Recycling Liquid Chromatography for Polymer Analysis

CAST member Leon Niezen developed a method termed recycling liquid chromatography for the analysis of the chemical composition distribution (CCD) of polymers.

Synthetic polymers play an important role in our current society and see use in an incredibly diverse number of materials. Examples include polyurethane foam cushions or the use of aramid in optical fiber cables and jet engine enclosures. To develop better materials without resorting to a “trial-and-error” approach of synthesizing new materials, we must know how a polymer’s molecular structures influence the properties of the material.

The first step in this process is the ability to analyze these molecular structures. We are not usually able to separate all of the individual structures, since polymers typically show dispersity in molecular weight and potentially in chemical composition. These are referred to as the molecular weight distribution (MWD) and chemical composition distribution (CCD), respectively.

Figure 1. A) Schematic illustration of the recycling-gradient set-up, B) Trace from the in-line DAD resulting from the recycling gradient with the switching moments of the valve indicated by the dotted lines, C) Data folded and aligned, displayed as stacked individual cycles (left) or as a surface plot (right). Reproduced with permission from [1].

When a synthetic polymer features both a broad MWD and a low average molecular weight (a type of polymer that is extremely common) it can be challenging to analyze the CCD since the most common method to do so is gradient-elution LC. Using this method the individual analytes elute based on their polarity, which is affected by both their composition and their molecular weight. As a result we typically measure a convoluted distribution.

Applying the theory of retention modelling to standards that feature a narrow MWD and a known molecular weight allows us to determine, approximately, what conditions are required to remove or mitigate the effect of one distribution. As it turns out effectively steep gradients are required to enhance the separation on chemical composition.

Figure 2. LCΠLC of copolymer MB4 using non-porous C18 particles with a 3-min 0-60% THF gradient in ACN. A) Front (blue) and tail (red) peak widths (in mL) as function of cycle number. B and C) Peak profiles after 1st and 20th cycle, respectively, with fractions taken indicated; dashed line under the peak indicates the background signal of the gradient. D and E) SEC chromatograms of the fractions indicated in B and C. Reproduced with permission from [1].

This has led to the development of a technique called recycling liquid chromatography, where the gradient is continuously recycled between two columns. Such a recycling allows analytes a longer time to travel through the gradient. Each successive cycle therefore increases the amount of the gradient the analytes get to experience before eluting from the column; i.e. the effective gradient steepness increases every cycle.  

With the addition of an in-line detector this allows us to assess directly how analytes travel through the gradient within a single experiment. The technique was shown to allow for a significantly better assessment of the CCD, unhampered by the underlying MWD, for several different types of styrene and acrylate-based copolymers.    

All of the conclusions are detailed in the open-access article that was recently published in Journal of Chromatography A. This means you can download and read it for free.

References

[1] Recycling gradient-elution liquid chromatography for the analysis of chemical-composition distributions of polymers, L.E. Niezen, B.B.P. Staal, C. Lang, H.J.A. Philipsen, B.W.J. Pirok, G.W. Somsen, P.J. Schoenmakers, J. Chromatogr. A2022, 463386, DOI: 10.1016/j.chroma.2022.463386

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