Poly(3-methylthiophene) conducting polymer solutions and films

Abstract

Poly(3-methylthiophene),P3MT, has been polymerized from 3-methylthiophene, 3MT, with AsF5 in AsF3 solution. Addition of excess AsF5 yields solutions of AsF5-doped P3MT in AsF3 from which electrically conductically conductive films are cast upon solvent removal. The chemistry and physics of these solutions and films are not clearly understood. We have characterized some of the chemistry and physics via solution and solid state NMR and EPR and SEM/EDX techniques. These P3MT solutions which exhibit three distinct stages in the polymerization and doping process contain different species in each stage some of which have been identified by NMR. One of the species identified at dopant levels below 1 AsF5 per monomer unit is AsF6- which has long been propssed as a dopant counterion. Above 2 AsF5 per monomer units, sulfur-centered radicals are observed in solution while carbon-centered radicals are observed in the corresponding cast films. These radicals in both the solution and solids state do not appear to correlate with the conductivity. Consequently this data suggests that the radicals observed do not participate in the conduction mechanism. The electrical and mechanical properties of the electrically conductive films cast from solution depend on the sample morphology. The method of film casting controls the morphology and thus the final sample properties. Hard compact films are approximately 100 times more conducting than brittle porous films suggetting a decrease in interparticle resistivity in the compact films. This work clearly illustrates the complexity of the chemical and physical interactions which govern polymer solubility and final film properties and the need for further spectroscopic studies in order to better understand the chemistry and physics of the systems. © 1989.