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 methacrylat e) has been investigated in detail by multidimensional C-13 solid-stat e NMR of the carboxyl moiety. In the glassy state the underlying molec ular motion is anisotropic and involves a pi flip of the side group co upled to a rocking motion around the local chain axis with a +/-20-deg rees amplitude. Above the glass transition (T(g)) the molecular motion remains highly anisotropic. The geometry of the molecular motion is s imilar to that in the glass; however, the rocking amplitude increases upon raising the temperature above T(g). This is indicative of a prono unced influence of the alpha main-chain motion on the beta side-group motion which manifests itself by a marked increase of the rocking ampl itude to a value of +/-50-degrees at 365 K (T(g) + 27 K). It eventuall y leads to a locally anisotropic uniaxial chain motion at 395 K (T(g) + 57 K). This behavior differs significantly from that of other amorph ous polymers above T(g) where the molecular motions of both the main c hain and side groups are isotropic. The averaged correlation times ext racted from NMR experiments are in good agreement with data from diele ctric relaxation.