Swelling and contraction of ferrogels based on polyacrylamide in a magnetic field

Abstract

The equilibrium swelling of magnetoactive ferrogels based on the copolymer of acrylamide with 10% potassium acrylate in water is studied. The gels are filled with strontium ferrite and/or magnetite taken in amounts of 20, 40, 60, and 80 parts per 100 parts of the polymer by weight. In the absence of a magnetic field, as the content of the filler is increased, the degree of swelling tends to increase for the gels containing strontium ferrite and to decrease for the magnetite-containing gels. In the structure of hydrogels, strontium ferrite particles possessing a permanent magnetic moment form a microscopic network from chain aggregates, while the particles of magnetite, a magnetically soft material, give rise to disordered aggregates. The enthalpy of hydration of ferrogel polymeric matrices weakly depends on the nature and amount of the introduced filler: The interaction parameter takes small positive values in the range 0.10–0.18. In a homogeneous magnetic field of 365 mT, the degree of swelling decreases at a small filler content for both types of ferrogels. This effect is accompanied by elongation of a ferrogel sample along the field direction and contraction in the transverse direction. With an increase in the filler content, the inversion of both effects occurs. The applicability of concepts about the homogeneously magnetizable continuous medium to the process of magnetostriction of ferrogels is analyzed.