Relationship Between Structure and Rheology of Constrained Geometry Catalyzed and Metallocene Polyethylenes

Abstract

Constrained geometry catalyzed and metallocene polyethylenes, referred to as mPEs, have narrow molecular weight distributions (MWDs) with polydispersity indexes of approximately 2. In the case of ethylene α-olefin copolymers produced with these catalysts, the short chain branches (SCBs) are distributed randomly and uniformly along the backbone and homogeneously among the molecules. Within the general class of mPE, there are two subclasses: linear and branched mPEs. The linear mPEs, have either no branches or only short chain branches but no long chain branches (LCBs). The branched mPEs, which are produced by INSITE™ technology, have precisely controlled levels of LCB. These materials are referred to as linear to distinguish them from highly branched low-density polyethylene (LDPE). The LCB found in mPEs produced with constrained geometry catalysts is a very important molecular characteristic. One of the most reliable analytical methods for detecting and quantifying LCB is nuclear magnetic resonance (NMR). This chapter presents a thorough study of the effect of molecular structure on the rheology of mPEs with a focus on the effect of LCB. Because of the precise control of molecular structure that is possible with the constrained geometry catalysts, one is able to study several sets of materials that differ in only one molecular characteristic.