A critical review of recent experimental studies on elongational flow behavior of polymer melts is presented. Discussion is limited only on the uniaxial elongational flow at a constant rate of strain and at a constant stress.
Three types of change in transient elongational viscosity μ(
t, ε) with time t have been observed for polystyrene (PS) melts, depending on their molecular weight distribution, where is the rate of strain. The first type is characterized by a smooth and slow growth in μ(
t, ε). Polystyrene melts with fairly narrow molecular weight distribution (
Mw/Mn=1.2-2.3) show this type of behavior. The second type shows a rapid growth in μ(
t, ε) which was obtained for PS melts with broader distribution (
Mw/Mn=3.0-4.7). A two-step growth in μ(
t, ε) at high rates of strain is characteristic to the third type, which was observed for a bimodal molecular weight distribution.
It is shown that two types of elongational flow behavior have been observed in high density polyethylene melts with broad molecular weight distribution. One is characterized by monotonic increase in μ(
t, ε) with t and by monotonic decrease in steady elongational viscosity μ(
ε) with ε. On the other hand, a two-step growth in μ(
t,ε) at high rates of strain and a maximum followed by a remarkable decrease in μ(
ε) are observed as the other type. The difference between these two types of behavior seems due to the molecular weight of high density polyethylene.
Two types of elongational flow behavior have been found for polypropylene melts depending on the molecular weight distribution. The characteristic behavior of samples with relatively narrow distribution is summarized by (i) monotonic increase in μ(
t,ε) with t toward μ(
ε), (ii) increase in μ(
ε) with a at high ε, and (iii) uniform elongation up to abrupt fracture. The characteristic features of broader distribution polypropylenes are (i) two-step rapid increase in μ(
t, ε) with t, (ii) maximum of μ(
t, ε) and decrease in the value with increasing ε, and (iii) necking just after the maximum of μ(
t, ε) followed by ductile failure.
For low density polyethylene melts, effects of branching on elongational flow behavior are not always clear due to the extremely broad molecular weight distribution. There are some measurements which suggest that chain branching contributes to increasing the value of steady elongational viscosity. The elongational flow measurements on model branched polymers with narrow distribution are required to give a quantitative conclusion for this problem. Elongational stress overshoot for a low density polyethylene melt is also discussed.
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