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  • 1
    Electronic Resource
    Electronic Resource
    The relaxation of polymers with linear flexible chains of uniform length (1990)
    Springer
    Rheologica acta 29 (1990), S. 400-408 
    Details
    ISSN: 1435-1528
    Keywords: Relaxation spectrum ; monodisperse polymers ; scaling ; glass transition ; plateau modulus ; recoverable compliance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The analysis of dynamic mechanical data indicates that linear flexible polymer chains of uniform length follow a scaling relation during their relaxation, having a linear viscoelastic relaxation spectrum of the formH(λ) = n 1 G N 0 × (λ/λ max) n1 forλ≤λ max. Data are well represented with a scaling exponent of about 0.22 for polystyrene and 0.42 for polybutadiene. The plateau modulusG N 0 is a material-specific constant and the longest relaxation time depends on the molecular weight in the expected way. At high frequencies, the scaling behavior is masked by the transition to the glassy response. Surprisingly, this transition seems to follow a Chambon-Winter spectrumH(λ) = Cλ−n2, which was previously adopted for describing other liquid/solid transitions. The analysis shows that the Rouse spectrum is most suitable for low molecular-weight polymersM ≈ M c , and that the de Gennes-Doi-Edwards spectrum clearly predicts terminal relaxation, but deviates from the observed behavior in the plateau region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01376790
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Rotation of liquid crystalline macromolecules in shear flow and shear-induced periodic orientation patterns (1996)
    Springer
    Rheologica acta 35 (1996), S. 160-167 
    Details
    ISSN: 1435-1528
    Keywords: Rheology ; liquid crystal ; monodomain ; instability ; conoscopy ; shear
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract We have measured the shear-induced rotation of the nematic director in a liquid crystalline polymer using poly benzyl glutamate (PBG) as model system. PBG is a well characterized synthetic poly (α amino acid) with rigid chain architecture and well defined conformations. For the experiments it is important to start out with a sample in which the molecules are highly aligned with a uniform director. This so-called monodomain morphology is obtained by use of strong magnetic fields and surface modifications of the sample holders. When shearing the monodomain at a constant rate, the macromolecules rotate initially homogeneously until a periodic director pattern develops. These spatially periodic structures emerge in a narrow range of shear strain and, as shearing continues, disintegrate into a chaotic texture. By varying the initial monodomain director with respect to the flow direction (but within the shear planes) we could show that the periodic patterns do not depend on the shear direction; they are governed by the director of the initial monodomain. We observe conoscopically that at high shear rates the texture becomes uniformly aligned. The molecules are aligned preferentially with an angle of about 4° to the shear direction (against vorticity direction). Interestingly, this agrees very well with predictions made by Larson (1990).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00396043
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    Paper (German National Licenses)
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  • 3
    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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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Mapping of the relaxation patterns of polymer melts with linear flexible molecules of uniform length (1997)
    Springer
    Rheologica acta 36 (1997), S. 330-344 
    Details
    ISSN: 1435-1528
    Keywords: Relaxation time spectrum ; monodisperse polymers ; hypergeometric function ; incomplete gamma function ; linear viscoelasticity ; polymer dynamics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The linear viscoelastic material functions of linear flexible polymers of uniform length are calculated from the BSW spectrum (Baumgaertel et al., 1990, 1992), and explicit analytic expressions are presented for several of the most common material functions for transient and dynamic experiments. However, numerical calculations are presented whenever needed. The BSW spectrum was determined from experimental G″, G″ data of two sets of molten polymers of narrow molecular weight distribution, polystyrene and polybutadiene. The purpose of the mapping is to show a wide range of viscoelastic behavior which otherwise is not available in such comprehensive form. Experimental check of these predictions is still needed in most cases. Also, some insight into the predictions for the non-linear (including the non-equilibrium) viscoelastic behavior is achieved by studying two particular experiments: the start-up of uniaxial extension at constant rate and the start-up of shear flow at constant rate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00366674
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    Paper (German National Licenses)
    Fulltext
  • 5
    Electronic Resource
    Electronic Resource
    Mapping of the relaxation patterns of polymer melts with linear flexible molecules of uniform length (1997)
    Springer
    Rheologica acta 36 (1997), S. 330-344 
    Details
    ISSN: 1435-1528
    Keywords: Key words Relaxation time ; spectrum ; monodisperse polymers ; hypergeometric function ; incomplete gamma function ; linear viscoelasticity ; polymer dynamics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The linear viscoelastic material functions of linear flexible polymers of uniform length are calculated from the BSW spectrum (Baumgaertel et al., 1990, 1992), and explicit analytic expressions are presented for several of the most common material functions for transient and dynamic experiments. However, numerical calculations are presented whenever needed. The BSW spectrum was determined from experimental G′,G″′ data of two sets of molten polymers of narrow molecular weight distribution, polystyrene and polybutadiene. The purpose of the mapping is to show a wide range of viscoelastic behavior which otherwise is not available in such comprehensive form. Experimental check of these predictions is still needed in most cases. Also, some insight into the predictions for the non-linear (including the non-equilibrium) viscoelastic behavior is achieved by studying two particular experiments: the start-up of uniaxial extension at constant rate and the start-up of shear flow at constant rate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00366674
    Library Location Call Number Volume/Issue/Year Availability
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    Paper (German National Licenses)
    Fulltext
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