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Synthesis, characterisation and transport properties of layered conducting electroactive polypyrrole membranes
Journal of Membrane Science 148 (1998) 161±172
Synthesis, characterisation and transport properties of layered conducting electroactive polypyrrole membranes H. Zhao1, W.E. Price*, G.G. Wallace Department of Chemistry, Intelligent Polymer Research Institute, University of Wollongong, Wollongong, NSW 2522, Australia Received 16 October 1997; accepted 20 April 1998
Abstract A novel method for producing asymmetric membranes based on conducting polymers is described. Two layers of different polypyrrole ®lms (PPy±p-toluene sulphonate and PPy±dodecyl sulphate) were electrodeposited onto an electrode to form a sandwich or layer structure. The ®lms produced could be removed from the electrode and had suf®ciently good mechanical properties to be used as free-standing membranes in simple transport experiments. Using electrochemically induced transport utilising technology described previously it was shown that a highly asymmetric membrane had been formed with a ratio of up to 35:1 in terms of the ¯ux in one direction compared with another. This was for the transport of simple salts such as KCl and NaCl. In mixtures of these salts it was still possible to derive some reasonable selectivity between cations with selectivity of K over Na in ratios up to 4.5:1. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Electrochemistry; Ion-exchange membranes; Polypyrrole
1. Introduction Conducting electroactive polymer membranes such as polypyrrole membranes have attracted much interest over the past decade because of their dynamic chemical/electrochemical properties. Electrochemically controlled transport of ionic species across conducting electroactive polymer membranes have been demonstrated by us previously [1±8]. It has been shown that the transport of ionic species across the membrane can be switched on and off by means of application of an appropriate electrical potential
[2,8,9]. In addition, the rate of transport and the selectivity of the membrane can be dynamically controlled in situ by application of different electrical potential waveforms. Furthermore, membranes synthesised using different counterions show a remarkable in¯uence not just on the electrical conductivity, mechanical property and electroactivity of the membrane but also on the transport and dynamic control transport properties [6,7,10]. A free-standing conducting polypyrrole membrane can be electrochemically synthesised according to the following reaction:
*Corresponding author. Tel.: +61-242-213529; fax: +61-242214287; e-mail: [email protected] 1 Present address. Faculty of Environmental Sciences, School of Applied Sciences, Griffith University, Qld 4217, Australia. 0376-7388/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00158-6
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where, Cÿ is the counterion. Although a wide range of anions can in principle be used as the counterion for polypyrrole synthesis, the special requirements for stand-alone membrane application, in terms of, for example, mechanical properties, electrical conductivity, electroactivity and stability of the resultant polymer need to be considered. When this is done the number of anions that can be used as counterions is substantially restricted. Thus many anions with very attractive chemical functions are unsuitable for synthesis of stand-alone membranes. This poses a limitation on the development of new conducting polymer membrane materials for various applications. For example, dodecylsulphate (DS) is a surfactant and has very attractive surface chemistry, and being reasonably large possesses good cation exchangeability with little counterion leaching [5,11,12]. Unfortunately, the polymer prepared using DS as the counterion cannot be used as a free-standing membrane since this polymer has poor uniformity and mechanical properties, and is thus not strong enough to separate two solutions. To overcome this dif®culty, we have reported a membrane synthesis method where mixed counterions are employed [5]. This approach enabled DS to be incorporated into the polymer together with p-toluenesulphonate sodium salt (pTS) producing a mixed counterion polypyrrole membrane. It was found that incorporation of DS markedly affected the electrochemically controlled ion-exchange characteristics of the membrane and this is in turn affected by the dynamic transport properties of the membrane. In developing alternative synthetic approaches, other possibilities may be envisaged. One of these is the use of conducting polymer membranes composed of layers with different counterions being used utilising particular properties in each to produce a membrane with an enhanced performance. The major objective of the current study was to synthesise and characterise a layered conducting electroactive polypyrrole free-standing membrane. The layered conducting polymer membrane is prepared by electrochemically depositing a primary layer, and then the surface of this polymer is modi®ed by electrochemical deposition of a second layer of conducting polymer. The primary conducting polymer layer acts as a substrate membrane. Consequently, it must have good mechanical properties, electrical conductivity, uniformity and be able to be peeled off the substrate
electrode very easily. In the course of this work, pTS and DS were selected as the counterions for the synthesis of the primary and second layer polymers, respectively. NaCl and KCl were used as the source solutions to determine the electrochemically controlled transport properties. 2. Experimental 2.1. Chemicals All inorganic reagents used were of AR grade purity unless otherwise stated. Pyrrole was obtained from Sigma and was distilled prior to use. Dodecylsulphate (sodium salt, DS) and p-toluenesulphonate (sodium salt) were obtained from Sigma and Merck, respectively. All solutions were prepared with deionised Milli-Q water (18 M cm). 2.2. Instrumentation A Princeton Applied Research (PAR) model 363 potentiostat/galvanostat was employed for both electropolymerisation and controlled transport. For the controlled transport studies, it was combined with a home-made signal generator (made in the Science Faculty workshop in University of Wollongong, Australia). Cyclic voltametric experiments were performed using a BAS CV-27 (Bioanalytical System, USA). Electrochemical quartz crystal microbalance (EQCM) experiments were carried out using homemade equipment. The 10 MHz AT-cut crystals were sandwiched between two vapour deposited gold electrodes obtained from the International Crystal Manufacturing Company (ICM) in USA. All electrochemical data were recorded using MacLab Analog/ Digital interfaced with a Macintosh computer with Chart 4 software. All electrochemical experiments were carried out using a single compartment cell with a three electrode system. In the case of membrane synthesis, a stainless steel plate (10.06.0 cm2, mirror ®nish surface) working electrode and a reticulated vitreous carbon plate (1071 cm3) auxiliary electrode were used. For cyclic voltametric experiments, the working electrode was a piece of free-standing layered polymer membrane (3 mm in diameter) and the auxiliary electrode
H. Zhao et al. / Journal of Membrane Science 148 (1998) 161±172
used was a platinum mesh (100 line/inch). A specially designed disposable electrode unit was employed to hold the free-standing membrane for electrochemical measurements. In the case of EQCM experiments, the working electrode used was a vapour deposited gold electrode on one side of the crystal (0.24 cm2). All electrode potentials were measured relative to a Ag/ AgCl (BAS, 3M NaCl) reference electrode. A standard four-point probe (made at the University of Technology, Sydney, Australia) was used to obtain the DC-conductivities of membranes. Studies of the mechanical tensile strength of the membrane were carried out using an Instron Dynamic analyser model 4303. SEMs were obtained using either a Hitachi S450 or a Leica S440 electron microscope. These studies were carried out to determine surface morphology and for measuring the thickness of the membrane. In this case, the membrane sample was placed in a specially designed sample tray that enabled cross-section photographs to be taken. The dynamic contact angle measurements were performed using a Cahn DCA 322 dynamic contact angle analyser interfaced with an IBM-compatible computer. The liquid probe used was deionised water throughout the experiments. The surface tension used for all calculations was 72.6 dynes/cm. The platform speed was 24 mm/s for all experiments. 2.3. Layered polypyrrole membrane preparation Polymerisation was carried out galvanostatically using a three electrode single compartment electrochemical cell. The primary layer of the membrane was prepared from a polymerisation solution containing 0.20 M pyrrole and 0.05 M pTS using a current density of 2 mA/cm2 for 3.0, 5.0 or 7.0 min, respectively. After polymerisation, the polymer was thoroughly rinsed with distilled water. Then, this electrode was placed in a polymerisation solution containing 0.20 M pyrrole and 0.05 M DS. The second polymerisation step was also carried out at 2.0 mA/cm2 for 3.0, 5.0 or 7.0 min. The charge density during preparation of all membranes was 1.2 C/cm2. For example, when the primary polymer layer was prepared using a current density of 2.0 mA/cm2 for 3.0 min (with a charge density during the polymerisation of 0.36 C/cm2), the subsequent modi®cation polymerisation was carried out at 2.0 mA/cm2 for 7.0 min (charge density
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during the polymerisation is 0.84 C/cm2) which makes the total charge density of 1.2 C/cm2. The resultant polymer was again washed thoroughly with distilled water and was then peeled off the electrode. It should be noted that the polymerisation solutions for both the primary and modi®cation polymerisation were deoxygenated with nitrogen before use. All membranes used for transport studies and cyclic voltameter experiments were prepared using a current density of 2.0 mA/cm2, and 5 min deposition time for both PP/pTS and PP/DS layers. 2.4. Transport experiments The transport cell setup was as described previously [2]. The electrical connection to the membrane is illustrated in Fig. 1. Deionised water was used as the receiver solution throughout the transport experiments. The source solutions used were 0.20 M KCl or NaCl for transport studies, while the source solution used for separation experiments was 0.10 M KCl and NaCl. The volume of both cell compartments was 600 ml. A square-waveform was applied to the transport cell during an electrochemically controlled transport experiment. The pulse width was 50.0 s and the potential range was 0.70 to ÿ1.00 V vs. Ag/AgCl reference electrode. It should be noted that the different sides of the membrane have designated nomenclature; plate side of the membrane (plate side ± being the polymer surface that was in contact with the stainless steel substrate electrode during polymerisation, in this case, PP/pTS) and the solution side of the membrane (the membrane surface exposed to the polymerisation solution during polymerisation, for this layered membrane it was PP/DS). In different
Fig. 1. Depiction of the ring connection system for the membrane.
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transport experiments, these sides were alternately exposed to the source solution. During the transport experiments content was determined. Samples were taken every 20 min from the receiver solution. The cation content was determined using atomic absorption spectrometry. The ¯uxes reported in this work were average ¯uxes measured when the electrical stimulus was applied. For the EQCM ion ¯ux studies, the polymer ®lms were electrochemically deposited onto the EQCM electrode. This consisted of a thin layer of gold vapour deposited on the quartz crystal surface. The polymerisation solutions were the same as used for layered membrane preparation. In the case of PP/pTS coated QCM electrode, the polymerisation was carried out using 2.0 mA/cm2 for 60 s. For PP/DS coated QCM electrode, a layer of PP/pTS (2.0 mA/cm2 for 30 s) was ®rstly deposited and then a layer of PP/DS was coated on top of the PP/pTS layer using a current density of 2.0 mA/cm2 for 30 s. 3. Results and discussion 3.1. Layered membrane preparation and characterisation Polypyrrole/p-toluenesulphonate±Polypyrrole/dodecylsulphate (PP/pTS±PP/DS) layered membranes were successfully synthesised by the application of a two step polymerisation method. The PP/DS deposited readily on the pre-deposited PP/pTS layer. The membranes obtained were ¯exible, uniform and pinhole free.
DC-conductivity and mechanical tensile strength of PP/pTS±PP/DS membranes were measured (see Table 1). A range of polymerisation conditions were tried for the layer membrane and compared with single PP/DS and PP/pTS membranes. The lowest conductivity and tensile strength were observed with a PP/DS membrane (sample 1). The tensile strength obtained in this case was only about 3 MPa which is not even strong enough to separate two solution for transport experiment. The best conductivity and tensile strength were obtained from a PP/pTS membrane (sample 5). It was for this reason that PP/pTS was selected for the substrate layer in the preparation of layered membranes. It can be seen in Table 1 that both conductivity and tensile strength increased as the polymerisation time for preparation of PP/pTS layer was increased (samples 2±4). Values of the magnitude of sample 4 were found to be quite adequate for electrochemically controlled transport studies (see later). The thickness of the resultant membranes decreased as the polymerisation time for preparation of the PP/pTS layer increased. Since the charge density passed during polymerisation was the same for all the membranes, if it is assumed that the polymerisation ef®ciency is approximately independent of the counterion used, then the decrease in thickness of the membrane suggests that the PP/DS layer is much less dense than the PP/pTS layer. Figs. 2±4 show scanning electron micrographs of PP/pTS, PP/DS and PP/pTS±PP/DS layered membranes. A smooth surface morphology was observed for the PP/pTS side (plate side ± a term used to show the side of the membrane which was adjacent to the electrode during polymerisation) of the layered mem-
Table 1 Conductivity and tensile strength measurement of layered polypyrrole membranes Specifications
1
2
3
4
5
Conductivity (S/cm) Tensile strength (MPa) Thickness (mm)
409 32 9.00.4
6311 2510 6.20.4
719 337 5.30.4
797 475 4.70.4
10510 706 4.00.3
Note: All membranes were prepared from the polymerisation solution containing 0.20 M pyrrole and 0.05 M counterion using a current density of 2.0 mA/cm2. Sample preparation methods as shown below: 1. PP/DS membrane, polymerisation time: 10.0 min. 2. PP/pTS±PP/DS layered membrane, polymerisation time: 3.0 min for PP/pTS and 7.0 min for PP/DS. 3. PP/pTS±PP/DS layered membrane, polymerisation time: 5.0 min for both PP/pTS and PP/DS. 4. PP/pTS±PP/DS layered membrane, polymerisation time: 7.0 min for PP/pTS and 3.0 min for PP/DS. 5. PP/pTS membrane, polymerisation time: 10.0 min.
H. Zhao et al. / Journal of Membrane Science 148 (1998) 161±172
Fig. 2. SEM of (a) plate side and (b) source side of PP/pTS single membrane.
brane (see Fig. 4(a)). This morphology is very similar to the morphology observed from the plate side of a simple PP/pTS membrane (see Fig. 2(a)) indicating that the plate side surface morphology of PP/pTS layer was not affected by the subsequent PP/DS deposition. The PP/DS side of the layered membrane (``solution side'' that which is exposed to the solution during polymerisation) revealed a rough surface with a ``cauli¯ower'' morphology (see Fig. 4(b)). It was found that the nodule size was larger than that observed on the solution side of a PP/pTS membrane but smaller than the nodules on the solution side of a simple PP/DS membrane (see Fig. 2(b), Fig. 3(b) and Fig. 4(b)). Clearly, the PP/DSsurface morphologyof the layered membrane is affected by the surface morphology of the pre-de-
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Fig. 3. SEM of (a) plate side and (b) source side of PP/DS single membrane.
posited PP/pTS layer. PP/DS growing on a metallic substrate has a very porous open structure (less dense) and a rough surface on both sides of (Fig. 3 (a) and (b)). By growing it on a PP/pTS surface, the roughness of the solution side of the membrane is decreased. This may suggest that the PP/DS growing on a PP/pTS is less porous (more dense structure) than when it is grown on a metallic substrate. This was also supported by the thickness measurement (Table 1), where, for example, if PP/DS growing on both PP/pTS and metallic substrates has the same density, then the thickness for sample 3 should be about 6.5 mm, but the measured value was only 5.3 mm. In order to obtain further information on the surface properties of the membrane, dynamic contact angle
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Fig. 4. SEM of (a) plate side and (PP/pTS side) (b) source side (PP/DS side) of PP/pTS±PP/DS layer membrane.
(DCA) analysis (Wilhelmy's plate technique) was employed. Both the advancing (a) and receding (r) contact angles for each side of the layered membranes were recorded (Table 2). It can be seen from the larger advancing contact angle for the PP/DS side that this surface is more hydrophobic than the PP/pTS side in its dry state (see
Table 2). As a surfactant, DS has a long hydrophobic chain and a charged, polar end. On a dried PP/DS membrane, there is likely to be a layer of DS molecules present on the surface either by incorporation or adsorption. Since a polar surface is provided by the positive charged polymer backbone, it is likely that these positive charges will attract the negative charged polar end on the DS and result in some preferred orientation for the DS molecules. Generally, receding angle (r) and hysteresis value () can be used to determine the wettability of the surfaces under examination. The results shown in Table 2 reveal that the r value obtained for PP/DS side of the membrane was much lower than that for the PP/pTS side of the membrane. This indicates that compared to PP/pTS, the PP/DS surface has better wettability and much stronger water adhesion. The results also reveal that the value for the PP/DS is much greater than that of PP/pTS also suggesting that PP/DS surface has better wetting properties and is a very dynamic surface. This is almost certainly due in part to the rough morphology of the PP/DS surface. The distinctive cauli¯ower type nodular surface will produce a very dynamic surface. However, the magnitude of the wettability changes observed here are unusual. We have observed the rough cauli¯ower morphology previously using polypyrrole with other dopants, but these have not exhibited the dynamic wettability properties shown here [4±6]. A complementary explanation for the above results may be that when PP/DS surface is immersed in water a strongly polar solvent), the solvation/wetting processes taking place results in rearrangement of the polymer surface structure. That is, the charged end of the surfactant on the polymer surface now faces the water interface. This would result in the orientation of the surfactant in the wet state being different to the dried state. Consequently the hydrophobic dry polymer surface changes into a hydrophilic wet surface as re¯ected in the DCA results by the low r value and large
Table 2 Dynamic contact angle measurement of layered polypyrrole membranes Specifications
Advancing contact angle (a)
Receding contact angle (r)
Hysteresis value ()
PP/PTS side of the membrane PP/DS side of the membrane
878118 948128
49898 11868
388 738
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Fig. 5. Cyclic voltammogram of free-standing layer membrane (PP/pTS±PP/DS) sample 3, with curve 1. pTS side exposed to solution and curve 2. DS side exposed to solution. Scan rate50 mV/s.
value observed. The difference in the surface characteristics of the layered membrane is likely to be of signi®cance as far as the transport behaviour is concerned. Cyclic voltameter techniques were then employed to ascertain the electrochemical properties of the membrane. This was done to see if there were differences in the redox capabilities of the two sides of the membrane and to determine optimal potential ranges to carry out controlled transport experiments. To obtain this information a free-standing membrane was used as the working electrode. Cyclic voltammograms in Fig. 5 reveal that both the PP/pTS side and the PP/DS side of the membrane were electroactive. With the PP/DS side of the membrane (see curve 1), during the cathodic potential scan the current response in the potential range 0.70 to ÿ0.25 V was due to anion expulsion as described in the following equation: Reduced
PP Clÿ ! PP0 Clÿ
(1)
where PP and PP0 are the oxidised and reduced forms of polypyrrole, respectively, and Clÿ is the anion in the solution. A broad current peak appears at ÿ0.95 V when the potential was scanned towards more negative potentials. This current peak was due to cation incorporation (see following equation): Reduced
PP DSÿ K ! PP0 DSÿ K
(2)
where DSÿ is the counterion incorporated during synthesis and K is cation in the solution. The current response during the anodic potential scan reveals that a broad peak appears at ÿ0.15 V corresponding to the cation expulsion process (see Eq. (3)) and the current response obtained from further anodic potential scan (>0.0 V) was due to anion incorporation as shown by Eq. (4): Oxidised
PP0 DSÿ K ! PP DSÿ K Oxidised
PP0 Clÿ ! PP Clÿ
(3) (4)
With the PP/pTS side of the membrane (see curve 2), during the cathodic potential scan two broad current peaks appear at about ÿ0.6 and ÿ0.9 V, respectively. These two peaks were not very well resolved. The ®rst peak at about ÿ0.6 V corresponds to the anion expulsion reaction (see Eq. (1)) and the second peak at about ÿ0.9 V was due to cation incorporation (see Eq. (5)). During an anodic potential scan, only one broad current peak was observed with a maximum around ÿ0.05 V. This may be attributed to both cation expulsion (Eq. (6)) and anion incorporation (Eq. (4)). There is clearly a substantial difference in ion movement for different sides of the membrane: Reduced
PP pTSÿ K ! PP0 pTSÿ K Oxidised
PP0 pTSÿ K ! PP pTSÿ K
(5) (6)
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3.2. Transport properties of the membrane Electrochemically controlled transport of electroinactive species across conducting electroactive membranes has been reported by us previously [1±8]. The results have shown that transport across membranes can be switched on and off, and the rate of transport can be controlled in situ by application of appropriate electrical stimuli. We have also shown that the transport behaviour of the membranes is markedly in¯uenced by the counterion incorporation during membrane synthesis [5,6]. For example, when more DS was used in a mixed counterion system the membrane displayed asymmetric transport behaviour [5]. In the present communication a new approach to membrane preparation utilising a layered membrane technique has been investigated. A typical example for transport of K across a PP/ pTS±PP/DS layered membrane is shown in Fig. 6. This membrane was prepared using the method for sample 3 (see Table 1). It displays the same electrochemical controlled transport behaviour as for other conducting polymer membranes in terms of being able to switch the transport on and off in situ [9]. The major difference between this layered membrane and others is that a signi®cant ¯ux can be obtained only when the PP/pTS side of the membrane was exposed to the source solution. Compared to the case of PP/pTS/DS
membranes prepared using the mixed counterion approach, [5] the asymmetric transport in the case of PP/pTS±PP/DS layered membrane is much more appreciable, with almost no K transport across the membrane into the receiving solution being observed when the PP/DS (solution) side of the membrane was exposed to the source solution. Table 3 shows the effect of the side of the membrane exposed to the source solution on the ¯uxes of K and Na transported across the membrane. Asymmetric transport properties of the membrane were quanti®ed by the asymmetric factor as shown in Table 3. It can be seen that the asymmetric factor for K was 20. This value is about ®ve times higher than that obtained using a PP/ pTS/DS mixed counterions membrane [5]. With Na an even higher value of 36.5 was obtained. The selectivity of the transport was then tested using a source solution containing a mixture of 0.1 M KCl and NaCl. The concentration±time pro®les of K and Na are given in Fig. 7. The ¯uxes and selectivity factors calculated from this set of experiments are shown in Table 4. It was found that when the PP/pTS side of the membrane was exposed to the source solution, the separation of K and Na can be achieved with a separation factor of 4.5. The separation factor for the PP/DS side was much lower, at 1.5. It may be noted that the concentration of K and Na in the receiving solution was very close to the detec-
Fig. 6. Electrochemically controlled transport of K across PP/PTS/PP/DS membrane sample 3. Source solution: 0.2 M KCl; receiving solution: deionised water. Asymmetric pulsed potential with 50 s pulse width and 0.70 to ÿ1.00 V pulse range applied between point A and point B. Curve 1: PP/pTS side of the membrane exposed to the source solution; curve 2: PP/DS side of the membrane exposed to the source solution.
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Table 3 Effect of the side of the membrane exposed to the source solution on the fluxes of K and Na transported across a PP/PTS/PP/DS membrane Specifications
FPP/PTS 2
Flux of K (mol/cm s) Flux of Na (mol/cm2 s)
FPP/DS ÿ9
3.010 1.910ÿ9
Asymmetric factor (FPP/PTS/FPP/DS) ÿ10
1.510 5.210ÿ11
20 36.5
Note: FPP/pTSflux obtained when PP/pTS side of membrane was exposed to source solution; FPP/DSflux obtained when PP/DS side of membrane was exposed to source solution.
are compared to those obtained previously with simple conducting polymer membranes [4±8]. Two conclusions can be made from these simple demonstration experiments. Firstly these novel membranes are very asymmetric. Only with the substrate side (PP/pTS) exposed to the source solution is any signi®cant ¯ux obtainable. In addition, in this con®guration, the membrane shows good selectivity for similar ionic species. This type of layered membrane shows much potential as a novel separation material. Conducting polymer materials may be made with greatly varying surface chemistries and transport properties via using different dopant species during polymerisation. This is one of their great attractions as materials in a wide range of applications including separation science. 3.3. A possible transport mechanism for the layered membrane
Fig. 7. Electrochemically controlled transport of K and Na across PP/PTS/PP/DS membrane sample 3. Source solution: 0.1 M KCl0.1 M NaCl; receiving solution: deionised water. Asymmetric pulsed potential with 50 s pulse width and 0.70 to ÿ1.00 V pulse range applied between point A and point B. (a) PP/ pTS side of the membrane exposed to the source solution; curve 1: K and curve 2: Na; (b) PP/DS side of the membrane exposed to the source solution; curve 1: K and curve 2: Na.
tion limit of the analytical method in for experiments involving exposure of the PP/DS side of the membrane was exposed to the source solution. These selectivities
As proposed previously the transport mechanism for conducting electroactive polypyrrole membranes is an electrochemically controlled ion-exchange (ion incorporation/expulsion) process. In the case of the PP/pTS±PP/DS layered membrane, the two sides of the membrane are made of different materials (and each of these materials have different surface chemistry and electrochemistry). This results in different transport behaviour dependant on the side of the membrane exposed to the source solution. The electrodynamic behaviour of the polymer was investigated using electrochemical quartz crystal microbalance (EQCM). On reduction the PP/pTS ®lm initially loses mass, caused by expulsion of dopant anions (see Eq. (1)). At about ÿ0.3 V the mass then increases. This may be attributed to cation incorporation to balance the charge for sites where the anion has not been expelled (Eq. (2)). On the accompanying cyclic voltammogram (CV) in Fig. 8, peaks A and
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Table 4 Effect of the side of the membrane exposed to the source solution on the selectivity factors Specifications PP/pTS side exposed to the source solution PP/DS side exposed to the source solution
Flux of K (mol/cm2 s) ÿ9
2.210 1.710ÿ10
Flux of Na (mol/cm2 s) ÿ10
4.910 1.110ÿ10
Selectivity factor FK =FNa 4.5 1.5
Fig. 8. The current and frequency responses of a PP/pTS coated QCM electrode obtained from a cyclic voltammetric experiment in an aqueous solution containing 0.20 M KCl. Scan rate20.0 mV/s. Note: the appearance of the CV trace in this figure is quite different from those obtained in Fig. 5. This is because the latter was conducted using a thick free-standing film of about 5 mm whilst the QCM studies used thin (<0.1 mm) deposited layers.
B represent the cation incorporation and expulsion couple. On reoxidation of the ®lm, cation expulsion occurs ®rst followed by anion incorporation. The mass ratio of anion to cation movement is about 2:1. Thus there is a good amount of both. We have previously shown that induced movement of both cation and anion species is necessary to produce electrochemically controlled transport across such a ®lm [9]. This EQCM may be contrasted with that for a PP/ DS ®lm as shown in Fig. 9. On the reduction cycle, the extent of anion expulsion is very small compared to the amount of cation insertion. The mass ratio of anion to cation movement is around 1:3. This may be attributed to the relative immobility of DS in the ®lm. It should be noted that calculated ratios ignore any mass change due to movement of the solvent. Thus the reduction cycle is dominated by facile cation motion. It should be noted in looking at the CV that the cation and anion couples are much better resolved than in the PP/pTS ®lm. On reoxidation, the incorporated cation is expelled. The extent of the mass loss is in excess of the amount occurring on reduction. This may be due to salt-draining [13±15] of cation±anion pairs from the
®lm. Anion re-incorporation occurs subsequently. Part of the shape of the curve may be attributed to a phase lag between mass change and electronic (redox) changes on the polymer backbone. However, it is certainly clear that in this ®lm anion movement is considerably more dif®cult than it is in PP/pTS. This dramatic difference in ion movement for the two ®lms is the basis of the asymmetric transport observed in the layer membrane. When the PP/pTS face of the layer membrane was exposed to a source solution and subjected to a pulsed potential waveform, substantial ¯ux is observed across the membrane (Fig. 6). During the reduction±oxidation cycle signi®cant amounts of both mobile cations and anions are incorporated and expelled from the ®lm. On re-oxidation, any cations incorporated at polymer sites in the ®lms are released. Some of these will be expelled back into the source solution, however, for a signi®cant amount it will be easier to be expelled into the PP/DS side of the ®lm. The porous nature of the PP/DS may play a role here. Clearly, in order to maintain charge neutrality if an ion is expelled then it will be accompanied by an anion. Once into the
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Fig. 9. The current and frequency responses of a PP/pTS±PP/DS coated QCM electrode obtained from a cyclic voltammetric experiment in an aqueous solution containing 0.20 M KCl. Scan rate20.0 mV/s. Curve 1: current responses and curve 2: frequency responses. See note in Fig. 8.
porous DS part of the ®lm, any redox changes in the ®lm will lead to expulsion of ions into the receiving solution. In this manner the salt from the source solution is transported across the ®lm. This has been discussed previously in more detail. By contrast the different surface chemistry and morphology of the DS side of the layer membrane produces no signi®cant transport when it is exposed to the source solution (Fig. 7). On reduction of the membrane, there is signi®cant cation incorporation into the porous PP/DS part of the membrane. From the EQCM study this is a rapid facile process. The ease of this process may be in part attributable to the hydrophilic nature of the PP/DS ®lm. On reoxidation, the ions are expelled back into the source side of the solution. Due the porous nature of this side of the ®lm, it is a much more favourable process for ions to move back into the source solution. Thus virtually no measurable salt ¯ux results. 4. Conclusions A novel kind of membrane has been synthesised based on two types of polypyrrole. The different surface chemistries and morphologies of the two sides result in very asymmetric transport behaviour. This layer type of membrane system shows promise in applications of ion separation. It also highlights the advantage of using conducting polymer systems. A great variety of surface chemistries and morphologies
may be introduced by using different dopant counterions and novel approaches such as that adopted here. References [1] A. Mirmohseni, W.E. Price, G.G. Wallace, Electrochemically controlled transport of small charged organic molecules across conducting polymer membranes, J. Membr. Sci. 100 (1995) 239±248. [2] H. Zhao, W.E. Price, G.G. Wallace, Electrochemically controlled transport of potassium chloride across a conducting electroactive polymer membrane, J. Electroanal. Chem. 334 (1992) 111±120. [3] H. Zhao, W.E. Price, G.G. Wallace, Transport of Cu(II) across conducting polypyrrole membranes: the effect of applied potential waveforms, Polymer 34 (1993) 16±20. [4] H. Zhao, Electrochemically controlled transport across conducting electroactive polypyrrole membranes, Ph.D. thesis, Wollongong University, 1993. [5] H. Zhao et al., Transport across stand-alone conducting polypyrrole membranes containing dodecylsulfate counterions, React. Polym. 23 (1994) 213±220. [6] H. Zhao, W.E. Price, G.G. Wallace, Effect of the counterion employed during synthesis on the properties of polypyrrole membranes, J. Membr. Sci. 87 (1994) 47±56. [7] H. Zhao et al., Parameters affecting the electrochemical control across conducting polymer membranes, J. Membr. Sci. 119 (1996) 199±212. [8] A. Mirmohseni, W.E. Price, G.G. Wallace, The effect of different electrical stimuli on the oxidation/reduction behaviour of polypyrrole±pTS: a study using EQCM, Synthetic Metals 84 (1997) 823±824. [9] W.E. Price et al., Electrochemical control in conducting polymer membrane system designs, in: Proceedings of Euromembranes 95, University of Bath, 1 (1995) 301±306.
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[10] L.J. Buckley, D.K. Roylance, G.E. Wrek, Influence of dopant ion synthesis variables on mechanical properties of polypyrrole films, J. Polym. Sci., Part B 251 (1987) 2179. [11] K. Naoi, M. Lien, W.H. Smyrl, Quartz crystal microbalance study: ionic motion across conducting polymers, J. Electrochem. Soc. 138 (1991) 440±445. [12] C.K. Baker, Y.-J. Qiu, J.R. Reynolds, Electrochemically induced charge and mass transport in polypyrrole/poly(styrenesulfonate) molecular composites, J. Phys. Chem. 95 (1991) 4446±4452.
[13] Q. Pei, O. InganaÈs, Electrochemical applications of the bending beam method. 1. Mass transport and volume changes in polypyrrole during redox, J. Phys. Chem. 96 (1992) 10507± 10514. [14] Q. Pei, O. InganaÈs, Electrochemical applications of the bending beam method. 2. Electroshrinking and slow relaxation in polypyrrole, J. Phys. Chem. 97 (1993) 6034±6041. [15] M.R. Gandhi, P. Murray, G.M. Spinks, G.G. Wallace, Mechanism of electromechanical actuation in polypyrrole, Synthetic Metals 73 (1995) 247±256.
Synthesis, characterisation and transport properties of layered conducting electroactive polypyrrole membranes H. Zhao1, W.E. Price*, G.G. Wallace Department of Chemistry, Intelligent Polymer Research Institute, University of Wollongong, Wollongong, NSW 2522, Australia Received 16 October 1997; accepted 20 April 1998
Abstract A novel method for producing asymmetric membranes based on conducting polymers is described. Two layers of different polypyrrole ®lms (PPy±p-toluene sulphonate and PPy±dodecyl sulphate) were electrodeposited onto an electrode to form a sandwich or layer structure. The ®lms produced could be removed from the electrode and had suf®ciently good mechanical properties to be used as free-standing membranes in simple transport experiments. Using electrochemically induced transport utilising technology described previously it was shown that a highly asymmetric membrane had been formed with a ratio of up to 35:1 in terms of the ¯ux in one direction compared with another. This was for the transport of simple salts such as KCl and NaCl. In mixtures of these salts it was still possible to derive some reasonable selectivity between cations with selectivity of K over Na in ratios up to 4.5:1. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Electrochemistry; Ion-exchange membranes; Polypyrrole
1. Introduction Conducting electroactive polymer membranes such as polypyrrole membranes have attracted much interest over the past decade because of their dynamic chemical/electrochemical properties. Electrochemically controlled transport of ionic species across conducting electroactive polymer membranes have been demonstrated by us previously [1±8]. It has been shown that the transport of ionic species across the membrane can be switched on and off by means of application of an appropriate electrical potential
[2,8,9]. In addition, the rate of transport and the selectivity of the membrane can be dynamically controlled in situ by application of different electrical potential waveforms. Furthermore, membranes synthesised using different counterions show a remarkable in¯uence not just on the electrical conductivity, mechanical property and electroactivity of the membrane but also on the transport and dynamic control transport properties [6,7,10]. A free-standing conducting polypyrrole membrane can be electrochemically synthesised according to the following reaction:
*Corresponding author. Tel.: +61-242-213529; fax: +61-242214287; e-mail: [email protected] 1 Present address. Faculty of Environmental Sciences, School of Applied Sciences, Griffith University, Qld 4217, Australia. 0376-7388/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00158-6
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where, Cÿ is the counterion. Although a wide range of anions can in principle be used as the counterion for polypyrrole synthesis, the special requirements for stand-alone membrane application, in terms of, for example, mechanical properties, electrical conductivity, electroactivity and stability of the resultant polymer need to be considered. When this is done the number of anions that can be used as counterions is substantially restricted. Thus many anions with very attractive chemical functions are unsuitable for synthesis of stand-alone membranes. This poses a limitation on the development of new conducting polymer membrane materials for various applications. For example, dodecylsulphate (DS) is a surfactant and has very attractive surface chemistry, and being reasonably large possesses good cation exchangeability with little counterion leaching [5,11,12]. Unfortunately, the polymer prepared using DS as the counterion cannot be used as a free-standing membrane since this polymer has poor uniformity and mechanical properties, and is thus not strong enough to separate two solutions. To overcome this dif®culty, we have reported a membrane synthesis method where mixed counterions are employed [5]. This approach enabled DS to be incorporated into the polymer together with p-toluenesulphonate sodium salt (pTS) producing a mixed counterion polypyrrole membrane. It was found that incorporation of DS markedly affected the electrochemically controlled ion-exchange characteristics of the membrane and this is in turn affected by the dynamic transport properties of the membrane. In developing alternative synthetic approaches, other possibilities may be envisaged. One of these is the use of conducting polymer membranes composed of layers with different counterions being used utilising particular properties in each to produce a membrane with an enhanced performance. The major objective of the current study was to synthesise and characterise a layered conducting electroactive polypyrrole free-standing membrane. The layered conducting polymer membrane is prepared by electrochemically depositing a primary layer, and then the surface of this polymer is modi®ed by electrochemical deposition of a second layer of conducting polymer. The primary conducting polymer layer acts as a substrate membrane. Consequently, it must have good mechanical properties, electrical conductivity, uniformity and be able to be peeled off the substrate
electrode very easily. In the course of this work, pTS and DS were selected as the counterions for the synthesis of the primary and second layer polymers, respectively. NaCl and KCl were used as the source solutions to determine the electrochemically controlled transport properties. 2. Experimental 2.1. Chemicals All inorganic reagents used were of AR grade purity unless otherwise stated. Pyrrole was obtained from Sigma and was distilled prior to use. Dodecylsulphate (sodium salt, DS) and p-toluenesulphonate (sodium salt) were obtained from Sigma and Merck, respectively. All solutions were prepared with deionised Milli-Q water (18 M cm). 2.2. Instrumentation A Princeton Applied Research (PAR) model 363 potentiostat/galvanostat was employed for both electropolymerisation and controlled transport. For the controlled transport studies, it was combined with a home-made signal generator (made in the Science Faculty workshop in University of Wollongong, Australia). Cyclic voltametric experiments were performed using a BAS CV-27 (Bioanalytical System, USA). Electrochemical quartz crystal microbalance (EQCM) experiments were carried out using homemade equipment. The 10 MHz AT-cut crystals were sandwiched between two vapour deposited gold electrodes obtained from the International Crystal Manufacturing Company (ICM) in USA. All electrochemical data were recorded using MacLab Analog/ Digital interfaced with a Macintosh computer with Chart 4 software. All electrochemical experiments were carried out using a single compartment cell with a three electrode system. In the case of membrane synthesis, a stainless steel plate (10.06.0 cm2, mirror ®nish surface) working electrode and a reticulated vitreous carbon plate (1071 cm3) auxiliary electrode were used. For cyclic voltametric experiments, the working electrode was a piece of free-standing layered polymer membrane (3 mm in diameter) and the auxiliary electrode
H. Zhao et al. / Journal of Membrane Science 148 (1998) 161±172
used was a platinum mesh (100 line/inch). A specially designed disposable electrode unit was employed to hold the free-standing membrane for electrochemical measurements. In the case of EQCM experiments, the working electrode used was a vapour deposited gold electrode on one side of the crystal (0.24 cm2). All electrode potentials were measured relative to a Ag/ AgCl (BAS, 3M NaCl) reference electrode. A standard four-point probe (made at the University of Technology, Sydney, Australia) was used to obtain the DC-conductivities of membranes. Studies of the mechanical tensile strength of the membrane were carried out using an Instron Dynamic analyser model 4303. SEMs were obtained using either a Hitachi S450 or a Leica S440 electron microscope. These studies were carried out to determine surface morphology and for measuring the thickness of the membrane. In this case, the membrane sample was placed in a specially designed sample tray that enabled cross-section photographs to be taken. The dynamic contact angle measurements were performed using a Cahn DCA 322 dynamic contact angle analyser interfaced with an IBM-compatible computer. The liquid probe used was deionised water throughout the experiments. The surface tension used for all calculations was 72.6 dynes/cm. The platform speed was 24 mm/s for all experiments. 2.3. Layered polypyrrole membrane preparation Polymerisation was carried out galvanostatically using a three electrode single compartment electrochemical cell. The primary layer of the membrane was prepared from a polymerisation solution containing 0.20 M pyrrole and 0.05 M pTS using a current density of 2 mA/cm2 for 3.0, 5.0 or 7.0 min, respectively. After polymerisation, the polymer was thoroughly rinsed with distilled water. Then, this electrode was placed in a polymerisation solution containing 0.20 M pyrrole and 0.05 M DS. The second polymerisation step was also carried out at 2.0 mA/cm2 for 3.0, 5.0 or 7.0 min. The charge density during preparation of all membranes was 1.2 C/cm2. For example, when the primary polymer layer was prepared using a current density of 2.0 mA/cm2 for 3.0 min (with a charge density during the polymerisation of 0.36 C/cm2), the subsequent modi®cation polymerisation was carried out at 2.0 mA/cm2 for 7.0 min (charge density
163
during the polymerisation is 0.84 C/cm2) which makes the total charge density of 1.2 C/cm2. The resultant polymer was again washed thoroughly with distilled water and was then peeled off the electrode. It should be noted that the polymerisation solutions for both the primary and modi®cation polymerisation were deoxygenated with nitrogen before use. All membranes used for transport studies and cyclic voltameter experiments were prepared using a current density of 2.0 mA/cm2, and 5 min deposition time for both PP/pTS and PP/DS layers. 2.4. Transport experiments The transport cell setup was as described previously [2]. The electrical connection to the membrane is illustrated in Fig. 1. Deionised water was used as the receiver solution throughout the transport experiments. The source solutions used were 0.20 M KCl or NaCl for transport studies, while the source solution used for separation experiments was 0.10 M KCl and NaCl. The volume of both cell compartments was 600 ml. A square-waveform was applied to the transport cell during an electrochemically controlled transport experiment. The pulse width was 50.0 s and the potential range was 0.70 to ÿ1.00 V vs. Ag/AgCl reference electrode. It should be noted that the different sides of the membrane have designated nomenclature; plate side of the membrane (plate side ± being the polymer surface that was in contact with the stainless steel substrate electrode during polymerisation, in this case, PP/pTS) and the solution side of the membrane (the membrane surface exposed to the polymerisation solution during polymerisation, for this layered membrane it was PP/DS). In different
Fig. 1. Depiction of the ring connection system for the membrane.
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transport experiments, these sides were alternately exposed to the source solution. During the transport experiments content was determined. Samples were taken every 20 min from the receiver solution. The cation content was determined using atomic absorption spectrometry. The ¯uxes reported in this work were average ¯uxes measured when the electrical stimulus was applied. For the EQCM ion ¯ux studies, the polymer ®lms were electrochemically deposited onto the EQCM electrode. This consisted of a thin layer of gold vapour deposited on the quartz crystal surface. The polymerisation solutions were the same as used for layered membrane preparation. In the case of PP/pTS coated QCM electrode, the polymerisation was carried out using 2.0 mA/cm2 for 60 s. For PP/DS coated QCM electrode, a layer of PP/pTS (2.0 mA/cm2 for 30 s) was ®rstly deposited and then a layer of PP/DS was coated on top of the PP/pTS layer using a current density of 2.0 mA/cm2 for 30 s. 3. Results and discussion 3.1. Layered membrane preparation and characterisation Polypyrrole/p-toluenesulphonate±Polypyrrole/dodecylsulphate (PP/pTS±PP/DS) layered membranes were successfully synthesised by the application of a two step polymerisation method. The PP/DS deposited readily on the pre-deposited PP/pTS layer. The membranes obtained were ¯exible, uniform and pinhole free.
DC-conductivity and mechanical tensile strength of PP/pTS±PP/DS membranes were measured (see Table 1). A range of polymerisation conditions were tried for the layer membrane and compared with single PP/DS and PP/pTS membranes. The lowest conductivity and tensile strength were observed with a PP/DS membrane (sample 1). The tensile strength obtained in this case was only about 3 MPa which is not even strong enough to separate two solution for transport experiment. The best conductivity and tensile strength were obtained from a PP/pTS membrane (sample 5). It was for this reason that PP/pTS was selected for the substrate layer in the preparation of layered membranes. It can be seen in Table 1 that both conductivity and tensile strength increased as the polymerisation time for preparation of PP/pTS layer was increased (samples 2±4). Values of the magnitude of sample 4 were found to be quite adequate for electrochemically controlled transport studies (see later). The thickness of the resultant membranes decreased as the polymerisation time for preparation of the PP/pTS layer increased. Since the charge density passed during polymerisation was the same for all the membranes, if it is assumed that the polymerisation ef®ciency is approximately independent of the counterion used, then the decrease in thickness of the membrane suggests that the PP/DS layer is much less dense than the PP/pTS layer. Figs. 2±4 show scanning electron micrographs of PP/pTS, PP/DS and PP/pTS±PP/DS layered membranes. A smooth surface morphology was observed for the PP/pTS side (plate side ± a term used to show the side of the membrane which was adjacent to the electrode during polymerisation) of the layered mem-
Table 1 Conductivity and tensile strength measurement of layered polypyrrole membranes Specifications
1
2
3
4
5
Conductivity (S/cm) Tensile strength (MPa) Thickness (mm)
409 32 9.00.4
6311 2510 6.20.4
719 337 5.30.4
797 475 4.70.4
10510 706 4.00.3
Note: All membranes were prepared from the polymerisation solution containing 0.20 M pyrrole and 0.05 M counterion using a current density of 2.0 mA/cm2. Sample preparation methods as shown below: 1. PP/DS membrane, polymerisation time: 10.0 min. 2. PP/pTS±PP/DS layered membrane, polymerisation time: 3.0 min for PP/pTS and 7.0 min for PP/DS. 3. PP/pTS±PP/DS layered membrane, polymerisation time: 5.0 min for both PP/pTS and PP/DS. 4. PP/pTS±PP/DS layered membrane, polymerisation time: 7.0 min for PP/pTS and 3.0 min for PP/DS. 5. PP/pTS membrane, polymerisation time: 10.0 min.
H. Zhao et al. / Journal of Membrane Science 148 (1998) 161±172
Fig. 2. SEM of (a) plate side and (b) source side of PP/pTS single membrane.
brane (see Fig. 4(a)). This morphology is very similar to the morphology observed from the plate side of a simple PP/pTS membrane (see Fig. 2(a)) indicating that the plate side surface morphology of PP/pTS layer was not affected by the subsequent PP/DS deposition. The PP/DS side of the layered membrane (``solution side'' that which is exposed to the solution during polymerisation) revealed a rough surface with a ``cauli¯ower'' morphology (see Fig. 4(b)). It was found that the nodule size was larger than that observed on the solution side of a PP/pTS membrane but smaller than the nodules on the solution side of a simple PP/DS membrane (see Fig. 2(b), Fig. 3(b) and Fig. 4(b)). Clearly, the PP/DSsurface morphologyof the layered membrane is affected by the surface morphology of the pre-de-
165
Fig. 3. SEM of (a) plate side and (b) source side of PP/DS single membrane.
posited PP/pTS layer. PP/DS growing on a metallic substrate has a very porous open structure (less dense) and a rough surface on both sides of (Fig. 3 (a) and (b)). By growing it on a PP/pTS surface, the roughness of the solution side of the membrane is decreased. This may suggest that the PP/DS growing on a PP/pTS is less porous (more dense structure) than when it is grown on a metallic substrate. This was also supported by the thickness measurement (Table 1), where, for example, if PP/DS growing on both PP/pTS and metallic substrates has the same density, then the thickness for sample 3 should be about 6.5 mm, but the measured value was only 5.3 mm. In order to obtain further information on the surface properties of the membrane, dynamic contact angle
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Fig. 4. SEM of (a) plate side and (PP/pTS side) (b) source side (PP/DS side) of PP/pTS±PP/DS layer membrane.
(DCA) analysis (Wilhelmy's plate technique) was employed. Both the advancing (a) and receding (r) contact angles for each side of the layered membranes were recorded (Table 2). It can be seen from the larger advancing contact angle for the PP/DS side that this surface is more hydrophobic than the PP/pTS side in its dry state (see
Table 2). As a surfactant, DS has a long hydrophobic chain and a charged, polar end. On a dried PP/DS membrane, there is likely to be a layer of DS molecules present on the surface either by incorporation or adsorption. Since a polar surface is provided by the positive charged polymer backbone, it is likely that these positive charges will attract the negative charged polar end on the DS and result in some preferred orientation for the DS molecules. Generally, receding angle (r) and hysteresis value () can be used to determine the wettability of the surfaces under examination. The results shown in Table 2 reveal that the r value obtained for PP/DS side of the membrane was much lower than that for the PP/pTS side of the membrane. This indicates that compared to PP/pTS, the PP/DS surface has better wettability and much stronger water adhesion. The results also reveal that the value for the PP/DS is much greater than that of PP/pTS also suggesting that PP/DS surface has better wetting properties and is a very dynamic surface. This is almost certainly due in part to the rough morphology of the PP/DS surface. The distinctive cauli¯ower type nodular surface will produce a very dynamic surface. However, the magnitude of the wettability changes observed here are unusual. We have observed the rough cauli¯ower morphology previously using polypyrrole with other dopants, but these have not exhibited the dynamic wettability properties shown here [4±6]. A complementary explanation for the above results may be that when PP/DS surface is immersed in water a strongly polar solvent), the solvation/wetting processes taking place results in rearrangement of the polymer surface structure. That is, the charged end of the surfactant on the polymer surface now faces the water interface. This would result in the orientation of the surfactant in the wet state being different to the dried state. Consequently the hydrophobic dry polymer surface changes into a hydrophilic wet surface as re¯ected in the DCA results by the low r value and large
Table 2 Dynamic contact angle measurement of layered polypyrrole membranes Specifications
Advancing contact angle (a)
Receding contact angle (r)
Hysteresis value ()
PP/PTS side of the membrane PP/DS side of the membrane
878118 948128
49898 11868
388 738
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167
Fig. 5. Cyclic voltammogram of free-standing layer membrane (PP/pTS±PP/DS) sample 3, with curve 1. pTS side exposed to solution and curve 2. DS side exposed to solution. Scan rate50 mV/s.
value observed. The difference in the surface characteristics of the layered membrane is likely to be of signi®cance as far as the transport behaviour is concerned. Cyclic voltameter techniques were then employed to ascertain the electrochemical properties of the membrane. This was done to see if there were differences in the redox capabilities of the two sides of the membrane and to determine optimal potential ranges to carry out controlled transport experiments. To obtain this information a free-standing membrane was used as the working electrode. Cyclic voltammograms in Fig. 5 reveal that both the PP/pTS side and the PP/DS side of the membrane were electroactive. With the PP/DS side of the membrane (see curve 1), during the cathodic potential scan the current response in the potential range 0.70 to ÿ0.25 V was due to anion expulsion as described in the following equation: Reduced
PP Clÿ ! PP0 Clÿ
(1)
where PP and PP0 are the oxidised and reduced forms of polypyrrole, respectively, and Clÿ is the anion in the solution. A broad current peak appears at ÿ0.95 V when the potential was scanned towards more negative potentials. This current peak was due to cation incorporation (see following equation): Reduced
PP DSÿ K ! PP0 DSÿ K
(2)
where DSÿ is the counterion incorporated during synthesis and K is cation in the solution. The current response during the anodic potential scan reveals that a broad peak appears at ÿ0.15 V corresponding to the cation expulsion process (see Eq. (3)) and the current response obtained from further anodic potential scan (>0.0 V) was due to anion incorporation as shown by Eq. (4): Oxidised
PP0 DSÿ K ! PP DSÿ K Oxidised
PP0 Clÿ ! PP Clÿ
(3) (4)
With the PP/pTS side of the membrane (see curve 2), during the cathodic potential scan two broad current peaks appear at about ÿ0.6 and ÿ0.9 V, respectively. These two peaks were not very well resolved. The ®rst peak at about ÿ0.6 V corresponds to the anion expulsion reaction (see Eq. (1)) and the second peak at about ÿ0.9 V was due to cation incorporation (see Eq. (5)). During an anodic potential scan, only one broad current peak was observed with a maximum around ÿ0.05 V. This may be attributed to both cation expulsion (Eq. (6)) and anion incorporation (Eq. (4)). There is clearly a substantial difference in ion movement for different sides of the membrane: Reduced
PP pTSÿ K ! PP0 pTSÿ K Oxidised
PP0 pTSÿ K ! PP pTSÿ K
(5) (6)
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3.2. Transport properties of the membrane Electrochemically controlled transport of electroinactive species across conducting electroactive membranes has been reported by us previously [1±8]. The results have shown that transport across membranes can be switched on and off, and the rate of transport can be controlled in situ by application of appropriate electrical stimuli. We have also shown that the transport behaviour of the membranes is markedly in¯uenced by the counterion incorporation during membrane synthesis [5,6]. For example, when more DS was used in a mixed counterion system the membrane displayed asymmetric transport behaviour [5]. In the present communication a new approach to membrane preparation utilising a layered membrane technique has been investigated. A typical example for transport of K across a PP/ pTS±PP/DS layered membrane is shown in Fig. 6. This membrane was prepared using the method for sample 3 (see Table 1). It displays the same electrochemical controlled transport behaviour as for other conducting polymer membranes in terms of being able to switch the transport on and off in situ [9]. The major difference between this layered membrane and others is that a signi®cant ¯ux can be obtained only when the PP/pTS side of the membrane was exposed to the source solution. Compared to the case of PP/pTS/DS
membranes prepared using the mixed counterion approach, [5] the asymmetric transport in the case of PP/pTS±PP/DS layered membrane is much more appreciable, with almost no K transport across the membrane into the receiving solution being observed when the PP/DS (solution) side of the membrane was exposed to the source solution. Table 3 shows the effect of the side of the membrane exposed to the source solution on the ¯uxes of K and Na transported across the membrane. Asymmetric transport properties of the membrane were quanti®ed by the asymmetric factor as shown in Table 3. It can be seen that the asymmetric factor for K was 20. This value is about ®ve times higher than that obtained using a PP/ pTS/DS mixed counterions membrane [5]. With Na an even higher value of 36.5 was obtained. The selectivity of the transport was then tested using a source solution containing a mixture of 0.1 M KCl and NaCl. The concentration±time pro®les of K and Na are given in Fig. 7. The ¯uxes and selectivity factors calculated from this set of experiments are shown in Table 4. It was found that when the PP/pTS side of the membrane was exposed to the source solution, the separation of K and Na can be achieved with a separation factor of 4.5. The separation factor for the PP/DS side was much lower, at 1.5. It may be noted that the concentration of K and Na in the receiving solution was very close to the detec-
Fig. 6. Electrochemically controlled transport of K across PP/PTS/PP/DS membrane sample 3. Source solution: 0.2 M KCl; receiving solution: deionised water. Asymmetric pulsed potential with 50 s pulse width and 0.70 to ÿ1.00 V pulse range applied between point A and point B. Curve 1: PP/pTS side of the membrane exposed to the source solution; curve 2: PP/DS side of the membrane exposed to the source solution.
H. Zhao et al. / Journal of Membrane Science 148 (1998) 161±172
169
Table 3 Effect of the side of the membrane exposed to the source solution on the fluxes of K and Na transported across a PP/PTS/PP/DS membrane Specifications
FPP/PTS 2
Flux of K (mol/cm s) Flux of Na (mol/cm2 s)
FPP/DS ÿ9
3.010 1.910ÿ9
Asymmetric factor (FPP/PTS/FPP/DS) ÿ10
1.510 5.210ÿ11
20 36.5
Note: FPP/pTSflux obtained when PP/pTS side of membrane was exposed to source solution; FPP/DSflux obtained when PP/DS side of membrane was exposed to source solution.
are compared to those obtained previously with simple conducting polymer membranes [4±8]. Two conclusions can be made from these simple demonstration experiments. Firstly these novel membranes are very asymmetric. Only with the substrate side (PP/pTS) exposed to the source solution is any signi®cant ¯ux obtainable. In addition, in this con®guration, the membrane shows good selectivity for similar ionic species. This type of layered membrane shows much potential as a novel separation material. Conducting polymer materials may be made with greatly varying surface chemistries and transport properties via using different dopant species during polymerisation. This is one of their great attractions as materials in a wide range of applications including separation science. 3.3. A possible transport mechanism for the layered membrane
Fig. 7. Electrochemically controlled transport of K and Na across PP/PTS/PP/DS membrane sample 3. Source solution: 0.1 M KCl0.1 M NaCl; receiving solution: deionised water. Asymmetric pulsed potential with 50 s pulse width and 0.70 to ÿ1.00 V pulse range applied between point A and point B. (a) PP/ pTS side of the membrane exposed to the source solution; curve 1: K and curve 2: Na; (b) PP/DS side of the membrane exposed to the source solution; curve 1: K and curve 2: Na.
tion limit of the analytical method in for experiments involving exposure of the PP/DS side of the membrane was exposed to the source solution. These selectivities
As proposed previously the transport mechanism for conducting electroactive polypyrrole membranes is an electrochemically controlled ion-exchange (ion incorporation/expulsion) process. In the case of the PP/pTS±PP/DS layered membrane, the two sides of the membrane are made of different materials (and each of these materials have different surface chemistry and electrochemistry). This results in different transport behaviour dependant on the side of the membrane exposed to the source solution. The electrodynamic behaviour of the polymer was investigated using electrochemical quartz crystal microbalance (EQCM). On reduction the PP/pTS ®lm initially loses mass, caused by expulsion of dopant anions (see Eq. (1)). At about ÿ0.3 V the mass then increases. This may be attributed to cation incorporation to balance the charge for sites where the anion has not been expelled (Eq. (2)). On the accompanying cyclic voltammogram (CV) in Fig. 8, peaks A and
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Table 4 Effect of the side of the membrane exposed to the source solution on the selectivity factors Specifications PP/pTS side exposed to the source solution PP/DS side exposed to the source solution
Flux of K (mol/cm2 s) ÿ9
2.210 1.710ÿ10
Flux of Na (mol/cm2 s) ÿ10
4.910 1.110ÿ10
Selectivity factor FK =FNa 4.5 1.5
Fig. 8. The current and frequency responses of a PP/pTS coated QCM electrode obtained from a cyclic voltammetric experiment in an aqueous solution containing 0.20 M KCl. Scan rate20.0 mV/s. Note: the appearance of the CV trace in this figure is quite different from those obtained in Fig. 5. This is because the latter was conducted using a thick free-standing film of about 5 mm whilst the QCM studies used thin (<0.1 mm) deposited layers.
B represent the cation incorporation and expulsion couple. On reoxidation of the ®lm, cation expulsion occurs ®rst followed by anion incorporation. The mass ratio of anion to cation movement is about 2:1. Thus there is a good amount of both. We have previously shown that induced movement of both cation and anion species is necessary to produce electrochemically controlled transport across such a ®lm [9]. This EQCM may be contrasted with that for a PP/ DS ®lm as shown in Fig. 9. On the reduction cycle, the extent of anion expulsion is very small compared to the amount of cation insertion. The mass ratio of anion to cation movement is around 1:3. This may be attributed to the relative immobility of DS in the ®lm. It should be noted that calculated ratios ignore any mass change due to movement of the solvent. Thus the reduction cycle is dominated by facile cation motion. It should be noted in looking at the CV that the cation and anion couples are much better resolved than in the PP/pTS ®lm. On reoxidation, the incorporated cation is expelled. The extent of the mass loss is in excess of the amount occurring on reduction. This may be due to salt-draining [13±15] of cation±anion pairs from the
®lm. Anion re-incorporation occurs subsequently. Part of the shape of the curve may be attributed to a phase lag between mass change and electronic (redox) changes on the polymer backbone. However, it is certainly clear that in this ®lm anion movement is considerably more dif®cult than it is in PP/pTS. This dramatic difference in ion movement for the two ®lms is the basis of the asymmetric transport observed in the layer membrane. When the PP/pTS face of the layer membrane was exposed to a source solution and subjected to a pulsed potential waveform, substantial ¯ux is observed across the membrane (Fig. 6). During the reduction±oxidation cycle signi®cant amounts of both mobile cations and anions are incorporated and expelled from the ®lm. On re-oxidation, any cations incorporated at polymer sites in the ®lms are released. Some of these will be expelled back into the source solution, however, for a signi®cant amount it will be easier to be expelled into the PP/DS side of the ®lm. The porous nature of the PP/DS may play a role here. Clearly, in order to maintain charge neutrality if an ion is expelled then it will be accompanied by an anion. Once into the
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Fig. 9. The current and frequency responses of a PP/pTS±PP/DS coated QCM electrode obtained from a cyclic voltammetric experiment in an aqueous solution containing 0.20 M KCl. Scan rate20.0 mV/s. Curve 1: current responses and curve 2: frequency responses. See note in Fig. 8.
porous DS part of the ®lm, any redox changes in the ®lm will lead to expulsion of ions into the receiving solution. In this manner the salt from the source solution is transported across the ®lm. This has been discussed previously in more detail. By contrast the different surface chemistry and morphology of the DS side of the layer membrane produces no signi®cant transport when it is exposed to the source solution (Fig. 7). On reduction of the membrane, there is signi®cant cation incorporation into the porous PP/DS part of the membrane. From the EQCM study this is a rapid facile process. The ease of this process may be in part attributable to the hydrophilic nature of the PP/DS ®lm. On reoxidation, the ions are expelled back into the source side of the solution. Due the porous nature of this side of the ®lm, it is a much more favourable process for ions to move back into the source solution. Thus virtually no measurable salt ¯ux results. 4. Conclusions A novel kind of membrane has been synthesised based on two types of polypyrrole. The different surface chemistries and morphologies of the two sides result in very asymmetric transport behaviour. This layer type of membrane system shows promise in applications of ion separation. It also highlights the advantage of using conducting polymer systems. A great variety of surface chemistries and morphologies
may be introduced by using different dopant counterions and novel approaches such as that adopted here. References [1] A. Mirmohseni, W.E. Price, G.G. Wallace, Electrochemically controlled transport of small charged organic molecules across conducting polymer membranes, J. Membr. Sci. 100 (1995) 239±248. [2] H. Zhao, W.E. Price, G.G. Wallace, Electrochemically controlled transport of potassium chloride across a conducting electroactive polymer membrane, J. Electroanal. Chem. 334 (1992) 111±120. [3] H. Zhao, W.E. Price, G.G. Wallace, Transport of Cu(II) across conducting polypyrrole membranes: the effect of applied potential waveforms, Polymer 34 (1993) 16±20. [4] H. Zhao, Electrochemically controlled transport across conducting electroactive polypyrrole membranes, Ph.D. thesis, Wollongong University, 1993. [5] H. Zhao et al., Transport across stand-alone conducting polypyrrole membranes containing dodecylsulfate counterions, React. Polym. 23 (1994) 213±220. [6] H. Zhao, W.E. Price, G.G. Wallace, Effect of the counterion employed during synthesis on the properties of polypyrrole membranes, J. Membr. Sci. 87 (1994) 47±56. [7] H. Zhao et al., Parameters affecting the electrochemical control across conducting polymer membranes, J. Membr. Sci. 119 (1996) 199±212. [8] A. Mirmohseni, W.E. Price, G.G. Wallace, The effect of different electrical stimuli on the oxidation/reduction behaviour of polypyrrole±pTS: a study using EQCM, Synthetic Metals 84 (1997) 823±824. [9] W.E. Price et al., Electrochemical control in conducting polymer membrane system designs, in: Proceedings of Euromembranes 95, University of Bath, 1 (1995) 301±306.
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[10] L.J. Buckley, D.K. Roylance, G.E. Wrek, Influence of dopant ion synthesis variables on mechanical properties of polypyrrole films, J. Polym. Sci., Part B 251 (1987) 2179. [11] K. Naoi, M. Lien, W.H. Smyrl, Quartz crystal microbalance study: ionic motion across conducting polymers, J. Electrochem. Soc. 138 (1991) 440±445. [12] C.K. Baker, Y.-J. Qiu, J.R. Reynolds, Electrochemically induced charge and mass transport in polypyrrole/poly(styrenesulfonate) molecular composites, J. Phys. Chem. 95 (1991) 4446±4452.
[13] Q. Pei, O. InganaÈs, Electrochemical applications of the bending beam method. 1. Mass transport and volume changes in polypyrrole during redox, J. Phys. Chem. 96 (1992) 10507± 10514. [14] Q. Pei, O. InganaÈs, Electrochemical applications of the bending beam method. 2. Electroshrinking and slow relaxation in polypyrrole, J. Phys. Chem. 97 (1993) 6034±6041. [15] M.R. Gandhi, P. Murray, G.M. Spinks, G.G. Wallace, Mechanism of electromechanical actuation in polypyrrole, Synthetic Metals 73 (1995) 247±256.