Magnetic response of CoFe2O4 nanoparticles confined in a PNIPAM microgel network.

The paper addresses coupling of magnetic nanoparticles (MNPs) with the polymer matrix of temperature-sensitive microgels and their response to magnetic fields. Therefore, CoFe2O4@CA (CA = citric acid) NPs are embedded within N-isopropylacrylamid (NIPAM) based microgels. The volume phase transition (VPT) of the magnetic microgels and the respective pure microgels is studied by dynamic light scattering and electrophoretic mobility measurements. The interaction between MNPs and microgel network is studied via magnetometry and AC-susceptometry using a superconducting quantum interference device (SQUID). The data show a significant change of the magnetic properties by crossing the VPT temperature (VPTT). The change is related to the increased confinement of the MNP due to the shrinking of the microgels. Modifying the microgel with hydrophobic allyl mercaptan (AM) affects the swelling ability and the magnetic response, i.e. the coupling of MNPs with the polymer matrix. Modeling the AC-susceptibility data results in an effective size distribution. This distribution represents the varying degree of constraint in MNP rotation and motion by the microgel network. These findings help to understand the interaction between MNPs and the microgel matrix to design multi responsive systems with tunable particle matrix coupling strength for future applications.

Goethite Nanorods: Synthesis and Investigation of the Size Effect on Their Orientation within a Magnetic Field by SAXS.

Goethite is a naturally anisotropic, antiferromagnetic iron oxide. Following its atomic structure, crystals grow into a fine needle shape that has interesting properties in a magnetic field. The needles align parallel to weak magnetic fields and perpendicular when subjected to high fields. We synthesized goethite nanorods with lengths between 200 nm and 650 nm in a two-step process. In a first step we synthesized precursor particles made of akaganeite (ß-FeOOH) rods from iron(III)chloride. The precursors were then treated in a hydrothermal reactor under alkaline conditions with NaOH and polyvinylpyrrolidone (PVP) to form goethite needles. The aspect ratio was tunable between 8 and 15, based on the conditions during hydrothermal treatment. The orientation of these particles in a magnetic field was investigated by small angle X-ray scattering (SAXS). We observed that the field strength required to trigger a reorientation is dependent on the length and aspect ratio of the particles and could be shifted from 85 mT for the small particles to about 147 mT for the large particles. These particles could provide highly interesting magnetic properties to nanocomposites, that could then be used for sensing applications or membranes.

Distribution of CoFe2O4 Nanoparticles Inside PNIPAM-Based Microgels of Different Cross-linker Distributions.

The aim of this study is to tailor the inner structure of positively charged poly-( N-isopropylacrylamid- co-allylamine) (P(NIPAM- co-AA)) microgels for a better control of the distribution of negatively charged magnetic cobaltferrite (CoFe2O4@CA) nanoparticles (MNPs) within the microgels. Therefore, two different strategies are followed for the microgel synthesis: the (one pot) batch method which leads to a higher cross-linker density in the microgel core and the feeding method which compensates different reaction kinetics of the cross-linker and the monomers. The latter one is expected to result in a homogeneous cross-linker distribution. Information about the cross-linker distribution is indirectly gained by measuring the elastic modulus via indentation experiments with an atomic force microscope. While the batch method results in a higher elastic modulus in the center of the microgel indicating a core/shell structure, the feeding method leads to a constant elastic modulus over the whole microgel. The loading with MNPs and their distribution are studied with transmission electron microscopy (TEM). The TEM images show a large difference in the MNP distribution which is correlated to the cross-linker distribution of both types of microgels. The batch method microgel has a low MNP concentration in the core. The feeding method microgel shows a much more homogeneous distribution of MNPs across the microgel. The latter one also shows a stronger charge reversal which is a hint for a higher loading of the feeding method microgel. Dynamic light scattering and electrophoretic mobility measurements demonstrate that for both types of microgels, the temperature sensitivity is preserved after loading with MNPs.

Tuning the Size of Thermoresponsive Poly(N-Isopropyl Acrylamide) Grafted Silica Microgels.

Core-shell microgels were synthesized via a free radical emulsion polymerization of thermoresponsive poly-(N-isopropyl acrylamide), pNipam, on the surface of silica nanoparticles. Pure pNipam microgels have a lower critical solution temperature (LCST) of about 32 °C. The LCST varies slightly with the crosslinker density used to stabilize the gel network. Including a silica core enhances the mechanical robustness. Here we show that by varying the concentration gradient of the crosslinker, the thermoresponsive behaviour of the core-shell microgels can be tuned. Three different temperature scenarios have been detected. First, the usual behaviour with a decrease in microgel size with increasing temperature exhibiting an LCST; second, an increase in microgel size with increasing temperature that resembles an upper critical solution temperature (UCST), and; third, a decrease with a subsequent increase of size reminiscent of the presence of both an LCST, and a UCST. However, since the chemical structure has not been changed, the LCST should only change slightly. Therefore we demonstrate how to tune the particle size independently of the LCST.

Cambios en la organización del trabajo: dirección orientada a los resultados y sus implicaciones para los empleados / Changes in the organisation of work: output-oriented control and the implications for employees

Objetivos: Este estudio indaga cómo las condiciones de trabajo están relacionadas con problemas psicosociales de los empleados en sistemas con dirección orientada a los resultados. Método: 134 trabajadores de un banco alemán respondieron un cuestionario sobre las condiciones de trabajo (exigencias, estresores y recursos), las características de los objetivos y la escala Irritación (Mohr et al. 2005). En base a esta última se formaron dos grupos, uno de trabajadores con “Irritación baja” y otro con “Irritación alta”. Los grupos fueron comparados en cuanto a las condiciones de trabajo y las características de los objetivos percibidas. Resultados: los resultados indican que los empleados en sistemas de trabajo con dirección orientada a los resultados y con altos coeficientes de Irritación presentan un nivel significativamente mayor de estresores, menor nivel de recursos, así como características de los objetivos más desfavorables. A la vez, y contrariamente a nuestras expectativas, este grupo de empleados también reporta mayores exigencias que el grupo con Irritación baja. Conclusiones: los resultados indican que las condiciones de trabajo del grupo de trabajadores con Irritación alta son más desfavorables que aquellas del grupo con Irritación baja. Adicionalmente, los resultados obtenidos apuntan a que el hecho de que la dirección del sistema de trabajo esté orientada a los resultados no implica necesariamente una deteriorización de las condiciones de trabajo. Estas pasan, más bien a ser favorables o no para los trabajadores y su bienestar psicosocial dependiendo del diseño conjunto de las condiciones de trabajo: especialmente las características de los objetivos a alcanzar parecen relevantes.

Objective: The study examines which patterns of working conditions in output-controlled working systems are associated with high and with low well-being of employees. Method: To test this, “low Irritation” and “high Irritation” groups (Mohr et al. 2005) of output-controlled employees were compared in regard to different aspects of their working situation. Data from 134 salesmen were obtained by questionnaire. The questionnaire asked about the perceived working conditions (requirements, stressors, resources, and goal quality) and employees’ Irritation. Results: As expected, the results show that employees in output controlled work systems high Irritation, report significantly higher stressors, mostly lower resources and lower goal quality. At the same time and contrary to our expectations this group reports higher requirements. Conclusions: In general, it could be shown, that the working conditions of employees with “high Irritation” are more unfavourable than those of the “low Irritation” group. Further, the results show that output controlled working systems do not necessarily implicate a deterioration of the working situation of employees. Whether the output-oriented control of work processes turns out to be favourable or unfavourable for the psychosocial well-being of the employees depends on the whole constellation of working conditions. Thereby, the quality of goals which employees have to achieve might be of particular importance.