ИНФОРМАЦИОННАЯ БЕЗОПАСНОСТЬ СЕТЕЙ СВЯЗИ

Современная информационно-телекоммуникационная система представляет собой сложную систему, состоящую из большого числа компонентов различной степени автономности, которые связаны между собой и обмениваются данными. Практически каждый компонент может подвергнуться внешнему воздействию или выйти из строя. Появление на отечественном рынке и внедрение в эксплуатацию современных импортных цифровых АТС приводят к повышению уязвимости телефонных сетей связи и информационных ресурсов организаций

Stochastic lattice models for the dynamics of linear polymers

Linear polymers are represented as chains of hopping reptons and their motion is described as a stochastic process on a lattice. This admittedly crude approximation still catches essential physics of polymer motion, i.e. the universal properties as function of polymer length. More than the static properties, the dynamics depends on the rules of motion. Small changes in the hopping probabilities can result in different universal behavior. In particular the cross-over between Rouse dynamics and reptation is controlled by the types and strength of the hoppings that are allowed. The properties are analyzed using a calculational scheme based on an analogy with one-dimensional spin systems. It leads to accurate data for intermediately long polymers. These are extrapolated to arbitrarily long polymers, by means of finite-size-scaling analysis. Exponents and cross-over functions for the renewal time and the diffusion coefficient are discussed for various types of motion.Comment: 60 pages, 19 figure

Molecular dynamics of polymers by means of NMR field cycling relaxometry

The molecular dynamics of polymer melts and networks with molecular weight well above the critical value were studied based on a power law dependence of the NMR spin-lattice relaxation time (T1) as a function of the Larmor frequency, predicted by Kimmich and Fatkullin, which were developed from the renormalized Rouse formalism. Three different regions of power laws explaining the relaxation dispersion for the chain mode dynamics of polymer melts can be distinguished and have been observed experimentally over a range of molecular weights and temperatures for several types of polymer melts. In these studies, the T1 relaxation dispersions of polymer melts and networks were obtained over a wide range of frequencies and temperatures employing NMR field cycling relaxometry, and WLF master curves were reconstructed to describe the dispersion over more than nine orders of magnitude. Influences of chemical and physical constraints on the relaxation dispersion of polymers were investigated. This included the introduction of chemical cross-links and filler particles to produce elastomers, for which further constraints were imposed by mechanical deformation. As an alternative means to affect molecular mobility, investigations were also carried out for polystyrene-polybutadiene (PS-PB) copolymers. In all cases, the T1 relaxation dispersions directly reflect the molecular dynamics which were altered in different ways by the inflicted restrictions. Moreover, solvent induced molecular mobility in both polymer melts and networks were studied and additional experiments measuring the transverse relaxation rate and the dipolar coupling strength via double quantum encoding were conducted. The polymer networks used in these studies were natural rubber, butadiene rubber, and polydimethylsiloxane rubber samples. The results obtained here are interpreted based on the molecular dynamics described for the polyisoprene (PI), polybutadiene (PB), and polydimethylsiloxane (PDMS) melts of higher molecular weight (Mw > Mc), where only the physical entanglements are present. In terms of field cycling relaxometry, the power-law relations, which describe the chain mode dynamics for the polymer melts, were used to analyze the experimentally obtained data. The results obtained from the PI melts were somewhat different from those of the other polymers and, therefore, partially deuterated PI melts were investigated employing proton and deuteron experiments to elucidate whether the chemical structure or the intermolecular interaction play the significant role for this particular behavior

Dependence of Order and Dynamics in Polymers and Elastomers under Deformation Revealed by NMR Techniques

Uniaxial stretching and swelling are considered as two limiting cases of deformations of elastomers. Under both conditions, the molecular dynamics is changed with respect to the behavior that describes the undisturbed, equilibrium elastomer. Particularly the spectrum of segmental motions, which reveals itself in the frequency-dependence of the longitudinal NMR relaxation time, is discussed in this study, but also order effects expressed via the dipolar coupling strength are investigated. For stretched elastomers, a significant change of the relaxation dispersion is found for three different types of rubber; it is a consequence of a change of the mode spectrum of segmental motions that becomes obvious at low frequencies (below 1 MHz at room temperature). In swollen elastomers, on the other hand, a cross-over towards a behavior expected for semi-dilute solutions is found, and a comparison to solutions of uncross-linked polymers reveals a significant effect of the cross-links only in the kHz range. A much more pronounced difference between elastomers and polymer solutions, however, is found from double-quantum encoded NMR measurements where the residual order introduced by the presence of permanent cross-links is maintained even in the presence of solvent

Western Star (Corner Brook, N.L.), 1914-06-03

The Western Star began publication on Newfoundland's west coast on 4 April 1900, appearing weekly with brief semiweekly periods up to 1952, when it became a daily. The current collection contains 21 April 1900 - 31 December 1952