Stress growth and relaxation of a molten polyethylene in a modified weissenberg rheogoniometer

A Weissenberg rheogoniometer was modified 1‐3 to improve sample temperature uniformity and constancy (to within ±0.5°C) and to give a quicker response to normal thrust changes (estimated gap change ≤0.1 μm/kg thrust; gap angle = 8.046°; gap radius = 1.2 cm; servomechanism replaced by an open‐loop cantilever spring of 10 kg/μm stiffness). Low‐density polyethylenes (IUPAC samples A and C, melt index at 190°C = 1.6) at 150°C were used in step‐function shear rate experiments. Inspection of marked sectors in the samples showed substantial uniformity of shear at values of Ṡ = 0.1, 2, and 5 sec −1 ; for Ṡ = 10 sec −1 and S ≤ 2 shear units ( S = Ṡ t ), the shear was highly nonuniform at and near the free boundary. Using selected premolded samples A, scatter in seven replicate tests at Ṡ = 1.0 sec −1 did not exceed ±6% for N 1 ( t ) and ±5% for σ( t ) ( N 1 = primary normal stress difference; σ = shear stress; t = time of deformation from the initiation of experiment at zero time). N 1 ( t ) and σ( t ) data agreed with Meissner's 1 ; for Ṡ = 0.1, 2.0, 5.0, and 10.0 sec −1 , torque maxima occurred at S = 6 shear units, and thrust maxima occurred in the range of 10 to 20 shear units. σ( t ) and N 1 ( t ) data do not satisfy the van Es and Christensen 4 test for rubber‐like liquids with strain rate invariants included in the memory function. On cessation of shear (after a shear strain S at constant shear rate Ṡ), initial values of − d σ( t )/ dt and − dN 1 ( t )/ dt were found to depend strongly on S , in some cases passing through maxima as S was increased. After shearing at Ṡ = 0.1 sec −1 for 500 sec, such that stresses became constant, stress relaxation data satisfied Yamamoto's 5 equation of dN 1 ( t )/ dt = −2Ṡσ( t ).