bluuebunny/crossref_metadata_2025_split huggingface dataset:
Free radical copolymerization of electron‐rich vinyl monomers with acrylonitrile complexed to zinc chloride
Authors: Wang H. , Chu G. , Srisiri W. , Padias A. B. , Hall H. K. 
Published: April 2003

AbstractThe spontaneous free radical copolymerizations of a series of electron‐rich olefins with acrylonitrile (AN) complexed to zinc chloride (ZnCl2) have been investigated in solution. With styrene (St) and 2‐vinylnaphthalene, an alternating copolymerization occurred without added initiator at 40°C. Using p‐methylstyrene as the electron‐rich olefin, no‐initiator spontaneous alternating copolymerization was observed at 30°C. Copolymer formed in an amount equal to that of the ZnCl2 used. With p‐methoxystyrene (pMeOSt) and with isobutyl vinyl ether (IBVE), the copolymerizations were conducted in the presence of ethyldiisopropylamine (EDIA) to inhibit cationic homopolymerization of these monomers. The copolymerization with pMeOSt proceeded to high yield, but the copolymers were rich in pMeOSt and the rates were slow presumably due to competitive complexation of ZnCl2 with the methoxy‐substituent. Using IBVE only low yields of a random copolymer were obtained, and again competitive coordination with ZnCl2 appears responsible. The small molecules accompanying these spontaneous copolymerizations were shown by GC/MS to possess cyclobutane and Mayo adduct structures. The mechanism of these spontaneous copolymerizations cannot be decided on the basis of the small molecules formed, but the increases of copolymer molecular weight with time at constant yield speaks for a diradical mechanism.
Synthesis and characterization of copolyamides with rigid and flexible segments
Authors: Fischer H. , Berger W. , Scheller D. 
Published: April 2003

AbstractCopolyamides with rigid and flexible chain segments were prepared using the reaction between diacids and diisocyanates. Segmented copolymers of these copolyamides are formed directly and spontaneously. UV‐ and NMR‐spectroscopy exclude the formation of homopolyamides only. The copolyamides with a content of rigid chain units between 20% nd 40% show thermotropic liquid crystalline behavior. Polarization microscopy and X‐ray diffraction investigations indicate the presence of nematic phase.
Preferential wetting of oligomeric ethylene glycol/propylene glycol blends on solid surfaces
Authors: Muniz E.C. , Pellegrini M.C. , Nunes S. Pereira 
Published: April 2003

AbstractThe phase separation of oligomeric ethylene glycol/propylene glycol systems was investigated by light scattering in the presence of a solid surface. Beside the spinodal decomposition in the bulk of the system, a wetting transition was observed when phase separation occurred at conditions near the maxima of these phase diagrams. The transition was detected as a light scattering halo, which is more intense at a composition which does not coincide with the critical one. This composition does not depend on the surface but the halo intensity is higher when glass is used instead of PVC.
Dichroism in photoinduced infrared spectra of polyalkylthiophenes
Authors: Veronelli M. , Gallazzi M.C. , Zerbi G. 
Published: April 2003

AbstractThe photoinduced infrared spectra of stretch‐oriented polyhexylthiophene and polyoctylthiophene are reported as a function of the polarization both of the pump light and of the probe IR beam. There is no change in the intensity of photoinduced IR absorption due to the polarization of the pump; however, the spectra show parallel dichroism when the probe is polarized. The experimental results are analyzed on the basis of the “effective conjugation coordinate” theory (ECC).
Application of a new generation of computerized apparatus for the study of oxygen uptake and production of CO and CO2 during photo‐(thermal) oxidation of polymers
Authors: Czekajewski J. , Nennerfelt L. , Kaczmarek H. , Rabek J. F. 
Published: April 2003

AbstractIn an attempt to measure simultaneously oxygen uptake and CO and CO2 production during photo‐(thermal) oxidation of polymers, an apparatus was constructed for sensitive measurements of changes in oxygen, CO and CO2 concentrations of 5–10 parts per million. The oxygen uptake and CO and CO2 production profiles show first‐order kinetics of observed photooxidative processes in polyethylene, poly(phenylene oxide) and cellulose. The experimental results suggest that this apparatus has a certain potential for the study of photo‐(thermal), bio‐ and environmental oxidation of polymers.
Synthesis of critically cross‐linked microgels by dispersion polymerization
Authors: Antonietti M. , Bremser W. , Pakula T. 
Published: April 2003

AbstractMicrogels with cross‐link densities close to the critical value of gel transition are synthesized via polymerization in microemulsion stabilized with a polymeric surfactant. The resulting structures are characterized by static and dynamic light scattering, viscometry, small angle neutron scattering and dynamic‐mechanical shear experiments.The proximity of the gel point influences the different properties in a different way. Global properties such as particle shape are closely connected with the gel point whereas the local structure and relaxation show critical behavior even quite far from the gel point.
Electrical noise in aqueous solutions of gelatin containing KCL during the sol–gel transition
Authors: Quadrat O. , Klason C. , Kubát J. , McQueen D. H. 
Published: April 2003

AbstractElectrical current noise and electrical resistivity as well as viscosity were measured in aqueous solutions of gelatin containing KCl during the sol—gel transition. While the viscosity rises toward infinity at the transition, the current noise power and the resistivity are continuous functions of the time and are therefore potentially useful probes for characterizing polymers undergoing gelation.
Fibers from blends of PBT and thermotropic liquid crystalline copolyester
Authors: Mehta S. , Deopura B. L. 
Published: April 2003

AbstractBlends of poly(ethylene terephthalate‐co‐p‐oxybenzoate), PET/PHB, with poly(butylene terephthtalate), PBT, have been studied in the form of as‐spun and drawn fibers. DSC melting and crystallization studies of blend fibers show a single melting endotherm, corresponding to PBT crystallites and a decrease in the crystallization of PBT by the addition of LCP in the matrix. X‐ray diffraction patterns of the blends exhibit mainly all the reflections of the homopolymer component (PBT). However, the diffused meridional reflections in the blend fibers show that the crystal structure of the matrix is affected by the addition of LCP and there is possible formation of LCP oriented domains. SEM studies of drawn blend fibers show that the blends up to 10 wt.% LCP content contain the LCP domains in the size range of 0.1‐–0.3 μm and are well distributed in the PBT matrix. Mechanical properties of drawn PBT/LCP blend fibers show an increase in modulus with the increase in LCP content. Values of the modulus are predicted from composite theory by assuming the blend system to consist of LCP domains dispersed in PBT matrix.
Local motions in polymeric and low molar mass supramolecular hydrogen bond assemblies: One‐ and two‐dimensional 2H‐NMR studies, full papers
Authors: Dardin A. , Boeffel C. , Spiess H. W. , Stadler R. , Samulski E. T. 
Published: April 2003

AbstractSupramolecular structures based on 4‐(3,5‐dioxo‐1,2,4‐triazolidin‐4‐yl) benzoic acid (U4A) units act as effective junction zones in functionalized polybutadienes, resulting in the formation of thermoplastic elastomers. The dynamics on a molecular scale of the phenyl rings have been probed by 2H‐NMR. In the polymeric system, there are three spatially separated environments, which are reflected in the mobility of the polar units. Phenyl rings which are incorporated in the structures are either rigid or undergo 180° phenyl flips. The small fraction of free functional groups move isotropically and their mobility is coupled to the dynamics of the polybutadiene matrix. In two model compounds, where the polymer backbone is replaced by a 1‐butenyl (tBu‐dU4A) or a C12H24‐chain (C12‐dU4A), the packing of the single molecules is more perfect, resulting in better‐defined dynamics of the molecules. The one‐dimensional 2H‐NMR spectra can be described quantitatively assuming a distribution of correlation times over 3 decades for the C12‐dU4A and 2 decades for the tBu‐dU4A, respectively. From the average jump rates the activation energies are determined as 101 kJ/mol and 102 kJ/mol. The geometry of the motional processes is defined by the environment in the clusters up to the order‐disorder transition temperature as determined by DSC. The two‐dimensional spectra of the model compounds show an elliptical exchange pattern, indicating well‐defined slow 180° phenyl flips on a time scale of 100 ms up to 3 s.
Evaluation of equation‐of‐state parameters from PVT data
Authors: Rudolf B. , Kressler J. , Shimomai K. , Ougizawa T. , Inoue T. 
Published: April 2003

AbstractThe characteristic parameters for three different equation‐of‐state (EOS) theories have been determined for polystyrene and low‐molecular weight polyisoprene using pressure‐volume‐temperature (PVT) data. Basically two procedures for the calculation of characteristic parameters were applied. The characteristic parameters were calculated, firstly, from coefficients of expansion, compressibilities and densities, and secondly, by fitting the corresponding EOS to PVT data minimizing the sum of the squares of the differences between experimental and calculated pressures. The characteristic parameters obtained by the two methods are compared with regard to their ability to describe PVT data. A comparison of the three different EOS theories reveals that, for describing polymeric liquids, the modified cell model of Dee and Walsh is superior to the theory of Flory, Orwoll and Vrij as well as to the lattice fluid theory of Sanchez and Lacombe. Finally, formulae are given to convert the characteristic parameters from one theory into those of another.
Complex formation between poly(oxytrimethylene) and cyclodextrins
Authors: Harada A. , Okada M. , Kamachi M. 
Published: April 2003

AbstractPoly(oxytrimethylene) (POT) has been found to form inclusion complexes with cyclodextrins to give crystalline compounds in high yields. The yields of the complexes of POT with α‐cyclodextrin increased with increase in the molecular weight of POT, reached a maximum at a molecular weight of about 1000, and then decreased with the molecular weight. The stoichiometries are 1:1.5‐2 (CD:monomer unit) when the molecular weight of POT is less than 2000. β‐Cyclodextrin gave complexes with POT in a 1:2‐2.5 ratio when the average molecular weights were less than 2000. When the molecular weight of the polymer was higher than 3000, the ratio of the monomer unit to cyclodextrin increased with increase in the molecular weight. The binding modes are discussed.
Retardation experiments in glass‐forming liquids and their description in terms of the conformon model
Authors: Kilian H. G. , Glöckle W. G. 
Published: April 2003

AbstractRelaxation in glass‐forming liquids as measured by dielectric, by light‐ and by neutron‐scattering experiments can be described with the aid of the conformon model. Conformons behave like bosons which display a photon statistics. Conformons are excited and can also be annihilated. They show a universal reduced energy density distribution, revealing their thermodynamic roots. Each relaxation mode is assumed to be directly connected with a distinct conformon. The coupling is described by an empirical power law. The actual relaxation time distribution is then obtained by scaling each relaxation mode with the same kinetic factor. The relaxation behavior of liquids is therefore self‐similar, characterized by individual factors such as the relaxation power, the coupling parameter and the temperature of pressure dependence of the kinetic factor (“shift factor”).
Polymerizable dendrimers: 2. Mono‐methacryl modified dendrimers containing up to 16 ester functions via stepwise condensations of L‐aspartic acids
Authors: Niggemann M. , Ritter H. 
Published: April 2003

AbstractA convergent synthesis of polymerizable dendrimers consisting of one methacrylic function combined with an alkylene spacer and several condensed L‐aspartic acid components is described. The highly functionalized monomers were prepared from N‐methacryloyl amino undecanoic acid (9) via condensation with free amino groups of L‐aspartic acid dimethyl ester (2), α, β‐bis‐(L‐dimethoxyaspartyl)‐L‐aspartic acid hydrobromide (4) and α, β‐bis‐(L‐aspartyl‐α, β‐bis‐(L‐dimethoxyaspartyl))‐L‐aspartic acid hydrobromide (6). The number of ester groups increased from 2 to 4 and 8, up to 16, depending on the dendritic generation of the condensed amino acids. The monomers were homopolymerized and copolymerized with styrene through a free radical mechanism.
Multilayer barrier films on polyethylene polymerized from CF3H/C2H4 plasmas
Authors: Walker M. , Baumgärtner K. M. , Räuchle E. 
Published: April 2003

AbstractThe barrier properties of thin fluorocarbon films toward toluene are studied. Mixtures of trifluoromethane and ethylene are used as monomers in an electron cyclotron resonance plasma discharge. The measured permeation fluxes of toluene through polyethylene show that thin fluorocarbon layers strongly reduce the permeation, e. g. by a factor of about 3000 when an appropriate tapered layer is deposited on polyethylene foils. Different tapering of the barrier layers was investigated with respect to low permeation and good adhesion of the films. The amount of permeated toluene through the untreated and treated polyethylene foils are measured as a function of time using a gas chromatograph with a flame ionization detector.
Degree of branching in hyperbranched polymers. 2. Enhancement of the db: Scope and limitations
Authors: Frey H. 
Published: April 2003

AbstractThe degree of branching (DB) as well as the ANB (average number of branches) of polymers obtained from polycondensation of AB2 and ABm‐type monomers in general is analyzed in detail on the basis of kinetic considerations. Various possibilities for the enhancement of the DB of hyperbranched polymers beyond the limit for a random ABm polycondensation (0.5 for AB2, 0.44 for AB3) are considered, i.e., (i) enhanced reactivity of linear vs. terminal units; (ii) polymerization of prefabricated, perfect dendritic units and (iii) the slow addition technique. For AB2 monomers, the formation of terminal (T), linear (L) and perfect dendritic (D) units in the course of the random one‐pot polymerization is calculated. Furthermore, the evolution of the DB and ANB with conversion is calculated for ABm monomers. It is shown that the definition of the DB for AB2 systems (DB = 2D/(2D+L)) derived in the first paper on the DB (Acta Polym. 1997, 48, 30) in the case of a random one‐pot polycondensation exhibits the same conversion dependence as the branching parameter α = 1/2PA, defined by Flory for AB2 polycondensations. The dilution principle is introduced, which is based on slow addition of ABm monomers to a core molecule Bf or an ABm‐type hyperbranched molecule with an arbitrary DB. It is shown that the maximum DB value obtainable from such a dilution/slow addition process is 0.67 for AB2 systems. Thus, the DB is considerably higher than in the case of a random one‐step polymerization (0.5). The DB for ABm‐type hyperbranched polymers prepared according to the dilution principle has been calculated to be m/(2m–1).
Pattern formation in the Drosophila wing: The development of the veins
Authors: de Celis Jose F. 
Published: April 2003

AbstractThe veins are cuticular structures that differentiate in precise patterns in insect wings. The genetic and molecular basis of vein pattern formation in Drosophila melanogaster is beginning to be unravelled with the identification and characterisation of the gene products that position the veins and direct their differentiation. Genes affecting the veins fall into two groups: transcriptional regulators that specify individual veins, and members of signalling pathways involved in patterning and differentiation of the veins. The elaboration of the vein pattern is progressive in time and requires the coordinated activities of these signalling pathways and the transcription factors regulated by them. Although the network of genetic interactions that determine vein cell fate is well understood, very little is known about the cellular biology underlying the acquisition of vein histotype. BioEssays 25:443–451, 2003. © 2003 Wiley Periodicals, Inc.
Fine tuning the HIF‐1 ‘global’ O2 sensor for hypobaric hypoxia in Andean high‐altitude natives
Authors: Hochachka Peter W. , Rupert Jim L. 
Published: April 2003

AbstractIncluded in the acute response of lowlanders exposed to reduced oxygen availability is an elevated red blood cell count due to increased erythropoietin (Epo) synthesis. According to current thinking, hypoxia is “sensed” by hydroxylases that permit Hypoxia Inducible Factor 1α (HIF‐1α) to complex with HIF‐1β to form a transcriptional activator (HIF‐1) that drives expression of hypoxia‐sensitive genes (such as EPO) under hypoxic conditions. In altitude‐adapted Andean natives, the Epo hypoxic response may be blunted; however, our data indicate that the DNA sequences of the genes encoding Epo (including the 3′ regulatory region) and HIF‐1α appear to be conserved. Hence, adaptive changes in the Andean hypoxic response are not a consequence of changes in the primary sequence of these proteins or of known transcriptional regulatory domains of EPO. These results suggest that the altered erthropoietic response in Andean natives reflects adaptations in hypoxia sensing, rather than hypoxia response, mechanisms. BioEssays 25:515–519, 2003. © 2003 Wiley Periodicals, Inc.
Tumor progression: Small GTPases and loss of cell–cell adhesion
Authors: Lozano Encarnación , Betson Martha , Braga Vania M.M. 
Published: April 2003

AbstractTumor progression involves the transition from normal to malignant cells, through a series of cumulative alterations. During this process, invasive and migratory properties are acquired, enabling cells to metastasize (reach and grow in tissues far from their origin). Numerous cellular changes take place during epithelial malignancy, and disruption of E‐cadherin based cell‐cell adhesion is a major event. The small Rho GTPases (Rho, Rac and Cdc42) have been implicated in multiple steps during cellular transformation, including alterations on the adhesion status of the tumor cells. This review focuses on recent in vivo evidence that implicates RhoGTPases in epithelial tumor progression. In addition, we discuss different hypotheses to explain disruption of cadherin‐mediated cell–cell adhesion, directly or indirectly, through activation of Rho GTPases. Understanding the molecular mechanism of how cadherin adhesion and RhoGTPases interplay in normal cells and how this balance is altered during cellular transformation will provide clues as to how to interfere with tumor progression. © 2003 Wiley Periodicals, Inc. BioEssays 25:452–463, 2003. © 2003 Wiley Periodicals, Inc.
A hypothesis for chromatin domain opening
Authors: Xin Li , Liu De‐Pei , Ling Chih‐chuan 
Published: April 2003

AbstractThe eukaryotic genome is organized into different domains by cis‐acting elements, such as boundaries/insulators and matrix attachment regions, and is packaged with different degrees of condensation. In the M phase, the chromatin becomes further highly condensed into chromosomes. The first step for transcriptional activation of a given gene, at a particular time during development, in any locus, is the opening of its chromatin domain. This locus needs to be kept in this state in each early G1 phase during every cell cycle. Certain distal enhance elements, including locus control regions (LCRs) and enhancers, are believed to perform this target chromatin domain opening process and several models have been proposed to explain distal enhance action. But they did not explain precisely how a given chromatin domain is opened. Based on various studies, we propose a hypothesis for the mechanism of opening chromatin on a large scale. One important mechanism may involved breaking one or two DNA strands and reducing the linking numbers within chromatin domain. The topological changes can overpass some complexes formed on DNA strands and can be transmitted from specific localized points over a broad region, until boundary elements or insulators are reached. These may initiate downstream events such as propagation of histone acetylation and the binding of transcription factors to proximal promoters and may further augment the action mediated by distal enhancer elements. BioEssays 25:507–514, 2003. © 2003 Wiley Periodicals, Inc.
Renin: from ‘pro’ to promoter
Authors: Morris Brian J. 
Published: April 2003

AbstractRenin is the rate‐limiting enzyme in a cascade that leads to production of angiotensin II, which is perhaps our most important regulator of salt and water balance and blood pressure. In this personal perspective, I describe how I entered the renin field 33 years ago by discovering that proteases increased the level of renin activity in biological fluids, so revealing the existence of a ‘pro’ form of the molecule. This led me on a journey that encapsulated all of the major milestones in molecular discovery for renin. These included (1) the elucidation of the steps in renin biosynthesis, (2) the cloning of renin cDNA and its gene, (3) demonstration of the structure of the renin protein, (4) using the renin gene in the first genetic studies in hypertension, (5) finding the mechanism by which the major controller, cyclic AMP, regulates the promoter, (6) showing that a strong enhancer and its weak promoter control this physiologically regulatable gene in accord with the variegation (on/off switching) model, and (7) being the first to identify molecules involved in posttranscriptional control. The renin molecule, its gene and molecular control are now very well understood, but more fine details on the topic of renin continue to emerge to delight ‘reninologists’ and others. BioEssays 25:520–527, 2003. © 2003 Wiley Periodicals, Inc.
Subversion of the chemokine world by microbial pathogens
Authors: Liston Adrian , McColl Shaun 
Published: April 2003

AbstractIt is well known that microbial pathogens are able to subvert the host immune system in order to increase microbial replication and propagation. Recent research indicates that another arm of the immune response, that of the chemokine system, is also subject to this sabotage, and is undermined by a range of microbial pathogens, including viruses, bacteria, and parasites. Currently, it is known that the chemokine system is being challenged by a number of mechanisms, and still more are likely to be discovered with further research. Here we first review the general mechanisms by which microbial pathogens bypass mammalian chemokine defences. Broadly, these can be grouped as viral chemokine interacting proteins, microbial manipulation of host chemokine and chemokine receptor expression, microbial blockade of host chemokine receptor signalling, and the largely hypothetical mechanisms of microbial enhancement of host anti‐chemokine networks (including digestion, antagonism, and neutralisation of host chemokines and chemokine receptors). We then discuss the potential results of these interactions in terms of outcome of infection. BioEssays 25:478–488, 2003. © 2003 Wiley Periodicals, Inc.