Synthesis and characterization of a new soluble conducting polymer
and its electrochromic device
Serhat Varis, Metin Ak, Cihangir Tanyeli, Idris Mecidoglu Akhmedov, Levent Toppare
∗Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received 16 February 2006; received in revised form 22 June 2006; accepted 27 July 2006 Available online 23 October 2006
Abstract
A mixture of isomers 2,5-di(4-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole, 2-(4-methyl-thiophen-2-yl)-5-(3-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole and 2,5-di(3-methyl-thiophen-2-yl)-2-(4-methyl-thiophen-2-yl)-5-(3-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole (Me-SNS(NO2)) were synthesized.
Result-ing monomers were polymerized chemically, producResult-ing soluble polymers in common organic solvents. The average molecular weight has been determined by gel permeation chromatography (GPC) as Mn= 5.6 × 103for the chemically synthesized polymer. The monomers were also electrochemically polymerized in the presence of LiClO4, NaClO4(1:1) as the supporting electrolyte in acetonitrile solvent. Resulting polymers
were characterized via CV, FTIR, NMR, SEM and UV–Vis spectroscopy. Spectroelectrochemistry analysis of polymer revealed –∗ transi-tion below 300 nm, with an electronic band gap of 2.18 ev. Switching ability of the polymer was evaluated by kinetic study measuring percent transmittance (%T ) at the maximum contrast point, indicating that poly(Me-SNS(NO2)) is a suitable material for electrochromic devices. ©2006 Elsevier Masson SAS. All rights reserved.
Keywords: Electrochemical polymerization; Conducting polymers; Electrochromic properties
1. Introduction
Polymers of thiophene and pyrrole based compounds are environmentally and thermally stable materials; therefore they can be used as non-linear optical devices [1], polymer light emitting diodes [2], gas sensors [3], organic transistors [4], and electrochromic devices[5]. Electrochromism is a phenom-enon where a material shows reversible and visible change in color that is associated with an electrochemically induced oxidation–reduction reaction[6].
Design and synthesis of new conjugated polymers are sig-nificant part of the conducting polymer research[7]and has at-tracted great attention since 1977[8]. Among conducting poly-mers, polythiophene and its derivatives have become a sub-ject of considerable interest as electrochromic materials due to their chemical stability, ease of synthesis and structural versa-tility[9].
* Corresponding author. Tel.: +903122105148; fax: +903122101280. E-mail address:toppare@metu.edu.tr(L. Toppare).
One of the most important characteristics exhibited by the polymers from the polythiophene family is their enhanced electrochromism upon application of a small voltage. A vari-ety of conjugated polymers have colors both in the oxidized and reduced states, since the band gap is in the visible re-gion [10]. After oxidation, the intensity of the –∗ tran-sition decreases, and two low energy trantran-sitions emerge to produce a second color. Therefore, there are many absorption changes in the visible region of the spectrum, making them use-ful in the construction of electrochromic devices[11].
Insolubility is the major limitation of conducting polymers. Synthesis of soluble conducting polymers enables direct appli-cation of these polymers to any substrate using conventional printing techniques[12].
In the present work, new isomers were synthesized. Both chemical and electrochemical polymerization of Me-SNS(NO2)
were studied. We investigated the optoelectrochemistry and measured the L, a, b values of the polymer coated on ITO via constant potential electrolysis by using LiClO4(0.1 M) and
NaClO4 (0.1 M) as the supporting electrolyte. The resultant
conducting polymer was characterized by the CV, FTIR, NMR, SEM, and conductivity measurements.
1293-2558/$ – see front matter © 2006 Elsevier Masson SAS. All rights reserved.
In recent years there has been a growing interest in applica-tion of conducting polymers in electrochromic devices. Thus, in the second part of the study dual type electrochromic device was constructed with poly(Me-SNS(NO2)) (anodically
color-ing polymer) and poly-3,4-ethylenedioxythiophene (PEDOT) (cathodically coloring polymer).
2. Experimental 2.1. Materials
AlCl3 (Aldrich), 3-methylthiophene (Aldrich), succinyl
chloride (Aldrich), dichloromethane (DCM) (Merck), p-toluene sulfonic acid (PTSA) (Sigma), 4-nitroaniline (Sigma), toluene (Sigma), nitromethane (Aldrich), methanol (Merck), ferric(III) chloride (Aldrich), acetonitrile (AN) (Merck), NaOH (Merck), LiClO4(Aldrich), NaClO4(Aldrich), propylene carbonate (PC)
(Aldrich) and poly(methyl methacrylate) (PMMA) (Aldrich) were used without further purification. 3,4-ethylenedioxythio-phene (EDOT) (Aldrich) were used as received.
2.2. Equipments
NMR spectra of the monomers were recorded on a Bruker-Instrument-NMR Spectrometer (DPX-400) by using CDCl3
as the solvent. The FTIR spectrum was recorded on a Nico-let 510 FTIR spectrometer. Mn was measured by gel per-meation chromatography (PL220). The surface morphologies of the polymer films were analyzed by using JEOL JSM-6400 scanning electron microscope. Solartron 1285 potentio-stat/galvanostat was used to supply a constant potential dur-ing electrochemical synthesis and cyclic voltammetry experi-ments. Varian Cary 5000 UV–Vis spectrophotometer was used in order to perform the spectroelectrochemical studies of poly-mer and the characterization of the devices. Colorimetry mea-surements were done via Minolta CS-100 spectrophotome-ter.
2.3. Synthesis of monomers Me-SNS(NO2)
The starting material, 1,4-di(3-methyl-2-thienyl)-1,4-butane-dione, 1-(3-methyl-2-thienyl)-4-(4-methyl-2-thienyl) butane-dione and 1,4-di(4-methyl-2-thienyl)-1,4-butanebutane-dione were
synthesized according to procedure in literature[13]. To a sus-pension of AlCl3(16 g, 0.12 mol) in CH2Cl2(15 ml), a solution
of 3-methylthiophene (9.61 ml, 0.12 mol) and succinyl chlo-ride (5.51 ml, 0.05 mol) in CH2Cl2were added dropwise. The
red mixture was stirred at room temperature for 4 h. This was then poured into ice and concentrated HCl (5 ml) mixture. The dark colored organic phase was washed with concentrated NaHCO3(3× 25 ml) and brine, and then dried over MgSO4.
Evaporation of the solvent yielded yellowish solid, which was suspended in ethanol. Filtration and washing with ethanol af-forded the desired compound.
The 2,5-di(4-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole, 2-(4-methyl-thiophen-2-yl)-5-(3-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole and 2,5-di(3-methyl-thio-phen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole (Me-SNS(NO2))
iso-mer monoiso-mers were synthesized from 1,4-di(3-methyl-2-thienyl)-1,4-butanedione, 1-(3-methyl-2-thienyl)-4-(4 -methyl-2-thienyl) butanedione, 1,4-di(4-methyl-2-thienyl)-1,4-butane-dione and 4-nitroaniline in the presence of catalytical amount of p-toluene-sulphonic acid (PTSA) [13]. A round-bottomed flask equipped with an argon inlet and magnetic stirrer was charged with the 1,4-di(3-methyl-2-thienyl)-1,4-butanedione, 1-(3-methyl-2-thienyl)-4-(4-methyl-2-thienyl) butanedione, 1,4-di(4-methyl-2-thienyl)-1,4-butanedione isomers (5 mmol, 1.25 g), 0.97 g (7 mmol) 4-nitroaniline, 0.1 g (0.58 mmol) PTSA and 20 ml of toluene were added. The resultant mixture was stirred and refluxed for 24 h under argon. Evaporation of the toluene, followed by flash column chromatography (SiO2
column, elution with dichloromethane:hexane (1:1)) afforded the desired compounds as pale brown powder. The isomers could not be separated into components no matter the type of column or solvent were used. The synthetic route of the monomer is shown inScheme 1.
2.4. Chemical polymerization of Me-SNS(NO2) with iron(III) chloride
A typical chemical polymerization of Me-SNS(NO2) was
fulfilled using iron(III) chloride as the oxidant. To carry out the oxidative polymerization, Me-SNS(NO2) (1×10−3M) was
dissolved under a blanket of N2in nitromethane (15 mL). A
so-lution of iron(III) chloride (2×10−3M) in nitromethane (5 mL)
was dropwise added to the monomer solution. The reaction was
carried out for 5 min with constant stirring. The dark blue oxi-dized polymer was first washed with methanol, filtered, com-pensated with 30% NaOH, and dried under vacuum for 1H
NMR analyses.
2.5. Cyclic voltammetry (CV)
Investigation of electroactivities of monomers is possible with cyclic voltammetry, consisting of a potentiostat, an XY recorder and a CV cell containing Pt working electrode, plat-inum wire counter electrode and Ag/Ag+ reference electrode. The measurements were carried out at room temperature under nitrogen atmosphere. NaClO4(0.1 M) and LiClO4(0.1 M)/AN
solvent-electrolyte couple was used.
2.6. Electrochemical polymerization of Me-SNS(NO2)
Electrochemical polymerization of Me-SNS(NO2) was
per-formed by sweeping the potential between 0.0 and 1.1 V with 500 mV/s scan rate, in the presence of 50 mg Me-SNS(NO2)
in NaClO4(0.1 M) and LiClO4(0.1 M)/AN electrolyte-solvent
couple. The working and counter electrodes were Pt wire and the reference electrode was Ag/Ag+ (a silver wire 1 mm thick, 5 cm long is dipped into the cell). Poly(Me-SNS(NO2))
was washed with AN in order to remove excess dopant ions and unreacted monomer after the potentiodynamic electro-chemical polymerization. Analogous method was employed to synthesize the polymers on an ITO coated glass plate (indium tin oxide coated glass, Delta Technologies, Min-nesota, USA).
2.7. Preparation of the gel electrolyte
Gel electrolyte was prepared by using NaClO4:LiClO4:
AN:PMMA:PC in the ratio of 1.5:1.5:70:7:20 by weight. Af-ter dissolving NaClO4/LiClO4 in AN, PMMA was added into
the solution. In order to dissolve PMMA, vigorous stirring and heating was needed. Propylene carbonate (PC), as a plasticizer, was introduced to the reaction medium when all of the PMMA was completely dissolved. The mixture was stirred and heated until the highly conducting transparent gel was obtained[14].
3. Results and discussion
3.1. NMR spectra of Me-SNS(NO2) and poly(Me-SNS(NO2))
1H NMR spectra of monomers and polymers were
inves-tigated on Bruker-Instrument-NMR Spectrometer (DPX-400) monomers with CDCl3as the solvent and chemical shifts (δ)
are given relative to tetramethylsilane as the internal standard. The isomers could not be separated into components no matter the type of column or solvent were used.
Pale yellow powder; mp 155◦C; 1H NMR spectrum of monomers (Fig. 1): δH (CDCl3): 1.83, 1.84, 2.03, 2.04 ppm (s, aliphatic methyl protons), 6.33, 6.34 ppm (s, pyrrolyl), 6.40 ppm (d, J = 5.4 Hz, pyrrolyl), 6.44 ppm (d, J= 3.5 Hz, pyrrolyl), 6.60 ppm (s, 3-thienyl), 6.67 ppm (d, J= 5.0 Hz, 4-thienyl), 6.64 ppm (s, 5-thienyl), 7.04 (d, J= 5.2 Hz, 5-thienyl), 7.06 (d, J = 8.9 Hz, o-phenyl), 7.94 (d, J= 8.9 Hz, m-phenyl), 7.31 (d, J = 1.8 Hz, o-phenyl), 8.15 (d, J= 2.0 Hz, m-phenyl), 7.17 (d, J = 7.4 Hz, o-phenyl), 8.03 (d, J = 2.0 Hz, m-phenyl).
1H NMR spectrum of polymer (Fig. 2): C
18H13NS2, δH
(CDCl3): 1.84 (s, aliphatic methyl), 1.83 (s, aliphatic methyl),
2.03 (s, aliphatic methyl), 2.04 (s, aliphatic methyl), 6.20–6.50 (broad m, pyrrolyl-H ), 6.50–7.10 (broad m, thienyl-H ), 7.10– 8.30 (broad m, o- and m-phenyl-H ). Broadening of the peaks and the decrease in the intensity of thienyl peaks confirms the polymerization.
GPC data revealed Mn= 5.6 × 103for Me-SNS(NO2)
pre-pared via chemical oxidation.
3.2. FTIR spectra
FTIR spectrum of the Me-SNS(NO2) shows the
follow-ing absorption peaks: 3093 cm−1 (aromatic C–H stretch-ing), 1513 cm−1(asymmetric ArNO2 stretching), 1338 cm−1
(symmetric ArNO2 stretching), 840 cm−1 (C–N stretching
for Ar–NO2), 3020 cm−1 (C–Hα stretching of thiophene),
1494–1340 cm−1(aromatic C=C, C–N stretching due to pyr-role and benzene), 1035 cm−1 (C–H in plane bending of benzene), 773 cm−1 (C–Hα out of plane bending of
thio-phene).
Most of the characteristic peaks of the monomer remained unperturbed upon chemical polymerization. The intensity ab-sorption bands of the monomer at 3020 cm−1 and 774 cm−1 arising from C–Hα stretching of thiophene moiety, disappeared
completely. This is an evidence of the polymerization from 2,5 positions of thiophene moiety of the monomer. Whereas, two new bands related to C–Hβ out-of-plane bending of 2,5
disub-stituted thiophene and C–S stretching appeared at 779 and 632 cm−1, respectively. The broad band observed at around 1649 cm−1proves the presence of polyconjugation and the new peak at 696 cm−1indicates the presence of the dopant ion (Cl−).
FTIR spectra of electrochemically synthesized poly(Me-SNS(NO2)) showed the characteristic peaks of the monomer.
The peaks related to C–Hαstretching of thiophene disappeared
completely. The new broad band at around 1640 cm−1 was due to polyconjugation. The strong absorption peak at 1113, 1087 and 620 cm−1were attributed to the incorporation ClO4−
ions into the polymer film during doping process. Results of the FTIR studies clearly indicated the polymerization of the monomer.
3.3. Cyclic voltammetry
When redox behavior of Me-SNS(NO2) was investigated
via cyclic voltammetry, an electrochromism between yellow and blue colors was observed, while a greenish cloud was formed around the electrode due to the partial dissolution of linear oligomers. First run of the cyclic voltammogram of Me-SNS(NO2) in AN showed two oxidation peaks at +0.55
and+0.76 V and a reduction peak at +0.28 V. After subsequent runs electroactivity increases with increasing scan number. The
Fig. 1.1H NMR spectrum of monomer.
Fig. 2.1H NMR spectrum of chemically prepared poly(Me-SNS(NO2)).
peak at 0.76 V decreases as a result of monomer consumption in the diffusion layer (Fig. 3).
3.4. Conductivities of the films
The conductivities of electrochemically and chemically pre-pared poly(Me-SNS(NO2)) were measured as 2.2× 10−4and
4.1× 10−5S cm−1respectively via four probe technique, prov-ing that the polymers are semiconductors.
3.5. Scanning Electron Microscopy (SEM)
Surface morphology of polymer was examined by em-ploying Scanning Electron Microscope. SEM micrograph of poly(Me-SNS(NO2)) designates the excellence of synthesized
monomer in film formation ability. The film also exhibited ho-mogeneous and compact structure (Fig. 4).
3.6. Electrochromic properties of conducting polymer
Spectroelectrochemistry is a technique that examines the changes in optical properties of conducting polymers upon volt-age change. It also provides useful data about the electronic structure of the polymer such as band gap (Eg) and the
inter-gap states that appear upon doping. Poly(Me-SNS(NO2)) film
was potentiodynamically synthesized on ITO electrode in the presence of 0.01 M Me-SNS(NO2).
The spectroelectrochemistry studies of the polymer were studied by applying potentials ranging between 0.2 and 1.0 V in monomer free AN/NaClO4/LiClO4(0.1 M) medium.
The wavelength at which polymer shows π –π∗transition is defined as maximum wavelength (λmax). This was determined
to be less than 300 nm. The band gap (Eg) was calculated as
Fig. 3. Cyclic voltammogram of Me-SNS(NO2) in ACN/LiClO4/NaClO4.
Fig. 4. SEM micrograph of the poly(Me-SNS(NO2)).
Fig. 5. Spectroelectrochemistry of the poly(Me-(SNS(NO2)).
Table 1
Colorimetric properties of polymer and device
Material Applied potential Color L a b Poly(Me-SNS(NO2)) −0.4 yellow 73 −13 35 0.5 green 53 −6 17 0.65 gray 44 −2 2 1.1 blue 30 29 −20 Poly(Me-SNS(NO2))/ PEDOT −0.2 yellow 67 −12 26 2.6 blue 63 4 −31
Fig. 6. Electrochromic switching of the poly(Me-(SNS(NO2)).
Colorimetry measurements were performed to define the col-ors of the electrochromic polymer in an exact way. CIE system was used as a quantitative scale to define colors scientifically. Three attributes of color; hue (a), saturation (b) and luminance (L) were measured and recorded. The poly(Me-SNS(NO2))
film shows different colors in the fully reduced state (−0.4 V), yellow, half reduced state (0.5 V), green, half oxidized state (0.65 V), gray, and fully oxidized state (1.1 V), blue (Table 1).
3.7. Electrochromic switching
The ability of a polymer to switch without delay and to ex-hibit a sharp color change is very significant. Double potential step chronoamperometry was carried out to estimate the re-sponse time of the device. The potential was stepped between 0.2 and 1.0 V with a residence time of 5 s. During the ex-periment, the % transmittance at the wavelength of maximum contrast was measured by using a UV–Vis spectrophotometer. The optical contrast was monitored by switching the polymer between 0.2 and 1.0 V at 345 and 1049 nm (Fig. 6). The opti-cal contrast was found to be 15% and the switching time was 2 s representing a fast response with a moderate contrast at 1049 nm.
3.8. Spectroelectrochemistry of Electrochromic Devices (ECDs)
A dual-type ECD consists of two electrochromic mate-rials (one anodically coloring, the other cathodically
color-Fig. 7. Spectroelectrochemistry of the poly(Me-(SNS(NO2))/PEDOT device.
ing) deposited on transparent ITO, placed in a position to face each other and a gel electrolyte was applied in between. The anodically coloring polymer film poly(Me-SNS(NO2)) was
fully reduced and the cathodically coloring polymer (PEDOT) was fully oxidized prior to construction of electrochromic de-vices (ECD).
Optoelectrochemical spectra of the dual type ECD as a func-tion of applied potential (from −0.2 to 2.6 V) are given inFig. 7. Maximum absorption at 360 nm revealing green color was observed due to π –π∗ transition upon application of neg-ative voltages. At that state, PEDOT did not reveal an obvious absorption at the UV–vis region of the spectrum and device re-vealed green color. When the applied potential increased, due to reduction of PEDOT layer, blue color became dominant and a new absorption was observed at 590 nm.
Upon application of a voltage, one of the polymer films is oxidized, whereas the other is reduced, resulting in a color change. The observed colors with the colorimetry parameters
L, a, b values are shown inTable 1.
3.9. Switching of ECDs
One of the most important characteristics of ECDs is the re-sponse time. It is the time needed to perform switching between the two colored states. For this purpose, chronoabsorptome-try was employed by stepping the potential between−0.2 and 2.6 V with a residence time of 5 s. During the experiment, the % transmittance (T %) at the wavelength of maximum con-trast was measured by a UV–vis spectrophotometer. For the de-vice, maximum contrast (T %) at 590 nm was measured as 18% (Fig. 8).
3.10. Stability of ECDs
Redox stability is a significant characteristic of electro-chromic devices with long lifetimes. The chief causes of device failure are different applied voltages and environmental con-ditions. Cyclic voltammetry method was used to estimate the long-term stability of the devices. After 500 cycles almost all electroactivity of the device retains intact (Fig. 9). These results
Fig. 8. Electrochromic switching of the poly(Me-(SNS(NO2))/PEDOT device.
Fig. 9. Redox stability of the poly(Me-(SNS(NO2))/PEDOT device.
showed that both ECDs have good environmental and redox sta-bility.
3.11. Open circuit stability
The color continuity in the electrochromic devices is a sig-nificant feature because it is directly connected to aspects involved in its employment and energy consumption during use[15]. The optical memory of an EC material is defined as the time during which the material maintains its color without applying potential[16]. We applied a pulse (−0.2 or +2.6 V)
for 1 s and held the cell in an open circuit condition for 200 s while the transmittance was monitored as a function of time.
Fig. 10shows the optical spectrum at 590 nm as a function of time at open circuit conditions. Poly(Me-SNS(NO2))/PEDOT
device is reasonably good at open circuit conditions.
4. Conclusions
The synthesis of a new monomers: 5-di(4-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole,
2-(4-Methyl-thiophen-2-Fig. 10. Open circuit stability of the poly(Me-(SNS(NO2))/PEDOT device.
yl)-5-(3-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyrrole and 2,5-di(3-methyl-thiophen-2-yl)-1-(4-nitrophenyl)-1H-pyr-role (Me-SNS(NO2)) were successfully fulfilled.
Poly(Me-SNS-(NO2)) has been synthesized by both chemical and
electro-chemical oxidative polymerizations. Chemically synthesized polymer of Me-SNS(NO2) is soluble in common organic
sol-vents. This property provides several applications such as direct application of polymers to any substrate using conventional printing techniques. Conducting polymer, poly(Me-SNS(NO2))
was synthesized potentiodynamically in AN/NaClO4/LiClO4
(0.1 M) solvent-electrolyte couple. Spectroelectrochemical analyses revealed that poly(Me-SNS(NO2)) has an electronic
band gap of 2.18 eV.
In the second part of the study, dual-type complementary colored polymer ECD made up of poly(Me-SNS(NO2))/PEDOT
was constructed and their characteristics were examined. A
po-tential range from −0.2 to 2.6 V was found suitable for op-erating the device. The color changes were distinctive and aesthetically pleasing. Good switching time and optical con-trast values were obtained. In addition, the device showed good environmental and redox stability. Considering these results, polymer of Me-SNS(NO2) monomer is a good candidate for
electrochromic layers in ECDs.
Acknowledgements
Authors are gratefully thank to DPT2002K120510-IM-4, to DPT-2005K120580, BAP2005-01-03-06 and TUBA grants.
References
[1] W.K. Chan, Y.M. Chen, A. Peng, L. Yu, J. Am. Chem. Soc. 115 (1993) 11735.
[2] M.R. Andersson, M. Berggren, G. Gustafson, T. Hjertberg, O. Inganas, O. Wennerstrom, Synthetic Metals 71 (1995) 2183.
[3] Y. Ohmori, K. Muro, K. Yashio, Synthetic Metals 57 (1993) 4111. [4] G. Horowitz, Adv. Mater. 10 (1998) 365.
[5] R.J. Mortimer, A.L. Dyer, J.R. Reynolds, Displays 27 (2006) 2. [6] P. Camurlu, A. Cirpan, L. Toppare, Mater. Chem. Phys. 92 (2005) 413. [7] M. Ertas, A. Cirpan, L. Toppare, Synthetic Metals 143 (2004) 49. [8] T.A. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of
Conducting Polymers, third ed., Marcel Dekker, New York, 1998. [9] D.R. Rosseinsky, R.J. Mortimer, Adv. Mater. 13 (11) (2001) 783. [10] I. Kerman, L. Toppare, F. Yilmaz, Y. Yagci, J. Macromol. Sci., Pure Appl.
Chem. 42 (2005) 509.
[11] K.S. Ryder, L.F. Schweiger, G. Andrew, J.M. Cooper, J. Mater. Chem. 10 (2000) 1785.
[12] R.C. Foitzik, A. Kaynak, J. Beckmann, F.M. Pfeffer, Synthetic Met-als 155 (1) (2005) 185.
[13] P.E. Just, K.I. Chane-Ching, P.C. Lacaze, Tetrahedron 58 (2002) 3467. [14] A. Cirpan, A.A. Argun, C.R. Greiner, B.D. Reeves, J.R. Reynolds,
J. Mater. Chem. 13 (2003) 2422.
[15] M.A. De Paoli, W.A. Gazotti, J. Braz, Chem. Soc. 13 (4) (2002) 410. [16] M. Ak, A. Cirpan, F. Yılmaz, Y. Yagcı, L. Toppare, Europ. Polymer J. 41
