Rheology of Metallocene-Catalyzed Polyethylenes. The Effects of Branching

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). In particular, LDPE (which is highly branched) exhibits a significant degree of extensional strain-hardening, whereas LLDPE exhibits none and HDPE can exhibit some as a result of a broad molecular weight distribution (MWD). LDPE also exhibits an onset of shear-thinning at very low shear rates, usually less than 0.1 s‑1, whereas LLDPE does not shear-thin readily. With the advent of metallocene-catalyzed polyethylenes (m-PEs), some of the problems associated with LLDPE—such as processing stability and the lack of shear thinning—readily arise as a result of the narrow MWD possible with these systems. To improve on the processing performance of m-PEs, researchers have indicated that branching can be incorporated into m-PEs by forming copolymers containing an alpha-olefin such as octene. However, it is not clear what the effect of branching is on the flow behavior of these systems. There is some indication that the addition of a few long chain branches can promote shear-thinning behavior. Furthermore, it is extremely difficult to identify branching analytically without assuming that it is present. This chapter presents studies concerned with the extensional and shear viscosity behavior, supercooling effect, and other stress–strain relationships of the three PEs which have apparently different degrees of long chain branching but similar melt flow indexes.