bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polymer‐solvent co‐crystallization during thermoreversible gelation in solutions of the helical polypeptide PBLG
Authors: Tadmor Rafi , Dagan Anca , Cohen Yachin 
Published: March 2011

AbstractMacromolecular aggregation during thermoreversible gelation in solutions of the helical polypeptide poly(γ‐benzyl‐L‐glutamate) [PBLG] in benzyl alcohol [BA] were studied by small angle neutron and small angle X‐ray scattering. Gelation is apparent as a large increase in the intensity scattered at low angles, signifying formation of a microfibrillar PBLG network. The aggregated phase in isotropic gels from semidilute solutions contains about 28% solvent. A periodic structure is observed when gelation is induced by rapid cooling to a low temperature, but not by slow cooling or gelation at a higher temperature. In gels from concentrated liquid crystal solutions, two crystalline structures are observed, depending on whether the solution is rapidly quenched and then annealed or slowly gelled at an elevated temperature. A phase diagram for the PBLG/BA system is presented and the observed microstructural transitions are rationalized in terms of a gelation mechanism involving a combination of liquid‐liquid phase separation and crystallization in the form of polymer‐solvent co‐crystals.
Syndiotactic polystyrene/ethylbenzene complex: Chain conformations and phase behaviour
Authors: Spells Stephen J. 
Published: March 2011

AbstractSyndiotactic polystyrene (sPS) has been studied, primarily in the δ (solvent‐stabilised) and γ phases. Crystallisation from dilute ethylbenzene solution at 40°C leads to a lamellar morphology, with a similar unit cell for the polymer/solvent complex as that proposed by Chatani et al for the sPS/toluene complex [1]. As the temperature is raised, the X‐ray long period increases in the temperature range of decomplexation, becoming constant in the y phase before increasing again on annealing in the α phase.Neutron scattering measurements show negligible isotopic fractionation and only a small increase in the radius of gyration in the plane of the sedimented δ phase mats (Rxy) with increasing molecular weight. Together with model simulations which reproduce satisfactorily both the form of Kratky plots and the values for Rxy, this evidence points strongly towards a sheetlike structure (as is obtained for other non‐complexing solution‐grown polymer crystals). Increases in Rxy and changes in the scattering curves from those characteristic of sheetlike structures in the γ phase are interpreted as indicating movement of crystal stems out of the sheets.
Thermoreversible gelation of syndiotactic polystyrene: Effect of solvent type
Authors: Daniel C. , Menelle A. , Brulet A. , Guenet J.M. 
Published: March 2011

AbstractThe temperature‐concentration phase diagrams of syndiotactic polystyrene (sPS)/benzene gels and sPS/toluene gels establish the existence of two polymer‐solvent compounds in both systems. In particular it is found that the sto ichiometries of the compounds differ with the type of solvent: The stoichiometrics, expressed as solvent molecules/monomer are respectively for the benzene 4/1 and 1/1 and for the toluene 0.8/1 and 1/4. Neutron diffraction patterns allow us to confirm the existence of differents compounds in sPS/toluene gel and sPS/benzene gel.
Transient electric birefringence study of agarose aggregates in water‐dimethyl sulphoxide mixtures
Authors: Ramzi M. , Mendes E. , Rochas C. , Guenet J.‐M. 
Published: March 2011

AbstractThe dynamical behavior of agarose aggregates in a mixture of water and dimethyl sulphoxide (DMSO) has been investigated using a Transient Electric Birefringence (TEB) set up. Different agarose concentrations for different water/DMSO proportions were considered. The relaxation of the birefringence curves can be fitted with a stretched exponential of the form Δn(t) = Δn0 exp(‐(Dt)0.5). The exponent 0.5 suggests a high polydispersity of the aggregates. The rotational diffusion coefficient, D, depends strongly on the water/DMSO proportion. For small values of the applied electric field it presents a maximum for a water volume fraction equal to 0.5. A strong dependence of D on the applied electric field is also observed. However, the same kind of behavior as for weak fields is recovered when a subtle representation of the data is used. Differential Scanning Calorimetry (DSC) results also show a maximum in the temperature of fusion of the aggregates for a water volume fraction of 0.5. This set of results suggest strongly that « complexes » of water/DMSO/agarose are formed in the mixture and that the process of aggregate formation is more « efficient » for a water/DMSO mixture of same proportions.
The solvation of poly(vinyl alcohol)
Authors: Assender Hazel E. , Windle Alan H. 
Published: March 2011

AbstractThe possible incorporation of water molecules within the crystal structure of poly(vinyl alcohol) is discussed. Modelling of the crystal structure suggested that water could be incorporated without severe disruption, and the effect on the X‐ray powder diffraction trace was simulated. The effect of variation in tacticity is discussed in terms of the nature of the hydrogen bonding. Simulated traces are compared with experimental data from atactic samples in which a change in the diffraction peak intensities is observed for samples crystallised with water present. This is compared with samples produced from nonaqueous solutions.
The effect of temperature on the curing of unsaturated polyester resins modified with a liquid polymer
Authors: de la Caba Koro , Guerrero Pedro , Franco Merche , Mondragon Iñaki 
Published: March 2011

AbstractThe effect of temperature on the cure kinetics of two different unsaturated polyester resins, Estratil 1.112 and G102, modified with a liquid polymer, has been studied by Fourier transform infrared (FTIR) spectroscopy in the range of 60–80°C and 70–90°C, respectively. The experimental results have been related to the rheological changes during cure.
Acrylonitrile‐sodium methallylsulfonate copolymer solutions. NMR approach to phase diagrams and ordering processes
Authors: Margulies Monique Marie , Viallat Annie , Addad Jean‐Pierre Cohen 
Published: March 2011

AbstractThe physical behavior of solutions of an acrylonitrile‐sodium methallylsulfonate copolymer in dimethyl sulfoxide is described from NMR and calorimetric observations by varying concentration, temperature and thermal history of the systems. Both the solvent and the polymer can crystallize in the studied range of temperature. A method is proposed to determine the composition of the phases present in solution. At high polymer concentration, we observe that the physical state of a given solution depends on its thermodynamic history and that three distinct phases can coexist over a large temperature interval.
Complex formation in poly(vinyl chloride) thermoreversible gels
Authors: Reinecke H. , Guenet J.‐M. , Mijangos C. 
Published: March 2011

AbstractThe molecular structure of gels formed by poly(vinyl chloride) with different solvents is analyzed by neutron diffraction and neutron scattering techniques. Structure parameters are determined from the scattering functions and discussed in the light of the fibrillar model. Arguments are given in favour of the formation of PVC‐solvent complexes.
The role of triplet repulsion in elementary steps of cyclic chain termination of oxidizing polymers and other substances
Authors: Denisov Evguenii T. 
Published: March 2011

AbstractParabolic model of the transition state was used for analysis of activation energies of reduction of benzoquinone (Q), aminyl (Am*), and nitroxyl (AmO*) radicals by peroxyl radicals of different structure. It was proved that radicals HO2* and R2C(OH)O2* reduce Am*, and AmO* with zero activation energy due to low triplet repulsion in the transition state. On the contrary alkylperoxyl radicals reduce Am* and AmO* slowly due to high triplet repulsion and sufficiently high activation energy. That is why AmH and AmOH perform cyclic chain termination in oxidizing substances where peroxyls with OH‐group are generated.
Interactions of stabilizers during oxidation processes
Authors: Habicher Wolf D. , Bauer Ingmar , Scheim Kerstin , Rautenberg Claudia , Loßack Annett , Yamaguchi Kazuo 
Published: March 2011

AbstractThe antioxidative action of mixtures of phenols, phosphites, HALS, a) and some of their transformation products in various compositions has been studied in the thermo‐ and photo‐oxidation of hydrocarbons and polypropylene under different conditions. In the AIBN‐initiated oxidation of hydrocarbons at low temperatures (< 80°C), hindered phenols, hindered aryl phosphites and the nitroxyl derivatives of HALS act antioxidatively when used individually in appropriate concentrations. Secondary HALS do not show any induction period, but a certain retardation of the oxidation process after some reaction time. The inhibiting efficiency of nitroxyls observed cannot be explained completely by the currently accepted action mechanisms of HALS, but is also related to the reaction of the nitroxyls with alkylperoxyl radicals.In mixtures with hindered phenols, HALS have almost no influence on the rate of thermooxidation at low temperatures. Their nitroxyl derivatives, however, always exhibit synergism, most pronounced when both stabilizers are used in equimolar ratios. During the photooxidation phenols lower the efficiency of HALS.The influence of mixtures of stabilizers on the oxidative stability of polypropylene is rather different and depends on the oxidation conditions, the structure, the concentration and the ratio of the stabilizers. Synergistic as well as antagonistic effects are observed. Both aliphatic and aromatic phosphites studied act synergistically when used together and with phenols. This demonstrates that for acting as synergist for phenols, the hydrogen peroxide decomposing capability of the phosphites, but not their chain breaking activity is important. HALS‐phosphites and phosphonites, containing amine and phosphorus units in one molecule, are highly effective inhibitors of photo‐ and thermooxidation and exhibit lower critical antioxidant concentrations and longer induction periods than phosphites alone. They even exceed the efficiency of phenols in many cases.Transformation products of phenolic antioxidants investigated act differently and in many cases contrarily under photo‐ and thermooxidative conditions. Therefore, they influence the efficiency of stabilizer mixtures also in a different way.
Stabilization beyond the year 2000
Authors: Zweifel Hans 
Published: March 2011

AbstractA variety of chemical reactions lead to the oxidative deterioration of polymers upon processing and during end‐use of plastic articles. To inhibit the autoxidation cycle, stabilizers interfering at the various steps of the degradation are needed. Complex stabilizer systems which are able to suppress oxidative degradation have been developed recently. Special emphasis is given to radical scavenging, including alkyl radicals. UV absorbers with inherent photostability have led to an enhanced stability of polymers, such as engineering thermoplastics.
Fluorescence‐resolved hole burning of squaraine dyes in polymer matrices at low temperature
Authors: Wolf Joachim , Myers Anne B. 
Published: March 2011

AbstractA new technique that combines nonphotochemical hole burning with multichannel detected fluorescence line narrowing has been used to obtain vibrationally resolved fluorescence spectra of squaraine chromophores in polymer matrices at 1.4 K. At a fixed excitation frequency, the intensities of the zero‐phonon lines decay with time due to nonphotochemical hole burning, leaving behind a broader background attributed to emission from molecules excited into phonon sidebands. Subtracting the spectrum of the hole‐burned sample from the initial one leaves predominantly a zero‐phonon line excited spectrum exhibiting enhanced vibrational structure. Spectra of the same squaraine in polystyrene and polyethylene matrices show differences in the frequencies and intensities of the phonon sidebands, indicating differences in the frequencies and strengths of the matrix modes coupled to the electronic transition of the chromophore. The phonon densities of states inferred through different measurement techniques are compared and related to electronic dephasing rates.
Pyridine‐based conjugated polymers: Photophysical properties and light‐emitting devices
Authors: Epstein Arthur J. , Wang YunZhang , Jessen Scott W. , Blatchford James W. , Gebler Darren D. , Lin Liang‐Bih , Gustafson Terry. L. , Swager Timothy M. , Macdiarmid Alan G. 
Published: March 2011

AbstractWe study the photophysical properties of the pyridine‐based polymers: poly(p‐pyridyl vinylene) (PPyV) and poly(p‐pyridine) (PPy). The primary photoexcitations in the pyridine‐based polymers are singlet excitons. We observe direct intersystem crossing (ISC) on picosecond time scales with the volume density of triplet excitons varying with the sample morphology (film or powder). These effects are demonstrated clearly by examining the millisecond photoinduced absorption characteristics of powder and film forms of PPyV. The pyridine‐based polymers have been shown to be promising candidates for polymer light‐emitting devices, both “conventional” diode device and symmetrically configured ac light‐emitting (SCALE) device. Here we examine the role of “insulating” layers and their interfaces with the emitting layer and electrodes in the SCALE device operation, with emphasis on the central role of the polymer‐polymer interfaces.
Photoinduced charge transfer in poly(phenylene‐vinylene) and its derivatives
Authors: Conwell Esther M. 
Published: March 2011

AbstractPhotoinduced absorption measurements on poly(phenylenevinylene), PPV, or its derivatives, illuminated with photons of energy past the absorption edge revealed that some percentage of the photons create a long‐lived excitation rather than the expected excitons. We present the evidence that these excitations are polaron pairs, which are essentially excimers. The reasons why this percentage varies from sample to sample are discussed. Also discussed are the reasons why polaron pairs in some derivatives of PPV emit light while in others they do not. Calculations with a relatively simple Hamiltonian can account well for the peaks in the photoinduced absorption produced by these pairs.
Hole transport in triarylamine doped polymers
Authors: Borsenberger Paul M. , Magin Edward H. , Heun Susanne 
Published: March 2011

AbstractHole mobilities have been measured in a series of triarylamine molecules doped into a poly(styrene) host. The results are described by a formalism based on disorder, due to Bässler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameter of the formalism is the energy width of the hopping site manifold. The width is described by a model based on dipolar disorder. According to the model, the width is comprised of a dipolar component and a van der Waals component. The dipolar component is in good agreement with the recent analysis of Young. The results show that the van der Waals component increases with decreasing dopant concentration. This effect, which is not predicted from first order treatment arguments, is attributed to increasing structural randomness with dilution.
Femtosecond photoexcitation dynamics in polydiacetylene 4BCMU film
Authors: Zheng L. X. , Feng Z. G. , Knopf F. Carl 
Published: March 2011

AbstractFemtosecond pulses were used to study photoexcitation dynamics in polydiacetylene 4BCMU film. Ultrafast photoinduced dichroism and optical Kerr gate responses were observed. These measurements allowed estimation of the diffusion constant (D ∼ 0.1 cm2/s), the nonlinear index of refraction (n2 ∼ 10−8 esu), and third‐order susceptibility (δχ(3) ∼ 6 × 10−10 esu), in the film. We also found that the sign of the photoinduced transmission signal was dependent on the pump energy at the low energy absorption edge of the film. Possible explanations are proposed.
Charge‐transfer interaction in liquid‐crystalline materials and their application to photonics
Authors: Tsutsumi Osamu , Kanazawa Akihiko , Shino Takeshi , Ikeda Tomiki 
Published: March 2011

AbstractPhotochemical phase transition of push‐pull type azobenzene liquid crystal (LC), which contains both an electron donor and an acceptor in both ends of the azobenzene moiety, has been explored. Polymer azobenzene LCs were prepared, which show nematic (N) LC behavior in the trans form while no LC phase in the cis isomer. Photoirradiation of a very thin film of the azobenzene LCs (∼ 200 nm) in the trans form resulted in disappearance of the N phase due to transcis photoisomerization of each mesogen, and the N phase recovered quickly when the irradiated sample was kept in the dark because of cis‐trans thermal isomerization and reorientation of trans‐azobenzenes. Time‐resolved measurements by the use of a laser pulse (355 nm; 10 ns FWHM) revealed that the N to isotropic (I) phase transition took place in 200 μs. The thermal I‐N phase transition of the push‐pull type azobenzene LC occurred in 800 ms at 135 °C. This response is faster by one order of magnitude than the response of azobenzene LCs without charge‐transfer interaction.
Photo‐fabrication of surface relief gratings on polymer films
Authors: Kim Dong‐Yu , Lee Taek‐Seung , Tripathy Sukant K. , Li Jiang Xin , Li Lian , Kumar Jayant 
Published: March 2011

AbstractSurface relief gratings were optically produced on a number of azobenzene‐based polymer films. The surface grating formation was investigated by monitoring the diffraction efficiency and using atomic force microscopy. The effect of the structure of the chromophores on surface modulation was investigated. The surface deformation process depended on the polarization state of the writing beams. The localized variations of the light intensity and alteration of the resulting electric field polarization were essential writing conditions to the formation of the surface relief gratings. The surface pattern from straight edge diffraction established that the surface profile of the recorded gratings is proportional to negative gradient of the intensity pattern incident on the polymer film.
Charge transfer interactions in polymers and the fabrication of high frequency electro‐optic modulators
Authors: Dalton Larry R. , Harper Aaron W. , Sun Shajing , Steier William H. , Mustacich Robert V. , Jen Alex K‐Y. 
Published: March 2011

AbstractThe synthesis and processing of second order nonlinear optical polymeric materials for application in electro‐optic devices is described with particular emphasis on (1) the synthesis and incorporation into hardened polymer lattices of chromophores characterized by large μβ (where μ is the dipole moment and β is the molecular first hyperpolarizability) values; (2) the synthesis and incorporation into hardened polymer lattices of chromophores which are capable of undergoing photo‐induced conformational changes and which can be processed by new multi‐color photolithography techniques into buried channel active waveguides; and (3) the processing of polymeric nonlinear optical materials appropriate for the realization of full integration with very large scale integration (VLSI) drive electronics and with silica fiber optic transmission lines. Prototype modulators have been fabricated and shown to be suitable for broadband operation from 0 to 113 GHz. Optical losses associated with waveguide wall roughness and with mode mismatch in transitioning between silica and polymeric waveguides are reduced by control of the conditions of reactive ion etching and by use of the new technique of multi‐color photochemical lithography.
The challenge of assembling molecules into materials
Authors: Stupp Samuel I. 
Published: March 2011

AbstractLinear organic chains and their architectural derivatives such as branched and crosslinked macromolecules have been the main focus of polymer science for many decades. A new direction which should be pursued in the field is the exploration of polymers as “nanostructures”. This exploration would study synthetic polymers as shape invariant objects with nanoscale dimensions which resemble the folded proteins of nature. Such synthetic macromolecules would have well defined shapes which may even include topographical features. Ideally, the definition of shape could include well defined dimensions as well, but molecular objects of common shape but varying dimensions could also exhibit interesting properties. One could for example envision polymers as flat plates (two‐dimensional polymers), discs, ellipsoids, spheres with cavities, parallelepipeds, among many other possible shapes. Such objects must have fairly high molar masses, possibly in the range of hundreds to thousands of kilodaltons, and would therefore not be accessible as chemical compounds synthesized by conventional methods. At the same time, the prospects of learning “folding rules” for synthetic chemical sequences in a biomimetic approach to molecular nanostructures seems to be an extremely difficult goal at the present time. This is suggested given our contemporary difficulties in understanding the protein folding problem.
Coupling and memory in liquid crystal elastomers
Authors: Mitchell Geoffrey R. , Roberts Philip M.S , Ahn Kyung‐hwa , Davis Frederick J. , Hasson Craig , Hirschmann Harald , Pople John A. 
Published: March 2011

AbstractThe polymer backbone of a side‐chain liquid crystal polymer exhibits an anisotropic shape due to the coupling of the liquid crystal orientational order of the mesogenic side‐chains to the backbone. The magnitude and sign of this coupling may be controlled by chemical design. The introduction of chemical cross‐links in to such a system provides both a memory of the anisotropic organisation and a mechanism by which the microscopic anisotropy can be realised at a macroscopic level. We show how this anisotropic network structure yields new phenomena when electric or mechanical fields are applied.
Non‐rigid liquid crystalline networks based on linear, segmented‐chain mesogenic polymers
Authors: Caruso Ugo , Iannelli Pio , Pragliola Stefania , Roviello Antonio , Sirigu Augusto 
Published: March 2011

AbstractFive linear mesogenic polyesters, containing terephthalic groups, and bearing different amounts of unsaturated substituents were prepared and their phase behaviour characterised by calorimetric and X‐ray diffraction means. Networks based on these polymers were obtained by thermally activated radical polymerisation. Some optical and structural features of these networks were investigated both in the bulk and in the swollen state.
Ferroelectric LC‐elastomers
Authors: Brehmer M. , Zentel R. 
Published: March 2011

AbstractFerroelectric liquid crystalline elastomers combine the rubber elasticity of polymer networks with liquid crystalline phases and ferroelectric ordering. Ferroelectric switching leads therefore to a deformation of the polymer network and an elastic stress. The coupling between both effects can be varied by changing the topology of the netpoints.
Polysoaps: The signatures of intrachain self assembly in θ solvents
Authors: Borisov Oleg. V. , Halperin Avraham 
Published: March 2011

AbstractPolysoaps are linear hydrophilic chains that incorporate, at intervals, covalently bound amphiphilic monomers. In water, the amphiphilic monomers self assemble into intrachain micelles thus endowing the chain with a secondary structure. Long polysoaps adopt a variety of configurations involving many intrachain micelles. While the stable configuration is that of a dense globule of closely packed micelles, it is also important to consider metastable configurations such as linear and branched strings of micelles. Theoretical discussion of these issues is presented for the case of a θ solvent for the hydrophilic backbone.