Strain-induced crystallization of natural rubber as studied by high-resolution solid-state C NMR spectroscopy

A series of high-resolution solid-state ¹³C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the ¹³C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the ¹³C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. ¹³C spin-lattice relaxation time (T₁) and ¹H spin-spin relaxation time (T₂) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative ¹³C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.     

Quantitative Si MAS NMR spectroscopy of cement and silica fume containing paramagnetic impurities

The low natural abundance and the long spin lattice relaxation time of ²⁹Si lead to long measurement times and/or low signal-to-noise ratios using ²⁹Si magic angle spinning NMR spectroscopy. By contrast, samples containing paramagnetic iron ions have much shorter relaxation times, making measurements up to seven times more efficient, but at the same time making quantitative analysis unreliable. To solve the problem, the spin-lattice relaxation times of ordinary Portland cement (opc) and silica fume with and without iron content has been determined with inversion recovery experiments. The effect of varying the spectrum repetition time on the quantitative analysis is demonstrated for mixtures of opc with silica fume. For opc and silica fume with iron impurities repetition times as short as 5 s has permitted accurate quantitative analysis of the silicates present in these materials.     

Solution O NMR study of thermal hydrolysis in nylon 6,6

The thermal hydrolysis of nylon 6,6 between 338 and 398 K was investigated using solution ¹⁷O NMR spectroscopy. By performing the hydrolysis with isotopically ¹⁷O-enriched H₂O, it is possible to easily identify the non-volatile oxygen-containing degradation products formed during the hydrolysis of nylon. For the aging temperature range investigated, the dominant oxygen-containing degradation species are carboxylic acids, consistent with the hydrolytic cleavage of the amide bond. These ¹⁷O NMR studies allowed the temperature variation for the hydrolysis of the amide bond in nylon 6,6 to be determined with of the initial rate of carboxylic acid concentration production giving an energy of activation of ∼87±1 kJ/mol.     

Solid-state nuclear magnetic resonance (NMR): Application to precursors of ceramics—Polycarbosilane

The chemical structures of oxidation- and electron irradiation-cured polycarbosilane fibers has been studied by IR and chemical analysis, but its structure has not been identified in detail. In this work, the chemical structure and curing mechanism was examined by solid-state high-resolution NMR spectroscopy. From the analysis of NMR spectra, it is explained that (i) in oxidation curing of PCS fibers, oxygen attacks first the SiC₃H bond and forms the SiC₃O bond and, next, the SiC₄ backbone bond and forms the SiC₂O₂ units; (ii) in electron irradiation curing, the signal intensity of SiC₃H units decreases with the increase in dose, the increase in the signal being due to the formation of SiC₄ units; (iii) solid-state²⁹Si high-resolution (CP/MAS) NMR spectroscopy is a powerful tool for investigating the chemical structure and curing mechanism of PCS fibers complementing infrared and solid-state¹³C high-resolution NMR spectroscopy; and (iv)¹H CRAMPS NMR spectroscopy is very useful for investigating the chemical structure and curing mechanism of PCS fibers.     

On the Quantification of Lignin Hydroxyl Groups With 31P and 13C NMR Spectroscopy

Factors affecting the accuracy of the analysis of lignin hydroxyl and carboxyl groups with ³¹P NMR have been further elucidated. Two modifications of ³¹P NMR analysis of lignin, namely the protocols using 1,3,2-dioxaphospholane (PR-I) and 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (PR-II) as phosphorylation reagents with different internal standards, were studied. The previous ³¹P NMR standard protocol with PR-II underestimated OH groups by about 30%, whereas the ³¹P NMR standard protocol with PR-I tended to produce overestimated data. It has been shown that cholesterol is not an appropriate internal standard, resulting in underestimated values for OH groups due to incomplete baseline resolution. The best internal standard has been found to be endo-N-hydroxy-5-norbornene-2,3-dicarboximide. Strong care should be taken related to the stability of the internal standards to avoid inflated results due to IS degradation. Under modified optimized conditions, both methods show a good correlation with the ¹³C NMR protocol in the quantification of hydroxyl groups as average, with the variability between the methods in the range of 5–15%. However, the ³¹P NMR protocols report COOH content that is twice as low as that of ¹³C NMR data. Finally, the best approach for the use of the ³¹P and ¹³C NMR methods in lignin analysis is discussed.     

Niobium oxide acidity studied by proton broad-line NMR at 4 K and MAS NMR at room temperature

The quantitative study of the Brønsted acidity of niobic acid (Nb₂O₅·xH₂O) using broad-line¹H NMR at 4 K has been performed by interacting niobic acid, pretreated at 573 K under vacuum, with water molecules. The number of oxyprotonated species (H₃O⁺ and H₂O...HO species formed, unreacted acidic OH groups or excess H₂O molecules) deduced from the simulations of the broad-line¹H NMR spectra shows a continuous increase in the number of H₃O⁺ species with adsorbed water molecules. This increase may be due to a classical dilution effect or to a synergistic interaction between Brønsted and Lewis acid sites. These results are compared with those of some HY zeolites with or without framework defects.     

High-resolution F and H NMR of a vinylidenefluoride telomer

The reaction products of vinylidenefluoride (VDF) with methanol as a telogen have been analysed in the solution state by ¹H and ¹⁹F nuclear magnetic resonance (NMR) spectroscopy. High-resolution ¹⁹F and ¹H NMR spectra were achieved using high-power ¹H and ¹⁹F decoupling, respectively, giving superior resolution and revealing previously unresolved signals of the vinylidenefluoride telomer (VDFT). ¹H and ¹⁹F homo- and hetero-nuclear scalar coupling constants are presented and the spectra of functional groups and reverse units (including the identification of short-chain structures) are discussed. Furthermore, the application of ¹⁹F or ¹H decoupling for the correct assessment of reverse-unit content and degree of polymerisation is demonstrated. This work highlights the need for high-resolution NMR spectroscopy to determine both the chemical structure and the composition of these important fluoropolymers.     

Dynamics of novel hydrogen-bonded acidic fluorinated poly(amide-imide-silica) hybrids studied by solid-state NMR

Novel hydrogen-bonded acidic fluorinated poly(amide-imide-silica) hybrid materials, FPAI-SiO₂ (6E and 6F) series, were synthesized by a sol-gel process. The structures and spin relaxation of the hybrids were characterized by infrared (IR), and ²⁹Si and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The abundant Q⁴ structures implied that in free catalyst the degree of condensation of tetramethoxysilane was enhanced by hydrogen-bonded acidic fluorinated poly(amide-imide). The dynamics on the local mobility of the hybrids was investigated by the time constant for energy exchange between ¹H and ²⁹Si spin system (T_SiH) and spin-diffusion path length (L) measurements. It was found that the faster T_SiH of 6E and 6F hybrids compared with the previous study of similar 6C and 6D hybrids implied that 6E and 6F hybrids had more aggregated structures even though the organic terminal segment changed from rigid imide to more flexible amide. The interactions of the charge transfer between donor and acceptor molecules or π-π aromatic stacking may be the dominant factors to affect the structures of 6E and 6F hybrids. Moreover, M¹ and D² segments of 6F hybrids had the same level mobility and the mobility of the 6F hybrids was little improved as the soft and flexible 1,3-bis(3-aminopropyl)-tetramethyl-disiloxane segment was incorporated in the dense structures of 6F hybrids. All of the L values of 6E and 6F hybrids were on the scale of 3.5-4.0 nm. The result also suggested that 6E and 6F hybrids had similar denser structures as 6D hybrids.     

Quantitative Analysis of Dairy Phospholipids by 31P NMR

³¹P NMR analysis of samples prepared in a sodium cholate detergent system was assessed as a method for the quantitative measurement of dairy phospholipids. Major phospholipid (PL) classes measured included: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), sphingomyelin (SM) and dihydrosphingomyelin (DHSM). The ³¹P NMR method was validated by comparison with a quantitative two-dimensional thin-layer chromatography (2D-TLC) technique. The 2D-TLC system was more sensitive, able to detect some minor compounds not observed by ³¹P NMR. However, ³¹P NMR is more suited to routine analysis, with sample analysis taking 36 min. The method was also more versatile and sample analysis was possible on high phospholipid containing materials without prior lipid extraction (e.g. buttermilk protein concentrate, beta serum liquid).     

MAS NMR studies of ZSM-5 zeolites: Correlation to para selectivity and SEM observations

²⁷Al and²⁹Si MAS NMR studies of various ZSM-5 zeolites of different Si/Al ratio, crystal size synthesized with and without alkali metal salt addition by the hydrothermal technique were carried out. The NMR observations clearly showed that the technique can be employed to differentiate Si/Al ratio, framework tetrahedral content and the crystal size. The last observation has been confirmed by SEM and catalytic activity test for para ethylation of ethylbenzene. A good correlation was found between the catalytic activity and the data from NMR and SEM.     

Use of the 15N natural abundance technique to quantify biological nitrogen fixation by woody perennials

Biological nitrogen fixation (BNF) associated with trees and shrubs plays a major role in the functioning of many ecosystems, from natural woodlands to plantations and agroforestry systems, but it is surprisingly difficult to quantify the amounts of N₂ fixed. Some of the problems involved in measuring N₂ fixation by woody perennials include: (a) diversity in occurrence, and large plant-to-plant variation in growth and nodulation status of N₂-fixing species, especially in natural ecosystems; (b) long-term, perennial nature of growth and the seasonal or year-to-year changes in patterns of N assimilation; and (c) logistical limitations of working with mature trees which are generally impossible to harvest in their entirety. The methodology which holds most promise to quantify the contributions of N₂ fixation to trees is the so-called `¹⁵N natural abundance' technique which exploits naturally occurring differences in ¹⁵N composition between plant-available N sources in the soil and that of atmospheric N₂. In this review we discuss probable explanations for the origin of the small differences in ¹⁵N abundance found in different N pools in both natural and man-made ecosystems and utilise previously published information and unpublished data to examine the potential advantages and limitations inherent in the application of the technique to study N₂ fixation by woody perennials. Calculation of the proportion of the plant N derived from atmospheric N₂ (%Ndfa) using the natural abundance procedure requires that both the ¹⁵N natural abundance of the N derived from BNF and that derived from the soil by the target N₂-fixing species be determined. It is then assumed that the ¹⁵N abundance of the N₂-fixing species reflects the relative contributions of the N derived from these two sources. The ¹⁵N abundance of the N derived from BNF (B) can vary with micro-symbiont, plant species/provenance and growth stage, all of which create considerable difficulties for its precise evaluation. If the%Ndfa is large and the ¹⁵N abundance of the N acquired from other sources is not several ¹⁵N units higher or lower than B, then this can be a major source of error. Further difficulties can arise in determining the ¹⁵N abundance of the N derived from soil (and plant litter, etc.) by the target plant as it is usually impossible to predict which, if any, non-N₂-fixing reference species will obtain N from the same N sources in the same proportions with the same temporal and spatial patterns as the N₂-fixing perennial. The compromise solution is to evaluate the ¹⁵N abundance of a diverse range of neighbouring non-N₂-fixing plants and to compare these values with that of the N₂-fixing species and the estimate of B. Only then can it be determined whether the contribution of BNF to the target species can be quantified with any degree of confidence. This review of the literature suggests that while the natural abundance technique appears to provide quantitative measures of BNF in tree plantation and agroforestry systems, particular difficulties may arise which can often limit its application in natural ecosystems.     

13C NMR analysis and gas uptake measurements of pure and mixed gas hydrates: Development of natural gas transport and storage method using gas hydrate

¹³C NMR spectra were obtained for pure CH₄, mixed CH₄+THF, and mixed CH₄+Neohexane hydrates in order to identify hydrate structure and cage occupancy of guest molecules. In contrast to the pure CH₄ hydrates, the NMR spectra of the mixed CH₄+THF hydrate verified that methane molecules could occupy only the small portion of 5¹² cages because the addition of THF, water-soluble guest component, to aqueous solution prevents the complete filling of methane molecules into small cages. Furthermore, from these NMR results one important conclusion can be made that methane molecules can’t be enclathrated at all in the large 5¹²6⁴ cages of structure II. In addition, gas uptake measurements were carried out to determine methane amount consumed during pure and mixed hydrate formation process. The moles of methane captured into pure CH₄ hydrate per mole of water were found to be similar to the full occupancy value, while the moles of methane captured into the mixed CH₄+THF hydrate per moles of water were much lower than the ideal value. The overall results drawn from this study can be usefully applied to storage and transportation of natural gas.     

Interactions between fullerene(C60) and poly(ethylene oxide) in their complexes as revealed by high-resolution solid-state C NMR spectroscopy

A series of poly(ethylene oxide) (PEO)/fullerene(C₆₀) complexes are prepared by lyophilization. The intermolecular interaction and molecular motion in the complex are investigated by solid-state ¹³C NMR spectroscopy. An intense C₆₀ signal due to the intermolecular cross-polarization is observed in the ¹³C CP/MAS spectra of the complex samples, indicating a high degree of dispersion of C₆₀s in the complexes. By measuring the ¹³C spin-lattice relaxation times and ¹H transverse relaxation times of the complex sample and by comparing the static ¹³C spectrum of the pure C₆₀ sample with that of the complex sample, it is demonstrated that there exist n-π interactions between the n-orbitals of the PEO ether oxygen and the π-system of C₆₀. The C₆₀ molecules act as physical cross-links in the amorphous region of PEO, which greatly inhibit the mobility of the surrounding PEO chains, while the rapid isotropic rotation of C₆₀ molecules is also reduced to some extent due to the interactions with the polymer chains.     

Characterization of siloxane copolymers by solution O NMR spectroscopy

Solution ¹⁷O NMR spectroscopy was used for structure elucidation of siloxane copolymers with the natural abundance of ¹⁷O, i.e. without any enrichment prior to spectroscopy. Homo, co, and terpolymers, as well as linear chains, cyclic oligomers, and graft polymers were investigated. All relevant chemical shifts and corresponding linewidths were reported for siloxane polymers substituted with methyl, phenyl, 3-cyanopropyl, 2-cyanoethyl, 3,3,3-trifluoropropyl, and polyethylene glycol ligands and with the backbone stiffening groups tetramethyl-p-silphenylene, tetramethyl-p,p′-sildiphenylene ether, and m-carborane. An increment system was extended to predict the chemical shifts of substituted siloxane copolymers. ¹⁷O NMR spectroscopy of polysiloxanes provided information concerning their chemical composition, average molecular weight, and microstructure.     

Fully quantitative carbon-13 NMR characterization of resol phenol-formaldehyde prepolymer resins

Different resol phenol-formaldehyde prepolymer resins have been synthesized with different Formaldehyde/Phenol (F/P) ratios or different catalysts and characterized by ¹³C NMR spectroscopy in solution. A fast quantitative measuring protocol is proposed based on the use of chromium(III)acetylacetonate as a relaxation agent. APT (attached proton test) and DEPT (distortionless enhancement by polarisation transfer) spectra were acquired to enable proper resonance assignments, especially in the regions with severe signal overlap. Equations are presented in which the methylene bridges (MB), the methylol groups (MG) and the dimethylene ether bridges (DMEB) of resol resins are quantitatively taken into account. Important structural factors determined quantitatively for resol prepolymer resins are the F/P ratio after reaction, the degree of polymerization (n), the number average molecular weight (M_n) and the content of free ortho and para positions.     

Solid-state V NMR and its potentiality in investigation of vanadia systems with paramagnetic centres

In this article we will discuss the potentiality of solid-state ⁵¹V NMR technique to characterize heterogeneous catalysts containing paramagnetic cations located on the surface or incorporated in the bulk of heterogeneous catalysts. ⁵¹V NMR data for a number of 3d and rare earth vanadates, where paramagnetic effects are caused by the presence of paramagnetic cations, are considered. We also summarize results available for vanadium bronzes, where some vanadium is in a paramagnetic V⁴⁺ state, and for some vanadium compounds, where closely spaced paramagnetic centers form diamagnetic pairs. The comprehensive ⁵¹V chemical shift scale (database) for paramagnetic solids was compiled.     

FTIR spectroscopic and1 H MAS NMR studies of the influence of lattice chemistry and structure on Brønsted acidity in zeolites

The influence of the Si/Al ratio, of the nature of the T-atom and of the pore size on the acidic strength of Brønsted sites in zeolites has been investigated using changes of the vibrational properties of Brønsted OH(OD) groups and a shift change of Brønsted protons in nuclear magnetic resonance upon adsorption of weak bases. Deuterated acetonitrile and trichloro-acetonitrile have been chosen to probe the acidic strengths of ZSM-5, FeZSM-5, mordenite and zeolite Y, which are often used as catalysts. From the results of the FTIR and 1 H MAS NMR studies it can be concluded that the chemical composition of the lattice dominates the acidic strength of the Brønsted sites in zeolites. Differences in structure or pore size play a much smaller role.     

Two-dimensional correlation relaxometry studies of cement pastes performed using a new one-sided NMR magnet

We present preliminary results of the first NMR T₁-T₂ two-dimensional relaxation correlation experiments performed using a one-sided NMR system in cement based materials. Two-dimensional correlation relaxometry has itself only recently been demonstrated in cement paste where it proved to be a particularly sensitive probe of pore-water dynamics providing direct information on exchange of water between the gel and capillary pore networks. Further to this we have observed differences in the structural development of a selection of cement pastes throughout the early stages of hydration and verified the theoretical frequency dependence of the ratio T₁ / T₂. When coupled with instrumentation developments in one-sided NMR magnets the way is opened to detailed, spatially resolved studies of the development of hydration and porosity in the surface layers (top 50 mm) of cementitious materials. A new magnet, suitable for such applications, is discussed.     

Gasoline composition determined by H NMR spectroscopy

A fast and simple ¹H NMR spectroscopic method was developed for the concentration measurement of aromatic, olefinic and aliphatic fractions in gasoline samples. Simultaneously, individual components such as benzene, methyl tert-butyl ether and several dienes could be determined. The method relies on only a few, well-established assumptions about the molecular compositions and is therefore applicable to a broad range of gasolines. It is well suited for laboratories with high sample throughput since measurement time is short and all concentrations are determined within one NMR experiment. The method was applied to commercial gasolines and samples used in European round robin tests. Comparisons of NMR and round robin test results showed excellent agreement.