Good adhesion between chemically oxidised titanium and electrochemically deposited polypyrrole

Electrochimica Acta 45 (2000) 2121 – 2130 www.elsevier.nl/locate/electacta

Good adhesion between chemically oxidised titanium and electrochemically deposited polypyrrole Katrin Idla a,*, Olle Ingana¨s b, Marek Strandberg c a

Laboratory of Physical Chemistry, Tallinn Technical Uni6ersity, Ehitajate tee 5, 19086, Tallinn, Estonia b Laboratory of Applied Physics, IFM, Linko¨ping Uni6ersity, Linko¨ping, Sweden c Edison Centre, Pikk 39, 10133, Tallinn, Estonia Received 7 September 1999; received in revised form 30 November 1999

Abstract A method for producing extremely adhesive polypyrrole (Ppy) films is described. The electrochemical synthesis of Ppy on thin chemically pre-oxidised Ti layers produces a mechanically strong, shiny polymer film with extremely good adhesion. Adhesion of Ppy films on Ti metal without chemical pre-oxidation is very weak. Two multilayer systems are described with Ppy as an electrochemically active layer, chemically oxidised Ti (Tix Oy ) as a thin adhesive layer, and either a Si-wafer or Al foil as a substrate. Ppy films survive more than 6000 reduction – oxidation cycles in aqueous electrolyte without delamination. The possible mechanisms of enhanced adhesion are discussed. Those are: (1) increased adhesion due to changes in the chemical composition and surface structure of the pre-oxidised Ti; (2) the possibility of the chemical oxidation of pyrrole on the metal surface in addition to the electrochemical polymerisation; (3) the adsorption of pyrrole molecules onto pre-oxidised Ti surface by interaction with Ti hydroxides on surface; and (4) the simultaneous growth of Tix Oy and Ppy. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polypyrrole; Electrochemical synthesis; Adhesion; Titanium; Artificial muscle; Coating

1. Introduction The adhesion of electrically conductive polymers (ECP) to a metal substrate is of prime importance in many applications where good electrical contact and durability of the system is required. The use of polypyrrole (Ppy) as an active component of bilayer or multilayer strips in artificial muscles and micromachined structures is one example [1–3]. Those strips consist of the electroactive polymer film (polypyrrole, polyaniline) and the supporting electrochemically inactive layer (metal foil, plastic) with or without adhesive layer between. In general, this kind of ‘artificial mus* Corresponding author. Tel.: +372-6202806; fax: +3726202020. E-mail address: [email protected] (K. Idla)

cles’ can be constructed in two principal ways; by in-situ electropolymerisation of ECP onto the substrate which is electrochemically inactive part of the ‘muscle’ (one-step preparation, ECP/metal foil or ECP/metal foil/non-conductive polymer structures) [1,3] or by previous electrochemical preparation of polymer films onto metal plate electrodes followed by peeling-off the polymer film from the metal with flexible adherent polymer (two-step preparation, ECP/non-conductive polymer structures) [4]. In this application the muscle movement relies on redox processes in ECPs [5 – 9] and is initiated by applying external voltage. The external voltage will cause reduction or oxidation of Ppy, which activates simultaneous volume change in the polymer layer. This volume change generates synchronous reversible bending of the whole multilayer strip because Ppy film is in

0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 4 3 3 - 8

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strong contact with the substrate. The direction of the movement depends on structural changes in the Ppy, insertion of ions causes the increase in volume and leads to expansion while migration of ions out from the electroactive polymer matrix results in contraction. Besides of the reversible electrochemical activity of the polymer the effective bending of the whole multilayer system depends on the strong and stable contact between the active polymer layer, converting electrochemical energy into mechanical energy, and the supporting metal layer. In previous studies of the bending bi(Au/Ppy)- or multilayer (polyethylene/Cr-Au/Ppy) systems, the delamination of Ppy film from Au is found to be a serious concern and a cause of short lifetime [10]. Bending of the PE/Au/Ppy layers is reported to be reversible over a short time but a decay in the bending degree was observed after running of five redox cycles in the aqueous solution [1]. This might be due to the degradation of the polymer film as well as to weak contact between the two layers. The research group at Linko¨ping University has been working intensely with the aim of finding the solutions to the adhesion problem. The chemical surface treatment with a monolayer of thiol modified pyrrole was studied for extending the operating lifetime of the microfingers [11]. But still, this solution seems to be not sufficient for long-term applications. Our study was motivated from the necessity to find a key for the synthesis of strongly adhesive electroactive polymer films, which would enhance the longterm stability of the multilayer devices. From our previous studies we know that gold does not always give strong and reproducible adherence to Ppy. Therefore, we focused on other metals. The useful properties of the metal for a given application is a combination of the following: (a) stable in different conditions; (b) non-toxic for natural environment; (c) with the low value of Young’s modulus and light weight (for effective bending) and (d) biocompatible (for artificial muscle and cell research). The biocompatibility was the key property that made us think about Ti as the best metal for our application. Al was also of interest because of the low value of the modulus of elasticity, lightweight, and availability. Both of these metals show high stability in various environments thanks to their native thin protective oxide layer. The electrochemical synthesis of polypyrrole onto different metals has been studied in some works. From the viewpoint of industrial preparation as well as application the main requirement is to perform the synthesis of Ppy from aqueous solution. In the case of inert metals substrate (Pt, Au) there are no problems in electrochemical deposition of Ppy. In the case of oxidisable metals (Fe, Al, Ti) there is always a competition between two simultaneous oxidation processes, formation of the Ppy and the metal oxide. The latter process may inhibit the electropolymerisation of Ppy by passi-

vating the metal surface. Electrodeposition of Ppy films onto thin (2000 – 3000 A, ) evaporated metal films was studied by Prejza et al., they could not grow Ppy films onto Al and other metals from organic solvents because the metal films were dissolved before the electropolymerisation process [12]. Ppy films can be electrochemically synthesised onto Al and Ti metal plates from organic solvents (acetonitrile, propylene carbonate) [13 – 15]. Investigation of the electrocoating process of Ti, Al and other metals in aqueous and non-aqueous electrolytes has been reported by Beck et al. [16,17]. Well adhering Ppy layers can be electrodeposited on Al from aqueous solutions of acids (H2C2O4, HNO3, H2SO4) [18]. Lacaze et al. investigated the pretreatment of iron with dilute nitric acid and reported the strong adhesion of Ppy [19,20]. The same research group synthesised adhesive Ppy films on oxidisable metals (iron, zinc, copper, nickel or aluminum) also without the pretreatment of the substrate using an aqueous or aqueous-alcoholic electrolytic solution containing salicylate ions [21]. We have reported the deposition of adhesive Ppy films onto steel from an aqueous-alcoholic electrolyte solution. These films exhibited good corrosion protective properties [22]. For our application the counter anion in Ppy matrix must be a large organic anion which is trapped into the polymer structure, which allows elastic deformation of the film and does not itself participate in ion-exchange in the course of redox processes. Amongst the studied organic anions, so far dodecylbensenesulfonate gives the best results in the bending property. Our experiments with a polymeric anion, polystyrenesulphonate, did not give the bending of the film in the same conditions [23]. In this paper we report the results of the studies of the galvanostatic growth of PPy on chemically pre-oxidised Ti surface. This new method for the electrochemical synthesis of adhesive Ppy films results not only in strong bonds between the metal and polymer, but also a polymer with improved mechanical strength and remarkable scratch-safe properties compared to the polymer film on Ti. We present scanning electron microscopy (SEM), atomic force microscopy (AFM), secondary ion mass spectrometry (SIMS) and auger electron spectroscopy (AES) results to characterise the metal surfaces and the interface. In addition, we present the multilayer systems consisting of Si/Tix Oy /Ppy and Al foil/Tix Oy /Ppy.

2. Experimental A Ti layer with the thickness of 3000 – 3500 A, was vacuum evaporated (evaporation rate 10 A, /s) either onto Si-wafer (MEMC Electronic Materials) or 10-mmthick Al foil (Al \99%, Skultuna). During the evapora-

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tion, the pressure in the system was in the range of 7× 10 − 7 torr. A working electrode with the dimensions of (0.5×2) cm was cut from these pieces. The Ti layer was oxidised by dipping the working electrode into boiling solution of 25% NH3 +H2O/30% H2O2/H2O mixture with the volume ratio of 1:1:5 (known as SC1). The oxidation time strongly depends on the freshness of the solution, in the case of freshly mixed SC1 the time is about 5–10 s. The electrode is taken out from the solution at the moment when the visible darkening of the surface from metallic to violet-brown occurs. After that the working electrode was washed in the flow of deionised water and dried in a stream of nitrogen. It is very important to take the Ti out of the SC1 at the moment when the surface is violet or brown because after those visible colour changes the surface turns back to metallic looking. In our text we mark pre-oxidised surface as Tix Oy and the surface without chemical oxidation as Ti. An Au-wire of about 1 cm length was used as the counter electrode and potential was measured with reference to the Ag/AgCl (in 3M NaCl) electrode (from BAS). The distance between the working and the counter electrodes was about 4 cm. Pyrrole (Aldrich) was distilled and then stored in the solid state at − 30°C. Sodium dodecylbenzenesulfonate, C12H25C6H4SO3Na (NaDBS) from Aldrich was stored in laboratory conditions and used without previous purification. Thirty percent hydrogen peroxide, H2O2 and 25% ammonium hydroxide, NH3xH2O were supplied from Riedel-de Hae¨n. Deionised water, produced by Millipore, Milli-Q plus 185 apparatus, was used for the preparation of aqueous solutions. Polypyrrole films were galvanostatically (0.2–1 mA/cm2) deposited from 0.25 M pyrrole and 0.1 M NaDBS aqueous solution. Nitrogen was bubbled through the solution for 40–60 min just prior to use. Varying the deposition time grew polymer films of different thickness and the thickness was calculated from the charge consumed during the synthesis [24]. In some cases the thickness was further estimated from the profile studies of the working electrode by JEOL JSM-T220A Scanning electron microscope and these values were smaller compared to those calculated from the charge. Electrochemical measurements were performed using a Autolab potentio/galvanostat 10 (Eco Chemie) and the General Purpose Electrochemical System (GPES) software. Adhesion of Ppy to the substrate was tested using Scotch™ Magic™ Tape 810 (3M). Ppy surface was always washed with deionised water and dried in the stream of nitrogen prior to test. The surface structures were studied and the roughness of the surfaces was measured using a Digital Instruments Nanoscope III Scanning Probe Microscope run in AFM Tapping Mode. The mean surface roughness (Ra) was calculated by Nanoscope software, the reported values are the average from several images.

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The chemical composition and the depth distribution of the elements in the metal films were analysed using AES and SIMS. The Auger spectra were recorded with a 0.5 mA current of primary electrons and the tilt angle was 30°. Sputter depth profiling was performed with Ar+ ions. SIMS analyses were performed using a Cs+ ion beam. 3. Results

3.1. Electrochemical synthesis of Ppy films The illustration of the galvanostatic polymerisation process of Ppy films onto different substrates is presented in Fig. 1. It is common knowledge that the synthesis at high current densities (oxidation potentials) modifies the Ppy structure and may produce overoxidised form of the polymer. In order to suppress overoxidation the syntheses were performed at low or moderate current densities up to 1 mA/cm2. It can be seen that the oxidation potential of Ppy on Tix Oy depends on the material of the underlying substrate (Al foil or Si-wafer). Differences (sharp peak in the case of Al) in the potential as a function of time can be observed within the first 25 s. This time corresponds to the nucleation and initial growth steps of the polymer film but the differences can be also caused by other factors (the influence of the back side of the Al-foil, for example). No marked differences were observed afterwards. The polymerisation potential stabilises within : 1.5 – 2 min, reaching a value of 0.620 – 0.650 V (versus Ag/AgCl) for Ppy synthesis onto Tix Oy /Al and staying at 0.550 V for Ppy on Tix Oy /Si. The attempts to deposit Ppy film onto pure Al foil (without Ti interlayer) from the aqueous NaDBS solution were not successful. Only some random black dots of Ppy on the surface or stripes at the edges of the working electrode were observed. This is explained with the formation of Al2O3, which passivates the metal surface and acts as a barrier for electron transfer during the electrochemical polymerisation. Ti is also an oxidisable metal but the electrochemical oxidation potentials of pure metals in aqueous solutions show that Ti is more difficult to oxidise compared to Al. According to our results, the oxide which forms on pure Ti due to the polarisation of the electrode in the course of electrochemical polymerisation as well as the oxide layer (Tix Oy ) produced by chemical pre-oxidation in SC1 do not block electron transfer to such an extent that pyrrole electrooxidation is inhibited. However, it should be noted that the time of chemical oxidation of Ti in SC1 should not be extended because during the long term oxidation an oxide layer with a metallic appearance and different properties is formed which will either slow the rate of pyrrole oxidation or will reduce the adhesion of the polymer film.

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Fig. 1. Potential/time responses recorded during galvanostatic synthesis of polypyrrole on different substrates, i =1 mA/cm2.

3.2. Adhesion test Adhesion of the Ppy films of the thickness from 500 nm to 4 mm was examined by peel-off test performed within some hours after the electrochemical synthesis. In order to compare the results of the test on Tix Oy identical Ppy films at the same polymerisation conditions were synthesised on Ti (without chemical oxidation) and Au (chemically oxidised in SC1). The results are presented in Table 1. In the case of Ppy films on Tix Oy we see 100% adhesion, the results did not depend on the thickness of the polymer film. It was impossible to peel off even 4-mm thick Ppy films from the chemically oxidised Ti surface. In the case of evaporated Ti (without oxidation) the test results were quite different; in all cases Ppy film is completely removed from the surface, which gives 0% adhesion in our measurements scale. The third substrate Au, chemically oxidised in SC1 shows different results, too. This surface seems to be not well defined or homogenous in its properties. In some cases we could not remove Ppy films but sometimes 50% of the polymer was peeled off with the tape. Overall, we got 75% adhesion in the scale of our measurements. Gold was only a reference in our study and therefore we do not concentrate on the reasons of such behaviour but point on [25], which presents an overview on the electrochemistry of gold, including recently discovered unusual behaviour of this metal. Those results gave the motivation to study whether electrochemical oxidation of the Ti electrode prior to the polymerisation of pyrrole produced an oxide layer with similar adhesive properties as chemically grown

oxide. A Ti electrode was polarised in the synthesis solution without pyrrole for different period of time at 2 mA/cm2. It was possible to grow Ppy onto the electrode after 5 s of polarisation but adhesion of those films was not better compared to pure Ti. After 20 s of polarisation, the electrode surface was passivated and Ppy could not be grown. The next step was the peel-off test after electrochemical redox cycles in 0.1 M NaDBS aqueous solution. Table 1 Scotch tape test after the electrochemical polymerization of Ppy filmsa Sample

Number of samples

Result

1. Ppy on Ti (without any treatment)

10

100% Ppy film off (10 cases)

12

0% Ppy film off (12 cases) 0% Ppy film off (3 cases)

2. Ppy on preoxidised Ti (Tix Oy ) 2.1. Preoxidised in SC1 solution 2.2. Preoxidised in SC1 vapour

3

3. Ppy on Au (preox- 8 idised in SC1)

a

Thickness of the Ppy is 1 mm.

0% Ppy film off (5 cases) 2% Ppy film off (1 case) 50% Ppy film off (2 cases)

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Table 2 Scotch tape test after red–ox cycles in 0.1 M NaDBS aqueous solutiona Sample

1. Ppy on preoxidised Ti (SC1 solution) 2. Ppy on preoxidised Ti (SC1 vapour) 3. Ppy on preoxidised Au (SC1 solution) a

Number of red–ox cycles 250

970

2000

3000

6000

0% off 5% off 95% off

0% off 35% off Failure 100% off

0% off Failure 100% off

0% off

0% off

Pulses of 5 s between −0.9 and +0.2 V vs. Ag/AgCl.

The external voltage was switched between − 0.9 and +0.2 V versus Ag/AgCl, the period of each pulse was 5 s so that the period for each redox cycle ( − 0.9U +0.2) was 10 s. In these measurements the behaviour of Tix Oy was compared with Au (oxidised in SC1) and Ti (oxidised in SC1 vapour for 5 min). The pre-oxidation in vapour phase was examined as a possible variation of the method. The results of those measurements are listed in Table 2. Ppy film on Tix Oy was evidently strongly attached onto the surface and was not removed after as much as 6000 redox cycles in aqueous solution. In the case of Au 95% of the polymer started to become detached after 250 redox cycles, and total failure of the adhesion was identified after 970 redox cycles. In the case of Ppy on Ti, treated in SC1 vapour, 35% was removed after 970 cycles and total failure was recorded after 2000 cycles.

3.3. Surface morphology The surfaces of Ti, Tix Oy, Au and Ppy were examined by AFM. After the results of adhesion measurements it was interesting to examine the roughness of the different substrates, especially to establish the effect of the chemical pre-oxidation on the surface.

As can be seen from Table 3, the pre-oxidation of Ti had a strong effect on the roughness of the metal surface. Comparison of the mean roughness values (Ra) shows that the surface roughness of Ti after the chemical treatment is increased more than three times compared to the evapourated Ti surface. Using SEM, we did not observe remarkable differences between Ti and Tix Oy surfaces at the micrometer scale. While comparing the mean roughness values (Ra) of the Ppy films on the different surfaces one should keep in mind that the thickness of the films is slightly different which may have certain effect on the roughness, previously shown in the AFM study [26]. In general, thicker films have higher values of Ra. But, there is one very interesting observation, which seems to be not common for the electrochemically prepared Ppy films. From our experience, electrodeposited Ppy films follow the surface structure of the substrate and the roughness of the polymer films is always higher than the roughness of the substrate. We observed this for Fe before [26] and the same for Ti and Au here, but surprisingly we did not observe this for Tix Oy. In contrast, the roughness of Ppy film (11 nm) grown onto oxidised Ti surface was lower compared to the substrate ( 32 nm). This is a ‘levelling’ effect, where Ppy grows into or

Table 3 The roughness of the surfaces by AFM Sample

1. Ti (on Si)

Scan size (mm)

Mean roughness, Ra (nm)

Max. height difference, Rmax (nm)

2×2

9.577

102.08

2×2 206×206

32.075 32.068

338.15 257.25

3. Au (on Si)

2×2

2.243

4. Ppy (1 mm) on Ti/Si

2×2 9×9

18.243 19.091

119.22 219.73

2×2 206×206

11.338 10.112

114.32 76.825

2×2 9×9

5.723 6.282

55.003 88.044

2. Preoxidised Ti (Tix Oy on Si)

5. Ppy (500 nm) on preoxidised Ti (Tix Oy on Si) 6. Ppy (400 nm) on Au/Si

24.640

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Fig. 2. SEM photos of the cross-sections of two interfaces. (a) Polypyrrole/Au/Si and (b) polypyrrole/oxidised Ti (Tix Oy )/Si.

from the pores of the oxidised layer and creates a smoother surface. The fact that Ra value is area independent [compare Ra of (2 ×2) mm scan and (206 × 206) mm scan] indicates that the polymer film is homogenous and smooth. The results of the adhesion tests and the ‘levelling effect’ observed by AFM indicates that the interface of Ppy/Tix Oy should be different from Ppy/Ti as well as from Ppy/Au. SEM photos of the cross-section of Ppy/Tix Oy /Si and Ppy/Au/Si are presented in Fig. 2a and b. Indeed, there is an obvious difference between these interfaces. It seems that Ppy film on Au is not attached to the surface at all, the clear separation of two films can be observed. This separation may also be a result of the stress caused by the cutting procedure, but in this case it also shows very weak adhesion. Closer examination of the interface in Fig. 2b confirms that Ppy is grown into the Tix Oy layer and there is no separation between the metal and polymer films. Thus, the photos support the adhesion test results and indicate that either Ppy is rather precipitated than bonded to Au or that the interaction between Au and Ppy is very weak.

slowly decreases as a function of the depth of the profile reaching the value of 45.2 atm% at the centre of the film. Those values do not fit the formula of welldefined Ti oxide with one certain oxidation state. This may indicate that Ti is in the form of mixed valence states, that Ti is not fully oxidised or that the proportion of the components changes, all this indicates that instead of well-characterised substance we have nonstoichiometric compound. Therefore, the formula for the chemically oxidised layer would be Tix Oy where the valence states as well as proportion of the components may change (according to Smart and Moore [27] it is quite common for the atomic ratios to be non-integral, titanium monoxide for instance can range in composition from TiO0.65 to TiO1.25). Based only on the quantitative data, the following formulas for the characterisation of the chemically oxidised layer can be written: Ti2O3 and TiO (most probable at the surface); TiO0.8 (the centre of the layer). Table 4 Chemical composition and thickness of Ti (oxide) films before and after different treatmentsa Sample

Thickness of Ti (oxide) film, A,

Chemical composition oxygen, atm%

1. Evaporated Ti (Ti film on Siwafer) 2. Ti after the preoxidation in SC1 solution 3. Ti after the preoxidation in SC1 vapor 4. Ti after electrochemical oxidation at 2 mA/cm2 for (a) 5 s (b) 20 s

3500

1520

2640

4047

3060

25

3450 3550

2023 2025

3.4. Chemical composition of Ti films The chemical composition of the evapourated Ti film without treatment and after different treatments (chemical treatment in SC1 solution and vapour, electrochemical oxidation) was examined and some results are shown in Tables 4 and 5. Oxidation reduces the thickness of Ti film from original 350 to 264 nm, which probably indicates the dissolution of Ti. There is an obvious difference in the composition of Ti film after the chemical oxidation. All the other treatments produce minor changes. The amount of oxygen in the case of chemical treatment in SC1 solution is increased more than two times, from 15 20 to 4047 atm%. AES results support SIMS results and show even higher proportion of oxygen at the surface of the film, 50 60 atm% which

a

The results of SIMS analyses.

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Table 5 Chemical composition and thickness of Ti (oxide) films before and after different treatmentsa Sample

Chemical composition (AES) At the surface

1. Evaporated Ti (Ti film on Si-wafer) 2. Ti after the preoxidation in SC1 solution a

Centre of film

Oxygen atm%

Ti atm%

Oxygen atm%

Ti atm%

2025 6050

8075 4050

15 45.2

85 54.8

The results of AES analyses.

In addition to the increased amount of oxygen, the chemical oxidation is only treatment which substantially reduces the thickness of Ti layer which we think is the result of dissolution of Ti. The same trend, but to a lesser extent can be seen for Ti pre-oxidised in SC1 vapour but not in the case of electrochemical oxidation.

3.5. Other changes in the properties of Ti surface after chemical treatment in SC1 The chemical treatment of Ti in SC1 produces two visible changes at the electrode surface. The colour changes from silvery to violet-brownish and the wettability of the surface changes from hydrophobic to hydrophilic. The latter is an important change as strong adsorption of molecules may be important mechanism for improved adhesion. Molecules, which wet the surface, are assumed to interact more strongly with it, and this is likely to improve adhesion. The colour change may be simply the interference, which can be seen in thin films but it may also indicate something else. The results of chemical analysis indicate the possible existence of different oxidation states of the surface atoms. The colour changes at the surface may support this observation as Ti compounds (oxides, hydroxides) are coloured [28]. The existence of surface atoms with low valence state would cause different oxidative properties of this surface compared to the stable Ti surface with + 4 Ti oxidation state (TiO2). A simple experiment was performed to show the different oxidative properties of the chemically treated surface [29]. A piece of Ti coated Si-wafer and a piece of the same sample after chemical treatment were placed into the acidic KMnO4 solution and the change in optical density of the solution was recorded with a photometer. The fast decrease in the optical density of the solution vs time was measured in the case of Tix Oy but with Ti the optical density remained nearly constant. The difference was visible, in the first case the original light violet colour of KMnO4 solution become less intensive with time. The explanation is the reduction of KMnO4 + − 2+ to Mn2 + (MnO− +4H2O) 4 +8H +5e =Mn which involves reaction of the active surface. Thus,

those measurements confirm the difference between oxidative properties of the Ti surface before and after the chemical treatment.

3.6. Artificial muscle Multilayer strips consisting of Al foil, Tix Oy adhesive interlayer and polypyrrole were tested for artificial muscle application. Thin strips with the dimensions of (2 × 0.1) cm were cut from the working electrode. This strip was placed into the NaDBS aqueous solution and subsequent pulses of − 0.9 and +0.2 V versus Ag/ AgCl were applied. The time of the pulses was varied from 5 to 60 s in order to find the minimal time sufficient for realising the maximum degree of bending. The maximum degree of bending varied slightly from sample to sample being in all cases at least or more than 80°; in some cases the muscle turned out from the solution and curled in the air. A decrease of the degree of bending was visible after about 3.5 h of continuous work which we think is due to irreversible changes in the structure of the polymer film. After 11 h of continuous work the muscle still moved but the degree of bending was substantially decreased (about 1/3 of the initial value). To estimate the mechanical strength of

Fig. 3. Illustration of the construction and movement of ‘artificial muscle’.

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those stripes, metal piece was fixed in a free end of the muscle and pulses were applied (Fig. 3). The muscle with the weight of 1.72 mg was able to lift weight of 14.6 mg but the degree of bending was significantly reduced and did not recover after the weight was removed. This was the only test, which caused the failure in the adhesion of Ppy film (the polymer film was easily removed from the Al foil). The lighter weights did not cause dramatic decrease in the degree of bending and the movement of the muscle was reproducible for more than hundreds of continuous cycles.

4. Discussion The most interesting result in this study is the fact that Ppy adheres so well onto chemically oxidised Ti but not onto pure Ti itself. Our measurements confirm that the strong adhesion of Ppy/DBS films is the result of the chemical oxidation of Ti in alkaline peroxide solution (Ti “ Tix Oy ). Without surface treatment adhesion strength of Ppy/DBS films onto evapourated Ti is zero, as measured by Scotch tape. There may be different explanations for such an effect. As shown, chemical oxidation in SC1 modifies the topography and chemical

composition of the substrate. At the moment, the reasons of improved adhesion are not clear and research is ongoing [30,31]. Thus, in the following part only hypothetical explanations are presented. We observe significant increase in the roughness of the metal surface. As a result of the chemical oxidation the roughness increases three times, shown with AFM measurements. Thus, there is more electrochemically active and inactive centres on the surface as schematically shown on Fig. 4a and b. The increased number of the nucleation sites for electrochemical polymerisation extends the substrate area Ppy is attached to, and therefore may be considered as a precondition for the improved adhesion. In addition to the existence of numerous electrochemically active sites the next requirement for the strong adhesion is the chemical state of the surface which is able to form strong bonds with the polymer. The chemical composition of the Ti layer and the oxidation state of the surface atoms change, suggested by surface analysis. Oxidation states such as +2, + 3 and + 4 are most common for Ti, + 4 being the most stable one. In compounds with oxygen Ti may exist in mixed oxidation states [32]. During the pre-treatment of Ti there is clear visible change of the surface from silver-metallic to violet-brown which may indicate the change of the valence state of Ti atoms. The other possibility is that we observe interference colours, which indicate the growth of the oxide layer. According to the literature violet colour corresponds to Ti3 + compounds (hydroxides, oxides) [28]. The analysis of the chemical composition of Ti surface after the chemical treatment showed an essential increase of the content of oxygen compared to the initial state of evapourated metal layer. SIMS indicated the possible existence of surface hydroxides on Tix Oy. Ti3 + hydroxides have strong reductive properties and therefore, it is possible that in the aqueous solution in the presence of oxygen Ti3 + will oxidise to Ti4 + with formation of hydrogen peroxide on the surface according to the following scheme: Ti(OH)3 + H2O+ O2 “ Ti(OH)4 + H2O2

Fig. 4. Schematical illustration of polymer formation on Ti substrate during electrochemical synthesis; (a) before and (b) after chemical oxidation.

This reaction would give the most stable Ti4 + oxidation state and produce hydrogen peroxide. The formation of the hydrogen peroxide, which is strong oxidiser, leads to the different possibilities: (1) that H2O2 will initiate chemical oxidation of pyrrole on the Ti surface in addition to the electrochemical polymerisation. In this way, there would be two initiators of pyrrole polymerisation: electrochemical; induced by current and chemical; induced by the highly oxidative properties of Tix Oy surface. If this scheme is true, we get the model of the formation of Ppy, which is quite different from the common picture for the electrochemical growth. Electrochemical polymerisation starts from the

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electrochemically active sites (n1) on the surface (Fig. 4a). Chemical polymerisation would initiate polymer growth also from another sites (n2), in the bottom of the pores for example (Fig. 4b). In this way, we would get a more dense distribution of the nucleation centres and the initial Ppy layer covering Ti. From that, the subsequent growth of the polymer film would proceed from more active sites (n1 +n2) and the resulting polymer would be more compact and of higher density compared to the polymer film grown only from electrochemically active sites (n1). The second possibility is that (2) the formation of hydrogen peroxide may also generate further oxidation of Ti surface and release of Ti ions, which is observed in Ti implants [33]. Another hypothetical explanation to the increased adhesion is the possible adsorption of pyrrole molecules onto surface. An adsorbed pyrrole monolayer would probably enhance bond between the substrate and the polymer film formed during electrochemical polymerisation. Pyrrole molecule has proton donor as well as proton acceptor properties; chemisorption of pyrrole onto some metals have been reported in [34,35]. One interesting feature we observe is that the resistance to abrasion of the Ppy surface, grown onto Tix Oy is very high, compared to the Ppy grown onto Ti, Au or steel. It is very easy to scratch Ppy film on the later mentioned substrates with a scalpel; moreover, pieces of the polymer are removed from the surface during this procedure. In the case of Ppy on Tix Oy only very thin line of the scratch can be seen in the case of the films with the thickness in 1 mm range. In the case of thicker films it is hard, if not impossible, to scratch those films (especially those grown on Al foil) and reach shiny metallic surface. By our experience this procedure leads to the breaking of the whole multilayer (we cut 8 mm a Ppy film together with 10 mm an Al foil under it, but could not separate the layers). This observation shows that the structure of Ppy grown by our method is different from those grown in the conventional manner. This may be due to the higher density of the Ppy already discussed. The oxidation potentials are rather low and therefore the question of overoxidation, which may modify the structure of the polymer, can be discounted. During the electrochemical polymerisation of pyrrole there is also a possibility of simultaneous growth of Ti-oxide, which leads to the formation of the composite structure with the hypothetical structure of Ppy(DBS)/TiO2 or Ppy+ (DBS−)/Tix O− y . Also, a graded transition from oxide/ polymer composite to common Ppy/DBS may occur. This kind of simultaneous growth would result in different mechanical properties as well as in a very good adhesion.

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5. Conclusions “ “

“ “

Extremely adhesive Ppy films have been produced on thin chemically pre-oxidised Ti layers. These Ppy films survive at least 6000 electrochemical reduction oxidation cycles in aqueous solution without delamination. Hypothetical reasons of the excellent adhesion were discussed. This new preparation method may find use in different applications where the durability of conducting polymer coating is of prime importance.

Acknowledgements A grant from Johnson Foundation (Sweden) enabled K. Idla to perform the experiments in the very stimulating atmosphere of the Laboratory of Applied Physics at Linko¨ping University. Dr. E. Smela within the group of polymer physics, Linko¨ping (presently SFST, Santa Fe, USA) is acknowledged for co-operation, suggestions and sharp comments. We would like to express our gratitude to P. Lo¨nn for technical assistance and C. Engstro¨m for taking the SEM pictures, both at Linko¨ping University. M. Yamaguchi at CASIO Computer, Hachioji Research Centre is acknowledged for performing the AES and SIMS analysis.

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