bluuebunny/crossref_metadata_2025 huggingface dataset:
In Situ Preparation of a Compatibilized Poly(cis‐1,4‐butadiene)/Poly(ε‐caprolactone) Blend
Authors: Friebe Lars , Nuyken Oskar , Windisch Heike , Obrecht Werner
Published: June 2003

The ternary Ziegler‐Natta‐type catalyst system based on neodymium versatate (NdV), diisobutylaluminium hydride (DIBAH) and ethylaluminium sesquichloride (EASC) was used for the in situ preparation of a compatibilized blend consisting of poly(cis‐1,4‐butadiene) (BR = butadiene rubber) and poly(ε‐caprolactone) (PCL). Poly(cis‐1,4‐butadiene)‐block‐poly(ε‐caprolactone) which acts as compatibilizer for the two immiscible polymers BR and PCL was obtained by a two step sequential polymerization with the preparation of a living cis‐1,4‐BR building block in the first stage and the subsequent polymerization of CL during the second stage. This preparation method resulted in a polymer blend comprising the homopolymers BR and PCL as well as the block copolymer BR‐block‐PCL. For detailed characterization the block copolymer was separated from the respective homopolymers BR and PCL by means of fractionation with the binary solvent mixture dimethylformamide/methylcyclohexane (DMF/MCH) which mixes well at elevated temperature and exhibits phase separation at ambient temperature. 1H NMR, IR, SEC and TEM were used for characterization of the block copolymer.

TEM of BR‐block‐PCL.

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TEM of BR‐block‐PCL.
Application of Small‐Angle X‐Ray Scattering as a Tool for the Structural Analysis of Industrial Polymer Dispersions
Authors: Bolze Joerg , Ballauff Matthias , Kijlstra Johan , Rudhardt Daniel
Published: June 2003

Small‐angle X‐ray scattering (SAXS) was applied for the structural analysis of an industrial polymer dispersion in water (synthetic latex). For the preparation of the spherical latex particles under investigation, butadiene, styrene, and acrylic acid were used as monomers in a seeded emulsion polymerization process. The product is widely being used as a film‐forming agent for coatings in the paper‐making industry. It is demonstrated that by measuring the SAXS curves at different contrasts the overall particle size, mass density, polydispersity, and degree of heterogeneity can be estimated with a good accuracy, even without the necessity of a detailed fitting procedure. In particular, one obtains information about the spatial distribution of the various monomer units within the particles. Five isoscattering points could be observed in the contrast variation measurements and the scattering curves at all contrasts could be modeled with a fully consistent set of fit parameters. It is thus safe to conclude that the contrast agent sucrose does not affect the particle structure. A quantitative fitting of the experimental data set revealed that a significant amount of the poly(acrylic acid) is preferentially located in a thin shell of ca. 2 nm thickness around the core of the particles, that is mainly formed by poly(styrene‐co‐butadiene). The obtained results are fully consistent with additional measurements of the particle mass density, of the hydrodynamic particle radius by dynamic light scattering, and of the particle surface charge by potentiometric titration. It is concluded that SAXS is a highly useful tool for characterizing the structure of industrial latexes.

SAXS data of a BAYSTAL® latex measured at various contrasts, the weight fraction of sucrose in the dispersion medium is given in the legend.

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SAXS data of a BAYSTAL® latex measured at various contrasts, the weight fraction of sucrose in the dispersion medium is given in the legend.
Effect of Thermal History on Structural Changes in Melt‐Intercalated Poly(ε‐caprolactone)/Organoclay Nanocomposites Investigated by Dynamic Viscoelastic Relaxation Measurements
Authors: Kwak Seung‐Yeop , Sei Oh Kwang
Published: June 2003

Poly(ε‐caprolactone) (PCL) nanocomposites were prepared using two different types of organically modified nanosilicates by melt intercalation with an internal mixer. Dynamic mechanical analysis revealed possible structural changes in the nanocomposites even during the small deformation occurring during shear oscillatory measurements, as evidenced by a V‐shaped modulus change in the plot of the dynamic storage modulus as a function of stepwise increased temperature. X‐ray diffraction patterns were recorded at different simulated temperatures during the various stages of dynamic measurements. The X‐ray data indicate that the structural changes can be ascribed to a further intercalation of the PCL matrix chains into the silicate layers. This further intercalation is a consequence of the heat treatment during the dynamic mechanical measurements. Furthermore, there is a considerable vertical shift in addition to the horizontal shift in the higher temperature regime, which allows the mapping of a master curve through the application of the time‐temperature superposition principle to the dynamic storage and the loss modulus data obtained at various isothermal temperatures. The present study is also concerned with the relative molecular mobility of both PCL nanocomposites in the given experimental conditions considering the Williams‐Landel‐Ferry (WLF) equation and the Arrhenius relationship between the horizontal shift factor and the activation energy of flow. Moreover, the extent of the vertical shift as a function of temperature made it possible to determine the apparent activation energy of the further intercalation of PCL into the silicate layers. This intercalation is caused by the additional exposure to heat during the dynamic mechanical measurements after mixing, which led to a comparison of the relative diffusivity of the PCL matrix in the two nanocomposites.

Dynamic shear storage moduli G′ of PCLOC25A and PCLOC30B as a function of temperature with increase increments of 20 °C from 60 to 260 °C. The G′ data were obtained from isothermal frequency sweep G′(ω) data at ω = 1 rad · s−1 at the corresponding temperatures.

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Dynamic shear storage moduli G′ of PCLOC25A and PCLOC30B as a function of temperature with increase increments of 20 °C from 60 to 260 °C. The G′ data were obtained from isothermal frequency sweep G′(ω) data at ω = 1 rad · s−1 at the corresponding temperatures.
Kinetic and Thermodynamic Studies of Emulsion Polymerization of a Sugar Monomer by Calorimetry with Compensation Heating and Differential Cooling
Authors: Dobre Adrian Cosmin , Yaacoub Emile‐Joseph , Schumpe Adrian
Published: June 2003

The kinetic and thermodynamic features of free‐radical batch emulsion polymerization of a sugar monomer (3‐MDG) and butyl acrylate (BA) were investigated in a power compensation calorimeter. The homopolymerizations as well as the copolymerization have been studied. The overall activation energy of 3‐MDG homopolymerization was 140 ± 3.8 kJ · mol−1, the polymerization enthalpy was ΔHMDG = −51.6 ± 1.9 kJ · mol−1 and the calculated adiabatic temperature rise was ΔTad = 78.5 K. The effects of the initiator and the emulsifier concentrations on the 3‐MDG/BA batch copolymerization kinetics and on the colloidal properties of the final sugar latexes were studied at 60 °C. At higher emulsifier and initiator concentration, respectively, the polymerization rate increases and the particle size decreases, but the trends do not conform to the Smith‐Ewart theory. Polydisperse sugar latex particles with a mean diameter in the range of 50–67 nm were obtained.

Relationship between the activation energy and the conversion for BA (open symbols) and 3‐MDG (solid symbols).

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Relationship between the activation energy and the conversion for BA (open symbols) and 3‐MDG (solid symbols).
The Kinetics of the Thermal Decomposition of Thermoplastic Polyurethane Elastomers under Thermoplastic Processing Conditions
Authors: Endres Wolfgang , Lechner M. Dieter , Steinberger Rolf
Published: June 2003

A theoretical approach to the kinetics of the thermal decomposition of molten thermoplastic polyurethane elastomers, under conditions of thermoplastic processing, is described. On the basis of these considerations, the thermal decomposition in different instruments (melt index analyser and measuring extruder) can be described quantitatively and the various results can be compared. As a result, identical conditions of decomposition of the melt can be defined accurately, thus opening up the possibility of combining experimental values from different instruments. The fundamental kinetic equation obtained for the kinetics of the thermal decomposition of thermoplastic polyurethanes describes the decomposition reaction and the reverse reaction (formation reaction) – which is dependent on the system of measurement and processing – as a function of the molar mass (end‐group concentration) of the original product, determined from the velocity constants for the decomposition reaction and back reaction. The consideration of the limiting value for t → ∞ is in agreement with the equilibrium constant. Consequently, the development of physical characteristic functions of thermoplastic polyurethane elastomers – independent of the system of measurement – is possible.

Experimental values and calculated curves for the thermal decomposition of PUR‐Et in a melt index analyser.

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Experimental values and calculated curves for the thermal decomposition of PUR‐Et in a melt index analyser.
A transmission problem with non‐linear internal damping in elasticity
Authors: Oquendo Higidio Portillo
Published: June 2003

We consider an anisotropic body constituted by two different types of materials: a part is simple elastic while the other has a non‐linear internal damping. We show that the dissipation caused by the damped part is strong enough to produce uniform decay of the energy, more precisely, the energy decays exponentially when the dissipation is linear with respect to the velocity. For a non‐linear class of dissipations we prove that the energy decays polynomially. Copyright © 2003 John Wiley & Sons, Ltd.
Homogenizing the acoustic properties of a porous matrix containing an incompressible inviscid fluid
Authors: Ferrin J. L. , Mikelić A.
Published: June 2003

We undertake a rigorous derivation of the Biot's law for a porous elastic solid containing an inviscid fluid. We consider small displacements of a linear elastic solid being itself a connected periodic skeleton containing a pore structure of the characteristic size ε. It is completely saturated by an incompressible inviscid fluid. The model is described by the equations of the linear elasticity coupled with the linearized incompressible Euler system. We study the homogenization limit when the pore size εtends to zero. The main difficulty is obtaining an a priori estimate for the gradient of the fluid velocity in the pore structure. Under the assumption that the solid part is connected and using results on the first order elliptic systems, we obtain the required estimate. It allows us to apply appropriate results from the 2‐scale convergence. Then it is proved that the microscopic displacements and the fluid pressure converge in 2‐scales towards a linear hyperbolic system for an effective displacement and an effective pressure field.

Using correctors, we also give a strong convergence result. The obtained system is then compared with the Biot's law. It is found that there is a constitutive relation linking the effective pressure with the divergences of the effective fluid and solid displacements. Then we prove that the homogenized model coincides with the Biot's equations but with the added mass ρa being a matrix, which is calculated through an auxiliary problem in the periodic cell for the tortuosity. Furthermore, we get formulas for the matricial coefficients in the Biot's effective stress–strain relations.

Finally, we consider the degenerate case when the fluid part is not connected and obtain Biot's model with the relative fluid displacement equal to zero. Copyright © 2003 John Wiley & Sons, Ltd.
Solution of axisymmetric Maxwell equations
Authors: Assous Franck , Ciarlet Patrick , Labrunie Simon
Published: June 2003

In this article, we study the static and time‐dependent Maxwell equations in axisymmetric geometry. Using the mathematical tools introduced in (Math. Meth. Appl. Sci. 2002; 25: 49), we investigate the decoupled problems induced in a meridian half‐plane, and the splitting of the solution in a regular part and a singular part, the former being in the Sobolev space H1 component‐wise. It is proven that the singular parts are related to singularities of Laplace‐like or wave‐like operators. We infer from these characterizations: (i) the finite dimension of the space of singular fields; (ii) global space and space–time regularity results for the electromagnetic field. This paper is the continuation of (Modél. Math. Anal. Numér. 1998; 32: 359, Math. Meth. Appl. Sci. 2002; 25: 49). Copyright © 2003 John Wiley & Sons, Ltd.
A constitutive model for bonded geomaterials subject to mechanical and/or chemical degradation
Authors: Nova R. , Castellanza R. , Tamagnini C.
Published: June 2003

The mechanical behaviour of bonded geomaterials is described by means of an elastoplastic strain‐hardening model. The internal variables, taking into account the ‘history’ of the material, depend on the plastic strains experienced and on a conveniently defined scalar measure of damage induced by weathering and/or chemical degradation.

For the sake of simplicity, it is assumed that only internal variables are affected by mechanical and chemical history of the material. Despite this simplifying assumption, it can be shown that many interesting phenomena exhibited by weathered bonded geomaterials can be successfully described. For instance, (i) the transition from brittle to ductile behaviour with increasing pressure of a calcarenite with collapsing internal structure, (ii) the complex behaviour of chalk and other calcareous materials in oedometric tests, (iii) the chemically induced variation of the stress and strain state of such kind of materials, are all phenomena that can be qualitatively reproduced. Several comparisons with experimental data show that the model can capture the observed behaviour also quantitatively. Copyright © 2003 John Wiley & Sons, Ltd.
Finite element formulation and algorithms for unsaturated soils. Part I: Theory
Authors: Sheng Daichao , Sloan Scott W. , Gens Antonio , Smith David W.
Published: June 2003

This paper presents a complete finite‐element treatment for unsaturated soil problems. A new formulation of general constitutive equations for unsaturated soils is first presented. In the incremental stress–strain equations, the suction or the pore water pressure is treated as a strain variable instead of a stress variable. The global governing equations are derived in terms of displacement and pore water pressure. The discretized governing equations are then solved using an adaptive time‐stepping scheme which automatically adjusts the time‐step size so that the integration error in the displacements and pore pressures lies close to a specified tolerance. The non‐linearity caused by suction‐dependent plastic yielding, suction‐dependent degree of saturation, and saturation‐dependent permeability is treated in a similar way to the elastoplasticity. An explicit stress integration scheme is used to solve the constitutive stress–strain equations at the Gauss point level. The elastoplastic stiffness matrix in the Euler solution is evaluated using the suction as well as the stresses and hardening parameters at the start of the subincrement, while the elastoplastic matrix in the modified Euler solution is evaluated using the suction at the end of the subincrement. In addition, when applying subincrementation, the same rate is applied to all strain components including the suction. Copyright © 2003 John Wiley & Sons, Ltd.
Finite element formulation and algorithms for unsaturated soils. Part II: Verification and application
Authors: Sheng Daichao , Smith David W. , W. Sloan Scott , Gens Antonio
Published: June 2003

The finite‐element formulation and integration algorithms developed in Part I are used to analyse a number of practical problems involving unsaturated and saturated soils. The formulation and algorithms perform well for all the cases analysed, with the robustness of the latter being largely insensitive to user‐defined parameters such as the number of coarse time steps and error control tolerances. The efficiency of the algorithms, as measured by the CPU time consumed, does not depend on the number of coarse time steps, but may be influenced by the error control tolerances. Based on the analyses presented here, typical values for the error control tolerances are suggested.

It is also shown that the constitutive modelling framework presented in Part I can, by adjusting one constitutive equation and one or two material parameters, be used to simulate soils that expand or collapse upon wetting. Treating the suction as a strain variable instead of a stress variable proves to be an efficient and robust way of solving suction‐dependent plastic yielding. Moreover, the concept of the constitutive stress is a particularly convenient way of handling the transition between saturation and unsaturation. Copyright © 2003 John Wiley & Sons, Ltd.
Novel synthesis of reactive polycarbonates with pendant chloromethyl groups by the polyaddition of bis(oxetane)s with 2,2′‐bis[(4‐chloroformyl)oxyphenyl]propane
Authors: Yashiro Takao , Saithou Takashi , Kameyama Atsushi , Nishikubo Tadatomi
Published: June 2003

The polyaddition of 1,4‐bis[(3‐ethyl‐3‐oxetanyl)methoxymethyl]benzene with 2,2′‐bis[(4‐chloroformyl)oxyphenyl]propane was examined with quaternary onium salts as catalysts. When the polyaddition was carried out with tetrabutylphosphonium bromide in chlorobenzene at 120 °C for 24 h, the corresponding poly(alkyl aryl carbonate) with a high molecular weight (number‐average molecular weight = 16,700) was obtained in an almost quantitative yield. It was found from the 1H NMR and 13C NMR spectra of the obtained polymer that the addition reaction proceeded without any side reactions, providing the polycarbonate with pendant chloromethyl groups in the side chain. The polyaddition of bis{[3‐(3‐ethyloxetanyl)]methyl}terephthalate also proceeded smoothly and gave the corresponding polycarbonate with high molecular weight in a good yield. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2304–2311, 2003
Synthesis, characterization, and antimicrobial activity of some novel poly(ether ketone)s
Authors: Patel Samir A. , Patel Milan V. , Ray Arabinda , Patel Rajni M.
Published: June 2003

Low molecular weight poly(ether ketone)s were synthesized from phenol, 1,4‐phenylenedioxy diacetylchloride, chloroacetylchloride, and dichloroalkanes [1,2‐dichloroethane and dichloromethane] by a Friedel–Crafts reaction with anhydrous aluminum chloride as a catalyst and carbon disulfide as a solvent. The conditions for the preparation of the poly(ether ketone)s and the chlorine contents obtained with the Carius method were examined, and a reaction scheme for each resin was established. The molecular weights and polydispersities of the resins were obtained by gel permeation chromatography. The polyketones were characterized by IR spectroscopy. The characteristic frequencies due to different functional groups were assigned. The thermal properties of the resins were studied with thermogravimetry and differential scanning calorimetry. The characteristic temperatures of thermal degradation for the poly(ether ketone)s were evaluated with thermogravimetric analysis. The kinetic parameters for the decomposition reactions of the resins were obtained with Broido and Doyle's method, and the heats of fusion were obtained from differential scanning calorimetry thermograms. The polyketones were thermally stable up to 200 °C. All the polyketones were tested for their microbial properties against bacteria, fungi, and yeast. The effect of poly(ether ketone)s on the growth of these microorganisms was investigated, and the polyketones were found to inhibit the growth of the microorganisms to a considerable extent. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2335–2344, 2003
Poly(arylether amides) and poly(aryletherketone amides) via aromatic nucleophilic substitution reactions of self‐polymerizable AB and AB2 monomers
Authors: Baek Jong‐Beom , Harris Frank W.
Published: June 2003

Two new extended self‐polymerizable AB monomers, N‐(4‐fluorobenzoyl)‐4‐amino‐4′‐hydroxydiphenylether and N‐(4‐fluorobenzoyl)‐4‐amino‐4′‐hydroxybiphenyl, were prepared. The monomers were homopolymerized and copolymerized to high‐molecular‐weight, linear poly(arylether amides) in N‐methylpyrrolidone (NMP)/toluene in the presence of potassium carbonate at elevated temperature. The polymers retained NMP up to 200 °C. Samples containing small amounts of the solvent (5–10 wt %) were soluble in polar aprotic solvents. However, after complete removal of the NMP, the polymers were only soluble in strong acids such as sulfuric acid and methanesulfonic acid (MSA). The polymers, which had intrinsic viscosities of 0.57–1.49 dL/g (30.1 ± 0.1 °C in MSA), were semicrystalline with melting temperatures above 400 °C. Two new self‐polymerizable AB2 amide monomers, N,N′‐bis(4‐fluorobenzoyl)‐3,4‐diamino‐4′‐hydroxydiphenylether and N,N′‐bis(4‐fluorobenzoyl)‐3,5‐diamino‐4′‐hydroxybenzophenone, were also prepared and polymerized to give a hyperbranched poly(arylether amide) and a hyperbranched poly(aryletherketone) amide. The arylfluoride‐terminated, amorphous polymers had intrinsic viscosities of 0.34 and 0.24 dL/g (30.0 ± 0.1 °C in m‐cresol), glass‐transition temperatures of 210–269 °C, and were soluble in a wide variety of organic solvents. Matrix‐assisted laser desorption/ionization time‐of‐flight analysis indicated that the components of the low‐molecular‐weight fractions contained cyclic structures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2374–2389, 2003
Porphyrin‐based, light‐activated antimicrobial materials
Authors: Bozja Jadranka , Sherrill Jennifer , Michielsen Stephen , Stojiljkovic Igor
Published: June 2003

New light‐activated antimicrobial materials with a potentially wide range of possible uses in civilian settings were synthesized by the grafting of protoporphyrin IX and zinc protoporphyrin IX to nylon fibers. These fibers were shown to be active against Staphylococcus aureus at light exposures of 10,000 lux and greater and against Escherichia coli at 60,000 lux. They were ineffective against both strains in the absence of light. At 40,000 lux, these fibers showed increased antimicrobial activity against S. aureus with increasing exposure time. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2297–2303, 2003
Linear soluble polybenzyls
Authors: Som Abhijit , Ramakrishnan S.
Published: June 2003

Linear soluble polybenzyls, although deceptively simple in structure, have been strangely elusive. We report for the first time the synthesis of perfectly linear soluble polybenzyls by the polycondensation of 1,2,4,5‐tetrasubstituted benzenes with formaldehyde using CHCl3/trifluoroacetic acid (TFA) as the medium, wherein TFA served both as an acidic catalyst as well as a cosolvent. The number‐average molecular weights (Mn's) of the polymers, as determined by gel permeation chromatography, varied from about 1000 to 37,000, depending on the nature of the substituent on the benzene ring; Mn was highest when all four substituents were alkoxy groups and was lowest when they were all alkyl groups. This correlated well with susceptibility of the aromatic ring toward electrophilic aromatic substitution, which is the underlying polymerization mechanism. Differential scanning calorimetry of the polymers showed that most of the samples were amorphous with glass‐transition temperatures ranging from about −80° to +80 °C, whereas a few that were either symmetrically substituted or possessed a long alkyl substituent were partially crystalline. Preliminary studies suggested that the methylene unit linking the phenyl rings in these polybenzyls could be readily oxidized to generate conjugated polymers that may be perceived as carbon analogues of polyaniline–poly(arylmethine)s. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2345–2353, 2003
Semicontinuous seeded cationic emulsion polymerization of styrene: The effects of the concentration and type of cationic surfactant
Authors: Ramos Jose , Forcada Jacqueline
Published: June 2003

The objective of this work was to analyze the effects of the concentration and type of cationic surfactant on the kinetic features (instantaneous and overall conversions) and colloidal characteristics [mean particle diameter, particle size distribution (PSD), and surface charge density] in the semicontinuous seeded cationic emulsion polymerization of styrene. 2,2′‐Azobis(N,N′‐dimethyleneisobutyramidine)dihydrochloride was used as an initiator. The surfactants were dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (HDTAB). So that the evolution of some polymeric and colloidal characteristics of the synthesized latices could be followed, the overall and instantaneous conversions were defined and determined gravimetrically. The PSDs and average particle diameters were determined by transmission electron microscopy and photon correlation spectroscopy. The surface charge density was determined by conductimetric titration. The evolution of the instantaneous conversions, the total number of particles, and the PSDs of the different reactions were related to the nucleation, growth, and coagulation processes taking place in the semicontinuous seeded emulsion polymerizations. The PSDs obtained from the reactions carried out with the emulsifier DTAB, at a concentration equal to its critical micelle concentration (cmc) and at a concentration twice its cmc, presented more and smaller particles than those obtained by the addition of HDTAB to the polymerization recipe. At lower emulsifier concentrations equal to half of the cmc, the system had lower colloidal stability with DTAB. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2322–2334, 2003
Photopolymerization of 2‐hydroxyethyl methacrylate: Effect of the medium properties on the polymerization rate
Authors: Valdebenito A. , Encinas M. V.
Published: June 2003

The effect of solvent properties on the polymerization rate of 2‐hydroxyethyl methacrylate (HEMA) was examined with the photoreaction of 4,4′‐azobis(2‐amidinopropane) and photoinduced electron transfer of a thioxanthone derivative and triethanolamine as the radical source. The polymerization rate of HEMA was markedly affected by pH and the medium polarity. The rate increased over a pH range of 6–8. The dependence of the polymerization rate on the amine concentration photoinitiated by the bimolecular system was different in water and acetonitrile as solvents. In aqueous medium, pH 9.5, the rate increased with the amine concentration reaching a constant value at 0.025 M amine; further amine addition inhibited the polymerization. In organic media the inhibition effect was not observed. Triethanolamine addition did not change the polymerization rate photoinitiated by the azo compound. Photochemical studies of the thioxanthone were carried out under the polymerization conditions. These studies allowed us to simulate the dependence of the polymerization rate on the amine concentration. The results are explained in terms of the interaction of the ketone excited states with the amine in the different media. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2368–2373, 2003
Thermal properties and fracture toughness of epoxy resins cured by phosphonium and pyrazinium salts as latent cationic initiators
Authors: Park Soo‐Jin , Heo Gun‐Young , Suh Dong‐Hack
Published: June 2003

In this work, the latent thermal cationic initiators triphenyl benzyl phosphonium hexafluoroantimonate (TBPH) and benzyl‐2‐methylpyrazinium hexafluoroantimonate (BMPH) were newly synthesized and characterized with IR, 1H NMR, and P NMR spectroscopy. The thermal and mechanical properties of difunctional epoxy [diglycidyl ether of bisphenol A (DGEBA)] resins cured by 1 phr of either TBPH or BMPH were investigated. The DGEBA/TBPH system showed a higher curing temperature and a higher critical stress intensity factor than the epoxy/BMPH system. This could be interpreted in terms of the slow thermal diffusion rate and bulk structure of the four phenyl groups in TBPH. However, the decomposition activation energy derived from the Coats–Redfern method was lower for epoxy/TBPH. This result was probably due to the fact that a broken short‐chain structure was developed by the steric hindrance of TBPH in the difunctional epoxy resin. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2393–2403, 2003
Preparation, thermal properties, and flame retardance of epoxy–silica hybrid resins
Authors: Liu Ying‐Ling , Wu Chuan‐Shao , Chiu Yie‐Shun , Ho Wen‐Hsuan
Published: June 2003

A novel epoxy system was developed through the in situ curing of bisphenol A type epoxy and 4,4′‐diaminodiphenylmethane with the sol–gel reaction of a phosphorus‐containing trimethoxysilane (DOPO–GPTMS), which was prepared from the reaction of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) with 3‐glycidoxypropyltrimethoxysilane (GPTMS). The preparation of DOPO–GPTMS was confirmed with Fourier transform infrared, 1H and 31P NMR, and elemental analysis. The resulting organic–inorganic hybrid epoxy resins exhibited a high glass‐transition temperature (167 °C), good thermal stability over 320 °C, and a high limited oxygen index of 28.5. The synergism of phosphorus and silicon on flame retardance was observed. Moreover, the kinetics of the thermal oxidative degradation of the hybrid epoxy resins were studied. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2354–2367, 2003
Fibrillar structure of resorbable microblock copolymers based on 1,5‐dioxepan‐2‐one and ε‐caprolactone
Authors: Andronova Natalia , Finne Anna , Albertsson Ann‐Christine
Published: June 2003

The copolymerization of 1,5‐dioxepan‐2‐one (DXO) and ε‐caprolactone, initiated by a five‐membered cyclic tin alkoxide initiator, was performed in chloroform at 60 °C. Copolymers with different molar ratios of DXO (25, 40, and 60%) were synthesized and characterized. 13C NMR spectroscopy of the carbonyl region revealed the formation of copolymers with a blocklike structure. Differential scanning calorimetry measurements showed that all the copolymers had a single glass transition between −57 and −49 °C and a melting temperature in the range of 30.1–47.7 °C, both of which were correlated with the amount of DXO. An increase in the amount of DXO led to an increase in the glass‐transition temperature and to a decrease in the melting temperature. Dynamic mechanical thermal analysis measurements confirmed the results of the calorimetric analysis, showing a single sharp drop in the storage modulus in the temperature region corresponding to the glass transition. Tensile testing demonstrated good mechanical properties with a tensile strength of 27–39 MPa and an elongation at break of up to 1400%. The morphology of the copolymers was examined with polarized optical microscopy and atomic force microscopy; the films that crystallized from the melt showed a short fibrillar structure (with a length of 0.05–0.4 μm) in contrast to the untreated solution‐cast films. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2412–2423, 2003
Amino‐functionalized latex particles obtained by a multistep method: Development of a new immunoreagent
Authors: Ramos Jose , Martín‐Molina Alberto , Sanz‐Izquierdo M. Pilar , Rus Antonio , Borque Luis , Hidalgo‐Álvarez Roque , Galisteo‐González Francisco , Forcada Jacqueline
Published: June 2003

Cationic latex particles with surface amino groups were prepared by a multistep batch emulsion polymerization. In the first one, two or three steps, monodisperse cationic latex particles to be used as the seed were synthesized. In the third and fourth steps, the amino‐functionalized monomer aminoethylmethacrylate hydrochloride was used to synthesize the final functionalized latex particles. Three different azo initiators 2,2′‐azobisisobutyramidine dihydrochloride, 2,2′‐azobisdimethylenisobutyramidine dihydrochloride, and 2,2′‐azobisisobutyronitrile were used as initiators. Hexadecyltrimethylammonium bromide was the emulsifier. To characterize the final latices, conversions were obtained gravimetrically, and particle size distributions and average particle diameters were determined by transmission electron microscopy and photon correlation spectroscopy. The amount of amino groups was determined by conductimetric titrations. Colloidal aspects were ascertained by measuring the electrophoretic mobilities. Activation of these particles with glutaraldehyde produced an efficient reagent for latex‐enhanced immunoassay. The covalent coupling efficiency (protein covalently bound with respect to the total amount of protein adsorbed) was compressed between 50 and 80%. The developed immunoreagent was applied to the measurement of serum ferritin concentration in a new turbidimetric procedure that was compared with a commercial nephelometric method; the results obtained with both methods demonstrated that the two procedures correlated well (r = 0.992). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2404–2411, 2003
Reaction mechanism of caspases: Insights from QM/MM Car–Parrinello simulations
Authors: Sulpizi M. , Laio A. , VandeVondele J. , Cattaneo A. , Rothlisberger U. , Carloni P.
Published: June 2003

Caspases are fundamental targets for pharmaceutical interventions in a variety of diseases involving disregulated apoptosis. Here, we present a quantum mechanics/molecular mechanics Car–Parrinello study of key steps of the enzymatic reaction for a representative member of this family, caspase‐3. The hydrolysis of the acyl–enzyme complex is described at the density functional (BLYP) level of theory while the protein frame and solvent are treated using the GROMOS96 force field. These calculations show that the attack of the hydrolytic water molecule implies an activation free energy of ca. ΔFA ≈ 19 ± 4 kcal/mol in good agreement with experimental data and leads to a previously unrecognized gem‐diol intermediate that can readily (ΔFA ≈ 5 ± 3 kcal/mol) evolve to the enzyme products. Our findings assist in elucidating the striking difference in catalytic activity between caspases and other structurally well‐characterized cysteine proteases (papains and cathepsins) and may help design novel transition‐state analog inhibitors. Proteins 2003;52:212–224. © 2003 Wiley‐Liss, Inc.
Free energy calculations for the relative binding affinity between DNA and λ‐repressor
Authors: Saito Minoru , Sarai Akinori
Published: June 2003

The change in the binding free energy between DNA and λ‐repressor resulting from a base substitution, thymine (T)→deoxyuracil (abbreviated as U), was evaluated by the free energy perturbation method on the basis of molecular dynamics simulations for the DNA–λ‐repressor complex in water with all degrees of freedom and including long‐range Coulomb interactions. The binding free energy change that we calculated (1.47 ± 0.40 kcal/mol) was in good agreement with an experimental value (1.8 kcal/mol). We clarified why the small difference between T and U (CH3 in T is replaced with H in U) caused such a significant change in the binding free energy: The substitution of CH3 in T with H in U lowered the dissociated‐state free energy level due to the gain of the hydration free energy. Furthermore, the T→U substitution raised the free energy level in the associated state due to the loss of the favored van der Waals (vdW) interactions with the λ‐repressor amino acid residues. In other words, the amino acid residues of λ‐repressor can recognize the CH3 in T through the vdW interactions with the CH3. This recognition is enhanced in a water environment, because the hydrophobic CH3 prefers the amino acid residues of λ‐repressor to water molecules. Proteins 2003;52:129–136. © 2003 Wiley‐Liss, Inc.
A new bioinformatic approach to detect common 3D sites in protein structures
Authors: Jambon Martin , Imberty Anne , Deléage Gilbert , Geourjon Christophe
Published: June 2003

An innovative bioinformatic method has been designed and implemented to detect similar three‐dimensional (3D) sites in proteins. This approach allows the comparison of protein structures or substructures and detects local spatial similarities: this method is completely independent from the amino acid sequence and from the backbone structure. In contrast to already existing tools, the basis for this method is a representation of the protein structure by a set of stereochemical groups that are defined independently from the notion of amino acid. An efficient heuristic for finding similarities that uses graphs of triangles of chemical groups to represent the protein structures has been developed. The implementation of this heuristic constitutes a software named SuMo (Surfing the Molecules), which allows the dynamic definition of chemical groups, the selection of sites in the proteins, and the management and screening of databases. To show the relevance of this approach, we focused on two extreme examples illustrating convergent and divergent evolution. In two unrelated serine proteases, SuMo detects one common site, which corresponds to the catalytic triad. In the legume lectins family composed of >100 structures that share similar sequences and folds but may have lost their ability to bind a carbohydrate molecule, SuMo discriminates between functional and non‐functional lectins with a selectivity of 96%. The time needed for searching a given site in a protein structure is typically 0.1 s on a PIII 800MHz/Linux computer; thus, in further studies, SuMo will be used to screen the PDB. Proteins 2003;52:137–145. © 2003 Wiley‐Liss, Inc.
Interaction of agitoxin2, charybdotoxin, and iberiotoxin with potassium channels: Selectivity between voltage‐gated and Maxi‐K channels
Authors: Gao Ying‐Duo , Garcia Maria L.
Published: June 2003

To gain insight into the molecular determinants that define the specificity of interaction of pore‐blocking peptides, such as agitoxin 2 (AgTX2), charybdotoxin (ChTX), and iberiotoxin (IbTX) with the Shaker‐type voltage‐gated potassium channel Kv1.3, or the large‐conductance Ca2+‐activated K+ (Maxi‐K) channel, homology models of these channels were generated based on the crystal structure of the bacterial, KcsA, potassium channel. Peptide–channel complexes were analyzed to evaluate the predicted interaction interfaces between the peptides and the channels' outer vestibules. The docking model, for either AgTX2 or ChTX with the Kv1.3 channel, predicts a novel hydrogen bonding interaction between the Asn30 side‐chain of the peptide and the Asp381 side‐chain of the channel. This interaction is consistent with the >500‐fold decreased potency of both AgTX2 and ChTX mutants at position 30 for the Shaker channel [(Ranganathan et al., Neuron 1996;16:131–139); (Goldstein et al., Neuron 1994;12:1377–1388)]. This hydrogen bonding interaction also suggests that Gly30 in IbTX may be the critical determinant for its lack of activity against Shaker Kv channels. The model of the Maxi‐K channel reveals a narrower and more structurally restrained outer vestibule in which the aromatic residues Phe266 and Tyr294 may stabilize binding of IbTX and ChTX by π–π stacking with the aromatic residues Trp14 and Tyr36 of the peptides. This study also suggests that the extra net negative charge of IbTX is not related to the selectivity of this peptide for the Maxi‐K channel. Proteins 2003;52:146–154. © 2003 Wiley‐Liss, Inc.
Conserved residue clustering and protein structure prediction
Authors: Schueler‐Furman Ora , Baker David
Published: June 2003

Protein residues that are critical for structure and function are expected to be conserved throughout evolution. Here, we investigate the extent to which these conserved residues are clustered in three‐dimensional protein structures. In 92% of the proteins in a data set of 79 proteins, the most conserved positions in multiple sequence alignments are significantly more clustered than randomly selected sets of positions. The comparison to random subsets is not necessarily appropriate, however, because the signal could be the result of differences in the amino acid composition of sets of conserved residues compared to random subsets (hydrophobic residues tend to be close together in the protein core), or differences in sequence separation of the residues in the different sets. In order to overcome these limits, we compare the degree of clustering of the conserved positions on the native structure and on alternative conformations generated by the de novo structure prediction method Rosetta. For 65% of the 79 proteins, the conserved residues are significantly more clustered in the native structure than in the alternative conformations, indicating that the clustering of conserved residues in protein structures goes beyond that expected purely from sequence locality and composition effects. The differences in the spatial distribution of conserved residues can be utilized in de novo protein structure prediction: We find that for 79% of the proteins, selection of the Rosetta generated conformations with the greatest clustering of the conserved residues significantly enriches the fraction of close‐to‐native structures. Proteins 2003;52:225–235. © 2003 Wiley‐Liss, Inc.
Electrostatic properties in the catalytic site of papain: A possible regulatory mechanism for the reactivity of the ion pair
Authors: Dardenne Laurent E. , Werneck Araken S. , de Oliveira Neto Marçal , Bisch Paulo M.
Published: June 2003

We present an analysis of the electrostatic properties in the catalytic site of papain (EC 3.4.22.2), an archetype enzyme of the C1 cysteine proteinase family, and we investigate their possible role in the formation, stabilization and regulation of the Cys25(−)…His159(+) catalytic ion pair. The electrostatic properties were computed using a reassociation method based in multicentered multipolar expansions obtained from ab initio quantum calculations of overlapping protein fragments. Solvent effects were introduced by coupling the use of multicentered multipolar expansions to two continuum boundary element methods to solve the Poisson and the linearized Poisson‐Boltzmann equations. The electrostatic profile found in the proton transfer region of papain showed that this enzyme has a well‐defined electrostatic environment to favor the formation and stabilization of the catalytic ion pair. The papain catalytic site electrostatic profile can be considered as an electrostatic fingerprint of the papain family with the following characteristics: (i) the presence of a net electric field highly aligned in the (Cys25)‐SG→(His159)‐ND1 direction; (ii) the electrostatic profile has a saddle‐point character; (iii) it is basically a local environmental effect. Furthermore, our analysis describes a possible regulatory mechanism (the ESG→ND1 attenuation effect) controlling the ion pair reactivity and permits to infer the Asp57 acidic residue as the most probable candidate to act as the electrostatic modulator. Proteins 2003;52:236–253. © 2003 Wiley‐Liss, Inc.
Assembly of protein tertiary structures from secondary structures using optimized potentials
Authors: Hoang Trinh Xuan , Seno Flavio , Banavar Jayanth R. , Cieplak Marek , Maritan Amos
Published: June 2003

We present a simulated annealing‐based method for the prediction of the tertiary structures of proteins given knowledge of the secondary structure associated with each amino acid in the sequence. The backbone is represented in a detailed fashion whereas the sidechains and pairwise interactions are modeled in a simplified way, following the LINUS model of Srinivasan and Rose. A perceptron‐based technique is used to optimize the interaction potentials for a training set of three proteins. For these proteins, the procedure is able to reproduce the tertiary structures to below 3 Å in root mean square deviation (rmsd) from the PDB targets. We present the results of tests on twelve other proteins. For half of these, the lowest energy decoy has a rmsd from the native state below 6 Å and, in 9 out of 12 cases, we obtain decoys whose rmsd from the native states are also well below 5 Å. Proteins 2003;52:155–165. © 2003 Wiley‐Liss, Inc.
Molecular dynamics simulations of a helicase
Authors: Cox Katherine , Watson Tim , Soultanas Panos , Hirst Jonathan D.
Published: June 2003

Helicases are ubiquitous enzymes involved in nucleic acid metabolism. The PcrA DNA helicase is an essential bacterial protein involved in rolling circle plasmid replication and DNA repair. Recent crystal structures of PcrA bound to DNA indicate that a flexible loop mediates a functionally important rigid‐body‐domain rotation. In this study, we report stochastic boundary molecular dynamics simulations focused on this region for wild‐type and mutants designed to increase the rigidity of the region. Residues in the region that were helix‐disfavoring, such as glycine, threonine, and others, were mutated to alanine. The simulated dynamics, analyzed with a variety of measures of structure and mobility, indicate that a few point mutations will substantially increase helix formation in this region. Subnanosecond stochastic boundary molecular dynamics simulations at several temperatures offer a rapid protocol for assessing large numbers of mutants and provides a novel strategy for the design of experiments to test the role of this flexible loop region in the function of PcrA. Proteins 2003;52:254–262. © 2003 Wiley‐Liss, Inc.
Genome‐based identification of a carbohydrate binding module in Streptococcus pneumoniae hyaluronate lyase
Authors: Rigden Daniel J. , Jedrzejas Mark J.
Published: June 2003

Hyaluronate lyase enzymes degrade hyaluronan, the main polysaccharide component of the connective tissues of higher animals, thereby destroying the normal connective tissue structure and exposing the host tissue cells to various endo‐ and exogenous factors, including bacterial toxins. The 3D crystal structures of functionally active but truncated Streptococcus pneumoniae and S. agalactiae hyaluronate lyases, along with their substrate and product complexes, have been determined. The enzymes are multidomain proteins with helical barrel‐like catalytic domains and two types of β‐sheet domains. Here, through genome‐based bioinformatics studies we identify an additional β‐sheet domain present in the most N‐terminal part of streptococcal hyaluronate lyases. Fold recognition and modeling studies show that the domain is structurally similar to carbohydrate binding modules and is therefore likely to be directly involved in hyaluronan binding. Likely carbohydrate binding residues were identified and electrostatic complementarity of the hyaluronate lyase domain with hyaluronan demonstrated. The newly identified presumed hyaluronan binding domain likely improves catalytic efficiency by colocalizing the enzyme and its substrate. Other possible functions are discussed. Two contacting aromatic residues are conserved in the hydrophobic core of the hyaluronate lyase domain and in many, perhaps all, families in the superfamily in which they may be placed. This observation may help the identification and classification of other carbohydrate binding modules. Proteins 2003;52:203–211. © 2003 Wiley‐Liss, Inc.