Phenyl group motions

The model which has been developed (2>1) to fit these results involves formation of excimers via thermally activated phenyl group motion. Such motion is restricted in a rigid matrix, and "pre-form-ed" excimer sites, where little phenyl motion would be required for collapse to the excimer, are evidently rare. 

Table II Phenyl Group Motion Simulation Parameters Three Bond Jump Model Using the...

PPO dPS DNMR Phenyl deuterated PS employed to study phenyl group motion in blends. PS phenyl group motion unchanged by blending 259... 

Polycarbonate (PC) serves as a convenient example for both, the direct determination of the distribution of correlation times and the close connection of localized motions and mechanical properties. This material shows a pronounced P-relaxation in the glassy state, but the nature of the corresponding motional mechanism was not clear 76 80> before the advent of advanced NMR techniques. Meanwhile it has been shown both from 2H NMR 17) and later from 13C NMRSI) that only the phenyl groups exhibit major mobility, consisting in 180° flips augmented by substantial small angle fluctuations about the same axis, reaching an rms amplitude of 35° at 380 K, for details see Ref. 17). 

Fig. 22. Deuteron NMR spectra at room temperature for the phenyl groups in polycarbonate itself and in mixtures of polycarbonate and para-dichlorobenzene and polychlorinated biphenyls as indicated. Note that the additives block the phenyl motion for a substantial fraction of the monomer units...

Further evidence for isotropic chemical shifts was found in variable temperature studies. The aromatic carbon ortho to the oxygen resolved into two peaks at low temperatures. The aromatic carbons meta to the oxygen have two peaks, but these peaks are not as well resolved. As the temperature was raised, the splittings in both coalesced into one peak, indicating that there is rapid sampling from two magnetic environments. This is indicative of the motion of the phenyl group with respect to the backbone. At lower temperatures, the methyl resonance broadens and eventually... 

As mentioned, the dielectric j3 peak is related to carbonate motions however, it is important to check whether the adjacent phenyl groups are involved or not in such motions. Concerning this feature, it is interesting to consider the dielectric behaviour of TMBPA-PC. The temperature dependence of the... 

Actually, the methyl and carbonyl dynamics within the BPA-PC repeat unit (Fig. 54) can be conveniently analysed by considering the methyl motion of DPP and the carbonyl motions of DPC. However, in the case of phenyl ring motions, neither DPP nor DPC correspond to the situation within BPA-PC. Indeed, in the latter the isopropylidene and the carbonate groups bonded to the phenyl rings are para to one another, with the Ci - Cp bonds lying almost on the same axis as the O - Cp bonds. 

Inhomogeneous local packing, which leads to a broad distribution of the activation energy of both phenyl ring and carbonate group motions. This... 

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