Structure and dynamic behaviour of anisotropic magnetic particles in complex matrices

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Aim of the project is the investigation of mesoscale structure and dynamics in composites consisting of anisotropic, magnetic nanoparticles and thermosensitive polyelectrolyte-hydrogels. These matrices are prepared by co-polymerization of NIPAM and ionic or ionogenic co-monomers. Their viscoelastic properties are influenced by the polymer volume fraction, the cross-linking density and the charge density which can be varied by the molar fraction of co-monomers. In the case of ionogenic co-monomers, the charge density additionally depends on the pH dependent protolysis of acidic or basic groups of the co-monomers. In addition, repulsive interactions between equal charges in the polymer network can be influenced by the Debye-length depending on the ionic strength. Spindle-shaped hematite particles with widely tunable aspect ratio can be prepared with considerably small polydispersity. These in aqueous suspension by negative surface charges stabilized particles align in presence of moderate external magnetic fields in first approximation perpendicular to the field direction. Therewith, in external fields the rotational mobility of these particles is confined and their translational mobility, caused by anisotropic friction coefficients, is direction-dependent. Field-induced changes of their rotational and translational mobility lead due to particle-matrix interactions to an influence on viscoelastic properties of particle-hydrogel composites. Due to attractive, electrostatic interactions between negatively charged hematite particles and positively charged polyelectrolyte gels, the exceptionally strong particle-matrix coupling in these systems causes significant field-induced changes of their viscoelastic properties. In the project's focus are correlations between field-induced changes of mesoscale structure and dynamics of such composites and related changes of their macroscopic, viscoelastic properties. Due to the significantly larger electron density of hematite particles compared to the polymer matrix, by means of static and quasielastic X-ray scattering experiments (SAXS, XPCS) selectively the structure and dynamics of embedded hematite particles can be addressed. The rotational motion of anisotropic hematite particles under shear can be investigated by means of time-resolved, coupled rheological and X-ray scattering experiments (RheoSAXS). The structure and dynamics of the hydrogel matrix is probed by means of light scattering experiments or neutrons. Macroscopic viscoelastic properties of gels and composites are investigated by means of oscillatory shear experiments in presence of external magnetic fields. Herewith, complex shear moduli are determined in dependence on the frequency and deformation. These quantities characterize the nonlinear viscoelastic properties of both, gels and composites.