Molecular Imprinting in Sol-Gel Materials: Recent Developments and Applications

Since its inception five decades ago, imprinted sol-gel materials went practically unnoticed, until in the 1970s the conceptual introduction of molecular imprinting in synthetic polymers triggered a new interest in this field. The recent growth in interest in organic–inorganic hybrid materials prepared by sol-gel chemistry and the development of a variety of new strategies for imprinting polymeric matrices have led to a growing activity in what became known as molecularly imprinted sol-gel materials. This paper intends to give an overview of recent progress in molecular imprinting in sol-gel matrices, the potential analytical applications of these tailor-made materials and their limitations, with the aim of drawing attention to useful information and to enhancing interest in this practically unexplored but promising field.     

Thermochemical transformations of polycarbosilane precursors into a ceramic matrix

The possibility of preparing polymeric ceramic-forming precursors from polycarbosilane and oligosilazane was examined. Oligosilazane as a component of polycarbosilane formulations acts as a curing agent, ensuring curing at 150–300°C in the absence of oxygen and modifier. Pyrolysis of the cross-linked copolymer in argon yields an inorganic matrix whose yield is 1.3 times higher than that of pyrolyzates of the starting components. The major phase of the ceramic products obtained, according to the results of X-ray diffraction and chemical analyses, is X-ray amorphous silicon carbide. The ceramic obtained is highly resistant to thermal oxidation (up to 1400°C). The composition of the ceramic and the heat resistance and service characteristics of ceramic-matrix composite materials prepared on its basis using the “polymer technology” can be controlled by varying the ratio of the initial oligomers.     

An investigation of the effect of Rolivsan on the process of synthesis of ceramic matrix from polycarbosilane precursor

Effect of Rolivsan MV-1 on the process of curing and pyrolysis of polycarbosilane PCS-M in argon and on the properties of the ceramic matrix being formed. Use of Rolivsan in polycarbosilane formulations provides effective curing of the latter to give network block-copolymers. Pyrolysis of the resulting precursors in the atmosphere of argon yields ceramic matrices, with the yield of these being 1.3–1.4 times that of the pyrolyzate of the starting components. A study of the elemental and phase composition demonstrated that the matrices belong to the class of amorphous ceramics of nonstoichiometric composition, which contain microcrystalline silicon carbide SiC. The ceramic obtained has a high thermo-oxidative stability (up to 1400°C) and changing the relative amounts of the starting oligomers enables a purposeful variation of its composition, temperature stability, and service characteristics of ceramic-matrix composite materials obtained on its basis by the “polymer technology.”     

On the normality a posteriori for exponential distributions,using the Bayesian estimation

Summary The Bayesian estimation problem for the parameter θ of an exponential probability distribution is considered, when it is assumed that θ has a natural conjugate prior density and a loss-function depending on the squared error is used. It is shown that, with probability one, the posterior density of the Bayesian—centered and scaled parameter converges pointwise to the normal probability density. The weak convergence of the posterior distributions to the normal distribution follows directly. Both correct and incorrect models are studied and the asymptotic normality is stated respectively.     

Computer-aided designing of simple electron-optical systems for electron image tubes

We consider the possibility of designing time-analyzing electron-optical systems for electron image tubes with subpicosecond resolution, containing no spherical electrodes, fine-structure grids, or narrow slots (i.e., extremely simple systems with high spatial and temporal resolutions).     

Log-Convexity of Counting Processes Evaluated at a Random end of Observation Time with Applications to Queueing Models

We consider a counting processes with independent inter-arrival times evaluated at a random end of observation time T, independent of the process. For instance, this situation can arise in a queueing model when we evaluate the number of arrivals after a random period which can depend on the process of service times. Provided that T has log-convex density, we give conditions for the inter-arrival times in the counting process so that the observed number of arrivals inherits this property. For exponential inter-arrival times (pure-birth processes) we provide necessary and sufficient conditions. As an application, we give conditions such that the stationary number of customers waiting in a queue is a log-convex random variable. We also study bounds in the approximation of log-convex discrete random variables by a geometric distribution.     

Organocatalytic enantioselective (3+2) cycloaddition using stable azomethine ylides

We have developed a highly efficient procedure for carrying out the catalytic enantioselective (3+2) cycloaddition between enals and stable azomethine ylides such as isoquinolinium and phthalizinium methylides. Under the optimized reaction conditions highly substituted chiral pyrroloisoquinolines and pyrrolophthalazines have been obtained in high yields and excellent diastereo- and enantioselectivities.     

Computer analysis of the source image structure in 3D electron-optic systems: II. A stationary electromagnetic system

Computer analysis of the image of an arbitrary (point or extended) source obtained in 3D electron-optic systems is performed. The systems involve magnetostatic and electrostatic fields, which, respectively, focus and deflect the electron beams. Two approaches to image scanning are considered where the scan potentials are applied in two (symmetric and asymmetric) modes. It is shown that the spread function of the source, which characterizes the imaging quality of the system most fully, may serve as a primary computational criterion.     

Viscoelastic behaviour of model polyurethane networks in the main transition zone

Starlike poly (oxypropylene) molecules (M=2630 resp. 710 g/mol), each with three OH-end-groups, are crosslinked with 4,4-diphenylmethane diisocyanate in one stoichiometric and several nonstoichiometric reactions. Dynamic mechanical (frequency 10^–4 to 10² Hz) and also some dielectric (10^–2 to 10⁶ Hz) measurements were made on these networks in a wide temperature range. The time-temperature-superposition principle was used to obtain master curves. Two large relaxation processes were detected (separated by many decades of frequency in some samples). The high frequency process seems to correspond to the glass-rubber transition in linear polymers, the low frequency process is probably due to the relaxation of dangling chains.