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

ND+4 and NH3D+ dynamics in ammonium persulphate. II. Transition from low-to-high-temperature regime

The transition of the dynamics of ND4 1 ions in ammonium persulphate, dominated at low temperatures by coherent uniaxial rotational tunnelling about one specific N–D bond ~the preferred bond! and, at high temperatures, by frequent stochastic jumps about all N–D bonds is elucidated with deuteron spin-lattice relaxation measurements, selective saturation experiments and deuteron NMR line shape analyses. Between 20 and 35 K, the coherent uniaxial tunnelling is superseded by thermally activated stochastic jumps about the same bond with kinetic parameters kdyn 0 510(11.560.5) s21 and Edyn a 5(3.660.3) kJ/mol. At higher temperatures, thermally activated stochastic jumps about the other N–D bonds set in. Their kinetic parameters are kst 0 510(12.260.5) s21 and Est a5(7.860.5) kJ/mol. From the primary and secondary tunnelling observed at low temperatures we infer the heights of the potentials which hinder rotations of the ND4 1 ions about the preferred and any other N–D bond. These heights, minus the rotator’s ground state energy, are about 25% larger than, respectively, Edyn a and Est a . The kinetic parameters of the two stochastic processes are such that the essentially uniaxial coherent and then incoherent dynamics at low temperatures is superseded at the decomposition temperature of the compound by stochastic reorientational jumps which reflect the basic tetrahedral symmetry of the ammonium ion

Estimation de fonctions de green dans les milieux complexes par décomposition de l'opérateur retournement temporel (application à l'imagerie médicale et à la correction d'aberration)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Location of the H atoms in ammonium persulphate by deuteron NMR. Verification by X-ray diffraction

It is demonstrated that H atoms can be located by the spectroscopic method of deuteron NMR. The requirement is that the heavy-atom positions are known from diffraction studies. The technique allows an accuracy of the order of 0.01 Å. The compound studied is ammonium persulphate (APS), (NH4)2S2O8. APS crystallizes in space group P21/n with lattice parameters a = 6.1340 (2), b = 7.9324 (3), c = 7.7541 (3) Å and β=94.966 (1)° at T = 118 K. In perdeuterated crystals of APS, only one of the deuterons of every ND +4 ion becomes localized at low temperatures. Therefore, most of this work uses samples with 9% deuteration. In such crystals, most of the ammonium ions containing deuterons come in the form of NDH +3 ions. At T < 25 K, the single deuteron of these ions becomes localized in one of four equilibrium sites. The deuteron site occupancies differ from each other and are measured at 17 K. The deuterons are located in three steps. (i) The deuteron quadrupole-coupling (QC) tensors are measured at 17 K. Their unique principal directions are identified, as is well justified, with the N-D bond directions. (ii) The fine structure of a deuteron NMR line is analyzed in terms of the magnetic dipole-dipole interactions between all nuclei in an NDH+3 ion to obtain the N-D and D-H internuclear distances. (iii) An empirical relation between deuteron QC constants and D…O distances in N-D…O hydrogen bonds is exploited to assign the N-D bond vectors to the appropriate N atom of which there are four in the unit cell. The results are highly relevant for an understanding of the complex tunnelling and stochastic reorientation dynamics of the ammonium ions in APS. They are verified by a complementary X-ray diffraction study