New effect in ionic polymeric gels: the ionic flexogelectric effect

Reported are a number of quantitative observations and analytical modeling of a new effect in ionic polymeric gels such as poly(2-acrylamido-2-methylpropanesulfonic acid) or PAMPS, polyacrylic acid plus sodium acrylate cross-linked with bisacrylamide (PAAM), or various chemically doped combinations of polyacrylic acid plus polyvinyl alcohol (PAA-PVA). This new effect, hereafter, referred to as `flexogelectric effect' is basically the inverse of the effect originally reported in 1965 by three GE researchers, namely, Hamlen, Kent, and Shafer in which the imposition of an electric field on an ionic polymeric gel fiber produced extension or contraction. Here it is shown, both theoretically and experimentally, that mechanically induced nonhomogeneous deformations, and in particular bending of strips of such ionic gels, can produce an electric field and the associated voltage. For typical samples of such gels (4 X 4 X 40 mm) with copper or platinum foil electrodes snugly contacting a pair of opposite sides (4 X 40 mm) of the strip, the difference in voltage measured between the electrodes for extreme bending configurations of the gel is typically in the 10s of millivolts range. This voltage difference which is quite significant for many engineering applications, such as large strain and deformation sensing, is still an order of magnitude smaller than the voltage necessary to induce similar deformations in the gel itself. A plausible explanation is also presented for such discrepancies.