Particle-crosslinked polymer composites and gels have recently been shown to possess novel or improved properties due to a covalent particle–matrix interaction. We employ spindle-like hematite particles as exclusive crosslinkers in poly(acrylamide) gels, and exploit their extraordinary magnetic properties for the realization of ferrohydrogels with a perpendicular orientation of the preferred magnetic and geometric axes of the particles. The angle-dependent magnetic properties of uniaxially oriented gels are investigated and interpreted with respect to particle–matrix interactions. The impact of the particle orientation on the resulting angle-dependent magnetic performance reveals the presence of two different contributions to the magnetization: a hysteretic component ascribed to immobilized particles, and a pseudo-superparamagnetic, non-hysteretic component due to residual particle mobility. Furthermore, a plastic reorientation of magnetic particles in the matrix when subjected to a transversal field component is observed.

While it is traditionally accepted that the chain interactions responsible for the elastic response in an elastomeric network are ideally permanent and instantaneously active, the ongoing investigation of self-healing materials reveals that the introduction of self-healing properties into elastomers requires high mechanical integrity under dynamic load conditions, while on long timescales (or at extended temperatures), the chain and bond dynamics must allow for an intrinsic repair of micro cracks occurring during operation and aging. Based on an acrylate-based amorphous ionomer model system with pendant carboxylate groups allowing the systematic variation of the composition and the nature of the counter ion, we demonstrate the interrelation between the morphological, thermal, and mechanical properties, and identify the prerequisites and tools for property adjustment and optimization of self-healing efficiency. While the ion fraction is directly related to the effective network density and elastic performance, the crossover frequency between viscous and elastic behavior is influenced by the nature of the counter ion. In order to achieve reliable elastic response and optimal damage repair, the ion fraction in these systems should be in the range of 5 mol% and the chain dynamics should be appropriate to allow for excellent self-healing behavior at moderate healing temperatures.

In this work, we correlate network dynamics, supramolecular reversibility and the macroscopic surface scratch healing behavior for a series of elastomeric ionomers based on an amorphous backbone with varying fractions of carboxylate pendant groups completely neutralized by Na+, Zn2+ or Co2+ as the counter ions. Our results based on temperature dependent dynamic rheology with simultaneous FTIR analysis clearly indicate that the effective supramolecular bond lifetime (τb) is an important parameter to ascertain the ideal range of viscoelasticity for good macroscopic healing. The reversible coordination increased with higher valence metal ions and ionic content. Both rheological and spectroscopic analyses show a decrease in supramolecular assembly with temperature. The temperature dependent τb was used to calculate the activation energy (Ea) of dissociation for the ionic clusters. According to self-healing experiments based on macroscale surface scratching, a supramolecular bond lifetime between 10 and 100 s results in samples with complete surface scratch healing and good mechanical robustness.

•We present a concept for on-demand self-healing elastomeric composites.•Magnetic nanoparticles serve as nano-heaters in a self-healing ionomer matrix.•The self-healing effect is activated by a selective induction process in a high-frequency alternating magnetic field.•Magnetic nanoparticles with different composition, size and shape are employed.•An accelerated effective recovery of the mechanical properties within short time of irradiation is observed after damage.Polymers that are crosslinked exclusively by dynamic bonds are recently shown to be of potential for autonomous or on-demand self-healing in flexible and elastic materials. In the case that the dynamic crosslinks are thermally reversible, a local heating process by inductive heating of magnetic nanoparticulate fillers considerably accelerates the healing process. We demonstrate the feasibility of this concept by introducing magnetic nanoparticles of different size, shape, and magnetocrystalline anisotropy into self-healing ionomeric elastomers. While the magnetically activated healing effect in these materials is quantified in tensile experiments, appropriate consideration is also given to the influence of the filler particles on the dynamic and mechanical properties of the nanocomposites.

•We investigate the superstructure formation of ferromagnetic cobalt nanoparticles.•An interplay of attractive/repulsive forces is observed by combined analytical methods.•Dominant dipolar forces are counteracted by a repulsive force in the presence of TOPO.•Potential profiles are in agreement with the experimental trends in chain formation.Control over the self-assembly of magnetic nanoparticles (MNP) into superstructures due to different types of coupling is of interest in the development of “bottom-up” fabrication schemes. Here we realize a simple strategy for the systematic variation of particle interaction potential in magnetic nanoparticles. This is achieved by varying the effective surface potential by means of a co-surfactant introduced in the course of the synthesis process. As a consequence, the ability to form chain-like assemblies is affected by the resulting balance of attractive and repulsive forces. We use electron microscopy, electron diffraction, and light scattering methods to study a series of cobalt nanoparticles as a characteristic example of ferromagnetic MNP. We demonstrate experimentally and substantiate theoretically that the observed behavior results from a balance between magnetic dipole–dipole, steric, and electrostatic interactions.

The incorporation of surface-functionalized spindle-like hematite nanoparticles as particulate cross-linkers in poly(acrylamide) hydrogel matrices delivers ferrogels with a covalent type of particle–matrix interaction. By systematic investigation of the stability and the internal architecture of the resulting gels, the regime of resilient gels is identified. The swelling properties and the rheological behavior are in accordance with a network structure based on particle nodes interlinked by long polymer segments and a significant fraction of loops.

Magnetic iron oxide, maghemite (Fe2O3) nanoparticles with covalent surface-bound CO-releasing molecules (CORMs) can be triggered to release CO through heating in an alternating magnetic field. In the proof-of-concept study the rate of CO-release from [RuCl(CO3)(μ-DOPA)]@maghemite nanoparticles was doubled upon exposure to an external alternating magnetic field (31.7 kAm−1, 247 kHz, 25 °C, 39.9 mTesla, DOPA = dioxyphenyl-alaninato).

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.

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.

Organic–inorganic nanocomposite materials combine the advantages of organic polymers and inorganic nanoparticles. Due to their high application potential, the investigation of these materials is of great interest. In this study we examine the dielectric response of CoFe2O4@PU nanocomposites subjected to an external electric field. Dielectric properties were investigated in a frequency range of 10–2 Hz to 1 MHz and over a temperature range of 213 to 333 K, while varying the content of ferromagnetic particles. The experimental results provide insight to the structure–property relationship of nanocomposites subjected to an alternating electric field. We discuss two relaxation processes (β- and α-processes) that show a dependence on the particle content. For the β-process we observe an Arrhenius-like behavior with temperature, and the related activation energies decrease with increasing particle content. Evidently, the particles' presence facilitates the β-process relaxation in the polymer matrix, resulting in higher dielectric losses. The α-process is attributed to a glass transition of the matrix, and the corresponding transition temperature Tg is compared to Tg obtained from differential scanning calorimetry (DSC).

Nanocomposites of magnetic nanoparticles and polymer matrices combine the properties of their components, and as such are good examples of functional nanomaterials with excellent application potential. Against this background, experimental and theoretical studies of such composites are of great interest. In this study we aim to provide insight into the static and dynamic magnetic response, as well as the dielectric response, of magnetic nanocomposites subjected to external magnetic and electric fields. We directly compare the behavior of polyurethane films doped with superparamagnetic Fe3O4, and blocked ferromagnetic CoFe2O4 nanoparticles. While a reversible, Langevin magnetization curve is observed for Fe3O4@PU films, hysteretic magnetic behavior is found in case of CoFe2O4@PU films. The hysteresis observed for CoFe2O4 nanoparticles can be explained by interactions at the interface between particles and polymer matrix in conjunction with its ferromagnetic nature. The results of dielectric spectroscopy experiments revealed different effects of Fe3O4 and CoFe2O4 nanoparticles on polymer dynamics.

Magnetic iron oxide, maghemite (Fe2O3) nanoparticles with covalent surface-bound CO-releasing molecules (CORMs) can be triggered to release CO through heating in an alternating magnetic field. In the proof-of-concept study the rate of CO-release from [RuCl(CO3)(μ-DOPA)]@maghemite nanoparticles was doubled upon exposure to an external alternating magnetic field (31.7 kAm−1, 247 kHz, 25 °C, 39.9 mTesla, DOPA = dioxyphenyl-alaninato).

Particle-crosslinked polymer composites and gels have recently been shown to possess novel or improved properties due to a covalent particle–matrix interaction. We employ spindle-like hematite particles as exclusive crosslinkers in poly(acrylamide) gels, and exploit their extraordinary magnetic properties for the realization of ferrohydrogels with a perpendicular orientation of the preferred magnetic and geometric axes of the particles. The angle-dependent magnetic properties of uniaxially oriented gels are investigated and interpreted with respect to particle–matrix interactions. The impact of the particle orientation on the resulting angle-dependent magnetic performance reveals the presence of two different contributions to the magnetization: a hysteretic component ascribed to immobilized particles, and a pseudo-superparamagnetic, non-hysteretic component due to residual particle mobility. Furthermore, a plastic reorientation of magnetic particles in the matrix when subjected to a transversal field component is observed.

In this work, we correlate network dynamics, supramolecular reversibility and the macroscopic surface scratch healing behavior for a series of elastomeric ionomers based on an amorphous backbone with varying fractions of carboxylate pendant groups completely neutralized by Na+, Zn2+ or Co2+ as the counter ions. Our results based on temperature dependent dynamic rheology with simultaneous FTIR analysis clearly indicate that the effective supramolecular bond lifetime (τb) is an important parameter to ascertain the ideal range of viscoelasticity for good macroscopic healing. The reversible coordination increased with higher valence metal ions and ionic content. Both rheological and spectroscopic analyses show a decrease in supramolecular assembly with temperature. The temperature dependent τb was used to calculate the activation energy (Ea) of dissociation for the ionic clusters. According to self-healing experiments based on macroscale surface scratching, a supramolecular bond lifetime between 10 and 100 s results in samples with complete surface scratch healing and good mechanical robustness.

While it is traditionally accepted that the chain interactions responsible for the elastic response in an elastomeric network are ideally permanent and instantaneously active, the ongoing investigation of self-healing materials reveals that the introduction of self-healing properties into elastomers requires high mechanical integrity under dynamic load conditions, while on long timescales (or at extended temperatures), the chain and bond dynamics must allow for an intrinsic repair of micro cracks occurring during operation and aging. Based on an acrylate-based amorphous ionomer model system with pendant carboxylate groups allowing the systematic variation of the composition and the nature of the counter ion, we demonstrate the interrelation between the morphological, thermal, and mechanical properties, and identify the prerequisites and tools for property adjustment and optimization of self-healing efficiency. While the ion fraction is directly related to the effective network density and elastic performance, the crossover frequency between viscous and elastic behavior is influenced by the nature of the counter ion. In order to achieve reliable elastic response and optimal damage repair, the ion fraction in these systems should be in the range of 5 mol% and the chain dynamics should be appropriate to allow for excellent self-healing behavior at moderate healing 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.