bluuebunny/crossref_metadata_2025_split huggingface dataset:
Oxygen and water‐vapor diffusion through biaxially oriented poly(ethylene terephthalate)
Authors: Orchard G. A. J. , Spiby P. , Ward I. M. 
Published: April 2003

AbstractThe gas barrier properties of a wide range of biaxially oriented polyethylene terephthalate films have been investigated. The permeability and diffusion coefficients for oxygen and water vapor transmissions were determined for films prepared by both simultaneous and sequential biaxial stretching. The effect of annealing was also studied and a comparison made with previous results for uniaxially oriented films. It was found that the permeability correlated well with the density of the sample, but that the dependence on the gauche/trans conformer ratio shown for uniaxial material was not so clear. A good empirical correlation was also obtained between permeability and a proposed measure of molecular orientation obtained from refractive index measurements. A more detailed interpretation, based on infrared measurements of orientation, showed that there is a systematic reduction in permeability as the planes of the terephthalate residues become parallel to the film surface.
Gas transport and diffusive resistance in highly oriented high‐density polyethylene
Authors: Chau C. C. , Raspor O. C. 
Published: April 2003

AbstractHigh density polyethylene was biaxially oriented by high‐pressure forging and subsequently low‐pressure thermal forming. The permeability of O2, CO2, and CH4 was reduced slightly by biaxial orientation, the diffusivity of the gases was found to increase with the degree of orientation. The solubility decreased markedly with orientation. Density measurements indicated that crystallinity increases with the extent of biaxial deformation. Based on the concept of particle flow in viscous media, the variation of gas diffusivity with orientation is viewed as a frictional resistance effect. The diffusivity is inversely proportional to a phenomenological friction coefficient; which can be related to orientation by a shape factor s. The diffusivity for the oriented state, D, is then related to that of the nonoriented state, D0, as D = D0/(1 + sϕ). The variation of diffusivity selectivity with orientation is also formulated based on this consideration. Results on both biaxially and uniaxially oriented HDPE are examined in light of this model.
Dielectric and conductivity relaxations in poly(hema) and of water in its hydrogel
Authors: Pathmanathan K. , Johari G. P. 
Published: April 2003

AbstractThe dielectric permittivity ε′ and loss ε″ of anhydrous poly(2‐hydroxyethyl methacrylate) and its 38.6 w/w% hydrogel have been measured in the frequency range from 12 Hz to 200 kHz and the temperature range from 77 to 273 K. The former has a sub‐Tg relaxation with a half‐width of 4.5 decades for the loss spectra, whose strength increases with temperature, and an activation energy of 62.5 kJ/mol. The dielectric relaxation time of the α process of supercooled water in the hydrogel is 53 s at its calorimetric Tg of 135 K. The half‐width of the relaxation spectrum is 2.85 decades and, in the narrow temperature range, its apparent activation energy is 60.8 kJ/mol. Heating of the hydrogel causes crystallization of water which begins at about 207 K and becomes readily detectable as a second dielectric loss peak at about 230 K. For each temperature between 207 and 267 K, supercooled water in the hydrogel coexists with its crystallized form, with the amount of the crystallized solid increasing with increasing temperature. These results are discussed in terms of “bound” and “free” states of water in the hydrogel.
Bubble nucleation in polymeric liquids. I. Bubble nucleation in concentrated polymer solutions
Authors: Han James H. , Dae Han Chang 
Published: April 2003

AbstractLaser light scattering, with the aid of Mie's scattering theory, was used to investigate bubble nucleation in concentrated polymer solutions. Solutions with 40, 50 and 60 wt % polystyrene in toluene were used. A test solution in a high‐pressure optical cell made of strain‐free quartz was heated to a predetermined temperature under pressure. Upon release of the pressure in the cell, both scattered and transmitted light fluxes were measured with photomultipliers, and the variation of system pressure with time was measured using a piezoelectric pressure transducer. The measurement of the light scattering flux and control of the experiment were performed by means of a microcomputer with a general‐purpose data acquisition interface. Data reduction was done using the same microcomputer. The critical bubble size was determined by obtaining a one‐to‐one correspondence between the extrema of the experimental and theoretical scattering curves. While the Mie scattering theory is for monodisperse particles, the experimental scattering curves indicated that the bubbles had a distribution of sizes. Therefore, the log‐normal distribution function was used to represent the size distribution; and theoretical scattering curves were computed by varying the breadth parameter in the log‐normal distribution function, until we had a one‐to‐one correspondence between the extrema of the experimental and theoretical scattering curves. In this way, we were able to determine (a) the size distribution of bubbles in the optical cell, (b) the critical bubble size, (c) the total number of bubbles nucleated, and (d) the critical pressure for bubble nucleation, as functions of temperature, the initial equilibrium pressure in the optical cell, and the concentration of the polymer solution.
Graft interpenetrating polymer networks of polyurethane and epoxy. II. Toughening mechanism
Authors: Hsieh K. H. , Han J. L. 
Published: April 2003

AbstractGraft interpenetrating polymer networks (graft‐IPNs) of polyurethane (PU) and the diglycidyl ether of bisphenol A (epoxy) were prepared by first grafting excess PU prepolymer to the epoxy and then simultaneously polymerizing the PU prepolymer and epoxy. The fracture properties, at high shear rate (e.g., impact) and low shear rate (e.g., pseudostatic tensile fracture energy measurement) of these graft‐IPNs exhibit opposite behavior. Although dispersed rubber particles can enhance the Izod impact strength, toughening of the matrix of graft‐IPNs was found to be the main contribution. In contrast, it was found that a heterogeneous morphology with suitably dispersed rubber domains of appropriate size as well as the toughness of the matrix are requirements for effectively increasing the fracture energy at low shear rate. A reinitiating crack in the plastic matrix is proposed as the main toughening mechanism and can be invoked to interpret the fracture behavior at high and low shear rates of the graft‐IPNs.
Diffusion after chain fragmentation in the glassy state: A SANS study
Authors: Meurer E. , Kuhn K. J. , Hellmann G. P. , Rennie A. R. 
Published: April 2003

AbstractA new method to measure the extremely slow chain diffusion in polymer systems at and below the glass transition temperature is proposed. It requires polymers that can be fragmented in a well‐defined manner in the glassy state. The idea is first to split all chains, while their conformations are frozen in, and then to measure at some higher temperature the diffusion of the chain fragments by small‐angle neutron scattering. The structure factor changes in the course of the fragment diffusion from that of the initial long chains to that of the uncorrelated fragments. Preliminary results from experiments on blends of thermosensitive copolycarbonates and tetramethylpolycarbonate are presented. They demonstrate the viability of the technique.
Mass diffusion of camphorquinone in poly(methyl methacrylate) hosts with different molecular weights
Authors: Xia J. L. , Wang C. H. 
Published: April 2003

AbstractDiffusion coefficients of camphorquinone (CQ) and its photoproduct (CQP) in two poly(methyl metbacrylate) (PMMA) samples with different molecular weights are measured as a function of temperature. Above the glass transition, the temperature dependence is not Arrhenius and can be described by the Williams‐Landel‐Ferry equation. It is also shown that the difference in the temperature dependence of the diffusion coefficients of CQ in the two PMMA samples is attributable to the difference in glass transition temperatures.
A free‐volume‐based approach to modeling thermoset cure behavior
Authors: Sanford W. M. , McCullough R. L. 
Published: April 2003

AbstractThe free‐volume theory for the temperature dependence of transport properties of glass‐forming polymers is extended to obtain their relationship to the extent of cure. This treatment centers on the unifying role of molecular mobility and yields a model which connects extent of reaction, viscosity, diffusivity, ionic conductivity and dipole relaxation time. The temporal dependence of these properties is expressed by coupling the extended free‐volume model with a relationship for the rate of cure, which included diffusional limitations. Analyses based on this model are applied to the observed behavior of a model epoxy‐amine resin system. The intrinsic kinetics of this model system are shown to be first order. It is shown that diffusional limitations strongly affected the progress of the reaction in the final stages of cure. The diffusion‐modified rate expression predictions agree with extent of reaction versus time data over the range of experimental temperatures. The temporal dependence of viscous behavior of the curing resin is measured. The extended free‐volume model accurately describes the evolution of resin viscosity during cure. The dielectric behavior is similarly characterized and is in close agreement with the predictions of the general free‐volume expression. The results of this study indicate that the free‐volume theory modified to account for molecular weight effects allows prediction of resin properties with a two‐parameter model. The results show that a power‐law relationship exists between viscosity and ionic conductivity. This result suggests that electrical properties may be used for on‐line measurement of resin viscosity during cure.
Dynamic x‐ray diffraction studies of liquid‐crystalline polyesters
Authors: Ober C. K. , Delvin A. , Bluhm T. L. 
Published: April 2003

AbstractMesophase transitions in liquid‐crystalline (LC) polyesters were studied by dynamic x‐ray diffraction using a synchrotron radiation source. Powder and fiber samples were examined by continuous heating from 50°C to 270°C in a hot stage. The polymer systems consisted of two types of thermotropic polyesters with mesogenic cores composed of combinations of substituted terephthalate, oxybenzoate and hydroquinone units combined with aliphatic spacers placed in the main chain. One of these samples was a chemically homogeneous LC polyester (HTH12) while the other LC polyester possessed chemical heterogeneity (BP6). BP6 could also be processed to form fibers which showed thermal transition behavior by x‐ray diffraction and no detectable melting or clearing transition by thermal measurements. LC textures were observed using polarized light microscopy. Results of the dynamic x‐ray diffraction studies of these two LC polymers are described.
Acoustic emission during crystallization of polymers
Authors: Galeski A. , Piorkowska E. , Koenczoel L. , Baer E. 
Published: April 2003

AbstractUltrasonic emission was found to accompany spherulitic crystallization of polymers from the melt. The source of acoustic waves was identified as an abrupt negative pressure release in regions of melt occluded by spherulites (weak spots) in the course of crystallization. The buildup of a negative pressure in occluded areas is a result of a density change during crystallization. When the level of stress approaches the limit related to melt cohesion, cavitation occurs, the stress is released, and acoustic wave is emitted. On the basis of a statistical approach to the description of spherulitic structure formation, it was predicted that most acoustic events should occur at a high degree of conversion—close to the end of crystallization. The ultrasonic emission was found during isothermal crystallization of isotactic polypropylene and poly(methylene oxide). In the case of Nylon 6, acoustic emission was attributed to environmental stress cracking and degradation. No acoustic events were recorded during crystallization of high‐density polyethylene and during similar thermal treatment of a series of noncrystallizable polymers. Addition of a nucleating agent to crystallizable polymers drastically reduces the acoustic emission during crystallization, which indicates that the spherulite size is one of the most important parameters. Depending on the size and geometry of weak spots, melt properties, and mechanical properties of spherulites, the stress in weak spots is either released by cavitation or remains frozen in the material. It was found that the time distribution of acoustic emission is located between the time distributions of the formation of weak spots and the formation of quadruple boundary points between spherulites.
Dynamic bulk modulus of various elastomers
Authors: Burns J. , Dubbelday P. S. , Ting R. Y. 
Published: April 2003

AbstractThe dynamic bulk modulus of elasticity has been measured for 14 different rubbery elastomers: three natural rubbers, five neoprenes, three polyurethanes, and one each of butyl, nitrile, and butadiene types. The measurements ranged in temperature from −10 to +40°C, at frequencies from 5 to 3000 Hz, but mostly in the range 100–1000 Hz, at 2.5 MPa pressure. Values of the real (storage) part of the modulus fell within 35% of the mean value of 2.9 GPa for all elastomers, whereas loss moduli were a few percent of the storage moduli. Master curves were obtained for two neoprenes, a polyurethane, and a butyl rubber. These were fitted by hyperbolic functions with four adjustable parameters. Effects of room‐temperature aging in artificial sea water were also studied. Aging versus time profiles fell into two distinct forms. Natural rubbers were least stable, neoprenes were intermediate, and urethanes proved most stable in bulk modulus.
Formation of ringed spherulites in polyethylenes
Authors: Chiu G. , Alamo R. G. , Mandelkern L. 
Published: April 2003

AbstractThe effects of molecular weight, molecular weight distribution, crystallization temperature, quenching medium, and sample preparation on the formation of ringed spherulites in linear polyethylenes were studied by polarized light microscopy and small‐angle light scattering. When the samples were crystallized at a predetermined temperature, ringed spherulites were formed over a narrow range of temperature and molecular weight with both fractionated and unfractionated polymer samples. Quenching the samples in air at room temperature considerably extended the range of molecular weights for the formation of ringed spherulites. Minor modification of an airquench method further extended the range and yielded better‐defined structures. The results are interpreted in terms of the anisotropy of the melt, the thermal conductivity of the quenching medium, and the shear stress applied during the crystallization process. That highly specific conditions are necessary for spherulite formation, of both the conventional and ringed type, is a major conclusion of this study.
Dynamic parameters for some poly (n‐alkyl methacrylates)
Authors: Havriliak S. 
Published: April 2003

AbstractThe dielectric relaxation data of poly (n‐hexyl methacrylate) and poly (n‐octyl methacrylate) by Strella and Chinai are represented in terms of the relaxation function proposed by Havriliak and Negami using the multiresponse techniques developed by Havriliak and Watts to evaluate the parameters and their dependence on temperature. The parameter α which represents the width of the distribution of relaxation times was found to be dependent on temperature whereas the parameter β representing the skewness of the distribution function was found to be independent of temperature. The relaxation process parameters are discussed in terms of Mansfield's jump model.
Prediction of microstructures for polydisperse block copolymers, using continuous thermodynamics
Authors: Spontak Richard J. , Williams Michael C. 
Published: April 2003

AbstractA generalization of an earlier theory (Leary–Henderson–Williams) developed for microphase separation in monodisperse block copolymers is made for copolymers having moderate degrees of polydispersity and illustrated for the Schultz molecular weight distribution (MWD). First, an explicit study is made of molecular weight (M) effects for monodisperse poly (styrene–butadiene) diblock (SB) and triblock (SBS) copolymers. For a fixed temperature, it is shown how the critical molecular weight (Mc)—above which the copolymer is phase‐separated at equilibrium —varies with molecular composition (ϕS, volume fraction of S component) for both molecular architectures. Also predicted are the microstructural parameters ΔT(M) andf(M)—interphase thickness and volume fraction, respectively—and the high‐Mlimiting functions ΔT∝︁Mα2,f∝︁Mα3,D∝︁Mα4(Dis domain repeat distance) andTs∝︁Mα5(Tsis separation temperature). Then, for polydisperse systems in the range 1 ≲p≲ 3 ( where\[ P = \bar M_w /\bar M_n \]) corresponding predictions at constant\[ \bar M_n \]are made after identifying the mixture free‐energy‐minimum state with a weight average of the free energy minima of each fraction of the MWD. Calculations are made specifically for ϕS= 0.50 andTs= 298 K. It is shown that, even when\[ \bar M_n < M_c \], polydispersity can induce microphase separation ifpis sufficiently large. Good success is obtained in comparisons ofDpredictions with data on blends of two polydisperse diblock samples.
Physical and chemical aging in PMMA and their effects on creep and creep rupture behavior
Authors: Crissman John M. , McKenna Gregory B. 
Published: April 2003

AbstractSamples of PMMA were aged at 80°C for between 1 h and 254 days and tested in creep in uniaxial extension at large stresses and at room temperature. Both chemical and physical aging effects were found to be important in determining the failure behavior of the samples. This behavior was found to be describable within the context of a commonly used failure criterion that the time to failure multiplied by the strain rate at failure is constant. The creep behavior was found to follow classical time–aging time and time–stress correspondence principles.
Polyurethane cationomers. I. Structure‐property relationships
Authors: Chen Show‐An , Chan Wu‐Chung 
Published: April 2003

AbstractPolyether polyurethane cationomers are prepared using poly (tetramethylene oxide) of molecular weight 2000 as soft segments, N‐methyl‐diethanolamine as chain extender, glycolic acid as quaternization agent, methyl ethyl ketone as solvent, and three different diisocyanates. The three diisocyanates are 4,4′‐diphenylenemethylene diisocyanate (MDI), hexamethylene diisocyanate (HDI), and toluene diisocyanate (TDI). Properties of the films cast from solutions of the three series of ionomers are studied by infrared spectroscopy, dynamic mechanical analysis, thermogravimetric analysis, differential scanning calorimetry, wide angle x‐ray diffraction, and tensile elongation testing. In the un‐ionized and ionized systems, the hard segments exhibit disordered and ordered arrangements, respectively. Ionization disrupts the order and produces increased cohesion in the hard domains, which have opposing effects on the tensile elongation properties. In the MDI and TDI systems, cohesion is predominant, leading to an increased tensile strength and modulus and decreased elongation at break. But in the HDI system, the disruption of the order is predominant, leading to decreased tensile strength and only insignificant reduction in the elongation at break. In the TDI system, the tensile strength is rather low, which is attributed to the poor order in the hard domains resulting from the high content of the asymmetric 2,4‐isomer of the urethane.
PMMA stereocomplex: Study of the aggregation process by electron microscopy
Authors: Quintana J. R. , Stubersfield R. , Price C. , Katime Issa 
Published: April 2003

AbstractThe structure of stereocomplex aggregates formed from isotactic and syndiotactic poly (methyl methacrylate)s was studied in tetrahydrofuran (THF) by transmission electron microscopy (TEM). We have found that the primary stereocomplex particles are nearly spherical and, as the complexation advances, the particles lengthen. Eventually aggregates branch to form a three‐dimensional network.
Improving tensile mechanical properties of aromatic polyimides by thermal imidization after cold drawing of poly (amic acids)
Authors: Kochi Masakatsu , Yokota Rikio , Iizuka Taka Yuki , Mita Itaru 
Published: April 2003

AbstractA new technique of thermal imidization after cold drawing of poly (amic acid) films is used to improve the tensile mechanical properties of various aromatic polyimide films. By this method PI (BDA/PPD) with linear, rigid main chains can be obtained as a high‐modulus and high‐strength material with Young's modulus of ca. 60 GPa and tensile strength of ca. 1.2 GPa. The enhancement of mechanical properties is favored by the rapid development of molecular order during imidization. The tensile properties of flexible polyimides such as PI (BDA/DDE) and PI (PMDA /DDE) are not significantly improved, because the orientation of cold‐drawn poly (amic acid) chains is easily relaxed at the temperature of imidization.
Local order and elasticity of polymer networks with nematiclike interactions
Authors: Walasek Janusz 
Published: April 2003

AbstractThe influence of nematiclike interactions on local order and elasticity of crosslinked polymer systems is considered. A molecular mean‐field approach is used to describe interactions between system segments. The orientation‐dependent potential is assumed to be proportional to the square of the cosine of the angle between the direction of the orienting field and the chain segment vector. The orientation factor is calculated from a self‐consistent equation, with no restrictions on the strength of intersegmental interactions. Isotropic (slightly oriented) and nematic (highly oriented) phases are observed. The influence of local order on the stress‐strain relation is derived for various values of network structural parameters (contraction coefficient, length of network chains) and the strength of segment‐segment interaction. For some values of these parameters, local order can modify considerably the elastic properties of the network system.