Poster: Chemical Composition Distribution in Polyolefins, a key microstructural property

By Alberto Ortín, Tonica González, Benjamín Monrabal. Polymer Char, Valencia, Spain.

Chemical composition techniques can reveal critical information on the microstructure of polyolefins beyond their molar mass distribution (MMD).

Abstract:

Chemical composition techniques can reveal critical information on the microstructure of polyolefins beyond their molar mass distribution (MMD). The most discriminating structural property in some polyolefin materials is missing in a Gel Permeation Chromatography (GPC) analysis, even if using multiple detection methods. Resins that seem nearly identical by a GPC analysis can only be fully differentiated when adding short chain branching information.

Chemical composition distribution (CCD) is a critical dimension for the study of the microstructure of polyolefins, but it is often overlooked because its impact is not widely known. Short chain branching distribution determines the crystallinity and morphology of the materials and therefore on many of their macroscopic (mechanical, optical ,…) properties. Here we present several method for obtaining information on chemical composition distributions in polyolefins and some selected applications. Two basic approaches for analyzing the CCD can be used: high-temperature GPC-IR, and the most powerful techniques based on crystallization or adsorption chromatography: TREF, CRYSTAF, CEF, and TGIC.

The first approach, GPC-IR, has  proven to be robust and straightforward with modern filter-based IR detectors and flow tough heated cell incorporated as the main detector in HT-GPC instruments. It provides direct access to data relating to the compositional distribution as a function of molecular weight. This information is vital to understanding the attributes and performance of the polymer sample for several classes of polyolefins, such as bimodal HDPE and ethylene-propylene copolymers.

However, most comprehensive information on the CCD is gathered after a separation process according to chemical composition, short chain branching or comonomer content. Separation by composition by CEF or other CCD techniques allows to differentiate for instance PP from PE based on their respective elution temperatures, which is not the case in a separation by GPC, and of course discriminate homogenous copolymers form heterogeneous copolymers made by multiple reactor processes pr using multisite catalysts. The online detection of methyl group frequency by IR complements the CCD separation identifying unequivocally each separated component, which is specially interesting in the case of heterogeneous high impact ethylene propylene copolymers.

Recycled materials are blends and more than one type of polyolefin is found. In that sense the chemical composition distribution tends to be more useful to identify and quantify those components, for instance a small fraction of PP in a PCR PE sample or vice-versa (REF Rafa). Moreover, the CCD can reveal the complexity of a PCR material which may be taken as a simple random copolymer by GPC analysis even if using IR detection.