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Characterization of ionic polymers : towards applications as soft sensors in medicine Dobashi, Yuta

Abstract

A phenomenon termed the piezoionic effect is described and characterized in various ionic polymers including polymer networks containing aqueous electrolytes (hydrogels) and organic electrolytes. Initial observations suggest that when an ion containing polymer is compressed, a concentration gradient is induced by the pressure differential, leading to an electrical potential difference detectable at electrodes placed at compressed and uncompressed portions of the polymer. The work focuses on the fundamental characterization of the nature of the piezoionic transduction to probe the effects of relative mobilities of the ions present in the system. The effective ion radii due to ion-solvent interactions and electrostatic ion-polymer interactions have been investigated for their contribution in dictating the piezoionic behavior by NMR measurements of the self-diffusion coefficients. The results are qualitatively correlated to the voltage response to mechanical compression of the polymer samples. Following the experiments, a numerical model is developed which incorporates a number of contributing events believed to be taking place in a concerted manner to cause the piezoionic effect. The deformation induced solvent flow is modeled by means of Biot’s constitutive equations on poroelasticity, a combination of thermodynamic equilibrium and Darcy’s law. The Darcy’s flow induced is then used as the input to model transport of dilute species. Here, the convective factor is being continuously modulated by Darcy’s flow, while Fickian diffusion concurrently takes place. The ionic species experience different displacements due to Stokes' drag experienced by the solvation spheres of the ionic species and solvent molecules and the electrostatic interactions between the charged polymer chains and the mobile ions. Furthermore, this non-homogeneous ionic charge distribution yields a voltage distribution via the Poisson’s equation. This voltage distribution is used to account for the migration of ionic species. The following chapter is dedicated to a novel electrochemical method and modelling approach designed to probe various ionic polymers, some electronically conductive and others interpenetrated, to determine the phase-wise contributions to ionic conductivities. Finally, potential applications of the piezoionic polymers as soft sensors in medicine, particularly in unobtrusive and longitudinal monitoring of physical parameters, are discussed and some preliminary prototypes are introduced and ultimate feasibility is assessed.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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