Synthesis and Characterization of Soluble and Thermally Stable Polypyrrole-DBSA Salts.
Summary: In the present study an attempt has been made to synthesize soluble PPy via inverse emulsion polymerization pathway using benzoyl peroxide as an oxidant and dodecylbenzenesulfonic acid (DBSA) as dopant as well as a surfactant. A mixture of chloroform and 2 -butanol was used as dispersion medium for the first time. Pyrrole polymerized into PPy inside the micelle under these conditions. The influence of synthesis parameters such as amount of pyrrole benzoyl peroxide (BPO) and DBSA on the percent yield and other properties of the resulting PPy was studied. T he synthesized PPy was found to be completely soluble in DMSO DMF T HF chloroform and m-cresol etc. T he structure of PPy and incorporation of DBSA in the polymer backbone was confirmed by FT -IR. UV-Vis spectra of PPy showed correspondence with the electrical conductivity of the prepared salts. SEM micrographs showed the typical granular morphology of PPy-DBSA. T GA and XRD of the prepared PPy samples were also studied and explained.
Keywords: Conducting polymers; Soluble polypyrrole; FT-IR; TGA; SEM; XRD. Introduction Inherently conducting polymers (ICPs) with large p-conjugated system have attracted considera-ble interest due to their prospective wide applications in light emitting diodes (LEDs) chemical and bio-logical sensors photovoltaic cells rechargeable bat-teries electrochromic devices corrosion inhibition microactuators antielectrostatic coatings and func-tional membranes  etc. Polypyrrole (PPy) is one of the most familiar conducting polymers due to its fac-ile synthesis good electrical conductivity biocom-patibility and environmental stability . PPy can be synthesized easily either by chemical or electrochem-ical polymerization. A number of papers discuss the synthesis and properties of PPy films via electro-chemical polymerization. However very little is known about the chemical synthesis of PPy in ho-mogenous solution. Chemical polymerization pro-ceeds readily in the presence of various oxidants like K 2 S 2 O 8
FeCl 3 (NH 4 ) 2 S 2 O 8 and CuCl 2. There are considerable reports on the polymerization of pyrrole in different textile composites  and on printed circuit boards . The strong interchain interactions in PPy films make the polymer insoluble in common sol-vents rendering it highly improcessable thereby de-creasing its scope of applications. Many techniques have been used to deal with these intractable prob-lems. It has been reported that polymerization of sub-stituted pyrrole with long-chain alkyl and sulphonic groups leads to formation of polypyrrole derivatives soluble in some common organic solvents . How-ever a significant decrease in the electrical conduc-tivity of the polymer is observed in this case which is due to the decrease of p-conjugation along the poly-mer chain caused by an increase in torsion angle be-tween the pyrrole rings.
Moreover this type of polymerization is tedious and leads to low efficiency and high cost. Investigations have shown that PPy synthe-sized in colloidal dispersion in the presence of exter-nal stabilizer shows good solubility and exhibits unique physicochemical characteristics that can wide-ly be used in sensors biomedicines supercapacitors and carbon nanomaterial precursors . Colloidal dispersions can be prepared by emulsion microemul-sion and inverse emulsion techniques. Emulsion polymerization is applied where water soluble PPy is required . Microemulsion polymerization is used to prepare nanostructures of PPy. This method per-mits particles to transfer into the spherical aggregates by surfactant templates . Inverse emulsion polymerization is a recent technique used for the syn-thesis of high molecular weight and highly soluble polymer. This method improves the physical proper-ties of polymer such as solubility in common organic solvents stability and processibility .
The use of surfactants like dodecylenzenesulfonic acid (DBSA) in emulsion polymerization has greatly increased the solubility of polymers in polar/non- polar solvents . The use of surfactants in polymerization bath is believed to create a reactor vessel in the dispersion medium via micelle formation in which the mono-mer is confined in a limited environment and to im-prove the polymer's physical properties like solubili-ty in organic solvents conductivity stability and pro-cessibility . We have recently reported the syn-thesis of completely soluble and highly thermally stable polyaniline via a new inverse emulsion path-way . Inverse microemulsion route has been adopted for a number of PPy nanocomposites like PPy-Pd nanospheres  graphite/AgCl/PPy nanosheets  and Carbon Nanotube/ PPy CoreShell . There is no exclusive report available on the synthesis and characterization of
PPy using in-verse emulsion polymerization. In the present paper synthesis of PPy via inverse emulsion polymerization and further characterization of the synthesized mate-rials are reported. The solubility of the synthesized polymer was investigated in various common organic solvents. The synthesized polymer was found to be completely soluble in a so far large number of com-mon organic solvents. The effect of various reaction parameters on the percent yield intrinsic viscosity morphology structure and crystallinity of PPy has also been investigated. TGA data showed high ther-mal stability of the polymer up to ~ 450C.
Results and Discussion Percent Yield High percent yield of the product reflects the optimization of various parameters and the potential of the procedure applied. The present technique was found to be effective in this regard. The effect of amount of monomer and the oxidant (BPO) on the % yield of PPy was studied as given in Table-1 and shown in Fig. 1. It was found that high pyrrole con-centration leads to a high yield of the polymer. It in-dicates that large numbers of monomer radicals get the chance of polymerization inside the micelle and thus enough quantity of the polymer is produced
Table-1: Percent Yield of PPy. Solubility
S. No###Sample ID###Pyrrole Conc###B PO Conc###PPy % Yield
1###PPy 1###1.2 x 10 -3 mol###0.012 mol###52
2###PPy 2###1.5 x 10 -3 mol###0.012 mol###80
3###PPy 3###1.5 x 10 -3 mol###0.0012 mol###98.5
The presence of long chain conjugated struc-ture make conducting polymers insoluble in aque-ous/non-aqueous solvents. One of the main objectives of this study was to synthesize soluble PPy that can easily be processed in its various applications. The solubility of PPy was checked in different common organic solvents. PPy solutions were prepared in s e-lected solvents like DMSO DMF THF m-cresol chloroform and mixtures of other organic solvents. Homogenous solutions were obtained at ambient emperature that determine the complete s olubility of PPy in these solvents studied so for as given in
Table-2. Chemical structure of PPy determines its solubility in various solvents.
S. No###O rganic solvent###Minimum solubility (wt./vol %)
Also PPy morphology at molecular level has a strong influence on the kin-ematics of its dissolution. Higher diffusion rates and swelling power of the solvent molecules accelerate the solubility of PPy. A two-step process for the dis-solution of PPy has also been proposed. First swell-ing of the polymer below temperature corresponds to the gradual dispersion of the PPy side chains and then the complete dissolution above temperature corresponds to the gradual dispersion of the main chains at molecular level. These dispersions reflect the fact that the cohesion interactions among side chains or main chains of PPy are weakened by the solvent molecules. The effect of relatively polar sol-vents like DMSO THF DMF and m-cresol on the solubility is reasonably greater because of dipole-dipole interactions [Error! Bookmark not defined.]. Fig. 2 shows that the solubility of PPy is also influ-enced by the amount of DBSA.
As the DBSA amount increases the solubility of PPy also increases which is attributed to the fact that high concentration of the dopant in the dispersion medium hinders the cross-linking among PPy chains [Error! Bookmark not defined.]. It has been found that these polar solvents build up strong hydrogen bonds with PPy and as a consequence the polymer dissolves completely in these solvents. The polar oxygen of SO 3 group in DBSA also increases the solubility of PPy in polar solvents . Solubility of PPy in these solvents is also governed by the free energy of mixing. Equation
where G m is the free energy change on mixing H m is enthalpy change on mixing T is the absolute tem-perature and S m is the entropy change on mixing. A negative value of G m means that the mixing process occurs spontaneously. FTIR Analysis The FT-IR spectra of PPy 1 (a) PPy 2 (b) and PPy 3 (c) is given in Fig. 3 and respective as-signments are presented in Table-3. It shows the typi-cal characteristic peaks of PPy which are consistent with that reported in literature [17-19] which means that the prepared salt is that of PPy-DBSA. The peaks in PPy 3 are at a little higher frequency than those of PPy 1 and PPy 2. It is due to the degree of oxidation and length of conjugated chain that influence the doping level of the PPy which in turn affects the ratio of transmittance maxima . The characteristic vi-bration peaks of PPy-DBSA are in the range of 700-1800 cm -1 . The peaks at 1544-1550 cm -1 and 1460 cm -1 are attributed to C=C/CC and CN stretching vibrations in pyrrole rings respectively. The broad peaks at around 1160-1180 cm -1 are associated with the breathing vibration of the pyrrole ring in the PPy chain.
The peaks at 1304-1320 cm -1 correspond to N-H bending. The bands at 1030-1040 cm -1 are assigned to the in-plane deformation vibration of C-H and N-H while the out-of-plane deformation vibration band for C-H is located at about 906 cm -1 . The peaks at 920-960 cm -1 in all the three samples (PPy 1 PPy 2 and PPy 3) correspond to bipolaron bands. The broad peak at 1182 cm -1 is related to the S=O stretching vi-bration of SO 3- of the DBS -1 in PPy. It has been found that the skeletal vibrations of PPy that involve the p-electrons delocalization are influenced by the doping of PPy. The typical peaks at 2352-2364 cm -1 are considered to be due to aromatic and aliphatic C-H stretching vibration. It means that the IR peaks above 2000 cm -1 are dominated by the DBS units while the IR absorption below 2000 cm -1 is affected by the pyr-role units in the PPy-DBSA. Thus the FT-IR analysis confirms the formation of
PPy and the presence of pyrrole and DBSA in this synthesized material. The assignments given in Table-3 are in agreement with that given in literature
Table-3: Assignment of the bands in IR spectra of different PPy samples.
S. No###IR Frequency of absorption (cm -1)###Assignment
###PPy 1###PPy 2###PPy 3
8###1160###1165###1160###b Pyrrole ring
Morphology SEM was performed to analyze the mor-phology of PPy in various amounts of the monomer. The results are shown in Fig. 4. SEM analysis at higher magnification reveals the homogeneous distri-bution of DBSA in PPy chains. It is seen that the clusters and granular structure of PPy is sustained in all the three samples with varying concentration of the monomer . Fig. 4 (a) shows smooth and bead-ing surface morphology of PPy with obvious crakes. The uniform structure assists the arrangement of elec-trical conductive network within the PPy chain .
Fig. 4 (b) and (c) show that PPy has globular struc-ture due to the formation of aggregates which results in a uniform surface morphology known as cauli-flower" . It is also called broken eggshell" like morphology due to phase segregation. The porous na-ture of all these samples can be applied in adsorption and gas sensing technologies while the pellet (Fig 4 (a)) like behavior is best fit for the use in biosensors .
UV-Vis spectra of the PPy correspond well with the conductivity of the polymer. UV-Vis meas-urements of PPy indicate three possible states of PPy existing all at the same time. These states are neutral PPy polaron (radical cation) and bipolaron (dica-tion). UV-Vis spectra of PPy solutions prepared in dimethylsufoxide (DMSO) are given in Fig. 5. All the three spectra show three characteristic bands. Band (a) around 256-270 nm is assigned to pp transition of the benzenoid ring in the PPy chains. Band (b) around 345-370 nm is attributed to polaron transition. Band (c) in the range of 630-650 nm is as-sociated with bipolaron transition (np) in the large p conjugated system of the polymer chain . The last two bands are assigned to the doping level and introduction of polaron and bipolaron lattices which represent the protonation stages of PPy chain.
This behavior of the doped PPy causes an easier transition of electrons due to small energy gap and thus the conductivity of the polymer increases . Consider-ably; a free carrier-tail broad band at about 700 nm is due to the extended coil of PPy chains showing a high degree of conjugation and thus high degree of doping and conductivity. Band (c) shows the spec-trum of PPy 3 where the concentration of BPO was kept lower (1.2 x 10 -3 mol) as compared to PPy 1 and PPy 2 (BPO = 1.2 x 10 -2 mol). The change in the trend in PPy 3 spectrum as compared to other two samples is attributed to be due the frequent reduction of the BPO and hence the over oxidation of the pol-ymer chain is reduced. The degree of oxidation and length of conjugated chain influence the doping level of the PPy that in turn affects the ratio of two absorp-tion maxima .
The TGA characteristic of PPy was studied from room temperature to 650C under N 2 (100cc/min) atmosphere. All the PPy samples were observed to exhibit three distinct weight losses as shown in Fig. 6. The first stage of weight loss (~5%) at about 80-100C is associated with the evaporation of solvents moisture and oligomers as well as un-reacted monomers elimination. At further high tem-perature (170C) a weight of about 35-40% occurs due to the loss of DBS - component of the PPy. The drop in weight (~55-60%) observed at 200-400C is due to the degradation of the PPy itself [Error! Bookmark not defined.]. PPy samples are thermally stable in the temperature range of 25-400C and be-yond this range; the decomposition route becomes very rapid. The residue weight of the PPy is about 40% in the nitrogen atmosphere which is the carbor-ized form (graphite structures) of the polymer.
This indicates that PPy does not completely decompose in N 2 even at high temperature
XRD analysis XRD pattern indicates that the synthesized PPy powders exhibit a broad peak 2 between 20-30 which indicates some crystalline order in the bulk polymer powders. The two relatively broad peaks at around 21 and 23 illustrate the presence of crystal-line domain in the amorphous PPy . The appear-ance of these peaks in all the three PPy samples (Fig. 7) is caused due to the long DBSA alkyl chains that are non-uniformly spaced in PPy polymer main chains . Also the presence of DBSA causes con-formational change to an ordered layer structure of PPy. A dramatic change in the crystallinity of PPy doped with DBSA depends on whether the polymer is cast from chloroform or m-cresol. PPy cast from chloroform are disordered as in the present investiga-tion . Sharp peaks at low angle (2 = 10-20) are assigned to the highly amphiphilic and aromatic sul-phonate dopants (DBSA) and high angle 2 is associ-ated with the
amorphous PPy chains. Sharps peaks show that the DBSA has successfully integrated in the polymer chain and gives a great degree of order-ing to the PPy salts. The diffracting plane spacing (d) average chain separation (S) were calculated using the modified forms (2) and (3) of Bragg's equation and average crystallite size (t) by using Scherrer equation (4)  as given below: Formula
The comparative data obtained from these relations for the three PPy samples is summarized in Table-4. The results are in agreement with that re-ported in literature. These results confirmed the s em-icrystalline nature of PPy-DBSA salts.
Table-4: Parameters from XRD data.
###Spacing`d' (A )###(A )###`t' (nm)
Pyrrole (Fluka Chemie) was distilled under vacuum. DBSA (Sigma Aldrich) Chlorofom (Shar-lau) 2-butanol (Aldrich) Acetone (Scharlau) DMSO (Acros) THF (Sharlau) m-cresol (BDH Chemicals) and DMF (Acros) were used as received. The emulsion polymerization was carried out in a 200 mL round bottom flask by taking 0.43 mol of chloroform in the flask. Then 0.012 mol of benzoyl peroxide was added to it under slow mechan-ical stirring. A white colour solution appeared within two minutes. Then 0.107 mol of 2-butanol 3.9 x 10 -3 mol of DBSA and 1.2 x 10 -3 mol of pyrrole were add-ed to the above solution. After adding 0.27 mol of distilled water to the above mixture a milky white viscous emulsion was formed. The mixture turned brown in about thirty minutes and it was allowed to proceed for 24 hours upon vigorous stirring. At the end a black organic layer containing polypyrrole (PPy) was separated through a separating funnel.
Then the PPy was washed three times with 20 mL of acetone and distilled water subsequently. PPy re-mained on the filter paper in the form of a thin layer which was allowed to dry for 12 hours at 70C in an electric oven. As a result black powder of PPy was obtained. Similarly other samples like PPy 2 and PPy 3 were synthesized with varying amounts of pyr-role (1.2 x 10 -3 and 1.5 x 10 -3 mol) and BPO (0.012 and 0.0012 mol) respectively. FTIR analysis was carried out by using Shimadzu (IR Prestige-21) spectrometer (Japan). The FTIR spectrum of PPy was obtained by preparing a thin potassium bromide (KBr) pellet containing the sample. UV-Vis spectroscopy was done using Perkin Elmer UV-Visible spectrophotometer (USA). The XRD patterns of PPy were studied by JDX-3532 (JOEL Japan) X-ray diffractometer having a fixed ra-diation wavelength of =1.54A. Thermal gravimetric analysis was carried out using Diamond TG/DTA (Perkin Elmer USA) analyzer. Scanning Electron Mi-croscope Model JSM-5910 (JOEL Japan) was used for the analysis of surface morphology of PPy. Conclusions Soluble PPy was successfully synthesized via new inverse emulsion polymerization pathway. The formation of PPy was confirmed by FTIR. The data indicate incorporation of DBSA in the polymer backbone. TGA shows high thermal stability of the synthesized PPy up to ~ 450 C. SEM and XRD analysis confirmed surface morphology and semi-crystalline nature of PPy respectively. The solubility of PPy in a variety of solvents and hence its process i-bility make it relatively good material for studying hitherto unavailable properties of the doped PPy.
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|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Dec 31, 2014|
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