Visualizing Ion and Solvent Transfer Processes in Electroactive Polymer Films

Chapter 13

Visualizing Ion and Solvent Transfer Processes in Electroactive Polymer Films STANLEY BRUCKENSTEIN, A. ROBERT HILLMAN and HELEN L. BANDEY

We describe the use of the electrochemical quartz crystal microbalance (EQCM) to study electroactive polymer film dynamics. Crystal impedance measurements provide a diagnostic of film (non)rigidity. Under deposition conditions in CH2CI2, oxidized poly(vinylferrocene) films are viscoelastic. Upon transfer to aqueous media, they show rigid film characteristics. Under the latter conditions, the EQCM can be used to follow redoxdriven ion and solvent population changes within the film. Coulometric and gravimetric responses imply that film redox switching involves coupled electron/anion transfer, solvation changes and polymer configuration changes. The latter two processes have both reversible and irreversible components. This mechanistic complexity can be rationalized using a scheme of cubes model. Redox cycling of poly(thionine) films in aqueous acetic acid buffers involves coupled electron/proton transfer, film solvation changes and acetic acid coordination state changes. Again, the mechanistic complexities of this ECC system can be readily visualized using a scheme of cubes approach. In general, the scheme of cubes is well suited to explaining and visualizing a range of electroactive polymer film characteristics, notably those associated with "break-in", overpotential, electrode history and experimental time scale phenomena. This approach should be of particular value when using non-electrochemical population probes in conjunction with electrochemical control functions.

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Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

13.1 INTRODUCTION

13.1.1 Background The modification of electrode surfaces with electroactive polymer films provides a means to control interfacial characteristics. With such a capability, one can envisage numerous possible applications, in areas as diverse as electronic devices, sensors, electrocatalysis, energy conversion and storage, electronic displays, and reference electrode systems [1, 2], With these applications in view, a wide variety of electroactive polymeric materials have been investigated. These include both redox polymers (by which we imply polymers with discrete redox entities distributed along the polymer spine) and conducting polymers (by which we imply polymers with delocalised charge centres on the polymer spine). The key to success is the ability to control and manipulate interfacial structure and properties in a predictable manner. As a first order strategy, the choice of polymer for a particular application is based upon the properties of a monomeric analog in solution. In general terms, it is recognized that (i) attachment of the redox entity to a polymer chain and (ii) immobilization of that chain at a solid/liquid interface under external potential control are both likely to modify the properties of the redox entity to some extent. However, there is little detailed understanding of the underlying reasons for these effects and little insight into how these phenomena might be controlled. At an operational level, the number of successful applications of polymer modified electrodes is disappointingly small. This is particularly surprising, given the wide range of materials available and the many ways in which one may synthetically derivatise parent compounds. It is a common anecdotal observation that qualitative characteristics of a given electrode/polymer system are reproducible. However, quantitative electrode performance is variable between measurements on the same electrode, between nominally "identical" electrodes, and in the hands of different operators. This is generally fatal to applications other than those in which a highly skilled operator is allowed to make calibration measurements under highly controlled conditions. In this work we explore some fundamental mechanistic issues that help explain why this is so. These considerations may help to delineate conditions under which electrode performance may be optimized or, at worst, made reproducible.

Introduction

491

Our approach to this problem involves a detailed mechanistic study of model systems, in order to identify the (electro)chemical parameters and the physicochemical processes of importance. This approach takes advantage of one of the major developments in electrochemical science over the last two decades, namely the simultaneous application of now-electrochemical techniques to study interfaces maintained under electrochemical control [3-5]. In general terms, spectroscopic methods have provided insight into the detailed structure at a variety of levels, from atomic to morphological, of surface-bound films. Other in situ methods, such as ellipsometry [6], neutron reflectivity [7] and the electrochemical quartz crystal microbalance (EQCM) [8-10], have provided insight into the overall penetration of mobile species (ions, solvent and other small molecules) into polymer films, along with spatial distributions of these mobile species and of the polymer itself. Of these techniques, the one upon which we rely directly here is the EQCM, whose operation and capability we now briefly review.

13.1.2 The electrochemical quartz crystal microbalance {EQCM) The EQCM is a sensitive in situ technique able to monitor changes in the mass of an electrode, including any surface film. These mass changes may be associated, for example, with the deposition or dissolution of a film, or with the exchange of ions and/or solvent between the film and the bathing electrolyte. In the present context, we will primarily use the technique as a gravimetric probe of the populations of ions and solvent within the film, as a consequence of exchange processes with the bathing solution. In particular, we seek to determine these ion and solvent populations as functions of potential and/or time in both long time scale ("equilibrium") and short time scale (dynamic) experiments. The EQCM comprises a quartz crystal oscillator, in which one of the Au exciting electrodes is also exposed to the solution and acts as the working electrode in a conventional (here, three electrode) cell. Provided any surface film is rigidly coupled to the underlying electrode changes in inertial mass (Am) of the electrode result in crystal resonant frequency changes (A/) that are described by the Sauerbrey equation [11]:

492

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

A/=(-^^)Am,

(13.1)

where/o is the unperturbed frequency (we used 10 MHz AT-cut crystals), Pg is the density of quartz, and Pg is the wave velocity within quartz. Thus, the EQCM can be used to monitor mass changes due to film deposition or due to redox driven ion and solvent exchange between the film and solution provided that the film remains rigid [8-10]. In the event that the film is not rigid, the EQCM response is a function of both the film mass and its rheological characteristics. Application of the Sauerbrey equation under these circumstances is inappropriate; it underestimates the mass change, to an extent that is dependent on the viscoelastic properties of the film. Under these circumstances, there are two questions to be addressed: first, how does one diagnose film (non-)rigidity and, second, how does one interpret responses from a non-rigid film? The answers to both questions can be found from crystal impedance measurements. This is a technique in which one determines the admittance (or impedance) of the loaded crystal as a function of frequency in the vicinity of resonance. Effectively, one determines the shape (width and height) and position (on the frequency axis) of the resonance, rather than just its position (as in the simple EQCM technique). The principle of the crystal impedance technique is illustrated in Fig. 13.1. Purely gravimetric changes result in a shift in resonant frequency, with no change in shape (see Fig. 13.1(a)). This would be the case for deposition of a rigid film, or exchange of ions and solvent between a rigid film and its bathing solution. Purely viscoelastic changes result in a change in peak shape (see Fig. 13.1(b)). In the general case of both gravimetric and viscoelastic changes, peak position and shape change (see Fig. 13.1(c)). This would be the case for deposition of a non-rigid film or for ion/solvent exchange that resulted in a change in film viscoelastic properties, e.g., solvent plasticisation. In this work, we use the crystal impedance method as a diagnostic of film rigidity. Where the film is rigid, we can use the Sauerbrey equation to interpret frequency changes. Where it is not, we use a more complex analysis to parameterize film rheological behaviour in terms of shear moduli.

493

Visualizing redox switcliing processes

u

\j E •a < ^

•

^

\

A A

a

c

Frequency — •

Fig. 13.1. Schematic crystal impedance responses, plotted as admittance vs. frequency, for (a) purely gravimetric, (b) purely viscoelastic and (c) simultaneous gravimetric and viscoelastic changes in resonator loading. Curves 1 represent the bare crystal and curves 2 the coated crystal responses.

13.2 VISUALIZING REDOX SWITCHING PROCESSES

13,2,1 Objective and strategy Our goal is to develop a method for visualizing extremely complicated electroactive polymer film redox switching mechanisms. To exemplify the problem and our proposed solution, we discuss the mechanisms of two

494

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

electrochemically driven processes: the redox cycling of poly(thionine) films in acetic acid buffer, and the **break-in" process and subsequent redox chemistry of poly(vinylferrocene) films. First we review processes that ordinarily accompany the redox switching of an electroactive polymer. The key step is coupled electron/ion transfer, which converts one or more reduced forms of the polymer to one or more oxidized forms of the polymer. Solvent and neutral species transfers and polymer structural changes (reconfigurations or coordination state changes) accompany the switching process under permselective conditions. Under nonpermselective conditions, salt transfer also occurs [12]. 13.2,2 Scheme of cubes As a first step in visualizing the steps in redox switching, we assume that both totally oxidized and totally reduced states may exist in different solvated and structural (configurational or coordination) forms. Although we consider a small number of discrete states, we recognize that reality may correspond to a continuum of all these kinds of polymer forms. As a first step, we assume that only two kinds of each state exist, i.e., both the totally oxidized and totally reduced forms of the polymer exist in only two solvated and two configurational (or coordination) forms, totalling eight chemically distinct species. Consequently, chemical transformations before and after electron transfer may involve paths in which a solvation step (chemical step C) precedes a configurational step (chemical step C ) , or vice versa. In other words, we will deal only with ECC mechanisms, and consider all the permutations of E, C and C steps. This leads to a 3D cube model of such systems. Extensions of this approach to higher dimensions are given elsewhere [13]. Figure 13.2 shows the basic 1 x 1 x 1 cube; the insert is the 3D axis system which generates the cube. O and /?, respectively, represent the oxidized and reduced forms of the polymer, the superscript "5" the more solvated forms of the polymer, and the subscripts "«" and "fc" the two structural forms (configurational states) of the polymer. This cube is useful in describing the redox cycling of many electroactive polymers. The FrankCondon principle makes electron/ion transfers along horizontal cube edges the most energetically favoured. Diagonal transfers across cube faces or through the cube are energetically far less favourable. Rather, the solvation and reconfiguration steps either precede or follow electron/ion transfer on a different time scale.

Visualizing redox switching processes

495

Fig. 13.2. Cube representing an electroactive polymer with two oxidation states (R and O), each having two solvation and two configuration states. Cube corners represent the eight possible species. Left/right, front/back and top/bottom planes, respectively, differ with respect to oxidation state, solvation state and configuration state. Insert shows 3D axes for the three elementary steps.

13.2.3 Overpotential effects A key issue in the redox switching of electroactive polymers relates to the value of the potential at which coupled electron/ion transfer occurs. The importance of this issue is illustrated in Fig. 13.3. The six cubes in

496

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

High overpotential Electron/ion transfer first %•

y

\A

ECC*

A

ECC

Low overpotential Solvent transfer first Si

s

CEC*

CC'E

Polymer reconfiguration first

pr C'EC

C'CE

Fig. 13.3. Dependence of redox switching mechanism on overpotential and the rates of chemical steps.

this figure have species at their corners that represent the ones on the corners of the cube in Fig. 13.2. In the six cubes shown in Fig. 13.3, we assume that initially only the reduced species {R^ exists at equilibrium, and when it is oxidized only a single product (O^) will eventually exist at equilibrium. First we consider application of a large instantaneous high overpotential step. As seen in Fig. 13.3, there are two possible high overpotential paths.

Visualizing redox switching processes

497

one involving solvation (step C) as the first step after electron/ion transfer, followed by reconfiguration (step C ) as the second step after instantaneous electron transfer, and vice versa. Accordingly, the two allowed mechanisms for the conversion of /?« to Ol are ECC and EC'C. Next we consider the case where a low overpotential exists at some point in the electrochemical switching process, as is necessarily the case in cyclic voltammetric or chronopotentiometric experiments. Now, four additional mechanisms become possible on the time scale of the electron/ion transfer process. This is so because the electron/ion transfer step, E, is no longer constrained to be the first. Consequently, either one or both of the chemical steps may precede the coupled electron/ion transfer. During the oxidation of Ra to Ol, if solvation of /?« is more rapid than its reconfiguration, the two mechanisms, CEC and CC'E may occur. If reconfiguration of Ra is more rapid than its solvation, two additional mechanisms, CEC and C'CE, become possible. Analogous to the oxidation process discussed above, the conversion of of, back to Ra may follow six different reduction paths, depending on the overpotential. Consequently, a redox cycle may involve any of 36 possible mechanisms for the hypothetical case of a single start state and a single end state. This simple example, involving one start state, illustrates the importance of the chosen electrochemical technique when studying a redox polymer film. It illustrates why different workers, using the same polymer and only slightly different electrochemical control functions (which necessarily have different time scales), may reach different mechanistic conclusions concerning a redox switching process. The practical reality is that there may not be a single start state for two reasons. First, thermodynamics may dictate significant equihbrium populations of more than one reactant solvation or configurational state. Second, kinetic effects may mean that attaining these equilibrium populations is not a rapid process, so that a considerable effort is required to reproduce a start state. The common practice of potential cycling to obtain reproducible current-potential curves does not accomplish this. It merely creates a steady state distribution of populations that is a function of the conditioning protocol. An example of a more complex situation involving multiple starting states is presented at the end of this work, and a fuller exposition is given elsewhere [14]. Quite generally, large overpotential techniques, followed by instantaneously open circuiting the filmed electrode, will greatly simplify solving

498

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

the kinetic problem. This approach corresponds to a coulostatic experiment in which the charge injection process instantaneously converts every species in one oxidation state to its corresponding solvation and configuration form in the other oxidation state. The simplification results from assuring that the electron/ion transfers (E) are first. Under these conditions (see Fig. 13.3), starting with a single reactant (Ra), solvation and reconfiguration processes can only occur from O^. This decreases the number of possible mechanisms for a redox half-cycle from six to two. If similar considerations apply to the reduction of Of (i.e., it must proceed via /?f), the number of possible mechanisms for a full redox cycle is decreased from thirty six to four. We show elsewhere how to disentangle overlapping solvation and configurational processes, and how to measure their rates [14].

13.3 POLYTHIONINE (PTh)

13.3,1 Background The monomer (in the oxidised form) can be electropolymerized as illustrated in Scheme 13.1 [15-17]. The subsequent redox chemistry of poly(thionine) is illustrated in Scheme 13.2. The polymer has voltammetric and spectroscopic signatures that broadly mirror those of the monomer,

Scheme 13.1. Electrochemical polymerization of thionine.

Scheme 13.2. Poly(thionine) film redox chemistry.

Polythlonine

499

consistent with the view that the polymer behaves as a collection of independent thionine-like redox moieties [16, 17]. In the light of their mediated charge transfer characteristics towards solution redox species, polythionine modified electrodes have attracted considerable interest as possible selective electrodes in photogalvanic energy conversion cells [15]. Here we use this system as an archetypal organic polymer, in that it involves coupled electron and proton transfers, for mechanistic studies. 13.3.2 Experimental observations and mechanistic interpretation Polythionine films behave as rigidly coupled systems, allowing interpretation of EQCM responses in purely gravimetric terms. Figure 13.4 shows EQCM current and mass responses accompanying redox switching of a polythionine film exposed to an aqueous solution of a weak acid, acetic acid. Slow scan voltammetric experiments established the overall normalised mass change, M^FIQ = 16gmor^ [18, 19]. Our published studies [18, 19] of poly(thionine) redox switching showed that, in acetic acid buffer, the leuco (reduced) form (L) is oxidized, losing two electrons and two protons, to generate the oxidised form {T). Furthermore, acetic acid is coordinated only to the T-form so that reduction causes T to lose an acetic acid molecule. However, reduction of T to L follows a different path than the reverse of the oxidation path. Water transfer is also involved in the redox cycle. We can (in simplified form) write the overall redox switching process as: [THM~.HA.H20]p + 2f/^ + 26.=^[LHr(/t")2]p + H2O5 (13.2) where A represents the acetate ion, and subscripts P and S, respectively, denote polymer and solution phases. This overall process (gain of one water and loss of two protons on oxidation) is consistent with the measured overall normalized mass change (see above). Our problem is to assign a mechanism to the implied combination of electron/proton, solvation and acetic acid coordination steps. The mechanistic details of this redox cycle were given in Figs. 6 and 7 of Ref. [19]. As we now show, the scheme of cubes is a vastly superior and more concise way to present the details of the complete redox cycle for this process. Figure 13.5 shows the 3D-rectangular axis system that applies to the cube in Fig. 13.6, representing the poly(thionine) redox system. The species in

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Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

Q/mC

0.2

0

-0.2

-0.4

Potential (V vs SCE) Fig. 13.4. Current vs. potential, charge vs potential and mass change vs potential curves for redox switching of a poly(thionine) film (F = 7.7 nmol cm~^) in aqueous 0.1 mol dm-3 acetic acid (pH = 2.9). The electrode was subject to a cyclic potential scan at 5mVs~*. (Adapted from Ref. [18] with permission.)

501

Polythionine

2e . 2H*

Coordination with acetic acid resident in polymer film

Fig. 13.5. 3D axes showing elementary steps associated with poly(thionine) redox switching in aqueous acetic acid buffer: coupled electron/ion (proton) transfer, solvation and acetic acid coordination.

the top, rear left corner corresponds to the reduced leuco form L. We show its oxidation (along the top, rear horizontal edge) to T, which then coordinates acetic acid (from the H^ and A~ released from L upon its oxidation; see equation (13.2)) to form T^A (the right, rear vertical edge). The latter then reacts with water (5) in the rate determining step and forms T^HA (bottom, right horizontal edge), to complete the oxidation half cycle. Reduction immediately converts 7^^ to L^HA (bottom, front horizontal edge). Next, L%A de-coordinates HA to produce L^ (front, left vertical edge). Finally, the reduction half cycle is completed by a de-solvation step that regenerates the starting species, L (top, left horizontal edge). At this point, the reader should accept that looking at the cube is far superior to a reading a description of the redox cycle.

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Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

- 2e - 2H*

+2e + 2H+ Fig. 13.6. Cube representing the poly(thionine) redox mechanism. Shaded (white) circles at the corners of the cube indicate species that are (are not) accessed in the redox cycle.

13.4 POLYVINYLFERROCENE (PVF)

13,4,1 Background The chemical stability and electrochemical reversibility of PVF films makes them potentially useful in a variety of applications. These include electrocatalysis of organic reductions [20] and oxidations [21], sensors [22], secondary batteries [23], electrochemical diodes [24] and non-aqueous reference electrodes [25]. These same characteristics also make PVF attractive as a model system for mechanistic studies. Classical electrochemical methods, such as voltammetry [26-28] chronoamperometry [26], chronopotentiometry [27], and electrochemical impedance [29], and in situ methods, such as spectroelectrochemistry [30], the SECM [26] and the EQCM [31-38] have been employed to this end. Of particular relevance here are the insights they have provided on anion exchange [31, 32], permselectivity [32, 33] and the kinetics of ion and solvent transfer [34-

503

Polyvinylferrocene

36]. One general observation from these studies, upon which our present studies shed light, is that mobile species movement can be markedly dependent upon film history [37, 38]. Thin films of PVF exposed to non-swelling solvents are anticipated to behave as rigidly coupled masses, so that the Sauerbrey equation (equation (13.1)) accurately describes the EQCM responses. However, experimental conditions may not always conform to this situation. For example, an early EQCM study of PVF electroprecipitation yielded film *'mass" data that were not consistent with coulometric assay of the film [39]; non-rigidity of PVF films in the CH2CI2 deposition solution was suggested. Also, Oyama et al, have studied thermoresponsive A^-isopropylacrylamide-vinylferrocene copolymers [40] and found them to exhibit potential dependent viscoelastic characteristics. In the light of these observations, we use the crystal impedance technique (illustrated schematically in Fig. 13.1) to guide our study of PVF films. 13.4,2 PVF deposition The deposition procedure is illustrated schematically in Scheme 13.3. It relies upon the difference in solubility between the reduced (uncharged) and oxidized (positively charged) forms of the polymer in dichloromethane solutions containing a tetraalkylammonium salt, such as tetraethylammonium tetrafluoroborate (TEAT). Upon the application of a potential step to 0.7 V (SCE), PVF^ is oxidized to PVF^, which deposits upon the

0.7V

+ nA'

•lectroprecipitation

PVF^

(solution)

f^

•¥ ne

Fe*.A"

PVF*

(film)

Scheme 13.3. Electrochemically driven precipitation of poly(vinylferrocene).

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Visualizing Ion and solvent transfer processes in electroactive polymer films Ch. 13 5.0

9.84

9.86

9.88

9.90

9.92

9.94

frequency/ MHz Fig. 13.7. Crystal impedance spectra acquired during the deposition of a PVF film from CH2CI2/O.I mol dm~^ TEAT. Inset shows the potential program. Numbers indicate deposition time. (Reproduced from Ref. [42] with permission.)

electrode surface. Polymer coverage can be controlled via the deposition time (a few minutes to 5 hours). Figure 13.7 shows a set of crystal impedance spectra as a function of time (coverage) during electroprecipitation of a PVF film. These measurements were made using a network analyzer in reflectance mode, as described previously [41]. For the long deposition times employed here, the film is relatively thick. Furthermore, there is appreciable solvent incorporation. These two factors mean that there will be considerable shear deformation of the film as a result of crystal oscillation. Quantitatively, we proceed via the use of equivalent circuit models. The most general model is the distributed transmission line model of Fig.

505

Polyvinylferrocene

air

quartz resonator

load layer

solution

(piezoelectric)

Fig. 13.8. Equivalent circuit models for crystal impedance responses: (a) transmission line model; (b) lumped element (modified Butterworth van Dyke) model.

13.8(a). Since one face of the crystal is exposed to air, the loading at that face of the crystal is negligible; the other face of the crystal has a surface mechanical impedance characteristic of the polymer and electrolyte. One can show [42] that, when the surface mechanical impedance is not large, the distributed model in the vicinity of resonance (where we make measurements) can be reduced to the simpler lumped-element model of Fig. 13.8(b). This modified Butterworth —van Dyke (BVD) electrical equivalent circuit comprises parallel "static" and "motional" arms. The static

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Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

arm consists of a capacitance (Co) associated with the two Au electrodes on either side of the quartz and stray capacitance associated with the measurement fixture. The motional arm is associated with the electromechanical coupling of the piezoelectric quartz and the loading (polymer film and electrolyte solution). Capacitive (C), inductive (L) and resistive (/?) elements, respectively, represent mechanical elasticity, inertial mass and energy dissipation within the various components of the loaded resonator. The electrical impedance (Z^) of the lumped element model is related to the surface mechanical impedance (Z^) and the quartz shear wave impedance (Zg) by

Z, = -IpL— f ^ U /? -h j
(13.3)

where A^ is the harmonic number (here A^= 1), K^ is the piezoelectric coupling constant for quartz, and (o^ = 27ifs (where/^ is the series resonant frequency). The resistive and inductive contributions to R and L from the crystal, film and solution are assumed to be additive (a reasonable description when the viscoelastic losses are small). Thus, rigid films contribute zero resistance to the system, and changes in inductance associated with film mass changes cause shifts in resonant frequency with no change in peak admittance. This corresponds to the situation illustrated in Fig. 13.1(a), described by the Sauerbrey equation. Simultaneously increasing resistance corresponds to increasing energy loss, as would occur in a viscoelastic polymer; this corresponds to the situation in Fig. 13.1(c). Inspection of the raw data in Fig. 13.7 immediately provides the qualitative deduction that depositing PVF films in dichloromethane are described by this latter case. Fitting to the BVD equivalent circuit (Fig. 13.8(b)) yields substantial and systematically increasing resistance values that confirm this result. Hence we conclude that the Sauerbrey equation cannot be employed to interpret the data. The above provides a qualitative diagnostic showing the growing films to be non-rigid, but provides no direct physical insight. We therefore now extend the method to determine the shear modulus for the viscoelastic polymer film by relating the measured surface mechanical impedance (Z,) to the appropriate physical parameters. We need to consider three types of loading: ideal (rigid) mass, viscoelastic material and Newtonian fluid.

Polyvinylferrocene

507

These correspond, respectively, to polymer or electrolyte entrapped within surface features, the polymer film, and the solution. The first of these is a minor effect when using polished crystals; the surface mechanical impedance of this contribution is Z^ = jiop^, where / = V - 1 , ca = lirfo, and ps is the areal mass density of the entrapped material. For finite and semiinfinite viscoelastic layers, the surface mechanical impedance is given by Z, = (GpfY^^ and Z^ = (Gp/)*^^ tanh(y/z/), respectively, where p4 and hf are the film density and thickness and y =ya>(py/G)^^^. For the solution, Zs = {(ops'r]J2y'^ (1 +y), where p^ and 17^ are the density and viscosity of the solution. When rigid mass, finite viscoelastic film and semi-infinite liquid loadings are all present, as in the experiment of Fig. 13.7, one can show that [42]:

\aco%h{yhf) + 6sinh(y/i/)/ where a = {GpfY'^ and b = ((opsVs/2f^ (1 -^y). For the films and conditions we have used, the transmission line and lumped element models give indistinguishable results. Fitting of the data of Fig. 13.7 yields G as a function of time. These values increase at short times (due to nucleation phenomena) to long time limiting values of G' = 1.9 X 10^ dyne cm"^ and G" = 3.0 x 10^ dyne cm~^ These values of the shear modulus components show that, in dichloromethane, the PVF film is a very rubbery polymer in which there is considerable viscoelastic loss when the film thickness exceeds 1 ^tm. 13.4.3 PVF redox cycling After the desired coverage has been achieved (via deposition time), the PVF"*^-coated electrode can be rinsed with water and transferred to aqueous 0.1 mol dm~^ sodium perchlorate for characterization. Since both oxidation states of the film are insoluble in aqueous media, we are able to investigate the redox switching process of surface-confined material, as schematically illustrated in Scheme 13.4. The polymer coverage (F/mol cm~^, expressed in terms of immobilised ferrocene sites) is obtained coulometrically by integration of the current response to a slow voltammetric scan. We now discuss the behaviour of thin (<0.1 |xm) PVF films after

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Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

+ ne :=;=::

PVF*

KJT +

nA"

PVF*"

Scheme 13.4. Poly(vinylferrocene) film redox chemistry.

transfer to aqueous solution. The film viscoelastic characteristics in the two media differ markedly: in aqueous solutions the PVF films we will discuss behave as rigid solids with a shear modulus on the order of 10*^ dyne cm~^. Hence, we are able to use the Sauerbrey equation to interpret EQCM responses. 13,4.3,1 Poly(vinylferrocene) break-in: the double cube When a freshly prepared poly(vinylferrocene) film is first oxidized in an aqueous electrolyte using cyclic voltammetry, the first cycle is remarkably different from subsequent ones [37]. Figure 13.9 shows a typical "breakin" set of current-potential and mass-potential curves obtained during an EQCM experiment. Several points are obvious. First, the potentials of the first and subsequent cycle oxidation current peaks are different. Second, the mass change during the first oxidation cycle is much larger than in subsequent cycles. Comparison of charge data with mass change data shows that the difference in mass just noted corresponds to an increased flux of water that enters the polymer when it is first oxidized. This water is retained within the film upon subsequent redox cycling. Figures 13.10 to 13.16 present a visual explanation for the pattern of mass- and current-potential curves that is obtained in this system. They are based on a cube-system in which the three elementary steps are coupled electron/ion transfer, solvation and polymer reconfiguration. Explanation of the phenomena observed in Fig. 13.9 requires the use of two cubes, joined at the bottom front edge of one and the top back edge of the other,

509

Polyvinylferrocene

700

E/mV Fig. 13.9. EQCM data obtained during the "break-in" of a poly(viny!ferrocene) film (F = 23.7 nmol cm ^ ) . Frames a and b, respectively, correspond to the current- and mass changepotential curves for the first two redox cycles in aqueous 0.1 mol dm""* NaCI04 immediately following deposition from CH2CI2. In frame a, the current (electron flux) was converted to the equivalent mass flux of counter ions, for subsequent correlation with the total (observed) gravimetric response of frame b. Potential scan rate: 5 m V s ' ' . The star symbol denotes the first anodic scan responses. (Adapted from Ref. [37] with permission.)

as shown in Fig. 13.10. This double cube configuration, which we present for the first time here, allows the possibility of three solvation and three configurational states for both the totally oxidized and reduced forms of poly(vinylferrocene). The superscripts S and S\ respectively, represent states more and much more solvated than the starting state. The subscripts a, b, c represent the three configurational states for this system.

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Visualizing ion and solvent transfer processes in electroactive polymer fllnis Ch. 13

Fig. 13.10. Double cube used to explain "break-in'' phenomena observed for poly(vinylferrocene) films in aqueous solution.

13.43,2 First oxidation half cycle Figure 13.11 shows the creation of Oa from Ra by the initial electron/ion transfer step. Figure 13.12 illustrates the second step, a reconfiguration to form Ofr. The third step is a solvation step in the upper cube to form Ol, as seen in Fig. 13.13. The next step forms a product in the lower cube, the reconfigured, solvated species. Of (Fig. 13.14). This step is followed

ill

Polyvlnylferrocene

/

0 0) f R.. V

m "s'<

/ ^.s\ 10,;

o

(Ofi

le) R

iof

Fig. 13.11. Poly(vinylferrocene) break-in: first oxidation half cycle, first step (coupled electron/ion transfer). In this and subsequent figures, the darker arrow(s) represent(s) the step(s) under discussion and the lighter arrow(s) represent(s) the preceding step(s). Also, shaded circles represent species that either have been or are presently being accessed.

by a final solvation step to form the stable oxidized species O^c ^ ^ilso seen in Fig. 13.14. 13.4,3,3 First reduction half cycle The shape of the current-voltage curve for the first reduction half cycle depends on the time scale imposed on the system by the scan rate used in the cyclic voltammogram. We have not experimentally explored the scan

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Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

Fig. 13.12. Poly(vinylferrocene) break-in: first oxidation half cycle, second step (reconfiguration). Reconfiguration is assumed faster than solvation in the upper cube.

rate dependence of the break-in process, but suggest the following analysis of what one might expect. Figure 13.15 shows the cyclic reduction and oxidation processes that would occur when using a very high scan rate cyclic voltammogram. Of converts to /?f which reconverts to Of before /?f has time to undergo a de-solvation or reconfiguration reaction. Potential cycling is confined to the two species on the lower front edge of the lower cube. Figure 13.16 illustrates the situation for a slightly longer time scale cyclic

Polyvinylferrocene

M3

( Rr>

R \ Fig. 13.13. Poly(vinylferrocene) break-in: first oxidation half cycle, third step (solvation). Reconfiguration is assumed faster than solvation in the upper cube.

voltammogram in which the reduction product Rf has sufficient time to reconfigure to /?^ , but not to undergo further reconfiguration or solvation before oxidation to Of. Thus subsequent cyclic voltammograms would be confined to cycling through the species Of to Of to Rf to Rl to Of, i.e., to the front face of the lower cube. Figure 13.17 represents the expected path for intermediate time scales. During the first reduction half cycle, the electron/ion transfer process would produce /?f, which would reconfigure to Rf and de-solvate to Rl.

514

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

Fig. 13.14. Poly(vinylferrocene) break-in: completion of first oxidation half cycle, by a second reconfiguration followed by a second solvation step, to form the most stable reduction product (doubly solvated, doubly reconfigured 0^ ) .

This in turn would undergo oxidation to Of when the potential scan was reversed. Subsequent cyclic voltammograms would then involve a path confined to the lower cube. If progressively slower sweep rates are used, the first reduction cycle will follow the path illustrated in Fig. 13.18. Now, the system has time to explore parts of the upper (as well as lower) cubes, although the original unsolvated states (back face of the upper cube) are never accessed, consistent with the data of Fig. 13.9 and numerous similar experiments.

Polyvinylferrocene

515

Fig. 13.15. Redox cycling of a broken-in poly(viny!ferrocene) film on very short time scales.

The formal potential for the first oxidation process, R^ to O^, differs from that of the second, Rf to Of, and subsequent ones, Rl to Of, Rf to of, Ra to Oa [43], which explains why the peak potential seen in Fig. 1 of reference [37] changes between the first and subsequent cycles. Continued cycling to steady state would populate many of the species, leading to the result shown in Fig. 13.19. Here there are four redox couples accessed {Ra'Ot RllOl, Rf/Ol', and Rf/Of, whose distinct standard potentials are related by the equilibrium constants around the cube faces [43]. This will result in a set of overlapping waves on the i-E curves, and corresponding mass changes on the AM-E curves, which will be practically

516

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

Fig. 13.16. Redox cycling of a broken-in poly(vinylferrocene) film on short time scales. Reconfiguration is assumed faster than (de-)solvation in the lower cube.

impossible to separate. We suggest that arbitrary mixtures of these processes, governed by different experimental procedures, may be the cause of differing responses for PVF and analogous redox polymer films. Therefore, we emphasize the importance of using carefully designed film history and experimental time scale protocols in order to generate defined start and end states, for which individual responses may be measured.

Conclusions

517

Fig. 13.17. Redox cycling of a broken-in poly(vinylferrocene) film on intermediate time scales. Reconfiguration is assumed faster than (de-)solvation in the lower cube.

13.5 CONCLUSIONS

The electrochemical quartz crystal microbalance is a versatile technique for studying several aspects of electroactive polymer film dynamics. For rigid films, it is a sensitive probe of mobile species (ion and solvent) population changes within the film in response to redox switching. For non-rigid films, it can be used to determine film shear moduH. In the former case, one simply follows changes in crystal resonant frequency. In the latter case, the frequency dependence of resonator admittance in the

518

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

Fig. 13.18. Redox cycling of a broken-in poly(vinylferrocene) film on long time scales. Reconfiguration is assumed faster than (de-)solvation in both cubes.

vicinity of resonance is required; this also gives a diagnostic for film (non-)rigidity. During deposition, poly(vinylferrocene) films are non-rigid. Analysis of crystal impedance data yields shear moduli that are typical of a rubbery material; this contrasts with the rigid film characteristics observed in aqueous media. Redox cycling of poly(thionine) films in aqueous acetic acid solution involves not only electron transfer (coupled with proton transfer to maintain electroneutrality), but also film solvation and acetic acid coordination

Conclusions

519

Fig. 13.19. Redox cycling of a broken-in poly(vinylferrocene) film after multiple cycles, generating a mixture of "start'' states.

State changes. Mechanistic complexities associated with this ECC system can be rationalized and readily visualized using a scheme of cubes approach. Redox cycHng of poly(vinylferrocene) films involves not only coupled electron/anion transfer, but also solvation and polymer configuration changes. The situation is further complicated by both reversible and irreversible elements of solvation and configuration changes, commonly referred to as '*break-in", immediately following deposition. Here, a dou-

520

Visualizing ion and solvent transfer processes in electroactive polymer films Ch. 13

ble cube model, allowing for three solvation and three configuration states, is able to rationalize break-in and reversible behaviour of the film. We conclude that the scheme of cubes is well suited to explaining and visualizing a range of electroactive polymer film characteristics, notably those associated with "break-in", overpotential, electrode history and experimental time scale phenomena. This approach should be of particular value when using non-electrochemical population probes in conjunction with electrochemical control functions.

ACKNOWLEDGEMENTS

We thank the EPSRC (GR/K29982) and the NSF (CHE-9616641) for financial support, and HLB thanks the University of Leicester for a studentship.

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