A multivariate approach to investigate the NMR CPMG pulse sequence for analysing low MW species in polymers

Detection and quantification of low molecular weight components in polymericsamples via nuclear magnetic resonance (NMR) spectroscopy can be difficultdue to overlapping signal caused by line broadening characteristics ofpolymers. A way of overcoming this problem could be the exploitation of thedifference in relaxation between small molecules and macromolecular species,such as the application of a T2 filter by using the Carr–Purcell–Meiboom–Gill(CPMG) spin-echo pulse sequence. This technique, largely exploited in metabolomicsstudies, is applied here to material sciences. A Design of Experimentsapproach was used for evaluating the effect of different acquisitionparameters (relaxation delay, echo time and number of cycles) and sample-relatedones (concentration and polymer molecular weight) on selectedresponses, with a particular interest in performing a reliable quantitative analysis.Polymeric samples containing small molecules were analysed by NMRwith and without the application of the filter, and analysis of variance wasused to identify the most influential parameters. Results indicated that increasingthe polymer concentration, hence sample viscosity, further attenuates polymersignals in CPMG experiments because the T2 of those signals tends todecrease with increasing viscosity. The signal-to-noise ratio measured for smallmolecules can undergo a minimum loss when specific parameters are chosenin relation to the polymer molecular weight. Furthermore, the difference indynamics between aliphatic and aromatic nuclei, as well as between mobileand stiff polymers, translates into different results in terms of polymer signal reduction, suggesting that the relaxation filter can also be used for obtaininginformation on the polymer structure.