Effects of Moisture and High Electric Fields on Conductivity in Alkali-Halide-Doped Cellulose Acetate

Abstract

Several problems associated with ionic conduction in amorphous hydrophilic polymers have been investigated from a theoretical point of view and then discussed in the light of experiments with alkali-halide-doped cellulose 2.5 acetate. The main ideas emphasized are: (1) The major effects of moisture on conductivity σ in this class of polymers are to increase the dielectric constant ε′ and therefore the concentration of ions by reducing the effective dissociation energy from its maximum U0′ (≈ 140 kcal/mole) to a lower value ≈U0′/ε′ such that σ≈σ1exp(−U0′/2RTε′), where σ1 depends on the square root of salt concentration n0; and (2) departures from Ohm's law in high fields E would occur as a result of activation barrier perturbation even in the absence of thermal and chemical effects. The above equation for σ was checked by starting with ``dry'' disks of CA and simultaneously measuring ε′ and σ (dc) after periods of moisture sorption. The evidence suggests that the micro- and macroscopic values for ε′ are proportional and of the same magnitude and that approximations valid for weak electrolytic solutions are applicable to ionic salts in moist solid polymers. For the C2.5A samples of the present study (≈ 270μ thick), the contribution of barrier perturbation to non-Ohmic behavior is larger than the effects of internal I2R heating or Onsager's theory of field-enhanced dissociation.