共查询到20条相似文献,搜索用时 31 毫秒
1.
A report is given of progress achieved during a 5-year period for describing the solidification of polymers by crystallization. Emphasis is placed on the fact that the classical quasi-equilibrium treatment of solidification under heat transfer conditions (as known under the heading of moving boundary value problems) is unable to predict any texturing in one-component systems. For polymers which are notorious for their slow approach to thermodynamic equilibrium, the local equilibrium theories, which are popular in metallurgy, are not discussed. In the present paper we show how to cope with nucleation under changing temperature conditions. Use is made of an adaptation of Avrami's theory to nonisothermal conditions, as recently proposed. Experimental work is extended to conditions of flow, where nucleation is enhanced. We show the possibility of theoretically describing this latter phenomenon. 相似文献
2.
A. Sahuquillo R. Rubio G. Rauret B. Griepink 《Fresenius' Journal of Analytical Chemistry》1995,352(6):572-576
The determination of chromium by flame atomic absorption spectroscopy (FAAS), e.g. as performed for environmental matrices, is still a controversial matter as can be concluded from a literature review. Most controversy is due to the measurement conditions or the effect of the interferences; other conditions such as digestion seem to be less critical. Consequently, this paper reports a systematic study of the instrumental settings for the two flame modes that are most widely used: air-acetylene and nitrous oxide-acetylene. In connection with the study of experimental conditions, possible interferences are investigated involving nineteen cations and some anions in different ratios Cr:interferent. Different releasers to overcome interferences are considered. The results are critically compared with current literature. For the operational conditions the quality parameters such as the linear range, detection limits, precision and accuracy are established. Although the nitrous oxide-acetylene flame is mostly recommended, this study reveals that a better choice can be the use of a fuel-rich air-acetylene flame with the addition of 1% 8-hydroxyquinoline (oxine) as protective agent. The method is applied to and validated by four sediment certified reference materials. 相似文献
3.
The interaction energies between gelatin-coated surfaces at various electrolyte and pH conditions are reported. The surfaces are of glass and are negatively charged under all conditions used here. Gelatin is a polyampholyte, with an isoelectric pH (IEP) of approximately 4.9. At low pH the gelatin molecules have a net positive charge, and thus the polyampholyte tends to adsorb with a relatively flat conformation. As the pH is increased the strong attractive interaction between the surface and the polyampholyte decreases as more negative charges and then fewer positive charges appear on the polyampholyte, and so the gelatin extends away from the surface. On changing electrolyte concentrations after adsorption no effect was seen at the IEP, but the layer was observed to swell at more alkaline pHs. This is consistent with the net minimum charge situation on the polymer under these conditions. Changing the adsorption conditions was seen to have an effect, and this is attributed to the different affinities of the gelatin chain to the surface depending on the solution chemistry. Results obtained when the gelatin was initially adsorbed on one surface or two were similar, suggesting that the gelatin transfers rapidly from one surface to another. The importance of adsorption conditions as well as current conditions is discussed. Copyright 1999 Academic Press. 相似文献
4.
《Spectrochimica Acta Part B: Atomic Spectroscopy》2006,61(3):284-300
This paper is a summary of the current knowledge of laser-generated aerosols under atmospheric conditions. It is restricted to typical laser sampling conditions as they are used in LA-ICP spectrometry. Published experimental evidence and proposed models are reviewed and critical summarized. The collected works show that a certain agreement exists that independently of the sample two size fractions with different chemical composition are found. The mechanism generating the different particle fractions are currently not clear. Possible sources of particle generation are described and critically reviewed. Fundamentally three distinguishable modes (gas, liquid, solid) can be described that can appear: gas-to-particle conversion, hydrodynamic sputtering, mechanical spallation/exfoliation. More recently explosive boiling as a mechanism of liquid expulsion has been discussed as a further possible source under certain conditions. Particle conditioning during transport is discussed as a source for agglomeration. The correlation between size distribution and laser parameters is discussed. 相似文献
5.
6.
A. Knöchel 《Fresenius' Journal of Analytical Chemistry》1990,337(6):614-621
Summary At first glance X-Ray fluorescence analysis seems to be a very sensitive and effectful method to detect element traces in the multielement mode. But in praxi the application range is restricted, if X-Ray tubes are used as excitation sources. To overcome this situation, it is necessary to improve the conditions of excitation and to reduce the background, produced by different scattering effects. TXRF, PIXE and SYXRF, which allow multielement analysis in the trace- and ultratrace region are using this strategy. In the case of TXRF a remarkable background reduction is achieved if the sample is prepared as a thin amorphous film on a planar sample holder and the excitation beam of a X-Ray tube is totally reflected on its surface. In the case of PIXE a particle beam of high intensity is used as excitation source, improving the conditions of excitation and giving the opportunity of spatial resolved analyses. In the case of SYXRF the X-Ray fraction of synchrotron radiation is used as excitation source, giving the opportunity, to improve the conditions of excitation as well as to reduce the background by using the high polarisation of the beam. In this case, too, spatial resolved analysis are possible. The principles of the three methods are described, their advantages and disadvantages are critically compared and advanced applications from different analytical fields are presented. 相似文献
7.
《Electroanalysis》2004,16(10):832-842
A comparative study of different types of cathodic stripping reactions under conditions of square‐wave voltammetry is presented. Cathodic stripping processes involving reactions of second order as well as reactions coupled by adsorption of the reacting ligand are analyzed The inherent parameters, controlling the overall voltammetric behavior of each cathodic stripping electrode reaction are derived. The criteria for qualitative distinguishing of each mechanism are established as well as a methodology for redox kinetic measurements is proposed. The influence of the parameters of the excitation signal on the properties of the voltammetric response is analyzed in order to find optimal conditions for analytical application. The theoretical results are illustrated by the experiments with a series of uracil derivatives. 相似文献
8.
《Macromolecular theory and simulations》2017,26(6)
Forward and inverse artificial neural network (ANN) models are used to describe ethylene/1‐butene copolymerization with a model catalyst having two site types. The forward ANN predicts number and weight average molecular weights, average comonomer content, and polymer yield as a function of a set of polymerization conditions, while the inverse model estimates polymerization conditions needed to produce copolymers with desired microstructures. The forward model is found to be robust and resilient to random noise introduced into the datasets. The inverse model, however, leads to multiple solutions (several polymerization conditions can produce polymers with similar microstructures) and is sensitive to random noise in the data. Although the polymerization conditions estimated from inverse ANN are different from the model data, the estimated polymerization conditions are found to provide similar microstructures even with the random noise. 相似文献
9.
《Journal of separation science》2018,41(17):3352-3359
Amphiphilic di‐ and tri‐block copolymers based on poly(ethylene oxide) as a hydrophilic segment and poly(ε‐caprolactone) as a hydrophobic part are synthesized by the ring‐opening polymerization of ε‐caprolactone while using poly(ethylene glycol)s and methoxy poly(ethylene glycol)s of varying molar masses as macro‐initiators. The synthesized block copolymers are characterized with respect to their total relative molar mass and its distribution by size exclusion chromatography. Liquid chromatography at critical conditions of both blocks is established for the analysis of individual block lengths and tracking presence of unwanted homopolymers of both types in the block copolymer samples. New critical conditions of polycaprolactone on reversed phase column are reported using organic mobile phase. The established critical conditions of polycaprolactone extended the applicable molar mass range significantly compared to already reported critical conditions of polycaprolactone in aqueous mobile phase. Block copolymers are also analyzed at critical conditions of poly(ethylene glycol). Complete analysis of the di‐ and tri‐block copolymers at corresponding critical conditions provided a fair estimate of molar mass of non‐critical block besides information regarding presence of homopolymers of both types in the samples. 相似文献
10.
11.
Chromatographic sorbents used within the purification of peptide or protein based active pharmaceutical ingredients (APIs) are commonly subjected to caustic regeneration procedures, so-called CIP treatments. While polymeric materials remain unaffected by this treatment, silica-based sorbents are at an intrinsic risk of dissolution under high pH conditions, such as, e.g. 0.1M NaOH. It is common misconception that silica-based materials simply cannot be subjected to alkaline conditions above pH 9. Moreover, most studies covering the chemical stability of HPLC sorbents above pH 9 have been limited to the chromatographic conditions used for the separations themselves. Such studies have used buffered mobile phases up to pH 11 or 12. Very little focus has been put on the stability of the stationary phases when subjected to shorter but harsher pH conditions required for regeneration purposes, such as 0.1M NaOH (pH 13). Knowledge about the amount of so-called leachables, degradation products originating from the stationary phase, is of growing importance for the registration of pharmaceuticals for human use and is addressed in this work. This study compares the chemical stability of different commercially available reversed phase silica materials (C18) that are used in industrial scale preparative HPLC. The silica materials were subjected to NaOH regeneration conditions and it is shown that some materials are able to withstand 0.1M NaOH conditions without significant harm. It is demonstrated that contaminants present in the effluent in the range of 10-50 microg/mL can lead to significant contamination of API product fraction. 相似文献
12.
Tölle P Köhler C Marschall R Sharifi M Wark M Frauenheim T 《Chemical Society reviews》2012,41(15):5143-5159
The conventional polymer electrolyte membrane (PEM) materials for fuel cell applications strongly rely on temperature and pressure conditions for optimal performance. In order to expand the range of operating conditions of these conventional PEM materials, mesoporous functionalised SiO(2) additives are developed. It has been demonstrated that these additives themselves achieve proton conductivities approaching those of conventional materials. However, the proton conduction mechanisms and especially factors influencing charge carrier mobility under different hydration conditions are not well known and difficult to separate from concentration effects in experiments. This tutorial review highlights contributions of atomistic computer simulations to the basic understanding and eventual design of these materials. Some basic introduction to the theoretical and computational framework is provided to introduce the reader to the field, the techniques are in principle applicable to a wide range of other situations as well. Simulation results are directly compared to experimental data as far as possible. 相似文献
13.
G. S. Anagnostatos 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,19(4):121-124
An alkali-atom shell model for alkali microclusters is introduced, where the quantum constituent is the neutral atom itself considered as a heavy fermion in a central potential created by all atoms in each shell of the cluster. Thus, here the magic numbers are due to atoms and not to electrons as in jellium models. The model is applicable under conditions (of production, temperature, size, etc.) not favoring delocalization of valence electrons. For conditions favoring electron delocalization the jellium model is applicable. New experiments are suggested via nuclear methods. 相似文献
14.
Hamid Bozorgzadeh Mahmood Nickkho-Amiry John M. Winfield 《Journal of fluorine chemistry》2003,121(1):83-92
The preparation and characterisation of two series of fluorinated chromia aerogel materials, lightly doped with zinc(II) or magnesium(II), are described. They behave as heterogeneous catalysts for transformations of 1,1,2-trichlorotrifluoroethane under HF-free conditions and at moderate temperatures. Product distributions depend critically on the contact time. When the latter is very long (static conditions) the surface becomes chlorinated, notwithstanding its fluorinated nature. Rather surprisingly, in view of previous work, the catalytic behaviour of the materials is almost identical to that of the undoped fluorinated chromia aerogel. The nature of reactions occurring under HF-free conditions and low levels of doping achievable are discussed as possible factors in determining catalytic behaviour. 相似文献
15.
Mirella Pérez Noelia Zambrano Marta Ramirez Eric Tyrode Jean-Louis Salager 《Journal of Dispersion Science and Technology》2013,34(1-3):55-63
Emulsion drop size depends on the both formulation and composition of the surfactant-oil-water system, as well as on the stirring conditions prevailing during emulsification. General trends versus formulation or composition changes are presented. However, it is shown that the effects are not independent and that a proper combination of these parameters allows the attainment of very small drop size, even at low stirring energy. An overall phenomenology is presented on a two-dimensional formulation-composition map from which it is easy to select the best emulsification conditions. 相似文献
16.
Functionalized indoles are synthezised under mild conditions in a tartaric acid-dimethylurea melt. The melt serves as the solvent and as the catalyst. Under these reaction conditions, sensitive functional groups such as N-Boc, N-Cbz, or azides are stable, and indolenines are obtained regioselectively in excellent yields. The practical use of the method is demonstrated in the synthesis of the hormone melatonin. 相似文献
17.
In practical applications biosensors are often forced to operate under less than optimal conditions. Because of their construction, and the physical processes and chemical reactions involved in their operation, compromise conditions are frequently required to synchronize all events taking place. In this communication it is shown Flow Injection Analysis offers itself as a complementary facility to augment the performance of biosensors, and in many cases as an attractive alternative. Based on the argument that instead of striving to force different reactions to work under compromise conditions, it is much better to separate them, optimize them individually and let them interact with each other in harmony, selected flow-injection applications are discussed. Initially, comments are made on operating sensors in the FIA mode, while the following sections deal with selected examples of enzymatic assays, aimed either at measuring substrate concentrations, including the use of enzyme amplification schemes, or determining enzyme activities. 相似文献
18.
19.
Spin locking of the nuclear magnetization of a spin with S=1 such as deuterium in the presence of a radio-frequency field under magic angle spinning (MAS) is described in terms of adiabatic modulations of the energy levels. In a brief initial period, part of the initial density operator nutates about the Hamiltonian and is dephased. The remaining spin-locked state undergoes persistent oscillatory transfer processes between various coherences with a periodicity given by the rotation of the sample. While all crystallites in the powder undergo such periodic transfer processes, the phases of the oscillations depend on the angle gamma of the crystallites. The angle gamma is the azimuthal angle defining the orientation of the unique axis of the quadrupolar interaction tensor in a rotor-fixed frame. The theory is extended to describe cross-polarization between spins S=1 and I=12 under MAS. There are four distinct Hartmann-Hahn matching conditions that correspond to four zero-quantum matching conditions, all of which are shifted and broadened compared to their spin S=12 counterparts. These matching conditions are further split into a family of sideband conditions separated by the spinning frequency. The theory allows the calculation of both shifts and broadening factors of the matching conditions, as verified by simulations and experiments. 相似文献
20.
Á. Alberto Magreñán Ioannis K. Argyros Lara Orcos Juan Antonio Sicilia 《Journal of mathematical chemistry》2018,56(7):1935-1957
We present a local as well a semilocal convergence analysis of secant-like methods under g eneral conditions in order to approximate a locally unique solution of a nonlinear equation in a Banach space setting. The new conditions are more flexible than in earlier studies. This way we expand the applicability of these methods, since the new convergence conditions are weaker. Moreover, these advantages are obtained under the same conditions as in earlier studies. Numerical examples are also provided in this study, where our results compare favorably to earlier ones. 相似文献