bluuebunny/crossref_metadata_2025_split huggingface dataset:
Characterizing the kinetics of volume recovery in glasses by instantaneous temperature‐jump experiments
Authors: Lagasse R. R. , Cohen R. E. , Letton A. 
Published: April 2003

AbstractThis article proposes a temperature‐jump (T‐jump) approach for characterizing the kinetics of volume recovery in glassy materials. The kinetic characterization is based on the Kovacs‐Aklonis model. This incorporates a retardation‐time spectrum which shifts according to both the temperature and the instantaneous volume. The proposed experiments involve measuring the change in recovery rate caused by an abrupt temperature jump. Although an analogous procedure has been used to determine the activation energy for linear viscoelastic creep, the analysis for volume recovery is complicated by its inherent nonlinearity. Nevertheless, accounting for the nonlinearity by a reduction of the time scale permits the T‐jump results to be analyzed. In particular, the T‐jump approach can be used to: (i) test a particular functional form for the shift factor and (ii) determine the previously unmeasurable parameter x, which defines the relative importance of the temperature dependence and the volume dependence in this function. In addition, numerical simulations indicate that the proposed method can be implemented in the laboratory.
Transient decay studies of photophysical processes in aromatic polymers. V. Temperature dependence of excimer formation and dissociation in copolymers of 1‐vinylnaphthalene and methyl methacrylate
Authors: Phillips D. , Roberts A. J. , Soutar I. 
Published: April 2003

AbstractThe use of pulsed laser excitation upon a series of 1‐vinylnaphthalene/methyl methacrylate co‐polymers in which the intramolecular chromophore concentration is varied over a wide composition range has allowed the individual rate constants governing intramolecular excimer formation to be determined at various temperatures between 205 and 295 K. Triple exponential functions are necessary for adequate description of the photophysical kinetics over the entire temperature range. It is proposed that the kinetic and temperature behavior is consistent with a kinetic scheme involving two kinetically distinct monomeric species and an excimer in which energy may be transferred from the second monomer M*2 to the first monomer M*1 which can interact with the excimer by a conventional migration mechanism.
Determination of branching distributions in polyethylene and ethylene copolymers
Authors: Wild L. , Ryle T. R. , Knobeloch D. C. , Peat I. R. 
Published: April 2003

AbstractA description is given of an analytical temperature‐rising elution fractionation (TREF) system for the purpose of determining short‐chain branching (SCB) or copolymer distributions in poly‐ethylenes and ethylene copolymers. The system achieves fractionation on the basis of crystallizability and is shown to be very little influenced by molecular weight in the normal high polymer range. Sample preparation by slow cooling from relatively dilute solution followed by continuous elution with a simultaneous and fairly rapid rate of temperature rise proves to be an efficient fractionation process. An on‐line detector and data system allows application of a calibration curve to give realistic SCB distribution data in a convenient manner. The potential value of the TREF technique for providing structural information is illustrated by examples which include low‐density high‐pressure resins made by both tubular and autoclave reactors, high‐ and low‐density resins made by low‐pressure processes, and copolymers of ethylene with vinyl acetate and ethyl acrylate.
Model networks of end‐linked polydimethylsiloxane chains. XIV. Stress‐strain, thermoelastic, and birefringence measurements on the bimodal networks at very low temperatures
Authors: Zhang Z.‐M. , Mark J. E. 
Published: April 2003

AbstractBimodal networks consisting of very short and relatively long poiydimethylsiloxane (PDMS) chains were studied from 30 to −52°C in an attempt to elucidate the anomalous increases in modulus [f*] exhibited by such elastomeric materials at high elongations. Temperature was found to have very little effect on (i) the elongation at which the upturn in [f*] becomes discernible, (ii) the elongation at which rupture occurs, and (iii) the total increase in [f*] up to the rupture point. The standard force‐temperature (“thermoelastic”) plots were linear, but gave values of the energetic contribution to the total force which were significantly smaller than those universally obtained on unimodal, long‐chain PDMS networks. Birefringence‐temperature relations were also found to be linear, and yielded values of the optical‐configuration parameter and its temperature coefficient which were in satisfactory agreement with the corresponding values reported for unimodal PDMS networks. These results indicate that even at very low temperatures the observed increases in modulus (and consequent improvements in ultimate strength) are due to non‐Gaussian effects arising from limited chain extensibility, rather than from intermolecular reinforcing effects such as strain‐induced crystallization.
Kinetics of microstructure rearrangement during annealing of cold‐drawn isotactic polypropylene
Authors: Babajko Suzanne , Schultz J. M. 
Published: April 2003

AbstractThe microstructure of polypropylene, annealed after cold drawing to an oriented state, was examined using density measurements and small‐ and wide‐angle x‐ray scattering (SAXS and WAXS) techniques. SAXS patterns were obtained after annealing (in an oil bath), and during annealing (of samples annealed in a furnace placed directly in the x‐ray beam). Data, for isothermal annealing, showed an increase in SAXS intensity, in crystal perfection, and in density over the observed time range, but no change in long spacing during the same period. Long spacing, SAXS intensity, and density were all strongly dependent upon annealing temperature, increasing at higher temperatures. Upon annealing the elastic modulus and yield strength dropped below the as‐drawn values in an immeasurably short time, and did not appear to change thereafter during the time range examined. The decrease was more marked for higher‐temperature annealing. The kinetics of the micro‐structural changes are compared qualitatively to the predictions of nucleation and growth theory and of spinodal decomposition. The defects in the microstructure are considered as the “solute molecules” of the spinodal model. The experimental results do not agree neatly with either model. However, a nonequilibrium thermodynamic approach appears to be the more promising. It is suggested that the kinetics of mechanical property change may be due to different rates of migration for different defect species.
The crystal structure of the alternating copolymer of hexafluoroisobutylene and vinylidene fluoride
Authors: Weinhold S. , Litt M. H. , Lando J. B. 
Published: April 2003

AbstractThe crystal structure of the alternating copolymer of hexafluoroisobutylene and vinylidene fluoride, P(HFIB/VF2), was determined using x‐ray diffraction techniques. Oriented specimens of the copolymer were produced by drawing melt‐pressed film at 300°C. The unit cell of P(HFIB/VF2) is orthorhombic with dimensions at a = 1.064, b = 1.837, and c = 0.783 nm. The chain conformation is a 21 helix with an approximate trans‐trans‐gauche‐gauche backbone dihedral‐angle sequence. Severe intrachain steric crowding exists; as a consequence, the chain is quite rigid and some of the backbone bond angles are highly extended. The unit cell contains four such chains packed according to the symmetry of space group Iba2. Neighboring chains in a crystal are mechanically interlocked; in combination with the rigidity of the chains, this is the basis for the superior mechanical properties of the copolymer compared to PTFE. A structure containing up to 20‐30% of head‐to‐head residues was found to be statistically indistinguishable from a completely head‐to‐tail structure.
A neutron scattering analysis: How does the conformation in polyethylene crystals reflect that in the melt?
Authors: Sadler D. M. , Harris R. 
Published: April 2003

AbstractNeutron scattering data for melt‐crystallized polyethylene have been analyzed in order to clarify to what extent the chain folding is randomly reentrant. No attempt has been made to specify the molecular conformation in every detail, and the emphasis is on distinguishing between different classes of conformation. The most random folding corresponded to a model where the folding is imposed solely by the criterion of the chain segments moving the least possible distance during the crystallization process (a “freezing‐in” model). This has been shown not to be compatible with published data. For this model analytic calculations are possible based on the projection of a three‐dimensional Gaussian distribution onto a plane. A subunit model is then proposed which requires substantial local rearrangement of the chain as it folds during crystallization, but where the distribution of the subunits within the whole molecule is imposed by the preexisting Gaussian chain of the melt. Arguments based on space filling considerations are invoked, with the postulate of a surface structure which is neither crystalline nor truly amorphous. Anything approaching a random switchboard model (e.g., the freezing‐in model which we consider) is contrary to both space filling considerations and to the comparison of observed and calculated neutron scattering. The analytical calculation which was performed for the freezing‐in model was employed so as to simplify calculations for the subunit model. For scattering intensities over a wide range of scattering angle it is deduced that only the structure within the subunit need be considered. Numerical computer calculations involving only a small number of stems were then carried out for a number of different subunit structures, and some general features are noted which restrict the type of model which can explain the data. As in previously published analyses, a very high proportion of adjacent folds is not compatible with the results. A row model for the stems within a molecule can achieve good agreement, either with straight rows or with a certain amount of “stagger” incorporated. Up to about 40% of the folds could be adjacent. Models based on two‐dimensional random walks did not give good agreement.
High‐resolution dark‐field imaging in epoxy and polyimide systems
Authors: Oberlin A. , Ayache J. , Oberlin M. , Guigon M. 
Published: April 2003

AbstractThe feasibility of molecular imaging in polymers by electron microscopy is discussed. The tilted dark‐field method is specifically chosen for studying ultramicrotome thin sections. One system studied here is bisphenol‐A diglycidyl ether epoxide monomer (DGEBA) amine cured either by dicyandiamine or by diaminodiphenyl sulfone under various conditions. The second sample is a polyimide prepared from maleic anhydride and diaminophenyl methane. The distribution of the molecules is consistent with the random‐coil model when neither etching nor strains are applied. Bisphenol‐A groups distributed at random in the polymer are imaged in DGEBA as units 10 Å long and 5 Å wide, whereas larger isometric diffracting domains (10–15 Å) are imaged in polyimides.
Thermal expansion of ultradrawn polystyrene
Authors: Wang Li‐Hui , Choy C. L. , Porter Roger S. 
Published: April 2003

AbstractMeasurements on the linear thermal expansivities α∥ and α⊥ parallel and perpendicular to the extrusion direction, respectively, have been carried out between −160 and 30°C for atactic polystyrene (aPS) with extrusion ratio 1 ≤λ≤ 15 and isotactic polystyrene (iPS) with 1 ≤λ≤ 7.5. For both aPS and iPS, α∥ decreases sharply with increasing λ whereas α⊥ shows only a slight increase. Below λ = 5 the anisotropy α⊥/α∥ is nearly the same for both, but α⊥/α∥ for iPS becomes much larger at higher λ. This is accompanied by an abrupt rise in crystallinity and probably results from the increase in the number and tautness of intercrystalline tie molecules. The birefringences of aPS and iPS increase with λ and have nearly the same values for λ < 5. At higher λ, however, there is a sharp rise in the birefringence of iPS which is probably associated with the sudden increase in crystallinity. With an aggregate model, the chain orientation function for aPS calculated from thermal expansivity is found to be in reasonable agreement with the corresponding value obtained from birefringence.
Spin‐lattice relaxation in the rotating frame in solutions of poly(2,6‐dimethyl‐1,4‐phenylene oxide)
Authors: Jones Alan Anthony , Shea Francis P. 
Published: April 2003

AbstractMethyl‐proton spin‐lattice relaxation times in the rotating frame T1ρ are reported for 10‐wt % solutions of poly(2,6‐dimethyl‐1,4‐phenylene oxide) in CDCl3. These measurements were made as a function of molecular weight and at two Larmor frequencies: 30 and 90 MHz. The T1ρs are substantially shorter than the spin‐lattice relaxation times T1, indicating conditions far from the extreme narrowing limit. However, the observed values of T1ρ correspond very well to predictions from an earlier interpretation of T1s. The interpretation was based on four motions likely in this polymer: overall rotatory diffusion, segmental motion, phenyl group rotation, and methyl group rotation. All parameters in the interpretation such as correlation times had been set in the previous interpretation of T1s, leaving none to be adjusted to account for the T1ρs. Nevertheless, agreement between the predicted and observed T1ρs is excellent; this is apparently the first such quantitative interpretation of T1ρs in a dissolved polymer far from the extreme narrowing limit. This agreement at high molecular weight points to the adequacy of the description of local motion from high frequencies to relatively low frequencies and to the improved utility of T1ρ data versus T2 data as a check of low‐frequency contributions to the spectral density. The agreement at lower molecular weights indicates that the description of long‐range motions is sufficient though clearly oversimplified by the omission of the Rouse–Zimm modes.
Studies of magnetic resonance phenomena in polymers. I. The effects of free volume and segmental mobility on the motion of nitroxide spin probes and labels in poly(methyl methacrylate)
Authors: Tsay Fun‐Dow , Hong Su Don , Moacanin Jovan , Gupta Amitava 
Published: April 2003

AbstractThin films of spin‐probed and spin‐labeled poly(methyl methacrylate) (PMMA) have been examined by electron spin resonance (ESR) at X‐band frequency (9.2 GHz) and at various temperatures. Direct spectral evidence is presented to indicate that the composite ESR spectra observed in a certain temperature range originate from two states of distinctly different mobility, one with slow motions corresponding to a glassy state and the other with fast motions corresponding to a liquidlike state. The coexistence of these two states at temperatures considerably below the glass transition temperature can be explained as a result of the effect of free volume in a solid polymeric glass.
Vapor‐liquid equilibria for polyisobutylene solutions
Authors: Saeki S. , Holste J. C. , Bonner D. C. 
Published: April 2003

AbstractVapor sorption isotherms in binary solutions of polyisobutylene (PIB), (Mη = 4.7 × 106 g/mol) in hydrocarbons (cyclopentane; cyclohexane; n‐heptane; 2,2‐dimethyl butane; and 2,2,4‐trimethyl pentane) and chlorinated methanes [carbon tetrachloride (CCI4) and chloroform (CHCI3)] have been determined at 23.5°C using the piezoelectric sorption method. The polymer‐solvent interaction parameter χ obtained agrees with previously published values determined by using gas‐liquid chromatography and a quartz‐helix vapor sorption apparatus. The Flory theory of corresponding‐states has been applied to the experimental results through the χ parameter and affords a good prediction of the concentration dependence of χ for solutions of chloroform, carbon tetrachloride, n‐heptane, and 2,2‐dimethyl butane in PIB. The experimental values of \documentclass{article}\pagestyle{empty}\begin{document}$ [\partial (a_1 /\psi _1 )/\partial a_1 ]_{T,P} $\end{document} for the PIB solutions are constant over the measured concentration range, for example \documentclass{article}\pagestyle{empty}\begin{document}$ [\partial (a_1 /\psi _1 )/\partial a_1 ]_{T,P} $\end{document} = −4.1 for CCI4, −3.65 for CHCI3, −3.0 for 2,2‐dimethyl butane and n‐heptane, −2.7 for 2,2,4‐trimethyl pentane, −2.7 for cyclohexane, and −1.7 for cyclopentane, where a1 is the solvent activity and ψ1 is the solvent segment fraction. The correlations between the values of \documentclass{article}\pagestyle{empty}\begin{document}$ [\partial (a_1 /\psi _1 )/\partial a_1 ]_{T,P} $\end{document} and the theories of Guggenheim, Miller, Huggins, and Flory are discussed.
Studies on mechanical and swelling behavior of polymer networks on the basis of the scaling concept. 3. The osmotic compressibility of gels
Authors: Zrinyi M. , Horkay F. 
Published: April 2003

AbstractThe osmotic compressibility of chemically crosslinked poly(vinyl acetate) (PVAc) gels in equilibrium with pure diluent has been determined by the method of decreasing equilibrium swelling in different swelling agents. The concentration dependence of the osmotic compressibility has been found to follow a scaling law with an exponent which was greater than that predicted by the mean‐field theory. A comparison has been made between the elastic moduli determined by unidirectional deformation measurements and the compressional moduli obtained from the osmotic compressibility data.
Electrical conduction in composites of a TCNQ anion‐radical salt in a polymer matrix
Authors: Kishimoto Yoshio , Shimotsuma Wataru 
Published: April 2003

AbstractThe conduction behavior of composites of the anion‐radical salt NaTCNQ (sodium 7,7,8,8‐tetracyanoquinodimethanide) in poly(vinyl chloride) plasticized by polyurethane has been studied. TCNQ salt‐polymer composites that have good moldability and flexibility are characterized by molecular or granular dispersion of the TCNQ salt in a polar polymer matrix. The conduction mode changes from one due to molecularly dispersed sites (CMDS) to one due to granularly dispersed sites (CGDS) with increasing NaTCNQ content. In the CMDS region, the predominant conduction is ionic; TCNQ anion‐radicals migrate toward the anode under a dc biasing voltage and a high‐resistivity layer is formed near the cathode. The fixation of TCNQ salt at sites in the polymer matrix is believed to be important for the stabilization of electronic conduction under a dc electric field. In the CGDS region, the conduction is electronic and the current‐voltage characteristics of the composite are nonohmic, which indicates that carrier generation depends on the Poole‐Frenkel effect.
Concentration effects on partitioning of dextrans and serum albumin in porous glass
Authors: Brannon J. H. , Anderson J. L. 
Published: April 2003

AbstractPartitioning of three dextrans and bovine serum albumin (BSA) in controlled‐pore glass beads was measured by a batch mass balance technique. For all solutes the measured partition coefficient increased with bulk phase concentration, and the relationship appeared to be linear up to 0.05 g/mL for the dextrans. The measured slopes of partition coefficient versus concentration were compared with predictions using a hard‐sphere theory, and while the agreement was tolerable for BSA, the dextran results were much lower than the theory dictated except for very small pores, in which case it was speculated that molecular weight polydispersity resulted in anomalous partitioning of the smaller components.
Nematic and cholesteric thermotropic polyesters with azoxybenzene mesogenic units and flexible spacers in the main chain
Authors: Blumstein A. , Vilasagar S. , Ponrathnam S. , Clough S. B. , Blumstein R. B. , Maret G. 
Published: April 2003

AbstractProperties of linear polyesters based on azoxybenzene and 2,2′‐methylazoxybenzene moieties with linear, flexible spacers based on mixtures of dodecanedioic acid (DDA) and methyladipic acid (MAA), chiral or racemic, of various compositions (system MAA/DDA‐8 and MAA/DDA‐9, respectively) have been described. Substitution of methyl groups in the 2,2′ or 3,3′ positions of the mesogenic core leads to soluble and relatively low‐melting‐point polyesters. The viscosity law for (MAA/DDA‐9) polyesters in 1,1,2,2 tetrachloroethane gives an exponent 0.76, indicating well‐sol‐vated, coiled chain conformations in dilute solution. Calorimetric data show an increase in isotropization entropy ΔSNI with increasing average length of the spacer. This suggests a nonrandom conformation of the spacer in the nematic melt with a degree of order superior to that of low‐molecular‐weight analogs. X‐ray data obtained with an oriented nematic glass quenched from the nematic melt of DDA‐9 subjected to a magnetic field of 10–12 T also support the extended‐chain model in the nematic phase of DDA‐9. Oriented fibers can be produced by subjecting nematic melts of polyesters 8 and 9 either to magnetic fields of high intensity or to shear fields. The x‐ray data obtained from these fibers also support the extended‐chain model. Cholesteric systems do not orient in the magnetic field of 10–12 T. The study of mesophases of systems 8 and 9 indicates a dramatic influence of the position of the ester group on the stability of the mesophase in the azoxybenzene polyesters. The results are interpreted in terms of geometric factors influencing the colinearity of the mesogenic core and of the extended spacer.
Phase relationships in a ternary solution composed of a solvent and two monodisperse homologous polymers
Authors: Fujita Hiroshi , Teramoto Akio 
Published: April 2003

AbstractExact expressions are derived for the spinodal, critical conditions, and separation factor of a ternary solution consisting of a pure solvent and two monodisperse homologous polymers in which the Flory‐Huggins interaction parameter χ depends separately on the concentrations of the polymer components. The results allow one to see the difference from previous expressions obtained with χ depending on the total concentration of the polymer, and are expected to be useful for experimental determination of the Gibbs free energy of ternary solutions.
Effect of molecular entanglements on craze microstructure in glassy polymers
Authors: Donald Athene M. , Kramer Edward J. 
Published: April 2003

AbstractThin films of ten glassy polymers are bonded to copper grids and strained in tension to produce crazes, which are then examined in the transmission electron microscope. The average craze fibril extension ratio λ for each polymer is determined from microdensitometer measurements of the mass thickness contrast of the crazes. The extension ratio λ is found to increase approximately linearly with the chain contour length le between entanglements, as determined from melt elasticity measurements of the entanglement molecular weight of these polymers. These results are analyzed by comparing them with λmax, the maximum extension ratio of an entanglement network in which polymer chains neither break nor reptate (i.e., permanent entanglement crosslinks are assumed). The values of λmax are given by le/d where d, the entanglement mesh spacing in the unoriented glass, is computed from d = k(Me)1/2 with k determined either from small‐angle neutron scattering results on isolated chains in the glass or from coil size measurements in dilute solutions of a θ solvent. The craze extension ratios fall somewhat below λmax at low λ but increase to well above λmax for polymers with high le. This comparison suggests a significant contribution due to chain breakage (or reptation) in the higher‐λ crazes of large‐le polymers, which may arise from the higher true stresses in the craze fibrils (which for a given applied stress increase proportionally to λ). The results also imply that a useful way to increase the “brittle” fracture stress and decrease the ductile‐to‐brittle transition temperature of a glassy polymer is to decrease its entanglement contour length le.