Extensional viscosity from entrance pressure drop measurements

Extensional rheological properties are important in characterization and processing of polymeric liquids. The use of entrance pressure drop to obtain extensional viscosity is particularly attractive because it can be applied to both low and high viscosity liquids using the Bagley correction obtained from a conventional capillary rheometer. Low density polyethylene of three different melt index values, including IUPAC-X (a different batch of IUPAC-A), and a high density polyethylene were tested using a commercial capillary rheometer. The entrance pressure drop (ΛP en ) was obtained with a “zero-length” orifice die with an abrupt contraction. The contraction ratio was 12:1. Predictions from several approximate analyses to calculate the uniaxial extensional viscosity ηu (using an axisymmetric contraction) from ΔP en were compared. These comparisons are summarized in the appendices. Due to the transient nature of contraction flows, η u is also a function of the strain (ɛ). This was examined by comparing η u from ΔP en (Cogswell's analysis was chosen for convenience) with transient extensional viscosity (η u +) at different magnitudes of ɛ from fiber-windup technique (Padmanabhan et al., 1996). η + at ɛ≈ 3 was found to be close to η u from ΔP en (using Cogswell's analysis) for two LDPE samples that had fiber-windup data available. The magnitude of the strain in the contraction did not vary with strain rate.