The molecular motions underlying the dielectric and dynamic-mechanical 8 relaxation in poly-(methyl methacrylate) (PMMA) have been elucidated in detail by means of two-dimensional (2D) and threedimensional (3D) 13C exchange NMR of the carboxyl moiety and 2D 2H exchange NMR of the methoxy group. The identity of the motions observed by NMR and the ^-relaxation dynamics is proved by the agreement of the measured correlation times. The selective-excitation "3D" NMR spectrum proves that, for every mobile side group, a relatively well-defined motion between two potential-energy minima occurs. The 2D spectral pattern shows that the OCO plane of the side group undergoes 180°(±20°) flips. Experiments with multiple exchange and selective saturation for analysis of the growth of exchange signals (MESSAGE) prove that the molecular motions responsible for the 8 relaxation are associated with a distribution of correlation times, which appears to be bimodal with both mobile and trapped side groups. Consistently, analysis of the integral 2D exchange intensity shows that around 330 K only about 50% of the side groups participate in the large-amplitude dynamical process on the time-scale of the ^-relaxation correlation time. The 2D 2H NMR spectra, while exhibiting narrowing due to methyl-group rotation around the 0-CH3 bond, exclude any significant motion of the methoxy group around the C-OCH3 bond. Both the 13C and the 2H 2D NMR spectra provide compelling evidence that the side-group flip is accompanied by a main-chain rearrangement which can be characterized as a random rotation around the local chain axis with a 20°r oot-mean-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-mechanical 8 relaxations of PMMA.

The occurrence and rate of 180° chain-flip motions in the crystalline regions of two polyethylenes were studied by 13C NMR. In high-density polyethylene (HDPE) and in ultradrawn ultrahigh molecular weight polyethylene (UHMWPE) fibers, the changes in the 13C−13C dipolar couplings brought about by the reorientations of the 13C−13C internuclear vectors in the crystallites were observed. In the HDPE sample, which was labeled with 4% of 13C−13C pairs, the rotational motion was observed directly via two-dimensional exchange spectroscopy, stimulated-echo decays, and 1D line shape changes monitoring the 13C−13C dipolar coupling. The data show that the jumps occur between two sites, with a rotation angle of 180° and with a jump rate of ∼10/s at ambient temperature. The correlation function of the motion was found to be slightly nonexponential, with a stretched-exponential β parameter of 0.8 ± 0.1. The data yield an activation energy of 93 ± 10 kJ/mol for the 180° chain flips. In the fibers, the narrowing of natural-abundance 13C−13C dipolar satellites is a clear NMR signature of the chain motion, indicating a jump rate of 150/s at 360 K, which is 20 times slower than in the unoriented HDPE. The correlation time dependence of the 1H T 1 ρ relaxation time, which probes the modulation of H−H dipolar couplings in the crystallites, was determined directly. Relations between the chain flip motion, the dynamic-mechanical α-relaxation, creep, and drawability are discussed.

Stiff macromolecules with flexible side chains are investigated by proton spin diffusion experiments with 13C detection and by a recently developed wideline separation 2D lH-13C NMR experiment (WISE-NMR spectroscopy). The conformational order and the molecular mobility of the alkyl side chains (CieHsa) are characterized for samples with polyester, polyamide, and polyimide main chains. The side chains, which are phase-separated from the main chain in a layer-type structure, can form crystalline as well as amorphous phases. The sizes of these domains depend on the nature of the main chains and their organization.In the polyimide and the polyester with regular main-chain packing, crystalline as well as amorphous regions are observed extending over more than one layer spacing. The heterogeneity observed in the polyamide is only of the order of the layer spacing. The polyester can also be obtained in a modification with uniformly ordered but anisotropically mobile side chains and conformationally disordered main chains. These results indicate coupling between the main-chain and side-chain packing in the investigated stiff macromolecules with flexible side chains.