UC Santa Barbara

Polymers, large molecules made up of repeating units, can be tailored to achieve specific functionality. This thesis presents the work done on the molecular design and synthesis of polymers and subsequent structure-property relationship studies. Two categories of polymers are targeted: intrinsically conducting polymers and ion conducting polymers. Both categories comprise materials capable of conducting electricity which makes them useful for energy applications.In the first chapter, a conjugated polyelectrolyte is used as an active layer in organic electrochemical transistors (OETCs). Conjugated polyelectrolytes (CPE), which contain a conjugated backbone with pendant ionic functionalities can get doped and transport electrons, coupling ionic and electronic charges. A self-doped CPE comprising cyclopentadithiophene-alt-benzothiadiazole (CPE-Na) as donor-acceptor units is used in this work and the chain length between the pendant sulfonate group and the backbone is modulated do understand the impact on material and devices properties. CPE-Na is a self-doped polymer due to the stabilizing effect of the negative anion on the positively charged polaron. Increasing alkyl chain distances from 2 to 5 methylene units were used. A wide range of characterization techniques, combined with theory, were used to understand the structure-property relationship between the side chain length and ease of doping, and the impact on transconductance on the device. In the second chapter, the same concept of investigating the influence the alkyl chain length on material and devices properties is extended to an ionically conducting polymers for solid state electrolytes. Polyvinylmethylsiloxane (PVMS) is functionalized via click-chemistry with a cyano ligand, which can interact with Li+ from LITFSI (lithium bis(trifluoromethanesulfonyl)imide) and dissociate the salt. The chain length between the cyano ligand and the PVMS backbone is varied between 5,9 and 16 units and the impact in both Li+ conductivity and mobility are investigated. Solid state electrolytes are a safer alternative for the electrolyte system used in current batteries since the flammability and toxicity of liquid electrolyte are eliminated. However, their limited conductivity and Li+ mobility remains a challenge that require further research into new materials. Hence, in chapter 2, the structure-property relationship between chain length and transference and conductivity of PVMS-based solid electrolytes containing a cyano ligand are studied. Building on the findings from chapter 2, the third chapter present a sequential study that delvers deeper into the intriguing results obtained with PVMS functionalized with cyano ligands. Astonished by the high transference number of PVMS functionalized with cyano, and good conductivity, chapter 3 proposes to investigate the impact of small polar ligands on the conductivity and mobility of Li+. Methoxy, cyano, carbonyl, sulfonamide and methyl groups are used to functionalized PVMS and investigated in their ability to coordinate to Li+, facilitating the dissolution of LiTFSI and promoting conductivity in the system. We found that a polar group is indeed required for salt dissolution, by the low conductivity of methyl group. Also, the conductivity of the three monodentate ligands coalesced when the conductivity is normalized by the glass transition temperature and these findings are explained bringing new insights into the field of solid state electrolytes.