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  • 1990-1994  (5)
  • Polymer and Materials Science  (3)
  • Liquid crystal  (1)
  • critical gel  (1)
  • 1
    Electronic Resource
    Electronic Resource
    The effect of entanglements on the rheological behavior of polybutadiene critical gels (1994)
    Springer
    Rheologica acta 33 (1994), S. 220-237 
    Details
    ISSN: 1435-1528
    Keywords: Gelation ; critical gel ; entanglement ; relaxation time spectrum
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract We investigated the stress relaxation behavior of critical gels originating from six nearly monodisperse, highly entangled polybutadiene melts of different molecular weight from 18000 to 97 000 g/mole. The polymers were vulcanized by a hydrosilation reaction which takes place nearly exclusively at the pendant 1,2-vinyl sites distributed randomly along the polybutadiene chain. The BSW spectrum represents the relaxation of the initial uncrosslinked precursor. A characteristic parameter is the longest relaxation time of the precursor. Crosslinking increases this longest time even further. Surprisingly, the relaxation spectrum of the precursor is not altered much by the crosslinking except for an additional long time behavior. At the gel point (critical gel), this long time behavior is self-similar. It follows the typical power law as described by the Chambon-Winter gel equation, G(t) = St −n , in the terminal zone. The critical relaxation exponent was found to be close to n = 0.5 over a range of stoichiometric ratios and for all precursor molecular weights analyzed. A new scaling relationship was found between the gel stiffness, S, and the precursor molecular weight of the form: S ∼ M w zn , where exponent z from the zero shear viscosity-molecular weight relationship, η0 ∼ M w z , is commonly found to be z = 3.3 – 3.6.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00437307
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  • 2
    Electronic Resource
    Electronic Resource
    Director dynamics of uniformly aligned nematic liquid crystals in transient shear flow (1994)
    Springer
    Rheologica acta 33 (1994), S. 473-484 
    Details
    ISSN: 1435-1528
    Keywords: Liquid crystal ; monodomain ; conoscopy ; Leslie angle ; flow alignment ; director diffusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract We have developed a modular rheo-optical apparatus to study the flow properties of liquid crystals. Its main components are shearing device, strong magnetic field, and optical microscope. We performed experiments on well defined initial morphologies with uniform molecular alignment. The monodomains were achieved with strong magnetic fields (4.7T). Time-resolved conoscopy is the primary optical technique in our investigation. We propose a simple relation between the distribution of alignment angles over the sample thickness and the conoscopically measured angle, to quantitatively measure the alignment angle in shear flow. We followed the relaxation of a shear-induced splay deformation in small molecule model systems (N-(p-methoxybenzylidene) p-butylaniline (MBBA), pentyl-cyano-biphenyl (5 CB) and a commercially available mixture OM14244). We define a rotational director diffusivity $$D_R = \frac{{K_s }}{{\eta _s }}$$ (K s splay elastic constant. i7s splay viscosity) from the relaxation process and devised a model, based on the diffusion equation to determine their values. The director alignment behavior of the small molecule liquid crystals (SMLC's) in shear flow is well described by the two-dimensional Leslie-Ericksen model. The effect of director elasticity can clearly be seen in our experiments, resulting in a decrease of the steady state alignment angle at smaller Ericksen numbers. We found that there is no strain rate dependence of the director vorticity from 0.002/s to 2/s for poly-(γ-benzyl-D/L-glutamate) (PBG). We determined α2/α3 = −44 for a 2007o solution of 280000 molecular weight PBG in m-cresol at 20°C. The conoscopic interference pattern vanished after 8 strain units from an initially planar alignment and shearing could be reversed up to 10 strain units to completely recover the initial monodomain.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00366333
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  • 3
    Electronic Resource
    Electronic Resource
    Metallocene catalysts for olefin polymerizations. XXIV. Stereoblock propylene polymerization catalyzed by rac-[anti-ethylidene(1-η5-tetramethylcyclopentadienyl)(1-η5-indenyl)dimethyltitanium: A two-state propagationSee Reference 1. (1992)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part A: Polymer Chemistry 30 (1992), S. 2601-2617 
    Details
    ISSN: 0887-624X
    Keywords: Ziegler-Natta catalysts ; ansa-metallocene catalyst ; thermoplastic elastomeric poly(propylene) ; Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Racemic-anti-[ethylidene(1-η5-tetramethylcyclopentadienyl) (1-η5-indenyl)dimethyltitanium (6) has been synthesized and its molecular structure determined by x-ray diffraction methods. The two Ti=Me(1) and Ti=Me(2) units have bond distances differing by 0.08 Å and their proton NMR resonances are separated by over 1 ppm. Using this compound and methylaluminoxane (MAO) as the activator, at 25°C the 6/MAO catalyst produced polypropylene having crystalline domain with physical crosslinks. The polymers obtained at lower polymerization temperatures are rheologically liquids. The behaviors of this catalyst system resembles closely the previously reported rac-[anti-ethylidene(1-η5-tetramethylcyclopentadienyl) (1-η5-indenyl)dichlorotitanium (4)/MAO system. The structure of 6 determined here furnishes tangible support for the proposed two-state (isomeric)-switching propagation mechanism. Addition of MAO to 6 causes broadening of the Me(1) resonance in the 1H-NMR spectra, and 6 is decomposed by Ph3C+B(C6F5)-4. © 1992 John Wiley & Sons, Inc.
    Additional Material: 19 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/pola.1992.080301215
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  • 4
    Electronic Resource
    Electronic Resource
    Optical anisotropy of a thermotropic liquid-crystalline polymer in transient shear (1990)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 28 (1990), S. 1571-1588 
    Details
    ISSN: 0887-6266
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: A rheo-optical apparatus, based on a linear shear rheometer, has been constructed to study the deformation of liquid-crystalline polymers. This apparatus uses optical techniques such as flow birefringence, small-angle light scattering, and optical microscopic image analysis. The rheological responses were simultaneously measured under varying temperatures and deformation conditions. The modified Debye-Bueche equation for scattering, in the nonspherically symmetrical form, was adapted to analyze small-angle light-scattering data. The orientation correlation lengths, determined by this method, reveal the deformation mechanism in nematic melts. Flow birefringence results are in agreement with the proposed mechanism.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/polb.1990.090280912
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  • 5
    Electronic Resource
    Electronic Resource
    Synthesis and properties of an easy-processable bismaleimide thermoset resin (1992)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 45 (1992), S. 451-459 
    Details
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    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: Reaction of methylenedianiline and maleic anhydride in acetone, followed by cyclodehydration in the presence of acetic anhydride and 1,4-diazabicyclo [2.2.2] octane as a catalyst, affords a mixture of compounds, Desbimid, with maleimide, isomaleimide, and acetamide groups. Dissolution of this mixture in styrene and 2-hydroxyethyl methacrylate results in clear liquid resins. The viscosity of the formulated resins ranges from 100-1700 mPas at 25°C depending on the concentration of Desbimid. These systems can be processed and cured at ambient temperatures until demoulding and postcured at temperatures up to 200 or 250°C. The flexural modulus, flexural strength, and elongation at break of a number of cured formulations are found between 3500-3800 N/mm2, 90-115 N/mm2, and 2.7-3.5%, respectively.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/app.1992.070450310
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