Hindered nematic alignment of hematite spindles in poly(N-isopropylacrylamide) hydrogels: a small-angle X-ray scattering and rheology study

Abstract

Field‐induced changes to the mesostructure of ferrogels consisting of spindle‐shaped hematite particles and poly(N‐isopropylacrylamide) are investigated by means of small‐angle X‐ray scattering (SAXS). Related field‐induced changes to the macroscopic viscoelastic properties of these composites are probed by means of oscillatory shear experiments in an external magnetic field. Because of their magnetic moment and magnetic anisotropy, the hematite spindles align with their long axis perpendicular to the direction of an external magnetic field. The field‐induced torque acting on the magnetic particles leads to an elastic deformation of the hydrogel matrix. Thus, the field‐dependent orientational distribution functions of anisotropic particles acting as microrheological probes depend on the elastic modulus of the hydrogel matrix. The orientational distribution functions are determined by means of SAXS experiments as a function of the varying flux density of an external magnetic field. With increasing elasticity of the hydrogels, tuned via the polymer volume fraction and the crosslinking density, the field‐induced alignment of these anisotropic magnetic particles is progressively hindered. The microrheological results are in accordance with macrorheological experiments indicating increasing elasticity with increasing flux density of an external field.

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…Texture analysis of the WAXS data further confirmed that the magnetic easy axis is located in the basal plane of the hematite crystal lattice [55]. The field-induced orientation of hematite particles can be used to probe the viscoelastic response of a gel such as that formed by poly(N-isopropylacrylamide) (PNIPAM) and correlate with the microrheological parameters [53]. With increasing elasticity of the gel, the transition to the nematic order occurred at progressively large value of the magnetic field.…”

“…The phase behavior of anisotropic colloids is very sensitive to applied fields such as electric or magnetic fields [53][54][55]. A field-induced isotropic to nematic transition can be observed in these suspensions.…”

Using the singular value decomposition to extract 2D correlation functions from scattering patterns

“…The dominance of the magnetocrystalline anisotropy leads to a net magnetic moment in the basal plane of the hematite spindles, i.e. perpendicular to the long particle axis (Reufer et al, 2010;Hoffelner et al, 2015;Nack et al, 2018). Therefore, the long spindle axes are oriented within the plane perpendicular to the field direction, which explains the anisotropy of both the scattering pattern and the extracted correlation function.…”

In situ magnetorheological SANS setup at Institut Laue-Langevin

“…When a magnetic field is applied, anisometric magnetic nanoparticles align with their magnetic easy axis along the field [29][30][31][32][33][34][35][36]. If the long nanoparticle axis aligns perpendicular to the flow direction, the aligned nanoparticles oppose resistance to the flow resulting in an increase fluid viscosity.…”

“…If the long nanoparticle axis aligns perpendicular to the flow direction, the aligned nanoparticles oppose resistance to the flow resulting in an increase fluid viscosity. In contrast to common ferromagnetic nanorods, the easy magnetic axis of hematite spindles lies in the basal plane (i.e., the equatorial direction) such that the particles typically orient with their long axis perpendicular to an applied field [30,[32][33][34] with the peculiar results that a negative magnetorheological effect is observed in a magnetic field oriented perpendicular to the shear direction [37]. Hence, the application of magnetic field allows either enhancing alignment or overcoming shear alignment of magnetic fibers during sheardominated processing techniques [38].…”