Gel permeation chromatography (room temperature)

Gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC), is a standard method for determining the distribution of the molar mass and averages derived from it. Instead of the term “molar mass”, the term “molecular weight” is also commonly used. The molar mass distribution and the average molar masses give information about central properties of a polymer/plastic. Among these are the toughness, the melt viscosity and the elastic properties of the melt. By comparing the molar mass distribution of different samples, it is, furthermore, possible to draw conclusions about degradation and cross-linking processes. Thus, GPC is one of the most important characterization techniques for polymers.  

Solvent selection

GPC is a liquid chromatographic method. Thus, it is essential to dissolve a sample prior to its characterization. The selection of a solvent for a GPC analysis is mostly dependent on the solubility of the polymer. The application of THF (used in the analysis of polystyrene (PS) or polyethylene glycol (PEG) among others) is very common, for example. On the other hand, the application of hexafluoroisopropanol (HFIP), which is used in the characterization of polyethylene terephthalate (PET) or polyamide (PA), is much less common. More detailed considerations in solvent selection may be necessary due to attractive or repulsive interactions. To counteract these, additives (acids, bases or salts) may be applied. In effect, it is fortunately possible to realize suitable conditions for GPC of most classes of polymers.

Hyphenation with other methods

The information provided by gel permeation chromatography (molar masses and their distribution) can already help to answer many important questions. One example is the amount of low molar mass molecules (amounts < 1000 g/mol) in a sample. The molar mass is, however, always linked to other characteristics of the polymer as well. For example, the composition of the polymer may vary or components with a specific molar mass may influence certain characteristics more strongly. To gather information about the linkages between the molar mass and additional properties, hyphenated methods are essential. One approach is Two-dimensional liquid chromatography (2D-LC).

To hyphenate GPC with spectroscopic (IR, NMR, …) or thermal (DSC, TGA, …) methods, it is, however, necessary to employ semi-preparative fractionation of the sample. Through this approach, parts (fractions) of the sample which are narrowly distributed with regards to molar mass can be obtained. For each fraction, several milligrams of material can be gathered. The fractions can then be characterized further by just about any method. Thus, it is possible to determine the correlation between molar mass and a multitude of other metrics. Furthermore, the approach can be transferred to other modes of separation (e.g., Liquid chromatography (HPLC, from room temperature up to 200°C), allowing for the elucidation of additional correlations.

Fields of application

• Elucidation of the molar mass distribution (also called molecular weight distribution and of averages derived from that (M_z, M_w, M_n, Ð)
• Damage analysis on the molecular level
• Investigation of batch variations and effects
• Answering regulatory questions (e.g., amount < 1000 g/mol)
• Prefractionation for detailed chemical analysis

The upper image shows a fraction collector which was developed and constructed at Fraunhofer LBF. Unlike commercial instruments, this model allows to heat the fractionation valve and inlet capillaries actively. In effect, fractions can be collected at variable temperatures. Even polyolefins can be fractionated in this way (c.f. Gel permeation chromatography (high temperature, up to 200°C)). Depending on necessity, additional commercial and non-commercial instruments are available at Fraunhofer LBF.

Publications:
 

• F. Malz, J. H. Arndt, J. Balko, B. Barton, T. Büsse, D. Imhof, R. Pfaendner, K. Rode, R. Brüll, Analysis of the molecular heterogeneity of poly(lactic acid)/poly(butylene succinate-co-adipate) blends by hyphenating size exclusion chromatography with nuclear magnetic resonance and infrared spectroscopy, J. Chrom. A, 1638 (2021) 461819, DOI: 10.1016/j.chroma.2020.461819
• N. Apel, E. Uliyanchenko, S. Moyses, S. Rommens, C. Wold, T. Macko, R. Brüll, Separation of Branched Poly(bisphenol A carbonate) Structures by Solvent Gradient at Near-Critical Conditions and Two-Dimensional Liquid Chromatography, Anal. Chem. 90 (2018) 5422–5429, DOI: 10.1021/acs.analchem.8b00618
• N. Apel, E. Uliyanchenko, S. Moyses, S. Rommens, C. Wold, T. Macko, K. Rode, R. Brüll, Selective chromatographic separation of polycarbonate according to hydroxyl end-groups using a porous graphitic carbon column, J. Chrom. A, 1488 (2017) 77–84, DOI: 10.1016/j.chroma.2017.01.075