bluuebunny/crossref_metadata_2025_split huggingface dataset:
Exploitation of electric field in controlling of nanofiber spinning process
Authors: Heikkilä Pirjo , Söderlund Lasse , Uusimäki Jari , Kettunen Lauri , Harlin Ali 
Published: November 2007

AbstractNanofibers can be prepared from a polymer solution utilizing electrospinning, a method exploiting an electrostatic field. In this work the electric field of the electrospinning process was modeled for better understanding and controllability of the fiber deposition in the electrospinning process. The model of the electric field was compared with experimental observations. The model of the electric field explained many phenomena in electrospinning, but did not provide an exclusive control tool for the process. POLYM. ENG. SCI., 47:2065–2074, 2007. © 2007 Society of Plastics Engineers
Shrinkage behavior and mechanical performances of injection molded polypropylene/talc composites
Authors: Shelesh‐Nezhad K. , Taghizadeh A. 
Published: November 2007

AbstractIn this research, the influences of adding talc mineral particles of 10 μm particle size on the shrinkage and the mechanical properties of injection molded polypropylene (PP)/talc composites were investigated. PP has a crystalline molecular structure and hence it possesses nonisotropic shrinkage along and across the flow directions. Addition of the talc mineral filler to PP induced an isotropic shrinkage in the molded part because of the nonisotropic shape of talc particles. The results of experiments indicated that the maximum flexural strength, maximum impact strength, and isotropic shrinkage were achieved by adding 10, 20, and 30 by weight percent of talc respectively. By incorporating of 10 wt% of talc particles into the PP matrix, the tensile strength was hardly affected but the occurrence of cold drawing phenomena in the tensile test was hindered considerably. The flake‐shape structure of talc filler played an important role in determining the molded part shrinkage and mechanical properties. POLYM. ENG. SCI., 47:2124–2128, 2007. © 2007 Society of Plastics Engineers
Thermolyzable polymer networks and star polymers containing a novel, compact, degradable acylal‐based dimethacrylate cross‐linker: Synthesis, characterization, and thermolysis
Authors: Themistou Efrosyni , Kanari Amelia , Patrickios Costas S. 
Published: November 2007

AbstractA compact, cleavable acylal dimethacrylate cross‐linker, 1,1‐ethylenediol dimethacrylate (EDDMA), was synthesized from the anhydrous iron(III) chloride‐catalyzed reaction between methacrylic anhydride and acetaldehyde. The ability of EDDMA to act as cross‐linker was demonstrated by using it for the preparation of one neat cross‐linker network, four star polymers of methyl methacrylate (MMA), and four randomly cross‐linked MMA polymer networks using group transfer polymerization (GTP). For comparison, the corresponding polymer structures based on the commercially available ethylene glycol dimethacrylate (EGDMA) cross‐linker (isomer of EDDMA) were also prepared via GTP. The number of arms of the EDDMA‐based star polymers was lower than that of the corresponding EGDMA polymers, whereas the degrees of swelling in tetrahydrofuran of the EDDMA‐based MMA networks were higher than those of their EGDMA‐based counterparts. Although none of the EDDMA‐containing polymers could be cleanly hydrolyzed under basic or acidic conditions, they could be thermolyzed at 200 °C within 1 day giving lower molecular weight products. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5811–5823, 2007
Preparation of the amphiphilic macro‐rings of poly(ethylene oxide) with multi‐polystyrene lateral chains and their extraction for dyes
Authors: Pang Xinchang , Wang Guowei , Jia Zhongfan , Liu Chao , Huang Junlian 
Published: November 2007

AbstractA novel method for synthesis of amphiphilic macrocyclic graft copolymers with multi‐polystyrene lateral chains is suggested, by combination of anionic ring‐open polymerization (AROP) with atom transfer radical polymerization (ATRP). The anionic ring‐opening copolymerization of ethylene oxide (EO) and ethoxyethyl glycidyl ether (EEGE) was carried out first using triethylene glycol and diphenylmethylpotassium (DPMK) as coinitiators; the monomer reactivity ratio of them are r1(EO) = 1.20 ± 0.01 and r2(EEGE) = 0.76 ± 0.02 respectively. The obtained linear well‐defined α,ω‐dihydroxyl poly(ethylene oxide) with pendant protected hydroxylmethyls (l‐poly(EO‐co‐EEGE)) was cyclized by reaction with tosyl chloride (TsCl) in the presence of solid KOH. The crude cyclized product containing the extended linear chain polymer was hydrolyzed and then purified by treat with α‐CD. The pure cyclic copolymer with multipendant hydroxymethyls [c‐poly(EO‐co‐Gly)] was esterified by reaction with 2‐bromoisobutyryl bromide, and then used as macroinitiators to initiate polymerization of styrene (St), and a series of amphiphilic macrocyclic grafted copolymers composed of a hydrophilic PEO as ring and hydrophobic polystyrene as side chains (c‐PEO‐g‐PS) were obtained. The intermediates and final products were characterized by GPC, NMR and MALDI‐TOF in detail. The experimental results confirmed that c‐PEO‐g‐PS shows stronger conjugation ability with the dyes than the corresponding comb‐PEO‐g‐PS. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5824–5837, 2007
Mechanisms of n‐butanol formation in butyl acrylate latexes
Authors: Goikoetxea Monika , Barandiaran María J. , Asua José M. 
Published: November 2007

AbstractThe mechanisms involved in the formation of n‐butanol during the synthesis of butyl acrylate containing latices were investigated. The experimental results showed that neither the hydrolysis of butyl acrylate nor of the ester bond in the butyl acrylate segments of the polymer played a major role in the formation of n‐butanol, which was mainly generated from the polymer backbone, by transfer reactions to polymer chain followed by cyclization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5838–5846, 2007
Influence of stereoerrors on the formation of helices during early stage crystallization of isotactic polyproyplene
Authors: Chen Xiaofeng , Ozisik Rahmi , Kumar Sanat K. , Choi Phillip 
Published: November 2007

AbstractCoarse‐grained, on‐lattice Monte Carlo simulations are performed to investigate the role of stereotacticity defects along an isotactic polypropylene chain on the formation of 31‐helices, which form the basic crystalline order within the chain. For this reason, systems with various stereoerror configurations are studied and are compared to neat isotactic polypropylene. All systems are equilibrated above the melting temperature and are cooled to lower temperatures in a stepwise manner, making sure each system is equilibrated at every temperature. Results indicate that chain ends have the lowest probability of being found in helices. Addition of a single stereoerror (mrm) decreases the probability of five repeat units' participation in helices (the repeat unit that contains the stereoerror and two nearest repeat units on both sides). The probability profile becomes more complicated when the number of stereoerrors increases, however, the results indicate that the effects of many stereoerrors can be explained by a simple addition of the effect of each stereoerror considered individually. The results also indicate that the presence of even a single stereoerror eliminates (within the temperature range studied) the transition to longer, more stable helices observed in neat isotactic polypropylene. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3349–3360, 2007
Conformation of a polymer chain in explicit solvent: A solvation potential approach
Authors: Taylor Mark P. , Ichida Sayuri 
Published: November 2007

AbstractIn much of the theoretical work on polymers in dilute solution the effects of solvent are treated in an implicit fashion: thus one studies an isolated chain interacting via an effective site–site potential. Although a formally exact mapping is possible between the chain‐in‐solvent system and a corresponding isolated effective‐potential‐chain, this mapping involves a many‐site solvation potential which is not practical to compute. Thus, one generally resorts to a two‐site potential approximation. In this article we test the validity of this two‐site approximation for a short hard‐sphere (HS) n‐mer chain (n ≤ 5) in a HS solvent. This test involves computing a set of “exact” site–site solvation potentials for an isolated chain. These potentials are shown to reproduce the conformation of a chain in explicit solvent. We make use of these exact short‐chain results (combining them with the solvent potential of mean force) to construct approximate two‐site solvation potentials for long chains. Monte Carlo simulations have been performed for HS chain‐in‐solvent systems and for the corresponding isolated effective‐potential n‐mer chain with n ≤ 50. These simulations show that our solvation potentials provide a quantitatively accurate description of the conformation of a HS chain in explicit (hard sphere) solvent. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3319–3326, 2007
Dendritic growth of poly(ε‐caprolactone) crystals from compatible blends with poly(t‐butyl acrylate) at the air/water interface
Authors: Li Bingbing , Marand Hervé , Esker Alan R. 
Published: November 2007

AbstractThermodynamic analyses of surface pressure‐area (Π‐A) isotherms and Brewster angle microscopy (BAM) reveal that poly(ε‐caprolactone) (PCL) with a weight average molar mass of Mw = 10 kg mol−1 and polydispersity index of Mw/Mn = 1.25 and poly(t‐butyl acrylate) (PtBA, Mw = 25.7 kg mol−1; Mw/Mn = 1.07) form compatible blends as Langmuir films below the dynamic collapse transition for PCL at Π = 11 mN m−1. For PCL‐rich blends, in situ BAM studies reveal growth of PCL crystals for compression past the PCL collapse transition. PCL crystals grown in the plateau regime of the Π‐A isotherm exhibit a dendritic morphology presumably resulting from the rejection of PtBA from the growing PCL crystals and hindered diffusion of PCL from the surrounding monolayer to the crystal growth fronts. The ability to transfer the PCL dendrites as Langmuir–Schaefer films onto silicon substrates spincoated with a polystyrene layer facilitates detailed morphological characterization by optical and atomic force microscopy (AFM). AFM reveals that the dendritic branching occurs along the {100} and {110} sector boundaries and is essentially independent of composition. AFM also reveals that the average thickness of PCL dendrites formed at room temperature (22.5 °C), ∼7–8 nm, is comparable with that of PCL crystals grown from single‐component PCL Langmuir films and spincoated thin films. In contrast, for PtBA‐rich blend films PCL crystallization is suppressed. These findings establish PCL blends as an ideal system for exploring the interplay between chain diffusion and crystal growth in a two‐dimensional confined geometry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3300–3318, 2007
Mechanical hole burning spectroscopy in an SIS tri‐block copolymer
Authors: Qin Qian , Shi Xiangfu , McKenna Gregory B. 
Published: November 2007

AbstractWe have previously developed a mechanical hole burning spectroscopy (MSHB) technique that promises to be a powerful tool for the investigation of dynamic heterogeneity of polymeric materials. This is because, unlike its dielectric analogue, MSHB can be used to characterize materials having a weak dielectric response, a particular feature of polymeric materials. However, while both mechanical and dielectric hole burning show behaviors that are consistent with dynamic heterogeneity in the materials, it is still unclear what the relationship between the hole properties, for example, frequency and amplitude, and the actual nature of the heterogeneity and particularly the length scale being probed. Here, we provide first evidence that a known length scale can be probed by the MSHB method by using a tri‐block copolymer as a “calibration” sample. The heterogeneity of a styrene‐isoprene‐styrene copolymer was investigated in the vicinity of the order‐disorder transition temperature (ODT). It was found that the amplitudes of the mechanical holes gradually decrease as the order‐disorder transition is traversed from the region with ordered structures. In the disordered state of the block copolymer, no apparent mechanical holes were detected. In contrast, mechanical holes were burned in the heterogeneous region and the hole amplitude increased as the depth into the ordered structure increased. Hence, the MSHB technique provides a qualitative correlation between the amplitude of the burned holes and the corresponding heterogeneity. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3277–3284, 2007
Control of the block copolymer morphology in templated epoxy thermosets
Authors: Hermel‐Davidock Theresa J. , Tang H. Sean , Murray Daniel J. , Hahn Stephen F. 
Published: November 2007

AbstractIt has been found that by the addition of low concentrations of an amphiphilic block copolymer to an epoxy resin, novel disordered morphologies can be formed and preserved through curing. This article will focus on characterizing the influence of the block copolymer and casting solvent on the templated morphology achieved in the thermoset sample. The ultimate goal of this work is to determine the parameters that would control the microphase morphology produced. Epoxy resins blended with a series of amphiphilic block copolymers based on hydrogenated polyisoprene (polyethylene‐alt‐propylene or PEP) and polyethylene oxide (PEO), specifically, were investigated. In this article, the cure‐induced order–order phase transition from the spherical to wormlike micelle morphology will also be discussed. It is proposed that the formation of the wormlike micelle structure from the spherical micelle structure is similar to the phase transition behavior that occurs in dilute block copolymer solutions as a function of the influence of the solvent on micelle morphology. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3338–3348, 2007
An efficient conformational sampling method for homology modeling
Authors: Han Rongsheng , Leo‐Macias Alejandra , Zerbino Daniel , Bastolla Ugo , Contreras‐Moreira Bruno , Ortiz Angel R. 
Published: November 2007

AbstractThe structural refinement of protein models is a challenging problem in protein structure prediction (Moult et al., Proteins 2003;53(Suppl 6):334–339). Most attempts to refine comparative models lead to degradation rather than improvement in model quality, so most current comparative modeling procedures omit the refinement step. However, it has been shown that even in the absence of alignment errors and using optimal templates, methods based on a single template have intrinsic limitations, and that refinement is needed to improve model accuracy. It is thought that failure of current methods originates on one hand from the inaccuracy of the effective free energy functions adopted, which do not represent properly the energetic balance in the native state, and on the other hand from the difficulty to sample the high dimensional and rugged free energy landscape of protein folding, in the search for the global minimum. Here, we address this second issue. We define the evolutionary and vibrational armonics subspace (EVA), a reduced sampling subspace that consists of a combination of evolutionarily favored directions, defined by the principal components of the structural variation within a homologous family, plus topologically favored directions, derived from the low frequency normal modes of the vibrational dynamics, up to 50 dimensions. This subspace is accurate enough so that the cores of most proteins can be represented within 1 Å accuracy, and reduced enough so that Replica Exchange Monte Carlo (Hukushima and Nemoto, J Phys Soc Jpn 1996;65:1604–1608; Hukushima et al., Int J Mod Phys C: Phys Comput 1996;7:337–344; Mitsutake et al., J Chem Phys 2003;118:6664–6675; Mitsutake et al., J Chem Phys 2003;118:6676–6688) (REMC) can be applied. REMC is one of the best sampling methods currently available, but its applicability is restricted to spaces of small dimensionality. We show that the combination of the EVA subspace and REMC can essentially solve the optimization problem for backbone atoms in the reduced sampling subspace, even for rather rugged free energy landscapes. Applications and limitations of this methodology are finally discussed. Proteins 2008. © 2007 Wiley‐Liss, Inc.
Comparison of vertebrate and invertebrate CLIC proteins: The crystal structures of Caenorhabditis elegans EXC‐4 and Drosophila melanogaster DmCLIC
Authors: Littler Dene R. , Harrop Stephen J. , Brown Louise J. , Pankhurst Greg J. , Mynott Andrew V. , Luciani Paolo , Mandyam Ramya A. , Mazzanti Michele , Tanda Soichi , Berryman Mark A. , Breit Samuel N. , Curmi Paul M. G. 
Published: November 2007

AbstractThe crystal structures of two CLIC family members DmCLIC and EXC‐4 from the invertebrates Drosophila melanogaster and Caenorhabditis elegans, respectively, have been determined. The proteins adopt a glutathione S‐transferase (GST) fold. The structures are highly homologous to each other and more closely related to the known structures of the human CLIC1 and CLIC4 than to GSTs. The invertebrate CLICs show several unique features including an elongated C‐terminal extension and a divalent metal binding site. The latter appears to alter the ancestral glutathione binding site, and thus, the invertebrate CLICs are unlikely to bind glutathione in the same manner as the GST proteins. Purified recombinant DmCLIC and EXC‐4 both bind to lipid bilayers and can form ion channels in artificial lipid bilayers, albeit at low pH. EXC‐4 differs from other CLIC proteins in that the conserved redox‐active cysteine at the N‐terminus of helix 1 is replaced by an aspartic acid residue. Other key distinguishing features of EXC‐4 include the fact that it binds to artificial bilayers at neutral pH and this binding is not sensitive to oxidation. These differences with other CLIC family members are likely to be due to the substitution of the conserved cysteine by aspartic acid. Proteins 2008. © 2007 Wiley‐Liss, Inc.
Characterization of local geometry of protein surfaces with the visibility criterion
Authors: Li Bin , Turuvekere Srinivasan , Agrawal Manish , La David , Ramani Karthik , Kihara Daisuke 
Published: November 2007

AbstractExperimentally determined protein tertiary structures are rapidly accumulating in a database, partly due to the structural genomics projects. Included are proteins of unknown function, whose function has not been investigated by experiments and was not able to be predicted by conventional sequence‐based search. Those uncharacterized protein structures highlight the urgent need of computational methods for annotating proteins from tertiary structures, which include function annotation methods through characterizing protein local surfaces. Toward structure‐based protein annotation, we have developed VisGrid algorithm that uses the visibility criterion to characterize local geometric features of protein surfaces. Unlike existing methods, which only concerns identifying pockets that could be potential ligand‐binding sites in proteins, VisGrid is also aimed to identify large protrusions, hollows, and flat regions, which can characterize geometric features of a protein structure. The visibility used in VisGrid is defined as the fraction of visible directions from a target position on a protein surface. A pocket or a hollow is recognized as a cluster of positions with a small visibility. A large protrusion in a protein structure is recognized as a pocket in the negative image of the structure. VisGrid correctly identified 95.0% of ligand‐binding sites as one of the three largest pockets in 5616 benchmark proteins. To examine how natural flexibility of proteins affects pocket identification, VisGrid was tested on distorted structures by molecular dynamics simulation. Sensitivity decreased ∼20% for structures of a root mean square deviation of 2.0 Å to the original crystal structure, but specificity was not much affected. Because of its intuitiveness and simplicity, the visibility criterion will lay the foundation for characterization and function annotation of local shape of proteins. Proteins 2008. © 2007 Wiley‐Liss, Inc.
Structural analysis of protein dynamics by MD simulations and NMR spin‐relaxation
Authors: Trbovic Nikola , Kim Byungchan , Friesner Richard A. , Palmer Arthur G. 
Published: November 2007

AbstractMolecular dynamics (MD) simulations and nuclear magnetic resonance spin‐relaxation measurements provide detailed insights into ps‐ns structural dynamics of proteins. An analysis of discrepancies between the two methods is presented for the B3 immunoglobulin‐binding domain of streptococcal protein G. MD simulations using three MD force fields (OPLS‐AA, AMBER ff99SB, and AMBER ff03) overestimate the flexibility of backbone NH vectors at the borders of secondary structure and in loops when compared with experimentally determined backbone amide generalized order parameters (Hall and Fushman, J Am Chem Soc 2006; 12:7855–7870). Comparison with a previous study of residual dipolar coupling constants (Bouvignies et al., Proc Natl Acad Sci USA 2005;102:13885–13890) indicates that slower timescale motions do not account for the discrepancies. Structural analysis reveals that relative imbalance between the description of hydrogen bonding and other terms of modern force fields may be responsible for disagreement. Proteins 2008. © 2007 Wiley‐Liss, Inc.
Exploring the molecular basis of heme coordination in human neuroglobin
Authors: Nadra Alejandro D. , Martí Marcelo A. , Pesce Alessandra , Bolognesi Martino , Estrin Darío A. 
Published: November 2007

AbstractNeuroglobin (Ngb), a recently discovered ancient heme protein, presents the typical globin fold and is around 20% identical to myoglobin (Mb). In contrast with Mb, however, its heme is hexacoordinated (6c). It is expressed in the nervous system and has been the subject of numerous investigations in the last years, but its function is still unclear. The proposed roles include oxygen transport, reactive oxygen species (ROS) detoxification, hypoxia protection, and redox state sensing. All proposed functions require distal histidine dissociation from the heme to yield a reactive iron. With the aim of understanding the 6c to 5c transition, we have performed molecular dynamics simulations for ferrous Ngb in the 6c, 5c, and oxy states. We also computed free energy profiles associated with the transition employing an advanced sampling technique. Finally, we studied the effect of the redox state of CysCD7 and CysD5, which are known to form a disulfide bridge. Our results show that protein oxidation promotes a stabilization of the pentacoordinated species, thus favoring the protein to adopt the more reactive state and supporting the existence of a molecular mechanism whereby O2 would be released under hypoxic conditions, thereby suggesting an O2 storage function for Ngb. Taken together, our results provide structural information not available experimentally which may shed light on the protein proposed functions, particularly as a redox sensor. Proteins 2008. © 2007 Wiley‐Liss, Inc.