Polyolefin microstructural characterization techniques

The previous discussion demonstrates the relevance of accurate microstructural characterization to understand existing polyolefin resins and to develop new ones. This has been, indeed, a very fertile research area since the very beginning of olefin polymerization technology. 

The MWD is the most fundamental microstructural distribution of any polymer because it has such a large influence on the polymer s mechanical and rheological properties. GPC is the most widely used technique for MWD determination of polymers. Most commercial polyolefins are only soluble at temperatures above 120 C in chlorinated solvents such as trichlorobenzene (TCB) and orthodichlorobenzene (ODCB) and, therefore, require a high-temperature GPC for MWD analysis. 

If the polymer chains are linear, there is a direct relationship between molecular weight, volume in solution and elution time for a given polymer type. This relation is used to create a calibration curve relating elution time to molecular weight. In addition, the universal calibration curve can be used to extend this relation to linear polymers of aU types, provided that the relation between intrinsic viscosity and molecular weight of the polymer is known (using, for instance, the Mark-Houwink equation) or measured using an on-line viscometer (GPC/RI-VISC). 

TREF was developed before Crystaf. In TREF, a very dilute polymer solution (TCB is generally the solvent of choice) is transferred at high temperature to a column packed with an inert support. The polymer solution is then cooled very slowly, typically from 120-140°C to room temperature. As the temperature decreases, chains with higher crystallization temperatures crystallize and precipitate, followed by chains with lower crystallization temperatures. Crystallization is the most important step in TREF. A slow cooling rate (2.0-6.0°Ch is a recommended range) allows the polymer chains to crystallize near thermodynamic 

High-temperature GPC, TREF and Crystaf are used almost exclusively to analyze polyolefins. Other more general polymer analytical techniques are also commonly used for polyolefin analysis. Because they are less specific to polyolefins, they will be described only very briefly in the remaining part of this section. 

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Despite their versatility, both polyethylene and polypropylene are made only of carbon and hydrogen atoms. We are so used to these remarkable plastics that we do not often stop to ask how materials made out of such simple building blocks can have this extraordinary range of properties and applications. The answer to this question lies on how the carbon and hydrogen atoms are connected to define the molecular architecture or microstructure of polyolefins. Because microstructure plays such a relevant role in polyolefin properties, several characterization techniques have been specifically developed to measure different aspects of their molecular architectures. Section 2.1 classifies the different types of commercial polyolefins according to their microstructures, discusses several microstructural characterization techniques developed for these polymers, and demonstrates how they are essential to understand polyolefins. 

Figure 2.5 depicts the bivariate distribution of molecular weight and chemical composition of another LLDPE resin. This rather appealing tri-dimensional plot summarizes the complexity inherent to most commercial polyolefin resins. It also shows that microstruc-tural characterization techniques, such as the ones used to generate Figures 2.4 and 2.5, are indispensable tools to understand polyolefins. The most important techniques for polyolefin microstructural characterization will be reviewed later in this section. 

Cong et al (2012) Determination of the microstructure of polyolefins using thermal gradient interaction chromatography and its hyphenated techniques. In Proceedings 4th international conference on polyolefin characterization, Houston, October 2012... 

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