The Column - December 2008 - (Page 14)

Tips & Tricks: GPC/SEC The Column www.thecolumn.eu.com December 2008 Don Tremain/Getty Images Tips & Tricks: GPC/SEC A Development in High Temperature (HT) GPC/SEC Peter Montag, PSS Polymer Standards Service GmbH, Mainz, Germany. Size exclusion chromatography (SEC) is a powerful method for analysing molar mass distributions. It is essential that the GPC/SEC eluent is appropiate for the analysed polymers. Some kinds of polymers (polyolefins and their copolymers) are not soluble at room temperature. HT-GPC/SEC applications always run at temperatures between 130–160 °C. Common solvents in HT-GPC/SEC are 1,2,4-trichlorobenzene, 1,2-dichlorobenzene and decaline. Because of the temperature and the chlorinated solvents, special instrumentation and detectors need to be used and the column set should have temperature stability to ensure long performance (e.g., PSS POLEFIN). automated filtering system with cleaning by backflushing the filter is a very helpful tool. Concentration effects: For an accurate molar mass distribution measured by GPC/SEC, the injected mass needs to be as low as possible. If the injected polymer mass is too high, the columns could be overloaded. Polymers, in particular, with higher molar mass or higher crystallinity, have a so-called viscous fingering effect. The results are influenced by this effect. Figure 1 shows molar mass distributions for the same polypropylene injected with different concentrations. For lower concentrations around 1 g/L, the distributions are similar. At higher concentrations, the viscosity of the injected polymer influences the separation, the polymer elutes later and the molar masses seem to be lower than expected. What is special in sample preparation? The first step in GPC/SEC is to use a representative sample preparation method without polymer degradation. In HT-GPC/SEC particularly, the polymer chain is stressed by temperature and “moving” of the solvent. High molar mass polymers will degrade significantly if stirred for hours at 160 °C. The best way to dissolve polyolefins is by shaking slightly until full dissolution has occurred. Older autosampler systems, such as the WATERS 150C, stored the samples until injection at a high temperature. Within longer serial runs, some polymers are stored for 20 hours or more and in these instances, the reproducibility of molar mass is very low because of uncontrolled degradation. Newer systems such as the WATERS 2000, PL 210 and Polymer CHAR GPCIR use individual temperature programmes for each sample. Some samples need to be filtered to remove filling materials such as fibreglass or black carbon. In these instances, an Authors: Peter Montag E-mail: PMontag@polymer.de Which detectors are best for HT-GPC/SEC? In applications at room temperature the most common detectors are refractive index detectors and UV detectors. However, polyolefins have a very low dn/dc value and the RI-signal is very temperature sensitive. As a result, the signal could be very noisy (WATERS 150C) or temperature equilibration times (WATERS 2000, PL 210) could interrupt the analysis for hours or days. UV detection is impossible because of the UV-inactive polymers. ELS detectors are sometimes helpful but the usable concentration range is smaller than for other detectors. Therefore, IR-detection is a powerful alternative for polyolefins. Detection of the C-H-vibration in the 3000 cm 1 range gives a linear concentration signal with a good signal-to-noise ratio. Figure 2 shows the excellent linearity for a typical GPC/SEC concentration range of the IR4 detector used in the PSS HT-GPC/SEC system. Note that the IR-detection with 14 http://www.thecolumn.eu.com

Table of Contents for the Digital Edition of The Column - December 2008

The Column - December 2008
Contents
Do Wrinkle Creams Really Work?
News
Incognito
Market Trends and Analysis
Tips and Tricks: GPC/SEC
UHPLC for the Analysis of Paracetamol
Q&A

The Column - December 2008