Nematic ordering in porous glasses: A deuterium NMR study

Qualitatively and Quantitatively Different Configurations of Nematic-Nanoparticle Mixtures

We consider the influence of different nanoparticles or micrometre-scale colloidal objects, which we commonly refer to as particles, on liquid crystalline (LC) orientational order in essentially spatially homogeneous particle-LC mixtures. We first illustrate the effects of coupling a single particle with the surrounding nematic molecular field. A particle could either act as a "dilution", i.e., weakly distorting local effective orientational field, or as a source of strong distortions. In the strong anchoring limit, particles could effectively act as topological point defects, whose topological charge q depends on particle topology. The most common particles exhibit spherical topology and consequently act as q = 1 monopoles. Depending on the particle's geometry, these effective monopoles could locally induce either point-like or line-like defects in the surrounding LC host so that the total topological charge of the system equals zero. The resulting system's configuration is topologically equivalent to a crystal-like array of monopole defects with alternating topological charges. Such configurations could be trapped in metastable or stable configurations, where the history of the sample determines a configuration selection.

Impact of random-field-type disorder on nematic liquid crystalline structures

We study bicomponent systems where one component represents a liquid crystalline (LC) phase, and the other component randomly perturbs the LC order. Such systems can serve as a testbed to systematically analyse the impact of qualitatively different types of random-type sources of perturbation on the orientational and/or translational order. This mini-review presents typical representatives of such systems, where orientational and translational order is probed in nematic and smectic A LCs, respectively. As a source of perturbation, we consider either different porous matrices (control-pore glass, aerogels) or aerosil nanoparticles, which can form in LCs' different fractal-like network organizations. In such complex systems, LC ordering fingerprints the interplay among LC elastic forces, interfacial forces, and randomness. The resulting LC behaviour could be characterised by either long-range, quasi long-range, or short-range order. We demonstrate under which conditions random-field-like phenomena or interfacial effects dominate. However, these effects are relatively strongly entangled in most experimental systems, and individual impacts cannot be precisely identified.

Memory effect in composites of liquid crystal and silica aerosil

Aerosil silica nanoparticles dispersed in a liquid crystal (LC) possess the interesting property of keeping memory of an electric- or magnetic-field-induced orientation. Two types of memory have been identified: thermally erasable memory arising from the pinning of defect lines versus a "permanent" memory where the orientation persists even after thermal cycling the samples up to the isotropic phase. To address the source of the latter type of memory, solid-state nuclear magnetic resonance spectroscopy and conventional x-ray diffraction (XRD) were first combined to characterize the LC orientational order as a function of multiple in-field temperature cycles. Microbeam XRD was then performed on aligned gels of different concentrations to gain knowledge of the structural properties at the origin of the memory effect. No detectable anisotropy of the gel or significant breaking of silica strands with heating ruled out the formation of an anisotropic silica network as the source of the permanent memory as previously proposed. Instead, support for a role of the surface memory effect, well known for planar substrates, in stabilizing the permanent memory was deduced from "training" of the composites, that is, optimizing the orientational order through the thermal in-field cycling. The ability to train the composites is inversely proportional to the strength of the random-field disorder. The portion of thermally erasable memory also decreases as the silica density increases. We propose that the permanent memory originates from the surface memory effect operating at points of intersection in the silica network. These areas, where the LC is strongly confined with conflicted surface interactions, are trained to achieve an optimized orientation and subsequently act as sites from which the LC orientational order regrows after zero-field thermal cycling up to the isotropic phase.

Competition between local disordering and global ordering fields in nematic liquid crystals

We study the influence of external electric or magnetic field B on orientational ordering of nematic liquid crystals or of other rod-like objects (e.g. nanotubes immersed in a liquid) in the presence of random anisotropy field type of disorder. The Lebwohl–Lasher lattice type of semi-microscopic approach is used at zero temperature. Therefore, results are valid well below the transition into the isotropic phase. We calculate the correlation function of systems as a function of B, concentration p of impurities imposing random anisotropy field disorder, the disorder strength W and system dimensionality (2D and 3D systems). In order to probe memory effects we calculate correlation length ξ for random and homogeneous initial configurations. We determine the crossover fields B_c(p) separating roughly the ordered and disordered regime. Memory effects are apparent only in the latter case, i.e. for B < B_c.

PACS numbers: 47.51.+a, 47.54.-r, 07.05.Tp, 61.30.-v

The influence of disorder on thermotropic nematic liquid crystals phase behavior

We review the theoretical research on the influence of disorder on structure and phase behavior of condensed matter system exhibiting continuous symmetry breaking focusing on liquid crystal phase transitions. We discuss the main properties of liquid crystals as adequate systems in which several open questions with respect to the impact of disorder on universal phase and structural behavior could be explored. Main advantages of liquid crystalline materials and different experimental realizations of random field-type disorder imposed on liquid crystal phases are described.

Influence of confinement in controlled-pore glass on the layer spacing of smectic- a liquid crystals

A detailed x-ray scattering study has been performed in the temperature range of the smectic- A phase for the liquid crystal compounds dodecylcyanobiphenyl (12CB) and octylcyanobiphenyl (8CB) confined in different controlled-pore glasses (CPGs) characterized by their average void radius R . On decreasing the temperature in bulk samples the layer thickness is increasing for 12CB and decreasing for 8CB, respectively. In nontreated CPG samples the layers dilate significantly with respect to the bulk liquid crystal. In order to explain the layer thickness behavior on varying temperature and R , one has to take into account molecular details of the liquid crystalline samples as well as memory effects.

An alternative NMR method to determine nuclear shielding anisotropies for molecules in liquid-crystalline solutions with (13)C shielding anisotropy of methyl iodide as an example

An alternative NMR method for determining nuclear shielding anisotropies in molecules is proposed. The method is quite simple, linear and particularly applicable for heteronuclear spin systems. In the technique, molecules of interest are dissolved in a thermotropic liquid crystal (LC) which is confined in a mesoporous material, such as controlled pore glass (CPG) used in this study. CPG materials consist of roughly spherical particles with a randomly oriented and connected pore network inside. LC Merck Phase 4 was confined in the pores of average diameter from 81 to 375 A and LC Merck ZLI 1115 in the pores of average diameter 81 A. In order to demonstrate the functionality of the method, the (13)C shielding anisotropy of (13)C-enriched methyl iodide, (13)CH(3)I, was determined as a function of temperature using one dimensional (13)C NMR spectroscopy. Methane gas, (13)CH(4), was used as an internal chemical shift reference. It appeared that methyl iodide molecules experience on average an isotropic environment in LCs inside the smallest pores within the whole temperature range studied, ranging from bulk solid to isotropic phase. In contrast, in the spaces in between the particles, whose diameter is approximately 150 microm, LCs behave as in the bulk. Consequently, isotropic values of the shielding tensor can be determined from spectra arising from molecules inside the pores at exactly the same temperature as the anisotropic ones from molecules outside the pores. Thus, for the first time in the solution state, shielding anisotropies can easily be determined as a function of temperature. The effects of pore size as well as of different LC media on the shielding anisotropy are examined and discussed.

Liquid crystal 8CB in random porous glass: NMR relaxometry study of molecular diffusion and director fluctuations

We present the measurements of the proton spin-lattice relaxation time T1 of liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB) confined into randomly oriented approximately 15 nm pores of untreated porous glass. In the low kilohertz range the spin-lattice relaxation rate in the nanoconfined 8CB is about ten times larger than in the bulk. We show that the increase is mainly due to molecular reorientations mediated by translational displacements (RMTD). In the paranematic phase the power law describing the RMTD dispersion, (T1(-1))RMTD proportional, omega(-p), is well characterized by the exponent p=0.5+/-0.06 and suggests an equipartition of diffusion modes with different wavelengths. The largest distance related to the decay of the orientational correlation function is about twice the diameter of the cavity. The situation is different in the nematic phase, where the orientational correlation is eventually lost at approximately 60 nm in the direction along the pore, a distance corresponding roughly to the length of a pore segment in the glassy matrix. The exponent p is between 0.65 and 0.9, depending on the temperature, which implies that in the nematic phase long wavelength modes are relatively more important--a consequence of the uniform director field along the pore. These observations are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. We point out that in strongly confined liquid crystals the proton NMR relaxometry does not provide the evidence of director fluctuations correlated over micrometer distances as was suggested earlier. The local translational diffusion of molecules within the cavities is found about as fast as in bulk.

Effect of suspended clay particles on isotropic-nematic phase transition of liquid crystal

Sterically stabilised nano-platelets were prepared by treating montmorillonite clay with both a surfactant and a polymeric stabiliser. These nano-platelets formed stable suspensions in a thermotropic liquid crystal, 5CB. This is in marked contrast with previous work on preparing liquid crystal suspensions of either spheres, which formed gels on cooling the solvent into the nematic phase, or clay platelets stabilised only with low molecular weight surfactant, which tended to aggregate. In the isotropic state of the liquid crystal, static light scattering showed that the clay nano-platelets were freely suspended, and no aggregation was detected even after repeated temperature cycling into and out of the nematic phase. Small-angle X-ray scattering showed that the clay was delaminated nearly completely in the liquid crystal, with some stacks of a few clay nano-platelets having formed. Differential scanning calorimetry of the liquid crystal/clay suspensions showed a small but non-monotonic shift of the transition temperature compared to that of the pure liquid crystal. This behaviour is similar to that of liquid crystal confined in porous media, with an initial increase of the transition temperature on adding clay being ascribed to the effect of surface anchoring facilitating the formation of the nematic phase, whereas a decrease at higher clay concentrations (or equivalently, for smaller pores) is ascribed to confinement effects frustrating the formation of the nematic phase. This interpretation is supported by polarising light microscopy which showed the nematic domain size becoming smaller on increasing the clay concentration.

Memory effect in the adsorption phenomena of neutral particles

The adsorption-desorption phenomenon of neutral particles dissolved in an isotropic fluid is investigated by using a nonsingular kernel in the kinetic equation at the limiting surfaces. To account for the relevance of a memory effect, three types of kernels in the kinetic equation are considered. Similar kernels have been used to investigate nonexponential relaxation including several contexts such as dielectric relaxation, diffusion-controlled relaxation in liquids, liquid crystals, and amorphous polymers. A suitable choice for a temporal kernel can account for the relative importance of physisorption or chemisorption, according to the time scale governing the adsorption phenomena, and can be the key mechanism to understand the specific roles of both processes. By using a general procedure, the time evolution of the density of particles is determined in closed analytical form. The analysis is relevant in the description of the adsorption phenomena in general.

Behavior of a Thermotropic Nematic Liquid Crystal Confined to Controlled Pore Glasses as Studied by 129Xe NMR Spectroscopy

The behavior of nematic liquid crystal (LC) Merck Phase 4 confined to controlled pore glass (CPG) materials was investigated using 129Xe nuclear magnetic resonance (NMR) spectroscopy of xenon gas dissolved in the LC. The average pore diameters of the materials varied from 81 to 2917 A, and the measurements were carried out within a wide temperature range (approximately 185-370 K). The spectra contain lots of information about the effect of confinement on the phase of the LC. The theoretical model of shielding of noble gases dissolved in liquid crystals on the basis of pairwise additivity approximation was applied to the analysis of the spectra. When pore diameter is small, smaller than approximately 150 A, xenon experiences on average an isotropic environment inside the pore, and no nematic-isotropic phase transition is observed. When the size is larger than approximately 150 A, nematic phase is observed, and the LC molecules are oriented along pore axis. The orientational order parameter of the LC, S, increases with increasing pore size. In the largest pores, the orientation of the molecules deviates from the pore axis direction to magnetic field direction, which implies that the size of the pores (approximately 3000 A) is close to magnetic coherence length. The decrease of magnetic coherence length with increasing temperature is clearly seen from the spectra. When the sample is cooled rapidly by immersing it in liquid nitrogen, xenon atoms do not squeeze out from the solid, as they do during gradual freezing, but they are occluded inside the solid lattice, and their chemical shift is very sensitive to crystal structure. This makes it possible to study the effect of confinement on the solid phases. According to the measured 129Xe NMR spectra, possibly three different solid phases are observed from bulk liquid crystal in the used temperature region. The same is also seen from the samples containing larger pores (pore size larger than approximately 500 A), and the solid-solid phase-transition temperatures are the same. However, no first-order solid-solid phase transitions are observed from the smaller pores. Melting point depression, that is, the depression of solid-nematic transition temperature observed from the pores as compared with that in bulk LC, is seen to be very sensitive to the pore size, and it can be used for the determination of pore size of an unknown material.

Field-cycling NMR relaxometry of a liquid crystal above in mesoscopic confinement

We measured the proton spin-lattice relaxation times in the isotropic phase of liquid crystal 4-n-pentyl-4-cyanobiphenyl (5CB) confined into porous glass (CPG) with the average pore diameter approximately 72 nm. The analysis of T1(-1) frequency dispersions, spanning over four decades, shows that the main relaxation mechanism induced by the ordered surface layer are molecular reorientations mediated by translational displacements (RMTD). The RMTD contribution to T1(-1) is proportional to the inverse square root of Larmor frequency, a consequence of the equipartition of diffusion modes along the surface. Low and high frequency cutoffs of the RMTD mechanism clearly reveal that the surface alignment of liquid crystal is random planar with the size of uniformly oriented patches approximately 5 nm, depending on the treatment of the CPG matrix. According to the size of the uniformly oriented patches varies also the thickness of the ordered surface layer and its temperature behavior. The surface-induced order parameter is found to be temperature independent and determined by the local short range surface interactions.

Proton spin-spin relaxation study of hydration of a model nanopore

The hydration pattern of controlled pore glass, with pore diameter of 237 A, was investigated using nuclear magnetic resonance. Water proton spin-spin relaxation decay curves were monitored and modeled as two-component exponential decays as a function of hydration. The results are consistent with a geometric model involving a surface water layer and a bulk-like liquid fraction in the form of a plug. The amount of surface water increases as the sample hydrates, until hydration reached approximately a monolayer, at which point a water plug starts to form in the pore, and grow in length at the expense of the surface layer. The results are also analyzed in terms of, and compared to, a recently developed puddle pore-filling model [S.G. Allen, et al. J. Chem. Phys. 106 (1997) 7802-7809].

Dynamics of propylene glycol and its oligomers confined to a single molecular layer

The dynamics of propylene glycol (PG) and its oligomers 7-PG and poly-propylene glycol (PPG), with M(w) = 4000 (approximately 70 monomers), confined in a Na-vermiculite clay have been investigated by quasielastic neutron scattering. The liquids are confined to single molecular layers between clay platelets, giving a true two-dimensional liquid. Data from three different spectrometers of different resolutions were Fourier transformed to S(Q,t) and combined to give an extended dynamical time range of 0.3-2000 ps. An attempt was made to distinguish the diffusive motion from the methyl group rotation and a fast local motion of hydrogen in the polymer backbone. The results show that the average relaxation time tau(d) of this diffusive process is, as expected, larger than the relaxation time tau averaged over all dynamical processes observed in the experimental time window. More interesting, it is evident that the severe confinement has a relatively small effect on tau(d) at T = 300 K, this holds particularly for the longest oligomer, PPG. The most significant difference is that the chain-length dependence of tau(d) is weaker for the confined liquids, although the slowing down in bulk PG due to the formation of a three-dimensional network of OH-bonded end groups reduces this difference. The estimated average relaxation time tau at Q = 0.92 Angstroms(-1) for all the observed processes is in excellent agreement with the previously reported dielectric alpha relaxation time in the studied temperature range of 260-380 K. The average relaxation time tau (as well as the dielectric alpha relaxation time) is also almost unaffected by the confinement to a single molecular layer, suggesting that the interaction with the clay surfaces is weak and that the reduced dimensionality has only a weak influence on the time scale of all the dynamical processes observed in this study.

Influence of finite size and wetting on nematic and smectic phase behavior of liquid crystal confined to controlled-pore matrices

The high-resolution calorimetric study was carried out on octylcyanobiphenyl liquid crystal (LC) confined to various controlled-pore glass (CPG) matrices with silane-treated surface. The diameter of the voids cross section ranged between 23.7 and 395 nm. The results are compared to those obtained previously on CPG voids nontreated with silane. We found a striking similarity between the shifts in the isotropic to nematic and nematic to smectic-A phase transition temperatures as a function of the void radius in which order parameter variations at the LC-void interface play the dominant role. Weaker temperature shifts are observed in silane-treated samples, where surface ordering tendency is larger. In nontreated samples, a finite-size scaling law in the maximum value of the heat capacity at the nematic to smectic-A transition was observed for void diameters larger than 20 nm. In silane-treated samples, this behavior is considerably changed by surface wetting interactions.

Liquid crystals in random porous media: disorder is stronger in low-density aerosils

The nature of glass phases of liquid crystals in random porous media depends on the effective disorder strength. We study how the disorder strength depends on the density of the porous media and demonstrate that it can increase as the density decreases. We also show that the interaction of the liquid crystal with random porous media can destroy long-range order inside the pores.

Random anisotropy nematic model: connection with experimental systems

We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.

Calorimetric study of octylcyanobiphenyl liquid crystal confined to a controlled-pore glass

We present a calorimetric study of the phase behavior of octylcyanobiphenyl (8CB) liquid crystal confined to a controlled-pore glass (CPG). We used CPG matrices with characteristic void diameters ranging from 400 to 20 nm. In bulk we obtain weakly first-order isotropic to nematic (I-N) phase transition and nearly continuous character of the nematic to smectic-A (N-SmA) phase transition. In all CPG matrices the I-N transition remains weakly first order, while the N-SmA one becomes progressively suppressed with decreasing CPG pore radius. With decreased pore diameters both phase transition temperatures monotonously decrease following similar trends, but increasing the stability range of the N phase. The heat-capacity response at the weakly first order I-N and continuous N-SmA phase transitions gradually approaches the tricritical-like and three-dimensional XY behavior, respectively. The main observed features were explained using a bicomponent single pore type phenomenological model.

Influence of surface treatment on the smectic ordering within porous glass

The influence of the surface treatment on the Sm-A-N phase transition of the 8CB (octylcyanobiphenyl) liquid crystal confined to controlled pore glass (CPG) matrices is studied. The characteristic linear size of voids in the chosen CPG matrix is 0.2 &mgr;m. The voids' surface was either nontreated or silane treated enforcing tangential or homeotropic anchoring, respectively. In both cases the x-ray measurements reveal a qualitative change of the temperature dependence of the smectic order-parameter correlation length in comparison to the bulk sample. In addition, the apparent smectic pretransitional ordering is observed for the silane-treated sample. A theoretical description based on the Landau-de Gennes type approach is developed to explain the experimental data. The surface positional anchoring strength of the silane-treated sample is estimated to be of the order of 10(-4) J/m(2) and at least 100 times weaker for the nontreated case.