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ABSTRACT Active polymer gels can achieve large, reversible deformations in response to environmental stimuli, such as the application of an electric field or a change in pH level. Consequently, great interest exists in using these gels as actuators and artificial muscles. The goal of this work is to characterize the mechanical properties of ionic polymer gels and to describe how these properties evolve as the gel actuates. Experimental results of uniaxial tests on poly(vinyl alcohol)-poly(acrylic acid) gels are presented for both acidic and basic environments. These materials are shown to be to be slightly viscoelastic and compressible and capable of large recoverable deformations. The gels also exhibit similar stress in response to mechanical deformation in both the acid and the base.

INTRODUCTION An active polymer gel is composed of a crosslinked polymer network and interstitial fluid. Environmental changes, such as a change in the pH of the fluid or the application of an electric field, can result in a shrinking or swelling of the gel. Large volume changes (as large as 500 percent for some gels) accompanied by relatively low densities (near that of water) have made these gels of great interest in a variety of applications. Many devices have been developed which incorporate a polymer gel as an actuating device. Examples of these include finger-like gripping devices and camera lens dust-wipers.'. 2 Polymer gels are also being developed for 3 internal drug delivery systems and other medical applications. Much research has been conducted on active polymer gels, especially in the area of swelling kinetics. 4' 5 Some constitutive models have been developed to describe the mechanical properties of these materials. However these models are typically limited to small deformation 6 theory and swelling from the dry state or only in response to concentration changes. The goal of this work is to characterize the mechanical properties of polymer gels undergoing large elastic deformations, and to describe the coupling of this behavior with the actuation (shrinking/swelling) response due to environmental changes. The specific polymer gel considered is poly(vinyl alcohol)-poly(acrylic acid) (PVA-PAA), and actuation is induced through changes in the pH of the interstitial fluid.

POLYMER GEL FABRICATION The polymer gel used in this work is fabricated from poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). PVA-PAA gels have been studied previously by a number of researchers including Shin et al.,7 Chiarelli et al.,8 Genuini et al.,9 Caldwell and Taylor,' 0 and 6 Chiarelli and De Rossi. PVA-PAA gel fabrication is conducted on-site and begins by separately dissolving 1 gram each of PVA and PAA in deionized water. Once both polymers are dissolved, the solutions are combined and mixed thoroughly. The combined solution is then poured into a clean glass pan where it remains until all of the water has evaporated. A thin (