Comparison of the rheology of polymer melts in shear, and biaxial and uniaxial extensions

Abstract

Abstract:The experimental properties of different polymer melts, polystyrene, high density polyethylene and low density polyethylene are compared for the first time in three different deformations: step shear, step biaxial extension and steady uniaxial extension. Properties of three other melts are also studied in step biaxial and shear experiments. For our comparative purposes some data of Laun and Winter from the literature are used, as well as new data reported here. In all the step strain experiments, the stresses can be factored into a time dependent relaxation modulus and a strain dependent damping function. The data are interpreted using a differential constitutive equation of Larson which satisfies this time-strain separability and has a single parameter that describes the strain softening character of the material. Results show that differences in the properties of the melts are most pronounced in uniaxial extension and least in biaxial extension. All melts follow the Doi-Edwards prediction relatively closely in biaxial extension. In uniaxial extension, the branched material shows a strong strain hardening effect although its shear and biaxial properties are similar to the other melts. The constitutive model gives a reasonably good fit to the data in all three deformations for unbranched materials for the same value of the adjustable parameter; the model, however, fails for the branched low density polyethylene.

A differential constitutive equation for polymer melts

“…The Giesekus equation is known to predict, both qualitatively [2] and quantitatively [5,7], material functions for steady and non-steady shear and elongational flows. However, the equation does suffer from two drawbacks: it predicts that the viscosity is inversely proportional to the shear rate in the limit of infinite shear rate (i.e., the shear stress is independent of shear rate at large shear rates), and it is unable to predict any decrease in the elongational viscosity with increasing elongation rate in uniaxial elongational flow.…”

Simple constitutive modelling of nonlinear viscoelasticity under general extension

“…This general rule also applies here with few exceptions, as well as for two additional polymers from Ref. [17], PS.2 for which n b = 1.02 and LLDPE.2 for which n b = 0.96. Increased MW polydispersity may explain the lesser n value for PS.606 in comparison to PS.50124; nevertheless, despite its narrower MWD, HDPE.S shows less stress damping than HDPE.II and it may therefore be suspect for the presence of some long chain branching.…”

Damping Behavior of Highly Entangled High Density Polyethylene after Uniaxial Step Strains

“…[1][2][3][4][5][6][7][8] The experimental results, which are mostly obtained in the deformation mode of shear, have shown that at long times the relaxation modulus G(t, g) at time t and strain g can be written in a factorable form as [1][2][3][4][5][6][7][8] (1) …”