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The main chain dynamics of amorphous poly(ethyl methacrylate) (PEMA) and poly(methyl methacrylate) (PMMA) below and above their respective glass transition temperatures T g are analyzed by two-dimensional solid-state exchange 2H NMR spectroscopy. In both polymers, a restricted mobility of the polymer backbone is already present in the glassy state, as is directly demonstrated and quantified using samples deuterated at the methyl and methylene moieties of the polymer main chain. The unusual main chain mobility below T g is coupled to the β-relaxation process, which involves 180° flips of the carboxyl side groups. At their respective glass transition temperatures, the coupling of the β-process to the main chain motions manifests itself differently in both polymers; the smaller ester side group reorients comparatively fast in PMMA, whereas in PEMA, the reorientation of the bulkier side group remains anisotropic and the correlation times are slower by about 1 order of magnitude. Therefore, in PMMA, the β-relaxation predominantly influences the time scale of the α-relaxation, leading to a particularly high mobility of the main chain itself. In contrast, in PEMA, a slow uniaxial diffusion of the main chain around its local axis sets in at T g, the β-process thus affecting mainly the geometry of backbone motions, as is further corroborated by comparing one-dimensional 13C NMR spectra with two-dimensional exchange 2H NMR spectra at higher temperatures. In summary, the coupling of the α- and β-processes leads to longer mean correlation times for the α-relaxation in PEMA than in PMMA.

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We present a new multidimensional NMR exchange experiment, three dimensional difference correlated exchange spectroscopy ͑DICO͒, which allows one to extract detailed information about the time scale as well as the geometry of rotational processes simultaneously. In contrast to experiments presently available the geometric information specified by a dynamic order parameter is independent of a specific model. Among others full isotropisation can be directly seen from the spectrum via a symmetry criterion. We apply this method to study the chain motion of polystyrene slightly above the glass transition. The DICO spectra prove that a typical chain segment performs small angle fluctuations as well as large angle jumps which can neither be described as random jumps nor tetrahedral jumps. A typical jump angle can be estimated to be of the order of 30°Ϫ40°.