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Structure–property relationship in mechanochemically prepared polyaniline
Synthetic Metals 160 (2010) 462–467
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Structure–property relationship in mechanochemically prepared polyaniline O. Yu. Posudievsky a,∗ , O.A. Goncharuk a , R. Barillé b,1 , V.D. Pokhodenko a a b
L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, 31 prospekt Nauki, Kiev 03028, Ukraine Laboratoire POMA, UMR-CNRS 6136, Universite d’Angers, 2 Boulevard Lavoisier, 49045 Angers, France
a r t i c l e
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Article history: Received 10 September 2009 Received in revised form 15 October 2009 Accepted 23 November 2009 Available online 23 December 2009 Keywords: Polyaniline Mechanochemical preparation Conductivity Structure–property relationship
a b s t r a c t The comparative analysis of the physicochemical properties of polyaniline prepared in the conditions of solventless mechanochemical treatment in the ball mill (PAni|mch ) and polyaniline synthesized by the usual oxidative polymerization in the solvent (PAni|c ) is carried out. Conductivity of PAni|mch substantially exceeds that of PAni|c . As molecular weights of PAni|mch and PAni|c are comparable, the observed difference could be connected with the influence of mechanical stress which affects the polymer during its mechanochemical preparation. The increased conductivity of polyaniline obtained by post-synthesis mechanochemical treatment of PAni|c (PAni|mt ) confirms such explanation. It could be concluded from the results of the structural investigations as well as of spectral and electrochemical studies that the shear stress which exerts influence on the polymer during mechanochemical synthesis or mechanochemical treatment leads to the increase of the interchain – interaction and, consequently, to the efficient improvement of conductivity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polyaniline (PAni) as a representative of the conducting polymers permanently attracts great attention of researchers due to its high conductivity, electronic, optical, thermoelectric and other properties which combine with good environmental stability [1–5]. Usually, PAni is prepared by chemical oxidative polymerization of aniline in acid aqueous solutions in the form of powder or electrochemically as a film on the electrode surface [6,7]. The polymer is here in the state of emeraldine salt – stable conducting state of PAni (PAni has many intrinsically inherent oxidation states: from completely reduced leucoemeraldine to totally oxidized pernigraniline [8]) – and as a result of doping (protonation of imine fragments of the emeraldine base) it usually possesses conductivity no more than ∼10 S/cm [6]. Conductivity of PAni could be sharply increased by secondary doping to the level of 200–400 S/cm [9], and it may achieve more than 1000 S/cm by using the method of self-stabilized dispersive polymerization [10]. PAni is probably the most studied conducting polymer. At present, the effects of solution, doPAnit and oxidant on the properties of PAni prepared in liquid media could be considered well established now [4,6,11,12]; methods of its preparation by vaporphase polymerization [13], as well as in conditions of ultrasonic treatment [14] or under the action of plasma [15] are known. It
was also shown that PAni could be obtained mechanochemically without the use of solvents [16–18]. But Ref. [16] was devoted to polyaniline/montmorillonite nanocomposite and the polymer was not isolated from the nanocomposite and characterized as an individual material. In Ref. [17] conductivity of the prepared polymer was not considered at all. Besides, hand-grinding in a mortar used in [16,17] seems to be an insufficiently suitable preparation technique. In Ref. [18] PAni was prepared in a planetary ball mill and the paper was mainly connected with analysis of the influence of monomer/oxidant ratio on the yield of the product, its molecular weight distribution and optical properties. However the polymer yield was no more than 65% and the conductivity of PAni was at the level of 10−2 S/cm. At the same time, we have established that PAni with conductivity above 20 S/cm could be prepared mechanochemically using other synthesis conditions [19]. In the present paper we show the results of studies of such highly conducting PAni obtained by solventless solid-state mechanochemical route. The data about its structure, molecular weight distribution, spectral properties and conductivity are presented, and the relationship between them is analyzed. On the basis of comparative analysis of characteristics of the different samples of PAni, the structural and spectral polymer features stipulated by the mechanochemical method of its preparation are shown. 2. Experimental
∗ Corresponding author. Tel.: +380 44 525 6672; fax: +380 44 525 6216. E-mail addresses: [email protected] (O.Yu. Posudievsky), [email protected] (R. Barillé). 1 Tel.: +33 02 4173 5364; fax: +33 02 4173 5216. 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.11.032
2.1. Preparation of polymer samples The samples of PAni were prepared by chemical polymerization of anilinium salts in two different conditions: in ball mill without
O.Yu. Posudievsky et al. / Synthetic Metals 160 (2010) 462–467
using of any solvent (PAni|mch ) and in aqueous acid solution using procedure analogous that in [20] (PAni|c ). Anilinium chloride and ammonium persulphate of analytical grade were used. The mechanochemical method for preparation of PAni|mch was as the following. The reaction mixture – the definite amount of ammonium persulphate and anilinium chloride in mole ratio (r) from 0.25 to 1.25 (polymerization at r = 0.25 does take place within 1 h) – was placed in agate grinding bowl of the planetary ball mill Pulverisette 6 (Fritsch). The reaction mixture/ball weight ratio was 1:15. That value was chosen as it is inside the range of the powder to ball mass ratio (1:5 to 1:50) typical in mechanical activation and mechanosynthesis [21]. Mechanochemical treatment of the reaction mixture was conducted at rotation rate of 300 rpm during 1 h in argon atmosphere. The choice of the reaction time was done to achieve nearly full polymerization of the monomer and also it was equal to that in Ref. [18]. The obtained product was right away thoroughly washed with ethanol to remove residual anilinium chloride and subsequently rinsed with 0.01 M HCl for removal of inorganic residues and then it was dried in vacuum at 60 ◦ C until permanent weight was achieved. The maximum yield of PAni|mch was 98% at r = 0.5. Solid-sate CP/MAS 13 C NMR of the dedoped PAni|mch (r = 0.5), ı: 114 (CH); 123 (CH); 137 (CH); 141 (C); 147 (C); 158 (C). Anal. Calcd C6 NH5 [Cl0.46 (HSO4 )0.04 ]: C, 64.7; H, 4.53; N, 12.6. Found C, 64.3; H, 4.71; N, 12.7. For preparation of PAni|c the reaction between anilinium chloride and ammonium persulphate (mole ratio 1:0.5) was carried out in 1 M aqueous solution of hydrochloric acid at 0–5 ◦ C during 3 h. The product was isolated by filtration, thoroughly washed with ethanol and subsequently rinsed with 0.01 M HCl for removal of inorganic residues and then it was dried in vacuum at 60 ◦ C until permanent weight was achieved. The yield of PAni|c was equal to 76%. Solid-sate CP/MAS 13 C NMR, ı: 114 (CH); 122 (CH); 136 (CH); 142 (C); 146 (C); 156 (C). Anal. Calcd C6 NH5 [Cl0.48 (HSO4 )0.02 ] 0.3H2 O: C, 62.4; H, 4.87; N, 12.1. Found C, 62.1; H, 5.14; N, 12.3. Another sample of PAni, PAni|mt , was prepared by subjecting PAni|c , synthesized by the usual chemical route in solution as described above, to post-synthesis mechanochemical treatment in the ball mill at the same conditions as PAni|mch (rotation rate of 300 rpm during 1 h in argon atmosphere and polymer to ball mass ratio of 1:15). 2.2. Characterization The yield of the polymer was calculated as percent of original amount of anilinium chloride on weight basis. C,H,N analysis was conducted using Carlo Erba 1106. The content of hydrogen sulphate in PAni samples was determined gravimetrically. Solid-sate CP MAS 13 C NMR spectra were registered on Bruker 400 spectrometer with TMS as a standard. X-ray diffraction and TEM were used to study the structure of the different samples of PAni. X-ray powder diffraction was performed on diffractometer D8 ADVANCE (Bruker) using filtrated Cu K␣ irradiation in the range of 2 = 1–60◦ with 0.05◦ increment. Crystallinity degree, Xcr was calculated as ratio of the integral intensities of the crystal reflexes to the total area under the curve of coherent scattering using the methods of Matthews [22] and Ruland [23,24]. Electron micrographs were obtained using microscope TEM125K (SELMI) worked at the voltage of 100 kV with resolution of 0.3 nm. Amorphous carbon film which covered the copper grid was used as a carrier for the polymer samples. SAED patterns were also obtained on this facility. Molecular weight of the polymers was estimated by means of viscosimetry using the Mark–Houwink equation: = KM˛ ( is an intrinsic viscosity measured in dL/g, K = 3.33 × 10−4 (1.95 × 10−6 ) and ˛ = 0.78 (1.36) for flexible(rigid)-chain limit [25]), for solutions of the polymer samples in concentrated sulphuric acid (1.8 g/cm3 ). Intrinsic viscosity of PAni was determined at 25 ◦ C using Ubbelo-
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hde viscosimeter. Before dissolution in the concentrated sulphuric acid, the base of PAni was washed with tetrahydrofuran on porous glass filter and then with methanol and diethyl ether to remove oligomers. Specific conductivity () was measured using the standard fourprobe method with an accuracy of 10%, and the ohmic character of the contacts with tablets of the polymer samples was achieved by the gold plating of the probe tips. Cyclic voltammetry studies were performed by electrochemical analyzer AUTOLAB III/FRA2 (ECO CHEMIE) in the potential range of 2.0–4.0 V vs. Li/Li+ at potential scan rate of 0.1 mV/s in a two-electrode electrochemical cell using 1 M solution of LiClO4 in ethylene carbonate/dimethyl carbonate mixture (1:1 vol.) as an electrolyte. The samples of PAni for NMR, viscosimetry, and electrochemical studies, were dedoped in 3% solution of ammonium hydroxide and then they were dried in vacuum at 60 ◦ C up to the permanent weight. UV–vis spectra were measured on double beam spectrophotometer 4802 (UNICO) with a resolution of 2 nm using dispersions of the polymer samples in the concentrated sulphuric acid/water mixture (1:1 vol.). FTIR spectra of PAni samples in KBr tablets were registered by SPECTRUM ONE (PerkinElmer) with an accuracy of 2 cm−1 . ESR spectra were obtained on spectrometer E-9 (Varian) utilizing a sample of Mn2+ isomorphically substituted in the crystal lattice of MgO as a standard for determining the value of g-factor and linewidth (Hpp ), the accuracy of calculation being equal to 0.0003 and 0.1 G respectively. 3. Results and discussion As a result of mechanochemical treatment of the anilinium chloride and ammonium persulphate mixture we prepared powder materials of deep-green color characterizing the doped PAni in the state of emeraldine salt as in the case of usual chemical polymerization in aqueous medium. The highest conductivity is obtained for the lowest achieved value of the reagent mole ratio with nearly linear dependence (r) (Fig. 1). Such behaviour of the mechanochemically prepared PAni in the studied range of r differs from practically invariable conductivity of the polymers synthesized in solution [20,26]. Our mechanochemical method for preparation of PAni appears more productive with environmental facility (its realization does not require usage of acid solutions) in comparison with the usual procedure of polymer preparation in the solutions as it provides greater polymer yield at less spent time. It is also more effective than the procedure used in [18] and that fact is probably connected with the twice lower rotation rate (300 rpm vs. 600 rpm) of the ball mill.
Fig. 1. Conductivity of PAni|mch as a function of oxidant/monomer mole ratio.
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O.Yu. Posudievsky et al. / Synthetic Metals 160 (2010) 462–467 Table 1 Crystallinity degree (Xcr ) and parameters of the unit cell (a, b, c) for samples of PAni. Sample
Xcr a (%)
a (nm)
b (nm)
c (nm)
␦ (◦ )
PAni|c PAni|mch
18 (23) 33 (46)
0.42 0.41
0.60 0.62
1.04 0.96
141 121
a
Fig. 2. FTIR spectra of PAni|mch (a) and PAni|c (b).
The formation of the polymer is confirmed by the data of FTIR spectroscopy (Fig. 2). In the spectrum of PAni|mch (here and below PAni|mch means the most conducting sample prepared using r = 0.5) there are characteristic bands of the polymer (1243, 1300, 1488 and 1574 cm−1 ), which correspond to vibrations of the bonds in benzenoid and quinonoid fragments [27]. Besides these bands, an intense wide band at 1120 cm−1 is present in the spectrum. It corresponds to the so-called “conductivity band” with a high delocalization of electrons in the macromolecules of PAni [15]. In agreement with the data of 13 C solid-sate CP/MAS NMR, our studies did not reveal the presence of carbonyl defects (the band about 1700 cm−1 ), cross-links between the chains of the conducting polymer (the band about 1450 and 916 cm−1 corresponding to 1,3,5-substituted rings [14]), phenazine-like fragments (the most strong feature of which is the band about 1414 cm−1 [28]), ortholinkage of the rings in the chains (the band about 860 cm−1 [28]), as well as covalently bonded sulpho-groups (the bands about 1070 and 1040 cm−1 [28]) which could led to self-doping effect. In this respect our PAni|mch coincides with the polymer from [18]. The data of UV–vis spectroscopy (Fig. 3) present another confirmation of the polymer formation as a result of the solventless mechanochemical process. For both spectra of PAni|mch and PAni|c , there is a peak at 360 nm with a shoulder about 440 nm. It corresponds to a spectral window in the range of green color, as well as a wide and intense polaron band in the long-wave region [22]. In comparison with PAni|c , the band about 350 nm in PAni|mch is
Fig. 3. UV–vis spectra of PAni|mch and PAni|c (solid and dot lines) normalized relative to the band about 360 nm.
The method of Matthews (Ruland).
blue shifted for ∼7 nm, the shoulder about 450 nm is characterized by the same tendency, while polaron band in PAni|mch is shifted in the red region for ∼18 nm (from 732 to 750 nm) and is simultaneously slightly widened (Fig. 3). Position of the band about 350 nm corresponds to interband –* transitions and depends on polymerization degree [23,29]. Therefore it could be concluded that in accord with the data of UV–vis spectroscopy the average polymerization degree in PAni|mch is slightly less than in PAni|c . At the same time, relative red shift of the polaron band in PAni|mch could argue in favor of comparatively greater delocalization of polarons in it [9,23]. In connection with the stated spectral differences it was interesting to compare the molecular weight characteristics of PAni|mch and PAni|c . In this goal we ascertain the molecular weight of the prepared polymers using the method of viscosimetry. In correspondence with the measured data the characteristic viscosity of PAni|mch and PAni|c is respectively equal to 1.56 and 2.06 dL/g. So, accordingly to the Mark–Houwink equation, the viscosity average molecular weight of PAni|mch in the flexible(rigid)-chain limit is equal to 51(22) kDa, while that of PAni|c –73(27) kDa. The obtained data testify to lower molecular weight of PAni|mch in comparison with PAni|c . That observation coincides with the conclusion reached above on the basis of the analysis of the UV–vis spectra. Also, it agrees with better solubility of PAni|mch relative to PAni|c in such organic solvents as NMP and DMF: correspondingly 14 and 9, as well as 21 and 18 mass % dissolves in these solvents at room temperature. The preparation of PAni|mch was repeated four times and our measurements showed that the average conductivity of PAni|mch samples is several times greater than that of PAni|c (correspondingly 23 and 5 S/cm). As it is known [23], conductivity of the conjugated polymers is determined by a series of factors: length of macromolecules, degree of their doping, as well as by type of packing of the chains in polymer matrix. We think that the differences in the molecular weight (polymerization degree) of PAni|mch and PAni|c marked above could not be the reason for greater conductivity of PAni|mch in comparison with PAni|c . Besides, the doping degree of the prepared PAni samples is identical in accord with the data of elemental analysis. Therefore we studied the structure of the polymers by means of X-ray diffraction and TEM. The powder diffractograms of the different samples of PAni are presented in Fig. 4, and the values of their crystallinity degree (Xcr ) calculated by the method of Matthews (Ruland) – in Table 1. It follows from the data of Table 1 that the value of Xcr for PAni|mch is greater than for PAni|c in correspondence with the higher conductivity. Taking into account the fact that the preparation of PAni|c was carried out without substantial mechanical effect it should be probably concluded that the increase of PAni|mch crystallinity is the consequence of the effect of mechanical forces, in particular, shear stress generated during treatment of the material in the ball mill. The influence of these forces leads to ordering of the polymer chains packing due to increase of the interchain – interaction between aromatic rings of the macromolecules thereby promoting increase of conductivity of the whole material [10]. Diffractograms in Fig. 4 allow us also to conclude that in all cases PAni is formed in the pseudo-orthorhombic lattice (Fig. 5) which is usually denoted in literature as ES-I accordingly to [30]. However the parameters of the unit cell (a, b, c, Fig. 5) in the polymer samples prepared by
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Fig. 5. Schematic view for the unit cell of ES-I on (a, b) and (c, b) planes of projection.
Fig. 4. X-ray powder diffractograms of PAni|c (а), PAni|mch (b) and PAni|mt (c) (the contribution of amorphous part of the polymers used to calculate the crystallinity is outlined by hatching).
different methods differ. In particular, the increase of conductivity is accompanied by decrease of the parameter c that imply the decrease of dihedral angle ␦ in the polymer macromolecules (Fig. 5) according with the conclusion from [31]. Investigations of the different polymer samples by means of TEM have allowed us to obtain additional information which proves an increase of the polymer ordering in PAni|mch . According to TEM data, PAni|c consists of aggregates of nanoparticles (Fig. 6a) which do not possess obvious crystalline order (Fig. 6b). At the same time,
in aggregates which form PAni|mch (Fig. 7a) there are nanoparticles with the size of ∼50 nm (Fig. 7b), which possess quasi-crystalline ordering evidenced by spot electron diffraction patterns one of which is shown in Fig. 7c. It should be noted that mechanochemical reaction could be considered as a cumulative effect of two processes: (1) solid-state (neglecting local melting) chemical reaction and (2) influence of mechanical forces on the parent reagents and reaction products [32,33]. To clarify how the conditions of the mechanochemical preparation of PAni influence the conductivity and structure–property relationship in the prepared polymer, we tried to separate these two processes. To achieve this aim, we prepared one more polymer, PAni|mt , by subjecting PAni|c to postsynthesis mechanochemical treatment in the ball mill at the same conditions as PAni|mch . We think the properties of PAni|mt should reflect the effect of only second effect mentioned above. We neglect side chemical reactions, such as formation of cross-links between the chains as well as destruction and dedoping of macromolecules,
Fig. 6. Bright-field image (a) and ring electron diffraction pattern (b) of the aggregates of PAni|c nanoparticles.
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Fig. 7. Bright- (a) and dark-field (b) images of the aggregates of nanoparticles in PAni|mch ; electron diffraction pattern of the nanoparticle in projection on (0 1 0) zone axis (c); electron diffraction pattern of the aggregate of nanoparticles.
Fig. 8. Bright- (a) and dark-field (b) images of PAni|mt nanoparticle; electron diffraction pattern of the nanoparticle in projection on (0 0 1) zone axis (c); electron diffraction pattern of the aggregate of nanoparticles (d).
their oxidation [32–36]) which are possible during mechanochemical treatment, because our analysis of FTIR spectrum of PAni|mt proves the absence (similarly to PAni|mch ) of cross-links and carbonyl defects in its structure, and high conductivity of 27 S/cm (a value averaged over four prepared samples) could indicate insignificance of the negative influence of the processes of destruction and dedoping. It is worthwhile to note that well-defined reflexes (0 1 0) and (1 0 0) characteristic of PAni|c and PAni|mch (Fig. 4a and b) are absent in the diffractogram of PAni|mt shown in Fig. 4c. Besides, general intensity of X-ray scattering in PAni|mt is substantially increased. So it could be supposed that the absence of (0 1 0) and (1 0 0) reflexes in Fig. 4c is probably caused by such broadening that become practically inconspicuous on the background of the intensive (1 1 0) reflex. Its integral intensity increases in comparison with PAni|c in approximately eight times. The diffractogram form of PAni|mt is very close to the diffractogram of PAni possessing record high conductivity [10]. In comparison with PAni|c the increase of (1 1 0) reflex is also observed in PAni|mch . Such increase proves the improvement of ordering in macromolecular packing in the polymer as result of mechanochemical impact in the ball mill. It reflects the growth of the effective size of charge carrier delocalization area leading to increase of conductivity in PAni|mch and PAni|mt as compared with PAni|c . According to TEM data the size of area in PAni|mt , for which the crystalline ordering (spot electronogram in Fig. 8c) is typical, is rather less than in PAni|mch (Fig. 7c). Also, the distinguishing features of PAni|mt are the patterns with characteristic arcs (Fig. 8d) that proves the presence of texture in PAni|mt , when crystallographic axes of the polymer nanoparticles which form aggregates line up along a definite preferable direction (Fig. 8a). So, the data of Fig. 8 are consistent and substantially supplement the data of X-ray diffraction in respect of relatively greater ordering of PAni|mt in comparison with PAni|mch . Namely, the decreased size of crystallinity areas leads, as it was previously mentioned, to a broadening of (1 0 0) and (0 1 0) reflexes that they become weakly resolved
in the diffractogram of PAni|mt (Fig. 4c); texturing stipulated by mechanochemical treatment leads to increase of the intensity of (1 1 0) reflex which corresponds to the distance between the plains, in which nitrogen atoms of the adjacent polymer chains lie. ESR spectra of the studied polymer samples PAni|mch and PAni|c are represented by strong singlets with practically identical values of g-factor equal to 2.0025 (Fig. 9). The numerical calculations show that integral intensity of the signal from PAni|c is ∼2.5 times greater. The measurement of the ESR spectra was carried out by us in the same conditions, so the decrease of the signal intensity for PAni|mch in comparison with that for PAni|c could be interpreted as a consequence of decrease of the number of polarons. This could be due to formation of spinless bipolarons and/or dimer radical-cations [37], the formation of which is probably connected with increased
Fig. 9. ESR spectra of PAni|mch and PAni|c (solid and dot lines). Inset: I , intensity of the ESR spectrum for PAni|mch ; H, magnetic field strength (G); H0 , center of the ESR spectrum for PAni|mch (G).
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such explanation. It is important to mark that conductivities of PAni|mch and PAni|mt are close to each other that allows us to connect the increase of their conductivity in comparison with that of PAni|c with the influence of the mechanical stress on the ordering of the spatial packing of the macromolecules of the conjugated polymer. The carried out structural investigations as well as the results of spectral and electrochemical studies show that the shear stress of the mechanical forces which exerts influence on the polymer during mechanochemical synthesis or mechanochemical treatment leads to the increase of the interchain – interaction and, consequently, to the efficient increase of the macroscopic charge transfer, i.e. conductivity. Fig. 10. Cyclic voltammograms of PAni|mch and PAni|c (solid and dot lines).
ordering of macromolecules in PAni|mch and possibility of a more rapid electron exchange. Thereby the decrease of the signal width takes place in PAni|mch and its form corresponds to Lorenz curve (Fig. 9, inset). Cyclic voltammograms (CVA) of PAni|mch and PAni|c are presented in Fig. 10. The differences between them is obvious: the peaks of the CVA for PAni|mch are clearer, and the distance between anode and cathode peaks is twice less (correspondingly 0.15 and 0.30 V); the area of the CVA for PAni|mch exceeds that for PAni|c in ∼10% (Fig. 10). The revealed differences of the electrochemical characteristics of the analyzed samples of PAni, in our opinion, could be connected with the mentioned structural features and conductivity of PAni|mch : the increased ordering of this polymer has to satisfy greater uniformity in size and energy of ionic diffusion paths and consequently results in greater discharge capacity, more narrow peaks on the CVA, and improved reversibility of redox processes taking place in the polymer during cycling of the potential [38,39]. 4. Conclusions Thus, the comparative analysis of the physicochemical properties of PAni|mch prepared in the conditions of solventless mechanochemical treatment in the ball mill and PAni|c synthesized by the usual oxidative polymerization in the solvent was carried out. Investigations by means of various physicochemical methods prove that the formation of polyaniline is possible without using solvents. High productivity is an inherent characteristic of the mechanochemical preparation of polyaniline because the polymerization proceeds are sufficiently quicker and with higher yield (even after 15 min from the start of the mechanochemical synthesis the polymer yield is 83% whereas the time of induction period for the beginning of polymerization in the aqueous medium is about 10 min [26]). We established that conductivity of PAni|mch substantially exceeds that of PAni|c . As molecular weights of PAni|mch and PAni|c are comparable, the observed difference, in our opinion, could be connected with the influence of the preparation conditions for these polymer samples, namely, with mechanical stresses which affect the polymer during its mechanochemical preparation. The increased conductivity of PAni|mt obtained by post-synthesis mechanochemical treatment of PAni|c is the fact which confirms
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Structure–property relationship in mechanochemically prepared polyaniline O. Yu. Posudievsky a,∗ , O.A. Goncharuk a , R. Barillé b,1 , V.D. Pokhodenko a a b
L.V. Pisarzhevsky Institute of Physical Chemistry of the National Academy of Sciences of Ukraine, 31 prospekt Nauki, Kiev 03028, Ukraine Laboratoire POMA, UMR-CNRS 6136, Universite d’Angers, 2 Boulevard Lavoisier, 49045 Angers, France
a r t i c l e
i n f o
Article history: Received 10 September 2009 Received in revised form 15 October 2009 Accepted 23 November 2009 Available online 23 December 2009 Keywords: Polyaniline Mechanochemical preparation Conductivity Structure–property relationship
a b s t r a c t The comparative analysis of the physicochemical properties of polyaniline prepared in the conditions of solventless mechanochemical treatment in the ball mill (PAni|mch ) and polyaniline synthesized by the usual oxidative polymerization in the solvent (PAni|c ) is carried out. Conductivity of PAni|mch substantially exceeds that of PAni|c . As molecular weights of PAni|mch and PAni|c are comparable, the observed difference could be connected with the influence of mechanical stress which affects the polymer during its mechanochemical preparation. The increased conductivity of polyaniline obtained by post-synthesis mechanochemical treatment of PAni|c (PAni|mt ) confirms such explanation. It could be concluded from the results of the structural investigations as well as of spectral and electrochemical studies that the shear stress which exerts influence on the polymer during mechanochemical synthesis or mechanochemical treatment leads to the increase of the interchain – interaction and, consequently, to the efficient improvement of conductivity. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polyaniline (PAni) as a representative of the conducting polymers permanently attracts great attention of researchers due to its high conductivity, electronic, optical, thermoelectric and other properties which combine with good environmental stability [1–5]. Usually, PAni is prepared by chemical oxidative polymerization of aniline in acid aqueous solutions in the form of powder or electrochemically as a film on the electrode surface [6,7]. The polymer is here in the state of emeraldine salt – stable conducting state of PAni (PAni has many intrinsically inherent oxidation states: from completely reduced leucoemeraldine to totally oxidized pernigraniline [8]) – and as a result of doping (protonation of imine fragments of the emeraldine base) it usually possesses conductivity no more than ∼10 S/cm [6]. Conductivity of PAni could be sharply increased by secondary doping to the level of 200–400 S/cm [9], and it may achieve more than 1000 S/cm by using the method of self-stabilized dispersive polymerization [10]. PAni is probably the most studied conducting polymer. At present, the effects of solution, doPAnit and oxidant on the properties of PAni prepared in liquid media could be considered well established now [4,6,11,12]; methods of its preparation by vaporphase polymerization [13], as well as in conditions of ultrasonic treatment [14] or under the action of plasma [15] are known. It
was also shown that PAni could be obtained mechanochemically without the use of solvents [16–18]. But Ref. [16] was devoted to polyaniline/montmorillonite nanocomposite and the polymer was not isolated from the nanocomposite and characterized as an individual material. In Ref. [17] conductivity of the prepared polymer was not considered at all. Besides, hand-grinding in a mortar used in [16,17] seems to be an insufficiently suitable preparation technique. In Ref. [18] PAni was prepared in a planetary ball mill and the paper was mainly connected with analysis of the influence of monomer/oxidant ratio on the yield of the product, its molecular weight distribution and optical properties. However the polymer yield was no more than 65% and the conductivity of PAni was at the level of 10−2 S/cm. At the same time, we have established that PAni with conductivity above 20 S/cm could be prepared mechanochemically using other synthesis conditions [19]. In the present paper we show the results of studies of such highly conducting PAni obtained by solventless solid-state mechanochemical route. The data about its structure, molecular weight distribution, spectral properties and conductivity are presented, and the relationship between them is analyzed. On the basis of comparative analysis of characteristics of the different samples of PAni, the structural and spectral polymer features stipulated by the mechanochemical method of its preparation are shown. 2. Experimental
∗ Corresponding author. Tel.: +380 44 525 6672; fax: +380 44 525 6216. E-mail addresses: [email protected] (O.Yu. Posudievsky), [email protected] (R. Barillé). 1 Tel.: +33 02 4173 5364; fax: +33 02 4173 5216. 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.11.032
2.1. Preparation of polymer samples The samples of PAni were prepared by chemical polymerization of anilinium salts in two different conditions: in ball mill without
O.Yu. Posudievsky et al. / Synthetic Metals 160 (2010) 462–467
using of any solvent (PAni|mch ) and in aqueous acid solution using procedure analogous that in [20] (PAni|c ). Anilinium chloride and ammonium persulphate of analytical grade were used. The mechanochemical method for preparation of PAni|mch was as the following. The reaction mixture – the definite amount of ammonium persulphate and anilinium chloride in mole ratio (r) from 0.25 to 1.25 (polymerization at r = 0.25 does take place within 1 h) – was placed in agate grinding bowl of the planetary ball mill Pulverisette 6 (Fritsch). The reaction mixture/ball weight ratio was 1:15. That value was chosen as it is inside the range of the powder to ball mass ratio (1:5 to 1:50) typical in mechanical activation and mechanosynthesis [21]. Mechanochemical treatment of the reaction mixture was conducted at rotation rate of 300 rpm during 1 h in argon atmosphere. The choice of the reaction time was done to achieve nearly full polymerization of the monomer and also it was equal to that in Ref. [18]. The obtained product was right away thoroughly washed with ethanol to remove residual anilinium chloride and subsequently rinsed with 0.01 M HCl for removal of inorganic residues and then it was dried in vacuum at 60 ◦ C until permanent weight was achieved. The maximum yield of PAni|mch was 98% at r = 0.5. Solid-sate CP/MAS 13 C NMR of the dedoped PAni|mch (r = 0.5), ı: 114 (CH); 123 (CH); 137 (CH); 141 (C); 147 (C); 158 (C). Anal. Calcd C6 NH5 [Cl0.46 (HSO4 )0.04 ]: C, 64.7; H, 4.53; N, 12.6. Found C, 64.3; H, 4.71; N, 12.7. For preparation of PAni|c the reaction between anilinium chloride and ammonium persulphate (mole ratio 1:0.5) was carried out in 1 M aqueous solution of hydrochloric acid at 0–5 ◦ C during 3 h. The product was isolated by filtration, thoroughly washed with ethanol and subsequently rinsed with 0.01 M HCl for removal of inorganic residues and then it was dried in vacuum at 60 ◦ C until permanent weight was achieved. The yield of PAni|c was equal to 76%. Solid-sate CP/MAS 13 C NMR, ı: 114 (CH); 122 (CH); 136 (CH); 142 (C); 146 (C); 156 (C). Anal. Calcd C6 NH5 [Cl0.48 (HSO4 )0.02 ] 0.3H2 O: C, 62.4; H, 4.87; N, 12.1. Found C, 62.1; H, 5.14; N, 12.3. Another sample of PAni, PAni|mt , was prepared by subjecting PAni|c , synthesized by the usual chemical route in solution as described above, to post-synthesis mechanochemical treatment in the ball mill at the same conditions as PAni|mch (rotation rate of 300 rpm during 1 h in argon atmosphere and polymer to ball mass ratio of 1:15). 2.2. Characterization The yield of the polymer was calculated as percent of original amount of anilinium chloride on weight basis. C,H,N analysis was conducted using Carlo Erba 1106. The content of hydrogen sulphate in PAni samples was determined gravimetrically. Solid-sate CP MAS 13 C NMR spectra were registered on Bruker 400 spectrometer with TMS as a standard. X-ray diffraction and TEM were used to study the structure of the different samples of PAni. X-ray powder diffraction was performed on diffractometer D8 ADVANCE (Bruker) using filtrated Cu K␣ irradiation in the range of 2 = 1–60◦ with 0.05◦ increment. Crystallinity degree, Xcr was calculated as ratio of the integral intensities of the crystal reflexes to the total area under the curve of coherent scattering using the methods of Matthews [22] and Ruland [23,24]. Electron micrographs were obtained using microscope TEM125K (SELMI) worked at the voltage of 100 kV with resolution of 0.3 nm. Amorphous carbon film which covered the copper grid was used as a carrier for the polymer samples. SAED patterns were also obtained on this facility. Molecular weight of the polymers was estimated by means of viscosimetry using the Mark–Houwink equation: = KM˛ ( is an intrinsic viscosity measured in dL/g, K = 3.33 × 10−4 (1.95 × 10−6 ) and ˛ = 0.78 (1.36) for flexible(rigid)-chain limit [25]), for solutions of the polymer samples in concentrated sulphuric acid (1.8 g/cm3 ). Intrinsic viscosity of PAni was determined at 25 ◦ C using Ubbelo-
463
hde viscosimeter. Before dissolution in the concentrated sulphuric acid, the base of PAni was washed with tetrahydrofuran on porous glass filter and then with methanol and diethyl ether to remove oligomers. Specific conductivity () was measured using the standard fourprobe method with an accuracy of 10%, and the ohmic character of the contacts with tablets of the polymer samples was achieved by the gold plating of the probe tips. Cyclic voltammetry studies were performed by electrochemical analyzer AUTOLAB III/FRA2 (ECO CHEMIE) in the potential range of 2.0–4.0 V vs. Li/Li+ at potential scan rate of 0.1 mV/s in a two-electrode electrochemical cell using 1 M solution of LiClO4 in ethylene carbonate/dimethyl carbonate mixture (1:1 vol.) as an electrolyte. The samples of PAni for NMR, viscosimetry, and electrochemical studies, were dedoped in 3% solution of ammonium hydroxide and then they were dried in vacuum at 60 ◦ C up to the permanent weight. UV–vis spectra were measured on double beam spectrophotometer 4802 (UNICO) with a resolution of 2 nm using dispersions of the polymer samples in the concentrated sulphuric acid/water mixture (1:1 vol.). FTIR spectra of PAni samples in KBr tablets were registered by SPECTRUM ONE (PerkinElmer) with an accuracy of 2 cm−1 . ESR spectra were obtained on spectrometer E-9 (Varian) utilizing a sample of Mn2+ isomorphically substituted in the crystal lattice of MgO as a standard for determining the value of g-factor and linewidth (Hpp ), the accuracy of calculation being equal to 0.0003 and 0.1 G respectively. 3. Results and discussion As a result of mechanochemical treatment of the anilinium chloride and ammonium persulphate mixture we prepared powder materials of deep-green color characterizing the doped PAni in the state of emeraldine salt as in the case of usual chemical polymerization in aqueous medium. The highest conductivity is obtained for the lowest achieved value of the reagent mole ratio with nearly linear dependence (r) (Fig. 1). Such behaviour of the mechanochemically prepared PAni in the studied range of r differs from practically invariable conductivity of the polymers synthesized in solution [20,26]. Our mechanochemical method for preparation of PAni appears more productive with environmental facility (its realization does not require usage of acid solutions) in comparison with the usual procedure of polymer preparation in the solutions as it provides greater polymer yield at less spent time. It is also more effective than the procedure used in [18] and that fact is probably connected with the twice lower rotation rate (300 rpm vs. 600 rpm) of the ball mill.
Fig. 1. Conductivity of PAni|mch as a function of oxidant/monomer mole ratio.
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O.Yu. Posudievsky et al. / Synthetic Metals 160 (2010) 462–467 Table 1 Crystallinity degree (Xcr ) and parameters of the unit cell (a, b, c) for samples of PAni. Sample
Xcr a (%)
a (nm)
b (nm)
c (nm)
␦ (◦ )
PAni|c PAni|mch
18 (23) 33 (46)
0.42 0.41
0.60 0.62
1.04 0.96
141 121
a
Fig. 2. FTIR spectra of PAni|mch (a) and PAni|c (b).
The formation of the polymer is confirmed by the data of FTIR spectroscopy (Fig. 2). In the spectrum of PAni|mch (here and below PAni|mch means the most conducting sample prepared using r = 0.5) there are characteristic bands of the polymer (1243, 1300, 1488 and 1574 cm−1 ), which correspond to vibrations of the bonds in benzenoid and quinonoid fragments [27]. Besides these bands, an intense wide band at 1120 cm−1 is present in the spectrum. It corresponds to the so-called “conductivity band” with a high delocalization of electrons in the macromolecules of PAni [15]. In agreement with the data of 13 C solid-sate CP/MAS NMR, our studies did not reveal the presence of carbonyl defects (the band about 1700 cm−1 ), cross-links between the chains of the conducting polymer (the band about 1450 and 916 cm−1 corresponding to 1,3,5-substituted rings [14]), phenazine-like fragments (the most strong feature of which is the band about 1414 cm−1 [28]), ortholinkage of the rings in the chains (the band about 860 cm−1 [28]), as well as covalently bonded sulpho-groups (the bands about 1070 and 1040 cm−1 [28]) which could led to self-doping effect. In this respect our PAni|mch coincides with the polymer from [18]. The data of UV–vis spectroscopy (Fig. 3) present another confirmation of the polymer formation as a result of the solventless mechanochemical process. For both spectra of PAni|mch and PAni|c , there is a peak at 360 nm with a shoulder about 440 nm. It corresponds to a spectral window in the range of green color, as well as a wide and intense polaron band in the long-wave region [22]. In comparison with PAni|c , the band about 350 nm in PAni|mch is
Fig. 3. UV–vis spectra of PAni|mch and PAni|c (solid and dot lines) normalized relative to the band about 360 nm.
The method of Matthews (Ruland).
blue shifted for ∼7 nm, the shoulder about 450 nm is characterized by the same tendency, while polaron band in PAni|mch is shifted in the red region for ∼18 nm (from 732 to 750 nm) and is simultaneously slightly widened (Fig. 3). Position of the band about 350 nm corresponds to interband –* transitions and depends on polymerization degree [23,29]. Therefore it could be concluded that in accord with the data of UV–vis spectroscopy the average polymerization degree in PAni|mch is slightly less than in PAni|c . At the same time, relative red shift of the polaron band in PAni|mch could argue in favor of comparatively greater delocalization of polarons in it [9,23]. In connection with the stated spectral differences it was interesting to compare the molecular weight characteristics of PAni|mch and PAni|c . In this goal we ascertain the molecular weight of the prepared polymers using the method of viscosimetry. In correspondence with the measured data the characteristic viscosity of PAni|mch and PAni|c is respectively equal to 1.56 and 2.06 dL/g. So, accordingly to the Mark–Houwink equation, the viscosity average molecular weight of PAni|mch in the flexible(rigid)-chain limit is equal to 51(22) kDa, while that of PAni|c –73(27) kDa. The obtained data testify to lower molecular weight of PAni|mch in comparison with PAni|c . That observation coincides with the conclusion reached above on the basis of the analysis of the UV–vis spectra. Also, it agrees with better solubility of PAni|mch relative to PAni|c in such organic solvents as NMP and DMF: correspondingly 14 and 9, as well as 21 and 18 mass % dissolves in these solvents at room temperature. The preparation of PAni|mch was repeated four times and our measurements showed that the average conductivity of PAni|mch samples is several times greater than that of PAni|c (correspondingly 23 and 5 S/cm). As it is known [23], conductivity of the conjugated polymers is determined by a series of factors: length of macromolecules, degree of their doping, as well as by type of packing of the chains in polymer matrix. We think that the differences in the molecular weight (polymerization degree) of PAni|mch and PAni|c marked above could not be the reason for greater conductivity of PAni|mch in comparison with PAni|c . Besides, the doping degree of the prepared PAni samples is identical in accord with the data of elemental analysis. Therefore we studied the structure of the polymers by means of X-ray diffraction and TEM. The powder diffractograms of the different samples of PAni are presented in Fig. 4, and the values of their crystallinity degree (Xcr ) calculated by the method of Matthews (Ruland) – in Table 1. It follows from the data of Table 1 that the value of Xcr for PAni|mch is greater than for PAni|c in correspondence with the higher conductivity. Taking into account the fact that the preparation of PAni|c was carried out without substantial mechanical effect it should be probably concluded that the increase of PAni|mch crystallinity is the consequence of the effect of mechanical forces, in particular, shear stress generated during treatment of the material in the ball mill. The influence of these forces leads to ordering of the polymer chains packing due to increase of the interchain – interaction between aromatic rings of the macromolecules thereby promoting increase of conductivity of the whole material [10]. Diffractograms in Fig. 4 allow us also to conclude that in all cases PAni is formed in the pseudo-orthorhombic lattice (Fig. 5) which is usually denoted in literature as ES-I accordingly to [30]. However the parameters of the unit cell (a, b, c, Fig. 5) in the polymer samples prepared by
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465
Fig. 5. Schematic view for the unit cell of ES-I on (a, b) and (c, b) planes of projection.
Fig. 4. X-ray powder diffractograms of PAni|c (а), PAni|mch (b) and PAni|mt (c) (the contribution of amorphous part of the polymers used to calculate the crystallinity is outlined by hatching).
different methods differ. In particular, the increase of conductivity is accompanied by decrease of the parameter c that imply the decrease of dihedral angle ␦ in the polymer macromolecules (Fig. 5) according with the conclusion from [31]. Investigations of the different polymer samples by means of TEM have allowed us to obtain additional information which proves an increase of the polymer ordering in PAni|mch . According to TEM data, PAni|c consists of aggregates of nanoparticles (Fig. 6a) which do not possess obvious crystalline order (Fig. 6b). At the same time,
in aggregates which form PAni|mch (Fig. 7a) there are nanoparticles with the size of ∼50 nm (Fig. 7b), which possess quasi-crystalline ordering evidenced by spot electron diffraction patterns one of which is shown in Fig. 7c. It should be noted that mechanochemical reaction could be considered as a cumulative effect of two processes: (1) solid-state (neglecting local melting) chemical reaction and (2) influence of mechanical forces on the parent reagents and reaction products [32,33]. To clarify how the conditions of the mechanochemical preparation of PAni influence the conductivity and structure–property relationship in the prepared polymer, we tried to separate these two processes. To achieve this aim, we prepared one more polymer, PAni|mt , by subjecting PAni|c to postsynthesis mechanochemical treatment in the ball mill at the same conditions as PAni|mch . We think the properties of PAni|mt should reflect the effect of only second effect mentioned above. We neglect side chemical reactions, such as formation of cross-links between the chains as well as destruction and dedoping of macromolecules,
Fig. 6. Bright-field image (a) and ring electron diffraction pattern (b) of the aggregates of PAni|c nanoparticles.
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Fig. 7. Bright- (a) and dark-field (b) images of the aggregates of nanoparticles in PAni|mch ; electron diffraction pattern of the nanoparticle in projection on (0 1 0) zone axis (c); electron diffraction pattern of the aggregate of nanoparticles.
Fig. 8. Bright- (a) and dark-field (b) images of PAni|mt nanoparticle; electron diffraction pattern of the nanoparticle in projection on (0 0 1) zone axis (c); electron diffraction pattern of the aggregate of nanoparticles (d).
their oxidation [32–36]) which are possible during mechanochemical treatment, because our analysis of FTIR spectrum of PAni|mt proves the absence (similarly to PAni|mch ) of cross-links and carbonyl defects in its structure, and high conductivity of 27 S/cm (a value averaged over four prepared samples) could indicate insignificance of the negative influence of the processes of destruction and dedoping. It is worthwhile to note that well-defined reflexes (0 1 0) and (1 0 0) characteristic of PAni|c and PAni|mch (Fig. 4a and b) are absent in the diffractogram of PAni|mt shown in Fig. 4c. Besides, general intensity of X-ray scattering in PAni|mt is substantially increased. So it could be supposed that the absence of (0 1 0) and (1 0 0) reflexes in Fig. 4c is probably caused by such broadening that become practically inconspicuous on the background of the intensive (1 1 0) reflex. Its integral intensity increases in comparison with PAni|c in approximately eight times. The diffractogram form of PAni|mt is very close to the diffractogram of PAni possessing record high conductivity [10]. In comparison with PAni|c the increase of (1 1 0) reflex is also observed in PAni|mch . Such increase proves the improvement of ordering in macromolecular packing in the polymer as result of mechanochemical impact in the ball mill. It reflects the growth of the effective size of charge carrier delocalization area leading to increase of conductivity in PAni|mch and PAni|mt as compared with PAni|c . According to TEM data the size of area in PAni|mt , for which the crystalline ordering (spot electronogram in Fig. 8c) is typical, is rather less than in PAni|mch (Fig. 7c). Also, the distinguishing features of PAni|mt are the patterns with characteristic arcs (Fig. 8d) that proves the presence of texture in PAni|mt , when crystallographic axes of the polymer nanoparticles which form aggregates line up along a definite preferable direction (Fig. 8a). So, the data of Fig. 8 are consistent and substantially supplement the data of X-ray diffraction in respect of relatively greater ordering of PAni|mt in comparison with PAni|mch . Namely, the decreased size of crystallinity areas leads, as it was previously mentioned, to a broadening of (1 0 0) and (0 1 0) reflexes that they become weakly resolved
in the diffractogram of PAni|mt (Fig. 4c); texturing stipulated by mechanochemical treatment leads to increase of the intensity of (1 1 0) reflex which corresponds to the distance between the plains, in which nitrogen atoms of the adjacent polymer chains lie. ESR spectra of the studied polymer samples PAni|mch and PAni|c are represented by strong singlets with practically identical values of g-factor equal to 2.0025 (Fig. 9). The numerical calculations show that integral intensity of the signal from PAni|c is ∼2.5 times greater. The measurement of the ESR spectra was carried out by us in the same conditions, so the decrease of the signal intensity for PAni|mch in comparison with that for PAni|c could be interpreted as a consequence of decrease of the number of polarons. This could be due to formation of spinless bipolarons and/or dimer radical-cations [37], the formation of which is probably connected with increased
Fig. 9. ESR spectra of PAni|mch and PAni|c (solid and dot lines). Inset: I , intensity of the ESR spectrum for PAni|mch ; H, magnetic field strength (G); H0 , center of the ESR spectrum for PAni|mch (G).
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such explanation. It is important to mark that conductivities of PAni|mch and PAni|mt are close to each other that allows us to connect the increase of their conductivity in comparison with that of PAni|c with the influence of the mechanical stress on the ordering of the spatial packing of the macromolecules of the conjugated polymer. The carried out structural investigations as well as the results of spectral and electrochemical studies show that the shear stress of the mechanical forces which exerts influence on the polymer during mechanochemical synthesis or mechanochemical treatment leads to the increase of the interchain – interaction and, consequently, to the efficient increase of the macroscopic charge transfer, i.e. conductivity. Fig. 10. Cyclic voltammograms of PAni|mch and PAni|c (solid and dot lines).
ordering of macromolecules in PAni|mch and possibility of a more rapid electron exchange. Thereby the decrease of the signal width takes place in PAni|mch and its form corresponds to Lorenz curve (Fig. 9, inset). Cyclic voltammograms (CVA) of PAni|mch and PAni|c are presented in Fig. 10. The differences between them is obvious: the peaks of the CVA for PAni|mch are clearer, and the distance between anode and cathode peaks is twice less (correspondingly 0.15 and 0.30 V); the area of the CVA for PAni|mch exceeds that for PAni|c in ∼10% (Fig. 10). The revealed differences of the electrochemical characteristics of the analyzed samples of PAni, in our opinion, could be connected with the mentioned structural features and conductivity of PAni|mch : the increased ordering of this polymer has to satisfy greater uniformity in size and energy of ionic diffusion paths and consequently results in greater discharge capacity, more narrow peaks on the CVA, and improved reversibility of redox processes taking place in the polymer during cycling of the potential [38,39]. 4. Conclusions Thus, the comparative analysis of the physicochemical properties of PAni|mch prepared in the conditions of solventless mechanochemical treatment in the ball mill and PAni|c synthesized by the usual oxidative polymerization in the solvent was carried out. Investigations by means of various physicochemical methods prove that the formation of polyaniline is possible without using solvents. High productivity is an inherent characteristic of the mechanochemical preparation of polyaniline because the polymerization proceeds are sufficiently quicker and with higher yield (even after 15 min from the start of the mechanochemical synthesis the polymer yield is 83% whereas the time of induction period for the beginning of polymerization in the aqueous medium is about 10 min [26]). We established that conductivity of PAni|mch substantially exceeds that of PAni|c . As molecular weights of PAni|mch and PAni|c are comparable, the observed difference, in our opinion, could be connected with the influence of the preparation conditions for these polymer samples, namely, with mechanical stresses which affect the polymer during its mechanochemical preparation. The increased conductivity of PAni|mt obtained by post-synthesis mechanochemical treatment of PAni|c is the fact which confirms
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