Enhanced Thermal Stability of Polyaniline with Polymerizable Dopants

Article pubs.acs.org/Macromolecules

Enhanced Thermal Stability of Polyaniline with Polymerizable Dopants Yeol Kyo Choi,† Hyeong Jun Kim,‡ Sung Ryul Kim,‡ Young Min Cho,† and Dong June Ahn*,†,‡ †

Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Korea KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Korea

‡

S Supporting Information *

ABSTRACT: Diacetylene (DA) is an amphiphilic structure and has been studied as a variety of PDA-based chemosensors. However, the prospect of using diacetylene (DA) as dopant of polyaniline (PANI) is yet to be reported. In this study, new amphiphilic PCDA-taurine and PCDA-pBzS dopants were synthesized by changing the primary functional group to a sulfonic group. These polymerizable dopants are photopolymerized by UV irradiation in PANI solution. Thereby we expect to enhance the thermal stability and sustain the conductivity of PANI. The polymerizable dopants were characterized by FT-IR, NMR, and GC-MS. PANI with polymerizable dopants was analyzed by resonance Raman spectroscopy (RRS). The thermal stability and conductivity of PANI were characterized by thermogravimetric analysis (TGA). Comparing the TGA results of PANI doped with general dopants with PANI doped with polymerizable dopants, we found that PANI with polymerizable dopants showed enhanced thermal stability.

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INTRODUCTION Conjugated polymers are very unique polymers owing to a backbone with an extended π-conjugated system. The extended π-bonds contain delocalized electrons, which result in unique optical and electronic properties. Hence, electroactive polymers are used in a variety of applications, including light-emitting diodes,1 batteries,2 electromagnetic shielding,3 antistatic agents,4,5 gas sensors,6−8 and activators.9 Polydiacetylene (PDA) is one of the most attractive conjugated polymers for biosensor applications because it can be readily prepared by photopolymerization after self-assembly of diacetylene (DA) molecules.10−16 In addition, PANI shows promising potential due to its ease of synthesis, environmental stability, and high electrical conductivity.17 PANI is generally used as an antistatic agent in the plastics to prevent electrostatic discharge.5,18−21 The molding temperature of plastics such as polyethylene (PE), polypropylene (PP), and polystyrene (PS) is approximately 300 °C. However, when PANI is doped with general dopants such as dodecylbenzenesulfonic acid (DBSA) and exposed to higher temperatures, especially above 250 °C, complete deprotonation and degradation of PANI were observed.22 When PANI was dedoped, the conductivity decreased sharply.22 Thus, there is the need to enhance the thermal stability of the doped PANI for use as antistatic agents. In this study, we developed diacetylenes as dopants by the changing carboxyl group to a sulfonic group and denoted polymerizable dopants as PCDAtaurine or PCDA-pBzS. Upon exposure to 254 nm UV, the polymerizable dopants underwent photopolymerization via a © XXXX American Chemical Society

1,4-addition reaction to form an ene−yne conjugated backbone.23 By polymerization, we expect to enhance the thermal stability and sustain the conductivity of PANI with PCDAtaurine or PCDA-pBzS as the dopant.

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EXPERIMENTAL SECTION

Materials and Instrumentation. 10,12-pentacosadiynoic acid (PCDA) was purchased from GFS Chemicals. N-Hydroxysuccinimde (NHS), N,N′-dicyclohexylcarbodimide (DCC), 2-aminoethanesulfonic acid, 4-aminobenzenesulfonic acid, triethylamine, and the necessary solvents were purchased from Aldrich, Korea. Aniline, ammonium persulfate (APS), and 4-dodecylbenzenesulfonic acid were also purchased from Aldrich, Korea. 1H NMR spectra were recorded on a Varian Unitylnova (500 MHz) using DMSO-d6 and CDCl3 as the solvent. Fourier transform infrared (FT-IR) spectroscopy analysis was performed using a PerkinElmer Spectrum GX1 instrument. Morphological properties were observed by scanning electron microscopy (SEM). Gas chromatography−mass spectrometry data were recorded on a JMS-600W (ionization mode is fast atom bombardment). Thermal gravimetric analysis of PANI powders was carried out on a thermal gravimetric analysis Q 50 system. For conductivity measurements, the polymer samples were pressed into a 10 mm diameter disk and analyzed using four-probe conductivity instrument (FPP-RS 8 resistivity meter model). Experimental Concept. Commercially available PCDA has an amphiphilic structure with a carboxyl functional group (−COOH) and Received: November 30, 2016 Revised: March 14, 2017

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Figure 1. Synthesis of sulfonic acid series: (a) 10,12-pentacosadiynoic acid (PCDA), NHS, DCC, dichloromethane, 25 °C, 6 h; (b) 2aminoethanesulfonic acid (taurine), triethylamine (TEA), tetrahydrofuran (THF), 25 °C, overnight; (c) 4-aminobenzenesulfonic acid (sulfanilic acid), TEA, THF, 25 °C, overnight.

Figure 2. FT-IR spectra of (a) 4-(pentacosa-10,12-diynamido)benzene-1-sulfonic acid (PCDA-pBzS), (b) 2-(pentacosa-10,12-diynamido)ethane-1sulfonic acid (PCDA-taurine), (c) 10,12-pentacosadiynoic acid (PCDA), and (d) dodecylbenzenesulfonic acid (DBSA). pKa of 4−5. When DA was used as the dopant, dedoped PANI could be synthesized by decreasing the ionization efficiency of aniline. In order to use DA as a dopant for PANI, the principal functional group of DA needs to be changed to a sulfonic group (−SO3H) by the EDC/ NHS reaction (Figure 1) because the sulfonic group has a pKa of −2. Doped PANI was synthesized by the initial polymerization of aniline via the chemical oxidative polymerization method and subsequent doping by protonic acids such as DBSA or camphorsulfonic acid (CSA).24,25 Synthesis of PCDA-taurine and PCDA-pBzS (Dopant). PCDANHS. To a solution containing 1.00 g (2.67 mmol) of PCDA in 20 mL of dichloromethane were added 0.38 g (3.47 mmol) of NHS and 0.75 g (3.47 mmol) of DCC at room temperature. The mixed solution was stirred at room temperature for 4 h. The solvent was removed under

vacuum, and the residue purified by column chromatography with dichloromethane to give 1.08 g (86.2%) of the desired diacetylene monomer PCDA-NHS as a white solid (Figure 1a). 1H NMR (CDCl3, 500 MHz) δ: 0.81 (t, 3H), 1.19−1.69 (m, 36H), 2.17 (t, 4H), 2.53 (t, 2H), 2.77 (d, 4H) (Figure S2). 2-(Pentacosa-10,12-diynamido)ethane-1-sulfonic Acid (PCDAtaurine). To a solution containing 0.34 g (2.73 mmol) of 2aminoethanesulfonic acid in 5 mL of tetrahydrofuran (THF) was added 0.83 g (8.19 mmol) of triethylamine, and then 5 mL of deionized water (di-water) was added for complete dissolution. 1.0 g (2.1 mmol) of PCDA-NHS in 10 mL of THF was added dropwise to the solution. The resulting solution was stirred overnight at room temperature. The solvent was removed under vacuum, and the residue was purified by extraction with ethyl acetate to give 0.78 g (78%) of B

DOI: 10.1021/acs.macromol.6b02586 Macromolecules XXXX, XXX, XXX−XXX

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Figure 3. SEM images of (a) PANI (DBSA), (b) PANI (DBSA/PCDA-taurine), and (c) PANI (DBSA/PCDA-pBzS) (scale bar is 1 μm).

Figure 4. Optical (a, d) and fluorescence images (b, c, e, f) of PANI doped with DBSA/PCDA-taurine (upper panel) and with DBSA/PCDA-pBzS (lower panel): (a), (b), (d), and (e) before UV irradiation and (c) and (f) upon annealing at 110 °C after UV irradiation. the color changed from white, through blue, and finally green. The polymerization further preceded upon stirring at 25 °C for 12 h, after 30 mL of ethanol was added to terminate the reaction. Then PANI solution was divided in two by the same amount. PANI solution was not exposed to UV irradiation (PANI with PCDA-taurine/DBSA). PANI solution was exposed to 254 nm UV irradiation for 30 min (PANI with polymerized PCDA-taurine/DBSA). Another PANI solution was not exposed to UV irradiation (PANI with PCDAtaurine/DBSA). The precipitate was filtered through vacuum filtration and washed with di-water and ethanol repeatedly until the filtrate was colorless. Finally, the product was collected and dried at 60 °C for 24 h. Then, PANI with PCDA-taurine/DBSA and PANI with polymerized PCDA-taurine/DBSA powders were obtained. The image of PANI powders is shown in Figure S7. Figure S8 shows the SEM micrograph of PANI with DBSA. FT-IR: 3302 cm−1 (vsN−H), 1557 cm−1 (CC quinoid rings), 1494 cm−1 (CC benzenoid rings), 1194 cm−1 (B−NH+Q), 1048 cm−1 (vsSO), 745 cm−1 (vsSO)22 (Figure S9).

the desired diacetylene monomer PCDA-taurine as a white solid (Figure 1b). 1H NMR (CDCl3, 500 MHz) δ: 0.86−0.89 (t, 3H), 1.25−1.74 (m, 32H), 2.22−2.25 (m, 2H), 2.29−2.32 (m, 4H), 3.12 (m, 2H), 3.42 (s, 1H), 3.46 (m, 2H), 7.46 (s, 1H) (Figure S3). 4-(Pentacosa-10,12-diynamido)benzene-1-sulfonic Acid (PCDApBzS). To a solution containing 0.47 g (2.73 mmol) of 4aminobenzenesulfonic acid in 5 mL of THF was added 0.83 g (8.19 mmol) of triethylamine, and then 5 mL of di-water was added for complete dissolution. 1.0 g (2.1 mmol) of PCDA-NHS in 10 mL of THF was added dropwise to the solution. The resulting solution was stirred overnight at room temperature. The solvent was removed under vacuum, and the residue was purified by extraction with ethyl acetate to give 0.81 g (74%) of the desired diacetylene monomer, PCDA-pBzS, as a white solid (Figure 1c). 1H NMR (CDCl3, 500 MHz) δ: 0.86−0.89 (t, 3H), 1.25−1.72 (m, 32H), 2.18−2.27 (m, 4H), 2.29−2.31 (m, 2H), 7.35 (s, 1H), 7.57 (s, 1H), 7.67 (d, 2H), 7.80 (d, 2H) (Figure S4). Synthesis of PANI with Polymerizable Dopants. An aqueous micellar dispersion was prepared by mixing DBSA (5.24 g) and PCDA-taurine (0.64 g) in distilled water (50 mL) and stirring for 1 h (the molar ratio of PCDA-taurine to DBSA is 1:10). Aniline (1.0 g) was then added to the solution. Ammonium persulfate ((NH4)2S2O8), used as the oxidant (initiator), was separately dissolved in 10 mL of diwater added to the solution and stirring for 1 h.25 After the micellar dispersion was stirred for 1 h, the color of the solution changed with the progress of polymerization step. As the polymerization proceeded,

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RESULTS AND DISCUSSION Chemical Structure Analysis of PCDA-taurine and PCDA-pBzS. We first observed the chemical structure of polymerizable dopants PCDA-taurine and PCDA-pBzS. The FTIR spectra of PCDA, PCDA-taurine, PCDA-pBzS, and DBSA in Figure 2 show peaks at 2922 and 2847 cm−1 due to C

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Figure 5. Raman spectra with laser excitation of 633 nm of (a) PCDA-taurine, (b) PCDA-pBzS, (c) polyaniline with DBSA/PCDA-taurine, and (d) polyaniline with DBSA/PCDA-pBzS.

Figure 4 are fluorescence images obtained immediately after synthesis, after polymerization and subsequent annealing (110 °C), respectively. There was no fluorescence in the monomeric state, whereas, upon thermal annealing, red fluorescence appeared and distributed throughout the whole area, for both cases of the dopants. Hence, we can state that diacetylene moieties mixed in PANI host rather than separated. Next, we observed resonance Raman spectroscopy to confirm that diacetylenic dopants can successfully be polymerized within the PANI host. The Raman spectra of blue phase PCDA-taurine and PCDA-pBzS, in solid state, recorded using a 633 nm light source comprise three major features: the carbon−carbon single bond stretch, (ν(CC)), carbon−carbon double bond stretch (ν(CC)), and carbon− carbon triple bond stretch (ν(CC)). Figure 5a shows that when PCDA-taurine is polymerized, strong bands appear at 703 cm−1 for the ν(CC) band, at 1460 cm−1 for the ν(CC) band, and at 2087 cm−1 for the ν(CC) band compared to the monomeric state. In the case of PCDA-pBzS, the results were similar to that of PCDA-taurine. When PCDA-pBzS is polymerized, strong bands are seen at 678 cm−1 (ν(C−C)), 1438 cm−1 (ν(CC)), and 2058 cm−1 (ν(CC)). (See peak assignments of Raman spectra listed in Table S2.) It is noted that resonance Raman intensified when the conjugation length is increased upon polymerization.30 In order to confirm the polymerization of diacetylene dopants within the PANI host, Raman analysis of PANI doped with diacetylene is shown in Figure 5c,d. The ν(CC) band is inadequate to determine the

the asymmetric and symmetric stretching vibrations of the CH2 groups of PCDA side chains.27 The PCDA-pBzS spectrum in Figure 2a shows the hydrogen-bonded amide and carbonyl stretching bands at 3326 and 1627 cm−1, respectively, along with a ring stretching band at 1578 cm−1 as well as the symmetric and antisymmetric stretching vibrations of the sulfonic acid at 1244 and 1089 cm−1.28 The hydrogen-bonded amide and carbonyl stretching bands observed for PCDAtaurine are observed at 3327 and 1627 cm−1, respectively, as seen in Figure 2b. The symmetric and antisymmetric stretching vibrations of the sulfonic acid group are seen at 1243 and 1088 cm−1, respectively.28 FTIR spectra of DBSA in Figure 2d show the symmetric and antisymmetric stretching vibrations of the sulfonic acid at 1251 and 1084 cm−1.29 The FTIR spectra data are listed in Table S1. Morphological and Resonance Raman Scattering Analysis of PANI with Polymerizable Dopants. SEM measurement was performed for morphological analysis of synthesized PANI. SEM images of PANI particles are shown in Figure 3. The shape and size of some isolated particles seem to be round shaped, and small particles have a size of a few hundreds of nanometers and large particles have a size of a few micrometers. There also exist some particulate aggregates. In the images from Figure 3a to Figure 3c, there shows little significant difference in shape or size depending on the kind of the dopants. Optical microscopy and fluorescence image analysis of synthesized PANI were performed to investigate the organization of diacetylene in the PANI host. Given in D

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Figure 6. TGA thermograph for PANI powder with polymerizable dopants. (a) PANI with PCDA-taurine, (b) PANI with DBSA/PCDA-taurine, and (c) PANI with DBSA/PCDA-pBzS (the molar ratio of DBSA to PCDA-taurine is 10:1), and (d) PANI with DBSA/PCDA-pBzS (the molar ratio of DBSA to PCDA-pBzS is 10:1).

Thermal Stability Analysis of PANI with Polymerizable Dopants. The TGA curve of PANI with polymerizable dopants powder are shown in Figure 6. We first synthesized PANI with only PCDA-taurine as dopant and measure TGA for it. Figure 6a shows that PANI with only PCDA-taurine starts to lose weight from 150 °C compared to PANI with DBSA maintaining thermal stability of up to 250 °C. These results were similar to those of undoped PANI. The reason for not doping was thought to be that the proportion of PCDA-taurine used as dopants was relatively small compared to aniline. Next, we synthesized PANI using HCl, general dopant, and PCDAtaurine. Figure 6b shows that thermal stability of PANI with PCDA-taurine and HCl and PANI with PCDA-taurine and HCl (UV) were significantly lower than those of PANI with DBSA. PANI doped with DBSA is known to show excellent thermal stability. So we synthesized PANI with DBSA and PCDA-taurine as dopants (the molar ratio of PCDA-taurine to DBSA is 1:10). Figure 6c shows that PANI with DBSA/PCDAtaurine and PANI with DBSA/PCDA-taurine (UV) degraded at ∼270 °C. Comparing these results with thermal stability of PANI with DBSA, it was found that the thermal stability was increased. In Figure 6d, it could be seen that PANI with DBSA/ PCDA-pBzS and PANI with DBSA/PCDA-pBzS (UV) underwent similar degradation at ∼270 °C. In addition, the thermal stability of PANI with DBSA/PCDA-pBzS (UV) was better than that of PANI with DBSA in the range 250−600 °C. These results indicated that polymerizable dopants, PCDA-taurine

presence of polymerization because the ν(CC) band of polymerized diacetylene overlaps with that of the benzenoid ring of PANI. Thus, we considered the triple bond as a measure of the presence or absence of polymerization and proceeded with the analysis. The Raman spectrum of PANI doped with DBSA (black line) is shown in Figure 5c. The Raman band at 1149 cm−1 is assignable to the out-of-plane C−H bending. The 1320 cm−1 band is assigned to the C−N•+ radical cations. The intense band near 1469 cm−1 occurs mostly due to the benzenoid C−C ring stretching vibration, while the band near 1562 cm−1 is attributed to the quinoid C−C stretching mode of the polymer chain.31 Figure 5c shows the Raman spectrum of PANI with PCDA-taurine (blue line). The peak of PCDAtaurine is seen at 2123 cm−1 (ν(CC)), while the peaks of PANI are observed at 1174 cm−1 (the out-of-plane C−H bending), 1324 cm−1 (C−N•+ radical cations), 1491 cm−1 (the benzenoid C−C ring stretching vibration), and 1589 cm−1 (the quinoid C−C stretching mode).31 After UV irradiation of PANI with PCDA-taurine the ν(CC) band at 2123 cm−1 was clearly appeared, indicating that PCDA-taurine was polymerized. In the case of PANI with PCDA-pBzS, the results were similar to that of PCDA-taurine. The Raman spectrum of PANI with PCDA-pBzS (red line) is shown in Figure 5d which showed the ν(CC) band at 2054 cm−1, indicating that PCDA-pBzS was polymerized after UV irradiation of PANI with PCDA-pBzS. Hence, it can be stated that diacetylene moieties are organized in the PANI host. E

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the primary functional group needs to be changed to a sulfonic group (−SO3H). The new amphiphilic 2-(pentacosa-10,12diynamido)ethane-1-sulfonic acid (PCDA-taurine) and 4(pentacosa-10,12-diynamido)benzene-1-sulfonic acid (PCDApBzS) dopants are synthesized by the EDC-NHS reaction. Then PANI was synthesized with the polymerizable dopants. As a result, when using diacetylene as dopant for PANI, enhanced thermal stability of PANI was observed in the range 250−600 °C. The sheet resistance of PANI with polymerizable dopants was nearly maintained when compared to those of PANI with DBSA. This study signifies a new approach of using DA as a novel conjugated surfactant-type dopant of PANI.

and PCDA-pBzS, enhanced the thermal stability of PANI by reinforcing NH+...SO3− interactions between the PANI chain and the dopants. The weight loss of PANI with DBSA, PANI with DBSA/ polymerized PCDA-taurine, and PANI with DBSA/polymerized PCDA-pBzS is shown in Figure 7. At 250 °C, these PANI

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02586.

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(PDF)

AUTHOR INFORMATION

Corresponding Author

Figure 7. TGA weight (%) of PANI with DBSA(black), PANI with DBSA/polymerized PCDA-taurine (10:1), PANI with DBSA/ polymerized PCDA-taurine (5:1), PANI with DBSA/polymerized PCDA-pBzS(10:1), and PANI with DBSA/polymerized PCDA-taurine (5:1). The white numbers of bar charts indicate the weight percent of PANI.

*(D.J.A.) E-mail: [email protected]. ORCID

Yeol Kyo Choi: 0000-0002-4218-7139 Dong June Ahn: 0000-0001-5205-9168 Author Contributions

D.J.A. designed and supervised the project. Y.K.C. and H.J.K. contributed equally to this work.

have similar weight percent. But as the temperature increased above 250 °C, the weight percent of PANI with DBSA/ polymerized PCDA-taurine and PANI with DBSA/polymerized PCDA-pBzS is higher than those of PANI with DBSA. These results show that the thermal stability of PANI with DBSA/ polymerized dopants is better than those of PANI with DBSA. Sheet Resistance Analysis of PANI with PCDA-taurine and PANI with PCDA-pBzS. PANI doped with DBSA is generally known to exhibit the sheet resistance of 10−2−102 Ω/ sq.32−34 Analysis of the surface resistance results showed that there was no significant difference between PANI (PCDAtaurine/DBSA) before and after photopolymerization. It was found that the sheet resistance of PANI (PCDA-taurine/ DBSA) was about 30−40 Ω/sq. PANI (PCDA-pBzS/DBSA) also had sheet resistivity of about 20−30 Ω/sq. But the sheet resistances of PANI with DBSA/PCDA-taurine and PANI with DBSA/PCDA-pBzS are lower than that of PANI with DBSA.35 The sheet resistance data are listed in Table 1.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (MSIP 2017R1A2B3006770 and 2015M3C1A3002152), LOTTE CHEMICAL CORPORATION, KU-KIST School Project of Converging Science and Technology (R1309521), and a Korea University Grant.

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REFERENCES

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CONCLUSION In this study, we have developed a new concept dopant using diacetylene, a photopolymerizable material, to improve the thermal stability of PANI and to maintain electrical conductivity. In order to introduce DA as a dopant of PANI, Table 1. Sheet Resistance of PANI with Polymerizable Dopants before UV irradiation

sheet resistance (Ω/sq)

after UV irradiation

PANI with DBSA

PANI with DBSA/PCDAtaurine

PANI with DBSA/PCDApBzS

PANI with DBSA/PCDAtaurine

PANI with DBSA/PCDApBzS

10−2−102

32.2 ± 1.5

38.1 ± 1.8

23.1 ± 0.8

25.9 ± 1.1

F

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DOI: 10.1021/acs.macromol.6b02586 Macromolecules XXXX, XXX, XXX−XXX