bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of methyl substituents on permeability and permselectivity of gases in polyimides prepared from methyl‐substituted phenylenediamines
Authors: Tanaka Kazuhiro , Okano Masaaki , Toshino Hiroyuki , Kita Hidetoshi , Okamoto Ken‐Ichi 
Published: April 2003

AbstractPermeability and solubility coefficients for H2, CO2, O2, CO, N2, and CH4 in polyimides prepared from 6FDA and methyl‐substituted phenylenediamines were measured to investigate effects of the substituents on gas permeability and permselectivity. The methyl substituents restrict internal rotation around the bonds between the phenyl rings and the imide rings. The rigidity and nonplanar structure of the polymer chain, and the bulkiness of methyl groups make chain packing inefficient, resulting in increases in both diffusion and solubility coefficients of the gases. Polyimides from tetramethyl‐p‐phenylenediamine and trimethyl‐m‐phenylenediamine display very high permeability coefficients and very low permselectivity due to very high diffusion coefficients and very low diffusivity selectivity, as compared with the other polyimides having a similar fraction of free space. This suggests that these polyimides have high fractions of large‐size free spaces.
A study of shear‐stress relaxation anomalies in binary mixtures of monodisperse polystyrenes
Authors: Morrison Faith A. , Larson Ronald G. 
Published: April 2003

AbstractWe report measurements of the nonlinear relaxation moduli after a step‐shear strain of polystyrene solutions with nearly monodisperse and with bidisperse distributions of molecular weight. We find, as have others, that for monodisperse solutions with M/Me > 60, there are anomalies, such as an unusually low nonlinear modulus and a kink in a plot of shear stress versus time after the step strain. Here M is the polymer molecular weight and Me is the entanglement molecular weight. We find that in the bidisperse solutions the anomalies persist as long as Mw/Me > 60, where Mw is the weight‐averaged molecular weight of the bidisperse solution. The persistence of the anomalies in bidisperse solutions disagrees with a theory of Marrucci and Grizzuti that attributes the anomalies to strain inhomogeneities similar to shear banding. The Marrucci‐Grizzuti theory predicts that as little as 10% short chains in the bidisperse mix should eliminate the anomalies, whereas in the experiments reported here at least 30% is required. Nevertheless the way in which the anomalies disappear at high strains when one increases the fraction of low‐molecular‐weight component is qualitatively similar to the theoretical predictions and supports the notion that strain inhomogeneities occur in these systems. © 1992 John Wiley & Sons, Inc.
Sorption and dilation of silicone elastomer composites at high gas pressures: The role of interfacial quality
Authors: Briscoe B. J. , Zakaria S. 
Published: April 2003

AbstractIn elastomers the crosslink density, the presence of filler particles, and the volumetric confinement toward sorptive dilation can influence the extent of gas mass uptake. In this study the effects of filler particles on the high‐pressure gas mass sorption and the volumetric dilation of a silicone elastomer matrix has been investigated. Glass beads, ca. 30 μm radii, with different surface treatments were incorporated as inclusions in various specimens at relatively low concentrations of ca. 10% by volume. The high‐pressure gases used were N2 and CO2 up to a maximum ambient pressure of ca. 25 MPa at ca. 20°C and 42°C, respectively. The gas mass sorption was determined by a vibrating reed technique. The sorptive dilation was measured by an ultrasonic transducer operating as a displacement probe. In certain systems the absorbed CO2 gas was able to disrupt the internal interfaces. This led to an increased gas mass uptake in the corrupted specimen. The N2 gas did not affect the interfacial bonds. The amount of penetrant uptake was found generally to be reduced when the internal interfaces were not disrupted. The presence of various internal interfaces restrained the sorptive dilation of the elastomeric matrix. These hindrances to the natural sorptive dilation of the elastomer network suppressed the extent of the gas sorption process. This effect has also been investigated separately in detail using model ‘poker chip’ type of specimens of various aspect ratios. The sorptive dilational characteristics have been correlated with the mechanical properties of similar specimens. The influence of an almost complete volumetric confinement on the gas sorption capacity of the silicone elastomer specimen has also been studied. © 1992 John Wiley & Sons, Inc.
Characterization of the molecular weight distribution of isotactic polypropylene by turbidity at the lower critical solution temperature (LCST)
Authors: Bohossian Takouhi , Delmas Geneviève 
Published: April 2003

AbstractThe molecular weight distribution (MWD) of high‐molecular‐weight isotactic polypropylene is measured by a new method developed for polyethylene (PE) using the turbidity at the lower critical solution temperature (LCST). The attenuation of transmitted light hi is measured at Ti as each fraction i is separated from the solution during a temperature increase ΔTi. From the thermogram, the set of hi (Ti), the MWD can be calculated through a relation Mi = f(Ti). The solvent chosen, a mixture of butane and 2‐methylbutane, optimizes resolution and relatively rapid dissolution. Homogenization and absence of degradation are verified by different methods including calorimetry in solution. © 1992 John Wiley & Sons, Inc.
Separation of small molecules using novel rolltruded membranes. I. Apparatus and preliminary results
Authors: Ciora R. J. , Magill J. H. 
Published: April 2003

AbstractA novel membrane production technique, rolltrusion, has been developed for the preparation of permselective polymeric membranes for both gas and vapor separations and pervaporation applications. Pinhole‐free thin films with a regulated three‐dimensional morphology comprising crystalline and amorphous regions have been produced with improved mechanical properties in three dimensions. Because rolltrusion is a solid‐state processing technique, it is not restricted by solubility constraints inherent in the common solvent‐based thinfilm production techniques. Consequently it can be extended to include commercially available engineering plastics that have not been used as membranes previously because of their limited solvent solubility. These polymers are interesting because of their applicability to temperatures in excess of 200 to 300°C in harsh chemical environments, compared with the ca. 150°C ceiling usually encountered in the most rugged of solvent‐cast polymers. In the first part of this series we detail an automated experimental permeation apparatus that permits testing of both single‐component and multicomponent gases and vapors over a temperature range of −73 to 273°C. Currently, modifications to the permeation cell are underway to permit pervaporation studies in the apparatus. To illustrate the operation of the apparatus, and to some degree the effect of rolltrusion processing, experimental permeability, diffusivity, solubility, and actual gas separation factors (via gas chromatography) are detailed for several light gases in rolltruded isotactic poly(propylene) (iPP) and in the thermally and chemically resistant thermoplastic, poly(ether ether ketone) (PEEK). Permeation temperatures to at least 225°C have been studied using PEEK, representing the first published experimental permeability and gas‐separation results for such engineering polymers at temperatures above 200 °C. © 1992 John Wiley & Sons, Inc.
Structure and properties of blends containing ethylene‐methacrylate copolymers
Authors: Peiffer D. G. 
Published: April 2003

AbstractThe preparation, melting point, degree of crystallinity, mechanical properties, and morphology of a family of blends composed of a transition‐metal‐neutralized carboxylate semicrystalline ionomer (metal‐neutralized ethylene‐methacrylate copolymer) and an amorphous copolymer (styrene‐4‐vinyl pyridine copolymer) are described. These polymeric materials contain low levels (≤ 10.0 mol %) of interacting groups which are capable of forming interpolymeric complexes. These interactions are best described as transition metal‐pyridine coordination complexes. A general characteristic of these blend systems is that the mechanical properties and morphology are directly influenced by the nature of the counterion and the specific composition ratio of amorphous to semicrystalline component. A nontransition metal counterion (sodium) is weakly interacting at best, while a transition metal counterion (zinc) is strongly interacting. Morphological studies (polarized‐light microscopy and small‐angle light‐scattering measurements) confirm that the glassy component, if nonassociating, resides primarily in the interspherulite region, while the associating species will behave in a similar manner only after the stoichiometric ratio is reached. The morphology directly influences the stress‐strain behavior of these blends. It is noteworthy that the spherulite size remains unchanged with nonassociating blends while a 50% reduction is noted in the associating blends. Thermal and wide‐angle x‐ray scattering measurements confirm the lamellar structure is unaffected by these associations. © 1992 John Wiley & Sons, Inc.
Surface characterization of laser‐treated poly(ethylene terephthalate) by optical profilometry and scanning tunneling microscopy
Authors: Wefers Lothar , Schollmeyer Eckhard 
Published: April 2003

AbstractCharacteristic changes in the surface topography due to UV laser treatment are of high significance for the determination of material destruction thresholds and surface structure development. E.g. irradiation of poly(ethylene terephthalate) (PET) film with pulsed UV laser light of 248 nm modifies the smoth surface of the polymer into a well oriented structured surface. The development of these structures were studied with scanning tunneling microscopy (STM) and high resolution optical profilometry. Three dimensional data of the surface were taken from the samples after each laser pulse. A change of topographic data was found in relation to fluence and number of pulses applied. © 1993 John Wilcy & Sons, Inc.
Computer simulation of the mechanical alpha relaxation in isothermally crystallized high‐density polyethylene
Authors: González‐Orozco J. A. 
Published: April 2003

AbstractThe influence of thermal history on the dynamic mechanical behavior in the alpha relaxation region of high‐density polyethylene (HDPE) is predicted using computer modeling. The model considers the mechanical alpha relaxation as a succession of cooperative single movements occurring within the defect region of the crystal lattice that finally produce a relative displacement between lamellae. A distribution function of the number of such single steps is postulated considering topological properties of the sample: branch content and molecular weight. To carry out the simulation, the model uses the spectrum of the sample subjected to fast cooling as a reference system. In addition, the crystallinity and the segregated material content for the particular thermal history of the object of study are also required. Loss modulus spectra are generated using balance of energy considerations, and a few examples are presented together with several experimental results. © 1993 John Wiley & Sons, Inc.
Evaluation of free‐volume theories for solvent self‐diffusion in polymer–solvent systems
Authors: Vrentas J. S. , Vrentas C. M. 
Published: April 2003

AbstractAn evaluation of free‐volume theories for solvent self‐diffusion is carried out using recent comprehensive data sets for penetrant self‐diffusion in polymer solutions. Different theories are compared, and free‐volume theories in the prediction of penetrant self‐diffusion coefficients in glassy polymer systems is also evaluated. © 1993 John Wiley & Sons, Inc.
Atomic force microscopy of extended‐chain crystals of polyethylene
Authors: Annis Brain K. , Reffner John R. , Wunderlich Bernhard 
Published: April 2003

AbstractExtended‐chain crystals of polyethylene grown at elevated pressure and temperature were analyzed for the first time by atomic force microscopy. It was possible to compare the typical fracture surface striation features with those obtained earlier by electron microscopy. High resolution atomic force microscopy on flat surfaces enabled the recording of an atomic scale regularity that could not be fully indentified. © 1993 John Wiley & Sons, Inc.
A rheological, optical, and x‐ray study of the relaxation of orientation of nematic PBZT
Authors: Odell J. A. , Ungar G. , Feijoo J. L. 
Published: April 2003

AbstractThe ultimate properties of liquid crystalline polymers depend upon the achievement of high orientation, usually by means of flow fields. The properties are limited by disorientation which can occur before the product is solidified. Such cooperative orientation and disorientation phenomena also underlie the complex fluid rheology and product microstructure of these materials. The orientation and subsequent disorientation can be followed dynamically by optical and x‐ray techniques. Normally, monitoring of orientation is possible only by “fast” techniqes, such as birefringence, these are not applicable to opaque and strongly scattering liquid‐crystalline systems. To enable examination of the full dynamic response of concentrated nematic solutions of poly(1,4‐phenylene‐2,6 benzobisthiazole) (PBZT) the Daresbury Synchrotron Radiation Source was used. PBZT is among the most rigid macromolecules and serves as a good model for other materials of its class. The orientation process determined optically and from x‐rays is correlated with fluid rheology and availabel theoretical approaches. During relaxation from near perfect mesogen alignment three principal stages of the disorientation process were identified corresponding to solvent disorientation (first stage of stress relaxation), banding (slow stress relaxation and mesogen disorientation), and finally a very slow banding to polydomain transition. © 1993 John Wiley & Sons, Inc.
Characterization of polyethylenes by x‐ray diffraction and 13C‐NMR: Temperature studies and the nature of the amorphous halo
Authors: Mcfaddin D. C. , Russell K. E. , Wu Gang , Heyding R. D. 
Published: April 2003

AbstractX‐ray diffraction patterns of linear and branched polyethylenes typically show two sharp reflections and an amorphous halo. The position of the halo depends on branch content and temperature. A single curve describes the position of the halo maximum (2θhalo) for a range of liquid hydrocarbons and polyethylenes in the 20–140°C range. At temperatures well below their melting point, branched polymers give 2θhalo values which differ significantly from those observed for the liquid Linear polymers show a greater divergence, indicating that some of the material giving rise to the halo is much better packed than in the liquid.Parallel 13 C NMR spin‐lattice relaxation studies suggest that this relatively ordered material has a trans conformation but a low average T1c value. © 1993 John Wiley & Sons, Inc.
An investigation of the yield and postyield behavior and corresponding structure of poly(methyl methacrylate)
Authors: Hasan O. A. , Boyce M. C. , Li X. S. , Berko S. 
Published: April 2003

AbstractThe mechanical behavior of glassy polymers is time and temperature dependent as evidenced by their viscoelastic and viscoplastic response to loading. The behavior is also known to depend strongly on the prior history of the material, changing with time and temperature without chemical intervention. In this investigation, we examine the effects of this process of physical aging on the yield and postyield behavior and corresponding evolution in the structural state of glassy polymers. This has been achieved through a systematic program of uniaxial, isothermal, constant strain–rate tests on poly(methyl methacrylate) (PMMA) specimens of different thermal histories and by performing positron annihilation lifetime spectroscopy (PALS) measurements prior to and after mechanical deformation. PALS is an indicator of the free volume content, probing size and density of free volume sites and can be considered to be a measurement of structural state. The results of the mechanical tests show that aging acts to increase both the initial yield stress and the amount of strain softening which occurs subsequent to yield. Moreover, the amount of strain softening was found to be independent of strain rate indicating that softening is related to an evolution in structure as opposed to deformation kinetics. Furthermore, after sufficient inelastic straining, the initial thermal history is completely erased as evidenced by identical values of flow stress following strain softening, for both annealed and quenched polymer. Strong confirmation of the structural state or free volume related nature of the strain softening process is obtained by our companion PALS measurements. PALS detects an increase in the size of free volume sites following inelastic deformation and finds the initially annealed and quenched specimens to posses the same post‐deformation distribution. The size of sites is found to evolve steadily with inelastic strain until it attains a steady‐state value. This evolution of free volume with strain follows the observed softening of the flow stress to a steady‐state value. These results provide experimental evidence that an increase in free volume with inelastic straining accompanies the strain softening phenomenon in glassy polymers and that strain softening is indeed a de‐aging process. Based on our experimental results a mechanistically based constitutive model has been formulated to describe the effects of thermal history on the yield and postyield deformation behavior of glassy polymers up to moderate strains. The model is found to successfully capture the effects of physical aging, strain softening, strain rate, and temperature on the inelastic behavior of glassy polymers when compared with experimental results. © 1993 John Wiley & Sons, Inc.
Calorimetric determination of vitrification time and heat capacity of a thermosetting polymer
Authors: Cassettari M. , Salvetti G. , Tombari E. , Veronesi S. , Johari G. P. 
Published: April 2003

AbstractFirst observations of a decrease in heat capacity on isothermal vitrification of a thermosetting polymer are reported. The method developed for this study allows simultaneous measurements of both the heat capacity of a material and the enthalpy released as chemical reactions occur under virtually isothermal conditions. Control experiments have been done on a nonstoichiometric composition which does not vitrify and for which total enthalpy released can be measured. During isothermal curing, the heat capacity of a thermoset first increases slightly, undergoes an abrupt decrease in a narrow range of time, and thereafter slowly decreases. The abrupt decrease represents the transition of the liquid to a glassy solid, which is irreversible and which shifts to shorter times as the curing temperature is increased. This transition occurs at different extents of cure for different temperatures. The thermodynamic consequence of kinetic slowing on irreversible conversion of a molecular liquid to a macromolecule under isothermal conditions is similar to that observed on reversible clustering, as a result of decrease in the thermal energy on supercooling a molecular liquid. © 1993 John Wiley & Sons, Inc.
Conformation of the ground‐state dimer in poly(ethylene terephthalate)
Authors: Dodge Robert W. , Mattice Wayne L. 
Published: April 2003

AbstractAbsorbance, excitation, and emission measurements have been performed with methyl benzoate and five model compounds, C6H5COO (CH2)xOOCC6H5, x = 2–6. Under appropriate conditions, three of the model compounds (those with x = 3, 4, 5) show evidence for the formation of intramolecular ground‐state dimers. The model compound with x = 5 can form two types of dimers which emit with different energies. The model compound with x = 3 forms one of these dimers, and the model compound with x = 4 prefers the other ground‐state dimer. Molecular modeling of the dimers suggests that the two conformations of the ground‐state dimers differ in the orientation of the two CO bonds. In the one dimer these two bonds are nearly parallel, but in the other they make an angle of about 120°. © 1993 John Wiley & Sons, Inc.
The ratio of excimer to monomer emission in diesters of 1‐pyrenoic acid and five glycols
Authors: Mendicuti Francisco , Saiz Enrique , Mattice Wayne L. 
Published: April 2003

AbstractThe ratio of the intensities of excimer and monomer emission, ID/IM, has been measured for PyCOO(CH2)mOOCPy (m = 2–6, Py = 1‐pyrenyl) in solvents of different viscosity, η. These compounds are dimeric models for the corresponding polymers, selected to eliminate the influence of energy migration on the population of an excimer. The solvents are H(CH2)nOH, n = 1–7, and ethylene glycol. The values of ID/IM decrease as η increases, and the rate of this decrease is a function of m. Therefore the qualitative dependence of ID/IM on m is a function of η. In the limit as η → ∞, ID/IM is an odd–even function of m for m = 2–5, and the values at m = 5 and 6 are nearly identical. A rotational‐isomeric state analysis of the conformations of the diesters can rationalize the appearance of the odd–even effect. This result is insensitive to assumptions about the extent of overlap of the two pyrene ring systems that is required for the production of emission in the excimer band. © 1993 John Wiley & Sons, Inc.
Measurement of the interfacial tension between polyethersulfone and a thermotropic liquid‐crystalline copolyester
Authors: James S. G. , Donald A. M. , Miles I. S. , Mallach L. , Macdonald W. A. 
Published: April 2003

AbstractDue to the increasing interest in forming blends of liquid‐crystalline polymers with conventional thermoplastics, it becomes important to determine the interfacial tension between two such polymers. A method for evaluating the interfacial tension between a thermotropic copolyester based on hydroxybenzoic and hydroxynaphthoic acid residues, and polyethersulfone is presented, based on the Fort and Patterson method. It is found that the value of the interfacial tension in the melt is much higher than is the case between conventional polymer pairs. It is suggested that this high value reflects an entropic effect due to the strong exclusion of the flexible coil polymer from the nematic melt. © 1993 John Wiley & Sons, Inc.
Stress experiences of parents with premature infants in a special care nursery
Authors: Lau Rosalind , Morse Carol A. 
Published: April 2003

AbstractA controlled prospective longitudinal study was carried out to determine the stress experiences of parents whose premature infants were admitted to a tertiary‐based special care nursery in Melbourne, Australia. The control group consisted of parents of term infants matched on maternal age, parity and socioeconomic status. Sixty mothers and 59 fathers of premature infants and 60 mothers and fathers of term infants were followed from birth to 16 weeks after discharge home. A range of repeated self‐reports was carried out regarding feelings, moods, marital/partner relationships, availability and satisfaction with perceived social support and salivary markers of stress (cortisol and tribulin) were collected. Assessments occurred on five occasions with parents of premature infants and with parents of healthy term infants, data were collected on three occasions. Findings revealed that parents of premature infants reported higher subjective stress levels than parents of term infants within the first week of their infant's birth but returned lower objective markers of stress. There was no correlation between self‐reported stress and the biochemical markers of stress. The stress levels for parents of premature infants reduced over time. Issues for future research are discussed. Copyright © 2003 John Wiley & Sons, Ltd.