Solid-State Ordering and Potential Dependence of Conductivity in Poly(2,5-dialkoxy-p-phenyleneethynylene)

Solution-cast films of four different poly(2,5-dialkoxy-p-phenyleneethynylene) molecules, I-IV, with varying backbone chain lengths and varying alkoxy substituent chain lengths, and an alternating copolymer of 1,4-diethynyl-2,5-dihexadecyloxybenzene and 9,10-dibromoanthracene, V, have been characterized electrochemically and by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). XRD and DSC show that the polymers have varying degrees of order and crystallinity based on long-range lamellar structure. Cyclic voltammetry in liquid SCVelectrolyte shows that the onset of oxidation for I-IV occurs at ~1.05 V vs SCE with the more crystalline polymers having slower electrochemical response than the less crystalline ones. In situ characterization of the potential dependence of conductivity in the same medium shows that the maximum conductivities of I-IV range from ~0.2 to ~5 ^-1 cm^-1, suggesting that higher conductivity is associated with lower longrange order in the polymer films but showing little dependence on average polymer chain length. I-IV all have maximum conductivity at ~1.6 V vs SCE and finite potential windows of high conductivity ~0.55 V wide, indicating that the potential of maximum conductivity and the width of the window of high conductivity are determined by molecular rather than bulk properties. For V, the onset of oxidation occurs at ~0.8 V vs SCE, the potential of maximum conductivity is ~1.5 V vs SCE, and the width of the potential window of high conductivity is ~0.85 V.