Using OMNISEC to observe sample degradation
The molecular properties of macromolecules often directly influence the physical properties of these materials in their final form. Therefore, it is important that researchers and manufacturers can understand and control various aspects of a sample to ensure it is appropriate for its end-product. Samples used in applications where performance depends on molecular weight need to avoid degradation during the entirety of the production process.
Degradation can occur when a sample is exposed to excessive heat, certain chemical environments, physical stress, or other taxing conditions. A multi-detector GPC/SEC instrument, such as OMNISEC, is a valuable tool for monitoring the molecular weight of a product. The refractive index (RI) detector will show changes in a sample’s molecular weight distribution, while the light scattering detector will be most responsive to changes in the sample’s molecular weight, particularly on the high molecular weight end of the distribution.
In the example discussed in this post, a polystyrene sample was analyzed and then passed through a capillary rheometer four times to simulate four processing cycles. The polystyrene was analyzed using OMNISEC after each processing cycle in order to monitor changes in its molecular weight.
Sample chromatograms
The RI and low angle light scattering (LALS) chromatograms of four injections of each cycle are shown in the images below, respectively.
The RI detector responds primarily to sample concentration and shows that the sample is changing over the course of the four processing cycles. There is an observable shift of the left, high molecular weight side of the peak to greater retention volumes, indicating that the sample’s molecular weight is changing.
While the RI chromatograms clearly show the sample is changing, the LALS chromatograms make it evident that the sample’s molecular weight is decreasing with each successive processing cycle. The light scattering detector is most responsive to the sample’s molecular weight: lower intensity equates to lower molecular weight.
It is interesting to note that the majority of the change is on the left, high molecular weight side of the peaks. The right, low molecular weight side of each chromatogram overlays nicely with the others, suggesting the changes are occurring within the highest molecular weight portions of the sample.
Calculated data
The molecular weight moments measured for the initial sample and then each of the four subsequent processing cycles all trend downward. The largest difference is observed in the Mz value, with a moderate difference in the Mw, followed by the smallest change in the Mn. This is consistent with the chromatograms, where the largest shift was observed on the high molecular weight side of the peaks. The largest, highest molecular weight fractions of the sample are the ones most affected by degradation.
Mark-Houwink plot
There are different ways samples can break down and lose molecular weight. One way is that the pendant functionality of the polymer can change and mass can be lost during this transformation. An example of this would be the hydrolysis of the pendant acetate group to convert polyvinyl acetate to polyvinyl alcohol. Another way a sample can lose molecular weight is the scission of bonds comprising the backbone of the polymer. In this example, the molecular structure would remain consistent, the difference would be the length of the chains.
The Mark-Houwink plot provides a quick and easy way to visually investigate if the structure of the sample has changed. The Mark-Houwink plot displays intrinsic viscosity (IV) as a function of molecular weight, both on log scales. Changes in molecular structure will affect the molecular size, and thus IV, at a given molecular weight. Linear samples that have the same molecular structure will fall along the same line, with only the range of the plot differing, based on the specific molecular weights of the samples.
When all four injections of the initial material and four subsequent processing cycled samples are overlaid on the Mark-Houwink plot, the resulting lines overlay. This is evidence that the molecular structure of the samples remains consistent – the broken bonds exist in the backbone – even though the processing cycles have reduced the molecular weight.
Final thoughts
I hope this post has increased your understanding of how a GPC/SEC system such as OMNISEC can be employed to help you study potential sample degradation in your samples. If you have any questions, please don’t hesitate to contact us or email me directly at kyle.williams@malvernpanalytical.com.
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