(Keynote) Nanoscale Structuring of Aniline-Based Electroactive Polymer Films By Co-Polymerisation and Particulate Inclusion
Authors: Hillman A. Robert , Ryder Karl , Ismail Hani K. , Unal Asuman , Burrell Robert , Efimov Igor , Chilton Morgan 
Published: July 2021

Electroactive polymer films derived from aniline and substituted derivatives have attracted huge interest, as a consequence of their electronic, optical, chemical and charge storage properties. Surprisingly, there has been limited translation of these characteristics into practical devices. Two factors that have impeded progress here are an inability to deliver and optimise multiple characteristics in a single material and the absence of control of internal film structure. The latter is particularly difficult to maintain for soft matter, whose molecular geometry and spatial disposition at the mesoscopic scale necessarily changes in response to solvent influx/efflux during redox switching. These solvation phenomena have been variously described in terms of activity (thermodynamic) effects, volume constraints (polymer mechanical properties) and mobility (kinetic) effects. Quite generally, the dominant factor depends on the timescale, a feature of the experiments reported here.

In this presentation, we explore strategies aimed at understanding and controlling internal film structure and solvation for polymer films based on aniline-type monomers. From a chemical perspective, we explore the effects of three factors: the nature of the solvent / electrolyte, extension from homopolymers to co-polymers, and the inclusion of geometrically distinct particulate species to generate hybrid organic/inorganic materials.

In the first instance, we compare film deposition and subsequent redox chemistry when exposed to a conventional protic solvent (water) or to the aprotic ionic liquid Ethaline (a mixture of choline chloride and ethylene glycol). As an intermediate case, we consider film deposition and redox chemistry in the protic ionic liquid Oxaline (a mixture of choline chloride and oxalic acid). Nanogravimetric (QCM) measurements during deposition from ionic liquid reveal that the polymer film has substantially greater solvent content than if deposited from aqueous medium. A recognised phenomenon for electroactive polymer films is an anomalous response for the first redox cycle; this is generally attributed to solvation effects. This phenomenon is manifested quite differently for polyaniline films in water and Ethaline. In water, film evolution occurs over a number of cycles, while in Ethaline (in which the film is more highly solvated) the transition is complete with a single cycle.

In the second instance, we consider copolymerization of aniline with o-toluidine or with o-aminophenol. Counter ion coordination by the hydrogen bond donors results in substantive differences in the effectively transferred entity. In the cases of Ethaline and Oxaline, the counter ion is no longer a small, mobile chloride ion, but rather a large hydrogen bond donor coordinated species. In pursuit of possible environmental applications, we consider the extreme case of fluoride as a counter ion. Surprisingly, the relatively small changes in polymer composition effected by co-polymerisation with o-toluidine or o-aminophenol result in substantial changes in fluoride uptake.

Finally, we explore the effect of inclusion of particulates, chosen so as to influence different aspects of film behaviour. Specifically, we chose multi-walled carbon nanotubes (MWCN), graphite flakes and molybdenum oxide as examples of 1D, 2D and 3D entities, respectively. Data will be presented to demonstrate their substantive effects on film structure and dynamics.

The outcomes of these studies provide insights relevant to practical applications. A particular case is that of medium transfer experiments, in which a film is deposited from one medium (aqueous or ionic liquid), redox cycled in the other medium and then returned to the first medium. We discuss the responses in terms of a “nature vs nurture” competition, in which the “nature” of a film is dictated by the conditions (medium) of deposition, and “nurture” is represented by the conditions to which it is exposed. Nanogravimetric (EQCM) responses of films subjected to water / ionic liquid / water and ionic liquid / water / ionic liquid transfer experiments reveal that films do retain some memory of their deposition conditions, but that extended redox cycling results in slow evolution that reflects the immediate medium of exposure. We speculate that this is a consequence of slow polymer structural relaxation.