Solid-state nuclear magnetic resonance spectroscopy of phosphazene polymers
Date
2009
Authors
Borisov, Alexey S.
University of Lethbridge. Faculty of Arts and Science
Journal Title
Journal ISSN
Volume Title
Publisher
Lethbridge, Alta. : University of Lethbridge, Dept. of Chemistry and Biochemistry, c2009
Abstract
High-resolution one-dimensional 1H, 19F, 31P and 13C MAS NMR experiments
were used in a morphological study of solvent-cast and heat-treated
poly[bis(trifluoroethoxy)phosphazene] (PBFP). Deconvolution analyses performed on all
Nuclear Magnetic Resonance (NMR) spectra are presented. These results suggest the
presence of broad and narrow overlapping components at ambient temperature, which
were assigned to the crystalline, amorphous and the mesophase regions within the
polymer, respectively. The number of signals in the spectra was independently verified
using 1H, 19F and 13C Discrimination Induced by Variable Amplitude Minipulses
(DIVAM) nutation experiments. Deconvolution analyses showed that heat-treatment
increases the overall crystallinity of the solvent-cast PBFP. Further studies conducted on
two preparations of the polymer showed significant differences in crystallinity due to
variations in the reaction conditions. Magic-Angle Spinning (MAS) NMR spectra of
PBFP obtained via living cationic polymerization at ambient temperature indicated that
the polymer contains mostly amorphous and mesophase regions with only a small
contribution from the crystalline domain.
Variable-temperature 31P NMR experiments suggested that the thermotropic
transition occurs in a temperature range of 80ºC to 90ºC, where the crystalline signal
disappears and a new signal due to a liquid crystalline phase emerges. Spin-lock 31P
experiments provided rates of the transverse relaxation in the rotating frame for each
signal, showing that the crystalline and the amorphous regions within the polymer are
characterized by significantly different mobilities at ambient temperatures, while the
v
comparable degree of motion occurs between the amorphous and mesophase
environments at temperatures above 90ºC.
The process of thermal ring-opening polymerization of
hexachlorocyclotriphosphazene was monitored using one-dimensional 31P MAS NMR at
different stages of the reaction. The ratio between cyclic species and the high molecular
weight poly(dichlorophosphazene) was seen to change over time. 31P NMR was seen to
be a potentially valuable tool in monitoring rates of chain propagation, branching and
cross-linking. Two-dimensional 31P homonuclear Radio-Frequency Driven Recoupling
(RFDR) and Incredible Natural Abundance Double Quantum Transfer (INADEQUATE)
MAS NMR experiments were first tested on the partially phenoxy-substituted
hexachlorocyclotriphosphazene, and subsequently applied in the study of a preparation of
the partially trifluoroethoxy-substituted poly(dichlorophosphazene). Very high resolution
was obtained in the direct dimension due to the presence of low molecular weight
species. Preliminary spectral assignments of all of the observed signals were made on the
basis of both known chemical shifts of the related species, and the through-space and
through-bond phosphorous-phosphorous connectivities.
Description
xiii, 188 leaves : ill. ; 29 cm
Keywords
Polyphosphazenes , Nuclear magnetic resonance spectroscopy , Polymerization , Ring-opening polymerization , Dissertations, Academic