NMR studies of polymer dynamics and conformation

The normal case with a molten or dissolved polymer is that its conformational jumps are quite rapid, compared with the tumbling and looping motions of entire chains. A typical barrier to a conformational jump in a vinyl polymer chain without external constraints is only approximately 12-20 kJ mol and is therefore readily surmounted at room temperature, typically at least 10 times per second. In this case, the local motions will contribute to the NMR relaxation parameters of the polymer, such as T, T2 and NOE. Similar motions will occur in the gel state and in the flexible solid, above Tg. 

However, the relaxation parameters, in particular T2 and NOE, are also sensitive to the slower molecular motions, such as chain reptation, and also to chain-chain interactions. It is not a trivial matter to disentangle the contributions of different motions to each overall relaxation parameter, but this separation can be achieved if sufficient data are available. A full relaxation and conformational analysis of this type can yield a wealth of motional and averaged structural information. This chapter sets out the relationships between all these properties in a simplified way, which nevertheless relates to physical fundamentals. 

The experimental data which the NMR experiment can readily obtain are as follows. 

the spin-spin relaxation time. With many homogeneous polymers above their glass transition temperature, this simply equals 1 / n x width at half peak height in Hz) (see chapter 1) and it is easily measured. But when the linewidth is a composite of contributions from both relaxation and chemical shift variation, then T2, the contribution from relaxation, must be measured 

Much more informative Overhauser enhancements may also be observed between like (or unlike) nuclei of different chemical shift, especially protons, provided the irradiation is selective, and does not affect the spins under observation. (Selective contributions can be measured similarly, but less conveniently or accurately). As will be seen below, the presence and size of such pairwise NOEs is a valuable source of averaged structural information. 

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Liquid state NMR studies of polymer dynamics and conformation... 

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