Molecular dynamics simulations of liquid crystal molecules on a polyimide monolayer

Preliminary results are presented on the molecular dynamics simulations of alignment of the liquid crystal molecule, 4-n-octyl-4'-cyanobiphenyl (8CB), on a polyimide (pyromelltic dianhydride-p-phenylene diamine) oligomer monolayer. We actually simulated a three-layer system, i.e., liquid crystal molecule/polyimide oligomer/a basal plane of graphite. First, simulations of the oligomers adsorbed on graphite were done in order to obtain reasonable adsorption structures, as the pre-stage simulation of the three-layer system. Then, by placing a liquid crystal layer on top, the three-layer system was simulated. The stable liquid crystal alignment direction on the polyimide monolayer was found roughly to be the polyimide chain direction with zero pretilt in this combination of liquid crystal and polymer materials. The calculated adsorption energy of an 8CB molecule to the polyimide monolayer was 128 kJ mol^-1 and the carbonyl group of the polyimide was the main adsorption site.     

Photo-reversible liquid crystal alignment using azobenzene-based self-assembled monolayers: comparison of the bare monolayer and liquid crystal reorientation dynamics

Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.     

Physisorbed water layer formation on fully hydroxylated mesoporous silicas

Kinetic adsorption isotherms were obtained by gravimetric determination of water adsorption into fully hydroxylated mesoporous silicas using samples exposed to controlled humidity air at 22+/-2 degrees C. Twenty kinetic isotherms at several relative humidities (11, 33, 43, 51, 75, and 85%) were obtained with 11 different batches of silica using this simple procedure to obtain quantitative information on the formation of H2O adsorbates. The H2O surface concentrations obtained from the plateau data of individual kinetic adsorption isotherms at 43 and 51% RH, typically precise to about +/-1%, show that a complete monolayer is formed with negligible second-layer adsorption at these relative humidities. This monolayer has a surface concentration of 7.68+/-0.30 micromol H2O/m2, which is lower than the quasi-equilibrium concentration at these relative humidities obtained by the conventional equilibrium-isotherm procedure. Comparison with the Kiselev-Zhuravlev concentration of silanol groups on fully hydroxylated silicas (7.6+/-0.8 micromol SiOH/m2) confirms 1:1 H2O:SiOH stoichiometry of this monolayer. The presence of partial-layer structures at 2.85+/-0.1 and 5.7+/-0.1 micromol H2O/m2 is suggested by isotherms at 11 and 33% RH, respectively, while a bilayer at approximately 14+/-1 micromol H2O/m2 is suggested by kinetic isotherms at 75 and 85% RH.     

Thermal properties of some fully aromatic thermotropic liquid crystal polyesters

Wholly aromatic liquid crystalline main chain polyesters derived from terephthalic acid, phenyl- or (1-phenylethyl)hydroquinone modified with either 3,4′- or 4,4′-dicarboxydiphenylether and p-hydroxybenzoic acid, have been prepared by acidolysis and thermally investigated. All prepared polyesters exhibit excellent thermal stability up to about 400°C, however, the (1-phenylethyl)hydroquinone polyesters generally showed lower stability. Melting points could be decreased to around 200°C without any decrease in the thermal stability or the nematic range.     

X-ray investigations of Langmuir-Blodgett multilayer films of side-chain liquid crystal copolymers

X-ray reflectivity studies on Langmuir-Blodgett multilayer films of side-chain liquid crystal polymers are reported. The films have a high degree of lamellar order. The layer periodicity is independent of the type of monolayer deposition, implying a reorientation of the side group mesogens following the deposition process. X-ray reflectivity from thin films displays subsidiary maxima permitting a quantitative measure of the change in side-chain density between multilayer and monolayer. A unit cell density profile is calculated for thick films assuming a symmetric unit cell.     

A novel photoalignment film for ferroelectric liquid crystal based on self-assembled monolayer method

A novel strategy based on self-assembly technology was devised for design of photosensitive material as a ferroelectric liquid crystal (FLC) alignment layer. This development offers new tools for the study and control at the molecular level of the interaction of FLCs with solid surfaces. The photoreactive material was self-assembled to the substrate by covalent bond linkage due to a special chemical adsorption reaction. Through ester bond linkage, a cyano group with strong polarity was introduced to be terminus of the film. Under irradiation of linearly polarised ultraviolet light, an optically anisotropic self-assembled film was easily obtained. The irradiated film was demonstrated to result in homogenous alignment of FLC by optical transmittance measurements and polarising optical microscopy images of a FLC cell at different rotation angles. The alignment quality of the FLC on this self-assembled monolayer film is comparable to that of commercial rubbed polyimide film. Furthermore, it was also found that the fine alignment of the FLC may be related to the smoothness of the self-assembled film surface owing to its polar end.     

Properties of a water layer on hydrophilic and hydrophobic self-assembled monolayer surfaces: A molecular dynamics study

The microscopic behaviors of a water layer on different hydrophilic and hydrophobic surfaces of well ordered self-assembled monolayers (SAMs) are studied by molecular dynamics simulations. The SAMs consist of 18-carbon alkyl chains bound to a silicon(111) substrate, and the characteristic of its surface is tuned from hydrophobic to hydrophilic by using different terminal functional groups ( CH 3 , COOH). In the simulation, the properties of water membranes adjacent to the surfaces of SAMs were reported by comparing pure water in mobility, structure, and orientational ordering of water molecules. The results suggest that the mobility of water molecules adjacent to hydrophilic surface becomes weaker and the molecules have a better ordering. The distribution of hydrogen bonds indicates that the number of water-water hydrogen bonds per water molecule tends to be lower. However, the mobility of water molecules and distribution of hydrogen bonds of a water membrane in hydropho- bic system are nearly the same as those in pure water system. In addition, hydrogen bonds are mainly formed between the hydroxyl of the COOH group and water molecules in a hydrophilic system, which is helpful in understanding the structure of interfacial water.     

Orientational control of liquid crystal molecules via carbon nanotubes and dichroic dye in polymer dispersed liquid crystal

Polymer dispersed liquid crystals (PDLCs) using nematic liquid crystal and photo-curable polymer (NOA 65) were prepared by polymerisation-induced phase separation technique, in equal ratio (1:1) of polymer and liquid crystal (LC). We demonstrate that doping of small amount (0.125%, wt./wt.) of multiwall carbon nanotubes (CNTs) and orange azo dichroic dye in PDLC generously controlled the molecular orientation, dynamics of LC in droplet and size of droplets. The effects of multiwall CNTs and dye on PDLCs were studied in terms of transition temperature, droplet morphology, transmittance characteristic, contrast ratio and response time. The results exhibited that the values of the threshold electric fields were reduced from 8 V/µm (pure PDLC) to 1.18 and 1.72 V/µm, doped with multiwall CNTs and dye, respectively. The CNTs-doped PDLC shows faster switching response as compared with pure PDLC and dye-doped PDLC. However, dye-doped PDLC shows much higher contrast among all PDLC samples. Further, the results also illustrate that the birefringence value of LC in PDLCs was changed with doping of CNTs and dye.     

AFM investigations of a glassy cholesteric liquid crystal with hydrophobic aerosil particles

AFM investigations of a glassy heterogeneous system consisting of an oligomeric cholesteric liquid crystal and the hydrophobic aerosil R812 were carried out. With increasing aerosil concentration, a suppression of the characteristic cholesteric surface pattern was observed. Typical separated aerosil aggregates appear in the samples. Their size and form change from small lumps through bigger rod-like entities to large crystallite-like aggregates of aerosil particles. This matches with observations of light scattering of systems with low molecular mass liquid crystals and of the memory effect. The pitch of the cholesteric fingerprint pattern slightly decreases with increase in the aerosil concentration.     

A novel process for holographic polymer dispersed liquid crystal system via simultaneous photo-polymerization and siloxane network formation

Formation of transmission holographic polymer dispersed liquid crystal gratings was studied for matrix components of trimethylolpropane triacrylate:trimethoxysilylmethyl methacrylate:1-vinyl-2-pyrrolidone (reactive diluent) in the range from 80:10:10,wt% to 10:80:10,wt% and E7 as nematic liquid crystal under the irradiation with Nd-YAG laser (532,nm). The optimum concentration of E7 in the recording solution was 35,wt% (65,wt% of matrix components) in presence of small amounts of radical photo-initiator system (Rose Bengal 0.05,wt%, NPG 0.1,wt%). When the concentration of methacrylate in matrix components was low (< 30%), gratings with apparently high diffraction efficiency could be fabricated both for trimethoxysilylmethyl methacrylate (cross-linkable by hydrolysis) and trimethylsilylmethyl methacrylate (non-cross-linkable by hydrolysis), although transiently high initial diffraction efficiency was observed by the non-equilibrium initial photo-polymerization of cross-linking components. Distinct difference was seen at higher concentration (> 50,wt%) of the methacrylates. Contrary to that gratings with reasonably high and stable diffraction efficiency were successfully fabricated with trimethoxysilylmethyl methacrylate, gratings with only low diffraction efficiency were obtained for non-cross-linkable trimethylsilylmethyl methacrylate. With higher concentration of photo-sensitizer and photo-initiator (0.2,wt%; 1.0,wt%), and shorter irradiation time, gratings with high diffraction efficiency could be fabricated only for the photo-initiator system of 3,3′-carbonylbis(7-diethylaminocoumarin) and diphenyliodonium hexafluorophosphate with shorter induction period (∼174,s). Grating with diffraction efficiency of 72% was obtained with trimethoxysilylpropyl acrylate (80,wt% in the matrix component) and 35,wt% E7. By increasing the concentration of diphenyliodonium hexafluorophosphate to 2,wt%, diffraction efficiency increased to 85% and induction period was shortened to 129,s, and low volume shrinkage of 8% was attained via simultaneous radical cross-linking of trimethylolpropane triacrylate and siloxane network formation of trimethoxysilyl groups of trimethoxysilylmethyl acrylate by atmospheric moisture catalyzed by cationic species produced from the initiator system. In SEM morphology, although gratings formed with high concentration of trimethoxysilylpropyl acrylate had some cracks in polymer matrix, the largest grating spacing indicating the lowest volume shrinkage and very regular and well-defined gratings were observed.     

The wetting-dewetting transition of monolayer water on a hydrophobic metal surface observed by surface-state resonant second-harmonic generation

The isothermal adsorption and desorption of monolayer water on a Ag(110) surface in the temperature range of 130-137 K were characterized by monitoring second-harmonic (SH) generation from the silver surface. The SH intensity resonantly enhanced by the silver surface-state transition is highly sensitive to the amount of silver surface area covered by water and allows the observation of an abrupt change in the adsorption/desorption behavior at 133.5 K. At temperatures below 133.5 K water wets the Ag surface in a two-dimensional structure with a measured desorption energy of 25.0 (+/-3.3) kJ/mol. At temperatures greater than 133.5 K water desorbs from three-dimensional clusters with a measured desorption energy of 48.3 (+/-2.2) kJ/mol, in agreement with temperature-programmed desorption measurements. This wetting-dewetting transition of water adsorbed on the silver surface at 133.5 K is supported by classical nucleation theory calculations.     

Thermodynamics of micelle formation in water, hydrophobic processes and surfactant self-assemblies

The critical micelle concentration (c.m.c.) for four cationic surfactants, alkyl-trimethyl-ammonium bromides, was determined as a function of temperature by conductivity measurements. The values of the standard free energy of micellisation DeltaG degrees(mic) at different temperatures were calculated by using a pseudo-phase transition model. Then, from the diagram (-DeltaG degrees(mic)/T)=f(1/T), the thermodynamic functions DeltaH(app) and DeltaS(app) were calculated. From the plots DeltaH(app)=f(T) and DeltaS(app) = f(ln T) the slopes DeltaC(p) = n(w(H))C(p,w) and DeltaC(p)=n(w(S))C(p,w) were calculated, with the numbers n(w(H)) and n(w(S)) negative and equal and therefore defined simply as n(w). The number n(w)<0, indicating condensed water molecules, depends on the reduction of cavity that takes place as a consequence of the coalescence of the cavities previously surrounding the separate aliphatic or aromatic moieties. The analysis, based on a molecular model consisting of three forms of water, namely W(I), W(II), and W(III), respectively, was extended to several other types of surfactants for which c.m.c. data had been published by other authors. The results of this analysis form a coherent scheme consistent with the proposed molecular model. The enthalpy for all the types of surfactant is described by DeltaH(app)= -3.6 + 23.1xi(w)-xi(w)C(p,w)T and the entropy by DeltaS(app)= +10.2+428xi(w)-xi(w)C(p,w) ln T where xi(w)= |n(w)| represents the number of molecules W(III) involved in the reaction. The term Deltah(w)= +23.1 kJ mol(-1) xi(w)(-1) indicates an unfavourable endothermic contribution to enthalpy for reduction of the cavity whereas the term Deltas(w)= +428 J K(-1) mol(-1) xi(w)(-1) represents a positive entropy contribution for reduction of the cavity, what is the driving force of hydrophobic association. The processes of non polar gas dissolution in water and of micelle formation were found to be strictly related: they are, however, exactly the opposite of one another. In micelle formation no intermolecular electronic short bond is formed. We propose, therefore, to substitute the term "hydrophobic bond" with that of "hydrophobic association".     

Hydrogel formation via hybridization of oligonucleotides derivatized in water-soluble vinyl polymers

Succinimido-copolymers, poly(N,N-dimethylacrylamide-co-N-acryloyloxysuccinimide)s, were coupled with 5′-terminal-amino-modified oligodeoxyribonucleotides (ODNs) to produce water-soluble copolymers partially derivatized with ODNs in their side chains. The mixing and thermal melting measurements of dilute mixed aqueous solutions of an ODN-derivatized copolymer and their complementary ODN and mixed solutions of complementary ODN-derivatized copolymers were monitored by ultraviolet spectroscopy. The results showed that hybrids were formed with their complementary ODNs at room temperature, but dissociated at high temperature. Based on the hybridization between complementary base pairs of nucleic acids and its thermal dissociation characteristics, two types of thermoresponsive hydrogels were prepared: (1) a hydrogel formed via hybridization between an oligodeoxythymidylate (oligoT)-derivatized copolymer and an oligodeoxyadenylate (oligoA), and (2) a hydrogel formed by hybridization between complementary oligoT- and oligoA-derivatized copolymers. Thus, selfassociation due to specific intermolecular hydrogen bonding between nucleic acid base pairs enabled the preparation of a novel thermoresponsive hydrogel.     

Weakly polar liquid crystal dispersed with hydrophobic and hydrophilic aerosils: a broadband dielectric study

Complex dielectric permittivities for parallel orientations of the director n, with the probing electric field E (E || n), of the weakly polar liquid crystal (LC) 4,4'-di-n-hexylazoxybenzene (D6AOB) dispersed with hydrophobic and hydrophilic aerosils, have been measured in the frequency range 20 Hz to 30 MHz. In the nematic phase of D6AOB dispersed with hydrophobic and hydrophilic aerosils, significant changes in the dielectric properties are observed. An additional slow process due to the restricted motion of the molecules in surface layers is observed for hydrophilic samples. The slow process is absent in the hydrophobic samples suggesting that in the LC-hydrophilic aerosils a surface layer is formed at the aerosil interface. The process does not depend on temperature and has a wide distribution of relaxation times. The bulk-like rotation around the short axis for D6AOB dispersed with hydrophilic aerosils has been modified significantly. It seems that this could be due to an overlap between the main bulk-like process and a hindered rotation of molecules around their long axes for molecules that are oriented perpendicular to the probing electric field.     

Characterisation of polymer wall formation in nematic liquid crystal devices

A phase-sensitive measurement technique is proposed to characterise polymer wall formation in a nematic liquid crystal cell. The impact of photopolymerisable monomer concentration and curing time on the electro-optical properties of polymer-wall LC cells was studied. The experimental results indicate that optimum curing conditions for constructing the polymer wall in an LC cell can be easily achieved by means of the proposed method. The structure of the polymer walls was also investigated using scanning electron microscopy and a polarising optical microscope.     

Dynamic in situ characterization of organic monolayer formation via a Novel substrate-mediated mechanism

Ultrahigh vacuum scanning tunneling microscopy data investigating octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are presented. The irregular monolayer pattern exhibits a 60 A length scale. Formation of the octylsilane monolayer relaxes the Au(111) 23 x square root3 surface reconstruction and ejects surface Au atoms. Au adatom diffusion epitaxially extends the Au(111) crystal lattice via step edge growth and island formation. The chemisorbed monolayer covers the entire Au surface at saturation exposure. Theoretical and experimental data suggest the presence of two octylsilane molecular adsorption phases: an atop site yielding a pentacoordinate Si atom and a surface vacancy site yielding a tetracoordinate Si atom. Theoretical simulations investigating two-phase monolayer self-assembly dynamics on a solid surface suggest pattern formation results from strain-induced spinodal decomposition of the two adsorption phases. Collectively, the theoretical and experimental data indicate octylsilane monolayer pattern formation is a result of interfacial Au-Si interactions and the alkyl chains play a negligible role in the monolayer pattern formation mechanism.     

Pattern formation and non-linear phenomena in stretched discotic liquid crystal fibres

This paper presents a non-linear numerical and bifurcation analysis of pattern formation phenomena in a discotic nematic liquid crystal confined to annular cylindrical cavities and subjected to extensional deformations. The results are of direct relevance to understanding the industrial melt spinning of mesophase carbon fibres, using discotic nematic liquid crystals precursor materials. Three types of orientation patterns are identified in this study: spatially constant (radial), monotonic (pinwheel), and oscillatory (zigzag). Numerical and closed form analytical results predicting continuous transformations between the radial, pinwheel, zigzag radial orientation modes are presented. The bifurcation analysis provides a direct characterization of the parametric dependence and the transitions between these three basic patterns, and provides a complete understanding of the multistability phenomena that is present in the oscillatory orientation patterns. In general it is found that small fibres of nearly elastically isotropic discotic nematic liquid crystals tend to adopt the classical ideal radial texture, while larger fibres with anisotropic elastic moduli tend to yield the zigzag texture. Fixed arbitrary surface orientation of intermediate size and anisotropy tend to adopt the pinwheel texture. The theoretical results are able to explain the main features and mechanisms that lead to the commonly observed cross-section textures of industrially spun mesophase carbon fibres.     

Crystalline ice growth on Pt(111) and Pd(111): nonwetting growth on a hydrophobic water monolayer

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20-155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of approximately 14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111).