NMR Study of Chain Motion in Atactic Polypropylene at High Pressure

Deuteron solid-state NMR techniques at high pressure are used to study the chain dynamics in the amorphous polymer atactic polypropylene. The arrest of the structural relaxation above the glass-transition temperature Tg is investi- gated using one- and two-dimensional deuteron NMR spectra. The slow reorien- tation of the main chain segments is identified with the :-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 temperatures. Along isobars, at pressure values up to 5 kbar, the temperature dependence of the logarithmic average correlation time is very well described by a Vogel Fulcher function. The motion of the main chain is strongly dependent on the pressure, while its character is determined mainly by the distance to Tg . The introduction of the equation of state allows the investigation of the dynamic behavior on isothermal and isochoric paths on approaching Tg . It is found that along an isotherm the mobility as a function of the density is also of the Vogel Fulcher form.