Molecular Dynamics in Polymers from Multidimensional NMR

Abstract

Relaxation processes in polymers [1–3] reflect the various degrees of disorder present in these materials, which range from highly crystalline to completely amorphous. However, most relaxation techniques are relatively insensitive to the disorder involved. The most common manifestations of the disorder are non-Arrhenius behaviour of the temperature dependence of mean correlation times and nonexponential relaxation. In the early mechanical and dielectric experiments, e.g., it was common to work at constant frequency and to assign various processes (a, β,γ...) to the relaxation maxima observed on changing the temperature [1–3]. Relaxation times τ _c and apparent activation energies E_a were obtained by varying the frequency and noticing the temperature shift of the relaxation maxima. The early ¹H-NMR experiments provided similar information [4]: correlation times of relatively slow motions could be determined from the inverse width of the broad line NMR spectrum at the temperature where motional narrowing was observed, those of faster motions from the inverse Larmor frequency at the minimum of the spin-lattice relaxation time T ₁ in the laboratory, or T _1ρ in the rotating frame. The marked increase in the accessible frequency range as well as improvements in instrumentation and automatisation under computer control recently led to a revival of dielectric relaxation techniques, see Chapter 10. Likewise the high precision with which light scattering can be measured today made this technique a powerful tool to probe collective dynamic phenomena over an extremely broad range of relaxation rates.