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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