Structure and Dynamics of Ionically Crosslinked Low Methoxyl Pectin (LMP) Hydrogels: Effect of Ca2+ and Fe3+ Ions on Rheological and Dielectric Relaxation Behaviour

Abstract

Investigations on structure and dynamics of LMP based ionically crosslinked hydrogels important for designing multifunctional materials for applications in energy storage devices. Low Methoxyl Pectin (LMP) solutions were prepared using solution casting technique. Stoichiometric ratios (SR) of 0.5, 1, 1.5 and 2 were selected for Ca²⁺ and Fe³⁺ ion doping. Rheology, electrochemical impedance spectroscopy (EIS), SEM studies were performed. The storage modulus (G’) and loss modulus (G”) with Ca²⁺ and Fe³⁺ ions showed a power law relationship, which are in good agreement to morphological changes in hydrogels. Rheology results showed a power law relationship with increasing Ca²⁺ and Fe³⁺ ion concentration. LMP/Ca²⁺ hydrogels showed dc conductivity (0.38 S/cm-1.26 S/cm), whereas LMP/Fe³⁺ hydrogels (0.37 S/cm–0.81 S/cm). Kramer’s Kronig relation was used to derive imaginary part \(({\varepsilon {^{\prime \prime}}_{der}})\) of permittivity to avoid conductivity contributions. Debye model function was used for fitting dielectric relaxation (segmental relaxation) peak. The dc conductivity of LMP was ≈1.52 S/cm with dipolar relaxation time of ≈1.95 × 10⁻⁴ s. Dielectric relaxation times were (1.9 × 10⁻⁴ – 7.1 × 10^− 4) for LMP/Ca²⁺ hydrogels, whereas, greater change (7.9 × 10^− 4 – 2.5 × 10⁻³) in relaxation time was observed for LMP/Fe³⁺ hydrogels. Electrode polarization was higher for LMP/Fe³⁺ over LMP/Ca²⁺ hydrogels, which can be used in energy storage devices.