bluuebunny/crossref_metadata_2025_split huggingface dataset:
Photoelastic behavior of poly(methacrylamide) and poly(methacrylamide‐co‐2‐hydroxyethyl methacrylate) gels
Authors: Ilavský M. , Dušek K. 
Published: June 2010

AbstractThe photoelastic behavior of swollen gels prepared by the crosslinking copolymerization of methacrylamide (MA), or of MA and 2‐hydroxyethyl methacrylate (HEMA), with methylene bisacrylamide as the crosslinking agent was investigated in water and in aqueous solutions of urea and magnesium perchlorate. Stress and birefringence could be separated into a time‐ and deformation‐dependent part; in all cases the stress‐optical coefficient,Ce, was independent of time and deformation. The characteristic features of the photoelastic behavior may be summarized as follows: (1) Time dependences of stress and birefringence at constant deformation increase with increasing MA content in the copolymer and are not reduced to any considerable degree even by compounds which break the water structure; (2) The equilibrium modulus related to the dry stateGedincreases with increasing MA content owing to an increase in the Mooney–Rivlin constantC2; the increase in swelling due to an addition of urea or Mg(ClO4)2does not affectGedfor MA‐rich gels; (3)Cedecreases with increasing content of MA, increasing content of the crosslinking agent, and decreasing dilution during the polymerization; (4) Mg(ClO4)2and urea have only a small effect on the optical anisotropy of the statistical segment δα. The above results are interpreted by a decrease in the importance of correlation structures originating in hydrophobic interactions, and by the increase in the role of hydrogen bonds in the transition from poly‐HEMA to poly‐MA.
The dependence of dielectric properties on crosslinking density of rubbers
Authors: Bakule R. , Havránek A. 
Published: June 2010

AbstractThe dielectric properties of natural rubber, synthetic polyisoprene, and polybutadiene samples crosslinked with sulfur and of natural rubber samples crosslinked simultaneously with sulfur and dicumyl peroxide were studied. In the investigated systems, the number of polar groups in unit volume and the crosslinking density may be changed independently in relatively wide ranges. The measurements were performed in the frequency range from 101 to 105 Hz at various temperatures, and macroscopic parameters describing the dielectric properties of the samples were evaluated. The position and intensity of the dielectric dispersion peak in the main transition zone is only slightly dependent on the crosslinking density of the samples. These two values are mainly influenced by the amount of combined sulfur, or more generally, by the number and the dipole moments of polar groups in the sample. The influence of crosslinking density on the width of the absorption curve is very strong; the width increases with increasing crosslinking density. The possibility of explaining this effect in terms of the dependence of the free volume distribution function on crosslinking density is discussed.
Retardation spectra of various cross‐linked rubber systems
Authors: Havránek A. , Bakule R. 
Published: June 2010

AbstractRetardation spectra of crosslinked natural rubber samples were obtained from creep measurements performed in the time range 10−2‐103 sec and in the temperature range 200–360°K. The samples were crosslinked only with sulfur, with sulfur in the presence of antioxidant and accelerators, and simultaneously, with sulfur and dicumyl peroxide. Thus, we have a set of samples with a wide range of crosslinking densities (v = 0.1 – 7 × 10−4 mole/cm3) in which the same crosslinking density may be obtained for samples with different sulfur content. The results of our measurements are as follows: The friction coefficient, standard reference temperature Ts and time position of the spectrum depend on sulfur content, and not on crosslinking density in the system natural rubber‐sulfur‐dicumyl peroxide. The addition of accelerators diminish the friction coefficients, decrease Ts temperatures, and shift the spectrum maxima to shorter times. The spectrum width depends on crosslinking density and not on sulfur content in the measured set of samples. The spectra of more crosslinked samples are narrower. These results are compared with the results of dielectric measurements performed on an analogous set of samples.
Kinetics of heat precipitation of proteins
Authors: Macritchie F. 
Published: June 2010

AbstractThe precipitation of lysozyme at 60°C in pH 9.2 buffer solution followed first‐order kinetics. The first‐order rate constants varied with initial protein concentration possibly due to nonpurity. Heat precipitation of bovine serum albumin (BSA) in pure water did not follow any simple rate law but in pH 5.8 buffer obeyed first‐order kinetics in the initial stages.The extent of aggregation of BSA in the precipitate was followed by dissolving the precipitate in acid solution with subsequent characterization by gel‐permeation chromatography. Addition of small amounts of the sulfhydryl blocking agent, iodoacetamide (twelve times the concentration of protein on a molar basis), reduced the aggregation. Heat precipitation of proteins appears to be governed by the favorable entropy change when interactions between water molecules and nonpolar groups of the protein are decreased.
Polymer photostabilization by surface grafting
Authors: Burchill P. J. , Pinkerton D. M. 
Published: June 2010

AbstractVarious means by which photostabilization of polymers can be achieved are described, and work on a method of photostabilization by surface screening on a substrate polymer is presented. The surface screen is obtained by graft copolymerization of a polymerizable hydroxybenzophenone ultraviolet absorber. The results show that the application of this method to polypropylene and poly(vinyl chloride) is an effective stabilization treatment.
Photooxidation of polystyrene–the role of impurities as prodegradants
Authors: George G. A. , Hodgeman D. K. C. 
Published: June 2010

AbstractThe role of in‐chain peroxides and phenyl alkyl ketone end groups in the initiation of photooxidation of polystyrene is discussed. Using quantitative phosphorescence spectroscopy and viscometric methods, the photolysis and thermolysis of small amounts of in‐chain peroxides present in polystyrene prepared under different oxygen concentrations was investigated. It was shown that in‐chain peroxides are unlikely initiators of photooxidation in polystyrene prepared in high‐temperature commercial syntheses, but will act as the precursor of the photoinitiator, the phenyl alkyl ketone end group.
Radiation‐induced polymerization of vinyl and diene monomers in deoxycholic acid inclusion compounds
Authors: Miyata Mikiji , Takemoto Kiichi 
Published: June 2010

AbstractA series of deoxycholic acid inclusion compounds with vinyl and diene monomers were prepared, and their radiation‐induced polymerization was studied. In the case of 2,3‐dimethyl‐and 2,3‐dichlorobutadienes, highly 1,4‐trans tactic polymers were obtained, while in the case of acrylonitrile, an atactic polymer resulted. Stability of the deoxycholic acid adducts as well as polymerization characteristics are described.
Novel sequential copolymers by elucidating the mechanism of initiation and termination of carbocationic polymerizations
Authors: Kennedy Joseph P. 
Published: June 2010

AbstractElucidation and understanding of the fundamentals of the initiation and termination of cationic olefin polymerization led to the control of these elementary events and in turn to the synthesis of novel sequential (block and graft) copolymers exhibiting unique physical properties. Most recent research concerns olefin polymerizations with BCl3 coinitiator. For example, the polymerization of isobutylene with the H2O/BCl3 initiator system produces macromolecules with tertiary chlorine termini. These end groups, in conjunction with certain alkylaluminum compounds, can be used to initiate the polymerization of a second olefin and thus can lead to new block copolymers. The synthesis of poly(isobutylene‐b‐styrene) by this technique will be discussed.
Study of the thermal oxidation of polyolefins. IV. Formation, characterization, and decomposition of polyethylene hydroperoxide
Authors: Iring M. , Kelen T. , Tüdös F. , László‐Hedvig S. 
Published: June 2010

AbstractA study was made on the structure and decomposition rate of polyethylene hydroperoxide obtained by the initiated oxidation of low density polyethylene. It was established that the decomposition is not a simple unimolecular process. The rate was found to be dependent on the amount of carbonyl groups formed during the synthesis.
Activation energy and critical concentration of peroxy radicals in low‐temperature oxidation of natural rubber and 1,4 cis‐polyisoprene
Authors: Tkáč A. 
Published: June 2010

AbstractA method is presented for studying the mechanism of carbonyl group formation in the low‐temperature oxidation of natural rubber and 1,4 cis‐polyisoprene, which does not require the investment of activation energy for the thermal or light scission of the polymer chain. Precise concentrations of t‐butyl peroxy radicals stabilized by coordination on Co (III), are used for initiation of the primary polymer radicals ˜Ṙ˜ after abstraction of an α‐methylene‐hydrogen. The “critical” threshold levels of peroxy radicals before and during the course of the oxidative degradation is discussed. The sensitivity for radical attack increases with the number of aliphatic double bonds; this is demonstrated using polypropylene and paraffin oil.
Recent advances in mechanisms for the stabilization of polyolefins
Authors: Hawkins W. L. 
Published: June 2010

AbstractOver the past several decades, there has developed a comprehensive mechanism for the stabilization of polyolefins. Degradation can be inhibited at the propagation stage through the use of chain terminators and in the chain‐branching stage with peroxide decomposers. Combinations of chain terminators with peroxide decomposers result in synergism, and mechanisms have been proposed to account for this phenomenon. Recently it has been demonstrated that the catalytic effect of transition metals in promoting oxidation of polyolefins at a polymer‐metal interface is initiated at the metal surface. Metal deactivators, which react by chelating active metal species, also function first at the interface, although in subsequent stages of oxidation, soluble metal salts may penetrate into the matrix and catalysis then may be inhibited in the bulk polymer.Reexamination of the role of carbon black in the stabilization of polyethylene against photo‐oxidation has shown that in the presence of this light screen, reactions of degradation and stabilization more closely resemble those found in thermal oxidation.
Time controlled stabilization of polyolefins
Authors: Scott Gerald 
Published: June 2010

AbstractHydroperoxides are key initiators in both thermal and UV initiated oxidation processes in polymers. The control of hydroperoxide concentration and hence the rate of the oxidation process can be achieved either directly by means of antioxidants which destroy hydroperoxides in nonradical generating reactions (preventive antioxidants) or indirectly by means of compounds which trap free radicals (chain‐breaking antioxidants) leading to a slower rate of hydroperoxide buildup.Transition metal ions which catalyze the breakdown of hydroperoxides to free radicals lead to the rapid achievement of a stationary hydroperoxide concentration and a rapid and initially linear rate of thermal oxidation. Some metal complexing agents delay the achievement of this linear rate.The effect of UV light in photooxidation of polymers has a similar effect to metal ions in thermal oxidation and the combined effect of both transition metal ions and UV irradiation can be catastrophic in polyolefins. However, certain metal ion complexing agents behave both as thermal oxidative stabilizers and as UV stabilizers due to a combination of different kinds of activity in the same molecule. These include metal deactivation, UV absorption, peroxide decomposition, and radical trapping. The photolytic stability of these complexes ultimately determines the length of the induction period which precedes polymer degradation. In general, nickel and cobalt complexes are UV stable and behave as long term stabilizers, whereas iron complexes are photolytically unstable and although they give rise to a period of initial stabilization which is related to their concentration, they photolyze to become effective photoinitiators.
Uni‐ and biaxial orientation of polymer films and sheets
Authors: de Vries A. J. , Bonnebat C. , Beautemps J. 
Published: June 2010

AbstractUni‐ and biaxial stretching of various polymer films has been studied under well‐defined conditions of temperature and elongational strain rate in order to determine the relationship between stress and recoverable strain for both modes of deformation. The extent of molecular orientation has been investigated with the aid of stress optical methods: In amorphous polymers birefringence was found to be directly proportional to the frozen‐in internal entropic stress irrespective of the latter's relationship with recoverable strain. In a first approximation, molecular orientation can be understood in terms of deformation of a rubberlike network with temporary junction points. The total internal stress in an oriented glassy polymer may be significantly larger than the entropic stress. Experimental methods based on retractive force measurements, able to distinguish between internal stresses of different nature, are described.In uni‐ and biaxially drawn films of polyethylene terephtalate preferred planar orientation of (100) planes has been studied by means of X‐ray diffraction and measurement of the three principal refractive indices. The intrinsic birefringence of completely oriented PET films has been estimated from these measurements with the aid of the Lorentz‐Lorenz theory. The effect of draw ratio and temperature on the rate of crystallization and its consequences (increase of stretching stress and density, decrease of shrinkage) has also been studied.Finally, the effect of molecular orientation on various mechanical properties: modulus, tensile strength, impact resistance, creep compliance, is discussed for both amorphous and semicrystalline polymers, with special emphasis on the predominant influence of amorphous phase orientation.
Direct observation of structure in high‐modulus aromatic fibers
Authors: Dobb M. G. , Johnson D. J. , Saville B. P. 
Published: June 2010

AbstractA detailed structural characterization of three aromatic polyamide (PPT) fibers has been carried out by means of direct electron microscope observations and X‐ray and electron diffraction analysis. Data obtained by the diffraction techniques are supplemented by high‐resolution images recorded from fragmented specimens, which provide both a qualitative assessment and quantitative information in terms of crystallite perfection and the distribution of crystallite size. Dark‐field images suggest major differences between the macrostructures of various PPT fibers; indeed some specimens are composed of long‐range axially periodic assemblies, while others appear to be homogeneous. Such differences in organization may well account for variations in tensile behavior.
Small‐angle light scattering from polymer films with rod‐like superstructure—effects of volume fraction and orientation
Authors: Chu W. H. , Yoon D. Y. 
Published: June 2010

AbstractThe light scattering patterns from anisotropic rods have been calculated as a function of the degree of volume filling by these rods, and also as functions of their orientation distribution and the resulting birefringence effect in biaxially oriented polymer films. The resultant theoretical calculations compare well with available experimental results and, thus, demonstrate their applicability to the study of the course of crystallization and the structural characterization of biaxially oriented polymer films.
Computer analysis of light scattering data. II. Critical assessment of molecular parameters
Authors: Roberts Kenneth R. , Torkington John A. , Gordon Manfred , Ross‐Murphy Simon B. 
Published: June 2010

AbstractThree different Fortran computer programs based upon different regression models have been applied to the computer analysis of light scattering data by variously weighted linear and nonlinear least‐squares methods to calculate the molecular parameters, viz., (MW)w,<S2>z. etc. for standard polystyrene samples in good solvents (benzene and toluene). Comparison of the results from the computer programs provides greater insight into the theoretical validity of these parameters, and leads to the conclusion that standardization of the data processing is an essential part of any “Round Robin” experiment to evaluate the molecular weight of standard polymer samples in different laboratories.
Anisotropic light scattering by solutions of large macromolecules oriented in the intense electric fields of laser beams
Authors: Dȩbska‐Kotłowska M. , Kielich S. 
Published: June 2010

AbstractLight scattering by monodisperse solutions of rigid anisotropic rod‐like macromolecules of lengthl, of the same order as the wavelength λ, reoriented in the strong electric field of linearly polarized laser light is discussed. This discussion is based on numerical calculations of the relative variations in intensity δH, δHδV, δVof the light scattering components, induced by a laser beam of high intensityIpolarized horizontally or vertically to the plane of observation. Calculations are made for various values of q(ωL) = [(a–a)/a)I/2kT] (the reorientation parameter of the optical polarizability ellipsoid with principal polarizabilitiesa,afor the laser frequency ωL) and for the three valuesl/δ = 0.5, 1, and 2.
Thermodynamics of polymer mixtures
Authors: Koningsveld R. , Kleintjens L. A. 
Published: June 2010

AbstractThe compatibility of mixtures of homopolymers, mixtures of the same polymer differing in chain length, and mixtures of random copolymers is discussed from a thermodynamic standpoint. Small deviations from the Flory–Huggins combinatory entropy of mixing reveal themselves markedly in phase diagrams and second virial coefficients. Suitable experimental methods such as Pulse Induced Critical Scattering, neutron scattering, and low‐speed analytical centrifugation are discussed briefly. The molecular origins of phenomena discussed can at present best be traced for mixtures of homopolymers. The mutual influence the constituents exert on each other's chain flexibility seems to manifest itself in the shape of the miscibility gap. The other types of mixture allow less well developed theoretical description, due to lack of experimental data.
Hydrodynamics of block copolymers
Authors: Price Fraser P. 
Published: June 2010

AbstractIn order to gain insight into the viscosity behavior of linear block copolymers, formulas have been developed for the calculation of radii of gyration of block copolymers whose blocks deviate from the theta condition by any given degree. In the course of this study the radii of gyration of sets of centrosymmetric arrays have been studied and the mean separation between the center of mass and the end of a random chain has been calculated. The results show that for AB block copolymers of fixed degree of polymerization (DP), the intrinsic viscosity varies in a monotonic, sigmoidal fashion when the composition is varied from homopolymer A to homopolymer B. The magnitude of the effect depends upon the relative stiffness of the A and B segments and upon the total DP. In ABA copolymers, on the other hand, at constant DP, varying the composition from all A to all B usually leads to a composition‐intrinsic viscosity relationship containing maxima and minima. As A becomes increasingly stiff compared to B, this relationship tends towards a monotonic one. This behavior is surprising and should be checked experimentally.
The search for spinodal decomposition in binary systems of high molecular weight polymers
Authors: Price Fraser P. 
Published: June 2010

AbstractPhase transformations associated with spinoidal decompositions are reviewed. Such phenomena associated with inorganic glasses are considered for the case of amorphous binary systems of organic high polymers. The thermodynamic and experimental considerations are projected and sample systems selected and tested. Results are presented for poly 1, 4(2, 6 dimethylphenylene oxide) plus a copolymer of styrene and orthochlorostyrene and also for this plasticized binary system.
Blends of poly(ϵ‐Caprolactone) with poly(vinyl chloride). II. Crystallization kinetics
Authors: Ong C. J. , Price F. P. 
Published: June 2010

AbstractCrystallization kinetics of poly(e‐caprolactone) from the blends of poly(e‐caprolactone (PCL) and poly(vinyl chloride) (PVC) were investigated. The isothermal spherulitic growth rates were measured with an optical microscope. Linear growth rates were obtained for all samples at temperatures between 20° and 35°C. The introduction of PVC into PCL resulted in a reduction in spherulitic growth rate. The overall crystallization rates were measured with a density gradient column. The Avrami equation was employed to analyze the isothermal crystallization kinetics. It was found that the value of Avrami constant k decreases markedly with increasing PVC concentration. However, the Avrami constant n has a value in the neighborhood of 3, almost independent of the composition. In light of these results, it is suggested that, for these blends, the crystallization proceeds by heterogeneous nucleation, followed by a three‐dimensional growth.
Mean chain length distributions in finite polycondensing systems
Authors: Donoghue Edward , Gibbs Julian H. 
Published: June 2010

AbstractThe current expressions for the molecular size distributions in certain polycondensing systems of linear or branched‐chain polymers were derived by Flory and by Stockmayer in the limit of infinitely large systems. These expressions are experimentally verified in the sol. But the expressions for the distributions of nonlinear polymers, although manifesting the observed gel point, do not describe a gel phase. With a view towards ultimately deriving expressions that do describe the gel, a probabilistic theory of condensation polymerization in finite systems is introduced here. The theory is based on Stockmayer's statistical weight factor, ω, and, as presented, applies to the elementary system composed of chemically identical structural units, each bearing two or more chemically identical functional groups. The probability density function for molecular size distributions is constructed from ω, and expressions for the mean number of k‐mers and the variance about this mean are derived from it. A generating function formalism for evaluating these expressions exactly is exhibited in the elementary case of linear polymers. Asymptotic expansions of the exact expressions for the means and variances are then obtained in the large system limit. The expansion for the means shows that the Flory‐Stockmayer expression represents the distribution of linear polymers accurately—in finite systems—only at low molecular weights. The expansion for the variances justifies—at low molecular weights—the expectation that observed distributions differ negligibly from the mean distribution, but shows that populations of higher molecular weight polymers deviate markedly from their means.
Inorganic fragments in graft and block copolymers: A review of solved and unsolved problems
Authors: Haiduc Ionel 
Published: June 2010

AbstractIn the first part of the paper some problems related to the formation of inorganic chains in polymeric backbones are discussed and the difficulties in the use of polymerization reactions in inorganic polymer chemistry are reviewed: the lack of suitable inorganic monomers; the tendency of inorganic ring formation; and the tendency of structure unit redistribution. In the second part several possibilities of introducing inorganic fragments in block and graft copolymers are reviewed.