A study of the role of dopant counter-anion on morphology and conductivity of polyaniline salts and blends.

Abstract

The changes in the molecular conformation, electronic structure, morphology and conductivity of polyaniline as a function of molar doping ratio, y, and dopant counter-anion functionality in a single solvent, hexafluoroisopropanol (HFIP) have been investigated. Upon doping, polyaniline emeraldine base (PANI-EB) in HFIP, an increase in intrinsic viscosity ($\eta$) and an absorption into the near IR region confirm that the polymer is becoming more electronically ordered as a function of y. When cast from HFIP solution, the conjugated salt retained its expanded conformation as evident by the absorption into the near IR region. As a result of the more rigid nature of the salt chains, a moderate crystallinity was observed by wide angle X-ray scattering (WAXS). Small angle X-ray scattering (SAXS) revealed that the packing tightness of the salt chains was influenced by dopant counter-anion size. With the exception of PANI-0.5-HCSA, electrical conductivity of the cast PANI-ES decreased as counter-anion size increased. Upon blending PANI-ES with nylon 6 or nylon 12, highly conducting films have been cast from HFIP. The immiscible nature of these blends was confirmed by FTIR, X-ray scattering, TEM and density measurements. Measurable onset of electrical conductivity of these blends appeared at very low values, approximately 0.5% by volume, of the salt. At this threshold value of electrical conductivity, a network was established through the nylon host. Lower onsets of electrical conductivity in the PANI-0.5-HMSA/nylon 6, PANI-0.5-HDBSA/nylon 12 and PANI-0.5-HCSA/nylon 6 blends paralleled a finer more branched network whereas the lower conductivities accompanied a coarser, more globular morphology in the PANI-0.5-HMSA/nylon 12, PANI-0.5-HDBSA/nylon 6 blends. At concentrations above the threshold value, the network became extremely dense and the electrical conductivity showed a smaller sensitivity to dopant functionality and host matrix. Transport of electrical charge through the salt domains was determined to be governed by a three dimensional variable range hopping mechanism and was observed to be independent of dopant anion, host matrix and geometry of the conducting phase at concentrations above the threshold value for conduction.