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91.
Polyimides can be processed by sintering or injection moulding, depending on their molecular structure. Raman spectroscopy shows differences in imide I (CO stretch), imide II (CNC axial vibration) and imide III (CNC transverse vibration) bands for both polyimide types, indicating that (i) further imidisation happens during sintering, while being amorphous and (ii) crystallisation happens during melt processing. Thermogravimetric analysis indicates no glass transition or melting temperatures until degradation at 592 °C after sintering, while moulded polyimides show a glass transition temperature at 250 °C. A dehydration reaction at 180 °C will importantly influence the sliding properties of both sintered and injection moulded polyimides. For sintered polyimide, friction and wear in cylinder-on-plate tests is mechanically controlled and mainly determined by normal loads. Overload for sintered polyimides is due to brittleness above 150 N, as also observed in debris morphology. For moulded polyimide, both normal loads and sliding velocity cause a decrease in friction and increase in wear. Overload for moulded polyimide is thermo-mechanically controlled. Sliding stability is controlled by smooth transfer, which is influenced by thermal action, plastic deformation or shear and promoted by elongation at break. Overload conditions are further discussed in relation to deformation of the Hertz line contact, recovery after sliding and the pv-limit (contact pressure × sliding velocity) related to frictional heating. 相似文献
92.
93.
Frontispiece: Photon Energy Becomes the Third Dimension in Crystallographic Texture Analysis 下载免费PDF全文
94.
95.
Henriette de Loor Ruben Poesen Wout De Leger Wim Dehaen Patrick Augustijns Pieter Evenepoel Björn Meijers 《Analytica chimica acta》2016
Chronic kidney disease (CKD) is associated with an increased risk of mortality and cardiovascular disease, which is, at least partly, mediated by the accumulation of so-called uremic retention solutes. Although there has been an increasing interest in the behavior of these solutes, derived from both the endogenous and colonic microbial metabolism, methods to simultaneously and accurately measure a broad panel of relevant uremic retention solutes remain scarce. 相似文献
96.
Bergmann U Di Cicco A Wernet P Principi E Glatzel P Nilsson A 《The Journal of chemical physics》2007,127(17):174504
We report the nearest-neighbor oxygen-oxygen radial distribution function (NN O-O RDF) of room temperature liquid water and polycrystalline ice Ih (-16.8 degrees C) obtained by x-ray Raman based extended x-ray absorption fine structure (EXAFS) spectroscopy. The spectra of the two systems were taken under identical experimental conditions using the same procedures to obtain the NN O-O RDFs. This protocol ensured a measurement of the relative distance distribution with very small systematic errors. The NN O-O RDF of water is found to be more asymmetric (tail extending to longer distances) with longer average distance (2.81 A for water and 2.76 A for ice) but a slightly shorter peak position (2.70 A for water and 2.71 A for ice). The refinement also showed a small but significant contribution from the linear O-H-O multiple scattering signal. The high sensitivity to short range distances of the EXAFS probe will set further restrictions to the range of possible models of liquid water. 相似文献
97.
Cocquyt J Saveyn P Van der Meeren P De Cuyper M 《Langmuir : the ACS journal of surfaces and colloids》2007,23(4):1959-1964
The sorption behavior of the amine containing beta-receptor blocking agent propranolol (Ppn) in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) vesicles was investigated. Both protonated and unprotonated Ppn were measured in the continuous phase after removal of the vesicles containing sorbed Ppn by centrifugation. In contrast, by analyzing the surface charge density, deduced from electrophoretic mobility measurements, only the sorbed protonated Ppn was determined. A partitioning model was used to describe the sorption behavior. Sensitivity analysis revealed that sufficiently reliable and independent parameters were obtained. The partition coefficient of the unprotonated Ppn was about 22 times higher than that of the protonated analogue. Statistical analysis revealed a significant increase in the intrinsic partition coefficients of both Ppn analogues with an increase in the salt concentration. 相似文献
98.
Saveyn P Van der Meeren P Cocquyt J Drakenberg T Olofsson G Olsson U 《Langmuir : the ACS journal of surfaces and colloids》2007,23(21):10455-10462
Lipid freezing in dilute sonicated vesicular dispersions was studied using differential scanning calorimetry (DSC) and 1H NMR. For charged, anionic, or cationic lipids, approximately half of the lipids remain in a fluid state when cooled 20 degrees C below the main chain melting temperature. With a zwitterionic phospholipid, on the other hand, essentially no supercooling of the liquid state was observed. The observations are analyzed in terms of the nucleation and growth of flat solid domains in originally fluid spherical vesicles. As the solid domains grow, the remaining fluid domain is deformed, resulting in a curvature stress. Depending on the vesicle size and the bilayer bending rigidity, the solid domain growth may terminate as the gain in cohesive free energy is balanced by the curvature stress of the remaining fluid domain. It is argued that high bending rigidities are required for having a significant supercooling, which is why it is only observed for charged lipids. 相似文献
99.
Saveyn P Cocquyt J Bomans P Frederik P De Cuyper M Van der Meeren P 《Langmuir : the ACS journal of surfaces and colloids》2007,23(9):4775-4781
Extruded vesicles, which are often used as models for living cells, can change their morphology when they are diluted into a hyperosmotic medium. Different morphological changes were observed with extruded dioctadecyldimethylammonium chloride (DODAC) vesicles after dilution with a nonionic (sucrose) and ionic (CaCl2) osmotic agent above and below the gel-to-liquid crystalline transition temperature. By means of turbidimetry, dynamic light scattering, and cryo-transmission electron microscopy, it was seen that the vesicles only deflated when they were in the gel state, whereas in the liquid crystalline state, an ionic osmotic agent could induce twinning of the vesicles, reminiscent to endocytosis. The latter could occur as a result of the combined effects of reduced repulsion, local dehydration, and reduced bending rigidity induced by the ionic agent. 相似文献
100.
Ramona A. Orza Pieter C.M.M. Magusin Victor M. Litvinov Martin van Duin M.A.J. Michels 《Macromolecular Symposia》2005,230(1):144-148
Peroxide-cross-linked EPM and EPDM rubbers have been investigated with magic-angle spinning 1H NMR spectroscopy and static 1H NMR relaxometry. The results yield a consistent nanoscale picture of the chemical and physical network properties in terms of chemical cross-links formed via macro-radical combination, chemical cross-links formed via addition to the double bonds, and physical cross-links resulting from chain entanglements. 相似文献