Amorphous polymer films containing hyperpolarizable dye molecules are poled by an electric fieldapplied parallel to the plane of the thin film. The resultant second harmonic generation is much stronger than predicted by the standard model of dipolar interactions, and furthermore indicates asymmetry both parallel and perpendicular to the poling field. These results are attributed to a new, efficient mechanism for the production of optical nonlinearity in poled films in which asymmetric charge injection into the polymerand its trapping by dye species sets up a charge gradient perpendicular to the direction of the external poling field.
Perfluorocarbon macroemulsions were stabilized by phospholipids which were composed mainly of phosphatidylcholine and phosphatidylethanolamine. Emulsions which were prepd. with a phosphate buffer as the external phase, were more stable than those prepd. with water only, in spite of the very high ionic strength (I = 0.4) in the former. It was found that hydrolysis of the phospholipids caused formation of lysophosphatidylcholine, and a significant decrease in the pH, in absence of the buffer. The decrease in pH led to a decrease in ζ potential, and hence decreased stability. This effect was not significant in the buffer-emulsions. However, oxidn. of the surfactant was detected for both systems, a process which resulted in oil sepn. after prolonged time (>200 days) also in the oil in buffer emulsion. [on SciFinder(R)]
CHox-cad is a chicken homeobox gene whose homeodomain is homologous to the Drosophila caudal and the murine Cdx1 genes. Based on sequence analysis of a 2.5 kb CHox-cad cDNA clone, we deduced that the primary translation product consists of 248 amino acids. Comparison between the cDNA and genomic clones revealed the presence of an intron within the CHox-cad homeodomain between amino acids 44 and 45. The onset of CHox-cad transcription correlates temporarily with the beginning of gastrulation. During primitive streak stages CHox-cad exhibits a caudally localized pattern of expression restricted to the epiblast and the primitive streak. At these stages, CHox-cad transcripts can also be detected in the definitive endoderm cells. Later in embryogenesis CHox-cad is expressed in the epithelial lining of the embryonic gut and yolk sac. After four days of chicken development, no CHox-cad transcripts could be detected. The early CHox-cad posterior expression in the germ layer undergoing gastrulation and its continuous expression in the early endodermal lineage raise the possibility of CHox-cad involvement in the establishment of the definitive endoderm.
CHox E is a novel chicken homeogene that belongs to the H2.0 family of homeodomains. Its homeobox sequence is interrupted by an intron between amino acids 44 and 45. Expression of CHox E during embryogenesis is localized to the central nervous system. The anterior boundary of CHox E expression can initially be localized to rhombomere number 1, later in development this boundary reaches up to the rhombencephalic isthmus. CHox E expression in the spinal cord localizes dorso-ventrally to the dorsal half of the basal plate. CHox E expression is always restricted to the proliferating region, the ventricular zone. As the ventricular zone becomes restricted laterally, so does the CHox E expressing region. Once this region of the ventricular zone ceases to exist, CHox E specific transcripts become undetectable. The site and time of CHox E expression suggest a very early function in the differentiation of the cells derived from that region of the ventricular zone.
Within the framework of two complementary models, we show that the densities and patterns of defects in amphiphile-water systems with lamellar organization are coupled to the strength of the bilayer-bilayer interactions and hence to the overall surfactant concentration. We consider defects which introduce curvature (i.e., larger head-group area per molecule) while preserving the integrity of stacked bilayers at surfactant volume fractions of several tenths. These features are favored if the molecules comprising the lamellae are preferentially packed with a nonplanar aggregate-water interface: curvature defects lower the local free energy in systems constrained by aggregate-aggregate interactions to lamellar geometry. As the amphiphile volume fraction is increased-and the bilayer-bilayer spacing thereby decreased-we predict phase transitions between lamellar phases of different defect patterns on the bilayer surface, with concurrent decrease in the defect area fraction per bilayer. Specifically, there is a progression from a stripe-like pattern of parallel channels to a random network of line defects to a pore phase, with the latter appearing at the highest amphiphile concentrations but characterized by the lowest density of defects. Connection is made with experimental work which has recently suggested various departures from classical lamellar structure.
The dielectric properties of a liquid-crystalline (LC) copolymer containing photochromic spiropyran groups have been determined over wide ranges of frequency and temperature. The observed loss curves contain a component from the dipole reorientations of the mesogenic groups and a further component from a conductivity-related process. An electrical cleaning method was used to reduce the contribution from the latter process and a subtraction method was devised to separate the overall loss curves into the two components. Using electrical/thermal treatments it was found possible to align the copolymer homeotropically but not planarly. The dielectric loss spectra for unaligned and homeotropically aligned samples were very different. The data were analysed to yield two relaxation parameters for unaligned and aligned samples and were interpreted in terms of the anisotropic motions of dipolar mesogenic groups as have been discussed previously. In addition, the temporal stability of alignment of homeotropic samples at temperatures close to the clearing temperature was determined using the dielectric method.
The photodissociation of HCl in the Ar-HCl cluster has been studied theoretically using a classical trajectory method to explore the effect of a ``solvent'' atom on the process. The calculated kinetic energy and angular distributions of the photofragments show a ``cage'' effect, due to collisions between H and the heavy atoms before H leaves the cluster. Also, significant differences are seen depending on whether a classical or a quasiclassical distribution is used to describe the initial Ar-HCl bending distribution.
The effect is studied of drag-reducing additives (e.g., polyacrylamide) on the drop size and drop-size distribution in turbulent flow of oil-water emulsions. A high-mol.-wt. polymer that is sol. in the continuous phase alters the mean drop size and drop-size distribution. The results can be used in the control of the drop size and drop-size distribution in the emulsion. [on SciFinder(R)]
During a study of the electrooxidn. of SnCl2 at Au and Pt electrodes by cyclic voltammetry, a remarkable hysteresis was assocd. with the oxidn. of SnCl2 that is attributed to the adsorption of SnCl2 on the Au electrode. The adsorbed SnCl2 inhibits the oxidn. of the bulk SnCl2 and therefore shifts the potential for oxidn. to more pos. values. However, once the adsorbed SnCl2 is oxidized, the dissolved SnCl2 is oxidizable at less pos. potentials. This inhibition by adsorbed SnCl2 also affects other inner-sphere electrode reactions (e.g., oxidn. of I-), but not outer-sphere reactions. This inhibition is explained in terms of the need of a bridging ligand to facilitate electron transfer in inner sphere reactions. This inhibition is even more pronounced on Pt electrodes, where no oxidn. wave for SnCl2 is obsd. However, adsorption of Cl- ions, 2-mercaptoethanol, or iodide causes desorption of the SnCl2 and the appearance of an irreversible wave for oxidn. of the Sn(II) in soln. [on SciFinder(R)]
