Synthesis and characterization of poly(3-thiophene acetic acid)

Abstract. Poly(3-thiophene acetic acid) (P3TAA) was synthesized using chemical procedures. The structure, surface and morphology of these water-soluble polythiophenes were determined by introducing pH-responsive carboxylic acid groups at the 3-position of thiophene ring and doing FT-IR spectra, SEM and TGA analyses. Based on the neutralization reaction obtained when using sodium hydroxide at various concentrations, we suggest that the local conformational changes that occur from the aggregated state to the extended state of the polymer main chain of P3TAA can be attributed to the pH change in the solution. The observed results are explained in terms of the rigid hydrophobic main chain, the interaction between hydrophilic charged groups and the nature of the conjugated polymer.

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JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2014, Vol. 59, No. 9, pp. 17-24 This paper is available online at SYNTHESIS AND CHARACTERIZATION OF POLY(3-THIOPHENE ACETIC ACID) Nguyen Ngoc Linh1, Dang Thi Hoa1, Tran Thi Thuy Duong1, Jiri Pfleger2 and Vu Quoc Trung1 1Faculty of Chemistry, Hanoi National University of Education 2Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic Abstract. Poly(3-thiophene acetic acid) (P3TAA) was synthesized using chemical procedures. The structure, surface and morphology of these water-soluble polythiophenes were determined by introducing pH-responsive carboxylic acid groups at the 3-position of thiophene ring and doing FT-IR spectra, SEM and TGA analyses. Based on the neutralization reaction obtained when using sodium hydroxide at various concentrations, we suggest that the local conformational changes that occur from the aggregated state to the extended state of the polymer main chain of P3TAA can be attributed to the pH change in the solution. The observed results are explained in terms of the rigid hydrophobic main chain, the interaction between hydrophilic charged groups and the nature of the conjugated polymer. Keywords: Poly(3-thiophene acetic acid), polyelectrolyte, conjugated polymer, water-soluble polythiophene. 1. Introduction Conjugated polyelectrolytes (CPEs) are polymers that feature hydrophobic -conjugated backbones and hydrophilic ionic side groups [8, 13]. They show unique optical and electrical properties together with good solubility in water making it possible to process these polymers from aqueous solution. Because CPEs exhibit both electronic and ionic conductivity, they can be used as active materials in the development of electrochromic devices and electronic devices, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic solar cells [5], and also as new solid polyelectrolytes with increased room-temperature electrical conductivity [2, 16]. Another possible CPE field of application is use as water-soluble sensing agents for the detection of DNA, protein, small bioanalytes, metal ions and surfactants due to the changes in UV-Vis and photoluminescence spectra that are induced by conformational Received October 13, 2014. Accepted November 12, 2014. Contact Vu Quoc Trung, e-mail address: trungvq@hnue.edu.vn 17 Nguyen Ngoc Linh, Dang Thi Hoa, Tran Thi Thuy Duong, Jiri Pfleger and Vu Quoc Trung changes of CPE macromolecules caused by their complexion with oppositely charged analyte species [1, 6, 7, 11, 15]. During the last two decades, water-soluble polythiophenes (PTs) and their derivatives are particularly important CPEs due to a unique combination of high conductivity, environmental stability and structural versatility allowing a derivatization of the -conjugated backbone in view of numerous technological applications. A lot of work in this field has been done on anionic CPEs with polythiophene main chains carrying short side alkyl chains with the first report of CPE published by Wudl, Heeger and coworkers in 1987 [12]. This group reported the synthesis and characterization of the conductivity of films of PTs substituted with ethyl sunfonate and butyl sulfonate side chains. McCullough et al. recently showed that regioregular CPEs with a propionic acid at the 3-position of a thiophene ring can switch from purple to yellow phases by increasing the amount of NH4OH in aqueous solution [4, 14]. In addition, they reported a cation size-dependent chromaticity due to a sterically induced disruption of the aggregated phase. In the present communication, we present a synthesis using a chemical oxidative coupling polymerization reaction and the properties of polythiophene-based anionic polyelectrolyte: poly(3-thiophene acetic acid) (P3TAA). Based on the neutralization reaction using sodium hydroxide at various concentrations to convert the carboxylic acid group in P3TAA, the properties and structure of new solutions were studied. It was found that the local conformational changes of P3TAA depend on pH changes and can be associated with the rigid conjugated structure of the main chain and interactions between charged groups. 2. Content 2.1. Experiment * Materials 3-thiopheneacetic acid (3TAA), anhydrous iron(III) chloride (98%) (FeCl3) and sodium hydroxide (NaOH) were all used as received (obtained from Merck), without further purification. Chloroform (CHCl3) and methanol (CH3OH) were dried overnight under a N2 atmosphere and distilled prior to use. * Synthesis - Synthesis of poly(3-thiophene acetic acid) (P3TAA) The monomer 3TAA was polymerized by chemical oxidative coupling reaction in dry chloroform using anhydrous iron(III) chloride. First, 2 g of 3TAA was dissolved in 30 mL of anhydrous chloroform under a nitrogen atmosphere in a 50 mL round-bottom flask with a condenser. In a 100 mL round-bottom flask with condenser, 5 g of iron(III) chloride was dissolved in about 30 mL of dry chloroform in a nitrogen atmosphere to which a solution of monomer was added dropwise. The molar ratio of oxidant to monomer was 4:1 in all cases. The mixture was stirred for 2 days at room temperature under nitrogen in the dark. The obtained black product was purified by washing with fresh methanol 18 Synthesis and characterization of poly(3-thiophene acetic acid) and deionized water to remove the iron(III) chloride and oligomers, then washed for two days using a Soxhlet extraction with 200 mL of methanol to eliminate the unexpected low molecular weight materials. Finally, the P3TAA was carefully washed repeatedly with methanol and vacuum-dried for 2 days to get the dark red-colored powder of the polymer. - Synthesis of poly(3-thiophenene sodium acetate) P3TSA 35 mg of P3TAA and NaOH solution (with a molar ratio of 1:0.5) were dissolved in 15 mL of distilled water under stirring. The suspension was then refluxed at about 60 ◦C for 2 days. The suspension was separated from the aqueous solution and dried in an oven overnight at 50 ◦C. This P3TSA is denoted by N1. The synthesis process is briefly summarized in Scheme 1. The other neutralization reactions were carried out in a manner identical to that of the N1 sample with the molar ratio of P3TAA and NaOH being: N2 (1:0.75), N3 (1:1) and N4 (1:1.25). Scheme 1. Synthesis process of P3TAA and P3TSA * Instrumentation The thermal stability was determined by thermogravimetric analysis (TGA, Shimadzu Simultaneous Measuring Instrument, DTG-60/60H) at the Department of Organic Chemistry, Hanoi National University of Education. The TGA thermograms were recorded at a heating rate of 10 ◦C/min in the temperature range of 30 - 880 ◦C under a nitrogen atmosphere. Scanning Electron Microscope (SEM) analysis was performed using a Hitachi S-4800 Field Emission SEM at the Faculty of Physics, Vietnam National University, Hanoi. Fourier transform infrared (FT-IR) spectra of the samples were recorded with a Nicolet Impact 410 FTIR Spectrometer at the Institute of Chemistry, Vietnam Academy of Science and Technology. 2.2. Results and discussion 2.2.1. FT-IR analysis Figure 1 shows the FT-IR spectrum of the P3TAA neutral polymer and P3TSA at various pH. The spectrum has the absorption band at 3090 - 3000 cm−1 assignable to the stretching vibration of the C-H bond on the thiophene ring. Occurring in the same region, the most characteristic presence of a stretching in the 3500 - 3100 cm−1 region due to the O-H bond indicates a strong hydrogen bonding of the dimers. The C=O stretching 19 Nguyen Ngoc Linh, Dang Thi Hoa, Tran Thi Thuy Duong, Jiri Pfleger and Vu Quoc Trung vibrations at 1732 - 1727 cm−1 are assigned to the aliphatic C-H bonds at 3000 - 2850 cm−1 and the thiophene ring bonds at 1400 cm−1. After the neutralization reaction, the absence of a stronger and wider broad stretching in the 3500 - 3000 cm−1 region due to the O-H bonds indicates clearly the good conversion of the carboxylic acid groups of P3TAA to carboxylate groups of P3TSA. Figure 1. FT-IR spectrum of P3TAA and P3TSA at various pH Table 1. FT-IR spectrum of P3TAA and P3TSA Materials OH (cm−1) C=O (cm−1) C=C (cm−1) CH (saturated) (cm−1) P3TAA 3443 1730 1630 2950 P3TSA - N3 3442 1727 1629 2960 2.2.2. SEM analysis Figure 2. SEM micrographs of P3TAA and P3TSA 20 Synthesis and characterization of poly(3-thiophene acetic acid) The morphology and size distribution of P3TAA and P3TSA polymers analyzed using SEM are presented in Figure 2. The polymer morphology is affected by regiochemistry. With both polymers, the morphology is amorphous indicating clearly the regioirregular structure of the polymer chain synthesized by the chemical oxidation polymerization reaction. P3TAA of smaller size, more uniform distribution and tight links between the particles can be attributed to the hydrogen bond of the carboxylic acid group. However, P3TSA seems to be more hydrophilic which could be an advantage in industrial fabrication. 2.2.3. TGA analysis (a) (b) Figure 3. TGA and DTA thermograms of P3TAA (a) and P3TSA (b) TGA and DTA analysis data for P3TAA are summarized in Figure 3 and Table 2. The approximately 14% initial weight loss observed up to 200 ◦C in the P3TAA polymer sample is possibly due to the evaporation of absorbed water on the hygroscopic polymer. P3TAA is quite stable until about 200 ◦C. From 200 ◦C to 480 ◦C, 84% of the initial weight of the polymer was lost. After the decomposition of all polymers at about 500 ◦C, the 13% remaining weight of the sample could consist of residual catalyst. Because of the TGA curve with a few conversion steps and the low remaining weight, the polymer had no more monomers and was almost completely clean. Table 2. Thermal analysis data of P3TAA and P3TSA P3TAA Temperature  200 ◦C 200 - 480 ◦C Remaining weight % ∆m 14.8 84.4 13.3 P3TSA Temperature  600 ◦C 600 - 880 ◦C Remaining weight %∆m 44.7 69.0 17.1 DTA analysis of P3TAA also indicates the decomposition of the polymer starting at a temperature above 200 ◦C, with the most decomposition occurring between 270 ◦C and 370 ◦C (which corresponds to the observed nonlinear weight loss which starts at around 21 Nguyen Ngoc Linh, Dang Thi Hoa, Tran Thi Thuy Duong, Jiri Pfleger and Vu Quoc Trung 250 ◦C in TGA). The peak of the DTA curve at 306 ◦C shows the neutral state of the polymer with the strongest exothermic. After a few conversion steps in the TGA line, the P3TSA polymer no longer consisted of monomers. Around 45% of the weight loss observed up to 600 ◦C is possibly due to the evaporation of the absorbed water and the initiation of decomposition of the polymer chain. The weight loss of P3TSA was strongest from 600 ◦C to 880 ◦C. With both the TGA and DTA curves, a decomposition of more than 25% of the weight of the sample was observed at 640 ◦C. Compared with previous studies of polythiophene, the P3TAA polymer has a higher decomposition temperature due to the stable bridge structures of the main chain formed by the hydrogen bonds between the thiophene rings. In addition, owing to the changing carboxylic acid group on the side chain of the carboxylate group in the neutralization reaction, the P3TSA polymer has a higher decomposition temperature than P3TAA polymer when in the acid form. 2.2.4. The local conformation of P3TAA depended on pH According to Katchalsky and Spitnik [15] and Leyte and Mande [16], the pH of many polyelectrolytes is well-expressed by the extended Henderson-Hasselbach equation over a wide dissociation range: pH = pKa+ nlog 1 where pKa is the intrinsic dissociation constant and is the degree of dissociation. The n parameter indicates the extent of interaction between the neighboring ionized groups of polyelectrolytes. When the main chain is not very hydrophobic, it corresponds to the electrostatic repulsion between charged groups. In this P3TAA, based on the Henderson-Hasselbach equation and Figure 4 showing the relationship between the amount of NaOH and the pH of polymer solutions, the pH values (from N1 to N3) varied from 10.65 to 11.38, directly proportional to the moles 22 Synthesis and characterization of poly(3-thiophene acetic acid) of NaOH added. This means that the neutralization reaction of P3TAA with NaOH is almost complete when the molar ratio of each reactant is 1:1. The increase in pH could be connected with the presence of the rigid coplanar structure of the main chain which might stabilize the ionized carboxylate and un-ionized carboxylic acid groups, for example, with a structure as shown in Figure 5. To prove this experimentally, IR spectra of P3TSA polymer solutions were taken at various pH and compared with P3TAA.With the increase of the amount of NaOH in aqueous solution to increase the amount of ionized carboxylate groups, stronger broad peaks in the 3500 - 3000 cm−1 region of hydrogen bonds can be seen as the evidence of the formation of the bridge structures. However, the increase in pH might also to be connected with an increased difficulty in dissociation on the rigid main chain of the thiophene ring and not simply that of the electrostatic repulsion of carboxylic groups. When the molar ratio of P3TAA and NaOH solution (N4) is 1:1.25, pH values increase more slowly and do not represent the linearity corresponding to the growth rate of the initial pH. It suggests that, at that time, the increase in pH depends only on the amount of NaOH that no longer depends on the reaction of carboxylic acid group in the polymer solution. At present, we attempt to explain only the initial conformation and properties of P3TAA which depend on pH. However, the pH changes can certainly be investigated to determine their association with the local conformational changes of P3TAA. 3. Conclusion We have successfully synthesized P3TAA using the chemical oxidative coupling reaction. An amorphous morphology affected by the irregularity structure of the main polymer chain was confirmed by SEM. The conjugated structure and properties of P3TAA after neutralization reactions using different amounts of NaOH in aqueous solution was suggested to be due to the ionized carboxylate group on the new polymer solution. However, we explain only the local conformation and properties of P3TAA that depend on pH. The increase in decomposition temperature can be explained by the FT-IR results, suggesting stable bridge structures of the main chain formed by hydrogen bonds between thiophene rings and the interaction between charged groups. 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