Industrious Innovations in Polymer Analysis

Q. You recently developed an innovative selective derivatization technique followed by size‑exclusion chromatography (SEC) to analyze polymeric carboxylic acid functionality for heterogeneity analysis studies (1). Why is this type of analysis important?

A: In the field of polymers – as in society at large – we need to make more sustainable choices. There is an ever-increasing shift from traditional solvent-borne coating systems towards more “ecologically‑friendly” water‑borne polymers. In many water-borne polymer systems, carboxylic acid‑functional monomers are incorporated to develop water-dispersibility of non‑water‑soluble polymer particles. These acid functionalities in the polymeric backbone are mainly responsible for the polymer particle size, shape, and stability and are thus a very important parameter in the development of a coating application. Although there is a vast field of polymer analysis or polymer separation techniques available, little is known about the actual incorporation of such functional monomers. If we can understand the distribution of these monomers along the polymer chain better, we have tools to steer the development of new and more efficient water-borne polymer systems. The technique we devised here is one of the tools that we recently developed to obtain more insights into the incorporation of acid‑functionality in these polymer types.

Q. What is innovative about this approach and what benefits does it offer the analyst? What aspect was most challenging when developing this method?

A: The key aspect of this approach is the specific derivatization of the acid functionality in the polymers. Many analysts in the field of chromatography use derivatization for certain applications, mainly in the field of small molecules. Derivatization of polymers is rather challenging with respect to derivatization efficiency over the full molar mass range, and therefore, this approach is less frequently practiced for large polymers. We have used a class of chemicals, phenacylbromides, to selectively transform the carboxylic acid-functionality in the polymers to phenacylesters. These phenacylester moieties show a very distinct UV‑absorption. By combining a SEC setup with both refractive index (dRI) and UV detection, we have two concentration traces: dRI for the polymer concentration, and UV for the (derivatized) acid concentration. Next to the usual molecular weight distribution we would get from a traditional SEC‑analysis, we can also obtain the content of incorporated acidic monomer and the distribution of these monomer types over the molecular weight. This gives us more insight in the actual incorporation of these monomers, and can possibly link polymer synthesis conditions to polymer properties. A clear advantage is that this approach can be performed in any polymer analysis laboratory because it uses generic chemicals and LC equipment.

Ideally, a derivatization step in any kind of analysis should be selective, complete, reproducible and fast. It has proven to be quite a challenge to find the right chemical approach to tick all these boxes, but the phenacylbromide reagent with a tertiary amine catalyst using N-methyl pyrrolidone as solvent quickly showed its value. The derivatization agent specifically reacts with carboxylic acid functionality, has more than 99% efficiency which is reached directly upon simple mixing of the dissolved polymer with the reagent/catalyst. The ease of use of this approach is another clear advantage for polymer analysts. The general idea of this selective derivatization approach can be extended for other polymer functionalities, provided that the appropriate reagent (conditions) can be identified.

Q. You also use targeted labelling of carboxylic acid functionality followed by pyrolysis gas chromatography (Py-GC) for acid monomer analysis in water‑borne polymers. What advantages does Py-GC offer here (and in general)?

A: Pyrolysis-GC is a very powerful characterization technique, which is able to provide detailed compositional information of known or unknown polymers by thermally breaking down polymers into their monomeric units (or derivatives thereof), which are often volatile enough to be separated and characterized by GC. Even solid samples may be introduced into the pyrolysis chamber, eliminating the need for sample solubilization. Pyrolysis-GC can be performed fast, is reproducible, and can reveal even low concentrations of incorporated species.

The mentioned derivatization protocol in the SEC approach is also of value in the analysis of polymer using pyrolysis‑gas chromatography mass spectrometry (Py-GC–MS) (2). Under pyrolysis conditions, materials which bear active hydrogens, such as amine-, hydroxyl- and carboxylic acid functionalities have the tendency to undergo unwanted side-reactions such as dehydration and decarboxylation. In many cases, these side-reactions result in the formation of non-volatile residues, resulting in a loss of information from the pyrolysis analysis because you are no longer looking at the complete composition. If one is able to derivatize these active hydrogen sites with a non-functional group, such as an alkyl-, phenacyl- or trimethylsilyl group, these side reactions can be negated. Due to the water-borne character of many of the samples we analyze, many alkylation- or silylation approaches were found to be less suitable, but by applying the described phenacylation, monomers that were once not analyzable by pyrolysis now appear in the pyrogram.Next to the overall composition, we can now discern different acidic monomer types at low levels of incorporation.

Q. Do you use Py-GC in other applications in your lab?

A: Most certainly! It is one of our essential tools that is used for applications like market research. Specifically, the double-shot pyrolysis approach is used on a daily basis. In this approach a sample is injected using sub-pyrolysis injection temperatures, followed by a GC analysis. This gives information about the volatile fraction of a polymer sample. After this volatile analysis, the polymer residue remains in the GC liner. This residue can subsequently be pyrolyzed, followed by a secondary GC analysis. This provides detailed compositional information. We use this technique in conjunction with spectroscopic techniques such as nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, the combined information of these techniques usually gives sufficient information to formulate more specific analysis questions or techniques.

Q. You also devised an inventive approach to separate synthetic macromolecules by non-aqueous ion‑exchange chromatography (NAIEC) (3)? Why did you select ion-exchange chromatography (IEC) in this case?

A: The presence of acidic monomers in polymer systems and the deprotonation of these moieties in aqueous environments essentially results in a polyelectrolyte. Our plan was to exploit the presence of these charged groups to separate synthetic macromolecules much like one would separate proteins (3). Obviously, our polymer systems are not water-soluble, which posed some serious analytical challenges. For instance, pK_a of the acidic monomers is shifted to considerably higher values in organic solvents compared to water, which results in essentially neutral carboxylic acids in non-aqueous conditions. The combination of N-methyl pyrrolidone solvent (which has a high dielectric constant, ε = 32.0, enabling the dissociation of ion pairs into free ions) with a so-called superbase (tetramethylguanidine) results in dissolved macromolecules with anionic functionalities. Using a polymeric strong anion exchange (SAX) high performance liquid chromatography (HPLC) column, combined with high-temperature evaporative light scattering (HT-ELSD) as detector, we were able—for the first time ever—to separate these macromolecules selectively by their number of incorporated anionic groups (4).

Q. What benefits does NAIEC provide in this application?

A: Chromatographic techniques that are used to characterize synthetic macromolecules (aside from SEC/hydrodynamic chromatography [HDC]) separate mainly on enthalpic differences between mobile and stationary phase. Most commonly, normal or reversed‑phase liquid adsorption chromatography (LAC) approaches are used for the characterization of compositional differences between samples (5–7). A major drawback of these approaches is that this enthalpic difference applies to the complete polymer—all monomer types—or polarities will have their own contribution to the overall enthalpy of the system. Although very useful for copolymers, these separations fail to accurately describe compositional separations for more complex polymer systems (and most industrial copolymer systems are very complex). The ion-exchange approach we describe selectively separates macromolecules on their (deprotonated) acid monomer units, regardless of the polarity of the comonomers. This can reveal more information on polymer heterogeneity, specifically separating polymers on a single functionality: something that was not possible before. It is therefore a very useful addition to the portfolio of polymer separation techniques.

Q. Could NAIEC be useful in other polymer applications?

A: As single-parameter polymer separations are very rare, the ion‑exchange approach can provide different distribution information than is currently available. However, like SEC or LAC, it is still a one‑dimensional approach. The coupling of this technique with other polymer separation techniques could further enhance its value. We are working towards incorporating this technique in two‑dimensional LC approaches, such as NAIEC×SEC and NAIEC×LAC. This two-dimensional approach can possibly unravel more information on the polymer microstructure.

Q. From an analytical perspective, are there any developments in polymer analysis and in separation science generally, that you are finding exciting at the moment?

A: We need to understand our increasingly complex copolymers better, especially if we want to accelerate the transition into more sustainable materials, such as replacing oil-based monomers with bio-based alternatives, reducing carbon footprint and more. Hyphenation is the magic word, and especially the coupling of so-called single-parameter separations (that is, NAIEC×SEC) which reveals insight into the correlation of specific polymer functionality distributions. Over the last few years, tremendous advances have been made in solving fundamental difficulties and exploring new separations and their coupling in the applicability of multi‑dimensional separation methodologies (8,9). These approaches will conquer more and more ground in many laboratories in the years to come. In conclusion, the continuous development in the field of multidimensional separation techniques is expected (at least in the field of polymer analysis) to become an indispensable tool to make sure we are able to further develop quality coating solutions with a favourable ecological profile.

References

1. T. Brooijmans, R.A. Okhuijsen, G.M.M. Oerlemans, B.W.J. Pirok, P.J. Schoenmakers, and R.A.H. Peters, Anal. Chim. Acta. 1072, 87–94 (2019). doi: 10.1016/j.aca.2019.04.051
2. T. Brooijmans, R. Okhuijsen, I. Oerlemans, P.J. Schoenmakers, and R.A.H. Peters, J. Chromatogr. A. 1560, 63–70 (2018). doi: 10.1016/j.chroma.2018.05.024
3. S. Fekete, A. Beck, J.L. Veuthey, and D. Guillarme, J. Pharm. Biomed. Anal. 113, 43–55 (2015). doi: 10.1016/j.jpba.2015.02.037
4. T. Brooijmans, P. Breuer, P.J. Schoenmakers, and R.A.H. Peters, J. Chromatogr. A. 1626, 461351 (2020). doi: 10.1016/j.chroma.2020.461351
5. P.J.C.H. Cools, F. Maesen, B. Klumperman, A.M. Van Herk, and A.L. German, J. Chromatogr. A. 736(1–2), 125–130 (1996). doi: 10.1016/0021-9673(95)01369-5.
6. H.J.A. Philipsen, B. Klumperman, F.A.M. Leermakers, F.P.C. Wubbe, and A.L. German, Chromatographia 55, 533–540 (2002). doi: 10.1007/BF02492898
7. B. Klumperman, P. Cools, H. Philipsen, W. Staal, P.O. Box, Macromol. Symp. 110, 1–13 (1996). doi: 10.1002/masy.19961100102
8. B.W.J. Pirok, A.F.G. Gargano, and P.J. Schoenmakers, J. Sep. Sci. 41(1), 68–98 (2018). doi: 10.1002/jssc.201700863
9. B.W.J. Pirok, D.R. Stoll, and P.J. Schoenmakers, Anal. Chem. 91(1), 240–263 (2019). doi: 10.1021/acs.analchem.8b04841

Ton Brooijmans is an analytical scientist in the research department of DSM Coating Resins (The Netherlands). After obtaining his B.S. from the Fontys University of Applied Sciences in Eindhoven, The Netherlands, in 2003, he worked at an environmental laboratory until he joined the research department of DSM Coating Resins in 2006. Ton’s activities are on the structure elucidations (both identification and quantification) of complex copolymeric systems (applied as waterborne coatings) by different chromatography and mass spectrometry approaches, and include advanced analytical method development for copolymer analysis. Next to his activities for DSM, he is also an external Ph.D. candidate at the University of Amsterdam (van ’t Hoff Institute for Molecular Science), studying under Prof. Dr. Peter Schoenmakers and Prof. Dr. Ron Peters.