bluuebunny/crossref_metadata_2025_split huggingface dataset:
Dielectric relaxation in styrene‐related polymers at low temperatures
Authors: Irvine J. D. , Work R. N. 
Published: April 2003

AbstractThe complex dielectric constants ϵ* = ϵ′ − jϵ″ of each of several members of a system of copolymers of 4‐chlorostyrene and 4‐methylstyrene have been measured from 1.6°K to 300°K and from 0.1 kHz to 20 kHz. The principal experimental findings are: the strength of the relaxation process which occurs near 50°K at 1 kHz varies linearly with changing copolymer composition; both the apparent activation energy (H = 2.7 ± 0.7 kcal/mole) and the shape of the relaxation curve are independent of the composition variable and of the temperature (or frequency) within the ranges studied; and the ratio of the relaxation strength of poly‐4‐methylstyrene to that of poly‐4‐chlorostyrene in the 50°K process is about 25 times the corresponding ratio for the primary relaxation process that occurs in the neighborhood of the glass‐transition temperature. These findings suggest that in the 50°K process the phenyl groups relax independently of one another; that the apparent activation energy and the shape of the relaxation spectrum are determined primarily by the nature of the intrachain forces; and that the strength of the relaxation process depends primarily on effects of intermolecular forces that are governed by the molar “free volume” of the copolymers.
Invariance of the long spacing–crystallization temperature dependence of polyamides precipitated from solution
Authors: Dreyfuss P. , Keller A. 
Published: April 2003

AbstractPreviously it was found that the lamellar thickness of solution‐grown polyamide crystals as assessed by low‐angle x‐ray diffraction appeared to be unaffected by crystallization conditions. This point has now been re‐examined in detail on nylon 6.6 with crystallization temperature Tc as the main variable. Over a very wide range of Tc the lamellar thickness was invariant; a notable increase could be produced only at the highest Tc. The “invariant” lamellar thickness occurs at larger supercoolings than can be realized in the much‐studied polyethylene, which may explain why it has not yet been reported for polyethylene. The existence of this minimum invariant value raises important questions concerning the applicability of the current kinetic theories over such a wide range of supercoolings. Several other polyamides also revealed invariant long spacings after crystallization from solution. This provides the justification sought for comparing, in the paper which follows, the long periods observed for a large number of polyamides.
Survey of the long spacing of polyamides crystallized from solution
Authors: Dreyfuss P. 
Published: April 2003

AbstractA series of linear, aliphatic polyamides in which the number of carbon atoms in the repeat unit ranged from three to twenty‐four was crystallized from solution. All gave lath‐shaped crystallization products (usually aggregated in the form of sheaves) that were unmistakable lamellar. Sedimented mats of the crystals were examined by lowangle and wide‐angle x‐ray diffraction. Each polyamide had a characteristic layer thickness (fold length) which was determined by the length of the repeat unit and the number of hydrogen bonds in the lamella. The thickness was independent of other variables examined including crystallization conditions. The polyamides studied cover a wide range: they border on polypeptides at the one extreme and approach polyethylene at the other. For all these materials there emerged a unifying pattern which relates chemical structure directly to chain folding.
Kinetics of craze formation in glassy polymers
Authors: Narisawa I. , Kondo T. 
Published: April 2003

AbstractCraze initiation and propagation near a Griffith crack were studied in poly(methyl methacrylate) and polycarbonate. Craze propagation results either in eventual catastrophic failure or in termination of propagation. Photoelastic observations indicate that high stress concentration at the initial crack tip is relieved immediately after craze initiation and craze propagation introduces no new stress concentration at the craze front. These results lead to the conclusion that the criteria for craze initiation and propagation are described fairly well by the stress intensity factor. A fracture mechanics model proposed by Marshall et al. can explain the kinetics of craze propagation in poly(methyl methacrylate). However the growth rates are lower in polycarbonate than in poly(methyl methacrylate). This is attributed to the difference in a penetrant activity.
Relation between hydraulic permeability and diffusion in homogeneous swollen membranes
Authors: Paul D. R. 
Published: April 2003

AbstractThe question of viscous flow versus molecular diffusion mechanisms for pressure‐induced liquid transport through membranes is critically examined for the specific case of homogeneous swollen membranes. It is shown that previous attempts to compute diffusion coefficients from hydraulic permeabilities for such systems have used an equation which is grossly in error. It estimates diffusion coefficients which are orders of magnitude too high and often exceed self‐diffusion coefficients. This has frequently led to the conclusion that viscous flow predominates. The origins of the errors in this equation are indicated, and a substitute equation is developed which gives diffusion coefficients well below that for self‐diffusion when applied to literature data. As a result it is concluded that molecular diffusion is the dominant mechanism in homogeneous systems.
Stress–strain behavior in extension of elastomer networks with crosslinks of different lenghts
Authors: Hasa J. , Van Der Hoff B. M. E. 
Published: April 2003

AbstractStress–strain behavior in extension and the swelling of polymer networks with different lenghts of crosslinks is reported. These networks were prepared by copolymerization of butyl acrylate with different amounts of various difunctional comonomers which yield crosslinks of 4, 7, 10, and 16 bonds in length. The efficiency of the comonomers in crosslinking is low, improving with increasing length of the chain between their unsaturated endgroups. Analysis of a large number of stress–strain data obtained at elongations between 2 and 8% elongation showed that in this deformation range the stress–strain relation based on the statistical theory of elasticity represents the data better than does Hooke's law or the Mooney‐Rivlin relation. It was found that the relation between the modulus at small deformations and the swelling ratio of the various samples inindependent of the length of the crosslinks. Also the shapes of the Mooney‐Rivlin curves are the same for all networks. Furthermore, the creep behavior of various networks with different crosslink lengths is the same for networks compared at the same elastically effective chain concentration. It is concluded that the lenght of the crosslinks, at least up to 16 bonds, does not affect the elastic response of polymer networks.
Internal motions in an alternating copolymer of ethylene and tetrafluoroethylene
Authors: Starkweather Howard W. 
Published: April 2003

AbstractInternal motions in an alternating copolymer of ethylene and tetrafluoroethylene were investigated by dynamic mechanical and dielectric measurements and by nuclear magnetic resonance. At 1 Hz the α, β, and γ relaxations were observed at 110, −25, and −120°C in a quenched sample. The activation energy was 76 kcal/mole for the α relaxation and 10.6 kcal/mole for the γ relaxation. These relaxations are attributed to the motion of long and short segments in the amorphous regions, respectively. The β relaxation, which was observed only in the dynamic mechanical experiments, appears to occur in the crystalline regions. The copolymer is isomeric with poly(vinylidene fluoride), but it has a higher melting point and a much lower dielectric loss.
Small‐angle scattering of circularly polarized light from an anisotropic sphere
Authors: Holoubek J. 
Published: April 2003

AbstractA theory of the small‐angle scattering of circularly polarized light from an anisotropic sphere has been derived. The validity of the theory has been verified, and a relationship between the structural information thus obtained and the structural information obtained with linearly polarized light has been demonstrated.
Hydrogen bonding effects in the glassy state of random copolyamides
Authors: Hatakeyama Tatsuko , Kanetsuna Hisaaki 
Published: April 2003

AbstractThe thermal behavior of random copolyamides which are used as model polymers with hydrogen bonds has been investigated by differential scanning calorimetry (DSC), x‐ray diffraction, and infrared spectroscopy. The quenched copolyamides have only halo patterns in their x‐ray diffraction photographs. A random copolymer of nylons 6, 66, and 610 (in a composition ratio of 3: 4: 3) was found to have 20% of unbonded amide groups immediately after quenching. When the sample was kept at the glass transition temperature (20°C), no change in x‐ray diffraction was observed after the treatment. The free amide band in the infrared spectrum at 3450 cm‐1, however, was decreased in intensity by keeping the sample at the glass transition temperature. The transition peak height observed in a DSC curve also increased in the same experiment. Large glass transition peaks were found in DSC curves after annealing of the random copolyamides in the vicinity of the glass transition temperature. It is probable that the free amide groups in the amorphous chains were rearranged and formed new hydrogen bonds during the heat treatment at the glass transition temperature. Packing and restriction of the amorphous chains due to the increase in hydrogen bonding seemed to increase the height of the transition peak in a DSC curve. It is inferred from the above results that in the case of the random copolyamide, structures corresponding to a given enthalpy of the glassy state can be related to the number of hydrogen bonds.
Conformational and packing stability of crystalline polymers. II. Several isotactic helical polymers
Authors: Tadokoro Hiroyuki , Tai Kazuo , Yokoyama Masaaki , Kobayashi Masamichi 
Published: April 2003

AbstractThe most stable conformations of typical isotactic helical polymers, polypropylene (PP), poly‐4‐methyl‐1‐pentene (P4MP), poly‐3‐methyl‐1‐butene (P3MB), polyacetaldehyde (PAA), and poly(methyl methacrylate) (PMMA), were analyzed without fixing the fiber identity period by taking into account the intramolecular potential energy due to the internal rotation barriers, and van der Waals and electrostatic interactions. According to the results of the calculations, the number of chemical units per turn N in the stable conformations is 2.91 for PP (value determined by x‐ray diffraction: 3.0), 3.52 for P4MP (3.5), 4.17 for P3MB (4.0), and 3.94 for PAA (4.0). This indicates that the molecular conformations of these four polymers are governed primarily by intramolecular interactions, i.e., steric hindrance of side chains, and are additionally modified by the intermolecular interactions in the crystal. For PMMA, the lowest minimum in the potential map corresponds to N = 12, but the energy difference between the minima of N = 12 and N = 5 (corresponding to the actual one) is only 3 kcal/mole of monomer unit. The (5/1) helix of PMMA may be reasonably interpreted in terms of the intermolecular interactions mainly due to the polar and bulky side groups. The intermolecular interaction energy of isotactic PAA in the crystal lattice is also calculated and discussed.
Feasibility of determination of heterogeneity in copolymer composition by light scattering
Authors: Vorlíc̆ek Jir̆í , Kratochvíl Pavel 
Published: April 2003

AbstractThe sensitivity of the light‐scattering method for the determination of the composition heterogeneity of copolymers is discussed in terms of the accuracy of measurements of the intensity of scattered light. When the composition is independent of molecular weight, a nomographic solution has advantages. The nomograph is used to investigate the feasibility (a) of the determination of heterogeneity by a single measurement at zero refractive index increment of the copolymer and (b) of the determination of the heterogeneity from two measurements. The results show that in the case of low heterogeneities the light‐scattering method can be used only at higher molecular weights. By using the heterogeneities calculated elsewhere, the possibility of determining them for the individual types of copolymers is discussed. It appears that reported discrepancies between measured and calculated heterogeneities are mostly found in cases where the sensitivity of the light‐scattering method for the determination of the composition heterogeneity is low.
Effect of polymerization conditions on the morphology of as‐polymerized poly(vinylidene chloride)
Authors: Wessling R. A. , Oswald J. H. , Harrison I. R. 
Published: April 2003

AbstractPoly(vinylidene chloride) (PVDC) separates from the reaction mixture during the free‐radical polymerization of vinylidene chloride. The morphology of the “as‐polymerized” powder is determined by the degree of undercooling existing in the reaction medium at the time of crystallization. Crystallization occurs simultaneously in the process of polymerization and subsequently by the precipitation of polymer which formed in solution. As the medium becomes more favorable, an increasing portion of the polymer forms in solution and then crystallizes. The morphology developed by polymerization in borderline solvents such as dioxane can be nearly duplicated by recrystallization under similar conditions. The simultaneous occurrence of solution and solid phase polymerization leads to a double DTA endotherm. The upper peak at ca. 200°C represents the melting point of PVDC. The size and temperature of the lower peak vary with polymerization conditions. The results can be correlated with the degree of under‐cooling as estimated by the difference between the polymerization temperature and the temperature at which the crystalline polymer dissolves in the reaction mixture.
Crystal structure of an alternating copolymer of ethylene and tetrafluoroethylene
Authors: Wilson F. C. , Starkweather H. W. 
Published: April 2003

AbstractThe unit cell of an alternating copolymer of ethylene and tetrafluoroethylene was determined by x‐ray diffraction. In spite of uncertainties due to irregularities in the chain structure and a low level of crystallinity, a reasonable unit cell and structure was derived which gives a calculated crystalline density of 1.9 g/cm3. The unit cell is believed to be either orthorhombic or monoclinic with the following parameters: a = 9.6 Å, b = 9.25 Å, c = 5.0 Å, (γ = 96°). The molecular conformation is that of the extended zigzag, and the molecular packing appears to be orthorhombic, each molecule having four nearest neighbors with the CH2 groups of one chain adjacent to the CF2 groups of the next.
Effect of N‐substitution on the crystallinity of polylaurolactam
Authors: Shalaby S. W. , Fredericks Robert J. , Pearce Eli M. 
Published: April 2003

AbstractPoly(N‐ethyl laurolactam) and poly(N‐benzyl laurolactam) were prepared from the corresponding monomers by hydrolyic polymerization. Unlike the partially crystalline poly(N‐methyl laurolactam), these two homopolymers were completely amorphous by x‐ray diffraction. Diffraction patterns of copolymers of N‐ethyl laurolactam or N‐benzyl laurolactam with laurolactam were shown to be composition‐dependent. For N‐ethyl laurolactam copolymers, crystallinity developed with 20% laurolactam as a comonomer and increased steadily with a subsequent change in the x‐ray pattern, up to 50% laurolactam. Higher laurolactam percentages resulted in copolymers having a nylon 12 x‐ray pattern. N‐Benzyl laurolactam copolymers with 30% laurolactam showed only 6% crystallinity. The x‐ray patterns of N‐benzylated nylon 12 made with more than 50% laurolactam showed patterns similar to that of nylon 12. Differential scanning calorimetry data of all these polymers substantiate the x‐ray findings. The effect of type and concentration of the N‐substituent on the glass transition, melting, and crystallization temperatures of the polymers is discussed.
Texture of injection‐molded polypropylene
Authors: Mencik Z. , Fitchmun D. R. 
Published: April 2003

AbstractInjection‐molded specimens of polypropylene were examined by x‐ray diffraction for texture and orientation patterns as a function of the distance from the surface and of molding conditions. At least three distinct layers in the surface region whose texture patterns differ and an isotropic core of the moldings have been identified. These layers show preferred orientations; the axes of symmetry of the oriented textures lie in a plane passing through the direction of flow and perpendicular to the mold surface, but are inclined to the mold surface at a small angle. Some of these layers may be missing in conditions of either very high or very low shear during the injection‐molding flow. A tentative explanation of the formation of the texture patterns is proposed.
Small‐angle x‐ray scattering of semicrystalline polymers. II. Analysis of experimental scattering curves
Authors: Crist B. , Morosoff N. 
Published: April 2003

AbstractThe small‐angle x‐ray scattering (SAXS) patterns of a number of linear polyethylene (PE) and polyoxymethylene (POM) samples have been measured and compared to the intensity functions of one‐dimensional paracrystalline lattices. It was found that the ratio of the angular positions of the second and first scattering maxima (θ2/θ1) is generally less than or equal to 2.0, implying that the paracrystalline lattice statistics are symmetric or moderately skewed to larger periods. The Bragg spacing (“long period”) of such samples is within 3% of the identity period of the macrolattice. With quenched POM the ratio θ2/θ1 is substantially larger than 2.0, which indicates either extremely asymmetric lattice statistics or coexisting structures within the material. From consideration of the reduced widths of the first scattering maxima, it was found that some broadening is present in addition to that from the paracrystallinity. This excess broadening could result from a finite lattice length of ∼1000 Å. The need for careful experimental technique for obtaining the actual position of the scattering maximum is emphasized. In addition, it is demonstrated that the scattering curve and the correlation function of the system yield essentially the same apparent structural periods.
Crystallization during polymerization of poly‐p‐xylylene. II. Polymerization at low temperature
Authors: Treiber Gert , Boehlke Klaus , Weitz Alexander , Wunderlich Bernhard 
Published: April 2003

AbstractThe polymerization of p‐xylylene was followed with a newly designed differential thermal analysis system at temperatures between −196°C and −20°C. It was found that at the lower temperatures the monomer condenses first to the crystalline monomer before simultaneous polymerization and crystallization. At the higher temperatures, polymerization and crystallization are successive. The data are in agreement with the morphology and crystal structure data derived in Part I of this series of papers on crystallization during polymerization of poly‐p‐xylylene.
Relation between dielectric behavior and structure in some solid polypeptides
Authors: Tanaka Akira , Ishida Yōichi 
Published: April 2003

AbstractAn attempt to interpret the prominent dielectric relaxations of poly(γ‐benzyl L‐glutamate) and poly(γ‐methyl L‐glutamate) in terms of their structures was made by applying the barrier theory of Hoffman. Potential energy maps for the rotation of the polar side group, which are required in this application, were calculated by taking account of molecular environment of the polar side group. The dichroic ratios from infrared measurements were satisfactorily calculated, based on the maps. This provides evidence that the maps are reliable. In applying the barrier theory, it was modified by the assumption that the conformations of the side groups are distributed according to the Boltzmann law. On the basis of the maps, the magnitude of the dielectric absorption and the mean relaxation time were calculated in terms of the modified barrier theory; these were in fairly good agreement with the experimental data.
Molecular weight determinations by gel‐permeation chromatography and viscometry
Authors: Spatorico A. L. , Coulter B. 
Published: April 2003

AbstractThe hydrodynamic volume concept can be used effectively with gel‐permeation chromatographic (GPC) and viscosity data to estimate the molecular weight of a variety of polymers. Agreement is within ±5–10% of the absolute values and thus is satisfactory for many purposes. An iterative computer technique and a method developed by Funt and Hornof for analyzing GPC–viscosity data were found to be equivalent with respect to estimating the molecular weights for the five cases studied. The latter is easily employed but restricted to the case where the sample of interest and the GPC calibration standards have approximately equal Mark‐Houwink parameters. Since GPC measurements are commonly performed in thermodynamically good solvents, the general applicability of the method is not impaired. Using the unperturbed dimensions of the polymer chain to estimate the molecular weight of a variety of polymers was not as satisfactory as the above techniques. This approach generally gave biased molecular weight values (consistently low or consistently high). Agreement with the absolute values ranged from 10 to 30%. We therefore believe that either of the techniques based on the hydrodynamic volume concept can be used more effectively to estimate the molecular weight of a series of polymers than the treatment based on the unperturbed dimension.
Electron spin resonance studies of spin‐labeled polymers. IV. Slow‐motional effects in solid polystyrene
Authors: Bullock A. T. , Cameron G. G. , Smith P. M. 
Published: April 2003

AbstractShape changes in the electron spin resonance spectrum of spin‐labeled polystyrene have been studied as a function of temperature in the range 77–340°K. Rotational correlation times have been calculated by using recent theories of slow‐motional effects on ESR spectra. Three models were used, namely, Brownian, moderate jump diffusion, and large jump diffusion. The moderate jump model gave correlation times in good accord with those found by other techniques for the δ relaxation in polystyrene. The relaxation was found to be independent of molecular weight.
Calculations of the strength of the polyethylene molecule
Authors: Boudreaux D. S. 
Published: April 2003

AbstractA theoretical determination of the elastic modulus, the breaking strength, and breaking strain is made for a single infinitely long molecule of polyethylene (CH2)∞. These results are compared with those of a similar calculation on a molecule of polyethylene with a side chain attached. It is found that the presence of a side chain weakens the molecule by approximately 17%.
Relation between refractive index and density of polymer solutions
Authors: Looyenga H. 
Published: April 2003

AbstractThe relation between the fractive index n and the density ρ of a liquid mixture is formulated as

where wi and Rsi are the weight fraction and the specific refraction, respectively, of component i. The calculation of the specific refraction of straight‐chain polyethylene and polystyrene from data for pure compounds of low molecular weight is discussed. The result is applied to dilute solutions of polystyrene in toluene. The calculated values of (dp/dw)0 and (d2ρ/dw2)0 at three different temperatures are compared with measured values. The agreement is satisfactory.