Structure and dynamic behaviour of anisotropic magnetic particles in complex matrices

该项目的目的是研究由各向异性、磁性纳米颗粒和热敏性聚电解质水凝胶组成的复合材料的中尺度结构和动力学。这些基质是由NIPAM和离子或离子性共聚单体共聚合制备的。聚合物的体积分数、交联密度和电荷密度对它们的粘弹性性能有影响，而交联密度和电荷密度随共聚物摩尔分数的变化而变化。在离子性共聚单体的情况下，电荷密度还取决于共聚单体的酸性或碱性基团的pH依赖性水解。此外，基于离子强度的德拜长度也会影响聚合物网络中相等电荷之间的排斥相互作用。可制备长宽比可广泛调节的纺锤形赤铁矿颗粒，其多分散性相当小。这些在水悬浮液中由负表面电荷稳定的粒子在垂直于场方向的中等外部磁场存在下排列。因此，在外场中，这些粒子的旋转迁移率受到限制，而由各向异性摩擦系数引起的平移迁移率则与方向有关。由于颗粒-基质相互作用，它们的旋转和动动率的场诱导变化影响了颗粒-水凝胶复合材料的粘弹性。由于带负电的赤铁矿颗粒和带正电的聚电解质凝胶之间的静电相互作用，这些体系中异常强的颗粒-基质耦合导致了它们的粘弹性性能发生了显著的场致变化。该项目的重点是这些复合材料的中尺度结构和动力学的场致变化与其宏观、粘弹性特性的相关变化之间的相关性。由于赤铁矿颗粒的电子密度明显大于聚合物基体，通过静态和准弹性x射线散射实验（SAXS， XPCS）可以选择性地研究嵌入赤铁矿颗粒的结构和动力学。利用时间分辨、流变学和x射线散射耦合实验（RheoSAXS）研究了各向异性赤铁矿颗粒在剪切作用下的旋转运动。利用光散射实验或中子对水凝胶基质的结构和动力学进行了研究。采用振荡剪切实验研究了外加磁场作用下凝胶和复合材料的宏观粘弹性。据此，确定了与频率和变形有关的复合剪切模量。这些量表征了凝胶和复合材料的非线性粘弹性特性。

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.