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  • 1
    ISSN: 1435-1528
    Keywords: Key words Mesophase pitches ; discotic mesophases ; rheological properties of mesophase pitches ; mesophase carbon fibers
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The rheological material functions predicted by a previously selected constitutive equation (CE) for discotic mesophases are presented. The predicted relations among rheological properties, shear-induced microstructure, processing conditions and material parameters of discotic mesophases are characterized and discussed. The first and second normal stress differences corresponding to planar (i.e., 2-D orientation) microstructure mode of discotic nematics are found to be qualitatively similar to those for rod-like nematics despite the existing differences in flow-orientation characteristics. The first (second) normal stress difference for discotic mesophases corresponding to non-planar (i.e., 3-D orientation) microstructure mode is always positive (positive or negative depending on viscous effects) and is found to be due to flow-induced biaxiality. The effect of change in nematic potential (or temperature) on rheological properties of discotic mesophases is also presented. The apparent shear viscosities for various microstructure modes and material properties are also presented and shown to agree qualitatively with the available experimental data. Though only restricted validation of the predicted results with the actual experimental data of discotics is possible, the present study provides essential theoretical feedback to the on-going experimental work being pursued in understanding the processing behavior of mesophase pitches.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Rheologica acta 37 (1998), S. 399-405 
    ISSN: 1435-1528
    Keywords: Key words Fourier-transform ; rheology ; non-linear properties ; higher harmonics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Oscillatory shear of polymeric liquids in the non-linear regime generates higher harmonic contributions in the shear stress response. These non-linear contributions are analyzed in Fourier space with respect to the different frequencies and intensities. Simulated and experimental Fourier rheology spectra for atactic poly(propylene) melts are shown.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Rheologica acta 37 (1998), S. 406-413 
    ISSN: 1435-1528
    Keywords: Key words Thermal diffusion ; thermal diffusion dyadic ; elastic dumbbells ; Rouse coils
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract In this paper diffusion of a dilute solution of elastic dumbbell model macromolecules under non-isothermal conditions is studied. Using the center of mass definition for the local polymer concentration, the diffusive flux contains a thermal diffusion dyadic d T .  To get some idea of thermal diffusion d T is evaluated for steady state isothermal conditions. Explicit results are presented for some homogeneous flows. It is shown that if the polymeric number density is defined via the beads (of the dumbbell) – termed n b – then the diffusive flux j contains , where τ c is the intramolecular contribution to the bulk stress. Though the form of the diffusion equation for n b thus differs from the corresponding one for n, it is shown that for essentially unbounded systems differences between n and n b are small. Since the results involve the translational diffusion coefficient they can readily be taken over for Rouse coils.
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  • 4
    ISSN: 1435-1528
    Keywords: Key words Actin ; rheology ; diffusing wave spectroscopy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract We monitor the time-dependent shear compliance of a solution of semi-flexible polymers, using diffusing wave spectroscopy (DWS) and video-enhanced single-particle-tracking (SPT) microrheology. These two techniques use the small thermally excited motion of probing microspheres to interrogate the local properties of polymer solutions. The solutions consist of networks of actin filaments which are long semi-flexible polymers. We establish a relationship between the mean square displacement (MSD) of microspheres imbedded in the solution and the time-dependent creep compliance of the solution, 〈Δr 2(t)〉=(k B T/πa)J(t). Here, J(t) is the creep compliance, 〈Δr 2(t)〉 is the mean-square displacement, and a is the radius of the microsphere chosen to be larger than the mesh size of the polymer network. DWS allows us to measure mean square displacements with microsecond temporal resolution and Ångström spatial resolution. At short times, the mean square displacement of a 0.96μm diameter sphere in a concentrated actin solution displays sub-diffusion. 〈Δr 2(t)〉∝t , with a characteristic exponent =0.78±0.05, which reflects the finite rigidity of actin. At long times, the MSD reaches a plateau, with a magnitude that decreases with concentration. The creep compliance is shown to be a weak function of polymer concentration and scales as J p ∝c –1.2±0.3. This exponent is correctly described by a recent model describing the viscoelasticity of semi-flexible polymer solutions. The DWS and video-enhanced SPT measurements of the compliance plateau agree quantitatively with compliance measured independently using classical mechanical rheometry for a viscous oil and for a solution of flexible polymers. This paper extends the use of DWS and single-particle-tracking to probe the local mechanical properties of polymer networks, shows for the first time the proportionality between mean square displacement and local creep compliance, and therefore presents a new, direct way to extract the viscoelastic properties of polymer systems and complex fluids.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1435-1528
    Keywords: Key words Interfacial stick-slip transition ; wall ship ; spurt flow
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract A systematic experimental investigation is carried out to clarify the nature of a well-known capillary flow phenomenon in linear monodisperse polybutadienes (PBd). By varying the surface condition and the die diameter, it is alluded that a spurt-like stick-slip transition actually results from a breakdown of chain entanglement between adsorbed and next-layer unbound chains. In other words, the transition is not a manifestation of any constitutive properties, as previously asserted by Vinogradov and coworkers (1984). The melt viscosity dependence of the transition amplitude agrees with a Navier-de Gennes type analysis of wall slip. A comparison between the capillary flow and dynamic shear behavior of the same monodisperse PBd reveals that the interfacial stick-slip transition occurs at a stress level that is only a third of the plateau stress given by the elastic plateau modulus G N 0=1.0 MPa at 40°C. The molecular weight independence of the critical stress for the transition provides a striking contrast with the transition characteristics observed in linear polyethylenes and suggests a different state of PBd chain adsorption on steel surfaces.
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  • 6
    ISSN: 1435-1528
    Keywords: Key words Interfacial flow hysteresis ; wall slip ; self oscillation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract When a stick-slip transition of an extremely large magnitude occurs in capillary flow of a highly entangled melt, a significant flow can be generated in the barrel. At a constant piston pressure, this barrel flow reduces the actual pressure available at the die entry that produces the capillary flow in the die. Experiments on linear monodisperse polybutadienes (PBd) show that a significant correction due to this barrel flow must be made to obtain a reliable characterization of the interfacial stick-slip transition. The interfacial flow discontinuity transition exhibits hysteresis behavior similar to that of linear polyethylenes. When a large pressure loss occurs in the barrel due to a massive stick-slip transition, the die wall stress may drop below the hysteresis loop, causing a new self-regulated flow oscillation to take place at a constant applied pressure.
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  • 7
    ISSN: 1435-1528
    Keywords: Key words Entry flow method ; extensional viscosity ; extensional strain ; polymer melts
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The extensional viscosity of some flexible chain polymers and a thermotropic liquid crystalline polymer was measured in uniaxial extensional flow at constant extension rate. Power law functions were found for the dependence of the extensional viscosity at constant accumulated strain on strain rate. The stress growth curves were compared with measurements in axisymmetric entry flow, where both elongation and shear occur. The comparison showed that the values of the extensional viscosity calculated from the measurements in the entry flow correspond to the ones calculated from the viscosity growth measured in uniaxial elongation and averaged over extensional strain equal to what is accumulated on the fluid as it flows from the barrel into the capillary.
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  • 8
    ISSN: 1435-1528
    Keywords: Key words Bagley correction ; end effects ; K-BKZ ; and PTT constitutive models
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The excess pressure losses due to end effects in the capillary flow of two linear low-density polyethylene resins (LLDPE) were studied. These losses were first determined experimentally by using two methods: 1) by extrapolating experimental data of pressure drop versus length-to-radius ratios (L/R) to zero capillary length and 2) by means of using orifice dies (L/R≅0). Both methods resulted in practically the same end corrections. Numerical simulation was also used to model this important aspect of experimental rheology. The constitutive equations used in the simulations are a multimode K-BKZ equation, a multimode Phan-Thien/Tanner, and finally a purely viscous Carreau equation. It was found that the numerical predictions agreed qualitatively but underestimated the experimental data for the various geometries used to determine the end effects. Furthermore, it is demonstrated that the entrance pressure loss is also insensitive to extensional rheology, while it depends more strongly on the shear rheology. This finding raises doubts as to the usefulness of end pressure (known also as Bagley correction) as a method of determining the extensional viscosity of polymer melts at high rates.
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  • 9
    ISSN: 1435-1528
    Keywords: Key words Linear polydimethylsiloxanes ; molecular weight distribution ; polymer relaxation mechanisms ; chain reptation with contour length fluctuations ; tube constraint release
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract Although it is known that chain reptation with contour length fluctuations and tube constraint release are the relevant mechanisms of chain relaxation, only applications of the theory to specific polymers could allow one to determine the relative importance of these relaxation modes. With this purpose we study the relation between the shear relaxation modulus of entangled, linear and flexible polydimethylsiloxane (PDMS) samples with narrow molecular weight distribution (MWD) and some of their blends. The linear PDMS are synthesized by anionic polymerization. Binary blends having different concentrations of the linear PDMS with narrow MWD are also prepared.  A theory developed by some of the authors has been proposed previously and is based on the double reptation mixing rule and a law for the relaxation time of the chains. Since, calculations in the theory must be carried out with rheometric data of linear viscoelasticity, the time-temperature superposition principle is used at a reference temperature of 20°C to obtain the and master curves. We conclude that the PDMS chains in a matrix with 1.5 relax predominantly by reptation while the tube renewal follows Rouse mode, which is proportional to chain molecular weight raised to the square power. It is also shown that dilution effects due to short chains start to be important in the binary blends of PDMS samples due to the presence of Zimm mode of relaxation, which is suggested by the theory for linear polymers of relatively high polydispersity.
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  • 10
    ISSN: 1435-1528
    Keywords: Key words Polymeric liquid crystals ; anisotropy ; fillers ; string phases ; negative normal stresses
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Abstract The effect of fillers on the flow curves of polymeric liquid crystals is investigated. Suspensions of polystyrene particles in liquid crystalline solutions of hydroxypropylcellulose (HPC) in water are used. By reducing the HPC concentration an isotropic solution can be prepared. It serves as a reference to isolate the effect of the isotropic/anisotropic structure of the suspending medium on the rheological behaviour. Suspensions in the isotropic solution behave as expected for filled viscoelastic matrices in general. In the anisotropic medium the shear rate rather than the shear stress seems to govern the changes in the relative viscosity. This behaviour is clearly different from isotropic viscoelastic media. The most dramatic effect however is that even small amounts of particles eliminate or drastically shift the region of negative normal stress differences. As far as the structure is concerned, microscopic observations show that particles align in anisotropic as well as in isotropic media. At rest or at relatively low shear stresses the liquid crystalline structure is, in the present case, hardly affected by the presence of the particles. If anything, it becomes more homogeneous.
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