Solid-state Proton Nuclear Magnetic Resonance Research Articles

Oxide Acidity Modulates Structural Transformations in Hydrogen Titanates during Electrochemical Li-Ion Insertion.

Hydrogen titanates (HTOs) form a diverse group of metastable, layered titanium oxides with an interlayer containing both water molecules and structural protons. We investigated how the chemistry of this interlayer environment influenced electrochemical Li+-insertion in a series of HTOs, H2TiyO2y+1·nH2O (y = 3, 4, and 5). We correlated the electrochemical response with the physical and chemical properties of HTOs using operando X-ray diffraction, in situ differential electrochemical mass spectroscopy, solid-state proton nuclear magnetic resonance, and quasi-elastic neutron scattering. We found that the potential for the first reduction reaction trended with the relative acidity of the structural protons. This mechanism was supported with first-principles density functional theory (DFT) calculations. We propose that the electrochemical reaction involves reduction of the structural protons to yield hydrogen gas and formation of a lithiated hydrogen titanate (H2-xLixTiyO2y+1). The hydrogen gas is confined within the HTO lattice until the titanate structure expands upon subsequent oxidation. Our work has implications for the electrochemical behavior of insertion hosts containing hydrogen and structural water molecules, where hydrogen evolution is expected at potentials below the hydrogen reduction potential and in the absence of electrolyte proton donors. This behavior is an example of electrochemical electron transfer to a nonmetal element in a metal oxide host, in analogy to anion redox.

Effect of Composition on the Molecular Dynamics of Biodegradable Isotactic Polypropylene/Thermoplastic Starch Blends

Polyolefins such as polypropylene are used in an immensely broad range of commodity products and account for the largest volume of synthetic polymers generated worldwide. For this reason, this family of thermoplastics contributes significantly to solid waste both on land and in the ocean. One viable approach to mitigate this growing problem and simultaneously reduce the cost of and dependence on petroleum-based polymers relies on blends wherein an added biopolymer can promote natural biodegradation. Due to their chemical dissimilarity, however, nonpolar polyolefins and polar biopolymers tend to phase-separate, in which case a fundamental, molecular-level understanding of the role of polymer/polymer interfaces on chain mobility in blends differing in composition is needed. In the present study, the molecular dynamics of blends composed of isotactic polypropylene (iPP) and glycerol-plasticized thermoplastic starch (TPS) are investigated by solid-state proton nuclear magnetic resonance and dielectric relaxat...

Substitution of strontium and boron into hydroxyapatite crystals: Effect on physicochemical properties and biocompatibility with human Wharton-Jelly stem cells

Hydroxyapatite (HA) enriched with strontium and boron ions was synthesized using two different methods: the precipitation method (Sr,B-HAw) and the dry method (Sr,B-HAd). Additionally, for the sake of comparison, the “pure” unsubstituted HA was prepared together with HAs substituted only with one type of a foreign ion. The obtained materials were subjected to physicochemical analysis with the use of various analytical methods, such as powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), inductively coupled plasma optical emission spectroscopy (ICP-OES), Fourier transform infrared spectroscopy (FT-IR) and solid-state proton nuclear magnetic resonance (1H ssNMR). All the obtained materials were also biologically tested for their potential cytotoxicity. The obtained materials (Sr,B-HAw and Sr,B-HAd) were homogeneous and respectively showed nano- and microcrystal apatitic structures. The simultaneous introduction of Sr2+ and BO33− ions turned out to be more effective in respect of the dry method. Of importance, doped materials obtained using both synthesis routes have been demonstrated to be biocompatible, opening the way for medical applications.

Solid-state 1H-NMR relaxation behavior for ultra-high-molecular-weight polyethylene reactor powders with different morphologies

The relaxation behaviors of several commercial ultra-high-molecular-weight polyethylene (UHMW-PE) reactor powders were compared. The surface and internal morphologies of these powders were characterized by scanning and transmission electron microscopy observations. On the basis of these morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. Molecular motions were detected by solid-state proton nuclear magnetic resonance (1H-NMR) techniques. The temperature dependence of the molecular motion characterizes the structural differences between these powders. The intermediate region between crystal/amorphous phases first relaxes during gradual heating from room temperature for all reactor powders examined in this study. Such a release of constraint for boundary chains induces structural reorganization beyond the critical temperature before melting. This trend was also confirmed by an increase in crystallinity beyond the critical temperature. In contrast, the results of differential calorimetry (DSC) analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. A relationship between the morphological and 1H-NMR relaxation characteristics was interpreted for several reactor powders. The relaxation behaviors for several commercial ultra-high molecular weight polyethylene reactor powders were compared. On the basis of the morphological analyses, the reactor powder consisted of particles and fibrils between them, the relative amounts of which depended on the powder preparation conditions. The results of differential scanning calorimetry analysis could not distinguish these powder characteristics because of the remarkable reorganization of these reactor powders during heating scans. In contrast, molecular motions detected by solid-state proton nuclear magnetic resonance techniques characterize the structural differences between these powders.

Nanopumpkins and a sunscreen agent: the inclusion complex of cucurbituril and Tinosorb S

The commercial sunscreen agent Tinosorb S (TS) is a promising oil-soluble broad spectrum UV filter. Here, we report for the first time a water-dispersible inclusion complex of TS using cucurbituril. The synthesis was carried out via a supramolecular approach in the solid state by using the high-speed vibration milling technique. The formation and the photophysical properties of the solid inclusion complex were investigated by spectroscopic techniques such as UV–vis spectrophotometry, Fourier-transform infrared and solid-state proton nuclear magnetic resonance spectroscopy as well as by thermal gravimetric analysis and X-ray diffractometry. An increase in the absorbance and a shift of the maximum absorption from 380 nm to 345 nm for the inclusion complex were observed in comparison to the precursor complex TS. The stochiometric ratio of cucurbituril and TS in the inclusion complex was found to be 2:1

Thin-film solid-state proton NMR measurements using a synthetic mica substrate: Polymer blends

Solid-state proton nuclear magnetic resonance (NMR) measurements are performed successfully on polymer blend thin films through the use of synthetic mica as a substrate. When used as a substrate, synthetic fluorophlogopite mica with its proton-free, diamagnetic character, allows for adequate measurement sensitivity while minimally perturbing the proton thin-film spectra, especially relative to more commonly available natural micas. Specifically, we use multiple-pulse techniques in the presence of magic-angle spinning to measure the degree of mixing in two different polymer blend thin films, polystyrene/poly(xylylene ether) and poly(1-methyladamantyl methacrylate) (PMAdMA)/triphenylsulfonium perfluorobutanesulfonate (TPS-PFBS), spin-coated onto mica substrates. Our earlier studies had focused on bulk systems where NMR signals are stronger, but may not be representative of thin films of the same systems that are relevant to many applications such as photoresist formulations in the electronics industry. The superiority of synthetic over natural paramagnetic mica is demonstrated by the maintenance of resolution and spinning sideband intensities (relative to bulk samples) for the synthetic mica samples. In contrast, degraded resolution and large spinning sidebands are shown to typify spectra of the natural mica samples. This approach can be applied to many other proton measurements of solid thin films, thereby greatly extending the types of systems to be investigated. Magnetic susceptibility measurements are also reported for all micas used.

NMR Characterization of Low Hard Segment Thermoplastic Polyurethane/Carbon Nanofiber Composites

Solid-state proton nuclear magnetic resonance (NMR) has been used to investigate the structure and dynamics of a thermoplastic polyurethane elastomer (TPE) filled with carbon nanofibers (CNF’s) for shape-memory applications. The TPE soft segments are above their glass transition temperature (Tg) at ambient temperature and give rise to relatively narrow (∼2 kHz) signals in the solid-state proton spectrum. The introduction of CNF’s leads to a concentration-dependent shifting and broadening of the signals, while the proton spin−lattice and spin−spin relaxation times are not significantly altered, showing that the broadening is inhomogeneous and related to the difference in magnetic susceptibility between the TPE and the CNF’s. Proton spin diffusion experiments reveal the onset of stress-induced crystallinity as the samples are stretched to 60%, and stretching to 1000% leads to crystallization at the CNF surface and increased separation between the CNF’s and the mobile amorphous phase of the TPE. The implications for the mixing of polymers and CNF’s are considered.

Characterization of the phase transitions of ethyl substituted polyhedral oligomeric silsesquioxane

This study describes the synthesis and molecular mobility of both partially deuterated and fully protonated ethyl polyhedral oligomeric silsesquioxane (POSS) crystals. The primary phase transitions were identified with differential scanning calorimetry at ∼257 and ∼253 K for partially deuterated and fully protonated ethyl POSS, respectively. A change in entropy between ∼47 and 28 ± 2 J mol −1 K −1 was observed for these transitions. At high temperature the unit cells are rhombohedral, while triclinic unit cells are observed at temperatures below the phase transition point. The crystallographic transition to the low temperature phase, 110 K, is marked by an abrupt increase in density (1.31 to 1.43 ± 0.05 g cm −3) and decrease in symmetry ( R-3 to P-1). Additionally, the crystallographic transition results in abrupt changes in the spin lattice relaxation and linewidth as detected with solid-state proton nuclear magnetic resonance (NMR) spectroscopy. This NMR behavior suggests a transition in molecular mobility of both ethyl derivatives. Both POSS derivatives exhibit an increase in correlation time and activation energy. For deuterated ethyl POSS, the motions became increasingly anisotropic after the temperature is lowered past its transition point.

Enhancement of solid-state proton NMR via the spin-polarization-induced nuclear Overhauser effect with laser-polarized xenon

We have successfully transferred the spin polarization of laser-polarized Xe-129 to the proton spins of solid-state (HCl)-H-1 via spin polarization-induced nuclear Overhauser effect (SPINOE). The key steps include mixing the laser-polarized Xe-129 gas with the (HCl)-H-1 gas and cooling them to their condensed state in a flow system. The solid-state nuclear magnetic resonance (NMR) signal enhancement factor of 6 for H-1 was observed, compared with the Boltzmann polarization signal at 1.879 T and 142 K. This method may be valuable for applications in both NMR spectroscopy and chemical physics.

Microstructural investigation of high-speed melt-spun nylon 6 fibers produced with variable spinning speeds

The structural details of high-speed melt-spun nylon 6 fibers at spinning speeds ranging from 4500 to 6100 m/min were investigated by solid-state proton nuclear magnetic resonance (1H NMR) spectroscopy, density and birefringence measurements, differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The analyses of the proton spin-lattice relaxation times in the rotating frame and correlation times confirmed the existence of three different phases, the immobile crystalline, intermediate rigid amorphous, and mobile amorphous regions, in the fiber sample. At spinning speeds lower than 5200 m/min, the portion of the crystalline phase increased at the expense of the rigid amorphous region and then reached a plateau afterward, from which the mobile amorphous portion increased. Combined analyses of density and birefringence measurements, DSC, and XRD in conjunction with NMR results indicated that the formation of the γ crystal became predominant compared to that of the α crystal. The orientation factor of the crystalline phase increased slightly with increasing spinning speed, whereas the amorphous orientation factor decreased because of the increase of the purely amorphous region. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1285–1293, 2000

Solid-state multipulse proton Nuclear Magnetic Resonance (NMR) characterization of self-assembling polymer films

Multipulse solid-state proton Nuclear Magnetic Resonance (NMR) has been used to study the domain structure in poly(styrene– b-isoprene– b-styrene) triblock copolymers in clear and self-assembled polymer films. Films containing ordered arrays of microcavities (3–5 μm) were obtained by casting the polymer from carbon disulfide solution in a moist environment, while clear films were obtained by solvent evaporation under nitrogen. The domain sizes for the polystyrene and polyisoprene blocks were measured by proton spin diffusion using the dipolar filter pulse sequence. The domain sizes for the dispersed phase and the long period were measured to be in the range of 3–6 nm, depending on the polymer molecular weight, and no differences domain size were observed for the clear and the self-assembled polymer films.

Nuclear magnetic resonance studies of lipid hydration in monomethyldioleoylphosphatidylethanolamine dispersions

Solid-state proton nuclear magnetic resonance has been used to examine surface hydration in suspensions of monomethyldioleoylphosphatidylethanolamine (MeDOPE). The magic-angle spinning (MAS) 1H spectra for aqueous suspensions of MeDOPE in the L alpha phase exhibited two resonances of roughly equal intensity that could be ascribed to water protons, but both their spin-lattice relaxation times and chemical shifts converged upon conversion to the hexagonal phase. Only a single water peak was observed for analogous samples of dioleoylphosphatidylcholine (DOPC). MAS-assisted two-dimensional nuclear Overhauser effect spectroscopy (NOESY) was conducted for multibilayers of both MeDOPE and DOPC. Through-space interactions were identified between pairs of lipid protons, as expected from their chemical structure. For lamellar suspensions of MeDOPE, positive NOESY cross-peaks were observed between the downfield-shifted water resonance (only) and both CH2N and NH2CH3+ protons of the lipid headgroup. These cross-peaks were not observed in the NOESY spectra of MeDOPE in its hexagonal or cubic phases or for lamellar DOPC reference samples. Taken together, the observation of two water peaks, spin-lattice relaxation behavior, and NOESY connectivities in MeDOPE suspensions support the interpretation that the low-field water peak corresponds to hydrogen-bonded interlamellar water interacting strongly with the lipid. Such a population of water molecules exists in association with MeDOPE in the lamellar phase but not for its inverted phases or for lamellar dispersions of DOPC.

Solid-State Proton Nuclear Magnetic Resonance of a Glassy Epoxy Exposed to Water

A cured epoxy originating from a stoichiometric mixture of a DGEBA and 1,3-phenylenediamine has been studied by proton NMR including multiple-pulse techniques. In order to address the question of the uniformity of water distribution based on possible variations in crosslink density, spin diffusion experiments on a dry epoxy were performed over a wide temperature range in order to probe the distance scale of cross-link heterogeneity. The actual experiments measured the minimum distance scale (4 nm) within which sample-wide variations in the multiple-pulse relaxation time, T 1xz , are included. Experiments were performed to indicate that these variations in T 1xz , were not primarily associated with aliphatic versus aromatic protons. Whether the variations in T 1xz , indicate variations in cross-link density versus fluctuations in local packing in the glass remains an open question. Water chemical shift versus water content was also measured. It was inferred from these data that the majority of the water was molecularly dispersed as opposed to being aggregated into voids. Given that the existence of voids is often invoked in the epoxy literature, the NMR data would limit the volume of any voids to a volume where only one or two water molecules would fit in each void. No evidence was seen in the chemical shift data which would support the idea that there were two different sites with substantially different affinities for water. Also, water line widths were measured at 75 o C for samples of different average water concentrations and relatively uniform concentration profiles. It was anticipated that this measurement would provide a calibration curve whereby the existence of water concentration gradients could be identified for samples of known average water content. This anticipated result was not demonstrated conclusively. Finally, the line-width results are considered as they apply to the behavior of water in the later stages of water uptake