Polymer-Solvent Interactions in Phosphazene Materials As Observed by Solid- and Liquid-State NMR Spectroscopy

The chemical nature of the interactions between materials and occluded small molecules have been the subject of intense study. In this paper, we demonstrate the utility of NMR spectroscopy for examining dynamic systems. Phosphazenes are inorganic polymers with an alternating nitrogen-phosphorus backbone that may be tailored by substitution of differing organic pendant groups, thus assuming a variety of physical and chemical characteristics. We examine two examples of phosphazenes and determine that solvents that penetrate and swell will influence the material at the molecular level. It has been determined that solvation in swollen materials is complete and can be observed by NMR spectroscopy. Variable-temperature NMR spectroscopy and nuclear relaxation phenomena demonstrate motional dynamics within the polymer matrix. In addition, reorientation rates are enhanced and the free volume within the matrix is increased with the inclusion of solvent. Additional evidence for solvation is observed through the use of heterogeneous-phase NOE difference spectroscopy where a dipolar interaction between solvent and substrate yields internuclear distances of less than 5 Å. These data lend support to the solution-diffusion model as a mechanism for permeation and transport of small molecules within a polymer matrix.