posted on 2007-08-09, 00:00authored byKevin D. O'Neil, Bryan Shaw, Oleg A. Semenikhin
The mesoscopic inhomogeneity of conducting polymer films obtained by electropolymerization and spin-coating was studied using Kelvin probe force microscopy (KFM) and current-sensing atomic-force microscopy
(CS-AFM). A well-pronounced correlation was established between the polymer morphology, on the one
hand, and its local work function (which is related to the polymer oxidation degree) as well as polymer
conductivity, on the other. The most conducting regions were associated with the tops of the polymer grains
and showed Ohmic behavior. They were surrounded first by semiconducting and then by insulating polymer.
The conductivity of the grain periphery could be lower by as much as 2 orders of magnitude. The grain cores
also showed consistently higher values of the local work function as compared to the grain periphery. This
fact suggested that the grain cores were more oxidized and/or more ordered as compared to the grain periphery,
which is in good agreement with the local conductivity data. More uniform morphology corresponded to less
variability in the other properties of the polymer. A model is proposed that relates the observed inhomogeneity
to preferential deposition of polymer molecules with higher molecular weight at the early stages of the polymer
phase formation. The polymer deposition in either electropolymerization or various solution-casting techniques
involves the nucleation of a new phase from a solution containing polymer fractions of different molecular
weights. The driving force of the nucleation process depends on the solubility of the polymer fractions, which
decreases with an increase in the molecular weight. This gives rise to preferential deposition of more crystalline,
higher molecular weight polymer at the early stages of the polymer deposition to form the cores of the polymer
grains. The fractions with lower molecular weights are deposited later and form less ordered/less conducting
grain periphery. On the basis of this model, we conclude that, to ensure the formation of materials with low
inhomogeneity and high quality, one should use the starting polymer with as narrow molecular weight
distribution as possible. Yet another possibility is to use solvents which would reduce the differences in the
solubilities of polymer fractions with different molecular weight.