Electric Field Control of Mechanical and Electrochemical Properties of Polyelectrolyte Gel Membranes

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Abstract

Electric fields applied across polyelectrolyte gel membranes can modulate the mechanical and electrochemical properties of such membranes. Experiments have been performed to demonstrate the feasibility of electric field control of polyelectrolyte membrane properties using crosslinked poly(methacrylic acid) membranes. An applied current density of 3.0 ma/cm² changed the total thickness of a crosslinked PMAA membrane subjected to a pressure of 3-8 kPa of pressure by over 20%, from 1000 μ to over 1200 μ.

A model has been developed to describe the steady state and transient swelling behavior of polyelectrolyte gel materials. This model includes the mechanical and electromechanical behavior of the membrane through Darcy's law for fluid flow and a membrane constitutive law relating swelling stress to membrane charge, local ionic environment and local hydration. The chemical and transport behavior of the membrane are included through charging and reaction isotherms, and through the Nernst-Planck equation of ionic flow, including convective flow. Equations describing continuity of fluid and of ionic species are written in the frame of the (expanding/ shrinking) solid. The parameters used in the constitutive laws in these equation are calculated from independent steady state experiments.

Experiments were done to characterize the dynamic response of PMAA membranes to step changes in applied load, bath [Naci], bath pH and applied current density. Based on these experiments, the approximation of mechanical quasi-equilibrium was made, and convective flow was assumed to contribute negligibly to the ionic profiles.

The steady state form of the model equations was solved numerically for a number of different boundary conditions, using these two approximations. The transient form of the model equations were then solved numerically, using the steady state solution as the initial condition. The trends of predictions of this model for the transient behavior of membrane thickness is in agreement with those of the experiments performed.

Electrodiffusion control of membrane thickness has been demonstrated, and the extension to electrodiffusion control of membrane permeability to solutes is discussed. Alternate membrane configurations which may decrease the response time while allowing finer control of membrane permeability are also discussed.

Cited Works (2)

Year	Entry
1980	Mow VC, Kuei SC, Lai WM, Armstrong CG. Biphasic creep and stress relaxation of articular cartilage in compression: theory and experiments. J Biomech Eng. February 1980;102(1):73-84.
1986	Weiss AM. Real-Time Control of the Permeability of Crosslinked Polyelectrolyte Membranes to Fluorescent Solutes [PhD thesis]. Cambridge, MA: Massachusetts Institute of Technology; 1986.

Cited By (3)

Year	Entry
1990	Frank EH, Grodzinsky AJ, Phillips SL, Grimshaw PE. Physicochemical and bioelectrical determinants of cartilage material properties. In: Mow VC, Ratcliffe A, Woo SL-Y, eds. Biomechanics of Diarthrodial Joints. Vol 1. New York, NY: Springer-Verlag; 1990:261-282.
1987	Frank EH. Electromechanics of Normal and Degenerated Cartilage: Poroelastic Behavior and Electrokinetic Mechanisms [PhD thesis]. Cambridge, MA: Massachusetts Institute of Technology; May 1987.
1989	Grimshaw PE. Electrical Control of Solute Transport Across Polyelectrolyte Membranes [PhD thesis]. Cambridge, MA: Massachusetts Institute of Technology; February 1989.