bluuebunny/crossref_metadata_2025_split huggingface dataset:
Extraction and sorption studies using linear polymer chains and model networks
Authors: Garrido L. , Mark J. E. 
Published: April 2003

AbstractModel networks have been prepared by tetrafunctionally endlinking linear polydimethylsiloxane (PDMS) chains having molecular weights Mn in the range 2000–15,000 g mole−1. The first series of networks were prepared from mixtures containing known amounts of unreactive linear PDMS chains with molecular weights Md between 1000 and 16,000 g mole−1. Rates of extraction were used to estimate diffusion coefficients; as expected, they were found to increase with increase in molecular weight Mc = Mn between crosslinks, but to decrease with increase in Md. The ease with which all of such a diluent could be removed showed the same dependence on Mc and Md. A second series of networks was prepared from the same reactive PDMS chains without diluents. Sorption and extraction studies using the same diluents were then carried out. The diffusion coefficients for sorption were found to be in the range (1.7–15.0) × 10−12 m2 s−1 and depended on both Mc and Md. The amount of diluent absorbed at equilibrium was between 10 and 70%, which is in good agreement with predictions from the Flory equation for dilation in networks, with account of constraints on crosslink fluctuations.
Phase separation phenomena in solutions of poly(2,6‐dimethyl‐1,4‐phenyleneoxide) in mixtures of trichloroethylene, 1‐octanol, and methanol: Relationship to membrane formation
Authors: Wijmans J. G. , Rutten H. J. J. , Smolders C. A. 
Published: April 2003

AbstractThe phase boundaries in the quaternary system consisting of the polymer poly(2,6‐dimethyl‐1,4‐phenyleneoxide) (PPOPPO is a registered trademark of the General Electric Company.
), the solvent trichloroethylene (TCE), and the nonsolvents 1‐octanol (OcOH) and methanol (MeOH) are determined. The kinetics of crystallization are investigated by pulse‐induced critical scattering. The formation and properties of PPO membranes are discussed in relation to the phase separation phenomena.
Evidence for sorption–desorption phenomena during thermal cycling in highly hydrated perfluorinated membranes
Authors: Pineri M. , Volino F. , Escoubes M. 
Published: April 2003

AbstractPerfluorinated ionomer membranes can absorb large amounts of water. DSC experiments show the existence of exothermic or endothermic peaks for water contents larger than 8 wt % at room temperature in Nafion 1200 EW in the acid form. Changes in the number of mobile protons versus temperature have been measured by NMR spectroscopy. A desorption process with an activation energy of 6 kcal/mol is proposed during the sample cooling. Such an explanation quantitatively explains both the DSC and NMR results and is also consistent with the other results obtained from mechanical experiments, low‐temperature sorption isotherms, and ESR spectroscopy.
Determination of the geometric structure of three substituted polyacetylenes: Polymethylacetylene, polypentylacetylene, and poly(tbutylacetylene)
Authors: Leclerc Mario , Prud'homme Robert E. , Soum Alain , Fontanille Michel 
Published: April 2003

AbstractThe geometric structure of polymethylacetylene (PMA), polypentylacetylene (PPA), and poly(t‐butylacetylene) (PTA) was investigated by 1H NMR, 13C NMR, and IR spectroscopies. It was shown that both NMR techniques can be used to determine the trans isomer content of PPA and PTA, whereas the 1H NMR and IR methods can be used for PMA. A calibration curve was constructed by using the 965‐ and 720‐cm−1 bands of the IR spectrum of PPA, and could be used in future work for the same purpose if the samples had molecular weights similar to that of the one used in this study. The isomerization kinetics of PTA was investigated and cis→ trans activation energies of 88 and 121 kJ/mol were calculated in solution and in the solid state, respectively. Heat treatment of the PMA and PPA samples always leads to a cis→ trans isomerization with a 100% trans content under extreme conditions. Moreover, a cis→ trans isomerization of PTA was induced in CCl4, CDCl3, toluene, and benzene, but a trans→ cis isomerization was induced in decalin. The reversible isomerization of PTA covered a trans isomer concentration ranging form 25 to 60%.
Spectroscopic study of α‐ and β‐phase isotactic polypropylene and of atactic polypropylene in solution
Authors: Merajver Sofia D. , Wunder Stephanie L. , Wallace William 
Published: April 2003

AbstractRaman spectroscopy is used to investigate the conformation and packing of isotactic crystalline α‐phase polypropylene compared with lower‐order β‐phase isotactic polypropylene and to study the solution behavior of atactic polypropylene. The high‐frequency region of the spectrum is analyzed in light of a normal‐mode calculation that takes into account the methyl‐group vibrations. This region is sensitive to both chain conformation and packing, and because of the high intensity of the methyl and methylene high‐frequency stretching modes, it can be used to probe small changes in intermolecular or intramolecular order. Differences in the thermal behavior between the two solid isotactic polypropylene samples are explained interms of packing defects which exist in the β‐phase form. In the solution study, we demonstrate that, for molecules in which bands sensitive to intermolecular interactions exist, as is the case of the methyl and methylene vibrations of polypropylene, spectroscopic techniques can be used to estimate the minimum overlap concentration.
The effect of spherulitic truncation on small‐angle light scattering
Authors: Tabar R. J. , Stein R. S. , Rose D. E. 
Published: April 2003

AbstractThe effects of spherulitic truncation on the Hv small‐angle light‐scattering (SALS) patterns are determined by computer simulation of spherulite nucleation and growth. The simulation is carried out for simultaneous and sporadic nucleation of two‐dimensional spherulites and simultaneous nucleation of three‐dimensional spherulites. The scattered intensity differences between truncated spherulites and round spherulites are determined as functions of the type of growth and the volume (or area) fraction of spherulites. Methods for the determination of certain geometrical characteristics of spherulites systems by SALS are developed. These characteristics include the volume (or area). fraction of spherulites, the average spherulite radius, and the average spherulite volume (or area). The results of this study are essential in the quantitative analysis of Hv SALS from spherulitic systems. The simulation process is readily extendable to the examination of other morphological phenomena by SALS.
Melting of water/poly(ethylene oxide) systems
Authors: Bogdanov B. , Mihailov M. 
Published: April 2003

AbstractThe transition temperatures of poly(ethylene oxide) (PEO)/water systems were studied over the temperature range from −90 to 110°C by differential scanning calorimetry. The composition of the systems and the average molecular weight of the PEO were varied over a wide range. The systems under study tend to supercool and become partially or completely glassy depending on the cooling rate, composition, and PEO molecular weight. The glassy phases crystallize on being heated above their glass transition temperature. Systems containing about 60 wt % of PEO are most readily super cooled without crystallization. The dependence of melting temperatures and enthalpies on composition was studied and their isobar phase diagrams were drawn. They show that at a definite polymer content (about 50 wt %), which is molecular weight dependent, the systems behave as binary eutectic mixtures. The results confirm the idea that PEO forms quite stable molecular complexes with water.
Glass and subglass transitions in a series of poly(itaconate ester)s with methyl‐terminated poly(ethylene oxide) side chains
Authors: Cowie John M. G. , Ferguson Roderick 
Published: April 2003

AbstractA series of poly(itaconate ester)s containing methyl‐terminated poly(ethylene oxide) side chains with lengths ranging from 1 to 5 ethylene oxide units has been synthesized. Both heat capacity Cp and dynamic mechanical measurements have been carried out on these polymers using differential scanning calorimetry (DSC) and torsional braid analysis (TBA), respectively. The resulting data for this polymer series are discussed, and comparisons are made with work previously published for the corresponding di‐n‐alkyl itaconate ester polymers where appropriate.
Electron‐beam‐induced fracture of polymers
Authors: Dickinson J. T. , Klakken M. L. , Miles M. H. , Jensen L. C. 
Published: April 2003

AbstractWhen a notched polymeric material is stressed, the notch opens into a wide crack tip, exposing a region of high stress concentration. The consequences of electron bombardment of the tip of such a stressed material under vacuum are explored here for the first time. Evidence is presented for electron‐induced crack growth at stress far below that needed for crack growth due to stress alone. The electron current densities used in these experiments are sufficiently small that thermal heating of the zone near the crack tip is minimal. To provide information on the phenomena involved, we present simultaneous measurements of electron current, gas pressure, and sample load in response to periodic bombardment of the sample. Experiments involving the bombardment of un‐notched polymers under stress are also described. Fractography of the unique structures obtained by fracture due to the combination of electron bombardment and stress are presented and interpreted in terms of a crosslinking mechanism.
The application of gas barrier and thermal expansion techniques to study structural changes on annealing
Authors: Holden P. S. , Orchard G. A. J. , Ward I. M. 
Published: April 2003

AbstractChanges occur in the structure of drawn polyethylene when it is annealed at temperatures close to that of the drawing process. Measurements have been made of the oxygen barrier properties and of the thermal expansion coefficient in the draw direction on samples over a wide draw ratio range both before and after annealing at various temperatures. The results are augmented by density, creep modulus, and shrinkage observations. All samples show a drop in the barrier properties and an increase in the thermal expansion after annealing. However, whereas high‐draw‐ratio samples show relatively small effects, the changes observed at low draw ratios are very large, with some barrier properties even lower than those of the isotropic feedstock. The effects are larger at the higher annealing temperatures. The diffusion and expansion measurements are greatly influenced by the amorphous regions in the polymer; these results are interpreted as a relaxation of internal stresses giving rise to greater accessibility of the intercrystalline regions. Further experimental work is required to exploit these techniques.
Spectroscopic analysis of poly(p‐phenylene benzobisthiazole) films
Authors: Chang Chih , Hsu S. L. 
Published: April 2003

AbstractInfrared spectroscopy has been used to determine the relative amount, location, and orientation of residual water and acid molecules in poly(p‐phenylene benzobisthiazole) films. By analyzing the relative absorbance of reflected polarized infrared radiation from highly oriented films, the indices of refraction parallel and perpendicular to the chain axis were also obtained.
Polydimethylsiloxane–polyurethane elastomers: Synthesis and properties of segmented copolymers and related zwitterionomers
Authors: Xue‐Hai Yu , Nagarajan M. R. , Grasel T. G. , Gibson P. E. , Cooper S. L. 
Published: April 2003

AbstractA series of polyurethane block polymers based on hydroxybutyl‐terminated polydimethyl‐siloxane soft segments of molecular weight 2000 were synthesized. The hard segments consisted of 4,4′‐methylenediphenylene diisocyanate (MDI) which was chain extended with either 1,4‐butanediol (BD) or N‐methyldiethanolamine (MDEA). The MDEA‐extended materials were ionized by using 1,3‐propane sultone. The weight fraction of hard segments was in the range 0.13–0.39. The morphology and properties of these polyurethane elastomers were studied by a variety of techniques. All of these short‐segment block copolymers showed nearly complete phase separation. The zwitterionomer materials exhibited ionic aggregation within the hard domains. Hard‐segment crystallinity or ionic aggregation did not affect the morphology. Hard‐domain cohesion was found to be a more important factor than hard‐domain volume fraction in determining the tensile and viscoelastic properties of these elastomers.
Polyethylene fiber formation in tubular flow. I. Observations of growth pattern and fiber properties
Authors: Rietveld J. , Mchugh A. J. 
Published: April 2003

AbstractThe process of seeded growth of fibrillar polyethylene crystals has been studied in a tubular flow geometry for 0.01‐wt % solutions of a high‐molecular‐weight polyethylene in xylene. The transformation sequence has been followed visually by using polarized‐light illumination in conjunction with a video camera. Data are presented to show that transformation is initiated by the formation of a concentrated, unoriented, amorphous precursor fiber within which oriented birefringent crystals subsequently grow in consequence of the stresses transmitted by the flowing solution. Time constants for the precursor formation, birefringence initiation, and completion of birefringence were measured as functions of temperature and flow rate over a range of growth conditions. Wide‐angle x‐ray diffraction, overall birefringence, and optical hot‐stage melting data were also obtained on the grown fibers. The net result of these observations is to conclude that fibrillar crystal growth during flow is always preceded by the formation of a liquidlike phase transformation which produces the concentrated, unoriented precursor. Subsequent orientation is in consequence of stress‐induced crystallization with overall fiber orientation showing an increase with solution flow rate at a fixed temperature and a decrease with temperature at a fixed flow rate. At higher temperatures and lower flow rates, birefringence develops in an oscillatory fashion, indicating a remelting process possibly due to slippage of trapped chain entanglements formed by flow. A discussion is given of the implications of these observations for the understanding of flow‐induced structure development, phase transformation, and oriented crystallization; this is expanded upon in a companion paper, Part II.
Estimation of the equilibrium modulus of rubber networks by nonlinear regression analysis
Authors: Scholtens Boudewijn J. R. , Hillegers L. Tom 
Published: April 2003

AbstractA nonlinear regression analysis is presented to estimate the equilibrium modulus of relaxing rubber networks from G'(ω) or G(t) data. Various semiempirical curve‐fitting equations are tested with two‐function minimization algorithms.
Small‐angle light scattering from a distribution of spherulite sizes
Authors: Wissler G. E. , Crist B. 
Published: April 2003

AbstractDepolarized small‐angle light scattering from spherulites in semicrystalline polymers gives rise to a characteristic cloverleaf pattern. For scattering from a single spherulite, the position of the maximum in scattered intensity is readily related to the spherulite radius. For a distribution of spherulites, the maximum should be related to some characteristic measure of the distribution. It is shown for a wide variety of distributions that this characteristic radius is a ratio of high moments of the size distribution, specifically R* ≈ 〈R7〉/〈R6〉. The shape of the light‐scattering profile should in principal be related to the nature of the spherulite distribution. Calculations of scattering profiles from a variety of distributions fail to demonstrate this, owing to the strong dependence of scattering power on spherulite size. Exceptions are noted for the case of certain bimodal distributions.
Temperature dependence of the crystal structures of nylon 11
Authors: Kim K. G. , Newman B. A. , Scheinbeim J. I. 
Published: April 2003

AbstractCareful examination of x‐ray diffraction patterns from melt‐crystallized nylon 11 films show significant discrepancies with the proposed α‐form structure. These discrepancies do not disappear after the samples have been annealed. The temperature dependence of the d spacings of the two strongest peaks show further evidence that the melt‐crystallized and solution‐cast films (α form) possess different crystal structures. These results suggest a different crystal structure for the melt‐crystallized films; this would help explain the rather low piezoelectric response of these films and also the failure to observe a rapid decrease in polarization at the transition temperature.
The influence of morphology on the dielectric and dynamic mechanical behavior of a linear low‐density polyethylene
Authors: Jang Y. T. , Parikh D. , Phillips P. J. 
Published: April 2003

AbstractThe dielectric and viscoelastic relaxation behavior of linear low‐density polyethylene has been investigated. All three usual relaxation regions occur; however, the α process is more dominant than in conventional low‐density polyethylene made dielectrically active through the introduction of carbonyl groups. The observed behavior is very sensitive to orientation and morphology. There are significant differences between oriented samples (stretching or extrusion) and a single‐crystal texture specimen. Effects are caused by dipolar orientation and dipolar immobilization, the relative importance of each effect varying with specimen preparation. Active dipolar groups are not carbonyls, or any common organic species, but appear to be due to chain ends having residual dipoles caused by the catalyst used in synthesis.
Crazing as quasifracture in an oriented viscoelastic medium
Authors: Chern S. S. , Zhang Z. D. , Hsiao C. C. 
Published: April 2003

AbstractCrazes of different types occur in polymeric systems. Long, sparse crazes develop in less oriented molecular systems, while fine, short, dense crazes occur in highly oriented molecular systems. Different crazing mechanisms and different models may have to be studied for a better understanding of these differences. However, in this report, using one model and one theory, an analysis has been made of the differences in the geometry of craze development. By emphasizing the mechanism of molecular orientation, it is found that the basic differences are essentially attributable to the variation of the anisotropy of the material system as a result of large deformation rather than to any fundamental differences in the crazing mechanism.
Studies on comblike polymers. X. Characterization of comblike polyesters by 13C NMR spectroscopy
Authors: Segre Anna Laura , Andruzzi Fulvio , Barone Corrado , Paci Massimo , Magagnini Pier Luigi 
Published: April 2003

AbstractThe 13C NMR spectra of aromatic comblike polyesters obtained from 1,2‐icosanediol and isomeric phthalic acids are reported and discussed. The analysis shows that the most informative spectral regions are those of the carbonyl and quaternary carbon resonances, whose peak multiplicities have been interpreted in terms of dyad and triad sequences. The results indicate that the extent of structural characterization possible for the polymers examined is dependent on the type of isomerism of the aromatic acid involved.
Estimating correlation by using a general linear mixed model: evaluation of the relationship between the concentration of HIV‐1 RNA in blood and semen
Authors: Chakraborty Hrishikesh , Helms Ronald W. , Sen Pranab K. , Cohen Myron S. 
Published: April 2003

AbstractEstimating the correlation coefficient between two outcome variables is one of the most important aspects of epidemiological and clinical research. A simple Pearson's correlation coefficient method is usually employed when there are complete independent data points for both outcome variables. However, researchers often deal with correlated observations in a longitudinal setting with missing values where a simple Pearson's correlation coefficient method cannot be used. General linear mixed models (GLMM) techniques were used to estimate correlation coefficients in a longitudinal data set with missing values. A random regression mixed model with unstructured covariance matrix was employed to estimate correlation coefficients between concentrations of HIV‐1 RNA in blood and seminal plasma. The effects of CD4 count and antiretroviral therapy were also examined. We used data sets from three different centres (650 samples from 238 patients) where blood and seminal plasma HIV‐1 RNA concentrations were collected from patients; 137 samples from 90 different patients without antiviral therapy and 513 samples from 148 patients receiving therapy were considered for analysis. We found no significant correlation between blood and semen HIV‐1 RNA concentration in the absence of antiviral therapy. However, a moderate correlation between blood and semen HIV‐1 RNA was observed among subjects with lower CD4 counts receiving therapy. Our findings confirm and extend the idea that the concentrations of HIV‐1 in semen often differ from the HIV‐1 concentration in blood. Antiretroviral therapy administered to subjects with low CD4 counts result in sufficient concomitant reduction of HIV‐1 in blood and semen so as to improve the correlation between these compartments. These results have important implications for studies related to the sexual transmission of HIV, and development of HIV prevention strategies. Copyright © 2003 John Wiley & Sons, Ltd.
From single‐race reporting to multiple‐race reporting: using imputation methods to bridge the transition
Authors: Schenker Nathaniel , Parker Jennifer D. 
Published: April 2003

AbstractIn 1997, the Office of Management and Budget issued revised standards for the collection of race information within the Federal statistical system. One revision allows individuals to choose more than one race group when responding to Federal surveys and other Federal data collections. This paper explores methods that impute single‐race categories for those who have given multiple‐race responses. Such imputations would be useful when it is desired to conduct analyses involving only single‐race categories, such as when trends over time are being examined by race group so that data collected under the old and new standards are being combined. The National Health Interview Survey has allowed multiple‐race responses for several years, while also asking respondents to specify one race as their primary race. Exploratory analyses of data from the survey suggest that imputation methods that use demographic and contextual covariate information to predict primary race can have advantages with respect to lower bias and improved variance estimation compared to simpler methods discussed by the Office of Management and Budget. It also appears, however, that the relationships between primary race and covariates might be changing over time. Thus, caution should be exercised if an imputation model fitted to data from one time period is to be applied to data from another time period. Published in 2003 by John Wiley & Sons, Ltd.