Deformation Behavior of Polymer Gels in Electric Field
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DEFORMATION BEHAVIOR OF POLYMER GELS IN ELECTRIC FIELD Toshio Kurauchi, Tohru Shiga, Yoshiharu Hirose and Akane Okada TOYOTA Central Research and Development Laboratories Inc. 41-1, Nagakute, Aichi, 480-11, JAPAN ABSTRACT The deformation of poly(acrylic acid)-co-(acrylamide) gels and poly(vinyl alcohol)-poly(acrylic acid) gels under an electric field was investigated. Bending of these ionic gels was induced by an electic field. Using this deformation, we constructed a prototype of a robot hand having soft fingers, and an artificial fish able to swim. INTRODUCTION A polymer gel is a crosslinked polymer network swollen in a liquid medium. Polymer gels, "solid-liquid coexistant materials", are candidate "biomimetic materials". Recently, their mechanical strength becomes very close to living muscle. In a soft structure of gels, a motion of polymer network and a diffusion of ions take place easily by an external stimulus. Therefore, polymer gels have various possibilities as advanced functional polymers. A typical function of a gel containing ionic groups is to bend reversibly under the influence of an electric field'", making it useful in some actuators driven by an electric field. In this paper, deformation of the gels under an electric field is presented. DEFORMATION OF POLYMER GELS UNDER AN ELECTRIC FIELD Acrylic acid-acrylamide copolymer gels (PAAm gels) swollen in aqueous electrolyte solution show three types of deformation under the influence of a d.c. electric field ; shrinking, swelling and bending, as shown in Fig.l.' 2 The type of deformation depends on the fraction of sodium acrylate (AANa) in the PAAm gel, the shape of the PAAm gel and the position of the gel between the positive and the negative electrodes. A PAAm gel with a low fraction of AANa shrinks on the surface facing to the positive electrode (Fig.l(a)) and a gel with a high fraction of AANa swells on the positive electrode side (Fig. 1(b)). When a rectangular gel is placed parallel to the electrodes, bending of the gel takes place toward the positive electrode (Fig.l(c)) or toward the negative electrode (Fig.l(d)), according to the fraction of AANa. The critical fraction of AANa is about 25mo1%. This bending behavior is similar to the buckling of a bimetallic strip submitted to a variation of temperature. Because we observed the bending deformation with gels having different ionic groups, such as SO3Na and NR3CI , the phenomenon is considered to be a general property of ionic Mat. Res. Soc. Symp. Proc. Vol. 171. '1990 Materials Research Society
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E22i EZ7E-
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Fig.l Deformations of PAAm gels under d.c. electric field. (a) and (c):for small fraction of AAna (b) and (d):for large fraction of AAna
gels. Of course the direction of the bending is the sign of the ionic groups is changed.
inversed,
when
BENDING OF PAAm GELS WITH HIGH FRACTION OF AANa In Fig.2, the weight gain of a PAAm gel (rectangular bar 8x~x8Omm) is plotted as a function of the strain in bending. When the PAAm gel bends semicircular, the strain
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