Anomalous Magnetorheological Response for Carrageenan Magnetic Hydrogels Prepared by Natural Cooling

The effect of cooling rate on the magnetorheological response was investigated for magnetic hydrogels consisting of carrageenan and carbonyl iron particles with a concentration of 50 wt.%. For magnetic gels prepared by natural cooling, the storage moduli at 0 and 50 mT were 3.7×104 Pa and 5.6×104 Pa, respectively, and the change in the modulus was 1.9×104 Pa. For magnetic gels prepared by rapid cooling, the storage moduli at 0 and 50 mT were 1.2×104 Pa and 1.8×104 Pa, respectively, and the change in the modulus was 6.2×103 Pa, which was 3.1 times higher than that for the magnetic gel of rapid cooling. The critical strains, where G‘=G“ on the strain dependence of the storage modulus, for magnetic gels of natural cooling and rapid cooling were 0.023 and 0.034, respectively, indicating that the magnetic gel of rapid cooling has a hard structure compared to that of natural cooling. Oppositely to this, the change in the storage modulus at 500 mT for the magnetic gel of rapid cooling was 1.6×105 Pa, which was 2.5 times higher than that of natural cooling. SEM images revealed that many small aggregations of carrageenan network were found in the magnetic gel of natural cooling and continuous phases of carrageenan network with large sizes were found in the magnetic gel of rapid cooling. It was revealed that, magnetic particles in the magnetic gel of rapid cooling can move and form a chain structure at high magnetic fields by breaking the restriction from the continuous phases of carrageenan.