bluuebunny/crossref_metadata_2025 huggingface dataset:
Large Poisson-Voronoi cells and Crofton cells
Authors: Hug Daniel , Reitzner Matthias , Schneider Rolf
Published: August 2004

It is proved that the shape of the typical cell of a stationary Poisson-Voronoi tessellation in Euclidean space, under the condition that the volume of the typical cell is large, must be close to spherical, with high probability. The same holds if the volume is replaced by the surface area or other suitable functionals. Similar results are established for the zero cell of a stationary and isotropic Poisson hyperplane tessellation.
Random minimal directed spanning trees and Dickman-type distributions
Authors: Penrose Mathew D. , Wade Andrew R.
Published: August 2004

In Bhatt and Roy's minimal directed spanning tree construction for n random points in the unit square, all edges must be in a south-westerly direction and there must be a directed path from each vertex to the root placed at the origin. We identify the limiting distributions (for large n) for the total length of rooted edges, and also for the maximal length of all edges in the tree. These limit distributions have been seen previously in analysis of the Poisson-Dirichlet distribution and elsewhere; they are expressed in terms of Dickman's function, and their properties are discussed in some detail.
On estimation of the Euler number by projections of thin slabs
Authors: Rataj J.
Published: August 2004

A stereological formula for the Euler number involving projections of the set in thin parallel slabs is considered. Sufficient conditions for the validity of this formula are derived.
An interference problem with application to crystal growth
Authors: Knight F. B. , Steichen J. L.
Published: August 2004

We model diffusion-controlled crystal growth as an interference problem. The crystal layers grow by nucleation (initiation of crystallization centers) followed by attachment of molecules to the nucleus. A forming crystal layer completes by either spreading across the length of the crystal or by colliding with another spreading crystal layer. This model differs from the classical Johnson-Mehl-Kolmogorov model in that nucleation happens only on boundaries of a ‘seed’ crystal as opposed to nucleation from random points in a given region. Our results also differ from the limiting results found for this classical model. We use the invariant measure of an embedded Markov process to find the growth rate of the crystal in terms of the nucleation rates. Ergodic theorems are then used to derive explicit formulae for some stationary probabilities.
The log-normal approximation in financial and other computations
Authors: Dufresne Daniel
Published: August 2004

Sums of log-normals frequently appear in a variety of situations, including engineering and financial mathematics. In particular, the pricing of Asian or basket options is directly related to finding the distributions of such sums. There is no general explicit formula for the distribution of sums of log-normal random variables. This paper looks at the limit distributions of sums of log-normal variables when the second parameter of the log-normals tends to zero or to infinity; in financial terms, this is equivalent to letting the volatility, or maturity, tend either to zero or to infinity. The limits obtained are either normal or log-normal, depending on the normalization chosen; the same applies to the reciprocal of the sums of log-normals. This justifies the log-normal approximation, much used in practice, and also gives an asymptotically exact distribution for averages of log-normals with a relatively small volatility; it has been noted that all the analytical pricing formulae for Asian options perform poorly for small volatilities. Asymptotic formulae are also found for the moments of the sums of log-normals. Results are given for both discrete and continuous averages. More explicit results are obtained in the case of the integral of geometric Brownian motion.
The identifiability problem for repairable systems subject to competing risks
Authors: Bedford Tim , Lindqvist Bo H.
Published: August 2004

Within reliability theory, identifiability problems arise through competing risks. If we have a series system of several components, and if that system is replaced or repaired to as good as new on failure, then the different component failures represent competing risks for the system. It is well known that the underlying component failure distributions cannot be estimated from the observable data (failure time and identity of failed component) without nontestable assumptions such as independence. In practice many systems are not subject to the ‘as good as new’ repair regime. Hence, the objective of this paper is to contrast the identifiability issues arising for different repair regimes. We consider the problem of identifying a model within a given class of probabilistic models for the system. Different models corresponding to different repair strategies are considered: a partial-repair model, where only the failing component is repaired; perfect repair, where all components are as good as new after a failure; and minimal repair, where components are only minimally repaired at failures. We show that on the basis of observing a single socket, the partial-repair model is identifiable, while the perfect- and minimal-repair models are not.
Interacting reinforced-urn systems
Authors: Paganoni Anna Maria , Secchi Piercesare
Published: August 2004

We introduce a class of discrete-time stochastic processes generated by interacting systems of reinforced urns. We show that such processes are asymptotically partially exchangeable and we prove a strong law of large numbers. Examples and the analysis of particular cases show that interacting reinforced-urn systems are very flexible representations for modelling countable collections of dependent and asymptotically exchangeable sequences of random variables.
Maxima of continuous-time stationary stable processes
Authors: Samorodnitsky Gennady
Published: August 2004

We study the suprema over compact time intervals of stationary locally bounded α-stable processes. The behaviour of these suprema as the length of the time interval increases turns out to depend significantly on the ergodic-theoretical properties of a flow generating the stationary process.
Perturbation analysis for denumerable Markov chains with application to queueing models
Authors: Altman Eitan , Avrachenkov Konstantin E. , Núñez-Queija Rudesindo
Published: August 2004

We study the parametric perturbation of Markov chains with denumerable state spaces. We consider both regular and singular perturbations. By the latter we mean that transition probabilities of a Markov chain, with several ergodic classes, are perturbed such that (rare) transitions among the different ergodic classes of the unperturbed chain are allowed. Singularly perturbed Markov chains have been studied in the literature under more restrictive assumptions such as strong recurrence ergodicity or Doeblin conditions. We relax these conditions so that our results can be applied to queueing models (where the conditions mentioned above typically fail to hold). Assuming ν-geometric ergodicity, we are able to explicitly express the steady-state distribution of the perturbed Markov chain as a Taylor series in the perturbation parameter. We apply our results to quasi-birth-and-death processes and queueing models.
Response times in M/M/s fork-join networks
Authors: Ko Sung-Seok , Serfozo Richard F.
Published: August 2004

We study a fork-join processing network in which jobs arrive according to a Poisson process and each job splits into m tasks, which are simultaneously assigned to m nodes that operate like M/M/s queueing systems. When all of its tasks are finished, the job is completed. The main result is a closed-form formula for approximating the distribution of the network's response time (the time to complete a job) in equilibrium. We also present an analogous approximation for the distribution of the equilibrium queue length (the number of jobs in the system), when each node has one server. Kolmogorov-Smirnov statistical tests show that these formulae are good fits for the distributions obtained from simulations.
On the subexponential properties in stationary single-server queues: a Palm-martingale approach
Authors: Miyoshi Naoto
Published: August 2004

This paper studies the subexponential properties of the stationary workload, actual waiting time and sojourn time distributions in work-conserving single-server queues when the equilibrium residual service time distribution is subexponential. This kind of problem has been previously investigated in various queueing and insurance risk settings. For example, it has been shown that, when the queue has a Markovian arrival stream (MAS) input governed by a finite-state Markov chain, it has such subexponential properties. However, though MASs can approximate any stationary marked point process, it is known that the corresponding subexponential results fail in the general stationary framework. In this paper, we consider the model with a general stationary input and show the subexponential properties under some additional assumptions. Our assumptions are so general that the MAS governed by a finite-state Markov chain inherently possesses them. The approach used here is the Palm-martingale calculus, that is, the connection between the notion of Palm probability and that of stochastic intensity. The proof is essentially an extension of the M/GI/1 case to cover ‘Poisson-like’ arrival processes such as Markovian ones, where the stochastic intensity is admitted.
Network decomposition in the many-sources regime
Authors: Eun Do Young , Shroff Ness B.
Published: August 2004

We derive results that show the impact of aggregation in a queueing network. Our model consists of a two-stage queueing system where the first (upstream) queue serves many flows, of which a certain subset arrive at the second (downstream) queue. The downstream queue experiences arbitrary interfering traffic. In this setup, we prove that, as the number of flows being aggregated in the upstream queue increases, the overflow probability of the downstream queue converges uniformly in the buffer level to the overflow probability of a single queueing system obtained by simply removing the upstream queue in the original two-stage queueing system. We also provide the speed of convergence and show that it is at least exponentially fast. We then extend our results to non-i.i.d. traffic arrivals.
On measuring fairness in queues
Authors: Avi-Itzhak Benjamin , Levy Hanoch
Published: August 2004

The issue of ‘fairness’ is raised frequently in the context of evaluating queueing policies, notably in relation to telecommunications and computer systems where it may be of no lesser importance than the conventional measures of performance. Comparisons of the fairness of various systems and policies are often awkward due to lack of generally accepted definitions and measures for this important property. The purpose of this work is to propose possible fairness measures enabling us to quantitatively measure and compare the level of fairness associated with G/G/R queueing systems. We define and discuss order (of service) fairness and use an axiomatic approach for developing a measure for it in the G/D/1 case. The measure obtained for the G/D/1 system is then generalized and applied to the G/G/R class of systems. A practical implication of this work is that, for a wide class of service disciplines, the variance of the waiting time can be used as a yardstick for comparing fairness levels.
On the martingale property of stochastic exponentials
Authors: Wong Bernard , Heyde C. C.
Published: August 2004

We present a necessary and sufficient condition for a stochastic exponential to be a true martingale. It is proved that the criteria for the true martingale property are related to whether a related process explodes. An alternative and interesting interpretation of this result is that the stochastic exponential is a true martingale if and only if under a ‘candidate measure’ the integrand process is square integrable over time. Applications of our theorem to problems arising in mathematical finance are also given.
Ruin probabilities for competing claim processes
Authors: Huzak Miljenko , Perman Mihael , Šikić Hrvoje , Vondraček Zoran
Published: August 2004

Let C1, C2,…,Cm be independent subordinators with finite expectations and denote their sum by C. Consider the classical risk process X(t) = x + ct - C(t). The ruin probability is given by the well-known Pollaczek–Khinchin formula. If ruin occurs, however, it will be caused by a jump of one of the subordinators Ci. Formulae for the probability that ruin is caused by Ci are derived. These formulae can be extended to perturbed risk processes of the type X(t) = x + ct - C(t) + Z(t), where Z is a Lévy process with mean 0 and no positive jumps.
Space filling and depletion
Authors: Baryshnikov Yuliy , Coffman E. G. , Jelenković Predrag
Published: August 2004

For a givenk≥ 1, subintervals of a given interval [0,X] arrive at random and are accepted (allocated) so long as they overlap fewer thanksubintervals already accepted. Subintervals not accepted are cleared, while accepted subintervals remain allocated for random retention times before they are released and made available to subsequent arrivals. Thus, the system operates as a generalized many-server queue under a loss protocol. We study a discretized version of this model that appears in reference theories for a number of applications, including communication networks, surface adsorption-desorption processes, and reservation systems. Our primary interest is in steady-state estimates of the vacant space, i.e. the total length of available subintervalskX- ∑ℓi, where the ℓiare the lengths of the subintervals currently allocated. We obtain explicit results fork= 1 and for generalkwith all subinterval lengths equal to 2, the classicaldimercase of chemical applications. Our focus is on the asymptotic regime of large retention times.
Limiting properties of Poisson shot noise processes
Authors: Lund Robert , McCormick William P. , Xiao Yuanhui
Published: August 2004

This paper studies limiting properties of discretely sampled Poisson shot noise processes. Versions of the law of large numbers and central limit theorem are derived under very general conditions. Examples demonstrating the utility of the results are included.
Cellular composition and ultrastructure of the gill epithelium of larval and adult lampreys
Authors: Bartels Helmut , Potter Ian C.
Published: August 2004

Lampreys, one of the only two surviving groups of agnathan (jawless)vertebrates, contain several anadromous species that, during their life cycle,thus migrate from fresh to seawater and back to freshwater. Lampreys have independently evolved the same overall osmoregulatory mechanisms as the gnathostomatous (jawed) and distantly related teleost fishes. Lamprey gills thus likewise play a central role in taking up and secreting monovalent ions. However, the ultrastructural characteristics and distribution of their epithelial cell types [ammocoete mitochondria-rich (MR) cell, intercalated MR cell, chloride cell and pavement cell] differ in several respects from those of teleosts. The ultrastructural characteristics of these cells are distinctive and closely resemble those of certain ion-transporting epithelia in other vertebrates, for which the function has been determined. The data on each cell type, together with the stage in the life cycle at which it is found, i.e. whether in fresh or seawater, enable the following proposals to be made regarding the ways in which lampreys use their gill epithelial cells for osmoregulating in hypo- and hypertonic environments. In freshwater, the intercalated MR cell takes up Cl– and secretes H+,thereby facilitating the uptake of Na+ through pavement cells. In seawater, the chloride cell uses a secondarily active transcellular transport of Cl– to provide the driving force for the passive movement of Na+ through leaky paracellular pathways between these cells.
The role of discontinuous gas exchange in insects: the chthonic hypothesis does not hold water
Authors: Gibbs Allen G. , Johnson Robert A.
Published: August 2004

Insects breathe through valved openings (spiracles) in their cuticle. Many insects open and close their spiracles in a cyclic pattern (discontinuous gas-exchange cycles, or DGC). These cycles were observed over half a century ago, their hypothesized function being to minimize loss of water from the tracheal system. However, numerous recent studies have found that respiration accounts for a small fraction of total water loss, and that insects stop performing DGC at times when this pattern would be most useful. Thus, the importance of cyclic gas exchange for water conservation has been challenged. The leading alternative is the chthonic hypothesis, which proposes that DGC originated in insects from hypercapnic (high CO2) environments(e.g. burrows) to aid in release of carbon dioxide. By keeping the spiracles closed, insects would concentrate CO2 and increase the gradient for outward diffusion of CO2. CO2 would be released rapidly when the spiracles opened, and respiratory water loss would be reduced. The chthonic hypothesis therefore predicts that DGC minimizes the ratio of respiratory water loss to CO2 release relative to other modes of gas exchange. We tested the chthonic hypothesis by simultaneously measuring water loss and CO2 release in reproductive females (queens) of the seed-harvester ant Pogonomyrmex barbatus, a burrowing species from North American deserts. Queens used one of three patterns of gas exchange,discontinuous, cyclic and continuous. We resolved the problem of separating cuticular transpiration and respiratory water loss for individuals that used continuous gas exchange by developing a regression method that can be used across all patterns of gas exchange. The ratio of respiratory water loss to CO2 release did not differ among ants using different patterns of gas exchange, in contrast to the expectation of the chthonic hypothesis. Metabolic rate, however, varied with gas-exchange pattern, and was lowest for individuals that used discontinuous gas exchange, intermediate for individuals using cyclic gas exchange, and highest for individuals using continuous gas exchange.
Recognition of calls with exceptionally fast pulse rates: female phonotaxis in the genus Neoconocephalus (Orthoptera:Tettigoniidae)
Authors: Deily Joshua A. , Schul Johannes
Published: August 2004

Male Neoconocephalus robustus and Neoconocephalus bivocatus produce remarkably fast calls, with pulse rates of approximately175–200 Hz. The temporal call patterns differ significantly between the two species. Male N. robustus produce calls with a single pulse rate of 200 Hz. In N. bivocatus, pulses are repeated with alternating periods, resulting in distinct pulse pairs: approximately 175 pulses s–1 are grouped into 87 pulse pairs s–1. In order to identify the temporal parameters used to recognize calls with such fast pulse rates, female call recognition in both species was tested during phonotaxis on a walking compensator. Female N. robustus were attracted to calls without amplitude modulation. Amplitude-modulated signals were equally attractive, as long as the silent intervals were short enough. The maximally tolerated interval duration varied with pulse duration. Female N. bivocatus did not require the paired-pulse pattern but were attracted to call models in which each pulse pair was merged into one long pulse. Females used the pulse rate to recognize such signals: pulse rates close to 87 Hz were attractive, largely independent of the duty cycle. Thus, females of the sibling species N. robustusand N. bivocatus used qualitatively different call recognition mechanisms.
Substantial changes in central nervous system neurotransmitters and neuromodulators accompany phase change in the locust
Authors: Rogers Stephen M. , Matheson Thomas , Sasaki Ken , Kendrick Keith , Simpson Stephen J. , Burrows Malcolm
Published: August 2004

Desert locusts (Schistocerca gregaria) can undergo a profound transformation between solitarious and gregarious forms, which involves widespread changes in behaviour, physiology and morphology. This phase change is triggered by the presence or absence of other locusts and occurs over a timescale ranging from hours, for some behaviours to change, to generations,for full morphological transformation. The neuro-hormonal mechanisms that drive and accompany phase change in either direction remain unknown. We have used high-performance liquid chromatography (HPLC) to compare amounts of 13 different potential neurotransmitters and/or neuromodulators in the central nervous systems of final instar locust nymphs undergoing phase transition and between long-term solitarious and gregarious adults. Long-term gregarious and solitarious locust nymphs differed in 11 of the 13 substances analysed: eight increased in both the brain and thoracic nerve cord (including glutamate,GABA, dopamine and serotonin), whereas three decreased (acetylcholine,tyramine and citrulline). Adult locusts of both extreme phases were similarly different. Isolating larval gregarious locusts led to rapid changes in seven chemicals equal to or even exceeding the differences seen between long-term solitarious and gregarious animals. Crowding larval solitarious locusts led to rapid changes in six chemicals towards gregarious values within the first 4 h(by which time gregarious behaviours are already being expressed), before returning to nearer long-term solitarious values 24 h later. Serotonin in the thoracic ganglia, however, did not follow this trend, but showed a ninefold increase after a 4 h period of crowding. After crowding solitarious nymphs for a whole larval stadium, the amounts of all chemicals, except octopamine, were similar to those of long-term gregarious locusts. Our data show that changes in levels of neuroactive substances are widespread in the central nervous system and reflect the time course of behavioural and physiological phase change.
Testing olfactory foraging strategies in an Antarctic seabird assemblage
Authors: Nevitt Gabrielle , Reid Keith , Trathan Phil
Published: August 2004

Procellariiform seabirds (petrels, albatrosses and shearwaters) forage over thousands of square kilometres for patchily distributed prey resources. While these birds are known for their large olfactory bulbs and excellent sense of smell, how they use odour cues to locate prey patches in the vast ocean is not well understood. Here, we investigate species-specific responses to 3-methyl pyrazine in a sub-Antarctic species assemblage near South Georgia Island(54°00 ′ S, 36°00 ′ W). Pyrazines are scented compounds found in macerated Antarctic krill (Euphausia superba), a primary prey item for many seabird species in this region. To examine behavioural attraction to this odour, we presented birds with either scented or`unscented' vegetable oil slicks at sea. As a positive control for our experiments, we also compared birds' responses to a general olfactory attractant, herring oil. Responses to pyrazine were both highly species specific and consistent with results from earlier studies investigating responses to crude krill extracts. For example, Cape petrels (Daption capense), giant petrels (Macronectes sp.) and white-chinned petrels (Procellaria aequinoctialis) were sighted at least 1.8–4 times as often at pyrazine-scented slicks than at control slicks. Black-browed albatrosses (Diomedea melanophris) were only sighted at pyrazine-scented slicks and never at control slicks. Wilson's storm-petrels(Oceanites oceanicus), black-bellied storm-petrels (Fregetta tropica), great shearwaters (Puffinus gravis) and prions(Pachyptila sp.) were sighted with equal frequency at control and pyrazine-scented slicks. As expected, responses to herring oil were more common. With the exception of great shearwaters (Puffinus gravis),each of these species was sighted up to five times as often at slicks scented with herring oil compared with control slicks. Together, the results support the hypothesis that Antarctic procellariiforms use species-specific foraging strategies that are inter-dependent and more complex than simply tracking prey by scent.
Dietary ecospace and the diversity of euprimates during the Early and Middle Eocene
Authors: Gilbert Christopher C.
Published: August 2004

This study examined adapoid and omomyoid euprimate dietary and body size diversity from the Eocene of North America and Europe. Estimates of body weights and shearing quotients calculated from lower molars were plotted on a coordinate graph as a representation of dietary niche space (dietary ecospace) occupied by extinct species. By computing the areas, average intertaxon distances, and average distances from the centroid of the resulting polygons, comparisons of Eocene euprimate dietary and body size diversity were made. Results indicate that euprimate dietary niche space expanded significantly in North America from the Early to Middle Eocene, and at all times during the Early and Middle Eocene, the niche space occupied by North American euprimates exceeded that of corresponding European euprimates. These results confirm that fossil euprimate diversity, as measured by diet and body size, significantly differed across biogeographic areas. There are many possible explanations as to why North American euprimates were significantly more diverse in terms of diet and body size than their European counterparts. The explanation advocated here as most responsible for the increased diversity during the Early and Middle Eocene relates to the existence and increased sampling of more ecologically diverse environments, such as basin margins in the western interior of North America. These diverse environments could have promoted biological processes that led to the generation of increased diversity in North America compared to the isolated island refugia of Western Europe during this time. Am J Phys Anthropol, 2004. © 2004 Wiley‐Liss, Inc.
Metallocene‐based functionalized polyolefins as compatibilizers in polyolefin nanocomposites
Authors: Motha Kshama , Hippi Ulla , Hakala Kimmo , Peltonen Maija , Ojanperä Vuokko , Löfgren Barbro , Seppälä Jukka
Published: August 2004

This article reports a study of some functionalized polyolefins evaluated as compatibilizers in polyethylene nanocomposites. The functionalized polymers were prepared by direct metallocene‐mediated copolymerizations of ethylene and a functional comonomer. The prepared nanocomposites were evaluated for mechanical and barrier property enhancement. A good combination of mechanical and barrier properties was obtained with the metallocene‐based functionalized polyethylene. The toughness–stiffness balance was better than or comparable to that achieved with conventional functionalized polymers such as maleic anhydride grafted polyethylene. The results also indicated that these metallocene‐based functionalized polyolefins, when used as compatibilizers, could have relatively higher molar masses and lower functionality than those of conventional post‐reactor‐modified compatibilizers, and so the drawbacks associated with the latter could be avoided. Their inherent properties could also further improve the final nanocomposite properties. This was attributed to the more homogeneous nature of metallocene‐catalyzed polymers in comparison with post‐reactor‐modified products. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1094–1100, 2004
Molecular weight dependency of refractive index increment of polystyrene determined by uniform oligomers
Authors: Itakura Masanao , Sato Keisuke , Lusenkova Marina A. , Matsuyama Shigetomo , Shimada Kayori , Saito Takeshi , Kinugasa Shinichi
Published: August 2004

Molecular weight dependency of a specific UV absorption coefficient of polystyrene in solution was determined by supercritical fluid chromatography applied to equimolar and equimass mixtures of uniform polystyrenes whose degree of polymerization was n = 1 to 40. From the ratio between the response from a UV detector and that from a refractometer, we determined the refractive index increment dn/dc as a function of molecular weight for polystyrene in tetrahydrofuran at the wavelength λ = 633 nm. We found that dn/dc is well expressed as a linear function of the reciprocal of molecular weight for uniform polystyrenes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1101–1106, 2004
Characterization by Fourier transform infrared spectroscopy of polyethylene adipate/cholesteryl palmitate blends
Authors: Popescu Maria‐Cristina , Vasile Cornelia , Filip Daniela , Macocinschi Doina , Singurel Gh.
Published: August 2004

FT‐IR spectroscopy has been employed to study compatibility, melting, and crystallization of the polyethylene adipate (PEA)/cholesteryl palmitate (CP) blends. The changes in FT‐IR spectra were followed by controlled rate of heating and cooling. The bands corresponding to the crystalline structure have been assigned. The accuracy of the transition temperature determination has been improved by fitting the curve of the integral absorbance dependence on temperature with a Boltzmann function. From dependence of the transition temperatures on the composition of the blend, it has been established that for each mixing ratio a certain mass fraction of CP should be dissolved in the PEA matrix. The PEA/CP blends behave as a phase‐separated system with partial miscibility. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1156–1163, 2004
Structural relaxation and morphology of the rubber–urethane composites
Authors: Danch Adam , Sułkowski Wieslaw W. , Moczyński Marek , Radon Agnieszka , Stelzer Franz , Jurga Stefan
Published: August 2004

The supermolecular structures and structural relaxation of the waste rubber–urethane composites are discussed on the basis of results of calorimetric measurements and mechanical spectroscopy. The results give a broader understanding of the relationship between processing history and morphology in the resultant engineering products. The effect of different kinds of urethane prepolymer on the thermomechanical properties of commercial composite products is presented. The idea of reaction between rubber granulate and urethane prepolymer is under consideration. The shear and tensile modes were applied to better explore relaxations that occur in rubber grains and in an interphase area created by the urethane agents. The rubber waste management problem was taken into account also. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1186–1193, 2004
Morphology and fracture behavior of intercalated epoxy/clay nanocomposites
Authors: Liu Tianxi , Tjiu Wuiwui Chauhari , Tong Yuejin , He Chaobin , Goh Sok Sing , Chung Tai‐Shung
Published: August 2004

Epoxy/clay nanocomposites were prepared by swelling organoclay in an epoxy resin, diglycidyl ether of bisphenol A, followed by curing with an aromatic hardener, diethyltoluenediamine. A combination of X‐ray diffraction with transmission electron microscopy showed the coexistence of intercalated and exfoliated clay morphologies throughout the matrix. In addition, a microscopic and homogeneous dispersion of clay agglomerations, with sizes ranging from about 2 to 5 μm, was revealed by optical microscopy. Dynamic mechanical analysis indicated a steady increase in storage modulus and a gradual decrease in high glass‐transition temperature as the clay loading increased. The fracture toughness of the nanocomposites significantly increased with increasing clay concentration, suggesting a toughening effect from the clay particles. Scanning electron microscopy and optical microscopy observations of the epoxy/clay nanocomposites suggested that shear yielding of the matrix, crack deflection, voiding, and debonding of clay particles and epoxy matrix are among the operative toughening mechanisms observed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1236–1244, 2004
An investigation of fiber splitting of bicomponent meltblown/microfiber nonwovens by water treatment
Authors: Zhang Dong , Sun Christine (Qin) , Song Hua
Published: August 2004

Meltblowing is a most versatile and cost‐effective process commercially available worldwide to produce microfiber nonwovens directly from thermoplastic resins. The new bicomponent (bico) meltblown technology opens a great possibility to make even finer microfibers by subsequently fiber splitting. Water‐dispersive Eastman AQ polymers were initially introduced to the meltblown process to make the mono‐ and bicomponent meltblown webs at Textiles and Nonwovens Development Center (TANDEC), University of Tennessee, Knoxville. The postwater treatment was performed on the fabrics, which resulted in the dispersive part (AQ polymer) being dispersed in water and only the other part remaining in the bico web. A process–structure–property study is provided toward the research reported in this article. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1218–1226, 2004
Benzene physical and chemical organogels: Effect of network scaffolding on the thermodynamic behavior of entrapped solvent molecules
Authors: Marković Nov , Ginić‐Marković Milena , Dutta Naba K.
Published: August 2004

This paper presents a thermodynamic investigation of the benzene physical and chemical organogels, using differential scanning calorimetry (DSC) and intends to draw an appropriate relationship between the gel network structure and the properties. Physical gels, formed by an aluminium soap of fatty acid, and chemical gels, created by in situ cross‐linking of a siloxane copolymer are investigated. The effects of the type and quantity of the gelators and their corresponding network mesh size distribution in the gels on crystallization, melting, and their kinetics are examined. It appears that the kinetics of crystallization of the entrapped solvent is significantly affected by the quality of the gel network scaffolding and can be treated successfully by the Avrami equation of crystallization. From the melting behavior of the entrapped solvent crystallites, quantitative information about the number of solvent molecules bound per molecule of the gelator has been extracted. DSC proves to be a reliable technique to evaluate the population distribution of solvent molecules trapped in the physical and chemical organogel network scaffolding. The state of the solvent may be treated as a probe to understand the structure of the gels. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1253–1264, 2004
Selective repartition of in situ generated silica produced during the evolution of an epoxide network from a homogeneous precursors mixture and effects on properties
Authors: Mascia Leno , Heath R J. , Ng Vincent S. Y.
Published: August 2004

Phase separation of organic networks derived from mixtures of reactive monomers and/or oligomers can take place through the formation of IPNs or precipitation of particles. The latter systems are widely used as a means of increasing the fracture toughness of thermosetting resins, and particularly for products obtained from mixtures of epoxy resins and functionalized aliphatic oligomers. In the present work several mixtures comprised of difunctional epoxy resins, silane functionalized perfluoroether oligomers, prehydrolyzed tetraethoxysilane, and an aromatic amine hardener were examined in a variety of compositions and preparative procedures. The aims were to control the kinetics of phase separation and the repartition of the different components in the two phases, so that the silica domains could be preferentially located within the precipitated soft particles. It was found that the silane functionalization of the perfluoroether oligomer provided an effective mechanism for the localization of the siloxane networks within the precipitated particles. However, phase separation by the precipitation of particles would only take place for systems in which the perfluoroether oligomer could be reacted with an excess of epoxy resin prior to adding the alkoxysilane solution and the hardener. Moreover, it was difficult to achieve the total localization of the siloxane component into the perfluoroether network. The siloxane species remaining dissolved in the resin gave rise to severe embrittlement of the products through reactions with the epoxy chains. Using mixtures of suitably functionalized perfluoroether oligomers it was possible to produce the conditions by which the localization of the silica domains into the precipitated particles could be maximized. This has resulted also in the formation of graded interphase regions and to a global improvement in mechanical properties, manifested as a concomitant increase in modulus, strength, and toughness. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1279–1290, 2004
Synthesis and Molluscicidal Activity of 5‐oxo‐5,6,7,8‐Tetrahydro‐4H‐Chromene Derivatives
Authors: Abdelrazek Fathy M. , Metz Peter , Farrag Ebtehal K.
Published: August 2004

Furfurylidenemalononitriles and thienylidenemalononitriles were treated with 1,3‐cyclohexanediones to afford 2‐amino‐4‐hetaryl‐5‐oxo‐5,6,7,8‐tetrahydro‐4H‐chromene‐3‐carbonitrile derivatives. The molluscicidal activity of these compounds was investigated.
Stahlverbundkonstruktionen im Parkhausbau
Authors: Kurz Wolfgang
Published: August 2004

Die Stahlverbundbauweise hat sich aus technischen und wirtschaftlichen Erwägungen zu einer Regelbauweise für offene Parkhäuser entwickelt. In diesem Beitrag werden die Randbedingungen und Besonderheiten der Ausführung von Stahlverbundparkhäusern erläutert. Gebräuchliche und bewährte Tragwerke und Deckensysteme werden beschrieben. Besonderheiten hinsichtlich der Dauerhaftigkeit der Konstruktionen werden erläutert.
Two‐dimensional magic angle spinning NMR investigation of naturally occurring chitins: Precise 1H and 13C resonance assignment of α‐ and β‐chitin
Authors: Kono Hiroyuki
Published: August 2004

13C homonuclear through‐bond correlations of α‐ and β‐chitin were determined by using two‐dimensional (2D) INADEQUATE spectra of these allomorphs purified from crab shell and squid pen, respectively. The 2D 13C–13C correlation spectra where two directly bonded carbons share a common double‐quantum frequency (DQ) enabled us to precisely assign all 13C resonances of the chitin allomorphs for the first time. Following the complete 13C assignment, 1H chemical shifts of protons attached to each carbon nuclei were assigned by 2D frequency‐switched Lee–Goldberg (FSLG) 1H–13C heteronuclear correlation (HETCOR) spectra of the chitin allomorphs, recorded with a short mixing time (60 μs) to provide isotropic 1H–13C chemical shift correlations between bonded pairs proton and carbon nuclei. From the 13C and 1H chemical shifts of chitin allomorphs, all 2‐deoxy‐2‐acetamide‐D‐glucose (N‐acetyl‐D‐glucosamine) monomer units in each allomorph were revealed to be an identical 13C–13C backbone conformation and magnetically equivalent. In addition, it was strongly suggested that there are two different hydrogen‐bonding patterns at the hydroxyl groups of α‐chitin by comparing 1H chemical shifts at the C6 site of α‐chitin with those at the same site of β‐chitin. © 2004 Wiley Periodicals, Inc. Biopolymers, 2004
Stimulation of methanol degradation in UASB reactors: In situ versus pre‐loading cobalt on anaerobic granular sludge
Authors: Zandvoort Marcel H. , Gieteling Jarno , Lettinga Gatze , Lens Piet N.L.
Published: August 2004

The effect of pre‐loading and in situ loading of cobalt onto a cobalt‐limited granular sludge on the performance of methanol fed bioreactors was investigated. One upflow anaerobic sludge bed (UASB) reactor was inoculated with cobalt pre‐loaded sludge (24h; 30°C; 1 mM CoCl2) and a second UASB with unloaded sludge. The UASB reactors (30°C; pH 7) were operated for 77 days at 8 h hydraulic retention time and organic loading rates ranging from 5 to 20 g COD · L reactor−1 · d−1. Cobalt pre‐loading clearly stimulated the methanogenic activity of the sludge with methanol as the substrate, e.g., after 30 days of reactor operation this activity was 5.8 times higher than that of the cobalt unloaded sludge. During the experiment, part of the cobalt leached from the pre‐loaded sludge, i.e., 54% of the cobalt content was lost during the 77 days of reactor operation. Sequential metal extraction showed that losses mainly occurred from the exchangeable and carbonate fraction and in the sludge remaining cobalt was mainly present in the organic/sulfide fraction of the sludge. In situ loading of cobalt in the unloaded UASB reactor on day 57 by adding 31 μM cobalt to the influent for a 24‐h period (16% of the cobalt present in the loaded sludge at day 11) resulted in a 4 time increase of the methanogenic activity of the sludge with methanol as the substrate at the end of the reactor experiment, while the accumulated amount of cobalt in the sludge only amounted to 6% of the cobalt accumulated in the loaded sludge (on day 11). This study showed that both pre‐loading sludge and in situ loading are adequate for achieving an increased reactor performance of methanol fed UASB reactors operating under cobalt limitation. However, the in situ dosing procedure needs substantially lower amounts of cobalt, while it also gives significantly smaller losses of cobalt with the effluent. © 2004 Wiley Periodicals, Inc.
Solid‐state fermentation: A continuous process for fungal tannase production
Authors: van de Lagemaat J. , Pyle D.L.
Published: August 2004

Truly continuous solid‐state fermentations with operating times of 2–3 weeks were conducted in a prototype bioreactor for the production of fungal (Penicillium glabrum) tannase from a tannin‐containing model substrate. Substantial quantities of the enzyme were synthesized throughout the operating periods and (imperfect) steady‐state conditions seemed to be achieved soon after start‐up of the fermentations. This demonstrated for the first time the possibility of conducting solid‐state fermentations in the continuous mode and with a constant noninoculated feed. The operating variables and fermentation conditions in the bioreactor were sufficiently well predicted for the basic reinoculation concept to succeed. However, an incomplete understanding of the microbial mechanisms, the experimental system, and their interaction indicated the need for more research in this novel area of solid‐state fermentation. © 2004 Wiley Periodicals, Inc.
Development and validation of a high‐sensitivity liquid chromatography/tandem mass spectrometry (LC/MS/MS) method with chemical derivatization for the determination of ethinyl estradiol in human plasma
Authors: Shou Wilson Z. , Jiang Xiangyu , Naidong Weng
Published: August 2004

An ultra‐sensitive liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for the analysis of oral contraceptive ethinyl estradiol (EE) was developed and validated over the curve range of 2.5–500 pg/mL using 1 mL of human plasma sample. Ethinyl estradiol and the internal standard, ethinyl estradiol tetra‐deuterated (EE‐d4), were extracted from the plasma matrix with methyl t‐butyl ether, derivatized with dansyl chloride and then back‐extracted into hexane. The hexane phase was evaporated to dryness, reconstituted and injected onto the LC/MS/MS system. The chromatographic separation was achieved on a Luna C18 column (50 × 2 mm, 5 µm) with an isocratic mobile phase of 20:80 (v/v) water:acetonitrile with 1% formic acid. The ofﬂine derivatization procedure introduced the easily ionizable tertiary amine function group to EE. This greatly improved analyte sensitivity in electrospray ionization and enabled us to achieve the desired lower limit of quantitation at 2.5 pg/mL. This high sensitivity method can be used for therapeutic drug monitoring or supporting bio‐equivalence and drug–drug interaction studies in human subjects. Copyright © 2004 John Wiley & Sons, Ltd.
Variations in content and structure of glycosaminoglycans of the vitreous gel from different mammalian species
Authors: Noulas Argiris V. , Skandalis Spyros S. , Feretis Elias , Theocharis Dimitrios A. , Karamanos Nikos K.
Published: August 2004

The vitreous of all species is composed of essentially the same type of extracellular matrix macromolecules organized to a transparent gel. In this study, the composition and ﬁne chemical structure of the glycosaminoglycans (GAGs) in the vitreous gel from sheep and goat were determined and compared with those of human and pig vitreous gels. The results showed that, in all examined species; hyaluronan (HA) was the predominant GAG, whereas chondroitin sulphate (CS) was the minor one. In the vitreous gel of the most relative species, i.e. sheep and goat, higher amounts of both of HA and CS were estimated as compared with pig and human tissues. The distribution of hydrodynamic sizes of HA and CS was signiﬁcantly differed among different species. All HA preparations consisted of molecules with great variability in hydrodynamic sizes. The relative proportions of the large HA molecules (size >1.8 × 106 kDa) were signiﬁcantly higher in sheep and goat as compared with human and pig vitreous gel. The length of CS chains was also of larger size in sheep and goat (50 and 58 kDa, respectively) than the respective chains in human and pig vitreous gel (38 and 28 kDa, respectively). The sulphation patterns of CS preparations were determined following enzymic treatments, HPLC and capillary electrophoretic analyses. The human vitreous‐derived CS chains showed quite different sulphation proﬁle than that of CS isolated from other species, since 4‐sulphated disaccharides were identiﬁed as the dominant moiety. In conclusion, signiﬁcant compositional and structural variations between the vitreous matrixes of different species at the GAG level were identiﬁed. The functional signiﬁcance of these species‐dependent variations is discussed. Copyright © 2003 John Wiley & Sons, Ltd.
Dynamic change in the distribution of alpha5beta1 integrin on isolated ventral membrane: Effect of divalent cation species
Authors: Hirata Hiroaki , Ohki Kazuo , Miyata Hidetake
Published: August 2004

We investigated the spatial distribution of α5β1 integrin in isolated ventral plasma membranes (VPMs) of human foreskin fibroblasts in order to elucidate how the interaction of integrin with cytoskeletal and extracellular matrix proteins might affect the integrin distribution. Cells were exposed to the jet of buffer to remove the apical surface and most of cellular organelles. After this treatment VPMs, which adhered to the glass surface, possessed the cellular structures such as fibronectin (FN) fibrils and actin stress fibers. The isolated VPMs thus prepared were employed without fixation to investigate the change in the integrin distribution. In isolated VPMs, α5β1 integrin, labeled with Cy3‐tagged anti‐integrin antibody, was found to accumulate not only at the tips of stress fibers but also along FN fibrils extending from there. When divalent cations were removed with EDTA, the accumulated integrin was dispersed, and the original pattern of distribution was recovered upon restoration of divalent cations. Talin, an integrin‐actin cytoskeleton linker protein, was found to accumulate only at the tips of stress fibers in isolated VPMs, but α5β1 integrin did not exhibit strong accumulation there, indicating that talin played little role in integrin distribution in isolated VPMs. The amount of α‐actinin associated with stress fibers was found to drastically decrease in isolated VPMs, which was presumably related to the failure of localization of integrin at the tips of stress fibers. It was also shown that the association of stress fibers to isolated VPMs seemed to be independent of accumulation of integrin. Cell Motil. Cytoskeleton 59:131–140, 2004. © 2004 Wiley‐Liss, Inc.