Phase diagram and glass transition of confined benzene

We used differential scanning calorimetry, neutron scattering, and proton NMR to investigate the phase behavior, the structure, and the dynamics of benzene confined in a series of cylindrical mesoporous materials MCM-41 and SBA-15 with pore diameters, d, between 2.4 and 14 nm. With this multitechnique approach, it was possible to determine the structure and, for the first time to our knowledge, the density of confined benzene as a function of temperature and pore size. Under standard cooling rates, benzene partially crystallizes in SBA-15 matrixes (4.7 相似文献   

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.     

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials

Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.     

Elevation of melting temperature for confined palmitic acid inside cylindrical nanopores

High resolution 13C nuclear magnetic resonance was employed to study the phase behavior of the amphiphilic long-chain palmitic acid (PA) confined inside the cylindrical nanopores in the matrix of titanate nanotubes. For a series of mixtures of titanate nanotubes and palmitic acid at various mass ratios, it was shown that annealing at the bulk melting temperature (approximately 335.5 K) of PA induced fast chemisorption of PA on the nanotube surface followed by slow physical trapping of PA into the cylindrical nanopore. It was found that the trapped PA remained solidlike substantially above the bulk melting temperature. Contrary to the bulk neat PA, for the trapped PA, the isotropic molecular-chain reorientation was shown to remain arrested even above the bulk melting temperature. When destabilized at approximately 349 K, the trapped PA deserted the nanopore and formed bulk PA, which could be retrapped into the nanopore upon annealing at the bulk melting temperature. The entire process was shown as reversible.     

Molecular dynamics study of translational and rotational diffusion in liquid ortho-terphenyl

NVT molecular dynamics simulations were performed on liquid o-terphenyl as a function of temperature in the range 320-480 K. Computed translational diffusion coefficients displayed the non-Arrhenius behavior expected of a fragile glass-forming liquid and were in good, semiquantitative agreement with experimental results. Rotational correlation functions calculated for various vectors within the molecule exhibited a very short time (0-1 ps) initial decay, followed by a reversal, which corresponds to free reorientation within the "solvent" cage prior to collision with a wall. Rotational correlation times of three orthogonal vectors fixed on the central benzene were close to equal at all temperatures, indicating nearly isotropic overall molecular reorientation. The average correlation times exhibited a non-Arrhenius temperature dependence and were in very good agreement with experimental values derived from 2D and 1H NMR relaxation times. Correlation times of vectors located on the lateral phenyl rings were used to calculate the "spinning" internal rotation diffusion coefficients, which were approximately twice as great as the overall rotational diffusion constants, indicating rapid internal rotation of the phenyl side groups over wide ranges of angle in the liquid.     

Molecular dynamics in paramagnetic materials as studied by magic-angle spinning 2H NMR spectra

A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M=Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 degrees flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 10(2) s(-1)相似文献   

Nuclear magnetic relaxation and diffusion study of the ionic liquids 1-ethyl- and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide confined in porous glass

The molecular dynamics of the room-temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (Bmim Tf2N) confined in porous glass is studied by nuclear magnetic resonance (NMR) relaxometry and diffusometry and is compared with the bulk dynamics over a wide temperature range. The molecular reorientation processes for anions and cations alike are found to be significantly affected by the presence of the glass interface at high temperatures. In this respect, the ionic liquid behaves similarly to polar liquids where proton NMR relaxation is governed by reorientations mediated by translational displacements (RMTDs). This process becomes less significant towards lower temperatures when the characteristic translational correlation times of the ions approach a timescale comparable with those of the RMTD process, and the relaxation dispersions in bulk and in confinement become similar below a temperature corresponding to about 1.2T_g, a value where the onset of dynamic heterogeneity has been observed before. The self-diffusion coefficient, on the other hand, is found to be strongly reduced than the bulk within the accessible temperature range of 248 K and above and is significantly slower than expected from the tortuosity effect, suggesting that ion–surface interactions affect the macroscopic properties.     

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。     

Nuclear spin relaxation in poly(diethylsiloxane)

Earlier work showed that heating causes poly(diethylsiloxane) to undergo a first-order transition from a semicrystalline solid to a more mobile viscous—crystalline material. The latter is composed of two phases and analogies between polymer and liquid crystal morphology and behavior have been made. The viscous—crystalline phase in PDES appears to be unique since the literature is devoid of other documented examples. In this study, spin—lattice and spin—spin relaxation times were measured over a wide temperature range. They show a glass transition at 138°K, a crystal—crystal transition at 206°K, and a transition around 250°K which results from translational motion of the polymer chains with respect to each other. This motion is observed in the amorphous phase at a lower temperature than in the crystalline phase. Translational motion in the crystalline phase is observed on melting of the crystallites. The spin—spin data permitted monitoring of the molecular motions in each phase and the data suggest that these phases exert some influence on the molecular motions of each other. The viscous—crystalline phase in PDES may represent a unique model for studying and understanding “precrystalline” behavior and structure in amorphous solids.     

Molecular motions and ion diffusions of the room-temperature ionic liquid 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide (DMPImTFSA) studied by 1H, 13C, and 19F NMR

The diffusive properties of an imidazolium room-temperature ionic liquid (RTIL), 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)amide (DMPImTFSA), are studied from the ionic conductivity and the ion diffusion coefficients measured by pulsed field gradient spin echo NMR. The temperature-dependent (1)H, (19)F, and (13)C NMR spin-lattice relaxation time T(1) values were observed, and the (1)H T(1) for DMPIm showed T(1) minima for various protons. According to the Bloemberger-Purcell-Pound (BPP) equation, the correlation time tau(c) values were directly calculated from (1)H NMR. By using the (1)H tau(c) values, an evaluation of the (13)C T(1) was attempted for the carbons having protons. The tau(c) estimated for molecular motions of DMPIm changes from 1.3 ns at 253 K to 72 ps at 353 K. The Stokes-Einstein-Debye (SED) model suggests that the tau(c) is too short for the overall molecular reorientation near room temperature. Consequently, the possibility of small-angle molecular rotation is proposed and tentative flip angles are calculated by using the translational diffusion coefficient, the bulk viscosity measured in this study, and the tau(c) obtained from (1)H T(1) data in the temperature range between 283 and 353 K. The flip amplitude increases with the temperature. DMPIm has isotropic reorientational motions with temperature-dependent amplitude, in addition to fast intramolecular motions such as methylene segmental motions, methyl rotational motion, and conformational exchange of the imidazolium ring. The existence of fast motions of TFSA is also shown. The translational diffusion of the ions is the slowest dynamic process in the present RTIL. Ab initio molecular orbital calculations are performed to understand the geometries of stable complexes of DMPIm(+) and TFSA(-), and the formation energies from the isolated ions are evaluated. The computed results are important for interpreting the (1)H T(1) behaviors observed for the imidazolium ring protons.     

Dynamics of benzene guest inside a self-assembled cylindrical capsule: a combined solid-state 2H NMR and molecular dynamics simulation study

The reorientational dynamics of benzene-d(6) molecules hosted into the cavity of a cavitand-based, self-assembled capsule was investigated by Molecular Dynamics (MD) simulations and temperature-dependent solid-state (2)H NMR spectroscopy. MD simulations were preliminarily performed to assess the motional models of the guest molecules inside the capsules. An in-plane fast reorientation of the benzene guest around the C(6) symmetry axis (B1 motion), characterized by correlation times of the order of picoseconds, was predicted with an activation barrier ( approximately 8 kJ/mol) very similar to that found for neat benzene in the liquid state. An out-of-plane reorientation corresponding to a nutation of the C(6) symmetry axis in a cone angle of 39 degrees (B2 motion, 373 K) with an activation barrier ( approximately 39 kJ/mol) definitely larger than that of liquid benzene was also anticipated. In the temperature range 293-373 K correlation times of the order of a nanosecond have been calculated and a transition from fast to slow regime in the (2)H NMR scale has been predicted between 293 and 173 K. (2)H NMR spectroscopic analysis, carried out in the temperature range 173-373 K on the solid capsules containing the perdeuterated guest (two benzene molecules/capsule), confirmed the occurrence of the B1 and B2 motions found in slow exchange in the (2)H NMR time scale. Line shape simulation of the (2)H NMR spectral lines permitted defining a cone angle value of 39 degrees at 373 K and 35 degrees at 173 K for the nutation axis. The T(1) values measured for the (2)H nuclei of the encapsulated aromatic guest gave correlation times and energetic barrier for the in-plane motion B1 in fine agreement with theoretical calculation. The experimental correlation time for B2 as well as the corresponding energetic barrier are in the same range found for B1. A molecular mechanism for the encapsulated guest accounting for the B1 and B2 motions was also provided.     

Proton spin relaxation in liquid polydimethylsiloxane: Molecular motion and network formation

Proton spin relaxation time measurements are reported for a series of liquid polydimethylsiloxanes covering the temperature range from ?50 to 200°C. These include fractionated and whole polymer samples (M _w from 10₄ to 10₆), binary mixtures, and crosslinked material. This paper is mainly devoted to the behavior of the spin-spin relaxation (T₂) which is sensitive to the low-frequency long-range configurational motions occurring in the liquid phase. These motions determine the time scale over which final motional averaging of the nuclear dipolar interactions occur, prior to the onset of normal liquid-like behavior of T₂. When these motions are too slow (e.g., at low temperatures of high molecular weight) to lead to appreciable averaging of the small residual static dipolar interactions present in the liquid phase, the spin-spin relaxation exhibits pseudo-solid-like behavior. A further aspect of this work concerns the observed sensitivity of T₂ to network formation which considerably restricts the long-range motions that determine T₂. It is possible to determine the fraction of material present in the form of a network from the relative proportions of network and non-network T₂ components. This is not only true for a permanent crosslinked network but also for a network arising from transient entanglement couplings. The dynamic nature of the latter type of a network is clearly revealed in the proton nuclear magnetic resonance measurements reported here.     

13C NMR spectroscopy of 13C1-labeled octanethiol-protected Au nanoparticles: shifts,relaxations, and particle-size effect

We report here an investigation of metal-ligand interactions in nanoparticles with 13C NMR, using a labeled 13C1-octanethiol, a protecting ligand for self-assembled monolayer (SAM) systems, in which close proximity of the 13C1 to the metal surface serves as an effective probe for the changing electronic environment. Several remarkable results have been obtained: as the metal core size increases from 1.6 to 4.0 nm, the 13C1 spectrum is downshifted from 40.5 to 53 ppm, and the spin-spin relaxation rate, T2-1, increases while the spin-lattice relaxation ratio decreases. Although the spin-lattice relaxation may be due to particle tumbling and ligand motion in the liquid state, the main source of the spin-spin relaxation, and NMR shift, is most possibly due to the changing electronic properties of the metal core.     

H NMR study of hydrogen bonding and molecular dynamics of 5CB confined to molecular sieves

It has been found that the main mechanism of ¹H protons spin–lattice relaxation of bulk 5CB at 200 MHz is its intramolecular motion, namely, the reorientation of CH₂ and CH₃ groups of its alkyl chain. Activation parameters of such motions have been estimated.

Drastic decrease in proton spin–lattice relaxation times at the nematic-to-isotropic phase transition can be explained by the activation of molecular translational diffusion and reorientations around long and short molecular axes of bulk 5CB.

Our NMR analysis revealed the slowing-down of molecular dynamics of confined 5CB molecules and their fragments. This can be explained by the interaction of some part of 5CB molecules with the surface active Si(Al)–OH centers of MCM matrix via hydrogen bonds of Si(Al)–OHN≡C-type.     

NMR studies of molecular motion of ultradispersed polytetrafluoroethylene

Molecular motion in ultradispersed polytetrafluroethylene obtained by special gas-phase technology has been studied experimentally and theoretically based on a temperature dependence of the second moment of ¹⁹F NMR spectra and the time of spin-lattice relaxation. The results of observations are interpreted as the consequence of reorientation motion of CF₂ groups around the axis of macromolecules at low temperature and of translational motion of macromolecules in the high temperature region. Qualitative differences from the molecular motion in industrial polytetrafluoroethylene (teflon-4) were detected and parameters of dynamic processes determined.     

Confinement of a liquid crystal to small droplets and its effect on nuclear magnetic relaxation

Abstract

The spatial dependence of the orientation of the molecular director and of the nematic order parameter is obtained by minimization of the Landau–de Gennes free energy of the nematic liquid crystal confined in a spherical droplet. Special attention is given to the vicinity of the nematic–isotropic transition. The influence of the resulting nematic structure, large liquid crystal–polymer interface and restricted molecular diffusion on the nuclear magnetic relaxation is analysed. The translationally-induced molecular reorientation and the liquid crystal–polymer cross relaxation are discussed in particular. The possibility of an indirect study of the molecular anchoring on the polymer surface is demonstrated.     

Dynamics of macromolecular chains. VI. Carbon 13 and proton nuclear magnetic relaxation of polystyrene in solution

Spin-lattice ¹H and ¹³C nuclear magnetic relaxation (NMR) times T₁ have been measured for solutions of polystyrene in hexachlorobutadiene at two different frequencies. Some nuclear Overhauser enhancements and linewidths have also been determined. At 15 and 25 MHz the relaxation times T₁ of the ortho and meta carbons show two different dependences on temperature. These measurements indicate internal motion of phenyl groups around the C_α—C_para axis. A single isotropic correlation time is inadequate to explain the relaxation data for the para carbon. Use of a diamond-lattice motional model reveals that segmental reorientation of the chain backbone of polystyrene can be described in terms of two correlation times, ρ characterizing the three-bond motion process, and θ reflecting either isotropic motions of subchains or departure from an ideal lattice. Data on low-molecular-weight polystyrene indicate the participation of overall rotatory diffusion in the relaxation process. This motion is no longer efficient in high-molecular-weight polymers, where relaxation is due to segmental reorientation.     

Columnar phases of achiral vanadyl liquid crystalline complexes

Two liquid crystalline vanadyl complexes have been studied by frequency domain dielectric spectroscopy over the range 10 mHz to 13 MHz. The materials exhibit two or three columnar phases denoted Col_ro, Col_rd, and Col_hd that were identified by X-ray diffraction. In the higher temperature Col_rd phase, a relaxation process in the kHz range is observed that is attributed to the reorientation about the molecular short axis. A pronounced dielectric relaxation process shows up in the low temperature Col_ro phase at hertz and sub-hertz frequencies. This slow relaxation is assigned to reorientation of the molecular dipoles within the polar linear chains, which are aligned along the column's axis. Triangular wave switching studies at low frequency reveal processes inside the Col_ro phase which are most probably due to ionic/charges relaxations but a ferroelectric switching for an achiral discotic system cannot be ruled out completely. Below the Col_ro phase there is an orientationally disordered crystalline Cr_x phase with disordered side chain dipoles. A dielectric relaxation process connected with the intramolecular relaxation of the alkoxy side chains, similar to the beta-process of polymers, has been found in the lower temperature Cr_x phase.     

Dynamic light scattering in liquid and supercooled diphenylmethane

We use a dynamic light scattering technique to measure both polarized (VV) and depolarized (VH) spectra of liquid diphenylmethane (DPM) between 288 and 362 K, covering both normal and supercooled liquid ranges. Our results allow extracting information on structural relaxation processes, rotational motions, rotation-translation couplings, and molecular reorientation phenomena in liquid DPM. The VV spectra are modeled according to the microscopic theory of Wang, which assumes that a structural relaxation process dominates the spectrum. We find that the relaxation time of the structural relaxation in DPM follows an Arrhenius behavior. The Rayleigh dip was observed in the VH spectra, which are described using the Andersen-Pecora theory. Our results are discussed in terms of the rotation-translation coupling parameter, which we find independent of temperature over the experimental range. The collective reorientation time also follows an Arrhenius behavior with temperature. Finally, we calculate the hydrodynamic volumes for the reorientation process from geometric molecular models in two hydrodynamic limits: slip and stick boundary conditions. Our results suggest that the DMP molecule reorientates in quasi-slipping conditions in the bulk liquid.