Introduction
Acrylic polymer emulsions and other water-based acrylic polymer products are important materials for the graphic arts and industrial coating industries, and are commonly the main ingredient in latex paints. As these samples often contain ultra-high molar mass polymers, they are challenging to separate and accurately characterize by chromatographic methods such as Size Exclusion Chromatgraphy (SEC). An alternative technique is Asymmetrical Flow Field-Flow Fractionation (AF4), which uses an open channel architecture for size separation instead of a packed column, avoiding the loss of large emulsion materials on the column packing material. [1,2] Thus the separation range of Field-Flow Fractionation (FFF) is larger than the one of SEC (see Figure 1).
Figure 2 illustrates the principle of AF4: in the open channel a combination of cross flow and channel flow causes size separation over the course of analysis, with smaller molecules eluting before larger molecules. In this work, AF4 coupled on-line to Refractive Index (RI) and Multi-Angle Light Scattering (MALS) detection was used to measure the molar mass and radius of three polyacrylate samples. Plotting these measurands against each other on a logarithmic scale provides information about polymer branching (conformation plot).
Experimental Details and Results
Three aqueous acrylate emulsion samples of varying size, molar mass, and degree of branching were analyzed by AF4-RI-MALS. A 50 μL aliquot was diluted in 10 mL of nanopure water and rotated overnight to disperse. An injection volume of 20 μL was separated by AF4 with a detector flow of 0.5 mL/min.
Figure 3 shows replicate fractograms for the 90° MALS signal vs time, and molar mass measurements for each sample. The molar masses range from ~25 MDa to ~90 MDa illustrating the wide range separable by AF4, and also AF4’s ability to separate molar mass species, which will likely be lost on an SEC column’s packing material. Sample 1 elutes first and has the smallest molar mass; samples 2 and 3 elute at approximately the same time, however MALS-analysis reveals that sample 3 obviously contains a component of much larger molar mass. Figure 4 shows replicate fractograms for the 90° MALS signal vs time, and radius measurements for each sample. Sample 1 is the smallest sample in size; samples 2 and 3 are similar in size with sample 3 containing a component of slightly larger measured radius. The radius for the three samples range from ~16 nm to ~27 nm, or about 32 nm to 54 nm in diameter.
Plotting the log R vs the log M results in a conformation plot, shown in Figure 5. This provides a means to compare changes in size with changes in molar mass; a shallower slope on this graph indicates that a polymer’s molar mass is increasing with relatively little increase in size, and is likely to be more highly compact or branched. A slope of 0.5 - 0.6 is common for polymers with a random coil geometry, and sample 1’s conformation plot slope of 0.54 indicates this. Sample 2 and 3’s slopes of 0.45 and 0.44, respectively, indicate that they are more compact and likely more branched than sample 1.
Conclusion
AF4-RI-MALS was used to separate and characterize three polyacrylate emulsions by radius, molar mass, and degree of branching. Even for samples with radii up to ~30 nm and MW up to ~100 MDa, AF4 provides robust separation, making it an ideal technique for analysis of such large macromolecules.
References
[1] Makan A.C., Williams R.P., Pasch H., Macromolecular Chemistry and Physiscs, 2016, 217(18), 2027-2040.
[2] Makan A.C., Spallek M.J., du Toit M., Klein T., Pasch H., Journal of Chromatography A, 2016, 1442, 94-106.
