Factors Affecting Electro-Actuation Rate in Partially Neutralized Polyelectrolytes Gels

Polyelectrolyte networks composed of poly(acrylic acid) (PAAc) cross-linked with poly(ethylene glycol dimethacrylate) (PEGDA) are sensitive to electrical stimuli, allowing potential use as soft actuators. In our studies, PAAc was prepared via radical polymerization using UV irradiation from pregel solutions with independent variation of the crosslinking density, the extent of neutralization, as well as the water content after polymerization. The networks thus formed underwent a swelling process that consists of, first, a soaking in deionized water to allow for the removal of the unreacted species, followed by several soakings in deionized water to reach swelling and pH equilibrium. The bending behavior of these swollen PAAc hydrogels was studied under the influence of a DC electric field when applied either in deionized water, or in solutions of varying pH. We find that the curvature observed for these hydrogels depends strongly on all of the chosen variables. However, an unexpected phenomenon is also observed: the bending behavior seems to be composed of three stages that are active over differing time-scales. The earliest stage consists of a curvature toward the anode, followed by an intermediate stage that reverses the bending direction to be toward the cathode. Finally, the last stage features hydrogel rod shrinkage in all the directions. Depending on the conditions of our electromechanical analyses, the earlier and intermediate stages are more or less improved in terms of fast response and/or higher curvature attained. Rheological measurements have also been performed and further correlated to the electroactive response observed. We find that for high modulus specimens, the compliance and electromechanical characteristics are strongly related to each other. INTRODUCTION Chemomechanical systems capable of responding to an external stimulus have been increasingly studied for the past two decades. Such a stimulus can be either changes in ionic forces[1], changes in temperature, or application of an electric field.[2] Soft actuators have been postulated as novel solutions for biomedical engineering applications; namely, artificial muscles, chemical valves, and drug delivery systems.[35] Their salient advantages over hard actuators include a close matching of stiffness with biological tissue coupled with large recoverable deformations in excess of 100%. Although a great deal has been done for applications in the medical area, such actuators are not only limited to that field. Indeed, we envision their incorporation in such diverse applications as Braille display or deformable mirror devices in telescopes, for example. Despite these attractive features of soft actuators, in general, and hydrogel actuators, in particular, there is still a need for a quantitative physico-chemical model that describes and predicts their response to specific external stimuli. The lack of such a model is a Mat. Res. Soc. Symp. Proc. Vol. 698 © 2002 Materials Research Society