Redox active conjugated polymers are a class of electrically conducting or semiconducting organic materials with promise in energy conversion, energy storage and bioelectronics applications. Characteristic of these materials is their facilitation of âmixedâ ion and electron/hole conductivity, where ion transport is enabled principally by chain segmental mobility and solvent affinity, and electron/hole transport by ð-orbital delocalization and local ordering between chains. While it is understood that electrolyte sorption and physical swelling have strong implications for mixed conductivity in conjugated polymers, only a few design principles exist to relate polymer chemical structure to this crucial electrolyte-polymer interaction. Here we discuss a family of poly (3,4-propylenedioxythiophene) (âProDOTâ) polymers and copolymers, functionalized with polar and/or apolar side chains of varying length and linearity. We illustrate that it is not only the polarity, but also the relative size and density of neighboring side chains, and their symmetry/asymmetry of attachment, that affect the extent, rate, and reversibility of organic electrolyte uptake during electrochemical doping. Stark differences between polymers with neighboring sidechains of similar and contrasting sterics and polarity are connected to transport-related factors, e.g.
, chain segmental mobility, and the sensitive thermodynamic interplay between osmotic pressure and the loss of conformational entropy due to chain extension during swelling. We also show that the character of these polymer-polymer and polymer-solvent interactions can be manipulated with careful selection of side chains, allowing us to establish structure-property relations towards the rational design of electroactive organic materials for devices from supercapacitors to soft electrodes for biosensing and stimulation of excitable living tissues.
Figure caption: Family of poly(3,4-propylenedioxythiophene) âProDOTâ polymer and copolymers arranged by side chain polarity (x-axis) and steric similarity (y-axis). Top left: ProDOT(EtHex)2; top right: ProDOT (OE3)-EtHex. Bottom left: ProDOT(HD)-DMP; bottom right: ProDOT(OE3)-DMP.