Dielectric Relaxation and Ionic Conductivity of a Chitosan/Poly(ethylene oxide) Blend Doped with Potassium and Calcium Cations

Abstract

Chitosan (CS) and poly(ethylene oxide) (PEO) are miscible with each other, because of the hydrogen bonding between CS and PEO that becomes stoichiometric at the PEO content of w_PEO = 20 wt%. This study analyzed the previously measured dielectric data of this stoichiometric CS/PEO blend being doped with two kinds of salt, potassium sulfate (K₂SO₄) and calcium sulfate (CaSO₄). Specifically, the casting solvent, acetic acid, was intentionally left in the blend to an amount almost equimolar to K and Ca, so that the doped blend contained two types of cations, H⁺ and K⁺ or Ca²⁺. The target of this study was to examine a synergetic effect of H⁺ and the metal cation, if any, on the dielectric behavior and ionic conduction. Comparison of the complex dielectric permittivity ε* of the K₂SO₄-doped and neat (undoped) blends suggested that K⁺ largely increased ε* by governing the electrode polarization at low frequencies (ω) and the hopping conduction at high ω. In contrast, doping of CaSO₄ considerably decreased ε* of the blend at low ω, even though the total concentration of the cationic species (H⁺ and Ca²⁺) was larger for the doped blend. This unusual, anti-synergetic effect was discussed in relation to constraint for the segmental motion of CS due to Ca²⁺ and the resulting suppression of segmental-motion-aided fast hopping and slow diffusion of H⁺.