Solid-state nuclear magnetic resonance studies of phosphazene inorganic polymers

Abstract

Polyphosphazenes are inorganic polymers which are based on chains of alternating phosphorus and nitrogen atoms. These materials are of considerable interest because of their mechanical and thermal properties, solvent resistance, and potential for providing biocompatible materials. This dissertation describes solid-state NMR studies of several important problems in polyphosphazene chemistry including hydrolysis, cross-linking, morphology, and molecular dynamics. Also, instrumentation and methodology are described which make variable-temperature (VT) magic-angle spinning (MAS) NMR reliable and routine at temperature between 77 K and 540 K. The role of the hydrolysis products of hexachlorocyclo-triphosphazene as catalysts for the melt polymerization was investigated using [^1]H MAS NMR. In an effort to assign the resonances in the [^1]H MAS spectra a novel NMR experiment was developed in which the magnetization from low-[gamma] nuclei is transferred (via cross polarization) to protons. [^31]P MAS NMR was used to examine hydrolysis and cross-linking in 2 and also the role of hydrolysis in the thermal degradation of poly [bis(ethoxy)phosphazene] (PBEP). The results of those studies indicated that both hydrolysis and cross-linking sites in polyphosphazenes can be observed and quantified using [^31]P MAS NMR. Several MAS NMR techniques, in conjunction with differential scanning calorimetry (DSC), have been employed in the study of morphology in polyphosphazenes. The thermal transition behavior of poly [bis(4-isopropylphenoxy)phosphazene] (PBIP) was examined using high temperature cross polarization (CP)/MAS NMR. The NMR spectra acquired verified the existence of two distinct crystalline phases in that polymer and indicated that conversion from one crystalline phase to the other occurs through a disordered, metastable state. The morphology of poly [bis(3-methylphenoxy)-phosphazene] (PB3MP) was studied using [^31]P CP/MAS NMR and [^31]P 2-D solid-state NMR. Those studies indicated that spin and/or chemical exchange occurs between the crystalline phase and mesomorphic phase in PB3MP.