Chain Motion of Partially Deuterated Polystyrene above the Glass Transition Temperature As Revealed by Deuterium NMR Spectroscopy

Deuterium NMR spectra of polystyrene containing deuterium in 10–20 % of the center of the polymer chain were examined in the temperature range 380–470 K. The results coincide with those of Spiess and co-workers for polymers containing deuterium along the entire polymer chain. Below 390 Κ there is no motional averaging of the deuterium spectrum although it is possible to measure NMR correlation functions sensitive to molecular reorientation with appropriate three-pulse sequences. Above 430 Κ the spectrum is averaged into a Lorentzian line that narrows as the temperature is raised. Above 430 Κ the pseudo-solid echo that results in some other deuterated polymer melts from a pair of 90° pulses in quadrature was not formed. It appears that, in the presence of the spectral averaging produced by motion between chain entanglements, reptative motion out of the entanglements is fast enough at temperatures above 430 Κ to average any residual couplings that might otherwise exist. Nevertheless, this motion would be much too slow to average the complete spectrum by itself. The results suggest that deuterium NMR spectroscopy may not be as useful for the study of very slow reorientation in polymer melts as are forms of NMR spectroscopy that have a longer effective time scale such as ¹³C and ¹H NMR.