The development of polymers with switchable glass transition temperatures (T) can address scientific challenges such as the healing of cracks in high-T polymers and the processing of hard polymers at room temperature without using plasticizing solvents. Here, we demonstrate that light can switch the T of azobenzene-containing polymers (azopolymers) and induce reversible solid-to-liquid transitions of the polymers. The azobenzene groups in the polymers exhibit reversible cis-trans photoisomerization abilities. Trans azopolymers are solids with T above room temperature, whereas cis azopolymers are liquids with T below room temperature. Because of the photoinduced solid-to-liquid transitions of these polymers, light can reduce the surface roughness of azopolymer films by almost 600%, repeatedly heal cracks in azopolymers, and control the adhesion of azopolymers for transfer printing. The photoswitching of T provides a new strategy for designing healable polymers with high T and allows for control over the mechanical properties of polymers with high spatiotemporal resolution.

Oily wastewater is an extensive source of pollution to soil and water, and its harmless treatment is of great importance for the protection of our aquatic ecosystems. Membrane filtration is highly desirable for removing oil from oily water because it has the advantages of energy efficiency, easy processing and low maintenance cost. However, membrane fouling during filtration leads to severe flux decline and impedes long-term operation of membranes in practical wastewater treatment. Membrane fouling includes reversible fouling and irreversible fouling. The fouling mechanisms have been explored based on classical fouling models, and on oil droplet behaviors (such as droplet deposition, accumulation, coalescence and wetting) on the membranes. Membrane fouling is dominated by droplet-membrane interaction, which is influenced by the properties of the membrane (e.g., surface chemistry, structure and charge) and the wastewater (e.g., compositions and concentrations) as well as the operation conditions. Typical membrane antifouling strategies, such as surface hydrophilization, zwitterionic polymer coating, photocatalytic decomposition and electrically enhanced antifouling are reviewed, and their cons and pros for practical applications are discussed.

We study the magneto-elastic coupling behavior of paramagnetic chains in soft polymer gels exposed to external magnetic fields. To this end, a laser scanning confocal microscope is used to observe the morphology of the paramagnetic chains together with the deformation field of the surrounding gel network. The paramagnetic chains in soft polymer gels show rich morphological shape changes under oblique magnetic fields, in particular a pronounced buckling deformation. The details of the resulting morphological shapes depend on the length of the chain, the strength of the external magnetic field, and the modulus of the gel. Based on the observation that the magnetic chains are strongly coupled to the surrounding polymer network, a simplified model is developed to describe their buckling behavior. A coarse-grained molecular dynamics simulation model featuring an increased matrix stiffness on the surfaces of the particles leads to morphologies in agreement with the experimentally observed buckling effects.

To describe many-particle systems suspended in incompressible low-Reynolds-number fluids, effective hydrodynamic interactions can be introduced. Here, we consider particles embedded in elastic media. The effective elastic interactions between spherical particles are calculated analytically, inspired by the approach in the fluid case. Our experiments on interacting magnetic particles confirm the theory. In view of the huge success of the method in hydrodynamics, we similarly expect many future applications in the elastic case, e.g. for elastic composite materials. planes, vehicles, ships, and propellers [4]. All these processes are described by the Navier-Stokes equations [5] or variants thereof. This set of equations typically poses significant challenges during solution due to a convective nonlinearity reflecting inertial effects. Basically, turbulence is driven by the inertial term. It often renders analytical solutions impossible.The situation changes for small dimensions and velocities or high viscosity. Then, the relative strength of inertial effects, measured by the Reynolds number, is low. The nonlinearity can be neglected. A Green's function in terms of the so-called Oseen matrix is then available, which formally solves the problem analytically [6,7]. In this way, semi-dilute colloidal suspensions, i.e. the dispersion of nano-to micrometer-sized particles in a fluid [7,8], or microswimmer suspensions [9] are described effectively. The explicit role of the fluid is eliminated and replaced by effective hydrodynamic interactions between the suspended particles [6,7].Despite the success of this theoretical approach for colloidal suspensions, hardly any investigations consider a surrounding elastic solid instead of a suspending fluid. This is surprising, since, as we show below, the formalism can be adapted straightforwardly to linearly elastic matrices and is confirmed by our experiments. Our approach will, for instance, facilitate describing the response of elastic composite materials to external stimuli. Such materials consist of more or less rigid inclusions embedded in an elastic matrix. They are of growing technological interest and may serve, e.g., as soft actuators or sound attenuation devices [10].In previous theoretical studies, the physics of one single rigid or deformable inclusion was addressed [11,12], also under acoustic irradiation [13]. For more than a single inclusion, mainly the so-called load problem was analyzed theoretically for a pair of rigid inclusions: one prescribes displacements of two rigid inclusions in an elastic matrix, and then determines the forces necessary to achieve these given displacements [14].Here, we take the converse point of view, based on the cause-and-effect chain in our experiments: external forces are imposed onto the inclusions, or mutual forces between the inclusions are induced, for example to actuate the material or to tune its properties. In response to the forces, the inclusions are displaced. Since they cannot penetrate through the surrounding elastic ma...

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Magnetorheological elastomers are a type of smart hybrid material where elastic properties of a soft elastomer matrix are combined with magnetic properties of magnetic micro particles. This combination leads to a complex interplay of magnetic and elastic phenomena, of which the magnetorheological effect is the best described. In this paper, magnetically hard NdFeB-particles were used to obtain remanent magnetic properties. X-ray microtomography has been utilised to analyse the particle movement induced by magnetic fields. A particle tracking was performed; thus, it was possible to characterise the movement of individual particles. Beyond that, a comprehensive analysis of the orientation of all particles was performed at different states of magnetisation and global particle arrangements. For the first time, this method was successfully applied to a magnetorheological material with a technically relevant amount of magnetic NdFeB-particles. A significant impact of the magnetic field on the rotation and translation of the particles was shown.

Reversible assembly of Janus particles was manipulated by host-guest interaction of β-cyclodextrin (β-CD) and azobenzene. One side of every Janus particle was modified with β-CD. Superstructures of Janus particles were formed by adding azobenzene-containing polymers to the dispersion of Janus particles. The superstructures were reversibly disassembled by adding α-CD or light irradiation.

Photoisomerization in some azobenzene‐containing polymers (azopolymers) results in reversible solid‐to‐liquid transitions because trans‐ and cis‐azopolymers have different glass transition temperatures. This property enables photoinduced healing and processing of azopolymers with high spatiotemporal resolution. However, a general lack of knowledge about the influence of the polymer structure on photoinduced reversible solid‐to‐liquid transitions hinders the design of such novel polymers. Herein, the synthesis and photoresponsive behavior of new azopolymers with different lengths of spacers between the polymer backbone and the azobenzene group on the side chain are reported. Azopolymers with no and 20 methylene spacers did not show photoinduced solid‐to‐liquid transitions. Azopolymers with 6 or 12 methylene spacers showed photoinduced solid‐to‐liquid transitions. This study demonstrates that spacers are essential for azopolymers with photoinduced reversible solid‐to‐liquid transitions, and thus, gives an insight into how to design azopolymers for photoinduced healing and processing.