Enhanced Frank elasticity and storage modulus in a diamagnetic liquid crystalline ferrogel

Abstract

Combining the magnetic properties of metal particles and the orientational anisotropy of a liquid crystal has been of interest owing to potential technological and bioengineering applications. The target is to achieve the ferromagnetic state while retaining the fluid environment of a nematic liquid crystal (NLC). Such a realization in a low molecular weight material which undergoes physical gelation is all the more advantageous since it can exploit the electrical switching, the concomitant birefringence change, etc. in the sol state as well have the structure frozen in the gel state. We report here such an attempt on a NLC doped with FePt nanoparticles and an organogelator by performing calorimetry, XRD, dielectric, elastic, rheological and magnetic characterization in the anisotropic sol and gel states. While the XRD data suggest that the orientational correlations of NLC are slightly strengthened by the presence of the particles and the gelator, the Frank bend elastic constant increases by two orders of magnitude indicating the mechanical rigidity of the gels. The magnetic measurements reveal that the superparamagnetic feature of the FePt particles is weakened in these ferrogels. This is surprising since it has recently been shown that a polymer ferronematic retains the ferromagnetic characteristics of the doped metal particle. These results suggest that the local environment of the particles has an important role in quenching the thermal fluctuations which in turn influences their magnetic interaction. Despite the bulk viscosity of the gelated system being high due to the trapping of NLC by the gelator, the immediate neighbourhood of the particles is still a fluid with enough orientational and translational freedom. This results in an ineffective quenching of the thermal fluctuations and consequently weak magnetic interactions.