D. Schaefer

Two‐dimensional exchange nuclear magnetic resonance of powder samples. IV. Distribution of correlation times and line shapes in the intermediate dynamic range

The two‐dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster chain motions in amorphous polymers in the glass transition range. Acquisition of the time domain data with a four‐pulse sequence leads to new characteristics in the corresponding 2H 2D line shapes if the correlation times of the motion are in the intermediate dynamic range. From the asymmetric 2D line shapes, the width of the correlation time distribution can be determined with higher accuracy than through conventional 1D NMR methods. Experimental data are presented on two amorphous polymers—atactic polypropylene and cis‐1,4‐polyisoprene—and are analyzed in terms of isotropic rotational diffusion. Deviations from this simple model due to the presence of conformational transitions within the polymer backbone are detected. Close to Tg, the mean correlation times extracted from 2D exchange NMR exhibit strongly nonArrhenius behavior usually described by the Williams–Landel–Ferry (WLF)...

Two-dimensional nuclear magnetic resonance with sample flip for characterizing orientation distributions, and its analogy to x-ray scattering

A two‐dimensional (2D) exchange NMR (nuclear magnetic resonance) experiment with sample flip during the mixing time (direction exchange with correlation for orientation‐distribution evaluation and reconstruction, DECODER) is presented, which allows investigation of molecular order in great detail. For anisotropic NMR interactions with axial symmetry (η=0), the features of the orientation distribution of certain molecular axes are reflected directly in the distribution of the intensity in the 2D spectrum. The orientation distribution in terms of two angles can be largely reconstructed by a simple rearrangement of the spectral intensity. Analogies between this NMR experiment and wide‐angle x‐ray scattering (WAXS) techniques for pole figure analysis are identified. Practical aspects of the DECODER method are considered in detail, e.g., sample orientations that are particularly suited for elucidating biaxiality with high resolution, or that are convenient in terms of sample preparation. Even with spectral ove...

Molecular dynamics in glassy polyelectrolytes: a NMR study

Abstract The segmental dynamics of a special “ionene” polyelectrolyte, poly[(dimethylimino) decamethylene-tetrafluoroborate] ([—(CH 2 ) 10 —N + (CH 3 ) 2 —] n · n BF − 4 ), have been investigated by applying one-and two-dimensional deuteron NMR techniques. Selective deuteration in the methyl groups and in the α-, β- and γ-positions of the alkyl chain allowed for investigation of the molecular mobility of each site. Due to strong Coulombic interactions between the cationic centres, the ammonium groups are essentially static below the glass transition, whereas the alkyl chains undergo trans-gauche isomerizations. At T g the ammonium groups become mobile, and we discuss here the coupling of this glass transition process to the local methylene unit dynamics.

Dielectric and electric relaxation in ionene-glasses

Abstract Ionene-glasses are considered as being a new class of materials, the properties of which are investigated in detail by thermoanalytical, solid state NMR and dielectric techniques. The glass transition in these materials is characterized by the softening of a quasi-lattice formed by the N-cations, while the organic chain segments are mobile already well-below Tg. Ionic conductivity in the glassy state is based on thermally activated anion transport. The influence of anion size and flexibility of the organic chain segments on the conductivity are discussed.

2D-solid state NMR studies of ultraslow motions: phenylflips and chain motions in the glassy state

Abstract The deuteron 2D exchange NMR experiment was applied to study ultraslow molecular reorientations in polymers below the glass transition. Experimental spectra are presented for phenyldeuterated polycarbonate (PC-d 4 ) and an ionene polyelectrolyte ([poly-( N , N -dimethylimino)decamethylenetetrafluoroborate] (I-10-Me-BF 4 ), deuterated in the α-position of the alkylene chain. In PC the phenylflip motion in the glassy state does not involve rotations by exactly 180°. Instead the reorientational angle distribution (RAD) of this local process is rather ill-defined and changes when mechanical stress is applied to the sample. The chain dynamics in the solid polyelectrolyte can be considered as a motion between fixed ends, due to the strong Coulomb interaction of the ionic centres. No evidence is found for trans-gauche isomerization; instead the spectra exhibit the characteristics of a restricted diffusive motion.

Two-Dimensional Solid State NMR Studies of Ultraslow Molecular Reorientation in Solid Polymers

Abstract Slow molecular reorientations in solid polymers have been investigated by means of two-dimensional solid state 2H- and 13C-NMR.