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Improving polyolefin analysis by gel permeation chromatograpy

30th May 2014

Posted By Paul Boughton


The GPC-IR is setting new standards for gel permeation chromatography
Sequential injections of SRM 1475 linear polyethylene (x2) and SRM 1476 branched polyethylene (x2) at 150ºC in 1,2,4-trichlorobenzene, injection volume 200µL, concentration 0.9mg/ml. A low molar mass hydrocarbon is added to every injection as flow rate marker
Analysis of two ethylene-propylene copolymers with similar MMD but distinct ethylene distributions, as seen with IR detection
Effect of degradation of higher molar mass chains due to the excessive exposure to high temperature, and benefit of vials nitrogen purge. Polypropylene in 1,2,4-Trichlorobenzene

How to set a higher standard in gel permeation chromatography analysis.

Polyolefin (PO) is the largest volume industrial polymer in the world. It is used to produce a wide range of commercial products that touch nearly every aspect of our daily lives; such as car parts, pipes, packaging films and so on.

Polyolefin products derive their broad range of end-use properties from their semi-crystalline structure. The ability to incorporate comonomers in Polyethylene (PE) and Polypropylene (PP) makes it possible to control the polymer crystallinity, and thus control the rigidity and flexibility of polyolefin products.

The controlling factor also depends to a great extent on the comonomer variations across the molar mass distribution (MMD). In today's competitive marketplace products are targeted to specific applications by tailoring both MMD and comonomer incorporation. This requires a reliable and fast analytical tool for studying comonomer incorporation or chemical composition along the MMD in all kinds of polyolefins.

The high-temperature GPC-IR by Polymer Char is a compact and fully automated instrument that is setting a new standard in gel permeation chromatography (GPC) for polyolefins, and is becoming the preferred choice in the industry.

Optimum detection

The IR detector has been shown to be the most appropriate detector for gel permeation chromatography of polyolefins. Flow rate, temperature and pressure variations have little influence on the IR absorbance, and so an excellent baseline stability is delivered by this detector (Fig. 1).

In addition, IR shows no injection peaks and the chromatograms are easily integrated, resulting in reliable and precise molar mass averages. What makes the IR detector particularly suited for this application is the new dimension it provides without any analytical complexity.

The sample absorbance at several IR bands is acquired as the polymer traverses the flow-through cell. It is thus possible to investigate chemical composition along the MMD for each material analysed. When GPC is performed with an IR detector, a wealth of useful information is obtained for catalyst development, product control or technical service support (Fig. 2).

The simple hardware set-up makes IR detectors very rugged. They can withstand high pressures and need no equilibration time. The detector signal is stable even before the samples are ready for injection or the columns are equilibrated.

For high-level triple detector capability, the IR detector complements the instrument's integrated full bridge viscometer and multiple-angle laser light scattering (Wyatt Heleos II), allowing a deeper insight into molecular architecture and study of short-chain branching, long-chain branching and absolute molar mass of homopolymers and copolymers.

Sample Care

A major innovation is provided via the GPC-IR's full automation of sample preparation, meaning it does not require any external device or manual work. Once the analysis begins via computer control, the instrument fills the vials by means of an integrated syringe pump. Nitrogen is directed into the vial through an intelligent valve system to purge the vial's internal atmosphere, preventing oxidative degradation.

Table 1 shows the benefit of the automated nitrogen purge applied in the GPC-IR and the negative effect of excessive heating.

The GPC-IR allows accurate control of the dissolution time for each vial to be analysed. As soon as it is fully dissolved, the polymer is injected without any additional exposure to high temperature. A durable filtration system is also incorporated in the instrument. The filter is rinsed in back-flush with clean, hot solvent after every injection.

This fact, together with its optimised design, allows for very long continuous operation before replacement.

Conclusion

With the introduction of GPC-IR, a higher standard has been set for analysis of polyolefins. It has been proven that optimum detection is achieved through IR given its stability and sensitivity while providing chemical composition information along the MMD.

For more information at www.scientistlive.com/eurolab

Alberto Ortín is with Polymer Char in Valencia, Spain.





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