

Synthesis of copolymers
Copolymers of aniline with o-anisidine were synthesized in various molar fractions using ammonium persulphate as oxidizing agent and H 2 SO 4 as acid medium.A typical procedure for the preparation of copolymers 50:50 ratio of aniline and o-anisidine is as follows:

Synthesis of copolymers, poly (aniline-co-oanisidine) (PAPOA)
An aqueous solution of ammonium persulphate (0.1 M) was added drop wise to 1 M H 2 SO 4 solution containing o-anisidine (0.05 M) and aniline (0.05 M) maintained at 0°C, stirred for 4 h and then kept at room temp for about 15 h.The green precipitate of copolymer salt obtained was filtered and washed with distilled water several times and then washed with acetone and subsequently dried at 100 °C till a constant mass (Scheme 1).
The copolymer base was obtained by stirring 1 g of salt in 100 mL of 1.0 M NH 4 OH for 12 h.The resultant solid was filtered and washed with water, followed by acetone and finally dried at 100 0 C till a constant mass.
Characterization techniques and studies used A weighed amount (10 mg) of the homopolymer and copolymer was added separately to 2 mL of the solvent with stirring.Additional solvent was added at the rate 1 mL per 10 min up to 10 ml, the copolymer completely dissolved was taken as soluble.; the polymers which did not dissolve completely during this period were taken as "partially soluble".
The FT-IR spectra of the polymers were recorded on a JASCO FTIR-5300 instrument in the range 4000-400 cm -1 at a resolution of 4 cm -1 by making KBr pellets.The XRD patterns were obtained employing a JEOL JDX-8p spectrometer using Cu Ka radiation (l = 1.54 Å).The X-ray generator was operated at 30 kV and 20 mA.The scanning range, 2è/è was selected.The morphologies of the polymers were studied by using coupling JSM-840A scanning electron microscope.The electron microscope was operated at 20 kV.The thermogravimetric analysis (TGA) measurements were made using a Mettler Toledo Star System at a heating rate of 10°C per min under nitrogen atmosphere.Conductivity measurements were done at room temperature by two-probe method on pressed pellets obtained by subjecting the powder to a pressure of 50 kN.The error in resistance measurements under galvanostic conditions with a Keithley model 220 programmable current sources and a Keithely model digital 195A voltammeter was less than 2%.

RESULTS AND DISCUSSION
Homopolymers of aniline is denoted as PANI and of o-anisidine as POA and copolymer, poly (aniline-co-o-anisidine) (as PAPOA), synthesized with various molar fractions of aniline and o-anisidine in the feed ranging from 0.25, 0.5 and 0.75 M are denoted here as PAPOA13, PAPOA11 and PAPOA31 respectively.

Yield
Homopolymers, PANI, POA and copolymers were obtained in good yields (90% to ) by keeping an oxidant to monomer ratio of 1:1.Yields of the copolymers were found to increase with an increase in the amount of aniline in the feed for most of the copolymers.The steric hindrance in anisidine which predominates over the effect of electron-donating groups may reduce the yield of the copolymers slightly when the amount of substituted polyaniline in the monomer feed ratio is higher¹³.

Solubility
PANI is soluble with dark bluish violet colour in polar solvents like DMSO, NMP, DMF, and THF and partially soluble with light bluish colour in less polar solvents like chloroform and insoluble in ethyl alcohol and benzene.Substituted polyaniline homopolymer, POA were clearly soluble in all above mentioned solvents as well as common solvents like ethyl alcohol, chloroform and other solvents with viscous dark bluish violet colour to non viscous blue colour.Among copolymers, PAPOA31 is sparingly soluble because of high feed of aniline ratio, PAPOA11, PAPOA13 are clearly soluble.Good copolymer solubility results from the presence of a large number of methoxy substituent on the aniline ring and an amorphous structure, which increases the distance between the macromolecular chains and then significantly reduces the interaction between the copolymer chains. 11In addition, PANI is insoluble in ethyl alcohol, POA is soluble in ethyl alcohol and copolymer PAPOA11 andPAPOA13 are completely soluble, better solubility is evidence that the polymerization product produced is indeed copolymer containing two monomers rather than a simple mixture of two homopolymers 14 .

FT-IR spectroscopy studies
The characteristic IR peaks of PANI, POA and the copolymer salts PAPOA11 are shown in Figure 1.An accurate quantitative determination of the compositions of the copolymers is rendered difficult due to the overlap of the absorption bands where the spectra of the polyanisidine differ from that of PANI 12 , especially in the case of copolymers which have low anisidine contents.The characteristic bands in the IR spectrum of the PANI salt occur at 1562, 1487, 1302 1240, 1107 and 798 cm "1 .A broad band at 3440 cm -1 was assigned to the free N-H stretching vibration.The bands at 2920 and 2850 cm -1 was assigned to vibration associated with NH part in C 6 H 4 NH 2 C 6 H 4 .The high-frequency bands at 1562 and 1487 cm "1 are assigned to the C=C ring stretching vibrations of the benzenoid ring and the C-N stretching of the quinoid ring, respectively.The bands at 1302 and 1240 cm "1 correspond to the N- The remaining bands at 1107 and 798 cm -1 could be attributed to the in-plane and out-of plane C-H bending modes respectively.The C __ H out-of-plane bending mode has been used as a key to identify the type of substituted benzene.For polyaniline salt, this mode was observed as a single band at 798 cm -1 , which was almost nearer to the range 800-860 cm -1 reported for 1,4-substituted benzene. 16The IR spectra of POA similar to that of the PANI salt with the bands showing slight shifts to higher frequencies.A new band which is not present in PANI appears in the spectra of POA at 1167 cm -1 , which could be attributed to the -OCH 3 rocking mode.Approximate estimations of the copolymer compositions can be made by utilizing the intensity of this new band at around 1160 cm -1 .Thus, the infrared spectra of the copolymer salts help us to estimate roughly, the amounts of anisidine present in the copolymers.The spectral characteristics of the copolymers are similar to those of polyaniline and polyanisidine (Fig 1).The IR absorptions at 1487-1574 cm -1 are associated with aromatic ring stretching.The peak at 1574 cm -1 assigned to the quinoid ring and peak at 1487 cm -1 to the benzenoid ring and the intensity of the peak near 1160 cm -1 increases as the amount of o-anisidine in the feed increases.Our findings are consistent with the findings of Umare et al 11 .

X-ray diffraction studies
Figure 2 shows the X-ray diffraction patterns of homopolymers and copolymers.The PANI exhibits three broad peaks at 2è angles around 10.3 0 , 19.5° and 25.5°, 2θ=25.5° is characteristics of the vander Waals distances between stacks of phenylene rings (polyaniline ring). 15,17 hese broad peaks indicate crystalline domains in the amorphous structure of PANI .X-ray diffractograms of copolymers show broadening of peaks, which indicates copolymers are of amorphous nature while PANI is crystalline.It is observed that there is increase in d spacing and decrease in coherence length as the fraction of o-anisidine increases in the copolymer composition.On increasing the amount of o-anisidine in the copolymer the crystallite size decreases, which may be due to the presence of methoxy group on the aromatic ring which increases disorder and decreases crystal size.Thus, in copolymers the increased charge localization may be due to the reduction of interchain diffusion of charge, decrease of interchain band width which is caused by the large transverse unit cell length and decrease in coherence between the chains caused by greater disorder in interchain separation within crystalline region, due to the existence of the side group (-OCH3) along the main copolymer chain.

Morphological studies
SEM images of homopolymers PANI, POA and their copolymer salt (PAPOA11) are shown in Figure 3 (a-c) respectively.In Figure 3a, bundles of agglomerated nanorods of PANI with typical sizes around 100 nm to 200 nm observed.POA and copolymer salts exist as highly agglomerated globular particles with typical sizes around 100-500 nm.

Thermogravimetric analysis
Thermogravimetric analysis of PANI, POA and their copolymers were performed in an air, employing a heating rate of 10 0 C min -1 .The TGA curve for the PANI salt shows a three-step weight loss.The weight loss of 6% up to 150 0 C is due to the loss of moisture.The weight loss of 4.5% occurring up to 395 0 C is attributed to the loss of the dopant H 2 SO 4 .The final step starts at around 400 0 C, leads to the complete degradation of the polyaniline salt. 9Figure 4 shows the representative TGA trace of the copolymer PAPOA11.Thermograms of the POA and copolymer salts show similar three-step degradation, but at lower temperatures than that of PANI.

Conductivity measurements
The conductivities of the homopolymers and copolymers were measured by using the twoprobe method.Figure 5 shows the conductivity measurement of PANI, POA and PAPOA11.It was noticed that as the amount of anisidine increases in the copolymers, the conductivity decreases.The presence of even 0.25 M anisidine in the feed decreases the conductivity.However copolymers show moderately high conductivity when compared to that of POA, The higher conductivity of the copolymers could be explained as being due to the formation of block copolymers which facilitates faster charge transport by bipolarons. 10However at higher temperatures all polymers tend to have almost same conductivity.

CONCLUSION
In order to make soluble and processable PANI, substituted polyaniline homopolymers and copolymers were synthesized by chemical polymerization with easily available reagents which in addition assures a good reaction yield.The resulting product exhibits reasonable conductivity with excellent solution processing properties.However, conductivity of copolymers is lower compared to PANI but higher than that of POA.At higher temperatures, all polymers tend to have almost same conductivity.Morphology of PANI salt showed nanorods with average diameter of 100-200 nm size, whereas copolymers showed 100-500 nm size particles.This processable form of polyaniline salt and its copolymers could be widely applicable to coatings, to making thin films, preparation of clay composites and solution blending with other commodity polymers.Still much research work is necessary to improve the quality of the materials to make commercially viable products.