Unravelling main- and side-chain motions in polymers with NMR spectroscopy and relaxometry: The case of polyvinyl butyral

Polyvinyl butyral (PVB) is an amorphous polymer employed in many technological applications. In order to highlight the relationships between macroscopic properties and dynamics at a microscopic level, motions of the main-chain and of the propyl side-chains were investigated between Tg − 288◦ C and Tg + 55◦ C, with Tg indicating the glass transition temperature. To this aim, a combination of solid state Nuclear Magnetic Resonance (NMR) methods was applied to two purposely synthesized PVB isotopomers: one fully protonated and the other perdeuterated on the side-chains.1 H time domain NMR and1 H field cycling NMR relaxometry experiments, performed across and above Tg, revealed that the dynamics of the main-chain corresponds to the α-relaxation associated to the glass transition, which was previously characterized by dielectric spectroscopy. A faster secondary relaxation was observed for the first time and ascribed to side-chains. The geometry and rate of motions of the different groups in the side-chains were characterized below Tg by2 H NMR spectroscopy

Deuteron spin-lattice relaxation in the presence of an activation energy distribution: Application to methanols in zeolite NaX

A new method is introduced for analyzing deuteron spin-lattice relaxation in molecular systems with a broad distribution of activation energies and correlation times. In such samples the magnetization recovery is strongly non-exponential but can be fitted quite accurately by three exponentials. The considered system may consist of molecular groups with different mobility. For each group a Gaussian distribution of the activation energy is introduced. By assuming for every subsystem three parameters: the mean activation energy E-0, the distribution width sigma and the pre-exponential factor tau(0) for the Arrhenius equation defining the correlation time, the relaxation rate is calculated for every part of the distribution. Experiment-based limiting values allow the grouping of the rates into three classes. For each class the relaxation rate and weight is calculated and compared with experiment. The parameters E-0, sigma and tau(0) are determined iteratively by repeating the whole cycle many times. The temperature dependence of the deuteron relaxation was observed in three samples containing CD3OH (200% and 100% loading) and CD3OD (200%) in NaX zeolite and analyzed by the described method between 20 K and 170 K. The obtained parameters, equal for all the three samples, characterize the methyl and hydroxyl mobilities of the methanol molecules at two different locations. (C) 2012 Elsevier Inc. All rights reserved

Translational and rotational mobility of methanol-d(4) molecules in NaX and NaY zeolite cages: A deuteron NMR investigation

Nuclear magnetic resonance (NMR) provides means to investigate molecular dynamics at every state of matter. Features characteristic for the gas phase, liquid-like layers and immobilized methanol-d(4) molecules in NaX and NaY zeolites were observed in the temperature range from 300 K down to 20 K. The NMR spectra at low temperature are consistent with the model in which molecules are bonded at two positions: horizontal (methanol oxygen bonded to sodium cation) and vertical (hydrogen bonding of hydroxyl deuteron to zeolite framework oxygen). Narrow lines were observed at high temperature indicating an isotropic reorientation of a fraction of molecules. Deuteron spin-lattice relaxation gives evidence for the formation of trimers, based on observation of different relaxation rates for methyl and hydroxyl deuterons undergoing isotropic reorientation. Internal rotation of methyl groups and fixed positions of hydrogen bonded hydroxyl deuterons in methyl trimers provide relaxation rates observed experimentally. A change in the slope of the temperature dependence of both relaxation rates indicates a transition from the relaxation dominated by translational motion to prevailing contribution of reorientation. Trimers undergoing isotropic reorientation disintegrate and separate molecules become localized on adsorption centers at 166.7 K and 153.8 K for NaX and NaY, respectively, as indicated by extreme broadening of deuteron NMR spectra. Molecules at vertical position remain localized up to high temperatures. That indicates the dominating role of the hydrogen bonding. Mobility of single molecules was observed for lower loading (86 molecules/uc) in NaX. A direct transition from translation to localization was observed at 190 K. (C) 2012 Elsevier Inc. All rights reserved

Local and nonlocal hydrogen dynamics in α-oxalic acid dihydrate. A 1D and 2D single crystal deuteron NMR study

We report on an investigation of the hydrogen dynamics in crystals of α-oxalic acid dihydrate by 2H-NMR line shape analysis and 2H—2D-exchange spectroscopy. We identify and characterize three types of hydrogen motions: (1) flips of water molecules, (2) exchange of a carboxylic deuteron with the deuterons of the nearest, hydrogen bonded water molecule, and (3) exchange of a carboxylic deuteron with the deuterons of a distant, non-hydrogen bonded water molecule. In addition, we observe hydrogen diffusion over macroscopic distances. At least the first two types of motions are thermally activated. They constitute local processes. We report the respective activation energies. The third process has not been observed before. Its rate at 333 is only about 25 s~1. It enables the deuterons to leave their initial immediate neighbourhood, but, alone, does not account for the observed macroscopic diffusion.Wir berichten über eine Untersuchung der Dynamik der Wasserstoffe in Kristallen von α-Oxalsäure-Dihydrat mittels Deuterium-NMR-Linienformanalyse und zweidimensionaler Deuterium-Austauschspektroskopic. Wir identifizieren und charakterisieren drei Arten von Wasserstoffbcwegungen: (1) Flips von Wassermolekülen, (2) Austausch des Deuterons einer Carboxylgruppe mit den Deuteronen des nâchsten Wassermoleküls (dieses ist an die Carboxylgruppe über eine Wasserstoffbrücke gebunden) und (3) Austausch des Deuterons einer Carboxylgruppe mit den Deuteronen eines weiter entfernt gelegenen Wassermoleküls, mit dem die betreffende Carboxylgruppe keine Wasserstoffbrücken-Bindung eingeht. Zusätzlich beobachten wir Diffusion von Wasserstoffen über makroskopische Distanzen. Zumindest die beiden ersten Arten von Bewegungen sind thermisch aktiviert. Es handelt sich bei ihnen um lokale Prozesse. Der dritte Prozeß ist bislang noch nicht beobachtet worden. Seine Geschwindigkeitskonstante beträgt bei 333 K, d.h. etwa 40 unterhalb der Zersetzungstemperatur, nur etwa 25 s-1. Er ermöglicht es den Wasserstoffen, die unmittelbare Umgebung ihrer Anfangspositionen zu verlassen. Für sich allein genommen vermag er jedoch nicht die beobachtete makroskopische Diffusion von Wasserstoffen zu erklären

Deuteron NMR relaxation, spectra, and evidence for the order-disorder phase transition in (ND4)2PtCl6

Deuteron NMR relaxation and spectra were studied at the resonance frequency of 46 MHz in polycrystalline (ND(4))(2)PtCl(6) between 300-5 K. The relaxation rate maximum near 50 K is about 53% smaller than the calculated maximum related to 120 degrees rotations about the threefold symmetry axes of the ammonium ion. The difference is explained by assuming for a N-D vector a total of 24 equilibrium directions, which in groups of six deviate from the nearest Pt-N vector by a certain angle Theta. So-called limited jumps between the directions of each group take place much more frequently than the large-angle rotations, thus rendering a fraction of the deuteron quadrupole coupling ineffective in relaxation. A motional model is presented, which takes into account both these motions simultaneously. A comparison with experimental data leads to Theta=26.0 degrees , in reasonable agreement with earlier neutron diffraction data. A sharp decrease found in the relaxation rate at the order-disorder phase transition temperature of 27.2 K is related to the fact that one of the six equilibrium directions becomes preferred. This leads to a formation of ordered domains, in which the active motion driving the relaxation is 120 degrees rotations. Two components in the spectra found below 55 K are related to domains (broad) and transition regions between domains (narrow). Reasons for the nonexponentiality observed below 20 K are discussed, the most likely explanation being that limited jumps dominate within transition regions and make the corresponding deuterons relax faster than those in domains