bluuebunny/crossref_metadata_2025_split huggingface dataset:
Changes in crystal structure parameters and thermal mechanical properties of poly(p‐phenylene terephthalamide) fibers under different annealing conditions
Authors: Wu Zongquan , Zhang Anqiu , Cheng Stephen Z. D. , Huang Bing , Qian Baojun 
Published: April 2003

AbstractAs‐spun poly(p‐phenylene terephthalamide) (PPTA) fibers have been annealed by three different methods: (1) the relaxation mode; (2) the constant temperature mode; and (3) the constant tension mode. Wide‐angle x‐ray diffraction (WAXD) was used with all three modes to characterize the changes in three crystal structure parameters: the degree of crystal orientation, the apparent crystal size, and the lattice strain along the c‐axis of the crystal unit cell. It was found that internal stresses in the fibers, which are frozen‐in during either the fiber spinning process or in later annealing, is the key factor causing these structure parameter changes. Our thermal shrinkage stress measurements may reasonably represent the fiber's internal stresses. The thermal shrinkage modulus and the tensile modulus of the PPTA fibers under these annealing modes have been determined. Based on a serial model, quantitative structure–property relationships can be established. The effects of annealing temperature and applied tension on both the microscopic crystal structures and the macroscopic properties are also discussed.
Probe diffusion in poly (vinyl acetate) / toluene solutions
Authors: Lodge T. P. , Lee J. A. , Frick T. S. 
Published: April 2003

AbstractThe technique of forced Rayleigh scattering has been used to examine the tracer diffusion of probe photochromic dyes, azobenzene and methyl red, in poly (vinyl acetate)/ toluene solutions, as a function of polymer concentration (0–96% by weight) and temperature (5–45°C). It is demonstrated that the dye diffusivity can be quantitatively scaled to the solvent diffusivity, thus permitting direct application of predictions for two‐component systems. The results have been interpreted using the free volume theories of Fujita and Vrentas and Duda. While both approaches are able to describe the concentration dependence extremely well, the Vrentas‐Duda expression has the virtue of being employed without any freely adjustable parameters. However, neither theory is particularly successful in predicting the observed temperature dependence.
Probe diffusion in polystyrene/toluene solutions
Authors: Frick T. S. , Huang W. J. , Tirrell M. , Lodge T. P. 
Published: April 2003

AbstractThe technique of forced Rayleigh scattering has been used to measure the tracer diffusion coefficient, Dt, for two photochromic dyes, azobenzene and Aberchrome 540, in polystyrene / toluene solutions, as a function of polymer concentration (79–90% by weight) and temperature (−15–+60°C). The dye diffusivity can be scaled quantitatively to the solvent diffusivity, permitting direct application of the Vrentas‐Duda free volume predictions for binary systems. However, the scaling factors are not in agreement with the model assumptions. The temperature and concentration dependence of Dt above the solution glass transition temperature, Tgs, is in good agreement with the model, without any freely adjustable parameters. The value of Dt at Tgs is a very strong function of concentration, increasing by a factor of 103 as the polymer concentration is reduced from 90 to 80%; this is also in quantitative agreement with the model. It is demonstrated that the temperature dependence of Dt exhibits a distinct break in the vicinity of Tgs, with both the existence and the character of the break in qualitative agreement with the extension of the Vrentas‐Duda approach to glassy solutions. However, a fundamental difficulty in the calculation of the polymer contribution to the solution free volume at temperatures well below the bulk polymer Tg is exposed.
Synthesis and characterization of polydimethylsiloxane polyurea‐urethanes and related zwitterionomers
Authors: Yang C. Z. , Li C. , Cooper S. L. 
Published: April 2003

AbstractA series of segmented polyurea urethane and polyurea block copolymers based on a hexane diisocyanate (HDI) modified aminopropyl terminated polydimethylsiloxane soft segment was synthesized. The hard segments consisted of 4,4′‐methylene diphenylene diisocyanate (MDI) which was chain extended with 1,4‐butanediol (BD), N‐methyldiethanolamine (MDEA), or ethylene diamine. Zwitterionomers were prepared by quaternizing the tertiary amine of the MDEA extended material with γ‐propane sultone. The effect of chemical structure on the extent of phase separation and physical properties was studied using a variety of techniques including thermal analysis, dynamic mechanical spectroscopy, tensile testing, and small‐angle x‐ray scattering. It was observed that the compatibility between the nonpolar polydimethylsiloxane soft segments and the polar urethane hard segments was improved by inserting HDI linkages into the polydimethylsiloxane soft segments. The aggregation of hard segments was enhanced by increasing hard‐segment content or by the introduction of ionic functionality. The tensile strength and modulus of these materials was higher than those of polyurethanes containing soft segments based on polydimethylsiloxane and its derivatives.
Fiber formation via solution spinning of the cellulose/ammonia/ammonium thiocyanate system
Authors: Liu Cheng‐kung , Cuculo John A. , Allen Thomas C. , Degroot A. Willem 
Published: April 2003

AbstractFiber formation via the cellulose/ammonia/ammonium thiocyanate system by wet spinning has been investigated. This report presents a characterization of the structure and tensile properties of fibers spun under various coagulation conditions. Microscopic observations showed that the molecular size of coagulant was the dominant factor governing the crosssectional shape of the fibers. Density, birefringence, and crystallinity data indicated that a higher cellulose concentration and lower coagulation temperature favored development of a fiber with a denser and more oriented structure. Under optimum conditions, a welldefined fibrillar structure was obtained. Fiber tensile property measurements suggested the existence of a linear relationship between the fiber breaking tenacity and the product of the square of the Hermans' orientation factor and the infrared crystallinity index.
Complexation thermodynamics of free and graft oligomers with complementary polymers
Authors: Scranton Alec B. , Klier John , Peppas Nikolaos A. 
Published: April 2003

AbstractThe dilute solution complexation equilibrium between linear macromolecules and smaller complementary oligomers is considered when: (1) the oligomers are free in solution; and (2) the oligomers are covalently attached at one end to the polymer. A general statistical mechanical framework is developed and is illustrated using a simple random walk model for polymer conformation. The statistical mechanical partition functions are formulated using a generating function technique, allowing thermodynamic averages in the complexed state to be calculated. Loops, trains, and tails of all possible length are allowed in the conformation of a complexed oligomer. Simulation results for the free oligomer case are compared with those obtained for oligomers covalently attached to the polymeric molecular. The model provides a theoretical explanation for the experimentally observed enhancement of complexation of oligomers grafted to the complementary polymers.
Argon sorption and partial molar volume in poly(ethyl methacrylate) above and below the glass transition temperature
Authors: Kamiya Yoshinori , Mizoguchi Keishin , Naito Yasutoshi , Bourbon Dominique 
Published: April 2003

AbstractSorption and dilation isotherms for argon in poly(ethyl methacrylate) (PEMA) are reported for pressures up to 50 atm over the temperature range 5–85°C. At temperatures below the glass transition (Tg=61°C), sorption isotherms are well described by the dual‐mode sorption model; and isotherms above Tg follow Henry's law. However, isotherms for dilation due to sorption are linear in pressure at all temperatures over the range investigated. Partial molar volumes of Ar in PEMA are obtained from these isotherms. The volumes are approximately constant above Tg (about 40 cm3/mol), whereas the volumes below Tg are smaller and dependent on both temperature and concentration (19–26 cm3/mol). By analyzing the experimental data according to the dual‐mode sorption and dilation model, the volume occupied by a dissolved Ar molecule and the mean size of microvoid in the glass are estimated to be 67 129 Å3, respectively. The cohesive energy density of the polymer is also estimated as 61 cal/cm3 from the temperature dependence of the dual‐mode parameters.
Structure‐property relationships in poly(n‐vinyl pyrrolidone)‐phenoxy‐water gels
Authors: Tsang Cynthia J. , Clark John C. , Wellinghoff Stephen T. , Miller Wilmer G. 
Published: April 2003

AbstractMiscible 80/20 and 90/10 by weight blends of poly (n‐vinyl pyrrolidone) [PNVP] and phenoxy polymer were swollen with 73–85% water to produce rubbery hydrogels with shear moduli as high as 103dyne/cm2. Small‐angle x‐ray scattering, rheological, and calorimetric measurements showed that the gel consisted of tie chains, most probably of PNVP embedded in glassy phenoxy particles with radius of gyration 50–200 Å. A ternary phase diagram was calculated assuming athermal mixing between PNVP–H2O and PNVP and the known endothermic interaction between H2O–phenoxy. Phase separation into a phenoxy phase containing minimal H2O and PNVP and a water‐swollen PNVP phase was predicted in accordance with experimental results. Phase separation was suspected as forming first an interconnected phenoxy phase which was later dispersed by swelling forces transmitted through PNVP tie chains. The presence of localized internal stress in the swollen network was thought to be responsible for the irreversible decrease in the shear moduli observed at shear strain above 10%.
Effects of blending on the polymorphic behavior of melt‐crystallized syndiotactic polystyrene
Authors: Guerra Gaetano , De Rosa Claudio , Vitagliano Vincenzo M. , Petraccone Vittorio , Corradini Paolo 
Published: April 2003

AbstractThe complex polymorphic behavior of syndiotactic polystyrene (s‐PS) in melt‐crystallized samples is altered by blending with poly (2,6‐dimethyl‐1,4‐diphenylene oxide) (PPO). In particular, to render the beta form in these blends, starting with samples containing the α or γ froms, requires much lower temperatures and shorter melting times than for pure s‐PS. On the basis of the results, it is suggested that this phenomenon is due to more rapid loss of the memory of the α form, for the same temperature and time, in the melt in the presence of PPO molecules.
The solubility of Carbon Dioxide and Methane in polyimides at elevated pressures
Authors: Hirose T. , Mi Y. , Stern S. A. , St. Clair Anne K. 
Published: April 2003

AbstractThe solubility of CO2 and CH4 in five polyimides was measured at 35.0°C and at pressures up to 10 atm (147 psia). The concentration of the penetrant gases dissolved in the polymers can be represented satisfactorily as a function of penetrant pressure by the “dual‐mode sorption” model. The solubility coefficients for CO2 and CH4, S(CO2) and S(CH4), increase in the polyimide order:

The magnitude of the solubility coefficients appears to depend primarily on the intermolecular forces between the penetrant gases and the polymers. The values of these coefficients are greater for the polyimides with larger mean interchain spacings, but no one‐to‐one correspondence appears to exist in this respect. The lower solubility of CO2 in PMDA‐4,4'‐m‐APPS compared with that in the 6FDA polyimides may be due to the lower “excess” free volume of the former polymer. The ratio S (CO2)/S (CH4) varies relatively little for a variety of PMDA and 6FDA polyimides.
Biaxial extension measurements on bimodal elastomeric networks
Authors: Xu P. , Mark J. E. 
Published: April 2003

AbstractBimodal elastomeric networks were prepared by tetrafunctionally endlinking mixtures of short and long hydroxyl‐terminated poly (dimethylsiloxane) chains having number‐average molecular weights of 500 and 18,000 g/mol−1, respectively, over a composition range of 0–98 mol % of the short chains. Biaxial extension (compression) measurements were carried out by inflation of circular sheets of these materials at 23°C. The unimodal network (0 mol % short chains) showed the same behavior reported by other workers for noncrystal‐lizable networks: as the compression increased, the reduced stress or modulus [f*] went through a rather slight maximum followed by a gradual leveling off to the rupture point. The bimodal networks, however, showed much more pronounced maxima with another, significant increase in [f*] at very high compressions. This final increase is presumably due to non‐Gaussian effects from the very limited extensibility of the short chains, and thus parallels the upturns in [f*] frequently reported for bimodal networks at very high elongations.
ESR and electrical conduction in pyrolyzed polyimides
Authors: Noro Sumiko , Yamadaya Tokio 
Published: April 2003

AbstractElectrical conductivity and electron spin resonance (ESR) of pyrolyzed polyimides change drastically in air after passage of a characteristic time, which depends on the temperature and time of pyrolysis. Samples must be kept from air throughout the measurements. ESR and resistivity measurements in vacuum show that there are two types of magnetic species: localized and delocalized. A. variable‐range hopping model is proposed as a possible mechanism for conduction.
Polyurethane network structure from tensile tests
Authors: Pegoraro M. , Di Landro L. , Severini F. , Cao N. , Donzelli P. 
Published: April 2003

AbstractThe mechanical behavior of polyurethane networks based on polyether (PE‐PU) and polyester (PES‐PU) diols is studies in light of recent molecular theories of rubber elasticity. The relationship between reduced stress (or modulus), extension ratio α, and network structure is discussed. In the range of low extensions, the deformation behavior of PESPU appears to be more affinelike than PE‐PU networks. When crosslinking agents with higher functionality are used, behavior closer to the affine limit is observed. Data in the low deformation range (α → 1) allow the estimation of network molecular weights predicted by the limiting phantom and affine networks. However, an exact determination of the true chain length cannot be obtained. The uncertainty in the molecular weight is due to the uncertainty in the theoretical structural parameter A'ϕ, which can assume values 1–2/ϕ ≤ A'ϕ ≤ 1. The range of possible network chain molecular weights can be narrowed when the real network, which always presents defects, is formed starting from building blocks of known molecular weight Ms, as in our case. In these real cases the molecular weight is between the value Ma Predicted by the affine model and the stoichiometric value Ms.
Isothermal heat treatment of a thermotropic LCP fiber
Authors: Sarlin Juha , Törmälä Pertti 
Published: April 2003

AbstractThe effect of the heat treatment on as‐spun fibers made of a commercial liquid crystalline polymer, Vectra®, was examined through tensile properties and differential scanning calorimetry (DSC) measurements. The heat treatment increased the fiber strength considerably, and it also increased the elongation at break. On the other hand, the treatment had a weaker effect on the modulus. In general, the effects of treatment depended on the temperature and on the treatment time used. DSC measurements showed the effect of the treatment on the behavior of the melting endotherm. In light of the development of this endotherm, it seems possible that the structure formed during treatment depends essentially on the treatment temperature. On the whole, an unambiguous correlation between tensile properties and calorimetric quantities could not be concluded.
Miscibility of solution‐chlorinated polyethylene with poly(α‐methylstyrene‐co‐acrylonitrile)
Authors: Cowie John M. G. , Elexpuru Elisa M. , McEwen Iain J. 
Published: April 2003

AbstractStatistical copolymers of α‐methylstyrene and acrylonitrile (MSAN) form single‐phase blends with solution‐chlorinated polyethylene within a restricted composition range, even though no combination of the corresponding homopolymers is miscible. The phase behavior can be successfully described using the general mean‐field expression which predicts miscibility on the basis of the intramolecular interactions of the comonomers. By deriving the absolute value of one of the six system interactions from a subsidiary determination of the miscibility limits of poly (methylmethacrylate) in MSAN, values of all the remaining interaction parameters have been determined using a novel phase boundary‐fitting technique.
Intersegmental interaction and critical concentrations in PET–HFIP solutions
Authors: Sonnenschein M. F. , Roland C. M. 
Published: April 2003

AbstractSpectroscopic determinations of the concentrations associated with the onset of intermolecular interaction in solutions of poly(ethylene terephthalate) (PET) are described. The intersegment forces perturb the electronic spectra and induce ground‐state aggregation. These phenomena become observable at different chain densities, reflecting the influence of concentration on the coil dimensions, as well as the dynamical nature of the interactions. An estimate of the equilibrium constant for PET dimer formation in hexafluoroisopropanol is reported.
Self‐diffusion of poly (ethylene oxide) fractions and its influence on the crystalline texture
Authors: Cheng Stephen Z. D. , Barley Jeffrey S. , Von Meerwall Ernst D. 
Published: April 2003

AbstractTwo somewhat separate issues have been delat with: the diffusion coefficients of various poly(ethylene oxide) (PEO) fractions at different temperatures; and morphological observations of spherulitic textures of a PEO mixture containing low‐ and high‐molecular mass components at different supercooling and cooling rates. We found that simultaneous knowledge of diffusion coefficient of PEO fractions and crystal growth rate of the mixture leads to a better description of the crystal growth. As observed, the spherulitic texture of the PEO mixture comprises three texture levels: the spherulite as a whole; the crystalgrowth unit comprised of different number of lamellae in a stack; and the individual lamellar crystal. Under certain crystallization conditions, the thickness of the lamellar stack is on the same order of magnitude as the parameter δ proposed by Keith and Padden.
Step‐strain deformations for viscoelastic fluids: Formulation of a strain‐coupling constitutive equation
Authors: Vrentas J. S. , Venerus D. C. , Vrentas C. M. 
Published: April 2003

AbstractAn integral constitutive equation which includes strain‐coupling effects is proposed. The constitutive equation is more general than the K‐BKZ equation, and it is shown that this equation can explain various important aspects of double‐step stress‐relaxation data.
Light‐scattering studies on solutions of cellulose in the ammonia / ammonium thiocyanate solvent system. II. Quasielastic light‐scattering and liquid crystal study
Authors: De Groot A. W. , Guinnup D. E. , Thiel M. H. , Cuculo J. A. 
Published: April 2003

AbstractThis is the second part of a two–part study of the NH3NH4SCN cellulose solvent system. Quasielastic light scattering was used to determine the diffusion coefficients of cellulose in solution and the effective hydrodynamic radius of the dissolved molecules. Additionally, the system was studied using light microscopy to determine the minimum critical volume fraction or liquid crystal formation. Very little change was found in the diffusion coefficients with change in cellulose concentration indicating little interaction between the chains in solution. Values of 7.69 and 2.66 × 108 cm2/s were measured for samples having a degree of polymerization of 153 and 969. The value of the coefficient relating the hydrodynamic volume to the radius of gyration was found to be in the range of 0.33 to 0.53, indicating an extended coil conformation according to the Kirkwood‐Riseman theory. The minimum critical volume fractions necessary for liquid crystal formation, υ2′ were 0.039, 0.038, and 0.048 for the three solvent compositions studied. The values calculated for υ2′ based on the measured persistence lengths were much larger than the predicted values, indicating strong deviation from theory or possible aggregation in the system.
Percolation effects in degradable polyethylene‐starch blends
Authors: Peanasky J. S. , Long J. M. , Wool R. P. 
Published: April 2003

AbstractThe accessibility of starch in polyethylene starch blends was investigated by computer simulation, percolation theory, and acid hydrolysis experiments. The object of this work was to model the bilateral invasion of microbes in polyethylene‐starch blends as a function of starch concentration (p), and thickness of the material. It was found that computer simulations in three dimensions were in agreement with both percolation theory and the acid digestion experiments. In computer simulation the accessibility is highly dependent on the percolation threshold concentration (pc), which is 31.17%. Similarly, the accessibility of starch is highly dependent on an apparent percolation threshold near 30% by volume or approximately 40% by weight of starch. At p < pc a small amount of starch is removed from the surfaces only, but at p > pc connected pathways existing throughout the bulk of the material facilitate large amounts of starch extraction. The sharpness of the transition at pc increases with the ratio of sample thickness to starch particle size. The results of this work have application to conduction and reacting systems where one component is dispersed in a matrix of the other.
Determination of chain stiffness and polydispersity from static light‐scattering
Authors: Denkinger Peter , Burchard Walther 
Published: April 2003

AbstractChain stiffness is often difficult to distinguish from molecular polydisperity. Both effects cause a downturn of the angular dependence at large q2 (q = (4π/λ)sin θ/2) in a Zimm plot. A quick estimation of polydisperity becomes possible from a bending rod (BR) plot in which lim (c → 0) qRθ/Kc is plotted against q(〈S2〉z)1/2 = u. Flexible and semiflexible chains show a maximum whose position is shifted from umax = 1.41 for monodisperse chains towards larger values as polydispersity is increased, while simultaneously, the maximum height is lowered. Stiff chains display a constant plateau at large q, its value is πML where ML is the linear mass density. Using Koyama's theory, the number of Kuhn segments can be determined from the ratio of the maximum height to the plateau height, if the polydispersity index z = (Mw/Mn − 1)−1 is known. Thus, if the weight‐average molecular weight Mw, is known, the contour length Lw, the number of Kuhn segments (Nk)w, the Kuhn segment length lk and the polydispersity of the stiff chains can be determined. The influence of excluded volume is shown to have no effect on this set of data. The reliability of this set can be cross‐checked with the mean‐square radius of gyration 〈s2〉z which can be calculated from the Benoit‐Doty equation for polydisperse chains. Rigid and slightly bending rods exhibit no maximum in the BR plot, and the effect of polydispersity can no longer be distinguished from a slight flexibility if only static scattering techniques are applied.