Electro‐Optical Properties of Polymer‐Dispersed Liquid Crystals

This paper deals with the electro‐optic (EO) properties of polymer‐dispersed liquid crystals (PDLCs). Several systems are considered to analyze the effects of preparation conditions and film characteristics on the EO response functions. Particular emphasis is put on systems based on mixtures of the commercial compound ASX‐95, the difunctional acrylate tripropylene diacrylate and the eutectic mixture of low‐molecular‐weight liquid crystals E7. Other systems are considered to assess the influence of monomer functionality on EO properties using for example the trifunctional glycerylpropoxytriacrylate. Various modes of preparing PDLCs are considered based on the mechanism of polymerization‐induced phase separation using either electron‐beam (EB) or UV radiation curing processes. The dose is changed in both techniques to improve film strength and determine which method leads to the best samples in terms of EO response functions. Other important parameters, such as film thickness, composition and applied voltage, are also considered to evaluate the impact on these functions. The article focuses on a comparison of EO performances of films elaborated by exposure to EB and UV radiations. Under similar conditions, one definitively finds a net superiority of the former technique. In addition this technique does not require any photoinitiator and leads to a higher conversion of the monomeric compounds, i.e. higher mechanical strength and less severe aging conditions.     

High‐performance holographic polymer‐dispersed liquid crystals by incorporating hyperbranched polymers

A minute amount (0.01–0.3 wt %) of ally isocyanate functionalized hyperbranched 2,2‐bis (hydroxymethyl) propionic acid (bis‐MPA) polyester‐16‐hydroxyl (HBP) was incorporated covalently into polyurethane acrylate‐based holographic polymer dispersed liquid crystals (HPDLCs), and its effects on the compound viscosity, grating kinetics, morphology, diffraction efficiency (DE), and electro‐optical properties of the HPDLC films were examined. HBP at low concentrations (0.01–0.05%) reduced the compound viscosity and domain size of liquid crystal (LC) significantly and augmented the cure rate and saturation DE by up to threefold compared to the HBP‐free compound. At high concentrations (0.10 and 0.30%), HBP increased the compound viscosity and decreased the rate of grating formation, giving rise to distorted LC‐polymer interfaces, which caused a significant decrease in the threshold and operating voltages. The rise and decay time showed a minimum and maximum, respectively, when the compound viscosity was a minimum at 0.03% HBP. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

全息聚合物分散液晶的结构调控与性能

全息聚合物分散液晶(HPDLCs)是由富聚合物相与富液晶相周期性排列而成的结构有序高分子复合材料.HPDLCs通过单体/液晶复合体系的光聚合诱导相分离而形成,如何调控并定量化描述复合体系的光聚合反应动力学、凝胶化行为和相分离程度,进而获得结构规整、电光性能优异的HPDLCs是关键难题.专论概述了光引发体系、单体结构、纳米无机材料掺杂对HPDLCs结构及性能的影响.光引发阻聚剂通过引发和阻聚的竞争与协同,降低了光聚合反应速率、延迟了凝胶时间,促进形成衍射效率达90%的HPDLCs.超支化单体降低了复合体系黏度和光聚合反应速率,延迟了凝胶时间,促使形成衍射效率达94%、具有一维光子晶体结构的HPDLCs.丙烯酰胺单体优化了相分离结构,将HPDLCs的衍射效率提升至98%.纳米硫化锌掺杂在保持规整结构和高衍射效率的同时,大幅降低了HPDLCs的驱动电压.研究还确定了HPDLCs的相分离程度与凝胶时间的函数关系.构建兼具高衍射效率与低驱动电压的HPDLCs,推进其在彩色3D图像存储等领域的应用仍是重要课题.     

Transmission Holographic Polymer Dispersed Liquid Crystals Based on a Siloxane Polymer

The low surface energy and the great immiscibility of poly (dimethylsiloxane) (PDMS) with liquid crystals (LCs) are used in the fabrication of holographic polymer dispersed liquid crystals (HPDLCs). By adding increasing amounts of PDMS, the extent of the phase separation between the polymer and the LC, the LC channel width, and—eventually—also the diffraction efficiency of the film can be increased, while keeping the droplet size essentially the same. In addition, the presence of PDMS causes a decrease in the switching voltage and an increase in the response time. At an optimum content of PDMS (PUA40), a minimum switching voltage of 4 V μm^?1, a rise time of 0.20 ms, and a decay of 14.75 ms were obtained. Regarding the effect of the LC content, an overshoot of the diffraction efficiency was observed when the amount of LC exceeded 35 %, which can be attributed to droplet coalescence.     

Application and New Developments in Polymer‐Dispersed Liquid Crystal Simulation Studies

Polymer‐dispersed liquid crystals (PDLCs) represent an important new class of materials with electro‐optic applications such as flexible displays, large‐area devices projection displays, electrically switchable windows, etc. On considering such applications of these materials, many studies have been performed on the experimental side. Similarly, research on the simulation side for the PDLCs is of the fundamental interest too. The present article contains a short review on the present and past simulation studies of PDLCs. Various molecular simulation approaches applied to PDLC systems by different groups are reviewed here. In addition some new findings of the bulk phases are also extensively described.

     

Effects of Monomer Functionality on Switchable Holographic Gratings Formed in Polymer‐Dispersed Liquid‐Crystal Cells

Switching characteristics: The 633 nm wavelength of an He–Ne laser is used to fabricate holographic transmission gratings in polymer‐dispersed liquid‐crystal cells. The microstructure and diffraction efficiency (η) both improve with increasing functionality (see figure). For functionalities of 2.5 or more, η approaches 34 % and can be switched off with electric fields of about 20 MV m^?1.

     

Broadband Reflection in Polymer‐Stabilized Cholesteric Liquid Crystals via Thiol–Acrylate Chemistry

Thiols are prone to react with a multitude of various functional groups in high yields, which has been widely used for surface‐ and particle‐patterning, bioorganic synthesis, polymer modification, imprint nanolithography, the fabrication of optical components, hydrogel synthesis, and the curing of hard protective coatings. In this work, a chiral thiol with a high helical twisting power was synthesized through a novel synthetic route with high selectivity, yield, and cost‐effectiveness. It was then used to fabricate a liquid‐crystal composite film with ultra‐wide broadband reflection via thiol click chemistry. Cholesteric liquid‐crystal materials with broadband reflection have many potential applications for broadband polarizers, polarizer‐free displays, organic optical data storage media, smart switchable reflective windows, and continuous waveband laser protection.     

Color Displays with Gel-Glass Dispersed Liquid Crystals

Liquid crystal microdroplets trapped into silica gel-glasses by sol-gel processes may be used for display applications. Gel-glass dispersed liquid crystals (GDLC) are switched between opaque and transparent states by applying external AC-voltage signals; no polarizers are required. The feasibility of applying these materials to colored displays has been explored. A comparative study of different dyes for GDLC color displays, which were either embedded in the sol-gel matrix or dissolved in the liquid crystal microdroplets is presented.     

Effect of Monomer Composition on the Performance of Polymer‐Stabilized Liquid Crystals with Two‐Step Photopolymerization

Polymer‐stabilized cholesteric liquid crystal (PSCLC) films with broadband reflection based on two‐step photopolymerization are fabricated. Owing to the helical twisting power (HTP) value of the chiral dopant (CD) decreasing with increasing temperature, PSCLC films with broadband reflection are obtained by two‐step polymerization anchoring helical pitch of different length at two different temperature points. The effect of monomer composition on the PSCLC reflection properties before and after polymerization was studied. The results show that the free‐radical monomers with appropriate concentration and cationic monomers with sufficient concentration are vital for the formation of PSCLC films with broadband reflection. In addition, the experiments show that the increase in the functionality and rigidity of the cationic monomer has a positive effect on the broadening of the reflection band. This study can provide guidance and reference for the selection of monomer species and concentration in PSCLC preparation. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1126–1132     

Novel Route to Immobilize Metallocenes within Silica‐Based Supports: Investigation of a Nonhydrolytic Sol–Gel Process

A novel approach to metallocene catalyst heterogenization is presented. Supported metallocenes are obtained by metallation of indenyl‐modified silica produced by a nonhydrolytic sol–gel process. The catalytic activities for ethylene polymerization are rather high for supported systems and may reach 190 grams of polyethylene per gram of catalyst per hour per bar. The influence of the preparation and composition of these supported metallocene systems on ethylene polymerization and polyethylene characteristics are reported.

Possible structure of the indenyl‐modified silicas.     

The Aspect Ratio and Gas Permeation in Polymer‐Layered Silicate Nanocomposites

Summary: Organophilized montmorillonite‐epoxy and ‐polyurethane nanocomposites, useful for packaging applications, were prepared and their oxygen permeability was measured. The composite morphology was mixed, exfoliated and intercalated, as shown by wide‐angle X‐ray diffraction (WAXRD) and transmission electron microscopy (TEM). The gas‐barrier performance of the polyurethane composites was better than that of the epoxy composites due to more exfoliation. The average aspect ratio of the montmorillonite platelets in the nanocomposites could be estimated from the reduction in permeability by a numerical finite element approach.

A computer model comprising 50 randomly distributed and oriented round platelets with an aspect ratio of 50 at 3 vol.‐% loading, periodic boundary conditions applied.     

Polymer‐ and Silica‐Supported Iron BPMEN‐Inspired Catalysts for CH Bond Functionalization Reactions

Direct catalytic C? H bond functionalization is a key challenge in synthetic chemistry, with many popular C? H activation methodologies involving precious‐metal catalysts. In recent years, iron catalysts have emerged as a possible alternative to the more common precious‐metal catalysts, owing to its high abundance, low cost, and low toxicity. However, iron catalysts are plagued by two key factors: the ligand cost and the low turnover numbers (TONs) typically achieved. In this work, two approaches are presented to functionalize the popular N¹,N²‐dimethyl‐N¹,N²‐bis(pyridin‐2‐ylmethyl)ethane‐1,2‐diamine (BPMEN) ligand, so that it can be supported on porous silica or polymer resin supports. Four new catalysts are prepared and evaluated in an array of catalytic C? H functionalization reactions by using cyclohexane, cyclohexene, cyclooctane, adamantane, benzyl alcohol, and cumene with aqueous hydrogen peroxide. Catalyst recovery and recycling is demonstrated by using supported catalysts, which allows for a modest increase in the TON achieved with these catalysts.     

Concurrent Solution‐Like Decoloration Rate and High Mechanical Strength from Polymer‐Dispersed Photochromic Organogel

To achieve a fast photochromic response in solid matrix, photochromic molecules/segments have been either dispersed into elastomers via physical doping or linked to glassy polymers by soft units through covalent bonding. However, the former is lack of high mechanical strength and the latter owes the drawback of time‐consumption of synthesis. Here, we propose a facile strategy of co‐solvent evaporation to prepare polymer‐dispersed photochromic organogel where both high mechanical strength of the glassy polymer matrix and solution‐like fast photochromism of the photochromic molecule within organogel can be retained concurrently. Glassy PVA matrix and dispersed organogel of 1,3:2,4‐di‐O‐benzylidene‐d ‐sorbitol/poly(propylene glycol) (DBS/PPG) provide high mechanical strength and sufficient free volume for intramolecular rotation of photochromic spiropyran (SP), respectively. Interestingly, these thin films behave a solution‐like decoloration the decay rate of which is 65–70 fold faster than that in the SP‐directly doped PVA film and only slightly slower than those in their corresponding PPG solutions.

     

Magnetic Field‐Induced Shape Transitions in Multiphase Polymer‐Liquid Crystal Blends

Summary: This paper presents a computational study of phase separation‐phase ordering‐texturing in blends of polymer coils and rod‐like nematic liquid crystals under the presence of magnetic fields, using an extended version of the Matsuyama‐Evans‐Cates model (Phys. Rev. E 2000 , 61, 2977). This work demonstrates that demixing in these blends leads to droplet morphologies with tunable droplet shapes and director textures. In contrast to filled nematics, where solids are suspended in a nematic liquid crystal matrix, demixing in coil‐mesogenic rods blends leads to nematic emulsions, in which the deformable viscoelastic polymer drops are suspended in a nematic matrix. Under strong anchoring conditions, the imposition of a magnetic field leads to a director re‐orientation that due to strong anchoring produces a droplet shape change. Magnetic field‐induced shape transitions in these blends are shown to be second order with a finite critical field threshold that diverges as anchoring strength vanishes. A morphological‐texture diagram summarizes the magnetic field‐anchoring conditions that promote anisotropic shapes. This work presents additional material processing routes to design and control bi‐phasic morphologies in polymer‐liquid crystal blend.

Computed morphology phase diagram in terms of magnetic field strength Λ_M and anchoring strength. Λ_ϕQ.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Graphene Oxide Facilitates Solvent‐Free Synthesis of Well‐Dispersed,Faceted Zeolite Crystals

Zeolites with molecular dimension pores are widely used in petrochemical and fine‐chemical industries. While traditional solvothermal syntheses suffer from environmental, safety, and efficiency issues, the newly developed solvent‐free synthesis is limited by zeolite crystal aggregation. Herein, we report well‐dispersed and faceted silicalite ZSM‐5 zeolite crystals obtained using a solvent‐free synthesis facilitated by graphene oxide (GO). The selective interactions between the GO sheets and different facets, which are confirmed by molecular dynamics simulations, result in oriented growth of the ZSM‐5 crystals along the c‐axis. More importantly, the incorporation of GO sheets into the ZSM‐5 crystals leads to the formation of mesopores. Consequently, the faceted ZSM‐5 crystals exhibit hierarchical pore structures. This synthetic method is superior to conventional approaches because of the features of the ZSM‐5 zeolite.     

Silica‐Based Liquid Marbles as Microreactors for the Silver Mirror Reaction

Little attention has been paid to the participation of the shell of silica‐particle‐based liquid marbles and their influence on chemical reactions. The fabrication of liquid marbles with the encapsulating particle shells not only act as protecting layers to provide a confined environment, but also provide the reactive substrate surfaces to regulate the classical silver mirror reaction. Fabrication of silver mirrors with different morphologies was achieved by modifying particle surface properties, which could further lead to Janus liquid marbles. The different evaporation behavior of microreactors was demonstrated. Micrometer‐sized silica particles were used for the preparation of monolayer‐stabilized liquid marbles, which show great potential in fabricating Janus particles from superhydrophobic particles that are not attainable from Pickering emulsions.