Extensional and Shear Rheology of Metallocene-Catalyzed Polyethylenes

Abstract

It is known that low-density polyethylene (LDPE), followed by high-density polyethylene (HDPE) and then linear low-density polyethylene (LLDPE), is the most stable in the film blowing process. The stability is related to the extensional viscosity exhibited in general by the three different types of polyethylenes (PEs). This chapter compares the extensional stress versus strain of a conventional LDPE (c-LDPE) to that of conventional LLDPE (c-LLDPE). The temperatures are selected so that the shear viscosity of c-LLDPE was similar to that of c-LDPE. There is a distinct difference in the growth of extensional stress for the two PEs. In particular, for c-LLDPE, the stress rises rapidly to steady state and tends to level off. The rapid rise to steady state is due to the short relaxation times, which are related to a somewhat narrow molecular weight density (MWD) for the c-LLDPE. On the other hand, the extensional stress of c-LDPE rises more slowly with strain (and hence, time) but continues to rise above that for c-LLDPE. This increase of extensional stress with increasing strain is referred to as strain-hardening. The transient response of the highly branched PE is related to a broad MWD, which provides longer relaxation times and the attempt of the fluid to rise to a higher steady state stress value, which is related to the presence of long chain branches. Hence, even though the shear viscosity of these two PEs is similar at these temperatures, the extensional viscosity of the branched PE is higher.