Geometry of Main Chain Motions in Poly(ethyl methacrylate) Monitored by 3D Difference Correlated NMR

Abstract

A systematic analysis of the rotational dynamics in poly(ethyl methacrylate) (PEMA) slightly above the glass transition temperature is performed with advanced multidimensional NMR spectroscopy focusing on a 3D difference correlated exchange experiment. This novel technique is capable of obtaining the detailed geometry of motion as well as the time scale of reorientation without employing a specific model. In PEMA, the coupling of the α- and β-process manifests itself in rotational dynamics that comprise both pronounced small angle fluctuations and large angle reorientations. The time evolution of the latter is characterized quantitatively by a dynamic order parameter, , a transient quantity which monitors the randomization of segmental orientation. The molecular orientations are found to remain correlated on a time scale exceeding the mean correlation time of rotational motion as obtained by 2D exchange NMR. A comparison with the results for polystyrene obtained previously indicates smaller effective jump angles and higher geometric restrictions in PEMA. Moreover, the fraction of segments performing large angle displacements is only slowly increasing with time, demonstrating that some segments stay trapped in their environment.