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  • 1
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 269 (1977), S. 396-397 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The secondary nucleation process controls the growth rate even after the primary crystal has grown macroscopi-cally large. Again the cause of this free energy increase is assumed to be the creation of new surfaces between the crystal and the liquid (or vapour). It has become traditional in such ...
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: A four-stage synthesis of molecular, micromechanical, and macromechanical models is used to predict the dependence of the longitudinal and transverse Young's moduli and the axial and transverse shear moduli of anisotropic polyethylene on percent crystallinity and the state of molecular orientation. Variational methods are employed to establish the upper and lower limits for anisotropic elastic response. The difference between lower and upper bound limits is interpreted as the potential for improving mechanical performance. A modified form of the Tsai-Halpin equation is used to examine parametric ranging (via a contiguity factor, ξ) between the lower and Tupper bound limits. In this application, the contiguity factor is interpreted as a characteristic of the internal stress-strain distribution which is dependent upon the size, shape, packing geometry, and elastic properties of the crystalline and amorphous regions. The potential for achieving high modulus polymeric materials is illustrated by treating percent crystallinity, molecular orientation, and contiguity as materials design variables subject to control by processing conditions. Optimum property trade-offs, necessary for balancing the over all mechanical behavior of anisotropic materials, are illustrated through the control of orientation and contiguity, The theoretical predictions for the moduli of anisotropic polyethylene are in good agreement with values reported for material processed by traditional procedures as well as ultra-oriented polyethylene.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 19 (1979), S. 45-50 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Statistical mechanical methods can be used to relate not only structure and thermodynamics but also structure and kinetics. Methods are developed for calculating the crystal growth rate kinetic models. When the usual macroscopic thermodynamics are applied, the resulting surface energies, σ and σe extracted from the experimental growth rates are quite comparable to those obtained from kinetic models. However, when one applies small system thermodynamics, a substantially smaller σ is obtained. This latter assumption has the advantage of explaining why extended chain seed crystals do not promote crystal growth and why folded chain crystals grow on such seeds. The statistical mechanical methods in both of these cases predict the observed decrease in the rate constant, Kg with decreasing molecular length without the necessity of a separate theory for varying σe with molecular length.
    Additional Material: 6 Ill.
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  • 4
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The methods of small systems thermodynamics have been applied to the calculation of the entropy barrier to the growth of polymer crystals. The height of this barrier at the equilibrium melting point of polyethylene molecules of typical lengths (X = 4,000 to 10,000 segments) is estimated to be of the order of 4 to 5 Kcal per molecule. This barrier increases linearly with ln X if the crystal is very large but retains the approximate values mentioned for crystals with the lamella thicknesses usually found. This entropy change causes the free energy to go through a maximum as in a nucleation phenomena but no new surface formation is involved. The existence of such a barrier explains why crystallization (or melting) never occurs at the equilibrium melting temperature.
    Additional Material: 3 Ill.
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  • 5
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The statistical mechanical methods developed in Parts I and II in this series permit the postulation of a thermodynamic criterion for the molecular fractionation which occurs during crystallization. Using this criterion we define a “local equilibrium” melting temperature as that temperature at which a polymer molecule (considered as a small thermodynamic system) has the same free energy when crystallized into the lowest possible free energy conformation on a given crystal surface (or surfaces) as it does in a completely molten state but still in contact with the same surfaces. This temperature will be a function not only of molecular length but also of the nature of the crystal surfaces to which it is exposed. Lowest “local equilibrium” melting temperatures occur on large flat crystal surfaces (secondary nucleation sites), higher temperatures result from the intersection of two crystal surfaces (tertiary nucleation sites). A number of such potential tertiary nucleation sites have been investigated and the resulting temperatures satisfactorily cover the range over which molecular fractionation has been observed.
    Additional Material: 5 Ill.
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  • 6
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 4 (1964), S. 157-164 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The relationship between crystallization and deformation is two-fold: (1) deformation or flow in polymers prior to crystallization has an important influence on the resultant crystalline morphology and (2) the morphology in turn largely determines the response of the structure to further deformation after crystallization. Thus, an understanding of deformation processes in crystalline polymers is intimately related to the crystallization processes and vice versa. The crystallization process may be approached by considering the observed morphological structures as being analogous to the usual crystal habits with the addition of various crystal-defects which add bending and twisting to the more common crystallographic variations. The crystal habit or morphology may be characterized by its surface to volume ratio. Crystallization of polymers may be considered as three steps (1) the formation of primary nuclei (2) crystallographic growth (also a nucleation controlled process) and (3) the introduction of crystal-defects which cause bending and twisting.
    Additional Material: 10 Ill.
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  • 7
    Electronic Resource
    Electronic Resource
    Stamford, Conn. [u.a.] : Wiley-Blackwell
    Polymer Engineering and Science 19 (1979), S. 386-390 
    ISSN: 0032-3888
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Polymer crystallization is recognized as a non-equilibrium process and the postulate of local equilibrium is combined with the formalism of small systems thermodynamics to provide a general method of relating structure, kinetics and thermodynamics. In this method a single polymer molecule is assumed to be in equilibrium with its environment and the appropriate partition function is factored into the bulk free energy and the remaining factors which are important only in small systems.
    Additional Material: 7 Ill.
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  • 8
    ISSN: 0022-3832
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: It has been possible to crystallize polyacrylonitrile from dilute solutions of propylene carbonate in the form of thin platelets. These platelets are ellipsoidal in shape and approximately 100 A. thick. Electron diffraction shows that the platelets are essential single crystals and the polymer axis is nearly perpendicular to the plane of the platelet. Five separate reflections were measured and indexed. The spacings calculated for these reflections agree with previously reported x-ray values obtained from drawn fiber. However, the present electron diffraction data require a different indexing of the reflections resulting in revised unit cell dimensions.
    Additional Material: 7 Ill.
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  • 9
    Electronic Resource
    Electronic Resource
    Hoboken, NJ : Wiley-Blackwell
    Journal of Polymer Science 57 (1962), S. 589-608 
    ISSN: 0022-3832
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: An investigation of the morphology of crystallization of fractionated linear polyethylene has shown that single crystal platelets may be obtained from dilute xylene solutions only in a limited region of temperature. The variation of this temperature region with molecular weight is greater than would be expected from the theoretical variation in equilibrium melting temperature. Uniformity of the crystallization conditions was sufficient to permit the measurement of size and thickness of the crystals as a function of both time and temperature. It was shown that the change in platelet thickness with temperature cannot be accounted from by the kinetics of nucleation with the assumption that the thickness is essentially equal to the critical nucleus dimension. The crystal growth rate data show that growth occurs by a nucleation process but the data are not compatible with the assumption that nucleation occurs through the formation of a homogeneous monomolecular layer on the growth surface. Evidence is presented to show that crystallization habit is dependent not only on the degree of supercooling but is also a function of some other factor that acts as a resistance or impedance to crystallization.
    Additional Material: 9 Ill.
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  • 10
    Electronic Resource
    Electronic Resource
    Springer
    Colloid & polymer science 231 (1969), S. 593-605 
    ISSN: 1435-1536
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Description / Table of Contents: Zusammenfassung Das Kristalldefektmodell polymerer Struktur wird kurz hinsichtlich der Evidenz des Auftretens, der Beweglichkeit und Erzeugung der Dislokationen dargestellt. Ausgehend von der Hypothese, daß vollkommen kristallisierbare Homopolymere kristallisieren, jeweils Molekül für Molekül, wird gezeigt, daß Kristallisation bei Temperaturen weit genug unter dem Schmelzpunkt zu kettengefalteten Kristallen führen sollte. Solche Kristalle repräsentieren metastabile Gleichgewichtszustände, die ihre Stabilität durch Ausscheiden oder Ausschließen von Molekülen niedrigen Molekulargewichts erhöhen können. Der Betrag der Molekulargewichtstrennung, der stattfindet, wird durch die Kinetik der Kristallisation bestimmt, und so kontrolliert die Kinetik, welche metastabilen Zustände, z. B. welche Faltungslänge bei einer gegebenen Zahl von Kristallisationsbedingungen auftritt. Wenn die Temperungstemperatur erhöht wird, steigt beides, die Entmischung und die Faltungsperiode. Wenn die Prozesse vollkommen ablaufen könnten, würde der Gleichgewichtskristall bei der Schmelztemperatur vollkommen entmischt sein und den vollkommen gestreckten molaren Kettenkristall bilden. In praktischen Fällen ist Entmischung unvollständig, so daß das teilweise geschmolzene oder gestörte Modell den Kristall repräsentiert, der im Gleichgewicht mit der Schmelze existiert.
    Notes: Summary The crystal-defect model of polymer structure is briefly reviewed along with the evidence for the existence, mobility and generation of dislocations. Starting with the hypothesis that a completely crystallizable homopolymer crystallizes one molecule at a time, it is shown that crystallization at temperatures well below the melting point should result in folded-chain crystals. Such crystals represent metastable equilibrium states which can increase their stability by rejecting or excluding the low molecular weight species. The amount of molecular species segregation which occurs is determined by the kinetics of crystallization and thus kinetics controls which metastable state (i. e. fold-period) occurs under a given set of crystallization conditions. As the annealing temperature is increased both molecular species segregation and molecular fold-period increase. If these processes could go to completion the equilibrium crystal at the melting temperature would be the completely segregated, completely extended-chain molecular crystal. In any practical case molecular species segregation is in complete so that the partial melted or “unpeeled” model represents the crystal which exists in equilibrium with the melt.
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