ИСТИНА

Nowadays much attention is paid to a development of a new type of magnetic elastomers (ME) demonstrating high responsiveness to external magnetic fields. They are based on rather soft polymer matrices filled with magnetic particles of nano- and/or micrometer size. Due to a coupling between polymer elasticity and magnetic forces acting between magnetic particles in a magnetic field these materials acquire a number of striking properties controlled by magnetic field. Particularly, ME are able to significantly change their rheological characteristics in presence of a homogeneous magnetic field of rather small density (0-600 mT). In this work we study and compare rheological characteristics of ME based on magnetically soft (carbonyl iron) and magnetically hard (NdFeB) fillers. Silicone rubber is used as a matrix. Two types of matrices were synthesized differing in the Young modulus (soft and hard matrices). The filler content was varied in the range of 70 – 82 mass %. Isotropic and anisotropic samples were synthesized. After curing, the samples based of NdFeB were magnetized in the homogeneous magnetic field of different strengths (0, 3, 6, 9, 12, 15 kOe). Rheological measurements were made using the commercial rheometer Anton Paar, model Physica MCR 301, with measuring “plate-plate” unit and a magnetic cell. Dependencies of the storage modulus G' and the loss modulus G'' on the oscillation frequency f at a constant strain amplitude as well as on the strain amplitude at a constant frequency were obtained in the absence and presence of magnetic field B. Moduli of magnetically soft ME become strain-dependent in the magnetic field. The maximum increase of the moduli (2-3 orders of magnitude for G' and 2-3 orders of magnitude for G'') is observed at small strains. Normal force significantly increases in the presence of magnetic field as well. These phenomena could be explained by the process of structuring of magnetic particles within the matrix. Particles form chains aligned to the direction of magnetic field and these chains form some kind of a net structure that makes sample stiffer. Magnetization of magnetically hard ME causes a change of their rheological behavior already without any additional magnetic field. In particular, their moduli increase with magnetization field and acquire well-pronounced strain dependence. Normal force in this case decreases with time, demonstrating a shape memory effect of ME induced by magnetic interactions. Financial support of the Russian Foundation for Basic Research (project 13-03-12147) and the International Bureau of the BMBF (grant Nr. 01DJ13006) is gratefully acknowledged.