Elastic excitations in polycarbonate membranes

Brillouin light scattering was used to probe acoustic waves propagating with both longitudinal and transverse polarizations in the surface and the bulk of self‐supported particle track‐etched polycarbonate membranes with 15‐ and 80‐nm nanopores. The recorded scattering line shape at gigahertz frequencies reveals changes in the surface waves of the membranes which are more pronounced for the 80‐nm nanopores despite the low porosity (0.7 and 0.05%). Because the measured elastic constants (1.2 and 6.2 GPa) were found to compare very well with the values for thick polycarbonate film, modifications of the elasto‐optical coefficients and/or the transparency might be the reason for the different scattering line shapes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3311–3317, 2004     

Self-rolled polymer microtubes with engineered hidden walls

Micro and nanotubes are manufactured by self-rolling of thin bilayer polymer films (polystyrene/poly(4-vinyl pyridine, PS/P4VP) gradually released from a solid substrate. Swelling of the P4VP layer in acidic water opposed by a stiff PS layer leads to rolling of the film into tubings of micrometer diameters. The position and the arrangement of the tubes on the substrate are defined by UV photolithography structuring of the polymer film. A few nanometer thick metallic patterns, such as arrays of conductive stripes and simple electrical circuits, are formed on the top of the polymer film by magnetron sputtering and transferred to the tube interior by rolling. Intra-tubular rolled-up microsolenoids are applied to generation of localized magnetic fields inside the tubes and in their vicinity.     

Influence of the conformation of polyethylene on wide-angle neutron scattering patterns

Wide-angle neutron scattering investigations of mixtures of deuterated and nondeuterated chains are well suited to detect sequences of regular adjacent reentry folding of a polymer chain in particular crystallographic directions. It is shown that the diffuse neutron scattering intensity is peaked at large scattering angles for (100) and (010) folding, whereas (110) folding is difficult to detect. Detailed calculations indicate that the position, intensity, and half-width of the diffuse peak depend strongly on the mode of folding as well as on the number of adjacent stems. By a comparison with experimental data adjacent reentry folding in (100) and (010) crystallographic planes with more than five adjacent stems can be excluded. The implications for other folding models containing regular folded chain sequences are discussed.     

Nucleophilic cleavage of 2,2-dimethylaziridines: competition between SN2 and postulated “SET” mechanism.

Regioselectivity of nucleophilic attack on 2,2-dimethylaziridines depends on the degree of leaving group activation: in highly activated aziridines it occurs at the methylene carbon and in less activated at the tertiary carbon. This latter abnormal ring opening is explained by an SET mechanism.     

Phase diagram and critical points for a metamagnetic Ising model using constant coupling approximation

We compute the isotherms and phase diagram in the constant coupling approximation for an Ising metamagnet with various values for the ratio of the ferromagnetic to the antiferromagnetic coupling parameters. The constant coupling method is set up entirely with the internal fields as the variation parameter. The search for the tricritical point, both directly and indirectly via the hessian of the internal fields led to the conclusion that this model has no tricritical point, but a critical end point. Comparing our computation with the experimental result for FeBr₂, we find that the value for the critical end point lies closer to the experiment than either the molecular field theory or the random phase approximation.     

The Study of Cavitation in HDPE Using Time Resolved Synchrotron X-ray Scattering During Tensile Deformation

Real time synchrotron Small-Angle and Wide-Angle X-ray Scattering was performed during the tensile deformation of a high-density polyethylene copolymer. The changes of the structure in the crystalline and in the amorphous domains were followed during the three characteristic stages of the load-displacement curves: The elastic stage and the plastic range composed of the stage of the lowering load in the force-displacement-curve (yielding) and the strain hardening. Competitive phenomena like crystallite fragmentation and cavitation were found to occur simultaneously in the phase of lowering the load but at different length scale. We prove that the void formation occurs mainly during yielding. During strain hardening there was no further increase of the void volume fraction, only changes in void size.     

Regularization of 2D frictional contacts for rigid body dynamics

Classic rigid body mechanics does not provide frictional forces acting in a 2D contact interface between two bodies during sticking. This is due to the statical undeterminacy related with this problem. Many technical systems, e.g. disk clutches, have such surface-to-surface contacts and it is sometimes desirable to treat them as rigid body systems despite the 2D contact. Alternatively it is possible to model the systems using elastic instead of rigid bodies, but this might lead to certain drawbacks. Here a new regularization model of such 2D contacts between rigid bodies is proposed. It is derived from a material model for elasto-plasticity in continuum mechanics. Only dry friction is taken into account. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A resonance effect of the atomic populations kinetics induced by a fluctuating plasma

The effect of fluctuations of the electron temperature on the population kinetics of a reduced atomic system is investigated in the case of a low density plasma, where the coronal approximation can be used. An exponential correlation function is assumed for the electron temperature fluctuations. A resonance of the average population as a function of the correlation time is observed and analyzed.