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  • polyethylene  (2)
  • phase separation  (1)
  • random copolymers  (1)
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  • 1
    Electronic Resource
    Electronic Resource
    Concentration and chain-length dependence of thermodynamic interactions in polyethylene isotope blends (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 35 (1997), S. 2889-2899 
    Details
    ISSN: 0887-6266
    Keywords: polyethylene ; polyolefins ; blends ; thermodynamics ; neutron scattering ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Small-angle neutron scattering (SANS) measurements of interactions in polymer blends, χNS, generally depend on blend concentration Φ, even though χNS is evaluated with a model that assumes that the thermodynamic interaction parameter χFH = χNS is independent of Φ. Londono et al. have reported χNS to increase by ∼ 4× when Φ drops below 0.05 in polyethylene isotope blends. The relation between scattering and thermodynamics is addressed with incompressible Flory-Huggins theory wherein the nthermodynamic interaction parameter χ may vary with concentration Φ and degree of polymerization N; here χNS(Φ) ≠ χ(Φ). For polyethylene isotope and similar polyolefin blends, the strong upward curvature of χNS implies a modest (ca. 30%) increase of χ. Macroscopic phase behavior is unaffected because the shape of the binodal remains essentially unchanged. The Φ-dependence of χNS in turn depends on N, leading to the following empirical expression for the thermodynamic interaction parameter: χ(Φ, N) = β - (2γ′/NΦ1Φ2)(Φ1 ln Φ1 + Φ2 ln Φ2). For polyethylene isotope blends at 155°C, β = 2.85 × 10-4 and γ′ = 0.15. Simple Flory-Huggins behavior with χFH = β is recovered when N approaches infinity. The source of the Φ- and N-dependent second term is not known. © 1997 John Wiley & Sons, Inc. J Polym Phys 35: 2889-2899, 1997
    Additional Material: 7 Ill.
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  • 2
    Electronic Resource
    Electronic Resource
    Recent developments in phase separation of polyolefin melt blends (1997)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 35 (1997), S. 2329-2353 
    Details
    ISSN: 0887-6266
    Keywords: phase separation ; polyolefins ; neutron scattering ; random copolymers ; thermodynamics ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Saturated hydrocarbon polymers may be differentiated by the relative amount and placement of methylene, methyl, methine, and quaternary carbon moieties. While it has been known or suspected for some time that polyolefins of conventional molecular weight (Mw ≈ 100 kg/mol) with dissimilar chemical microstructures are most often immiscible in the liquid state, recent experiments with binary blends of model polyolefins have increased greatly our understanding of thermodynamic interactions between unlike chains. Model systems with methyl (-CH3) and ethyl (-C2H5) short-chain branches give results, expressed as the Flory-Huggins interaction parameter χ, that are nearly universal; repulsive interactions (χ 〉 0) are more pronounced at low temperatures, leading to liquid-liquid phase separation at an upper critical solution temperature. Phase behavior of more complex systems (with distributions of chain microstructures and/or molecular weight) is generally consistent with predictions from model systems. An interesting exception is from work at Bristol on blends of lightly branched ethylene - α-olefin copolymers with unbranched polyethylene as the minority species. Here the presence of two liquid phases is inferred under conditions not expected from model studies; effects of copolymer composition and molecular weight are also unusual. Recent theoretical work points to the importance of chain stiffness (established by short-chain branching) in determining the thermodynamics of model blends. Nonrandom mixing of chains with different stiffness gives rise to an enthalpic χ, which may be negative under certain conditions. Other limitations of the Flory-Huggins approach to describing blend energetics are considered. At present there is no theoretical basis for liquid-liquid phase separation reported by the Bristol group. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2329-2353, 1997
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Molecular orbital studies of polyethylene deformation (1996)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 34 (1996), S. 449-457 
    Details
    ISSN: 0887-6266
    Keywords: polyethylene ; elastic modulus ; molecular orbital ; ab initio ; semiempirical ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Young's modulus E for polyethylene in the chain direction is calculated with molecular orbital theory applied to n-alkanes C3H8 through n-C13H28 and analyzed with the cluster-difference method. Semiempirical CNDO, MNDO, and AM1 models and ab initio HF/STO-3G, HF/6-31G, HF/6-31G*, and MP2/6-31G* models are used. Cluster-difference results, when extrapolated to infinite chain length, give E in good agreement with moduli evaluated with molecular cluster or crystal orbital methods, provided minimal basis sets are employed. E decreases from 495 GPa (CNDO) to 336 GPa (MP2/6-31G*) as the level of theory is improved, consistent with established behaviors of the various models. Our calculations do not reproduce earlier molecular cluster or crystal orbital results, which gave E 〈 330 GPa. The most rigorous MP2/6-31G* model is known to overestimate force constants by ∼ 11%; the scaled modulus E = 299 GPa is in good accord with E = 306 GPa from recent calculations based on experimental vibration frequencies. © 1996 John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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