bluuebunny/crossref_metadata_2025_split huggingface dataset:
The production and properties of poly(vinylidene fluoride) rods oriented by drawing through a conical die
Authors: Richardson A. , Hope P. S. , Ward I. M. 
Published: April 2003

AbstractHighly oriented poly(vinylidene fluoride) rods have been produced by drawing isotropic polymer through a conical die. A range of oriented products was obtained, depending on the drawing temperature and the deformation ratio achieved. At low draw temperatures the draw ratio and final modulus are comparatively low, but a high Form I crystal content is obtained. At high draw temperatures the Form I content varies greatly with draw ratio, reaching high values at large draw ratios, where the highest‐modulus samples were obtained.
Line broadening in the 13C NMR spectra of bulk polymers above Tg
Authors: Komoroski Richard A. 
Published: April 2003

AbstractThe effects of magic‐angle sample spinning (MAS), high‐power decoupling, and resonance frequency on the 13C NMR linewidths of bulk polyisobutylene and bulk trans‐polybutadiene are examined. The 13C linewidths increase with resonance frequency, are unaffected by high‐power decoupling, and are reduced to different extents by MAS. The dominant contribution of the natural linewidth of the polyisobutylene lines is confirmed. The two carbons of trans‐polybutadiene have approximately equal linewidths under all conditions, a result that eliminates residual chemical shift anisotropy as a major contributor to the linewidths. The large reduction of the trans‐polybutadiene linewidths with MAS, coupled with the above result, suggests that microscopic variation of magnetic susceptibility is the major factor for this semicrystalline polymer. Cross‐polarized 13C spectra of trans‐polybutadiene were obtained with and without MAS. With MAS, resonances due to the crystalline and amorphous components were resolved. The principal components of the chemical shift tensor of the vinylene carbons were obtained from the spectrum without MAS.
Studies of TSD of polyethylene electrets by a differential technique
Authors: Reboul Jean‐Pierre , Toureille Alain 
Published: April 2003

AbstractThe determination of charge storage and charge decay processes in polyethylene electrets by means of TSD techniques alone seems to be limited by deficiencies in resolving power and reproducibility. Therefore, in the present work, we describe a new differential technique which cancels the effects of parasitic currents and improves the reproducibility of the measurements. The dependence of the shape of thermograms on experimental poling conditions has been studied. The comparison of discharge currents from the differential technique and from the usual one has also been found to be an interesting means of investigating the charge decay processes.
Fourier‐transform infrared study of polystyrene/poly(vinyl methyl ether) blends
Authors: Garcia Dana 
Published: April 2003

AbstractFourier‐transform infrared (FTIR) studies of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends are presented. Both compatible (one‐phase) and phase‐separated blends were studied. In the case of compatible PS/PVME blends, there is strong evidence for molecular interactions. The interaction spectrum was obtained by digital subtraction techniques. In contrast, no interaction is detected for the phase‐separated blends. In view of these results, molecular interactions must play a role in the compatibility of the two polymers. The merits of factor analysis and least‐squares fit methods, as pertaining to our data, are also discussed.
NMR study of the tie‐chain length distribution in oriented polymers
Authors: Zhizhenkov V. V. , Egorov E. A. 
Published: April 2003

AbstractWide‐line NMR has been used in an investigation of noncrystalline (amorphous) regions in oriented semicrystalline polymers. Nylon 6 was chosen as a model material. The tie‐chain length distribution function, the fraction of tie chains in the total number of chains in the crystallite cross section, and the relative number of taut tie chains have been determined. The data on the tie‐chain length distribution are used in discussing specific features of vitrification of the amorphous regions in oriented polymers and in prediction macroscopic mechanical properties.
Application of the transverse differential swelling stress model to the interpretation of case II diffusion kinetics
Authors: Petropoulos J. H. 
Published: April 2003

AbstractAttention is called to an early observation of Case II diffusion kinetics during penetration of a liquid into a uniaxially prestretched polymer film along the axis of stretching. Application of a previously developed transverse differential swelling stress model to these experimental conditions is described. It is shown that the model can predict Case II diffusion kinetics in a physically meaningful manner and will also explain the transition of the kinetics of the above system into normal or hindered diffusion regimes as either the degree of stretching or its direction relative to that of penetration is changed.
Effect of diluents on poly(vinyl acetate) dynamics
Authors: Blum Frank D. , Dickson Jane E. , Miller Wilmer G. 
Published: April 2003

AbstractThe effect of diluents and temperature on segmental motion in poly(vinyl acetate) was investigated by both NMR and ESR spectroscopy. Three classes of diluents were studied: chloroform, a thermodynamically good solvent; water, a poor solvent which slightly swells the polymer and lowers its glass transition temperature; and decane, a nonsolvent so poor it does not appear to swell the polymer nor lower the calorimetric glass transition temperature. At all temperatures investigated each type of diluent increased the segmental motion of the polymer over that of the bulk sample. Under the conditions studied, 13C and 2H NMR and nitroxide spin‐label ESR data gave similar views of segmental motion of the polymer, indicating that in this amorphous polymer the segmental motion of the polymer may be safely inferred from spin‐label data.
Transport of hydrogen and carbon monoxide in highly crosslinked poly(propylene glycol) networks
Authors: Andrady A. L. , Sefcik M. D. 
Published: April 2003

AbstractPrevious interpretations of gas transport data in crosslinked networks have been hindered by an inability to accurately control and evaluate the network parameters. We have recently prepared a series of model networks by reacting poly(propylene glycol) with a triisocyanate crosslinking agent. The poly(propylene glycol)s had narrow molecular weight distributions and average molecular weights between 425 and 3000, so the resulting networks had uniform average molecular weights between crosslinks. Hydrogen and carbon monoxide permeabilities in membranes formed from these networks increase with decreasing crosslink density. These results indicate increased cooperative molecular motions in the networks with longer average chain lengths between crosslinks. Increasing the average molecular weight between crosslinks also reduces the discrimination between these two gases so that the separation factors decrease. For networks prepared from mixtures of poly(propylene glycol)s with different molecular weights the gas permeabilities (but not the separation factors) depend on the molecular weight distribution.
Chlorination mechanism of polyethylene crystals
Authors: Chang B. H. , Siegmann A. , Hiltner A. 
Published: April 2003

AbstractA new chlorination mechanism has been suggested for polyethylene crystals. Based on electron microsocopy and infrared spectroscopy of chlorinated high‐density polyethylene single crystals, it is proposed that beyond the initial chlorination of the lamellae surfaces, which accounts for only 2‐3% chlorine uptake, the reaction proceeds through the lamellar side faces from the periphery inward. Thus at intermediate degrees of chlorination two major phases coexist: the undamaged interior regions of the lamellae and the surrounding chlorinated material. This mechanism differs from that of oxidative degradation, and also from the generally Accepted mechanism of chlorination. Rather than proceeding through the reacted fold surface into the crystalline core, it is proposed that the direction of chemical attack is prependicular to that suggested previously. This mechanism probably describes any chemically nondestructive attack on the crystalline regions of polymers in general.
Formation, structure, and elasticity of loosely crosslinked epoxy‐amine networks. II. Mechanical and optical properties
Authors: Ilavský M. , Bogdanova L. M. , Dušek K. 
Published: April 2003

AbstractThe mechanical and optical behavior in the dry and swollen states of loosely crosslinked epoxy networks prepared from the diglycidyl ether of bisphenol A, phenylglycidyl ether, and 4,4′‐diaminodiphenylmethane was investigated, and the weight fraction of sol in the networks was determined. The crosslinking density was controlled by an excess of diamine and by the fraction of monoepoxide. The reaction proceeded to almost full conversion of epoxy groups. With increasing content of monoepoxide or with increasing excess of diamine, the main transition region is shifted to lower temperatures. The dependence of the viscoelastic modulus on temperature and the optical behavior indicate that the networks are homogeneous. In all cases, the sol fraction is adequately described by the theory of branching processes (cf. Part I). The equilibrium modulus related to the dry state is the same irrespective of whether it is obtained by measurements in the dry or swollen state. The mechanical behavior in the rubbery state can be described by the theory of phantom networks with fully suppressed fluctuations of crosslinking (front factor A = 1) or by the theory of phantom networks with fully released fluctuations of crosslinks (front factor) A = fe−2/fe] and contribution of trapped entanglements of the Langley‐Graessley type (cf. Part I). In the analysis of the equilibrium behavior, it is advantegeous to use the plot of superimposed dependences of Ge on the gel fraction, which considerably reduces the effect of experimental inaccuracy in determination of composition and degree of conversion.
Far‐infrared and x‐ray studies on poled semicrystalline poly(vinylidene fluoride)
Authors: Latour M. , Dorra H. Abo , Galigné J. L. 
Published: April 2003

AbstractFar‐infrared (FIR) Fourier‐transform transmission spectra and x‐ray diagrams of poly(vinylidene fluoride) films are examined in terms of poling‐field action on the crystalline phase (α and β forms) and on the amorphous phase. We show that the poling field induces an increase of crystallinity in the α phase as well as in the β phase, and for the latter an orientation of dipoles in the field direction. For samples containing a mixture of phases α and β we note an α to β phase transition, a rotation of the dipoles in the β phase, and an increase of the crystallinity of the sample. Part of the β phase is generated from the amorphous phase.
The thermodynamic properties of polytetrafluoroethylene
Authors: Fai Lau Suk , Suzuki Hidematsu , Wunderlich Bernhard 
Published: April 2003

AbstractPolytetrafluoroethylenes of different crystallinity were analyzed between 220 and 700 K by differential scanning calorimetry. A new computer coupling of the standard DSC is described. The measured heat capacity data were combined with all literature data into a recommended set of thermodynamic properties for the crystalline polymer and a preliminary set for the amorphous polymer (heat capacity, enthalpy, entropy, and Gibbs energy; range 0–700 K). The crystal heat capacities have been linked to the vibrational spectrum with a θ3 of 54 K, and θ1 of 250 K, and a full set of group vibrations. Cv to Cp conversion was possible with a Nernst–Lindemann constant of A = 1.6 × 10−3 mol K/J. The glass transition was identified as a broad transition between 160 and 240 K with a ΔCp of 9.4 J/K mol. The room‐temperature transitions at 292 and 303 K have a combined heat of transition of 850 J/mol and an entropy of transition of 2.90 J/K mol. The equilibrium melting temperature is 605 K with transition enthalpy and entropy of 4.10 kj/mol and 6.78 J/K mol, respectively. The high‐temperature crystal from is shown to be a condis crystal (conformationally disordered), and for the samples discussed, the crystallinity model holds.
Dynamic mechanical studies of α and β relaxations of polyethylenes
Authors: Popli R. , Glotin M. , Mandelkern L. , Benson R. S. 
Published: April 2003

AbstractThe α and β relaxations of a variety of polyethylenes have been extensively studied using lowfrequency dynamic mechanical methods. The main focus of this work has been both the control and the quantitative measurement of the key structural factors that describe semicrystalline polymer systems. The structural factors that have been examined in detail include the level of crystallinity, the crystallite thickness, the interfacial content, and the supermolecular structure. Consequently a variety of other types of supplementary measurements were made to accomplish the necessary characterization. The location of the α transition is found to depend primarily on the crystallite thickness. There also is the distinct possibility that the interfacial structure exerts an important influence. The level of crystallinity and the supermolecular structure do not play a significant role in the location of Tα. A strong correlation is found with the carbon‐13 NMR crystalline T1, which is reported in a separate paper. From analysis of the influence of the different structural factors on the β transition, it is concluded that this transition results from the relaxation of chain units which are located in the interfacial region. The elusiveness of this transition and the contradictory reports that have existed in the literature are given a ready explanation. The enhancement of this transition by branching and copolymerization follows naturally as does its invariance with counit content.
Isothermal melting behavior of poly(L‐lactic acid)
Authors: Kishore K. , Vasanthakumari R. , Pennings A. J. 
Published: April 2003

AbstractThe isothermal melting behavior of poly(L‐lactic acid) spherulites has been studied in order to understand the mechanism of melting using hot‐stage microscopy. When the periphery of the spherulite melts or shrinks, the central portion undergoes thickening as a result of which the melting or shrinking rate is not linear. Annealing studies at 448 K reveal that there is an increase in melting temperature and heat of fusion with annealing time, suggesting an increase in lamellar thickness. The high activation energy (828 kJ/mol) obtained in this investigation suggests the highly complex nature of the melting process wherein both melting and thickening occur.
The effects of morphology on 1H NMR spectra and relaxation in semicrystalline polyolefins
Authors: Packer K. J. , Pope J. M. , Yeung R. R. , Cudby M. E. A. 
Published: April 2003

AbstractA brief résumé is given of the role of structural heterogeneity, magnetic dipolar couplings, molecular structure, and molecular motion in determining the 1H NMR spectra and relaxation properties of heterogeneous solids such as semicrystalline polymers. Measurements of 1H spin‐lattice relaxation in laboratory (T1) and rotating frames (T) are reported for a number of solid polyolefin samples. These include solution‐crystallized and melt‐crystallized polyethylene, annealed and quenched isotactic polypropene, and isotactic polybut‐1‐ene. In addition, broad‐line 1H spectra, both normal and partially (T) relaxed, are reported for these materials as well as a number of pulsed NMR experiments having the philosophy of the so‐called Goldman–Shen experiment. Spin‐lattice relaxation (T1) for all samples is a single exponential process, whereas rotating‐frame relaxation comprises three exponential processes both on‐resonance (θ = 90°) and off‐resonance at the magic angle (θ = 54.7°), with the latter generally being much slower. The spectra show clearly the existence of components having differing degrees of mobility and, with the exception of the solution‐crystallized polyethylene, the partially (T) relaxed spectra indicate a correlation between breadth of resonance line and magnitude of T1ρ. The Goldman–Shen‐type experiments indicate a spin‐diffusional transport of magnetization between the different spectral and (T) components. A computer program has been used to simulate the NMR behavior of a three‐region system comprising repeating units of infinite lamellae of different widths, each region having different intrinsic relaxation times and spin diffusion coefficients. The results demonstrate that the observed 1H NMR behavior of these samples can be interpreted in terms of this model and that, inter alia, the long‐time T behavior reflects, qualitatively, the time taken for magnetization to diffuse a distance of the order of the dimensions of the region to which it corresponds.
Correction factors for sizing weakly anisotropic transparent fibers using light scattering
Authors: Sharma S. K. , Somerford D. J. 
Published: April 2003

AbstractCorrection factors defined as the ratio of the true size parameter and the size parameter deduced from Fraunhofer diffraction theory are presented for weakly anisotropic transparent fibers. These correction factors enable the Fraunhofer diffraction approximation to be used for sizing weakly anisotropic fibers from light‐scattering measurements. Fibers in the diameter range 4–50 μm and refractive index range 1.45–1.56 are considered. It is shown that for fibers having birefringence −0.005 ≤ Δm ≤ 0.01, the diameters may be obtained to within 9% accuracy if the fiber diameter lies between 4 and 50 μm.
Polypropylene spherulite morphology and growth rate changes in blends with low‐density polyethylene
Authors: Galeski A. , Pracella M. , Martuscelli E. 
Published: April 2003

AbstractMorphological studies of isotactic polypropylene/low‐density polyethlene blends revealed that the shape, size, and orientation of LDPE occlusions remain undisturbed during the crystallization of IPP spherulites. On the other hand, LDPE occlusions introduce large changes in the internal structure of IPP spherulites. It was found that many new boundaries similar to those between spherulites are fromed when the crystallizing front passes the LDPE occlusion. Dead‐ended boundaries with soft LDPE occlusions at the ends give rise to an improvement of the impact properties. It was shown that LDPE obstacles do not influence significantly the IPP spherulite growth rate in thin films or in the bulk.
Theory of the rod‐to‐coil transition in polydiacetylene
Authors: Berlinsky A. J. , Wudl F. , Lim K. C. , Fincher C. R. , Heeger A. J. 
Published: April 2003

AbstractA theory is proposed for the rod to coil transition in polydiacetylene 4BCMU and related polymers which is based on the hypothesis that the high‐temperature (yellow) phase consists predominantly of the cis structure, while the low‐temperature (red) phase is trans. Because the occurrence of a cis‐trans interface is energetically costly, the correlation length for either isomer remains long and the transition is sharp, much like the helix‐coil transition in the theory of Zimm and Bragg. The transition is driven by the higher entropy of the cis isomer, which is free to coil, unlike the trans or butatriene forms. The theory gives excellent agreement with optical absorption measurements and is consistent with all other experimental data for this system.
Effects of polymer chain disentanglement on NMR properties. II. 13C magnetic relaxation in polystyrene
Authors: Cohen‐Addad J. P. , Feio G. 
Published: April 2003

AbstractThe transverse magnetic relaxation of 13Cα nuclei has been studied in concentrated solutions of polystyrene. The magnetic relaxation rate was measured as a function of molecular weight at several temperatures (313,318, and 323 K) and at several concentrations (0.53, 0.43, and 0.34 g/cm3). The spin‐system response of these nuclei in natural abundance exhibits a characteristic evolution from pseudosolid properties to liquidlike one, induced by decreasing the molecular weight of polymer molecules. This evolution is analogous to that already observed in protons attached to polyisobutylene or polydimethylsiloxane chains; it is assumed to be induced by an increase of the disentanglement rate of polymer chains. The spin‐system response may be considered as reflecting single‐chain magnetic properties, because of the low concentration of 13CCα nuclei, although all chains are in dynamic interaction with one another. The NMR disentanglement transition is interpreted in terms of a two‐step motional averaging effect involving submolecules. A numerical analysis of NMR properties is given using a model of polymer chain relaxation based on a multiple‐mode relaxation process, characterized by (i)a terminal relaxation time τv1 depending upon M3, the molecular weight, and approximately proportional to the polymer concentration C (like the reptation time); (ii)a relaxation‐time spectrum analogous to a Rouse spectrum; (iii)a terminal relaxation time τv1 = 2.5 × 10−2s for M = 2.5 × 105, C = 0.53 g/cm3 in carbon tetrachloride at 313 K.
Molecular parameters for the prediction of polyurethane structures
Authors: Blackwell J. , Quay J. R. , Nagarajan M. R. , Born L. , Hespe H. 
Published: April 2003

AbstractRecent efforts to determine the structures of poly(MDI/diol) hard segments in polyurethane elastomers have relied on the structures determined by single‐crystal x‐ray methods for diphenylmethane urethane model compounds. We have surveyed the structure of six model compounds, and have derived average values for the bond lengths, bond angles, and bond torsion angles for use in future analyses. The applicability of these averages to polymer structures is discussed, and the data are used to derive models for the poly(MDI‐butanediol) chain which are found to be consistent with the fiber repeat determine by x‐ray methods.
An analysis of the crystallization behavior of poly(ethylene oxide)/poly(methyl methacrylate) blends by spectroscopic and calorimetric techniques
Authors: Li X. , Hsu S. L. 
Published: April 2003

AbstractInfrared spectra in conjunction with calorimetric measurements have been used to follow the crystallization process and microstructural changes of poly(ethylene oxide) (PEO) in poly(ethylene oxide) and poly(methyl methacrylate) (PMMA) blends. We have given particular attention to compositions containing low PEO concentrations. The crystallization behavior and the resultant microstructures of PEO are strongly perturbed by the presence of PMMA. In addition, we found phase separation and trans sequences of PEO to be present, especially at low PEO concentrations.