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.

Poly[(ethylene glycol)-rotaxa-(R-cyclodextrin)]s, (PEG-rotaxa-RCD)s, were prepared by a direct route from commercially available poly(ethylene glycol)s (PEG) with molecular weights from 1.5 to 20 kg/mol. The rotaxanated structure was verified by diffusion-ordered 2D NMR spectroscopy (DOSY). The self-diffusion coefficients for PEG-rotaxa-RCD and PEG in dilute DMSO-d6 scale with their respective molecular weights as DPEG-rotaxa-RCD ∼ Mn -0.60(0.05 and DPEG ∼ Mn -0.55(0.03 . With increasing temperature, the hydrodynamic radius of the PEG-rotaxa-RCD increases by the same slope as the unthreaded PEG backbone. These polyrotaxanes, in which up to 70% of the backbone is covered with cyclodextrins, behave as random coils in good solvents.

The coupling of the a and 8 processes in poly(ethyl methacrylate) has been investigated in detail by multidimensional 13C solid-state NMR of the carboxyl moiety. In the glassy state the underlying molecular motion is anisotropic and involves a it flip of the side group coupled to a rocking motion around the local chain axis with a ±20°a mplitude. Above the glass transition (Tg) the molecular motion remains highly anisotropic. The geometry of the molecular motion is similar to that in the glass; however, the rocking amplitude increases upon raising the temperature above Tt. This is indicative of a pronounced influence of the a main-chain motion on the 8 side-group motion which manifests itself by a marked increase of the rocking amplitude to a value of ±50°at 365 K (Tg+ 27 K). It eventually leads to a locally anisotropic uniaxial chain motion at 395 K (Tg + 57 K). This behavior differs significantly from that of other amorphous polymers above T( where the molecular motions of both the main chainandside groups are isotropic. The averaged correlation times extracted from NMR experiments are in good agreement with data from dielectric relaxation.

We report the identification and elucidation of the mechanistic role of molecular precursors and nanoscale (1-3 nm) intermediates with intrinsic curvature in the formation of single-walled aluminosilicate nanotubes. We characterize the structural and compositional evolution of molecular and nanoscale species over a length scale of 0.1-100 nm by electrospray ionization mass spectrometry, nuclear magnetic resonance spectroscopy ((27)Al liquid-state, (27)Al and (29)Si solid-state MAS), and dynamic light scattering. Together with structural optimization of key experimentally identified species by solvated density functional theory calculations, this study reveals the existence of intermediates with bonding environments, as well as intrinsic curvature, similar to the structure of the final nanotube product. We show that "proto-nanotube-like" intermediates with inherent curvature form in aqueous synthesis solutions immediately after initial hydrolysis of reactants, disappear from the solution upon heating to 95 °C due to condensation accompanied by an abrupt pH decrease, and finally form ordered single-walled aluminosilicate nanotubes. Detailed quantitative analysis of NMR and ESI-MS spectra from the relevant aluminosilicate, aluminate, and silicate solutions reveals the presence of a variety of monomeric and polymeric aluminate and aluminosilicate species (Al(1)Si(x)-Al(13)Si(x)), such as Keggin ions [AlO(4)Al(12)(OH)(24)(H(2)O)(12)](7+) and polynuclear species with a six-membered Al oxide ring unit. Our study also directly reveals the complexation of aluminate and aluminosilicate species with perchlorate species that most likely inhibit the formation of larger condensates or nontubular structures. Integration of all of our results leads to the construction of the first molecular-level mechanism of single-walled metal oxide nanotube formation, incorporating the role of monomeric and polymeric aluminosilicate species as well as larger nanoparticles.