Anopore Membranes Research Articles

X-ray and dielectric measurements of the smectic- A -hexatic- B transitionin bulk and confined geometries.

We report x-ray and dielectric measurements on a liquid crystal exhibiting a smectic- A -hexatic- B (Sm- A -Hex- B ) transition in bulk and confined geometries. The confined geometry experiments have been performed by filling the compound, n -butyl 4(') - n -hexyloxybiphenyl-4-carboxylate, into Anopore membranes with 20 nm pore size. The surprising result seen in the x-ray studies on both the bulk and the Anopore samples is the presence of unusually strong second harmonic reflection arising from the smectic layers in both the Sm- A and Hex- B phases. In fact, the third harmonic reflection is also observed in the entire temperature range of the Hex- B phase and close to the transition in the Sm- A phase of the bulk sample, with the ratio of I(2qo) / I(qo), and I(3qo) / I(qo) being approximately 1/40 and 1/470, respectively (here I(qo), I(2qo), and I(3qo) correspond to the peak intensities of the first, second, and third harmonic reflections). These features point to the fact that the layering is much better defined in this system, unlike the usual description of pure sinusoidal mass density wave with a typical value of 10(-4) for I(2qo) / I(qo). Another notable feature observed is that whereas the extent of positional correlations of the molecules within the smectic layer, is comparable in the Sm- A phase of the bulk and Anopore samples, the correlations are reduced in the Hex- B phase. In the bulk as well as the Anopore case, the critical behavior of the positional correlations is found to be well described by a power-law expression with the exponent alpha being slightly larger for the Anopore sample. It is also seen that the weak first order nature of the transition seen in the bulk, gets weakened further, perhaps becoming second order. Despite such significant differences between the bulk and Anopore samples, the dynamics associated with the director fluctuations is only slightly influenced by confinement.

Comparative x-ray and dielectric measurements of smectic A-smectic-C* transition in bulk and confined geometries.

Comparative x-ray and dielectric measurements have been made on a liquid crystal exhibiting a smectic-A-chiral-smectic-C (smectic-C*) transition in bulk and confined geometries. It is observed that confining the material in Anopore membranes having 200-nm pore size leads to the following features: (1) the temperature dependence of the x-ray layer spacing shows a qualitatively different behavior, (2) in the smectic-A phase the soft mode relaxation frequency increases by a factor of 2.5, and (3) in the smectic-C* phase the relaxation frequency of the Goldstone mode increases dramatically by as much as 400 times, perhaps owing to a partial unwinding of the helix by the surface induced field.

Direct observation of membrane cleaning via rapid backpulsing

Direct visual observation (DVO) was used to view membrane surfaces at different times in crossflow microfiltration of yeast suspensions with rapid backpulsing at varied backpulsing duration and pressure. The DVO photos show that the membranes are more effectively cleaned by longer backpulse durations and higher backpulse pressures. However, a tradeoff exists, as longer and stronger backpulses also have greater permeate loss during each backpulse. The net flux was increased two-fold and four-fold with backpulsing under optimum conditions for 0.2 μm Anopore and cellulose acetate membranes, respectively, fouled by 0.05 g/L washed yeast cells. Shorter, stronger backpulses resulted in higher net fluxes than did weaker, longer backpulses after 2 h of filtration.

Construction and Characterization of a Nanowell Electrode Array

Gold nanowell electrode arrays, with a nanowell depth of approximately 600 nm, have been prepared using commercially available anopore membranes as templates. Electrodes were prepared by forming a copper film on anopore, followed by electrodeposition of gold throughout the pore and finally removal of the copper to expose the nanowells. The electrodes were characterized by atomic force microscopy, scanning electron microscopy, and electrochemical methods. Dodecanethiol self-assembled monolayers were found to passivate the electrode. C. R. Martin coined the term “template synthesis” to describe a method in which the pores of a nanoporous membrane act as templates for the synthesis of nanostructures of a desired material. 1 As the pores within these membranes are nearly cylindrical and of uniform diameter, monodisperse nanocylinders of a desired material can be obtained. This method has been applied to the formation of nanowires, 2 metal nanostructures 3 and hollow metal tubules, 4 conductive 5 and insulating polymers, 6 and semiconductors. 7 (For a review of general template-based methods for the preparation of nanomaterials see ref 1.) The formation of gold metal nanostructures by Martin’s group using anopore disks as a template 3 was accomplished by sputtering a thin layer of Ag onto one side of the membrane in order to form an electrode, followed by the galvanostatic deposition of Ag to seal the pores and potentiostatic deposition of Au into the pores on top of the Ag microcylinders. Nitric acid was then used to dissolve away the Ag, leaving Au nanostructures. In this report we discuss a modification of this method which allows us to form an array of Au nanoelectrodes, 200 nm in diameter and which have a controlled depth, embedded within an anopore template. The nanoelectrode array was characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), and electrochemical methods. Passivation of the electrodes with alkanethiol molecules in a self-assembled monolayer (SAM) was also investigated. Anodisc membranes obtained from Whatman were used as the template material for gold nanoelectrode fabrication.

Behavior of mesogenic molecules deposited at the alumina-air interface: a deuteron NMR study.

Thin molecular depositions of 4(')-pentyl-4-cyonobiphenyl (5CB) mesogenic molecules are investigated via quadrupole-perturbed deuteron nuclear magnetic resonance (DNMR) spectroscopy. Uniform and controlled thickness molecular surface depositions are prepared on the inner cylindrical surfaces of Anopore membranes by the solvent-evaporation technique. As a result, 5CB molecules are found in two different configurations: a bulklike one with parallel axial arrangement, and a surface one with planar radial arrangement. If the 5CB surface coverage exceeds c approximately 0.35, only the bulk state is present. In the coverage range between 0.015 and 0.35, the bulklike state and the surface layer coexist, conforming to a typical dewetting scenario. Below c approximately 0.015, only the surface layer is present. The dilution of the surface deposition with decreasing coverage is manifested as an increase in the DNMR doublet frequency splitting. The surface orientational order parameter Q, the surface biaxiality eta, and the diffusion coefficient D(S) are determined from the DNMR spectral patterns obtained at different sample orientations in the external magnetic field. These angular patterns prove that in highly diluted surface depositions the molecules lie flat on the surface. However, they are not frozen and their molecular axes rapidly reorient on the DNMR measurement time scale, typically 10(-4) s, while remaining confined to the surface. Simultaneously, molecules diffuse over the surface with a surface diffusion constant on the order of 10(-11) m(2) s(-1). Such molecular diffusion is responsible for an effective biaxiality on the DNMR time scale. However, an inherent biaxiality cannot be completely ruled out and thus may play a minor role. The surface phase has a two-dimensional (2D) gas character with some (possible) indicators of 2D-liquid properties.

Dynamics of the two-dimensional melting transition of a liquid crystal confined in Anopore membranes

Dielectric measurements on a liquid crystal exhibiting the smectic A-crystal B transition and confined to Anopore membranes having 20 and 200 nm pore sizes are reported. The studies reveal that compared with the bulk, the confined material shows a decrease in transition temperature. More importantly, the confinement leads to a slowing of the relaxation mechanism by about three orders of magnitude.

Liquid Crystal Wetting in Cylindrical Pores

We have performed a calorimetry and deuterium nuclear magnetic resonance study for a member of the liquid crystal homologous series, n.O m, in bulk and confined to the 0.2 μm cylindrical pores of aluminum oxide Anopore membranes. It was shown in an ellipsometry study by Lucht and coworkers that this material shows a prewetting nematic transition at an air interface. Thermodynamically, at the Anopore solid pore walls these liquid crystals also exhibit unique wetting properties: a distinct signature in the specific heat, a “bump” at a temperature approximately 0.5 K above T NI is found. We believe that such signature is related to the manner that the nematic liquid crystal wets the solid surfaces.

Dynamics of Librational Mode of Nematic Liquid Crystal Confined in Cylindrical Pores

Dielectric spectroscopy was applied to investigate the dynamic properties of nematic liquid crystal - 5CB confined in 2000 cylindrical pores of Anopore membranes with the molecules oriented perpendicular to the pore wall with lecithin treated surface. This allows the investigation of the librational (tumbling) mode since the probing electric field is parallel to the pore axis. We found that the dynamics of the librational mode is totally different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. The interpretation of the temperature dependence of relaxation times in nematic phase needs the involvement of the temperature dependence of orientational order parameter. In the supercooled state, the temperature dependence of the librational mode did not follow the behavior observed in the nematic phase and shows a sharp increase while the temperature is decreased that is mainly determined by the variations of viscosity.

Comparison between chemical and isotopic measurements of biological nitrate utilization: further evidence of low new-production levels in the equatorial Pacific

New-production (nitrate uptake) rates in the equatorial Pacific were estimated by parallel measurements of nitrate disappearance from sea water using a colorimetric method and of 15N-labelled nitrate (15NO3 −) incorporation into particulate organic nitrogen (PON) collected on GF/F filters (net nitrate uptake, conventional 15N-tracer method) and Anopore (0.2 μm) membranes. Regression analyses of 74 sample pairs gathered during 12 and 24 h productivity experiments revealed a significant positive relationship between decreasing nitrate level and 15NO3 − accumulation into PON retained on GF/F filters, but the slopes of Model I and Model II regression lines were 1.18 and 1.29, respectively, suggesting that 15 to 22% of 15NO3 − removed from the dissolved fraction were lost to another N-pool. Two possible avenues for the missing 15NO3 − have been examined: uptake by submicron particles passed through the GF/F filters, and loss as dissolved organic nitrogen (DON). Nitrate uptake by small cells not recovered on GF/F filters, could be safely eliminated as a cause of loss, since 15NO3 − uptake rates obtained from 15N entering PON collected on GF/F filters agreed well with those obtained from 15N entering PON collected on Anopore membranes (32 sample pairs). Inspection of the DON pool of 0.2 μm filtrates for excess-15N enrichment (20 samples) revealed that in nitrate-rich waters (equatorial upwelling between 1°N and 10°S), loss of 15NO3 − as DO15N accounted for 2) were probably erroneous; (2) the use of GF/F filters does not result in an underestimation of new production, although it was found to underestimate PON concentrations by up to 60%; (3) in the equatorial upwelling area (1°N to 10°S), which has high ambient nitrate levels (>2000 nmol l−1) but only slight changes in concentration (0 to 80 nmol l−1 d−1), new production is more accurately estimated by the isotopic method than by the chemical method; (4) in subtropical oligotrophic waters (from 11°S southward) with low ambient nitrate levels (0 to 100 nmol l−1), both procedures are appropriate as long as nitrate removal per incubation period is >3 nmol l−1 (lower rates are only detectable with the isotopic method); (5) the traditional 15N-tracer technique does not substantially underestimate net new-production in the equatorial Pacific, and failure to account for the loss of 15NO3 − as DON, i.e. to estimate gross nitrate uptake (gross uptake = net uptake + 15N loss) tends to underestimate new production on an average by only 10%. Overall, the apparent low level of new production in the nitrate-rich area of the central equatorial Pacific seems to be a fact, and may be ascribable to other nutrient (macro and micro) deficiencies and/or to intense in situ recycling of ammonium and nitrate (regenerated production) rather than to inaccurate nitrate uptake rates measured with the classical 15N-tracer technique.

Substrate-induced order in the isotropic phase of a smectogenic liquid crystal: A deuteron NMR study

Using deuteron nuclear magnetic resonance, we study the pretransitional wetting behavior of the smectogenic and nematogenic liquid crystal 8CB in the isotropic phase confined to 200 nm diameter channels of Anopore membranes. The channels were treated with aliphatic acid to impose homeotropic alignment. Just above the nematic-isotropic transition we observe an enhanced line splitting compared to non-smectogenic homologs such as 5CB; moreover, the line splitting is characterized by a discontinuity of its slope at approximately 5 K above the transition temperature. This deviation from a purely nematic wetting scenario is attributed to the substrate-stabilized smectic order within the wetting film, which appears to be stimulated by the proximity of the smectic-$A$ phase: in 8CB, the smectic-$A$-nematic transition occurs approximately 7 K below the nematic-isotropic transition. We interpret the data in terms of a semimicroscopic Landau--de Gennes type model, which allows us to estimate the thickness of the presmectic film---which consists of 2 bilayers---as well as the parameters of the surface interaction and the effective smectic-nematic coupling.

The Influence of Boundary Conditions and Surface Layer Thickness on Dielectric Relaxation of Liquid Crystals Confined in Cylindrical Pores

AbstractDielectric spectroscopy was applied to investigate the dynamic properties of liquid crystal octylcyanobiphenyl (8CB) confined in 2000 Å cylindrical pores of Anopore membranes with homeotropic and axial (planar) boundary conditions on the pore walls. Homeotropic boundary conditions allow the investigation of the librational mode in 8CB by dielectric spectroscopy. We found that the dynamics of the librational mode is totally different from the behavior observed in investigations of relaxation due to reorientation of molecules around their short axis. The interpretation of the temperature dependence of relaxation times and of the dielectric strength of the librational mode needs the involvement of the temperature dependence of orientational order parameter. For samples with axial boundary conditions, layers of LCs with different thickness were obtained on the pore walls as a result of controlled impregnation of porous matrices with 8CB from solutions of different liquid crystal concentration. The process due to rotation of molecules around their short axis with single relaxation time observed for bulk 8CB is replaced by a process with a distribution of relaxation times in thin layers. This relaxation process broadens with decreasing layer thickness.

Deuteron NMR relaxometry applied to confined liquid crystals

We discuss the capability of deuteron nuclear magnetic resonance (NMR) spectroscopy and relaxometry to reveal molecular ordering and dynamics in confined liquid crystals. The attention is focused on the high-temperature phase above the nematic-isotropic transition, which is — in the absence of the long-range orientational order — very suitable for the study of surface interactions. Deuteron NMR spectra and relaxation rates are presented for two representatives of confined liquidcrystal systems: 8CB in cylindrical cavities of Anopore membranes and 5CB with an embedded polymer network. A substantial increase in the transverse spin relaxation rate, stimulated by the surface-induced order in enclosures, has been observed. In cylindrical cavities, it exhibits a strong temperature dependence on approaching the phase transition, whereas in the polymer network dispersion it is temperature-independent. The increase of T2−1 provides information on the effect of spatial constraints on molecular mobility and on the surface orientational order parameter. Using deuteron relaxometry, one can measure the degree of orientational order in the isotropic phase not only in cylindrical but also in spherical cavities and enclosures of irregular shape, where the standard approach based on quadrupolar splitting of the NMR spectrum fails.

Influence of Confinement on Dynamics of Molecular Motion and Collective Modes of Liquid Crystal Dispersed in Pores

We present the results of investigations of the influence of the confinement, interface, porous matrix structure, pore size and shape on the dynamic behavior of nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes) by photon correlation and dielectric spectroscopies. Investigations of LC in cylindrical pores together with studies in random porous matrices, makes it possible to separate the role of random structure and domain formation from the contributions due to existence of LC - solid pore wall interface and pure finite size effect in different relaxational processes. We suggest that slow relaxational process observed in photon correlation experiments is due to the formation of interfacial layers on the pore walls rather than due to the dynamics of domains.

The Dielectric Properties of Nematic Liquid Crystal, 5CB Confined to Treated and Untreated Anopore Membranes

In recent years a substantial amount of research has been devoted under - standing of anchoring and ordering mechanisms of liquid crystal molecules near solid surfaces. The interaction of a liquid crystalline compound with a solid surface depends on the nature of both the surface and liquid crystal material. The dielectric measurements in untreated Anopore membranes confirm the planar axial order of nematic molecules; in membranes treated by decanoic acid, a polar-radial or polar - polar orientation of the molecules is very likely, although a slight escaped director structure at least at low temperatures seems to be indicated in the dielectric data.

Application of Broad-Band Dielectric Spectroscopy for Investigations of Liquid Crystal - Porous Media Microcomposites

ABSTRACTWe applied ultra broad-band dielectric spectroscopy in the frequency range from 10–3 Hz to 109 Hz to investigate the effect of size, shape and volume fraction of the pores in the porous matrices on the dielectric properties of liquid crystals (LC) dispersed in these matrices. Measurements in such a broad frequency range make it possible to obtain detailed information on the important aspects of the electrical behavior of heterogeneous materials such as: conductivity, surface polarization, and influence of confinement on dynamics of molecular motion of polar molecules forming LC. We investigated alkylcyanobiphenyls in the isotropie, nematic and smectic phases dispersed in porous glasses (average pore sizes - 100 Å and 1000 Å) which have randomly oriented, interconnected pores, and anopore membranes (pore diameters - 200 Å and 2000 Å) with parallel cylindrical pores. Dispersion of LC resulted in qualitative changes of their dielectric properties. Analysis of broad-band dielectric spectra shows that in organic (LC) - inorganic (porous matrix) heterogeneous composites conductivity plays an important role at F <1 Hz. We observe the appearance of new dielectric modes: a very slow process with characteristic frequency ≃ (1 – 10) Hz and a second process in frequency range about (103 - 106) Hz. The slow process arises due to the relaxation of interfacial polarization at pore wall - LC interface. The origin of this could be due to absorption of ions at the interface. Another possibility is the preferential orientation of the permanent dipoles at pore surface. The second new mode is due to the hindered rotation of the molecules near the interface. Additionally we observed two bulk like modes due to the rotation of the molecules around their short and long axii which are modified.

Molecular ordering in microconfined liquid crystals: An infrared linear dichroism study

Abstract The molecular arrangement of 5CB confined within the cylindrical pores of Anopore membranes was characterized by means of the IR-order parameter obtained from linear dichroism measurements of selected IR absorption bands. The treatment of the experimental data includes a local field correction extended to the twisted nematic configuration, yielding order parameters increased by about 30% in comparison with the uncorrected data. The nematic director of 5CB aligns along the pore axes, whereas in lecithin coated Anopore channels, the local nematic director is oriented approximately radially due to the perpendicular anchoring of the 5CB molecules at the pore wall. Doping of 5CB with the chiral agent CB15 yields local nematic directors tilted with respect to the pore axes. The average tilt angle increases up to about 40° at a fraction of CB15, x cb15 = 0.25 (w/w). These results are discussed in terms of the conical helicoidal and alternatively the radially twisted axial arrangement of the LC molecules within submicrometer cylindrical cavities.

Optical Properties of Microconfined Liquid Crystals

This paper is concerned with studies of birefringence and optical activity of the twisted nematic mixture 5CB/CB15 confined to cylindrical pores of ANOPORE (AP) membranes. Both birefringence and op...

Atomic force microscope studies of membranes: Surface pore structures of Cyclopore and Anopore membranes

Non-contact atomic force microscopy has been used to investigate the surface pore structure of Cyclopore and Anopore microfiltration membranes in air. Three Cyclopore membranes and three Anopore membranes of different pore sizes were studied. Excellent high resolution images were obtained. Analysis of the images gave quantitative information on the surface pore structure, in particular the pore size distribution. Non-contact AFM is an excellent means of obtaining such information for microfiltration membranes.

Model Systems for Liquid Crystal Based Dispersed Heterogeneous Materials

AbstractPorous matrices with determined pore size, volume fraction and structure can be used as model systems to understand physical properties of dispersed liquid crystals (LC) - material important for different applications. Using photon correlation spectroscopy and dielectric spectroscopy we investigated dynamic properties of nematic liquid crystal dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses with average pore sizes of 100 Å and 1000 Å) and parallel cylindrical pores (Anopore membranes with pore diameters of 200 Å and 2000 Å). Since the structural characteristics of these matrices are nearly independent of the temperature, all observable effects due to temperature changes can be attributed to the change in the physical properties of the second component (bC). The spatial confinement and a highly developed interface in porous matrices have a strong influence on the optical and dielectric properties of confined LC which is resulted in: appearance of at least two new dielectrically active modes, absent in the bulk and existence of slow glass-like relaxational process detected in both dielectric and photon correlation experiments.

Photon Correlation Spectroscopy of Liquid Crystals Confined in Porous Matrices with Different Structure and Pore Size

ABSTRACTWe present the results of photon correlation spectroscopy investigations of the influence of confinement, interface, porous matrix structure, pore size and shape on the dynamic behavior of nematic liquid crystals (LC) dispersed in porous matrices with randomly oriented, interconnected pores (porous glasses) and parallel cylindrical pores (Anopore membranes). Investigations of LC in cylindrical pores together with studies in random porous matrices, makes it possible to separate the role of random structure and domain formation from the contributions due to existence of LC - solid pore wall interface and pure finite size effect in relaxation of order parameter or director fluctuations. In the temperature range below nematic - isotropie phase transition temperature we observed two overlapping relaxational processes which are satisfactorily described by the decay functionf(q,t) =a·exp(–t/τ1) + (1–a)·exp(–xz), wherex=ln(t/τ0)/ln(τ2/τ0) and τ0= 10−8s. For LC in 100 Å random pores the second term describing the slow process dominates, whereas for 200 Å and 2000 Å cylindrical pores as well as 1000 Å random pores the contribution from the first term (fast process) is more visible. Since the slow relaxational process which does not exist in the bulk LC and broad spectrum of relaxation times (10−6- 10)s appear not only for LC in random pores but in cylindrical as well, we conclude that differences in dynamical behavior of confined LC from that in the bulk are mainly due to the existence of the interface.