Synthesis and Multifunctionality of Self-Stabilized Poly

ARTICLE pubs.acs.org/JPCC

Synthesis and Multifunctionality of Self-Stabilized Poly(aminoanthraquinone) Nanofibrils Xin-Gui Li,*,†,‡ Hu Li,† Mei-Rong Huang,*,† and Mark G. Moloney*,‡ †

Institute of Materials Chemistry, Key Laboratory of Advanced Civil Engineering Materials, College of Materials Science and Engineering, Tongji University, 1239 Si-Ping Road, Shanghai 200092, China ‡ Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.

bS Supporting Information ABSTRACT: Intrinsically self-stabilized nanofibril bundles of poly(1-aminoanthraquinone) (PAQ) were facilely synthesized by the chemical oxidative polymerization of 1-aminoanthraquinone. Critical polymerization parameters such as the monomer concentration, medium, oxidant species, temperature, and time were studied to significantly optimize the synthesis, size, properties, and multifunctionality of the resulting nanofibrils by IR, UVvis, and fluorescence spectroscopy, X-ray diffraction, SEM, TEM, DSC, and thermogravimetry. It is found that the polymerization of 1-aminoanthraquinone with (NH4)2S2O8 as an oxidant in HClO4/acetonitrile without external stabilizer simply affords finer PAQ nanofibrils with an optimal combination of diameter of ca. 15 nm, a length of ∼6 μm, a higher preparation yield, a purer composition, a higher conductivity, and higher melting and decomposition temperatures than that with CrO3 and H2O2/Fe2þ. Furthermore, the polymer nanofibrils exhibit high self-stability, powerful redispersibility, and a clean surface because of the complete avoidance of contamination from an external stabilizer. PAQ exhibits remarkably good solubility in polar solvents, colorful solvatochromism, widely controllable conductivity moving across 10 orders of magnitude from 109 to 50 S/cm, fluorescence, lead-ion adsorbability, high thermostability in air, and a very high carbon yield in nitrogen at 1000 °C. In particular, the nanoeffect of the PAQ nanofibrils with a large specific surface area and aspect ratio further enhances their fluorescence, lead-ion adsorbability, and nanocomposite ability of facilely forming a unique nanonetwork. PAQ would be useful as advanced materials including fluorescent emitters, sorbents of toxic metallic ions, costeffective carbon foam precursors, and conducting nanocomposites with low percolation thresholds.

’ INTRODUCTION The discovery of nanoscopic conducting polymers with large π-conjugated structures has opened a very important field of modern functional materials science.15 Nanostructured polyaniline (PAN) is one of the most attractive advanced conducting polymers because of a novel combination of its electroconductivity, electroactivity, electrocatalysis, optical activity, heavy metal ion adsorbability, electrorheology, and environmental and thermal stability that have found wide potential applications in the fields of sensors,1,69 enzyme immobilization,10 light-emitting diodes,11 electrochromic films,12 transparent conducting films,13 electrorheological fluids,14,15 supercapacitors,16 and anticorrosion coatings for metals.17 Furthermore, the conducting polymers containing fused aromatic rings such as aminoquinoline and diaminonaphthalene have also received a great deal of attention because of their superior multifunctionalities and optimizable mechanisms by facilely regulating the polymerization and doping conditions.1820 However, poly(1-aminoanthraquinone) (PAQ) has attracted relatively less attention because it is very difficult to synthesize PAQ with a high molecular weight and high conductivity by electropolymerization or the chemical oxidative polymerization of 1-aminoanthraquinone in traditional reaction media. r 2011 American Chemical Society

In the past few years, it has been reported that poly(1,5-diaminoanthraquinone)21,22 and poly(2-aminoanthraquinone)23 have been synthesized and demonstrated some excellent properties, such as stable and strong electroactivity, high electrical conductivity, powerful specific electrocapacity, and fast reversible redox ability. Meanwhile, the electroconducting aminoanthraquinone polymers have been investigated for use as electrode materials for biosensors,23 secondary batteries, and electrochemical capacitors.21 The aminoanthraquinone polymers must contain a moiety of 1,4-benzoquinone groups and the PAN-like main chains, which should have higher electroactivity than PAN because of the hybridization of those two parts.24 That is to say, a hybridization moiety, for example, quinone (Q/Q•/Q2), in a π-conjugated system might give even higher electroactivity than just their simple mixture or the conducting polymer alone. Moreover, the aminoanthraquinone polymer is anticipated to be a multifunctional material that presents better properties than conventionally conducting polymers because the polymer contains Received: March 1, 2011 Revised: April 12, 2011 Published: April 26, 2011 9486

dx.doi.org/10.1021/jp201967n | J. Phys. Chem. C 2011, 115, 9486–9497

The Journal of Physical Chemistry C 1,4-benzoquinone groups in the aromatic rings as one more redox site than in PAN. Therefore, there is no doubt that these novel characteristics of aminoanthraquinone polymers will attract the attention of more scientists from chemistry and materials science. However, the electrosyntheses of PAQs such as poly(1,5-diaminoanthraquinone) and poly(2-aminoanthraquinone) have some inherent disadvantages such as very low productivity and high preparation cost.21,23 The chemical oxidative polymerization of 1,5-diaminoanthraquinone can occur only in nonaqueous sulfuric acid/N,N-dimethylforamide using CrO3 as an oxidant, leading to the formation of poly(1,5-diaminoanthraquinone) nanospheres with a moderate polymerization yield (52.4%).22 Furthermore, no poly(1,5-diaminoanthraquinone) was obtained if using (NH4)2S2O8 as an oxidant for the 1,5-diaminoanthraquinone polymerization. To the best of our knowledge, no other reports on the chemical oxidative polymerization of 1-aminoanthraquinone or 1,5-diaminoanthraquinone were found to date. Here, the chemical oxidative polymerization of 1-aminoanthraquinone (AQ) was carried out as one of the best methods to facilely and productively synthesize poly(1-aminoanthraquinone) (PAQ) nanofibril bundles with high purity, inherent self-stability, adjustable conductivity, extremely high thermostability, and a high conducting carbon yield. The presence of 1,4-benzoquinone groups in PAQ polymers would be vital to the formation and stabilization of the nanofibrils owing to their negative electrostatic repulsion and steric hindrance effect, thus leading to the in situ fabrication of pure self-stable nanofibril bundles. The significant effect of key polymerization parameters, including the oxidant species, polymerization temperature, and time on the yield, structure, properties, and multifunctionality of the PAQ polymers has been systematically investigated and optimized.

’ EXPERIMENTAL SECTION Reagents. 1-Aminoanthraquinone (AQ), perchloric acid, acetonitrile, propylene carbonate, (NH4)2S2O8, CrO3, H2O2, FeCl2, FeCl3, Pb(NO3)2, poly(vinyl alcohol) (DP = 1750), dodecylbenzenesulfonic acid (DBSA), concentrated H2SO4, N-methylpyrrolidone (NMP), formic acid, dimethyl sulfoxide (DMSO), chloroform, tetrahydrofuran (THF), and other solvents were commercially obtained and used as received. Synthesis of PAQ Polymers. The PAQ polymers were prepared by the chemical oxidative polymerization of AQ monomers. A typical polymerization procedure is as follows: 40 mL of 50 mM HClO4 in CH3CN containing a very small amount of water in a 150 mL glass flask in a water bath at 20 °C was added to 446 mg (2 mmol) of AQ and then stirred vigorously for half an hour. An oxidant solution was prepared separately by dissolving 456 mg (2 mmol) of (NH4)2S2O8 in 1 mL of deionized water. The AQ solution was then treated with the (NH4)2S2O8 solution by dropwise adding the (NH4)2S2O8 solution at a rate of 1 drop (around 60 μL) per 20 s. The reaction mixture was stirred continuously at 20 °C for 48 h together with an in situ measurement of the open-circuit potential (OCP) and temperature of the polymerization solution. After that, the polymer virgin salts as precipitates were isolated from the reaction mixture by filtration and washed with ethanol and excess distilled water in order to remove the remaining monomer, oxidant and its reduced product (SO42), and soluble oligomers until no BaSO4 precipitate was detected after adding a 1 mM BaCl2 aqueous solution. Part of the PAQ salt was subsequently stirred and undoped in 100 mL of 0.2 M NH4OH for 24 h for the preparation of PAQ

ARTICLE

dedoped base as an undoping material. However, the PAQ virgin salts were further redoped in a 1 M HClO4 aqueous solution for 24 h for the preparation of redoped salt samples. All of the resultant polymers were left to dry in ambient air for 3 days, obtaining the PAQ salt and base as fine dark-blue powders. Preparation of the PAQ Nanofibrils/Polyvinyl Alcohol Nanocomposite Films. The nanocomposite films were prepared by an ultrasonic dispersion of the PAQ nanofibrils in a 1 wt % poly(vinyl alcohol) aqueous solution for 1.0 h, followed by solution casting onto a 5 7 cm2 PTFE plate. After drying at 30 °C for 24 h, a 15-μm-thick film was peeled off of the substrate to form a freestanding film for performance evaluation. Adsorption Experiments. The adsorption of the Pb2þ ions onto the PAQ virgin salts was performed in a batch experiment. A Pb2þ aqueous solution (25 mL) at a concentration of 200 mg/L was incubated with a given amount of the PAQ powders at a fixed temperature of 30 °C. After the desired treatment period, the powders were filtered from the solution and then the concentration of metal ions in the filtrate after the adsorption was measured by EDTA titration and used to calculate the adsorbance and adsorptivity of Pb2þ ions on the PAQ. Measurements. The IR spectra were recorded on a Nicolet FTIR Nexsus 470 spectrophotometer in KBr pellets. UVvis spectra of the PAQ virgin salts in NMP and concentrated H2SO4 were recorded between 200 and 900 nm at a scanning rate of 600 nm/min on a U-3000 spectrophotometer (Hitachi Ltd., Tokyo, Japan). Wide-angle X-ray powder diffractograms for the samples were obtained at a scanning rate of 10°/min in reflection mode over a 2θ range from 3 to 70° using a D/max 2550 X-ray model diffractometer (Rigaku, Japan) with Cu KR radiation. The size and morphology of the resulting PAQ nanofibril bundles that have been thoroughly washed with water were analyzed by fieldemission scanning electron microscopy (SEM, Quanta 200 FEG) and transmission electron microscopy (TEM, Hitachi model H600). The samples for SEM observation were dispersed on a silicon wafer and then subjected to gold sputtering prior to observation. The samples for TEM observation were prepared by dropping their suspension in alcohol onto copper grids. The solubility of the PAQ powders was evaluated using the following method: 2 mg of each dried PAQ powder was added to 1 mL of solvent and dispersed thoroughly. After the mixture was shaken continuously for around 2 h at room temperature, the solubility and solution color of the polymers were evaluated. The bulk electrical conductivity of a pressed pellet with a thickness of 3040 μm for the PAQ powders was measured by a two-disk method at 20 °C.22 The fluorescence spectra were recorded on a Cary Eclipse fluorescence spectrophotometer (Varian Australia Pty Ltd., Victoria, Australia). Simultaneous thermogravimetry (TG)/differential scanning calorimetry (DSC) experiments were carried out in a Stanton-Redcroft STA 449 C Jupiter thermogravimetric analyzer in an air or pure nitrogen flow of 50 mL/min at heating rates of 20 and 40 °C/min with a sample size of 1.920.0 mg. An independent DSC measurement was performed in a TA DSC Q100 analyzer in a pure nitrogen flow of 50 mL/min with a sample size of 2.44.8 mg.

’ RESULTS AND DISCUSSION Synthesis of PAQ Nanofibril Bundles. The chemical oxidative polymerization process shown in Scheme 1 was investigated by tracing the open-circuit potential (OCP) and temperature of the polymerization solution (Figure S1). The OCP first rose 9487

dx.doi.org/10.1021/jp201967n |J. Phys. Chem. C 2011, 115, 9486–9497

The Journal of Physical Chemistry C

ARTICLE

Table 1. Effect of Oxidant Species on the Polymerization Yield, Bulk Electrical Conductivity, and Solubility of Poly(1-aminoanthraquinone) (PAQ) with an (NH4)2S2O8/AQ Molar Ratio of 1 in a 50 mM HClO4/CH3CN Solution at 15 °C for 48 h H2O2/Fe2þ

CrO3/H2O2

initial/top/final OCP of polymerization

668/912/861

671/991/905

(mV vs SCE) time at the top OCP (min)

7.1

11.4

polymerization yield (%)

6.2

18.5

oxidants

electrical conductivity (S/cm)

virgin PAQ salt redoped PAQ salt dedoped PAQ base

a

b

solubility and solution color of PAQ salts

c

(NH4)2S2O8 /H2O2

67.4

CrO3

(NH4)2S2O8

672/979/817

670/950/925

2.2

219

70.2

74.3

2.2 109

1.9 105

5.8 105

1.3 105

5.0 105

8

1.0 10

4

1.2 10

4

2.4 10

4

2.2 103

5.5 10

2.4 10

8

1.1 10

8

1.8 10

9

3.0 109

1.2 10

10

H2SO4 (101)

S, g

S, g

S, g

S, g

S, g

H2O (80)c

IS

IS

IS

IS

IS

HCOOH (58)c DMSO (47)c

PS, p S, bp

PS, p MS, b

WS WS

PS, p PS, bp

PS, p PS, bp

NMP (32)c

S, b

MS, b

S, b

S, b

S, b

m-cresol (12)c

MS, b

WS

WS

WS

MS, b

THF (7)c

WS

WS

WS

WS

WS

CHCl3 (5)c

PS, bp

WS

WS

WS

WS

aqueous HCl or NaOH

IS

IS

IS

IS

IS

a

IS = insoluble; MS = mainly soluble; PS = partially soluble; S = soluble; and WS = weakly soluble. The 1-aminoanthraquinone(AQ) monomer is completely soluble in H2SO4, HCOOH, DMSO, NMP, m-cresol, DMF, THF, and CHCl3 and displays a red solution but is insoluble in H2O, HCl, and NaOH. b b = blue; bp = bluish purple; g = green; and p = purple. c The dielectric constant of the solvents.

Scheme 1. Nominal Chemical Oxidative Polymerization of 1-Aminoanthraquinone and Possible Formation and SelfStabilized Mechanisms of the Poly(1-aminoanthraquinone) Nanofibrils

sharply and then decreased rapidly in the initial 2 min, experiencing a maximum of 742 mV versus SCE. After that, the OCP again increased to the second maximum of 950 mV in around 200 min and finally decreased slowly to a relatively stable equilibrium value in the next 200 min. These four stages could be attributed

to the sudden addition of oxidant, the formation of oligomers accompanied by the slow consumption of oxidant, chain propagation, and total consumption plus the completion of polymerization, respectively. It is obvious that the polymerization of the AQ monomer with great spatial hindrance is slower than the chemical oxidative polymerization of aniline and its derivatives.25,26 In the second stage, with the gradual oxidation of the protonated AQ monomer by the oxidant, the AQ cation radical could be slowly formed and then subsequent oligomerization would occur by an oxidative condensation between the cation radical and neutral AQ monomers via an electrophilic aromatic substitution mechanism. In the third stage, the oligomers formed during the second stage may further polymerize with each other and with the residual monomers and/or their cation radicals, resulting in persistent chain propagation to form a higher-molecular-weight polymer. This process is repeated again and again until most of the oxidant, monomer, and oligomer are consumed at the end of the plateau stage. However, a slight undulation of the polymerization solution temperature during polymerization was detected, which is also different from the polymerization of aniline and its derivatives with a huge exothermicity.27,28 This may be ascribed to weaker reactivity and much lower concentrations of AQ monomer and oxidant in this study than in refs 27 and 28. In other words, the AQ polymerization is smooth and the corresponding scale-up reaction may be simply designed and controlled. As listed in Table 1, the top and final OCPs of the polymerization solution significantly vary with the oxidant species despite approximately the same initial OCP. The polymerization induced by CrO3/H2O2 exhibits the highest top OCP of up to 991 mV versus SCE, indicating that the redox interaction between the AQ monomer as a reducer and the CrO3/H2O2 oxidant is the strongest. The polymerization by only CrO3 exhibits the top OCP at the shortest reaction time of down to 2.2 min, implying that the redox interaction between AQ and CrO3 is the fastest. On the contrary, the polymerization induced by H2O2/Fe2þ exhibits the lowest top OCP of 912 mV versus SCE, suggesting that the redox 9488



Figure 1. Effect of AQ monomer concentration on (a) the polymerization yield and bulk electrical conductivity and (b) the number-average diameter (Dn) and size polydispersity index (PDI) of redoped PAQ salt particles prepared at an (NH4)2S2O8/AQ molar ratio of 1 in 50 mM HClO4/CH3CN at 0 °C for a polymerization time of 48 h.

interaction between the AQ reducer and H2O2/Fe2þ oxidant is the weakest. The polymerization by (NH4)2S2O8 demonstrates the top OCP at the longest reaction time of 219 min, signifying that the redox interaction between AQ and (NH4)2S2O8 is the slowest or smoothest. The screening of the polymerization medium is of great importance to the oxidative polymerization. Acetonitrile and propylene carbonate have been utilized because they can dissolve the AQ monomer very well and also exhibit mutual solubility with acidic water. When a combination of H2O2/FeSO4(200/1 mol) in 6 M H2SO4 and AQ in propylene carbonate was used, no oxidative polymerization occurred. If 2 mmol of (NH4)2S2O8 in 1 M HClO4 was added to 2 mmol of AQ in propylene carbonate at 30 °C, then oxidative polymerization indeed happened after 17 h but the yield was very low (9.82%) even for a long polymerization time of up to 1 week. Consequently, it seems that propylene carbonate is not good enough for the oxidative polymerization of AQ. When CH3CN was chosen as the solvent dissolving AQ, some dark particles appeared just after 3 h of mixing AQ with (NH4)2S2O8 in HClO4, having a much higher yield of 59.2% after 48 h. Apparently, CH3CN/HClO4 is a good medium for AQ polymerization to synthesize PAQ with a high polymerization yield and a high conductivity, as discussed below. The remarkable dependence of the polymerization yield on the AQ concentration is illustrated in Figure 1a. As the AQ concentration rises from 0 to 100 mM in HClO4/CH3CN, the yield of redoped PAQ salt demonstrates a maximum of 41.5% at 50 mM. It is obvious that too low an AQ concentration means too few polymerizing active centers whereas the polymerizing efficiency at too high an AQ concentration was restricted by a limited solubility of AQ in HClO4/CH3CN, both of which result in a lower yield. Therefore, the optimal AQ concentration should be around 50 mM for the resultful synthesis of the PAQ. Several representative oxidants, including H 2 O2 /Fe 2þ, (NH4)2S2O8, CrO3, and FeCl3 with different standard reduction potentials (RP) of 2.8, 2.01, 1.35, and 0.77 V, respectively, were chosen for AQ polymerization. It seems that the resultant polymerization solution did turn dark when FeCl3 with the lowest RP was used, but no solid polymer was attained by centrifugation, suggesting the formation of some soluble oligomers. Another five oxidants or their combination, such as H2O2/Fe2þ, CrO3/H2O2, (NH4)2S2O8/H2O2, CrO3, and (NH4)2S2O8 with higher RPs

ARTICLE

Figure 2. Influence of the polymerization temperature on (a) the polymerization yield and bulk electrical conductivity and (b) adsorption capacity and adsorptivity of Pb2þ of PAQ polymers prepared by a chemically oxidative polymerization at a fixed (NH4)2S2O8/AQ molar ratio of 1 and an initial AQ concentration of 50 mM in 50 mM HClO4/ CH3CN for 48 h, with different polymerization temperatures in 25 mL of a Pb(NO3)2 solution at an initial Pb2þ concentration of 200 mg/L at 30 °C for 24 h.

from 2.8 to 1.35 V, can produce dark-blue precipitates as polymerization products, as listed in Table 1. Obviously, (NH4)2S2O8 is the optimal oxidant for preparing PAQ with the highest synthesis yield of 74.3%, the highest conductivity of 2.2 103 S/cm, and good solubility, indicating the effective oxidizability of (NH4)2S2O8 to the AQ monomer for its oxidative polymerization in HClO4/ CH3CN. The lowest yield and conductivity and the strongest solubility were observed when H2O2/Fe2þ with the highest RP was employed as an oxidant, which may be ascribed to the overoxidation by H2O2/Fe2þ, leading to the formation of some soluble oligomers in the polymerization medium and finally to the lowest yield and thus the highest solubility. Therefore, the RP of the oxidants has a significant influence on the AQ polymerization. RPs that are either too high or too low are not appropriate for this AQ polymerization. The strong effect of the polymerization temperature on the yield is revealed in Figure 2a. It can be seen that the yield of PAQ exhibits a maximal value of 74.3% at 15 °C. With increasing polymerization temperature from 15 to 50 °C, the yield gradually decreases to the lowest values, possibly because of the loss of more oligomers formed at higher temperature. The chemical oxidative polymerization of aniline and 4-sulfonic diphenylamine shows a similar relationship between polymerization temperature and yield.28 It is concluded that 15 °C is a favorable temperature for AQ polymerization because of the highest yield of PAQ, which should be attributed to the proper chain initiation and propagation rates. Too high a temperature would induce more chain termination, whereas too low a temperature might cause slower and less chain initiation and propagation. As shown in Figure 3a, the polymerization yield of the virgin PAQ salt is dependent on the polymerization time, rising from 48.5 to 67.2% with increasing polymerization time at 20 °C from 9 to 12 h and then rising slightly from 67.2 to 79.3% with increasing time from 12 and 72 h. During the initial 12 h of reaction, chain propagation was significantly, accompanied by the rapid consumption of the oxidant and the gradual precipitation of the polymers from the reaction medium. After 12 h, much lower concentrations 9489



Figure 3. (a) Effect of polymerization time on the polymerization yield and electrical conductivity of PAQ polymers prepared at a fixed (NH4)2S2O8/AQ molar ratio of 1 in 50 mM HClO4/CH3CN at a polymerization temperature of 20 °C. (b) Effect of polymerization temperature and time on the number-average diameter (Dn) and size polydispersity of redoped PAQ salt particles (in water) prepared at an (NH4)2S2O8/AQ molar ratio of 1 in 50 mM HClO4/CH3CN at 0 °C (dash lines) for the polymerization time of 48 h (solid lines).

of the oxidant and monomer will lead to much weaker polymerizability and then a slight variation in the polymerization yield with increasing polymerization time. Furthermore, it appears that the polymerization yield of the redoped PAQ salt with CrO3 as an oxidant at 15 °C linearly and remarkably rose from 11.1 to 70.2% with increasing polymerization time from 24 to 96 h because the PAQ that formed earlier would strongly adsorb the chromic ions in the polymerization medium in the later stage and the chromic ions adsorbed onto the PAQ particles would not be eliminated by rinsing with water and acid aqueous solution. This is another reason that CrO3 was not selected as an oxidant for the whole study. Structure of the PAQ Nanofibril Bundles. IR Spectra. The IR spectra of the AQ monomer and PAQ polymer bases obtained with different oxidants and polymerization temperatures are shown in Figure 4a. A strong doublet due to NH2 stretching at approximately 3430 and 3310 cm1 in the spectrum of the AQ monomer has turned into a broad singlet centered at around 3450 cm1 in three spectra of PAQs, which strongly suggests that almost all of the free NH2 groups in the monomer have changed to NH groups after polymerization. The IR absorption band of PAQs at (1) 1670 cm1 corresponds to the CdO stretching vibration of the quinone groups.24 Two bands at (2) 1580 and (3) 1490 cm1 are associated with the stretching of quinoid and benzenoid rings, respectively. The band at (4) 1270 cm1 can be attributed to the CN stretching vibration. The absorptions at 11701020 cm1 and around 723 cm1 could be attributable to the characteristic CH in-plane and out-of-plane bending vibrations of benzenoid rings, respectively.28 The three PAQs oxidized by three oxidants exhibit similar IR spectra, indicating their similar chain structures. However, some obvious differences could be found upon close comparison of the three spectra in Figure 4a.

ARTICLE

Figure 4. (a) IR and (c) UVvis spectra in concentrated H2SO4 of the AQ monomer and PAQ bases prepared with an oxidant/AQ molar ratio of 1 at 15 °C with the band assignments of 1, CdO; 2, CdC quinoid; 3, CdC benzenoid; and 4, CN; (b) IR and (d) UVvis spectra in H2SO4. (e) UVvis spectra in NMP of the PAQ virgin salts obtained with different polymerization temperatures at a fixed (NH4)2S2O8/AQ molar ratio of 1 in 50 mM HClO4/CH3CN for 48 h and fixed AQ and PAQ concentrations of 10 mg/L each for UVvis spectral analyses.

For example, if using either CrO3 or H2O2/Fe2þ as the oxidant, it seems that the peak at 1580 cm1 for quinoid rings is always slightly weaker than that at 1490 cm1 for benzenoid rings. In particular, the PAQ obtained by (NH4)2S2O8 exhibits a stronger peak at 1580 cm1 than at 1490 cm1, implying a higher content of quinoid rings in the polymer. In summary, a great difference between the IR spectra of the monomer and the oxidative products and the changes in the IR bands of quinoid and benzenoid structures of the oxidative products indicate that the oxidative products are real polymers rather than a simple complex or mixture of monomers with some oligomers. The IR spectra of PAQ salts oxidized by (NH4)2S2O8 at various polymerization temperatures are shown in Figure 4b. It is observed that a new band at around 1100 cm1 clearly appears and the absorption at 1400 cm1 becomes stronger because of the presence of perchlorate dopant anions,21 implying the incorporation of perchlorate groups into the PAQ chains. However, the band at 1100 cm1 obviously becomes weak when the polymerization temperature reaches 50 °C. This phenomenon reflects that the PAQ obtained at 50 °C has a weaker doping ability because of the existence of more oligomers. However, with the polymerization temperature increasing from 0 to 50 °C, the relative band intensity for the quinoid structure of the PAQ polymers seems to be maximal at 20 °C, which is coincident with the results of the UVvis spectra in Figure 4d,e and the electrical conductivity in Figure 2a. 9490



ARTICLE

Figure 5. X-ray diffractograms of AQ monomer and PAQ salts prepared with an oxidant/AQ monomer molar ratio of 1 in 50 mM HClO4/ CH3CN at 15 °C for 48 h. Thermal treatment: PAQ obtained with the oxidant of (NH4)2S2O8 was heated from 20 to 1000 at 40 °C/min under N2. Pb2þ adsorption: 50 mg PAQ obtained with the oxidant of (NH4)2S2O8 in 25 mL of 200 mg/L Pb(NO3)2 solution at 30 °C for 24 h.

UVVis Spectra. The UVvis absorption spectra of all PAQ salts in Figure 4ce exhibit three bands: the first band at 260280 nm owing to a ππ* transition,29,30 the second band at 420460 nm due to quinone groups, and the third band centered at 600650 nm from an nπ* excitation band from the benzenoid to the quinoid rings in the polymer chains,31 which are quite different from the UVvis spectra of the AQ monomer. The strong band of the monomer centered at 380480 nm due to quinone groups has become a moderate or even weak band for the three PAQs.32 Moreover, the PAQs display a unique band at 600671 nm corresponding to the excitation of the large π-conjugated system in their polyaniline-like units,33 but this band was not observed in the UVvis spectra of the AQ monomer. Therefore, such great differences between the UVvis spectra of AQ monomer and PAQ polymers must be attributed to a strong polymerization effect, and the obtained PAQs must be polymers. It can easily be found from Figure 4c that the intensity ratio of the exciton bands at 671 nm over 460 nm increases steadily as the oxidant changes from H2O2/Fe2þ to CrO3/H2O2 to (NH4)2S2O8/ H2O2 to CrO3 to (NH4)2S2O8. This increasing order coincides with the conductivity enhancement in Table 1. The much stronger exciton band of PAQs obtained by (NH4)2S2O8 and CrO3 verifies their much higher conductivity than that by the other three oxidants. Therefore, it is certain that (NH4)2S2O8 is the best oxidant for the formation of PAQ with the highest yield/conductivity and also the longest conjugation length. Meanwhile, by using (NH4)2S2O8 as the fixed oxidant, it is observed from Figure 4d,e that the PAQ salts obtained at 1015 °C have the largest intensity ratio of the exciton bands at 671 (or 584) nm over 460 (or 478) nm in H2SO4 and NMP. Furthermore, the exciton band of PAQ salts experiences a red shift with increasing polymerization temperature from 0 to 15 (or 20) °C, implying an increased extent of conjugation and a decreased band gap for the PAQ main chains. However, the PAQ salts formed at 35 (or 50) °C exhibit a dramatically hypsochromic shift or a considerably weak exciton band at 671 nm because of the formation of many oligomers at high temperature. This behavior is in agreement with the IR spectral results and further confirms its

Figure 6. Different magnification (a, c) TEM and (b, d) SEM images of PAQ polymer nanofibrils prepared with an (NH4)2S2O8/AQ molar ratio of 1 in 50 mM HClO4 in CH3CN at 20 °C for 48 h.

highest conductivity at 1520 °C (Figure 2a). The characteristics of the UVvis spectra vary significantly with testing solvents from H2SO4 to NMP, demonstrating stronger band to large π-conjugated chains in NMP but longer wavelength to large π-conjugated chains in H2SO4 because of a very strong redoping ability from H2SO4. X-ray Diffractograms. The wide-angle X-ray diffractograms in Figure 5 suggest that the PAQ virgin salts obtained with (NH4)2S2O8 and CrO3 have intermediate and higher crystallinity compared to polyaniline salts30 because a possible ordered or oriented PAQ chains exist in the PAQ nanofibril bundles illustrated in Scheme 1. In fact, the relatively high crystallinity of the PAQ salts obtained with (NH4)2S2O8 has been further confirmed by the relatively strong endothermicity due to crystallite melting in Figure 10 and high conductivity in Table 1. The PAQs show four diffraction peaks at Bragg angles 2θ of 8.9, 12.6, 18.4, and 25.8°. The two strongest peaks centered at 12.6 and 25.8° should be attributed to the periodicity parallel and perpendicular to the polyaniline-like main chains, respectively.30,33 The PAQ obtained with H2O2/Fe2þ appears to be the most amorphous and exhibits nearly the same weak intensity of the peaks at ca. 3 and 25°, suggesting the presence of larger ordered sizes. Although another two PAQ salts are moderately crystalline, as mentioned above, their crystallinity is still low relative to the highly crystalline AQ monomer (top curve in Figure 5). Therefore, it is concluded that the supramolecular structure of PAQ salts is quite different from that of the AQ monomer, which should be important evidence of the obtainment of a real PAQ polymer. Size and Morphology of the PAQ Nanofibril Bundles. The preparation of conducting polymers with nanostructures by conventional microemulsion and dispersion polymerization usually involves a large amount of external stabilizer, which leads to impure final nanoproducts. In this study, unexpected fine nanofibrils 9491



ARTICLE

Figure 7. (ac) SEM images of PAQ nanofibrils prepared at 15 °C and (d, e) TEM and (f) SEM images of PAQ nanofibrils prepared at 0 °C in the same 50 mM HClO4 in CH3CN for the same polymerization time of 48 h with the same (NH4)2S2O8/AQ molar ratio of 1.

of PAQ polymers were easily and directly obtained by chemical oxidative precipitation polymerization in an acidic organic medium without any external stabilizer. The size and morphology of the PAQ nanofibrils fabricated with (NH4)2S2O8 in 50 mM HClO4/CH3CN solution at 20 °C for 48 h were analyzed by TEM and FESEM techniques, as shown in Figure 6. The TEM and FESEM images show that the nanofibrils appear to gather into clusters. In Figure 6, a large number of nanofibrils have a diameter of around 15 nm and interpenetrate with each other to become single nanofibril bundles with a diameter of around 60 nm and larger nanofibril ribbons with the width of around 130180 nm. The PAQ synthesized at 15 and 0 °C can also form the nanofibril bundles, as shown in Figure 7. The nanofibrils merely look more irregular than those synthesized at 20 °C in Figure 6. It is interesting that the stable PAQ nanofibril colloids can clearly be observed when as-formed nanofibril bundles are washed and purified with water by centrifugation. The dispersion gradually becomes darker as the nanofibrils become purer with an increasing number of washing/centrifugation cycles. After three cycles of washing/centrifugation, the whole dispersion remains dark blue even after centrifugation at 3400 rpm for 20 min or longer, indicating that stable PAQ nanofibril colloids have formed because of self-stabilization through electrostatic repulsion. The size and distribution of the PAQ nanofibril colloids are shown in Figures 1b, 3b, and S2. The AQ monomer concentration and polymerization medium show a slight effect on the size and polydispersity of the PAQ colloids, but the polymerization temperature and time show much stronger effects. Only the polymerization at 020 °C for 4872 h could provide the PAQ colloids with a number-average diameter of around 200 nm and a size polydispersity index of 1.2. A similar phenomenon has been observed in the formation of other conducting polymer nanofibrils.34 It is apparent that the self-stabilization of the nanofibrils would be assigned to the existence of negatively charged quinone groups on the polymer units, as shown in Scheme 1. Because the AQ monomers also contain negatively charged quinone groups such as the PAQ nanofibrils, a statically repulsive interaction between AQ monomers and the as-formed nanofibrils would happen during

oxidative polymerization, avoiding the AQ monomer anchoring and thus PAQ secondary growth onto the as-formed nanofibrils. Furthermore, many charged quinone groups as internal stabilizers on the as-formed nanofibrils can provide strong static repulsion among the as-formed nanofibrils, thus efficiently stabilizing them and finally preventing them from coagulating. However, the polymer main chains have the tendency to force the nanofibrils to grow along the axial direction of the nanofibrils because of the high rigidity of the PAQ main chains. Therefore, the nanofibrils with high purity and a clean surface can facilely be obtained here because (1) no external stabilizer was added to the polymerization medium and (2) dissociative HCl, residual AQ monomer and oxidant, and its reducing product (NH4)2SO4 in or on the nanofibrils have been totally removed by centrifugation in water. It is concluded that the chemical oxidative polymerization of the AQ monomer in the HClO4/CH3CN medium can lead to the self-assemby of self-stabilized surfactant-free nanomaterials.35 Properties and Multifunctionality of the PAQ Nanofibril Bundles. Solubility. It can be seen from Table 1 that the solubility of the PAQ salts in some representative solvents with various dielectric constants depends significantly on the oxidants used for the polymerization. Generally, all polymers are soluble in concentrated H2SO4 and NMP and/or partially soluble in HCOOH, and the polymers obtained with H2O2/Fe2þ and (NH4)2S2O8 are mainly soluble in m-cresol. The PAQ obtained with H2O2/ Fe2þ is soluble in DMSO, but the PAQs obtained with CrO3/ H2O2 or (NH4)2S2O8 and CrO3 are only mainly or partially soluble in DMSO. Note that the PAQ salts are totally dissolved in concentrated H2SO4 without degradation, suggesting their extremely high chemical stability because of their wholly aromatic chain structure just like that of poly(p-phenylene terephthalamide). Furthermore, it seems that the PAQ oxidized by H2O2/Fe2þ exhibits a higher solubility in DMSO and CHCl3 than those oxidized by the other four oxidants, possibly because of the lower molecular weight caused by overoxidization from H2O2/Fe2þ with the highest RP. This result suggests that the solubility is primarily controlled by the molecular structure. All of the polymers are slightly soluble or even insoluble in THF, H2O, HCl, and NaOH 9492


The Journal of Physical Chemistry C aqueous solutions, signifying their good chemical resistance to strong acid/base aqueous media, which could be very beneficial to heavy metal ion sorption in wastewater containing strong acid/ base. Additionally, it should be noticed that the solubility of the PAQ polymers can also vary with the polymerization conditions. The PAQ salts obtained with (NH4)2S2O8 exhibit improved solubility in CHCl3, m-cresol, and DMSO with increasing polymerization temperature or shortening polymerization time owing to their decreasing molecular weight, though all PAQ salts synthesized at 050 °C for 972 h are constantly soluble in H2SO4 and NMP, partially soluble in HCOOH, and slightly soluble in THF. Solvatochromism. As listed in Table 1, the solution color of the virgin-doped PAQ salts depends significantly on the solvents used. The PAQ solutions are green in concentrated H2SO4 and become blue in NMP, in accordance with the results of UVvis spectra in Figure 4d,e. In other words, the green PAQ/H2SO4 solutions demonstrate a stronger band at longer wavelength up to 671 nm because of an additional doping effect from H2SO4 than do the blue PAQ/NMP solutions with a strong band at 584 nm. Moreover, the solution color of the polymers can turn bluish purple or even purple in m-cresol, DMSO, or formic acid. That is to say, PAQ might exist in different chain conformations or even configurations in various solvents. Nevertheless, the PAQ solution colors hardly ever change with polymerization conditions, including oxidant species, monomer concentration, polymerization temperature, and time. Bulk Electroconductivity. The bulk electrical conductivity of the PAQ strongly depends on polymerization conditions such as the AQ concentration, oxidant species, polymerization temperature, and time. As illustrated in Figure 1a, the redoped salt of PAQ that formed at an AQ concentration of 100 mM possesses a higher conductivity corresponding to a lower yield, but the redoped salt of PAQ that formed at an AQ concentration of 50 mM possesses the lowest conductivity corresponding to the highest yield. That is to say, the conductivity is inversely proportional to the yield with the AQ concentration for the polymerization. As summarized in Table 1, the conductivity of the PAQs remarkably changes with oxidant species. The PAQ formed with (NH4)2S2O8 exhibits the highest conductivity, but the polymer formed with H2O2/ Fe2þ shows the lowest conductivity. This phenomenon coincides with the results of IR and UVvis spectra, WAXD, and solubility (Figures 4 and 5). Furthermore, the conductivity varies dramatically over a wide range from 1010 to 103 S/cm with changing oxidant species with various standard RPs. Apparently, H2O2/Fe2þ is the worst oxidant but (NH4)2S2O8 is the best for the productive synthesis of the highly conducting PAQ polymer. This indicates that the standard RP of the oxidant plays an important role in the formation of the large π-conjugated main chains of the polymers. In particular, the conductivity of the PAQs obtained depends notably on the polymerization temperature (Figures 2 and 3). The most important finding is that the conductivity reaches a maximum of up to 3.36 103 S/cm at a polymerization temperature of 20 °C. A monotonically decreasing conductivity of the polymers with further increasing polymerization temperature from 20 to 50 °C can be ascribed to the formation of more polymers with a shorter π-conjugation length. The polymerization time is another important factor influencing the conductivity of the PAQs (Figure 3). A maximum conductivity appeared at a polymerization time of 48 h when (NH4)2S2O8 was used as the oxidant at a fixed oxidant/monomer ratio of 1 in 1 M HClO4/CH3CN solution at 20 °C. Normally, an increasing polymerization time would lead to a greater molecular mass and a longer π-conjugation length of

ARTICLE

Figure 8. Excitation (black lines) and emission (blue lines) scans for an AQ monomer solution in (a) NMP and (b) PAQ nanofibrils solvated in NMP and dispersed in (c) ethanol, (d) water, and (e) a 0.1 M DBSA aqueous solution at the same concentration of 10 mg/L. The spectral slit width when measuring the fluorescence intensity of PAQ nanofibrils dispersed in water is 10 nm; the others are 5 nm.

polymers, which will finally improve their conductivity. However, the conductivity of PAQs declines after a polymerization time of more than 48 h, possibly because of the secondary reaction of grafting onto the main chains after a certain degree of polymerization. After a comparison of the conductivitytime relation, it seems that 48 h is the optimal polymerization time for the synthesis of the PAQ with the highest conductivity and the second highest yield. By analogy, the conductivity of the redoped PAQ salt formed with CrO3 as the oxidant at 15 °C also displays a maximum of 2.4 104 S/cm at a polymerization time of 48 h. The enhanced chromic ion adsorbance onto the PAQ particles should be responsible for the lowered conductivity from 2.4 104 to 5.24 105 with increasing polymerization time from 48 to 96 h. It appears that the oxidative polymerization for chain propagation has stopped after 48 h. Note that the dependency of the conductivity on the polymerization time is different if the polymerization temperature was lowered from 15 to 0 °C with (NH4)2S2O8 as the oxidant. Therefore, the conductivity is very sensitive to the polymerization and doping conditions because the redox state, doping state, molecular weight, and π-conjugation length of the PAQs are dramatically influenced by the polymerization conditions. In other words, the conductivity could be efficiently optimized by controlling and regulating the polymerization and doping conditions. Fluorescence. The fluorescence characteristics, including the excitation and emission spectra of the AQ monomer and PAQ nanofibrils in several media, are shown in Figure 8. It is obviously found that all excitation spectra exhibited a strong band in the range of 230350 nm corresponding to π f π* electronic transitions. The triple bands in NMP are characteristics of many π-conjugated polymers and arise from a distribution of large π-conjugation lengths. The emission spectra, which are assigned to the radiative decay of excitons, have a more pronounced vibronic structure, exhibiting a maximum between 330 and 412 nm. Note that the shape, position, and intensity of the excitation and emission 9493


The Journal of Physical Chemistry C spectra of PAQs differ significantly from those of the AQ. The excitation and emission intensities of the PAQ/NMP solution due to fluorescence reinforcement by polyaniline-like π-conjugated chains are 2.7 times stronger than those of an AQ/NMP solution of the same concentration. In particular, the excitation and emission intensities of the PAQ nanofibril dispersion in 0.1 M DBSA are 3.2 times stronger than those of the AQ monomer. Apparently, the strongest fluorescence should be attributed to the best dispersion originating from the additional stabilization of DBSA toward the PAQ nanofibrils. Similarly, the second best and worst dispersions of the PAQ nanofibrils in ethanol and water, respectively, may be responsible for their second strongest and weakest fluorescence. These phenomena also reflect the fact that PAQ indeed has a very different molecular structure from the AQ monomer (i.e., PAQ is a real π-conjugated electroconducting polymer). Furthermore, the strongest nanoeffect of the PAQ nanofibrils in DBSA may be responsible for the strongest fluorescence, as discussed below. Some excitation and emission peaks of the PAQ in Figure 8 were blue- or red-shifted, accompanied by changes in the fluorescence intensity, distinctively depending on the medium around it. The excitation and emission are greatly affected by the solvent polarity because the solvent polarity might interfere with the fundamental and excited states of the polymer. Media of high polarity would stabilize the excited state with highly polar character on a low energy level, causing a red shift in the emission, whereas a blue shift is expected for solvents of low polarity. The fluorescence emission maximum of the PAQ dispersion shifts to longer wavelengths from 302 to 358 nm as the dielectric constant of the medium increases from 25 (ethanol) to 80 (water). The PAQ solution in NMP (Figure 8) presents three excitation bands at 274, 314, and 350 nm and two emission bands at 356 and 412 nm. In comparison to the PAQ solution in NMP, PAQ nanofibril dispersions in ethanol and water show very weak, blue-shifted fluorescence bands. However, the PAQ nanofibrils exhibit the strongest fluorescence band at 347 nm if dispersed in a 0.1 M DBSA aqueous solution. These results indicate that the dispersion state and microenvironment of the PAQ nanofibrils significantly influence the fluorescence properties. It is known that nanosized conducting polymers would produce a nanoeffect, shortening the π-conjugated bond length and leading to the localization of free electrons as a result of some lattice defects. However, the PAQ nanofibrils should have an expanded or “open” configuration when dispersed in the DBSA micelle solution because of the strong interaction between the micelle molecules and the functional groups on PAQ nanofibrils. Lead-Ion Adsorption. As shown in Figure 2b, the Pb2þ adsorbance and adsorptivity onto the PAQ nanofibrils increase first and then decrease with increasing polymerization temperature from 0 to 50 °C, displaying the respective maximal adsorbance and adsorptivity of 95.8 mg/g and 95.8% at 20 °C at the same time. In particular, this maximal Pb2þ adsorbability exactly corresponds to the minimal size polydispersity index of the PAQ fibrils formed at 20 °C in Figure 3b, possibly because of the presence of the most Nd/NH and CdO groups on the surface of the most uniform PAQ fibrils synthesized at 20 °C, which are small and have the smallest size polydispersity index (i.e., the largest specific surface area). Fewer Nd/NH and CdO groups and a much larger particle size of the PAQs obtained at lower and higher temperatures, respectively, have been revealed by the IR/UVvis spectral results in Figure 4 and laser particlesize analysis in Figure 3b, respectively. In short, the finest PAQ

ARTICLE

Figure 9. Effect of the PAQ polymer nanofibril content on the bulk electroconductivity of PAQ/poly(vinyl alcohol) nanocomposite films. The inset shows a photograph of the PAQ (0.1 wt %)/poly(vinyl alcohol) free-standing nanocomposite film with a thickness of around 15 μm.

fibrils synthesized at 20 °C possess the second highest polymerization yield, the highest conductivity, the longest UVvis absorbance wavelength, and the strongest Pb2þ adsorbability. The occurrence of Pb2þ adsorption on the PAQ nanofibrils has been further confirmed by the variation of the wide-angle X-ray powder diffractogram of the nanofibrils adsorbing Pb2þ (bottom curve in Figure 5). The PAQ salts adsorbing Pb2þ display a series of sharp, new diffraction peaks as compared to the original salts, indicating the existence of PbSO4 from a reaction between Pb2þ and SO42 groups as the dopant. This phenomenon is similar to Pb2þ sorption onto poly(m-phenylenediamine) microparticles,36 implying that the chelation and precipitation adsorptions of Pb2þ onto the PAQ salts occur simultaneously. Furthermore, the Pb2þ adsorption of PAQ nanofibrils could be attributable to a chelation between Pb2þ ions and the Nd groups through sharing the four lone pairs of electrons on the four nitrogen atoms.3739 In addition, other possible routes to Pb2þ adsorption could be an ion exchange of Pb2þ ions on the protonated Nd groups and the reduced quinone groups of polymer chains. In particular, PAQ exhibits a much stronger Pb2þ adsorbability than does poly(diaminoanthraquinone). Under the same adsorption conditions, poly(diaminoanthraquinone) synthesized with an oxidant (CrO3)/monomer (diaminoanthraquinone) molar ratio of 1 at 0 °C for 24 h has a low Pb2þ adsorbance and adsorptivity of 37.0 mg/g and 37.9%, respectively, suggesting that their macromolecular structures are obviously different. Conducting Nanocomposite Film of the PAQ Nanofibrils. Figure 9 reveals that the electroconductivity of poly(vinyl alcohol) nanocomposite films containing the PAQ nanofibrils rises dramatically from 2.14 1014 to 1.55 105 S/cm with increasing nanofibril content from 0 to 5.0 wt %. Although the composite films do not show any well-defined percolation threshold, the data have been fitted to the scaling law of percolation theory. The method yields a percolation threshold value of 0.1 wt % and a critical exponent of 1.86. The critical exponent is in good agreement with the predicted universal value (t = 1.72 for a 3D blend system).40 This percolation threshold is significantly lower than that of nanocomposite films of PAN nanofibrils in poly(methyl methacrylate).13 The very low percolation threshold in the PAQ nanofibril composites may be due to (1) the good dispersibility of the nanofibrils in poly(vinyl alcohol) aqueous solution and (2) the unique ability of the nanofibrils with a large 9494



Figure 10. TG and DTG thermograms of dedoped PAQ bases in air and nitrogen. The PAQ polymers were obtained with three oxidants at a fixed oxidant/AQ molar ratio of 1 in 50 mM HClO4/CH3CN at 15 °C for 48 h. Upper left inset: DSC thermograms of the as-polymerized PAQ base powders in nitrogen. The PAQ bases obtained by (NH4)2S2O8 were synthesized at 0, 15, and 20 °C. The heating rate is fixed at 20 °C/ min, except for the cyan line at 40 °C/min.

specific surface area and aspect ratio to connect facilely with each other to form a nanochannel or nanonetwork of effectively conducting electricity, as shown in Figures 6 and 7. The inset in Figure 9 is a photograph of the nanocomposite film containing 0.1 wt % PAQ nanofibrils in poly(vinyl alcohol). The nanocomposite film looks smooth and light blue and exhibits a conductivity of 7.76 109 S/cm. Melting and Degradation Accompanied by the Preparation of the Conducting Carbon Foam. The DSC scans of the PAQs synthesized with three oxidants at 0, 15, and 20 °C all show an endothermic peak centered at 7090 °C that is due to the evaporation of a little water trapped inside the polymers (Figure 10 (DSC) inset). In addition, respective strong and medium exothermic peaks at 264 and 361 °C were observed for PAQ oxidized by CrO3, possibly because of a series of complicated chemical reactions such as cross-linking. The PAQ oxidized by H2O2/Fe2þ displays very weak exothermic peaks at 244 and 309 °C. On the contrary, three PAQs oxidized by (NH4)2S2O8 at 0, 15, and 20 °C exhibit weak endothermic peaks at 325, 331, and 330 °C, respectively, probably originating from the melting of the crystalline PAQs. This means that the PAQs oxidized by CrO3 and H2O2/Fe2þ are thermally unstable compared to the PAQs oxidized by (NH4)2S2O8 because the former has a lower molecular weight and more impurities just like chromic or ferric/ferrous active ions, which was further proven by the following TG study in air. Figure 10 shows simultaneous TG/DTG/DSC curves of PAQs in air and nitrogen over a much wider temperature range. The polymers show a two-step weight-loss behavior in air. The first step is a 1020% weight loss at 200530 °C, which should be caused by the exclusion of the residual dopant, oxidants, and their reduced products trapped in the samples and the quinone and end-amino groups. The second-step major loss accompanied by tremendous

ARTICLE

exothermicity is attributed to the rapid decomposition of the polymer backbones. It is surprisingly noted that the PAQ oxidized by (NH4)2S2O8 at 15 °C exhibits a very high thermostability in air. The degradation of the PAQ main chains occurs up to 550 °C, and the temperature corresponding to the maximal weight-loss rate of only 0.8%/min dramatically increases to 625 °C. Apparently, the PAQ obtained with (NH4)2S2O8 as the oxidant possesses much better melting behavior, much higher thermostability, a much smaller burning enthalpy, much slower degradation, and a much lower char yield at 1000 °C than those obtained with CrO3 and H2O2/Fe2þ, indicating that the PAQ obtained with (NH4)2S2O8 has a higher molecular weight and a much purer composition because there should be impurities such as chromic and ferreous compounds in the PAQ obtained with the other two oxidants. It seems that this PAQ is more thermostable than most highly heatresistant polymers except for wholly aromatic polyimide, poly(3-ethynylphenanthrene), and poly(p-phenylene benzobisoxazole), which are very expensive (Table S1).4148 That is to say, PAQ could be one of the most thermostable, cost-effective polymers owing to a combination of its high aromaticity and the good π-conjugated structure of the polycyclic-type fused ring. Furthermore, it is surprisingly discovered that the PAQ powders oxidized by (NH4)2S2O8 at 15 °C exhibit a very high carbon yield in nitrogen (Table S1). After a slow weight loss of, at the utmost, 0.1%/min, at a relatively low temperature of 310 to 460 °C, a black pellet char yield at 1000 °C is up to ∼73.2%, that is, ∼96.3% of the theoretical carbon yield of 76.0%. The residual char pellet exhibits different X-ray diffraction (the second curve from the bottom in Figure 5) and a different density (0.9 g/cm3) but a much higher conductivity (50 S/cm) than initial PAQ base powders (3 109 S/cm in Figure 2) because the initial shaggy powders of the PAQ base (19.7 mg) in the ceramic crucible have converted or shrunk into a porous carbon foam pellet with a weight of 14.4 mg, a diameter of 4.1 mm, and a thickness of 1.2 mm in the inset of Figure 10 (TG) after an efficient melting and felting at an elevated temperature of 25 to 1000 °C. In other words, this PAQ could be a new potential carbon precursor because it can be facilely synthesized in acidic CH3CN/water at ambient temperature/pressure with low-cost dye intermediate AQ as a direct monomer and then directly carbonized at a low temperature of around 750 °C in nitrogen. However, it can achieve a higher carbon yield than most traditional flame-retardant polymers except for two ethynyl polymers and poly(p-phenylene), which can be obtained only at extremely high cost via a complicated synthesis process (Table S1). This carbon foam could act as an ultra-heatresistant shield when burning, protecting underlying substrates. PAQ presents new progress in advanced materials science because it offers a good combination of unique heat and flame resistance and high carbon yield with a simple synthesis route, a one-step carbonization procedure, and low cost.

’ CONCLUSIONS A new kind of intrinsically conducting polymer nanofibrils was successfully synthesized by the facile oxidative polymerization of the AQ monomer in an organic acidic medium in the absence of any external stabilizer. Five key synthesis parameters involving the monomer concentration, polymerization medium, oxidant species, polymerization, temperature, and time have been optimized for the formation of fine PAQ polymer nanofibrils with a high synthesis yield, fine fibril size, and optimal properties and functionalities including controllable electrical conductivity, strong 9495


The Journal of Physical Chemistry C fluorescence, good Pb2þ adsorbability, and high thermostability. The presence of quinone groups on the polymer chains is vital to the formation and self-stabilization of nanofibrils with a diameter of as small as 15 nm by means of the oxidative polymerization of the AQ monomer. The quinone groups act as a unique internal stabilizer that can engender strong electrostatic repulsion among the as-formed nanofibrils and further efficiently stabilize them. The polymer nanofibrils are good semiconductor with widely adjustable conductivity moving from 109 to 50 S/cm by simply regulating the polymerization parameters or carrying out reversible acid doping/alkali dedoping or heat treatment. The polymers exhibit good solubility in polar solvents such as concentrated H2SO4 and NMP and good chemical resistance in strong acid/ alkali solution. The nanocomposite film of PAQ nanofibrilsPVA exhibits a low percolation threshold of 0.1 wt % because the nanofibrils easily form a nanochannel or network that efficiently conducts electricity. Upon increasing the nanofibril loading from 0 to 5.0 wt %, the conductivity of the nanocomposites increases drastically from 2.14 1014 to 1.55 105 S/cm. These endow the nanofibrils with a wide variety of potential application as fluorescent and semiconducting materials and a powerful Pb2þ sorbent for the recovery and elimination of harmful heavy metal ions from wastewater. In particular, the PAQ nanofibrils possessing stronger nanoeffects demonstrate stronger fluorescence, more powerful Pb2þ adsorption, and a lower percolation threshold in their nanocomposite. Moreover, PAQ is a highly heat- and fireresistant material with a high carbon yield of 73.2% in nitrogen and very high cost efficiency because of its very cost-effective synthesis and carbonization route and the low cost of its monomer.

’ ASSOCIATED CONTENT

bS

Supporting Information. Dependence of the open-circuit potential and the temperature of the polymerization solution on the polymerization time, the size distribution of PAQ particles, and comparison of the thermal stability of traditional highly heatresistant polymers and the PAQ polymers. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (X.-G.L., M.-R.H.) þ86-21-65799455. Fax: þ86-2165799455. E-mail: (X.-G.L.) [email protected]. (M.-R.H.) [email protected]. (M.G.M.) mark.moloney@ chem.ox.ac.uk.

’ ACKNOWLEDGMENT This project was supported by the Royal Society, U.K., and the National Natural Science Fund of China (grant no. 20774065). ’ REFERENCES (1) McNeill, C. R.; Abrusci, A.; Hwang, I.; Ruderer, M. A.; M€uller-Buschbaum, P.; Greenham, N. C. Adv. Funct. Mater. 2009, 19, 3103–3111. (2) Tran, H. D.; Wang, Y.; D’Arcy, J. M.; Kaner, R. B. ACS Nano 2008, 2, 1841–1848. (3) Jang, J.; Ha, J.; Cho, J. Adv. Mater. 2007, 19, 1772–1775. (4) Nikiforov, M.; Liu, H. Q.; Craighead, H.; Bonnell, D. Nano Lett. 2006, 6, 896–900. (5) Roux, S.; Soler-Illia, G. J. A. A.; Demoustier-Champagne, S; Audebert, P; Sanchez, C Adv. Mater. 2003, 15, 217–221.

ARTICLE

(6) Willner, I.; Basnar, B.; Willner, B. Adv. Funct. Mater. 2007, 17, 702–717. (7) Bekyarova, E.; Kalinina, I.; Itkis, M. E.; Beer, L.; Cabrera, N.; Haddon, R. C. J. Am. Chem. Soc. 2007, 129, 10700–10706. (8) Wang, J.; Chan, S.; Carlson, R. R.; Luo, Y.; Ge, G.; Ries, R. S.; Heath, J. R.; Tseng, H.-R. Nano Lett. 2004, 4, 1693–1697. (9) Virji, S.; Huang, J.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491–496. (10) Rumbau, V.; Marcilla, R.; Ochoteco, E.; Pomposo, J. A.; Mecerreyes, D. Macromolecules 2006, 39, 8547–8549. (11) Yang, C. H.; Chih, Y. K. J. Phys. Chem. B 2006, 110, 19412–19417. (12) DeLongchamp, D. M.; Hammond, P. T. Adv. Funct. Mater. 2004, 14, 224–232. (13) Wang, Y. Y.; Jing, X. L. Mater. Sci. Eng., B 2007, 138, 95–100. (14) Park, J. H.; Lim, Y. T.; Park, O. O. Macromol. Rapid Commun. 2001, 22, 616–619. (15) Cho, M. S.; Choi, H. J.; To, K. W. Macromol. Rapid Commun. 1998, 19, 271–273. (16) Wu, Q.; Xu, Y.; Yao, Z.; Liu, A.; Shi, G. ACS Nano 2010, 4, 1963–1970. (17) Zhang, D. H. J. Appl. Polym. Sci. 2006, 101, 4372–4377. (18) ElRahman, H. A. A.; Schultze, J. W. J. Electroanal. Chem. 1996, 416, 67–74. (19) Won, M. S.; Yoon, W. H.; Shim, Y. B. Electroanalysis 2005, 17, 1952–1958. (20) Freire, J. A.; Dal Moro, G. A.; Toniolo, R.; Hummelgen, I. A.; Ferreira, C. A. Org. Electron. 2006, 7, 397–402. (21) Naoi, K.; Suematsu, S.; Hanada, M.; Takenouchi, H. J. Electrochem. Soc. 2002, 149, 472–477. (22) Li, X. G.; Li, H.; Huang, M. R. Chem.—Eur. J. 2007, 13, 8884–8896. (23) Ju, H. X.; Sun, H. B.; Chen, H. Y. Anal. Chim. Acta 1996, 327, 125–132. (24) Naoi, K.; Suematsu, S.; Manago, A. J. Electrochem. Soc. 2000, 147, 420–426. (25) Zhang, X. Y.; MacDiarmid, A. G.; Manohar, S. K. Chem. Commun. 2005, 5328–5330. (26) Pud, A. A.; Noskov, Y. V.; Kassiba, A.; Fatyeyeva, K. Y.; Ogurtsov, N. A.; Makowska-Janusik, M.; Bednarski, W.; Tabellout, M.; Shapoval, G. S. J. Phys. Chem. B 2007, 111, 2174–2180. (27) Li, X. G.; Huang, M. R.; Hua, Y. M. Macromolecules 2005, 38, 4211–4219. (28) Li, X. G.; Huang, M. R.; Li, F.; Cai, W. J.; Jin, Z.; Yang, Y. L. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 4407–4418. (29) Nguyen, M. T.; Kasai, P.; Miller, J. L.; Diaz, A. F. Macromolecules 1994, 27, 3625–3631. (30) Li, X. G.; Lu, Q. F.; Huang, M. R. Chem.—Eur. J. 2006, 12, 1349–1359. (31) Wei, Y.; Focke, W. W.; Wnek, G. E.; Ray, A.; MacDiarmid, A. G. J. Phys. Chem. 1989, 93, 495–499. (32) Yamamoto, T.; Etori, H. Macromolecules 1995, 28, 3371–3379. (33) Chaudhari, H. K.; Kelkar, D. S. J. Appl. Polym. Sci. 1996, 62, 15–18. (34) Li, D.; Kaner, R. B. Chem. Commun. 2005, 3286–3288. (35) Zhang, G. Z.; Niu, A. Z.; Peng, S. F.; Jiang, M.; Tu, Y. F.; Li, M.; Wu, C. Acc. Chem. Res. 2001, 34, 249–256. (36) Huang, M. R.; Peng, Q. Y.; Li, X. G. Chem.—Eur. J. 2006, 12, 4341–4350. (37) Li, X. G.; Feng, H.; Huang, M. R. Chem.—Eur. J. 2010, 16, 10113–10123. (38) Jin, L.; Bai, R. Langmuir 2002, 18, 9765–9770. (39) Gupta, R. K.; Singh, R. A.; Dubey, S. S. Sep. Purif. Technol. 2004, 38, 225–232. (40) McCullen, S. D.; Stevens, D. R.; Roberts, W. A.; Ojha, S. S.; Clarke, L. I.; Gorga, R. E. Macromolecules 2007, 40, 997–1003. (41) Bourbigot, S.; Flambard, X. Fire Mater. 2002, 26, 155–168. (42) Li, X. G.; Huang, M. R. Polym. Degrad. Stab. 1999, 64, 81–90. (43) Naffakh, M.; Gomez, M. A.; Marco, C.; Ellis, G. Polym. Eng. Sci. 2006, 46, 129–138. 9496



ARTICLE

(44) Li, X. G.; Huang, M. R.; Bai, H.; Yang, Y. L. Macromol. Mater. Eng. 2001, 286, 421–428. (45) Musikabhumma, K.; Masuda, T. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 3131–3137. (46) Choi, S. W.; Ohba, S.; Brunovska, Z.; Hemvichian, K.; Ishida, H. Polym. Degrad. Stab. 2006, 91, 1166–1178. (47) Wang, M. C.; Zhao, T. J. Appl. Polym. Sci. 2007, 105, 2939–2947. (48) Okuzaki, H.; Kubota, I. Synth. Met. 2005, 153, 161–164.

9497


Synthesis and Multifunctionality of Self-Stabilized Poly

Recommend Documents