Solvent swelling influences the electrochemical behavior and stability of thin films of nitrated poly (styrene)

The electrochemical behavior and stability of thin films of poly(nitrostyrene) (PNS) and a partially dinitrated derivative of PNS (PDNS) have been examined in acetonitrile. Qualitative information regarding the extent of solvent swelling of these polyelectrolyte films in non-aqueous solvents is obtained using the electrochemical quartz crystal microbalance (EQCM) technique, both by measuring the frequency changes due to solvent swelling and by measuring the conductance (impedance) spectra of the quartz crystal/polymer film composite resonator in the different oxidation states. Extensive solvent swelling as a function of the number of reductive scans is correlated with instability of the films toward dissolution or delamination and with subtle changes in electrochemical behavior. The effectiveness of thermal pretreatments (which presumably induce crosslinks of some type) for enhancing film stability by slowing the rate of swelling is examined. For certain types of films, it is shown that the transport processes which serve to achieve electroneutrality at high scan rates may occur on a different time scale (i.e. faster) than those which serve to attain thermodynamic equilibrium. The implication is that transient, non-equilibrium states may be prepared during switching whose composition is not necessarily that dictated by the equilibrium state, and that manipulation of these transient states to achieve faster switching rates may be done independently of equilibrium considerations, at least in some cases.