x 10'8 molcm'2. No voltammetry was performed at this stage of the experiment

Column A of Table 2 .4 summarises the characteristic features o f these admittance spectra. Fig. 2.9 illustrates the corresponding set of admittance spectra, obtained with the same layer, as the electrolyte concentration was decreased. The layer was

voltammetrically scanned at each concentration prior to the recording o f the open circuit crystal resonance.

The shift in resonant frequency, on increasing the HC104 electrolyte concentration, is summarised in Column A of Table 2.4. The data have again been corrected for the solution viscous load and reflect the increase in layer mass

associated with the protonation process. The relative shifts in resonant frequency are smaller than those observed for the OSPVP75 and 0sPVP67 polymers. This reflects the lower number o f protonable pyridine units and the lower requirement for

compensating counterions. This point is discussed in greater detail in Section 2.3.3.

In Fig. 2.8, with increasing concentration, the crystal resonance behaviour for this polymer is quite similar to that observed for the OsPVP67 polymer. At all

concentrations, with the possible exception of 0.0005M and 0.001M HCIO4, the crystal resonance shape becomes slightly sharper. Therefore, the increase in layer

Figs. 2.8. and 2.9. Admittance spectra for an OSPVP33 coated crystal in HCIO4. Gravimetric surface coverage is 1.34 x 10"8 molcm'2.

Fig. 2.8. Increasing electrolyte concentration, no voltammetry.

0.0038

Fig. 2.9. Decreasing electrolyte concentration, with voltammetry.

a

Table 2.4. Admittance data obtained at open circuit for an OSPVP33 coated crystal in HCIO4. Gravimetric surface coverage is 1.34 x 10'8 mol cm'2.

HC104

H20 0.342 0.337 5391 5504 0 -82

1 x 10'5 0.341 0.335 5446 5541 -19 -138

1 x 10~4 0.345 0.330 5372 5606 -38 -157

5 x 10'4 0.339 0.330 5456 5578 -19 -288

1 x 10‘3 0.338 0.326 5542 5681 -187 -439

5 x 10'3 0.343 0.333 5512 5652 -300 -664

0.01 0.343 0.335 5502 5365 -431 -644

0.1 0.346 0.338 5457 5587 -529 -727

1.0 0.338 0.333 5606 5700 -764 -756

(a) Data resolution is 20 Hz.

(b) Estimated from the shift in resonant frequency and corrected for the viscous load of electrolyte.

Column A is data obtained with increasing concentration, no voltammetry.

Column B is data obtained with decreasing concentration, following electrochemical cycling at each concentration.

mass associated with the protonation process does not result in a loss in layer rigidity.

The “apparent” broadening of the crystal resonance in 1 .OM HCIO⁴ again reflects the viscous load of the electrolyte and does not indicate swelling of the polymer layer. The slight broadening of the resonance in 0.0005M and 0.001M HCIO4 concentrations is not related to the viscous load o f the solution and may therefore indicate partial swelling of the polymer layer.

Comparison with the corresponding data obtained for the OSPVP75 and OsPVP67 coated crystals illustrates that the crystal resonance shape of the OSPVP33

coated crystal is sharper in all electrolyte concentrations. Consequently, as

anticipated, this OsPVP^{3 3} is more compact than the other polymers. This behaviour is likely a consequence of the reduced requirement for anion influx into this lower PVP content polymer and the more hydrophobic nature o f this higher styrene content polymer.

Fig. 2.9 illustrates the change in crystal resonance for this OSPVP33 layer as the electrolyte concentration is decreased. The layer was voltammetrically cycled at each concentration. Excluding 0.01M HCIO4, there is a slight broadening o f the resonance at all concentrations which suggests that there is slight swelling of the layer following electrochemical cycling. This behaviour is accompanied by a decrease in the resonant frequency and, by implication, an increase in resident layer mass. This behaviour is summarised in Table 2.4 and illustrated in Fig. 2.5. The increase in layer mass upon voltammetric cycling reflects the difficulty in attaining equilibrium levels of solvent and ions within this compact layer, which is only facilitated by the

extensive counterion and solvent transfer that occurs during electrochemical cycling.

This behaviour is referred to as a “break-in” effect and has been observed many times previously [31-34], It is analogous to the variation between the first and subsequent electrochemical responses which is frequently observed for electroactive polymers

[31-34], This “break-in” effect was not observed for the OSPVP75 and OsPVP67

polymers and thus illustrates the greater difficulty in attaining equilibrium solvent

levels within the more compact and hydrophobic OsPVP33 polymer. This point is further supported by the fact that equilibration times, on initial exposure to these electrolyte concentrations, were greater for the higher styrene content polymers.

Overall, as was observed for the OSPVP75 and OsPVP67 polymer coatings,

there are changes in the morphology o f OSPVP33 crystal coatings in HCIO4

electrolytes. However, these swelling phenomena are extremely small and constitute

only a 4% change in the PW H M . Whilst the absolute change in P W H M that a polymer must undergo before the rigid layer approximation is compromised, is

unknown, these changes are insignificant in comparison with the swelling o f polymer

layers in contact with organic solvents (see Chapter 6).

In summary, OsPVP75, OsPVP67 and OSPVP33 electrode coatings are rigid in all HCIO4 electrolyte concentrations under investigation. This rigidity is a

consequence of both the crosslinking properties of the perchlorate anion and the hydrophobic nature o f the styrene moiety o f the copolymer backbone. On increasing the styrene content o f the polymer backbone, the rigidity of the layer increases as a consequence of the reduced requirement for anion ingress and the enhanced

hydrophobicity of the polymer backbone.

For all polymers, an increase in layer mass is observed with increasing HC104 electrolyte concentration. This mass influx is associated with the influx o f counter­

anions to maintain electroneutrality within the protonated polymer matrix. Having demonstrated that these polymers are rigid at all HCIO4 electrolyte concentrations it is now possible to give a quantitative assessment o f the mass changes within the layer using the Sauerbrey equation.

Fig. 2.10 illustrates the change in layer mass, with increasing electrolyte pH, for OSPVP75, OsPVP67 and OsPVP33 polymer layers. At each concentration, the shift in resonant frequency resulting from the change in viscous load was corrected for, by subtracting the frequency shift observed for an uncoated crystal. Tables 2.5 - 2.7 give the corresponding sets of data. All mass changes were calculated using a mass

sensitivity o f 0.232 Hzcrrfng' 1 [35], It is anticipated that these mass changes result from the influx of perchlorate anions, in response to the electroneutrality constraints imposed by the protonated polymer backbone.

Columns A and B of each table are estimates o f the number of perchlorates required for electroneutrality, assuming the influx o f A) unhydrated perchlorate (H CIO4 ) and B) hydrated perchlorate (FrCKV 2.6H2O). Assuming the influx of hydrated perchlorate anions, the level of anion influx is less than that anticipated for protonation of the backbone. This suggests incomplete protonation o f the backbone, which is considered unlikely at concentrations so far to the acid side of the pKa of the pyridine units (pKa = 3.3 [36]). However, there would appear to be excellent

agreement between the unhydrated data and the electroneutrality requirement, assuming complete protonation of the polymer layers at pH 0. Taking the OSPVP33

polymer as an example, the OsPVP33 polymer unit [Os(bipy)2(PVP33)ioCl]+