QUANTITATIVE MEASUREMENT OF CHROMIUM'S ABILITY TO PROMOTE ADHESION

Using atomic force spectroscopy, we investigated the adhesion-promoting ability of chromium. An intermediate layer of chromium can overcome the low adhesion between metal films and silicon dioxide. For the first time, we quantitatively studied this experimentally well known fact. We compared the adhesion between chromium and different substrates such as gold, silver, mica, and silicon dioxide and, beyond that, the adhesion between silicon dioxide and the same substrates. To avoid additional effects due to water, we chose ethanol as a nonpolar solvent. Taking the interfacial energies of the surfaces with the liquid into account eliminates the direct influence of the fluid medium on the adhesion of the solid material. The results we obtained corroborate the experimental fact of higher adhesion of chromium with the chosen substrates, as well as substantiate the value of chromium as an adhesion promoter. The adhesion of chromium-coated probes on gold, silicon dioxide, and mica is higher than the adhesion of silicon dioxide probes on the same substrates.     

Stretching and deformation vibrations of CH2, C(CH3) and O(CH3) groups of poly(methyl methacrylate)

Infrared and Raman spectra of atactic PMMA and infrared spectra of stereoregular PMMA and of its four deuterated derivatives

, were measured. They were used to assign the bands of stretching and deformation vibrations of CH₂, CCH₃ and OCH₃ groups in the infrared and Raman spectra and to discuss the effect of stereoregularity on these bands in the infrared spectra.     

Enzymatic esterification of glycerol I. Lipase-catalyzed synthesis of regioisomerically pure 1,3-sn-diacylglycerols

Regioisomerically pure 1,3-sn-diacylglycerols are conveniently prepared in high yields (>80%) and in large quantities by enzymatic esterification of glycerol in the presence of various 1,3-selective lipases(Chromobacterium viscosum, Rhizopus delemar, Rhizomucor miehei) and a variety of different acyl donors like free fatty acids, fatty acid alkyl esters and vinyl esters. All reactions are carried out in aprotic organic solvents of low water content, namelyn-hexane, diethyl ether or tBuOMe. The creation of an artificial interphase between the solvent-immiscible hydrophilic glycerol and the hydrophobic reaction media by the adsorption of glycerol onto a solid support prior to use was essential for the success of these transformations. The effects of reaction conditions and the regioselectivities of the lipases on the product yields are described in detail.     

C-13 Rotating frame relaxation of solid polymers

Summary ¹³c T₁ of some polymers has been measured at various temperatures. This relaxation is sensitive to other motions than those determining the proton relaxation. The T₁'s of polyethylene and poly(ethylene-vinylacetate) show minima which correspond to second moments of 108 s^–2 and 4·10⁸ s^–2, respectively, in contrary to the theoretical value of 4·10⁹ s^–2. The reason is a strongly restricted motion — e. g. torsional oscillation — which cannot average out completely the dipolar interaction. In poly(methylmethacrylate) and in poly(vinylacetate) a decrease of the restriction with increasing temperature has been observed.Presented at the 22nd Microsymposium, Characterization of Structure and Dynamics of Macromolecular Systems by NMR Methods, Prague, CSSR, July 20–23, 1981     

Microchemical composition and cell dimensions of mullites from refractory-grade South American bauxites

Two different types of mullite could be determined in differently coloured fragments of South American bauxites: − Mullites, occuring in light bauxite fragments with low impurity contents (Σ wt % (Fe₂O₃ + TiO₂) 2.5), and cell dimensions close to that of 3/2 mullite, the b parameter being slightly shorter. − Mullites, occurring in grey to dark grey, and in brown bauxite fragments, with high impurity contents (Σ wt % (Fe₂O₃ + TiO₂) 4,0) and a and c cell dimensions close to those of 2/1 mullite. The b constant of these mullites is slightly expanded with respect to impurity free mullites. Increasing impurity contents in mullite are linearly correlated with a b expansion. The expansion was structurally explained with a substitution of Al³⁺ by Fe³⁺ and Ti⁴⁺ at octahedral lattice sites, which causes stretching of the AlO₆ octahedra along to the elastic Al₁---D_D bond in mullites which is 30° to either side of b. The substitution-produced expansion is superimposed by an expansion due to the change of the structural state of mullite from the 3/2 to the 2/1 type. The greatest variation in any cell parameter with the change of the structural state is the a constant.     

Living dispersion polymerization of styrene in the presence of polystyrene-block-poly(4-trimethylsilylstyrene)

Summary Living polystyrene microparticles with average particle diameters varying between 0.5 and 3.8 m were obtained via sec-butyl lithium-initated anionic polymerization of styrene in n-hexane diluent when polystyrene-block-poly(4-trimethyl-silylstyrene) was added as dispersing agent. The influence of block copolymer molecular architecture and concentration as well as polymerization temperature, monomer concentration and THF addition on polystyrene particle size, molecular weight and molecular weight distribution were investigated. In comparison to anionic styrene polymerization in cyclohexane solution, the anionic styrene dispersion polymerization in n-hexane was markedly slower.     

Synthesis and evaluation of dialkyl furan-2,5-dicarboxylates as plasticizers for PVC

Di(2-ethylhexyl), di(2-octyl), dihexyl, and dibutyl furan-2,5-dicarboxylate were synthesized from furfural and characterized for their plasticizing abilities toward PVC by dynamic mechanical thermal analysis (DMTA) with di(2-ethylhexyl) phthalate (DOP) as the standard of reference. DMTA gave values for the depression of the glass transition temperature (T_g) per mass fraction of plasticizer, and it also yielded a set of parameters, relative to those for DOP, which describe the compatibility of the furan diesters with PVC. The efficiency in lowering T_g as exhibited by di(2-ethylhexyl) furan-2,5-dicarboxylate is similar to that of its benzenoid analog, DOP, and was determined at 2.41 and 2.45°C per mass % plasticizer, respectively. All four furan diesters were found to be more compatible toward PVC than toward DOP, with dibutyl furan-2,5-dicarboxylate the most highly compatible. © 1994 John Wiley & Sons, Inc.     

Detection and evaluation of the fibers' deposition parameters during wet filament winding

Wet filament winding technology has been extensively used for the manufacture of rotationally symmetric parts made of fiber-reinforced plastics (FRP). As the design and modeling of FRP-parts require numerous assumptions, deviations between calculated and achieved mechanical properties are expected. One aspect that contributes to this discrepancy is the assumption of a homogeneous rectangular cross-section of the fiber-band. In this work, the fiber-band geometry in a wet-winding process of carbon fiber cylinders is analyzed. An Infrared-optical system for the detection of the fiber bandwidth and winding-angle is implemented. The influence of the winding speed and the resin temperature is analyzed. An image processing algorithm for the automatic measurement of the fiber's bandwidth and winding-angle is developed. Manual and adaptive gamma corrections are implemented to improve image quality. A parameter study for the suitable selection of image processing parameters is performed.     

Comparison of 2D simultaneous multi-slice and 3D GRASE readout schemes for pseudo-continuous arterial spin labeling of cerebral perfusion at 3 T

Objective

In this perfusion magnetic resonance imaging study, the performances of different pseudo-continuous arterial spin labeling (PCASL) sequences were compared: two-dimensional (2D) single-shot readout with simultaneous multislice (SMS), 2D single-shot echo-planar imaging (EPI) and multishot three-dimensional (3D) gradient and spin echo (GRASE) sequences combined with a background-suppression (BS) module.

Materials and methods

Whole-brain PCASL images were acquired from seven healthy volunteers. The performance of each protocol was evaluated by extracting regional cerebral blood flow (rCBF) measures using an inline morphometric segmentation prototype. Image data postprocessing and subsequent statistical analyses enabled comparisons at the regional and sub-regional levels.

Results

The main findings were as follows: (i) Mean global CBF obtained across methods was were highly correlated, and these correlations were significantly higher among the same readout sequences. (ii) Temporal signal-to-noise ratio and gray-matter-to-white-matter CBF ratio were found to be equivalent for all 2D variants but lower than those of 3D-GRASE.

Discussion

Our study demonstrates that the accelerated SMS readout can provide increased acquisition efficiency and/or a higher temporal resolution than conventional 2D and 3D readout sequences. Among all of the methods, 3D-GRASE showed the lowest variability in CBF measurements and thus highest robustness against noise.