The Importance of Proper Microstructural Determination and Control in Polyolefins

The Importance of Proper Microstructural Determination and Control in Polyolefins 

Since polyolefins are composed of only carbon and hydrogen atoms, the way these atoms are connected - in other words, the microstructure of a polyolefin -determines their properties. At the most elementary level, the microstructure of a polyolefin is defined by its distributions of molecular weight, chemical composition, long-chain branching, and stereoregularity. Therefore, determining the microstructure of polyolefins can be considered the first and most important step toward the understanding of their properties. 

High-temperature gel permeation chromatography (GPC) is the standard technique for measuring the molecular weight distribution of polyolefins. Since gel permeation chromatography is a very well-established technique it will not be discussed here. Several publications provide additional discussions on the GPC technique [7-9]. 

The chemical composition distribution of polyolefins is measured (indirectly) by either temperature rising elution fractionation (Tref) or crystallization analysis fractionation (Crystaf). These two techniques provide similar information on the chemical composition distribution of polyolefins and can be used interchangeably in the vast majority of cases. Both methods are based on the fact that the crys-tallizability of HOPE and LLDPE depends strongly on the fraction of a-olefin comonomer incorporated into the polymer chains, that is, chains with an increased a-olefin fraction have a decreased ciystallizability. A similar statement can be made for polypropylene and other polyolefin resins that are made with prochiral monomers resins with high stereoregularity and regioregularity have higher crystalliz-abilities than atactic resins. 

In Crystaf, polyolefin chains are crystallized from a dilute solution by slowly lowering the solution temperature. Chains with fewer a-olefin units crystallize at higher temperatures, while chains with a higher a-olefin fraction crystallize at lower temperatures. This information is used to generate a Crystaf profile relating crystallization temperatures to the fraction of polymer that crystallizes at those 

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