In material science, the molecular weight of polymers is an important property, which can relate to different physical properties. Our previous blog posts demonstrate that polymer molecular weight can be determined with benchtop NMR using two different methods: 1) end-group analysis, and 2) self-diffusion coefficient. End-group analysis can be performed through simple 1D experiments using 1H, 19F, 29Si, or other nuclei, if the signal from the end group is observable and well separated from that of the repeating unit. However, in situations where the signal of the end group cannot be detected either due to overlapping or simply because the signal is too small (polymers with large molecular weight), self-diffusion coefficients are recommended for the determination of molecular weight. While each method can be used in different situations, this blog post demonstrates that they can give accurate and comparable results for the same polymer.
The molecular weight of polyphenylsulfone (PPSU) polymer was first determined using end-group analysis through a 1D 1H spectrum measured on a Spinsolve 90 ULTRA (Fig. 1). The signal for the methoxy end group appears as a singlet at 3.85 ppm, while the signals for the repeating unit resonate in the 7 to 8 ppm range. The degree of polymerization (DP) is calculated from Eq. 1. The average molecular weight of this PPSU sample is determined to be 8200 g/mol using Eq. 2.
                               Figure 1. 1D 1H spectrum of PPSU (5 mg/mL in CDCl3) collected on a Spinsolve 90 Carbon ULTRA (32 scans, total measurement time of 4 minutes).
The polymer molecular weight can also be determined through self-diffusion coefficients measured with pulsed field gradient NMR, as discussed in our previous blog post. The diffusion coefficient (D) of the same PPSU sample was measured on the Spinsolve 90 ULTRA equipped with pulsed field gradient (0.5 T/m), which resulted in the average D value of 7.13 x 10-11 m2/s (Fig. 2).
Figure 2. Left: Superimposed plot of the 8 steps PGSTE experiment for PPSU (5 mg/mL in CDCl3). Right: Stejskal-Tanner plot, depicting the signal integral as a function of γ2g2δ2(Δ-δ/3). The data was fitted with a linear function to obtain the diffusion coefficient (D). The total time required to collect this experiment was 15 minutes.
Using Eq. 3, with the calibration value of -8.23 and 0.49 for log(b’) and v, respectively from the universal calibration curve from the publication [1] , an average molecular weight of 8167 g/mol was calculated for PPSU. This value nicely agrees with the result obtained from the end group analysis. As the end group analysis is an absolute method, the fact that the diffusion method delivers a similar molecular weight confirms that the universal calibration curve built-in [1] from different sets of polymers and on different NMR instruments is reliable.Â
While each method of molecular weight determination has its advantages and limitations, depending on the sample, the good agreement obtained in this application note for PPSU confirms that both approaches can be implemented in the Spinsolve benchtop NMR spectrometer to determine molecular weight in just a few minutes with minimal sample preparation and fewer consumables compared to the traditional method of size exclusion chromatography (SEC).Â
To Read the Full Application Note Click Here
References
1.    Solvent-Independent Molecular Weight Determination of Polymers Based on a Truly Universal Calibration; P.-J. Voorter, A. McKay, J. Dai, O. Paravagna, N. R. Cameron, T. Junkers; Angew. Chem. Int. Ed.; Volume 134, Issue 5 (2022) e202114536 (https://doi.org/10.1002/ange.202114536)