Two-Dimensional Solid-State NMR Studies of Ultraslow Chain Motion: Glass Transition in Atactic Poly(propylene) versus Helical Jumps in Isotactic Poly(propylene)

The deuteron 2D exchange experiment has been applied to study the ultraslow rotational chain dynamics of atactic (aPP) and isotactic poly(propylene) (iPP). The motional mechanism was found to be vastly different for these polymers, reflecting the different chain conformation and packing. Whereas in amorphous aPP in the vicinity of the glass transition a diffusive motion was observed, iPP was shown to perform helical jumps about the helix axis in the crystalline domains at temperatures above 360 K. The temperature variation of the mean correlation times, obtained from an analysis of the aPP spectra based on isotropic rotational diffusion with a distribution of correlation times, follows the WLF equation over 11 orders of magnitude between 10-10 and 10 s. The parameters extracted from this fit correspond to textbook values known from macroscopic measurements of the viscoelastic behavior in amorphous polymers. This shows that 2D NMR, although monitoring the chain dynamics via a localized probe, is able to follow the collective dynamics of the glass process. The discrete jump motion observed for iPP is caused by 120° rotations of the 31 helix about the helix axis. The relative angle of 113° between the methyl groups before and after the jump is determined from the 2D spectra directly without the need of interfacing a model and agrees with the value calculated from crystallographic data. © 1990, American Chemical Society. All rights reserved.