Poly ring-flips

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. 

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.

Hiraoki et al. investigated the phenyl ring dynamics of poly(L-phenylalanine) using H-NMR, showing that it is characterized by a fairly broad distribution of correlation times. The mean correlation time of this distribution was 1.2 x 10 Hz at 25°C, which is close to that of the fast motional component of the B. mori and S.c. ricini silk fibroins. The Tyr ring flip in the pentapeptide [Leu ] enkephalin was reported to be 5.6 x 10 Hz at 25°C, which is close to that of the slow motional component observed here for silk fibroin. On the other hand, the ring motion in crystalline N-acetyl-L-Asp-L-Pro-L-Tyr-N -methylamide was found to be 1.1 X 10 Hz at 27°C, close to that of the fast motional component of the silk fibroins. 

Ring-flip motion of poly(p-phenylene vinylene)... 

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.)...

The dielectric relaxation exclusively reflects the amide group motions, starting at - 120 °C. So, the mechanical loss, E", occurring at a lower temperature originates from motions of the CH2 units, as evidenced by 13C NMR measurements (Sect. 6.2). Furthermore, these latter show that, at higher temperatures, the phenyl ring motions (oscillations then 7r-flips) are coupled to the amide group motions, in a way similar to that observed for poly(fere-phthalate) (Sect. 4). 

The equilibrium constant of the interconversion (8-40) should depend on temperature. However, the EQ-H% index of poly-DBO, determined at different temperatures (room temp, to 60 °C) is constant, indicating that flipping of the THP ring does not occur within the temperature range studied. There is also a possibility that flipping occurs on the penultimate unit of the growing cation. 

Solid-state NMR was used by Choudry et al. to examine the site-specific chain dynamics, from the Hz to kHz, of films of poly(ethylene ter-ephthalate) (PET) and filled PET. The CODEX sequence, was used to determine the fraction of flipping phenylene rings in these polymers. 

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