Phenyl ring flips

Fig. 11. Calculated spectra of PBLG-d5 for the phenyl ring flipping as a function of temperature. Figures on the left- and right-hand side of the spectra indicate temperature and jump rate obtained from Tj, respectively.

The antiplasticiser additives do not affect the motions of the carboxyl groups, but they hinder the phenyl ring flips. 

The mean value of the energy barrier for the 38 resulting phenyl ring flips was 45 d= 28 kj mol-1. The peak energy barriers showed a broad distribution, which could be fitted to a Williams-Watts distribution function with a between 0.1 and 0.2. By applying the transition state theory, the distribution of ring flip frequencies could be derived, as shown in Fig. 59. 

Rocking main-chain motions, which occur in bulk BPA-PC when phenyl rings flip and carbonate groups change conformation, with rms averages around 13° and 11°, respectively. 

Finally, it is interesting to point out that, whereas for poly(ethylene tere-phthalate), bisphenol A polycarbonate and the aryl-aliphatic copolyamides (Sects. 4 to 6, respectively), the phenyl ring flips plays an important role in the p transition processes. In the case of the aryl-aliphatic epoxy networks such 7t-flips exist but they are only indirectly involved in the [J> transition. 

Indeed, whereas in these latter polymers the cooperativity of motions associated with the high temperature part of the ft transition is an intramolecular cooperativity, in the case of BPA-PC (besides an intrinsic cooperativity between phenyl ring flips attached to the same isopropylidene unit) there is an intermolecular cooperativity which develops in this high temperature part of the ft transition, associated mostly with the occurrence of phenyl ring n-flips. 

Comment For polystyrene, a /J-like relaxation process was attributed to phenyl-ring flips, while the main chain does not participate, see Section 3.3. Hence, when the backbone motion of a selectively labeled compound is studied, polystyrene can be regarded as a type A glass former. 

Fig. 2. The MAT experiment applied to poly(2-hydroxypropyl ether of bisphenol A)5 (top) to examine the 180° ring flips affecting 13C 4 and 5. (a) The complete two-dimensional MAT spectrum.5 The projection in f2 is effectively the lineshape that would be recorded for a powder sample. As this spectrum clearly shows, the chemical shift anisotropy powder patterns from the nine 13C sites in this polymer are extensively overlapped and would not be resolved without the aid of this MAT experiment, (b) The powder lineshapes for each 13C site taken from the two-dimensional spectrum in (a).5 Those for carbons 4 and 5 show distortions of the lineshape shoulders typical of motional averaging, in this case from 180° phenyl ring flips.

Fig. 3. Simulations of the lineshape expected for carbon 4 in Fig. 2 assuming phenyl ring flips in the fast-motion limit and a Gaussian distribution of flip angles centred at 180° with standard deviation a ...

Figure 55 Experimental H powder patterns, simulated spectra, and model phenyl-ring flip-rate distributions for quenched, phenylene-labeled poly (ethylene terephthalate) for (a) crystalline phase (b) mobile noncrystalline (c) total noncrystalline. (From Ref. 223, 1998 Elsevier Science Ltd.)...

Further examples of characteristic dynamical processes in liquid crystal polymers, discussed in detail elsewhere [19,31], include the phenyl ring flip in side chain liquid crystal polymers [100] and the conformational dynamics of the spacer in a main chain liquid crystal polymer [101]. 

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