Polyolefin Characterization by High Temperature Adsorption Liquid Chromatography: A Review of Solvent Gradient (SGIC) and Thermal Gradient Interaction Chromatography (TGIC)

Jun 06, 2014

This article provides an overview of solvent gradient interaction chromatography (SGIC) and thermal gradient interaction chromatography (TGIC) for polyolefin characterization.

Polyolefin resins account for more than half of the world's plastics demand and, in spite of their simple chemistry (carbon and hydrogen atoms), they can be made in a variety of complex microstructures that demand extensive characterization. Most polyolefins are copolymers of ethylene or propylene with the incorporation of other alpha olefin co-monomers that result in short chain branches, which may or may not be uniformly distributed between molecules. The analysis of the co-monomer distribution in the chain, also referred to as short chain branching (SCBD) or chemical composition distribution (CCD), is in many cases the most fundamental microstructure feature in the industrial resins.

The CCD is typically measured by established crystallization techniques: temperature rising elution fractionation (TREF),1 crystallization analysis fractionation (CRYSTAF),2 and crystallization elution fractionation (CEF).3 All of these techniques separate the polymer molecules according to crystallizability and provide a predictable separation of the polymer fractions depending on the presence of branches, irregularities, or tacticity differences. In recent years new copolymers of lower crystallinity have been developed, extending the polyolefin products into the elastomers region. Consequently, new characterization techniques also have to be developed to properly characterize those more amorphous resins. The most significant contribution has been the use of high temperature liquid chromatography in adsorption mode on graphitized carbon.4 The use of this adsorbent in the characterization of polyolefins has been intensively investigated in recent years by many researchers using a solvent gradient approach which has become known as solvent gradient interaction chromatography (SGIC), or a temperature gradient approach, known as thermal gradient interaction chromatography (TGIC).5

Solvent Gradient Interaction Chromatography (SGIC)

In the last decade the application of liquid chromatography (LC) to analyze polyolefins has been extensively investigated by Professor Pasch's group6 at the DKI (now Fraunhofer Institute) in Germany. High temperature is needed for the dissolution of the resin and the non-functionality of the polymer molecules make it difficult to use LC in interaction mode for polyolefin analysis.

Figure 1: Analysis of different ethylene copolymers by SGIC on a graphitized carbon column. Adapted and reprinted with permission from Analytical and Bioanalytical Chemistry 399(4), T. Macko, R. Brüli, R.G. Alamo, F.J. Stadler, and S. Losi, Separation of short-chain branched polyolefins by high-temperature gradient adsorption liquid chromatography, 1547–1556. © 2011 Springer (7).
An important turning point came with the finding of a graphitized carbon column by Macko and Pasch4 that could perform the separation of polyethylene and could handle the various polypropylene tacticity configurations using a gradient of decanol-trichlorobenzene in a very short analysis time. The most significant work came later, with the separation of ethylene-copolymers by the level of co-monomer incorporation,7 again using a graphitized carbon column, and resulting in a linear calibration as shown in Figure 1. Similar results were obtained by Miller et al.8 using the same carbon column. The presence of branches in the ethylene-copolymers reduced their surface area of interaction with graphite, and a linear correlation was obtained between the co-monomer mole percentage incorporated and the elution volume. These were similar results to that obtained by the crystallization techniques, but were capable of extending the analysis to the elastomers region.

Solvent gradient interaction chromatography (SGIC) can be used to analyze copolymers from 0% to 100% of co-monomer incorporation, which is not possible with crystallization techniques. SGIC has been used in the separation of ethylene-propylene copolymers9 and the analysis of ethylene propylene diene monomer (EPDM) resins.10 A drawback of the technique is the current lack of appropriate detectors.

To overcome detector limitations, the combination of SGIC with gel permeation/size-exclusion chromatography (GPC/SEC) in a second dimension (SGIC 2D) was proposed by Roy et al.11 using a gradient of decanol or ethylene-glycol mono-butyl-ether and trichlorobenzene (TCB) on a graphitized carbon column. A similar approach was also followed by Ginsburg et al.12 The GPC/SEC second dimension used infrared (IR) detection and, besides the convenience, the molar mass composition interdependence could also be analyzed.11

SGIC 2D has been utilized for the characterization of ethylene-propylene copolymers, EPDM resins,13 high impact polypropylene,14 and ethylene octene copolymers, alongside the addition of light scattering (LS) and viscosity detectors.15