Factors influencing electronic and ionic conductivity in ion-coordinating conjugated polymers

The study of conductive polymers is a large and active area of research. Conjugated polymers are able to conduct electronic charge carriers through their π electron systems, while those materials with ion-coordinating components, like poly(ethylene oxide) (PEO), facilitate the transport of ionic species.; In this study, polymers featuring PEO-like side chains attached to poly(thiophene) backbones were synthesized. The length of the side chains was found to have a significant effect on the solubility and film-forming properties of these materials. Those materials with short side chains were found to be soluble in chlorinated solvents and formed quality films only with great difficulty. In contrast, the polymers with longer side chains were soluble in more polar solvents and were easily cast as uniform films. In addition, various aspects of the polymerization reaction such as temperature, concentration, and purity were determined to be crucial to obtaining soluble materials.; Addition of lithium triflate to the polymers in a concentration range of [O]:[Li] = 10 to 100 led to ionic conductivity. This property was found to be dependent on the concentration of the added salt as well as the length of the side chains. The maximum ionic conductivity for each material was found at intermediate concentrations of salt. Through the use of infrared spectroscopy, it was determined that maximum concentrations of free ions were also attained at intermediate total salt concentrations. In general, increased length of the side chains led to increased conductivity. The maximum value of ionic conductivity at ambient temperature determined for these materials was 2 · 10−4 S · cm−1 with a lithium triflate concentration of 0.75 mmol LiOTf/g –C₂H₄O–. This value is competitive with many of the best known PEO-based polymer electrolytes.; The electronic conductivity of these materials when doped with NOBF ₄ was found to be quite sensitive to the presence of lithium triflate as well as length of the side chains. In their salt-free states, those materials with shorter side chains showed values of conductivity on the order of 10 −1 S · cm−1. Those polymers with longer side chains were much less conductive, with values of electronic conductivity only about 10−6 S · cm−1. Any added salt led to conductivity values of about 10−6 S · cm−1, regardless of the length of the side chains. Very little difference in conductivity between the various salt concentrations studied could be observed.