bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical foundations of interdiffusion
Authors: Hess Walter , Nägele Gerhard , Akcasu A. Ziya
Published: April 2003

Expressions for the interdiffusion and the tracer‐diffusion coefficients in terms of the time‐integrals of force‐correlation functions are obtained using a microscopic description of the inter‐diffusion and tracer‐diffusion processes in a two‐component liquid. The present approach provides a systematic and direct derivation, as well as an extension, of the results of Bearman et al. in their pioneering work in the late 1950s which have been used recently by Cohen and Foley to discuss the two possible relationships between the interdiffusion and tracer diffusion coefficients, referred to as the “fast‐mode expression” of Hartley and Crank and the “slow‐mode expression” based on the random phase approximation. In our analysis we find an additional three‐particle friction term in the exact expression for the interdiffusion coefficient, which is not present in Bearman's work. It is concluded that not much can be said in favor of the fast‐mode or slow‐mode expressions, unless this additional three‐particle friction is neglected. We also show that the center‐of‐volume reference frame, in which the hydrodynamic fluxes are measured, naturally follows from the microscopic description in the isothermal‐isobaric limit.
Diffusion of gases in inhomogeneous polymeric membranes
Authors: Higuchi Akon , Nakagawa Tsutomu
Published: April 2003

Fick's first law is modified to be applicable for the transport of gases in inhomogeneous polymeric membranes having continuous, concentration‐dependent, gradients of solubility and diffusivity along the flux direction (x axis). The membranes are considered to be in the rubbery state, and all gases in the membranes are assumed to be mobile. The modified Fick's law is derived from the consideration of standard chemical potential difference between the membrane and gas phases, and of fugacity coefficients in the membrane.
\documentclass{article}\pagestyle{empty}\begin{document} $J=-D\left\{dC/dx - C(d\ln {S/dx})\right\}/L$\end{document}
where J is the flux, D is the diffusion coefficient, C is the concentration in the membrane; x is the dimensionless space coordinate (0 ≦ x ≦ 1); S is the solubility; and L is the membrane thickness. The above equation is applicable for the cases where D and S are dependent on concentration. It is also solved analytically or numerically by a simulation method for some model membranes. Flux difference due to the flow direction and concentration profiles in the model membranes are discussed.
Permeability of carbon dioxide at elevated pressures in substituted polycarbonates
Authors: Jordan S. M. , Fleming G. K. , Koros W. J.
Published: April 2003

Past work has shown that hysteresis in sorption, volume dilation, and permeation result when standard bisphenol A polycarbonate is conditioned by exposure to 900 psia carbon dioxide at 35°C. This article reports hysteretic permeation results for the same polycarbonate exposed to other carbon dioxide conditioning pressures. Furthermore, permeation results are reported for tetramethyl and hexafluoropolycarbonate following conditioning with carbon dioxide. Permeation measurements with these structurally related polycarbonates show qualitatively similar, but quantitatively smaller long‐lived hysteretic effects compared with standard polycarbonate dilated to similar volume percentages. The percentage increase in permeability following conditioning treatments will be discussed in terms of fractional free volume considerations.
Isothermal refolding in crystals of long alkanes in solution. II. Morphological changes accompanying thickening
Authors: Organ S. J. , Ungar G. , Keller A.
Published: April 2003

The morphological changes accompanying isothermal thickening in C198H398 are studied by electron microscopy. The crystal morphologies confirm previous assertions that thickening occurs in a single step from once‐folded to extended chains and suggest solution‐mediated nucleation of the thickened crystal followed by rapid spreading via solid‐state diffusion.
Effect of bound water on electromechanical properties of modified PMLG films
Authors: Maeda Hideatsu , Ebihara Shigeyoshi
Published: April 2003

The complex piezoelectric, dielectric, and elastic constants of oriented poly(γ‐methyl L‐glu‐tamate) (PMLG) films, treated with ethylene diamine to introduce more hydrophilic groups into the side chains, are changed drastically by hydration. In dynamic mechanical measurements below 0°C, stiffening and plasticizing actions of bound water are observed in hydrated films. This seems to be caused by water bound particularly by the hydrophilic −NH2 groups in the noncrystalline region of the modified PMLG structure. The piezoelectric constant of hydrated modified film exhibits a steep increase beginning at about −50°C and passes through a maximum at about −10°C. The steep increase around −50°C may be caused mainly by increasing electric conduction (and also by the increase of the dielectric constant and the decrease of the elastic constant) in the noncrystalline region of the modified PMLG structure. On the other hand, the decrease above −10°C may be caused by increasing electric conduction, probably surface conduction of the dispersed crystalline (i.e., piezoelectric) regions and electrode polarization. These effects may be enhanced by bound water, particularly around the hydrophilic −NH2, groups.
Gradient energy parameters for polymer–polymer–solvent systems and their application to spinodal decomposition in true ternary systems
Authors: Ariyapadi M. V. , Nauman E. B.
Published: April 2003

The entropic gradient contributions to the free energy of nonuniform polymer–solvent systems and polymer–polymer–solvent systems have been obtained using a mean field approach. The results for a polymer–polymer–solvent system reduce to those of de Gennes for a polymer–polymer system at the appropriate limit. The binary results for a polymer–solvent system predict interfacial tensions that are systematically high but closer to experiment than those predicted by the earlier model of Balsara and Nauman. The ternary results have been applied to spinodal decomposition in polymer–polymer–solvent systems. The linear theory predicts a decrease in the domain size with increasing quench depth. Also, it predicts an increasing domain size with an increase in compatibility between the two polymers for any given quench depth.
Temperature dependence of the interaction between n‐heptane, carbon tetrachloride, and benzene and poly(vinyl methyl ether) between 120 and 180°C
Authors: Riedl Bernard , Prud'homme Robert E.
Published: April 2003

The corresponding‐states theory of Flory has been used to characterize the interaction between three different probes, n‐heptane, carbon tetrachloride, and benzene, with poly(vinyl methyl ether). Different results are obtained if the characteristic parameters are corrected for their temperature dependences, which are compared for the three different probes. It turns out that the variation with temperature of the interaction energy parameter X12 is also different when using temperature‐dependent or temperature‐independent reduction parameters. The interaction is weak and very sensitive to small deviations from ideality in the corresponding‐states theory. These results suggest that a weakening of polymer‐polymer interactions, and not free volume differences, is responsible for the phase separation in polystyrene/poly(vinyl methyl ether) blends.
Magnetic metal‐polymer composites: Thermal and oxidative decomposition of Fe(CO)5 and CO2 (CO)8 in a poly (vinylidene fluoride) matrix
Authors: Tannenbaum Rina , Flenniken Cindy L. , Goldberg Eugene P.
Published: April 2003

The aim of this study is the synthesis and characterization of polymer composites in which microscopic metal or metal oxide particles are incorporated by thermal or photolytic decomposition in solid solutions of organometallic complexes in polymers. In contrast to conventional composites or filled polymer compositions, which are prepared by dispersion of particles or fibers in a polymeric matrix, this study typically involves the preparation of metal carbonyl solid solutions in polymers and the phase‐separation decomposition of the organometallic complex to form uniform metal or metal oxide dispersions of very small particle size. Fourier transform infrared spectroscopy is used to monitor the rate of the in situ decomposition of the organometallic moiety in the polymer matrix. Moreover, this technique is very important in determining the changes that occur on the polymer chains due to metal‐polymer interactions. Electron diffraction and transmission electron microscopy studies are used to determine the nature, shape, and size of the particles formed by the decomposition of the organometallic complex within the polymer matrix. Finally, a general scheme of possible mechanisms involving metal–polymer interactions is presented.
Plastic deformation mechanisms in polyimide resins and their semi‐interpenetrating networks
Authors: Jang Bor Z. , Pater Ruth H. , Soucek Mark D. , Hinkley Jeff A.
Published: April 2003

The plastic deformation mechanisms in both thermoplastic and thermoset polyimide resins and their semi‐interpenetrating networks (semi‐IPNs) were investigated. The fundamental tendency to undergo strain localization (crazing and shear banding) as opposed to a more diffuse (or homogeneous) deformation in these polymers was evaluated. In situ polarized light microscopic observation of crack‐tip deformation mechanisms in solvent‐cast films was conducted with a miniature testing device attached to the microscope stage. NASA LaRC TPI, a thermoplastic polyimide, was found to exhibit combined shear yielding and crazing under plane‐stress loading conditions. Contrarily, NR‐150B2 thermoplastic polyimide exhibited diffuse shear yeilding; no shear banding or crazing was found near the crack tip. Adding a small amount of thermoplastic component, either NR‐150B2 or LaRC TPI, was found to raise considerably the fracture toughness of PMR‐15 thermoset PI. PMR‐15 thermoset films also showed diffuse shear yielding, albeit with a much smaller deformation zone. This was consistent with its low toughness and could be understood on the basis of a limited extensibility of a highly crosslinked network. Numerical calculations were performed to confirm this low value of network chain draw ratio. The dimensions of the deformation zone ahead of the crack tip in a semi‐IPN with a thermoset matrix were increased as a higher weight fraction of the thermoplastic component was added. The deformation zone size of a thermoplastic matrix was found to decrease with an increasing amount of thermoset PMR‐15. Deformation also became more diffuse with a higher PMR‐15 content in LaRC TPI. Fracture toughness variations can be correlated with deformation size changes in these semi‐IPNs.
Pressure dependence of the β molecular relaxation process and dielectric properties of polyvinylidene fluoride
Authors: Samara G. A.
Published: April 2003

The effects of hydrostatic pressure to 20 kbar on the β molecular relaxation process of polyvinylidene fluoride (PVDF) and on the dielectric properties in the neighborhood of this relaxation have been investigated. This relaxation has a strong influence on the electrical and mechanical properties of PVDF. Pressure causes a large shift to higher temperatures (∼ 10K/kbar) of the dielectric relaxation peak and a decrease in the width of the distribution of relaxation times. This slowing down of the relaxation process is discussed in terms of the Vogel–Fulcher equation and related models, and it results from an increase in both the energy barrier to dipolar motion and the reference temperature (T0) for the kinetic relaxation process which represents the “static” dipolar freezing temperature for the process. The general applicability of the Vogel–Fulcher equation to relaxional processes in polymers and other systems is briefly discussed. The pressure dependence of the dielectric constant both above and below the relaxation peak temperature (Tmax) is found to be dominated by the change in polarizability. The effect is larger above Tmax because of the relatively large decrease in the dipolar orientational polarizability with pressure.
Elastic shear modulus of biphasic gels
Authors: Nilsson S. , Muhrbeck P.
Published: April 2003

The effect on rheological properties on mixing one gel‐forming and one nongelling polymer is investigated. It is found that on addition of a nongelling polymer to a constant amount of gel‐forming polymer, the shear modulus of the resulting gel can either decrease or increase depending on the polymers. The results are interpreted within a simple qualitative model based on polymer incompatibility in combination with percolation theory and a uniform stress approximation.
Planar deformation of amorphous poly(ethylene terephthalate) by stretching and forging
Authors: Guan Jia Y. , Wang Li‐Hui , Porter Roger S.
Published: April 2003

The planar deformation of amorphous poly(ethylene terephthalate) (PET) was performed by stretching and by forging under comparable conditions at a series of constant temperatures, 80, 90, 100, and 110°C. The highest planar draw ratios of 4.5 × 4.5 and 3.5 × 3.5 were obtained by forging and stretching, respectively. Samples were studied before and after deformation by wide angle x‐ray scattering (WAXS), differential scanning calorimetry (DSC), density measurements, and elastic recovery at 100°C. A distinct difference in efficiency of draw between these two techniques is found, as judged mainly by the straininduced crystallization. The forging is more effective than stretching in achieving stabilized planar draw under comparable process conditions.
1H pulsed NMR study of bombyx mori silk fibroin: Dynamics of fibroin and of absorbed water
Authors: Asakura Tetsuo , Demura Makoto , Watanabe Yasuo , Sato Kazuo
Published: April 2003

The dynamical behavior of the Bombyx mori silk fibroin chain and of absorbed water in silk fiber, film, and powder has been studied by 1H pulsed nuclear magnetic resonance (NMR). Segmental motions do not occur and only the rapid rotation of the methyl groups of alanine residues is observed from −120 to 130°C. This is independent of the conformation or form of the silk fibroin samples. Magnetization of dry silk fibroin by the solid‐echo method shows a single Gaussian decay, while two components are observed in the solid‐echo signals of films containing 6–10 w/w% water. An immobile component with a T2 value of 11 μs is attributed to silk fibroin, and the mobile component to bound water. The T2 of the latter varies from 50 to 200 μs, depending on the sample. The dynamical behavior of water trapped in the film is discussed on the basis of these T2 values.
Diffusion of ethylene glycol accompanied by reactions in poly(ethylene terephthalate) melts
Authors: Lee K. J. , Moon D. Y. , Park O. O. , Kang Y. S.
Published: April 2003

Diffusion coefficients of ethylene glycol (EG) have been measured in poly(ethylene terephthlate) (PET) melts by a quartz‐spring sorption apparatus. A simple mathematical model was developed to investigate the sorption behavior accompanied by chemical reactions of EG and PET at high temperatures. Diffusion coefficients are deduced from experimental data for an asymptotically thin sample in order to minimize the effects of reactions. The diffusion coefficient of EG is strongly dependent on the vapor pressure of EG and temperature but not on the molecular weight of PET in this experimental range (degree of polymerization 80–120). The diffusion coefficient of EG in PET melt at 265°C is 2.58 × 10−7 cm2/s at the limit of zero concentration of EG. The activation energy for diffusion is 38.4 kcal/gmol, and the heat of solution for sorption is −44.9 kcal/gmol. The concentrations of the volatile materials resulting from reactions in PET‐EG system were analyzed with gas chromatography. In addition, a fit of the current model to experimental data yields frequency factors for the polymerization reaction (k1) and the acetaldehyde formation reaction (k2) to be 5.84 × 108 cm3/mol ċ min and 3.90 × 1011 min−1, respectively.
A general model for the release of active agents incorporated in swellable polymeric matrices
Authors: Petropoulos J. H. , Papadokostaki K. G. , Amarantos S. G.
Published: April 2003

A theoretical model, which takes proper account of the simultaneous uptake of a liquid leachant by a polymer matrix and the consequent release of a bioactive or other solute incorporated therein, is presented and the kinetics of release predicted under various conditions is investigated. This model is shown to be both more rigorous and much more widely applicable than previous attempts to approach the above problem. Furthermore, it can be easily modified or extended, in accordance with the information available about any particular system. It is, therefore, expected that the model introduced here will prove useful as a basis for the design of monolithic controlled‐release devices of this type or for the evaluation of the leachability of low‐level and medium‐level radioactive wastes “immobilized” in polymeric matrices.
Dynamical relaxations in mixtures of different polymers and nonsymmetrical copolymers in solution
Authors: Benmouna M. , Benmansour Z. , Fischer E. W. , Benoit H. , Vilgis T. A.
Published: April 2003

Dynamical relaxation properties of nonsymmetrical mixtures of homopolymers and of copolymers in solution are examined. In particular, examples of bimodal mixtures in solution, ternary mixtures of two different homopolymers and a solvent, and nonsymmetrical diblock copolymers in solution are examined. The frequencies of the eigenmodes Γ and Γ′, those of the cooperative mode ΓT and the interdiffuse mode ΓI are calculated for these systems. It is found that the variations of TT and TI, with the scattering wave number q are practically identical to those of ΓT and ΓI, respectively. This identity is rigorous only for mixtures or copolymers where the two polymer components are identical except for their contrast factors (i.e., indices of refraction or scattering lengths). Here, it is argued that the identification of the eigenmodes with the cooperative and interdiffuse modes can be extended qualitatively to mixtures of different polymers and to nonsymmetrical copolymers in solution.
The multiple melting endotherms from poly(butylene terephthalate)
Authors: Nichols Mark E. , Robertson Richard E.
Published: April 2003

The melting behavior of poly(butylene terephthalate) (PBT) has been investigated, and a simulation has been performed to determine whether the multiple melting endotherms observed during the thermal analysis of PBT can be explained by the simultaneous melting and recrystallization of an initial distribution of crystal melting temperatures that contains only one maximum and two inflection points. Specimens that were cooled at constant rates from the melt showed between one and three melting endotherms upon heating in a differential scanning calorimeter (DSC). The position and breadth of the crystallization exotherms upon cooling from the melt and small‐angle x‐ray scattering showed that as the cooling rate is increased, the distribution of melting temperatures broadens and shifts to lower temperatures. By combining temperature‐dependent recrystallization with an initial distribution of melting temperatures, simulated DSC curves were produced that agreed well with experimental DSC curves. In instances of triple peaked curves, the high temperature peak was due to crystals formed during the scanning process, and the middle and low temperature peaks were due to crystals originally present in the material. Satisfactory agreement between the experimental and simulated curves was found without considering additional crystallization from the amorphous regions during the scanning process.
Unimodal and bimodal networks of poly(dimethyl‐siloxane) in pure shear
Authors: Wang Shuhong , Mark J. E.
Published: April 2003

Model networks of poly(dimethylsiloxane) (PDMS) were prepared by tetrafunctionally endlinking hydroxyl‐terminated chains of various molecular weights. Some networks were prepared from mixtures of chains so as to yield a bimodal distribution of network chain lengths and, in some cases, these networks were prepared in solution. The stress–strain behavior of these unimodal and bimodal networks was studied in pure shear, which was imposed by stretching a sheet of the network having a large ratio of width to length in the direction perpendicular to the width. The pure‐shear moduli of both types of networks generally were found to depend markedly on strain. Stress–strain isotherms for unimodal networks prepared from chains of one or the other of two molecular weights were well interpreted using the constrained‐junction model of Flory and Erman. The bimodal networks showed large increases in the pure‐shear modulus at high strains which were similar to those reported for uniaxial extension and compression. Endlinking in solution decreases the modulus in general and its upturn in particular, presumably because of diminished chain‐junction entangling.
Crystallization of markoffian copolymers
Authors: Allegra Giuseppe , Marchessault Robert H. , Bloembergen Steven
Published: April 2003

An equilibrium theory is proposed for crystallization of (A, B) binary copolymers whose comonomeric unit sequences are statistically described by conditional pair probabilities PAA, PAB, PBA, and PBB. These are linked to the product of the reactivity ratios by r = rArB = (PAAPBB)/(PABPBA). Three cases are considered here, (i) B units are rejected from the crystals, (ii) cocrystallization of A and B comonomeric units is possible in the full range of compositions within a single crystal structure (copolymer isomorphism), (iii) cocrystallization takes place either in a poly(A)‐type or in a poly(B)‐type structure, depending on composition (copolymer isodimorphism). For case (i) crystallization the theory demonstrates, according to expectation, that alternating copolymers (r = 0) produce the largest melting point depression, whereas in case (ii) they give rise to the smallest composition difference between the crystals and the liquid. The theory developed here further illustrates that for binary copolymers which are isodimorphic (case iii), a phase diagram is obtained similar to that for a classical binary system of small molecules.
Gas transport properties of thermotropic liquid‐crystalline copolyesters. I. The effects of orientation and annealing
Authors: Weinkauf D. H. , Paul D. R.
Published: April 2003

Gas transport properties are reported for two series of films prepared from copolyesters of 73 mol % hydroxybenzoic acid (HBA) and 27 mol % 2,6‐hydroxynaphthoic acid (HNA) which systematically vary the degree of orientation and annealing time. Scanning electron microscopic (SEM) photomicrographs of the liquid‐crystalline polymer (LCP) films showed evidence of a skin‐core structure and polydomain texture. The degree of orientation in the films was quantified by analyzing the azimuthal intensity of the x‐ray reflection associated with the lateral packing of the nematic mesophase. Using heat of fusion data from differential scanning calorimetry (DSC), the films were found to contain low levels of crystallinity estimated to be in the range of 5 to 15 wt %, which increased with annealing time. Permeability measurements were made for He, H2, O2, N2, Ar, and CO2 at 35°C and the diffusivities were computed from time‐lag data. The films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. A moderate reduction in permeability was observed with increased orientation, which could be attributed directly to a decrease in the effective diffusivity. The effect of increased crystallinity from annealing on the permeability coefficients was smaller than would be expected for similar changes in the crystallinity of conventional polymers. Analysis using a simple two‐phase model suggests that a mechanism dominated by transport in a small volume fraction of boundary regions possibly could account for the bulk transport properties of these materials.
Gas transport properties of thermotropic liquid‐crystalline copolyesters. II. The effects of copolymer composition
Authors: Weinkauf D. H. , Paul D. R.
Published: April 2003

Gas transport properties are reported for a series of compression‐molded films prepared from copolyesters of hydroxybenzoic acid (HBA) and 2,6 hydroxynaphthoic acid (HNA) having 30/70, 58/42, 73/27, 75/25, and 80/20 mol % HBA/HNA. The mesomorphic and crystalline morphology of the materials was characterized using dynamic mechanical thermal analysis (DMTA), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and X‐ray diffraction As evidenced by DMTA, the phenyl and naphthyl moieties of the HBA/HNA materials exhibit a significant degree of segmental mobility below the glass transition temperature. The nonlinear nature of the naphthyl unit leads to a more hindered rotation about the chain axis. Permeability measurements were made for He, H2, O2, N2, Ar, and CO2 at 35°C and the diffusivities were computed from time‐lag data. As previously observed in these materials, the films exhibited excellent barrier properties resulting largely from very low gas solubility coefficients. The liquid‐crystalline copolyester: (LCP) materials with the highest HNA content exhibit the best barrier properties. It appears that the more hindered motions of the naphthyl unit restrict penetrant mobility. The reduction in permeability with increased naphthyl unit content is accompanied by a very dramatic increase in selectivity between gas pairs. Fractional free volume analysis was used to correlate the transport properties of the LCP materials and other conventional polymers. A “two‐phase” modification of the free volume correlation suggests that transport may likely occur in a small volume fraction of a less dense boundary phase.
Structure and phase transitions in poly[bis‐(2,2,3,3‐tetrafluoropropoxy) phosphazene]
Authors: Tsvankin D. Ya. , Gerasimov M. V. , Zhukov V. P. , Dubovik I. I. , Tur D. R. , Papkov V. S.
Published: April 2003

The structure and phase transitions in poly[bis‐(2,2,3,3‐tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x‐ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis‐trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.
Sliding‐friction electrification of polymers and charge decay
Authors: Kumar A. , Perlman M. M.
Published: April 2003

Linear low‐density polyethylene (LLDPE) and polypropylene (PP) were charged by sliding under a cylindrical aluminium contact. The surface charge accumulation on the polymer films was time dependent, and a function of the sliding velocity, metal contact force, and film type. The surface potential increased linearly with velocity in the range 0.33–0.75 m/s, and showed a square‐root dependence on the contact force up to 6.5 N. Thermally stimulated charge decay (TSCD) studies showed longer charge lifetimes in samples friction‐charged on one side than in corona‐charged samples. Friction charging creates deeper near‐surface traps than are normally present in the polymers. Charge stability is further imporved by simultaneously friction charging both surfaces of the films, increasing the half‐value charge decay temperature (T1/2) by 27°C and 37°C over that in corona‐charged samples (i.e., to 98°C and 120°C for LLDPE and PP, respectively). These monopolarity electrets should prove useful in charge storage devices.
Light absorption due to higher harmonics of molecular vibrations in transparent amorphous polymers for plastic optical fibers
Authors: Takezawa Yoshitaka , Taketani Noriaki , Tanno Seikichi , Ohara Shuichi
Published: April 2003

We have studied simple empirical equations to estimate light absorption loss αv due to harmonics of molecular vibrations of transparent amorphous polymers used in plastic optical fibers (POFs). In the visible region, absorption involves two losses. One is αv, and the other is the electronic transition absorption loss, αe. Of the two, αv is considerably larger than αe in the wavelength region used for optical communication with POFs. We have clarified relationships between chemical structure of repeat units of polymers and αv. We find that αv is proportional to the concentration of specific chemical bonds (CH, NH, and Obond;H bonds) in the polymer solid, and we propose empirical equations to estimate αv from the polymer density and the chemical structure of the repeat unit. These equations are used to estimate αv of several polymers [i.e., poly(methyl methacrylate), polystyrene, and polycarbonate]. The estimated values are nearly equal to the experimental or reference values. Furthermore, to minimize the attenuation in the POF, we conclude that the POF core polymer should have no NH, OH, or aliphatic CH bonds in its repeat unit.
Effect of methyl substituents on permeability and permselectivity of gases in polyimides prepared from methyl‐substituted phenylenediamines
Authors: Tanaka Kazuhiro , Okano Masaaki , Toshino Hiroyuki , Kita Hidetoshi , Okamoto Ken‐Ichi
Published: April 2003

Permeability and solubility coefficients for H2, CO2, O2, CO, N2, and CH4 in polyimides prepared from 6FDA and methyl‐substituted phenylenediamines were measured to investigate effects of the substituents on gas permeability and permselectivity. The methyl substituents restrict internal rotation around the bonds between the phenyl rings and the imide rings. The rigidity and nonplanar structure of the polymer chain, and the bulkiness of methyl groups make chain packing inefficient, resulting in increases in both diffusion and solubility coefficients of the gases. Polyimides from tetramethyl‐p‐phenylenediamine and trimethyl‐m‐phenylenediamine display very high permeability coefficients and very low permselectivity due to very high diffusion coefficients and very low diffusivity selectivity, as compared with the other polyimides having a similar fraction of free space. This suggests that these polyimides have high fractions of large‐size free spaces.
A study of shear‐stress relaxation anomalies in binary mixtures of monodisperse polystyrenes
Authors: Morrison Faith A. , Larson Ronald G.
Published: April 2003

We report measurements of the nonlinear relaxation moduli after a step‐shear strain of polystyrene solutions with nearly monodisperse and with bidisperse distributions of molecular weight. We find, as have others, that for monodisperse solutions with M/Me > 60, there are anomalies, such as an unusually low nonlinear modulus and a kink in a plot of shear stress versus time after the step strain. Here M is the polymer molecular weight and Me is the entanglement molecular weight. We find that in the bidisperse solutions the anomalies persist as long as Mw/Me > 60, where Mw is the weight‐averaged molecular weight of the bidisperse solution. The persistence of the anomalies in bidisperse solutions disagrees with a theory of Marrucci and Grizzuti that attributes the anomalies to strain inhomogeneities similar to shear banding. The Marrucci‐Grizzuti theory predicts that as little as 10% short chains in the bidisperse mix should eliminate the anomalies, whereas in the experiments reported here at least 30% is required. Nevertheless the way in which the anomalies disappear at high strains when one increases the fraction of low‐molecular‐weight component is qualitatively similar to the theoretical predictions and supports the notion that strain inhomogeneities occur in these systems. © 1992 John Wiley & Sons, Inc.
Sorption and dilation of silicone elastomer composites at high gas pressures: The role of interfacial quality
Authors: Briscoe B. J. , Zakaria S.
Published: April 2003

In elastomers the crosslink density, the presence of filler particles, and the volumetric confinement toward sorptive dilation can influence the extent of gas mass uptake. In this study the effects of filler particles on the high‐pressure gas mass sorption and the volumetric dilation of a silicone elastomer matrix has been investigated. Glass beads, ca. 30 μm radii, with different surface treatments were incorporated as inclusions in various specimens at relatively low concentrations of ca. 10% by volume. The high‐pressure gases used were N2 and CO2 up to a maximum ambient pressure of ca. 25 MPa at ca. 20°C and 42°C, respectively. The gas mass sorption was determined by a vibrating reed technique. The sorptive dilation was measured by an ultrasonic transducer operating as a displacement probe. In certain systems the absorbed CO2 gas was able to disrupt the internal interfaces. This led to an increased gas mass uptake in the corrupted specimen. The N2 gas did not affect the interfacial bonds. The amount of penetrant uptake was found generally to be reduced when the internal interfaces were not disrupted. The presence of various internal interfaces restrained the sorptive dilation of the elastomeric matrix. These hindrances to the natural sorptive dilation of the elastomer network suppressed the extent of the gas sorption process. This effect has also been investigated separately in detail using model ‘poker chip’ type of specimens of various aspect ratios. The sorptive dilational characteristics have been correlated with the mechanical properties of similar specimens. The influence of an almost complete volumetric confinement on the gas sorption capacity of the silicone elastomer specimen has also been studied. © 1992 John Wiley & Sons, Inc.
Characterization of the molecular weight distribution of isotactic polypropylene by turbidity at the lower critical solution temperature (LCST)
Authors: Bohossian Takouhi , Delmas Geneviève
Published: April 2003

The molecular weight distribution (MWD) of high‐molecular‐weight isotactic polypropylene is measured by a new method developed for polyethylene (PE) using the turbidity at the lower critical solution temperature (LCST). The attenuation of transmitted light hi is measured at Ti as each fraction i is separated from the solution during a temperature increase ΔTi. From the thermogram, the set of hi (Ti), the MWD can be calculated through a relation Mi = f(Ti). The solvent chosen, a mixture of butane and 2‐methylbutane, optimizes resolution and relatively rapid dissolution. Homogenization and absence of degradation are verified by different methods including calorimetry in solution. © 1992 John Wiley & Sons, Inc.
Separation of small molecules using novel rolltruded membranes. I. Apparatus and preliminary results
Authors: Ciora R. J. , Magill J. H.
Published: April 2003

A novel membrane production technique, rolltrusion, has been developed for the preparation of permselective polymeric membranes for both gas and vapor separations and pervaporation applications. Pinhole‐free thin films with a regulated three‐dimensional morphology comprising crystalline and amorphous regions have been produced with improved mechanical properties in three dimensions. Because rolltrusion is a solid‐state processing technique, it is not restricted by solubility constraints inherent in the common solvent‐based thinfilm production techniques. Consequently it can be extended to include commercially available engineering plastics that have not been used as membranes previously because of their limited solvent solubility. These polymers are interesting because of their applicability to temperatures in excess of 200 to 300°C in harsh chemical environments, compared with the ca. 150°C ceiling usually encountered in the most rugged of solvent‐cast polymers. In the first part of this series we detail an automated experimental permeation apparatus that permits testing of both single‐component and multicomponent gases and vapors over a temperature range of −73 to 273°C. Currently, modifications to the permeation cell are underway to permit pervaporation studies in the apparatus. To illustrate the operation of the apparatus, and to some degree the effect of rolltrusion processing, experimental permeability, diffusivity, solubility, and actual gas separation factors (via gas chromatography) are detailed for several light gases in rolltruded isotactic poly(propylene) (iPP) and in the thermally and chemically resistant thermoplastic, poly(ether ether ketone) (PEEK). Permeation temperatures to at least 225°C have been studied using PEEK, representing the first published experimental permeability and gas‐separation results for such engineering polymers at temperatures above 200 °C. © 1992 John Wiley & Sons, Inc.
Structure and properties of blends containing ethylene‐methacrylate copolymers
Authors: Peiffer D. G.
Published: April 2003

The preparation, melting point, degree of crystallinity, mechanical properties, and morphology of a family of blends composed of a transition‐metal‐neutralized carboxylate semicrystalline ionomer (metal‐neutralized ethylene‐methacrylate copolymer) and an amorphous copolymer (styrene‐4‐vinyl pyridine copolymer) are described. These polymeric materials contain low levels (≤ 10.0 mol %) of interacting groups which are capable of forming interpolymeric complexes. These interactions are best described as transition metal‐pyridine coordination complexes. A general characteristic of these blend systems is that the mechanical properties and morphology are directly influenced by the nature of the counterion and the specific composition ratio of amorphous to semicrystalline component. A nontransition metal counterion (sodium) is weakly interacting at best, while a transition metal counterion (zinc) is strongly interacting. Morphological studies (polarized‐light microscopy and small‐angle light‐scattering measurements) confirm that the glassy component, if nonassociating, resides primarily in the interspherulite region, while the associating species will behave in a similar manner only after the stoichiometric ratio is reached. The morphology directly influences the stress‐strain behavior of these blends. It is noteworthy that the spherulite size remains unchanged with nonassociating blends while a 50% reduction is noted in the associating blends. Thermal and wide‐angle x‐ray scattering measurements confirm the lamellar structure is unaffected by these associations. © 1992 John Wiley & Sons, Inc.
Surface characterization of laser‐treated poly(ethylene terephthalate) by optical profilometry and scanning tunneling microscopy
Authors: Wefers Lothar , Schollmeyer Eckhard
Published: April 2003

Characteristic changes in the surface topography due to UV laser treatment are of high significance for the determination of material destruction thresholds and surface structure development. E.g. irradiation of poly(ethylene terephthalate) (PET) film with pulsed UV laser light of 248 nm modifies the smoth surface of the polymer into a well oriented structured surface. The development of these structures were studied with scanning tunneling microscopy (STM) and high resolution optical profilometry. Three dimensional data of the surface were taken from the samples after each laser pulse. A change of topographic data was found in relation to fluence and number of pulses applied. © 1993 John Wilcy & Sons, Inc.
Computer simulation of the mechanical alpha relaxation in isothermally crystallized high‐density polyethylene
Authors: González‐Orozco J. A.
Published: April 2003

The influence of thermal history on the dynamic mechanical behavior in the alpha relaxation region of high‐density polyethylene (HDPE) is predicted using computer modeling. The model considers the mechanical alpha relaxation as a succession of cooperative single movements occurring within the defect region of the crystal lattice that finally produce a relative displacement between lamellae. A distribution function of the number of such single steps is postulated considering topological properties of the sample: branch content and molecular weight. To carry out the simulation, the model uses the spectrum of the sample subjected to fast cooling as a reference system. In addition, the crystallinity and the segregated material content for the particular thermal history of the object of study are also required. Loss modulus spectra are generated using balance of energy considerations, and a few examples are presented together with several experimental results. © 1993 John Wiley & Sons, Inc.
Evaluation of free‐volume theories for solvent self‐diffusion in polymer–solvent systems
Authors: Vrentas J. S. , Vrentas C. M.
Published: April 2003

An evaluation of free‐volume theories for solvent self‐diffusion is carried out using recent comprehensive data sets for penetrant self‐diffusion in polymer solutions. Different theories are compared, and free‐volume theories in the prediction of penetrant self‐diffusion coefficients in glassy polymer systems is also evaluated. © 1993 John Wiley & Sons, Inc.
Atomic force microscopy of extended‐chain crystals of polyethylene
Authors: Annis Brain K. , Reffner John R. , Wunderlich Bernhard
Published: April 2003

Extended‐chain crystals of polyethylene grown at elevated pressure and temperature were analyzed for the first time by atomic force microscopy. It was possible to compare the typical fracture surface striation features with those obtained earlier by electron microscopy. High resolution atomic force microscopy on flat surfaces enabled the recording of an atomic scale regularity that could not be fully indentified. © 1993 John Wiley & Sons, Inc.
A rheological, optical, and x‐ray study of the relaxation of orientation of nematic PBZT
Authors: Odell J. A. , Ungar G. , Feijoo J. L.
Published: April 2003

The ultimate properties of liquid crystalline polymers depend upon the achievement of high orientation, usually by means of flow fields. The properties are limited by disorientation which can occur before the product is solidified. Such cooperative orientation and disorientation phenomena also underlie the complex fluid rheology and product microstructure of these materials. The orientation and subsequent disorientation can be followed dynamically by optical and x‐ray techniques. Normally, monitoring of orientation is possible only by “fast” techniqes, such as birefringence, these are not applicable to opaque and strongly scattering liquid‐crystalline systems. To enable examination of the full dynamic response of concentrated nematic solutions of poly(1,4‐phenylene‐2,6 benzobisthiazole) (PBZT) the Daresbury Synchrotron Radiation Source was used. PBZT is among the most rigid macromolecules and serves as a good model for other materials of its class. The orientation process determined optically and from x‐rays is correlated with fluid rheology and availabel theoretical approaches. During relaxation from near perfect mesogen alignment three principal stages of the disorientation process were identified corresponding to solvent disorientation (first stage of stress relaxation), banding (slow stress relaxation and mesogen disorientation), and finally a very slow banding to polydomain transition. © 1993 John Wiley & Sons, Inc.
Characterization of polyethylenes by x‐ray diffraction and 13C‐NMR: Temperature studies and the nature of the amorphous halo
Authors: Mcfaddin D. C. , Russell K. E. , Wu Gang , Heyding R. D.
Published: April 2003

X‐ray diffraction patterns of linear and branched polyethylenes typically show two sharp reflections and an amorphous halo. The position of the halo depends on branch content and temperature. A single curve describes the position of the halo maximum (2θhalo) for a range of liquid hydrocarbons and polyethylenes in the 20–140°C range. At temperatures well below their melting point, branched polymers give 2θhalo values which differ significantly from those observed for the liquid Linear polymers show a greater divergence, indicating that some of the material giving rise to the halo is much better packed than in the liquid.Parallel 13 C NMR spin‐lattice relaxation studies suggest that this relatively ordered material has a trans conformation but a low average T1c value. © 1993 John Wiley & Sons, Inc.
An investigation of the yield and postyield behavior and corresponding structure of poly(methyl methacrylate)
Authors: Hasan O. A. , Boyce M. C. , Li X. S. , Berko S.
Published: April 2003

The mechanical behavior of glassy polymers is time and temperature dependent as evidenced by their viscoelastic and viscoplastic response to loading. The behavior is also known to depend strongly on the prior history of the material, changing with time and temperature without chemical intervention. In this investigation, we examine the effects of this process of physical aging on the yield and postyield behavior and corresponding evolution in the structural state of glassy polymers. This has been achieved through a systematic program of uniaxial, isothermal, constant strain–rate tests on poly(methyl methacrylate) (PMMA) specimens of different thermal histories and by performing positron annihilation lifetime spectroscopy (PALS) measurements prior to and after mechanical deformation. PALS is an indicator of the free volume content, probing size and density of free volume sites and can be considered to be a measurement of structural state. The results of the mechanical tests show that aging acts to increase both the initial yield stress and the amount of strain softening which occurs subsequent to yield. Moreover, the amount of strain softening was found to be independent of strain rate indicating that softening is related to an evolution in structure as opposed to deformation kinetics. Furthermore, after sufficient inelastic straining, the initial thermal history is completely erased as evidenced by identical values of flow stress following strain softening, for both annealed and quenched polymer. Strong confirmation of the structural state or free volume related nature of the strain softening process is obtained by our companion PALS measurements. PALS detects an increase in the size of free volume sites following inelastic deformation and finds the initially annealed and quenched specimens to posses the same post‐deformation distribution. The size of sites is found to evolve steadily with inelastic strain until it attains a steady‐state value. This evolution of free volume with strain follows the observed softening of the flow stress to a steady‐state value. These results provide experimental evidence that an increase in free volume with inelastic straining accompanies the strain softening phenomenon in glassy polymers and that strain softening is indeed a de‐aging process. Based on our experimental results a mechanistically based constitutive model has been formulated to describe the effects of thermal history on the yield and postyield deformation behavior of glassy polymers up to moderate strains. The model is found to successfully capture the effects of physical aging, strain softening, strain rate, and temperature on the inelastic behavior of glassy polymers when compared with experimental results. © 1993 John Wiley & Sons, Inc.