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Analysis of engineering plastics by gel permeation chromatography

3rd December 2013


Jens Reichenberger outlines new developments for efficient sample preparation and optimised separation conditions for the GPC analysis of various engineering plastics.

Various types of engineering plastics showing superior characteristics under high thermal or mechanical load have been developed recently. These materials are supposed to complementing traditional materials, such as wood, metals or ordinary plastics. As the applications of these high performance polymers increase, there is a need for methods to accurately and precisely characterise these materials. Some engineering plastics were analysed using a new GPC/SEC system and a series of GPC column designed for high temperature GPC/SEC analysis.

Engineering plastics, such as ultra-high molecular weight polyolefins or polyphenylene sulphides, excel other materials by their mechanical strength and their resistance to chemical and physical degradation. These favourable properties turn into hurdles when it comes to characterisation of these polymers. They are difficult to analyse because they are crystalline and often require elevated temperatures or special solvents for complete dissolution.

Gel permeation chromatography (GPC, also known as size exclusion chromatography, SEC) is an established technique used for the characterisation of polymers with regard to their molecular weight distribution. The GPC analysis of engineering plastics at high temperatures requires specialised instruments and columns. The high temperature is required throughout the whole experiment to avoid re-crystallisation and to ensure that the sample remains in solution.

We investigated separation conditions for various polymers. For these experiments a new type of high temperature GPC/SEC system, the HLC-8321GPC/HT (HT EcoSEC System) was used (Fig.1). The system provides stable thermostatisation up to 220°C, autosampler, pumps, and as integrated standard detector an extremely stable refractive index detector with an independent temperature control. With regard to the high temperatures applied various safety features such as auto-lock doors and gas sensors are built into the system.

The first step in high temperature polymer analysis is ensuring an efficient dissolution and filtration of samples prior to GPC/SEC analysis without risking any unwanted polymer degradation.

The polymer chain is stressed by heat and shaking. The best way to gently dissolve these polymers is moving the sample carefully and applying an individual temperature program until full dissolution has occurred.

The DF-8321 sample processing unit of the new system can process up to 24 samples with a frequency of 10 to 100 rounds per minutes at temperature programs ranging from 40 to 220°C. In addition samples can be filtered automatically to avoid contamination of the column and system.

The standard detection in GPC/SEC applications is refractive index (RI) detection. The data shown below are measured with a unique dual-flow RI detector, similar to the RI detector of the basic EcoSEC system which was described in detail in Eurolab June 2013.

The combination of a high performance heating system and the dual flow RI detector speeds up equilibration time and reduces the baseline noise in an effective way.

Fig.2 shows the equilibration of column temperature and detector signal at a flow rate of 1ml/min and a target temperature of 145°C. After 180 minutes both parameters show excellent stability. Fig.3 depicts the extremely low baseline noise of the RI signal for different solvents and temperatures.

Separation of polymers

The high temperature GPC/SEC system delivers consistent results over a broad range of conditions. For optimum performance the GPC column should be carefully selected to ideally fit the temperature and solvent conditions. The TSKgel family of GPC/SEC columns offers a wide range of GPC columns with individual pore sizes but also mixed bed and multi-pore columns exhibiting linear mass calibrations. The TSKgel GMHHR- HT2 series was developed to complement the new high temperature GPC system when using the maximum temperature range up to 220°C. Fig. 4 shows examples of polymer analysis with RI-detection at 145°C and at 220°C.

Conclusions

The combination of HLC-8321GPC/HT and TSKgel GMHHR HT2 is a powerful tool for GPC measurement of engineering plastics. An accurate sample preparation is just as important as a stable RI detector baseline. A stable RI detector baseline is required for successful experiments in particular for repeatable and reproducible molar mass average calculations.

The repeatability and reproducibility of the molar mass averages have been shown to significantly increase by replacing a conventional RI detector with a dual-flow RI detector.

For more information at www.scientistlive.com/eurolab

Jens Reichenberger is with Tosoh Bioscience GmbH, Stuttgart, Germany. www.tosohbioscience.de. Data generated at Tosoh Corporation and Tosoh Hitech by Fumiya Nakata; Shuji Kumagai, Kazunari Fukugawa and Satoshi Fujii.





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