固体核磁共振弛豫在高分子研究中的应用

本文综述了近年来固体核磁弛豫方法在高分子研究中的应用，共分５个部分加以介绍：（１）自旋－晶格弛豫过程；（２）在旋转坐标系中的＾１３Ｃ自旋－晶格弛豫过程；（３）交叉极化速率和旋转坐标系中的＾１Ｈ自旋－晶格弛豫过程；（４）自旋－自旋弛豫过程；（５）动态结构导致的线形变化。本文主要讨论磁性核的各种弛豫过程以及它们与分子结构和分子运动的关系。     

EPR and solid-state NMR studies of poly(dicarbon monofluoride) (C2F)n

Poly(dicarbon monofluoride) (C2F)n was studied by electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (NMR). The effects of physisorbed oxygen on the EPR and NMR relaxation were underlined and extrapolated to poly(carbon monofluoride) (CF)n and semi-covalent graphite fluoride prepared at room temperature. Physisorbed oxygen molecules are shown to be an important mechanism of both electronic and nuclear relaxations, resulting in apparent spin-lattice relaxation time and line width during NMR and EPR measurements, respectively. The effect of paramagnetic centers on the 19F spin-lattice relaxation was underlined in accordance with the high electron spin density determined by EPR. 19F magic angle spinning (MAS) NMR, 13C MAS NMR, and 13C MAS NMR with 19F to 13C cross polarization (CP) underline the presence of two types of carbon atoms, both sp3 hybridized: some covalently bonded to fluorine and the others linked exclusively to carbon atoms. Finally, a C-F bond length of 0.138 +/- 0.002 nm has been determined thanks to the re-introduction of dipolar coupling using cross polarization.     

CP/MAS Carbon-13 NMR Study of Spin Relaxation Phenomena of Cellulose Containing Crystalline and Noncrystalline Components

Abstract

Cross-polarization, ¹³C rotating frame spin-lattice relaxation and C laboratory frame spin-lattice relaxation processes have been studied for different cellulose samples by CP/MAS ¹³C NMR spectroscopy. It was found that the CP process can be described by a simple thermodynamic model and relative intensities of the respective resonance lines are consistent with the atomic ratios for the spectra obtained at a contact time of about 1 ms. The observed rotating frame spin-lattice relaxation times T^C ₁₀ were dominantly dependent on the time constant T^D _CH by which ¹³C nuclei were coupled to the ¹H dipolar spin system. It was, therefore, impossible to obtain information about molecular     

NMR study of styrene-butadiene rubber (SBR) and TiO2 nanocomposites

The chemical behavior of styrene-butadiene rubber (SBR) and of the SBR/TiO₂ and photodegraded SBR/TiO₂ nanocomposites was investigated through nuclear magnetic resonance spectroscopy (NMR) in the solid state with magic angle spinning (MAS). The ¹³C cross polarization/magic angle spinning (CP/MAS) routine spectrum allowed us to obtain information on the polymer microstructure and also to evaluate the domain mobilities. The variation contact time and the proton spin-lattice relaxation time in the rotating frame (T₁ρH) were determinant factors to evaluate the dynamic molecular motion. The NMR spectrum of the nanocomposites was dislocated 5 ppm to higher chemical shift, indicating the presence of a strong interaction between the polymer chains and the TiO₂ nanoparticles. The VTC experiment showed a rigid domain in the SBR/TiO₂ photodegraded nanocomposite due to cross-linking reactions.     

Entanglements and elasticity in polybutadiene networks

Polybutadiene networks were prepared by peroxide crosslinking of monodisperse 1,4-polybutadienes both in solution and in bulk. The effect of the entangled sol fraction on the elastic modulus of high-molecular-weight polybutadiene was observed in stress relaxation measurements. Sol fraction was shown to make a large contribution to the Mooney–Rivlin 2C₂ term. This effect was also observed on the molecular level in NMR spin-spin relaxation measurements. For networks crosslinked in bulk the stress relaxation measurements suggest the presence of trapped entanglements. The 2C₂ term is insensitive to sol extraction in these networks. NMR spin-lattice relaxation measurements in the rotating frame at 4.68 kHz verify the presence of additional effective crosslinks in these networks.     

Physical properties of the nonlinear optical material Li2B4O7 studied by static NMR and MAS NMR

The mechanisms behind the nonlinear optical (NLO) properties of Li₂B₄O₇ are characterized by ⁷Li static nuclear magnetic resonance (NMR) and magic-angle spinning (MAS) NMR. Furthermore, the structural nature of 3-coordinate BO₃ and 4-coordinate BO₄ groups is also characterized by the same method. For ⁷Li and ¹¹B, the spin-lattice relaxation time T₁ in laboratory frame gradually decreases with increasing temperature, whereas the spin-lattice relaxation time T_1ρ in rotating frame, which differs from T₁, is nearly constant. In addition, the activation energies of ⁷Li and ¹¹B, which are obtained via the values of T₁ and T_1ρ, are also compared.     

NMR measurement of the surface dynamics of poly(dimethylsiloxane) adsorbed on silica gel

CPMAS-DD ¹³C NMR spectroscopy was used to examine the mobility of poly(dimethylsiloxane) adsorbed on silica gel (PDMS/SiO₂) at submonolayer coverages. The spin-lattice relaxation time in the rotating frame (T_1ρH) decreased linearly with increasing loading. This is consistent with a decrease in the mobility of the polymer segments as the loading is increased. The decrease in mobility results from interpolymer interference. We propose a model that explains these results in terms of a surface intrinsic viscosity that incorporates the polymer-polymer interactions on the surface.     

NMR molecular dynamic study of high crystalline polymers

Analysis of the dynamic behaviour of two highly crystalline polymers, polyethylene (HDPE) and polypropylene (iPP) was carried out by solution and solid state nuclear magnetic resonance (NMR) to obtain the response of the behaviour of both polymers with respect to the molecular chain dynamics, the chain ordination and the molecular packing. The proton spin-lattice relaxation time in the rotating frame (T₁^Hρ) showed that HDPE is more rigid than iPP, because the chain orientation and the molecular packing are different. T₁^Hρ parameter also depends on the molecular chain dynamics, as a result of the different sequence distribution in the domains     

Solid-state (67)zn NMR spectroscopy in bioinorganic chemistry. Spectra of four- and six-coordinate zinc pyrazolylborate complexes obtained by management of proton relaxation rates with a paramagnetic dopant

Solid-state (67)Zn NMR spectra of model compounds for metalloproteins, such as [H(2)B(3,5-Me(2)pz)(2)](2)Zn (pz denotes pyrazolyl ring), have been obtained using low temperatures (10 K) to enhance the Boltzmann factor in combination with cross polarization (CP) from (1)H to (67)Zn. Attempts to observe spectra of other model compounds, such as [H(2)B(pz)(2)](2)Zn, were hindered by long relaxation times of the protons. To decrease the proton relaxation times, the high-spin six-coordinate complex [HB(3,4,5-Me(3)pz)(3)](2)Fe has been investigated as a dopant. NMR and EPR measurements have shown that this Fe(II) dopant effectively reduces the (1)H spin lattice relaxation time, T(1), of the zinc samples in the temperature range 5-10 K with minimal perturbations of the (1)H spin lattice relaxation time in the rotating frame, T(1)(rho). Using this methodology, we have determined the (67)Zn NMR parameters of four- and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful models for systems of biological importance. The (67)Zn NMR parameters are contrasted to the corresponding changes in the (113)Cd NMR parameters for the analogous compounds. Further, these investigations have demonstrated that a temperature-dependent phase transition occurs in the neighborhood of 185 K for [HB(3,5-Me(2)pz)(3)](2)Zn; the other poly(pyrazolyl)borate complexes we investigated did not show this temperature-dependent behavior. This conclusion is confirmed by a combination of room-temperature high-field (18.8 T) solid-state (67)Zn NMR spectroscopy and low-temperature X-ray methods. The utilization of paramagnetic dopants should enable low-temperature cross polarization experiments to be performed on a wide variety of nuclides that are important in bioinorganic chemistry, for example, (25)Mg, (43)Ca, and (67)Zn.     

类蜘蛛丝聚合物结构及分子运动的固体核磁共振研究

运用固体核磁共振(NMR)技术研究了聚丙氨酸多肽片段(Ala)5与高分子齐聚物聚苯乙烯(PS, 分子量2000)及聚异戊二烯(PI, 分子量2210)共聚而成的类蜘蛛丝蛋白聚合物——聚苯乙烯-co-聚丙氨酸聚合物(PS-co-PAL)和聚异戊二烯-co-聚丙氨酸聚合物(PI-co-PAL)的结构及分子运动. 聚合物13C CP/MAS NMR(交叉极化/魔角旋转核磁共振)谱及其旋转坐标系中自旋-晶格弛豫时间(T1ρ(13C))的结果表明, 此两种聚合物中多肽片段(Ala)5具有相同的化学位移, 即相似的化学环境和二级结构, 并具有相近的T1ρ(13C), 即类似的聚集态结构. 聚合物的宏观力学性质明显不同: 常温下, PS-co-PAL呈硬颗粒状, PI-co-PAL呈橡胶状且易拉伸. 结果说明聚合物力学性质与高分子链段的性质密切相关. PI-co-PAL聚合物的PI链段, 其骨架—CH2CH—的T1ρ为(5.3±0.4) ms, 而PS-co-PAL聚合物的PS链段, 其骨架—CH2CH—的T1ρ为(47.0±5.5) ms, 说明二聚合物中PI链段较PS链段更为柔软. 另外, 基于密度泛函理论(DFT)的化学位移计算证明, 聚合物PS-co-PAL和PI-co-PAL中多肽片段(Ala)5的二面角均为(-131°, 142°), 说明它们以β-折叠构象存在.     

Structure and dynamics of perfluoroalkane/beta-cyclodextrin inclusion compounds as studied by solid-state 19F MAS and 1H -->19F CP/MAS NMR spectroscopy

The molecular structure and dynamics of novel inclusion compounds (ICs) consisting of n-perfluoroalkane (PFA) guests and beta-cyclodextrin (beta-CD) host (PFA/beta-CD) have been investigated using 19F magic angle spinning (MAS) and 1H-->19F cross polarization (CP)/MAS NMR spectroscopy with the aid of thermal analyses, FT-IR spectroscopy, and X-ray diffraction method. The ICs of C9F20/beta-CD and C20F42/beta-CD were successfully obtained as precipitates from mixtures of respective PFAs and saturated aqueous solution of beta-CD. The wide-angle X-ray diffraction (WAXD) revealed that C9F20/beta-CD forms a channel-type crystallite, while C20F42/beta-CD is nearly amorphous at room temperature. The structural orders in both ICs increase at elevated temperatures. The 19F NMR signals obtained by the direct polarization (DP) method for PFA/beta-CD are resonated at higher frequencies than those for original PFA. This can be ascribed to the lower dielectric environment of the beta-CD cavity. Above 80 degrees C, 1H-->19F CP/MAS NMR technique revealed that C9F20 molecules undergo vigorous molecular motion and partly come out of the beta-CD channel. However, the guests hardly degrade or evaporate unless the host is pyrolytically decomposed above ca. 300 degrees C. The spin-lattice relaxation times in the laboratory frame for 19F (T1F) are almost identical for all the fluorines in PFA/beta-CD at each temperature, while significantly different values were observed for fluorines in neat PFA. This indicates that effective intramolecular spin diffusion occurs within a PFA molecule included in beta-CD.     

Structure and dynamics of MG rubber studied by dynamic mechanical analysis and solid‐state NMR

Methyl‐methacrylate‐grafted natural rubber was prepared by free radical polymerization of methyl methacrylate in natural rubber latex, and their structure and dynamics were investigated by dynamic mechanical analysis and solid‐state nuclear magnetic resonance (NMR). Samples were prepared by chemical initiation and high‐energy radiation. The changes of glass transition temperature and tan δ max with different total poly(methyl methacrylate (PMMA) content are reported. The effect of the change in composition in copolymers on tan δ peak width, tan δ max, and area under the tan δ curve are used to understand the miscibility and damping properties. Solid‐state ¹³C‐NMR measurements were carried out to determine several relaxation time parameters, such as rotating frame and laboratory frame proton and carbon relaxation times. Cross polarization times and carbon relaxation times were interpreted based on the changes in the molecular motion. Proton relaxation times were interpreted based on the heterogeneity of the matrix. Results confirmed phase separation and a presence of an interfacial region. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1141–1153, 1999     

New application of proton nuclear spin relaxation unraveling the intermolecular structural features of low-molecular-weight organogel fibers

Proton nuclear spin relaxation has been for the first time extensively used for a structural and dynamical study of low-molecular-weight organogels. The gelator in the present study is a modified phenylalanine amino acid bearing a naphthalimide moiety. From T(1) (spin-lattice relaxation time in the laboratory frame) and T(1ρ) (spin-lattice relaxation time in the rotating frame) measurements, it is shown that the visible gelator NMR spectrum below the liquid-gel transition temperature corresponds to a so-called isotropic compartment, where gelator molecules behave as in a liquid phase but exchange rapidly with the molecules constituting the gel structure. This feature allows one to derive, from accessible parameters, information about the gel itself. Nuclear Overhauser effect spectroscopy (NOESY) experiments have been exploited in view of determining not only cross-relaxation rates but also specific longitudinal rates. The whole set of relaxation parameters (at 25 °C) leads to a correlation time of 5 ns for gelator molecules within the gel structure and 150 ps for gelator molecules in the isotropic phase. This confirms, on one hand, the flexibility of the organogel fibers and, on the other hand, the likely presence of clusters in the isotropic phase. Concerning cross-relaxation rates, a thorough theoretical investigation in multispin systems of direct and relayed correlations in a NOESY spectrum allows one to make conclusions about contacts (around 2-3 ?) not only between naphtalimide moieties of different gelator molecules but also between the phenyl ring and the naphtalimide moiety again of different gelator molecules. As a result, not only is the head-to-tail structure of amino acid columns confirmed but also the entangling of nearby columns by the naphthalimide moieties is demonstrated.     

Rotating frame spin-lattice relaxation through anisotropic chemical shift in polycrystalline UF6

A minimum in the temperature dependence of the ¹⁹F rotating frame spin-lattice relaxation time T_1? in polycrystalline UF₆ has been observed which enables one to determine the correlation time for slow reorientations of the UF₆ octahedra directly. The dependence of the depth of the T_1? versus temperature minimum on the external magnetic field H₀ at a fixed value of the rotating field H₁ provides an independent proof that fluctuations of the ¹⁹F screening constant tensor significantly contribute to the fluorine spin-lattice relaxation rate in this system.     

Temperature dependence of the proton exchange time in pure water by NMR

The temperature dependence of the proton spin-spin relaxation rate 1/T₃ on 180° pulse spacing (Meiboom dispersion) was measured for pure water enriched at 4% ¹⁷O to obtain the proton exchange time. At 58°C, the dispersion of the proton spin-lattice relaxation in the rotating frame (T_1ρ) was shown to be explained by a comparable proton exchange time.     

High field carbon 13 relaxation of 5CB. Chemical shift anisotropy as a probe of nematic dynamics

Abstract

The carbon 13 spin-lattice relaxation times of the ring carbons at 22·6 MHz in the nematic phase and at 20·0 MHz in the isotropic phase of 5CB have been published previously. An analysis of the results in the nematic phase based on diffusion constants obtained from deuteron relaxation was presented. In this paper measurements of the carbon 13 spin-lattice relaxation times of the ring carbons of 5CB at 125·6 MHz are given along with an analysis which makes use of the wide variation in the field to interpret the relatively simple relaxation of the protonated aromatic carbons. An effort to interpret the more complicated relaxation behaviour of the unprotonated carbons using the simplification in relaxation behaviour at high fields where chemical shift anisotropy dominates the relaxation is less successful. However, the conclusion that order director fluctuations account for almost 50 per cent of the relaxation rate in the nematic phase at high fields suggests that study of high field rotating frame relaxation for these carbons might be a rich source of information on critical fluctuations.     

NMR investigation of the dynamics of banana shaped molecules in the isotropic phase: a comparison with calamitic mesogens behaviour

The first translational self-diffusion NMR measurements in the isotropic phase of banana-shaped liquid crystals are reported. In this paper, two banana-shaped mesogens, having a similar molecular structure and showing a nematic phase, have been investigated by means of translational self-diffusion NMR, (2)H NMR spin-spin and (1)H NMR spin-lattice relaxation measurements in the isotropic phase. While (1)H diffusion and (2)H relaxation times reveal a peculiar slow dynamic behaviour of banana-shaped mesogens compared with calamitic mesogens, the (1)H relaxation times seem to be affected by fast dynamics only. The origin of these dynamic features is discussed in terms of overall and internal molecular motions, in the frame of recent speculations concerning the formation of molecular clusters or aggregates in the isotropic phase of banana-shaped liquid crystals.     

Solid‐State NMR Study on Relationships between Miscibility and Chain Mobility in Poly(4‐Methylstyrene)/Poly(Cyclohexyl Methacrylate) Blend

The phase behavior and motional mobility in binary blends of poly(4‐methylstyrene) (P4MS) and poly(cyclohexyl methacrylate) (PCHMA) have been examined by ¹³C solid state NMR techniques. The blend miscibility was studied by measuring the ¹H spin‐relaxation times in the laboratory frame (T₁^H) and in the rotating frame (T_1ρ^H), respectively. Although intermolecular spin diffusion contributes to the proton relaxations in accordance with homogeneity, T_1ρ^H data shows signs of in complete averaging. The T_1ρ^H relaxation behavior indicates the existence of heterogeneous do mains with shortest dimensions in the nanometer range, which is also sup ported by the intermolecular cross polarization experiments with variable contact times. In addition, according to the resuits of carbon T_1ρ relaxation time measurements, it is concluded that mixing is intimate some what enough to cause a reduction in local chain mobility for P4MS and vice versa for PCHMA.     

Identification of halogen substituents in natural products by measurement of 13c spin-lattice relaxation times and integrated intensities

A method is proposed for differentiating brominated carbons from chlorinated carbons by means of natural-abundance ¹³C NMR spectroscopy. The basis of the method is that the spin-lattice relaxation behaviour of brominated carbons is influenced by carbon-bromine scalar interactions, which can lead to shortened ¹³C spin-lattice relaxation times and reduced values of the nuclear Overhauser enhancement. C-Cl scalar interactions make a negligible contribution to the spin-lattice relaxation of chlorinated carbons. These effects are illustrated by measurement of the ¹³C spin-lattice relaxation times and integrated intensities of chloro-, bromo and iodobenzene and chloro-, bromo- and iodocyclohexane. The method is then tested on four polyhalogenated marine natural products. The results indicate that ¹³C relaxation measurements can be used to distinguish brominated carbons from chlorinated carbons in the case of halogenated quaternary carbons, sp² hydridized methine carbons and some sp³ hydridized methine carbons, but not in the case of halogenated methylene carbons or gem-dihalo substituted methine carbons.     

Solid-state NMR study of biodegradable starch/polycaprolactone blends

Abundant literature exists on starch or modified starch blended with biodegradable polyesters to achieve good performance with cheap compost plastics. The level of miscibility in these blends is one of the most relevant parameters. In the present study, solid-state ¹H and ¹³C NMR spectra, as well as carbon spin-lattice relaxation times T₁(C) and proton spin-lattice relaxation times T₁(H) and proton spin-lattice relaxation times in the rotating frame T_1ρ(H) of biodegradable starch (or starch formate)/polycaprolactone (PCL) (or polyester (PE) oligomers) blends and samples of the neat components were measured. From the T_1ρ(H) and T₁(H) relaxation times it follows that blends starch/PCL, starch/PE-oligomers and starch formate/PE-oligomers are phase separated even on the scale of 20-110 nm. On the contrary starch formate/PCL blend is phase separated on the scale 2.5-12 nm but homogeneously mixed on the scale 20-90 nm. Moreover, shorter T₁(C) and especially T_1ρ(H) values found for the starch or starch formate component in all these blends in comparison with neat samples show that molecular mobility of starch and starch formate segments is affected by blending. This indicates some miscibility also in phase separated blends which can happen in amorphous channels of starch.