Depth profile study of polyaniline films by photoacoustic spectroscopy

Abstract
In this study, we carried out a depth profile analysis of the optical properties of undoped and doped polyaniline (PANI) films, using photoacoustic spectroscopy (PAS). As the thermal diffusion length varies inversely to chopper modulation frequency, then at high chopping frequencies information about the sample near the surface is obtained, while at low chopping frequencies the data come from deeper within the sample. The objective of this study is to analyze how the polyaniline doping process in acidic media occurs in the polymeric chain. The results showed that the undoped sample is, in reality, slightly doped and the polymeric chain of the doped samples is formed by undoped regions. On the other side, the polymeric chain of the undoped samples presents doped regions in lower degree. These conclusions are in concordance with published results which is the granular metal model of doped PANI. This model explains the polyaniline polymeric chain as it is formed by conducting islands on a sea of insulating material. Also, our experimental results showed that the doping process of PANI films, that is, immersing the films in HCl solution, occurs better in the external surface of the films.