bluuebunny/crossref_metadata_2025_split huggingface dataset:
Equilibria and kinetics of the cationic ring‐opening polymerization of permethylated 1,4‐dioxa‐2,3,5,6‐tetrasilacyclohexane. Comparison with cyclosiloxanes
Authors: Kurjata Jan , Chojnowski Julian 
Published: April 2003

AbstractEquilibrium constants were determined for the formation of cyclic oligomers of the general formula \documentclass{article}\pagestyle{empty}\begin{document}$ {\hbox{-\hskip-5pt-\hskip-5pt-}[({\rm CH}_3})_2 {\rm Si \hbox{---} Si(CH_3})_2\hbox{---} {\rm O\hbox{-\hskip-6.5pt-}]}_n , n = 2 - 6$\end{document}, in the polymerization of the n = 2 homologue, octamethyl‐1,4‐dioxa‐2,3,5,6‐tetrasilacyclohexane (1). The formation of a linear polymer is thermodynamically favoured in bulk and in concentrated solutions. The equilibrium concentration of cyclics is little dependent on temperature. The six‐membered ring, n = 2, used here as a monomer, is the most abundant cyclic in the equilibrium mixture. Kinetics of the conversion of 1 during the polymerization initiated with CF3SO3H, CH3SO3H and CF3COOH was studied in methylene dichloride solution. In its first stage, the reaction follows first order, but it slows down at higher monomer conversion. The order of the reaction with respect to the acid is two. Some kinetic features of the polymerization initated with CF3SO3H were similar to those exhibited by the polymerization of octamethylcyclotetrasiloxane (D4). In particular small amounts of additional water have rather small effect on the rate, but change to a considerable extent the entropy and enthalpy of activation. The concentration of cyclic oligohomologues passes through a maximum during monomer conversion, which indicates the kinetic enhancement of the cyclisation process. This observation is somewhat similar to those in the polymerization of hexamethylcyclotrisiloxane (D3). However, in contrast to the polymerization of D3 and similarly to the polymerization of D4, the excessive formation of oligomers being multiples of the monomer is not observed.
Sequential assignments of polymers by 2D NMR techniques: application to stereochemical configuration assignment of poly(acrylic acid)
Authors: Beshah Kebede 
Published: April 2003

AbstractTwo‐dimensional heteronuclear single quantum coherence — total correlation spectroscopy (2D HSQC‐TOCSY) was used to make end‐to‐end sequential assignments and determine stereochemical configuration at the triad, tetrad and pentad levels of poly(acrylic acid). The pseudo three‐dimensional experiment allows heteronuclear (13C‐1H) as well as homonuclear (1H‐1H) corelations in a single 2D NMR experiment. The stereochemical configuration assignment is based on sequential identification of neighboring species. For example, a mmr tetrad resonance (methylene‐13C) is determined by its correlations to the mm and mr triad resonances (methine) that constitute it. Hence, it is independent of chemical shift and peak intensity estimates that are requisites of the conventional method. This self‐consistent technique enables absolute assignments of all magnetically unique tetrads and some of the pentad resonances. In addition, some resonances in the methylene and methine regions that have been difficult to characterize by conventional techniques were unequivocally assigned to endgroup resonances and other polymerization by‐products of the low‐molecular‐weight poly(acrylic acid).
A posteriori error estimation for convection dominated problems on anisotropic meshes
Authors: Kunert Gerd 
Published: April 2003

AbstractA singularly perturbed convection–diffusion problem in two and three space dimensions is discretized using the streamline upwind Petrov Galerkin (SUPG) variant of the finite element method. The dominant convection frequently gives rise to solutions with layers; hence anisotropic finite elements can be applied advantageously.The main focus is on a posteriori energy norm error estimation that is robust in the perturbation parameter and with respect to the mesh anisotropy. A residual error estimator and a local problem error estimator are proposed and investigated.The analysis reveals that the upper error bound depends on the alignment of the anisotropies of the mesh and of the solution. Hence reliable error estimation is possible for suitable anisotropic meshes. The lower error bound depends on the problem data via a local mesh Peclet number. Thus efficient error estimation is achieved for small mesh Peclet numbers.Altogether, error estimation approaches for isotropic meshes are successfully extended to anisotropic elements. Several numerical experiments support the analysis. Copyright © 2003 John Wiley & Sons, Ltd.
Interaction of elementary waves for scalar conservation laws on a bounded domain
Authors: Liu Hongxia , Pan Tao 
Published: April 2003

AbstractThis paper is concerned with the interaction of elementary waves on a bounded domain for scalar conservation laws. The structure and large time asymptotic behaviours of weak entropy solution in the sense of Bardos et al. (Comm. Partial Differential Equations 1979; 4: 1017) are clarified to the initial–boundary problem for scalar conservation laws ut+ƒ(u)x=0 on (0,1) × (0,∞), with the initial data u(x,0)=u0(x):=um and the boundary data u(0,t)=u‐,u(1,t)=u+, where u±,um are constants, which are not equivalent, and ƒ∈C2 satisfies ƒ′′>0, ƒ(0)=f′(0)=0. We also give some global estimates on derivatives of the weak entropy solution. These estimates play important roles in studying the rate of convergence for various approximation methods to scalar conservation laws. Copyright © 2003 John Wiley & Sons, Ltd.
Presence and developmental regulation of serotonin N‐acetyltransferase transcripts in oocytes and early quail embryos (Coturnix coturnix japonica)
Authors: Oblap Ruslan , Olszańska Bożenna 
Published: April 2003

AbstractBy RT‐PCR two transcripts for arylalkylamine N‐acetyltransferase (AA‐NAT; serotonin N‐acetyltransferase; EC 2.3.1.87), the key enzyme in melatonin synthesis, were found, for the first time, in the oocytes and blastoderms from freshly laid eggs (323‐ and 238‐bp RT‐PCR products), and one (238‐bp product) in the pineal gland of Japanese quail (Coturnix coturnix japonica). The two products differed by an intron of 85‐bp present in the 323‐bp band and absent from the 238‐bp band. The identity of the products was confirmed by restriction analysis and sequencing. The ratio of the 323:238‐bp bands changed during oogenesis from ∼17:1 in small 3‐mm oocytes to ∼4:1 in immature vitellogenic oocytes and ∼1:1 in mature, preovulatory oocytes; it was reversed to ∼0.2:1 in blastoderms from fertile freshly laid eggs, corresponding to embryo of ∼40,000 cells. It is proposed that the longer 323‐bp product, containing an intron, represents a translationally inactive form of the transcript, stored in maternal RNA. The shorter 238‐bp product lacking an intron may represent the mature active AA‐NAT mRNA found in the pineal gland and in early embryos, and—to a lower proportion—in older oocytes. These data constitute the first direct proof of an intron sequence in maternal RNA of avian oocyte. It is possible that differential processing of the immature mRNA is part of a transcriptional regulation mechanism of AA‐NAT activity. A possible role of extrapineal melatonin in early avian development is discussed. Mol. Reprod. Dev. 65: 132–140, 2003. © 2003 Wiley‐Liss, Inc.
γ‐Ray exposure accelerates spermatogenesis of medaka fish, Oryzias latipes
Authors: Kuwahara Yoshikazu , Shimada Atsuko , Mitani Hiroshi , Shima Akihiro 
Published: April 2003

AbstractTo examine the spermatogenesis (and spermiogenesis) cell population kinetics after γ‐irradiation, the frequency and fate of BrdU‐labeled pre‐meiotic spermatogenic cells (spermatogonia and pre‐leptotene spermatocytes) and spermatogonial stem cells (SSCs) of the medaka fish (Oryzias latipes) were examined immunohistochemically and by BrdU‐labeling. After 4.75 Gy of γ‐irradiation, a statistically significant decrease in the frequency of BrdU‐labeled cells was detected in the SSCs, but not in pre‐meiotic spermatogenic cells. The time necessary for differentiation of surviving pre‐meiotic spermatogenic cells without delay of germ cell development was shortened. More than 90% of surviving pre‐meiotic spermatogenic cells differentiated into haploid cells within 5 days after irradiation, followed by a temporal spermatozoa exhaust in the testis. Next, spermatogenesis began in the surviving SSCs. However, the outcome was abnormal spermatozoa, indicating that accelerated maturation process led to morphological abnormalities. Moreover, 35% of the morphologically normal spermatozoa were dead at day 6. Based on these results, we suggest a reset system; after irradiation most surviving spermatogenic cells, except for the SSCs, are prematurely eliminated from the testis by spermatogenesis (and spermiogenesis) acceleration, and subsequent spermatogenesis begins with the surviving SSCs, a possible safeguard against male germ cell mutagenesis. Mol. Reprod. Dev. 65: 204–211, 2003. © 2003 Wiley‐Liss, Inc.
Regulation of the proliferation of cocultured gonocytes and sertoli cells by retinoids, triiodothyronine, and intracellular signaling factors: Differences between fetal and neonatal cells
Authors: Boulogne Barbara , Habert René , Levacher Christine 
Published: April 2003

AbstractThe regulation of early fetal germ cell growth has not been studied in cell culture, probably due to the poor survival of these cells. However, cell culture is the only system in which the control of cell growth can be studied independently of the influence of secreted testicular factors, which are diluted in the medium. We successfully cultured dispersed testicular cells from 16.5‐day‐old rat fetuses in defined medium and compared the growth of these cells with that of cells from 3‐day‐old neonates. In this system, fetal gonocytes displayed low levels of mitotic activity and their numbers remained stable. In contrast, neonatal gonocytes displayed high levels of mitotic activity and increased in number, these characteristics resembling those observed in vivo. We found that retinoic acid had deleterious effects on the number of gonocytes but did not affect Sertoli cell proliferation in fetal and neonatal cell cultures. Moreover, in fetal cell cultures, the decrease in the number of gonocytes resulted from a decrease in mitotic activity, probably due to a direct effect of retinoids on fetal gonocytes. Among the selective agonists for the retinoic acid receptor (RARα agonist, RARβ agonist, and RARγ agonist) and the retinoic X receptor (pan‐RXR agonist) tested, only the RARα agonist reproduced the effects of retinoic acid at concentrations lower than its Kd value in both fetal and neonatal cell cultures. As both RARα and RXRα are present in fetal and neonatal gonocytes, we suggest that retinoic acid exerts its effects on gonocytes via a RARα‐RXRα heterodimer, with RARα functioning as an active partner and RXRα as a passive partner. In this culture system, we show for the first time that triiodothyronine (T3) inhibits testicular fetal Sertoli cell and germ cell growth. We also tested intracellular signaling factors and found that a cAMP analog increased Sertoli cell proliferation and germ cell survival in both fetal and neonatal cells whereas phorbol esters (PMA) strongly inhibited the proliferation of fetal but not of neonatal gonocytes. None of the tested factors (T3, dbcAMP, and PMA) seemed to interact with the all‐trans retinoic acid pathway. Thus, fetal gonocytes and neonatal gonocytes differ in intrinsic properties, and their growth is not regulated in the same manner. Despite their low level of mitotic activity, fetal gonocytes were more sensitive to various factors than neonatal gonocytes. Mol. Reprod. Dev. 65: 194–203, 2003. © 2003 Wiley‐Liss, Inc.
Conventional DTI vs. slow and fast diffusion tensors in cat visual cortex
Authors: Ronen Itamar , Kim Keun‐Ho , Garwood Michael , Ugurbil Kamil , Kim Dae‐Shik 
Published: April 2003

AbstractDiffusion tensor imaging (DTI) uses water diffusion anisotropy in axonal fibers to provide a tool for analyzing and tracking those fibers in brain white matter. In the present work, multidirectional diffusion MRI data were collected from a cat brain and decomposed into slow and fast diffusion tensors and directly compared with conventional DTI data from the same imaging slice. The fractional anisotropy of the slow diffusing component (Dslow) was significantly higher than the anisotropy measured by conventional DTI while reflecting a similar directionality and appeared to account for most of the anisotropy observed in gray matter, where the fiber density is notoriously low. Preliminary results of fiber tracking based on the slow diffusion component are shown. Fibers generated based on the slow diffusion component appear to follow the vertical fibers in gray matter. DslowTI may provide a way for increasing the sensitivity to anisotropic structures in cortical gray matter. Magn Reson Med 49:785–790, 2003. © 2003 Wiley‐Liss, Inc.
Stereotaxic assembly and procedures for simultaneous electrophysiological and MRI study of conscious rat
Authors: Khubchandani M. , Mallick H.N. , Jagannathan N.R. , Mohan Kumar V. 
Published: April 2003

AbstractA stereotaxic restraining assembly was designed and developed for simultaneous electrophysiological recordings and functional MRI (fMRI) data acquisition from a conscious rat. The design of the nonmagnetic stereotaxic apparatus facilitated the restraining of head and body of the unanesthetized conscious animal during MRI experiments. The apparatus was made of Teflon and Perspex materials with an appropriate size and shape for a 4.7 T / 40 cm animal MRI scanner. Electrodes made from nonmagnetic silver wire were implanted on the skull for recording the electroencephalogram (EEG), the electro‐oculogram (EOG), and the electromyogram (EMG), while polycarbonate screws were used for anchoring the electrode assembly. There were no major distortions or artifacts observed in the electrophysiological tracings and MR images. Electrophysiological recordings during fMRI acquisitions are useful to study different neurophysiological mechanisms of sleep and pathophysiology of seizure activity. Integration of electrophysiological recordings (with their good temporal resolution) and MRI (with its superior spatial resolution) is helpful in characterizing the functional state of different brain regions. Magn Reson Med 49:962–967, 2003. © 2003 Wiley‐Liss, Inc.
A numerical model for the cyclic deterioration of railway tracks
Authors: Suiker Akke S. J. , de Borst René 
Published: April 2003

AbstractAn elasto‐plastic material model is proposed that can be used to simulate the cyclic deterioration of railway tracks. The model describes the envelope of the irreversible, plastic material response generated during a cyclic loading process, thereby distinguishing the mechanisms of frictional sliding and volumetric compaction. The reversible response is represented by a pressure‐dependent, hypo‐elastic material law. After the numerical integration of the model is specified, the model is calibrated on laboratory experiments and employed in a finite‐element case study of the long‐term settlement behaviour of a railway track. The main features of the model are illustrated by comparing the computed response with the response obtained by in situ track measurements. Copyright © 2003 John Wiley & Sons, Ltd.
An arbitrary Lagrangian–Eulerian finite element method for finite strain plasticity
Authors: Armero Francisco , Love Edward 
Published: April 2003

AbstractThis paper presents a new arbitrary Lagrangian–Eulerian (ALE) finite element formulation for finite strain plasticity in non‐linear solid mechanics. We consider the models of finite strain plasticity defined by the multiplicative decomposition of the deformation gradient in an elastic and a plastic part (F = FeFp), with the stresses given by a hyperelastic relation. In contrast with more classical ALE approaches based on plastic models of the hypoelastic type, the ALE formulation presented herein considers the direct interpolation of the motion of the material with respect to the reference mesh together with the motion of the spatial mesh with respect to this same reference mesh. This aspect is shown to be crucial for a simple treatment of the advection of the plastic internal variables and dynamic variables. In fact, this advection is carried out exactly through a particle tracking in the reference mesh, a calculation that can be accomplished very efficiently with the use of the connectivity graph of the fixed reference mesh. A staggered scheme defined by three steps (the smoothing, the advection and the Lagrangian steps) leads to an efficient method for the solution of the resulting equations. We present several representative numerical simulations that illustrate the performance of the newly proposed methods. Both quasi‐static and dynamic conditions are considered in these model examples. Copyright © 2003 John Wiley & Sons, Ltd.
A collocation meshless method based on local optimal point interpolation
Authors: Zuppa Carlos , Cardona Alberto 
Published: April 2003

AbstractThis paper deals with the use of the local optimal point interpolating (LOPI) formula in solving partial differential equations (PDEs) with a collocation method. LOPI is an interpolating formula constructed by localization of optimal point interpolation formulas that reproduces polynomials and verifies the delta Kronecker property. This scheme results in a truly meshless method that produces high quality output and accurate solutions. Copyright © 2003 John Wiley & Sons, Ltd.
A priori error estimator of the generalized‐α method for structural dynamics
Authors: Chung Jintai , Cho Eun‐Hyoung , Choi Keeyoung 
Published: April 2003

AbstractAn a priori error estimator for the generalized‐α time‐integration method is developed to solve structural dynamic problems efficiently. Since the proposed error estimator is computed with only information in the previous and current time‐steps, the time‐step size can be adaptively selected without a feedback process, which is required in most conventional a posteriori error estimators. This paper shows that the automatic time‐stepping algorithm using the a priori estimator performs more efficient time integration, when compared to algorithms using an a posteriori estimator. In particular, the proposed error estimator can be usefully applied to large‐scale structural dynamic problems, because it is helpful to save computation time. To verify efficiency of the algorithm, several examples are numerically investigated. Copyright © 2003 John Wiley & Sons, Ltd.
Sedimentation and diffusion of cellulose molecules
Authors: Hermans J. J. 
Published: April 2003

AbstractIt is shown that the sedimentation and diffusion constants of cellulose at infinite dilution conform with a formula derived on the assumption of randomly coiled structures. The principle is introduced that the sedimentation velocity of closely packed particles moving through a liquid exerts an influence on their motion, according to known hydrodynamic laws.
Silver dichroism of micellar systems with fibrous structures
Authors: Frey‐Wyssling A. , Wälchli O. 
Published: April 2003

AbstractBy embedding submicroscopic silver rodlets in micellar systems with fibrous structures, unexpectedly strongly anisotropic rodlet compounds bodies are formed. The observed dichroism, indigo blue–straw yellow, suggests the possibility that a particular anisotropic modification of silver is being dealt with, instead of cubic silver. On applying Wiener's compound body formulas, it appears, however, that in the short‐wave region the observed anisotropic effects (negative rodlet double refraction and dichroism) can be computed from the optical properties of isotropic silver. In the spectral region of the average and long waves, differences are found between the observed anisotropy and the value calculated by means of Wiener's formulas. These differences are, however, not of such a nature that it is necessary to assume an unknown allotropic modification of silver with particular optically anisotropic properties in order to explain them.
Determination of persulfate in emulsion polymerization latices
Authors: Kolthoff I. M. , Guss L. S. , May D. R. , Medalia A. I. 
Published: April 2003

AbstractIn the emulsion polymerization of butadiene, styrene, or both, potassium persulfate is often used as the catalyst. A convenient concentration of potassium persulfate in the aqueous layer of the charge is 0.10–0.2%; the method of analysis should therefore be suitable for very small persulfate concentrations. Three methods have been investigated. In the volumetric method the persulfate is allowed to react in an acid medium with an excess of standard ferrous iron, the excess being back‐titrated after a given period of time with standard ceric cerium solution. The recommended procedure was found accurate and precise within 0.5%, even at such small concentrations as 5 mg. of potassium persulfate per 100 ml. When latex is analyzed by this method, the polymer is first removed by coagulation with sulfuric acid and the persulfate determined in the filtrate. In order to obtain reliable results the reaction between the persulfate and ferrous iron in the filtrate must be carried out in the absence of oxygen, although this is not necessary in solutions of pure persulfate. The interference by oxygen is attributed to substances of peroxidic character in the fatty acid soaps used as emulsifiers in the emulsion polymerization. The colorimetric method (“Becco” method) is based upon the oxidation of the leuco base of commercial wool green BSMA to a green dye by the persulfate and (colorimetric) measurement of the dye formed. The various factors affecting the accuracy and precision of this method have been investigated and a procedure is given for the analysis of latex. For several reasons, the volumetric and polarographic methods are superior to the colorimetric method. Persulfate in a latex can be determined accurately by polarographic analysis of the latex filtrate after removal of the polymer with sulfuric acid. The polarographic determination can be carried out, in the presence of oxygen, with simple equipment. The method is rapid and precise.
Preparation and properties of rubberlike high polymers. II. Polymerization of mixtures in bulk
Authors: Koningsberger C. , Salomon G. 
Published: April 2003

AbstractMixtures of butadiene and dimethylbutadiene with varying quantities of styrene and acrylonitrile have been copolymerized under various catalytic conditions at 100°C. The formation of cyclic dimers from dienes and vinyl compounds is, in some cases, much more pronounced than the dimerization of the dienes alone. Since the latter is also a kind of Diels‐Alder reaction, a series with increasing dienophilic properties can be established from a comparison of the yields of cyclic dimers. From solubility and swelling experiments it is evident that the polymers prepared from butadiene and the vinyl compounds and those from dimethylbutadiene and acrylonitrile are true copolymers. The mechanical properties of the copolymers from butadiene and increasing amounts of a vinyl compound change gradually from those of a rubber to those of the polyvinyl compound. The copolymers from a mixture containing 30% styrene or acrylonitrile are very similar, respectively, to Buna S and Perbunan. Copolymers of the same composition from dimethylbutadiene and acrylonitrile, on the other hand, form a new class of thermoplastics with a much higher temperature of elasticity.