Figure 3
Performance evaluation. Five coating layers show only negligible impact on total maximum deflection of the actuator compared to an uncoated one (A). Error bars show one standard deviation of the mean of samples (ns = 4) and cycles (nc = 5). Starting from two coating layers there is no visible change in the maximum amplitude during operation in water over the first 10,000 cycles (B). Each dip-coated layer adds to the total coating thickness because the solvent dissolves previously added layers very slowly, as can be seen from SEM images of PPy actuator cross-sections with 1 to 5 added layers (C top (1 layer) to bottom (5 layers)). Monitoring electrochemical impedance through the actuator that is submerged in water (500 mL MilliQ) shows electrolyte leach out of the sample in case of uncoated samples and only a small increase in conductivity in case of 4 coating layers (D). Small increase in conductivity could be attributed to the reduced viscosity of the electrolyte due to water vapor leaching in the sample (D–F). Further studies of the coating permeability while the sample is submerged in a conductive liquid (phosphate-buffered saline) and electrochemical impedance against an external counter electrode (H) is measured indicate that three coating layers was sufficient to electrically seal the sample from external electrolytes (G), while a small amount of water is still able penetrate the coating due to the characteristic WVTR of SIBS (D–F). Sample size in all experiments was 20 (18) mm × 1 mm × 150 µm (L × W × T) with 2 mm mounted between gold contacts and 18 mm free to bend. All samples (excluding uncoated references) were coated with 5% (w/w) SIBS in 2,2,4-trimethylpentane in solvent atmosphere at 1 mm/s dipping and withdrawal speeds.
