Magneto-Sensitive Elastomers: An Experimental Point of View

Magneto-sensitive elastomers (MSEs) are smart materials changing their shape and mechanical properties in the presence of a magnetic field. Focussing on model systems, silicone based MSEs are prepared by a multi-step mixing process and characterised using a rotational rheometer (plate-plate). Data obtained by relaxation tests is used to set-up a material model coupling the theories of viscoelasticity and magnetoelasticity. The behaviour of MSEs in quasi-static and dynamic mechanical shear experiments can be successfully predicted by the analytical model using parameters received by fitting the transient experiments. The model is validated for small shear deformations (γ = 0.02) and low magnetic fields (𝔹 = 0.2 T). (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Implementing a discontinuous Galerkin method for the compressible,inviscid Euler equations in the DUNE framework

A discontinuous Galerkin scheme was implemented in the DUNE framework to solve the compressible, inviscid Euler equations in a multi-dimensional Cartesian grid. It uses a HLLC Riemann solver for the numerical fluxes in the interfaces, a total variation bounded limiter to handle discontinuities, a positivity preserving limiter for near vacuum conditions, and adaptive mesh refinement (AMR). (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Neutral Pentacoordinate Silicon(IV) Complexes with a Tridentate Dianionic O,N,O or N,N,O Ligand,an Anionic PhX Ligand (X = O,S, Se), and a Phenyl Group: Synthesis and Structural Characterization in the Solid State and in Solution

A series of neutral pentacoordinate silicon(IV) complexes with a SiO₃NC, SiO₂SNC, SiO₂SeNC, SiO₂N₂C, SiOSN₂C, or SiOSeN₂C skeleton was synthesized and structurally characterized by multinuclear NMR spectroscopy in the solid state and in solution and by single‐crystal X‐ray diffraction. The compounds studied contain a tridentate dianionic O,N,O or N,N,O ligand, an anionic PhX ligand (X = O, S, Se), and a phenyl group. The structures, NMR spectroscopic parameters, and chemical properties of these silicon(IV) complexes were compared with those of related compounds that contain a tridentate dianionic S,N,O ligand instead of the O,N,O or N,N,O ligand.     

F-(H2O)+CH3I Ligand Exchange Reaction Dynamics

Single hydration of the gas phase F\begin{document}$^-$\end{document}+CH\begin{document}$_3$\end{document}I\begin{document}$\rightarrow$\end{document} I\begin{document}$^-$\end{document}+CH\begin{document}$_3$\end{document}F reaction allows to probe solvent effects on a fundamental nucleophilic substitution reaction. At the same time, the addition of a solvent molecule opens alternative product channels. Here, we present crossed beam imaging results on the dynamics of the F\begin{document}$^-$\end{document}(H\begin{document}$_2$\end{document}O)+CH\begin{document}$_3$\end{document}I\begin{document}$\rightarrow$\end{document}[FCH\begin{document}$_3$\end{document}I]\begin{document}$^-$\end{document}+H\begin{document}$_2$\end{document}O ligand exchange pathway at collision energies between 0.3 and 2.6 eV. Product kinetic energies are constrained by the stability requirement of the weakly bound product complexes. This implies substantial internal excitation of the water molecule and disfavors efficient energy redistribution in an intermediate complex, which is reflected by the suppression of low kinetic energies as collision energy increases. At 0.3 eV, internal nucleophilic displacement is important and is discussed in light of the competing nucleophilic substitution pathways that form I\begin{document}$^-$\end{document} and I\begin{document}$^-$\end{document}(H\begin{document}$_2$\end{document}O).     

Highly phosphonated polypentafluorostyrene: Characterization and blends with polybenzimidazole

In this study we present results of the conductivity and resistance to thermooxidative and condensation reactions of a highly phosphonated poly(pentafluorostyrene) (PWN2010) and of its blends with poly(benzimidazole)s (PBI). This polymer, which combines both: (i) a high degree of phosphonation (above 90%) and (ii) a relatively high acidity (pK_a (–PO₃H₂ ↔ –PO₃H⁻) ∼ 0.5) due to the fluorine neighbors, is designed for low humidity operating fuel cell. This was confirmed by the conductivity measurements for PWN2010 reaching σ = 5 × 10⁻⁴ S cm⁻¹ at 150 °C in dry N₂ and σ = 1 × 10⁻³ S cm⁻¹ at 150 °C (λ = 0.75). Furthermore, this polymer showed only 48% of anhydride formation when annealing it at T = 250 °C for 5 h and only 2% weight loss during a 96 h Fenton test. These properties combined with the ability of the PWN2010 to form homogeneous blends with polybenzimidazoles resulting in stable and flexible polymer films, makes PWN2010 a very promising candidate as a polymer electrolyte for intermediate- and high-temperature fuel cell applications.     

Femtosecond dynamics of isolated phenylcarbenes

Understanding the primary photophysical processes in molecules is essential for interpreting their photochemistry, because molecules rarely react from the initially excited electronic state. In this study the ultrafast excited-state dynamics of chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), two species that are considered as models for carbene dynamics, were investigated by femtosecond time-resolved pump probe spectroscopy in the gas phase. Their dynamics was followed in real time by time-resolved photoionization and photoelectron imaging. CPC was excited at 265 nm into the 3 1A' state, corresponding to excitation from a pi-orbital of the aromatic ring into the LUMO. The LUMO contains a contribution of the p-orbital at the carbene center. Three time constants are apparent in the photoelectron images: A fast decay process with tau1 approximately 40 fs, a second time constant of tau2 approximatley 350 fs, and an additional time constant of tau3 approximately 1 ps. The third time constant is only visible in the time-dependence of low kinetic energy electrons. Due to the dense manifold of excited states between 3.9 and 5 eV, known from ab initio calculations, the recorded time-resolved electron images show broad and unstructured bands. A clear population transfer between the states thus can not directly be observed. The fast deactivation process is linked to either a population transfer between the strongly coupled excited states between 3.9 and 5.0 eV or the movement of the produced wave packet out of the Franck-Condon region. Since the third long time constant is only visible for photoelectrons at low kinetic energy, evidence is given that this time constant corresponds to the lifetime of the lowest excited A 1A' state. The remaining time constant reflects a deactivation of the manifold of states in the range 3.9-5.0 eV down to the A 1A' state.     

Emulsions stabilized by stimuli-sensitive poly(N-isopropylacrylamide)-co-methacrylic acid polymers: microgels versus low molecular weight polymers

Responsive polymer microgels can be employed for the preparation of stimuli-sensitive emulsions. The microgels used in this study are based on cross-linked copolymers including N-isopropylacrylamide and methacrylic acid. We conducted the synthesis under acidic and basic conditions to investigate the effect of changes of comonomer solubility on the microgel's composition and ability to stabilize emulsions. The synthesis product was partially divided into two fractions by centrifugation. Raw product, collected supernatant, and purified microgel were characterized by means of light scattering, titration, as well as electrophoretic mobility. The ability of the three components to act as stabilizers was investigated by preparing the octanol/water emulsions and looking at their response to pH and temperature changes. The interfacial activity of the three components was characterized by means of the pendent drop technique. Furthermore, we investigated the response of the interface to dilatational stress using a pendant drop tensiometer equipped with an oscillating drop module. The results demonstrate that the pH during synthesis has a significant impact on the composition and thus the properties of the microgel and its ability to be utilized as a stimuli responsive stabilizer for emulsions. We conclude that microgels can be used as stimuli-sensitive stabilizers for emulsions, if the charges are incorporated in the microgel itself.     

Microgels as stimuli-responsive stabilizers for emulsions

Temperature- and pH-sensitive microgels from cross-linked poly(N-isopropylacrylamide)-co-methacrylic acid are utilized for emulsion stabilization. The pH- and temperature-dependent stability of the prepared emulsion was characterized. Stable emulsions are obtained at high pH and room temperature. Emulsions with polar oils, like 1-octanol, can be broken by either addition of acid or an increase of temperature, whereas emulsions with unpolar oils do not break upon these stimuli. However, complete phase separation, independent of oil polarity, can be achieved by successive acid addition and heating. This procedure also offers a way to recover and recycle the microgel from the sample. Interfacial dilatational rheology data correlate with the stimuli sensitivity of the emulsion, and a strong dependence of the interfacial elastic and loss moduli on pH and temperature was found. The influence of the preparation method on the type of emulsion is demonstrated. The mean droplet size of the emulsions is characterized by means of flow particle image analysis. The type of emulsion [water in oil (w/o) or oil in water (o/w)] depends on the preparation technique as well as on the microgel content. Emulsification with high shear rates allows preparation of both w/o and o/w emulsions, whereas with low shear rates o/w emulsions are the preferred type. The emulsions are stable at high pH and low temperature, but instable at low pH and high temperature. Therefore, we conclude that poly(N-isopropylacrylamide)-co-methacrylic acid microgels can be used as stimuli-sensitive stabilizers for emulsions. This offers a new and unique way to control emulsion stability.