Effects of Spacers on Photoinduced Reversible Solid-to-Liquid Transitions of Azobenzene-Containing Polymers

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.     

Colloids in external electric and magnetic fields: Colloidal crystals, pinning, chain formation, and electrokinetics

The motion of dilute and concentrated dispersions of colloids by external electric or magnetic fields is discussed. Electrokinetics is studied for colloids in confinement, where the confining walls can be flat or rough. As an example for a rough wall superhydrophobic surfaces are chosen. It is shown that the reduced friction at the water-air interface is insufficient to enhance electro-osmosis. Magnetic particles are pulled through a crystalline matrix formed by nonmagnetic colloids to investigate local melting and recrystallization of a crystalline matrix. The average strain field is calculated and the reorganization processes are compared to those induced by shear fields. Using single domain, magnetically blocked particles of different shape and surface characteristics, the interplay between particles, their environment and an external field is investigated.     

Time‐resolved X‐ray microscopy of nanoparticle aggregates under oscillatory shear

Of all the current detection techniques with nanometre resolution, only X‐ray microscopy allows imaging of nanoparticles in suspension. Can it also be used to investigate structural dynamics? When studying the response to mechanical stimuli, the challenge lies in its application with a precision comparable with the spatial resolution. In the first shear experiments performed in an X‐ray microscope, this has been accomplished by inserting a piezo actuator driven shear cell into the focal plane of a scanning transmission X‐ray microscope. Thus shear‐induced re‐organization of magnetite nanoparticle aggregates could be demonstrated in suspension. As X‐ray microscopy proves suitable for studying structural change, new prospects open up in physics at small length scales.     

Frequency-dependent deformation of liquid crystal droplets in an external electric field

Nematic droplets suspended in the isotropic phase of the same substance were subjected to alternating electrical fields of varying frequency. To keep the system at a constant nematic/isotropic volume ratio with constant droplet size, we carefully kept the temperature in the isotropic/nematic coexistence region, which was broadened by adding small amounts of a non-mesogenic liquid. Whereas the nematic droplets remained spherical at low (in the order of 10 Hz) and high frequencies (in the order of 1 kHz), at intermediate frequencies we observed a marked flattening of the droplets in the plane perpendicular to the applied field. Droplet deformation occurred both in liquid crystals (LCs) with positive and negative dielectric anisotropy. The experimental data can be quantitatively modelled with a combination of the leaky dielectric model and screening of the applied electric field due to finite conductivity.     

Humidity influence on mechanics of paper materials: joint numerical and experimental study on fiber and fiber network scale

Cellulose - Paper materials are well-known to be hydrophilic unless chemical and mechanical processing treatments are undertaken. The relative humidity impacts the fiber elasticity, the interfiber...     

Mapping humidity-dependent mechanical properties of a single cellulose fibre

Cellulose - Modelling of single cellulose fibres is usually performed by assuming homogenous properties, such as strength and Young’s modulus, for the whole fibre. Additionally, the...     

Shear-induced undulation of smectic-$\mathsf{A}$: Molecular dynamics simulations vs. analytical theory

Experiments on a variety of systems have shown that layered liquids are unstable under shear even if the liquid layers are planes of constant velocity. We investigate the stability of smectic-A like liquids under shear using Molecular Dynamics simulations and a macroscopic hydrodynamic theory (including the layer normal and the director as independent variables). Both methods show an instability of the layers, which sets in above a critical shear rate. We find a remarkable qualitative and reasonable quantitative agreement between both methods for the spatial homogeneous state and the onset of the instability.Received: 2 October 2003, Published online: 9 March 2004PACS: 61.30.Dk Continuum models and theories of liquid crystal structure - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 05.70.Ln Nonequilibrium and irreversible thermodynamics - 83.10.Rs Computer simulation of molecular and particle dynamics - 83.50.Ax Steady shear flows, viscometric flowTh.Soddemann : Present address: Rechenzentrum der Max-Planck-Gesellschaft, Boltzmannstrasse 2, 85748 Garching, Germany