bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractDiffusion 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

AbstractA 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

AbstractDynamical 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

AbstractThe 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

AbstractModel 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

AbstractAn 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

AbstractGas 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

AbstractGas 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

AbstractThe 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

AbstractLinear 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

AbstractWe 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.