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排序方式: 共有116条查询结果,搜索用时 15 毫秒
41.
42.
Motivated by the celebrated example of Y. Kannai of a linear partial differential operator which is hypoelliptic but not locally solvable, we consider a class of evolution operators with real-analytic coefficients and study their local solvability both in L2 and in the weak sense. In order to do so we are led to propose a generalization of the Nirenberg-Treves condition (ψ) which is suitable to our study. 相似文献
43.
L. Posp?&#x;šil R. Trsková S. Záliš M.P. Colombini R. Fuoco 《Microchemical Journal》1996,54(4):367-374
The reduction of atrazine and terbutylazine was preceded by protonation equilibrium. Three protonation sites of thes-triazine molecule determined the structure of the final reduction product. Protonation was investigated by the change of UV–Vis spectra. Two slightly different pKs corresponding to protonation on N5 and N1 heteroatoms were evaluated. The principal reduction pathway involved the cleavage of a chlorine atom. A small quantity of desethylatrazine was detected in the most acidic media. 相似文献
44.
Didier Colombini Helen Hassander Ola J. Karlsson Frans H. J. Maurer 《Journal of Polymer Science.Polymer Physics》2005,43(17):2289-2306
The effects of thermal annealing on the viscoelastic properties and morphology of films prepared from bimodal latex blends containing equal weight fractions of soft and hard latex particles with controlled sizes were investigated. The thermal and viscoelastic properties of as‐dried and annealed samples were investigated with differential scanning calorimetry and dynamic mechanical analysis (DMA). Throughout the thermal annealing, the latex blend morphologies were also followed with atomic force microscopy and transmission electron microscopy (TEM). A particulate morphology, consisting of hard particles evenly dispersed in a continuous soft phase, was observed in the TEM micrographs of the as‐dried latex blends and resulted in an enhancement of the mechanical film properties at temperatures between the α relaxations of the soft and hard phases in the DMA thermograms. As soon as the thermal annealing involved temperatures higher than the glass‐transition temperature of the hard phase, the hard particles progressively lost their initial spherical shape and formed a more or less continuous phase in the latex blends. This induced coalescence of the hard particles was confirmed by the association of the experimental viscoelastic data with theoretical predictions, based on self‐consistent mechanical models, which were performed by the consideration of either a particulate or cocontinuous morphology for the bimodal latex blends. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2289–2306, 2005 相似文献
45.
46.
Gas chromatographic and mass spectrometric investigations of organic residues from Roman glass unguentaria 总被引:1,自引:0,他引:1
A combination of gas chromatographic (GC) and mass spectrometric (MS) techniques, including direct exposure-MS (DE-MS), high-temperature GC-MS (HTGC-MS) and GC-MS of neutral and acid fractions, was employed to study the composition and recognise origin of the organic materials used to manufacture balm residues surviving in a series of glass unguentaria recovered from excavations of a Roman villa (Villa B) in the ancient town of Oplontis (Naples, Italy). DE-MS provided comprehensive 'fingerprint' information on the solvent soluble components of the contents of the unguentaria, while GC-MS analyses provided detailed molecular compositions, highlighting the presence of a wide range of compound classes including mid- and long-chain fatty acids, long-chain hydroxy-acids, n-alkanols, alkandiols, n-alkanes, long-chain monoesters, phytosterols and diterpenoid acids. Characteristic biomarkers and their distributions indicate the presence of beeswax, Pinaceae resin and another wax, as the main organic constituents of all of the preparations examined. In particular, the occurrence of phytosterols and long-chain monoesters, in which the acyl moiety was not exclusively palmitic acid, suggested the presence of a second waxy-lipid constituent of plant origin. The results are consistent with beeswax being used in the preparation of the cosmetics preserved in the unguentaria, while the other lipids are most likely the residue of some as yet unidentified plant extract(s), possibly deriving from the cuticular waxes of flowers and/or leaves. The composition of the extracts are consistent with the ancient practices of maceration and/or "enfleurage", in which lipid-based materials, such as beeswax, animal fat or vegetables oils, were used to extract aromatic and fragrant substances from resin, flowers, spices and scented wood, in order to produce unguents and balms. 相似文献
47.
Ghezzi Lisa Duce Celia Bernazzani Luca Bramanti Emilia Colombini Maria Perla Tiné Maria Rosaria Bonaduce Ilaria 《Journal of Thermal Analysis and Calorimetry》2015,122(1):315-322
Journal of Thermal Analysis and Calorimetry - The thermal degradation of rabbit skin glue, a collagen-based proteinaceous material used as a paint binder in paintings, was investigated in this... 相似文献
48.
Jeannette J. Łucejko Magdalena Zborowska Francesca Modugno Maria P. Colombini Włodzimierz Prądzyński 《Analytica chimica acta》2012
The macromolecular complexity of wood limits the possibility of obtaining complete chemical information on its alteration in archaeological objects. This paper compares the results obtained in the characterisation of the components of archaeological wood by a classical wet chemical method and by an instrumental method based on pyrolysis in presence of hexamethyldisilazane coupled with gas chromatography/mass spectrometry, Py(HMDS)–GC/MS. We compare the results obtained with the two methods quantitatively. This enables us to evaluate the efficiency of Py(HMDS)–GC/MS in assessing the chemical composition and the state of conservation of degraded wood. The material analysed consisted of reference sound wood and waterlogged wood from the ?ó?te historical site, located on a small island on Lake Zarańskie in Poland. The samples are from the remains of settlements dating to a period between the 9th and the 12th centuries AD. 相似文献
49.
This review discusses the application of chromatographic techniques (GC, HPLC and Py-GC) for the characterisation of proteinaceous materials in artistic paintings. The focus is on the various analytical steps that are needed to determine these natural materials in paint samples, from sampling and sample pre-treatment, including various methods of hydrolysis and derivatisation for GC and HPLC, to approaches for data evaluation. 相似文献
50.
This paper describes an analytical approach to investigate the origin of oxalate films on marble. Calcium oxalate films were collected on buildings of historical importance in Lucca and Pisa (Italy) and characterised by optical microscopy (OM), scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDX), Fourier transform infrared (FTIR) spectroscopy (equipped with diamond cell), and gas chromatography-mass spectrometry (GC-MS). The morphology of the films was investigated by optical and electronic microscopy. FTIR analyses highlighted the presence of calcium oxalate (both as whewellite and weddellite), gypsum, calcite, nitrates, silicates and apatite, while EDX maps showed the distribution of elements. Several samples showed traces of organic compounds, identified by GC-MS as paraffin wax, lipids of animal origin and egg. The correlation between organic material and oxalate contents suggests the origin of the films from degradation processes of past surface treatment. 相似文献