Can I detect and characterize polymer branching with MALS?

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Introduction

Polymer branching is an important structural parameter that influences a number of mechanical and rheological properties in both synthetic and natural polymers, including many with pharmaceutical applications. Quantifying branching content is crucial to understanding the structure-property relationship of polymer-based materials and is only made possible with a multi-angle light scattering (MALS) detector like the DAWN®.

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Branching Ratio

To characterize branching ratio, information about the molar mass and the molecular size are needed—and MALS determines both of these absolutely, simultaneously, and independently from first principles! This is downstream of a size-based separation technique, such as SEC-MALS or FFF-MALS. The slope of the log-log plot of the root mean square (rms) radius and molar mass determines the branching ratio. A theoretical background is provided in the white paper, Branching Revealed: Characterizing Molecular Structure in Synthetic and Natural Polymers by Multi-Angle Light Scattering.

One of the few limitations of MALS is that the lower limit for determining the rms radius is about 10 nm. In these cases of smaller polymers, an alternative size determination technique can be used, such as intrinsic viscosity, dynamic light scattering, or SEC elution volume calibration. The addition of the ViscoStar provides both higher sensitivity for lower molar mass species and the ability to determine Mark-Houwink-Sakurada parameters.

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Branching Methods in ASTRA

Data from both a branched polymer of interest and a linear version can be analyzed in ASTRA to determine the branching ratio, which can be used for other branching analyses. Branching ratio is derived from either a conformation plot (MALS) or the Mark-Houwink-Sakurada plot (MALS + IV). An example of a conformation plot for polyethylene with rms radius vs. molar mass is provided to the right. Branching results in a more compact polymer structure, so reduces the size of a polymer at the same molar mass compared to a linear analog. This is readily apparent from the slope of the conformation plot.

Step-by-step instructions for utilizing these branching analysis methods in ASTRA can be found in TN1001 and TN1002 on the Support Center. There are three strategies in ASTRA for determining the branching ratio:

      Radius method from the conformation plot (log-log of radius vs. molar mass)
      Viscosity method from the Mark-Houwink-Sakurada plot (log-log of intrinsic viscosity vs. molar mass)
      Mass method from the plot of molar mass versus SEC elution volume.

Either experimental linear polymer data or modeled linear polymer data can be used for subsequent calculations. And ASTRA can report this data numerically and graphically in a number of different ways.

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Branching Analysis

ASTRA can determine the branching ratio, the branching per molecule, and long chain branching properties of non-linear polymers. The branching ratio is typically less than 1, where a lower branching ratio indicates a higher degree of branching—for example, a value of 0.1 indicates a highly branched polymer structure. The branching per molecule provides information about the number of branch points across the length of an entire polymer. An example plot of the branching units per molecule vs. molar mass from is shown to the right.

Additionally, ASTRA can also provide long chain branching graphs, or the number of branch points per 1000 repeat units when the repeat unit molar mass is input. All of these results can be displayed in the EASI Graph.

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Limitations of SEC for Branching Analysis

Certain branched macromolecules may exhibit an abnormal elution behavior in SEC. This may result in an upswing in the conformation plots and consequently incorrect branching ratio values. The root cause for this abnormal elution is anchoring of branched molecules in the column packing material. This can be completely eliminated by using asymmetric flow field-flow fractionation (FFF) which results in the expected conformation plots as shown to the right.

FFF-MALS with the Eclipse can be considered an orthogonal technique to SEC size separations and is uniquely suited for providing superior separation for polymers that may be affected by SEC column anchoring or interactions.

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Conclusion

SEC-MALS or FFF-MALS can directly determine polymer branching and the number of branch units per molecule. Branching analysis may be enhanced with an online viscometer to determine Mark-Houwink-Sakurada parameters, especially for smaller branched polymers. Although both techniques separate by size, FFF may offer superior separation for polymers that experience column pore anchoring or column interactions.

To learn more about what you can do with the branching analysis procedure and ASTRA 8, login to our Support Center for resources including technical notes, User Meeting presentations, and the ASTRA 8 User’s Guide.

Do you have a question? Contact our experts here in Customer Support. We’re happy to help! Call +1 (805) 681-9009 option 4 or email support@wyatt.com.