COUPLING OF THE ALPHA AND BETA-PROCESSES IN POLY(ETHYL METHACRYLATE) INVESTIGATED BY MULTIDIMENSIONAL NMR

The coupling of the alpha and beta processes in poly(ethyl methacrylate) has been investigated in detail by multidimensional C-13 solid-state NMR of the carboxyl moiety. In the glassy state the underlying molecular motion is anisotropic and involves a pi flip of the side group coupled to a rocking motion around the local chain axis with a +/-20-degrees amplitude. Above the glass transition (T(g)) the molecular motion remains highly anisotropic. The geometry of the molecular motion is similar to that in the glass; however, the rocking amplitude increases upon raising the temperature above T(g). This is indicative of a pronounced influence of the alpha main-chain motion on the beta side-group motion which manifests itself by a marked increase of the rocking amplitude to a value of +/-50-degrees at 365 K (T(g) + 27 K). It eventually leads to a locally anisotropic uniaxial chain motion at 395 K (T(g) + 57 K). This behavior differs significantly from that of other amorphous polymers above T(g) where the molecular motions of both the main chain and side groups are isotropic. The averaged correlation times extracted from NMR experiments are in good agreement with data from dielectric relaxation.