Second normal stress difference relaxation in a linear polymer melt following step-strain

Flow birefringence is used to study stress relaxation following step strain deformations of a well entangled polyisoprene melt. The optical method employs multiple light paths to fully sample the three-dimensional stress tensor, and hence provides measurements of all three independent shear material functions (shear stress and both first and second normal stress differences). Experiments are complicated by multiple orders in retardation. However, data show that the ratio of the second to the first normal stress difference is a strain thinning function, with magnitude intermediate between the predictions of the Doi-Edwards model with and without independent alignment. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2671–2675, 1998     

Diastereoselective Coordination of Chiral Phosphine Substituted Thioamide Ligands

Abstract

Homochiral thioamides are forming diastereomeric molybdenum and tungsten complexes in different ways (type 1–3) and different stereoselectivities (1 and 2: > 99%de, 3:0%de). The dissolved complexes 1 epimerized slowly at room-temperature.     

Orientation dynamics in commercial thermotropic liquid crystalline polymers in transient shear flows

In situ X-ray scattering measurements of molecular orientation under shear are reported for two commercial thermotropic liquid crystalline polymers (TLCPs), Vectra A950® and Vectra B950®. Transient shear flow protocols (reversals, step changes, and flow cessation) are used to investigate the underlying director dynamics. Synchrotron X-ray scattering in conjunction with a high-speed area detector provides sufficient time resolution to limit the total time spent in the melt during testing, whereas a redesigned X-ray capable shear cell provides a more robust platform for working with TLCP melts at high temperatures. The transient orientation response upon flow inception or flow reversal does not provide definitive signatures of either tumbling or shear alignment. However, the observation of clear transient responses to step increases or step decreases in shear rate contrasts with expectations and experience with shear-aligning nematics and suggests that these polymers are of the tumbling class. Finally, these two polymers show opposite trends in orientation following flow cessation, which appears to correlate with the evolution of dynamic modulus during relaxation. Specifically, Vectra B shows an increase in orientation upon flow cessation, an observation that can only be rationalized by the assumption of tumbling dynamics in shear. Together with prior observations of commercial LCP melts in channel flows, these results suggest that this class of materials, as a rule, exhibits director tumbling.     

Die Kautschukr?hren, Stopfen etc.

Ohne Zusammenfassung     

A hybrid hydrodynamic-Liouvillian approach to mixed quantum-classical dynamics: application to tunneling in a double well

The hybrid quantum-classical approach of Burghardt and Parlant [Burghardt, I.; Parlant, G. J. Chem. Phys. 2004, 120, 3055], referred to here as the quantum-classical moment (QCM) approach, is demonstrated for the dynamics of a quantum double well coupled to a classical harmonic coordinate. The approach combines the quantum hydrodynamic and classical Liouvillian representations by the construction of a particular type of moments (that is, partial hydrodynamic moments) whose evolution is determined by a hierarchy of coupled equations. For pure states, which are at the center of the present study, this hierarchy terminates at the first order. In the Lagrangian picture, the deterministic trajectories result in dynamics which is Hamiltonian in the classical subspace, while the projection onto the quantum subspace evolves under a generalized hydrodynamic force. Importantly, this force also depends upon the classical (Q, P) variables. The present application demonstrates the tunneling dynamics in both the Eulerian and Lagrangian representations. The method is exact if the classical subspace is harmonic, as is the case for the systems studied here.     

Precise rf tuning for cw dye lasers

Laser beams are rf amplitude modulated to generate side frequencies which are used to tune a cw dye laser with rf accuracy. In a laser spectroscopic test experiment a precision of ±15kHz has been realized.     

Gravitationsphysik und Lorentzraum

Gravitationphysics and Lorentz-space Coordinate invariant conservation laws for static and stationary gravitation fields can be formulated. Being seen as elements of Lorentz space the quantities to be conserved open to a physical interpretation. These quantities are field and matter terms. Some examples illustrate how useful this approach has turned out to be.     

The 3d states of23Na and7Li: determination of unresolved hyperfine splittings and radiative lifetimes

The electronic transitions from the ground state to the 3d states have been studied for²³Na and⁷Li atoms by two-photon Doppler-free laser spectroscopy. The positions of the resonances are used to determine the unresolved hyperfine structure of the 3d states from which the magnetic-dipole hyperfine interaction constantsA are derived. The results for theA factors are:A(3d ² D _3/2;²³Na)=+527(25) kHz;A(3d ² D _5/2;²³Na)=+108.5(2.4) kHz;A(3d ² D _3/2;⁷Li)=+843(41) kHz;A(3d ² D _5/2;⁷Li)=+343.6(1.0) kHz. For the fine structure intervals fs of the 3d doublets we obtain: fs(3d Na)=-1 494 444(44) kHz and fs(3d Li)=+1 083 936(60) kHz. The linewidths of the resonances are evaluated with respect to the natural lifetimes of the 3d states. For Na the result is τ(3d Na)=19.27(23) ns.     

Preparation of highly stable organic steps with a fullerene-based molecule

We report the formation of highly stable Langmuir-Blodgett (LB) organic steps made with a hexa-adduct fullerene-based amphiphile. This amphiphile forms films of excellent quality, with a very low roughness, that are structurally stable: X-ray reflectivity spectra recorded on fresh and 12-month-old samples are undiscernible. Such a behavior contrasts with that of more traditional amphiphiles, which are unfortunately well-known for their instability in time. The stability of the films stems, among others, from the spheroidal shape of the constitutive molecules. These experiments show that it is possible to circumvent the major drawback of LB films and to prepare materials more suited for applications. We show that the LB film prepared with this fullerene derivative can successfully be used as thickness gauges for atomic force microscopy or light microscopy studies.