Behavior of acetonitrile confined to mesoporous silica gels as studied by 129Xe NMR: a novel method for determining the pore sizes

129Xe NMR spectra of xenon dissolved in acetonitrile confined into three mesoporous silica gels with nominal pore diameters of 40, 60, and 100 A have been measured over the temperature range 170-245 K. The spectra consist of a number of lines, which contain detailed information on the system. The most interesting result is that the chemical shift of a particular signal observed below the melting point of confined acetonitrile is highly sensitive to the pore size, and hence its shape is sensitive to the pore size distribution function. This signal originates from the xenon atoms sited in very small cavities built up inside the pores during the freezing transition. It can be used to determine the size or even the size distribution function of the pores. In addition, the emergence of this signal reveals the phase transition temperature of acetonitrile inside the pores, which can also be used to determine the size of the pores. The difference in the chemical shifts of two other signals, which arise from xenon dissolved in bulk and confined acetonitrile, provides still another novel method for determining the size of the pores.     

2D 129Xe EXSY of xenon atoms in a thermotropic liquid crystal confined to a controlled-pore glass

Two-dimensional (129)Xe exchange spectroscopy (EXSY) NMR measurements are presented for xenon atoms dissolved in a thermotropic nematic Liquid Crystal (LC), Merck Phase 4, confined to a mesoporous Controlled-Pore Glass (CPG) material with an average pore diameter of 81 A. Experiments were carried out as a function of mixing time at two different temperatures in which Phase 4 appears in nematic and isotropic phases. The exchange rate constants of xenon atoms between two different sites were determined utilizing the intensities of diagonal and off-diagonal signals measured in the EXSY spectra. In the studied system, the sites are: (a) xenon dissolved in the bulk LC between the CPG particles; and (b) xenon in the LC confined inside the pores. The diffusion rate of xenon atoms between the various sites was observed to be very slow.     

Behavior of liquid crystals confined to mesoporous materials as studied by 13C NMR spectroscopy of methyl iodide and methane as probe molecules

The behavior of thermotropic nematic liquid crystals (LCs) Merck Phase 4 and ZLI 1115 confined to mesoporous controlled pore glass materials was investigated using 13C nuclear magnetic resonance spectroscopy of probe molecules methyl iodide and methane. The average pore diameters of the materials varied from 81 to 375 A, and the temperature series measurements were performed on solid, nematic, and isotropic phases of bulk LCs. Chemical shift, intensity, and line shape of the resonance signals in the spectra contain lots of information about the effect of confinement on the state of the LCs. The line shape of the 13C resonances of the CH3I molecules in LCs confined into the pores was observed to be even more sensitive to the LC orientation distribution than, for example, that of 2H spectra of deuterated LCs or 129Xe spectra of dissolved xenon gas. The effect of the magnetic field on the orientation of LC molecules inside the pores was examined in four different magnetic fields varying from 4.70 to 11.74 T. The magnetic field was found to have significant effect on the orientation of LC molecules in the largest pores and close to the nematic-isotropic phase transition temperature. The theoretical model of shielding of noble gases dissolved in LCs based on pairwise additivity approximation was utilized in the analysis of CH4 spectra. For the first time, a first-order nematic-isotropic phase transition was detected to take place inside such restrictive hosts. In the larger pores a few degrees below the nematic-isotropic phase transition of bulk LC the 13C quartet of CH3I changes as a powder pattern. Results are compared to those derived from 129Xe NMR measurements of xenon gas in similar environments.     

129Xe NMR spectroscopy study of porous cyanometallates

Zinc and cadmium hexacyanocobaltates(III) were prepared, and their porous networks were explored using 129Xe spectroscopy. The crystal structures of these two compounds are representative of porous hexacyanometallates, cubic (Fm-3m) for cadmium and rhombohedral (R-3c) for zinc. In the cubic structure, the porosity is related to systematic vacancies created from the elemental building block (i.e., the hexacyanometallate anion), whereas the rhombohedral (R-3c) structure is free of vacant sites but has tetrahedral coordination for the zinc atom, which leads to relatively large ellipsoidal pores communicated by elliptical windows. According to the Xe adsorption isotherms, these porous frameworks were found to be accessible to the Xe atom. The structure of the higher electric field gradient at the pore surface (Fm-3m) appears and is accompanied by a stronger guest-host interaction for the Xe atoms and a higher capacity for Xe sorption. For cadmium, the 129Xe NMR signal is typical of isotropic movement for the Xe atom, indicating that it remains trapped within a spherical cavity. From spectra recorded for different amounts of adsorbed Xe, the cavity diameter was estimated. For the zinc complex, 129Xe NMR spectra are asymmetric because of the Xe atom movement within an elongated cavity. The line-shape asymmetry changes when the Xe loading within the porous framework increases, which was ascribed to Xe-Xe interactions through the cavity windows. The Xe adsorption revealed additional structural information for the studied materials.     

129Xe NMR as a probe of polymer blends

The miscibility of two-component polymer blends has been investigated using xenon-129 (¹²⁹Xe) nuclear magnetic resonance (NMR) to probe the phase morphology. The chemical shift of ¹²⁹Xe dissolved in a given polymer is unique, thus heterogeneous blends with large domain sizes exhibit two ¹²⁹Xe NMR lines. When a single resonance is obtained, the data are consistent with miscibility, yielding an upper bound on the domain size. The temperature dependence of the relative solubilities and chemical shifts of ¹²⁹Xe dissolved in the pure components may allow a determination of the phase morphology in blends exhibiting a single resonance. The method is used to demonstrate that polychloroprene and 25% epoxidized 1,4-polyisoprene form a miscible blend.     

Silver nitrate-acetonitrile and iodine-benzene complexes as studied by NMR in thermotropic liquid crystals

Silver nitrate-acetonitrile and π iodine-benzene complexes in thermotropic liquid crystals have been studied by ¹H, ²H, and ¹³C NMR spectroscopy and by optical microscopy. Evidence for at least two silver complexes in each liquid crystal is presented.     

Internal molecular disorder in the nematic and smectic phases of thermotropic liquid crystals studied by NMR SPDE experiments

The recently developed NMR SPDE experiment is shown to provide a new and particularly convenient technique for probing the conformational dynamics of mesogens in thermotropic liquid crystals. Measurements have been made in the nematic and smectic phases of the 4,4′-di-n-alkoxyazoxybenzenes. It is shown for the first time that the internal disorder of the alkyl end chains is intimately related to the molecular organization of these mesophases.     

Recent advancements in understanding thermotropic liquid crystal structure and dynamics by means of NMR spectroscopy

Recent progresses of NMR spectroscopy to the study of liquid crystals and related ordered systems are surveyed. Particular attention will be devoted to review suitably tailored NMR experiments and their applications on those systems (e.g. biaxial nematics, chiral smectics, V shaped mesogens, liquid crystalline elastomers) which are presently subject of active and innovative research.     

The stability diagram of a nematic liquid crystal confined to a cylindrical cavity

Different nematic structures confined to a long cylindrical cavity with homeotropic surface anchoring are studied using a numerical minimization of the free energy of the uniaxial nematic liquid crystal. The stability of escaped radial structures and planar polar structures (with and without line defects) is analysed in terms of the ratio of elastic constants K₂₄/K₁₁, K₃₃/K₁₁, anchoring strength and external magnetic field applied perpendicular to the symmetry axis of the cylinder. We draw the analogy between the stability diagram of the cylindrical structures and structures in a spherical droplet. In particular, a simple way extracting the value of the saddle-splay elastic constant K₂₄ from the stability studies is discussed.     

NMR polarization echoes in a nematic liquid crystal

We have modified the polarization echo (PE) sequence through the incorporation of Lee-Goldburg cross polarization steps to quench the 1H-1H dipolar dynamics. In this way, the 13C becomes an ideal local probe to inject and detect polarization in the proton system. This improvement made possible the observation of the local polarization P(00)(t) and polarization echoes in the interphenyl proton of the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline. The decay of P(00)(t) was well fitted to an exponential law with a characteristic time tau(C) approximately 310 micros. The hierarchy of the intramolecular dipolar couplings determines a dynamical bottleneck that justifies the use of the Fermi Golden Rule to obtain a spectral density consistent with the structural parameters. The time evolution of P(00)(t) was reversed by the PE sequence generating echoes at the time expected by the scaling of the dipolar Hamiltonian. This indicates that the reversible 1H-1H dipolar interaction is the main contribution to the local polarization decrease and that the exponential decay for P(00)(t) does not imply irreversibility. The attenuation of the echoes follows a Gaussian law with a characteristic time tau(phi) approximately 527 micros. The shape and magnitude of the characteristic time of the PE decay suggest that it is dominated by the unperturbed homonuclear dipolar Hamiltonian. This means that tau(phi) is an intrinsic property of the dipolar coupled network and not of other degrees of freedom. In this case, one cannot unambiguously identify the mechanism that produces the decoherence of the dipolar order. This is because even weak interactions are able to break the fragile multiple coherences originated on the dipolar evolution, hindering its reversal. Other schemes to investigate these underlying mechanisms are proposed.     

An NMR study on the shielding of the 129Xe isotope of xenon dissolved in thermotropic liquid crystals

The variation of the nuclear shielding of the ¹²⁹Xe isotope in natural xenon dissolved in various liquid crystals and liquid crystal mixtures has been studied over the temperature range from 300 to 360 K. The temperature dependence is linear in the isotropic phase of the liquid crystals. An abrupt change in the shielding is observed when passing through the nematic-isotropic and smectic A-nematic phase transitions as well as when the liquid crystal director rotates by 90° in the so-called critical mixture of ZLI 1167 and EBBA. This is interpreted as being mainly the consequence of the shielding anisotropy of xenon arising from the deformation of its electronic distribution. The shift changes observed for 4,4'-di-n-heptylazoxybenzene at the nematic-isotropic phase transition on the one hand and at the smectic A-nematic phase transition on the other are found to be opposite in sign, reflecting the change in the liquid crystal structure.     

Surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses: curvature effects observed by 1H NMR

We have measured the thickness of the pre-molten surface layer that appears at the interface of octamethylcyclotetrasiloxane (OMCTS) to the matrix in controlled pore glasses with pore diameters ranging 7.5-73 nm. Except for the glass with the largest pores, the layer thickness data for different pore diameters fall on a single master curve when plotted versus Tm - T, where Tm is the size-dependent volume melting point of the pore-confined OMCTS. Hence, at a single temperature, the surface layer thickness depends strongly on the curvature of the pore wall and therefore that of the solid-liquid interface. For temperatures where it exceeds two monolayers, the layer thickness depends logarithmically on Tm - T; for the glass with the largest pores, this turns into a power law with the exponent -1/2. The results are interpreted in terms of a continuous model of the solid-liquid interface with an arbitrary curvature. Because OMCTS is a weakly polar molecule with close to spherical shape, our data also lend themselves to Lennard-Jones type simulations.     

Characterization of porosity in organic and metal-organic macrocycles by hyperpolarized 129Xe NMR spectroscopy

Hyperpolarized (129)Xe NMR spectroscopy is used to establish the solid-state porosity of shape-persistent macrocycles with either an organic or metal-organic framework. These studies show that even upon removal of cocrystallized solvent molecules, the macrocycles maintain a porous or channeled structure. The technique can provide valuable information about systems for which X-ray crystallographic analysis is not feasible. [structure: see text]     

Flow-induced structure in a thermotropic liquid crystalline polymer as studied by SANS

Small-angle neutron scattering is utilized to determine the flow induced alignment of a model thermotropic liquid crystalline polymer (LCP) as a function of shear rate and temperature. The results demonstrate that the flow-induced structures in thermotropic liquid crystalline polymers have similarities and differences to those in lyotropic liquid crystalline polymer solutions. The shear rate dependence of the alignment shows that the flow-induced alignment correlates very well to the viscosity behavior of the LCP in the shear thinning regime, while temperature variation results in a change in the extent of alignment within the nematic phase. Relaxation results also demonstrate that the flow-induced alignment remains essentially unchanged for up to an hour after the shear field has been removed. Last, there exists a regime at low shear rate and low temperature where alignment of the LCP molecule perpendicular to the applied shear flow is stable. These results provide important experimental evidence of the molecular level changes that occur in a thermotropic liquid crystalline polymer during flow, which can be utilized to develop theoretical models and more efficiently process thermotropic polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3017–3023, 1998     

In situ switching of sorbent functionality as monitored with hyperpolarized (129)Xe NMR spectroscopy

In this contribution, we demonstrate that a material (organic zeolite mimetic coordination polymer [CuL(2)], where L = L(-) = CF(3)COCHCOC(OCH(3))(CH(3))(2)) can be endowed with its functionality in situ under molecular-level control. This process involves the isomerization of the ligands followed by phase interconversion from a dense to an open, porous form. The porous (beta) form of the complex reveals zeolite-like behavior but, unlike zeolites and many other hard porous frameworks, porosity may be created or destroyed at will by the application of suitable external stimuli. Contact with methylene chloride vapor was used to switch on the sorbent functionality, whereas switching off was accomplished with a temperature pulse. The transformations between functionally inactive alpha and active beta forms, as well as the amount of vacant pore space, were monitored in situ by observing the NMR spectrum of hyperpolarized (HP) Xe atom probes. For methylene chloride, the chemical shift of the coabsorbed HP Xe correlated directly with the amount of adsorbate in the pore system of the open framework, illustrating the use of HP Xe for following sorption kinetics. The adsorption of propane, as an inert adsorbate, was also monitored directly with (1)H NMR, with HP Xe and by BET measurements, revealing more complex behavior.