bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dynamics of polypropene and propene‐ethylene copolymers at temperatures above ambient
Authors: Zeigler Robert C. 
Published: March 2011

AbstractSeveral applications of solid‐state nuclear magnetic resonance (NMR) spectroscopy to studying polyolefin mobility at temperatures ranging from room temperature to above the polymer melt are described. 13C NMR can be used with magic‐angle spinning and high‐power proton decoupling to determine the fraction of mobile polymer in polypropene and to characterize the nature of the polymer chain motions as a function of sample temperature. Similar techniques can be used to characterize the local motions of complex copolymer systems such as heterophasic ethylene‐propene copolymers. The practicality of low‐speed magic angle spinning to observe quantitative high‐resolution NMR spectra of neat, molten polymer samples is also described.
Nuclear spin relaxation times dispersion in disordered polymer systems
Authors: Chu Peter P. J. , Howard J. J. 
Published: March 2011

AbstractNuclear spin relaxation reveals a wealth of information concerning motional dynamics and morphology within a polymer system. The conventional breakdown of relaxation into discrete decay components is usually arbitrary and often ambiguous due to strong coupling and spin diffusion between domains. To avoid being limited by pre‐determined models, we have explored the utility of maximum entropy regularization scheme to reconstruct the complete NMR spin relaxation distribution continuum. Case studies showed that the three characteristics of the distribution correlate with various aspects of polymer morphology and with physical properties. These positive results suggested that this approach has the potential of yielding profiles on polymer dynamics and information concerning phonon coupling between domains in complex polymer systems which are not previously accessible. Furthermore, the nondestructive nature of NMR gives this approach an extra advantage over conventional tests for “in‐situ” studies of polymers.
NMR imaging of solid polymers: Strategies for chemical‐shift selectivity
Authors: Hepp Mark A. , Miller Joel B. 
Published: March 2011

AbstractOne reason that the solid‐state imaging field is exciting is because of its potential to provide spatially resolved chemical information from deep inside a solid sample. To achieve this goal, methods are needed which exploit the many NMR parameters to generate image contrast. We describe two strategies for employing the chemical‐shift anisotropy as a contrast mechanism in images of abundant nuclei in solid polymers. We illustrate these approaches with images which map spatial variations of molecular orientation in samples of polytetrafluoroethylene.
Structural characterization of langmuir‐blodgett films of a cellulose derivative by X‐ray photoelectron spectroscopy
Authors: Maaroufi Abdelkrim , Mao Lijun , Ritcey Anna M. 
Published: March 2011

AbstractThe method of variable angle x‐ray photoelectron spectroscopy has been applied to transferred monolayers of a new cellulose derivative. The results support a model for the monolayer in which the cellulose backbone lies flat on the substrate and the sidechains extend perpendicular to the surface. Furthermore, a significant portion of the substrate surface is found to be bare, indicating that the transferred monolayer contains major defects in the form of holes.
Structure and stability of Langmuir‐Blodgett‐Kuhn multilayers containing “hairy‐rod” copolymers
Authors: Foster Mark D. , Vierheller Timothy R. , Schmidt Albert , Mathauer Klemens , Knoll Wolfgang , Wegner Gerhard 
Published: March 2011

AbstractThe microstructure and thermal stability of Langmuir‐Blodgett‐Kuhn multilayers containing rod‐like polyglutamate copolymers having flexible aliphatic side chains have been studied with x‐ray and neutron reflectometry. Upon annealing at 84°C the multilayer structure changes in a manner which is dependent upon the degree of orientation of the rod‐like backbones and therefore upon the flow dynamics of the deposition. A sample deposited with a highly convergent flow loses the bilayer structure induced by deposition, whereas samples with less highly aligned backbones may not undergo this relaxation as readily.
Structure formation of organic multi‐layers
Authors: Isoda Seiji , Hashimoto Shugo , Itoh Toshihiko , Hoshino Akitaka , Kobayashi Takashi 
Published: March 2011

AbstractInterface structures of C60 and polymeric phthalocyanines (GeOPc and AlFPc) were analyzed by high resolution electron microscopy. C60 was crystallized making its {111} ‐plane parallel to the surface of tetragonal phthalocyanines as on other substrates. This may be due to strong tendency of C60 to make its closed‐pack layer parallel to the surface during crystallization. In addition to this plane orientation, an axial orientation was observed; The <110> and <112> directions of C60 orient along the <110> directions of phthalocyanines. It can be concluded from observations that the lattice‐matching between the corresponding repeating distances of C60 and phthalocyanines is an important factor governing the orientation.
NMR Imaging and materials research
Authors: Blümich B. , Blünder P. , Fülber C. , Scheler U. , Weigand F. 
Published: March 2011

AbstractThe use of nuclear magnetic resonance in materials research is elaborated. While NMR applications to materials are discriminated between rigidly solid, rubbery, and liquid matter, NMR methods are divided into spectroscopy and imaging. The connections between the different material and NMR categories are explained. Particular emphasis is placed on NMR with spatial resolution, which is illustrated by examples from imaging of solid polymers and elastomers.
Surface acidic sites of highly disperse magnesium chloride: IR and ESR spectroscopy studies
Authors: Zakharov V. A. , Paukshtis E. A. , Mikenas T. B. , Volodin A. M. , Vitus E. N. , Potapov A. G. 
Published: March 2011

AbstractExperimental data obtained by means of some high performance techniques allow to identify and register quantitatively the Lewis acidic sites on the surface of highly disperse magnesium chloride. Carbon monoxide adsorption on magnesium chloride has been studied with the IR diffuse reflectance spectroscopy (IRS‐DR). The interaction of some stable nitroxyl radicals with magnesium chloride has been investigated with the ESR technique.
Asymmetric Markovian analysis of the polymerization mechanism of propene by ziegler‐natta catalysis and external alkoxysilane donors
Authors: Chûjǒ Riichirǒ , Härkönen Mika , Seppälä Jukka V. 
Published: March 2011

AbstractThe formulae for conditional probabilities are derived with the term of NMR triad tacticities for the polymers whose polymerization process is described by an asymmetric Markovian process. The formulae obtained are applied to the polypropylene polymerized by Ziegler‐Natta catalysis with external alkoxysilane donors. For both, unfractionated samples and heptane‐insoluble fractions, four evidences are clarified as follows: Don the transformation from symmetric to (fluctuating) asymmetric sites the transformation of one (hereafter, L) site is much more enhanced than that of the other (D) site, 2) monotonous increase of isotacticity with an increase of the concentration of the donor is due to the increase of pLL, 3) aromatic donors are more effective than aliphatic ones for the improvement of isotacticity, and 4) the improvement of isotacticity after an addition of alkoxysilane is not due to the contribution from both of L and D sites, but the competition between the larger positive contribution from the former and the smaller negative one from the latter.
Symmetrical and lopsided zirconocene pro‐catalysts
Authors: Ewen John A. 
Published: March 2011

AbstractMe2C[1‐Cp‐9‐Flu]ZrCl2 derivatives with H‐, CH3‐and (CH3)3C‐ substituents β to the Cp bridgehead carbon atom as well as complexes having mono‐ or di‐substituted fluorenyl ligands (R = CH3‐, (CH3)3C‐, CH3O‐, CH3OCH2‐, (CH3)3CC≡C‐, (CH3)2N‐, F‐, and Cl‐) have been investigated as propylene polymerization pro‐catalysts. Steric effects from the β‐Cp substituents determine the iso‐, hemi‐iso‐, and syndio‐specificities of the catalysts. The relative stereospecificities of Me2Si[1‐Cp‐3t‐Bu‐9‐Flu]ZrCl2 and rac‐Me2Si[1‐Ind‐3‐t‐Bu]2ZrCl2 are in accord with molecular models. The more un‐symmetrical the catalysts are the more m and mm stereosequences there are in s‐PP. The m dyads also increase with the concentration of Me2C[1‐Cp‐9‐Flu‐2‐OMe]ZrCl2.
The geometry of the site and its relevance for chain migration and stereospecificity
Authors: Razavi Abbas , Peters L. , Nafpliotis L. , Vereecke D. , Dauw K. Den , Atwood Jerry L. , Thewald Ulf 
Published: March 2011

AbstractThe structurally well characterized members of the family of the bridged metallocene procatalysts with cyclopentadienyl‐fluorenyl ligand framework constitute excellent examples for the investigation of the interrelation between the catalyst geometry and stereospecificity from a dynamic chain perspective. We have thus carefully selected a set of metallocenes, based on the increasing size of their β‐substituents and the choice of Zr and Hf as transition metals. By their application as polymerization catalysts we have been able to investigate the impact of the substituent size on the migratory behavior of the chain and stereospecific control of the corresponding catalysts. Through these investigations it has been demonstrated that the degree of freedom for the chain's lateral displacements, and the frequency of its back and forth migrations, varies largely with and is dependent on the steric bulk of the β‐positioned substituent and to the degree of the geometrical (stereo‐electronic) flexibility of the catalytic site with a given symmetry. It has been inferred from the experimental results that substituents, depending on their voluminosity and spatial expansion, interfer differently with the migratory function of the chain and can bring its migration, temporarily or permanently, to a halt (chain stationary insertion). The ratio of chain migratory insertion to chain stationary insertion and their prevalence in each individual case has the final saying about the degree and the type of the microtacticity of the resulting polymer chains (e.g. syndiotactic, syndiotactic containing isoblock, syndio‐isoblock, isotactic, and isotactic with syndioblock.
AB‐initio molecular dynamics study of metallocene catalysed ethylene polymerisation
Authors: Iarlori Simonetta , Buda Francesco , van Doremaele Gerard , Meier Robert J. 
Published: March 2011

AbstractAb‐initio Molecular Dynamics simulations on ethylene insertion in a bridged di(cyclopentadienyl) methylzirconocene have revealed that the entire reaction path starting from the π‐coordinated ethylene‐zirconocene complex up to and including propyl formation takes place in about 150 femtoseconds, which is unexpectedly fast, and suggesting the absence of any significant barrier of activation. Starting from a reactant structure without α‐H agostic interaction, at T=400 K such an interaction evolves during the course of the insertion reaction and before the propyl is formed. The product state exhibits γ‐H agostic interaction.
New catalysts for the polymerization of cyclic olefins
Authors: Goodall Brian L. , McIntosh Lester H. , Rhodes Larry F. 
Published: March 2011

AbstractVarious routes to high Tg thermoplastic materials based on the Ziegler‐Natta catalyzed polymerization of norbornene‐type monomers are described. These routes involve both ring‐opening metathesis polymerization (ROMP) and addition polymerization. New catalysts, and a new catalytically‐active metal (cobalt), for both ROMP and the addition homopolymerization of norbornene‐type monomers are reported.
Regio‐ and stereocontrol in the alternating copolymerization of olefins with carbon monoxide
Authors: Amevor Esther , Bronco Simona , Consiglio Giambattista , Di Benedetto Silvia 
Published: March 2011

AbstractStrictly alternating copolymers between olefins and carbon monoxide are synthesized using cationic palladium catalysts modified by phosphorus or nitrogen ligands. Basic chelate diphosphines as the ligand allow the regioregular copolymerization of aliphatic olefins thus affording, e.g. in the case of propene, poly(1‐oxo‐2‐methyltrimethylene). Steric control of the copolymerization process towards the production of overwhelmingly isotactic copolymers is possible particularly when using atropisomeric ligands. In the case of styrene as the substrate and for all ligands employed the copolymerization process is regioregular. Prevailing syndiotactic structure is obtained with 1,10‐phenanthroline or 2,2′‐bipyridine as the ligand. Chelate thioether ligands allow the preparation of a completely atactic material. For 4‐tert‐butylstyrene an isotactic structure became accessible by using chiral bisoxazolidines. The prevailing structure of the copolymers of cyclopentene corresponds to a 1,3‐enchainment of the olefin units most probably associated with a prevailing diisotactic structure
The features of syndiotactic polypropylene
Authors: Kakugo Masahiro 
Published: March 2011

AbstractThe mechanism of stereoselectivity of propylene insertion in propylene‐ethylene copolymerization on a CS symmetrical zirconium complex i‐Pr(Cp) (Flu) ZrCl2 catalyst is discussed. Calculation results indicate that not only the β‐carbon in the growing chain end of the polymer but also the substituent of the β‐carbon play an important role in the selectivity of the prochiral face of the next‐coming propylene monomer. The stereoregularity of propylene units connected to an ethylene unit (PPE) in propylene‐ethylene copolymer was observed to be lower than that in propylene sequences (PPP) in the 13C NMR spectrum, which supports the calculation results. Furthermore, the structure and properties of propylene‐olefin (ethylene, 1‐butene, 1‐pentene, 1‐hexene, and 4‐methyl‐1‐pentene) copolymers prepared with the i‐Pr(Cp) (Flu) ZrCl2 catalyst system were studied. Propylene‐1‐butene copolymer exhibits peculiarly lower melting point depression because 1‐butene units enter into the unit cell of the crystal structure of syndiotactic polypropylene.
Syntheses and properties of syndiotactic polystyrene
Authors: Ishihara N. 
Published: March 2011

AbstractSyndiotactic polystyrene (SPS) is a new crystalline engineering thermoplastic. With a melting point of 270 °C and its crystalline nature, SPS has high heat resistance, excellent chemical resistance and water/steam resistance. Since SPS has excellent dielectric properties, it is useful as a capacitor insulation material. The rate of crystallization is very fast in comparison with isotactic polystyrene (IPS), thus, SPS can be used in a number of forming operations, including injection molding, extrusion and thermoforming.A system composed of a homogeneous titanium compound and methylaluminoxane (MAO) is an effective catalyst for syndiospecific polymerization of styrene. On the other hand heterogeneous titanium compounds containing halogen make a mixture of isotactic and syndiotactic components. The amount of syndiotactic polystyrene obtained is dependent on the mole ratio of Al to Ti. The result of ESR measurement suggests the Ti3+ species are important as a highly active site for producing syndiotactic polystyrene.A comparison of the stereoregularities of polypropylene and polystyrene formed by various metallocene catalysts is studied. The (C6H6)2C(η5‐C5H4)(η5‐C9H6)TiCl2/MAO system gives a homogeneous catalyst for the polymerization of propylene giving isotactic rich polypropylene and of styrene to give syndiotactic polystyrene.
Thermodynamics, dynamics and relaxation in glass‐forming liquids ‐ the conformon concept
Authors: Kilian Hanns‐Georg , Beyer Claus , Pietralla Martin 
Published: March 2011

AbstractAny interpretation of the dynamics in liquids requires understanding of fluctuating collective motions in the stationary limit. To this end, “eigen‐states” are defined, named conformons. Conformons are to be considered as a new type of bosons. Their properties characterize the typical features of the dynamics in disordered systems. Eigen‐energy and “eigen‐entropy” are adjusted to each other to minimize the free energy of each mode (saturation). The whole conformon ensemble exhibits a photon statistics with a broad universal mode spectrum. Analogues of Stefan‐Boltzmann's law and of Wien's displacements law are formulated.A crucial point of our treatment is that conformons should, in principle, only become excited above a system‐typical temperature To (Vogel‐Fulcher temperature). Equilibrium properties of supercooled liquids are necessarily related to To. Cp/Cv, of the conformon ensemble should, for example, show a typical ΔT3‐dependence (ΔT=T‐To). Experimental data give evidence in support of this behaviour. As a consequence of the excluded volume supercooled liquids already solidify about 30 to 40 degrees above To at least at the quasi‐static glass temperature Tg.Relaxation in the glass transition regime of glass formers can fully be described if relaxation modes are connected to each conformon. The description covers a set of experiments so different like mechanical and dielectric relaxation or quasi‐elastic light‐ and neutron scattering.
From chemical structure to viscoelastic properties of polymers
Authors: Ngai K. L. , Tsang Kwok Yeung 
Published: March 2011

AbstractThe basic result of the coupling model consists of a cross‐over from exp‐(t/τo) to exp‐(t/τ*)1‐n at a temperature insensitive microscopic time tc. We indicate how this basic result can be derived from chaotic, interacting Hamiltonian systems which include densely packed polymer molecules. Recent quasielastic neutron scattering experiments and molecular dynamics simulations are discussed and the results are shown to support this result as well. An application of the coupling model to find how the viscoelasticity of a polymer depends on the chemical structure of the monomer through the coupling parameter of the local segmental motion is given to illustrate the utility of the model.
Dielectric relaxations in bilayer structures of comb‐like copolymers: Perfluoralkyl and liquid‐crystalline side chains
Authors: Alig I. , Braun D. , Jarek M. , Hellmann G. P. 
Published: March 2011

AbstractAbstract: A series of comb‐like copolymers and homopolymers containing different liquid‐crystalline and perfluoralkyl side chains was studied by frequency and temperature dependent dielectric measurements. The structure and the phase behaviour of the systems were characterized by X‐ray scattering and differential scanning calorimetry. The dielectric relaxation spectra of these polymers reflect the molecular mobility in bilayer structures formed by the mesogenic or crystalline side chains. By changing temperature it was possible to investigate the molecular motions in the different phases of the copolymers (smectic‐crystalline, smectic‐isotropic, and isotropic). The homopolymer containing perfluoralkyl side chains and all copolymers show a β‐relaxation at low temperatures, which is assigned to local molecular motions. Above the glass transition temperature, all samples exhibit a segmental (α) relaxation with WLF‐like temperature dependence in the activation plots. For the polymers forming LC structures only one relaxation process was detected in all phases, i.e. in the smectic, nematic, and isotropic structure or, in case of the copolymers, in the smectic‐crystalline and smectic‐isotropic double structures. This process was attributed always to the segmental motions, assisted by tumbling motions of the mesogens. The phase transitions are clearly indicated by discontinuities in the dielectric relaxation times and changes in the relaxation strength Δϵ. The dipole reorientations of the mesogens seem to be more restricted by the crystalline layers in the copolymers as by the smectic order of the LC homopolymers.
Triblock copolymers in a selective solvent: Dilute and semi‐dilute solutions
Authors: Lairez Didier , Adam Mireille , Raspaud Eric , Carton Jean‐Pierre , Bouchaud Jean‐Philippe 
Published: March 2011

AbstractTriblock copolymers polystyrene‐polyisoprene‐polystyrene were studied in dilute and semi‐dilute solutions in a selective solvent, i.e. a non‐solvent for polystyrene and a good solvent for polyisoprene. The mass of the copolymer is equal to 1.6×105g/mol and it contains 70% of polyisoprene. At a concentration C equal to 1.6×10−3 g/cm3 triblock aggregation occurs.Experiments performed on dilute solutions1 plead in favor of loose and polydisperse aggregates rather than spherical micelles: 1) the form factor of the aggregates shows a smooth decrease at qR<1; 2) internal modes are observed by quasi‐elastic light scattering; 3) specific viscosity measurements show that the internal concentration of unimers and aggregates are identical; 4) the unimer concentration and the aggregation number depend on the copolymer concentration.In semi‐dilute solutions2 (C>3×10−2 g/cm3), dynamical properties are strongly affected by the temperature. Viscosity and longest relaxation time increase by a factor 105 as the temperature decreases from 50°C to 5°C, whereas the plateau modulus G remains independent of the temperature and proportional to the concentration. Small angle neutron scattering experiments reveal a cubic structure having a unit cell dimension d independent of the temperature and varying as C−1/3 leading to G∼1/d3. Quasi‐elastic light scattering experiments performed on this system show a wide relaxation function extending over 7 decades of time. These results argue for a lattice made of polystyrene nodes linked together by the polyisoprene middle blocks, the dynamics being governed by the glass transition of the nodes.
Mobility op polymer micelles
Authors: Anufrieva Elizaveta V. , Gromova Raisa A. , Lushchik Vanda B. , Nekrasova Tatyana N. , Krakoviak Mark G. 
Published: March 2011

AbstractThe nanosecond mobility of polymer micelles in aqueous solution of poly(N‐propylmethacrylamide) and that of reversed micelles of a surfactant in organic solvents were studied by polarized luminescence. The factors affecting the polymer micelles formation and the effect of polymers solubilized in systems of reversed micelles of a surfactant on their relaxation properties were established.
Structure and dynamics of copoly(oxyethylene‐oxypropylene‐oxyethylene) in o‐xylene/water mixtures
Authors: Chu Benjamin , Wu Guangwei 
Published: March 2011

AbstractAbstract: The structure and dynamics of supramolecules of a triblock copolymer, copoly(oxyethylene‐oxypropylene‐oxyethylene), also known as PLURONIC L64: PEO13PPO30PEO13, in o‐xylene/water mixtures were studied by means of dynamic light scattering, small‐angle X‐ray scattering and NMR. At low copolymer concentrations, micelles coexisted with unimers and the micellar size increased with increasing amount of water in the mixture. Large aggregates were formed at higher copolymer concentrations. The size and the amount of the large aggregates increased with increasing amount of water in the mixtures, and increasing copolymer concentration. The solubilized water can be classified into free‐water(forming hydrogen bond with water) and bound‐water(forming hydrogen bond with EO units). The environment of EO units changed with the amount of solubilized water. The conformation of the copolymer remained essentially the same before and after micellization and did not change with the amount of solubilized water.