Structural analysis of cellulose triacetate polymorphs by two-dimensional solid-state C-C and H-C correlation NMR spectroscopies

For the elucidation of the crystal structures of the two crystalline allomorphs of cellulose triacetate (CTA), namely CTA I and CTA II, two-dimensional (2D) solid-state through-bond ¹³C-¹³C and ¹H-¹³C correlations NMR techniques applied to the two crystalline allomorphs of CTA. As a result, the ¹³C and ¹H chemical shifts of the glucopyranose ring of CTA I and CTA II were completely assigned by the 2D NMR spectra of these allomorphs. On the 2D ¹³C-¹³C correlation spectrum of CTA II, two sets of the ¹³C-¹³C correlations from C1 to C6 were observed. This indicated that the CP/MAS ¹³C NMR spectrum of CTA II can be characterized by its overlapping of the ¹³C subspectra of two kinds of 2,3,6-triacetyl-anhydroglucopyranose units and that there are two magnetically non-equivalent sites in the unit cell of CTA II. In the case of CTA I, the numbers of respective ¹³C and ¹H shifts of CTA I agreed with the those of the glucopyranose residue in the allomorph, strongly suggesting that the asymmetric unit of CTA I is only one glucose residue. In addition, conformational differences in the exocyclic C5-C6 bonds between CTA I and CTA II were strongly suggested by the notable differences in the ¹H and ¹³C chemical shifts at the C6 sites of these allomorphs.     

Characterisation of the supramolecular structure of chemically and physically modified regenerated cellulosic fibres by means of high-resolution Carbon-13 solid-state NMR

Carbon-13 high-resolution solid-state NMR techniques have been invaluable in elucidating the structure of regenerated cellulosic materials. Studies of a range of fibres have shown systematic changes in chemical shifts, which can be related to the influences of physical processing or chemical modification. A constrained curve fitting method has been applied, where the C4 spectral envelope is represented as the sum of contributions from polymer in ordered, partially-ordered and disordered environments, associated with differing conformational arrangements of the cellulose hydroxymethyl and glycocidic bonds. The empirical gamma-gauche effect seems likely to provide the best rationalization for the relationship between C4 shifts and conformational order, taking into account the increased range of bond angles in disordered environments. The quantification of proportions of polymer units within different conformational groupings will provide new insights into the development of supramolecular texture. This will allow better appreciation of the relationships between fibre processing and ultimate fibre performance.     

High-resolution solid-state NMR structure of an anticancer agent

We demonstrate that solid-state NMR methods can be used to rapidly determine the high-resolution 3D structure of Epothilone B in the polycrystalline state. The solid-state NMR structures exhibit an average heavy atom RMSD to the mean structure of 0.14 A. The 3D-structural analysis leads to stereospecific assignments and provides insight into the influence of intermolecular interactions upon ssNMR chemical-shift parameters. Our results pave the way to the study of ligand-microtubule interactions in a noncrystalline and insoluble environment at atomic level.     

The study of water behaviour in regenerated cellulosic fibres by low-resolution proton NMR

Regenerated cellulosic fibres and comparative materials were studied in the hydrated state by low-resolution proton NMR. Experiments at variable pH and temperatures have shown that the shortened T₂ relaxation times of water within fully swollen cellulosic fibres are dominated by proton exchange with accessible cellulose hydroxyl groups. Proton exchange is accelerated by both acid and base catalysis, with relaxation data used to estimate rate constants for acid, base and neutral mechanisms. Complementary deuterium exchange measurements suggest that accessible cellulose regions below the immediate water interface may not contribute effectively to the proton exchange relaxation mechanism, with two-site relaxation models sensitive only to the direct pore surface area. Differences between surface-relaxing water and deuterium-exchanging water can therefore be used to determine an apparent depth of the accessible cellulose, which is greater for viscose and modal compared to lyocell. However, from relaxation data lyocell has a higher pore surface area. This work also confirmed that water interacting with accessible cellulose experiences motional restriction, allowing an intra-molecular dipolar contribution to relaxation. However, in the fully swollen state water molecules are diffusing rapidly between all internal fibre environments and there is no evidence of specific binding.     

An electron diffraction study of the crystal structure of native cellulose

The crystal lattice of native cellulose from four sources has been investigated by electron diffraction techniques. The four sources were: cotton, ramie, a bacterial cellulose (Acetobacter xylinum), and an algal cellulose (Valonia ventricosa). Evidence for the existence of at least two different unit cells is provided. There were no systematic absences of odd-order OkO reflections in any of the cellulose patterns, therefore, it was concluded that neither cellulose cell falls into the P2₁ space group.     

Atomic structure of chlorine containing calcium silicate glasses by neutron diffraction and 29Si solid-state NMR

Bioactive glasses are of great importance for medical and dental applications. In order to understand, model, and predict the behavior of these materials, and ultimately improve their design, it is important to understand the structure of these glasses. Ion dissolution is known to be the crucial first step in bioactivity and is strongly dependent upon the atomic-scale structure and network connectivity. While significant progress has been made understanding the structure of oxide-based glasses, relatively little is known about the structure of bioactive glasses containing halides. Recently, a series of novel chloride-based bioactive glasses has been developed. Chlorapatite converts to hydroxyapatite in water and these glasses are therefore of interest for novel toothpastes. This study reports the first detailed structural investigation of these bioactive chloride glasses using neutron diffraction and solid-state NMR. Chlorine was found to bond to calcium within the glass, and no evidence of Si-Cl bonding was detected. Furthermore, the absence of a chemical shift in the ²⁹Si NMR upon the addition of CaCl₂ helped confirm the absence of detectable amounts of Si-Cl bonding. Given that chlorine does not disrupt the Si-O-Si network, widely used network connectivity models are therefore still valid in oxychloride glasses.     

Probing the degree of crosslinking of a cellulose based superabsorbing hydrogel through traditional and NMR techniques

The network structure of a cellulose-based superabsorbing material has been probed by using three different techniques: ¹³C solid state NMR, free swelling in water and uniaxial compression of water swollen samples. A good agreement between the three apporaches has been found in terms of concentration of crosslinks per unit volume.The results have been discussed taking into account that NMR technique is able to detect only chemically effective crosslinks while free swelling and compression are sensitive to elastically effective physical and chemical crosslinks.A depression of swelling capacity and an apparent increase of degree of crosslinking with time, promoted by ageing of the cellulosic material, has been experimentally evidenced and discussed in terms of development of intermolecular physical interactions.     

Thermosetting mechanism study of silicon-containing polyarylacetylene via in situ FTIR and solid-state NMR spectroscopy

Silicon-containing polyarylacetylene (abbreviated as PSA) resins, which contain Si-(R₁, R₂) (R₁ and R₂ represent methyl or phenyl groups) and —CC—, exhibits high thermal stability upon curing up to 250 °C. The structure and thermosetting mechanism of PSA were characterized using FTIR, in situ FTIR, ¹³C and ²⁹Si CP-MAS spectroscopy, and thermogravimetric analysis. From the experimental results we can conclude that: (1) biphenyl and naphthalene rings are formed via a Diels–Alder reaction between the Ph—CC and CC groups at 210 °C, (2) the terminal alkyne mainly transforms into ethylenic bonds at 170 and 210 °C, and (3) an oxidation reaction occurs to give the oxide structure (Si—O—Si) and carbon dioxide at 250 °C. A new curing procedure has been proposed to maximize the T_d5 up to 635.2 °C on that basis. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47301.     

On the structure and oxygen transmission rate of biodegradable cellulose nanobarriers

ABSTRACT: Cellulose nanofibrils have been proposed for novel barrier concepts, based on their capability to form smooth, strong and transparent films, with high oxygen barrier properties. A series of cellulose-based films were manufactured and tested with respect to their oxygen transmission rate (OTR) capabilities. The obtained OTR levels were considerably better than the levels recommended for packaging applications. Part of the nanofibrillated material applied in this study was produced with 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) mediated oxidation as pretreatment. Films made of TEMPO-pretreated samples yielded lower OTR values. The minimum obtained OTR value was 3.0 mL m-2 day-1 atm-1 with a corresponding oxygen permeability of 0.04 mL mm m-2 day-1 atm-1, tested at 50% relative humidity. The good barrier properties are due to the compact and dense structure of the films, as revealed by field-emission scanning electron microscopy. A relationship between OTR and the structure of the corresponding nanofibril-based films was confirmed.     

FTIR spectroscopic study of the effect of microwave heating on the transformation of cellulose I into cellulose II during mercerization

Fourier transform infrared (FTIR) spectroscopy techniques were used in the study of the effect of microwave (MW) heating on the structural properties of cotton fibers and on the mercerization mechanism of these fibers. Samples of the fibers were microwave heated for different times and different microwave powers. Also, mixtures of cotton fibers and aqueous solution of NaOH with different concentrations were exposed to microwave radiation for different times and different powers. It was found that microwave heating of cotton fibers under these experimental conditions causes no observable changes in their spectral features apart from slight changes in the intensities of the absorption bands. The determined values of the absorbances ratio A1375 cm^?1/A2900 cm^?1 revealed that microwave heating for short periods and at low powers decreases the crystallinity of cotton as the result of the drying effect of microwave heating, while microwave heating for longer periods and at higher power results in recristallyzation of the fibers. The analysis of the experimentally obtained data revealed that microwave heating that causes molecular motions by migration of ions and rotations of the dipoles produces no considerable effects on the mechanism of mercerization but only reduces the concentration of NaOH in the solution and the time of treatment which are needed for the complete transformation of cellulose lattice type I into cellulose lattice type II without any heating. Also it was found that the magnitude of reductions depends on the applied power. Moreover the results proved that the deconvolution and the second derivatives of the FTIR spectra of cotton fibers can be used as a useful tool for distinguishing cellulose lattice type II from cellulose lattice type I. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and spectroscopic analysis of cellulose sulfates with regulable total degrees of substitution and sulfation patterns via C NMR and FT Raman spectroscopy

Synthesis and spectroscopic characterisation of cellulose sulfate (CS) were reported. Various CS exhibiting diverse degrees of sulfation (DS_S) were prepared through acetosulfation or direct sulfation of cellulose. During the acetosulfation, intermediate product - cellulose acetate sulfate (CAS) - was formed after the comparative esterification and subsequent deacetylation of CAS led to CS. The direct sulfation proceeded quasi-homogeneously and heterogeneously in N,N-dimethylformamide (DMF) or homogeneously in N,N-dimethylacetamide (DMAc)/LiCl mixture. The total DS_S between 0.21-2.59 and partial DS_S6 as well as DS_S2 of up to 1 were determined via elemental analysis and ¹³C NMR spectroscopy. Besides, solid-state CP/MAS ¹³C NMR could characterise CS regarding the sulfation. Subsequently, FT Raman investigation of obtained CS was conducted with the aim to establish analysis methods quantifying the total DS_S. The intensities of Raman bands ascribed to the vibrations of OSO and C-O-S groups were used as analysis parameters, yielding calibration curves with high correlation coefficients of more than 0.96.     

NMR study of the phosphonomethylation reaction on chitosan

N-Phosphonomethylation of chitosan reaction was studied and optimized using different reaction conditions. NMR spectroscopy was an important tool in this work to study this reaction and the α-aminomethylphosphonic acid function introduced onto chitosan was unequivocally characterized by ³¹P and ¹³C NMR analyses. But surprisingly, whatever the reaction conditions used, N-phosphonomethylation reaction of chitosan cannot be dissociated from a side reaction: the N-methylation reaction of chitosan. A mechanism was proposed to explain the formation of this side product.     

Characterisation of cellulose treated by the steam explosion method. Part 3: Effect of crystal forms (cellulose I,II and III) of original cellulose on changes in morphology,degree of polymerisaion,solubility and supermolecular structure by steam explosion

An attempt was made to clarify the effect of the crystal form of untreated cellulose on the morphological and structural changes of cellulose during steam explosion treatment (steam pressure P = 2.9MPa (T = 508K), treatment time t = 15-300 s). For this purpose, the crystal form of soft wood pulp (cellulose I) was converted by solid-to-solid transition, with minimal unavoidable change in other structural characteristics including morphology and average degree of polymerisation, into cellulose II or cellulose III. It was proved by both X-ray and solid-state cross-polarisation/magic-angle sample-spinning (CP/MAS) ¹³C NMR analyses that even a simple addition of water at room temperature brought about a significant structural change in the steam-untreated cellulose samples. The solubility towards 9.1 wt% aqueous sodium hydroxide, S_a, of the cellulose samples of crystal forms I and III could be improved from 31-33% up to almost 100% by selecting appropriate steam explosion conditions (for example, P = 2.9MPa, t = 30 s). Such a magnificent increase in S_a by the steam explosion treatment was not observed for the cellulose II sample, even under the rather severe conditions of the steam explosion treatment at which the cellulose III crystal was converted to a large extent to cellulose I, as confirmed by X-ray diffraction. X-ray diffraction analysis showed that crystallisation of samples with cellulose I or II crystal occurred to some extent during the steam explosion treatment. Contrary to this, the degree of breakdown of the intramolecular hydrogen bond O₃…O'₅, as estimated by CP/MAS ¹³C NMR analysis, significantly increased for cellulose I and I11 during the treatment. The decrease in the viscosity-average degree of polymerisation, P, observed for all treated samples can be roughly categorised into two or three steps of the first-order decomposition reaction with different reaction rates.     

X‐ray diffraction methods in the study of the effect of microwave heating on the transformation of cellulose I into cellulose II during mercerization

X‐ray diffraction (XRD) techniques were used in the study of the effect of microwave (MW) heating on the structural properties of cotton celluloses I and II and on the mercerization mechanism of cotton fibers. Samples of celluloses I and II were MW heated at 900 W for different times ranging from 10 to 40 min. The obtained data revealed that MW heating of cellulose II in opened glass tubes produces no significant effects on the resolution of its XRD patterns, whereas the most evident effects occur when cotton fibers (cellulose I) are heated in opened tubes at 900 W for 10 and 20 min. Also, mixtures of cotton fibers and aqueous solution of NaOH with different concentrations were exposed to MW radiation for different times and different powers. It was found that MW heating has no considerable effects on the mechanism of transformation of cellulose I into cellulose II during mercerization. On the other hand, MW heating of cotton fibers during mercerization reduces the values of concentration of NaOH in the aqueous solution and the time of treatment that are needed for the complete transformation of cellulose lattice type I into cellulose lattice type II without any heating. Also it was found that the magnitude of reductions depends on the applied power. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Network structure and reaction mechanisms in high pressure and peroxide vulcanization of polybutadiene: Microstructural changes studied by 13C solid-state NMR

This article is concerned with the microstructural changes during peroxide and high pressure vulcanization of polybutadiene that is unfilled and filled with carbon black. The main tool is ¹³C solid-state NMR; it shows that vinyl unsaturations are consumed in both filled and unfilled samples under peroxide and high pressure vulcanization. Chemical shift calculations of unvulcanized polybutadiene show good agreement with the observed peaks. Calculations of proposed structures, based on a possible reaction mechanism, suggest that a large number of peaks will appear, each at very low intensity. Nevertheless, some changes can be seen as a result of the crosslinking reaction, and the results provide support for the suggested reaction mechanism. Thus, the proposed addition crosslinking mechanism over vinyl unsaturations seems to be a reasonable explanation of the crosslink formation in high pressure vulcanization. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2799–2806, 1999     

Impact of magnesium on the structure of aluminoborosilicate glasses: A solid-state NMR and Raman spectroscopy study

Seven magnesium-containing aluminoborosilicate glasses, with three to five oxides, have been studied through comprehensive multinuclear solid-state NMR (¹¹B, ²⁷Al, ²⁹Si, ²³Na, ¹⁷O, and ²⁵Mg) and Raman spectroscopy. The progressive addition of cations and the substitution of sodium and calcium by magnesium illuminate the impact of magnesium on the glass structure. The proportion of tri-coordinated boron drastically increased with magnesium addition, demonstrating the poor charge-compensating capabilities of magnesium in tetrahedral boron units. Oxygen-17 NMR showed the formation of mixing sites containing both Na and Mg near nonbridging oxygen sites. Furthermore, a high magnesium content appears to result in the formation of two subnetworks (boron and silicon rich) with different polymerization degrees as well as to promote the formation of high-coordination aluminum sites (Al[V] and Al[VI]). Finally, magnesium coordination ranging from 4 to 6, with a mean value shifting from 5 to 6 along the series, suggests that magnesium might endorse an intermediate role in these glasses.     

Control of the crystal structure of microbial cellulose during nascent stage

The structure of the product, from an Acetobacter culture in the presence of Fluorescent Brightener, Direct Red 28, and Direct Blue 1, 14, 15 and 53, characterized by an X‐ray diffractormeter is a crystalline complex. On the other hand, solid‐state ¹³C‐NMR spectroscopy reveals that the product is noncrystalline. However, the X‐ray result of the product sample suggests that the dye molecule is included in the form of a monolayer between the cellulose sheets in the complex corresponding to the (11¯0) plane of microbial cellulose. But the celluloses regenerated from the Fluorescent Brightener product, the Direct Red 28 product, and the rest of the dye products are celluloses I, IV, and II, respectively. More specifically, the ¹³C‐NMR spectra revealed that the crystal types of cellulose from the Fluorescent Brightener and Direct Red 28 products are I_β and IV_I, respectively. Thus, the crystal structure of the product and the regenerated cellulose depends mainly on the position and number of the sulfonate groups in a direct dye and the interactions of the dye with the noncrystalline microbial cellulose in the nascent stage. The conformation and arrangement of the nascent cellulose chain changes when a direct dye adheres to it. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1726–1734, 2001     

A pulsed field gradient and NMR imaging investigations of the water retention mechanism by cellulose ethers in mortars

The study presented in this paper is devoted to improve the knowledge on the influence of cellulose ethers (CE) on the freshly-mixed mortars water retention. Indeed, this crucial property is the most important imparted by these polysaccharides. One of the assumptions proposed to explain this phenomenon is that CE acts as diffusion barrier to the water. To test this hypothesis, the CE effect on the self-diffusion coefficient of water in solution and on the water mobility between two fresh cement pastes was studied by Nuclear Magnetic Resonance. CE does not significantly modify the water self-diffusion coefficient in CE solution or in admixed cement pastes. Moreover the interdiffusion imaging experiments demonstrated that the water diffusion at the paste/paste interface is not affected by the presence of cellulosic admixture.     

Separation of Co(II) and Mn(II) from sulphate media via a HFSLM: Reaction flux model and experimental verification

Separation of Co(II) and Mn(II) from sulphate media through triple HFSLM in series using D2EHPA was explored. Mn(II) was extracted preferentially over Co(II) at the pH value of 5, 100 ppm of Co(II) and Mn(II), 5% (v/v) of D2EHPA, 0.2 M HCl. The highest extraction percentage of Mn(II) was 98.14%, and Co(II) remained at 94.05% in the raffinate stream. The reaction order (n) and the reaction rate constant (k_f) were found to be 1.00 and 0.0180 min⁻¹, respectively. Furthermore, the reaction flux model proved to be in good agreement with the experimental data at an average deviation of 2.24%.     

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.