The fact that a polymer consists of a number of chains of different lengths, each in turn consisting of a series of monomer units, means that the motion of one part of the polymer chain will profoundly affect the motion of other parts. Hence, for a given polymer, a description of microscopic processes occurring under a given flow field depends on hypotheses regarding the molecular structure and mechanisms of flow in the polymer. Today, it is well-known, gained from practical experience, that the molecular weight, the molecular weight distribution, and the degree of long-chain branching influence the rheological properties of polymeric liquids. Therefore, a better understanding of the relationship between molecular parameters and rheological properties is very important from the standpoints of both polymer synthesis and polymer processing. However, the theoretical development of this aspect of the problem is far from complete, although some important progress has been made. In the preceding chapter, we discussed the viscoelastic behavior of polymeric liquids from the phenomenological point of view, without associating the significance of theoretical predictions to molecular origin(s). Specifically, we have seen that the rheological equations of state contain parameters that vary from polymer to polymer. Since it has amply been demonstrated by experiment that the extent of a particular viscoelastic behavior is greatly influenced by the molecular parameters, such as molecular weight, molecular weight distribution, and the degree of long-chain branching, predictions of any viscoelastic behavior of polymers on the basis of phenomenological theory is of very limited use to either control the quality of polymers produced or improve the performance of polymers, unless the parameters appearing in various continuum constitutive equations are related to molecular parameters.
Complex interplay of short- and long-chain branching on thermal and rheological properties of ethylene/α-olefin copolymers made by metallocene catalysts with oscillating ligand structure
