MOLECULAR NATURE OF THE BETA-RELAXATION IN POLY(METHYL METHACRYLATE) INVESTIGATED BY MULTIDIMENSIONAL NMR

The molecular motions underlying the dielectric and dynamic-mechanical beta relaxation in poly-(methyl methacrylate) (PMMA) have been elucid ated in detail by means of two-dimensional (2D) and three-dimensional (3D) C-13 exchange NMR of the carboxyl moiety and 2D H-2 exchange NMR of the methoxy group. The identify of the motions observed by NMR and the beta-relaxation dynamics is proved by the agreement of the measure d correlation times. The selective-excitation ''3D'' NMR spectrum prov es that, for every mobile side group, a relatively well-defined motion between two potential-energy minima occurs. The 2D spectral pattern s hows that the OCO plane of the side group undergoes 180-degrees (+/-20 -degrees) flips. Experiments with multiple exchange and selective satu ration for analysis of the growth of exchange signals (MESSAGE) prove that the molecular motions responsible for the beta relaxation are ass ociated with a distribution of correlation times, which appears to be bimodal with both mobile and trapped side groups. Consistently, analys is of the integral 2D exchange intensity shows that around 330 K only about 50% of the side groups participate in the large-amplitude dynami cal process on the time-scale of the beta-relaxation correlation time. The 2D H-2 NMR spectra, while exhibiting narrowing due to methyl-grou p rotation around the O-CH3 bond, exclude any significant motion of th e methoxy group around the C-OCH3 bond. Both the C-13 and the H-2 2D N MR spectra provide compelling evidence that the side-group flip is acc ompanied by a main-chain rearrangement which can be characterized as a random rotation around the local chain axis with a 20-degrees root-me an-square amplitude. This is ascribed to the fact that the asymmetric side group, after the flip, does not fit into its original environment . These findings explain both the dielectric and the dynamic-mechanica l beta relaxations of PMMA.