Search

Characterizing elongational behavior of polymer melts in constant elongation rate or constant tensile stress experiments is hampered by the onset of 'necking' instabilities according to the Considère criterion: For an elastic filament, homogeneous uniaxial extension is not guaranteed for strains larger than the strain at which a maximum occurs in the force versus extension curve. Although simulations and experiments seem to indicate that in viscoelastic fluids viscosity effects delay the onset of necking to higher strains, integral measurements of elongational viscosities in Meissner- or Münstedt-type elongational rheometers are often affected by inhomogeneous deformations. A simple means of avoiding the consequences of the Considère criterion consists in operating at constant tensile force, but little attention has been paid so far to constant force elongational rheometry since the pioneering work of Raible et al. [J. Non-Newtonian Fluid Mech. 11, 239 (1982)]. Constant force elongation is also of great industrial relevance, since it is the correct analog of steady fiber spinning, as, e.g., exemplified by the so-called Rheotens test. We present experimental data for two long-chain branched polyethylene melts obtained in constant elongation rate and constant tensile force modes by use of a Sentmanat extensional rheometer in combination with an Anton Paar MCR301 rotational rheometer. The accessible experimental window and experimental limitations are discussed. Constant force elongation can be subdivided into three distinct deformation regimes: At small deformations (regime 1), constant force elongation is equivalent to creep at constant tensile stress. This is followed by regime 2, which is characterized by a steep increase in tensile stress at roughly constant strain rate, while regime 3 corresponds to elongation of a viscous power-law fluid. All three regimes can be modelled quantitatively by a single integral constitutive equation, and a constitutive analysis reveals substantial underestimation of the effective strain-hardening effect observed in constant strain-rate elongation in comparison to constant tensile force extension, which may be indicative of the effect of necking under constant elongation-rate conditions. [ABSTRACT FROM AUTHOR]

Two-step reversing flows are severe tests of constitutive equations for polymer melts and have attracted interest for testing the response of branched polymers. Here we report on single-step and three types (types B, C, and D) of reversing torsional flows on branched and linear polyethylenes (PEs). The responses were analyzed using Kaye–Bernstein–Kearsley–Zapas (K-BKZ) theory. Comparisons between data and theory in type B and C flows for the two branched PEs (LD 103 and LD 146) and a linear HDPE are presented. For the LD 146 material and to strains higher than previously examined, the results agree with prior studies, i.e., the K-BKZ theory provides a good description of the response of branched polymers in reversing flows while the opposite is the case for the linear polymers. Surprisingly, it is found that the other branched polymer (LD 103) behaves more like a linear polymer and not a branched polymer in that it does not follow the K-BKZ theory. In the type D flow, one which was not previously investigated, the K-BKZ theory describes the normal force (NF) responses for branched LD 146 but not for the linear PE. Also, isochronal derivatives of the strain potential function (W1 and W2) were calculated from torque (T) and NF responses in single-step tests. The damping function fits to a modified sigmoidal formula and is close to the BKZ limit as reported by Wagner (2004a) for branched polystyrenes. The normal stress difference ratio and damping function were extracted from W1 and W2. Finally, the Cox–Merz rule was examined for one of the branched PEs. [ABSTRACT FROM AUTHOR]