2D EXCHANGE NMR INVESTIGATION OF THE ALPHA-RELAXATION IN POLY(ETHYL METHACRYLATE) AS COMPARED TO POLY(METHYL METHACRYLATE)

The main chain dynamics of amorphous poly(ethyl methacrylate) (PEMA) a nd poly(methyl methacrylate) (PMMA) below and above their respective g lass transition temperatures T-g are analyzed by two-dimensional solid -state exchange H-2 NMR spectroscopy. In both polymers, a restricted m obility 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 un usual main chain mobility below T-g is coupled to the beta-relaxation process, which involves 180 degrees flips of the carboxyl side groups. At their respective glass transition temperatures, the coupling of th e beta-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 abou t 1 order of magnitude. Therefore, in PMMA, the beta-relaxation predom inantly influences the time scale of the alpha-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 ax is sets in at T-g, the beta-process thus affecting mainly the geometry of backbone motions, as is further corroborated by comparing one-dime nsional C-13 NMR spectra with two-dimensional exchange H-2 NMR spectra at higher temperatures. In summary, the coupling of the alpha- and be ta-processes leads to longer mean correlation times for the alpha-rela xation in PEMA than in PMMA.