Tips & Tricks: GPC/SEC Analysis of Aqueous Polymer Dispersions
Polymer dispersions are used in numerous applications including paints, binders, and adhesives. These complex mixtures usually contain polymeric components that contribute significantly to their physical and chemical properties. Therefore, the molecular weight distributions of the polymeric materials present in dispersions are a relevant matter of interest for characterization. But there are issues which need to be considered for GPC/SEC analysis of these special testing materials. This article will provide valuable guidance how to handle these type of samples.
A main parameter determining the application properties of polymeric materials is the chemical structure, that is, the type and ratio of the monomers applied in the polymerization process. In addition, the molar mass and molar mass distribution of the material are of high importance. For characterization of molar mass averages and molar mass distribution of polymers and macromolecules, gel permeation chromatography (GPC), also named size-exclusion chromatography (SEC) has become the major characterization technique, due to its simplicity, speed, and the amount of information that can be obtained. For GPC/SEC analysis, the sample is dissolved in a proper solvent, and its components are separated based on size in solution by columns filled with porous particles. Pure polymers are usually dissolved in a proper solvent, which is also used as mobile phase. For dispersions, the presence of the liquid used to disperse the polymer also needs to be considered to avoid damaging columns and other undesired additional chromatographic effects.
Solvent Selection
Typical aqueous polymer dispersions and latices consist of at least one (solid or liquid) dispersed polymeric component and water as surrounding dispersion medium. Thus, dispersions generally present physically heterogeneous mixtures.
The first idea that may come up is using the dispersion medium (water) as solvent for GPC/SEC analysis. But this is a fallacy, because GPC/SEC requires good solubility of the analyte in the applied eluent. However, the dispersed component that needs to be characterized is obviously not water soluble hence there will be no opportunity to get this component analyzed under aqueous conditions.
The choice of an appropriate solvent depends on the type of polymer to be investigated. The analyte type defines the applicable solvent, which in turn, determines the suitable stationary phase for the GPC/SEC column. This process mirrors a typical GPC/SEC analysis of a solid polymeric material. If the dispersion comprises multiple polymer types, it might be necessary to apply two or more different methods to characterize each individual polymer type separately. Therefore, keep in mind: The analyte type determines the applied GPC/SEC mobile phase. The column material needs to be chosen based on the eluent applied.
Navigating Sample Preparation Challenges
The next consideration is how to prepare our samples effectively. Dispersions usually contain a dispersion medium distinct from the solvent applied in the GPC/SEC analysis of the dispersed polymeric compound. When simply diluting the dispersion to the appropriate concentration using the eluent and injecting this mixture onto the column, the injected aqueous dispersion liquid may result in undesired consequences. In the worst-case scenario, the injection of the dispersion medium may irreversibly damage the column (1). This is particularly true for polymer-based column materials, where the swelling behaviour of polymer particles is strongly influenced by the solvent environment. To mitigate these challenges, it is recommended to remove the water forming the dispersion medium and redissolve the analyte in the GPC/SEC eluent used for analysis. This prevents damaging of the GPC/SEC columns and minimizes chromatographic artifacts introduced by the dispersion medium.
Applicable methods for removal of the dispersion media are evaporation at ambient or elevated temperatures, or by using reduced pressure techniques such as freeze-drying. The choice of the most suitable method depends on the specific sample under investigation. Solvent evaporation at elevated temperature may result in polymer degradation and therefore mild conditions should be applied. Slow solvent evaporation at ambient temperature might be a better option. However, even under mild conditions, crosslinking may occur, affecting the polymer’s solubility.
Freeze drying, lyophilization, is another gentle way to eliminate water from aqueous dispersions. However, volatile sample components, which need to be analyzed as well, might be lost during the freeze drying or evaporation process. This might also affect the results of the molar mass determination.
If none of the above strategies to remove the dispersant can be applied, diluting the dispersion with the eluent might be the remaining choice. Miscibility of dispersant and GPC/SEC eluent is required for this approach. Occurrence of two-phase mixtures is not acceptable, because a homogenous sample solution is an essential pre-condition for GPC analysis.
The dilution factor that needs to be applied depends on the concentration of the analyte in the original dispersion. As typical polymer concentrations applied in GPC/SEC are in the range of 1–5 g/L, the dilution of highly concentrated dispersions to the required polymer concentration often results in low concentrations of the dispersion medium. Concentrations of water below 1% in the injected sample solution are usually expected not to harm the columns. Your column supplier may provide information on the compatibility of the specific column material and applied dispersant.
At analysis of a diluted dispersion a huge peak resulting from the eluting dispersion medium is often observed at the end of the chromatogram, as the dispersion medium is usually present in similar or higher amounts than the dispersed polymer (2). An example is shown in Figure 1. The figure shows the chromatogram of an aqueous acrylate dispersion containing approximately 30% polymeric solids. The dispersion was diluted with THF to obtain a concentration of 4.0 g/L before injection onto a styrene/divinylbenzene-based GPC/SEC column. While the polymer itself provides a broad signal of low intensity, a strong negative sharp peak is observed, resulting from the amount of water introduced by diluting the dispersion.
Such strong peaks which result from injecting the dispersion medium might superimpose lower molar mass analyte peaks, preventing detection and quantitation of lower molar mass oligomers, which in turn affects molar mass determination. In addition, as the polymeric species are dissolved in a solvent differing in solvent quality from the eluent, the polymer size in the injected solvent is expected to differ from the size in the pure eluent. The swelling effect occurring during the chromatographic experiment may also affect the molar mass determination of high molecular components.
Figure 2 presents a comparison of the chromatograms obtained by the same polyacrylate dispersion using different sample preparations. While the general form of the sample chromatograms look similar, the major differences are observed in the low molar mass region. Therefore, it is recommended to use multiple sample preparations, to get a suitable indication on repeatability of the whole sample preparation procedure.
If the dispersant cannot be removed by evaporation and is not miscible with the GPC/SEC eluent, consider precipitating and separating the polymer, for example, by centrifugation or filtration, followed by redissolutions. However, this approach introduces additional complexity. The extra working steps—centrifugation, filtration, and redissolution—require careful execution and care must be taken not to lose low molar mass components and to skew the molar mass distribution, and thus molar mass averages. This is of particular concern if GPC/SEC results are needed for regulatory purposes.
Conclusions
GPC analysis of dispersions is possible, but several critical aspects need to be carefully considered to generate meaningful and reproducible results. The polymer moiety is the component that defines the solvent system to be applied. Suitable precautions for sample preparation are required to produce accurate results and avoid damaging GPC/SEC columns.
References
(1) Held, D.; Oleschko, K. Tips & Tricks GPC/SEC: Silica Versus Polymer-Based Columns. The Column 2020, 16 (1), 8–12.
(2) Held, D. Tips & Tricks GPC/SEC: How to Treat Your RI Detector. The Column 2017, 13 (17), 24–27.
About the Authors
Heidi Berg is a trained chemical laboratory technician with more than 20 years' experience in GPC/SEC analysis.
Michael Möller studied chemistry at the University of Münster (Germany). He is an application specialist in the GPC/SEC contract analysis department of Agilent Technologies.
Wolfgang Radke studied polymer chemistry in Mainz (Germany) and Amherst (Massachusetts, USA) and is head of the GPC/SEC contract analysis department of Agilent Technologies. Direct correspondence to: wolfgang.radke@agilent.com
