Experimental investigations into charge and spin carriers in polyaniline.

Abstract

Conductivity and electron spin resonance measurements have been performed on solution doped polyaniline (PANi). It is proposed that both camphor-sulphonic acid (CSA) and 2-acrylamido-2-methyl-l-propanesulphonic acid (AMPSA) doped PANi can be described by the same model. It is suggested that the polyaniline materials are composed of differently ordered layers, a highly ordered region forming the core of the crystallites. The core of the crystallites are believed to be encapsulated within a semi- ordered region, with the crystallites themselves being dispersed in an amorphous polymer matrix. The conductivity measurements and ESR results described in this work support the proposal that within the highly ordered region of doped polyaniline crystallites, a polaronic lattice exists. The polaronic lattice facilitates "free" carriers which are responsible for "metallic" conduction within the crystallites. Encapsulating the polaronic lattice is a semi-ordered region in which (partially) mobile polarons (and possibly bipolarons) are present. The highly conductive crystallites are randomly dispersed in a less conductive polymer matrix. Charge transport within this heterogeneous system is well described by a heterogeneous metal - fluctuation induced tunnelling (FIT) model. The differences in the temperature dependent conductivities of the PANi-CSA and PANi-AMPSA materials are attributed to the systems having layers of different relative sizes (in the above model). AMPSA doped polyaniline films had a maximum room temperature conductivity of ~100 Scm(^-1). This material also showed potential for use as an electrode layer in polymer LEDs, to replace ITO coated glass. The conductivity of PANi-AMPSA was measured to be 50 ± 10 Scm(^-1) at thickness' of ~30nm. Layers of this thickness provide >90% optical transmission between 450 and 675 nm (most of the visible spectrum). Faraday rotation measurements have shown that the recently reported large Faraday rotation of polyaniline can not be reproduced. The limited results of the Faraday rotation experiments described in this work provide support for the theory that charge carriers in polyaniline have an effective mass of at least 100 times that of a free electron. It has also been shown that the claims of a polyaniline derivative (namely the Marcoussis polymer) being an entirely organic ferromagnet are unsubstantiated, despite intense investigation.