| Abstract: | Variable-temperature magic-angle spinning (MAS) NMR has been used to study the morphology and molecular dynamics in semicrystalline polyphosphazenes. This research was motivated by the observation of two well-resolved peaks in the 31P MAS NMR spectra of several semicrystalline polyphosphazenes. These peaks were assigned to a relatively rigid crystalline phase and a highly mobile mesomorphic phase. The resolution of 31P chemical shifts allowed the morphology and dynamics to be investigated using a variety of NMR methods. 31P and 13C MAS NMR studies were performed in order to investigate the phase transitions in semicrystalline polyphosphazenes, the nature of the interaction between the phases, and the molecular motion of the polymer chain and side groups as well as the types of molecular motion occurring in the different phases. The molecular dynamics of NLO chromophores doped in glassy polymer hosts, poly(methyl methacrylate) and poly(vinyl cinnamate), were examined in detail using one- and two-dimensional 15N MAS NMR. This work was motivated by the need to characterize loss of chromophore orientation in polymers for nonlinear optical applications. Line shape analysis of one-dimensional NMR spectra was used to investigate rapid molecular motion occurring near the glass transition temperature. Two-dimensional 15N MAS NMR was used to characterize slow reorientation of the chromophore at temperatures well below the glass transition temperature. A simple mathematical model was developed for the quantitative analysis of integrated peak intensities in two-dimensional MAS spectra. This model demonstrated that the chromophore molecules experience random, isotropic reorientation at the longest mixing times used and also permitted a calculation of the fraction of molecules that do not experience complete reorientation at any intermediate mixing time. |
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