bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polypeptide helix lifetimes in the helix–random coil transition region. Application to NMR spectra
Authors: Ferretti J. A. , Jernigan R. L. , Weiss G. H. 
Published: June 2010

AbstractTwo rate processes characterize the helix‐random coil transition in synthetic polypeptides. A slow rate is obtained by an interpretation of separate helix and random coil peaks in the NMR spectra. A faster rate is obtained from relaxation time measurements which utilize rapid reaction techniques. The present model describes the time dependent behavior of polypeptides in the transition region by combining random walk probabilities with conventional equilibrium distributions of conformations. Average conformational lifetimes for each peptide unit were calculated for molecules containing up to 150 units. These results indicate that in the transition region no helical unit is sufficiently long lived to be individually distinguishable by NMR. Spectra are simulated using a two site model for exchanging systems. On the basis of these model calculations the two peaks observed in the NMR spectra are attributed to (1) the all random coil species and (2) all other conformations containing at least one helix unit.
Dilute solution viscosity and conformation of poly‐n‐hexyl isocyanate
Authors: Berger M. N. , Tidswell B. M. 
Published: June 2010

AbstractPoly‐n‐hexyl isocyanate, prepared according to the method of Shashoua, was fractionated by a coacervation technique. Light scattering and osmometric measurements show that the molecules are enormously extended in solution and can be described in terms of the Porod‐Kratky worm‐like model.Dilute solution viscosity measurements were carried out in a series of solvents and solvent/nonsolvent theta mixtures. Mark‐Houwink constants are reported in which the exponent is very high (a > 1) for both pure solvents and theta mixtures. In nonpolar solvents the polymer molecules are too stiff to respond to long‐range thermodynamic forces. The chains relax somewhat in chloroform and on the addition of trifluoroacetic acid due to a change in the effect of short‐range forces. [η] decreases with increasing temperature indicating that the more compact conformations of the molecules have higher energy states than the more extended ones.
Copolymerization and structure of copolymers of α, β‐unsaturated aldehydes with styrene
Authors: Kálal J. , Houska M. , Seyček O. , Adámek P. 
Published: June 2010

AbstractThe radical bulk copolymerization of styrene with acrolein, methacrolein, and α‐ethyl‐acrolein is studied and monomer reactivity ratios are determined. Employing the dilatometric method the overall rate of polymerization is measured and discussed with respect to the special structure of copolymers. Quantitative IR spectroscopy is used to elucidate the structure of copolymers, and the absorption behavior of copolymers is compared with the statistical theory on sequence length distribution.
Polypeptides at the air‐water interface and in solution
Authors: Birdi K. S. , Fasman Gerald D. 
Published: June 2010

AbstractMonomolecular films of various polypeptides spread at the air‐water interface have been investigated. The conformation in the solid state (e.g., a‐helical or β‐structure) was known for all the polymers studied. In the surface pressure‐area isotherms the a‐helical polymers gave a distinct flat plateau, whereas the β‐form polymers showed only an inflexion. A correlation between the collapse pressure and the stability in solution for β‐form polymers was found, while no similar relationship was observed for the a‐helical polymers. Surface entropy and enthalpy were determined for each polymer and these results indicated that the polypeptides retained the same conformation in the monolayer as in the solid state, i.e., a‐helical or β‐structure. For a‐form polymers a correlation exists between the intermolecular forces in solution and the cohesive forces at the interface. The lack of such a correlation for a‐helical polypeptides thus provides a means of determining the conformation of polypeptides and proteins at the interface.
Thermodynamic properties of liquids, including solutions. VIII. Comparison of properties of several polymer and solvent systems
Authors: Huggins Maurice L. 
Published: June 2010

AbstractThe writer's new theory of solutions has, so far, been applied in detail to experimental data from four types of polymer solutions: rubber plus benzene, polyisobutylene plus benzene, polyisobutylene plus cyclohexane, and poly(propylene oxide) plus carbon tetrachloride. The parameters deduced for these systems are here compared and interpreted in terms of molecular and intermolecular properties. The effects of these parameters on the magnitudes and concentration dependence of the related thermodynamic properties (especially the interaction coefficient χ and its enthalpy and entropy components) are discussed and illustrated.
Precipitation of polymers from solution in the presence of an electrical field
Authors: Krozer S. , Cantow H‐J. 
Published: June 2010

AbstractSome polymers and copolymers were precipitated from solutions by nonsolvent addition with and without stirring as well as by temperature lowering. It turned out that the precipitate is charged in all these cases. Stirring yields an increase of the charge quantity. The charging of the precipitate in experiments without stirring being a result of electron transfer between the solvent and polymer rich precipitate phase is probably due to the density fluctuations. The precipitation by stirring was used for preparative purposes to remove the precipitated drops from solution by fractionation. Deviation in composition observed in different fractions of copolymers could influence the molecular weight and molecular weight distribution measurements.
Kinetics of destruction of stressed polymers
Authors: Korsukov V. E. , Vettegren V. I. , Novak I. I. 
Published: June 2010

AbstractThe infrared spectroscopy technique (IRS) was used for the investigation of polymer film destruction under the action of tensile stress and temperature.The structure and concentration of the stable chemical compounds has been found. The regularities of their accumulation as a function of time, temperature, and the magnitude of the applied load has been studied.These compounds are assumed to be formed due to the decay of free radicals arising from the breakage of the polymer macromolecules under the action of mechanical stresses and thermal agitation. It has been shown that both at room and at elevated temperatures the kinetics of the accumulation is controlled by the rate of breakage of the macromolecules, but not by that of the occurrence of subsequent reactions.The activation energy of formation of new stable products has been found.
The hydrolytic degradation of polyurethanes
Authors: Cohen J. L. , Van Aartsen J. J. 
Published: June 2010

AbstractThe hydrolytic degradation of poly(ester)‐urethanes (PUR) has been studied by viscometry of solutions of films that were treated for various times with water of fixed temperature.On the basis of some simplifying assumptions a general expression was derived relating the decay of the limiting viscosity number (LVN) upon hydrolysis to the reaction rate constant of the hydrolysis reaction.For a homologous series of polymers, with decreasing molecular weight that eventually resulted from the measurements of the hydrolysis, the relation mechanical property versus molecular weight was obtained for four mechanical properties. These relations were interpreted in terms as described previously by Flory and many others.The mechanical properties of hydrolyzed films and those of redissolved and recast hydrolyzed films were compared and showed some interesting features.
Some aspects of γ‐radiolysis of polyisoprene in the form of natural rubber latex
Authors: Ambroź Hanna 
Published: June 2010

AbstractThe most important radiation‐induced reactions of polyisoprene in the form of natural rubber latex are cross‐linking and dislinking. They have been tested in the system with six methane halides at 77, 195, 273, and 328°K. Chlorides are sensitizers of the cross‐linking process within the whole range of temperatures studied. Bromine and iodine derivatives exhibit different features depending on temperature and composition. The activation energies of the cross‐linking process in sensitized and unsensitized systems have been evaluated within the range of high and low temperatures. The dislinking of polyisoprene estimated on the basis of the Charlesby‐Pinner theory varies from 0.3 to 5% of cross‐links, depending on experimental conditions.The role of oxygen in polyisoprene radiolysis and aging resistance of cross‐linked latex films has been also examined.
Oxidation of cellulose by acid‐sodium nitrite systems
Authors: Arendt J. H. , Carrière J. P. , Bouchez P. , Sachetto J. P. 
Published: June 2010

AbstractInvestigations on the oxidation of cellulose by phosphoric acid‐sodium nitrite systems have shown that the specificity of the reaction depends on the concentration of the acid. Supporting evidence has been given by chemical analysis, x‐ray diagrams, and identification of degradation products. The suggestion of a mechanism of oxidation by the nitrosonium ion instead of nitrous acid is supported by experimental results.
Concentration and size effect of microgel particles of acetylcellulose spinning solutions on physicomechanical properties of fibers
Authors: Klenin V. J. , Denisova G. P. , Lyutaya E. N. , Milevskaya R. A. , Kosimtseva L. V. 
Published: June 2010

AbstractThe correlation between physicomechanical properties of fibers and parameters (average size and concentration) of raicrogel particles (MGP) of acetylcellulose (AC) spinning solution has been studied.The average size λ and numerical concentration N (the number of particles in 1 cm3 of solution) of MGPs have been measured by the method of the turbidity spectrum. Fibers and solutions of specially selected industry batches of AC and their mixtures have been studied.The selection of AC batches has been done in the following manner. A chosen pair of batches has approximately equal molecular parameters (molecular weight, polymolecularity, degree of substitution) with different MGP parameters (the turbidity of equiconcentrated AC solutions is extremely varied). Mixtures of different compositions were prepared from batches thus chosen.Thus there was a number of AC samples with size and concentration of MGPs varying in their solutions while the molecular parameters remained constant. Several pairs of AC batches have been chosen. These pairs may vary between each other in their molecular characteristics.Physicomechanical properties of fibers were compared with MGP parameters in AC solutions within each number of samples which had either the same concentration and different sizes of MGPs (one series of comparison) or the same size MGPs but different concentrations (second series).Dependence of physicomechanical properties upon MGP parameters shows that the MGPs form the imperfect regions in fiber structure. This is why an increase of MGP size decreases both strength and breaking elongation of fibers.
The thermal breakdown mechanism of polybenzoxazoles and polybenzothiazoles
Authors: Ehlers G. F. L. , Fisch K. R. , Powell W. R. 
Published: June 2010

AbstractPolybenzoxazoles, when heated up to 660°C in vacuum, lose most of their oxygen as carbon monoxide and carbon dioxide and possibly form Schiff base‐type structures. The carbon required for the formation of carbon monoxide is predominantly supplied by the benzene rings. About one out of every six CN linkages is eliminated as hydrogen cyanide, and some free nitrogen from this group is reduced to ammonia. The elimination of about 1 hydrogen atom per polymer unit (in the form of free hydrogen, hydrogen cyanide, ammonia, and methane) results in free radical formation and crosslinking.Carbon monoxide and carbon dioxide are also some of the major decomposition products of the benzoxazole in poly‐2,2′‐(1,8‐perfluorooctane)‐6,6′‐bibenzoxazole. In addition, fluorine from the perfluoroalkyl chain combines with ring hydrogen to form hydrogen fluoride, which in turn attacks the quartz of the crucible and reaction vessel to form silicon tetrafluoride.A polybenzothiazole, on heating to 625°C, crosslinks with elimination of some hydrogen. One out of every ten sulfur atoms is removed, essentially as hydrogen sulfide, leaving a Schiff‐base linkage; very few of the CN‐groups are eliminated as hydrogen cyanide. Two less stable polybenzothiazole systems, heated to 660 and 670°C, lose about 50% of their sulfur, mainly as hydrogen sulfide, and 20% of their nitrogen as hydrogen cyanide.Flash pyrolysis of model benzoxazoles and benzothiazoles was tried as a means to provide supporting data.
The structural characterization of oxidized celluloses by thermogravimetric analysis
Authors: Pataky B. , Perczel S. , Sachetto J.‐P. 
Published: June 2010

AbstractThe oxidation of cellulose may lead to polymers of more or less complex structure according to the type of oxidizing agent used. No practical methods are available for a quick structural characterization of these compounds. The only reliable method is chemical in nature and very time‐consuming.The object of this paper is to present some results obtained by the thermogravimetric analysis of oxidized celluloses used as a means of characterization for these compounds.
Gel permeation chromatography: Efficiency and operational variables
Authors: Chuang Jau‐Yi , Cooper Anthony R. , Johnson Julian F. 
Published: June 2010

AbstractA variety of expressions for measuring efficiency and resolution in gel permeation chromatography have been used. These are compared and evaluated. In general, there are large differences for all of these methods as a function of molecular weight. Further, at the same molecular weights the various equations do not always rank columns in the same order. This complicates the problem of selecting the optimum operational variables. The various expressions are compared as a function of flow rate and temperature.
A method for characterization of long–chain branched polymers by GPC and intrinsic viscosity
Authors: Kraus Gerard , Stacy C. J. 
Published: June 2010

AbstractThe authors previously reported a method for molecular weight and long‐chain branching distributions using GPC and intrinsic viscosity measurements on whole polymer and fractions [1]. The method does not require prior knowledge of the functional relationship between branching frequency and molecular weight. Applications included “randomly” branched polymers, prepared by free radical initiation, and anionically polymerized macromolecules branched by various techniques leading to specific distributions of branches. Additional examples are given in the present report. The technique has been applied both without and with GPC band spreading correction. The effect of this correction on the resulting estimate of the long‐chain branching distribution is insignificant.When the method is applied to whole polymer only (without preparative fractionation), an average branching factor is obtained, which serves as a useful partial characterization of long‐chain branching. Use of this alternative is discussed and illustrated with examples of low and high density polyethylene, branched styrene‐butadiene block copolymers, star‐branched polybutadiene and others. For the special case of statistically random branching, the average branching factor may be converted to a branching frequency by a straightforward computer program, leading to a description of branching similar to that of Drott and Mendelson [2].
Supramolecular structural transitions in polypeptide solutions: An NMR study
Authors: Hines William A. , Samulski Edward T. 
Published: June 2010

AbstractProton NMR spin‐lattice relaxation times are reported for the solvent in the two component lyotropic liquid crystal, dichloromethane plus poly(γ‐benzyl‐L‐glutamate). The relaxation times become Larmor frequency dependent on passing from the isotropic solution to the liquid crystal. A simple model based on the supramolecular structure in the liquid crystal exhibits orientational fluctuations in the radio frequency region suggesting that spin‐lattice relaxation of the polypeptide protons should also show frequency dependence.
Structure of poly (n‐alkylacrylates) gels
Authors: Tal'Roze R. V. , Shibaev V. P. , Platé N. A. 
Published: June 2010

AbstractThe crystallization of polymers with long side chains proceeds in the form of hexagonal crystal lattice, regardless of the configuration of the backbone chain. For these polymers the ordered structure in the crystalline state as well as in the melt is typical. The tendency to form an ordered structure takes place in the solutions of branched polymers. At polymer critical concentration about 0.5 wt % the gelation process occurred.The structure and properties of gels of acrylic polymers (i.e., polycetylacrylate) in n‐aliphatic alcohols (C7H15OH–C12H25OH) and hydrocarbons (C10H22–C16H34) were studied. The x‐ray patterns show that all the gels have the same interplanar spacings at the wide angles which correspond to the hexagonal cell. The phase transitions and thermodynamic parameters of gel‐solution transition were studied by means of x‐ray and differential thermal analysis. The structure of gels in alcohols and hydrocarbons was found to be different. This difference was explained in terms of specific polymer‐solvent interactions.The kinetics of supermolecular structure formation in gels was investigated using small‐angle light scattering (SMAL) method. Rather good correlation between SMAL and electron microscopic data was found.Some conclusions about the character of the supermolecular structure‐forming process were made and the model of gel structure was proposed.
Studies on swelling and liquid‐induced crystallization of polycarbonates
Authors: Turska E. , Benecki W. 
Published: June 2010

AbstractRate of the diffusion front displacement, rate of sorption and rate of turbidity for the system of bisphenol A polycarbonate foil‐methylisobutylketone were determinated. The influence of the temperature on the observed phenomena was investigated. An attempt at apreliminary interpretation was done.
Principles regulating the morphology of crystallization nuclei in concentrated polymer solutions
Authors: Zurabian R. S. , Baranov V. G. , Frenkel S. 
Published: June 2010

AbstractGeneral expressions are obtained for the nucleation rate taking into account explicitly the contribution of the free energy change when nucleation follows two different mechanisms, both monomolecular and multimoleuclar, i.e., with folded and extended chains.Nucleation with folded chains is the more probable for flexible random coiled macromolecules. However, upon decreasing the degree of coiling without affecting the origin of the uncoiling, the probability of the formation of multimolecular nuclei increases while that of the formation of monomolecular nuclei decreases. At some critical degree of uncoiling a reverse situation as compared to the initial probabilities is attained.A general thermodynamic and thermokinetic interpretation is given of the influence of the equilibrium or induced rigidity on the morphology of crystallization of polymers from solutions and melts.
Angular dependence of the light scattering of polymer solutions at moderate concentrations especially near the consolute point
Authors: Vrij A. 
Published: June 2010

AbstractThe light scattering equation ℋ︁c2(1 + cos2 θ)/Rθ = dΠ/RTdc2 + (16 π2 〈r2〉/3λ2)M2−1 sin2θ/2 for moderately concentrated polymer solutions derived previously from a gradient dependent free energy will here be rederived with the help of the so‐called direct correlation function, C(s), first introduced by Ornstein and Zernike.The equation will be used to calculate 〈r2〉/M2 for polystyrene in cyclohexane near the consolute (or critical) point from light scattering measurements of Debye, Coll, and Woermann in combination with those of Scholte We found [〈r2〉/M2]1/2 = 3.5 (± 0.1) nm mol1/2 g−1/2 for M2 ranging from 8 × 104→ 106 g mol−1.
Polymerization in preoriented media: Polymerization rate of p‐styrene sulfonic acid on poly(cationic) backbones
Authors: Blumstein A. , Kakivaya S. R. , Shah K. R. , Wilkins D. J. 
Published: June 2010

AbstractMany important naturally occurring biochemical reactions such as synthesis of nucleic acids, proteins, and polysaccharides proceed through template polymerization. Such reactions are very complex, and the study of simpler systems is of great interest.A radical initiated polymerization of p‐styrene sulfonic acid anion attached to a poly(quaternary ammonium) backbone (ionene) was studied. The polycationic matrix was prepared from 1–4 diazobi‐cyclo (2.2.2.) octane and 1–4 dibromobutane. It was of low molecular weight (5–10 monomeric units), high rigidity, and could be considered to be in a rod‐like conformation in dilute water solutions. The polymerization of the pSSA‐ionene complex was followed by means of ultraviolet spectroscopy. The overall polymerization kinetics was studied as a function of temperature, concentrations of monomer and initiator, and of the “degree of filling” of the matrix at neutral pH. A rate increase of over two orders of magnitude was observed in the presence of the matrix. This effect was maximum for stoichiometric amounts of monomer and matrix and fell off rapidly for lower monomer‐to‐matrix ratios. The kinetic effect is accompanied by a large decrease in the overall activation energy by 6 kcal/mol.
Graft copolymers from cobalt‐60 pre‐irradiated starch and mixtures of acryl‐amide with N,N,N‐trimethylaminoethyl methacrylate methyl sulfate
Authors: Fanta George F. , Burr Robert C. , Doane W. M. , Russell C. R. 
Published: June 2010

AbstractStarch was preirradiated with cobalt‐60 and then allowed to react with mixtures of acrylamide and N,N,N‐trimethylaminoethyl methacrylate methyl sulfate (TMAEMA·MS) in a water solution. The graft copolymers resulting were purified and characterized, and the influence of a number of irradiation and reaction variables on graft copolymer composition was determined. Although the reaction of irradiated starch with monomers must be carried out with a monomer solution purged with nitrogen to remove dissolved oxygen, a nitrogen atmosphere over the starch sample during irradiation is not necessary to achieve graft polymerization. With monomer mixtures containing 2, 5, 10, 15, and 50 mol% TMAEMA·MS, the mole percentage of TMAEMA·MS in grafted branches was less than that in the initial monomer mixture, whereas ungrafted polymer generally contained a higher percentage of the cationic monomer than was present initially. A tenfold variation in irradiation dose (0.5–5.0 Mrad) caused only small variations in percent add‐on; however, intrinsic viscosities of grafted branches decreased with increasing irradiation dose. Substituting aqueous ethylene glycol for water as the reaction medium produced more frequent grafts of lower molecular weight. Graft polymerization onto partially degraded starches gave graft copolymers with better water solubility.
The solid‐state properties of ionomers, A comparison of ethylene and styrene based materials
Authors: Eisenberg A. 
Published: June 2010

AbstractA comparative review of properties of ionomers is presented, stressing primarily the similarities and contrasts between ethylene and styrene based materials. In particular, it is shown, that in the concentration region where ion clustering is encountered, the mechanical properties of the two ionomers resemble each other more closely than do those of the styrene ionomers and styrene. The similarity can be seen from a range of experimental results, and several different approaches are described.