bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theory of the instability and failure of viscoelastic materials in tension
Authors: Coleman Bernard D. , Zapas Louis J. 
Published: April 2003

AbstractIn a tensile test of a bar or fiber formed from a transversely isotropic viscoelastic material, the initial motion in regions away from the clamps is a homogeneous uniaxial extension. If the applied tensile load is “tame” in the sense that it is given by a piecewise smooth function of time, then during the early stages of loading, the homogeneous extension of the specimen is also tame. It is, however, often the case that at a time tc, previous to the instant of fracture, the motion departs from a tame homogeneous extension. The critical time tc is the “failure time” of the specimen, i.e., the time at which the motion first changes character; tc precedes, often by a constant factor, the time at which neck‐down is easily visible. The problem of calculating tc for a given loading program is here treated within the framework of the general theory of nonlinear simple materials with fading memory. For such materials the instantaneous tensile modulus, i.e., the derivative of the immediate change in tensile stress with respect to a sudden change in tensile strain, depends upon the previous history of the strain. Reasons are presented here for identifying tc with the earliest time t0 at which the instantaneous tensile modulus becomes zero. It is shown that at a time at which the instantaneous modulus vanishes one cannot arbitrarily assign the rate of change of tensile stress and have the motion remain in the class of tame homogeneous extensions.
Some phenomenological consequences of the Doi–Edwards theory of viscoelasticity
Authors: Graessley William W. 
Published: April 2003

AbstractDoi and Edwards have recently proposed a molecular theory for the dynamics of entangled polymer liquids based on a tube model to represent the mutual constraints on configurational rearrangement of the chains. Expressions for diffusion coefficient, plateau modulus, zero‐shear viscosity, steady‐state recoverable compliance, and terminal relaxation time can be devloped, and relations among these properties that depend only upon observable quantities can be obtained. Several such relations are derived and are compared with experimental observations.
Particle structure of suspension poly(vinyl chloride) and its origin in the polymerization process
Authors: Davidson J. A. , Witenhafer D. E. 
Published: April 2003

AbstractThe initial stage of the suspension polymerization of poly(vinyl chloride) (PVC) is characterized by the formation of colloidally stable micron‐sized grains of PVC inside the polymerizing ca. 150 μm vinyl chloride droplets. The fate of these micron‐sized PVC grains depends upon the agitation conditions. If no agitation is employed, they serve as growth centers for further polymerization to give a final particle possessing a uniform internal bead morphology. In agitated systems, these grains coagulate early in the conversion to give a more irregular structure in the interior of the PVC particle. The formation of these stable growth centers appears to be unique to PVC. The polymerization of acrylonitrile, also insoluble in its monomer, is characterized by rapid agglomeration of the precipitated polymer throughout the polymerization. In PVC, the colloidal stability of the polymerizing grains is demonstrated to be electrical in nature. A pericellular membrane or skin formed by polymerization in both the water and vinyl phase completely surrounds the polymerizing droplet after about (1–2)% conversion. This skin is responsible for the charge retention of the PVC grains inside the polymerizing monomer droplets.
Conformation of polystyrene with special emphasis to the near all‐trans extended‐chain model relevant in polystyrene gels
Authors: Atkins E. D. T. , Isaac D. H. , Keller A. 
Published: April 2003

AbstractIn light of novel and problematic diffraction patterns arising through the thermoreversible gelation of isotactic polystyrene, the conformational possibilities were examined with particular reference to other conformational works published recently on this polymer. The specific objective was to ascertain whether highly extended, helical backbone geometries such as are required by the new x‐ray patterns are possible or not with a purely isotactic arrangement of side groups. The first realization was that the recently announced low‐energy, near‐trans conformations are not relevant to the present issue as they do not provide the required extended geometry. Further, specific sampling of the conformational energy map revealed that the required high chain extension may be energetically feasible, contrary to traditional conceptions and to recent conformational calculations elsewhere. The divergences as regards the latter were traced to the specific choice of nonbonded atomic radii—all within generally accepted limits—which accordingly has a crucial effect even to the extent whether specific conformation types are forbidden or energetically stable. The implications this has for conformational analysis in general are being pointed out. While the novel crystal structure has not been solved, the present study at least eliminates the previously believed categoric objections to it being associated with isotactic chains and, beyond this, opens up new perspectives as regards conformational possibilities in polyolefins. The more general problems concerning the nature and origin of the gelation phenomenon nevertheless remain.
Small‐angle light scattering from thin polymer films: Multiple scattering from Debye–Bueche scatterers
Authors: Koberstein Jeffrey T. , Stein Richard S. 
Published: April 2003

AbstractAn extension of Hartel's theory for multiple scattering has been applied to the case of small‐angle light scattering from polymer films with random two‐phase morphology. The scattering is treated in terms of Debye–Bueche theory from which values of the correlation length, an average phase size, and mean‐square fluctuation in polarizability are determined. It is shown that multiple scattering leads to a reduction in the angular dependence of scattering due to an enhancement of scattering at high angles. This leads to an underestimate of the correlation length and an overestimate of the magnitude of the mean‐square polarizability fluctuation.
Small‐angle x‐ray scattering studies of melting
Authors: Schultz J. M. , Fischer E. W. , Schaumburg O. , Zachmann H. A. 
Published: April 2003

AbstractThe course of melting of melt‐crystallized polyethylene fractions and of a poly(ethylene oxide)‐polystyrene‐poly(ethylene oxide) triblock copolymer has been followed by small‐angle x‐ray scattering (SAXS). Changes in the intensity and shape of the SAXS curves indicated that both surface melting and melting over the full crystallite thickness (full‐strand melting) take place. Full strand melting is the final, irreversible process. Comparison with an analytical model indicates that in the earlier stages of the irreversible, full‐strand process the crystallites melt out randomly throughout the bulk. Later stages may occur by the simultaneous melting of a larger stack of crystallites.
Thermally stimulated current (TSC) study of the Tg and Tll transitions in anionic polystyrenes
Authors: Lacabanne Colette , Goyaud Patrice , Boyer Raymond F. 
Published: April 2003

AbstractThe thermally stimulated current (TSC) technique has been used to investigate three anionic polystyrenes of M̄n 17,000, 71,700, and 1.55 × 106, i.e., M < Mc, M > Mc, and M ≫ Mc, where Mc is the entanglement molecular weight. A current maximum near Tg designated TMg, has relaxation times which follow an Arrhenius equation. A second current maximum at T > Tg appears to be the Tll process and is designated TMll. Relaxation times for it follow a Vogel equation. TMg and TMll vary with molecular weight, increasing below Mc and leveling off above Mc at a temperature of about 170°C. Values of TMg and TMll are compared with values of Tg and Tll obtained from torsional braid analysis, which involves melt flow; and with differential‐scanning‐calorimetric values on fused films, where there is no transport of polymer. It is concluded from such cross‐comparisons that TSC, at least for polystyrene, is a quasistatic test which may involve microscopic viscosity. Macroscopic viscosity does not play a role. The ratio TMll/TMg is in the range 1.10–1.16, similar to Tll/Tg values by other methods. A few comments about Tll in atactic poly(methyl methacrylate) by the TSC method are given.
Drawing behavior of linear polyethylene. II. Effect of draw temperature and molecular weight on draw ratio and modulus
Authors: Capaccio G. , Crompton T. A. , Ward I. M. 
Published: April 2003

AbstractThe drawing behavior of linear polyethylene homopolymers with weight‐average molecular weights (M̄w) from 101,450 to ca. 3,500,000 has been studied over the temperature range 75°C to the melting point. In all cases 1‐cm gauge length samples were drawn in an Instron tensile testing machine at a constant cross‐head speed of 10 cm/min. With the exception of the lowest molecular weight polymer, it was found that increasing the draw temperature led to substantial increases in the maximum draw ratio which could be achieved, and that this increased monotonically with increasing draw temperature. Measurements of the Young's modulus of the drawn materials showed, however, that the unique relationship between modulus and draw ratio previously established for drawing at 75°C was not maintained to the highest draw temperatures. The highest draw temperature at which this relation held was found to be strongly molecular weight dependent, increasing from ca. 80 to ca. 125°C when M̄w increased from 101,450 to 800,000. In all cases conditions could be found for drawing samples to draw ratios of 20 or more with correspondingly large values of the Young's modulus.
Thermodynamic analysis of the plastic deformation of glassy poly(methyl methacrylate)
Authors: Haussy J. , Cavrot J. P. , Escaig B. , Lefebvre J. M. 
Published: April 2003

AbstractA thermodynamic analysis of the thermally activated yielding of glassy atactic poly(methyl methacrylate) has been carried out. Samples were deformed in compression at a constant strain rate in the temperature range 150–330 K. Two different deformation modes are rate controlling below and above a critical temperature Tc of the order of 200 K. The low‐temperature mode is characterized by a purely exponential stress dependence of the strain‐rate, and thus defines a true stress‐activation volume. The higher‐temperature mode reveals a stress dependence of the strain rate in the form of a power law with a constant activation energy.
Morphological studies on polybutylene terephthalate
Authors: Stein R. S. , Misra A. 
Published: April 2003

AbstractThe morphology of poly(butylene terephthalate) (PBT) crystallized from the melt at various temperatures was studied by small‐angle light scattering, polarizing microscopy, and wide‐angle x‐ray diffraction. Spherulites with a maltese cross at 45° to the polars formed at lower temperatures while spherulites having an apparently higher melting point with a maltese cross along the polars (0°–90°) formed at higher temperatures. The spherulite size and crystallinity increased with increasing crystallization temperature. The Hv scattering patterns arising from the spherulites formed at lower temperature showed intensity maxima at azimuthal angles of 0° and 90°, while those obtained at higher temperatures showed the more common 45° intensity maxima. Microtomed samples from molded PBT bars showed spherulites with a 45° maltese cross which changed to a 0°–90° maltese cross upon heating just prior to melting. The skin‐core effect due to varying thermal histories in these molded bars was clearly observed. Solvent crystallized films contained positive 0°–90° spherulites. Some changes occurring upon uniaxial stretching of PBT films are also discussed.
Hydrodynamically induced formation of cellulose fibers. I. Observations on the formation of cellulose fibers from stirred solutions
Authors: Kalb B. , Manley R. St. John 
Published: April 2003

AbstractLike synthetic polymers, a natural polymer such as cellulose may crystallize in fibrous form from stirred solutions. In the present work, it is demonstrated that cellulose fibers can be formed by precipitation from dimethyl sulfoxide/paraformaldehyde solutions by two methods that involve different mechanisms of fiber formation, viz., (A) precipitation of cellulose by addition of nonsolvent to the stirred cellulose solution, and (B) precipitation of cellulose by coagulation of droplets of cellulose solution in a stirred precipitant. Both processes yield fibers with properties depending on the stirring speed and the coagulant strength. The molecular orientation and tensile strength of the fibers produced by method A was low, but increased with the stirring speed, while some fibers formed by method B reached extremely high orientation, depending on the thickness of the fibers. The two mechanisms of fiber formation are discussed on the basis of the experimental observations.
Fourier transform infrared study of the gel form of isotactic polystyrene
Authors: Painter Paul C. , Kessler Richard E. , Snyder Randy W. 
Published: April 2003

AbstractInfrared spectra of isotactic polystyrene gel films are observed to be significantly different from those of amorphous and crystalline polymer, indicating that the gel form is in a different conformation than the usual 31 helical crystal structure. Computer‐assisted subtraction indicates that approximately 35% of the chains participate in the gel component, effectively ruling out structural defects as the origin of the ordered gel component.
Raman spectroscopic studies of the solid‐state polymerization of diacetylenes. II. Thermal polymerization of 1,6‐di‐p‐methoxybenzene‐sulfonyloxy‐2,4‐hexadiyne
Authors: Bloor D. , Ando D. J. , Hubble C. L. , Williams R. L. 
Published: April 2003

AbstractThe Raman spectra of 1,6‐di‐p‐methoxybenzene‐sulfonyloxy‐2,4‐hexadiyne (MBS) have been recorded during thermal polymerization. The spectra are similar to those of the related p‐toluene‐sulfonyloxy monomer but indicate a higher strain in the initially formed MBS polymer chains. Despite this similarity, the polymerization kinetics for the two monomers are markedly different. The polymerization behavior of MBS shows that the polymer chain initiation and propagation are practically independent of lattice strain. Possible causes for this independence are discussed.
Microkinetics of crystallization in poly(ethylene terephthalate)
Authors: Palys L. H. , Phillips P. J. 
Published: April 2003

AbstractThe lamellar growth kinetics and lamellar thickness of poly(ethylene terephthalate) crystallized from the glassy state have been determined as a function of crystallization temperature. Values of end and side surface free energies have been estimated as well as the residual lamellar thickness. Analyses carried out using secondary nucleation approaches indicate that the width of a critical nucleus is comparable to the effective substrate length for multiple nucleation in this and other slowly crystallizing polymers at high supercoolings. A “universal” critical value of T/T2ΔT below which the strip completion process ceases was found to exist. All crystallization must, therefore, occur through the deposition of critical nuclei. Models are proposed for this process which appear to be consistent with both neutron scattering and infrared experiments on quenched polyethylenes. Comparison of crystallization rates, expressed as “jump” rates, with relaxation frequencies suggest that in order for crystallization to occur at any given temperature the relaxation frequency must be at least two decades faster than the crystal “jump” rate.
Photoconductivity of poly(vinyl alcohol) films containing Cu2+ complexes
Authors: Sumita Osao , Fukuda Atsuo , Kuze Eiichi 
Published: April 2003

AbstractPhotoconductivity has been measured in well‐dried poly(vinyl alcohol) (PVAl) films containing Cu2+ complexes prepared by treatment with Cu(NO3)2, CuSO4, CuCl2, and CuBr2. When the ratio [Cu2+]/[MU] ([MU] denoting the concentration of PVAl monomer residues) is greater than 0.015, illumination at the charge‐transfer (UV) band produces a strong photocurrent in PVAl‐CuCl2 and PVAl‐CuBr2, but not in PVAl‐Cu(NO3)2 or PVAl‐CuSO4. Optical absorption spectra suggest that two halide ions enter the first coordination sphere in the cis configuration. The temperature dependence of ESR indicates the existence of antiferromagnetic superexchange interaction via intervening halide ions. Hence, there exists a network structure of the PVAl‐CuCl2 or PVAl‐CuBr2 complex. The dependence of the photocurrent on the polarity of the illuminated electrode shows that the majority of carriers are holes. Holes in the network structure produced by charge transfer from halogen to Cu2+ ions are concluded to be responsible for the photoconduction.
Rheo‐optical studies on the deformation mechanism of semicrystalline polymers. X. Orientation retardation spectrum of polyethylene
Authors: Kyu Thein , Suehiro Shoji , Nomura Shunji , Kawai Hiromichi 
Published: April 2003

AbstractIn order to explore the correlation between rheo‐optical and mechanical dispersions, orientation and lattice‐deformation retardation spectra are introduced in the phenomenological treatment of the stress‐orientation coefficient and crystal lattice compliance functions in dynamic x‐ray diffraction. A similar complementary treatment of the stress‐optical retardation spectrum is also carried out for the dynamic birefringence. To determine the mechanical and rheo‐optical retardation spectra, a deconvolution method based on the Fourier transform is developed. By this method, the mechanical and rheo‐optical retardation spectra of a low‐density polyethylene are computed from the imaginary components of the corresponding experimental functions; and subsequently, the real and imaginary components of the original functions are recalculated numerically from their respective retardation spectra and compared with the observed ones. The similarity of the normalized mechanical and rheo‐optical retardation spectra demonstrates that the α mechanical dispersion is related to orientation retardation of the crystal grains at their boundaries. Furthermore, separation of crystalline and amorphous contributions to the stress‐optical retardation spectrum is made on the basis of the two‐phase hypothesis for polyethylene. The amorphous contribution to the stress‐optical retardation spectrum is further resolved into two components corresponding to the α and β mechanical dispersions under the assumption that the resolved component for the α dispersion is symmetric in shape.
Phosphorescence studies of relaxation effects in bulk polymers. II. Emission depolarization study of relaxation mechanisms in poly(methyl methacrylate)
Authors: Rutherford Henry , Soutar Ian 
Published: April 2003

AbstractPhosphorescence depolarization measurements, under steady state polarized excitation, have been used to examine the relaxation behavior of bulk poly(methyl methacrylate) (PMMA). Poly(methyl methacrylate) bearing phosphorescent labels has been synthesized by copolymerization of small quantities of acenaphthylene (I), 1‐vinylnaphthalene (II), 2‐vinylnaphthalene (III), 1‐naphthyl methacrylate (IV), and 2‐naphthyl methacrylate (V), respectively, with methyl methacrylate. In no case was depolarization of emission due to probe rotation apparent below the onset of the β‐relaxation of the polymer. Rotation of label V was characterized by an activation energy of 94 kJ mole−1 in excellent agreement with that of the β relaxation measured by conventional relaxation techniques. This result clearly implicates ester motion in the β relaxation. No motion of label I, which cannot move independently of the polymer backbone, was evident in the vicinity of the β relaxation. Above 378 K the activation energy for rotational relaxation of label I of 460 kJ mole−1 is in excellent agreement with published data for the α transition in PMMA. This result is in accord with the general assumption that backbone segmental motion is involved in the α relaxation. However, backbone motion of lesser temperature dependence (Ea = 115 kJ mole−1) is apparent from depolarization behavior of probe I between 343 and 378 K. Label II shows three regions of relaxation behavior. In the temperature range above the β transition motion of the label independent of the polymer is evident (Ea = 44 kJ mole−1). At temperatures in excess of 343 K this motion becomes cooperative with that of the backbone yielding activation energies comparable to those obtained in system I. Label III, while exhibiting depolarization characteristics similar to those of label II in the vicinity of the β relaxation, emitted insufficient intensity to permit estimation of an energy of activation for the motion. The phosphorescence of label IV was completely depolarized over the entire temperature range studied. While phosphorescence intensity and lifetime data may be used to detect the existence of polymeric transitions, the photophysical behavior of the naphthalene species studied is independent of the attachment to the polymer and does not primarily yield information regarding the polymer relaxations.
Thermal and dynamic mechanical relaxation behavior of stereoregular poly(2‐hydroxyethyl methacrylate)
Authors: Russell Geoffrey A. , Hiltner P. Anne , Gregonis Donald E. , de Visser A. C. , Andrade Joseph D. 
Published: April 2003

AbstractThermal, dynamic mechanical, and dielectric relaxation techniques were used to determine the relaxation behavior of isotactic and syndiotactic poly(2‐hydroxyethyl methacrylate) (pHEMA). Activation energies Ea were determined for the dielectric γ relaxation and compared with those of poly(2‐methoxyethyl methacrylate) (pMEMA) to determine the influence of hydrogen bonding on side‐chain relaxation processes. No difference in Ea was observed between syndiotactic pHEMA and atactic (predominantly syndiotactic) pMEMA. Isotactic pHEMA, however, had Ea + 1 kcal/mole higher than that of syndiotactic pHEMA. This was attributed to improved side‐chain packing in the isotactic polymer.
Phase transitions in mesophase macromolecules. I. Novel behavior in the vitrification of poly(ethylene terephthalate‐co‐p‐oxybenzoate)
Authors: Menczel Jozsef , Wunderlich Bernhard 
Published: April 2003

AbstractA 40:60 mole ratio poly(ethylene terephthalate‐co‐p‐oxybenzoate) was studied by dynamic scanning calorimetry. Vitrification occurred in two steps, which could be linked by optical microscopy to a two‐phase structure. For the lower glass transition, hysteresis was absent on heating and cooling through the transition temperature. Since all prior studied glasses show such hysteresis, this may be linked to the mesophase structure and raises the question whether mesophases always display such glass‐transition behavior.