Orientation Relaxation in Sheared Polystyrene Melts Measured by 13C SMAS-DECODER NMR

The slow magic angle spinning DECODER NMR technique is used to study the evolution of the orientation distribution function of phenyl rings in polystyrene during stress relaxation following cessation of steady shear flow. Natural abundance 13 C is detected in commercial samples of polystyrene sheared at Weissenberg numbers of 33 and 3.3. Moments up to eighth order in a spherical harmonic expansion are used to characterize the steady-state distributions, and the orientational order is observed to decrease as the relaxation proceeds. Contour plots of the orientation distribution function as a function of time are reconstructed from the experimentally determined moments of the distribution. The results emphasize the resolution required to characterize fully the orientation distribution function in deformed polymers.