Flow and field induced instabilities in a smectic C liquid crystal

This paper discusses the possibility of flow induced instabilities occurring in smectic C liquid crystals by examining appropriate solutions of a recently proposed continuum theory for such materials. These preliminary calculations suggest that results rather similar to those obtained for nematics some twenty years ago are equally likely in smectics.Paper originally intended for publication in Vol. 31/5, Special Issue dedicated to Gianfranco Capriz, but omitted due to an unfortunate misunderstanding.     

Low-temperature polymerization of isobutyl vinyl ether

Isobutyl vinyl ether has been polymerized under conditions well known to yield isotactic polymer, viz., with boron trifluoride etherate at 78°C. in a nonpolar hydrocarbon diluent. A particular mixed solvent ratio and previous dissolution of catalyst enabled the polymerization to proceed homogeneously at the beginning. By following the temperature rise in an initially thermostatted system, we showed that the progress of the reaction eventually proceeded via a homogeneous phase to a gellike phase. Isotactic polymer is shown to be produced in both steps by a mechanism of slow chain propagation.     

The use of cross-spectral density measurements in partially reverberant sound fields

The cross-spectral densities of sound at two points in a partially reverberant space are shown to be sensitive to the power flow out of the space. A nearly homogeneous sound field is found to possess a co-spectral density which is nearly insensitive to the nature of the field. This spectrum is traditionally obtained as a cross-correlation and its analytical form has been well established. Perhaps less well known is the imaginary part of the cross-spectrum which is shown in this paper to be sensitive to the direction of propagation from a sound scattering enclosure. Specific analytical forms of the spectrum are derived for incidence included within certain cones by using a wavefunction model of the scattered sound field. In some cases the co- and quadrature-spectra are related to each other by the Hilbert transform.     

Simultaneous Amplification of Terahertz Difference Frequencies by an Injection-Seeded Semiconductor Laser Amplifier at 850 nm

Two-frequency operation of an 850 nm semiconductor optical amplifier was achieved by simultaneously injection seeding it with two diode lasers. The two frequencies could be independently amplified without strong interference when they were separated by more than 10 GHz, and the spectral purity was preserved by the amplification process. At frequency differences below 10 GHz, unbalanced two-frequency output was observed, which can be explained by a two-mode interaction driven by the refractive index modulation at the beat frequency. The laser system is suitable for the difference-frequency generation of coherent terahertz radiation in ultra-fast photoconductors or nonlinear optical media.     

Multinuclear (13C, 15N) NMR study of polyethyleneimine-based polymers

Representative polyethyleneimine-based polymers have been studied by ¹³C- and ¹⁵N-nuclear magnetic resonance spectroscopy and an extensive set of chemical shift assignments has been proposed for the complex spectra obtained. The samples in question represent a complex mixture of both protonated and nonprotonated primary, secondary, and tertiary as well as quaternary monomeric units.     

Additivity of the hydrogen bridges in several polymers of formamide

The purpose of the study was to examine the energy additivity of the hydrogen bridges in formamide polymers. Consideration was given to both the linear polymers observed in polypeptides, and to the cyclic polymers observed in the crystal structure. Additivity was found in several geometries. The linear polymers were considered as far as the tetramer, and the cyclic as far as the heptamer.Work supported in part by the Instituto Fisica, Universidad Nacional Autónoma de Mexico and Conacyt, México.