(255at) Diffusion NMR (DOSY) for Fast Absolute Molecular Weight Analysis of Polyethylene Furanoate (PEF)

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 Härd, 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.