Using well-established measurement techniques like transmission electron microscopy (TEM), dynamic light scattering (DLS), small and wide angle X-ray scattering (SAXS, WAXS), susceptometry, and magnetorelaxometry, the distribution of the physical and magnetic size (magnetic moments) and magnetic anisotropy of a variety of structurally different magnetic nanoparticle samples (MNPs) is analyzed and compared. A term which accounts for the presence of weak magnetic areas (WMAs) within the MNPs was introduced to the widespread analysis model for M(H) data, enabling a consistent interpretation of the data in most of the systems. A comparison of the size distributions as obtained for the physical and the magnetic diameter suggests a multidomain structure for three single core systems under investigation, in all probability evoked by the presence of a wustite phase, as identified by WAXS.Analyzing the relationship d < dm < dc between the average single core diameter d, the effective magnetic (domain) size dm and the cluster diameter dc quantitatively, two qualitatively different magnetic structures in multicore MNP (MCMNP) systems were identified: (i) The magnetic moments of single cores within the MCMNP of fluidMAG tend to build flux closure structures, driven by dipole-dipole interaction. (ii) The magnetic behavior of Resovist & REG; was attributed to the presence of domain sizes of about 12 nm within MCMNP, exceeding the single core diameters of 5 nm. Thereby, WAXS revealed a bimodal crystallite size distribution suggesting a crystallite merging process within the MCMNP. The value of the effective magnetic moment of these MCMNP could be explained within the presented "random moment cluster model" (RMCM).We conclude that the combination of physical and magnetic structure parameters obtained from complementary measurement methods allows a reliable assessment of the magnetic structure of single and multicore MNPs.

By merging soft, hydrogel-based matrices with nanoscopic inorganic nanoparticles to organic-inorganic hybrid materials, novel properties can arise from the unique interplay of the components 'properties. The introduction of magnetic nanoparticles of different size and shape into hydrophilic polymer network architectures leads to nano- or macroscopic hybrid gel structures that respond to magnetic fields in a predetermined way. A variety of complex gel structures are designed that allow a mutual interaction of their mechanical and thermal properties. In this review, we highlight recent accomplishments and trends in the field of magnetically active hybrid hydrogels, and conclude with an outline on future prospects in the design and application of magnetic soft matter with particle-matrix interaction.

In this study we investigated the mechanical properties of composite hydrogels based on a polyacrylamide (PAAm) matrix with embedded temperature sensitive poly(N-isopropylacrylamide) (PNiPAM) microgels. We analysed the mechanical properties of the composite material with tensile tests, shear and cavitation rheology. The results of the different experiments displayed an enhancement of mechanical stability with increasing concentration of incorporated microgels. The improved stability is related to an increase of physical cross-linking points due to the incorporation of the microgels. The incorporation of temperature responsive microgel particles introduces temperature sensitive mechanical behaviour of the composite hydrogels. The collapse of the microgels inside the polyacrylamide matrix leads to a change of the volume of the filler particles as well as to a change from a soft filler to a hard filler. The influence of the hard particles on the mechanical stability of the matrix is much stronger which leads to materials with enhanced mechanical properties at high temperatures.

Magnetically blocked CoFe2O4 nanoparticles are used as nanoscopic mechanical probes in PAAm solutions and gels with different architectures. From quasi-static magnetometry, we extract information on the mechanical feedback from the local restoring forces on the rotational remagnetization of the probes. While the hysteretic magnetic behavior of conventionally crosslinked PAAm ferrohydrogels can be explained by the predominantly viscous processes involved with particle rotation, a reversible, Langevin-like magnetization curve is found in particle-linked ferrohydrogels. From the local restoring force, we conclude on the elastic moduli of the underlying process, in accordance with expectations from statistical thermodynamics.

Ferrohydrogels are synthesized by incorporation of magnetic CoFe2O4 nanoparticles into a polyacylamide hydrogel network during the polymerization process by utilizing different cross-linking units. Conventional cross-linked ferrohydrogels, using a molecular cross-linker, are compared to those obtained by our new approach where the magnetic particles, surface-functionalized with methacylic groups, serve as sole, multifunctional cross-linkers. Both experimental series are analyzed with regard to their swelling behavior. The novel composite network is examined with respect to the cross-linkage, the network homogeneity, and the network architecture by various experimental techniques.

Significant advances in the field of responsive hydrogels have been achieved by the combination of soft, gel-based matrices with the unique functions of inorganic or biological nanostructures. Like in many biological tissues, the components of such hybrid materials often have converse, yet complementary properties. The possibility of forming self-assembled and supramolecular morphologies from organic polymers in combination with inorganic nanoparticles or biological motifs is of interest for gels with new response properties. A variety of complex gel structures with unique chemical, physical, and biological properties have been engineered or discovered at the nanoscale. In this review, we highlight recent accomplishments and trends in the field of hybrid polymer hydrogels with a focus on approaches towards soft, yet tough shape-changing and actuating materials. We conclude with an outline on future directions and challenges that have to be faced in the design and application of hybrid hydrogels.

We report on a new approach towards magneto-responsive hydrogels, showing a reversible gelation upon inductive heating via AC magnetic fields. An aqueous solution of a triblock terpolymer with a partially quaternized poly(2-vinylpyridine) (Pq2VP) outer block, a water soluble poly(ethylene oxide) (PEO) middle block and a thermo-sensitive poly(glycidyl methyl ether-co-ethyl glycidyl ether) (P(GME-co-EGE)) end block (Pq2VP-b-PEO-b-P(GME-co-EGE)) was mixed with a solution of sodium citrate stabilized superparamagnetic maghemite nanoparticles. Due to electrostatic interactions between the oppositely charged particles, surface and the Pq2VP block of the triblock terpolymer, well-defined hybrid micelles with a superparamagnetic core were formed. The number of triblock terpolymer chains stabilizing the nanoparticles was found to be constant above a critical triblock terpolymer/maghemite ratio, i.e. for higher polymer contents hybrid micelles and free (non-bound) triblock terpolymers are present in solution. Thermo-reversible hydrogels are formed via an open association of hybrid micelles at temperatures above the cloud point of the P(GME-co-EGE) corona blocks, which form the network junctions. The superparamagnetic character of the maghemite nanoparticles enables contactless heating, and thus gelation, by applying AC magnetic fields, as demonstrated by high frequency magnetocalorimetry. The thermo-reversible gelation and the dynamic-mechanical properties of the hydrogels were studied by rheology.

A general and versatile method for the functionalization and subsequent modification of single dispersed magnetite nanoparticles by direct reaction of alkoxysilanes on the particle surface is reported. Our data supports the formation of a dense monolayer that is controlled by sterical needs. By selecting the functionality of alkoxysilanes, the surface properties of the particles can be tailored. Depending on the surface functionality, the modified particles can be used as macrocomonomers or macrocrosslinkers, or as macroinitiators for surface-initiated polymerization and thus enable the covalent attachment of polymers.

Magnetocalorimetry is a new method to determine transition temperatures and the corresponding enthalpies in enthalpic phase transitions by an intrinsic heating process. We present first results on the magnetically induced melting process of a system of magnetically activated iron oxide nanoparticles embedded in an ice matrix in order to investigate the mechanisms involved in heat generation and heat transfer at the nanometer scale. By high frequency (HF) irradiation, magnetic field energy is transferred to heat locally in the particles, and the endothermic melting process of the ice matrix can be followed by recording temperature development throughout the matrix. Significant differences can be found when the results are compared to extrinsic melting processes in the absences of a field both at ambient temperature and in differential scanning calorimetry (DSC) experiments. By recording the temperature development throughout the ice matrix, first insights on the heat transfer process in magnetic heating are obtained.