Magnetically induced stress birefringence in stimuli-responsive hydrogels

We report the appearance of stress birefringence induced by an external magnetic field in stimuli-responsive hydrogels containing magnetic nanoparticles (Fe 3 O 4 ). The measurement of the magneto-optical Faraday effect confirmed the superparamagnetic behaviour of those nanoparticles (NPs), its magnitude depending on the NP concentration. The results obtained from both magneto-optical Faraday and Voigt effects showed that the magnetic field induced a stress birefringence in the non-rigid structure of the magnetic hydrogel. The additional ellipsometric characterization of the transmitted light allowed us to fully characterize its polarization state, revealing the magnetic field direction to be the optic axis of that birefringent system. These results are discussed in the frame of a local rearrangement and stretching of polymeric chains parallel to the magnetic coupling direction of NPs. Such an induced alignment would break the inherent optical isotropy of the hydrogel, providing an axis of anisotropy that is responsible for turning its disordered polymeric structure into a birefringent system.