High-Temperature Solid-State NMR of Cross-Linked, Insoluble, and Unswellable Polymers

Abstract

The use of solid-state magic-angle spinning (MAS) and variable temperature (VT) NMR techniques as routine molecular structure characterization tools to analyze insoluble, high M.W., and/or cross-linked polymers is demonstrated. Increasing the mobility in the polymers by either swelling in a solvent and/or by elevation of temperature leads to narrow resonances with isotropic chemical shifts in MAS NMR spectra. Current instrumental capabilities of solid-state NMR spectrometers to acquire MAS spectra at temperatures up to 250 °C enable NMR measurements at temperatures wherein the mobility of the polymers is high and the material is viscous. In a viscous state, the T1 time of a polymer is shorter than in a dilute solution or in an amorphous state. Combined with MAS, the viscous sample has long T2 time (narrow resonances) and significantly reduced chemical shift anisotropy (CSA) such that spinning side bands are often not significant. Such line narrowing allows rapid acquisition of reasonably well-resolved proton, fluorine, carbon, phosphorous, or silicon NMR spectra of insoluble materials for their molecular structure characterization.