Phenylene motion

Fig. 14.4. (a) 2D solid-state NMR spectra of a bundle of oriented, industrial PET fibers. The diagonal peaks are labeled according to Fig. 14.5. The 180° phenylene motion about the para-axis is reflected in the sharp exchange peak between positions 2 and 3. The high intensity along the diagonal is due to carbon atoms which have not changed their position and, therefore, their frequency during the mixing time r , 1 s. (b) Contour plot of the spectrum shown in Fig. 14.4(a). The 20 linearly spaced lines between 1.5 and 17% of the maximum height of the spectrum indicate clearly the 180° flip motion. In addition, a cut through the spectrum at peak 2 is shown.

Charati et al. [20] have reported dynamic-mechanical data for a number of different polyarylates. They concluded that the y relaxation originates from defects of the glass and is reduced through thermal annealing. A 5 relaxation (water sensitive) was attributed to phenylene motion in the bisphenol moiety and is shifted to high temperatures with... 

F. Horii, T. Beppu, N. Takaesu, M. Ishida, Selective excitation SASS C NMR smdy of the phenylene motion of glassy polymers, Magn. Reson. Chem. 32 (1994) S30. 

Following the above conclusion it is clear that the rather bizarre spectra of these polymers derive from the special features of 1,2-enchainment. Examination of molecular models reveals that runs of 1,2-enchained segments are considerably more restricted in their degrees of motional freedom than are runs of 1,4- enchained segments. The restriction arises partly from the absence of a "crankshaft" mode with 1,2-enchainment and partly from the steric interference of substituents on adjacent phenylene rings. 

The simplest motional description is isotropic tumbling characterized by a single exponential correlation time ( ). This model has been successfully employed to interpret carbon-13 relaxation in a few cases, notably the methylene carbons in polyisobutylene among the well studied systems ( ). However, this model is unable to account for relaxation in many macromolecular systems, for instance polystyrene (6) and poly(phenylene oxide)(7,... 

It was straightforward to apply the TRMC technique to study on-chain charge transport to ladder-type poly-phenylene (LPPP) systems because covalent bridging between the phenylene rings planarizes the chain skeleton, eliminates ring torsions, and reduces static disorder. One can conjecture that in these systems intra-chain motion should be mostly limited by static disorder and chain ends. To confirm this... 

Use of Proton and 13C NMR at temperatures from 27 to 400 °C provide very detailed information as to the nature of these motions [30], Thus, it has been shown that even at 300 °C the phenylene ring displays a rapid 180° flipping motion. Above the transition temperature of 350 °C the ester unit also begins to rotate in the form of 180° flips as a result of lattice expansion (see Fig. 7). Furthermore, the entire repeat unit participates in a synchronous motion. This should be interpreted as a jumping motion rather than free or random rotation. 

Garroway, Ritchie, and Moniz 70 continued the characterization of epoxy polymers with respect to molecular motion using variable temperature, solid state C-13 NMR. DGEBA was the epoxy of interest. The DGEBA was cured with piperidine, m-phenylene diamine, hexahydrophthalic anhydride and nadic methyl anhydride. The piperidine cured DGEBA had the best resolved polymer spectra. This system... 

One way of studying molecular motions involves monitoring the reduction of dipole-dipole couplings probed by DQ spinning sidebands. The site selectivity is particularly high for heteronuclear DQ coherences. In Fig. 8, simulated sideband patterns are plotted for the C-H group, which is a sensitive probe of phenylene rotational motions, often met in practice. At low temperatures, one would expect... 

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