Morphology, 31P spin diffusion, and phase transitions in a representative semicrystalline polyphosphazene by solid-state NMR

Abstract : Solid-state NMR spectroscopy was used to investigate the morphology and molecular dynamics of polyBIS(3-methylphenoxy)phosphazenel-PB3MP, a representative semicrystalline polyphosphazene. Variable temperature 31P magic-angle spinning (MAS) NMR spectra revealed two well resolved isotropic resonances for the crystalline and mesomorphic phases. The ability to resolve resonances for the two phases makes this class of polymers well suited for studying morphology and phase transitions using NMR. 31P spin-lattice relaxation (T1) behavior for PB3MP at 23 deg C suggested that spin diffusion occurs between the two phases, which was confirmed by the observation of cross peaks in a two-dimensional exchange experiment. The measured 31P spin diffusion rate is intermediate between the well established limits of 1H and 13C spin diffusion, and it appears as an apparent anomaly in the 31P spin-lattice relaxation behavior. Employing a model diffusion equation, the spin diffusion rate constant was used to estimate a characteristic diffusion distance of 4.5 nm, implying a lamellar thickness of ca. 9 nm. The amplitude of side-group motion in the crystalline and mesomorphic phases was investigated by the analysis of sideband patterns in 13C MAS spectra obtained at slow spinning speeds. Negligible motion occurs in the crystalline phase, whereas large amplitude motion occurs in the mesomorphic phase. Variable temperature 31P and 1H rotating frame spin-lattice relaxation (T1P) studies revealed activation energies of ca. 10 kcal/mol for motion in the mesomorphic phase.