Elongational Viscosity Measurements on Polymer Melts by a Meissner-type Rheometer

Abstract

The elongational flow behavior of various polymer melts has been measured using a new Meissner-type uniaxial extensional rheometer. The following results are obtained for polystyrene melts :(1) When the strain rate ε is sufficiently low, transient elongational viscosity function ηE+(t) agress with 3η+(t), where η+(t) is the corresponding function in shear. ηE+(t, ε) at high ε still agrees with 3η+(t) when the strain is smaller than a critical value. At large strains, ηE+(t, ε) becomes larger than 3η+(t) and approaches asymptotically to a steady value.(2) The steady value of ηE+ for a broad-distribution polystyrene is somewhat larger than 3η0, where η0, is the zero-shear viscosity. The steady elongational viscosity ηE(ε) of a narrow-distribution polystyrene is equal to 3η0, in the strain-rate region where the shear viscosity ε(γ) decreases with γ, while it begins to decrease with ε at high ε.(3) The nonlinear function ΛηNL(=ηE+(t, ε)/ηE+(t)) becomes more strongly dependent on strain ε(=εt) with increasing strain rate. For a low-density polyethylene melt, ηE+(t, ε)(or elongational stress σ) at low ε increases smoothly with time t (or ε), approaching asymptotically to a steady value. ηE+ (or σ) at high ε increases in two stages with t (or ε). Finally, it has been emphasized that the measurement of ε through observation of the sample diameter is very important, especially for polyolefin samples.