Effect of cross-linking on the molecular motion of water in polymer gel as studied by pulse 1H NMR and PGSE 1H NMR

¹H NMR measurements on the spin-spin relaxation time (T₂) and self-diffusion coefficient (D_H2O) of water in a poly(metacrylic acid) gel were carried out to clarify the molecular motion of water molecules as a function of the degree of cross-linking under a constant amount of water contained in the gel. From experimental results, it was found that ¹H T₂ and D_H2O decrease with an increase in the degree of cross-linking in the gel. It can therefore be said that an increase in cross-linking leads to a restraint of molecular motion of the water molecules in the gel.     

Dynamic and molecular association in premicellar aqueous solutions of dicarboxylate amino acid-based surfactant as studied by 1H NMR

We examined a series of amino acid-based surfactants with two carboxylic groups separated by a spacer of one, two, or three carbon atoms with sodium and calcium counterions in the premicellar concentration region near the CMC. ¹H nuclear magnetic resonance (NMR) spectroscopy and NMR diffusometry techniques were used to study the local environment, association, and translational dynamics of the surfactant's molecules. We measured the self-diffusion coefficients of the micelles, calculated the effective hydrodynamic radii, and determined the temperature region in which the premicelles exist. With an increase in temperature from 295 to 335 K, the premicellar state of the surfactant is replaced by the monomeric state.     

Acid-base titrations of schiff bases in deuteriomethanol as studied by1H NMR

The Schiff bases of salicylaldehyde and pyridoxal 5-phosphate with amino compounds were studied by¹H NMR in CD₃OD by using a glass electrode for the control of acidity. This study makes a new contribution for the elucidation of the structures of pyridoxylidene Schiff bases and gives additional data on the¹H NMR of the compounds that have not been available from the related studies in aqueous solutions.     

Aggregation of 1-dodecyl-3-methylimidazolium nitrate in water and benzene studied by SANS and 1H NMR

Aggregation of imidazolium-based ionic liquid, C(12)mim(+)NO(3)(-), in both polar solvent of water and nonpolar solvent of benzene was elucidated by electrical conductivity, small-angle neutron scattering (SANS), and (1)H NMR measurements. The electrical conductivities of C(12)mim(+)NO(3)(-)-water solutions at 298 K as a function of ionic liquid concentration showed a break point at 8.4 mmol dm(-3) as a cmc. However, those of C(12)mim(+)NO(3)(-)-benzene solutions drastically increase in accordance with a cubic function of concentration, but without a break point. The SANS profiles of both aqueous and benzene solutions obviously differ from each other. The profiles of the aqueous solutions indicated the formation of polydisperse spherical micelles. Those of the benzene solutions revealed Ornstein-Zernike behavior. Thus, C(12)mim(+)NO(3)(-) forms clusters in the benzene solutions, but the shape of clusters is indefinite. On the basis of the (1)H NMR chemical shifts of the aqueous solutions, the effect of nitrate on the formation of micelles was discussed on a microscopic scale. Furthermore, the interactions among C(12)mim(+), NO(3)(-), and benzene molecules in the benzene solutions were considered according to the (1)H NMR data.     

Water behavior in mesoporous materials as studied by NMR relaxometry

Water in mesoporous materials possessing a two-dimensional hexagonal structure has been studied by the variation of its NMR longitudinal relaxation time T(1) as a function of the static magnetic field value, or equivalently of the NMR measurement frequency. This technique, dubbed relaxometry, has been applied from 5 kHz (measurement frequency) up to 400 MHz with various instruments including a variable-field spectrometer operating between 8 and 90 MHz. Moreover, the range 0-5 kHz could be investigated by transverse relaxation, T(2) denoting the corresponding relaxation time, and relaxation in the rotating frame, T(1ρ) denoting the corresponding relaxation time. Measurements of proton relaxation rates (inverse of relaxation times) have been performed with H(2)O and HOD (residual protons of heavy water) at water volumes of 80%, 60%, and 40% relative to the porous volume. Comparison between H(2)O and HOD shows clearly that, above 1 MHz where both sets of data are superposed, relaxation is purely intermolecular and due to paramagnetic relaxation (dipolar interactions of water protons with unpaired electrons of paramagnetic entities). Below 1 MHz, it is possible to subtract the intermolecular contribution (given by HOD data) from H(2)O data so that one is left with intramolecular relaxation which is solely due to water reorientational motions. The analysis of these low-frequency data (in terms of Lorentzian functions) reveals two types of water within the pores: one interacting strongly with the surface and the other corresponding to a second layer. High-frequency data, which arise from paramagnetic relaxation, exhibit again two types of water. Due to their correlation times, one type is assigned to relatively free water within the pores while the other type corresponds to bulk (interparticular) water. Their proportions, given as a function of the volume fraction, are consistent with the above assignments.     

Structure,dynamics and interactions in polymer systems as studied by NMR spectroscopy

The possibilities of NMR spectroscopy in studies of interactions in polymer systems are demonstrated on the example of two types of macromolecular complexes: (i) By measuring ¹H NMR high resolution line intensities, the formation of ordered associated structures of syndiotactic (s) poly(methyl methacrylate)(PMMA) in mixed solvents was quantitatively characterized. The obtained results permit us to assume that the mechanism by which the solvent affects self-association of s-PMMA involves specific interactions of the solvent molecules with PMMA units. Solid state high resolution ¹³C NMR spectra of associated s-PMMA gels were also measured and compared with the spectra of a solid s-PMMA sample. (ii) By using ¹³C solid state NMR spectroscopy, the differences in the structure of the amorphous and crystalline phases in pure poly(ethylene oxide) and its complexes with p-dichlorobenzene or p-nitrophenol were characterized. Prounounced differences also in the dynamic structure of the crystalline phase in these systems are indicated by the relaxation times T₁(C), T_1ρ(C) and T_1ρ(H).     

Dynamic processes in polymeric networks as studied by NMR relaxation methods

By means of NMR pulse methods molecular motions of polymer subchains in crosslinked polystyrene gels are studied. The temperature dependence of the effective transverse relaxation timeT _{2 eff} is explained by a local reorientation process. The local configuration of the elastic chains of the network is shown to be mainly determined by polymer-solvent interaction. The results are compared with those of concentrated solutions. The influence of static-like contributions to the 2nd moments is discussed.     

Probing paeonol-pluronic polymer interactions by 1H NMR spectroscopy

By using a combination of 1H NMR spectroscopy, two-dimensional heteronuclear single-quantum coherence-resolved (1)H{(13)C} and homonuclear rotating-frame Overhauser enhancement NMR correlation experiments with diffusion ordered spectroscopy (DOSY), the location and distribution of a hydrophobic drug, paeonol, have been established with respect to the methyl groups of the poly(ethylene oxide)-poly(propylene oxide) -poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer. The interaction between them is adjustable according to the different temperature-dependent hydrophilicities or hydrophobicities of the triblock copolymer components. On the other hand, such interactions influence the self-assembly properties of the block copolymer amphiphiles in solution. The amount of anhydrous methyl groups of PPO segments shows an increase with increasing paeonol concentration. It was also demonstrated that the shell-crosslinking of the Pluronic polymer has an effect in increasing the amount of anhydrous methyl groups and thus increasing the hydrophobicity of Pluronic micelles. This might be the deeper reason underlying the increase in drug-loading capacity and prolongation in release time of Pluronic micelles for drug delivery after the shell-crosslinking. Changes in self-diffusion coefficients of paeonol with varying copolymer concentrations and types were also determined by the diffusion-based NMR DOSY technique, and values of K(a), DeltaG, and n were calculated.     

Aromatic imine stereochemistry as studied by 13C and 1H NMR of 15N-enriched materials

A series of thirteen ¹⁵N-enriched N-aryl imines derived from benzaldehydes and acetophenones have been prepared and examined by ¹³C and ¹H NMR. Preferred conformations of the C-arvl and N-aryl rings have been deduced from ¹³C chemical shifts and ¹³C-¹⁵N couplings. Evidence for steric inhibition of resonance in O-alkylated materials is presented. Relative signs of ¹J(CN) and ²J(CCN) have been determined in some compounds. An E: Z interconversion barrier of 21.6 kcal mol^?1 has been obtained from ¹H NMR coalescence data.     

Structure of water incorporated in sulfobetaine polymer films as studied by ATR-FTIR

The structure and hydrogen bonding of water in the vicinity of a thin film of a sulfobetaine copolymer (poly[(N,N-dimethyl-N-(3-sulfopropyl)-3'-methacrylamidopropanaminium inner salt)-ran-(butyl methacrylate)], poly(SPB-r-BMA)), were analyzed with band shapes of O-H stretching of attenuated total reflection infrared (ATR-IR) spectra. The copolymer could be cast as a thin film, of approximate thickness 10 microm, on a ZnSe crystal for the ATR-IR spectroscopy. At an early stage of sorption of water into the polymer film, the O-H stretching band of the IR spectra for the water incorporated in the film was similar to that for free water. This is consistent with the tendency for another zwitterionic polymeric material, poly[(2-methacryloyloxyethylphosphorylcholine)-ran-(butyl methacrylate)] (poly(MPC-r-BMA). It is, however, contradictory to the drastic change in the O-H stretching band for water incorporated into films of polymers such as poly(2-hydroxyethyl methacrylate), poly(methyl methacrylate) and poly(butyl methacrylate). These results suggest that polymers with a zwitterionic structure do not significantly disturb the hydrogen bonding between water molecules incorporated in the thin films. The investigation into the blood-compatibility of both the poly(SPB-r-BMA) and the poly(MPC-r-BMA) films indicate a definite correlation between the blood-compatibility of the polymers and the lack of effect of the polymeric materials on the structure of the incorporated water.     

Hydrogen bonding of water confined in mesoporous silica MCM-41 and SBA-15 studied by 1H solid-state NMR

The adsorption of water in two mesoporous silica materials with cylindrical pores of uniform diameter, MCM-41 and SBA-15, was studied by 1H MAS (MAS=magic angle spinning) and static solid-state NMR spectroscopy. All observed hydrogen atoms are either surface -SiOH groups or hydrogen-bonded water molecules. Unlike MCM-41, some strongly bound water molecules exist at the inner surfaces of SBA-15 that are assigned to surface defects. At higher filling levels, a further difference between MCM-41 and SBA-15 is observed. Water molecules in MCM-41 exhibit a bimodal line distribution of chemical shifts, with one peak at the position of inner-bulk water, and the second peak at the position of water molecules in fast exchange with surface -SiOH groups. In SBA-15, a single line is observed that shifts continuously as the pore filling is increased. This result is attributed to a different pore-filling mechanism for the two silica materials. In MCM-41, due to its small pore diameter (3.3 nm), pore filling by pore condensation (axial-pore-filling mode) occurs at a low relative pressure, corresponding roughly to a single adsorbed monolayer. For SBA-15, owing to its larger pore diameter (8 nm), a gradual increase in the thickness of the adsorbed layer (radial-pore-filling mode) prevails until pore condensation takes place at a higher level of pore filling.     

Nature of acidic protons in metallosilicate molecular sieves as studied by variable temperature 1H MAS NMR

The dynamic properties of protons in H-[Ga]-ZSM-5, H-[B]-ZSM-5 and H-[Al-B]-ZSM-5 were compared with that of protons in H-[Al]-ZSM-5 by temperature dependence of ¹H MAS NMR in the range of 298 k and 473 K. The temperature dependence of the line width of ¹H MAS NMR reveals that protons in H-[Ga]-ZSM-5 were more mobile than those in H-[Al]-ZSM-5 at temperature as low as 373 K. The protons in H-[B]-ZSM-5 were not mobile at 473 K and fixed in the zeolite frame work as the bridging hydroxyl groups, ≡B-OH-Si≡. The thermal motion of protons in ≡Al-OH-Si≡ was suppressed by introducing B³⁺ cations into the framework of H-[Al]-ZSM-5.     

Polycondensation behavior of methyltrimethoxysilane studied by NMR spectroscopy

Silicate gels are most often synthesized by hydrolyzing monomeric alkoxide precursors employing acid or base catalysts. The properties of final products are determined by the pathway of the reactions, which includes hydrolysis, condensation, and solvent exchange reaction. We analyzed a system with methyltrimethoxysilane as monomer and ethanol as solvent by using ¹H, ¹³C, and 2D heteronuclear correlated nuclear magnetic resonance (NMR) spectroscopy. Complete assignments of the proton and the ¹³C-NMR bands were established, which rendered the kinetic study feasible. An understanding of the behavior of the system was also attempted, especially at earlier stages. The results showed that hydrolysis and solvent exchange reactions occurred at early stage whereas condensation occurred slowly. © 1996 John Wiley & Sons, Inc.     

Formation and reaction of polymer ions in solutions studied by pulse radiolysis

Formation and reaction of polymer anions in solutions of poly(methyl methacrylate), poly(ethyl methacrylate), poly(n-butyl methacrylate), poly(isobutyl methacrylate) and poly(4-vinylbiphenyl) in hexamethylphosphorictriamide and 2-methyltetrahydrofuran were studied by nanosecond pulse radiolysis. The rate constants of the reactions of the polymers with solvated electrons were determined and they were compared with those of the corresponding low molecular-weight molecules. Besides, the decay reactions as well as the electron transfer reactions of the anion radicals of these polymers were investigated. The time-profile of the poly(4-vinylbiphenyl) anion observed in 2-methyltetrahydrofuran at room temperature showed a spike which was followed by a slow decay. The fast decaying component was tentatively attributed to geminate recombinations within micro domains in the solution where the polymers were entangled.     

Molecular dynamics in paramagnetic materials as studied by magic-angle spinning 2H NMR spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)相似文献   

Poly-1-oxy-2-phenyltrimethylene as studied by 1H pulsed low resolution NMR: a possible oxygen scavenger

Aromatic polymers can adsorb a large amount of oxygen on their aromatic rings. In semicrystalline aromatic polymers, the amount of adsorbed oxygen is much larger in the amorphous phase than in the crystalline counterpart. As a consequence, fully amorphous aromatic polymers are more suitable as oxygen scavengers than semicrystalline polymers.A pulsed low resolution ¹H-NMR relaxation study on semicrystalline and fully amorphous poly-1-oxy-2-phenyltrimethylene (St–CO), deuteriated and not deuteriated on the backbone, is reported. T₁ relaxation values were measured at 30 and 57 MHz and compared with the values of two aromatic polymers previously studied, syndiotactic polystyrene (s-PS) and polyphenyleneoxide (PPO), both patented as oxygen scavengers. For all these polymers, using a set of equations, at each temperature, the amount of adsorbed oxygen was calculated.Very short T₁ values are observed at the low temperature point of PPO which is the best oxygen scavenger at low temperature, while St–CO adsorbs oxygen efficiently at room temperature. Thus, St–CO might be suitable to be used as an oxygen scavenger.In St–CO, a ¹H T_1ρ relaxation study on the rotating frame has also been performed. In the atactic copolymer, in the temperature range 150–160 K, a sharp transition was observed only in the presence of oxygen. In agreement with a previously given interpretation for analogous data, the observed transition might be related to low frequency motions present in low molecular weight components.     

Gas diffusion in polycrystalline silicalite membranes investigated by 1H pulse field-gradient NMR

1H pulse field-gradient (PFG) spin-echo NMR was performed to measure the diffusivity of methane in a polycrystalline MFI-type silicalite membrane. Measured diffusivities decreased with an increase in the diffusion distance and converged to the constant value. This result suggests the presence of a transport barrier in the membrane. The long-time diffusivity in the membrane was 3.7 x 10(-9) m2/s, which was a factor of 3 smaller than reported values in a single crystal. The distance between the transport barriers was estimated to be much larger than 6 mum from the relationship of diffusivity with displacement. It should be noted that the estimated distances were larger than the smallest dimension of the crystals appearing in the membrane surface. Gas permeation and pervaporation tests were carried out on the same sample for which NMR measurements were taken. The estimated methane flux using measured long-time diffusivity by the permeation theory overestimated the experimental value, although it is closer to the experimental value than the value estimated using the short-time diffusivity. These results mean that the methane diffusivity in a silicalite membrane is much smaller than that in a single crystal.     

Cellulose–water interactions during enzymatic hydrolysis as studied by time domain NMR

The different states and locations of water within the cellulose matrix can be studied by the use of time domain low field NMR. In this work we show how the state and location of water associated with cellulose in filter paper fibers are affected by enzymatic hydrolysis. Three locations of water were identified in the filter paper; (1) bound water associated with the microfibril surfaces and (2) water in the cell wall or cellulose matrix and (3) capillary water in the lumens and between fibers. The different mechanisms of cellulase enzymes can be seen in their effect on the cellulose–water interactions and the synergistic effects between endo- and exo enzymes can be easily detected by time domain NMR. An interesting observation is that it is possible to link the state and location of water within the cellulose fiber with structural changes upon enzymatic hydrolysis.     

Structure of polymer networks in liquid crystals studied by 13C NMR

We report on 13C NMR measurements above and below the clearing temperature of the liquid crystal 4-n-hexyloxyphenyl 4-methoxybenzoate constrained to an oriented, low concentration polymer network. The network is obtained by UV-irradiation of the reactive monomer 1,4-di-[4-(6-acryloyloxyhexyloxy)benzoyloxy]-2-methylbenzene which is in admixture with the liquid crystal. The characterization of orientational order of the mixtures before UV-radiation, and hence before polymerization, reveals the high order of the components at the polymerization temperature. The chosen geometry explains the LC director orientation only by the aligned network. Above the nematic-isotropic transition a strong pretransitional order is detected. Fast molecular translational diffusion averages the order over dimensions smaller than 1 mum. The Landau-de Gennes theory predicts a relation between pretransitional order and the lateral dimension of the LC regions. The experimental data are successfully explained by pore diameters of 35 and 98 nm for concentrations of 20 and 8 mol% of monomer, respectively. The results support the model of nearly cylindrical shaped liquid crystal domains surrounded by thin walls of crosslinked network.