Further NMR evidence for various motions in polymers

In this section some further examples of conclusions reached from NMR studies of motion in polymers are given. The emphasis is on the results, rather than on the specific NMR techniques used. It is important to understand that, although in what follows only one type of relaxation mechanism is mentioned for any one polymer, several different relaxations can usually take place. 

In the previous section the helical jumping of the molecules in the crystallites of POM was discussed. Similar motions have been observed in a number of other crystalline polymers, including poly(ethylene oxide) (PEO), isotactic polypropylene (IPP) and polyethylene. At 240 K each helical chain in PEO makes about three jumps per second. In the crystalline regions of IPP helical jumps along the 3i helix are observed at temperatures in the range 340 380 K, whereas in syndiotactic polypropylene, which has a more complicated 4j helix, no jumps are observed. 

Rotational jumps not involving translations are observed in many polymers, particularly important types being the rotational jumps of methyl groups, as considered above for isotactic polypropylene and observable for a wide variety of polymers, and the 180° flips of para-disubsti-tuted benzene rings (phenylene rings, — — ). These flips have been observed in many polymers, including amorphous polymers such as bisphenol-A polycarbonate (PC), —C(CH3)2— —O—CO O,  

Such spectra often require a wide range of values of for their interpretation or the assumption of a non-exponential form of the correlation function C(t) in equation (5.13), such as the stretched exponential (see problem 5.10). Relaxations associated with the glass transition are discussed in more detail in chapter 7. Chapter 6 is concerned with the mechanical properties of polymers well away from the glass transition and from other relaxation regions. 

Motion Region Nature Typical activation energy (kJ morh Remarks 

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