bluuebunny/crossref_metadata_2025_split huggingface dataset:
Birefringence of high‐pressure injection‐molded high‐density polyethylene at 70.5 μm wavelength
Authors: Jacobsson Stellan , Hård Sverker , Boldizar Antal 
Published: April 2003

AbstractA newly constructed far‐infrared laser interferometer was used to measure the birefringence of 1‐mm‐thick injection‐molded high‐density polyethylene test bars, manufactured at unusually high molding pressures. The long wavelength used, 70.5 μm, allowed nondestructive tests to be made, demonstrating the usefulness of far‐infrared techniques for probing crystalline polymers. The birefringence was shown to increase with increasing molding pressure, supporting the belief that molecular orientation increases with increasing molding pressure. The tensile modulus was also measured and was found to increase linearly with molding pressure.
The drawing behavior of polyethylene copolymers
Authors: Capaccio G. , Ward I. M. 
Published: April 2003

AbstractThe tensile drawing behavior of a range of selected polyethylene copolymers has been studied. Sheets were prepared by quenching molten polymer into cold water. Two‐centimeter‐gauge‐length samples were then drawn in air at 75 or 115°C in an Instron tensile testing machine at a crosshead speed of 10 cm/min. It was found that even at the very low concentration of one side branch per 1000 carbon atoms there was a very marked effect on the strain hardening behavior and the maximum draw ratio that could be achieved. The reduction in draw ratio increased with increasing branch concentration, and long branches were more effective than short branches in limiting the draw ratios achieved. The similarity between these effects and the effects of increasing M̄w or radiation crosslinking is noteworthy. This suggests that even a very small concentration of branches can significantly reduces the moleculer motions required for the process of plastic deformation. The Young's modulus/draw ratio relationship follows a pattern virtually identical to that observed in the case of homopolymers.
Changes in superstructure of drawn isotactic polypropylene with annealing: A two‐dimensional small‐angle x‐ray analysis
Authors: Rognoni A. Ferrero , Ferracini E. , Čačkovic̀ J. Loboda , Čačkovic̀ H. 
Published: April 2003

AbstractTwo‐dimensional small‐angle x‐ray (SAXS) analysis is extended and applied to isotactic polypropylene cold‐drawn and annealed near the melting point. The SAXS intensity down to very small angles is detected by a high‐resolution point‐collimation camera. The SAXS fiber pattern, exhibiting only one meridional reflection and continuous scattering on the equator, shows that the microparacrystallites (“bricks” of the macrolattice) are densely packed together laterally into finite extended layers. The drawing process increases the ultrafibrillar characteristics of the samples and annealing increases their lamellar character. Annealing is carried out at a temperature near the melting point with the ends of the samples fixed so that recrystallization under strain takes place. The volume crystallinity increases on annealing from 32 to 57%. The SAXS results are in good agreement with NMR results; nevertheless the ideal paracrystal model assumption is, in the present case, not quite consistent with the results.
Model calculations of the distribution function of molecular weight and chemical composition of binary statistical copolymers
Authors: Procházka Ondřej , Kratochvíl Pavel 
Published: April 2003

AbstractModel calculations based on the kinetics of free‐radical copolymerization were carried out for the distribution functions of molecular weight and chemical composition, and for heterogeneity parameters of statistical copolymers. The calculations were conducted so as to depict the dependence of the heterogeneity parameters on the degree of conversion and to make possible an estimate of the maximal heterogeneity of statistical copolymers at given monomer reactivity ratios and rate constants. A comparison of the results obtained with a critical analysis of the feasibility of the determination of heterogeneity parameters by light scattering has revealed that, for statistical co polymers, this method can in principle be used with satisfactory accuracy only with samples of high degree of conversion and high molecular weight. The frequently used approximation P = 0 cannot be applied with out careful verification.
Crazing in polyethylene
Authors: Kamei Eiichi , Brown Norman 
Published: April 2003

AbstractThe tensile stress—strain curves of various types of polyethylene were compared from 77 to 298 K in nitrogen, isopentane, and the inert environment of helium at various strain rates. It was found that in general polyethylene crazes in a gas such as nitrogen at a temperature below 1.6 times its boiling point and in isopentane. Although the behavior of polyethylenes is similar to that of other polymers with regard to crazing in gases at low temperatures, they are in general less sensitive to the gas. The decrease in tensile strength of polyethylene in an environmental gas increases with crystallinity. The differences in the intrinsic low‐temperature brittle fracture stress are attributed to differences in the density of tie molecules. The intrinsic yield point at room temperature showed the usual increase with increasing crystallinity, but all the polyethylenes have the same yield point below the γ transition temperature.
Creep behavior of ultrahigh‐modulus polyethylene: Influence of draw ratio and polymer composition
Authors: Ward I. M. , Wilding M. A. 
Published: April 2003

AbstractThe creep behavior of ultrahigh‐modulus polyethylene monofilaments has been studied over the temperature range 20–70°C. A wide range of samples was examined in an attempt to determine the influence of draw ratio, molecular weight, copolymerization, and crosslinking by γ irradiation Prior to drawing. Results are also presented for a solution‐spun fiber. It is proposed that the permanent flow creep arises from a combination of two creep processes, one of which is associated with the crystalline regions of the oriented structure and the other with a molecular network.
NMR study of polyethylene crystallization kinetics under high pressure
Authors: Brown D. R. , Jonas J. 
Published: April 2003

AbstractCrystallization of polyethylene under hydrostatic pressures of 1–4.5 kbar is directly observed using pulsed proton NMR. The rate of growth of extended‐chain polyethylene crystals is measured over this pressure range and to a maximum temperature of 227°C. The observed crystallization isotherms are superimposable on a log time scale; this implies a consistent mechanism for extended‐chain growth over this pressure range. Avrami coefficients for high‐pressure extended‐chain crystallization are determined to be 1.3–1.7. A decrease of crystal nucleus surface free energies with increasing pressure is indicated. Findings are consistent with Wunderlich's model of initial folded‐chain crystallization followed immediately by chain extension. Future applications of this NMR technique are briefly considered.
High‐temperature phases of poly(p‐xylylene)
Authors: Isoda Seiji , Kawaguchi Akiyoshi , Katayama Ken‐Ichi 
Published: April 2003

AbstractThe crystal structure of poly(p‐xylylene), as polymerized, is the α form. This transforms irreversibly to the β from by annealing or drawing. To clarify the mechanism of this transition, structural changes of the α and β crystals were examined with a high‐temperature stage in the electron microscope. Two high‐temperature phases, β1 and β2, were found and their structures were analyzed. In these structures lattice distortions due to rotational and translational motions of chains are in troduced, especially in the β2 form. The α → β transition is induced through such a disordered phase. The statistical arrangement of a molecule in the β‐form unit cell results from freezing the disorder in the high‐temperature phases.
Structural relaxation in amorphous polymers
Authors: Chow T. S. 
Published: April 2003

AbstractA theoretical calculation based on the general solution of a multi‐ordering‐parameter model is found to be in good agreement with the measured volume relaxation of poly(vinyl acetate). This suggests that a limiting equilibrium state is eventually reached, which may resolve the disagreement between the behavior of the model and experiments discussed in the literature. In addition, the asymmetric character of the isothermal response and memory effect is satisfactorily calculated from the same basic equation. The distribution function and the temperature‐structure dependence of relaxation times are discussed.
Sorption and diffusion of toluene in highly oriented polypropylene
Authors: Choy C. L. , Leung W. P. , Ma T. L. 
Published: April 2003

AbstractThe sorption and diffusion of toluene vapor at 30°C in polypropylene with draw ratios from 1 to 18 have been studied. Drawing leads to the transformation of the initially spherulitic material into the fibrous structure, with many taut tie molecules lying mainly on the outer boundary of the microfibrils. The free volume and hence the sorption sites are thereby reduced, and the microfibrils become less and permeable as the draw ratio increases. As a result, the equilibrium concentration and the zero‐concentration diffusion coefficient drop by factors of 4 and 30, respectively. The diffusion coefficient increases exponentially with toluene concentration but the concentration dependence becomes weaker with increasing draw ratio, indicating that the severely constrained chain segments in the drawn samples have much less freedom to mix with penetrant molecules. Annealing relaxes the tie molecules and thus restores the sorption and diffusion properties to values corresponding to completely relaxed amorphous component, i.e., to values even higher than those of the undrawn but quenched material.
Crazing and shear deformation in crosslinked polystyrene
Authors: Henkee Chris S. , Kramer Edward J. 
Published: April 2003

AbstractThin films of polystyrene (PS) are bonded to copper grids and crosslinked with electron irradiation. When the films are strained in tension regions of local plastic deformation, either crazed or plane stress deformation zones (DZs), nucleate and grow from dust particles. the nature of the local deformation, as well as the local extension ration λ, is determined by transmission electron microscopy. The behavior of the PS glass is consistent with its being a network of molecular strands of total density v = vE + vX, where vE is the entangled strand density inferred from melt elasticity measurements of uncrosslinked PS and vX is the density of crosslinked strands determined from the ratio of the applied electron dose to the electron dose for gelation. when v is less than 4 × 1025 m−3 (<1.3vE), only crazes are observed whose microstructure is similar to those in uncrosslinked PS. As v increases from 4 × 1025 to 8 × 1025 m−3 (from 1.3vE to 2.5vE) shear deformation begins to compete with crazing. As v increases above 8 × 1025 m−3, only shear DZs are observed, the strain in which becomes progressively more diffuse as v increases. The λ in the crazes and DZs correlate well with λmax, the maximum extension ratio of a strand in a network of density v computed using the Porod×Kratky model. For crazes ln(λ) ≃ 0.9 ln(λmax) and for DZs ln(λ) ≃ 0.55 ln(λmax). The strain at which crack nucleation is first observed increases as v increases from <5% in uncrosslinked PS with v = 3.3 × 1025 m−3 to >20% in PS with v = 33 × 1025 m−3 (v = 10vE); crosslinking to still higher crosslink densities, e.g., v = 14vE, results in cracks which propagate in a catastrophic manner at low applied strains. An optimum v thus exists, one not too high to suppress local shear ductility but high enough to suppress crazes which can act as crack nucleation sites. these results are compared with previous results on a variety of linear homopolymers, copolymers, and polymer blends that are characterized by a wide range of v (v = vE). The transitions from crazing to crazing plus shear and from crazing plus shear to shear only take place at almost identical values of v. In addition the correlation between λ in the crazes and DZs and λmax for a single network strand is the same for both classes of polymers. This agreement implies that chain scission is the major mechanism by which strands in the entanglement network are removed in forming fibril surfaces. Craze suppression, by either increasing v in the crosslinked polymer or vE in the uncrosslinked ones, is due to the extra energy required to break more main‐chain bonds to form these surfaces.
Polymer fibers drawn to the limit
Authors: Slutsker Leonid I. , Utevskii Lev E. 
Published: April 2003

AbstractThe orientational drawing of polymers is known to be terminated because of sample rupture. The limiting draw ratio λlim reached may be different (either large or small) depending on the polymer and the actual drawing conditions. The purpose of the present work is to identify the change of supermolecular structure of polymer fibers which results in the termination of orientational drawing. Small‐and wide‐angle x‐ray diffraction were used to study the variation of geometrical parameters of this structure with increasing draw ratio λ. The geometrical parameters discussed are the dispersions (fluctuation) of long periods and of longitudinal sizes of crystalline as well as amorphous regions. In this study we used fibers of poly(vinyl alcohol), poly(ε‐caprolactam), polyoxymethylene, and poly(4,4′‐diphenyloxide) pyromellitimide. It is found that the long period dispersion of these polymers, drawn under different conditions, increases to approximately the same value for different samples drawn to the limit, this relative standard deviation δL of long periods being 0.30‐0.40. It is also found that the crystallite size dispersion does not increase with increasing λ; the increase of λL is due to increasing dispersion of the amorphous region lengths. For poly(vinyl alcohol) fibers drawn to the limit under different conditions and which have different λlim, the relative standard deviation of the sizes of amorphous regions δA turned out to be about the same (ca. 0.60). The latter evidence gives grounds to suggest that the rupture of polymers under drawing is associated with reaching a high degree of amorphous region size dispersion. In those regions which are considerably below the average size there probably will appear local overstress and molecular ruptures because the relative deformation of these regions is much larger than that of the adjacent regions in the cross section of the sample.
A spectroscopic study to interpret the increased piezoelectric effect at high temperature in poly(vinylidene fluoride)
Authors: Jei Lu Fei , Waldman David Allen , Hsu Shaw Ling 
Published: April 2003

AbstractSpectroscopic experiments were carried out to directly measure the electric‐field‐induced microstructural changes in PVF2 at high temperature (65°C). We found, in comparison with roomtemperature measurements, that the reduction of the coercive field is directly related to faster dipolar orientation at this temperature. Furthermore, the α to β or δ phase transformation can take place at a much faster rate and lower field strength than has been previously reported either theoretically experimentally.
Concentration dependence of the first cumulant for partially labeled chains in dilute solutions
Authors: Hammouda B. , Akcasu A. Z. , Benmouna M. 
Published: April 2003

AbstractWe use Zimm's single‐contact approximation to investigate the concentration dependence of the first cumulant for partially labeled chains in dilute solutions. This approximation allows us to express interchain properties (such as the interference part of the generalized mobility) in terms of static structure factors, which are then modeled using a Gaussian chain model. We present numerical results for three specific labeling sequences.
Effects of polymer chain disentanglement on NMR properties. I. Proton magnetic relaxation in polyisobutylene
Authors: Cohen‐Addad J. P. , Guillermo A. 
Published: April 2003

AbstractThe transition of NMR properties from a solidlike spin system response to a liquidlike one observed on molten polymers or on concentrated solutions is analyzed to characterize the slow diffusion process of long linear polyisobutylene (PIB) chains. It is first shown that the relaxation function of the transverse magnetic component of protons linked to PIB chains obeys a superposition property when the polymer concentration is varied from 0.5 to 1.0g/cm3; this is interpreted as reflecting the applicability of the viscoelastic submolecule concept to the description of NMR properties. The transition in NMR properties is then analyzed numerically, using a model of polymer chain relaxation based on a multiple‐relaxation‐mode process analogous to the Rouse model. The terminal relaxation time τv1 is shown to depend upon the chain molecular weight M as M3; it is found to be equal to 8.3 × 10−2 s for molecular weight M = 2.25 × 105 and concentration c = 0.47 g/cm3 in carbon disulfide at room temperature.
Temperature dependence of the thermal expansivity of polymer crystals: Linear chain model
Authors: Choy C. L. , Wong S. P. , Young K. 
Published: April 2003

AbstractA linear chain model is used to calculate the thermal expansivities of polymer crystals from room temperature down to liquid‐helium temperature. Because of restraint by strong covalent forces, vibrations along the chains give a negligible contribution. Vibrations transverse to the chains cause the length projected along the chain direction to decrease. The expansivity along the chain direction, αc∥is therefore negative, and has temperature dependence characterized by the low Debye temperature of transverse vibrations. These vibrations, through a cubic term in the interchain van der Waals potential, also cause a positive expansivity, αc⟂perpendicular to the chain direction. The theoretical predictions for αc∥and αc⟂agree quite well with experimental data over the wide temperature range under consideration.
Theoretical models for diffusion in glassy polymers. II
Authors: Cohen Donald S. 
Published: April 2003

AbstractThe author refines and generalizes a model for diffusion in glassy polymers which he previously introduced. The model unifies many diverse observations by explicity formulating the common property of a glassy polymer in all its various modes, namely the finite relaxation time due to its slow response to changing conditions. An integral approximation method is used to study the motion of the penetrant front and the glass‐gel interface and a useful polynomial approximation method is introduced for use in special simple situations.
The volume structural relaxation theory for amorphous polymers
Authors: Rostiashvilli V. G. , Nekhoda A. R. , Irzhak V. I. , Rozenberg B. A. 
Published: April 2003

AbstractAn interpolation model of density fluctuations in amorphous bulk polymers is suggested for the explanation of some peculiarities of structural relaxation of amorphous polymers volume. The main peculiarities of the model are (i) its account of the influence of fluctuations in the region where the WLF theory is true (the region of large fluctuations); (ii) its account of small fluctuations in the region where the dissipation mechanism appears to be considerably different from that in the region of large fluctuations; and (iii) its joining of the solutions of the resulting kinetic equations in the crossover region. On the basis of the suggested model we have obtained an equation of the Fokker‐Plank type for the distribution function for density fluctuations with nontrivial boundary conditions for large fluctuations, and a quasistatic solution for small ones. These equations are analyzed and numerically solved in the isothermal regime. The results show that the theory corresponds to the WLF theory at temperatures much higher than Tg and that memory effects are found in the Tg region. In addition, preliminary estimates show that near Tg an extreme form of temperature dependence on activation energy is observed. All these facts agree qualitatively with experiment.
Studies on polyethylene crystallized at unusually high supercoolings: Fold length, habit, growth rate, epitaxy
Authors: Martinez‐Salazar J. , Barham P. J. , Keller A. 
Published: April 2003

AbstractThe newly arisen possibility of crystallizing polyethylene at supercoolings much higher than were achievable previously has enabled the study of crystallization to be extended in several directions. Thus, fold length can be followed down to previously inaccessibly low crystallization temperatures, in the present case with sharp fractions, demonstrating the essential independence of the fold length of molecular weight. In this context the thinnest isolated crystal reported so far was obtained (ca. 6 nm). The faceted nature of crystals grown at such low temperatures and high rates has been noted, and is in line with new conceptions of polymer crystal growth. A previous observation of exceptionally high crystal growth rate (ca. 2 m/s) has been supplemented by measurements over a range of crystallization temperatures and the results found to be in good agreement with the predicated regime III behavior in the least theory of Hoffman. Observations of epitaxy on mica, while broadly in line with those by Lovinger, were revealing in several respects. Among these the observation that the substrate can influence the fold length when the chains are parallel to the substrate plane remains unexplained and puzzling.
Strain criterion in nonlinear creep and recovery in concentrated polymer solutions
Authors: Nakamura K. , Wong C.‐P. , Berry G. C. 
Published: April 2003

AbstractTransient and steady‐state rheological data are reported for several anionic polystyrene solutions in tritolylphosphate (1. 6 < cM/ρMc < 7). Here c is the concentration of the solution, M is the molecular weight, ρ the density of the undiluted polymer, and Mc the molecular weight between entanglements as determined from zero‐shear viscosity. The polystyrene used had Mw = 410,000 and Mw/Mn < 1.06. Data are also given for solutions of polyisobutylene and poly(vinyl acetate) with larger Mw/Mn. The results give a critical strain γ′ ∝ c−1 such that linear viscoelastic behavior was obtained in a simple shear deformation with shear less than γ′. A simplified version of the constitutive equation of Bernstein, Kearsley, and Zapas is used with an empirical strain function F (γ) which contains γ′ as a parameter to discuss transient and steady‐state behavior in terms of the distribution of relaxation (or retardation) times determined for linear viscoelastic responce. Features of the dependence of the steady‐state viscosity ηk, recoverable compliance Rk, the first‐normal stress function Nk(1) on shear rate k are discussed in terms of F (γ) and the distribution of relaxation times to conclude that the latter plays a dominant role in the behavior observed in the range of k usually studied. The results predict that the reduced functions ηk/η0, Rk/R0, and Nk(1)/N0(1) should depend on η0R0k, and that the functional form depends markedly on the distribution of relaxation times, at least in the range η0R0k < 102. Comparison with the mechanistic model of Doi and Edwards shows a similar F (γ) but substantial differences in the reduced functions caused by a very narrow distribution of relaxation times in the model.