The molecular weight of a polymer is one of its most important attributes, as it affects critical properties such as the glass transition temperature and viscosity. Chemists and material scientists use many complementary techniques to determine the molecular weight of a polymer, such as gel permeation chromatography (GPC), MALDI mass spectrometry, dynamic light scattering, viscometry, osmometry, and nuclear magnetic resonance spectrometry. Many of these techniques are destructive and may require a scientist to sacrifice amounts of their precious materials.
This article focuses on three non-destructive techniques: GPC, dynamic light scattering, and 1H NMR spectroscopy, and how they can complement each other given their individual benefits and shortcomings.
Gel Permeation Chromatography (GPC)
GPC is the most commonly used technique for determining a polymer’s molecular weight because it requires very little analyte and sample preparation time. This technique uses a porous column to elute a polymer solution, in which larger polymers are eluted before smaller polymers because they are not retained as long in the pores of the stationary phase. GPC uses a calibration curve generated by eluting nearly monodisperse polymer samples with structures similar to the polymer being investigated. As such, it provides a relative molecular weight, in contrast to other methods that provide an absolute molecular weight.
GPC Considerations
To ensure a successful GPC analysis, there are several critical factors to consider, including the elution time, injection volume, column length, solvent, detector type, column pore size, and column temperature. One of the most important of these is the temperature, as peaks become sharper above room temperature due to accelerated sample diffusion, which improves resolution. Therefore, many labs use a column oven to maintain their GPC columns at 40 oC.1
If a sample contains polymers whose molecular weight lies outside a GPC column’s specified range, the use of an individual column may not provide useful results, and it may be necessary to connect several columns in series to obtain useful information about the polymer. Most labs also require their users to filter their polymer solutions before injecting them into the instrument because any large undissolved particles may clog or damage the column.
While GPC has benefits such as rapid sample preparation for analysis, it also has several limitations. For example, it is a relative technique, and monodisperse standards similar to the analyte must be available, which may make GPC unsuitable for analyzing polymers without available standards. It also cannot determine the composition of random copolymers and consumes large volumes of high-purity solvents and requires long elution times.
Dynamic Light Scattering
Dynamic light scattering (DLS) is another relative method of determining a polymer’s molecular weight. In this technique, a monochromatic laser is shone through a sample containing the polymer of interest. Instead of directly measuring the molecular weight, the hydrodynamic radius of polymers in solution is measured.2 Similar to GPC, a calibration curve is used by plotting the hydrodynamic radius versus the molecular weight of known standards.
DLS Considerations
When performing DLS, it’s important to use the same solvent for the calibration curve and measurements because the hydrodynamic radius will vary depending on the solvent. A gradient dilution scheme is typically used to determine whether the solution is sufficiently dilute for measurements. If, after diluting several times, the measurement results are identical, the sample is generally considered to be sufficiently dilute. It is also best not to use too low of a concentration, as this will lead to a high signal-to-noise ratio.
The suspension should not be hazy or white to avoid the effects of multiple scattering, which is when light from one particle is then scattered by other neighboring particles. This also helps avoid mutual diffusion effects.3 Colored solutions that absorb the wavelength of the laser should also be avoided.
It is also critical to obtain a dispersion that is stable for the duration of the DLS measurements. If particles have settled to the bottom of the cuvette (or risen to the top in the case of less-dense particles), inaccurate results will be obtained because these particles will not scatter light, as they are not located along the laser’s propagation path. Similarly, it is best to avoid filtering the dispersion before measurement, as this may remove larger particles to be measured.
1H NMR Spectroscopy
Most chemists are familiar with the use of 1H NMR to determine the chemical structure of the small-molecule compounds they synthesize, but they may be less familiar with its ability to determine a polymer’s molecular weight. In contrast to GPC and DLS, 1H NMR provides an absolute measurement of a polymer’s molecular weight because it does not require the use of comparative standards.4 This makes the technique much faster than GPC and does not require the use of a calibration curve. Because this is a commonly-used technique by chemists and material scientists, it generally requires no additional staff training or equipment investments.
1H NMR Considerations
Determining a polymer’s molecular weight using 1H NMR requires comparing the endgroup signal to another polymer signal. Therefore, the endgroup signal must not overlap with any other signals or NMR solvent. As the endgroups may be vanishingly small compared with the peaks from the rest of the polymer, care must be taken to ensure that the endgroup signal is integrated properly and accurately assigned.
Compared with GPC and DLS, 1H NMR provides several advantages, most notably in terms of the information it provides. For example, it can be used to determine the relative composition of block copolymers and provides an absolute molecular weight. It also typically does not require additional training or equipment investments, as most chemistry labs already use 1H NMR as part of their routine chemical analysis.
However, it also comes with limitations, just like GPC and DLS. Importantly, it cannot be used for polymers in which the endgroup signal overlaps with other signals in the spectra. It is also possibly unsuitable for analyzing polymers with molecular weights > 25 kDa due to decreased resolution. Finally, the polymer sample must be soluble in a deuterated solvent whose signal is non-overlapping with the endgroup signal.
Although they each have their own benefits and limitations as discussed above, GPC, DLS, and 1H NMR each provide different types of information about a polymer’s molecular weight. By understanding the complementary aspects of GPC, DLS, and 1H NMR, researchers can leverage their individual strengths to obtain more comprehensive insights into a polymer’s molecular weight.
About the author
Brandon Sharp, Ph.D., is a freelance technical content writer with experience designing photoresists and other organic materials for advanced lithography applications.
References
1. Mabrouk, S. T.; Dark, W. A. .; Ellison, H. R. The Solvent-Temperature Dependence of GPC Separation of Organic Compounds. Journal of Chromatographic Science 1986, 24 (7), 293–301. https://doi.org/10.1093/chromsci/24.7.293.
2. Dynamic light scattering: a practical guide and applications in biomedical sciences - PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5425802/ (accessed 2023-07-24).
3. Why Are DLS Measurements in High Concentration Solutions Difficult? https://www.spectraresearch.com/wp-content/uploads/2019/08/DLS-High-Concentration.pdf (accessed 2023-07-14).
4. Izunobi, J. U.; Higginbotham, C. L. Polymer Molecular Weight Analysis by 1H NMR Spectroscopy. J. Chem. Educ. 2011, 88 (8), 1098–1104. https://doi.org/10.1021/ed100461v.