bluuebunny/crossref_metadata_2025_split huggingface dataset:
Optical properties of a shear‐deformed thermotropic cellulose derivative
Authors: Navard P. , Zachariades A. E. 
Published: April 2003

AbstractThe band texture of a thermotropic cellulose derivative, trifluoroacetoxypropylcellulose, was studied by optical microscopy. The texture consists of parallel, long, thin, black lines when viewed between crossed polarizers with one polarizer parallel to the shear direction. These lines are perpendicular to the shear direction. Their optical properties indicate that the molecules are cooperatively oriented and this orientation director alternates from 0° to ±45° from the shear direction, with most of them at ±45°. This band phenomenon is identical to that observed with thermotropic nematic copolyesters.
Molecular dynamics of a smectic liquid‐crystalline side‐chain polymer. The dielectric properties of aligned and nonaligned material studied over a wide range of frequency and temperature
Authors: Attard George S. , Moura‐Ramos J. J. , Williams Graham 
Published: April 2003

AbstractThe dielectric relaxation behavior of a nonaligned and an aligned liquid‐crystalline (LC) polymer are reported for the ranges 10−3.5 to 105 Hz and 274–363 K. Multiple processes (δ and α) are observed that follow a Vogel equation for the temperature dependence related to the apparent glass transition temperature. The occurrence of these processes and the variation in their relaxation strengths as sample alignment is changed is interpreted in terms of a molecular theory for the dielectric behavior of a LC polymer that involves the director order parameter Sd, the mesophase order parameter S, the dipole moment components of the mesogenic head groups, and their associated relaxation functions.
Thermal expansion of poly(vinylidene fluoride)
Authors: Nanayakkara S. J. , Orchard G. A. J. , Davies G. R. , Ward I. M. 
Published: April 2003

AbstractThe thermal expansion behavior of oriented poly(vinylidene fluoride) films has been studied over the temperature range −75 to +20°C. Representative high draw, low draw, and voided samples have been examined. For all samples at low temperatures the transverse thermal expansion coefficients, both in the plane of the sheet and perpendicular to it, are similar and have positive values of about 10−4 K−1. In the draw direction the thermal expansion coefficients are much smaller in magnitude and can be either positive or negative, the room temperature values varying in the range +4 × 10−6 K−1 for low draw samples to −14 × 10−6 K−;1 for high draw samples.As the temperature is raised the coefficients also increase but, above the glass transition temperature, the value in the draw direction, α1, shows a rapid fall in value. It is shown that this effect can be related quantitatively to the presence of an internal shrinkage stress. Differences between samples can then be primarily related to differences in the magnitude of this internal stress and to differences in the temperature dependence of the modulus of the sample.
Effect of crack tip sharpness on the strength of vulcanized rubbers
Authors: Lake G. J. , Yeoh O. H. 
Published: April 2003

AbstractThe effect of crack tip sharpness on crack propagation in vulcanized rubbers has been studied. For very sharp cracks, tearing is found to occur on a small scale at very low energies not far above the threshold required for the onset of mechanical crack growth. The “small‐scale” tearing energies show relatively little variation for rubbers that differ widely in tear strength as normally measured. Thus the latter property appears to be strongly influenced by variations in the ability of rubbers to promote tip blunting. The small‐scale tear behavior is of relevance to other fracture phenomena, including cutting by sharp objects and tensile failure. Natural variations in tip sharpness occur during cyclic or time‐dependent mechanical crack growth and influence the form of the crack growth characteristics.
Effect of structure on glass transition temperature of amine crosslinked epoxies
Authors: Bellenger V. , Verdu J. , Morel E. 
Published: April 2003

AbstractAbout 40 epoxide‐amine network polymers with glass transition temperatures ranging from 0 to 232°C were investigated, about one‐third being reported for the first time. The glass transition temperature Tgl of the corresponding linear copolymers was first calculated by use of an additivity law whose physical validity was carefully checked. Then the contribution of crosslink mers was determined, and various physical and empirical approaches of the effect of crosslinking on Tg were compared. The results gave evidence in favor of the DiMarzio approach. A predictive relationship based on these considerations is proposed.
A new microwave dielectric method for studying photoinitiated chain radical polymerization processes
Authors: Carlini Carlo , Ciardelli Francesco , Rolla Pier Angelo , Tombari Elpidio 
Published: April 2003

AbstractMeasurements of both real and imaginary parts of the dielectric constant at a fixed microwave frequency have been performed on n‐butyl acrylate/poly(n‐butyl acrylate) mixtures. The dielectric constant of the mixtures has been compared with that of neat n‐butyl acrylate during UV photoinitiated polymerization. The values of the imaginary part of the dielectric constant thus obtained have allowed determination of the instantaneous monomer concentration and verified the kinetic equation for the photoinitiated polymerization process. The limits of validity of the method as well as its capability of providing detailed kinetic information are also discussed.
Chemical healing in partially cross‐linked polyamides
Authors: Fakirov S. , Avramova N. 
Published: April 2003

AbstractEvidence is presented for chemical healing—healing resulting from chemical reactions between neighboring macromolecules located at the interface surface—which has recently been reported in linear polycondensates. In order to distinguish the contribution of the solid‐state reaction from the healing effect, samples (nylon‐11, nylon‐12, and nylon‐6,6) with strongly restricted chain mobility (by crystallization and chemical cross‐linking in the amorphous zones) have been used. It has been found that under the same healing conditions the cross‐linked samples display better welding than the un‐cross‐linked ones. The conclusion is drawn that the observed healing effect is due mainly to the solid‐state chemical reactions at the contact surfaces and not to the diffusion of macromolecules. The results obtained can be considered as an indication of the existence of “chemically released diffusion” in the solid state, i.e., motion of chain segments owing to the interchain and/or intrachain chemical reactions, mainly of the exchange type.
The solid‐state decomposition and oxidation of dicobaltoctacarbonyl in a polymer matrix
Authors: Tannenbaum R. , Flenniken C. L. , Goldberg E. P. 
Published: April 2003

AbstractThe decomposition of Co2(CO)8 in hydrocarbon solutions, to Co4(CO)12 and further to metallic Co and carbon monoxide under an inert atmosphere, has been extensively studied and is well documented in the literature. We report here a study of the solid‐state decomposition of Co2(CO)8 in a polymeric matrix under various conditions. Co2(CO)8 was incorporated into polystyrene by film casting from CH2Cl2 solutions under CO at room temperature. These films were decomposed at 90°C under N2. The decomposition rate of the Co2(CO)8 in polystyrene films was two orders of magnitude slower than for hydrocarbon solution decompositions.Conversely, the oxidation of Co2(CO)8 to CoO or Co2O3 in air at room temperature was observed to be two orders of magnitude faster in the polystyrene matrix as compared with solutions. When protective layers of polystyrene were cast on both sides of the Co2(CO)8‐polystyrene films, oxidation became slower as a function of the thickness of the protective polymer layers.These observations, supported by infrared spectra, TEM micrographs, and electron diffraction patterns, are discussed to compare the solution and solid‐state chemistry of Co2(CO)8.
Theoretical aspects of small‐angle neutron scattering from multiphase polymers
Authors: Quan X. , Koberstein J. T. 
Published: April 2003

AbstractA molecular theory for small‐angle neutron scattering from polymer mixtures is reviewed and extended to consider multiphase polymer systems such as block copolymers and their blends with homopolymers. Methods are developed for the isolation of scattering functions for individual components in these blends. These methods rely on two contrast‐matching techniques: the concept of “composition matching,” where a mixture of deuterium‐labeled and protonated species is used to match the contrast of a third component; and the synthesis of “phase‐matched” block copolymers, where the contrast of the block copolymer sequences are matched. Methods are discussed specifically for the isolation of single chain and single sequence scattering functions for diblock and triblock copolymers, their blends with homopolymers, and star copolymers.
Solution properties of ionomers. I. Counterion effect
Authors: Hara M. , Lee A. H. , Wu J. 
Published: April 2003

AbstractThe effect of counterions on the solution properties of two types of ionomers, one based on sulfonated polystyrene and the other based on styrene–methacrylic acid copolymer, was studied by viscosity and light scattering measurements. It was found that the order of counterion binding of ionomers in a polar solvent and the order of aggregation of ionomers in a low‐polarity solvent were the same for the same ionomer system. However, the order for the sulfonated ionomer was Li < Na < K < Cs, whereas that for the carboxylated ionomer was the opposite. This can be explained by a difference in desolvation during anion–cation interaction and by considering site‐binding in a polar solvent and the association of ion pairs in a low‐polarity solvent. These findings for ionomer systems are parallel to the association behavior of small ions in water, cation affinity in crosslinked resins, and counterion binding of polyelectrolytes in water.
The effects of polydispersity, entanglements, and crosslinks on pulsed‐gradient NMR diffusion experiments in polymer melts
Authors: von Meerwall E. , Palunas P. 
Published: April 2003

AbstractWe have used the pulsed‐gradient spin‐echo NMR method to measure diffusion in a highly polydisperse polymer, a hydroxyl‐terminated polybutadiene. We have modelled the expected spin‐echo attenuation assuming that the low‐molecular‐weight fractions are unentangled and constitute a Rouse‐like diluent to the entangled larger molecules. Fitting this simulation to our data is consistent with the model and permits the extraction of the range of diffusivities and a semiquantitative value for the molecular weight above which entanglements inhibit diffusion. The model is extended to include a monodisperse diluent present additionally, e.g., a curing agent. This extension is tentatively applied to polybutadiene resins undergoing curing and shows that the diffusivity of the remaining unreacted curing agent is little affected by network formation, whereas the retardation of the unreacted polymer molecules is clearly evident.
Miscibility of bisphenol‐A polycarbonate with poly(methyl methacrylate)
Authors: Chiou J. S. , Barlow J. W. , Paul D. R. 
Published: April 2003

AbstractThe miscibility of bisphenol‐A polycarbonate (PC) with poly(methyl methacrylate) (PMMA) has been reexamined using differential scanning calorimetry (DSC) and optical indications for phase separation on heating, i.e., lower critical solution temperature (LCST) behavior. Various methods have been used to prepare the blends including methylene chloride (CH2Cl2) and tetrahydrofuran (THF) solution casting, melt mixing, and precipitation of PC and PMMA simultaneously from THF solution by using the nonsolvents methanol and heptane. It is shown that the resulting phase behavior for PC/PMMA blends is strongly affected by the blend preparation method. However, these blends are miscible over the whole blend composition range (unambiguous single composition‐dependent Tg's and LCST behavior) when prepared by precipitation from solution using heptane as the nonsolvent. To the contrary, solution‐cast and melt‐mixed PC/PMMA blends were all phase separated, which may be attributed to the “solvent” effect and LCST behavior, respectively, not discovered in previous reports. Methanol precipitation does not lead to fully mixed blends, which demonstrates the importance of the choice of nonsolvent when using the precipitation method.
Translational diffusion of relatively short polystyrene chains in cyclohexane. II. An interpretation of hydrodynamic radius with the freely jointed chain model
Authors: Sun C. K. Jacob , Maa Y. F. , Chen S. H. 
Published: April 2003

AbstractThe translational diffusion coefficients for three relatively short, nearly monodisperse polystyrene chains (with molecular weights 2000, 4000, and 9000) in liquid cyclohexane have been measured up to 523.2 K using the extended Taylor dispersion method. The resultant hydrodynamic radii are successfully interpreted with the freely jointed chain model corrected for the approximations intrinsic to Kirkwood's theory with Stokes' radius as the only adjustable parameter, which is found to decrease with chain length but approach a constant value for long enough chains. The experimentally observed hydrodynamic radii of polystyrene standards with molecular weights 17,500, 50,000 and 100,000 at 308.2 K conform quite well to the values predicted with the approach formulated in this work.
Nonsteady‐state kinetics for anionic polymerization of polar monomer initiated instantaneously by electron transfer
Authors: Wang Zhiyu , Yan Deyue 
Published: April 2003

AbstractWith the help of the nonsteady‐state method, the kinetic differential equations for the anionic polymerization of polar monomer initiated instantaneously by electron transfer with monomer termination are treated rigorously. The expressions for the molecular weight distribution, the number‐average and weight‐average degrees of polymerization, and the functionality distribution are derived in closed form. A theoretical means is established by which all the molecular parameters of the polymer can be calculated from polymerization conditions, such as the reaction constants, the concentrations of initiator and monomer, and the monomer conversion.
On the modeling of small‐angle X‐ray scattering from systems of oriented fibrils
Authors: Cohen Yachin , Thomas Edwin L. 
Published: April 2003

AbstractA method is outlined for the simulation of the scattering pattern from systems of oriented fibrils using the radial distribution function of a hard‐disc fluid obtained under the Percus–Yevick approximation. In this manner both the diameter and the volume fraction of the fibrils may be estimated directly from the scattering pattern. The effect of polydispersity in the distribution of fibril diameters on the position of the scattering peak is discussed, suggesting that an estimate of the volume fraction from the peak position alone, such as in a modified Bragg's law, may be inaccurate.
Associating polymers in a well‐defined extensional flow field: Influence of solvent quality and polymer concentration
Authors: Peiffer D. G. , Kim M. W. , Schulz D. N. 
Published: April 2003

AbstractThe conformational response of an associating‐type random coil macromolecule in solution was investigated utilizing an opposing jet device. This device, capable of generating a well‐defined elongational flow field, is quite useful for probing intra‐ and intermolecular interactions of lightly sulfonated polystyrene ionomers in both nonpolar and polar solvent systems. Below a critical concentration in nonpolar media, such ionomers qualitatively follow trends predicted by dilute solution theory, although intramolecular ionic associations markedly increase the critical elongational shear rate. With further increases in concentration, the extensional behavior is determined by the initial formation of relatively strong intermolecular associations. At even higher polymer concentrations, a third regime is observed where the conformational relaxation process becomes even more facile. On the contrary, in a polar solvent, the conformational relaxation process is markedly enhanced (i.e., critical elongational shear rate is reduced) due to the polyelectrolyte effect, i.e., dissociation of a significant level of the counterions. The effect of this dissociation process influences the relaxation process over the entire concentration region examined. These findings are compared directly with solution rheology, where in low polarity solvents the reduced viscosity is markedly diminished by ion pair‐type interactions, and in more polar environments the reduced viscosity is enhanced due to the dissociation of the counterions from the vicinity of the chain backbone.
Chain orientation and intrinsic anisotropy in birefringence‐free polymer blends
Authors: Saito Hiromu , Inoue Takashi 
Published: April 2003

AbstractWe found 10 miscible pairs of dissimilar polymers that rendered birefringence‐free polymer blends. Each pair consists of a positive birefringence polymer and a negative one. Zero birefringence in the blend is attained as the result of compensation of positive and negative contributions to the overall birefringence. This concept was confirmed by characterizing the chain orientation of individual chains in the blend using polarized Fourier‐transform infrared spectroscopy and by estimating theoretically the intrinsic optical anisotropy of the chain molecules.
Crosslinking of poly(arylene ether ketones). II. Crystallization kinetics
Authors: Chan C.‐M. , Venkatraman S. 
Published: April 2003

AbstractResults of crystallization kinetics study and x‐ray diffraction are in good agreement with our previous rheological and mechanical results. Poly(arylene ether ketones), which are modified by sulfur at concentrations below approximately 2% by weight, have higher crystallization rates owing to the decrease in molecular weight caused by chain scission and/or the presence of sulfur that acts as a nucleating agent. However, all the observed differences in mechanical properties can be rationalized only by the nucleation effect of the sulfur. Upon aging of the sulfur‐modified samples, cross linking occurs, leading to decreases in crystallization rates.
Relating creep and creep rupture in PMMA using a reduced variable approach
Authors: Crissman J. M. , McKenna G. B. 
Published: April 2003

AbstractCreep and creep rupture of PMMA at high stresses have been characterized and found to be relatable by use of reduced variables. It is shown that when the creep compliances can be correlated by a superposition principle for which the vertical shift is the ratio of the applied stress to a reference stress and when strain at failure is a constant, a commonly used failure criterion (that the product of the strain rate at failure and the time to failure is constant) becomes valid. The reduced variables approach is found to apply to two greatly different thermal histories. Consistent with the concept of physical aging, the response of a quenched sample is simply shifted along the log time axis to shorter times relative to the response of the aged sample.
Low‐angle electron diffraction from high temperature polystyrene crazes
Authors: Berger Larry L. , Buckley Donald J. , Kramer Edward J. , Brown Hugh R. , Bubeck Robert A. 
Published: April 2003

AbstractLow‐angle electron diffraction (LAED) was used to study the microstructure of crazes produced at different temperatures T and strain rates in thin films of monodisperse polystyrene (PS). At a slow strain rate of 4.1 × 10−6 s−1 both the fibril diameter D and the fibril spacing D0 of crazes in 1800k molecular weight PS remained constant with temperature up to T ≈ 70°C and then sharply increased as T approaches Tg. At a higher strain rate of ∼ 10−2 s−1, both D and D0 increase only slightly with T. The values of D and D0 over a range of temperature are in very good agreement with those values obtained in bulk samples using small‐angle x‐ray scattering. The crazing stress was measured as a function of temperature in the thin films of the 1800k molecular weight PS strained at the same slow strain rate used for the LAED measurements. These measurements were analyzed using a simple model of craze growth to reveal the temperature and strain rate dependence of the craze surface energy Γ. At room temperature Γ ≈ 0.076 J/m2 (versus Γ ≈ 0.087 J/m2 predicted) and was observed to remain constant up to T ≈ 70°C and then decrease by approximately a factor of two at T = 90°C. This decrease in Γ is believed to result from chain disentanglement to form fibril surfaces at sufficiently high temperatures and occurs in the same temperature range in which the craze fibril extension ratio λ was observed to increase.
Gas transport in a thermotropic liquid‐crystalline polyester
Authors: Chiou J. S. , Paul D. R. 
Published: April 2003

AbstractA preliminary report is given on the gas transport characteristics of a thermotropic liquid‐crys talline copolyester that is fully aromatic. At 35°C the permeability coefficients for He, H2, O2, Ar, N2, and CO2 in this polymer are comparable to or smaller than those for polyacrylonitrile, which is one of the least permeable polymers known. This low permeability seems to stem from low solubility of these gases in the liquid‐crystalline polymer rather than low transport mobility. Activation energies for transport are somewhat larger than those found for conventional amorphous polymers. Determination of the detailed mechanism of transport in polymers with liquid‐crystalline order requires further study.
Submillimeter infrared absorption and chain conformations of poly(vinylidene fluoride) and copolymers. II. Phase transition analysis
Authors: Latour M. , Moreira R. L. 
Published: April 2003

AbstractAbsorption spectra, and hence the dielectric properties, of poly(vinylidene fluoride‐tri‐fluoroethylene) copolymers have been obtained by far infrared spectroscopy in the frequency range 15–55 cm−1. We have studied the evolutions of the absorption coefficient and imaginary part of the dielectric constant as a function of poling field and copolymer composition. During the phase transition from form II to form I, the whole absorption presents a strong increase. This behavior is explained by polarizability changes at phase transition.