Morphological characterization of innovative electroconductive polymers in early stages of growth

Polypyrrole (PPy), one of the most studied conducting polymers due to its ease of preparation and its interesting properties, is nowadays involved in a wide range of applications. Among the available techniques to realize PPy thin films, electrochemical polymerization allows an optimal control of the synthesis and a fine-tuning of the resulting properties. Electrodepositions of PPy are carried out using proper anions (doping anions). that are immobilized by the new-made polycationic PPy chains, in order to achieve electroneutrality. Small inorganic anions are usually used (such as perchlorate), but a new class of large organic dopants (i.e. heparin) is emerging, due their ability to confer to the final product a series of attractive properties, the most important being biocompatibility. When dealing with biomaterials, other important characteristics must be taken into account to ensure good properties to the final product. In particular, the control of surface morphology is of great importance for cellular adhesion and growth. In this work, we characterized in detail the morphology of two PPy-based thin films (synthesized with heparin and perchlorate as doping anions) grown on ITO substrates that are used as cell growth supports. In order to determine the dynamics of the deposition of these films, we followed their growth since the very first instants of deposition. Indeed, during these first steps, the film will deposit directly over the bare electrode, influencing the morphology of the growth and allowing comparing the growth dynamics of films produced using different synthesis conditions. We determined the features of these growing polymeric thin films at nanometer level using Atomic Force Microscopy (AFM), which is the ideal technique to image the real 3D structure of the surface of such specimens. In both cases, the PPy film was formed by an array of grains measuring tens of nanometers, but significant differences in grain size and arrangement on the substrate were observed. Mainly, PPy-heparin film appears to be formed by ordered superimposed layers of grains whose size distribution is peaked around a mean value, while PPy-perchlorate nuclei are arranged in a more disordered way on the substrate and do not have a preferred dimension. The presented results emphasize the importance of the investigation of the polymerization dynamics, especially in the first steps of the deposition, in this class of innovative and technologically relevant materials, and represent an optimal starting point to realize a study, combining AFM and electrochemical techniques. to characterize in detail the PPy mechanism of nucleation and growth. (c) 2012 Elsevier B.V. All rights reserved.