Surface-induced Order Research Articles

Field-cycling NMR relaxometry of a liquid crystal aboveTNIin 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.

Thermodynamic and Dielectric Studies Concerning the Influence of Cylindrical Submicrometer Confinement on Heptyloxycyanobiphenyl

Measurements of the specific heat and the static dielectric permittivity of heptyloxycyanobiphenyl (7OCB) confined to the 0.2 microm diameter parallel cylindrical pores of Anopore membranes in the isotropic phase and nematic mesophase, are presented. A comparison between the bulk and the confined 7OCB in treated and untreated pore wall surfaces using a chemical surfactant (HTBA) is performed. Both the treated and untreated membrane confinements seem to affect the nematic-to-isotropic phase transition by a downshift in transition temperature and some rounding at the specific-heat maximum, in a way similar to that which was earlier published for other liquid crystals confined in the same geometry. The static dielectric measurements clearly point out that untreated membrane confinement is axial, with the nematic director aligned parallel to the pore axis being homeotropic bulklike, i.e., with the nematic director aligned perpendicular to the electrode cell surfaces. After chemical surfactant treatment, the nematic director is constrained in a radial alignment being perpendicular to the pore walls. The dielectric measurements are revealed to be specially sensible to analyze the surface-induced nematic order due to the pore wall. The tricritical nature of the nematic-to-isotropic phase transition in bulk 7OCB as well as in treated and untreated Anopore confined geometries is discussed through both the specific heat and the static dielectric data.

Surface-induced orientational order in stretched nanoscale-sized polymer dispersed liquid-crystal droplets

We investigate orientational ordering in stretched polymer-dispersed liquid-crystal (PDLC) droplets using deuterium nuclear magnetic resonance, in the nematic and isotropic phases. In the latter case, we estimate the surface order parameter S(0) and the thickness of the interfacial layer from the temperature-independent surface ordering model for an elliptical cavity with a varying aspect ratio. A simple phenomenological model well describes the quadrupole splitting frequency of NMR spectra in the isotropic phase. The strain dependence of S(0) suggests that stretching-induced changes in the orientation of polymer chains in the PDLC matrix noticeably affect liquid-crystal surface anchoring. Experimental results are supported by simulated NMR spectra obtained as output from Monte Carlo simulations of paranematic ordering in ellipsoidal droplets based on the Lebwohl-Lasher lattice model.

Surface-Induced Conformational Changes in Poly(3-hexylthiophene) Monolayer Films

With the aim of elucidating the surface-induced molecular ordering in regioregular poly(3-hexylthiophene) (P3HT) monolayer films, we have controlled the intermolecular interactions at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with two kinds of groups (-NH2 and -CH3). We have found that, depending on the surface properties of such modified insulator substrates, the P3HT chains in the monolayer films can adopt two different conformations (edge-on and face-on). This surprising variation in chain conformation arises because of the specific interactions of the P3HT chains with the modified insulator substrates, which can be explained in terms of the following factors: the unshared electron pairs of the SAM end groups (in the -NH2 system), the pi-H interactions between the thienyl backbone bearing pi systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs (in the -NH2 system).

Structure formation in films of weakly charged block polyelectrolyte solutions

A mean-field dynamic density functional theory is used to describe a phase diagram of concentrated solutions of weakly charged flexible block polyelectrolytes in a film. Electrostatics is taken into account by applying the local electroneutrality constraint (the Donnan membrane equilibrium approach). In the Donnan limit it is assumed that a salt added to the solution perfectly screens long-range electrostatic interactions. The phase diagram of a solution of a triblock polyelectrolyte in a film as a function of the solvent concentration and the charge of the polyelectrolyte (solvophilic) block is calculated for a given film thickness. The phase behavior of the block polyelectrolyte film arises from the interplay between surface-induced alignment and the electrostatically-driven structure formation. The observed mesoscopic structures (lamellar, perforated lamellar, cylindrical, micellar, and mixed phases) are oriented parallel to the surfaces for the considered case of morphologies unfrustrated by the film thickness. Structures with connections between parallel layers (bicontinuous, etc.) are not formed. As a result of surface-induced ordering, the region of ordered phases in a film is wider than in bulk and the phase boundary between ordered and disordered phases is more diffuse. As in the case of unconfined block polyelectrolyte solution, the solution in a film does not follow the lyotropic sequence of phases of such a block copolymer upon increase in the charge of the polyelectrolyte block. Upon changing the charge of the solvophilic copolymer block, transformations of copolymer morphology take place via change in curvature of polymeric domains. Due to confinement of a polyelectrolyte film, no swelling of solvophilic domains is observed.

Surface-induced ordering of nematics in an external field: the strong influence of tilted walls.

Microscopic theory is used to investigate surface-induced order in a model nematic subjected to an external orienting field. The wall-particle interaction tends to orient particles perpendicular to the surface. It is shown that if the wall is tilted at approximately 45 degrees to the field, the reorientational effects can be an order of magnitude larger than those observed for perpendicular or parallel orientations. The surprising observation is associated with the breaking of a particular bulk symmetry. A possible practical application of the tilted geometry is briefly discussed.

Controlling disorder in liquid crystal aerosil dispersions.

The effect of disorder in the behavior of liquid crystal (LC) is assessed and controlled by dispersing known amounts of silica aerosil in the liquid crystal material. Using deuteron nuclear magnetic resonance, the director configuration and the orientational order was determined for hydrophilic aerosil dispersions in octylcyanobiphenyl. The confined liquid crystal exhibits a well-defined alignment as the silica spheres stabilize the molecular configuration. At low silica densities, a silica network is eventually established, forming a soft gel. When the sample orientation in the magnetic field is changed, a few silica strands links are broken and a fraction of the LC molecules is realigned. The field anneals the random disorder introduced by the aerosil up to a certain density beyond which, in the so-called stiff-gel regime, disordering effects completely dominate. At a fixed temperature in the isotropic phase, there is surface-induced order that is linearly proportional to the silica density.

Crystallization of Weakly Segregated Poly(styrene-b-ε-caprolactone) Diblock Copolymer in Thin Films

The surface morphology and crystallization behavior of a weakly segregated symmetric diblock copolymer, poly(styrene-b-ε-caprolactone) (PS-b-PCL), in thin films were investigated by optical microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM). When the samples were annealed in the molten state, surface-induced ordering, that is, relief structures with uniform thickness or droplets in the adsorbed monolayer, were observed depending on the annealing temperature. The polar PCL block preferred to wet the surface of a silicon wafer, while the PS block wet the air interface. This asymmetric wetting behavior led to the adsorbed monolayer with a PCL block layer having a thickness of around 4.0 nm. The crystallization of PCL blocks could overwhelm the microphase-separated structure because of the weak segregation. In situ observation of crystal growth indicated that the nucleation process preferred to occur at the edge of the thick parts of the film, that is, the relief structures or droplets. The crystal growth rate was presented by the time dependence of the distance between the tip of crystal clusters and the edge. At 22 and 17 °C, the average crystal growth rates were 55 ± 10 and 18 ± 4 nm/min, respectively. Nevertheless, at a low crystallization temperature (17 °C), the formation of a primary nucleus and subsequent growth in the adsorbed monolayer with a crystalline PCL layer of 4.0 nm were directly observed by real-time AFM. It was concluded that the primary nucleus formed at the polymer/substrate interface and the average growth rate for individual edge-on lamellae at 17 °C was about 12 nm/min. The nucleation mechanism in the adsorbed monolayer was discussed, and the influence of the polymer/substrate interface and weak segregation accounted for the nucleation process.

Structure and dynamics of a free liquid crystal surface at the N-to-SmA phase transition

Capillary wave dynamics at the free liquid crystal surface depends upon the viscosity which is anisotropic in the nematic phase and related to the structural order. We have compiled the dispersion relations governing the capillary wave dynamics on a liquid crystal surface. The surface-induced structural ordering of the liquid crystal compound 8OCB was determined by grazing incidence X-ray diffraction from the free surface in the vicinity of the nematic (N)-to-smectic A (SmA) phase transition. This allows us to identify the relevant dispersion relation, which is of decisive importance for the interpretation of dynamic X-ray photon correlation spectroscopy (XPCS) data taken at the N-to-SmA phase transition.

Continuous nematic anchoring transition due to surface-induced smectic order.

A continuous transition from tilted to homeotropic alignment at an interface is observed at a temperature T(a) for a nematic liquid crystal on cooling toward the nematic-smectic-A phase transition temperature. T(a) is found to depend on the treatment of the substrate. The behavior is examined theoretically in terms of a pair of competing easy axes (homeotropic and planar) and the tilt elasticity associated with the growth of surface-induced smectic order.

Surface ordering transitions at a liquid crystal-solid interface above the isotropic smectic-A transition.

The degree of orientational order induced by confining cylindrical surfaces is monitored via deuteron nuclear magnetic resonance linesplitting and linewidth above the smectic-A to isotropic phase transition. The orientational order strongly depends on the length of the surfactant coupling molecule, on the surface coverage, and on the liquid crystal. Continuous and stepwise growth of orientational order and surface-induced orientational order transitions found in the isotropic phase are explained in terms of a simplified model of surface-induced layering and molecular self-diffusion.

Observation of surface and bulk phase transitions in nematic liquid crystals.

The behaviour of liquid crystal (LC) molecules near a surface is of both fundamental and technological interest: it gives rise to various surface phase-transition and wetting phenomena, and surface-induced ordering of the LC molecules is integral to the operation of LC displays. Here we report the observation of a pure isotropic-nematic (IN) surface phase transition-clearly separated from the bulk IN transition-in a nematic LC on a substrate. Differences in phase behaviour between surface and bulk are expected, but have hitherto proved difficult to distinguish, owing in part to the close proximity of their transition temperatures. We have overcome these difficulties by using a mixture of nematic LCs: small, surface-induced composition variations lead to complete separation of the surface and bulk transitions, which we then study independently as a function of substrate and applied magnetic field. We find the surface IN transition to be of first order on surfaces with a weak anchoring energy and continuous on surfaces with a strong anchoring. We show that the presence of high magnetic fields does not change the surface IN transition temperature, whereas the bulk IN transition temperature increases with field. We attribute this to the interaction energy between the surface and bulk phases, which is tuned by magnetic-field-induced order in the surface-wetting layer.

Neutron magnetic tomography of ferrofluids

Magnetic flux distribution in ferrofluids has been evaluated by a new method based on Larmor precession of neutron spin and Fourier analysis of precession phases over the neutron wavelength spectrum. Using this method for a thin layer of magnetic fluid in an in-plane magnetic field, we detected an inhomogeneous distribution of induction. It is stronger near the non-magnetic fluid–metal interface. The surface layer covering ∼60% of the volume at low field H 1=10 Oe is shrunken to ∼25% at a stronger field H 2=200 Oe when the fluid is substantially magnetised. Microscopic mechanisms of surface-induced ordering are discussed.

Deuteron NMR study of molecular ordering in a holographic-polymer-dispersed liquid crystal.

Using deuteron nuclear magnetic resonance (NMR) and dynamic light scattering, we study the orientational order and dynamics of a BL038-5CB liquid-crystal mixture in a holographic polymer dispersed liquid-crystal material (HPDLC) as used for switchable diffractive optical elements. At high temperatures, where the liquid crystal is predominantly in the isotropic phase, the HPDLC deuteron NMR linewidth and transverse spin-relaxation rate T-12 are two orders of magnitude larger than in the bulk. The analysis shows that the surface-induced order parameter in HPDLC is significantly larger than in similar confining systems and that translational diffusion of molecules in the surface layer is at least two orders of magnitude slower than in the rest of the cavity. The unusual temperature dependence of T-12 upon cooling suggests the possibility of a partial separation of the 5CB component in the liquid-crystal mixture. The onset of the nematic phase in HPDLC occurs at considerably lower temperature than in the bulk and takes place gradually due to different sizes and different content of non-liquid-crystalline ingredients in droplets. Parts of the droplets are found isotropic even at room temperature and the structure of the nematic director field in the droplets is only slightly anisotropic. We point out the capability of NMR to detect the actual state of liquid-crystalline order in HPDLCs and to contribute in this way to the improvement of the switching efficiency of diffraction gratings.

Systematic calorimetric investigation of the effect of silica aerosils on the nematic to isotropic transition in heptylcyanobiphenyl.

The effect of confinement on liquid crystal phase transitions was investigated in mixtures of the liquid crystal heptylcyanobiphenyl with hydrophilic aerosils. The influence of the aerosil density on the nematic to isotropic transition was studied by adiabatic scanning calorimetry. Mixtures have been investigated with an aerosil content between rho(S)=0.1 and rho(S)=0.7, the latter being substantially higher than that investigated in previous studies with other liquid crystal-aerosil mixtures. The transitions in the examined mixtures exhibited an unusually large broadening, while the transition temperatures showed peculiar behavior. Notwithstanding, the transition (latent) heat behaved as could be expected on the basis of previous studies. The observed behavior can be explained, in the context of the elastic-strain approach usually employed to describe transition temperature shifts, by variations in the distribution of the radius of curvature R of the aerosil voids originating from sample preparation. It is important in this regard to separate the behavior of the transition temperatures and the transition (latent) heats, the former being influenced by the radius of curvature of the voids and the latter by the total void surface per unit volume. Three quantitative models were compared with experimental results. Both the pinned boundary layer and the random field model yield an evolution of transition temperatures not in agreement with experimental observations. More importantly, they predict a decrease of the pretransitional specific heat capacity, not supported by experiment. The difficulties with these models are avoided in a third, mean-field surface-induced order model.

Polymer network-induced ordering in a nematogenic liquid: a Monte Carlo study.

In this Monte Carlo study we investigate molecular ordering in a nematogenic liquid with dispersed polymer networks. The polymer network fibers are assumed to have rough surface morphology resulting in a partial randomness in anchoring conditions, while the fiber direction is assumed to be well defined. In particular, we focus on the loss of long-range aligning capability of the network when the degree of disorder in anchoring is increased. This process is monitored by calculating relevant order parameters and the corresponding 2H nuclear magnetic resonance spectra, showing that the aligning ability of the network is lost only for completely disordering anchoring conditions. Moreover, above the nematic-isotropic transition temperature surface-induced paranematic order is detected. In addition, for perfectly smooth fiber surfaces with homeotropic anchoring conditions topological line defects can be observed.

Disruption of surface-induced smectic order by periodic surface corrugations.

A Fréedericksz transition measurement is reported for a liquid crystal cell composed of surfactant-coated substrates. One substrate was locally scribed with the stylus of an atomic force microscope to create a nanoscopic grooved structure. The Fréedericksz threshold voltage was found to be smaller in the scribed region than in the unscribed region, indicating that the corrugated surface disrupts surface-induced smectic order, and that the effect grows toward the nematic-smectic-A transition temperature T(NA) in conjunction with the smectic correlation length.

Exact calculations of the paranematic interaction energy for colloidal dispersions in the isotropic phase of a nematogenic material.

In a recent paper [Phys. Rev. E 61, 2831 (2000)], Borstnik, Stark, and Zumer have studied the stability of a colloidal dispersion of micron-sized spherical particles in the isotropic phase of a nematogenic material. Close to the nematic transition, the attraction due to a surface-induced paranematic order can yield flocculation. Their calculation of the nematic-mediated interaction was based on an ansatz for the order-parameter profile. We compare it with an exact numerical calculation, showing that their results are qualitatively correct. Besides, we point out that in the considered regime, the exact interaction is extremely well approximated by a simple analytical formula which is asymptotically exact.

Surface-induced nematic and smectic order at a liquid-crystal-silanated-glass interface observed by atomic force spectroscopy and Brewster angle ellipsometry.

We have investigated nematic and smectic surface-induced ordering in the isotropic phase of the 4-cyano-4'-n-octylbiphenyl liquid crystal on silanated BK7 and LaSF glass using atomic force spectroscopy and Brewster angle ellipsometry. We have observed complete wetting of the silanated glass surfaces with the nematic phase when approaching the isotropic-nematic phase transition from above. Using the atomic force spectroscopy at the same interfaces, we have detected a significant presmectic ordering that is enhanced at the nematic-isotropic transition. We have observed the first, strongly adsorbed layer of liquid-crystalline molecules underneath this presmectic ordering. This first molecular layer is laterally inhomogeneous with typically 100 nm voids and covers approximately 70% of the surface. It is stable far above the clearing point and is responsible for the surface-memory effect. The results have been analyzed using Landau-de Gennes theory. The surface coupling energies of the nematic and smectic order have been determined, as well as the coupling energy between the nematic and smectic order.

Forces in the isotropic phase of a confined nematic liquid crystal 5CB.

Using a temperature controlled atomic force microscope, we have measured the temperature dependence of the force between a flat silanated glass surface and a silanated glass microsphere, immersed in the isotropic phase of the nematic liquid crystal 5CB (4'-n-pentyl 4-cyanobiphenyl). At separations of several nanometers, we observed a weak, short range attractive force of the order of 100 pN, which was increased by decreasing the temperature. The temperature dependence of the amplitude and the range of this attractive force can be described by a combination of van der Waals and a mean-field prenematic force due to the surface-induced nematic order. This is supported by ellipsometric study and allows for the determination of the surface coupling energy of 5CB on a silanated glass surface.