Original Article
Synthesis, characterization, and properties of aniline-pphenylenediamine copolymers
High Performance Polymers 25(3) 348–353 ª The Author(s) 2012 Reprints and permission: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954008312465451 hip.sagepub.com
Tingxi Li, Chengqian Yuan, Yuhua Zhao, Quanliang Chen, Meng Wei and Yanmin Wang
Abstract Polyaniline (PANI), poly(aniline-co-p-phenylenediamine) (P(Ani-co-pPD)), and poly(p-phenylenediamine) (PpPD) have been synthesized by oxidative polymerization using ammonium persulphate as an oxidant in hydrochloric acid medium. Several important properties such as solubility, electrical conductivity, and crystallinity of these polymers were comprehensively compared. The polymers were characterized by infrared spectroscopy, x-ray diffraction, thermogravimetry, scanning electron microscopy, and a four-point probe conductivity method. The results reveal that PANI shows the maximum conductivity and thermal stability among the polymers; P(Ani-co-pPD) exhibits the best solubility, PpPD presents the highest crystallinity, and all the polymers exist in granular morphology. Keywords poly(aniline-co-p-phenylenediamine), electrical conductivity, x-ray diffraction, thermogravimetry, scanning electron microscopy
Introduction In recent years, conducting polymers have attracted a lot of scientific and technological interest. Among these conducting polymers, polyaniline (PANI) is a very useful polymer in the fields of microelectronics, electrochromic displays, electromagnetic shieldings, and sensors,1,2 because of its high environmental stability, controllable electrical conductivity, and unique interesting redox properties controlled by its different oxidation and protonation states.3 However, the main disadvantages of PANI are its poor solubility and poor processability both in melt and solution due to its stiffness of the backbone,4 which limited its further extensive applications in many areas. An important approach to remarkably improve the processability of PANI is the copolymerization of aniline with the substituted anilines. Recently, a great deal of attention has been paid for the synthesis and characterization of copolymers of aniline and its derivatives.5–10 As one of the influential derivatives of PANI, poly(p-phenylenediamine) (PpPD) has been widely synthesized by electropolymerization and chemical oxidative polymerization.11 PpPD has demonstrated a great potentiality for use as electrochromic display materials, humidity sensors, electrode-modified materials, pH response, protection
against metal corrosion, and so on.10 Among the conducting copolymers, poly(aniline-co-p-phenylenediamine) (P(Ani-co-pPD)) has attracted much attention due to its film forming, electronic, and electrochromic properties.8 In our previous work, we have successfully synthesized PANI by microemulsion polymerization12 and electropolymerization13 and found that the film of PANI obviously slow down the rate of corrosion of stainless steels in NaCl. Although the chemical oxidative polymerization has previously been successfully used for the preparation of PANI, PpPD, and P(Ani-co-pPD), to the best of our knowledge, little attention has been paid to the comparative study on their properties systematically. The purpose of this report was to make a comparative study on several important properties, including solubility, electrical conductivity, crystallinity, thermostability, morphology, and molecular
School of Material Science and Engineering, Shandong University of Science and Technology, Qing Dao, China Corresponding author: Tingxi Li, School of Material Science and Engineering, Shandong University of Science and Technology, 579 Qianwangang Road Economic and Technical Development Zone, Qing Dao 266590, China. Email:
[email protected]
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349 H N
N N H
n
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(a) H N
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HN
NH
NH
NH
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H2N
H N
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H N
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N H
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N H
(c) Figure 1. The chemical structure of polymers: (a) PANI, (b) P(Ani-co-pPD), and (c) PpPD. PANI: polyaniline; P(Ani-co-pPD): poly(aniline-co-p-phenylenediamine); PpPD: poly(p-phenylenediamine).
structures, of these polymers synthesized through chemical oxidative polymerization in an acid medium.
DSC1/1600LF) from 50 to 650 C at a heating rate of 10 C min1 under nitrogen atmosphere. The morphologies and sizes of the resulting products were characterized by scanning electron microscopy (SEM; KYKY-2800B).
Experimental section Materials and measurements
Synthesis of P(Ani-co-pPD)s
Aniline, p-phenylenediamine (pPD), hydrochloric acid (HCl), ammonium persulphate (APS; (NH4)2S2O8), and ethanol were purchased as analytically pure reagents from Tianjin Damao Chemical Reagent Co. (Tianjin, China). Aniline was distilled twice under reduced pressure. APS, HCl, and ethanol were used as received without further purification. The solutions were prepared using doubledistilled water. The solubility of the polymers was characterized using the following method. Polymer powder sample of 10 mg was added to the solvent of 1 ml and dispersed thoroughly. After constant slight shaking of the mixture for 24 h at room temperature, the solubility of the polymers was determined. Room temperature conductivity of the pressed pellets was measured by a Hall effect measurement system (Ecopia, HMS-5000) using Van Der Pauw four-probe method. Fourier-transformed infrared (FT-IR) spectra of the polymers were obtained with a Nicolet 380 FT-IR spectrometer in the range of 4000–400 cm1 with 32 scans. The samples were dispersed in potassium bromide pellets. X-Ray diffraction (XRD) patterns were collected on a powder x-ray diffractometer (Rigaku D/MAX 2500PC) with Cu Ka radiation (l ¼ 0.1542 nm). Thermogravimetric analysis (TGA) was performed on a TA instruments (TGA/
The PANI, PpPD, and P(Ani-co-pPD) were synthesized by adopting the literature procedure of Rani et al.8 For the preparation of P(Ani-co-pPD), 0.01 M of pPD and 0.01 M of aniline were dissolved in acidic aqueous solution of about 160 ml of 1.0 M HCl. The solution was cooled in an ice bath and stirred well. The polymerization reaction was initiated by steadily adding a precooled solution containing 0.02 M of APS in 40 ml of 1.0 M HCl to the monomer solution that was kept under vigorous stirring. Dropwise addition of oxidant is beneficial for obtaining polymer with a relatively higher molecular weight because the oxidative polymerization of the aromatic diamines is highly exothermic. The addition was continued for about 3 h with constant stirring. The reaction mixture was stirred for 24 h in the ice-water bath. After the reaction was terminated by pouring 300 ml ethanol into the reactant solution, the obtained black-coloured copolymer–HCl salt was washed successively with deionized water and ethanol to remove the oxidant and oligomers until the filtrate became colourless. The copolymer was dried in an oven at a temperature between 45 and 55 C. A drying temperature of higher than 60 C might lead to the cross-linking of the polymer. The same procedure was adopted to synthesize PANI and PpPD, while keeping the amount of monomer
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Table 1. The conductivity and solubility of polymers.
PANI P(Ani-copPD) PpPD
Conductivity (S/cm)
NMP DMSO DMF THF Acetone
1.98 101 2.12 105
S S
PS S
MS S
SS MS
IS PS
4.17 109
S
MS
S
SS
IS
S: soluble; MS: mostly soluble; PS: partially soluble; SS: slightly soluble; IS: insoluble; PANI: polyaniline; P(Ani-co-pPD): poly(aniline-co-p-phenyleneia mine); PpPD: poly(p-phenylenediamine); DMF, dimethylformamide; NMP, N-methylpyrrolidone; THF, tetrahydrofuran; DMSO: dimethyl sulphoxide.
unchanged. The macromolecular structure of the polymers is shown in Figure 1.
(b) Transmittance (%)
Polymer
(a)
(c)
4000 3600 3200 2800 2400 2000 1600 1200
Results and discussion Conductivity and solubility The conductivity and solubility of polymers are shown in Table 1. It is clear that the conductivity of PANI is much higher compared with that of other polymers as shown in Table 1. It is because in PANI, all the benzene rings and nitrogen are expected to be in the same plane where p electron delocalization is very high leading to high electrical conductivity.14 For the copolymer and PpPD, the benzene rings may not be in the same plane, and as a result, the p electron delocalization is expected to be hindered leading to a decrease in conductivity. The most important factor,8 which determines the solubility of polymer, is the interaction between polymer chains and solvent molecules. As can be seen in Table 1, the copolymer and PpPD exhibit better solubility than PANI. This may be attributable to the distortions in the polymer chains and the increasing chain flexibility, which facilitate polymer–solvent interactions and overcome polymer–polymer interactions. Consequently, the copolymer and the PpPD have better solubility than PANI.
FT-IR spectra of P(Ani-co-pPD)s FT-IR spectroscopy was used to investigate the molecular structures of three polymers. As can be seen in Figure 2, the infrared (IR) spectra of the polymers synthesized using HCl showed the following characteristic bands. The longdescending baseline in the spectral region 2000–4000 cm1 is attributed to free-electron conduction in the doped polymers. In the IR spectra of PANI, the main peaks at 1560 and 1493 cm1 correspond to stretching deformations of quinone and benzene rings, respectively. The bands at 1293, 1122, and 880 cm1 can be assigned to C–N stretch in a secondary aromatic amine, the aromatic C–H in-plane bending modes, and the out-of-plane deformations of C–H in the 1,4-disubstituted benzene ring, respectively.
800
400
Wavenumber (cm ) –1
Figure 2. FT-IR spectra of (a) PANI, (b) P(Ani-co-pPD), and (c) PpPD. PANI: polyaniline; P(Ani-co-pPD): poly(aniline-co-p-phenylenediamine); PpPD: poly(p-phenylenediamine); FT-IR: Fouriertransformed infrared.
The observed IR bands confirmed that the PANI was in the state of protonated emeraldine salt. In the IR spectra of P(Ani-co-pPD), the peaks at 1576 and 1502 cm1 are related to C¼C stretching vibration of quinoid and benzenoid rings, respectively. The peak at 1315 cm1 is attributed to the C–N stretching vibration of secondary amine. The peaks at 1112 and 815 cm1 are associated with the aromatic C–H bending in the plane and out of plane of 1,4-disubstituted aromatic ring. The peaks in the frequency range of 3200–3500 cm1 correspond to the N–H stretching vibration of the secondary amine groups of the copolymer. The appearance of these IR bands should verify the formation of copolymer.8 In the IR spectra of PpPD, the main peaks at 1576 and 1493 cm1 can be ascribed to stretching deformations of quinone and benzene rings, respectively. The bands at 1293, 1122, and 880 cm1 can be assigned to C–N stretch in a secondary aromatic amine, the aromatic C–H in-plane bending modes, and the out-of-plane deformations of C–H in the 1,4-disubstituted benzene ring, respectively. The results correspond well with the previously published report.15
XRD patterns of P(Ani-co-pPD)s In order to compare the crystallinity of PANI, P(Ani-copPD), and PpPD, the XRD patterns of the polymers are presented in Figure 3. From these curves, one can find that the structures of the resulting polymers are amorphous. The spectra of polymers depict two broad peaks at the region of 2y ¼ 18.4 and 26.4 shown in Figure 3. The peak centred
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351 110 100 90 Weight loss (%)
Intensity (a.u.)
(a)
(b)
80 70 60 c
50
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b a
40
(c) 70
30
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at 2y ¼ 18.4 may be ascribed to the momentum transfer and the periodicity parallel to the polymer chain, while the latter peak may be caused by the periodicity perpendicular to the polymer chain.16 However, the copolymer exhibits the XRD patterns with another two diffraction peaks at 2y ¼ 11.9 and 24.0 and PpPD with another three diffraction peaks at 2y ¼ 14.5 , 16.5 , and 21.0 . The result indicates that PpPD shows the highest crystallinity among the polymers, the order of crystallinity is PpPD > P(Ani-copPD) > PANI. The differences in the crystallinity may be due to the different polymer chain structure. It is reported that PpPDs synthesized by chemically oxidative polymerization with APS as an oxidant exhibit ladder and ladderlike structures, which is corresponded to the molecular structure of PpPD shown in Figure 1(c). Meanwhile, incorporation of the substituent groups will induce distortions in the ladder chain, reducing the conjugation and increasing the chain flexibility. Consequently, P(Ani-co-pPD) reveals a lower crystallinity but has a better solubility as shown in Table 1.
Thermal stability of P(Ani-co-pPD)s The main purpose of the TGA experiments is to study the thermal degradation and stability of the polymers. Figure 4 demonstrates the thermal stability of polymers synthesized using HCl dopants. All the samples followed the similar decomposition trend with the exhibition of a gradual weight loss. P(Ani-co-pPD) exhibits three main weightloss stages. The first weight-loss stage in the TGA curve of the copolymer between 50 and 140 C could be attributed to the loss of moisture, volatilization of the solvent, and adsorbed HCl. The second stage between 170 and 260 C
400
500
600
700
Temperature (°C)
2θ (°)
Figure 3. XRD patterns of polymers: (a) PANI, (b) P(Ani-co-pPD), and (c) PpPD. PANI: polyaniline; P(Ani-co-pPD): poly(aniline-co-p-phenylenediamine); PpPD: poly(p-phenylenediamine); XRD: x-ray diffraction.
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Figure 4. TGA curves of polymers: (a) PANI, (b) P(Ani-co-pPD), and (c) PpPD. PANI: polyaniline; P(Ani-co-pPD): poly(aniline-co-p-phenylenediamine); PpPD: poly(p-phenylenediamine); TGA: thermogravimetric analysis.
may be attributed to the loss of dopant from deeper sites in the copolymer. The third stage occurring between 420 and 600 C is due to the final degradation of the polymers. However, it was obvious that the thermal stability of PANI is higher than the copolymer and PpPD between 420 and 600 C. Gradual weight loss over the wide range of temperature could be ascribed to the good thermal stability of polymers main chain.
Morphological study Scanning electron micrographs of the polymers provide a clear morphology. Figure 5 represents the morphologies of PANI, P(Ani-co-pPD), and PpPD. As can be seen in these images, all the polymers exist in granular morphology. The figure reveals crystalline as well as amorphous morphology with nonuniformity in the surface, which is in good agreement with the XRD patterns of the polymers. Figure 5(a) shows the spongy and porous structure of PANI, which could be due to the extension of polymer chain and porosity and the surface-to-volume ratio. From Figure 5(b), we have found that there is also a small portion of rod-shaped and sheet-shaped structures in addition to granular particle agglomerations, which may be ascribed to the linear nature of copolymer chains.17 PpPD exhibits a typical granular morphology in Figure 5(c). Apparently, the microparticles tend to aggregate into interconnected microparticles agglomerations, suggesting a strong interaction among the microparticles in the agglomerations.
Conclusions In summary, PANI, P(Ani-co-pPD), and PpPD have been synthesized and characterized. The comparative study
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Figure 5. SEM image of polymers: (a) PANI, (b) P(Ani-co-pPD), and (c) PpPD. PANI: polyaniline; P(Ani-co-pPD): poly(aniline-co-p-phenylenediamine); PpPD: poly(p-phenylenediamine); SEM: scanning electron microscopy.
reveals that PANI shows the maximum conductivity and thermal stability among the polymers; P(Ani-co-pPD) exhibits the best solubility, PpPD presents the highest crystallinity, and all the polymers exist in granular morphology. In addition, the study also confirms that the molecular structures and the interactions between the polymers have profound influences on the properties of polymers. Authors’ Note TL and CY contributed equally to this work.
Funding This work is supported by the Natural Science Foundation of Shandong Province (Nos. ZR2012E MM012 and ZR2012EMQ002) and the Scientific and Technical Innovation Fund for Graduates of Shandong University of Science and Technology (No. YCA120369).
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