Stability and mobility of ions

Although no special experiments were conducted, chemically synthesized electrode layers were stable at least for two years when stored under ambient conditions for both, chemically synthesized PPy and PEDOT. No significant differences in mechanical properties were observed and the conductivity remained sufficient for EC synthesis.

Since there are no metal electrodes involved and the various polymer layers are rather strongly attached to each other, there was no delamination observed, inherent to many other actuators. It is obvious that the complete elimination of the delamination problem cannot be proved in the same way as can be proved the existence of the delamination problem. During actuation experiments, no signs of delamination were noticed under any circumstances, including a large number of actuation cycles (in order of several thousand), aging during several years, or subjection to extreme strains, causing the curling of the actuator by several turns. Quantitative stability testing was complicated, as electrolytes, causing higher strains were based on the PC electrolyte, which evaporated

slowly during the experiments. It can be concluded that chemically synthesized electrodes ensured strong adhesion between the CP and PVdF membrane.

Considering the large size of DBS^– anions, cation-active actuation was expected for PPy/DBS^–actuators. Nevertheless, in 1 M LiTFSI in PC electrolyte and 1 M EMImBF₄ in PC electrolyte, after 10 … 20 cation-active cycles, the actuation became anion-active. The reversal of the actuation relative to voltage was attributed to the reduced mobility of Li⁺ due to the solvation with PC molecules, formation of Li⁺DBS^– ion pairs or LiDBS salt, which is insoluble in the PC electrolyte. Another reason could be the very different diffusion speed of Li⁺ cations on oxidation, when compared to that of anions, and Li⁺ diffusion on reduction. When the charge of doping DBS^– of the reduced CP is compensated by the not very mobile cation, the oxidation charge was compensated by the more mobile anions from the electrolyte. The latter reasoning (excluding the solvation shell) remains relevant also for DBS^–-doped CP with 1 M EMImBF4

electrolyte. After being kept in electrolyte overnight, the actuators behaved again as cation-active for 10 … 20 cycles and then became anion-mobile. The TFSI^–-doped actuators in EMImTFSI showed mixed behavior. At high frequencies cation-activity dominated, followed by mixed-type activity, where the movement of both ion types compensated each other, and the strain difference became low, finally anion-activity became dominant at low frequencies. The switching from one mobility type to another is clearly observable from the step responses depicted in Figure 3.12c,d. The actuators with 1 M EMImBF4 in PC electrolyte showed noticeable creep, which was attributed to the uneven accumulation of cations inside the CP (starting already from the first cycle).

All TFSI^–-doped CP combinations showed only anion-active behavior in 1 M LiTFSI in PC electrolyte.

For electrolyte-operated linear tri-layer PEDOT|PPy/DBS (Sample A on PVdF membrane and Sample B on CS film) actuators, actuated in 0.1 TBACF₃SO₃ in PC, the transition from one type of activity to another was also observed. Sample A (PVdF|PEDOT) transformed from anion-active to cation-active (Figure 3.15) at low scan rate, the transition from cation-cation-active to anion-active took place at high frequencies, introduced by potential steps (Figure 3.16a).

Figure 3.16. Response of current density (j) and strain to square wave potential (0.1 Hz,

±1.0 V, in 0.1 M TBACF3SO3 in PC electrolyte) recorded at cycles 10, 50, 100, and 150:

a) Sample A; b) Sample B (adapted from Figure 5 of paper III).

The transition period for Sample A lasted about 150 square wave cycles in the potential range E = 1 … –1 … 1 V vs. Ag/AgCl (3 M KCl) (Figure 3.16a). The actuation was restored to the initial cation dominated character after resting overnight. CV measurements at 10 and 50 mV s^–1 exhibited only cation dominated transport after 8 cycles. After 4 … 8 cycles at the scan rate of 5 mV s^–1, sample A become cation-active, while Sample B was cation-active from the very beginning (Figure 3.15, Figure 3.16b). Since the EC polymerization conditions and the CP for the chemically synthesized electrodes were identical, the different behavior of Sample A and Sample B was attributed to the less stretchable substrate of Sample B, preventing the accumulation of cations inside the CP. Similar behavior, where ion mobility type depends on applied stress has been observed by Kaneto, et al. [227]. Cation accumulation inside the electrode has been observed in many cases, but is still not a well explained phenomenon yet and cannot be covered within the scope of this work. Accumulation of cations in PPy films with less mobile anions has been studied by several authors, typically the recovery of the exchanged charge after resting overnight has been observed. The recovery is most probably related to the diffusion of the accumulated cations out of the film and various relaxation phenomena. An alternative explanation has been proposed by Yamato et al. [228]. They cycled PPy/TFSI in BMPTFSI in PC solution and observed a transition to anionic behavior, which they explained by the lubricating effect of PC, which allows doping and dedoping by the large TFSI^– anion. However, it was not clear from the paper if the moving TFSI^– anions originate from the electrolyte or were the dopant during the EC synthesis.

The chemical polymerization on various surfaces can be considered as interfacial or hard template synthesis, since the structure and appearance of the resulting CP is determined by the substrate. The appearance of the product of the chemical synthesis in micellar media (soft template synthesis) is determined by the properties of the micellar solution. Usually, the result is in form of powder or non-uniform aggregated clusters, but with carefully tuned conditions, also self-assembly of stable 3D structures is possible. The chemical synthesis of PPy hydrogels and the derived aerogels represent another type of chemical synthesis as well as another type of synthesis result.

3.2.1 Sodium dodecylbenzenesulfonate colloidal solution formation Colloidal solution formation was tested separately with 1 M SDBS solution and the oxidants SPS or APS at different concentrations (without Py). The results are summarized in Table 3.4.

Table 3.4. Oxidants and concentrations used for colloidal solution formation evaluation.

Oxidant and concentration Result

0.5 M APS Clear solution

0.1 M APS * Stable opalescent colloidal solution 0.15 M APS Visible sedimentation of micelles

0.1 M SPS ** Clear solution

0.15 M SPS Stable opalescent colloidal solution 0.1 M SPS, 0.1 M NaCl Stable opalescent colloidal solution 0.2 M SPS Visible sedimentation of micelles

* pH was measured to be 6.9

** Initial pH was 7.6, but it was lowered gradually to 6.5 by adding H₂SO₄

While blending 0.1 M SDBS with APS at different concentrations, there was a narrow concentration range around 0.1 M APS, where a stable opalescent colloidal solution was formed. At lower concentrations, no colloidal solution was formed, and at higher concentrations, visible sedimentation of micelles occurred. The suitable range appeared to depend on the chosen salt(s), their concentrations and to be independent of pH.

The presence of an optimal concentration range can explain the observation of DeArmitt et al. that by replacing the ammonium cation in SDBS and APS solution for sodium (using the same concentration of SPS instead of APS) prevented the formation of pre-polymerization micelles and colloidal particles [185]. Similarly, in the current work no colloidal solution was formed using