Measurement of surface area of polymer shavings

A modified BET apparatus has been developed to measure the surface area of polymer samples in the range 0.2–3.0 m². The sample holder is designed to permit easy entry for bulky samples, such as polymer shavings, which cannot be loaded through a capillary or exposed to the high temperature required to seal glass. Other apparatus modifications, which allow the accurate measurement of surface area, include the addition of a dibutyl phthalate differential manometer in conjunction with a pressure reservoir. Corrections for the dead volume, nonideality of nitrogen, and the thermal gradient in the sample holder are developed. Calibrations of the equipment with inorganic samples of known areas from 1.81 to 11.8 m² gave highly accurate values of 1.74 to 11.6 m². Results on high-density polyethylene shavings showed precision in determinations and very close agreement in the value of 49.7 for C in the BET equation with the published value, 47.7.     

Structural investigations of tungsten silver phosphate glasses by solid state NMR, vibrational and X-ray absorption near edge spectroscopies

Glasses were prepared in the pseudo-binary system (1 − x)AgPO₃-xWO₃ (0≤ × ≤ 0.6 mol%). The structural evolution of the vitreous network was studied as a function of composition by thermal analysis, Fourier Transform Infrared spectroscopy (FTIR), Raman scattering, high resolution ³¹P solid state NMR and XANES at the W-L₁ absorption edge. For compositions with x ranging from 0 to 0.5 a pronounced increase in the glass transition temperature is observed, suggesting a significant increase in the glass network connectivity. At the same time Raman spectra indicate that tungsten atoms are linked to non-bridging oxygen atoms (W-O- or W=O bonded species) whereas the ³¹P MAS-NMR spectra indicate the successive formation of new P-O-W linkages. The formation of some anionic tungsten sites (if these are revealed by the presence of W-O terminal bonds) implies an increase in the average degree of polymerization of the phosphate network, which is consistent with the compositional evolution of the ³¹P MAS-NMR spectra at low x values. For higher x-values, the Raman spectra indicate the presence of W-O-W linkages. W-L₁ XANES data indicate that at least 90% of tungsten atoms are octahedrally coordinated.     

Synthesis and mechanical properties of mechanically alloyed Al-Cu-Fe quasicrystalline composites

Al-Cu-Fe alloys were prepared by mechanical alloying starting from elemental powders in a high-energy planetary ball mill. Three different alloy compositions with the same c _Cu : c _Fe ratio of 2:1 but different aluminium contents, that is Al₅₅Cu₃₀Fe₁₅, Al₆₃Cu₂₅Fe₁₂ and Al₇₀Cu₂₀Fe₁₀, were investigated. A sequence of solid-state reactions resulting in quasicrystalline phase formation in Al₆₃Cu₂₅Fe₁₂ proceeds during milling and during annealing of the as-milled powder. These reactions were studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. In order to form an aluminium-matrix composite, Al₆₃Cu₂₅Fe₁₂ single-phase quasicrystalline powders were blended with different amounts of aluminium. In an intermediate milling step the powder blend was homogenized. The powders were consolidated by hot extrusion. The bulk samples revealed a homogeneous dispersion of the particles in the matrix but a rather heterogeneous size distribution. The mechanical properties at room temperature were tested by constant-rate compression tests. A rule-of-mixtures dependence of the ultimate strength and the yield strength on the volume fraction of the quasicrystalline particles was found.     

PHOTOCHEMISTRY OF THE REACTION CENTERS OF SYSTEM II UNDER REPETITIVE FLASH GROUP EXCITATION IN ISOLATED CHLOROPLASTS

Abstract— Absorption changes induced in isolated chloroplasts by excitation with repetitive flash groups have been measured at 690 nm, indicating the photochemical turnover of chlorophyll-a_II (Chl-αn), and at 480 nm and 513 nm respectively, reflecting via electrochromic effect the formation of a transmembrane electric field. The data are compared with measurements of oxygen evolution. In chloroplasts with practically fully intact oxygen evolving capacity it was found: 1. The initial amplitude of the 690 nm absorption change induced by the second flash as a function of the time t_v between the first and second flash of a group increases with a half life of about 35 µs. On the other hand, the average oxygen yield due to the second flash as a function of the time t_v rises with a half life of about 600 µs (and a kinetics in the ms-range of minor extent), confirming the data of Vater et al. (1968). 2. Under far red background illumination, where contributions due to PS I in the µs-range can be excluded, the difference spectrum in the red of the absorption changes induced by the first flash corresponds with that of the absorption changes induced by the second flash fired 200 µ after the first flash. 3. The pattern of the absorption changes at 690 nm induced by repetitive double flash groups at t_v= 200 µs does not markedly change in normal chloroplasts by the presence of DBMIB?. Similar 690 nm absorption changes occur in trypsin treated chloroplasts, independent of the presence of DCMU. 4. The fast regeneration in the µs-range of Chl-a_n is also observed in the third flash of a triple flash group at a time t_v= 200 µs between the flashes of the group. 5. The initial amplitudes of the absorption changes with a decay kinetics slower than 100 µs induced by the second flash at 480 nm and 513 nm, respectively, as a function of the time t, between the first and second flash of a group, are characterized by a recovery half-time of about 600 µs, confirming earlier measurements at 520 nm (Witt and Zickler, 1974). On the basis of these results it is inferred that there does exist a photoreaction of Chl-α_n., with an electron acceptor, referred to as X_a, other than the ‘primary’ plastoquinone acceptor X320, if X320 persists in its reduced state. Under conditions of X320 being in the reduced state, this photochemical reaction was shown to be highly dissipative with respect to charging up the watersplitting enzyme system Y. Furthermore, this Chl-a_n-photoreaction with X_a does not lead to a vectorial transmembrane charge separation, which is stable for more than a few microseconds. Different models for the functional and structural organization of PS II are discussed.     

Local polarity in CO2-expanded acetonitrile: a nucleophilic substitution reaction and solvatochromic probes

Many processes that use highly tunable gas-expanded liquids (GXLs) rely on the fact that CO2 addition can greatly affect the polarity of the solvent. We have examined several measures of bulk and local polarity in CO2-expanded acetonitrile to enable more effective exploitation of these polarity changes. The rate of the nucleophilic substitution reaction of tributylamine with methyl p-nitrobenzenesulfonate has been analyzed as a function of solvent composition by using in situ high-pressure UV/vis spectroscopy. We have also measured solvatochromic properties including the Kamlet-Taft pi* parameter and Kosower's Z-value. We correlate these local polarity-based kinetic and solvatochromic measures to develop a better understanding of these property changes as a function of bulk and local solvent composition. The data suggest that local composition enhancement in CO2-expanded acetonitrile has a significant impact on the reaction kinetics.     

A bridging hexazene (RNNNNNNR) ligand from reductive coupling of azides

This communication reports the first examples of transition metal complexes containing an RNNNNNNR 2- ligand. Addition of 1-azidoadamantane to the diiron(I) synthon LRFeNNFeL R (L R = HC[C(R)N(2,6- iPr 2C 6H 3)] 2; R = methyl, tert-butyl) leads to the diiron complexes L RFe(mu-eta2:eta2-AdN6Ad)FeLR, which are surprisingly thermally stable. Magnetic, M?ssbauer, and crystallographic data are consistent with pairs of high-spin iron(II) ions antiferromagnetically coupled through a dianionic AdN6Ad 2- bridge.     

Materials Chemistry in Münster

Ende August haben Sigma‐Aldrich High‐Technology und die NRW Graduate School of Chemistry ihren zweiten gemeinsamen Doktoranden‐Workshop in Materials Chemistry veranstaltet. Mehr als 60 Doktoranden aus dem In‐ und Ausland haben drei Tage in den Münsteraner chemischen Instituten über Materialwissenschaften diskutiert.