bluuebunny/crossref_metadata_2025_split huggingface dataset:
Hydrodynamic fluctuations and spinodal decomposition in ternary systems
Authors: Bobba Ratnaleela , Cohen Claude 
Published: April 2003

AbstractTime correlation functions of concentration fluctuations are obtained from hydrodynamic fluctuation theory extended to ternary systems. The coupling between the concentrations of the different species is accounted for and leads naturally to symmetric correlation functions unlike those previously reported. The time correlation functions of concentration fluctuations incorporating the effect of thermal noise are used to obtain the initial scattering intensity of ternary polymer‐polymer‐solvent systems undergoing spinodal decomposition. It is shown that the scattering function is represented by a sum of three exponentials. Single exponential growth is predicted under certain conditions which appear to have been met in the limited experimental data available. © 1994 John Wiley & Sons, Inc.
Thermal history and enthalpy relaxation of an interpenetrating network polymer with exceptionally broad relaxation time distribution
Authors: Sartor Günter , Mayer Erwin , Johari G. P. 
Published: April 2003

AbstractDifferential scanning calorimetry (DSC) of an interpenetrating network polymer of composition 25% polyurethane–75% poly(methyl methacrylate) shows a slowly increasing heat capacity, instead of the usual glass transition endotherm, whose onset temperature is not clearly discernible. On aging of the polymer at several temperatures between 193 and 333 K, an endothermic peak is observed whose onset is in the vicinity of the respective temperature of aging. The area under these peaks increases with increasing aging time at a fixed temperature. The effects are attributed to a very broad distribution of relaxation times, which may be represented by either a sum of discrete structural relaxation times of local network arrangement or by a nonexponential relaxation function which is equivalent to a distribution of relaxation times. In either view the vitrified state of the polymer can be envisaged as containing local structures whose own Tgs extend over a wide range of temperature. Aging decreases the enthalpy and produces an endothermic region which resembles an increase in Cp on heating because of relaxation of that local structure. The interpretation is supported by simulation of DSC scans in which the distribution of relaxation times is assumed to be exceptionally broad and in which aging introduced at several temperatures over a wide range produces endothermic effects (or regions of DSC scans) qualitatively similar to those observed for the interpenetrating network polymer. © 1994 John Wiley & Sons, Inc.
Effect of structure on the temperature dependence of gas transport and sorption in a series of polycarbonates
Authors: Costello Lora M. , Koros William J. 
Published: April 2003

AbstractThe permeabilities and solubilities of five gases are reported for bisphenol‐A polycarbonate (PC), tetramethyl polycarbonate (TMPC), and tetramethyl hexafluoro polycarbonate (TMHFPC) at temperatures up to 200°C. The temperature dependence of permselectivity is discussed in terms of solubility and diffusivity selectivity changes with temperature for CO2/CH4 and He/N2 gas separations. The activation energies for permeation and diffusion and the heats of sorption are also reported for each gas in the three polycarbonates. Analysis of these values provides a better fundamental understanding of the effect of polymer‐penetrant interactions and polymer backbone structure on the temperature dependence of the transport and sorption properties of gases in membrane separation processes. Important factors affecting the solubility and diffusivity selectivity losses or gains with increased temperature are also identified through correlation of these data with physical properties of the gases and polymers. These conclusions provide a framework for choosing the most promising membrane materials for particular gas separations at elevated temperatures. © 1994 John Wiley & Sons, Inc.
The chain conformations of a polyether based on bis(4‐hydroxyphenoxy)‐p‐xylylene and 1,11‐dibromoundecane (HPX‐c11) in its mesophase states, derived from 13C NMR
Authors: Cheng Jinlong , Jin Yimin , Chen Wei , Wunderlich Bernhard , Jonsson Håkan , Hult Anders , Gedde Ulf W. 
Published: April 2003

AbstractA thermotropic main‐chain polyether based on bis(4‐hydroxyphenoxy)‐p‐xylylene and 1,11‐dibromoundecane has been studied by variable‐temperature solid‐state 13C NMR. Between isotropization and glass transition temperatures, the material can be identified to be semicrystalline, consisting of two conformationally and motionally distinguishable phases. The more mobile component is liquid‐like and thus, can be attributed to an amorphous phase. In the more rigid component, the molecules have a conformationally disordered methylene sequence. In the low‐temperature ordered phase approximately ⅔ of the carbon‐carbon bonds are trans (t). Starting from the bond between the oxygen and the first methylene carbon, the bond conformations are: dtdttttttdtd, where d stands for disordered (i.e., it represents the common dynamic interchange between gauche and trans with an overall gauche content of perhaps 40%). The motion of the αα′‐diphenoxy‐p‐xylylene unit consists mainly of 180° ring‐flips, which cause no entropy increase relative to ordered phenylene groups in a crystal, but significantly changes the 13C NMR spectra. The central p‐xylylene ring starts its flipping motion at a lower temperature than the two phenoxy rings. The high‐temperature mesophase contains a methylene sequence of the bond conformations: dtdddttdddtd. Thus, the difference between the low‐temperature and high‐temperature mesophases consists of different degrees of conformational disorder. Thermal analysis seems to indicate that additional mesophases may be possible. © 1994 John Wiley & Sons, Inc.
Crystallization study of a thermoplastic polyimide (new‐TPI)
Authors: Hsiao Benjamin S. , Sauer Bryan B. , Biswas Amit 
Published: April 2003

AbstractCrystallization, melting, and morphology of a thermoplastic polyimide (New‐TPI) containing pyromellitic dianhydride (PMDA) and 3,3'‐bis(4‐aminophenoxy) biphenyl diamine (33BAPB) moieties have been studied. This material showed a glass transition temperature (Tg) of 250°C, an equilibrium melting temperature (T°m) of 406°C and a heat of fusion (ΔH) for 100% crystallinity of 6.38 kJ/mol. Measurements of the crystallization bulk rate (by DSC) and spherulite growth rate (by optical microscopy) indicated that the maximum crystallization temperature was about 320°C and the crystallization growth process was three‐dimensional under thermal nucleation (the Avrami exponent n ca. 4). The rate of nucleation density was estimated to decrease with increasing temperature, and the product of two crystal surface free energies σeσo was calculated to be 1176 erg2/cm4. The meltgrown spherulite consistently showed a Maltese cross pattern with negative birefringence under cross‐polars. The calculation of polarizability along the three unit cell axes suggested that the crystal b axis may be along the spherulite growth (radial) direction. Two scattering maxima were seen in small‐angle x‐ray scattering (SAXS) profiles. The dominant peak indicated a long period of ca. 20 nm which varied as a function of crystallization temperature. The weak peak at a d‐spacing of 2.5 nm was independent of temperature and has been attributed to the chemical repeat distance determined by Okuyama et al. (indexed as 001). The lamellar thickness lc, estimated by the correlation function analysis of the SAXS data, was found to be similar to that determined by the Scherrer analysis of the 001 reflection peak. © 1994 John Wiley & Sons, Inc.
Comparison of the molecular dimensions of di‐alkyl and aryl‐alkyl‐substituted polysilanes in solution using SEC/LS
Authors: Cotts Patricia M. 
Published: April 2003

AbstractPolysilanes provide an opportunity for exceptional control of the chain hindrances to rotation through the choice of substituents on each backbone silicon. Two alkyl substituents on each silicon result in a large characteristic ratio of at least 19 for poly (di‐n‐hexylsilane), determined by extrapolation of intrinsic viscosities. Bulky aromatic substituents provide even more hindrance to backbone rotations, and can be expected to result in a more extended polymer chain. Direct measurement of the dimensions of these polymers by scattering techniques has been limited by the small quantities available, and by the polydispersity of samples. The recent introduction of light‐scattering detectors for size exclusion chromatography enables the simultaneous measurement of light scattered at as many as 15 scattering angles as the fractionated polymer elutes from the column. Determination of both M and the root‐mean‐square radius of gyration Rg of narrow fractions eluting from a column allows determination of the Rg M relation over as much as a decade in M with less than a milligram of sample. Values of Rg and M across the distribution have been determined for alkyl and aryl substituted polysilanes with this technique. Estimation of Rg,0/M unperturbed by long‐range interactions is made by an extrapolation procedure. The dependence of Rg,0 on M across the distribution is compared among the different substituents and with other measurements reported for these polymers. © 1994 John Wiley & Sons, Inc.
Interdiffusion of PMMA and PVF2, studied by solid‐state NMR
Authors: Maas W. E. J. R. , Papavoine C. H. M. , Veeman W. S. , Werumeus Buning G. H. , Vankan J. M. J. 
Published: April 2003

AbstractA previously published new solid‐state nuclear magnetic resonance (NMR) method is applied to the interdiffusion of poly(methacrylate) (PMMA) and poly(vinylidene fluoride) (PVF2) above their Tg. Via a variation of the cross‐polarization technique magnetization is transferred from protons to fluorines. When this magnetization is made to disappear at the fluorine sites, only those protons that are distant from fluorines greater than the distance over which cross‐polarization functions will retain their magnetization. In this way we detect the fraction of PMMA near (ca. 20 Å) PVF2. Starting from sheets of PMMA and PVF2, which are then heated at 190°C for a variable time, and applying the above technique, we can determine the fractions of PMMA and PVF2 that have diffused within a distance of a few Å of each other. The intrinsic diffusion coefficients of PMMA and PVF2 determined from such experiments compare well with literature data. Initial attempts to fit the experimental data suggest that the concentration dependence of the diffusion coefficients cannot be neglected. © 1994 John Wiley & Sons, Inc.
Glass transition and melting behavior of poly(ether‐ester) multiblock copolymers with poly(tetramethylene isophthalate) hard segments
Authors: Phillips Roger A. , McKenna J. Michael , Cooper Stuart L. 
Published: April 2003

AbstractThe glass transition and melting behavior of poly(ether‐ester) multiblock copolymers with poly(tetramethylene isophthalate) (PTMI) hard segments and poly(tetramethylene oxide) (PTMO) soft segments are studied by differential scanning calorimetry (DSC) and small‐ and wide‐angle x‐ray scattering (SAXS and WAXS). Thermodynamic melting parameters for the PTMI homopolymer are estimated by WAXS and from the dependence of melting point on crystallization temperature. The melting behavior of PTMI is characterized by dual endotherms which are qualitatively representative of the original morphology, although reorganization effects are present. The composition dependence of the glass transition temperature parameters after rapid quenching from the melt are well described by mixed phase correlations for copolymers in the range 30‐100 wt% hard segment. Combined with SAXS characterization at melt temperatures, a single phase melt is suggested in these materials which extends to temperatures below the hard segment melting point. © 1994 John Wiley & Sons, Inc.
Mechanism of adhesion of polyurethane/polymethacrylate simultaneous interpenetrating networks adhesives to polymer substrates
Authors: Jia Demin , Pang Yongxin , Liang Xi 
Published: April 2003

AbstractPolyether‐based polyurethane/poly (methyl methacrylate‐co‐ethyleneglycol dimethacrylate) interpenetrating polymer networks [PU/P (MMA–co–EGDMA)‐IPNs] were synthesized and used as adhesives to adhere vulcanized natural rubber (NR) and soft polyvinyl chloride (PVC). The structure and morphology of the IPN adhesives in bulk and near the adhesive/substrate interfaces were investigated. A new mechanism of adhesion called conjugate interpenetration of networks across interfaces, which is suitable for IPN adhesives and polymer substrates, was put forward. According to this mechanism, while forming simultaneous interpenetrating networks in the adhesive, the monomers in the IPN adhesive can permeate polymer substrates and polymerize in situ to form gradient IPNs, thereby producing conjugate three‐component IPNs near the adhesive/substrate interfaces. It is the conjugate interpenetration of the networks across the interfaces that strengthens interfacial combination remarkably and results in high bond strength of IPN adhesives. © 1994 John Wiley & Sons, Inc.
The effect of orientation on thermal expansion behavior in polyimide films
Authors: Pottiger Michael T. , Coburn John C. , Edman James R. 
Published: April 2003

AbstractThe anisotropy of the thermal expansion of polyimide films was investigated . Out‐of‐plane or thickness direction coefficients of linear thermal expansion (CTE) were calculated from the difference between the coefficient of volumetric expansion (CVE) and the sum of the in‐plane or film direction coefficients of linear thermal expansion for commercial and spin‐coated PMDA//ODA and BPDA//PPD films and spin coated BTDA//ODA/MPD films. The CVEs were obtained from a pressure‐volume‐temperature (PVT) technique based on Bridgeman bellows. The CVE was shown to be essentially constant, independent of molecular orientation and thickness. A decrease in the in‐plane CTEs therefore occurs at the expense of an increase in the out‐of‐plane CTE. In all cases the calculated out‐of‐plane CTE was higher than the measured in‐plane CTE. The ratio of the out‐of‐plane CTE to the in‐plane CTE was 1.2, 3.8, and 49.3 for the spin‐coated BTDA//ODA/MPD, PMDA//ODA, and BPDA//PPD films, respectively. © 1994 John Wiley & Sons, Inc.
Effect of crystallization temperature on the crystalline phase content and morphology of poly(vinylidene fluoride)
Authors: Gregorio, Jr. Rinaldo , Cestari Marcelo 
Published: April 2003

AbstractThe effect of temperature on the crystallization of α, β, and γ phases of PVDF from dimethylacetamide (DMA) solution was studied. Variation in the crystallinity content of these three phases was obtained as a function of temperature using infrared spectroscopy, differential scanning calorimetry (DSC) and x‐ray diffraction techniques. Such variation is related to the dependence of the crystallization rate of each phase with temperature, and allowed a better understanding of some results found in the literature about the crystallization and interconversion of these phases. Micrographs of samples present morphologies that corroborate with the proposed explanations. © 1994 John Wiley & Sons, Inc.
The annealing and thermal analysis of poly(butylene terephthalate)
Authors: Kim Junkyung , Nichols Mark E. , Robertson Richard E. 
Published: April 2003

AbstractWhen poly(butylene terephthalate) (PBT) is annealed, a second endotherm is often displayed in a subsequent scanning thermal analysis at a temperature below that of the original endotherm, and this new endotherm appears to grow with annealing at the expense of the original. This growth is not due to chemical changes, because the thermogram obtained before annealing is recovered after complete melting. But a physical change would also seem unlikely because the transformation of higher‐melting into lower‐melting crystals is generally prohibited by thermodynamics. Two hypotheses to explain the result were tested. The first is that higher‐melting crystals are not transformed into lower‐melting crystals. Instead, because of recrystallization during thermal analysis, the single endotherm that results without annealing overestimates the population of high‐melting crystals present before the analysis. This hypothesis was tested by extending to annealing a mathematical analysis previously used to describe the thermal scanning behavior of specimens crystallized at different cooling rates. Though most features of the thermograms obtained after annealing were able to be described, the decrease in the higher‐temperature endotherm concomitant with growth of the lower endotherm was not. The second hypothesis is that the transformation of higher‐melting to lower‐melting crystals during annealing is allowed because it is coupled to the crystallization of formerly amorphous material. The amount of such crystallization observed for PBT was found to be sufficient to satisfy thermodynamic requirements, suggesting that this hypothesis is correct. © 1994 John Wiley & Sons, Inc.
A simulation model for long‐chain branching in vinyl acetate polymerization: 1. Batch polymerization
Authors: Tobita Hidetaka 
Published: April 2003

AbstractA new simulation model for the kinetics of long‐chain branching formed via chain transfer to polymer and terminal double‐bond polymerization is proposed. This model is based on the branching density distribution of the primary polymer molecules. The theory of branching density distribution is that each primary polymer molecule experiences a different history of branching and provides information on how each primary polymer molecule is connected with other chains that are formed at different conversions, therefore making possible a detailed analysis on the kinetics of the branched structure formation. This model is solved by applying the Monte Carlo method and a computer‐generated simulated algorithm is proposed. The present model is applied to a batch polymerization of vinyl acetate, and various interesting structural changes occurring during polymerization (i.e., molecular weight distribution, distribution of branch points, and branching density of the largest polymer molecule) are calculated. The present method gives a direct solution for the Bethe lattice formed under nonequilibrium conditions; therefore, it can be used to examine earlier theories of the branched structure formation. It was found that the method of moments that has been applied successfully to predict various average properties would be considered a good approximation at least for the calculation of not greater than the second‐order moment in a batch polymerization. © 1994 John Wiley & Sons, Inc.
Gel formation in free radical polymerization via chain transfer and terminal branching
Authors: Zhu S. , Hamielec A. E. 
Published: April 2003

AbstractGel formation in free‐radical polymerization via chain transfer to polymer, recombination termination, and terminal branching due to either chain transfer to monomer or disproportionation termination is investigated using the method of moments. It is found that no gel can possibly form in the systems consisting of initiation, propagation, and one of the above reactions. However, systems with the following combination of reactions are found to be capable of gelling. They are: chain transfer to polymer + recombination termination; chain transfer to polymer + terminal branching due to disproportionation termination; and terminal branching due to transfer to monomer + recombination termination. Systems with the following combination of reactions are incapable of gelling; transfer to polymer + terminal branching due to transfer to monomer; and terminal branching due to disproportionation termination + recombination termination. An examination of the gelation mechanisms reveals that the formation of multivinyl macromonomers during the course of polymerization is the reason that systems involving terminal branching gel. Sol/gel diagrams are generated to give critical kinetic parameters required for gelation. It is found that terminal branching does not always promote gelation due to the adverse effect on chain length through chain transfer to monomer and termination by disproportionation, reactions which generate terminal double bonds. © 1994 John Wiley & Sons, Inc.
Sol‐gel transition of polyacrylonitrile/propylene carbonate solution
Authors: Ko Moon Bae , Kwon Ick Hwan , Jo Won Ho , Son Tae Won 
Published: April 2003

AbstractThe thermoreversible gelation of polyacrylonitrile (PAN)/propylene carbonate (PC) solution was investigated by calorimetric, thermomechanical, optical, and rheological measurements. Two endothermic signals are observed for PAN/PC gels in DSC thermograms, and the peak temperature of the higher melting endotherm coincides with the gel melting temperature determined from the TMA thermogram. SEM micrographs show that the overall structure is separated into two phases and is composed of sperical domains, which may be formed upon passing through the thermodynamically unstable or metastable state via the liquid‐liquid phase separation. The gelation rate increases with the aging temperature. This is also discussed on the basis of the elasticity theory of the fishnet gel. © 1994 John Wiley & Sons, Inc.
Crystallization of polypropylene, nylon‐66 and poly(ethylene terephthalate) at pressures to 200 MPa: Kinetics and characterization of products
Authors: He J. , Zoller P. 
Published: April 2003

AbstractThe crystallization kinetics of polypropylene (PP), polyamide (PA66), and poly(ethylene terephthalate) (PET) were studied, using a pressure dilatometer (to 200 MPa) to follow the volume changes associated with the crystallization process. The commonly used Avrami equation fitted the isothermal/isobaric crystallization data of PP and PA66 well. The Avrami exponent n was between 1.3 and 1.7, independent of crystallization pressure and temperature. Lines of constant Avrami rate parameter Z in the P‐T plane were essentially parallel to the pressure dependence of the melting points and crystallization temperatures. However, the Avrami equation was not suitable for PET. The Malkin, Dietz, and Kim equations provided better fits. The crystallization half‐time of PET increased with pressure at constant supercooling, in contrast to PP and PA66, for which it remained essentially unchanged. X‐ray diffraction, differential scanning calorimetry, and pressure dilatometry were used to study the effect of formation pressure on the crystal structure, the melting point, and the density of products which were crystallized for short times (minutes) at various temperatures and pressures. No new crystal structures were found for PA66 and PET, but a mixture of monoclinic and triclinic crystals existed in PP above a formation pressure of 50 MPa. The melting points increased with formation pressure for PET, but remained unchanged for PP and PA66. Density at ambient conditions decreased with formation pressure for PP, but increased for PET and PA66. © 1994 John Wiley & Sons, Inc.