470766 Diffusion NMR (DOSY) for Fast Absolute Molecular Weight Analysis of Polyethylene Furanoate (PEF)

Thursday, November 17, 2016: 4:00 PM
Golden Gate 5 (Hilton San Francisco Union Square)
Jan-Georg Rosenboom1, Giuseppe Storti1 and Massimo Morbidelli2, (1)Institute of Chemical and Bioengineering, ETH Zurich, Zurich, Switzerland, (2)Institute for Chemical and Bioengineering, ETH Zurich, Zurich, Switzerland

Polyethylene furanoate (PEF) has the potential to replace polyethylene terephthalate (PET) with a bio-based and less permeable plastic for commercial application in bottles, packaging, textiles, medical grafts, etc. [1]. However, as promising the advantages and properties of this new type of polymer are, so difficult can be its characterization. As aromatic polyesters in general, PEF is insoluble in most common solvents. Established MW analysis of PEF and PET using size exclusion chromatography (SEC) requires hexafluoro-2-propanol (HFIP) as solvent, special HFIP-resistant equipment, solvent volumes up to liters, and analysis times up to hours. Besides, absolute molecular weights are essential for process and material development, as measurements relative to standards such as polystyrene can grossly overestimate the actual molecular weight. For absolute molecular weights, expensive multi-angle light scattering (MALS) equipment is required. In this work, we present diffusion NMR (DOSY) as an alternative and quick method to measure absolute molecular weight of PEF [2].

NMR DOSY applies pulsed field gradients to spatially-labelled molecules and measure signal intensity upon a magnetization de- and refocusing. The further a polymer molecule moves, as dictated by its size, during a given diffusion time (Δ) between de- and refocusing, the stronger the signal intensity decreases. Fitting of the decay over a sequence of pulse gradients of different strength with the Stejskal-Tanner equation [3] derives a diffusion coefficient D. The relationship between diffusion coefficient and molecular weight is readily calibrated using PET standards of known absolute molecular weight. Conduct of the measurement can be done even without greater knowledge with a pre-installed double stimulated echo pulse sequence (dstegpd2), which enables very reproducible analysis due to convection compensation.

DOSY performance is simply dependent on signal quality for the fitting, which can be tuned by sample concentration, as well as the number of repeated scans (ns) and pulse gradient increments. To study the effect of concentration, DOSY experiments were successfully performed in a range of 0.4 – 8 mg/mL of polymer in 0.75 mL deuterated trifluoroacetic acid (TFA-d), where Brownian motion regime without molecular interaction was always confirmed. However, sample concentration affects viscosity η and thus diffusion D, which can be avoided by staying within a 1 mg/mL concentration range, or compensated by a correction factor from a fit to the measured D-η dependence. Signal-to-noise increases with applied ns and gradient increments, but so does measurement time. This enables 1) the measurement of low concentration samples (e.g. 0.3 mg/mL) without losing data for fitting, and 2) very fast measurement of samples available at higher concentration (e.g. 5 mg/mL). In the latter case, DOSY analysis time per sample can be as short as 1 min without any significant change (<5%) in molecular weight estimation. Alongside with the little solvent use, this makes DOSY a unique and reliable method for semi-online measurement of kinetic samples or quality control during polymerization reactions. Finally, the results for PET and PEF derived from DOSY match those from laser scattering (MALS), and can therefore be considered absolute molecular weights (as shown in Figure 1) [4].

Besides molecular weight analysis, NMR DOSY can be used to assess further properties of polymer samples. In the case of ring-opening polymerization-based PEF, as synthesized in our labs, the cyclic monomers yield a different chemical shift than the PEF product (A1 vs A2 in Figure 2), which enables simultaneous molecular weight and conversion analysis via peak area comparison. Furthermore, DOSY can be used to analyze T1 and T2 relaxation to study flexible and rigid parts of the molecular backbone [5]. We are currently investigating this to compare with the predictions of molecular dynamics and quantum chemistry simulations.

References

[1] L Sipos, E De Jong, MA Dam, J M Gruter, ACS Symposium Series 1105, P Smith (Ed.), 2012, 1-11
[2] W Li, H Chung, C Daeffler, JA Johnson, RH Grubbs, Macromolecules 2012, 45, 9595-9603
[3] EO Stejskal, JE Tanner, J. Chem. Phys. 1965,  42, 1-5
[4] JG Rosenboom, G Storti, M Morbidelli
2016, in preparation
[5] H Berglund, H Baumann, S Knapp, R Ladenstein, T H
ärd, JACS. 1995, 117, 12883-12884

Figure 1: Absolute molecular weights determined with MALS and DOSY for PEF and PET samples.

Figure 2: Monomer (A1) and PEF polymer (A2) chemical shifts, enabling simultaneous conversion and molecular weight analysis.


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