The electro-optical properties of 'charged' PDLCs

The application of high intensity electric fields to polymer dispersed liquid crystal (PDLC) films can induce changes in their electro-optical properties and morphology. In particular, a quasilinear electro-optical response to an external electric field can be achieved if an internal built-in d.c. field is induced. In this work, we show how the liquid crystal/polymer weight ratio influences the electro-optical response of 'charged' PDLCs, i.e. of PDLC films after the application of a high intensity electric field. We observed that a quasilinear electro-optical response can be achieved in a well determined range of composition. Larger liquid crystal concentrations are unable to maintain the built-in field, while PDLCs with lower liquid crystal loadings do not allow the onset of a built-in d.c. field.     

Surfactant effects on morphology and switching of holographic PDLCs based on polyurethane acrylates.

Effects of octanoic acid (OA) on the morphology, diffraction efficiency, and electro-optic properties of the transmission mode of holographic polymer-dispersed liquid crystals (HPDLC) are studied. Droplet size decreases with increasing OA content (0-9 %), and this leads to a monotonic increase in off-state diffraction with increasing OA content. However, on-state diffraction decreases with increasing applied voltage and shows a minimum at 6 % OA, for which minimum switching voltage (5 V microm(-1)) and maximum contrast ratio (10) are obtained. Rise time and decay time decrease with increasing OA content. Interposition of OA between polymer and LC droplet is theoretically predicted by the spreading coefficient (lambda>0) calculated on the basis of the solubility parameter, while the coalescence behavior of droplets is described by a dimensionless group (gamma d rho / mu(2)) called coalescence number.     

Ultraviolet-curable epoxy acrylate dispersions: effect of urethane acrylate anionomers on stabilizing and film properties

 Epoxy acrylate dispersions stabilized using urethane acrylate anionomers were prepared for an application of ultraviolet (UV) curing. By observing the optical microscopy and colloidal stability for the epoxy acrylate dispersions, it was confirmed that the urethane acrylate anionomers incorporated have an interfacial activity in the interface between the epoxy acrylate oil and the water/ ethanol mixture (80/20, w/w). This was possible by the structurally designed urethane acrylate anionomers, containing a hydrophobic soft segment and two hydrophilic ionic sites in their molecules. In addition, when ultraviolet (UV)-cured, the urethane acrylate anionomers agglomerated to form the rubber domains in the epoxy acrylate film, which were induced by the ionic interaction. Consequently, this agglomerated rubber domains improved the final film properties. Received: 4 April 1998 Accepted: 1 July 1998     

The effect of fluorine-substituted acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystals

The effects of fluorinated acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystal (PDLC) films are reported. The partial fluorination of host polymer matrices resulted in improved optical properties and better defined morphologies. An enhancement in contrast ratio was observed for fluorinated systems containing trifluoroethyl acrylate (TFEA) and hexafluoroisopropyl acrylate (HFIPA). Conversely, the incorporation of methyl acrylate (MA), a chemically similar non-fluorinated acrylate, resulted in no appreciable change in contrast ratio and an increase in relaxation time. Scanning electron microscopy morphological studies were conducted to understand further the influence of fluorinated monomers in PDLC systems.     

The use of nonionic polymerizable surfactants in latexes and paints

In combination with anionic surfactants nonionic polymerizable surfactants lead to stable dispersions, which can be used as binders for paints. Generally the obtained results are worse than with a nonpolymerizable ethoxylated fatty alcohol. However the properties can be optimized through the right choice of the anionic surfactant. This further work is in progress.     

The effect of fluorine-substituted acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystals

The effects of fluorinated acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystal (PDLC) films are reported. The partial fluorination of host polymer matrices resulted in improved optical properties and better defined morphologies. An enhancement in contrast ratio was observed for fluorinated systems containing trifluoroethyl acrylate (TFEA) and hexafluoroisopropyl acrylate (HFIPA). Conversely, the incorporation of methyl acrylate (MA), a chemically similar non-fluorinated acrylate, resulted in no appreciable change in contrast ratio and an increase in relaxation time. Scanning electron microscopy morphological studies were conducted to understand further the influence of fluorinated monomers in PDLC systems.     

The effect of MWS polarisation on the morphology and electro-optical behaviour of normal-mode polymer-stabilised cholesteric textures

We investigate the process and the resulting properties of the curing electric-field-induced segregation of reactive monomers (containing polar groups) in a host liquid crystal. The obtained dielectric spectrum of these cholesteric liquid crystals (CLCs) reveals polarisation at low frequencies. This is a strong indication that charge carriers accumulate at the interface between the sample and the electrodes in these compounds and consistent with Maxwell–Wagner–Sillars (MWS) polarisation. A polymer film forms on the substrate with its thickness determined by the MWS polarisation strength. The SEM images show that the polymer network becomes less dense when MWS polarisation increases. The results are a higher operating voltage and faster response time.     

Use of surfactants in cellulose nanowhisker/epoxy nanocomposites: effect on filler dispersion and system properties

Cellulose nanowhiskers (CNWs) prepared via TEMPO mediated oxidation are used as biodegradable filler in an epoxy matrix. Since CNWs are hydrophilic and epoxy is hydrophobic, amphiphilic block copolymer surfactants are employed to improve the interactions between the filler and the matrix. The surfactants used are Pluronics, a family of triblock copolymers containing two poly(ethylene oxide) blocks and one poly(propylene oxide) block. In this study, Pluronic L61 and L121 with molecular weight of 2000 and 4400 g/mol and hydrophilic to lipophilic balance of 3 and 1 respectively, are used and their effect on the dispersion of CNWs in epoxy is discussed. The hydrophilic tails of Pluronics interact with the hydroxyl and carboxylic groups on the CNW surface and then these surfactant-treated CNWs are directly incorporated into epoxy by high speed mixing. The dispersion state of the surfactant-treated CNWs in epoxy is assessed by rheological measurements and the mechanical properties of the resulting composites are characterized by tensile test and dynamic mechanical thermal analysis. The Pluronic L61 treated CNW/epoxy composites show the highest storage modulus at high temperatures (about 77 % increases) indicative of improved interfacial interactions between the CNWs and the epoxy matrix. Also, an increase of around 10 °C in the glass–rubbery transition temperature of the L61 treated CNW/epoxy composite leads to potential application at higher service temperatures.     

Electrical and positron annihilation study on epoxy and epoxy acrylate composites

Epoxy acrylate resin was prepared by endcapping the acrylic acid to epoxy resin backbone in the presence of triphenyl phosphene as catalyst. The structure was elucidated by IR and NMR spectroscopy. Epoxy and epoxy acrylate composites were prepared by mixing different concentrations of mica, magnesium hydroxide and calcium silicate with each epoxy/hardener and epoxy acrylate/styrene mixtures, respectively. The permittivity ε′, dielectric loss ε′′ and loss tangent tan δ were measured for these composites in the frequency range (10²-10⁶ Hz) and at 30 °C. The data obtained were analyzed into two absorption regions related to Maxwell-Wagner effect and to some local molecular motions rather than the main chain motion. The higher values of ε′ and the lower values of tan δ given for the composites containing the epoxy acrylate resin indicate some improvement in the dielectric properties when compared with those containing the epoxy resin. The effect of filler type and filler content on the positron annihilation lifetime and its intensity as well as S-parameter for epoxy and epoxy acrylate composites were also studied. The high values of S-parameter noticed by with increasing filler content indicates some increase in free electrons which lead to an increase in electrical conductivity. The highest value of hardness was obtained in the case of calcium silicate followed by mica and magnesium hydroxide.     

Effects of the structures of epoxy monomers on the electro-optical properties of heat-cured polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) films were prepared from thermal polymerisation-induced phase separation in heat-curable monomers/nematic liquid crystal (LC) mixtures. For PDLCs with a certain amount of LCs, the microstructure and the refractive index of polymer networks could be influenced by the relative content of epoxy monomers, owing to their different chemical structures. The effect of these factors on the electro-optic properties of films was also investigated.     

Properties and morphologies of UV‐cured epoxy acrylate blend films containing hyperbranched polyurethane acrylate/hyperbranched polyester

The properties and morphologies of UV‐cured epoxy acrylate (EB600) blend films containing hyperbranched polyurethane acrylate (HUA)/hyperbranched polyester (HPE) were investigated. A small amount of HUA added to EB600 improved both the tensile strength and elongation at break without damaging its storage modulus (E′). The highest tensile strength of 31.9 MPa and an elongation at break around two times that of cured pure EB600 were obtained for the EB600‐based film blended with 10% HUA. Its log E′ (MPa) value was measured to be 9.48, that is, about 98% of that of the cured EB600 film. The impact strength and critical stress intensity factor (K_1c) of the blends were investigated. A 10 wt % HUA content led to a K_1c value 1.75 times that of the neat EB600 resin, and the impact strength of the EB600/HPE blends increased from 0.84 to 0.95 kJ m^?1 with only 5 wt % HPE addition. The toughening effects of HUA and HPE on EB600 were demonstrated by scanning electron microscopy photographs of the fracture surfaces of films. Moreover, for the toughening mechanism of HPE to EB600, it was suggested that the HPE particles, as a second phase in the cured EB600 film, were deformed in a cold drawing, which was caused by the difference between the elastic moduli of HPE and EB600. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3159–3170, 2005     

Synthesis and characterization of new cationic quaternary ammonium polymerizable surfactants

Two new polymerizable surfactants (surfmers), (11-acryloyloxyundecyl)dimethylethylammonium bromide (ethyl surfmer) and (11-acryloyloxyundecyl)dimethyl(2-hydroxyethyl)ammonium bromide (hydroxyethyl surfmer), were synthesized and characterized. The binary phase diagrams of both surfmer/water systems are described. Both surfmers can form isotropic solutions and lamellar lyotropic liquid crystalline phases. The hydroxyethyl surfmer/water system forms a lamellar phase for weight concentrations of surfmer between 70 and 90% relative to between 75 and 85% for the ethyl surfmer/water system. The differences in the self assembly of these surfmers were attributed to the ability of hydroxyethyl surfmer to form hydrogen bonds (between two head groups and with water), whereas no such interactions can occur with the ethyl surfmer system.     

Photo-oxidative stability of electron beam and UV cured acrylated epoxy and urethane acrylate resin films

The post-cured photo-oxidative stability of urethane and bisphenol-A epoxyacrylate resins in mixed compositions with triacrylate and amine diacrylate resins are examined using UV and reflectance infra-red absorption spectroscopic techniques. Overall, by measuring the growth in hydroxyl absorption at 3400 cm⁻¹, electron-beam cured resin films are more photostable than UV curedfilms, indicating the high photo-activity of residual photoinitiator in the latter case. Regarding the UV cured systems benzophenone is found to be a more photo-active residual photoinitiator than the benzoyl ester photofragmenting types. Films containing the amine diacrylate resin are more photostable than those containing the triacrylate resin. This stabilising effect is associated with the oxygen and radical scavenging ability of the terminal amine groups. Photo-yellowing, as measured by the growth in an absorption band at 280 nm, is observed only in resin films containing the amine diacrylate resin and is associated with the formation of unsaturated carbonyl groups. The latter are, however, photobleached on prolonged irradiation whereas with resins containing the bisphenol-A epoxy acrylate resin, a longer term photo-yellowing is observed due to oxidation of the bisphenol-A to give stilbene-quinone products.     

Dilatational properties and morphology of surface films spread from clinically used lung surfactants

The dilatational properties, structure, and morphology of the surface films spread at the air–water interface from complex lipid/protein systems were studied by measuring the surface pressure–area and surface potential–area isotherms, the surface rheological properties, and AFM images. The commercially available lung surfactants Alveofact, Curosurf, Survanta, and Exosurf were used as examples.The isotherms of the studied lung surfactant surface films are compared with model lipid and protein monolayers spread from bulk solutions. On the basis of a simple rheological model, the values for the elasticity and the specific time of relaxation related to the reorganization processes occurring in the monolayers were calculated. The spread films of natural surfactants Curosurf and Alveofact show a high effectiveness of spreading and respreading under the conditions of this study. These observations were confirmed by AFM imaging.     

Effects of monomer structure on the morphology of polymer networks and the electro-optical properties of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) films were prepared by photochemical polymerisation with a series of (meth)acrylate monomers. The effects of monomer structure on the morphology of polymer networks in the PDLC films were studied. The acrylate monomers without sidegroup chain formed uniform polymer networks. The methacrylate monomers with methyl as their sidegroup chains formed lace-like networks. The size of the LC droplets increased with increasing the length of the flexible chain of both methacrylate and acrylate monomers. Meanwhile, the effects of the morphology of the polymer network on the electro-optical properties of PDLC films were also investigated.     

Self-fixing medicinal films based on copolymers of N-vinylsuccinimide with n-butyl acrylate

The information on properties of films based on copolymers of N-vinylsuccinimide with n-butyl acrylate that are recommended as wound coverings was summarized. Atraumatic, gel-forming, gas-permeable elastic films that self-fix on skin were made from these copolymers by the method of pouring from solutions. As a result, the materials displaying antibacterial (dioxidin, furacylin, lincomycin, rifampicin) and anesthetic (trimecain, pyromecain, anesthesin, novocain) influences were prepared and investigated.     

Surface modification of TEMPO-oxidized cellulose nanofibers,and properties of their acrylate and epoxy resin composite films

Cellulose - The carboxy groups abundantly and densely present on 2,2,6,6-tetramehylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibers (TEMPO-CNFs) have been chemically modified to...     

Superior electro-optical performance in vertically aligned liquid crystal devices based on aluminum oxide films

We achieved vertically aligned (VA) liquid crystals (LCs) on aluminum oxide (Al₂O₃) films deposited via e-beam evaporation using a rubbing treatment. Uniform and vertical LC alignment was achieved and high thermal stability was obtained using these substrates. By analyzing measurements from optical retardations, we confirm that the LC orientation is adjustable using rubbing treatment. The superior electro-optical characteristics of the VA cells based on Al₂O₃ films are measured and compared with those based on polyimide layers, indicating that this approach will allow the fabrication of high-performance, advanced LC displays using a conventional rubbing process.     

Synthesis and characterization of UV-curable polydimethylsiloxane epoxy acrylate

UV-curable polydimethylsiloxane epoxy acrylate (PSEA) was synthesized by hydrosilylation of allyl glycidyl ether with hydrogen-containing polydimethylsiloxane to give polydimethylsiloxane-type epoxy resin which modified with acrylic acid. The curing speed and the double bond conversion in the UV cured film were influenced by the purity of PSEA with Fourier transform infrared spectroscopy (FT-IR) measurements. The influences of the synthetic process, such as, the reaction temperature, the concentration of reactants and the catalyst which determined the purity and activity of resins were discussed in detail. The structures of this resin were characterized by ¹H-NMR and FT-IR spectra. The molecular weight was checked by gel permeation chromatography, and M_n is 45,000. The properties of the cured film were also investigated by thermogravimetric analyzer, dynamical thermal mechanical analysis, and etc. For example, tensile strength (6.9 Mpa), elongation (20%), hardness (A; 18), water absorption (24 h; 2%), weight loss (40 min, 300 °C; 5%) and etc.     

Effects of chemical structure and composition of the polymer matrix on the morphology and electro-optical performance of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) systems on the basis of nematic liquid crystal E7 and amphiphilic binary copolymers of acrylic acid (AA) with such acrylates as 2-ethylhexyl acrylate (EHA), n-butyl acrylate (BA), and methyl acrylate (MA) are investigated. It is shown that the liquid crystal (LC) drops in the copolymer EHA–AA have submicrometre sizes, and their dependence on the composition of the photo-curable monomer mixture is described by a parabolic curve. The highly oriented domain structure in the same system is first revealed when electric field is applied. The threshold voltage for all systems begins to increase with some critical composition of a monomer mixture in which the longer the hydrocarbonic radical in an acrylate molecule, the higher the content of AA. The PDLC system based on the BA–AA copolymer with 30 wt% LC exhibits the least value of the driving voltage, 1 V μm^–1, and the lowest memory effect.