Pulsed Nuclear Magnetic Resonance Technique Research Articles

Solvothermal Template-Induced Hierarchical Porosity in Covalent Organic Frameworks: A Pathway to Enhanced Diffusivity.

The rapid advancement of covalent organic frameworks (COFs) in recent years has firmly established them as a new class of molecularly precise and highly tuneable porous materials. However, compared to other porous materials, such as zeolites and metal-organic frameworks, the successful integration of hierarchical porosity into COFs remains largely unexplored. The challenge lies in identifying appropriate synthetic methods to introduce secondary pores without compromising the intrinsic structural porosity of COFs. In this study, a template-induced synthetic methodology is realized to facilitate the construction of hierarchically porous COFs (hCOFs). This novel approach utilizes commercially available zinc oxide nanoparticles as a hard template, enabling to increase the total pore volume of a series of β-ketoenamine-linked COFs as well as an imine-based COF while preserving their surface areas. In addition to transmission electron microscopy and gas adsorption analyses, small-angle X-ray scattering and pulsed field gradient nuclear magnetic resonance techniques are employed to investigate the hierarchical porosity and diffusivity of guest molecules within hCOFs. This study demonstrates that the hierarchically porous nature of hCOFs significantly reduces diffusion limitations, thus leading to simultaneous enhancements in adsorption capacity, diffusivity, and catalytic performance.

Dynamics of fluorinated imide-based ionic liquids using nuclear magnetic resonance techniques.

There is increasing interest in studying molecular motions in ionic liquids to gain better insights into their transport properties and to expand their applications. In this study, we have employed the fast field cycling relaxometry and pulsed field gradient nuclear magnetic resonance techniques to investigate the rotational and translational dynamics of fluorinated imide-based ionic liquids (ILs) at different temperatures. We have studied a total of six ILs composed of the 1-butyl-3-methylimidazolium cation ([BMIM]+) combined with chemically modified analogs of the bis((trifluoromethyl)sulfonyl)imide anion ([NTf2]- or [TFSI]-). The primary objective of this paper is to broaden the understanding of how the anion's conformational flexibility, fluorination, and mass affect the molecular dynamics of cations and anions. Our results indicate that flexibility has the most significant impact on the rotational and translational motions of ions. Meanwhile, the effect of fluorination and mass is only relevant when conformational flexibility does not change significantly between the ILs being compared.

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation.

The molecular dynamics study of thermotropic mesogens exhibiting the crystal phases is valuable in unraveling the complex global (collective) and local (noncollective) motions executed by liquid crystal molecules, which would further advance the existing knowledge on orientationally disordered crystalline (ODIC) phases. Toward the fulfillment of such a task, a combined nuclear magnetic resonance (NMR) relaxometry approach employing the fast field cycling (FFC) NMR (10 kHz-30 MHz) and high-field pulsed NMR (400 MHz) techniques is utilized to sample the broad frequency range offered by molecular motions in the crystal phase of 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The validity of the observed relaxation data is tested and interpreted by the Bloembergen-Purcell-Pound (BPP) model involving the superposition of four mutually independent Lorentzian spectral densities, reflecting molecular dynamical processes on different time scales. The salient feature of the detailed analysis reveals that the lengthening of temporal dynamics in the crystal phase due to molecular rotations by jumps, which are of intermolecular origin, is evident and further supports the presence of collective-like local dynamics. The analysis does permit decoupling of the molecular reorientations about their short axes (∼100 ns) as well as long axes (∼50 ns) and methyl group rotations (∼0.5 ns) on distinct time scales. The activation energies for reorientations about the short axes and methyl group rotations are found to be 27.3 ± 2.7 and 15.8 ± 1.1 kJ/mol, respectively. The fast methyl rotations in the crystal phase of 6CHBT obtained from FFC NMR are further well complemented by high-field NMR, where 1H NMR line shapes are relatively narrow when compared to those of the nematic phase.

A neutron resonance spin flip device for sub-millitesla magnetic fields

A neutron resonance spin flip device is presented which efficiently induces spin flips of neutrons from a cold beam in static fields down to approximately 100μT. Detailed performance measurements show typical characteristics of the related pulsed nuclear magnetic resonance technique. The obtained results are in excellent quantitative agreement with model calculations which use a formalism based on time-evolution operators.

27Al pulsed nuclear magnetic resonance study of CeAl2

The magnetic properties and the electronic structures of a rare-earth aluminum intermetallic compound CeAl2 are investigated by magnetic susceptibility measurements and 27Al pulsed nuclear magnetic resonance (NMR) techniques. The magnetic susceptibility is strongly temperature-dependent, following a Curie–Weiss law down to ∼12 K, and shows an antiferromagnetic transition at 4 K. The 27Al NMR spectra show a typical powder pattern for a nuclear spin I of 5/2 with the second-order nuclear quadrupole interaction at high temperature and an additional large dipolar broadening between the 4f electron spins of cerium and the 27Al nuclear spins at low temperature. The 27Al NMR Knight shift follows the same temperature dependence as the magnetic susceptibility, suggesting that the 27Al NMR Knight shift originates from the transferred hyperfine field of the Ce 4f electron spins with the hyperfine coupling constant of A = +5.7 kOe/μB. The spin-lattice relaxation rate 1/T1 is roughly proportional to temperature, as with most non-magnetic metals at high temperature, and then strongly temperature-dependent, increasing rapidly with a peak near the antiferromagnetic transition temperature and decreasing at lower temperature. The temperature dependence of the Korringa ratio K, however, suggests that the antiferromagnetic spin fluctuation signature, which is an enhancement in the Korringa ratio, is washed out owing to the geometrical cancellation of Ce 4f fluctuations at the Al sites.

Comprehensive Study of Altered Amorphous Structure in Functionalized Polypropylenes Exhibiting High Tensile Strength

Polypropylene (PP) and its derivatives, poly(5-hexen-1-ol-co-propylene) (PPOH) and poly(1-hexene-co-propylene) (PPH), were comprehensively investigated by means of wide-angle X-ray diffraction, dynamic mechanical analysis (DMA), 1H pulsed nuclear magnetic resonance (NMR), infrared (IR), and positron annihilation lifetime (PAL) techniques. The deduced nanoscopic amorphous structure was discussed in comparison with the mechanical properties such as tensile strength. Introducing polar hexenol and nonpolar hexene comonomers into PP chains resulted in a significant reduction of the crystallinity for both the derivatives, which failed to explain the anomalous higher tensile strength of PPOH. The long decay T2 observation from NMR indicated a lower chain mobility in the amorphous region of PPOH compared to PPH, consistent with the DMA analysis, providing a higher glass transition temperature for the former polymer. The PAL and IR results signified reduced free-volume size along with hydrogen bond formation in the amorphous region of PPOH, illustrating an essential role of the nanoscopic amorphous structure in the superior mechanical strength of PPOH compared to the other polymers.

PFG-NMR and SANS Studies in Cation Exchange Membranes Based on Sulfonated Polyphenylene Multiblock Copolymers

We employed the pulsed field gradient nuclear magnetic resonance (PFG-NMR) technique to investigate the self-diffusion coefficient of water, D-NMR, in newly designed cation exchange membranes, made of sulfonated poly(p-phenylene)-poly(ether ketone) multiblock copolymers, which are denoted by S-6H (n) x:y. The D-NMR value was measured as a function of the gradient strength, g, from 0.1 to 11.0 T m-1. S-6H (14) 1:1 showed the D-NMR value of 2.35 x 10-10 m2 s-1 at 30 ºC and 90% RH, which are about 10 times lower than the value in bulk liquid water. Small angle neutron scattering (SANS) measurements showed two peaks in S-6H (14) 1:1. The low q peak is attributed to the formation of microdomains and the high q peak is due to the formation of ionic clusters in the swollen microdomains.

Diffusion of pentane isomers in faujasite-type zeolites : NMR and molecular dynamics study

Diffusion of pentane isomers in zeolites NaX has been investigated using pulsed field gradient nuclear magnetic resonance (PFG-NMR) and molecular dynamics (MD) techniques respectively. Temperature and concentration dependence of diffusivities have been studied. The diffusivities obtained from NMR are roughly an order of magnitude smaller than those obtained from MD. The dependence of diffusivity on loading at high temperatures exhibits a type I behavior according to the classification of Karger and Pfeifer [1]. NMR diffusivities of the isomers exhibit the order D(n-pentane)>D(isopentane)>D(neopentane). The results from MD suggest that the diffusivities of the isomers follow the order D(n-pentane)<D(isopentane)<D(neopentane). The activation energies from NMR show Ea(n-pentane)<Ea(isopentane)<Ea(neopentane) whereas those from MD suggest the order Ea(n-pentane)>Ea(isopentane)>Ea(neopentane). The latter follows the predictions of levitation effect whereas those of NMR appears to be due to the presence of defects in the zeolite crystals. The differences between diffusivities estimated by NMR and MD are attributed to the longer time and length scales sampled by the NMR technique, as compared to MD.

Mechanical properties of electron beam treated carboxylated nitrile butadiene rubber (XNBR) composites reinforced by organic/inorganic hybrid filler

Two types of carboxylated nitrile butadiene rubber (XNBR) composites were prepared; one of which was XNBR reinforced with organic filler-based olive solid by-product (OSB) and the other was XNBR reinforced with a combination of OSB and organo-layered nanoclay. To enhance the interaction between both the fillers and the matrix, the samples were exposed to electron beam (EB) treatment under ambient conditions. The samples were inspected with reference to their mechanical and dynamic mechanical (DMA) properties. It has been found that the tensile strength at break was enhanced after EB treatment for both composites. Such scenario was attributed to the irradiation-induced vulcanization. To support the observed trend, the pulse nuclear magnetic resonance technique was implemented. It has been found that the relaxation time ( T2) was shortened after EB treatment. As further evidence on the fact that EB treatment was effective, the attenuated total reflectance infrared spectra were utilized as well. The IR results showed an increment in the spectra intensities after EB treatments. Scanning electron microscope revealed that EB treatment has turned their surface rough with an improved interaction between the matrix and the filler. DMA results were in harmony with the tensile properties in the sense that increased elastic modulus ( E′) with filler incorporation. The intensity of the mechanical loss factor (tan δ) was a sign on the reinforcing capability of the fillers as well as improved interaction between the composite components.

A poroelastic model for ultrasonic wave attenuation in partially frozen brines

Although there are many possible mechanisms for the intrinsic seismic attenuation in composite materials that include fluids, relative motion between solids and fluids during seismic wave propagation is one of the most important attenuation mechanisms. In our previous study, we conducted ultrasonic wave transmission measurements on an ice-brine coexisting system to examine the influence on ultrasonic waves of the unfrozen brine in the pore microstructure of ice. In order to elucidate the physical mechanism responsible for ultrasonic wave attenuation in the frequency range of 350–600 kHz, measured at different temperatures in partially frozen brines, we employed a poroelastic model based on the Biot theory to describe the propagation of ultrasonic waves through partially frozen brines. By assuming that the solid phase is ice and the liquid phase is the unfrozen brine, fluid properties measured by a pulsed nuclear magnetic resonance technique were used to calculate porosities at different temperatures. The computed intrinsic attenuation at 500 kHz cannot completely predict the measured attenuation results from the experimental study in an ice-brine coexisting system, which suggests that other attenuation mechanisms such as the squirt-flow mechanism and wave scattering effect should be taken into account.

Quantification of oil binding capacity of structuring fats: A novel method and its application

A robust, well-defined and reproducible method to accurately measure the oil binding capacity (OBC) of structuring fats was developed. The method was validated using two oil/fat model systems, i.e., fully hydrogenated canola oil (FHCO) in canola oil (CO) (FHCO/CO) and fully hydrogenated soybean oil (FHSO) in CO (FHSO/CO). The mixtures were crystallized from the melt down to three different temperatures (15, 25 and 35 °C) at constant rates of cooling and the OBC was measured after different periods of storage time. The critical concentration of hard fat at which the solid fat network is stable and effectively binds oil has been also measured for mixtures crystallized at temperatures close to room temperature, i.e., 25 °C. Crystal structure, melting behavior, microstructure, and solid fat content of these binary systems have been investigated in relation to the OBC of the solid fat network using X-ray diffraction (XRD), differential scanning calorimetry (DSC), polarized light microscopy (PLM), and wide-line pulsed nuclear magnetic resonance (pNMR) techniques. The two model systems exhibited similar trends in OBC over time, a behavior attributed to their similar TAG composition and polymorphism. However, relatively smaller OBC values were achieved by the CO/FHSO compared to CO/FHCO samples, largely due to differences in their solid network structure. Four successive decreasing linear segments, identifying successive mechanisms of oil migration/binding, were observed in the experimental OBC versus fat weight fraction curves. The critical concentration of hard fat, at which the solid fat network is effective in binding oil, was also determined and found to be ∼6 wt% for both systems.

An NMR investigation on the phase structure and molecular mobility of the novel exfoliated polyethylene/palygorskite nanocomposites

AbstractNovel exfoliated polyethylene (PE)/palygorskite nanocomposites prepared by in situ polymerization are characterized by solid‐state nuclear magnetic resonance (NMR). The phase structure and molecular mobility are investigated by a combination of proton and carbon NMR. The results showed that incorporation of small amounts of palygorskite had great influence on the phase structure and molecular mobility. The incorporated palygorskite hindered the crystallization process and introduced motion‐hindered chains in the NMR crystalline and amorphous phase. 13C cross‐polarization and magic‐angle spinning NMR revealed two orthorhombic crystalline phase with different line‐width. The chain mobility of orthorhombic crystalline phase with broad resonance line is obviously hindered compared with the phase with narrow resonance line when the filler is introduced. Additionally, the results of pulsed field gradient NMR technique show those the tortuosities in the nanocomposites are much higher than that in the bulk PE. The self‐diffusion process of probe molecules is also influenced by the palygorksite load. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1363–1371, 2010

Design and Synthesis of Microporous and Micro/Mesoporous Silica Materials with Excellent Adsorption Properties via Self-Assembly of Silica Species with Tetraethyl Ammonium in Acidic Aqueous Media

The concentration dependence of tetraethyl ammonium cations (TEA+) in strongly acidic media was investigated through a pulsed field gradient nuclear magnetic resonance (PFG-NMR) technique. The results indicated that TEA+ is monodispersed in aqueous solution at relatively low concentration (<6 wt %), whereas they partially aggregate with each other at relatively high concentration (6−40 wt %). Different states of TEA+ cations are expected to be used as possible templates for design of micropores or micro/mesopores mixed materials. Accordingly, a series of microporous silica materials and hierachically micro/mesoporous silica materials have been synthesized via self-assembly of silica species with TEA+ cations in acidic solutions by simply controlling TEA+ concentration. N2 isotherms show that the samples synthesized at low TEA+ concentration (<6 wt %) have uniform micropores (0.64−0.65 nm) and the samples synthesized at high TEA+ concentration (6−16 wt %) exhibit both micropores (0.66−0.68 nm) and mesopores (3.5−3.6 nm). Furthermore, differential thermal analysis and thermogravimetric (DTA−TG) curves and infrared (IR) spectra indicated that TEA+ species were certainly included in as-synthesized samples, confirming that TEA+ species were the templates for the formation of porosity. Very importantly, these porous silica materials showed higher adsorption capacity of volatile organic compounds (VOCs) such as benzene and cyclohexane at low relative pressure than conventional zeolites (Y and ZSM-5) and mesoporous materials (SBA-15 and MCM-41), which would be potentially important for the removal of VOCs in the future.

Laboratory experiments on compressional ultrasonic wave attenuation in partially frozen brines

Often, the loss mechanisms responsible for seismic attenuation are unclear and controversial. We used partially frozen brine as a solid-liquid coexistence system to investigate attenuation phenomena. Ultrasonic wave-transmission measurements on an ice-brine coexisting system were conducted to examine the influence of unfrozen brine in the pore microstructure on ultrasonic waves. We observed the variations of a 150–1000 kHz wave transmitted through a liquid system to a solid-liquid coexistence system, changing its temperature from [Formula: see text] to –[Formula: see text]. We quantitatively estimated attenuation in a frequency range of [Formula: see text] by considering different distances between the source and receiver transducers. We also estimated the total amount of frozen brine at each temperature by using the pulsed nuclear magnetic resonance (NMR) technique and related those results to attenuation results. The waveform analyses indicate that ultrasonic attenuation in an ice-brine coexisting system reaches its peak at [Formula: see text], at which the ratio of the liquid phase to the total volume in an ice-brine coexisting system is maximal. Finally, we obtained a highly positive correlation between the attenuation of ultrasonic waves and the total amount of unfrozen brine. Thus, laboratory experiments demonstrate that ultrasonic waves within this frequency range are affected significantly by the existence of unfrozen brine in the pore microstructure.

Properties of unsaturated phospholipid bilayers: Effect of cholesterol

Properties of hydrated unsaturated phosphatidylcholine (PC) lipid bilayers containing 40 mol % cholesterol and of pure PC bilayers have been studied. Various methods were applied, including molecular dynamics simulations, self-consistent field calculations, and the pulsed field gradient nuclear magnetic resonance technique. Lipid bilayers were composed of 18:0/18:1(n-9)cis PC, 18:0/18:2(n-6)cis PC, 18:0/18:3(n-3)cis PC, 18:0/20:4(n-6)cis PC, and 18:0/22:6(n-3)cis PC molecules. Lateral self-diffusion coefficients of the lipids in all these bilayers, mass density distributions of atoms and atom groups with respect to the bilayer normal, the C-H and C-C bond order parameter profiles of each phospholipid hydrocarbon chain with respect to the bilayer normal were calculated. It was shown that the lateral self-diffusion coefficient of PC molecules of the lipid bilayer containing 40 mol % cholesterol is smaller than that for a corresponding pure PC bilayer; the diffusion coefficients increase with increasing the degree of unsaturation of one of the PC chains in bilayers of both types (i.e., in pure bilayers or in bilayers with cholesterol). The presence of cholesterol in a bilayer promoted the extension of saturated and polyunsaturated lipid chains. The condensing effect of cholesterol on the order parameters was more pronounced for the double C=C bonds of polyunsaturated chains than for single C-C bonds of saturated chains.

Transport Properties and Aggregation Phenomena of Polyoxyethylene Sorbitane Monooleate (Polysorbate 80) in Pig Gastrointestinal Mucin and Mucus

The aqueous environment in the gastrointestinal tract frequently requires solubilization of hydrophobic drug molecules in appropriate drug delivery vehicles. An effective uptake/absorption and systemic exposure of a drug molecule entails many processes, one being transport properties of the vehicles through the mucus layer. The mucus layer is a complex mixture of biological molecules. Among them, mucin is responsible of the gel properties of this layer. In this study, we have investigated the diffusion of polyoxyethylene sorbitane monooleate (polysorbate 80), a commonly used nonionic surfactant, in aqueous solution, in mucin solutions at 0.25 and 5 wt %, and in mucus. These measurements were done by using the pulsed field gradient spin echo nuclear magnetic resonance (PGSE-NMR) technique. We conclude that polysorbate 80 is a mixture of non-surface-active molecules that can diffuse freely through all the systems investigated and of surface-active molecules that form micellar structures with transport properties strongly dependent on the environment. Polysorbate 80 micelles do not interact with mucin even though their diffusion is hindered by obstruction of the large mucin molecules. On the other hand, the transport is slowed down in mucus due to interactions with other components such as lipids depots. In the last part of this study, a hydrophobic NMR probe molecule has been included in the systems to mimic a hydrophobic drug molecule. The measurements done in aqueous solution revealed that the probe molecules were transported in a closely similar way as the polysorbate 80 micelles, indicating that they were dissolved in the micellar core. The situation was more complex in mucus. The probe molecules seem to dissolve in the lipid depots at low concentrations of polysorbate 80, which slows down their transport. At increasing concentration of polysorbate 80, the diffusion of the probe molecules increases indicating a continuous dissolution of hexamethyldisilane in the core of polysorbate 80 micelles.

Sub- and superdiffusive molecular displacement laws in disordered porous media probed by nuclear magnetic resonance

Hydrodynamic dispersion of water flowing through porous glasses with nominal pore sizes in the range 40 to 160 micrometers was studied with the aid of a pulsed gradient nuclear magnetic resonance technique compensating for coherent flow velocities. The crossover from effectively subdiffusive mean square displacement, <Z2> proportional variant t0.84, in the absence of hydrodynamic flow to a superdiffusive, almost ballistic power law, <Z2> proportional, variant t1.95, at the highest flow rates was observed. At intermediate flow rates, a gradual conversion between these two limiting power laws occurs. As a function of the Péclet number, the effective dispersion coefficient is in accordance with a power law with an exponent 1.2.

The Concentration of (SiH2)n Sites in Low and High Defect Density a-Si:H

AbstractThe concentration of polysilane chains (SiH2)n, where n≥1, is estimated for higher quality hydrogenated amorphous silicon (a-Si:H) by pulsed proton nuclear magnetic resonance techniques (1H NMR). Our measurements indicate the minimum hydrogen content of approximately 10% of the total hydrogen is in the (SiH2)n configuration. Similar measurements in a high defect density sample (1017 silicon dangling bond defects cm-3) show that (SiH2)n sites account for ~ 15% of the total hydrogen. While the (SiH2)n infrared absorption (IR) modes are observed in the highly defective sample, no such modes are seen in the higher quality material. The results indicate that a significant amount of the total hydrogen content exists as (SiH2)n regardless of film quality.

Investigation of fine granular material flow through a packed bed

Three different methods cut-off, time-of-flight, and Pulsed Field Gradient Nuclear Magnetic Resonance were used to study downstream flow of fine granular material through the fixed bed reactor. For describing the transport of solid particles within a fixed granular bed, a model has been developed. In time-of-flight and cut-off techniques the highest average velocity of filtration is observed at the lowest mass flow rate in all experimental traces, while upon the flow rate increase it tends to an asymptotic value. Experimental results obtained by pulsed field gradient nuclear magnetic resonance technique have revealed the bimodal character of particles velocities distribution. The average filtration velocity has a maximum at an intermediate mass flow rate close to the bed flooding, in contrast to the results obtained by cut-off and time-of-flight methods. The velocities measured using all three techniques were compared by converting them into dimensionless values. From the experimental results, the values of model parameters have been evaluated which allowed us to describe particle velocities within a bed.

Ionic and molecular Self-Diffusion in Ion-Exchange materials for fuel energetics studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to investigate the self-diffusion mechanism of water, alcohol molecules and Li− counterions in sulfocation exchangers with different structures of the polymeric matrix. It was shown that in the homogeneous perfluorinated sulfocation exchange membranes the ionic and water translation motions are controled by the hydrogen bond network forming in ionogenic channels at the high water content. At the low solvent content, the self-diffusion coefficients of methanol and ethanol are higher than the water self-diffusion coefficients. The influence of non-ion-exchange sorbed electrolyte on Li+ self-diffusion coefficients was observed in the heterogeneous sulfocation exchanger KU-23.