Water-Dispersed Conductive Polypyrroles Doped with Lignosulfonate

Jun 16, 2009 - The water-dispersed conductive polypyrroles (PPys) were prepared via the chemical oxidative polymerization with lignosulfonate (LGS), a...
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Ind. Eng. Chem. Res. 2009, 48, 9498–9503

Water-Dispersed Conductive Polypyrroles Doped with Lignosulfonate and the Weak Temperature Dependence of Electrical Conductivity Chao Yang and Peng Liu* State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou 730000, People’s Republic of China

The water-dispersed conductive polypyrroles (PPys) were prepared via the chemical oxidative polymerization with lignosulfonate (LGS), an inexpensive byproduct from pulp processing industries, as a dopant and template for tuning the properties of polypyrrole nanoparticles. The lignosulfonate doped polypyrrole (PPy/LGS) nanoparticles were characterized with Fourier transform infrared (FTIR), UV-vis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electric conductivity. The diameters of the PPy/LGS nanoparticles decreased from 100 to 20 nm with the increasing LGS. Their electrical conductivities were affected markedly by the dopant amount added. The PPy/LGS have weakly temperature-dependent conductivity over the entire temperature interval from 283 to 423 K. Introduction Conducting electroactive polymers (CEP) remain a subject of intense investigation for many research groups worldwide. Among the conductive polymers investigated, polypyrrole (PPy) is one of the most extensively studied ones, because it possesses the characteristics of high conductivity, oxygen resistance, thermal and environmental stability, relative ease of synthesis, and nontoxicity, which are favorable for various applications such as metallization of dielectrics,1 batteries,2 anticorrosive coatings,3 shielding of electromagnetic interferences,4 sensors,5 actuators,6 microactuators,7 and so on. Nevertheless, a few applications have been reported because the conducting polymers based on PPy exhibit poor physical and mechanical properties and are not soluble in common solvents.8 Various sulfonates or organosulfonic acids have been widely used in the in situ doping polymerization to afford PPy with high conductivity, good solubility, and special morphology.8 Most recently, polymeric protonating agents have also been used for the synthesis of soluble, electroactive polymers.9-16 This research mainly focused on polyaniline.9-14 The natural polymeric acids, the derivatives of the natural polymers such as lignin14,16 and cellulose,17 have attracted more and more attention as the stabilizers of conducting polymers due to their biocompatibility and biodegradation. Lignosulfonate (LGS) (Figure 1), an inexpensive byproduct from pulp processing industries, produced by sulfonation or sulfomethylation of the lignin (the second most abundant biopolymer in nature; it is the component of plants and trees that gives them rigidity),18 it has become increasingly valuable for its versatility in performance for scale inhibitor,19 water reducer,20 drilling mud thinner,21 composites,22 building materials,23 dispersant,24 corrosion inhibitor,25 fuel cells,26 etc. The LGS doped polypyrrole had been prepared via the chemical oxidative polymerization of pyrrole with LGS as both polymeric template and dopant for corrosion protection.15,16 * To whom correspondence should be addressed. Tel.: 86-9318912516. Fax: 86-931-8912582. E-mail: [email protected].

In the present work, the effect of LGS on the morphology and electrical conductivity of polypyrrole was investigated. Experimental Section Materials. Pyrrole monomer (Acros Organics) was distilled under reduced pressure after being dehydrated with calcium hydride for 24 h. Sodium lignosulfonate (Yeats Chemical Reagent Co., Tianjin, China) as a dopant and iron(III) chloride hexahydrate (Tianjin Chemical Reagent Co., Tianjin, China) as an oxidant were used as received. The double distilled water was used throughout. Preparation of PPy/LGS. The sodium lignosulfonate (LGS) was added to 100 mL of double distilled water in a 250 mL round-bottom flask and stirred for 15 min. Freshly distilled pyrrole (1 mL, 14.4 mmol) was added to the above solution and stirred for 45 min in ice-water bath to obtain a brown solution. The conditions of the polymerizations are given in Table 1. Iron(III) chloride hexahydrate (4.152 g, 0.015 mol) in 10 mL of double distilled water was added drop by drop to the above solution, and stirring was continued for 10 h under icecold condition. The resulting polypyrrole was purified by

Figure 1. Typical monomer unit of LGS. Table 1. Polymerization Conditions PPy/LGS

pyrrole (mL)

LGS (g)

LGS (wt %)

S-1 S-2 S-3 S-4 S-5 S-6 S-7

1 1 1 1 1 1 1

0.1 0.3 0.5 0.7 1.0 1.5 2.0

9.35 23.64 34.03 41.94 50.78 60.75 67.36

10.1021/ie900189j CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

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Figure 2. FTIR spectra of the pure PPy, LGS, and PPy/LGS samples.

pouring into a large excess of distilled water, filtered, and washed with distilled water and methanol until the filtrate become colorless. The black powder sample was dried under vacuum (0.1 mm of Hg) for 24 h prior to further analysis. Analysis and Characterizations. The Fourier transform infrared (FTIR) measurements (Impact 400, Nicolet, Waltham, MA) were carried out with the KBr pellet method. Electronic absorption spectra of the PPy/LS were recorded in water solution in the wavelength range of 200-1200 nm at room temperature by using Lambda 35 UV-vis spectrometer (PerkinElmer, U.S.A.).Thermogravimetric analysis (TGA) was obtained with a TA Instrument 2050 thermogravimetric analyzer at a heating rate of 10 °C/min from 25 to 700 °C under a nitrogen atmosphere. The X-ray diffraction (XRD) patterns were recorded in the range of 2θ ) 5-60° by step scanning with a Shimadzu XRD6000 X-ray diffractometer. Nickel-filter Cu KR radiation (λ ) 0.15418 nm) was used with a generator voltage of 40 kV and a current of 30 mA. The morphologies of the PPy/LGS samples were characterized with a JEM-1200 EX/S transmission electron microscope (TEM; JEOL, Tokyo, Japan). The powders were dispersed in water in an ultrasonic bath for 30 min and, then, deposited on a copper grid covered with a perforated carbon film. The electrical conductivities of the PPy powders were measured using SDY-4 four-point probe meter (Guangzhou Semiconductor Material Academe) at ambient temperature. The pellets were obtained by subjecting the powder sample to a pressure of 30 MPa. The reproducibility of the result was checked by measuring the resistance three times for each pellet. The temperature dependence of conductivity was determined by WDJ-1 temperature change resistance measuring instrument (Institute of Chemistry the Chinese Academy of Sciences) at a heating rate of 10 °C/min from 25 to 150 °C. Results and Discussion Spectral Analysis. Figure 2 shows the FTIR spectra of the KBr pellets of the PPy/LGS samples in the region of 2000-400 cm-1. The spectra show a rich-band fingerprint region, revealing seven strong intensity bands. The peaks at 1540 and 1450 cm-1 could be attributed to CsN and CsC asymmetric and symmetric ring-stretching, respectively. Additionally, the strong peaks near 1160 and 890 cm-1 present the doping state of polypyrrole, the peak at 1030 cm-1 is attributed to CsH deformation and NsH stretching vibrations, and the broadband at 1300 cm-1 demonstrates the CsH and CsN in-plane deformation vibration, respectively. All results demonstrate almost the same peak positions of the main IR bands which are associated with the structure of the PPy.27 The absorbance of

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Figure 3. UV-vis absorbance spectra of PPy/LGS in water.

the OH bands at 3352, 1323, 1217, and 1033 cm-1, methoxy group bands at 2939, 2881, 1460, and 1425 cm-1, CdC vibration of aromatic ring at 1514 cm-1, and SdO stretching at 1030 cm-1 of LGS also appeared.28 Additionally, it was observed that the intensity of the absorption band at 1030 cm-1 increased with an increasing mass ratio of LGS and pyrrole. The PPy/LGS were freely suspended in water by stirring under ultrasonic vibration at room temperature. It was found that the samples which contain more LSG were more dispersible in water. Sedimentation was found in sample S-4 after two weeks. However, sample S-7 had been well-dispersed for more than months. UV-visible spectra of the samples were recorded in water and are given in Figure 3. The absorption spectra of LGS showed a strong transition at 280 nm corresponding to the π-π* transition band.29 The shift of the absorption band from 291 to 282 nm could be attributed to the polymer-solvent interaction. The nature of the solvent molecules influences the polymer chain conformation and the length of the conjugated segments, leading to a diference in the energy gaps.30 The UV-visible spectra showed two distinct bands at 460 nm and a free tail above 800 nm in the near infrared (NIR) region. These three transitions corresponded to the transitions from valence bond to bipolarons and antibipolarons of the oxidized form of polypyrrole. The increase in the intensity of the low-energy transition in the NIR for PPy/LGS samples S-4 and S-7 revealed that these were highly doped as compared to sample S-1. This indicated that polypyrrole had been doped by anionic lignosulfonate. Morphological Analyses. The TEM images of the PPy/LGS powders that were dispersed in water were given in Figure 4. The morphologies of the PPy/LGS powders became smaller with the increasing LGS. The PPy/LGS sample S-1 could not be dispersed. However, the dispersibility of the PPy doped with LGS increased with an increase of the LGS feed ratio. TEM images of the samples clearly indicate that the materials have a uniform solid nanoparticle (no hollow spheres or fibers) morphology and their diameters decreased from about 100 to 20 nm with the increase of the LGS feed ratio. This indicated that the LGS molecules acted as the templates as well as the dopant, as reported previously.16 Thermal Analysis. Figure 5 shows the TGA curves of the PPy/LGS samples. PPy/LGS samples containing 50-70% LGS showed a severe weight loss 7-9% from about 130 °C, while PPy/LGS samples S-1 and S-2 showed a slight weight loss. This may be due to the difference in the doping level of LGS which interacts with H2O through the formation of hydrogen bonds between the carbonyls or between the carbonyls and the hydroxyl groups present in the LGS, as compared to that of the organic sulfonate.31 The weight loss relative to the PPy/LGS composites about 130 °C could be indicating the hygroscopic

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Figure 4. TEM images of the PPy/LGS.

Figure 5. TGA curves of PPy/LGS samples. 32

character of lignin that is more pronounced than that of PPy. The second mass losses at about 175 °C are attributed to degradation of lignin and PPy. It was reported that the decomposition of lignin showed two maxima in the mass loss corresponding to the release of moisture at 92 °C and to lignin decomposition in a broad temperature range from 150 to 650 °C, respectively.33 Weight losses of PPy/LGS composites containing 50 and 70% LGS, when heated to 700 °C, are much less than that of PPy/LGS composites which contain less than 10% LGS. Free sulfonate groups are, in general, thermally active, and they possibly weaken the thermal degradation of the PPy composite as shown in Figure 5. However, the sulfonate group becomes much more thermally stable when it forms a complex with a countercation. Thus, the TGA curves of the PPy/LGS composites clearly demonstrated the increased thermal

Figure 6. Conductivities of various rates Py/LGS samples at room temperature.

stability on complexation of PPy with the polyelectrolyte, which in turn offers thermal processing advantages. Electrical Conductivity of PPy Samples at Room Temperature. Electrical conductivities were determined for each of the samples, and the results were shown in Figure 6. It was observed that conductivity was determined to be on the order of 2.70 S/cm without addition of any external dopants. Although the conductivity of the PPy doped with LGS increased with an increase of the LGS feed ratio, it decreased with increasing exorbitant amounts of LGS, similar to the results reported.34,35 Conductivity showed little change with the optimized feeding ratio range of LGS from 9.35% to 67.36%. Thus, we conclude that LGS plays the role of dopant as well as template, as shown in Figure 7.

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Figure 9. Temperature dependence of electrical conductivity of PPy/LGS based on the Greaves equation. Figure 7. Proposed schematic for the formation of the molecular complex PPy/LGS.

conductivity. All PPy/LGS samples follow the law σ(T) ) σo exp[-(To/T)m], which has been explained within the framework of the granular metals model (m ) 0.5) for PPy-DBSA and Mott’s variable range hopping model (m ) 0.25) for PPy-DBS-BF, respectively.39 The conduction mechanisms for conducting polymers have been investigated from various models related to the temperature dependence of electrical conductivity. According to work reported,40 for inhomogeneous samples where partial dedoping or strong morphological disorder dominates the sample properties, transport could be expressed as eq 1: log σ ) σ0 exp(-AT-1/2)

Figure 8. Temperature dependence of electrical conductivity of PPy/LGS samples.

Additionally, LGS is not an electric conductor, and an excessive feed ratio could induce a decline of the conductivity. The decrease of conductivity should be considered to result from the long chain characteristics of LGS possibly preventing a number of the sulfate anions in LGS from doping the PPy moiety, leaving many free sulfate anions in the composite. It is known that 25-30 mol % of PPy moiety is generally doped when synthesized by chemical polymerization.36 Therefore it must be assumed that the effective doping level of the PPy/LGS composite must be lower than its apparent doping level, even though the apparent doping level increased with the amount of LGS. Excessive amounts of LGS interact with PPy through the formation of hydrogen bonds between the PPy secondary amine groups and carbonyls or the hydroxyl groups present in the LGS as suggested in Figure 7. Nevertheless, we can speculate that the doping level of PPy/LGS composites is close to about 25% since the electrical conductivities of the composites are in the same range. Temperature Dependence of the Conductivity and XRD Analysis. The temperature dependence of the conductivity of PPy is characteristic of disordered metals near the metalinsulator transition.37 The temperature dependences of the conductivity, σ(T), of PPy/LGS samples are shown in Figure 8. This figure clearly indicates that, despite of rather similar sample preparation conditions and the same dopant, LGS, the temperature dependences of the conductivity shows that all the samples had a semiconducting property. For semiconductors, the higher the temperature, the higher the electrical conductivity.38 However, the samples which are in the low feed ratios have a strongly temperature-dependent conductivity over the entire temperature interval from 283 to 423 K. It is worth noticing that samples which are in the high feed ratios have a weakly temperature-dependent

(1)

It was found that the temperature dependence of electrical conductivity of water-dispersed polypyrroles followed the 3D variable range hopping model,41 as expressed in eq 2: σ ) A exp(-BT-1/4)

(2)

Greaves suggested that the phonon-assisted hopping mechanism applied for amorphous materials in general temperature ranges as in eq 3:42 σT1/2 ) exp(-BT-1/4)

(3)

To investigate the conduction mechanism, we plot the temperature dependence of electrical conductivity for PPy/ LGS. The results showed that the plot of Greaves’ model had most linear behavior over the entire measured temperature range, as shown in Figure 9. However, the samples which are at the low feed ratios have not in accord with Greaves’ model. It might be due to morphological disorder. When the feeding ratio of LGS is lower than 41.94%, strong morphological disorder dominates the sample properties. Increasing the feed ratio, the morphological disorder of samples is more and more weak. Moreover, those samples are shown in accordance with Greaves’ model. Namely, the samples show higher order. The polyelectrolyte LGS as a template has been known to have a profound effect on the ability to charge and preferentially align the monomers on the template prior to polymerization via the interaction between the template molecules and monomers.43 Pyrrole polymerization in the presence of polyelectrolyte templates shows a highly ordered conducting electroactive polymer as Figure 10. This was also substantiated by the XRD shown in Figure 11. The solid-state properties of PPy/LGS composites were studied for finely powdered samples using XRD. Polypyrrole is a highly rigid polymer because of its linear structure and less flexible chain folding to induce crystalline domain. As doping with LGS, the dopant-polymer undergoes various interactions, which tend to organize the polymer chains in three-dimensional ordered

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Figure 10. Proposed schematic for the polymerization of pyrrole in the presence of polyelectrolyte templates.

molecules will be utilized to fine-tune the polypyrrole nanostructured materials, which will be published elsewhere. Literature Cited

Figure 11. XRD patterns of PPy/LGS samples.

fashions. Decreasing the amount of LGS affects the nature of the peaks in the XRD plots. The peak at 2θ showed a shift toward a higher angle, whereas it was assigned to scattering from pyrrole-counterion or intercounterion interactions and the pyrrole-pyrrole interplanar distance. This indicates that the decrease in the feeding amount of LGS increases the interplanar distance in the PPy/LGS composites and produces a rather ordered polypyrrole. Moreover, the penetration of the surfactant molecule increases the interplanar distance, and therefore, the solvent molecules easily enter into the lattices to dissolve the polymer chain in water. Conclusion In conclusion, we have developed a renewable resource lignosulfonate and utilized it as a dopant for water-dispersed, less temperature-dependent, solid-state ordered, and uniform size polypyrrole nanoparticles. The present approach is demonstrated for a larger window of pyrrole/LGS: the feeding ratio of the dopant LGS in the PPy/LGS composites was changed from 9.35% to 60.75%, for the first time, to systematically control the aggregation of pyrrole/LGS for tuning the properties of polypyrrole. The present approach has many advantages: (i) a renewable resource anionic lignosulfonate was utilized as the structure-directing agent for tuning the properties of polypyrrole nanoparticles; (ii) the lignosulfonate is built for producing ordered polypyrrole, and its presence enhances the dispersibility of the nanomaterials in water; (iv) the PPy/LGS has weakly temperaturedependent conductivity over the entire temperature interval from 283 to 423 K, which is very rarely reported in the literature; (v) the lignosulfonate is very cheap and can be easily obtained from pulp processing industries. These

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ReceiVed for reView February 3, 2009 ReVised manuscript receiVed May 30, 2009 Accepted June 2, 2009 IE900189J