Crystalline order in polyethylene: A 13C NMR CP/MAS solid-state T1 study

The carbon-13 spin-lattice relaxation times T₁ of the crystalline components of four solid ethylene-octene copolymers have been studied as a function of thermal history, branching number, and branching distribution. Slowly cooled samples (1 deg/min from melt to room temperature) exhibited similar or longer T₁s with respect to the same sample quench cooled (from the melt into 20°C water). The greater the degree of branching and the more homogeneous the branching distribution, the shorter were the observed crystal lattice T₁s. Differences of up to a factor of 3 in T₁ were observed for the same sample undergoing the two thermal treatments. Different degrees of branching homogeneity (for the same total number of branches) resulted in differences approaching a factor of 7 for samples with the same thermal history. These variations were attributed to the differing effects of side-chain disruption of the crystal lattice.     

13C CP/MAS NMR studies of vitamin E model compounds

13C cross-polarization magic angle spinning (CP/MAS) NMR data for 2,2,5,7,8-pentamethylchroman-6-ol (2), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox c) (3) and its acetate (4), 2-methoxy-2,2,5,7,8-pentamethylchroman-6-ol (5), 2-hydroxy-2,2,5,7,8-pentamethylchroman-6-ol (6) and 2,2,5,7,8-pentamethylchroman (7) are reported. A deshielding of 7.7 ppm for the carboxylic carbon was observed in solid Trolox due to formation of intermolecular hydrogen bonds within cyclic dimers. Such crystal packing permits effective cross-polarization and fast relaxation (short T1rho(H)). The impact of the proton concentration on the CP dynamics is reflected by the longer T(CP) and T1rhoH for Trolox-d2 (deuterated at mobile proton sites). The calculated GIAO RHF shielding constants are sensitive to intramolecular effects: rotation around the C-6-O bond (changes of sigma up to 8 ppm) and conformation at C-2.     

Variable temperature CP/MAS13C NMR study of cyclodextrin complexes of benzaldehyde

Fully hydrated as well as dried benzaldehyde complexes of - and -cyclodextrins were studied by using CP/MAS¹³C NMR techniques. Variable temperature studies have shown that below 200 K the guest is rigidly held in the complex, whereas at 328 K, only the aromatic ring performs rapid two-fold flips about the C₁–C₄ axis. In the -Cd complex the benzaldehyde performs more general reorientation. Removal of water causes marked changes in both guest and host spectra, generally consistent with a loss of short-range order and increase in guest motional rate.NRCC No. 27826.     

CP/MAS (13)C NMR study of cellulose and cellulose derivatives. 1. Complete assignment of the CP/MAS (13)C NMR spectrum of the native cellulose

The precise assignments of cross polarization/magic angle spinning (CP/MAS) (13)C NMR spectra of cellulose I(alpha) and I(beta) were performed by using (13)C labeled cellulose biosynthesized by Acetobacter xylinum (A. xylinum) ATCC10245 strain from culture medium containing D-[1,3-(13)C]glycerol or D-[2-(13)C]glucose as a carbon source. On the CP/MAS (13)C NMR spectrum of cellulose from D-[1,3-(13)C]glycerol, the introduced (13)C labeling were observed at C1, C3, C4, and C6 of the biosynthesized cellulose. In the case of cellulose biosynthesized from D-[2-(13)C]glucose, the transitions of (13)C labeling to C1, C3, and C5 from C2 were observed. With the quantitative analysis of the (13)C transition ratio and comparing the CP/MAS (13)C NMR spectrum of the Cladophora cellulose with those of the (13)C labeled celluloses, the assignments of the cluster of resonances which belong to C2, C3, and C5 of cellulose, which have not been assigned before, were performed. As a result, all carbons of cellulose I(alpha) and I(beta) except for C1 and C6 of cellulose I(alpha) and C2 of cellulose I(beta) were shown in equal intensity of doublet in the CP/MAS spectrum of the native cellulose, which suggests that two inequivalent glucopyranose residues were contained in the unit cells of both cellulose I(alpha) and I(beta) allomorphs.     

CP/MAS ~(13)C NMR技术对木浆纤维微观结构的研究

利用交叉极化结合魔角旋转技术~(13)C核磁共振法(CP/MAS ~(13)C NMR)对桉木浆纤维的微观结构进行研究,为进一步研究木质纤维素材料开发过程中反应障碍特征奠定基础.通过对NMR光谱C1区(δ 102～108)进行洛仑兹拟合,得到桉木浆纤维中纤维素Iα的相对含量为26.92%,纤维素Iβ的相对含量为52.04%,主要以纤维素Iβ晶体形式为主.通过计算纤维素C4结晶区(δ 86～92)和非结晶区(δ 80～86)的相对含量得到桉木浆的纤维素结晶度为47%.通过洛仑兹和高斯函数的混合模型对NMR光谱C4区(δ 80～92)进行拟合得到基原纤尺寸和微原纤横向尺寸分别为4.0与17.9 nm,并通过计算不同形态的结晶纤维素的相对含量得到纤维素结晶度为51%,证实了在微原纤内部次晶纤维素的存在.     

Insights into the Lattice Structure of Cellulose II From the High Resolution CP/MAS Solid State 13C NMR Spectrum of Cellotetraose

Abstract

The chemical shifts and multiplicities of the high-resolution ¹³C CP/MAS NMR spectrum of cellulose II are quite diagnostic of the lattice structure of this polymorph. Particularly important is the chemical shift of C-1 and its clear splitting into two lines of equal intensity. Similar chemical shifts and multiplicities are seen in the spectrum of cellotetraose. Thus cellotetraose is considered to be a good model for the lattice structure of the polymer. A detailed investigation of the multiplicity of the C-1 resonance of cellotetraose shows that the two peaks are of equal intensity in this case also. Because of the limited number of repeat units in the tetramer, this observation implies that the unit cell contains two independent chains rather than a “double” repeat unit. This gives support for a similar lattice structure, with two independent chains, for cellulose II itself.     

Line shapes in CP/MAS (13)C NMR spectra of cellulose I

The CP/MAS (13)C NMR line shape of cellulose I has been qualitatively analyzed by direct simulations using the Ornstein-Uhlenbeck stochastic process and the Kubo model. Both approaches describe a anhydroglucose C4 carbon as a oscillator with fluctuating Larmor frequency. The NMR resonance frequency is written omega=omega +omega(t), where the fluctuating part with zero mean was modelled as a stationary Markov diffusion process. The simulation results both motivates the use of multiple line shapes when fitting CP/MAS (13)C NMR spectra recorded on cellulose I and gives some insights into why signals from crystalline cellulose I give rise to Lorentzian line shapes.     

Characterization of amorphous carbon films by 13C CP MAS NMR spectroscopy

¹³C CP MAS NMR spectroscopy was used to characterize α-C:H materials generated from methane and hydrogen mixtures using a microwave plasma. Dipolar dephasing experiments indicate a range of T_dd making quantification of quaternary “diamond”-like carbons difficult. Unconstrained lineshape analysis is not suitable for the deconvolution of the NMR spectra, but linewidth constrained analyses gave reasonable results. © 1993 John Wiley & Sons, Inc.     

13C CP MAS NMR and DFT Studies of 6-Chromanyl Ethereal Derivatives

Vitamin E consists of a group of compounds including α- β- γ- and δ-tocopherols and α- β- γ- and δ-tocotrienols, containing the chroman-6-ol system. The recognition of the structural and dynamic properties of this system, present in all vitamers, seems to be important for the full explanation of the mechanism of the biological activity of vitamin E. This paper presents results of the structural analysis of the chosen 6-chromanyl ethereal derivatives using experimental (¹³ C NMR-in solution and solid state, as well as variable temperature experiments; single crystal X-ray diffraction) and theoretical (DFT) methods. For one of the studied compounds, 2,2,5,7,8-pentamethyl-6-((tetrahydro-2H-pyran-2-yl)oxy) chroman, the splitting of some signals was observed in the ¹³C dynamic NMR spectra. This observation was explained by the application of a conformational analysis and subsequent DFT optimization, followed by the calculation of NMR properties.     

An X-ray and 13C CP/MAS NMR study of C,C-linked bipyrazoles and bispyrazolylmethanes

4,4′-Methylene-bispyrazole (1) crystallizes in the P2₁/n space group with Z = 4, a = 5.4199(8) Å, b = 16.194(6) Å, c = 8.381(5) Å, β = 98.18(5)°. 4,4′-Methylene-bis (3,5-dimethylpyrazole) (2) crystallizes in the Pbca space group with Z = 8, a = 8.350(3) Å, b = 16.078(3) Å and c = 17.154(5) Å. Finally, 3,3′-bipyrazole (4) crystallizes in the P2₁/n space group with Z = 2, a = 5.465(2) Å, b = 5.491(3) Å, c = 10.058(4) Å and sol; = 92.88(2)°. The packing adopted by molecules 1 and 2 is related to the disposition of the pyrazole rings. In compound 4, all the molecules are joined to their neighbors by two double hydrogen bond systems forming zigzag chains. Using solid state ¹³C NMR spectroscopy, no dynamic proton transfer was observed in these crystals, and not on those of 3,5,3′,5′-tetramethyl-4,4′-bipyrazole (3) either.     

Dynamic study of the noncrystalline phase of 13C-labeled polyethylene by variable-temperature 13C CP/MAS NMR spectroscopy

The ¹³C spin-lattice relaxation times T₁ of ¹³C-labeled polyethylene crystallized under different conditions were measured at temperatures from ?120 to 44°C by variable-temperature solid-state high-resolution ¹³C nuclear magnetic resonance (NMR) spectroscopy, in order to determine accurately the dynamics of the noncrystalline region of the polymer. From these results, it was found that the T₁ minimum for the CH₂ carbons in the noncrystalline region of solution-crystallized polyethylene with high crystallinity appears at higher temperature by about 20°C than that of melt-quenched polyethylene with low crystallinity. This means that the molecular motion of the CH₂ carbons in the noncrystalline regions is more constrained at a given temperature in the material of higher crystallinity. Furthermore, dynamics of the noncrystalline region is discussed in terms of the ¹³C dipolar dephasing times.     

Carbon-13 NMR and CNDO/2 studies of phenoxachalcogenins

The ¹³C chemical shifts of the carbon atoms in dibenzodioxin, phenoxathiin, phenoxaselenin and phenoxatellurin were determined in CDCl₃ solutions and assigned. The total (σ and π) charge densitites on the carbon atoms calculated by the CNDO/2 method without consideration of d-orbitals correlated well with the experimentally determined shifts. Rather good agreement was also found between experimental shifts and shifts calculated from ¹³C data for phenyl methyl chalcogenides on the assumption that a phenoxachalcogenin molecule can be assembled from C₆H₅O and C₆H₅X groups. Only the shifts of the carbon atoms bonded to the heavier chalcogen atoms show an upfield trend in the sequence O, S, Se, Te. All other shifts exhibit a downfield trend. These trends are rationalized in terms of the electronegativities, abilities to participate in π-interactions, and anisotropy effects of the chalcogen atoms.     

Blending to Make Nonhealable Polymers Healable: Nanophase Separation Observed by CP/MAS 13C NMR Analysis

Can commodity polymers are made to be healable just by blending with self-healable polymers? Here we report the first study on the fundamental aspect of this practically challenging issue. Poly(ether thiourea) (PTUEG₃; T_g=27 °C) reported in 2018 is extraordinary in that it is mechanically robust but can self-heal even at 12 °C. In contrast, poly(octamethylene thiourea) (PTUC₈; T_g=50 °C), an analogue of PTUEG₃, cannot heal below 92 °C. We found that their polymer blend self-healed in a temperature range above 32 °C even when its PTUEG₃ content was only 20 mol %. Unlike PTUEG₃ alone, this polymer blend, upon exposure to high humidity, barely plasticized, keeping its excellent mechanical properties due to the non-hygroscopic nature of the PTUC₈ component. CP/MAS ¹³C NMR analysis revealed that the polymer blend was nanophase-separated, which possibly accounts for why such a small amount of PTUEG₃ provided the polymer blend with humidity-tolerant self-healable properties.     

Binuclear Cadmium Dithiophosphate Crystals: 13C, 31P,and 113Cd CP/MAS NMR Studies

Binuclear diisopropyl and dicyclohexyl dithiophosphate cadmium complexes, namely, [Cd₂{S₂P(OR)₂}₄], were studied by high-resolution heteronuclear (¹³C, ³¹P, and ¹¹³Cd) NMR spectroscopy in the solid state in a temperature range from 295 to 378 K. ³¹P NMR signals for the terminal and bridging ligands of the complexes were differentiated. The experimental NMR spectra show ³¹–^111,113Cd and ¹¹³Cd–³¹ spin–spin couplings only for the terminal ligands. The chemical shift anisotropy _aniso and the asymmetry parameter were calculated for ³¹P and ¹¹³Cd NMR signals. It was found that the ³¹P chemical shifts for the terminal and bridging dithiophosphato groups differ in anisotropy character.     

CP/MAS NMR investigations of silica gel surfaces modified with aminopropylsilane

Summary Aminopropyl chemically bonded phases for high performance liquid chromatography (HPLC) have been prepared using mono- and trifunctional methoxyor ethoxysilanes. Three types of silica gel with different surface characteristics were used as support for the chemically bonded phases (CBPs). Surface characteristics of the packings before and after chemical modification were determined by porosity parameters, elemental analysis and CP/MAS NMR spectroscopy.²⁹Si and¹³C CP/MAS NMR investigations gave informations about different interactions between aminosilyl ligands and/or these ligands and/or water molecules condensed in the pores of the silica gel surface. With decreasing pore diameter of the silica gel the proportion of protonated aminopropyl ligand increases.     

Structure of ultrahigh-molecular-weight polyethylene in the solid state as studied by variable-temperature 13C CP/MAS NMR spectroscopy

In order to obtain some insight into the structure of an ultrahigh-molecular-weight polyethylene sample, ¹³C CP/MAS NMR experiments have been carried out at temperatures from 23 to ?108°C. The peak for the crystalline component moves upfield with a decrease in temperature, which is contrary to what was reported previously for a melt-crystallized polyethylene sample. On the basis of x-ray diffraction results and quantum-chemical calculations, it is suggested that the methylene carbons are in the distorted orthorhombic form at low temperatures.