bluuebunny/crossref_metadata_2025_split huggingface dataset:
Polydispersity effects in polymer self‐diffusion measured by small‐angle neutron scattering
Authors: Finerman T. , Crist B. 
Published: April 2003

AbstractThe small‐angle neutron scattering (SANS) method for measuring the self‐diffusion coefficient D has been analyzed for effects of polydispersity in degree of polymerization for the case of linear polymers diffusing by reptation. Polydispersities corresponding to Mw/Mn = 1.0−10 were considered. It is shown that in all cases a meaningful effective diffusion coefficient De can be obtained from the short time recovery of the SANS intensity. This quantity De ≤ 1.3 D(Mw), where D(Mw) is the diffusion coefficient of a monodisperse polymer having molecular weight M = Mw. The method relies on SANS intensities extrapolated to zero scattering angle; realistic extrapolation is shown to give rise to quite acceptable errors on the order of 0.05 De.
Fiber spinning from the nematic melt. IV. Effect of P oxybenzoate crystallinity on the spinning and fiber properties of the copolyester of polyethylene terephthalate and p‐oxybenzoate
Authors: Muramatsu Hiromochi , Krigbaum W. R. 
Published: April 2003

AbstractWe have previously reported two studies of the rheology and fiber properties of one sample of the copolymer of polyethylene terephthalate having 60 mol% of p‐oxybenzoate (PHB) units. The DSC curve of that sample exhibited crystalline melting transitions, and the sample appeared to contain PHB blocks. Here we compare those results with observations for a second sample that, although nominally the same polymer, appears to be more random because it exhibits little PHB crystallinity. We had previously reported that the flow of the copolymer containing PHB blocks was non‐Newtonian at all temperatures, and that it exhibited a thermal history effect. We find the flow of the more random polymer is Newtonian above the melting temperature, and the melt viscosity of the more random copolymer exhibits no thermal history effect. Fibers were spun from the more random copolymer with a capillary rheometer using a capillary having a length/diameter ratio of 14.1 and a shear rate at the wall of 6.4 sec−1. Spinning temperatures were 250, 260, and 280°C, and the spin draw ratio was examined as a variable. The initial modulus increased with spin draw ratio but exhibited no dependence upon the spinning temperature. For the copolymer containing PHB blocks, the initial modulus increased as the spinning temperature was raised. These differences are due to the larger amount of PHB crystallinity in the more blocky sample. When chips of the more random sample were heated for 1 h at 235°C, the melt viscosity increased and the initial modulus of the fibers decreased. These changes are due to the crystallization of longer PHB blocks produced by melt interchange.
Two‐dimensional NMR characterization of propylene copolymers
Authors: Cheng H. N. , Lee G. H. 
Published: April 2003

AbstractNewly developed two‐dimensional NMR techniques are used to characterize homopolypropylene and copolymers of propylene with ethylene and butylene. Both homonuclear and heteronuclear shift‐correlated experiments have been attempted. The most useful technique for the copolymers is 13C‐1H shift correlation, whereby the one‐dimensional 1H‐NMR spectra of these copolymers are completely interpreted for the first time. Although the 1H resonances are heavily overlapped, the use of a computerized “analytical” approach permits pertinent information to be obtained from the one‐dimensional 1H‐NMR spectra.
The onset of chain folding in ultralong n‐alkanes: An electron microscopic study of solution‐grown crystals
Authors: Organ S. J. , Keller A. 
Published: April 2003

AbstractCrystallization of extremely pure n‐C198H398 from dilute solution is investigated by electron microscopy and differential scanning calorimetry. This long‐chain alkane is capable of crystallizing in the extended form or of folding once or twice and thus provides useful insights into the fundamental process of chain‐folded crystal growth. Crystallization conditions that give rise to the different crystal types are identified by measuring the variation in dissolution temperature with crystallization temperature and crystallization time. Examples of each type of crystal are examined in the electron microscope using various techniques. From shadow length measurements we find the crystals to have thicknesses corresponding to integer fractions of the extended chain length, in agreement with previous findings on this and similar materials. However, contrary to all past experience with polyethylenes, refolding was found to occur while in the solvent. This refolding, which here takes place isothermally, also occurs in integral steps, with wider implications discussed. The crystal morphologies are similar in many respects to those obtained from polyethylene; in particular the crystals containing once‐folded chains are often lozenge shaped with the folds along {110} growth planes, thus producing clearly defined sectorization made visible by several techniques. Electron diffraction studies show that the chains are substantially perpendicular within the crystals in all cases. In addition, differences in surface regularity between the different crystal types revealed by decoration techniques are discussed.
Molecular weight relations for crosslinking of chains with length and site distribution
Authors: Miller Douglas R. , Macosko Christopher W. 
Published: April 2003

AbstractMany polymer networks are formed by crosslinked polymer chains through reactive sites distributed along the chains. How these sites are distributed as well as the chain length distribution can have a significant effect on properties like the gel conversion and molecular weight. Previous treatments have used simplifying approximations. In this paper we eliminate these approximations and derive computational formulae for weight average molecular weight and gel point for polymer chains of any length and reactive site distribution. Three types of crosslinking are considered: direct coupling of chains (homopolymerization), direct coupling through propagation, and coupling through copolymerization with small monomers.
Molecular weight dependence of mechanical properties of poly(p,p′‐oxydiphenylene pyromellitimide) films
Authors: Volksen W. , Cotts P. , Yoon D. Y. 
Published: April 2003

AbstractThe mechanical properties, i.e., Young's modulus, elongation, and tensile stress, were determined as functions of the molecular weight for films of poly(oxydi‐p‐phenylene pyromellitimide) prepared by thermal cyclization of the precursor poly(amic acid). The molecular weights of the samples were controlled by the monomer stoichiometry employed for the solution condensation of pyromellitic dianhydride and p,p′‐oxydianiline. Weight‐average molecular weights were determined by light scattering of the precursor poly(amic acid) as well as the fully cyclized polyimide. The elongation‐at‐break is most sensitive to the molecular weight, undergoing a rapid increase at Mw ⋍ 8000 and reaching a limiting value of about 60% for Mw > 20,000.
Craze growth and healing in polystyrene
Authors: McGarel Owen J. , Wool Richard P. 
Published: April 2003

AbstractThe kinetics of craze growth and craze healing were studied by dark‐field optical microscopy in monodisperse molecular weight polystyrene (PS) that varied in molecular weight from 88,000 to 1,334,000. The following observations were made. (1) G1 the virgin growth rate, decreased rapidly with increasing molecular weight until Mn ∼ 200,000 and then remained constant. (2) G1 decreased with increasing craze density. (3) The growth rates of approaching craze tips decreased when the craze tips overlapped, and the effect was less for crazes whose parallel growth paths were greater than 40 μm apart. (4) Complete craze healing was observed by comparison of the nucleation times, τ2, and growth rates, G2, of healed individual crazes with the craze kinetics of the virgin sample. (5) The extent of healing was characterized using four cases in which τ and G were measured as a function of healing time, temperature, constant stress, and molecular weight. (6) Craze healing times were found to increase with molecular weight and were analyzed in terms of the modified molecular weight of the craze zone. (7) Significant bond rupture was determined to occur during crazing by comparison of healing times with stress relaxation and diffusion data. (8) Craze healing studies provide insight into both crack healing and fracture of glassy polymers.
Network parameters for crosslinking of chains with length and site distribution
Authors: Miller Douglas R. , Macosko Christopher W. 
Published: April 2003

AbstractPolymer chains are crosslinked to form a network. The chains are polydispersed, with an arbitrary distribution of weight and functionality. Crosslinks may form in three different ways: direct coupling (homopolymerization), direct coupling with propagation, and coupling through copolymerization with small monomers. Various network parameters are calculated. We give computational formulae for gel point, weight fraction soluble, weight fraction pendant, weight fraction effective, concentration of effective junctions, concentration of effective network strands, and the entanglement trapping factor. These formulae give exact values for systems that previously have only been solved using simplifying approximations. Examples show that such simplification may lead to sizable errors in the computation of network parameters.
Thermodynamic and morphological studies of the solid‐state transition in copolymers of vinylidene fluoride and trifluoroethylene
Authors: Stack G. M. , Ting R. Y. 
Published: April 2003

AbstractCopolymers of vinylidene fluoride and trifluroethylene are presently being considered for many piezoelectric applications. In contrast to the poly(vinylidene fluoride) homopolymer, a transition from the room temperature ferroelectric phase to a paraelectric phase has been observed in these copolymers. The temperature of this transition was determined by measuring the position of the endothermic peak observed in the differential scanning calorimeter. The rate of cooling from the paraelectric to ferroelectric phase was found to affect the temperature at which the transition occurred on subsequent heating. Changing the conditions under which molten polymer initially crystallized into the paraelectric phase had an even greater effect on the resultant transition temperature. In addition to the DSC studies, wide‐angle x‐ray measurements were performed on samples subjected to different thermal treatments. No significant differences were found in the x‐ray scans of these copolymers.
Depression of the crystal‐nematic phase transition in thermotropic liquid crystal copolyesters
Authors: George Eric R. , Porter Roger S. 
Published: April 2003

AbstractThe crystal‐nematic phase transition of a copolyester consisting of 20 mol% poly(ethylene terephthalate) and 80 mol% p‐hydroxybenzoic acid (PHB) was characterized by depression of the crystal‐nematic transition by the addition of a liquid crystal diluent. This copolyester contains blocks of crystalline PHB. Its transition behavior was compared with thatrandom copolyester with diluent of the same composition. From the extrapolated transition temperature depression data, the heat of transition per mole of p‐oxybenzoate was calculated as about 1.3 kcal/mol, with an entropy of about 2 cal/deg mol. This assumes that only the p‐oxybenzoate unit crystallized from the nematic state. The validity of the Flory‐Huggins model for this transition point depression was confirmed graphically by comparison with two different thermotropic‐liquid crystal polyesters. These results may represent the first reported crystal‐nematic temperatures and heats generated by the dilution method for liquid crystal copolyesters of this type.
Thermodynamic properties of chain liquid mixtures with strong, local interactions
Authors: Durning C. J. , Secor R. M. 
Published: April 2003

AbstractThis paper addresses the effect in chain liquids on thermodynamic behavior of a large charge in the strength of local interactions on mixing. We classified real systems according to the ability of the Flory equation of state (FES) theory to prodict the excess volume. The FES theory enables reliable estimates provided the mixture is nearly athermal and the pure components obey the Flory equation of state. These requirements arise from Flory's approximations for the lattice part of the configuration integral, ZL, and the cell part, ZC, respectively. We revised ZL without adding adjustable parameters by the quasichemical perturbation method to account for local ordering in the mixture because of a large change in the strength of binary interactions on mixing. The modification improves the predictions of the thermodynamic functions for systems where Flory's ZL is inadequate.
Primary and secondary pyroelectricity in highly oriented polyvinylidene fluoride
Authors: Nix E. L. , Nanayakkara J. , Davies G. R. , Ward I. M. 
Published: April 2003

AbstractMeasurements of the pyroelectric response in both free and constrained conditions are presented for low‐draw ratio, high‐draw ratio and voided form I polyvinylidene fluoride sheets. The results are used together with related data for the piezoelectric response and for the thermal expansion behavior to estimate the primary and secondary components of the total macroscopic pyroelectricity. For the high‐draw ratio and voided sheets the primary response was approximately 90% of the total pyroelectricity compared with 40% for the low‐draw ratio nonvoided sheet. It is proposed that the role of reversible temperature‐dependent crystallinity is more pronounced in the high‐draw samples. Previous NMR studies provide support for this conclusion.
A study of the mechanical anisotropy of high‐draw, low‐draw, and voided PVDF
Authors: Humphreys J. , Lewis E. L. V. , Ward I. M. , Nix E. L. , McGrath J. C. 
Published: April 2003

AbstractThe mechanical anisotropy of oriented PVDF sheet is examined using a variety of experimental techniques. The mechanical behavior is similar to that observed previously for low‐density polyethylene and nylon and consistent with a parallel lamellar crystalline structure. The s31 compliance is reduced in magnitude by drawing to higher draw ratio, but the reduction in the piezoelectric coefficient d31 is less marked, suggesting that the piezoelectric response cannot be related solely to dimensional changes under stress. Drawing to high draw ratio increases the s33 compliance, and this is further increased by introducing voids. The corresponding d33 piezoelectric coefficient is not changed significantly by drawing to high draw ratio, or by the introduction of voids, again indicating that the piezoelectric behavior relates to factors other than dimensional changes.
A strip resonance technique for measuring the ultrasonic viscoelastic parameters of polymeric sheets with application to cellulose
Authors: Pankonin B. , Habeger C. 
Published: April 2003

AbstractAn acoustic strip resonance apparatus that measures the viscoelastic response of polymeric sheets over the temperature range of 100–400°K and at frequencies from 20 to 100 kHz is described. This is a computer‐controlled instrument which uses the resonant frequency and the quality factor of standing waves in strips to calculate the ratio of complex Young's modulus to density. Results are presented for cellophane and paper samples. These are consistent with measurements reported by others at lower frequencies. That is, (1) a single predominant secondary relaxation process which corresponds to the γ relaxation occurring at 200°K and 1 Hz is found in dry samples at 260°K and 60 kHz; (2) as moisture is added, the γ relaxation weakens and a higher temperature, β, relaxation appears; and (3) at low temperature, moisture addition initially increases the mass specific Young's modulus. The effects of methanol on the viscoelastic parameters of cellulose were also measured and found to be similar to those of water.
Interpretation of light scattering from supermolecular structures in liquid systems by master curves
Authors: Dautzenberg H. , Rother G. 
Published: April 2003

AbstractBy a detailed analysis of light scattering curves, a more complete characterization of colloidal particles may be achieved. For this purpose, a fitting procedure based on theoretical master curves of models of polydisperse systems of homogeneous spheres and Gaussian coils is presented. The use of a suitable logarithmic distribution function makes it possible to separate the influence of polydispersity from that of particle size on the shape of the scattering curve. A double logarithmic plot of master curves reduces the fitting procedure to translations of the experimental curves. The reliability and accuracy of this procedure are demonstrated by light scattering results on solutions of a polyelectrolyte complex with variation of salt content.
Optical properties of poly‐diacetylene monolayers
Authors: Collins Michael A. 
Published: April 2003

AbstractThe topochemical polymerization of diacetylene molecules as monolayers in situ on a Langmuir trough is described via the visible absorbance of the polymer products. Possible differences in the product structure between crystal or multilayer polymerizations and that in monolayers is investigated for two monomers. Significant differences in the visible, infrared, and resonance Raman spectra are reported and a strong dependence of visible absorbance on surface pressure is described.
Cholesteric hydroxypropylcellulose solutions: Microscopy and small‐angle light scattering
Authors: Meeten G. H. , Navard P. 
Published: April 2003

AbstractPolarized optical microscopy and small‐angle light scattering (SALS) are used to investigate the cholesteric phase of aqueous hydroxypropylcellulose (HPC) solutions. The results suggest a polygonal focal conic rather than the recently proposed parabolic focal conic morphology. Measurements of the polygonal domain size show it to decrease with increasing HPC concentration.Depolarized SALS gives clover‐leaf patterns, whose maximum of intensity at 45° is related to the domain size.
Latent energy of deformation of bisphenol A polycarbonate
Authors: Adams G. W. , Farris R. J. 
Published: April 2003

AbstractThe thermodynamic behavior of poly(bisphenol A carbonate) (PC) during uniaxial cold drawing and the properties of the drawn polymer were examined. Isothermal deformation calorimetric measurements were made during the drawing process. The deformation calorimeter measures heat, work, and internal energy changes for deformation. It was found that PC exhibited nonideal plasticity with approximately 50–80% of the work of deformation dissipated as heat. The remainder of the work of deformation was stored as a latent internal energy change. The value of the internal energy change was dependent on strain rate at 20°C but was not strongly dependent on temperature in the range 20–65°C. Thermomechanical measurements on cold‐drawn PC samples demonstrated striking behavior at temperatures far below the glass transition temperature Tg. Stress‐temperature experiments showed that the stress increased for uniaxially constrained samples, and this stress increase began at temperatures just above the deformation temperature. Additional experiments indicated that the changes which took place during cold drawing were physical in nature and were thermoreversible. These changes in physical properties are related to those which occur due to physical aging below Tg.
Effect of combined pressure and temperature changes on structural recovery of glass‐forming materials. I. Extension of the KAHR model
Authors: Ramos A. R. , Kovacs A. J. , O'Reilly J. M. , Tribone J. J. , Greener J. 
Published: April 2003

AbstractThe KAHR model of structural relaxation has been extended to include the effects of pressure upon the retardation times of glass‐forming materials. The previously used methodology is applied with a continuous distribution of retardation times of the fractional exponential form. Several forms of the pressure dependence are examined. The combined temperature and pressure changes on structural recovery of glasses are addressed in this paper.
Effects of ionizing radiation on epoxy, graphite fiber, and epoxy/graphite fiber composites. Part II: Radical types and radical decay behavior
Authors: Seo K. S. , Fornes R. E. , Gilbert R. D. , Memory J. D. 
Published: April 2003

AbstractElectron spin resonance (ESR) investigations of line shapes and radical decay behavior have been made on an epoxy based on tetraglycidyl diaminodiphenyl methane (TGDDM)/diaminodiphenyl sulfone (DDS), T‐300 graphite fiber, and T‐300/5208 (graphite fiber/epoxy) composites after irradiation with Co60 γ‐radiation or 0.5 MeV electrons. Two kinetically distinct radical species are found in the irradiated epoxy as the temperature is raised beyond 120 K following irradiation of samples at 77 K with Co60 γ. One has been termed a fast‐decaying species and the other a slow‐decaying species. The ratio of fast‐decaying/slow‐decaying radicals increases as the decay temperature rises. The fast‐decaying radicals at room temperature are attributed largely to alkyl type radicals residing in regions of relatively low crosslink density, while the long‐lived radicals are attributed to radicals residing in the highly crosslinked regions of the epoxy. A large concentration (ca. 1020 to 1021 spins/g) of unpaired electrons was found in unirradiated graphite fiber which masked the ESR spectral change in irradiated composites.
Effects of thermal treatment on biaxially oriented poly (ethylene terephthalate). III. Creep behavior following various thermal histories
Authors: Vallat M.‐F. , Plazek D. J. , Bhushan B. 
Published: April 2003

AbstractElongational creep measurements were carried out on a biaxially oriented poly (ethylene terephthalate) (PET) film parallel to, orthogonal to, and at 45° to the principal optic axis. Measurements made after various thermal treatments which were intended to stabilize the physical state of the PET were shown to be ineffective. Samples were annealed at 140°C for 12 days and aged at 95°C for over 24 days before measurement without success. Thermal cycling between 41 and 91°C which was also employed to stabilize the mechanical response also failed. Significant deceleration of the creep rate caused by densification of amorphous regions of the samples during storage below the glass temperature Tg is illustrated. Because of physical aging below Tg and morphological changes occurring above Tg during the various thermal treatments and histories, time‐scale shift factors were found to be not unique.
Simulations on radiation‐cured polyethylene
Authors: Galiatsatos V. , Eichinger B. E. 
Published: April 2003

AbstractSimulations of the simultaneous crosslinking and chain scission reactions that are induced in bulk amorphous polyethylene by irradiation have been implemented with a computer. A detailed study of the various defect structures in the network is reported, to show that dangling ends have the highest population among these defects, especially at higher crosslink densities. The calculated sol fractions compare very favorably with previously published experimental studies. We find that there is about 1 chain scission for every 9 crosslinked units, i.e., one scission per 4.5 crosslinks. This is fewer scissions than had previously been deduced by application of the Charlesby‐Pinner method.