bluuebunny/crossref_metadata_2025_split huggingface dataset:
Characterization of polymer molecular weight distributions by ratios of molecular weight averages
Authors: Pyun Chong Wha 
Published: April 2003

AbstractThe molecular weight (MW) distribution of a polymer is characterized by a hierarchy of average MWs and their appropriate combinations. For example, the ratio of the weight‐average to the number‐average MW is the most frequently used measure of the polydispersity of a polymer. As is well known the lower bound to this ratio is unity, and it has been shown that the upper bound is (m + 1)2/4m, where m = Mmax/Mmin is the ratio of the highest to the lowest MW of the MW species present in a given polymer. This upper bound corresponds to an extremely bimodal MW distribution of one half weight fraction with Mmin and the other half with Mmax. The behavior of the upper bound for two special unimodal distributions is investigated: one is the triangular distribution, the other the quadrilateral. The results suggest that the upper bound for all possible unimodal distributions is considerably less than the corresponding general case, especially for large values of m. For example, the maximum ratios for the quadrilateral distribution and the general upper bound are 1.04 and 1.125 for m = 2; 1.43 and 3.205 for m = 10; 2.56 and 25.5 for m = 100; 3.99 and 250.5 for m = 1000, respectively.
A dielectric study of poly(ethylene‐co‐vinylacetate)–poly(vinyl chloride) blends. I. Miscibility and phase behavior
Authors: Rellick G. S. , Runt J. 
Published: April 2003

AbstractMeasurements of the complex permittivity were used to study miscibility and phase behavior in blends of poly(vinyl chloride) (PVC) with two random ethylene—vinyl acetate (EVA) copolymers containing 45 and 70 wt % of vinyl acetate. The dielectric β relaxation of the pure polymers and blends was followed as a function of temperature and frequency for different blend compositions and thermal treatments. Blends of EVA 70/PVC were found to be miscible for compositions of about 25% EVA 70 and higher. Blends of lower EVA 70 content showed evidence of two‐phase behavior. EVA 45/PVC blends were found to be miscible only at the composition extremes; at intermediate compositions these blends were two‐phase, partially miscible. Both blend systems showed lower critical solution temperature behavior. Phase separation studies revealed that in the EVA 45/PVC blends, PVC was capable of diffusing into the higher Tg phase at temperatures below the Tg of the upper phase. In the blends, ion transport losses were significant above the loss peak temperatures, and in the two‐phase systems, often obscured the upper temperature loss process. It was shown possible, however, to correct the loss curves for this transport contribution.
An isochronous decoration method for measuring linear growth rates in polymer crystals
Authors: Dosiere M. , Colet M.‐Ch. , Point J. J. 
Published: April 2003

AbstractThe long spacing l of lamellar crystals of linear polyethylene increases with the crystallization temperature Tc. For low degrees of supercooling, the ratio Δl/ΔT is around 0.5 nm K−1 for PE single crystals obtained from solution in xylene. In the restricted situation where only conduction in the crystallization vessel is involved, a heat transfer analysis shows that about 20 s is needed to change by 5 K the crystallization temperature Tc in a cylindrical vessel of 1.5 mm radius. Such rapid change of the crystallization temperature induces a sharp increase or decrease of the thickness of the single crystals. After conventional shadowing with palladium–gold alloy, the steps on the crystals are observed by conventional bright‐field electron microscopy. A pioneering work was performed in this way by Bassett and Keller in 1962. Our technique allows one to determine both the shape and the dimensions of single crystals or twinned crystals of polyethylene as a function of the time of crystallization, and therefore give the quasi‐instantaneous growth rates at various times.
Gas sorption in poly(vinyl benzoate)
Authors: Kamiya Yoshinori , Mizoguchi Keishin , Naito Yasutoshi , Hirose Takuji 
Published: April 2003

AbstractHigh‐pressure sorption (up to 50 atm) for CO2, N2, and Ar in poly(vinyl benzoate) (PVB) was studied at temperatures from 25 to 70°C by a gravimetric method utilizing an electromicrobalance. The results are described by Henry's law above the glass transition temperature Tg for all gases. The dual‐mode sorption model, Henry's law plus a Langmuir isotherm, applies to the sorption isotherms of N2 and Ar in the glassy state, and the dual‐mode parameters are given. For CO2, a new type of sorption isotherm is observed below Tg. The isotherm is concave to the pressure axis in the low‐pressure region and turns into a straight line with increasing CO2 pressure which can be extrapolated back to the coordinate origin. The linear part of the isotherm is characteristic of the rubbery state, while the nonlinear part stems from glassystate behavior. The “glass transition solubility” of CO2, at which PVB film changes from the glassy to the rubbery state, decrease as the temperature increases. The disappearance of microvoids, that is, the decrease of the Langmuir capacity, may be due to a large plasticizing effect of sorbed CO2. The difference between the N2 and Ar isotherms and the CO2 isotherm is discussed from this standpoint.
Dynamic mechanical properties of plasticized polystyrene‐based ionomers. II. Melt rheology
Authors: Bazuin C. G. , Eisenberg A. , Kamal M. 
Published: April 2003

AbstractA high molecular weight styrene ionomer containing 5 mol% sodium methacrylate was blended with a styrene oligomer (MW 800) and investigated by dynamic mechanical techniques. The focus of the study is on the dynamic melt rheology of these materials, whose ratios by weight of ionomer to oligomer are 60/40, 40/60, and 25/75. The glass‐transition temperature and ionic transition are first characterized by torsion pendulum measurements as a function of temperature. It appears that a maximum level of plasticization is achieved for the ionic regions, the extent depending on sample history. Time–temperature superposition is obeyed by the blend of 60 wt% ionomer, but not by the other two blends. Relaxations due to the ionic regions are clearly evident in the relaxation spectra of all three blends. Above a particular temperature, the 25 wt% blend indicates an Arrhenius type of dependence.
A MNDO study of the structural and electronic properties of polysilane model compounds
Authors: Bigelow Richard W. , McGrane Kathleen M. 
Published: April 2003

AbstractThe results of MNDO geometry optimizations on selected H(SiH2)nH polysilane model compounds are presented. Near energetic degeneracy is indicated for all‐trans(T), alternating gauche–trans (GT), and all‐gauche (G+G+) models (n = 10). The most stable (T) and least stable (G+G+) conformations are separated by only ca. 0.11 eV. The existence of low‐energy barriers to moderate structural distortion is also suggested. Orbital localizations and charge density distributions along the “polymer” backbone are found to be sensitive functions of such distortion. The ground‐state electronic distribution of the saturated all‐trans silane chains are calculated to be considerably more polarizable than the fully conjugated H(CH)nH π‐electron framework of comparable length. The one‐electron HOMO → LUMO excitation can be viewed essentially as an in‐plane Si 3p → Si3s + H1s intramolecular charge transfer transition. The qualitatively different atomic orbital character of the HOMO and LUMO levels yields transition moment components for the separate repeat units which are relatively small. In the case of the rigidly trans conformation, the phase relationships of the transition moment terms are such as to constructively sum to a large net value reflecting strong optical absorption, as is observed experimentally.
Model MDI/butanediol polyurethanes: Molecular structure, morphology, physical and mechanical properties
Authors: Christenson C. P. , Harthcock M. A. , Meadows M. D. , Spell H. L. , Howard W. L. , Creswick M. W. , Guerra R. E. , Turner R. B. 
Published: April 2003

AbstractModel butanediol–MDI–polypropylene oxide polymers have been synthesized to explore the structure–property relationships in urethane/polyether polymers. The results of mechanical, thermal, and spectroscopic analyses agree remarkably well. The phase mixing in these polymers decreases with increasing hard‐segment length, while the hard‐segment stability increases with increasing hard‐segment length. This is demonstrated clearly by dynamic mechanical, differential scanning calorimetry, infrared, and NMR studies. The importance of hydrogen bonding to the stability of the hard segment is strongly supported by the variable‐temperature infrared experiments. The critical hard‐segment size for phase segregation was shown to be two butanediols per hard segment. The temperature limit of the flatness of the storage modulus was tied to the thermal stability of the hard‐segment hydrogen bonding which is controlled largely by the length of the hard segment and the extent of the hydrogen bonding.
Molecular orientation in biaxially oriented sheets of poly(ethylene terephthalate). I. Characterization of orientation and comparison with models
Authors: Bower D. I. , Jarvis D. A. , Ward I. M. 
Published: April 2003

AbstractA study of molecular orientation in a series of five one‐way‐drawn sheets of poly(ethylene terephthalate) is reported. Five orientation averages P which characterize the benzene‐ring orientation were determined directly by Raman spectroscopy. Two further averages P and P were determined from a combination of Raman spectroscopy and refractive index measurements on the basis of two different conformational models. As in a previous publication, the seven averages P were used to estimate all the P up to fourth order for the benzene rings on the assumption that in each case the actual distribution of orientations is close to the most probable distribution. The seven orientation averages P for each drawn sheet were also compared with those calculated on the basis of two simple models for the development of molecular orientation during drawing, the pseudoaffine deformation scheme and the rubber network model. It was shown that the orientation of the chain axes is closer to that predicted by the rubber network model, the pseudoaffine deformation scheme greatly overestimating the degree of chain biaxiality. The results also show that the benzene ring planes are preferentially oriented toward the planes of the sheets. The chain axis orientation is, however, nearly uniaxial and similar to that observed for uniaxially drawn samples, suggesting that there is no direct connection between the processes of chain axis orientation and preferential orientation of the benzene‐ring planes.
Refractometry of lyotropic mesophase systems: Ethyl cellulose/m‐cresol and ethyl cellulose/acetic acid
Authors: Suto Shinichi , Ise Hirotada , Karasawa Mikio 
Published: April 2003

AbstractAn Abbé refractometer with a rotatable polarizer mounted on the eyepiece is used for determining the two principal refractive indices of birefringent concentrated solutions of ethyl cellulose in m‐cresol and in acetic acid. At a certain concentration, birefringence appears for both systems, and the concentration agrees with that for the onset of iridescence (here defined as the critical concentration Ca for formation of the liquid‐crystalline phase). The birefringence of concentrated solutions depends strongly on concentration and temperature and can be generalized by a master curve. The refractive index increments for both systems are almost independent of temperature at high concentrations.
Crystallization in oriented poly(ethylene terephthalate) fibers. II. Consequences in subsequent deformation
Authors: Yoon K. J. , Desai P. , Abhiraman A. S. 
Published: April 2003

AbstractWhen a melt‐spun poly(ethylene terephthalate) (PET) fiber is heat treated at a temperature above its glass transition temperature, the relative rates at which the crystallization and major orientational relaxation processes occur have been shown to have a pronounced effect on the structure of the fiber and its deformability. The present study describes the consequences of this aspect, with examples from drawing of melt‐spun PET fibers subsequent to their crystallization by thermal annealing. Additional features of the highly ordered PET fibers which can be produced through a combination of oriented crystallization and drawing at high temperatures are also given.
Fiber spinning from the nematic melt. I. Copolyester of poly(ethylene terephthalate) and p‐oxybenzoate
Authors: Muramatsu Hiromochi , Krigbaum W. R. 
Published: April 2003

AbstractThe Tennessee Eastman copolyester of poly(ethylene terephthalate) with 60 mol % p‐oxybenzoate units was spun with various capillaries using a constant shear rate at the wall. Variables examined were the length‐to‐diameter ratio L/D of the capillary, the spin draw ratio Vf/V0, and the spinning temperature. Fibers spun at 260°C showed improved homogeneity of orientation through the cross section, better crystallite orientation, and higher initial moduli as L/D was increased. The spin draw ratio required to optimize these fiber properties decreases as L/D is increased. For example, when L/D = 49.44, the initial modulus has nearly reached its plateau value at a spin draw ratio of 10. However, in contrast to the results of Sugiyama, Lewis, White, and Fellers, we find that some spin draw is always required to optimize fiber properties. Fibers spun with a spin draw ratio of approximately unity showed very poor crystallite orientation and initial moduli. It is suggested that loss of orientation under these conditions may be due to the different velocity profiles in the spinneret and in the solidifed fiber. Fibers were also spun at five temperatures using a capillary having L/D = 49.44. Shear in the capillary is more effective in introducing orientation when the spinning temperature is 260°C or above. At spinning temperatures of 240 and 250°C, the initial modulus increases more slowly with spin draw ratio, and appears to have a lower plateau value. Acierno, La Mantia, Polizzotti, Ciferri, and Valenti spun the same polymer under conditions in which essentially all the orientation was introduced by spin draw. They used a very low extrusion velocity at the spinneret, a small L/D, and spin draw ratios up to 3000. They reported that the initial modulus increased with decreasing spinning temperature, in contrast to our results. Thus the optimum spinning conditions may depend upon whether most of the orientation is introduced by shear in the capillary, or by a high spin draw ratio.
Electron trapping in low‐density polyethylene
Authors: Markiewicz A. , Fleming R. J. 
Published: April 2003

AbstractThermoluminescence (TL) emission from low‐density polyethylene has been investigated. The glow curves of gas‐free samples x‐irradiated at −190°C and heated to room temperature were found to contain three peaks numbered I, II, and III in order of increasing temperature, in agreement with earlier results. The sites of all traps are accessible to absorbed gases; in the presence of air, O2, N2 or Ar, “gas” traps are formed, resulting in the appearance of an additional peak IV in the glow curve at a temperature between peaks I and II, large reductions in the intensities of peaks II and III, and various changes in peak I. The peak I, II, and III traps are formed from particular chain configurations occurring in the chain‐fold regions of the samples, these configurations being broken up by different forms of molecular motion within the chains. It is unlikely that the peak IV traps are just the gas molecules themselves; they are probably formed from new chain configurations occurring in the amorphous regions of the samples in the presence of the gas, the properties of the gas influencing the associated TL intensity and emission temperature. These traps are also broken up by molecular motion. The samples can be divided into two main types, differing mainly in the height of peak I relative to peak II, which is of nearly constant intensity in all samples. We suggest that two types of trap which are not interconvertible are associated with peak I, and that the dominant type in a given sample depends on the fine details of the sample fabrication process.
Conformational reorganization viewed by the raman spectrum of aromatic polyesters
Authors: Kim Jong S. , Lewin Menachem , Bulkin Bernard J. 
Published: April 2003

AbstractAnalysis is made of the asymmetry of the carbonyl stretching vibration of poly(ethylene terephthalate) and poly(propylene terephthalate) as a function of annealing frrom amorphous to crystalline polymer. An apparently anomalous behavior of carbonyl band half‐width as a function of density for PPT is shown to be due to this asymmetry. The results are interpreted in terms of the conformational redistribution which occurs upon annealing.
Field‐induced dipole reorientation and piezoelectricity in heavily plasticized nylon 11 films
Authors: Scheinbeim J. I. , Mathur S. C. , Newman B. A. 
Published: April 2003

AbstractNylon 11 films with very low initial crystallinity were made by dissolving the nylon 11 in 2‐ethyl‐1,3‐hexanediol at 150°C. Films were cast from the solution and excessive plasticizer was removed in a vacuum oven. Films were then melt pressed and quenched to yield heavily plasticized nylon 11 films containing ca. 30% by weight of the plasticizer. These films were poled under vacuum to allow the plasticizer to evaporate in the presence of an electric field. A high piezoelectric response (d31 = 7.1 pC/N) was observed for the films subjected to the maximum electric field (Ep = 350 kV/cm) while the sample contained a large fraction of plasticizer. Significant development of crystallinity was observed without apparent indication of orientation of the crystallites. These studies suggest that the observed piezoelectric response originates primarily from oriented hydrogen bonds in the amorphous regions of nylon 11.
Light scattering by optically heterogeneous materials: Structural aspects of dichromatic laser speckle patterns
Authors: Holoubek Jaroslav , Lednický Frantis̆ek , Sedlàc̆ek Blahoslav , Mikeš Josef 
Published: April 2003

AbstractAn analysis is given of dichromatic speckle patterns formed by small‐angle scattered light in the illumination of optically heterogeneous systems with light of two nearby wavelengths. The existence of the radial‐fibrillar appearance of the speckle patterns is explained; it is shown that the occurrence of the radial‐fibrillar “structure” corresponds to the measurable correlation of dichromatic speckle patterns as specified by the spectral spatial cross‐correlation function of intensity fluctuations. A measurement of speckle‐pattern correlation to determine σz (the standard deviation of optical path fluctuations due to the passage of coherent light through a layer of heterogeneous material) is described, and the relations between σz and the structural parameters for two simple types of heterogeneous materials are discussed. Experimental results obtained with a series of samples of polyethylene filled with CaCO3 are in comparatively good agreement with theoretical values.
Temperature and time dependence of diffusion of PCL into PVC
Authors: Parker M. A. , Vesely D. 
Published: April 2003

AbstractThe phenomenon of diffusion between two joined polymers [poly(vinyl chloride) and poly(ε‐caprolactone)] has been investigated by light microscopy and electron microscopy of thin sections taken perpendicular to the interface. It has been established that the diffusion is unidirectional, i.e., PCL into PVC, and that the solubility is limited and increases with temperature. Chlorine x‐ray analysis has shown that the concentration profile consists of a sharp change at the PCL boundary and a gradual change at the diffusion front. The diffusion region morphology has been found to be inhomogeneous and reveals density fluctuations within the PVC matrix. The diffusion behavior is therefore different from the classical Fickian diffusion previously noted and can be well described by a sorption mechanism.
Tests of a “free‐volume” model of gas permeation through polymer membranes. II. Pure Ar, SF6, CF4, and C2H2F2 in polyethylene
Authors: Stern S. A. , Sampat S. R. , Kulkarni S. S. 
Published: April 2003

AbstractPermeability coefficients for Ar, SF6, CF4, and C2H2F2 (1,1‐difluoroethylene) in polyethylene membranes were determined from steady‐state permeation rates at temperatures from 5 to 50°C, and at applied gas pressures of up to 15 atm. The temperature and pressure dependence of the permeability coefficients was represented satisfactorily by an extension of Fujita's free volume model of diffusion of small molecules in polymers. The parameters required by this model were determined from independent absorption (diffusivity) measurements with the above gases in polyethylene rods. The present work confirms the results of previous studies with CO2, CH4 C2H4 and C3H8 in polyethylene.
Theoretical study of sequence distributions in polydienes. IV. Experimental test
Authors: Wang Zhiyu , Yan Deyue , Wang Fusong 
Published: April 2003

AbstractThe effects of reaction time t, monomer conversion Y, and initial molar ratio of monomer to initiator M0/I0 on the microstructure of polybutadiene and polyisoprene generated by sec‐BuLi or sodium naphthalene were examined. The structural parameters determined by IR and NMR vary with t(Y) or M0/I0 monotropically and nonlinearly during the initial stage of the polymerization of dienes (or in the case of small monomer to initiator molar ratio), and then reach fixed values. The experimental data measured confirm the theoretical predictions for the unsteady stage after the beginning of the polymerization given in parts II and III of this series.
The effect of surface treatment on the alignment and phase transition of a polypeptide liquid crystal
Authors: Hakemi H. 
Published: April 2003

AbstractPoly(γ‐benzyl‐L‐glutamate) (PBLG), was utilized as a surface coating agent in order to obtain parallel molecular orientation in thin layers of PBLG liquid‐crystalline solutions confined between coated surfaces. It was found that in a thickness range much larger than the cholesteric pitch value, the texture was predominantly planar cholesteric. At film thickness comparable or smaller than the pitch, isotropic spherulites developed from a homeotropic nematic phase. Because of the coupling between the solution concentration and nematic director, as well as large differences in the elastic moduli of PBLG liquid‐crystalline solutions, the long‐range intermolecular interaction was unfavorable. Competition between parallel orientation induced by the surface coating of PBLG and inherent perpendicular orientation induced by PBLG molecules in the bulk would favor a short‐range helix‐coil transition mechanism, which results into a reentrant isotropic phase.
Analysis of trichroic infrared absorption in solids. II. One‐way drawn poly(ethylene terephthalate)
Authors: Fina L. J. , Koenig J. L. 
Published: April 2003

AbstractThe techniques of polarized infrared spectroscopy and sample tilting are applied to one‐way drawn poly(ethylene terephthalate) (PET) films to calculate the trichroic absorption as a function of frequency. Of specific interest in this study is the distribution of vibrational modes in the plane transverse to the draw direction. The rotational isomeric content of the films is found by the method of least squares and compared with a density‐method prediction of crystallinity. A two‐phase model to describe PET morphology in the oriented state is examined and found to be inadequate. The segmental orientation as a function of strain is followed with five conformationally sensitive infrared modes. High segmental orientation is found in the trans conformers of both amorphous and crystalline phases. Little net segmental orientation in the gauche conformers of the amorphous phase occurs.