bluuebunny/crossref_metadata_2025_split huggingface dataset:
Phase behavior and gelation of solutions of poly (vinyl alcohol)
Authors: Stoks W. , Berghmans H. 
Published: April 2003

AbstractThe phase behavior of poly (vinyl alcohol) dissolved in ethylene glycol and propylene glycol was studied. On cooling a solution in the low concentration range, solidification results from a combination of a liquid‐liquid demixing and a crystallization or liquid‐solid demixing. This conclusion is based on the results obtained from optical and calorimetric observations and is further supported by the influence of the molecular weight and the solvent quality. It also explains the melting point‐crystallization relationship observed after isothermal crystallization of the polymer in presence of the solvent. The mechanism proposed for the thermoreversible gelation of these solution is confirmed.
Glass transitions in hydrogen‐bonded polymer complexes
Authors: Wang L. F. , Pearce Eli M. , Kwei T. K. 
Published: April 2003

AbstractMutual precipitates of poly (N, N‐dimethyl acrylamide) and poly (4‐hydroxystyrene) were collected from dioxane, methanol, or acetone. The glass transition (Tg) temperatures of the precipitates are higher than the weight‐average values. Clear films cast from dimethylformamide solutions have lower Tg values. Complexation also occurred between poly (ethyl oxazoline) and poly (4‐hydroxystyrene) in dioxane and between poly (vinyl pyrrolidone) and poly (4‐hydroxystyrene) in methanol. Again, the glass transition temperatures of the precipitates are higher than the values for the blend films. The ΔCp values associated with the glass transitions of the complexes are smaller than those of the blends having the same compositions. Negative excess heat capacities of mixing have been observed for several precipitates.
Synthesis and characterization of an amphoteric ion‐exchange membrane
Authors: Elmidaoui Azzeddine , Boutevin Bernard , Belcadi Said , Gavach Claude 
Published: April 2003

AbstractAn amphoteric ion‐exchange membrane was prepared by chemical grafting of acrylic acid and dimethyl amino‐2‐ethyl methacrylate on ozonized polyethylene. The effects of the variation of external pH on the physicochemical properties of this membrane were studied (exchange capacity, electrical resistance, water and electrolyte content, and membrane potential). The determination of the mean activity coefficient of the NaCl in the membrane phase using the Donnan equilibrium equation reveals that this coefficient increases with the NaCl concentration in the external solution. Fundamental electrochemical properties of the amphoteric membrane are discussed on the basis of the variation of the membrane selectivity with the external electrolyte concentrations and the pH. The properties at neutral pH are accounted for by a mutual neutralization of some acidic and basic groups in the membrane phase.
A novel chromatographic method for determining the polydispersity index for polymers with a specified number of functional groups
Authors: Yu Wei‐Ching , Mattice Wayne L. 
Published: April 2003

AbstractA novel chromatographic method is proposed to determine the polydispersity index, without reference to standards, for polymers that contain a specified number of functional groups. Refractive index and ultraviolet‐visible (or infrared) detectors are employed for detecting the weight and number fractions, respectively, of the sample eluted in size exclusion chromatography (SEC). Pyrene‐labeled polystyrenes were used to demonstrate the validity of this method. The polydispersity index, Mw/Mn, measured by the new method underestimates the true value by 3%, whereas the application of the calibration curve overestimates the polydispersity index of the same sample by 6%. An illustrative SEC model including the effect of diffusion was used to explain the production of errors in different directions by the two methods.
Transport of gases in aromatic polyesters: Correlation with WAXD studies
Authors: Charati S. G. , Houde A. Y. , Kulkarni S. S. , Kulkarni M. G. 
Published: April 2003

AbstractPolymer packing density can be conveniently measured by either density‐based measurements (direct or estimated free volume) or wide‐angle X‐ray diffraction (WAXD) spectra. Previously reported diffusivity and permeability values for various polyarylates and polyimides are examined on the basis of the packing density measured by these different techniques. It is shown that in these rigid glassy polymers, the mean intersegmental distance, the d‐spacing, obtained from WAXD provides a better measure of the effect of polymer packing on diffusivity than either direct density measurements or estimated free volumes. It has been possible to correlate previously reported diffusivity and sorption data for various polyarylates and polycarbonate on the basis of the WAXD investigation, using the d‐spacing and the amorphous peak width, respectively. The effect of polyarylate structure on d‐spacing has been studied. A series of polyarylates have been synthesized with different substituents on the bridge carbon. In this series the d‐spacing remains constant while the carbonyl group density and the glass transition temperatures are varied. However, small substituents on the aromatic ring cause significant changes in the d‐spacing. These results can be used to tailor polyarylates with desired intersegmental distances and chain flexibility.
Viscoelastic properties of styrene‐co‐methyl methacrylate random copolymers
Authors: Halary J. L. , Oultache A. K. , Louyot J. F. , Jasse B. , Sarraf T. , Muller R. 
Published: April 2003

AbstractViscoelastic properties of styrene‐co‐methyl methacrylate random copolymers have been determined over a temperature range covering the glass transition, the rubbery plateau, and the terminal zone and compared with polystyrene and polymethyl methacrylate homopolymers. Nonlinear behavior was observed in the variations of most of the physical and rheological characteristics with the methyl methacrylate content in the copolymer. Results are interpreted in terms of the rupture of polar‐polar intermolecular interactions between ester groups due to the presence of styrene units.
The kinetics of flow‐induced crystallization from solution
Authors: McHugh A. J. , Spevacek J. A. 
Published: April 2003

AbstractIn situ birefringence measurements of the seeded growth in a tubular flow geometry of 0.01 wt% solution of a polyethylene fraction in xylene have been used to determine the flowinduced crystallization kinetics as a function of temperature and flow rate. In contrast to earlier reports on higher molecular weight polyethylene and polypropylene systems, orientational properties of the crystallized fibers do not show a clear correlation with growth conditions (i.e., temperature and flow rate). The combined kinetic data from these experiments and our earlier studies of higher molecular weight polyethylene—xylene and polypropylene—tetralin systems are analyzed in terms of a modified from of the Avrami equation which provides a basis for separately correlating temperature and flow rate effects. The observed temperature dependency of the crystallization process can be interpreted in terms of nucleation and growth models while the flow rate dependency can be interpreted on the basis of entanglement formation arguments. Results showing liquid phase precursor formation in an atactic polystyrene system are also presented to further document the liquidphase separation which can be induced in polymers under flow.
A dual‐mode interpretation of spin relaxation for 13CO2 sorbed in polycarbonate
Authors: Cain E. J. , Wen W.‐Y. , Jones A. A. , Inglefield P. T. , Cauley B. J. , Bendler J. T. 
Published: April 2003

AbstractCarbon‐13 spin‐lattice relaxation, spin‐spin relaxation and nuclear Overhauser enhancements (NOE) are reported for 13CO2 sorbed in glassy polycarbonate. These spin relaxation parameters were determined as a function of temperature and Larmor frequency. A quantitative interpretation is presented based on a two‐site, rapid exchange model. The model is related to the dual‐mode model developed to interpret sorption and permeability data for gases sorbed in glassy polymers. This model proposes the existence of two types of sorbed gas molecules: dissolved molecules and Langmuir sorbed molecules. The nuclear magnetic resonance (NMR) interpretation supports the conceptual features of the dualmode model. Gas molecules in the dissolved site have rotational, collisional, and translational dynamics typical of low molecular liquids. Langmuir sorbed gas molecules have much slower rotational and translational motions. The activation energy for motions in the dissolved site is about 1 kJ while the activation energy for the Langmuir site is about 10 kJ. The relative values of these activation energies are again consistent with the dual‐mode picture. The translational diffusion constants determined from the NMR data range from 10−7 to 10−13 cm2 s−1 and are consistent with the apparent diffusion constant from permeability measurements of 3x10−8 cm2 s−1. A relation between the permeability value and the NMR values is available from a theoretical description of the dualmode model in terms of a random walk on a lattice containing two sites.
Measurement of molecular transport in poly(vinyl chloride) by the step response technique
Authors: Wachi S. , Yates J. G. 
Published: April 2003

AbstractSorption and diffusion of methane and 1‐butene in poly(vinyl chloride) (PVC) powder have been investigated by the step‐response method using gas chromatography. The sorption equilibrium constant of 1‐butene was lower than that of chlorine, despite the higher critical temperature of 1‐butene, while the sorption of methane was negligibly small at temperatures much higher than critical. The diffusivity of 1‐butene in PVC showed a discontinuous dependence on temperature at the glass transition point, suggesting that the micro‐Brownian motion of the polymer chain enhances the diffusivity at the higher temperature. Below the glass transition temperature, the diffusivity of 1‐butene in PVC showed a lower value than has been measured for other smaller molecules such as chlorine and hydrogen chloride.
Measurement of thermal diffusivity of filler‐polymide composites by flash radiometry
Authors: Tsutsumi Naoto , Takeuchi Norihito , Kiyotsukuri Tsuyoshi 
Published: April 2003

AbstractThermal diffusivity D of filler‐polyimide composites was investigated using flash radiometry. The fillers used were: diamond, alumina, boron nitride, aluminium nitride, silicon carbide, and silicon carbide whiskers, Composite films were prepared by casting a polyamic acid solution with dispersed filler on a glass plate and then annealing. The effects of filler type, size, content, and shape on D were studied. D increased with increasing average filler particle diameter and filler content. For the composite filled with fine Al2O3 whose average particle diameter is about 0.2 μm, aggregates of filler were observed with dimensions up to a few microns. The formation of thermal paths through this aggregate enhances thermal diffusion. For the composites filled with high thermal conductivity filler such as BN and AIN, there was a strong increase in D above a filler content of 20 vol %. D was ten times larger than that of unfilled polyimide when the BN content was 54 vol %. D of the SiC‐whisker composite increased strongly up to a filler content of 18 vol %. Above this content D significantly decreased, probably because of interconnected voids formed by mold shrinkage.
Relaxations in thermosets. X. Analysis of dipolar relaxations in DGEBA‐based thermosets during isothermal cure
Authors: Mangion Michel B. M. , Joharis G. P. 
Published: April 2003

AbstractDielectric properties measured during isothermal curing of DGEBA‐based thermosets using a mixture of aromatic amines as curing agent are analyzed. The evolution of the dielectric features of thermosets during curing and after a time when their dc conductivity has reached a negligibly small value are phenomenologically similar to the dielectric features of physically and chemically stable dipolar liquids and solids observed with increasing frequency or decreasing temperature. This equivalence is a consequence of the invariance of the dynamic behavior of dielectric susceptibility with respect to either the frequency of measurement or the relaxation time of the substance and demonstrates that crosslinking of a thermoset causes its relaxation time to increase monotonically.It is shown that the stretched exponential relaxation function formalism satisfactorily describes the dielectric results and that the value of its distribution parameter initially decreases and, after gelation, reaches a constant value, which we denote γ, in the latter part of the cure. The value of the curing parameter, γ, which lies between 0.2 and 0.4, monotonically decreases with increasing curing temperature and tends to a limiting value characteristic of a thermoset at higher temperatures. This is in contrast with the increase found in the corresponding representation in the Kohlrausch‐Williams‐Watts parameter β with increasing temperature. The curing time dependence of the dipolar relaxation time ι has been determined and found to have the shape of an elongated S, with a well‐defined point of inflexion, as ι increases during the cure, from a value characteristic of a liquid to an ultimate value characteristic of a glass.
The equation of state of solid and molten poly (butylene terephthalate) to 300°C and 200 MPa
Authors: Fakhreddine Youssef A. , Zoller Paul 
Published: April 2003

AbstractThe volumetric behavior of poly (butylene terephthalate) (PBT) was studied from 30 to 303°C and pressures from atmospheric to 200 MPa. The pressure‐volume‐tempreature (PVT) data show behavior typical of other semicrystalline materials. The empirical Tait equation was used to fit the measured volumes in the solid and melt regions. The maximum deviations between the Tait fits and the measured volumes were 0.001 and 0.002 cm3/g in the semicrystalline and melt regions, respectively. The theoretical equation of Simha‐Somcynsky was used to represent the PVT behavior of this material in the equilibrium melt. Fitting the theory to the data involves a determination of the reducing parameters of the theory. We find P* = 1139 MPa, V* = 0.7838 cm3/g, and T* = 11133 K. From these parameters we calculate that the chemical repeat unit of PBT has 6.37 external degrees of freedom, in comparison with 4.97 degrees of freedom to the closely related poly(ethylene terephthalate), with two less (CH2) units in the backbone. The difference of 1.4 degrees of freedom may be compared with the 0.88 degrees of freedom of the polyethylene (C2H4) repeat unit itself.
The structure and properties of drawn blends of poly (vinylidene fluoride) and poly (methyl methacrylate)
Authors: Gallagher G. A. , Jakeways R. , Ward I. M. 
Published: April 2003

AbstractA blend consisting of equal proportions of poly (vinylidene fluoride) and poly (methyl methacrylate) has been prepared and drawn to draw ratios up to 7. The mechanical properties and the structure and morphology of the samples have been measured, the latter using differential scanning calorimetry, optical microscopy, and various x‐ray techniques. A structural model is proposed for the drawn materials which accounts for the mechanical properties and for the response of the crystalline regions of the material to an applied stress.
Transport properties of a side‐chain liquid‐crystalline polymer. Poly(4‐acryloyloxy biphenyl)
Authors: Vittoria V. , Russo R. , De Candia F. , Magagnini P. L. , Bresci B. 
Published: April 2003

AbstractThe transport properties of a side‐chain liquid‐crystalline polymer forming a smectic A phase have been investigated using dichloromethane as a permeant. Samples differing in the isotropization enthaoly were analyzed. A. Correlation between this thermal parameter and sorption has been found, while the diffusion coefficient is substantially the same in all samples. The results obtained can be explained in terms of a biphasis model in which a disordered permeable phase is present within the liquid‐crystalline phase, which is impermeable to the diffusant molecules.
Modified fuoss–kirkwood distribution function for interpretation of asymmetric nmr relaxation curves. I
Authors: Schneider Horst 
Published: April 2003

AbstractInterpretation of asymmetric representations of nuclear magnetic resonance (NMR) relaxation times versus reciprocal temperature requires a distribution function of correlation times that describes any relation between the slopes in the high‐ and low‐temperature regions. Although it can be shown that the Havriliak–Negami function fulfills this condition after slight modification, a more useful function (a generalization of the Fuoss—Kirkwood distribution) is introduced and discussed in detail. This new function allows for description of curves of different symmetry by an expression similar to that for a single correlation time and gives a simple rough estimation of the distribution parameters, as well as more exact computer fitting. This interpretative discussion of NMR relaxation data is purely phenomenological and has no direct connection with molecular properties.
A calorimetric investigation on high molecular weight polyethylene reactor powders
Authors: Ottani Stefano , Porter Roger S. 
Published: April 2003

AbstractReactor powders of high‐ and ultrahigh‐molecular weight polyethylene have been investigated. Two different Ziegler‐Natta synthesis processes were used: polymerization in a slurry and in the gas phase. Synthesis temperature range was 30–85°C. Monoclinic crystals were identified in samples synthesized at 30°C. Investigations of thermal parameters were carried out by differential scanning calorimetry. A range of heating rates (0.4–10.0°C/min) was used to obtain information on sample reorganization on heating. The corresponding melt‐crystallized samples were scanned and their thermal parameters were compared with those obtained from the original nascent powders. Percent crystallinity and average lamellar thickness, computed by the Thompson‐Gibbs equation, were found to be controlled by conditions of synthesis. For reactor powders, the fraction of crystallinity is found to be insensitive to synthesis temperature. Crystallinity is controlled mainly by the synthesis process type: slurry or gas phase. Lamellar thickness was found to decrease as synthesis temperature was increased. This trend is the opposite of what is obtained on melt crystallization and can be interpreted on the basis of Lauritzen and Hoffman's theory of crystal growth. Nascent reactor powders give experimental support for the dependence of lamellar thickness on crystallization temperature that follows the pattern described in the theory at high undercooling. The influence of molecular weight on crystallinity and lamellar thickness of both nascent powders and melt‐crystallized samples was also studied. Catalyst and synthesis conditions were found to control crystallinity and crystallite dimensions of the reactor powders. Thus, polyethylenes suitable for a specific purpose can be obtained directly on synthesis.
On orientational and distortional contributions to birefringence
Authors: Heymans Nicole 
Published: April 2003

AbstractA model for the effect of orientation dependence of polarizability anisotropy on birefringence is given and compared with literature data. If this dependence arises through bond angle distortion, the model shows a significant contribution to birefringence but a very small contribution to strain, accounting satisfactorily for differences in birefringence between hot‐drawn and cold‐drawn samples, for discrepancies between measurements of orientation obtained by different experimental methods and for previous observations on orientational behavior of preorientated polycarbonate at low additional strain.
Sub‐TG relaxations in LiClO4‐poly (propylene glycol)‐2000 solutions
Authors: Sidebottom D. L. , Johari G. P. 
Published: April 2003

AbstractThe dielectric permittivity and loss of LiClO4 solutions in poly (propylene glycol (PPG)), molecular weight 2000, have been measured over a concentration range up to a ratio of Li+ to oxygen atoms in PPG of 33.3:100, between 77 and 350 K. The data have been analyzed in both the permittivity and electrical modulus formalisms. Addition of LiClO4 to poly (propylene glycol) first increases the height of the β‐relaxation peak, and ultimately a second sub‐Tg relaxation peak at a higher temperature emerges. This is in addition to the β‐relaxation peak due to the reorientation of PPG dipoles, whose strength decreases from that in pure PPG‐2000. For a fixed temperature, the dc conductivity initially decreases with increasing Li+ concentration up to 20 Li+ per 100 O atoms and thereafter increases. This concentration corresponds to that at which the Tg of the solution reaches its limiting value of ca. 310 K. It is concluded that the formation of ion pairs causes a second and slower sub‐Tg relaxation process and that the increase in the efficiency of chain packing reduces the strength of the β‐relaxation of the polymer.
Calorimetric study of the room‐temperature transitions of polytetrafluoroethylene: The influence of thermal history
Authors: Villani Vincenzo , Pucciariello Rachele , Ajroldi Giuseppe 
Published: April 2003

AbstractThe trend of the room‐temperature transitions of native polytetrafluoroethylene versus annealing temperature has been studied. A method of deconvolution for the calorimetric curves Cp(T), based on Pearson VII type functions and the Marquardt's nonlinear leastsquares method, allowed the evaluation of the temperatures and of the heat of each transition. Annealing at 330°C determines the loss of the crystalline fraction responsible of the lowest room‐temperature transition. An interpretation of the lowest room‐temperature transition is suggested.
The combined effects of evaporation and quench steps on asymmetric membrane formation by phase inversion
Authors: Tsay C. S. , McHugh A. J. 
Published: April 2003

AbstractAn analysis of the effects of combined evaporation and quench periods on the formation of asymmetric membranes by the phase inversion technique is presented. The model for the evaporation period assumes binary diffusion and includes composition dependence in the mutual diffusion coefficient and free convection mass transfer in the gas phase. The quench period model includes the use of 4 composition‐dependent diffusion coefficients and incorporates a modified from of the interface jump mass balance which enables efficient numerical analysis of composition gradients in the film at the beginning of the quench period. Results for the effects of evaporation‐period variable, including: evaporation time, initial cast film thickness, casting surface dimension, and vapor‐phase composition, clearly indicate that significant effects of the evaporation period on membrane structure formation and its reproducibility occur within 20 seconds.