Ionic polymer gels, consisting of a polymer network with ionizable groups and a liquid phase with mobile ions, exhibit very good actuatoric capabilities due to their large swelling ratios. In this paper we investigate gels immersed in salt solution at different positions - in direct contact with the anode, the cathode or in the middle of the electric field. The concentrations of anions and cations in these gels as well as the electric potential inside and outside the gel are calculated for a given number of anionic groups fixed to the polymer. The applied chemo-electric formulation consists of a convection-diffusion equation for the chemical field and a Laplace equation for the electric field. The numerical simulation of the coupled formulation has been performed by using unconditionally stable space-time finite elements. Based on the results of the numerical simulation we compare the concentrations inside and outside the gel for the different test cases in order to optimize the position of the gel film. The highest swelling ratio of the gel has been taken as criterium for the optimization. The optimal condition is characterized by a maximum value of the concentration differences and of the Donnan potential.
Polyelectrolyte gels are distinguished by enormous swelling capabilities under the influence of external physical or chemical stimuli. No other kind of material attains similar volume expansiveness. These properties make them most attractive candidates for a new generation of pseudomuscular actuators. In contrast to chemical stimulations which are able to trigger large in-toto deformations, weak electric fields can only induce considerable bending strains in ionic polymer gels when confined to direct electrical effects. This, of course, restricts their potential for technical applications. To characterize their chemo-mechanical and electrical behavior and the underlying physico-chemical processes, experimental and theoretical findings are presented. Measurements of basic mechanical and electrical parameters on polyelectrolyte gels allow quantification of their electroactive responses, especially with respect to the direct effects of external electric fields on the Donnan potential inside the gels. Model calculations on the basis of a coupled chemo-electro-mechanical multi- field formulation are in good agreement with the experimental results. Although the emphasis of this study is given to various anionic and cationic gels of the polyacrylamide family, a new class of hydrogels based on the biopolymer chitosan is included. These natural polymers have excellent properties such as biocompatibility, biodegradability, non-toxicity etc. making them predestinate to biomedical applications.
In this paper, electrolyte polymer gels, consisting of a polymer network with ionizable groups and a liquid phase with mobile ions, are investigated. For these gels, we present a volume- and surface-coupled multi-field problem involving chemo-electro-mechanics. First, we derive a convection-diffusion equation for the ion concentrations inside and outside the gel as well as a Laplace equation for the electric field. Second, an equation of motion in order to simulate the unsteady swelling-behavior of the gels, is presented. For the chemo-electro-mechanical coupling, the equations as well as the solution scheme, are given. For the numerical simulation, unconditionally stable, higher order accurate, conservative and implicit space-time finite elements with interpolations - continuous in space and discontinuous in time - are used. We investigate the anionic and the cationic ion concentrations for a given fixed number of bound anionic groups as well as the electric potential inside and outside the gel at a given electric field. The resulting increase in the Donnan potential difference on the anode side of the gel, which represents the higher swelling rate, is in good agreement with experimental results. This shows the validity and the potential of the model.
Direct effects of electrical currents on polyelectrolyte gels are always associated with changes in their Donnan potential. Thus electrical stimulation of gels can be only completely understood if the direct effect of electric fields on the potential profile within the gels are known. The purpose of this study is to present recordings of Donnan potentials in electroactive gels of various compositions, especially under the influence of electric fields. An important finding is that opposite alterations in the Donnan potential simultaneously occur at the current inflow and outflow region of the gel. In anionic gels hyperpolarization, i.e. higher negativity, is induced on the anode-side of the gel, whereas depolarization is found on the cathode-side. As these shifts in the potential are supposed to affect swelling or deswelling of polyelectrolyte gels, they will primarily promote bending motions of the gel. To demonstrate the opposite bending behavior of anionic and cationic polymer gels under the influence of an electric field a short video sequence of an EAP gripper in action is presented. It is made exclusively of polyelectrolyte gel strips taking advantage of the fact that anionic and cationic polyacrylamide gels can be attached firmly to each other without any adhesive.
In order to gain more insight into basic principles of the nature of polymer hydrogels which are able to execute work by large deformations in electric fields, this study is mainly focused on those gels with a polyacrylamide backbone being very suitable for considerably varying their physicochemical properties simply by specific copolymerization. In the experimental part, the Donnan potential has been registered for the first time in PAAm/PAA gels in varying electric fields and different chemical environments with a new microelectrode technique. The mechanical properties of the gels have been characterized by measurements of swelling ratio, elastic modulus and being in dynamics under various stimuli. In the theoretical part, a model based both on this theory and the measured mechanical parameters, the bending dynamics of a polyelectrolyte gel in an electric field can be evaluated. Numerical simulations employing finite element discretization demonstrate the potential and the validity of the model. A promising correlation between theory and experiment could be shown.
Conference Committee Involvement (3)
Electroactive Polymer Actuators and Devices (EAPAD)
7 March 2005 | San Diego, California, United States
Electroactive Polymer Actuators and Devices (EAPAD)
15 March 2004 | San Diego, CA, United States
Electroactive Polymer Actuators and Devices (EAPAD)
3 March 2003 | San Diego, California, United States
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