MEASUREMENT OF BIAXIAL AND UNIAXIAL EXTENSIONAL FLOW BEHAVIOR OF POLYMER MELTS AT CONSTANT STRAIN RATES

Biaxial extensional flow behavior of polystyrene and polypropylene melts was measured utilizing the lubricated squeezing flow method under the condition of constant strain rate. The uniaxial extensional flow behavior of these samples was also measured using a Meissner-type rheometer for comparison with the biaxial behavior. The biaxial stress growth coefficient eta(B+) grows more slowly than the uniaxial stress growth coefficient eta(E+) at long times. Deviation of eta(B+) from the low strain rate asymptote is not so prominent as that of eta(B+). The deviation of eta(B+) and the upturn of eta(E+) from the low strain rate asymptotes are more apparent in broad distribution samples than in a narrow distribution sample. It is found that eta(B+) decreases with strain rate at long times near steady state. The upturn behavior of eta(E+) at high strain rates in a narrow distribution sample is discussed based on the Doi-Edwards theory and stress relaxation data. The upturn is clearly observed when the strain rate exceeds the contraction (retraction) rate tau(eq)-1, where the contraction time tau(eq) is evaluated from the relation data. Consideration on chain stretching in various multi-axial extensional flows gives a prediction for deviation of stress growth coefficients from the low strain rate asymptotes in uniaxial, biaxial, planar, and ellipsoidal extensional flows.