Bedeutung der GC-Detektoreigenschaften

Ohne Zusammenfassung

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Synthesis and inversion barriers of undeca- and dodeca-substituted saddle shaped porphyrin complexes

Synthesis and barriers to inversion of a series of highly saddle shaped complexes are reported. The ΔG^≠ has decreased by 8 kJ mol⁻¹ at 243 K when the meso phenyl groups are replaced by bulkier 2,6-dichlorophenyl groups, and by 17 kJ mol⁻¹ when one of the peripheral ethyl groups is removed.     

Thermal degradation of vinylidene chloride/vinyl chloride copolymers in the presence of N-substituted maleimides

As a consequence of their excellent barrier properties vinyl chloride/vinylidene chloride copolymers have long been prominent in the flexible packaging market. While these polymers possess a number of superior characteristics, they tend to undergo thermally- induced degradative dehydrochlorination at process temperatures. This degradation must be controlled to permit processing of the polymers. Three series of N-substituted maleimides (N-alkyl-, N-aralkyl, and N-aryl) have been synthesized, characterized spectroscopically, and evaluated as potential stabilizers for a standard vinyl chloride/vinylidene chloride (85 mass%) copolymer. As surface blends with the polymer, these compounds are ineffective as stabilizers. However, significant stabilization may be achieved by pretreatment of the polymer with N-substituted maleimides. The most effective stabilization of the polymer is afforded by N-aralkyl- or N-arylmaleimides, most notably, N-benzylmaleimide and N-p-methoxyphenylmaleimide.     

活性炭纤维对氙的动态吸附性能研究

采用动态吸附实验装置，研究了吸附温度、原料气浓度和原料气流量对活性炭纤维动态吸附氙性能的影响。结果表明：在低温条件下（≤273K)，随着温度的升高，活性炭纤维对氙的平衡吸附量明显下降／同时，活性炭纤维对氙的平衡吸附量分别随着原料气浓度及原料气流量的增大而增大。     

投料顺序对动态硫化PP／EPDM性能影响及其机理的研究

针对ＰＰ降解在ＰＰ／ＥＰＤＭ动态硫化中对体系产生的不利影响。考察了四种不同投料顺序对ＰＰ／ＥＰＤＭ动态硫化物的冲击韧性，加工流动性，交联程度和断面形貌等宏观和微观性能，并进行了不同ＥＰＤＭ含量的动态硫化和非硫化ＰＰ／ＥＰＤＭ的性能比较。结果表明，优化投料顺序能实现对动态硫化体系中化学反应对象和化学反应程度的有效干预。     

Optimizing constrained subtrees of trees

Given a treeG = (V, E) and a weight function defined on subsets of its nodes, we consider two associated problems. The first, called the rooted subtree problem, is to find a maximum weight subtree, with a specified root, from a given set of subtrees.The second problem, called the subtree packing problem, is to find a maximum weight packing of node disjoint subtrees chosen from a given set of subtrees, where the value of each subtree may depend on its root.We show that the complexity status of both problems is related, and that the subtree packing problem is polynomial if and only if each rooted subtree problem is polynomial. In addition we show that the convex hulls of the feasible solutions to both problems are related: the convex hull of solutions to the packing problem is given by pasting together the convex hulls of the rooted subtree problems.We examine in detail the case where the set of feasible subtrees rooted at nodei consists of all subtrees with at mostk nodes. For this case we derive valid inequalities, and specify the convex hull whenk 4.Research supported in part by Nato Collaborative Research Grant CRG 900281, Science Program SC1-CT91-620 of the EEC, and contract No 26 of the programme Pôle d'attraction interuniversitaire of the Belgian government.     

Simultaneous distillation-extraction and dynamic headspace methods in the gas chromatographic analysis of Parmesan cheese volatiles

Summary Dynamic headspace and simultaneous steam distillation-extraction techniques were used to isolate volatile components of Parmesan cheese. Identification of the substances was carried out by GC and GC-MS; 110 compounds were identified in the samples obtained using the headspace technique, 105 in the extracts; among them, about 50 compounds were isolated with both procedures. Mass spectral data showed evidence for a number of newly reported compounds such as 3-(methylthio)propanal, -tetradecalactone, 9-tetra-decenoic and 9-hexadecenoic acids. Mass spectra of some compounds are discussed and a comparison between the results obtained with the two sampling methods is given.     

Optimality of a Threshold Policy in theM/M/ queue with repeated vacations

Consider anM/M/1 queueing system with server vacations where the server is turned off as soon as the queue gets empty. We assume that the vacation durations form a sequence of i.i.d. random variables with exponential distribution. At the end of a vacation period, the server may either be turned on if the queue is non empty or take another vacation. The following costs are incurred: a holding cost ofh per unit of time and per customer in the system and a fixed cost of each time the server is turned on. We show that there exists a threshold policy that minimizes the long-run average cost criterion. The approach we use was first proposed in Blanc et al. (1990) and enables us to determine explicitly the optimal threshold and the optimal long-run average cost in terms of the model parameters.     

Thermogravimetric properties of inclusion complexes ofβ-cyclodextrin with benzene,acetylsalicylic acid and methyl salicylate

This paper reports TG analyses of inclusion complexes of-cyclodextrin with benzene, acetylsalicylic acid and methyl salicylate. The data were used for calculation of the compositions of the three body complexes and the apparent kinetic parameters of the thermal decompositions. Water exclusion proceeds as a reaction with ordern=1 and an activation energy about 20 kJ/mol. The expulsion of aromatic guest molecules follows ann=1 order process with the activation energy above 155 kJ/mol, except that for the methyl salicylate complex, which was found to be ca. 64 kJ/mol.     

Bond-length fluctuations in transition-metal oxoperovskites

Bond-length fluctuations in transition-metal oxoperovskites may give rise to two-phase fluctuations in what appears to be a single phase to a diffraction experiment. Orbital disorder at Jahn-Teller ions results in bond-length fluctuations that give 3D-ferromagnetic, vibronic Mn(III)-O-Mn(III) superexchange interactions and allow disproportionation into Mn(IV) and Mn(II) in LaMnO₃; where orbitally ordered and disordered phases coexist, an external magnetic field stabilizes the orbitally disordered, ferromagnetic phase relative to the orbitally ordered, antiferromagnetic phase. Spin-lattice interactions in the paramagnetic phase of charge-transfer compounds give bond-length fluctuations arising from the semicovalent component of the superexchange interactions. At the crossover from localized to itinerant electronic behavior, the coexistence of two-phase fluctuations has been demonstrated in both the single-valent RNiO₃ family (R=rare-earth, A=alkaline-earth) and the mixed-valent R_0.5A_0.5MnO₃ perovskites. “Bad-metal” behavior is found to be associated with bond-length fluctuations.