bluuebunny/crossref_metadata_2025 huggingface dataset:
Morphological heterogeneity of polymer fibers which do not reveal small‐angle x‐ray reflections
Authors: Slutsker Leonid I.
Published: April 2003

For many aliphatic and wholly aromatic polymer fibers, meridional small‐angle x‐ray reflections (SAXR's) are not observed. The morphological features which give rise to this effect have Received little attention in the literature. It is sometimes speculated that this is evidence of a morphological homogeneity along the fiber axis (no fluctuation of electron density–no long periods), ignoring other possible reasons (morphology is heterogeneous but very irregular or the maximum intensity of SAXR (Imax) is low and SAXR cannot be observed against the background of diffuse scattering.) Fibers with homogeneous crystalline morphology should have an elastic modulus Ef close to that of the crystallites Ecr along the polymer chains over a broad range of stresses. But, as was found in this work for the overwhelming majority of such nonreflection fibers (including Kevlar aramid fibers) there is a considerable difference between Ecr and Ef (for some nonreflection fibers the ratio Ecr/Ef is on the order of 10). This means that their morphology is heterogeneous along the fiber axis and contains “weaker” intercrystalline regions. This heterogeneity appeared to be regular enough to give rise to SAXR, as determined from the appearance of SAXR during elastic extension and after the beginning of pyrolysis in some nonreflection fibers. Thus, it was concluded that the lack of observable SAXR for these fibers is a result of the weakness of Imax relative to the diffuse scattering. Support for this statement is seen in the analysis of the decrease of Imax to zero with increasing draw ratio for poly(vinyl alcohol) fibers. The key factors influencing the decrease of Imax in this case were found to be the decrease of fibril diameter (fibril splitting), an increase of the dispersion of the long periods and some possible decrease in the density difference between crystalline and amorphous regions. The particular combinations of such factors can be different for each particular case of missing SAXR and are discussed in this paper.
Sorption of a finite amount of swelling solvent in a glassy polymer
Authors: Cohen Donald S. , Goodhart Charles
Published: April 2003

We study the time history of a diffusing front when a polymer is exposed to a finite amount of penetrant which becomes used up. A class of polymers is considered for which slow molecular relaxation occurs only at or near the glass‐gel interface with instantaneous relaxation both ahead of and behind the progressing front. We show that the position of the penetrant front versus time undergoes a long smooth transition from standard Fickian t1/2 behavior to exponential time decay onto a final equilibrium position attained when all the penetrant is used up.
Compatibility of coils and rods bearing flexible side chains
Authors: Ballauff M.
Published: April 2003

We present a theoretical treatment of nematic‐isotropic phase equilibria in mixtures which consist of random coils and comblike polymers, the latter components being composed of a rigid backbone and flexible side chains. The mixing partition function is evaluated by using the Flory lattice model. The comblike component is characterized by the axial ratio xr of its rigid main chain and the number of flexible side chains z, each containing m segments. The coiled component is described by its number of segments xc. The net exchange energy of mixing is assumed to be zero; i.e., we consider athermal solutions. It is shown that the flexible side chains attached to the rigid main chains markedly enhance the compatibility in the isotropic phase. If the ratio of the volume fraction of the side chains to the volume fraction of the main chains is high enough, there is even a finite range of concentration where the random coils mix homogeneously with the comblike component. This is in contrast to mixtures of rods and coils, which have been shown by Flory to be incompatible over nearly the full range of composition. These conclusions hold true only when ordered states are involved. For comblike polymers with flexible backbones mixed with random coils in isotropic melts, the resulting free energy of mixing is given by the familiar Flory‐Huggins expression.
Ultrasonic and hypersonic dispersion in polysiloxanes
Authors: Onabajo A. , Dorfmüller Th. , Fytas G.
Published: April 2003

We report ultrasonic attenuation and velocity measurements on poly(dimethylsiloxane) (PDMS), poly(phenylmethyl siloxane) (PPMS), and copolymer poly(dimethyl phenylmethyl siloxane) in the temperature range of 10–50°C and frequency 0.3–45 MHz. The present data complement previously reported Brillouin spectra at hypersonic frequencies. Whereas the ultrasonic velocity u0 is virtually independent of frequency, the ultrasonic absorption exhibits strong dispersion which can be ascribed to the viscoelastic normal mode relaxation. The ultrasonic attenuation data for PPMS at low temperatures display an additional relaxation process related to localized segmental motion. This mode is also responsible for the relatively large dispersion of the sound velocity and attenuation in the gigahertz frequency range accessible to the Brillouin scattering experiment. The extended information, which can be extracted by studying hypersonic dispersion, is discussed in detail.
A study of the acoustic mode structure of oriented poly(ethylene)
Authors: Cavanaugh David B.
Published: April 2003

The low‐frequency acoustic vibrational modes of solid poly(ethylene) (PE) oriented by stretching and hydrostatic extrusion are studied with Brillouin spectroscopy. The experimental results evidence a damping of the longitudinal acoustic mode propagating in the direction parallel to the orientation axis of the samples. The results are discussed in terms of the theoretically derived acoustic dispersion relations for the PE crystal.
Fiber spinning from the nematic melt. II. Effect of thermal history on spinning of the copolyester of polyethylene terephthalate and p‐oxybenzoate
Authors: Muramatsu Hiromochi , Krigbaum W. R.
Published: April 2003

Fibers were spun from the nematic phase of the copolymer of polyethylene terephthalate having 60 mol % of p‐oxybenzoate units. A capillary rheometer was used for spinning with a shear rate at the wall of 6.4 sec−1, and capillary (length/diameter) ratio of 14.1. The spinning temperature was varied from 250° to 300°C and, at each temperature, the spin‐draw ratio was examined as a variable. Spinning was performed under two conditions. When spinning from the melt without preheating, the initial modulus of the fibers increased with spin‐draw ratio and increased with increasing spinning temperature for a fixed spin‐draw ratio. In the second case, the melt was preheated and then cooled to the desired temperature before spinning the fibers. The preheating temperature was 280°C for spinning at 250°C, and 300°C for spinning at 280°C. Preheating increased the fiber modulus to the value obtained by spinning at the preheating temperature. A reduction of the viscosity due to the melting of poly(p‐hydroxybenzoic acid) (PHB) crystallites produces better orientation and higher modulus. However, with increasing spin‐draw ratio, the modulus of the preheated fibers decreased to the values expected for the spinning temperature. This decrease in modulus is due to recrystallization of PHB in the threadline.
Studies of magnetic resonance phenomena in polymers. II. Molecular motions in poly(methyl methacrylate) as studied by spin‐probe and spin‐label methods
Authors: Tsay Fun‐Dow , Gupta Amitava
Published: April 2003

Thin films of spin‐probed and spin‐labeled poly(methyl methacrylate) (PMMA) have been examined by electron spin resonance (ESR) in the temperature range of 77–520°K. The rotational correlation times of nitroxides used as spin probes and labels have been determined as a function of temperature from which activation energies are also determined. The nitroxide rotational times are found to strongly correlate with local segmental and side‐chain motions of the host PMMA matrix. Five discrete molecular motions are detected in PMMA along with their activation energies as measured: side‐chain CH3 rotation (1 Kcal/mol), main‐chain CH3 rotation (2 Kcal/mol), ester side‐chain COOCH3 rotation around the CC bond (4 Kcal/mol), main‐chain CC bond rotation (6 Kcal/mol), and side‐chain OCH3 rotation around the CO bond (18 Kcal/mol). The activation energies determined by ESR are consistent with the potential‐energy barriers calculated theoretically for various rotations in PMMA. It is concluded that the probe and label rotational motions do respond to localized, small‐scale segmental and side‐chain motions of host polymers but are relatively ineffective in response to the large‐scale segmental motion with an activation energy larger than 20 Kcal/mol in the case of PMMA.
Complex refractive indexes for polymers over the infrared spectral region: Specular reflection IR spectra of polymers
Authors: Pacansky J. , England C. , Waltman R. J.
Published: April 2003

A dispersion analysis is used to determine the complex refractive indexes over the infrared for the following polymeric systems: isotactic poly(methyl methacrylate), syndiotactic poly(methyl methacrylate), poly(2‐methyl‐1‐pentene)sulfone, poly(perfluoropropylene oxide), poly(pyromellitimide), poly(styrene), and poly(vinyl alcohol). These were combined with solutions of Maxwell's equations for electromagnetic radiation at interfaces in a FORTRAN computer code to model specular relection infrared spectra. Calculations and experimental data are compared to demonstrate the complicated nature of specular reflection and the ability of the modelling to simulate the observed spectra.
Transport of methylene chloride in poly(aryl‐ether‐ether‐ketone) (PEEK)
Authors: Grayson Michael A. , Pao Peter S. , Wolf Clarence J.
Published: April 2003

The sorption of methylene chloride in neat PEEK was investigated as a function of temperature, sample thickness, surface treatment, and thermal history. The solubility of H2CCl2 in neat PEEK is 23 wt.% and is independent of thickness. Both surface treatment and thermal annealing strongly affect the rate of penetration; the as‐Received material sorbs H2CCl2 more rapidly than abraded or annealed samples; however, the bulk solubility is independent of surface treatment. The sorption and desorption processes are considerably different, and the diffusion process is not simple Fickian Case I. The penetrant advances as a sharp front, suggesting a two‐step, relaxation‐controlled diffusion mechanism.
Effect of antiplasticization on gas sorption and transport. II. Poly(phenylene oxide)
Authors: Maeda Y. , Paul D. R.
Published: April 2003

The propensity for various diluents to cause antiplasticization of poly(phenylene oxide) was examined and compared with similar responses for polysufone. Of the diluents used, none resulted in significant stiffening of poly(phenylene oxide) at 35°C relative to what was observed for polysulfone in an earlier paper. However, these diluents did cause substantial reduction in gas sorption levels and permeability coefficients. The latter is consequently a more sensitive indicator of changes in molecular motions of glassy polymers caused by diluents. The permeability response of adding low molecular weight diluents is compared with that of adding a high molecular weight polymer, polystyrene, to poly(phenylene oxide). The data were analyzed and interpreted along lines used in the companion paper for polysulfone.
Effect of antiplasticization on gas sorption and transport. III. Free volume interpretation
Authors: Maeda Y. , Paul D. R.
Published: April 2003

Previous papers have shown that antiplasticization of polysulfone and poly(phenylene oxide) by low molecular weight diluents is accompanied by substantial reductions in permeability to gases like helium, carbon dioxide, and methane because of reduced mobility in the glass. These effects are not explained even qualitatively by the extent of volume contraction on mixing for these mixtures. However, as shown here, a simple free volume treatment provides an excellent correlation of these effects. Free volume was computed from measured specific volume and an estimate of the volume at the absolute zero of temperature of the material from a group contribution method. Application of this approach to the mechanical stiffening associated with antiplasticization is suggested. The basis and limitations of this analysis are discussed.
Role of matrix‐particle interface adhesion on fracture toughness of dual phase epoxy‐polyethersulfone blend
Authors: Raghava R. S.
Published: April 2003

Compatibility or miscibility of a polyethersulfone (ICI: Victrex 100P) and a tetrafunctional epoxy (Ciba‐Geigy: MY‐720), cured with an aromatic anhydride, has been studied using scanning electron microscopy, x‐ray microanalysis, and dynamic mechanical spectroscopy. Fracture toughness of epoxy and a blend of epoxy and polyethersulfone has been measured using three point bend tests (ASTM: E‐399‐81). Fracture surfaces were examined by x‐ray microanalysis for detecting concentration of sulfur, present in polyethersulfone, in the matrix and precipitated phase. The influence of the morphology of the epoxy/polyethersulfone blend on its fracture toughness and toughening mechanism has been studied. Fracture toughness values of unmodified and modified cured epoxy resin were evaluated as a function of test temperature.
Response of polyisoprene and polybutadiene vulcanisates to double‐step retractions: Analysis by a disengagement process of pendant chains
Authors: Thirion Pierre , Monnerie Lucien
Published: April 2003

The influence of the molecular weight of the primary chains and of crosslinking density on the viscoelasticity of polyisoprene and polybutadiene vulcanisates has been examined by subjecting these materials to double‐step extensions in opposite directions. The stress peaks caused by these sudden retractions are well described by a model derived from the Curro–Pincus treatment of the disengagement pendant chains from a net of obstacles. These topological interactions do not appear to be derived from discrete chain entanglements but from diffuse interspersion effects between the pendant chains and the chemical junctions of the networks.
Nematogenic aromatic block copolymers of rigid and flexible units. II. Phase equilibria
Authors: Krigbaum W. R. , Shufan Zhang , Preston Jack , Ciferri A. , Conio G.
Published: April 2003

The solubility and mesophase behavior are investigated for block copolymers of poly(p‐benzamide) (PBA), the polyterephthalamide of p‐aminobenzhydrazide (PABH‐T), and PBA and poly(m‐phenylene isophthalamide) (MPD‐I) dissolved in N,N‐dimethylacetamide (DMAc) containing 3% LiCl. The block copolymers, whose synthesis and characterization were described in the previous paper in this series, included samples prepared by the two‐step and multistep copolycondensations. The first of these methods yields a considerable amount of the flexible homopolymer (PABH‐T) and also some of the rigid homopolymer. The flexible homopolymer can be removed from the block copolymer by extraction with dimethyl sulfoxide (DMSO), whereas precipitation may offer a way to remove the rigid homopolymer. The results observed for the block copolymers are compared with those for the homopolymers and mixtures of homopolymers. The apparent solubility of the PBA/PABH‐T block copolymers obtained by the two‐step method is unusually large but decreases toward the value observed for mixtures after the flexible homopolymer had been extracted with DMSO. Labile adducts involving PABH‐T and/or the block copolymer appear to be capable of forming a single mesophase. This offers a most interesting approach to the preparation of composite materials involving rigid and flexible polymers.
Polyethylene–isotactic polypropylene epitaxy: Analysis of the diffraction patterns of oriented biphasic blends
Authors: Lotz B. , Wittmann J. C.
Published: April 2003

Diffraction patterns of oriented blends of isotactic polypropylene (iPP) and polyethylene (PE) published recently by several authors are analyzed on the basis of a unique epitaxial relationship between the iPP and PE crystal lattices. The contact planes are (100)PE and (010)iPP, and the PE chains lie at 50° to the iPP chain axis, parallel to the helical path of the iPP helices, which is formed by rows of side‐group methyls.
Optical properties of a shear‐deformed thermotropic cellulose derivative
Authors: Navard P. , Zachariades A. E.
Published: April 2003

The 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

The 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

The 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

The 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

About 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

Measurements 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

Evidence 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

The 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

A 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

The 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

We 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

The 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

The 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

With 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

A 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

The 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

We 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

Results 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

Creep 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

Low‐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

A 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

Absorption 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.
Rheological technique for determining the order–disorder transition of block copolymers
Authors: Han Chang Dae , Kim Jinhwan
Published: April 2003

A rheological technique is proposed for determining the thermally induced order–disorder transition of block copolymers. In the present investigation, a cone‐and‐plate rheometer was used to measure dynamic storage and loss moduli, G′(ω) and G″(ω), as a function of angular frequency ω of a commercial grade polystyrene‐block‐polyisoprene‐block‐polystyrene (SIS) tri‐block copolymer (KRATON D‐1107, Shell Development Company) in the temperature range from 140 to 240°C. For comparison purposes, dynamic viscoelastic properties of a commercial grade low‐density polyethylene (LDPE) were also determined in the temperature range from 160 to 238°C. We have found that log G′ versus log G″ plots for the LDPE show no temperature dependence, whereas log G′ versus log G″ plots for the SIS block copolymer do show systematic temperature dependence in the temperature range 140–230°C. This observation leads us to conclude that the order–disorder transition of the SIS block copolymer takes place gradually as the temperature is raised from 140 to 230°C. This conclusion is in good agreement with that drawn from the study of Roe (Ref. 33), who employed the same block copolymer using small‐angle x‐ray scattering. It is not possible to reach such a conclusion using log G′(ω) versus log ω, log G″(ω) versus log ω, or log η′(ω) versus log ω plots in which η′ is the dynamic viscosity. We have demonstrated further that the use of frequency‐temperature superposition is inappropriate for investigating the rheological behavior of block copolymer in the temperature range over which a thermally induced transition from an ordered structure to a disordered homogeneous phase occurs. We therefore suggest that when using information on dynamic viscoelastic properties, log G′ versus log G″ plots be used for determining the thermally induced order–disorder transition of block copolymers.
Studies of craze fibril deformation during fatigue in polystyrene
Authors: Brown Hugh R. , Kramer Edward J. , Bubeck Robert A.
Published: April 2003

Small‐angle scattering of synchrotron x‐ray radiation has been used to study the effects of fatigue on craze fibril microstructure. The results obtained during unloading and reloading during a single cycle have been compared with those predicted by a model of sinusoidally bent fibrils. In addition the total displacement of the craze boundaries was found from the change in the invariant on unloading. The mean fibril diameter D was measured at the maximum tensile strain in each cycle. Over 250 cycles, D increased by at least a factor of 2 from an initial value of 6.5 nm, with most of this change happening in the first few cycles. The increase in D must occur by fibril coalescence, a mechanism that requires that the material in craze fibrils have considerable molecular mobility, even at room temperature, 70°C below the glass transition temperature.
Thermal diffusivity of polymer films by the flash radiometry method
Authors: Choy C. L. , Leung W. P. , Ng Y. K.
Published: April 2003

We have developed a flash radiometry technique for the determination of the thermal diffusivity of polymer films that consists of flashing a laser pulse on the front surface of the sample and monitoring the thermal radiation from the rear surface as a function of time using an infrared detector. This method is applicable to polymer films of thickness 14–250 μm in the wide temperature range between 150 and 500 K. We have performed measurements on four polymers, including polycarbonate, polyimide, polyvinylidene fluoride, and polyethylene terephthalate. The temperature dependence of the thermal conductivity of polycarbonate can be understood by considering the phonon mean free path, and the crystallinity and orientation dependence of the thermal conductivity of polyethylene terephthalate can be explained in terms of the two‐phase model proposed by Choy and Young.
Solution properties and chain conformation characteristics of cellulose tricarbanilate
Authors: Daňhelka Jaroslav , Netopilíak Miloš , Bohdanecký Miloslav
Published: April 2003

Fractions of two cellulose tricarbanilate samples were characterized by light‐scattering (weight‐average molecular weight, second virial coefficient, mean‐square radius of gyration), gel permeation chromatography (polydispersity index), and viscometry (intrinsic viscosity) in tetrahydrofuran and acetone. The intrinsic viscosity data were analyzed in terms of the theory developed for the continuous wormlike cylinder model, and the chain parameters (Kuhn statistical segment length λ−1, chain diameter d, and shift factor ML) were evaluated. The molecular‐weight dependence of the mean‐square radius of gyration in tetrahydrofuran was calculated for the Kratky—Porod chain model and compared with the experimental results. Data on the intrinsic viscosity and radii of gyration for other solvents at temperatures from 0 to 100°C were analyzed in the same way, and the effects of solvent and temperature on the statistical segment length were evaluated. Polymer—solvent interaction parameters were estimated from the second virial coefficients.
Thermoluminescence and chain‐fold morphology in low‐density polyethylene
Authors: Markiewicz A. , Fleming R. J.
Published: April 2003

Further thermoluminescence data are presented supporting our earlier suggestion that the electron traps associated with the three peaks in the thermoluminescence glow curve of a low‐density polyethylene sample from which absorbed air has been removed are formed by the polymer chains themselves in the chain‐fold regions of the samples. These traps are shown to be sensitive to heating of the sample to temperature around its melting point; in particular, the lowest temperature peak disappears if the sample is held at 90°C in vacuum for 5 min. If the sample is maintained in vacuum at room temperature after such treatment, its modified glow curve remains unchanged for a period of at least 7 days; however, if the sample is exposed to air, nitrogen, or argon after such treatment, its gas‐free glow curve begins to change within 3 days, evolving toward a three‐peak form with the same peak temperatures but with relative intensities different from those observed before heating began. This suggests that the gas molecules “lubricate” the polymer chains, which then begin to move toward new equilibrium configurations. Immersion in n‐hexane at room temperature has little effect on the luminescence centers but disables the electron traps. Immersion in fuming nitric acid at room temperature for 2 days appears to destroy the electron traps permanently, as would be expected if the chain folds are digested by the acid; its effect on the luminescence centers is still to be determined.
Electron spin resonance and UV absorption of irradiated poly(vinyl alcohol)
Authors: Rao B. Sanjeeva , Murthy M. Ramakrishna
Published: April 2003

Electron spin resonance (ESR) of γ‐irradiated poly(vinyl alcohol) (PVA) at room temperature has been used to identify the radiation damage products. The ESR spectrum is analyzed by using least‐squares fits between observed and calculated spectra, assuming various free radical species. The best theoretical fit, together with UV absorption data, enabled the assignment of the observed ESR spectrum. The existence of carbonyl macrofree radicals, probably with aldehyde functional groups, is proposed in irradiated PVA.
Stress‐optical properties of poly(oxypropylene) networks
Authors: Andrady Anthony L. , Llorente Miguel A. , Saiz Enrique
Published: April 2003

Strain birefringence of poly(oxypropylene) was studied using several poly(oxypropylene) model networks of different crosslink densities. Most of the measurements were carried out in elongation, in both the unswollen and the swollen states, over the temperature range 10–70°C. The optical configuration parameter was found to be Δa = (4.33 ± 0.09) × 10−24 cm3 at 25°C with a very small temperature coefficient. Theoretical calculations based on rotational isomeric state theory were employed to interpret the experimental data. The results indicate the intermolecular correlations to be low for this polymer, and, contrary to other systems, the stress‐optical coefficient C decreased with increasing average molecular weight between crosslinks. This fact was attributed to the end‐group effect introduced by the crosslinking agent.
Viscometry and light scattering on moderately concentrated solutions of poly(vinyl chloride)
Authors: Lin Y. C. , Beeson J. J. , Berry G. C.
Published: April 2003

Moderately concentrated solutions (0.015–0.10 g/mL) of poly(vinyl chloride) have been studied in three different solvents by light scattering and viscometry. It is concluded that intermolecular association occurs in all solutions studied, including data over the temperature interval 25–110°C. The data can be interpreted in terms of a model with loci of association holding chains together in an aggregate that has an overall configuration similar to that of a randomly branched polymer.