The electromechanical response of ionic polymer gels in electric fields

The electromechanical responses of two anionic polymer gel systems, 1) crosslinked 50% sulfonated poly(styrene-b-ethylene- co-butylene-b-styrene) (S-SEBS) triblock copolymer gel, and 2) partially sulfonated randomly crosslinked polystyrene (S-XL-PS) gel, were studied in an 1.6 V/cm d.c. electric field in salt solutions. The crosslinked S-SEBS gel films were prepared using Co-60 gamma-ray irradiation. In this method, only the middle EB block was crosslinked. The crosslinking did not affect the morphology of the dry gel. The degree of crosslinking in the S-SEBS gel increased as the total absorbed dose increased. Both the S-SEBS and S-XL-PS swelled in aqueous solutions with swelling ratio about 1.2--1.5 and 10--30, respectively. The electromechanical behavior of both gels was studied in four salt solutions, namely Cs2SO 4, Na2SO4, tetramethylammonium hydrogen sulfonate (TMA) and tetrabutyl-ammonium hydrogen sulfate (TBA), at concentrations between 0.005 and 0.1 M.;When the gels were pre-equilibrated in the corresponding salt solution, both gels bent toward the cathode as the electric field was applied. Both gels responded to the electric field immediately. The bending angle increased linearly with time at the beginning, then leveled off to a steady state bending angle (thetass). The time needed to reach the steady state bending angle was equal to the time needed for the mobile cations to move across the gel slab. The time needed to reach the steady state bending angle depended on the mobility of the different mobile cations, the electric field strength and the thickness of the gel but did not vary with the salt concentration. Curves showing thetass and the initial speed of bending versus salt concentration were parallel to each other for both gels in all salts. Both theta ss and the initial speed of bending reached a maximum at the about the same intermediate salt concentration, C* in all cases. This concentration C* showed very little or no dependence on the type of the salt and the type of the gel within the experimental error. It was shown that some of the variables that control the gel bending are the size of the hydrated cations, the mobility of the cations, and the osmotic pressure difference between the interior of the gel and the exterior solution. The steady state bending angle increased as the size of the hydrated cation increased and the initial speed of bending increased as the mobility of the cation increased.;When the S-XL-PS gel was pre-equilibrated in DI water, the gel first bent toward the anode then reversed to the cathode in 0.065--0.08 M TBA solutions when the electric field was applied. This did not happen in the other salt solutions. The gel bending, both pre-equilibrated in salt solution and in DI water, was related to the ion motion-induced changes of osmotic pressure between the interior of the gel and the external solution at the anode and cathode side of the gel.;Some of the S-XL-PS gels were used as electromechanical actuators by using a gel slab to drag or lift various objects in the salt solutions. An inhomogeneous sandwich-like S-XL-PS gel appeared to have a higher flexural modulus and performed as a stronger actuator than a homogeneously sulfonated S-XL-PS gel. An Al2O3 nanoparticle filled S-XL-PS gel had an even higher flexural modulus. This gel exerted almost three times the force of the sandwich-like S-XL-PS gel.