High-temperature polyimide nanofoams for microelectronic applications

Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term ‘nanofoam’. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block, leaving pores of the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials were investigated as well as from a number of thermally labile polymers. The foams were characterized by a variety of experiments including TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and low dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.     

Studies of lipoproteins and fatty acids in maternal and cord blood of two racial groups in Trinidad

The high mortality rate from coronary heart disease (CHD) among Indians compared to Negroes in Trinidad led us to test plasma lipid profiles to see whether dietary or genetic factors might be involved. There were no interracial differences in the composition of plasma cholesterol ester fatty acids of the tested women and neonates. This finding suggests that dietary fat does not account for the interracial difference in CHD, nor does the cause appear to be due to genetic differences in lipid profiles, as there was no significant difference between values for plasma triglycerides, total cholesterol, high density lipoprotein (HDL) cholesterol, apo-I, apo-II, apo B or cholesterol ester fatty acids in the cord blood of each racial group. Blood samples were collected from 69 nonpregnant and 71 postpartum, fasted Negro and Indian women. Also taken were 71 umbilical cord blood samples. The mean triglyceride level was significantly lower in the Negro nonpregnant and postpartum women than in the Indians. HDL cholesterol and apo-I values were lower in the Indian women. There were no significant differences in the total cholesterol and apo B measurements. The triglyceride values for postpartum women were higher than those of the nonpregnant Negroes and Indians (75% and 47%, respectively), whereas the total cholesterol and HDL cholesterol, apo A-I and apo A-II ranged from 9% to 29% higher in the postpartum women. Apo B was about 40% higher postpartum in both ethnic groups. The high CHD rate of Indians in Trinidad cannot be explained by dietary factors, plasma total cholesterol or fatty acid composition. However, the lower level of HDL cholesterol and plasma A-I could play a role in the higher CHD rate in Indians.     

Thermal Conductivity/Diffusivity of Silicon Carbide Whisker Reinforced Mullite

The thermal conductivity and diffusivity of silicon carbide whisker reinforced mullite was shown to increase with whisker content. This effect was much greater for vapor-liquid-solid (VLS) whiskers than for rice-hull (RH) whiskers. This suggests that the thermal conductivity for the VLS whiskers was significantly higher than for the RH whiskers. Due to preferred orientation of the whiskers, thermal conductivity and diffusivity of the composite samples exhibited significant anisotropy.     

Comparison of capillary gas chromatographic method and automated spectrophotofluorometric methods for measuring content uniformity of conjugated oestrogens in pharmaceutical preparations

To measure the content uniformity of conjugated cestrogens tablets, the USP XXII monograph specifies a capillary gas chromatographic method. This involves separation of the trimethylsilyl derivatives of the various oestrogens found in a typical conjugated oestrogens tablet using a fused-silica 0V 225 bonded phase column. Hydrogen is used as the carrier gas with FID detection. A more rapid spectrophotofluorometric method of analysis has been developed in which the oestrogens are extracted from the dosage form into water, then transferred into organic solvent as the dicyclohexylamine complex. Fluorescence is developed by heating the solution of the complex in the presence of 70% sulphuric acid. The chemistry and detection are carried out using a segmented-flow analyser.The spectrophotofluorometric method is preferred as a routine control for content uniformity because considerable time is saved during sample preparation and analysis compared to gas chromatography. Coefficients of variation show that both methods produce acceptable results. These results fall well within the USP XXII [1] monograph, limits and also well within the tighter limits imposed by the USP XXII, section 905.     

Orientation effects during the flow of short-fibre reinforced thermoplastics

A study has been carried out to investigate molecular orientation in injection moulded bars of short glass fibre reinforced polypropylene and polyethylene. For the range of fibre concentrations encountered commercially, the fibres do not appear to have any direct effect on the matrix orientation. As the fibre concentration increases, however, the matrix orientation becomes dominated by the orientation of the fibres. These effects are interpreted in terms of current ideas of the rheology of polymer melts during injection moulding and the crystallization of polymers at fibre surfaces.     

Dressing plasmons in particle-in-cavity architectures

Placing metallic nanoparticles inside cavities, rather than in dimers, greatly improves their plasmonic response. Such particle-in-cavity (PIC) hybrid architectures are shown to produce extremely strong field enhancement at the particle-cavity junctions, arising from the cascaded focusing of large optical cross sections into small gaps. These simply constructed PIC structures produce the strongest field enhancement for coupled nanoparticles, up to 90% stronger than for a dimer. The coupling is found to follow a universal power law with particle-surface separation, both for field enhancements and resonant wavelength shifts. Significantly enhanced Raman signals are experimentally observed for molecules adsorbed in such PIC structures, in quantitive agreement with theoretical calculations. PIC architectures may have important implications in many applications, such as reliable single molecule sensing and light harvesting in plasmonic photovoltaic devices.     

Defining the nanostructured morphology of triblock copolymers using resonant soft X-ray scattering

The morphologies of a poly(1,4-isoprene)-block-polystyrene-block-poly(2-vinyl pyridine) (IS2VP) copolymer were investigated using resonant soft X-ray scattering (RSoXS) together with scanning force microscopy, small-angle X-ray scattering, and electron microscopy. Differences in the nanoscopic morphologies in the bulk and thin film samples were observed arising from the competition between segmental interactions between the blocks and the substrate and the surface energies of each block. Using soft X-rays at selected photon energies to isolate the scattering contribution from different polymer blocks, RSoXS unambiguously defined the complex morphology of the triblock copolymer. In the bulk sample, two nested, hexagonal arrays of P2VP and PI cylindrical microdomains residing in the PS matrix were observed. The cylindrical microdomains of one component were found to be located at the interstitial sites of the hexagonal array of the other component that has the larger d spacing. In solvent-annealed thin films with 40 nm in thickness, a hexagonal array of core-shell microdomains of P2VP cores with PS shells that reside in a PI matrix were observed.     

Effect of the dielectric-substrate interface on charge accumulation from vacuum ultraviolet irradiation of low-k porous organosilicate dielectrics

We compare the effect of various dielectric-substrate interfaces on charge accumulation during vacuum ultraviolet irradiation of capped low-k porous organosilicates to find that more charges are trapped in a dielectric stack deposited on silicon compared with the same stack deposited on copper. Insertion of a 5-nm interfacial thermal oxide layer further increases the amount of trapped charges in the dielectric. The difference between the photoemission and injection currents determines the number of charges trapped in the dielectric as a result of irradiation. Fewer charges are trapped when the injection current increases.