Fine structural analysis,formation of interfacial particle films,and accurate estimation of orientation in polyguanamine derivatives with a high refractive index

Precise analyses of the molecular arrangement of three‐dimensional crystals, two‐dimensional molecular films, and interfacial particle layers of polyguanamine derivatives with a high refractive index have been performed. The high refractive index of the polyguanamine derivatives is not due to the chemical structure of the molecule, but is based on the packing of molecular chains or the refraction of transmitted light due to the difference in electron density between the crystalline and amorphous regions. A highly crystalline polymer has been produced by polycondensation of guanamine derivatives bearing a triazine ring and phenyl rings. The packing models of molecular chains in the three‐dimensional crystal have been determined using wide‐angle X‐ray diffraction measurements and reciprocal lattice analysis. Highly hydrophobic polyguanamine derivatives undergo a transition from monolayer to single particle layer at the air/water interface. The π‐conjugated molecular plane in the two‐dimensional films is densely stacked. Multiparticle layers are formed with a highly ordered layered structure. Polymer nanoparticles are formed by the integration of units of the collapsed polymer monolayer folded along the height direction. Since this folding occurs within the amorphous region, formation of fine particles with a high refractive index and their integrated films with densely packed π‐conjugated planes is feasible. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 999–1009     

Adhesion of alumina surfaces through confined water layers containing various molecules

When two surfaces confine water layers between them at the nanoscale, the behavior of these confined water molecules can deviate significantly from the behavior of bulk water, and it could reflect on the adhesion of such surfaces. Thus, the aim of this study is to assess the role of confined water layers on the adhesion of hydrophilic surfaces and how sensitive this adhesion is to the presence of contaminants. Our methodology used under water AFM force measurements with an alumina-sputtered sphere-tipped cantilever and a flat alumina single crystal and then added fractions of ethanol, dimethylformamide, formamide, trimethylamine, and trehalose to water as contaminants. Such solutions were designed to illuminate the influences of dielectric constant, molecular size, refractive index, and number of hydrogen bonds from donors and acceptors of solutes to water. Apart from very dilute solutions of dimethylformamide, all solutions decreased the ability of confined water to give adhesion of the alumina surfaces. The predicted theoretical contribution of van der Waals and electrostatic forces was not observed when the contaminants distorted the way water organizes itself in confinement. The conclusion was that adhesion was sensitive mostly to the hydrogen-bonding network within water layers confined by the hydrophilic alumina surfaces.     

High on-state clarity polymer dispersed liquid crystal films

Abstract

Results are presented which show that the on-state clarity of a UV cured polymer dispersed liquid crystal (PDLC) film depends on the refractive index of the final polymer in the PDLC film, the ordinary refractive index of the liquid crystal, the solubility of the liquid crystal in the prepolymer and the rate at which the film is cured. Liquid crystal mixtures for use in PDLC films are chosen such that the ordinary refractive index of the liquid crystal is equal to the refractive index of the polymer matrix. It has been shown previously that a large quantity of liquid crystal remains dissolved in the polymer matrix, thus increasing the mismatch between the refractive index of the polymer and the ordinary refractive index of the liquid crystal and therefore reducing the on-state clarity. For liquid crystal mixtures which have high solubility in the prepolymer (>60 per cent) the mismatch in the refractive indices can be very large and the on-state clarity of the resulting film can be very poor (T _on<70 per cent). Results are presented which show that it is possible to increase the on-state clarity of such films by increasing the rate at which these films cure. If the liquid crystal is less soluble in the prepolymer (<45 per cent), a PDLC film formed from such a liquid crystal/ prepolymer system often has very good on-state clarity (T _on>75 per cent) be it cured slowly or quickly. Results are also presented which show that in order to achieve a true measure of on-state clarity it is necessary to use a small collection angle (<3°) in the detecting optics. If larger collection angles are used, the photodetector collects light which is scattered out of the specular beam, thus leading to a false measure of on-state clarity.     

Single-step fabrication and characterization of photonic crystal biosensors with polymer microfluidic channels

A method for simultaneously integrating label-free photonic crystal biosensor technology into microfluidic channels by a single-step replica molding process is presented. By fabricating both the sub-micron features of the photonic crystal sensor structure and the >10 microm features of a flow channel network in one step at room temperature on a plastic substrate, the sensors are automatically self-aligned with the flow channels, and patterns of arbitrary shape may be produced. By measuring changes in the resonant peak reflected wavelength from the photonic crystal structure induced by changes in dielectric permittivity within an evanescent field region near its surface, detection of bulk refractive index changes in the fluid channel or adsorption of biological material to the sensor surface is demonstrated. An imaging detection instrument is used to characterize the spatial distribution of the photonic crystal resonant wavelength, gathering thousands of independent sensor readings within a single fluid channel.     

Doped colloidal photonic crystal structure with refractive index chirping to the [111] crystallographic axis

A three-dimensionally ordered array of close-packed colloidal spheres, a photonic crystal structure in which the refractive index of the medium interstitial lattice in a colloidal crystal spatially changes in the [111] crystallographic axis, is demonstrated. The colloidal photonic crystal structure with refractive index chirping was produced by infiltration of a monomer and organic dopants with a high refractive index into a silica opal, followed by interfacial gel polymerization. The resulting photonic crystal structure has a gradually varying stop band at each different (111) plane in the face-centered cubic (fcc) crystal structure at a normal incidence. This novel structure exhibited optical characteristics that have band-gap broadening by the superposition of stop bands at each plane of the crystal with different dielectric functions. Moreover, the refractive index perturbation in the [111] fcc opal also showed a defect state within a pseudo-photonic band gap. This new type of photonic crystal structure should be useful for the band-gap engineering of photonic-band-gap materials.     

Spatiotemporal Control of Enzyme‐Induced Crystallization Under Lyotropic Liquid Crystal Nanoconfinement

Water nanoconfinement has important effects on the properties of biomolecules and ultimately on their specific functions. By performing experiments and molecular dynamic simulations, we show how intrinsic nanoconfinement controls the crystallization of small organic molecules converted by enzymatic reactions within the water nanochannels of lipid cubic phases (LCPs). By controlling the nanochannel size, enzymatic reactions in LCPs can be engineered to turn the same converted substrate into its soluble, microcrystal, or needle‐like crystal form due to the large variability in water dynamics. Differential scanning calorimetry studies, supported by molecular dynamics simulations, show that most of water within the mesophase nanochannels behaves differently due to interactions with the LCP interface, and that this mechanism has a larger impact for smaller channels. These findings suggest that the amount of free water in the core of the nanochannels is the key factor determining local substrate diffusion and self‐assembly within LCPs.     

Optical and morphological characterization of polyacrylamide hydrogel and liquid crystal systems

In this work, the thermotropic liquid crystal MBBA (N-(4-methoxybenzilidene)-4-butylaniline), entrapped on hydrogels, based on cross-linked polyacrylamide (PAAm), was studied. The liquid crystalline phases of system were characterized by polarized optical microscopy (POM), refractive index, optical transmittance, scanning electron microscopy (SEM) and water loss. It was verified the presence of birefringence on hydrogel + liquid crystal. The dynamic of formation of such birefringence finished 40 days after the hydrogel synthesis. The effective birefringence Δn, i.e., the difference on refractive index of polyacrylamide hydrogel to refractive index of hydrogel + liquid crystal (Δn₁) and the difference on refractive index of liquid crystal (MBBA) to refractive index of hydrogel + liquid crystal (Δn₂) are dependent of content of acrylamide (AAm) and MBBA on hydrogel. The increase on Δn₁ and Δn₂ with the polyacrylamide content on hydrogel was attributed to decreasing of the mobility liquid crystal inside the hydrogel. Also, an increase on MBBA concentration in the polymeric matrix provides a reduction in the values of optical transmittance in the system. The morphology observed by SEM shows that hydrogel + liquid crystal is more compact that PAAm hydrogels. The presence of MBBA causes an increase in hydrophobicity. The water loss speed is favored by the increase in the amount of MBBA present in the hydrogels.     

Speckle observation of pulsed laser-induced dynamics in a guest-host smectic A liquid crystal system

Using a pulsed, pump-probe experimental arrangement, we have investigated the speckle generated by a 100 μm path length cell of smectic A liquid crystal 4-cyano-4'-octylbiphenyl doped with 0·1 wt% dichroic dye. Upon irradiation, the guest-host system undergoes a phase transition from an initially transparent condition to a scattering state. We show that the statistical theory of speckle can be used to describe the evolution of scattering domains and estimate an rms refractive index variation of 0·0021 within the irradiated region for one case. Dynamic response as a function of molecular anchoring conditions was also investigated.     

Effect of internal field on the birefringence of polyethylene crystals and on the anisotropy of polymer chains. I. Birefringence effects

Values of the anisotropy of polarizability of methylene groups deduced from the refractive indexes of n-paraffin crystals differ appreciably from those obtained from gas- or liquid-phase measurements. The differences is attributed to the erroneous application of the Lorenz—Lorentz internal field to the anisotropic n-paraffin crystal. A more detailed calculation of the internal field is carried out by detailed summation of the dipolar field over an idealized n-paraffin crystal in which the molecular chains are replaced by anisotropic rods. With reasonable values of parameters and assumed crystal shape, the discrepancy in polarizability anisotropy is resolved.     

Water density and polarizability deduced from the refractive index determined by interferometric measurements up to 250 MPa

The refractive index of water is precisely determined in the visible light range as a function of the pressure until 250 MPa by means of a new measurement device that uses a special pipe tee included in an interferometer set. This technique allows revisiting the Bradley-Tait and Sellmeier equations to make them dependent on the wavelength and the pressure, respectively. The Bradley-Tait equation for the pressure dependence of the water refractive index is completed by a wavelength-dependent factor. Also, in the considered pressure and wavelength ranges, it is shown that the Sellmeier coefficients can be straightforwardly linked to the pressure, allowing the determination of the refractive index of water for either any wavelength or pressure. A new simple model allows the determination of the density of water as a function of the measured refractive index. Finally, the polarizability of water as function of pressure and wavelength is calculated by means of the Lorentz-Lorenz equation.     

Structures and phase transition of multi-layered water nanotube confined to nanochannels

We placed nanometer-scale water-tube clusters with phase transition within a porous crystal formed from molecular blocks specifically designed to investigate the molecular dynamics of confined water molecules.     

Refractive index of liquid water in different solvent models

We present a combined molecular dynamics/quantum chemical perturbation method for calculating the refractive index of liquid water at different temperatures. We compare results of this method with the refractive index obtained from other solvent models. The best agreement with the experimental refractive index of liquid water and its temperature dependence is obtained using correlated gas-phase polarizabilities in the classical Lorentz-Lorenz expression. Also, the iterative self-consistent reaction field approach in the semicontinuum implementation matches the experimental refractive index reasonably well.     

Speckle observation of pulsed laser-induced dynamics in a guest-host smectic A liquid crystal system

Abstract

Using a pulsed, pump-probe experimental arrangement, we have investigated the speckle generated by a 100 μm path length cell of smectic A liquid crystal 4-cyano-4′-octylbiphenyl doped with 0·1 wt% dichroic dye. Upon irradiation, the guest-host system undergoes a phase transition from an initially transparent condition to a scattering state. We show that the statistical theory of speckle can be used to describe the evolution of scattering domains and estimate an rms refractive index variation of 0·0021 within the irradiated region for one case. Dynamic response as a function of molecular anchoring conditions was also investigated.     

Effect of the internal field on the birefringence of polymer crystals

The discrepancy between the values of the anisotropy of methylene groups determined from crystal refractive indices, stress-optical data, and gas and liquid light-scattering depolarization is explained on the basis of the effect of the internal field on the polarizability of the isolated molecule. The internal field may arise from intermolecular or intramolecular interactions which depend upon molecular conformation and state of aggregation. A simple continuum calculation based upon an extension of the calculation of the Lorenz-Lorentz field is shown capable of accounting for the discrepancy.     

Characterization of the alignment of a nematic liquid crystal using half leaky guided modes

The excitation of half leaky guided optical modes to characterize fully the optical tensor profile in a thin liquid crystal layer has been used to evaluate the effect of rubbed polyimide aligning layers on the alignment of a nematic liquid crystal. A cell fabricated with rubbed polyimide alignment surfaces was studied at a wavelength of 632.8 nm. The liquid crystalline layer is sandwiched between a high refractive index top glass plate and a low refractive index glass substrate. Angular dependent reflectivities are recorded using a coupling prism and matching fluid with the same index as the top glass plate. Careful fitting of the predictions from multilayer optics theory to the observed angle dependent polarization conversion and reflectivity data yields the director profile within the liquid crystal layer in great detail.     

Crystal growth,structural, spectral,optical, DFT analysis and Z-scan analysis of pyridine-1-ium-2-carboxylatehydrogenbromide (PHBr) for optoelectronic and nonlinear optical applications

A single crystal of Pyridine-1-ium-2-carboxylatehydrogenbromide (PHBr) was grown using the Slow Evaporation Solution Technique. Using Single Crystal X-Ray Diffraction analysis, the crystal lattice characteristics and molecular structure of the grown crystal of PHBr were found and it corresponds to the Triclinic crystal system with space group Pī. Intra and intermolecular interactions were visualized using Hirshfeld surface analysis. The theoretical calculation conducted by Density Functional Theory (DFT) and it is well agreed with the experimental results. The Molecular optimized geometry, FT-IR and HOMO-LUMO energy gap were computed using the B3LYP level of theory with a 6-31 + G (d,p) basis set. The FT-IR spectrum studies are given here to look at the modes of vibration of numerous functional groups found in the PHBr crystal. The measurements of UV–visible NIR transmittance show that the crystal has a high transmittance over the whole visible spectrum. The Z-scan approach is used to perform third-order nonlinear optical (NLO) investigations on a PHBr crystal and optical properties such as linear and nonlinear refractive index are computed.     

Refratometric study of ternary mixtures formed by water,glucose and acetonitrile at temperatures ranging from 293 K to 333 K

The study of thermodynamic properties of solutions provides important information on existing molecular interactions between the components present in a solution. These studies are critical for testing, validation and development of theories and mathematical models. The refractometric study of a solution is a simple assessment that can contribute to the understanding of these interactions. In this context, the behaviour of the binary water and glucose mixture was studied as well as ternary mixtures of water, glucose and acetonitrile at five different temperatures in the range 293–333 K by the determination of the refractive index of the solution. Due to the weakening of the molecular interactions with the increasing of the temperature, a decreasing dependence of refractive index with temperature was observed. The addition of acetonitrile provides an increase in the refractive index indicating the formation of clusters in the solution.     

On crystal versus fiber formation in dipeptide hydrogelator systems

Naphthalene dipeptides have been shown to be useful low-molecular-weight gelators. Here we have used a library to explore the relationship between the dipeptide sequence and the hydrogelation efficiency. A number of the naphthalene dipeptides are crystallizable from water, enabling us to investigate the comparison between the gel/fiber phase and the crystal phase. We succeeded in crystallizing one example directly from the gel phase. Using X-ray crystallography, molecular modeling, and X-ray fiber diffraction, we show that the molecular packing of this crystal structure differs from the structure of the gel/fiber phase. Although the crystal structures may provide important insights into stabilizing interactions, our analysis indicates a rearrangement of structural packing within the fibers. These observations are consistent with the fibrillar interactions and interatomic separations promoting 1D assembly whereas in the crystals the peptides are aligned along multiple axes, allowing 3D growth. This observation has an impact on the use of crystal structures to determine supramolecular synthons for gelators.     

Ultrafast dynamics of a solution in spatially restricted environments studied by photothermal spectroscopies

The ultrafast dynamics of a solution in spatially restricted environments was studied by using the ultrafast transient lens (UTL) method. The UTL method is used to monitor the molecular dynamics of a solution by means of a change in the refractive index, which is advantageous for investigating the molecular dynamics of restricted systems. We investigated the photoisomerization of azobenzene derivatives in cyclodextrin nanocavities and revealed how the confinement affects the photoisomerization dynamics and yields. We also studied the relaxation dynamics of photo-excited auramine O (AuO) in a water/aerosol-OT/n-heptane reversed micelle. Both the perturbed properties of the included water and the interactions between AuO and the interface of the reversed micelle strongly appeared to affect the relaxation dynamics. At the same time, we observed a change in the refractive index suggesting a structural change of the micelles in the picosecond region that could not be detected by transient absorption spectroscopy. In addition, we developed the total internal reflection UTL (TIR-UTL) method to monitor the ultrafast molecular dynamics at the liquid interface. The relaxation dynamics of photoexcited AuO at the silica/water interface were observed with subpicosecond time resolution, and it was revealed that the interaction with the interface strongly inhibited the relaxation process. These results demonstrated the advantages of the UTL method for investigating the molecular dynamics of a solution in spatially restricted environments.     

Molecular dynamics investigation of the interaction between IR radiation and an ammonia-water medium

The frequency dependence of the absorption factor and refractive index of a dispersion water system absorbing ammonia are studied by means of molecular dynamics. It is found that the capture of the ammonia molecules by water clusters is accompanied by a substantial reduction in their ability to absorb IR radiation in the frequency range 0 ≤ ω ≤ 3500 cm^?1, and the refractive index of the ammonia-water system of clusters is lowered. It is shown that the maxima of the absorption spectra and the refractive index shift to higher frequencies. Starting from a certain concentration of ammonia in the clusters, however, the integral intensity of the spectra increases.