Micellar polymerization: Computer simulations by dissipative particle dynamics

Nowadays, micellar polymerization is widely used in different fields of industry and research, including modern living polymerization technique. However, this process has many variables and there is no comprehensive model to describe all features. This research presents simulation methodology which describes key properties of such reactions to take a guide through a variety of their modifications. Dissipative particle dynamics is used in addition to Monte Carlo scheme to simulate initiation, propagation, and termination events. Influence of initiation probability and different termination processes on final conversion and molecular‐weight distribution are presented. We demonstrate that prolonged initiation leads to increasing in polymer average molecular weight, and surface termination events play major role in conversion limitation, in comparison with recombination. © 2018 Wiley Periodicals, Inc.     

Direct observation of liquid crystal phases under nanoconfinement: A grazing incidence X-ray diffraction study

We have controlled the molecular orientation of nematic and smectic A liquid crystal (LC) phases in a porous anodic aluminium oxide (AAO) film, in which the pore diameter was varied from 20 nm to 100 nm. Surface anchoring to induce planar and homeotropic molecular arrangement was controlled by chemical modification of the AAO inner surface. Direct observation of the molecular orientation of LC phases was performed using grazing incidence X-ray diffraction technique, showing in-plane and out-of-plane molecular orientation, and the corresponding layer orientation. The systematic investigation of LC phases under nanoconfinement will be useful to design various kinds of physicochemical environments to control the orientation of other soft matters.     

Lasing in micro-grating liquid crystal systems

A waveguide lasing effect has been observed and investigated in a dye-doped layer of a nematic liquid crystal (NLC) between two quartz plates. One of the plates has an electrode micro-grating, which allows (i) creating the feedback, (ii) guiding a part of the lasing emission into the quartz substrate and (iii) modulating the NLC refractive index by an electric field. At 0 V, a single Transverse Magnetic mode (TM)-polarised mode lasing has been observed. The emission goes out from the edge of the quartz plate in a narrow angular sector (±1.5°) at an angle of about 67.0° with respect to the NLC layer normal. At voltage applied, a number of additional lasing modes propagating at the same angle, but located at different wavelengths, appear. The experimental results are interpreted on account of numerical simulations based on the finite difference time domain method.     

Self-Assembly Behaviors of Heterogemini Surfactant in Aqueous Solution Investigated by Dissipative Particle Dynamics

As a preliminary study, self-assembly behaviors of heterogemini surfactant in aqueous solution are explored tentatively by means of dissipative particle dynamics simulation. Five kinds of heterogemini molecules are involved, and a variety of novel morphologies have been obtained. Results based on detailed comparisons among themselves and with traditional symmetric gemini surfactant show the proportion of hydrophilic to hydrophobic chain lengths in one monomer is the most important, more difference between proportions in the two monomers can induce more diverse self-assembly morphologies. The second important is the hydrophilic chain length, in which a small change can lead to obvious difference in self-assembly behaviors. While the length of hydrophobic chain has a less important influence, only the concentration for self-assembly morphologies appearing can be affected by its change. The microscopic morphology of heterogemini surfactant in its self-assembly structure can be embodied through its radius of gyration. Our simulation results can undoubtedly provide a theoretical guide to further research towards self-assembly behaviors of heterogemini surfactants and practical applications of these matters.     

Liquid crystal phases in confined geometries

ABSTRACT

The orientation control of liquid crystal (LC) phases is essential for fundamental studies as well as practical applications. Surface treatment and topographic confinement have emerged as two of the most effective tools to control ordering and orientation of various types of LC phases. This review is intended to give an overview of the LC phases controlled in confined geometries at micro- and nanometre scales, in which the orientation control methods and the effective analytical techniques will be covered. Finally, the review closes with the applications using such confined LC phases.     

Synthesis of azobenzene-containing liquid crystalline gelator for use in liquid crystal gels

A liquid crystalline gelator containing the azobenzene chromophore was synthesized for the first time; it was used to form self- assembled network in nematic liquid crystals resulting in liquid crystal gels with distinct features.     

Dynamics of phase transitions in a liquid crystal probed by Raman spectroscopy

Raman spectra of the Schiff 's base liquid crystalline compound 5O5, N-(4- n-pentyloxybenzylidene)-4'-n-pentylaniline, have been recorded as a function of temperature from 22 to 80°C in the 1140-1220 cm^-1 and 1550-1640 cm^-1 spectral regions. From careful deconvolution of the spectral features using Lorentzian profiles, precise values of peak positions, integrated intensities and linewidths of some selected Raman bands were obtained. The variations of the Raman spectral parameters with temperature are discussed in terms of changes in the molecular alignment and its effect on intra-/inter-molecular interactions at the Cr-G, G-SmF, SmF-SmC and SmA-N phase transitions. From a detailed study, it is inferred that the increased orientational/vibrational freedom of the alkyl chains, as well as the delocalization of the electron clouds, is responsible for the spectral anomalies at the Cr-G transition. Loss of positional ordering and twist around the -C₆H₄-N= bond takes place at the SmF-SmC transition. In the SmA-N transition, some evidence for the formation of cybotactic clusters was obtained.     

Orientation of liquid crystal molecules in polydimethylsiloxane micro-channels

The orientation of liquid crystal (LC) molecules confined in polydimethylsiloxane micro-channels, which are produced using mould-replica technique, with a rectangular cross section, is studied. The rod-like molecules of the LC adopt the vertical anchoring at every surface of the channels. The macroscopic alignment of the confined LC can be changed by altering the aspect ratio of the channels. The change in the macroscopic alignment of the LC is attributed directly to the restructuring of the proportion of the number of molecules anchored to the respective surfaces due to the changes in specific area of the bounding surface of the channels. Solutions for the elastic energy function obtained using conformal mapping method were used to predict director profile of the LC in the confinement.     

Structure of methylenecyclopropane in a liquid crystal by 1H and 13C NMR

The ¹H and ¹³C NMR spectra of methylenecyclopropane, both in the isotropic phase and in the Merck IV nematic phase, were analysed. The relative signs of the ¹³C H scalar coupling constants were determined. The complete geometry deduced from this study was compared with that from a microwave study, and the discrepancies are discussed.     

Free energy in vertically aligned liquid crystal mode

Vertical alignment (VA) is a widely applied operation mode for liquid crystal displays. To achieve optimum brightness, the electrode of VA is often patterned with fish-bone fine slits to generate fringe field, so the negative liquid crystal aligns along the fine slits when the electric field is applied. VA is usually simply modelled by the bend geometry along the cell gap. However, defects, domain boundaries and periodical splay induced by the fine slits also exist in real pixels and disturb the liquid crystal alignment. Polymer-stabilised VA test cells with various fine slit pitches which lead to various strength of fringe field were fabricated to observe the deformation of liquid crystal. Then the models of liquid crystal deformation nearby the defects and in the fine slit area were proposed to calculate the electromagnetic (f_EM) and elastic free energy (f_elastic). The results show that the key factor to regulate f_EM and f_elastic is the pitch of the fine slits, and the optimum liquid crystal alignment is obtained when f_EM and f_elastic are equal. The models are useful for further investigation on the dynamics of liquid crystal alignment and applications in industrial products.     

A novel method for encapsulation of a liquid crystal in monodisperse micron-sized polymer particles

Liquid crystals (LCs) encapsulated in monodisperse micron-sized polymer particles were prepared to control the size and size distribution of LC droplets in polymer-dispersed LCs. The poly(methyl methacrylate) (PMMA) seed particles were swollen with the mixture of liquid crystal, monomers (methyl methacrylate and styrene) and initiator by using a diffusion-controlled swelling method. A single LC domain was produced by the phase separation between PMMA and LC through polymerization. The optical microscopy and scanning electron microscopy showed that the particles are highly monodisperse with core–shell structure. Moreover, monodisperse LC core domains were confirmed from polarized optical microscope observations. The final particle morphology was influenced by the cross-linking of the seed particle. When linear PMMA particles, which are not cross-linked, were used as a seed, the microcapsules were distorted after annealing for a few days; however, in the case of cross-linked PMMA particles, the core–shell structure was sustained stably after annealing. Received: 22 November 2000 Accepted: 12 March 2001     

Enhancement of flexoelectric ratio of nematic liquid crystal by doping calamitic ferroelectric liquid crystal

We measured the flexoelectric ratio e* of a nematic liquid crystal (NLC) doped with a calamitic ferroelectric liquid crystal (FLC). We doped two kinds of commercial FLC into the pure NLC at a weight concentration of 5 wt%. The absolute value of the flexoelectric ratio was increased up to 49% compared to the pure NLC. The greater transverse dipole moment and the elastic constant of FLC are thought to be related to the increase of the flexoelectric ratio.     

Calorimetric study of activated kinetics of the nematic and smectic phase transitions in an aligned nano-colloidal liquid crystal+aerosil gel

This study explores the calorimetric analysis of an aligned nano-colloidal aerosil dispersed octyl-cyanobiphenyl gel. This system was prepared by solvent dispersion method (SDM). Heating scans were performed at different heating rates from 20 to 1 K min⁻¹ using DSC. Aligned samples follow Arrhenius behavior and showed a temperature shift in SmA-N and N-I transitions towards lower temperature. These samples show a decreased activated kinetics and an interesting relationship with their enthalpy. This behavior can be explained in terms of surface and molecular interaction between aerosil nano-particles and 8CB molecules and produced strain in the system.     

Reduction of gamma distortions in liquid crystal display by anisotropic voltage-dividing layer of reactive mesogens

We demonstrate a new concept of reducing the off-axis gamma distortions in a liquid crystal display by the formation of an anisotropic voltage-dividing layer (AVDL) on the alignment layer. The AVDL was prepared using reactive mesogens (RMs) by simple spin-coating, followed by the photo-polymerisation under the exposure of ultraviolet light (UV) through a photomask and the removal of residual RMs. The UV exposure time for the photo-polymerisation was found to be critical for the uniform alignment of the liquid crystal molecules on the AVDL. Owing to the capacitance difference between the sub-domains produced by the AVDL, the threshold voltage shift was naturally appeared in sub-domain by domain so that the reduction of the gamma distortions in the off-axis was achieved in a simple way.     

Solubilization of oil in a mixed cationic liquid crystal

The mixing fraction of didodecyldimethylammonium bromide (DDAB) in dodecyltrimethylammonium bromide + DDAB to produce a lamellar liquid crystal (L _α) abruptly decreases upon addition of a small amount of m-xylene, whereas the mixing fraction becomes constant at high m-xylene content. Similar results were obtained in saturated hydrocarbon systems. It is considered that oil molecules in the surfactant palisade layer increases the effective cross-sectional area per surfactant head group, a_s, whereas a_s is constant if the oil molecules are solubilized in the core of the liquid crystal. The volume fraction of penetrating oil in the total solubilized oil is defined as a penetration parameter, Pe, which is calculated from small-angle X-ray scattering data. Pe is high in the m-xylene system, whereas it is low in the n-decane system. Even in the same oil system, Pe decreases dramatically with increasing solubilization. Hence, most of the oil added penetrates into a palisade layer at an early stage of oil addition. This causes a change in the mixing fraction of surfactant in the L _α phase. Thereafter the oil is solubilized in the core of the bilayer with further addition of oil. Received: 20 April 1998 Accepted: 16 July 1998     

Sol/gel transition and liquid crystal transition of HPC in ionic liquid

This work uses an ionic liquid (IL), 1-butyl-3-methylimidazolium acetate, as a solvent to study the sol/gel transition (SGT) and liquid crystal transition (LCT) of hydroxypropylcellulose (HPC) solution. The LCT concentration of HPC at room temperature is 7 wt%, which is slightly higher than its SGT concentration of 6 wt%. For HPC concentrations of over 7 wt%, three rheological approaches were utilized, and the parameters relaxation time, hysteresis ratio and loss modulus (G″) are measured to determine the LCT temperature. The relaxation study concludes that the LC-critical concentration of HPC is 7 wt%. When HPC exceeds 7 wt%, the LC transition temperatures, ranging from 45 to 51 °C, can be measured and are proportional to the HPC concentration. The rheological results are then confirmed by making observations under a polarized optical microscope. All results are highly mutually consistent. Most significantly, the rheological parameters adopted herein can be used as good indicators of the LC transition. Of those indicators, G″ changes distinctly at the LCT point, and can thus be suggested to be the most helpful indicator in determining the LCT temperature.     

Effect of SDBS on interfacial electron transfer at the liquid/liquid interface by thin layer method

The effect of an adsorbed anionic surfactant sodium dodecyl benzene sulfonate(SDBS) on electron transfer(ET) reaction between ferricyanide aqueous solution and decamethylferrocene(DMFc) located on the adjacent organic phase was investigated for the first time by thin layer method.The adsorption of SDBS at the interface resulted in a decay in the cathodic plateau current of bimolecular reaction with increasing concentrations of SDBS in aqueous phase.However,the rate constant of electron transfer(k_(et)) i...