Polyolefins structural changes

This chapter looks at the key structural changes in polyolefins, how the industry could make best use of attractive feedstock in the Middle East, and the implications for companies across the world (see also Chapter 6). 

The LCB structure of polyolefins obtainable with a single metallocene catalyst can be altered in several ways. For instance, two or more metallocenes can be used to control, simultaneously, the MWD and LCB of polyolefins [48,49]. If an even more drastic micro-structural change is required, one that would make the LCB structure of polyolefins made with coordination catalysts resemble that of LDPE, dienes can be copolymerized with ethylene and a-olefins [50]. The cited references provide some additional information on this subject. 

FIGURE 19.5 Orthorhombic crystal of PE (A) 2-D model and (B) 3-D model (adapted from White JL and Shan H. Deformation Induces Structural Changes in Crystalline Polyolefins Polymer-Plastics Technology and Engineering 2006 45 317-328). Image courtesy of Eric Wysocki, Exponent, Inc. 

In the debate about existence of pre-ordered states in the polymer melt, as advocated recently, polyolefins with chiral side chains may well become a major investigation tool. Indeed, the macromolecular amplification induces a pre-organization, or at least a preferred helical conformation in the polymer melt or solution. As such, these polymers display very precisely the behavior that is assumed by some of the recent crystallization schemes or scenarios. Furthermore, for the P4MH1 systems considered so far at least, the confor-mationally racemic character of the stable crystal structure implies that half of the stems must change their helical hands at some stage in the crystallization process - which may greatly delay the formation of this stable crystal structure, as illustrated by P(S)4MH1. 

A surface analysis technique that has the potential to detect structural chemical changes in polymer surfaces, including low-molecular weight material formation, is static SIMS. Its capabilities for characterizing polymers by virtue of their fingerprint spectrum nave been amply demonstrated in recent years (5 6). The technique is more surface sensitive than XPS and can detect structural differences, even in hydrocarbons (7). It is, therefore, highly complementary to XPS. Nevertheless, only very few applications to the study of modified polymer surfaces have been published. Among these are reports on SIMS analysis of flame-treated polypropylene and plasma-fluorinated polyolefin surfaces (8 9). 

Metallocenes are very versatile catalysts for the production of polyolefins, polystyrene and copolymers. Some polymers such as syndiotaetic polypropene, syndiotactic polystyrene, cycloolefin copolymers, optically active oligomers, and polymethylenecycloalkenes can be produced only by metallocene catalysts. It is possible to tailor the microstructure of polymers by changing the ligand structure of the metallocene. The effect and influence of the ligands can more and more be predicted and understood by molecular modeling and other calculations. 

Ductile with flow. These materials show still greater deformability than the typical ductile materials. Initially, the stress-strain dependence resembles that described for ductile resin, but before the rupture there is a zone of deformation where the stress remains about constant. Within this zone there is flow of material that usually leads to molecular alignment and/or to changes to the crystalline structure (viz. deformation of polyolefins). 

One way to achieve compatibilization involves physical processes such as shear mixing and thermal history, which modify domain size and shape. The second way is the use of physical additives to increase attraction between molecules and phases. The third method is reactive processing, which is used to change the chemical structure of one or more of the components in the blend and thus increase their attraction to each other. Table 1.5 contains a list of compatibilizers used in the formulation of polyolefin blends. As can be seen from Table 1.5, most of the compatibilizers used in the formulation of polyolefin blends contain compounds such as maleic anhydride, acrylic and methacrylic acid, glycidyl methacrylate, and diblock and triblock copolymers involving styrene, ethylene, and butadiene. 

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