Polymer additives mixture analysis using pulsed-field gradient NMR spectroscopy

The analysis of mixtures is a problem commonly faced in analytical chemistry. In this work, pulsed-field gradient NMR (PFG-NMR) experiments were used to analyze mixtures of polymer additives and simple polymer solutions. PFG-NMR experiments were also utilized to determine diffusion coefficients of the individual components of a mixture and in this way facilitate resonance assignments. This strategy is particularly useful for molecules containing isolated protons that are not amenable to the standard two-dimensional NMR experiments based on J-coupling. PFG-NMR was also used to edit the NMR spectra of polymer solutions by eliminating the resonances of fast-diffusing components such as low molecular weight additives or residual solvent. Diffusion ordered spectroscopy (DOSY) was used for the analysis of the PFG-NMR data. The DOSY method produces a two-dimensional spectrum that correlates chemical shift with the calculated diffusion coefficient, and simplifies the analysis of many complex mixtures. For well resolved resonances, the correct diffusion coefficient is unambiguously determined. The DOSY analysis was able to resolve the diffusion coefficients of most of the components in the mixtures examined. However, in some cases of overlapped resonances and components with similar diffusion coefficients, a single diffusion coefficient was determined which was the weighted average of the two components.