Atactic polypropylene at high pressure. III. Chain motion above the glass-transition temperature from one- and two-dimensional deuteron NMR spectra

Abstract One- and two-dimensional (2D) deuteron NMR experiments in high-pressure NMR equipment have been performed in an investigation of the effect of pressure (up to 5000 bar) on the slow chain dynamics in atactic deuterated polypropylene, above its glass-transition temperature Tg(p). The slow reorientation of the main chain segments, which can adequately be described as quasi-isotropic rotational diffusion, is identified with the α-process observed in mechanical relaxation experiments. On approaching the glass transition the time scale of the collective motion of the main chain becomes longer very rapidly at decreasing temperature. The motion of the main chain is strongly dependent on pressure. Along different isobars, the logarithmic average correlation time is shown to be determined completely by the distance to the glass transition and to follow the same Williams–Landel–Ferry (WLF)-function of T−Tg(p). On an isothermal approach to the glass transition, the chain dynamics as a function of density also follows a WLF function, or, equivalently, a Vogel–Fulcher type law.