Microscale evaluation of the viscoelastic properties of polymer gel for artificial muscles using transmission acoustic microscopy

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Abstract
Local viscoelastic properties and a micromechanical structure of polymer hydrogel were studied using transmission acoustic microscopy (AM) with a spatial resolution up to 5 μm. Utilizing various amplitude, phase, and pulse techniques provided by the transmission AM, a velocity and an attenuation of the acoustic wave in the local point of the polymer sample as well as a frequency dependence (dispersion) of these values were evaluated in the frequency range from 30 to 320 MHz. Using these facilities a poly(vinylalcohol) polymer hydrogel for the artificial muscle was studied at the succeeding steps of its manufacturing process of cyclic freezing–thawing. It was found that synchronously, with the increase of the compressional elasticity and acoustic attenuation, a micromechanical nonuniformity of the gel also profoundly enhances it. Fiber-like structures of 10–50 μm width possessing higher elastic modulus appear in the gel which can play a significant role in the macroscopic mechanical properties of the gel.