bluuebunny/crossref_metadata_2025_split huggingface dataset:
Rheology of diblock copolymer micellar systems
Authors: Watanabe H. 
Published: April 2003

AbstractThis article summarizes rheological features of moderately concentrated, diblock copolymer micellar solutions in selective solvents. In solvents of monomeric sizes, spherical micelles (with glassy cores) are arranged on cubic lattices to exhibit elasto‐plastic responses. These lattices are formed due to an osmotic constraint for the conformation of solvated corona blocks of the micelles. In polymeric solvents (homopolymers) chemically identical to the corona blocks, this constraint is screened and the micelles are randomly dispersed in the systems. In this type of blends (micellar dispersions) in short, nonentangling matrices the micelles have no plasticity but exhibit two‐step viscoelastic relaxation: The fast relaxation process corresponds to relaxation of individual corona blocks while the slow process is attributable to diffusion of the micelles. Corresponding to these molecular mechanisms, the fast process exhibits modest damping of the nonlinear relaxation modulus against large step strains while the slow process exhibits much stronger damping. Under steady flow, the micellar dispersion exhibits two‐step shear‐thinning with the molecular origin identical to that for the nonlinear damping. All these rheological features of the micellar lattices and dispersions are strikingly similar, in many aspects, to the features of colloidal crystals and hard‐sphere suspensions of solid particles.
Clear evidence of the sensitivity of the solvatochromic effect to side chain functionalization in 3‐hexyl‐substituted polythiophenes
Authors: Della‐Casa C. , Costa‐Bizzarri P. , Lanzi M. , Bertinelli F. 
Published: April 2003

AbstractThe solvatochromic properties of 3‐hexyl‐substituted polythiophenes functionalized with an ester, ether or hydroxy group at the end of the alkyl side chain are reported. An extraordinarily large shift, Δmax = 99 nm, and a dramatic change of the UV/ vis spectral profile are found when the carboxylate is replaced with the hydroxy group by functionalization exchange. The solvatochromic behavior of each polymer is believed to be strictly related to the type of functional group owing to different physical interactions (solvation) between groups different in polarity and solvent molecules. A unique and common chromophoric unit for the final conformation of the 3‐hexyl‐substituted polythiophenes, induced by the solvatochromic effect, is also suggested.
Two‐dimensional computer simulation of spherulite formation by branching lamellae
Authors: Kalinka G. , Hinrichsen G. 
Published: April 2003

AbstractThe structure formation of growing spherulites by branching lamellae is simulated by a computer model. In addition to the process of linear growth of lamellae, three different mechanisms are implemented in the simulation program: the fork‐like branching of lamellae at their growth front (“front branching”), the generation of secondary lamellae at the sides of already existing lamellae (“side branching”) and the crystallization of lamellae parallel to existing lamellae (“clustering”). The simulation is performed stepwise and the branching takes place in dependence on branching probability and angle, leading to a multiplication of the lamellae and to disc‐like superstructures. Moreover, the side branching as well as the clustering mechanisms result in a gradient of “density” (crystallized entities per area unit) inside the spherulites.
Diffusion of polystyrene micronetwork spheres in melts of polyvinylmethylether
Authors: Pies C. , Sillescu H. 
Published: April 2003

AbstractWe have determined by forced Rayleigh scattering the diffusion coefficients of photo‐labeled polystyrene micronetwork spheres (radii ≤ 10 nm) in melts of linear polyvinylmethylether (Mw ≤ 40 000 g/mol) at temperatures of 20−80°C. An expected slippage of the spheres through the meshes of the entanglement network appears possible but is still within the experimental uncertainty of our results.
Regioselective polycondensation of benzyl 2‐amino‐2‐deoxy‐α‐D‐glucopyranoside hydrochloride with carbon dioxide
Authors: Kadokawa J. , Horiguchi T. , Sunaga E. , Karasu M. , Tagaya H. , Chiba K. 
Published: April 2003

AbstractThis paper reports the polycondensation of benzyl 2‐amino‐2‐deoxy‐α‐D‐glucopyranoside hydrochloride (1) with carbon dioxide using the triphenylphosphine/carbon tetrachloride/DBU system as a condensing agent to give poly(urethane) (2). The polycondensation of 1 with CO2 was carried out in DMF solvent in the presence of the condensing agent under CO2 atmosphere. From the NMR and IR analyses of the product, the polymerization proceeded regioselectively to form 2. The molecular weight was estimated as 2000–3000 by GPC measurement after acetylation of 2, corresponding to DP = 5–8.
Measuring chain diffusion coefficients and activation energy during coalescence of fluorescence‐labeled hard latex particles
Authors: Canpolat M. , Pekcan Ö. 
Published: April 2003

AbstractThe steady‐state fluorescence (SSF) technique was used to study interdiffusion of polymer chains across the particle–particle junction during film formation from hard latex particles. The latex films were prepared from pyrene (P) and naphthalene (N) labeled poly(methyl methacrylate) (PMMA) particles and annealed for long time intervals above the glass transition temperature (Tg) between 170 and 220°C. A novel correction method was suggested and employed to eliminate changing of the optical density of latex films after such annealing processes. P and N fluorescence intensities solely from the energy‐transfer processes were monitored versus annealing time and were used to measure the polymer chain diffusion coefficients Dp and DN, which were found to be between 4.34 × 10–14 and 7.68 × 10–13 cm2/s for an annealing temperature between 170 and 220°C. Using these diffusion coefficients with the Arrhenius relation, the diffusion activation energy for the polymer backbone was found to be 28 kcal/mol.
Morphology of banded spherulites in poly(ε‐caprolactone)/poly(styrene‐ran‐acrylonitrile) blends
Authors: Wang Chun , Thomann R. , Kressler J. , Thomann Y. , Crämer K. , Stühn B. , Svoboda P. , Inoue T. 
Published: April 2003

AbstractThe occurrence of banded spherulites in blends of poly(ε‐caprolactone) with poly(styrene‐ran‐acrylonitrile) is studied by means of optical‐, scanning electron‐, environmental scanning electron‐, and atomic force‐microscopy as well as smallangle X‐ray scattering. The measurements reveal that lamellae are confined in fibrils that bend periodically from the center of spherulites in a radial direction. The fibrils show also a regular spacing in the lateral direction. Single bent lamellae can be observed by atomic force microscopy after permanganic etching. The enrichment of amorphous material near to the growth front of the spherulites can be directly observed by polarized light microscopy after temperature jump experiments. The excluded amorphous material on the surface of the growing spherulites has the shape of droplets. This might be the main reason for the initiation of the bending process because the result is a non‐uniform stress distribution acting on the growing lamellae at the interface between spherulites and the surrounding melt. The amorphous material of the blends can be found in three areas as observed by small‐angle X‐ray scattering experiments and microscopical methods: (i) between the lamellae, (ii) excluded from the interlamellar region but within the spherulite, and (iii) for high contents of non‐crystallizable material, it can be excluded from the spherulite. The banding periodicity of spherulites as a function of the crystallization temperature can be described in terms of two models, based either (i) on the inherent twisting of lamellae perpendicular to the axis of a screw dislocation or (ii) on the dependence of the diffusion of amorphous material away from the growth front of lamellae and the temperature dependence of the rate of crystallization. Both models fail at very low supercoolings near to temperatures where the banding disappears completely.
Combining oxidative addition and polymerization: A study towards the synthesis of macromonomers
Authors: Massa W. , Faza N. , Kang Hak‐Chul , Focke C. , Heitz W. 
Published: April 2003

AbstractOxidative addition to a metal in a low valent state produces catalysts that allow reporter or functional groups to be introduced to a polymer chain end. Substituted allyltrifluoroacetates were added to Ni(0)(COD)2. The resulting π‐allyl‐Ni trifluoroacetates are dimeric in the solid state as well as in solution. The crystal structure of bis[(η3‐2‐phenylallyl)(trifluoroacetato)‐nickel(II)], bis[(η3‐2‐trimethylsilylallyl)(trifluoroacetato)nickel(II)], are given and correlated with the reactivity as polymerization catalysts. Butadiene, styrene, and norbornene are used as monomers. The substituent of the allyl group is found quantitatively to be the end group of each macromolecule. Thus it is demonstrated that no chain transfer occurs.
Chirality induction in cyclopolymerization
Authors: Yokota K. , Kakuchi T. , Uesaka T. , Obata M. 
Published: April 2003

AbstractAn overview of the synthesis of optically active polymers due to main‐chain chirality based on the configurational arrangement in cyclopolymerization and cyclocopolymerization is presented. The template having chiral twist transmitted its chirality to the main chain in the intramolecular cyclization of divinyl monomers to form the enantiomeric racemo cyclic units. The asymmetric cyclocopolymerization produced the polymers which were optically active even after cleaving the template. The synthesis of optically active polymers from achiral monomers using a slight amount of chiral initiator was achieved by the enantioselective cyclopolymerization. The cyclopolymerization capable of controlling stereochemistry of the cyclic constitutional units was one of useful means for asymmetric polymerization.
Molecular motions in crystalline and mesomorphic forms of poly(diethylsiloxane)
Authors: Litvinov V. M. , Macho V. , Spiess H. W. 
Published: April 2003

AbstractMolecular motions in low molar mass (46200 g mol—1) poly(diethylsiloxane) (PDES) are studied by 2H NMR. From DSC measurements the existence of the crystalline phases α1, β1 and α2, β2, as well as a pronounced mesogenic phase αm, is clearly established. Based upon calculations of averaged field gradient tensors based on cone models a detailed motional analysis in the different phases has been conducted. At low temperatures the only motion present besides methyl rotation are librations of the ethyl groups with amplitudes below 25°. The motion in the conformational disordered phase is characterized by conformational changes within the side chains, accompanied by occasional rotations through angles larger than 70° within the backbone. The flexibility of the ethyl groups, however, is limited by steric hindrance, prohibiting a full rotation. This situation is observed also in the meso phase where there is an additional rotational degree of freedom around the main chain axis.
Dielectric anomalies in the β‐relaxation of glassy 1,4‐polybutadiene
Authors: Hansen C. , Richert R. 
Published: April 2003

AbstractWe have measured the dielectric relaxation of the polymeric glass‐former 1,4‐polybutadiene in the glassy and supercooled liquid state. The loss spectra ϵ″ of the β‐relaxation are measured between the lower detection limit at T = 125 K and T = 225 K > Tβ, with Tβ ≈ 210 K being the α‐β‐merging temperature. The related storage data ϵ′(ω) indicates that the β‐process is a signature of supressing the higher frequency polarization with increasing temperature, rather than giving rise to additional polarizability at frequencies below the average relaxation frequency of the β‐process. This observation, as confirmed by data obtained for o‐terphenyl and salol, contradicts the previous understandings of the mechanism underlying the Johari—Goldstein‐type β‐process in glasses. A tentative link between the β‐process and high frequency vibrational modes is established.
New synthesis, thermal properties and textures of cholesteric poly[oxy(1,2‐butene)oxycarbonyl‐1,4‐phenyleneoxycarbonyl‐1,4‐phenylenecarbonyloxy‐1,4‐phenylenecarbonyl], PTOBEE
Authors: Pérez‐Méndez M. , Marco C. 
Published: April 2003

AbstractLiquid crystal poly[oxy(1,2‐butene)oxycarbonyl‐1,4‐phenyleneoxycarbonyl‐1,4‐phenylenecarbonyloxy‐1,4‐phenylenecarbonyl], PTOBEE, has been synthesized in a different way than the previously reported nematic and it has been obtained as cholesteric, although synthesized from a racemic mixture of the 1,2‐butanediol involved. Stereoselective solubilization of one enantiomer during its decantation in toluene, at the reaction temperature, explains why this chiral polymer phase appears. Evidence of this could be precisely obtained when a white solid recrystallized within the same toluene after the filtration of the polymer, being identified as (—) PTOBEE with a value of [α] = −2.33 [0.0056 mol/1, toluene].
Order and molecular motion in wholly aromatic copolyesters
Authors: v. Stein F. , Martin A. , Gronski W. , Blackwell J. , Chvalun S. N. , Kricheldorf H. R. 
Published: April 2003

AbstractThe order and molecular dynamics of wholly aromatic thermotropic copolyesters prepared from hydroxybenzoic acid (HBA), hydroquinone (HQ) and phenoxyterephthalic acid (Ph‐O‐TPA) have been studied by X‐ray analysis and deuteron NMR spectroscopy of specimens containing either deuterated hydroquinone or deuterated phenoxy side‐chain units. X‐ray analysis shows the coexistence of nematic and three‐dimensionally ordered regions that are characterized by registry of adjacent chains with interleaved side groups. The molecular motions in these nematic/three‐dimensionally ordered polymers are compared to those of a highly crystalline copolyester prepared from deuterated TPA and HQ. In the fast motional limit the motion in the crystalline system is characterized by 180° flips about the TPA axis, whereas in the nematic/three‐dimensionally ordered copolyester the same type of axially symmetric spectrum is observed for both the nematic and the three‐dimensionally ordered regions. The motion of the HQ units is characterized by rotational diffusion about the C1C4 axis, augmented by fast rotational fluctuation of this axis about the main‐chain direction. A model of free rotation about the axis of the phenyl group augmented by librations of restricted amplitude about the PhO‐TPA bond for the ordered regions superimposed by slow axial motions about the main‐chain axis in the nematic phase is consistent with both the NMR data and X‐ray analysis.
Real‐space composition–depth profiling in polymeric samples to 3 nm resolution using the 2H(3He, 1H)4He nuclear reaction
Authors: Kerle T. , Scheffold F. , Losch A. , Steiner U. , Schatz G. , Klein J. 
Published: April 2003

AbstractDirect depth profiling techniques to date have largely lacked the necessary depth resolution to investigate interfacial phenomena of the order of the bulk correlation length (5—10 nm for a wide range of systems). Here we investigate the optimal spatial resolution and depth of probe that may be attained for composition — depth profiling of polymeric samples via nuclear reaction analysis (NRA) using the 2H(3He, 1H)4He reaction. We find that the spatial resolution can be greatly improved by using a grazing incidence geometry of the incident 3He beam on the sample, and analyzing the emitted protons in a backwards direction. This results in spatial resolutions down to about 3 nm at the sample surface, compared to a value of some 7 nm or more previously reported in earlier studies when emitted α‐particles were detected in the forward direction. At the same time the depth to which samples can be profiled via the backwards emitted protons may be considerably extended relative to the α‐particle detection mode, when the 3He beam impinges on the sample surface at normal incidence (up to about 4 μm into the sample for incident energies of 1.2 MeV in the proton‐detection mode compared to only 1 μm for the equivalent α‐particle detection mode).
Rubber toughening of polystyrene–acrylonitrile copolymers
Authors: Ramsteiner F. , McKee G. E. , Heckmann W. , Fischer W. , Fischer M. 
Published: April 2003

AbstractThe efficiency of rubber toughening of PSAN depends on the size of the rubber particles, their agglomeration, the deformation rate, the temperature, and the orientation of the polymer molecules. Large particles are more effective than small particles. By a suitable choice of processing, however, small particles can agglomerate, forming large soft units and improving in this way impact toughness. At high deformation rates crazing or/and stretching of the matrix wall between the rubber particles must be activated for ductility, otherwise the material is brittle. The temperature at impact must be above the glass transition temperature of the rubbery phase for toughening. Increasing the orientation of the material decreases the tendency for craze formation with the consequence of embrittlement, if stretching is not activated.
A new approach to polymer crystallization used in an analysis of data of syndiotactic polypropylene
Authors: Strobl G. 
Published: April 2003

AbstractUnder the conditions of “normal growth” on rough surfaces, crystallization rates follow from the balance between the attachment and detachment of the chain sequences building up the layer‐like crystallites. In the proposed model, detachment rates are related to the change in the mean‐field potentials experienced in the crystal and the melt; attachment rates are associated with the population number of straight sequences in the melt. Both rates show an exponential dependence on the sequence length. Combination of this picture with the basic kinetic criterion that the crystallite thickness found at a chosen crystallization temperature maximizes the growth rate yields equations that agree with the general observations. A detailed check of the model predictions is carried out in a discussion of data recently obtained for syndiotactic polypropylene. Attachment rates calculated using an RIS treatment are in fair agreement with those derived from the experimental data on the basis of the model.
N‐benzhydryl‐glycolamide: The first protecting group in peptide synthesis with a strong conformational bias
Authors: Crisma Marco , Formaggio Fernando , Ruzza Paolo , Calderan Andrea , Elardo Stefano , Borin Gianfranco , Toniolo Claudio 
Published: April 2003

AbstractWe have synthesized and examined the preferred conformation of a set of N‐benzhydryl‐glycolamide esters from Nα‐protected (or Nα‐blocked) α‐amino acids. Experiments were performed in CDCl3 solution by Fourier transform infrared absorption and 1H‐NMR techniques, and in the crystalline state by x‐ray diffraction. The results of our analysis strongly support the view that this type of Nα‐acylated α‐aminoacyl esters has a marked tendency to fold into a β‐turn conformation, the nature of which is dictated by the structural propensity of the amino acid constituent at the i+1 position. © 2003 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 71:17–27, 2003
Structure–activity relationships of de novo designed cyclic antimicrobial peptides based on gramicidin S
Authors: Lee Darin L. , Hodges Robert S. 
Published: April 2003

AbstractThe cyclic β‐sheet structure possessed by the 10‐residue antibiotic peptide gramicidin S was taken as the structural framework for the de novo design of biologically active peptides with membrane‐active properties. We have shown from previous studies that gramicidin S is a broad‐spectrum antibiotic effective against Gram‐positive bacteria, Gram‐negative bacteria, and fungi, but is toxic to human red blood cells. We tested the effect of ring size on antimicrobial activity and hemolytic activity on peptides varying from 4 to 16 residues. Interestingly, we were able to dissociate hemolytic activity and antimicrobial activity by increasing the ring size of the peptide to 14 residues (peptide GS14). Furthermore, we increased specificity for microbial membranes while decreasing toxicity to red blood cells by substituting enantiomers (D‐amino acids for L‐amino acids and vice versa) into the GS14 sequence. The enantiomeric substitutions all disrupted β‐sheet structure in benign medium and decreased peptide amphipathicity. The least amphipathic peptide, produced by substituting a D‐Lys at position 4 of GS14 (peptide GS14K4), also had the highest therapeutic index, i.e., highest degree of specificity for microbial cells over human cells. Solution structures of GS14 analogs solved by NMR spectroscopy showed that the D‐amino acid side chain was located on the nonpolar face of GS14K4. Another analog, a β‐sheet peptide with reduced amphipathicity (peptide GS14 K3L4), also had a lysine (lysine 3) on the nonpolar face as determined by the NMR structure. Both GS14K4 and GS14 K3L4 had reduced amphipathicity relative to GS14 and much higher therapeutic indices. Finally, the alteration of the nonpolar face hydrophobicity of GS14K4 analogs provided a range of activities and specificities, where the peptides with the intermediate hydrophobicities among the series had the highest therapeutic indices. The optimal peptide hydrophobicities varied depending on the microorganism being tested, with higher hydrophobicity requirements against Gram‐positive bacteria and yeast compared with Gram‐negative microorganisms. The net result of these studies suggests that it is possible to rationally design a cyclic membrane‐active antimicrobial peptide with high specificity towards prokaryotic (bacterial and fungal) membranes and minimal toxicity to eukaryotic (e.g., mammalian) membranes. © 2003 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 71: 28–48, 2003
Structure–activity relationships of dynorphin analogs substituted in positions 2 and 3
Authors: Schlechtingen Georg , DeHaven Robert N. , Daubert Jeffrey D. , Cassel Joel , Goodman Murray 
Published: April 2003

AbstractFollowing up on the observation that the dynorphin analog [Pro3]Dyn A(1–11)–NH2 2 possesses high affinity and selectivity for the κ opioid receptor, a number of related peptides were prepared and characterized by radioligand binding and [35S]GTPγS assays. While incorporation of 2‐azetidine carboxylic acid in position 3 led to the equally potent analog 3, the corresponding analog containing piperidine‐2‐carboxylic acid showed a nearly 90‐fold reduction in κ affinity. Differential preferred bond angles ϕ in the three building blocks might account for these observations. Compounds 2 and 3 were κ antagonists with IC50 values of 380 and 350 nM, respectively. The Sar3 analog 7 and the Sar2 analog 8 were κ agonists, with greater selectivity than Dyn A(1–11)–NH2 1. In view of their high κ affinities (8: Ki = 1.5 nM; 2: Ki = 2.4 nM), the new analogs were surprisingly weak κ agonists or antagonists, e.g., the EC50 value for the agonist 8 was 280 nM. Different κ receptor subtypes in binding vs functional assays can not account for these results, since both assays were performed using the same membrane preparation. © 2003 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 71: 71–76, 2003