Theoretical investigation of the interaction of a polymer film with a nanoparticle

The fundamental aim in our investigation of the interaction of a polymer film with a nanoparticle is the extraction of information on the dynamics of the liquid using a single tracking particle. In this work two theoretical methods were used: one passive, where the motion of the particle measures the dynamics of the liquid, one active, where perturbations in the system are introduced through the particle. In the first part of this investigation a thin polymeric film on a substrate is studied using molecular dynamics simulations. The polymer is modeled via a 'bead spring' model. The particle is spheric and non structured and is able to interact with the monomers via a Lennard Jones potential. The system is micro-canonical and simulations were performed for average temperatures between the glass transition temperature of the film and its dewetting temperature. It is shown that the stability of the nanoparticle on the polymer film in the absence of gravity depends strongly on the form of the chosen interaction potential between nanoparticle and polymer. The relative position of the tracking particle to the liquid vapor interface of the polymer film shows the glass transition of the latter. The velocity correlation function and the mean square displacement of the particle has shown that it is caged when the temperature is close to the glass transition temperature. The analysis of the dynamics at long times shows the coupling of the nanoparticle to the center of mass of the polymer chains. The use of the Stokes-Einstein formula, which relates the diffusion coefficient to the viscosity, permits to use the nanoparticle as a probe for the determination of the bulk viscosity of the melt, the so called 'microrheology'. It is shown that for low frequencies the result obtained using microrheology coincides with the results of the Rouse model applied to the polymer dynamics. In the second part of this investigation the equations of Linear Hydrodynamics are solved for a nanoparticle oscillating above the film. It is shown that compressible liquids have mechanical response to external perturbations induced with the nanoparticle. These solutions show strong velocity and pressure profiles of the liquid near the interface, as well as a mechanical response of the liquid-vapor interface. The results obtained with this calculations can be employed for the interpretation of experimental results of non contact AFM microscopy

Spectroscopic investigations of functional bulk and thin film Heusler alloys

Functional and smart materials have gained large scientific and practical interest in current research and development. The Heusler alloys form an important class of functional materials used in spintronics, thermoelectrics, and for shape memory alloy applications. An important aspect of functional materials is the adaptability of their physical properties. In this work functional polycrystalline bulk and epitaxial thin film Heusler alloys are characterized by means of spectroscopic investigation methods, X-ray magnetic circular dichroism (XMCD) and energy dispersive X-ray analysis (EDX). With EDX the homogeneity of the samples is studied extensively. For some cases of quaternary compounds, for example Co2(MnxTi1−x)Sn and Co2(Mn0.5Dy0.5)Sn, an interesting phase separation in two nearly pure ternary Heusler phases occurs. For these samples the phase separation leads to an improvement of thermoelectric properties. XMCD as the main investigation method was used to study Co2TiZ (Z = Si, Sn, and Sb), Co2(MnxTi1−x)Si, Co2(MnxTi1−x)Ge, Co2Mn(Ga1−xGex), Co2FeAl, Mn2VAl, and Ni2MnGa Heusler compounds. The element-specific magnetic moments are calculated. Also, the spin-resolved unoccupied density of states is determined, for example giving hints for half-metallic ferromagnetism for some Co-based compounds. The systematic change of the magnetic moments and the shift of the Fermi energy is a proof that Heusler alloys are suitable for a controlled tailoring of physical properties. The comparison of the experimental results with theoretical predictions improves the understanding of complex materials needed to optimize functional Heusler alloys.