UBC Theses and Dissertations
Investigation of polyelectrolyte gel based electronic devices Triandafilidi, Vasilii
We present a molecular dynamics (MD) and experimental study of polyelectrolyte (PE) gel based electronic devices such as sensors and diodes. We first perform an MD study of two PE gels with different degrees of ionization coupled in a slab geometry. Our simulations show that a pressure gradient emerges between the two gels that results in the buildup of a Nernst-Donnan potential. The Nernst-Donnan potential at the interface is found to scale linearly with temperature with the coefficient of proportionality given by the fraction of concentrations of the uncondensed counterions. We show that the potential difference can also be expressed as a linear function of the lateral pressure, thus providing a molecular interpretation of the piezo-ionic effect. These findings provide further insight onto the behaviour of soft-sensors in the equilibrium regime with no salt ion/solvent fluxes. We also perform an MD study of a junction of two oppositely charged PE networks, and compare the ion densities and electrostatic field to a corresponding continuum Poisson-Boltzmann (PB) model. At low electrostatic coupling strength, the PB model reproduces the MD simulation results for density and electric field throughout the gel very well. At higher electrostatic coupling and higher degrees of ionization, the standard PB fails to predict the MD profiles at the diode interface due to counterion condensation, network collapse and field-induced gel deformation. In fact, MD simulations predict that the rectifying behavior of diodes operating in such regimes will be much reduced. We develop a modified PB model that accounts for these effects, show that it produces better agreement with the MD results, and can be used for improved modeling of Polyelectrolyte Gel Diodes (PGDs). Additionally, we perform a systematic stress-test of the predictions of the (Yamamoto and Doi 2014) theory of the PGDs under linear sweep and step bias. We have found that the predictions of the functional forms of current-voltage (I-V) curves of the Yamamoto theory hold well. They predict an exponential increase in the regime of the forward bias as well as the square-root dependency for the regime of the reverse voltage.
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