Spin Diffusion NMR Investigations of Amorphous Polymer Organization in the Solid State

Abstract

We report a general method, based on intramolecular spin-diffusion, for the measurement and calculation of spin-diffusion coefficients in amorphous polymers and their blends using only NMR data. The basic structural unit that defines 1H polarization density in polymers is the monomer unit. Using appropriately selected internal reference distances calculated from energy-minimized chain dimension simulations, timescales for the redistribution of 1H polarization within amorphous homopolymers may be used to independently calculate maximum values of the spin diffusion coefficients D. This strategy represents an attractive alternative to current methods employed for domain size measurements in polymer blends, which require calibration of spin diffusion coefficients based on comparisons of similar NMR data obtained on model compounds analyzed previously using scattering or microscopy techniques. In this way, many more polymer systems become amenable to study by NMR spin-diffusion methods, since X-ray scattering or microscopy calibrations on representative standards are no longer necessary to define quantitative limits on spin-diffusion coefficients. Experimentally, the fate of proton magnetization is followed with high selectivity using 2D 1H-13C solid-state HETCOR sequences incorporating controlled periods of 1H-1H spin-diffusion.