Understanding Actuation Mechanisms of Conjugated Polymer Actuators: Ion Transport
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This dissertation explored ion transport in conjugated polymers. Study in this dissertation focused on following subjects: 1. Driving mechanisms (migration and diffusion) for ion transport. 2. Correlation among ions, charge, and volume change. 3. Effects of experimental situations (voltage, swelling of polymers, film thickness, ion barrier thickness, electrolyte, and temperature) on ion transport. 4. Developing a physics-based model and conducting numerical simulations for ion transport in conjugated polymers. The research results of this dissertation were summarized mainly in 3 articles and presented in Chapter 3, Chapter 4, and Chapter 5 respectively. Chapter 3 reported preliminary experimental and modeling results of cation ingress in PPy(DBS). Cation ingress in the polymer was displayed through phase front propagations that were formed by electrochromism. Migration was found to dominate ion ingress evidenced by a linear relationship between phase front velocity and reduction potentials. Chapter 4 is a full-scale experimental study of ion transport in PPy(DBS). Besides phase front propagation velocity and broadening, current data and actuation strains of PPy(DBS) were also collected. Comparisons among these data gave more insights of cation transport in PPy(DBS). Diffusion of ions in PPy(DBS) was found to be non-Fickian diffusion, which has not been included in models in the literature. Cation egress was found to be independent with applied potentials, suggesting a diffusion controlled process, while cation ingress was found to be dominated by migration. This difference between cation ingress and cation egress has not been realized before this dissertation. The effect of polymer swelling on cation ingress was characterized for the first time, which suggested an exponential relationship between ion mobility and ion concentration. Chapter 5 reported more advanced theoretical modeling and simulation results. Nernst-Planck-Poisson's equations were used to model hole transport, ion transport, and potential profiles in conjugated polymers. The model was able to explore ion transport with various experimental situations including changing of voltage, ion diffusivity, hole mobility, Einstein relation, electrolyte concentration, and film geometry. The model successfully predicted both ion ingress and ion egress features for PPy(DBS). Predictions of anion transport conjugated polymers such as PPy(ClO4) were also reported.