bluuebunny/crossref_metadata_2025_split huggingface dataset:
Small‐angle X‐ray scattering studies of microdomain structure in segmented polyurethane elastomers
Authors: Koberstein Jeffrey T. , Stein Richard S. 
Published: April 2003

AbstractThe small‐angle x‐ray scattering (SAXS) technique has been used to characterize the detailed microphase structure of two crosslinked segmented polyurethane elastomers. Both copolymers contain trifunctional polypropylene ether triols in the rubbery elastomeric block, but are synthesized with different hard segments: a symmetric 4,4′‐diphenylmethane diisocyanate (MDI) chain extended with butanediol (BD); and an 80/20 mixture of asymmetric 2,4‐toluene diisocyanate (TDI) and symmetric 2,6‐toluene diisocyanate (TDI) chain extended with ethylene glycol (EG). Calculations of SAXS invariants and determinations of deviations from Porod's law are used to examine the degree of phase segregation of the hard‐ and soft‐segment domains. Results show that the overall degree of phase separation is poorer in the asymmetric TDI/EG‐based copolymer than in the symmetric MDI/BD‐based copolymer. Determination of diffuse phase boundary thicknesses, however, reveals that the domain boundaries are sharper in the asymmetric TDI/EG system. The contrasting morphologies found in the two systems are interpreted in terms of differences in hard‐soft segment compatibility, diisocyanate symmetry, and diisocyanate length. Coupled with conformational considerations, this information is used to construct a new model for polyurethane hard‐segment microdomain structure. Important features of the model are that it takes into account the effects of hard‐segment sequence length distribution and allows for folding of the longer hard‐segment sequences back into the hard‐segment domain.
An analysis of local flow effects in flow‐induced orientation and crystallization
Authors: Dairanieh I. S. , McHugh A. J. 
Published: April 2003

AbstractAn analysis is presented of the effects of external flow kinematics on the so‐called local flow in seeded, flow‐induced crystallization and orientation. The flow field around a growing crystal or nucleation seed is modelled by the Stokes flow equations past a prolate ellipsoid of high aspect ratio. Exact solutions for various flow kinematics, worked out elsewhere by the singularity method, are applied here to the analysis of local gradients. The results show that along the symmetry axis of the spheroid, the extensional gradients which result for various free‐stream velocity fields are primarily the result of the constant‐velocity free‐stream component. However, free‐stream, extensional flow can significantly enhance the region of such high gradients. Along the symmetry plane of the spheroid, primarily shearing gradients result, with small extensional gradients occurring when the free‐stream flow has extensional components. Results of chain extension and birefringence calculations are also presented and discussed.
Influence of the chemical composition and comonomer distribution on the dipole moments of 3,3‐dimethyloxetane/tetrahydrofuran copolymers
Authors: Garrido L. , Riande E. , Guzmán J. 
Published: April 2003

AbstractExperimental values of the dipole moment ratio 〈μ2〉/nm2 and its temperature coefficient d ln 〈μ2〉2/dT, where 〈μ2〉 is the mean‐square dipole moment of a chain with n skeletal bonds and m2 is the mean square of the skeletal bond dipole moments, are reported for well‐characterized random copolymers of 3,3‐dimethyloxetane and tetrahydrofuran. The results are interpreted in terms of the rotational isomeric state theory in a manner consistent with that developed for the parent homopolymers. The theory gives a good account of the experimental results corresponding to copolymers in which the mole fraction of tetrahydrofuran lies in the range 0.11–0.89. It is found that whenever the copolymerization obeys Bernouillan statistics, the dipole moments are quite insensitive to the comonomer distribution. The theoretical analysis suggests, however, that the value of the dipole moment ratio of alternating copolymers of 3,3‐dimethyloxetane and tetrehydrofuran should be near that of the parent homopolymer of lower polarity.
Low‐frequency raman scattering from methylene‐oxyethylene‐methylene triblock oligomers
Authors: Swales T. G. E. , Teo H. H. , Domszy R. C. , Viras K. , King T. A. , Booth C. 
Published: April 2003

AbstractLow‐frequency Raman spectra are reported for monodisperse methylene‐oxyethylene‐methylene triblock oligomers. Assignments are made to the longitudinal acoustical modes (LAMs) of the oxyethylene block and the methylene block. The results for LAM‐1 can be modeled by the vibrations of rods with perturbing forces and masses. The results for LAM‐3 are not so readily modeled.
Flow‐induced crystallization by surface growth of polyethylene fibers
Authors: Rietveld J. , McHugh A. J. 
Published: April 2003

AbstractA series of experiments has been carried out investigating several features of the surface growth method for observing longitudinal growth of polyethylene fibers in Couette geometry. Attempts to obtain limiting steady‐state takeup rates using a Teflon rotor were hampered by fiber breakage; however, maximum growth rates before breakage were found to be considerably higher than those observed in previous studies. Growth rates were also obtained using a static method, and for the Teflon rotor indicated above a critical concentration a linear growth rate equal to the stirrer velocity with rates essentially independent of temperature. With a silanized glass rotor, the same method gave much lower growth rates at comparable stirrer speeds and temperatures and showed a temperature dependence suggestive of a nucleation‐controlled mechanism. The implications of these results for other studies of the mechanisms of growth by the surface method are also discussed.
Dynamics and mechanism of iodine sorption by polyacetylene
Authors: Danno Tetsuya , Miyasaka Keizo , Ishikawa Kinzo 
Published: April 2003

AbstractSorption of iodine by bulk polyacetylene was studied under various I2 gas pressures at 25°C. The sorption dynamics show that the penetration of iodine into PA is not Fickian and the diffusion coefficient increases with time of sorption. A discontinuous increase in the sorption isotherm is observed at P/P0 = 0.25 (P is the pressure of the I2 gas and P0 is the saturation value at 25°C). It is due to iodine penetration into PA crystals, as evidenced by x‐ray analysis. The distribution of iodine within crystals is apparently inhomogeneous: some unit cells are changed into “iodine‐PA” cells, while others remain unchanged. The electrical conductivity depends not only on the amount of iodine but also on the I2 gas pressure under which sorption is carried out. At given iodine content, the conductivity of a sample doped under higher I2 pressure is greater than that of a sample doped under lower pressure.
Spectroscopic characterization of solvent‐induced crystallization of PET
Authors: Lin Shaow‐Burn , Koenig Jack L. 
Published: April 2003

AbstractUsing recently developed quantitative FTIR methods, the gauche and trans conformer compositions have been measured for polyethylene terephthalate (PET) samples exposed to benzene and acetone. The kinetics of the primary and secondary isomerization stages of the solvent‐induced crystallization process have been studied and compared with the thermally crystallized system. The Arrhenius activation energies of isomerization in the primary stage in which Fickian diffusion behavior is followed for the benzene‐ and acetone‐induced PET systems are 8.2 ± 0.4 and 7.7 ± 0.4 kcal/mol, respectively.
Studies on dilute solutions of rodlike macroions. I. Light scattering, densitometry, and cryoscopy
Authors: Lee C. C. , Chu S.‐G. , Berry G. C. 
Published: April 2003

AbstractTotal integrated and photon correlation light scattering have been used to study two samples of poly(1,4‐phenylene‐2,6‐benzobisthiazole) (PBT) representing two methods of precipitation to recover the polymer from the polymerization solvent. Some details of the light‐scattering instrument are given. It is found that the PBT sample is rodlike, with persistence length of at least 50 nm, but that the postpolymerization processing method influences the state of interchain aggregation. The data are augmented by measurements of freezing‐point depression to estimate the degree of protonation of PBT in sulfuric acid, and by measurements of partial specific volume to provide an estimate for the geometric diameter of the chain in solution. The results show a degree of protonation of 2–4 protons per repeating unit, and partial specific volumes that are appreciably smaller than the specific volume determined for the polymers. Similar data and results are reported for poly(1,4‐phenyl‐ene‐2,6‐benzobisoxazole) and poly(1,4‐phenylene terephthalamide).
Polymer single crystals of poly(4‐hydroxybenzoate). I. Morphology
Authors: Lieser G. , Schwarz G. , Kricheldorf H. R. 
Published: April 2003

AbstractPoly(4‐hydroxybenzoate) was prepared from 4‐acetoxybenzoic acid, 4‐pivaloyloxybenzoic acid, and 4‐trimethylsiloxy benzoyl chloride by condensation without the addition of a catalyst. Although various reaction conditions were used the products were always crystalline. The morphology of the as‐polymerized crystals has been studied by transmission electron microscopy as a function of molecular weight and end‐group type. It was possible to detect morphological features during the course of reaction from oligomers to polymer which enabled us to determine the mechanisms that are likely to control chain growth for different regimes of polymerization kinetics.
Small‐angle x‐ray scattering study of density fluctuation in pressure‐densified polystyrene glasses
Authors: Curro J. J. , Roe Ryong‐Joon 
Published: April 2003

AbstractDensified polystyrene glasses, prepared by cooling from the liquid state under elevated pressure, were studied by small‐angle x‐ray scattering at ambient pressure. The density fluctuation, determined from the x‐ray data, showed a decrease with increasing pressure up to about 1.5 kbar, and then leveled off to a fairly constant value. The reduction in the density fluctuation produced by the pressure is much greater than the associated decrease in the specific volume. The observed change in density fluctuation is consistent with the view that the density fluctuation in glassy polymers consists of dynamic and quasistatic components and that the first of these can be correlated with the compressibility of the glass. The present data on the density fluctuation, in conjunction with the available data on volume and enthalpy, can be interpreted to mean that in pressure‐densified glasses unfavorable chain configurations are trapped in local energy minima, and the strain energy thus stored can promote segmental motion leading to volume expansion at temperatures far below Tg. Some preliminary evidence indicating the formation of microcavities in these pressure‐densified glasses is also presented.
Microstructure rearrangement during the heat‐treatment of melt‐drawn poly(ethylene terephthalate) fibers
Authors: Gupte K. M. , Motz Heike , Schultz J. M. 
Published: April 2003

AbstractMelt‐spun poly(ethylene terephthalate) fibers were isothermally heat‐treated at constant length. Microstructural changes occurring during the heat‐treatment were monitored using specific gravity, wide‐angle x‐ray scattering (WAXS), small‐angle x‐ray scattering (SAXS), optical birefringence, and static mechanical testing. Major changes in the density of the most highly oriented fiber examined occurred in times below 100 ms. For less oriented fibers, the time scale for significant density change increases to the 1–10 s range. The course of birefringence increase approximates that of the density. WAXS measurements show that crystallinity develops at essentially constant crystal perfection, but that the orientation of the crystallites first decreases and then increases with time. SAXS results show development of a four‐point pattern, the azimuthal angle of the lobes decreasing with initial orientation, with temperature, and with time. A streak transverse to the fiber axis develops more rapidly than do the lobes. A two‐stage transformation process is envisaged, the first stage being the formation of defective crystal fibrils and the second being internal rearrangement of the fibrils to form more perfect crystallites, separated by more amorphous zones. Changes in the crystallite orientation are related to constraints of the noncrystalline material on the crystallites.
Optimization of permanganic etching of polyethylenes for scanning electron microscopy
Authors: Naylor K. L. , Phillips P. J. 
Published: April 2003

AbstractThe permanganic etching technique has been studied as a function of time, temperature, and concentration for a series of polyethylenes. Kinetic studies show that a film of reaction products builds up on the surface, impeding further etching, an effect which is greatest for the lowest‐crystallinity polymers. SEM studies combined with EDS show that the film contains sulfur, potassium, and some manganese. An artifact is produced by the etching process which is impossible to remove by washing procedures if certain limits of time, temperature, and concentration are exceeded. For lower‐crystallinity polyethylenes multiple etching and washing steps were required for optimal resolution. Plastic deformation during specimen preparation, whether from scratches or freeze fracturing, enhances artifact formation. When appropriate procedures are used virtually artifact‐free surfaces can be produced allowing a combination of permanganic etching and scanning electron microscopy to give a rapid method for detailed morphological characterization of bulk specimens.
Nonlinear viscoelasticity of entangled polymer chains. I. Response of the slip‐link model to elongation strains
Authors: Thirion P. , Tassin J. F. 
Published: April 2003

AbstractThe theoretical stress relaxation of polymer melts is derived from a new solution of the response of Doi's slip‐link model in the plateau zone of the rubberlike part of the spectrum. The results of this treatment, based on the application of the statistical theory of elasticity to a virtual transient network of monodisperse entangled chains, are applied to the calculation of stress‐strain relations for uniaxial extension performed at constant strain rate and to that of stress relaxation after such an elongation.
Bisphenol‐A polycarbonate–diluent interactions
Authors: Belfiore L. A. , Cooper S. L. 
Published: April 2003

AbstractThe effect of low‐molecular‐weight miscible additives on the sub‐Tg (β) relaxation process in bisphenol‐A polycarbonate (BPAPC) was studied using high‐resolution carbon‐13 solid‐state NMR. The trend of the spin‐lattice relaxation times T1 at 50 MHz suggests that strong intermolecular interactions occur upon mixing when BPAPC is physically stiffened by the antiplasticizing diluent, diphenylphthalate. The values of 13C T1 at 15 MHz in d‐chloroform solutions for similar BPAPC‐diluent mixtures suggest that diluent effects on the megahertz mobility of the polymer occur exclusively in the solid state. These results are explained using equilibrium thermodynamics, in the Ehrenfest sense, at the second‐order glass transition temperature Tg. Theory predicts that the temperature dependence of the Flory–Huggins interaction parameter ∂χ/∂T changes abruptly as the polymer‐diluent blends are cooled below Tg from the molten state. The difference between ∂χ/∂T in the liquid and glassy states is the major factor which determines the diluent concentration dependence of Tg. A method is developed to estimate the relative magnitudes of χ for polymerdiluent blends in the glassy state.
Hard‐segment domain sizes in MDI/diol polyurethane elastomers
Authors: Blackwell John , Lee Chun Dong 
Published: April 2003

AbstractWide‐angle x‐ray line broadening methods have been used to determine the crystallite size and degree of distortion for the crystalline hard domains in MDI/diol polyurethane elastomers. Crystallite widths have been determined from the integral breadths of the 00l reflections following correction for distortions of the second kind according to the method Hosemann for polymers prepared using ethylene glycol, propandiol, butandiol, and hexandiol as the chain extender. These data relate to the width of the crystallites along the chain axis and thus can be interpreted in terms of the morphology of the hard domains. The crystallite sizes increase and the distortions decrease with increasing elongation and annealing time, and the sizes tend toward asymptotic values which correspond approximately to the average lengths of the hard‐segment chains. These results rule out any appreciable folding of the hard‐segment chains and point to a model for the hard domains formed by lateral aggregation of extended hard segments.
Halato‐telechelic polymers. VII. Viscoelastic behavior of α,ω‐divalent cation dicarboxylato polybutadiene
Authors: Broze G. , Jérôme R. , Teyssié Ph. , Marco C. 
Published: April 2003

AbstractBa, Ca, Mg, and Zn short‐length carboxylato‐telechelic polybutadienes (M̄n = 4,600) exhibit thermorheological simplicity. A secondary relaxation characteristic of ionic aggregates obeys an Arrhenius type of activation, the energy of which is in inverse proportion to the ionic radius of the cation, whereas the mean size of the ionic aggregates is proportional to it. The glass transition of the carboxy‐telechelic polybutadiene is quite independent of the degree of neutralization and of the subsequent phase separation by the metal cations. The increase in cation size favors the growth of the multiplets into layered structures. Two sets of relaxation times are reported for the smallest alkaline‐earth cation (Be). They suggest the existence of small multiplets unable to grow except into clusters.
An examination of specific interactions of esters and polyesters with chlorinated solvents by infrared and nuclear magnetic resonance spectroscopies
Authors: Garton Andrew , Cousin Patrice , Prud'homme Robert E. 
Published: April 2003

AbstractPoly(ϵ‐caprolactone) and low‐molecular‐weight esters (methyl acetate and propyl propionate) were dissolved in binary solvent mixtures. The relative ability of the two solvents to associate with the ester group was then determined by examining the equilibrium between the two solvation states of the ester using infrared and nuclear magnetic resonance spectroscopies. The ability of the solvents to associate with the chosen esters was found to be in the sequence α‐hydrogenated chlorocarbons > β‐hydrogenated chlorocarbon > carbon tetrachloride ≈ aliphatic hydrocarbons. The association‐state equilibrium was also shown to be concentration dependent and the polyester was shown to become involved in specific interactions to a lesser degree than the model esters. The relevance of these results to the nature of the specific interactions in polyester/halogenated polymer blends is discussed.
Comparison of the rheological properties of linear and star‐branched polyisoprenes in shear and elongational flows
Authors: Pearson Dale S. , Mueller Susan J. , Fetters Lewis J. , Hadjichristidis N. 
Published: April 2003

AbstractThe rheological properties of narrow‐molecular‐weight‐distribution linear and star‐branched polyisoprenes have been determined using both shearing and stretching deformations. At all strain rates studied the tensile stress measured under transient and steady‐state conditions did not increase above the linear viscoelastic value. The absence of an enhanced tensile stress for the branched polymer is in contrast to what is observed for branched low‐density polyethylene. An explanation for the difference is proposed. Additional remarks are made about the broad distribution of relaxation times observed for star‐branched polyisoprenes and about the approach to steady state in constant‐strain‐rate and constant‐stress tests.
Carbon‐13 NMR of polyethylenes: Correlation of the crystalline component T1 with structure
Authors: Axelson D. E. , Mandelkern L. , Popli R. , Mathieu P. 
Published: April 2003

AbstractThe carbon‐13 spin‐lattice relaxation times T1 of the crystalline portion of a set of polyethylenes have been studied. Chain structure and crystallization conditions have been varied over the widest possible extremes so that large differences are developed in the level of crystallinity, the supermolecular structure, and the crystallite thickness. Concomitantly, the observed crystalline T1 values cover the extraordinarily wide range of about 40–4500 s. They bear a one‐to‐one relation with the crystallite thickness, which is found to be the key structural variable determining this property. A correlation with the temperature for the α‐transition can be established, which implies a similar type of segmental motions for the two phenomena. Major changes in the interfacial structure can also have a drastic influence on the value for the crystalline T1. Analysis of the magnetization decay curve also allows for a quantitative determination of the degree of crystallinity, which is found to be in excellent agreement with the corresponding value found from Raman spectroscopy.
A kinetic analysis of the gauche‐trans isomerization in semicrystalline poly(ethylene terephthalate)
Authors: Lin Shaow‐Burn , Koenig Jack L. 
Published: April 2003

AbstractA series of kinetic measurements using FT‐IR have been carried out in order to clarify the mechanism of the gauche‐trans isomerization process and the time‐temperature‐transformation relationships in poly(ethylene terephthalate). Two‐stage isomerization isotherms are distinguished on the logarithmic scale of annealing time: a primary transformation stage with an activation energy of 40 kcal/mol characterized by a sigmoidal curve, followed by a linear secondary process. The activation energies of the secondary transformation obey an equation of the Arrhenius type InaT = B(1/T − 1/Tm) where 160 < T < 260°C ≈ Tm and can be used to describe the effects of annealing time and temperature on the isomerization process of PET in the secondary transformation region. On the basis of these analyses, the morphology and microstructure of PET in these temperature regimes of the isomerization process are proposed.
Epitaxial crystallization of polymers onto benzoic acid: Polyethylene and paraffins, aliphatic polyesters, and polyamides
Authors: Wittmann J. C. , Hodge A. M. , Lotz B. 
Published: April 2003

AbstractEpitaxial crystallization of polyethylene, n‐paraffins, aliphatic polyesters, and various polyamides has been achieved on benzoic acid crystal substrates, specifically on their (001) crystals faces, which are made up of the aromatic rings. The epitaxial relationship is very similar for all polymers investigated: it is characterized by a unique chain orientation (parallel to the b axis of the substrate) and a plane of contact which is either the b‐c, plane of polyethylene, or crystallographically similar ones for polyesters, or the plane of the hydrogen‐bonded sheet for polyamides. Since benzoic acid is structurally similar to its alkali‐metal salts, it is inferred that the nucleating efficiency of the latter toward chemically different but structurally similar polymers may well rest on an epitaxial relationship. Finally, the highly oriented morphology made it possible, for some polyesters with a monoclinic cell, to determine the angle between the chain axis and the Z axis of the optical indicatrix, a quantity not measured so far in polymers.
The crystallite orientation distribution functions of deformed polytetrafluoroethylene: Multiple‐pulse 19F NMR spectroscopy
Authors: Brandolini Anita J. , Alvey Mark D. , Dybowski Cecil 
Published: April 2003

AbstractA moment analysis of dipolar‐decoupled 19F NMR spectra of deformed polytetrafluoroethylene leads to an expansion which approximates the crystallite orientation distribution function. The resulting distributions are then interpreted in terms of this polymer's known mechanical properties, and an attempt is made to model these functions.