Effect of polymer structures on electro-optical properties of polymer stabilized liquid crystal films

The polymer stabilized liquid crystal (PSLC) film is a relatively novel electro-optical material, which is generally obtained by dissolving a small amount of a bifunctional photoreactive monomer in a low molecular mass liquid crystal. In this paper, the PSLC films were prepared with photoreactive biphenyl methacrylate monomers by photopolymerization induced phase separation. The effects of liquid crystal concentration, curing time, monomer structures and alignment layer on the electro-optical properties of PSLC films were investigated. The results show that the transmittance in the OFF state (T _OFF) increased with the liquid crystal concentration, but the driving voltage decreased. T _OFF was also influenced by the curing time. Furthermore, when polyimide was used as alignment layer, the films prepared from the bifunctional monomer shows a higher T _OFF, while those from the single functional monomer exhibited a deformed electro-optical curve due to the unsteady polymer networks. __________ Translated from Polymer Materials Science and Engineering, 2008, 24(1): 63–66 [译自: 高分子材料科学与工程]     

Effect of polymer structures on electro-optical properties of polymer stabilized liquid crystal films

The polymer stabilized liquid crystal (PSLC) film is a relatively novel electro-optical material, which is generally obtained by dissolving a small amount of a bifunctional photoreactive monomer in a low molecular mass liquid crystal. In this paper, the PSLC films were prepared with photoreactive biphenyl methacrylate monomers by photopolymerization induced phase separation. The effects of liquid crystal concentration, curing time, monomer structures and alignment layer on the electro-optical properties of PSLC films were investigated. The results show that the transmittance in the OFF state (T_OFF) increased with the liquid crystal concentration, but the driving voltage decreased. T_OFF was also influenced by the curing time. Furthermore, when polyimide was used as alignment layer, the films prepared from the bifunctional monomer shows a higher T_OFF, while those from the single functional monomer exhibited a deformed electro-optical curve due to the unsteady polymer networks.     

Rheology of sterically stabilized dispersions and latices

Steric stabilization is a method that is often used to properly disperse small particles. It can be applied in aqueous as well as non-aqueous media. The rheological properties of sterically stabilized dispersions are discussed here. The various controlling parameters and the physical mechanisms involved are reviewed. Brownian hard spheres are used as a reference. Scaling relations are presented that make it possible to reduce data sets and to predict properties. Viscosity, yield stress, shear thickening and viscoelasticity are included. The rheological properties are also related to the fundamental colloidal properties of the dispersions under consideration. Quantitative results are available for monodisperse spherical particles, although the effects of particle size distribution can sometimes be predicted also quite well. In other cases the procedures presented here can be used qualitatively to predict viscosities.     

Stable and scalable smart window based on polymer stabilized liquid crystals

A stable and scalable polymer-stabilized liquid crystal window which electrically switches from transparent to opaque has been fabricated. Scanning electron microscope measurement shows that higher polymer concentration will induce denser polymer network in polymer stabilized liquid crystal system and then stronger anchoring force between polymer network and liquid crystal molecules, which resulting in larger operating voltage. The cell with larger cell gap has a lower saturated transmittance in the voltage-on state, which attributed to a larger number of scattering domains in thick cell. The optimized cell exhibits a highly transparent voltage-off state (3.5% haze) and a voltage-on scattering state (98% haze) with the threshold voltage of around 20 V. The durability test shows that the optical device switches at least 100,000 times without degradation of optical contrast and shows a high temperature tolerance. Meanwhile, a 40 × 50 cm² window has been developed in an industrial production line showing the same optical properties. Our results demonstrate the fabrication of smart windows with a highly transparent rest state and high optical contrast on a commercial mass production scale, making them attractive for applications in buildings, automobiles, and switchable sunglasses for light management and potentially energy saving. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48917.     

The homogeneous nucleation of polymer colloids

A theory of particle formation based on a model involving initiation of free radicals in the continuous medium and self-nucleation or particle capture is proposed. Expressions for determining the number of particles are presented together with a consideration of limiting cases.     

On the potential of mean force of a sterically stabilized dispersion

The potential of mean force (PMF) is the total free energy of a many-body colloidal system, and consequently it includes all the interactions the colloids experience due to collisions with themselves and with the solvent. Here, the PMF of a colloidal dispersion under various circumstances of current interest, such as varying solvent quality, polymer coating thickness, and addition of electrostatic interaction, is obtained from radial distribution functions available from the literature. They are based on implicit-solvent, computer simulation studies of a model TiO₂ dispersion that takes into account three major components to the interaction between colloidal particles, namely van der Waals attraction, repulsion between polymer coating layers, and a hard-core particle repulsion. In addition, a screened form of the electrostatic interaction was included. It is argued that optimal conditions for dispersion stability can be derived from a comparative analysis of the PMF under the different situations studied. This thermodynamics-based analysis is believed to be more accessible to specialists working on the development of improved colloidal formulations than that based on the more abstract, radial distribution functions.     

Enhanced light scattering in coatings templated with interfacially stabilized colloids

Controlling inter-pore distances enables tuning the color or whiteness of microvoid coatings. While pore spacings have been modified in limited area inverse opal films, little work has occurred studying the feasibility of controlling pore spacings and thus the appearance of scalable, spray-on, microvoid inorganic coatings. In this work we investigated using interfacially stabilized colloidal templates to increase pore spacing and thus enhance Mie scattering in porous silica films. Coatings were fabricated by forming monodisperse colloids with or without a polyvinylpyrrolidone (PVP) interfacial stabilizing layer, dispersing them in a silica precursor solution, and spraying this suspension on a substrate. The films were cured and the colloids subsequently solution extracted at mild temperatures to create porous surfaces. Coatings made with PVP coated colloids had thicker pore walls and scattered light approximately 3× more efficiently than coatings made with bare colloids. Furthermore, a viewing angle dependent color shift was observed in the PVP colloid templated coatings. Side illumination of the samples with white light causes an orange appearance under angles of specular reflection, while a light blue appearance is observed out of these angles because of strong Mie scattering of short-wavelength radiation in both situations. Lastly, modeling based on Mie scattering confirms that it is the dominant optical effect in these coatings and explains the appearance of these coatings. The approach of using interfacially stabilized colloids to improve pore separation applies to many porous films and should be considered when increased light scattering is desired.     

Fluorescence techniques in the study of polymer colloids

Applications of fluorescence techniques to the study of polymer colloids are reviewed, with emphasis on excimer and exciplex forming probes and fluorescence energy transfer processes. In these experiments non-aqueous dispersions covalently labelled with naphthalene or anthracene were examined by a variety of techniques in order to elucidate aspects of core morphology with a resolution of about 10Å. The current “concentric sphere” or “core-shell” model of colloid structure was found to be inconsistent with the results of the luminescence experiments. A new “microphase” model is proposed which overcomes these difficulties. In this model, the core structure is seen to be made up of phase-separated microdomains of core and stabilizer chains.     

Phase separation and morphology in polymer modified colloids

The morphology of films of sterically stabilised polystyrene particles, PS, with added free poly(vinyl alcohol), PVA, and poly(vinyl pyrrolidone), PVP, was investigated with scanning electron microscopy and correlated to the phase behaviour and interactions. In the case of the PS/PVP system exhibiting depletion phase separation, large polymer domains (ca. 7 μm) were present in the solid state when free polymer concentration exceeded ca. 13%. This concentration was well within the two-phase region of the phase diagram. Similar polymer domains were also observed for the homogeneous system PS/PVA. The surface of the films was sealed at free polymer concentrations of ca. 20% and ca. 40% for PVP/PS and PVA/PS, respectively. The final film morphology reflects non-equilibrium conditions arising from the increase in system viscosity due to particle close packing and continuous phase thickening by the free polymer. In the case of PS/PVP systems, incomplete depletion phase separation and phase demixing and prevention of particle restabilisation gives rise to large free polymer domains in the final film. Bridging flocculation was argued to be responsible for the large polymer domains, some with internal structure, observed with the PVA/PS systems.     

Facile synthesis of metal nanoparticles using conducting polymer colloids

We report a simple method to synthesize Ag, Au, and Pt nanoparticles with a reasonable size dispersity using water-dispersible conducting polymer colloids composed of polyaniline (PANI) and conventional polyelectrolyte. This facile synthesis results in single crystalline metal nanoparticles that are stable in an aqueous solution for at least several weeks. The process involves incrementally adding a metal ion solution to aqueous conducting polymer colloids and does not require reducing agents such as NaBH₄. In addition, the complete synthetic and purification procedure is carried out in an aqueous solution; therefore, it is environmentally benign and potentially suitable for large-scale production. We have also demonstrated synthesis of larger nanoparticles and nanosheets by varying the experimental parameters. With the tunable oxidation states of conducting polymers, we expect this synthetic platform can synthesize a wide range of nanostructured metals with specific size, shape and properties. Finally, the nanoparticles embedded in the conducting polymer matrix, the metal-polyaniline nanocomposite itself may be interesting since it represents a type of materials where metallic nanoislands are embedded in a semiconducting matrix.     

Liquid crystalline polymer blends with stabilized viscosity

The rheology of blends of thermotropic liquid crystalline copolyester co(hydroxybenzoate-isophthalate-hydroquinone) (HIQ) and polyetherimide were studied at different temperatures. As a result of the anisotropic-to-isotropic transition, the viscosity of the biphasic HIQ was found to be able to compensate that of the host polyetherimide at proper composition when the temperature increased or decreased in the capillary rheometer study. The 33% HIQ blend had almost stabilized viscosity between 350 and 370°C, whereas the 15% HIQ blend did not have stabilized viscosity. For the nonisothermal spiral molding trial, the 33% HIQ blend stabilized at a mean length of 21 inches (53 cm) with a standard deviation of 2.1 inch (5.3 cm) when the melt temperature increased by 30 Celsius degrees from 340 to 370°C.     

Monodisperse polymer beads as packing material for high-performance liquid chromatography

A novel approach to monodispersed porous polymer beads allowing accurate control over a broad range of pore size distribution has been developed. It involves the use of monodispersed template particles which are used as polymeric porogens in the suspension polymerization of monomers such as styrene and divinylbenzene. The size uniformity of the template particles is retained by the final porous beads. The porous properties of the final beads are determined in large part by the characteristics of the porogenic mixture such as its composition, the molecular weight of the polymeric porogen, as well as the relative amount of monomers, polymeric and low molecular weight porogens used.     

UV-nanoparticles: Photopolymerized polymer colloids from aqueous dispersions of acrylated oligomers

Polymer colloids are usually prepared using thermally activated polymerization. Photo-induced techniques for the preparation of polymerized or crosslinked colloids have seldom been reported in the literature and to our knowledge none has found real application in industry. Here, we introduce a new approach to form insoluble crosslinked colloidal particles by in situ photopolymerization starting from aqueous dispersions of auto-dispersible acrylated oligomers originally developed for water-based UV-coating applications. Of particular interest is the relatively high rate at which particle polymerization takes place even in highly opaque conditions e.g. with particle loadings of more than 20 w/w% and particle sizes ranging from 40 up to 500 nm. In this paper, the preparation process is demonstrated with an immersion-type batch photoreactor and the properties of this new family of organic colloids are highlighted. Furthermore, we report the preparation of submicron sized containers by loading a hydrophobic compound (i.e. n-hexadecane) within a water-dispersible urethane acrylate shell followed by solidification of the container wall using radical photopolymerization with a suitable photoinitiator. The thermal and colloidal characteristics of these novel capsules are discussed in some detail and the potential application as nanoscaled phase-change material is highlighted.     

Photo-regulated metal coordination of azobenzene polymer having sterically controlled ion binding sites

Summary 2,2-Dimethylazobenzenes having metal chelating ligands at meta-positions were synthesized, and the interaction with metal ions were estimated for syn- and anti-ligands on thermal isomerization. The azobenzenes with the metal-interaction were transformed to vinyl azobenzenes and copolymerized with styrene. The metal extracting ability was found in photo-isomerized polymer having syn-bis(iminodiacetic acid) groups; the selectivity was high for Cu(II) ion.     

Influence of chromophoric electron-withdrawing groups on photoinduced deformation of azo polymer colloids

This study investigated the influence of chromophoric electron-withdrawing groups on photoinduced deformation behavior of colloidal spheres of three azo polymers. The colloidal spheres were prepared by using the epoxy-based azo polymers (BP-AZ-CA, BP-AZ-CN, and BP-AZ-NT) through gradual hydrophobic aggregation of the polymers in THF-H₂O media. The colloidal spheres were controlled to have similar average sizes by adjusting both the polymer concentrations in the initial THF solutions and water-adding rates in the preparation processes. The colloids were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The colloid deformation was investigated by irradiating the colloidal spheres in solid state with a linearly polarized Ar⁺ laser beam (488 nm, 130 mW/cm²). For comparison, the colloids were also studied by irradiation with a polarized diode solid state laser beam (532 nm, 130 mW/cm²). Upon the light irradiation, all the colloidal spheres were elongated along the polarization direction of the laser beams. The electron-withdrawing groups showed significant influence on the colloid deformation behavior related with the response to the light irradiation. The colloid deformation was more efficiently induced by irradiation with the laser beams having the intermediate wavelengths between the λ_max and the absorption band tails of the azo chromophores. When the hydrophilic carboxylic group was used as the electron-withdrawing groups, more significant deformation was induced under the same light irradiation condition, which could be attributed to the higher hydrophilicity of the polymer. Above observations can lead to a better understanding of the photoinduced deformation mechanism of azo polymer colloids.     

Enhancement of surface adhesion between thermoplastic resin and carbon fiber using polymer colloids

To enhance the interfacial adhesion between carbon fiber and thermoplastic resin, poly(methyl methacrylate) (PMMA) particles were adsorbed on the carbon fiber. It was found that positively charged PMMA particles were readily adsorbed on the carbon fiber, and the interfacial shear strength between the modified carbon fiber and the resin was enhanced. In addition, the interaction between the carbon fiber coated with particles to the PMMA resin would be improved, and the surface adhesion between them was strengthened.     

The magnetic assembly of polymer colloids in a ferrofluid and its display applications

Nonmagnetic polymer colloids have been assembled into colloidal photonic crystals in a ferrofluid by applying an external magnetic field based on the dipole-dipole interactions of "magnetic holes". The photonic crystal disassembles immediately when the magnetic field is removed. The mechanism of assembly can be explained by two simultaneous processes: phase separation and colloidal assembly. In this work, increasing the size of the building blocks still produces colorful photonic crystals due to their 2nd order diffraction. With a larger building block, the magnetic response between the polymer colloids is greatly enhanced so that an instant and reversible assembly/disassembly can be realized in a much weaker magnetic field and lower ferrofluid concentration. Based on these investigations, a magnetically controlled photonic display unit has been fabricated, which works in a weak magnetic field, has stable reflection signals and possesses fast and reversible on/off switching of reflections.     

Emulsifier-free synthesis of monodisperse core–shell polymer colloids containing chloromethyl groups

Both the synthesis of m,p-trimethyl(vinylbenzyl) ammonium chloride (TMVBAC) and the synthesis of polymer colloids consisting of a shell of poly(vinylbenzyl chloride) grafted onto a well-defined, monodisperse poly(styrene–divinylbenzene) core are described. The preparation of the polymer latices consists of a sequence of different emulsifier-free emulsion polymerizations. First, monodisperse poly(styrene–divinylbenzene) seed latices, with 2,2′-azobis(2-amidinopropane) hydrochloride (AIBA·2HCl) as cationic initiator, are prepared under batch conditions. These latices are used for the seeded polymerization of divinylbenzene (DVB), in order to synthesize monodisperse particles, with a well-defined spherical shape, which contain a sufficient amount of pendant vinyl groups for grafting of the vinylbenzyl chloride (VBC) monomer. After the graft polymerization, the chloromethyl groups can easily be modified with a postpolymerization reaction. As an example, the amination with trimethylamine (TMA) is described. © 1995 John Wiley & Sons, Inc.