DIFFUSION-SEPARATED NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY OF POLYMER MIXTURES

The potential of diffusion-ordered 2D NMR spectroscopy (DOSY) for the analysis of solutions of polymer mixtures and polymers with complex mo lecular mass distributions is investigated. Diffusion coefficient labe ling in NMR is generally achieved by stepwise ramping up of the amplit udes of pairs of pulsed field gradients (PFGs). After Fourier transfor mation in the acquisition dimension and an inverse Laplace transform ( ILT) with respect to the square of the gradient strength, 2D spectra a re obtained that show the chemical shift along one dimension and the t ranslational diffusion coefficient along the other. Since polymers may have broad, nonsymmetric or complex (bi- and multimodal) molecular we ight distributions (MWDs), the diffusion coefficient distribution shou ld follow the main features of the MWD. However, the calculation of th e diffusion coefficient distribution involves a numerically unstable d ata inversion (ILT), which limits the resolution in the diffusion dime nsion. The applications of DOSY NMR techniques to the study of polymer blends and for the determination of MWDs of polymers with PFG NMR are assessed. DOSY spectra recorded from industrial polypropylene and pol ystyrene samples and mixtures are shown and interpreted to illustrate the features of this technique.