SEPARATION OF 13C NMR RELAXATION MECHANISMS IN VISCOUS SOLUTIONS

The ¹³C NMR analysis of relaxation data from viscous solutions requires the application of complex mathematical functions. These equations can be combined and solved exactly using a series of iterative algorithms. Correct analysis of typical relaxation data obtained for several viscous ionic liquids provides rotational correlation times that describe the molecular dynamics of these and other viscous solutions. The ¹³C NMR relaxation data for 1-methyl-3-nonyl-imidazolium PF₆ ionic liquid is used to illustrate the details of this method using iterative techniques in a stepwise manner. This approach is outlined such that other investigators can easily duplicate the algorithms.     

Relaxation mechanism in several kinds of polyethylene estimated by dynamic mechanical measurements, positron annihilation, X-ray and C solid-state NMR

Relaxation processes of several kinds of polyethylene films and fibers with different molecular orientational degrees and crystallinities were extensively investigated by the dynamic mechanical relaxation, positron annihilation and ¹³C nuclear magnetic relaxation (¹³C NMR). From complex dynamic tensile modulus, the activation energies of α₁ and α₂ relaxations were determined to be 97-118 and 141-176 kJ/mol, respectively. The activation energy of β relaxation was 114-115 kJ/mol. These values were similar to those of α₁ relaxation reported already. For γ relaxation mechanisms, there existed two mechanisms, γ₁ and γ₂, the activation energies being 9-11 and 23-25 kJ/mol, respectively. The values were independent of the molecular orientation and crystallinity. The two local motions indicate that non-crystalline phase composes of two regions of non-crystalline phase, rubber-like amorphous phase and interfacial-like amorphous phase. From ¹³C NMR measurements of ¹³C longitudinal relaxation time for the non-crystalline phase, the activation energy was 20.7 kJ/mol. This value is close to the activation energy (23-25 kJ/mol) of the γ₂ relaxation estimated by the dynamic mechanical measurement. The result by ¹³C NMR did not provide two kinds of activation energy, indicating combined influence of the two correlation times. Even so, the activation energies obtained by ¹³C NMR indicated that the γ₂ relaxation mainly is due to the motion of the C-C central bond of a short segment (e.g. three to four CH₂) within interfacial-like amorphous phase. The γ and β relaxation peaks by the dynamic mechanical measurements corresponded to the first and second lifetime transition of ortho-positronium indicating, in turn, a drastic change in free volume by local mode relaxation.     

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.     

Ion and Molecular Transport in Solid Electrolytes Studied by NMR

NMR is the method of choice for molecular and ionic structures and dynamics investigations. The present review is devoted to solvation and mobilities in solid electrolytes, such as ion-exchange membranes and composite materials, based on cesium acid sulfates and phosphates. The applications of high-resolution NMR, solid-state NMR, NMR relaxation, and pulsed field gradient ¹H, ⁷Li, ¹³C, ¹⁹F, ²³Na, ³¹P, and ¹³³Cs NMR techniques are discussed. The main attention is paid to the transport channel morphology, ionic hydration, charge group and mobile ion interaction, and translation ions and solvent mobilities in different spatial scales. Self-diffusion coefficients of protons and Li⁺, Na⁺, and Cs⁺ cations are compared with the ionic conductivity data. The microscopic ionic transfer mechanism is discussed.     

Development and evaluation of methods for determining the pattern of functionalization in sodium carboxymethylcelluloses

Sodium carboxymethylcellulose (NaCMC) with varying degrees of substitution (DS) was investigated with different analytical methods in order to characterize the functional group distribution. The following methods were tested and adapted: high-resolution ¹³C NMR spectroscopy in the solid state (¹³C CP/MAS NMR) and ¹³C NMR spectroscopy on solutions of NaCMC samples with a reduced molar mass. Partial degradation was accomplished by ultrasonic means and with the enzyme endoglucanase. Combining the two techniques resulted in the greatest reduction in molar mass and hence in the best spectral resolution. Analysis of the NaCMC fragments following ultrasonic and/or endoglucanase degradation also reveals another interesting experimental finding. It appears that ultrasonic degradation is favored at unsubstituted areas near the center of the chain. These methods were compared with the following already familiar techniques of analysis: titrimetric techniques; ¹³C and ¹H NMR spectroscopy as well as HPLC on completely hydrolysed solutions (hydrolysis with perchloric acid, trifluoroacetic acid and sulfuric acid). All of the methods characterize the samples as a series with increasing DS, the values of which range from 0.9 to 2.4. Methods that permit analysis of the partial degree of substitution produced the distribution x₂ > x₆ > x₃. Therefore, they are in principle suited for determining the functionalization pattern of the NaCMC samples relative to one another. The most suitable method can therefore be selected according to the objectives and the apparatus available. However, the measured values do exhibit considerable spread, variances of approx. 20%, thus, place restrictions on using the values of DS or x_i in absolute comparisons beyond these methods.     

Ultrasound devulcanization of unfilled natural rubber networks,studied via component molecular mobility

¹³C NMR solids spectroscopy and transverse relaxation, and ¹H relaxation and pulsed‐gradient spin‐echo self‐diffusion measurements at 70 °C were used to study molecular and segmental mobilities in natural rubber before and after sulfur crosslinking, and after subsequent devulcanization using intense ultrasound. NMR relaxation does not clearly distinguish between entangled and crosslinked network mobility, but unentangled sol and oligomeric species are separable within the longer T₂ decay components. Ultrasound reactor settings affect the amount of extractable sol generated. Some two‐thirds of the sol is entangled, with number‐average molecular weights (M_n) above 10 000 g mol^?1. Samples also contain near 2 wt% of inert light species (M_n < 400 g mol^?1); ultrasound is relatively ineffective in producing additional oligomeric material. All proton mobilities increase as more sol is produced, but ¹³C relaxation, reflecting intramolecular effects, indicates a slight decrease in backbone mobility. In contrast with other rubbers, in natural rubber, neither the glass transition nor the sol diffusion rate is greatly affected by the extent of ultrasound exposure. Comparisons with previous similar work of this laboratory, particularly styrene‐butadiene rubber, are useful in confirming the molecular mechanisms involved. Copyright © 2007 Society of Chemical Industry     

Cationic polyelectrolytes from natural building blocks of chitosan and inulin

Novel chitosan-N-inulin graft copolymers with different degree of substitution (DS) of chitosan were synthesized via water-soluble 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide mediated reaction. Chemical structure and composition of the chitosan derivatives was confirmed by chemical analysis, FT-IR, XPS, ¹H and ¹³C NMR spectroscopy, and potentiometry. Chitosan–inulin copolymers were high-molecular-weight hydrophilic products soluble in water in a wide pH range forming extraordinary viscous solutions. Intrinsic viscosity of N-modified chitosans was sharply suppressed by added electrolyte and had tendency to decrease at higher DS of chitosan. pK_α values of the chitosan–inulin copolymers determined from potentiometric titration data using Henderson–Hasselbalch equation were in the range 6–7 slightly increasing at higher DS. Novel water-soluble chitosan copolymers retained cationic properties of chitosan and could be used as surface conditioners.     

Morphology and molecular mobility of plasticized polylactic acid studied using solid‐state 13C‐ and 1H‐NMR spectroscopy

MAS ¹³C‐NMR measurements were used for the study of morphology and molecular mobility in amorphous quenched and triacetine‐plasticized PLA samples and PLA samples which underwent cold crystallization during annealing at 80 and 100 °C. The single pulse MAS ¹³C‐NMR spectra indicate that plasticizer promotes cold crystallization which results in the decrease of the temperature of crystallization and formation of more perfect crystalline domains. The T₁(¹³C) spin‐lattice relaxation times show that the presence of plasticizer molecules leads to an increase of local mobility in PLA chains but plasticized PLA after annealing at 100 °C shows more rigid structure. The series of broad line ¹H‐NMR spectra performed at temperatures up to 100 °C provided information on the changes in relaxation processes and morphology of the studied samples. The interpretation of the results obtained using the techniques of NMR spectroscopy were supported by WAXD and DSC measurements. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43517.     

Anionic effect of chloride,fluoride, and sulfide ions on the viscosity of slag melt

The effect of the addition of CaX (X=Cl₂, F₂ and S) on the viscous behavior and structure of CaO–SiO₂–Al₂O₃–MgO–CaX slag was investigated by measuring its viscosity. The viscosity of the slag without CaX gradually decreased with an increase in the C/S ratio because of the depolymerization of the silicate groups in the slag. While the viscosity of the CaX‐bearing slag decreased with an increase in the CaX content, depolymerization was not observed in this case. Three distinct compositional regions for the activation energy of the viscous flow were observed because of the effect of the equilibrium of the polymeric silicate groups. The relaxation effect of the CaX groups on the activation energy was also observed. Raman spectroscopic analysis indicated that the relaxation in the viscosity and activation energy by CaX addition stemmed from the breaking of the NBO‐M²⁺‐NBO linkage to form NBO‐M²⁺‐F^?, NBO‐M²⁺‐Cl^?, or M²⁺‐S^2?. All these results are discussed in detail with the help of a viscous flow model based on the ionic interactions.     

NMR determination of crystallinity for poly(?-l-lysine)

Crystallinity of poly(?-l-lysine) (?-PL) was discussed by analyzing the differences in the ¹H spin-spin relaxation times (T₂^H), the ¹³C spin-lattice relaxation times (T₁^C), and the ¹³C NMR signal shapes between the crystalline and the non-crystalline phases. The observed ¹H relaxation curve (free induction decay followed by solid-echo method) showed the sum of Gaussian and exponential decays. Similarly, the observed ¹³C relaxation curves obtained from the Torchia method were double-exponential. The ¹³C NMR spectrum of ?-PL was divided into the narrow and the broad lines by utilizing the intrinsic differences in the ¹H spin-lattice relaxation times in the rotating-frame between them, which are attributed to the crystalline and the non-crystalline phases, respectively. Even though the crystallinity is obtained from the identical NMR measurements, the estimated values are different with each other. The crystallinity estimated from the T₂^H differences was 75.8 ± 0.1% at 333 K and 60.7 ± 0.4% at 353 K. From the T₁^C differences, the value was estimated to be 62 ± 11%. Furthermore, the value estimated from the NMR signal separation was 54 ± 5%. In this study we have explained these discrepancies by the difference in susceptibility among the experiments for the inter-phase, which exists in-between the crystalline and the amorphous phases. Furthermore, the estimated crystallinity was ascertained by the X-ray diffraction experiment.     

C NMR study of C-enriched single-wall carbon nanotubes synthesized by catalytic decomposition of methane

¹³C NMR spectra and spin-lattice relaxation times were measured for single-wall carbon nanotubes with 99.9 and 50.0% ¹³C enrichments and natural abundance (1.1% ¹³C) prepared by catalytic decomposition of CH₄. The ¹³C isotropic shift is about 116 ppm from tetramethylsilane, being estimated from magic-angle-spinning (MAS) spectra. The value does not depend on the degree of the ¹³C enrichment. The ¹³C MAS NMR spectra show two additional small peaks at 171 and 152 ppm, which are ascribed to carbon species at defects or edges. The line widths of the main isotropic peak in MAS spectra are about 30 ppm, the origin of which is mostly chemical shift dispersion, reflecting a distribution of diameter and helicity. The line width in the ¹³C static spectra originates from chemical shift dispersion, chemical shift anisotropy and dipole–dipole interactions between ¹³C spins as well as between ¹³C and ¹H spins at defects or edges. ¹H NMR spectra confirm the presence of H-containing species. The ¹³C spin-lattice relaxation is dominated presumably by interaction with magnetic impurities.     

Nondestructive NMR determination of oil composition in transformed canola seeds

Magic-angle spinning (MAS) ¹³C nuclear magnetic resonance (NMR) spectroscopy is a convenient method for nondestructive, quantitative characterization of seed oil composition. We describe results for intact hybrid and transformed canola seeds. The MAS ¹³C NMR technique complements and agrees with gas chromatography results. The spectral resolution approaches that of neat, liquid oils. MAS ¹³C NMR data allow quantitative analysis of major oil components, including saturates and oleic, linoleic, and linolenic acyl chains. ¹³C NMR directly and quantitatively elucidates, triglyceride regiochemistry and acyl chain cis-trans isomers that cannot be quickly detected by other methods. MAS ¹³C NMR can serve as the primary method for development of near-infrared seed oil calibrations. These NMR methods are nondestructive and attractive for plant-breeding programs or other studies (e.g., functional genomics) where loss of seed viability is inconvenient.     

Molecular motion of poly(methyl methacrylate), polystyrene and poly(propylene oxide) in solution studied by 13C n.m.r. spin-lattice relaxation measurements: effects due to distributions of correlation times

The ¹³C n.m.r. spin lattice relaxation times and Nuclear Overhauser Enhancements have been measured for solutions of poly(methyl methacrylate) (PMMA) in o-dichlorobenzene, polystyrene (PS) in pentachloroethane and poly(propylene oxide) (PPO) in CDCI₃ as a function of temperature. For PS and PMMA a T₁ minimum is observed close to ambient temperature. The single correlation time theory of relaxation is inadequate to explain the relaxation data, but within experimental error, the data can be interpreted in terms of either the Cole-Cole distribution of correlation times, the log ?χ² distribution or a conformational jump model of chain dynamics.     

Molecular motion of syndiotactic poly(α-methylstyrene) in solution studied by carbon-13 n.m.r. relaxation measurements

The carbon-13 n.m.r. spin-lattice relaxation times, nuclear Overhauser enhancement factor (NOE), and line widths have been measured for a syndiotactic poly(α-methylstyrene) in solutions in toluene-d₈ and o-dichlorobenzene-d₄ as a function of temperature from 40° to 163δC. The single correlation time model of relaxation is inadequate to explain the data of spin-lattice relaxation time and NOE. But, within experimental error, these relaxation data in two solvents over a full temperature range can be interpreted in terms of either the Cole-Cole or the log-χ² distributions of correlation times, or a conformational jump model proposed by Monnerie et al. The internal rotation of the methyl group is relatively rapid, while that of the phenyl ring is slow and practically overshadowed by the backbone segmental reorientation over the temperature range examined. The solvent dependence of relaxation data was discussed.     

13C NMR as a primary method for determining saturates,cis‐ and trans‐monounsaturates and polyunsaturates in fats and oils for nutritional labeling purposes

The suitability of ¹³C NMR as a primary method for the analysis of lipids to obtain nutritional labeling compositional data (NLCD), i.e. the percentages of saturated, cis‐monounsaturated, trans‐monounsaturated, and cis‐polyunsaturated fat, was assessed. The ¹³C NMR methodology was developed by using mixtures of pure triglycerides as model lipids to optimize and standardize scan conditions and spectral pre‐processing procedures, establish fixed integration limits for measurement of the ¹³C resonances used in the determination of NLCD, and evaluate the quantitative accuracy of the ¹³C NMR analysis. The standardized ¹³C NMR methodology allowed the NLCD of the model triacylglycerol mixtures to be determined within ?±1%. To further evaluate the methodology, two sets of validation samples, consisting of ten unhydrogenated oils from the American Oil Chemists' Society Laboratory Proficiency Program (AOCS‐LPP) and two trans‐containing AOCS‐LPP samples combined with three samples from a hydrogenation process, were analyzed. Good overall agreement between the NMR‐determined NLCD (in units of mol‐%) and the mol‐% NLCD calculated for these samples from fatty acid compositional data obtained by gas chromatography was found, including good tracking of the trans content in the second validation set. Given that the NLCD must be expressed on a wt‐% basis to be of practical utility, a means of mol‐% to wt‐% conversion was developed assuming all unsaturates to be C₁₈ and obtaining the weight‐average molecular weight of the saturated fatty acid contributions from the NMR data. This conversion was shown to be especially effective for oil blends, where errors become significant if unit conversion is not done. This work indicates that ¹³C NMR can provide excellent primary NLCD data, even in wt‐% terms, which can be used for calibrating simpler and automatable instrumental methods such as FTIR spectrometers to determine or screen for NLCD for fats and oils or lipids extracted from food on a routine basis.     

Degree of sulfonation and microstructure of post-sulfonated polyethersulfone studied by NMR spectroscopy

Commercial polyethersulfone (Udel^® 1800) based on 4,4′-dihydroxy-2,2-diphenylpropane (bisphenol-A) and dichlorodiphenylsulfone was post-sulfonated using trimethylsilyl chlorosulfonate under mild conditions. The partially sulfonated polyethersulfones were investigated by ¹H and ¹³C NMR in their acid and/or sodium salt form. Several ¹³C NMR signals could be assigned up to the triad level. ¹³C-T₁ relaxation times were determined by the inversion recovery method for all carbons to ensure conditions for quantitative ¹³C NMR measurements. Additionally, the nuclear Overhauser enhancements are given. The degree of sulfonation (DS) of the samples covers the range from 13.6 to 100% as determined both by ¹H and ¹³C NMR spectroscopy. The NMR spectra confirm that the sulfonation occurs solely as mono-substitution of each phenyl ring of the dioxy-2,2-diphenylpropane unit in ortho-position to the ether bond. The microstructure of all polymers was evaluated from the content of dioxy-2,2-diphenylpropane-based diads and diphenylsulfone-centred triads. From the development of non-, mono- and disulfonated dioxy-2,2-diphenylpropane units with increasing DS it can be concluded that the sulfonation of the first phenyl ring retards the sulfonation of the second one. Thus, the non-sulfonated units deplete faster and the monosulfonated units are enriched compared with a random sulfonation. There is no reactivity influence between dioxy-2,2-diphenylpropane units which are separated by a diphenylsulfone unit because the content of diphenylsulfone-centred units follows the random distribution.     

Nuclear magnetic resonance in solid ethylene/α-olefin copolymers: Relaxation,spin-diffusion and lamellar widths

¹H and ¹³C-n.m.r. spectra and spin-lattice relaxation behaviour (laboratory frame, T₁ (¹³C and ¹H), and rotating frame, T_1ρ (¹H)) are reported for a range of solid ethylene copolymers with α-olefins having different types and concentrations of branches. The spin-diffusion model for relaxation in semicrystalline polymers is summarised and some new theoretical results given. The ¹³C high-resolution n.m.r. spectra obtained using both cross-polarisation (c.p.) and single pulse excitation (s.p.e.) methods associate side chain resonances mainly with the mobile, short T_1ρ protons and, hence, the more disordered region of the solids, consistent with the short ¹³C T₁ components. ¹H laboratory-frame spin-lattice relaxation is single component whilst those for ¹³C and the ¹H on-resonance rotating-frame relaxation are both multiexponential processes, requiring a minimum of three components to describe them. Annealing of samples, which is known to increase the lamellar thicknesses of the crystalline region, causes large increases in the longer relaxation time components. The ¹H and ¹³C spin-lattice relaxation data for a set of annealed and quenched ethyl-branched materials having different branch contents are compared in detail with the predictions of the spin-diffusion relaxation model. The results are internally and semi-quantitatively consistent with this theory and it is concluded that for the ¹H spin-lattice relaxation the model is clearly appropriate and for ¹³C it is consistent with observations. Questions concerning the relevance of spin-diffusion for the magnetically dilute ¹³C nucleus at natural abundance are mentioned and possible alternative explanations for the relationships observed are referred to briefly.     

Alkyl Tail Segments Mobility as a Marker for Omega-3 Polyunsaturated Fatty Acid-Rich Linseed Oil Oxidative Aging

Omega-3 polyunsaturated fatty acid (PUFA)-rich linseed oil (LSO) is an important component in biological systems, foods, and many other industrial products. In recent years, LSO has attracted increased attention in the field of functional foods, which has highlighted its facile susceptibility to aging by autoxidation. Common colorimetric and a long list of spectral methodologies have been used to follow after and predict LSO shelf life's quality, especially in regards to aging by autoxidation. These standard methodologies are nevertheless limited, because of the complexity of the LSO's chemical and physical changes. The goal of the present study is to develop a sensorial ¹H LF-NMR energy relaxation time application based on monitoring primary chemical and structural changes occurring with time and temperature during oxidative thermal stress for better and rapid evaluation of LSO's aging process. Using ¹H low-field NMR, the different T₂ times of energy relaxations due to spin–spin coupling, and proton motion/mobility of LSO molecular segments were monitored. As previously reported, we characterized the chemical and structural changes in all phases of the autoxidation aging process. Starting from the initiation phase (abstraction of hydrogen radical, fatty acid chain rearrangement, and oxygen uptake yielding hydroperoxides products), through to the propagation phase (chain reactions resulting in tail cleavage to form alkoxy radicals, and alpha, beta-unsaturated aldehydes formation), and a termination phase (cross linking and production of polymerization end products). The ¹H LF NMR transverse relaxation approach, monitors both the covalent bond's strong forces (100–400 kJ mol⁻¹) in LSO oxidative aging decomposition, as well as secondary relatively weak interactive forces by hydrogen bonds (~70 kJ mol⁻¹), and electrostatic bonds (0–50 kJ mol⁻¹) contributing to secondary crosslinking interactions leading to a LSO viscous gel of polymerized products in the termination phase. In the present paper, we show that LSO tail segments mobility in terms of T₂ multi-exponential energy relaxation time decays, generated by data reconstruction of ¹H transverse relaxation components are providing a clear, sharp, and informative understanding of LSO sample's autoxidation aging processes. To support T₂ time domain data analysis, we used data from high-field band-selective ¹H NMR pulse excitation for quantification of hydroperoxides and aldehydes of the same LSO samples treated under the same thermal conditions (25, 40, 60, 80, 100, 120 °C) with pumped air for 168 hours. Peroxide value, viscosity, and self-diffusion analyses, as well as fatty acids profile and by-products determined by GC–MS on the same samples were carried out, and correlated with the LSO tail T₂ energy relaxation time results. From these results, it is postulated that selective determination of LSO tail T₂ time domain can be used as a rapid evaluation marker for following omega-3 PUFA-rich oils oxidative aging process within industrial and commercial products.     

Investigation of the cellulose/LiCl/dimethylacetamide and cellulose/LiCl/N-methyl-2-pyrrolidinone solutions by 13C NMR spectroscopy

The cellulose/lithium chloride/dimethylacetamide (DMAc) and cellulose/lithium chloride/N-methyl-2-pyrroilidinone (NMP) solutions were investigated by ¹³C NMR spectroscopy. Well-resolved spectra were obtained for both solutions and indicated that cellulose was present in these systems in the form of underivatized cellulose. The change in ¹³C chemical shifts of DMAc and NMP in the presence of LiCl and LiBr was compared with that of several salt/aprotic solvents, and the results point to the existence of a cellulose–LiCl–DMAc (or NMP) complex in which the lithium cation is strongly bound to the amide carbonyl oxygen and the chloride anion involved in the dissociation of the cellulose hydrogen bonds. Spin–lattice relaxation times (T₁ of the ¹³C carbons of the solvent molecules, DMAc and NMP, show a large decrease in T¹ for all solvent carbons upon addition of LiCl. Further decrease in T₁ is observed when cellulose is introduced to the LiCl/NMP but not to the LiCl/DMAc systems. These observations are attributed to slower molecular motions of DMAc and NMP in the presence of LiCl, and, in the case of NMP, in the presence of cellulose.     

Polymer chain mobility in polyelectrolyte multilayers

Free-standing polyelectrolyte multilayer membranes have been formed by the layer-by-layer technique using a dip-coating apparatus. The polymer-chain mobility has been studied by ¹H relaxation in the rotating frame T _1rho NMR with ¹³C chemical shift resolution. For each of the individual polymers a single relaxation component has been observed for all resolved signals. In the multilayer a significantly different relaxation time T _1rho has been observed with a minor second component. The interaction between the oppositely charged polyelectrolytes influences the molecular mobility.