Morphology and switching of holographic gratings containing an azo dye

The effects of an azo dye on the diffraction efficiency, morphology and electro-optic properties of the transmission mode of a holographic polymer dispersed liquid crystal (LC) have been studied. The azo dye induced an induction period which otherwise does not exist, followed by a gradual increase of the diffraction efficiency to a saturation value which increased with increasing azo dye content, as a result of the azo dye reorientating LC molecules within the droplet. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of the LC by the dye molecules, and to LC orientation induced by azo dye molecules giving a high refractive index contrast. The droplet size decreased with increasing dye content. The dye also lowered the threshold voltage due to the high dielectric anisotropy caused by the presence of a strong on-axis dipole moment and decreased the response time.     

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.     

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.     

Photoswitching of holographic polymer-dispersed liquid crystals doped with chiral dopant

Chiral dopants were added to the formulation of holographic polymer-dispersed liquid crystals and the effects studied in terms of grating formation dynamics, morphology, diffraction efficiency, contrast ratio and electro-optical properties of the films. A gradual increase of real-time diffraction efficiency, decrease of droplet size and increase of diffraction efficiency of the composite film were obtained with the addition and increasing content of chiral dopant, due to the increased viscosity of the liquid crystal (LC) doped with the chiral dopant leading to decreased droplet coalescence. The contrast ratio decreased with increasing content of chiral dopant due to the difficult orientation of LC molecules caused by the formation of a helical structure. Addition of a small amount of the chiral dopant increased the driving voltage slightly, whereas the decay time is decreased significantly as a result of the high twisting of the helical structure.     

Effects of silicone monomer on the grating formation of holographic PDLC

To augment polymer-LC phase separation and driving characteristics of holographic polymer dispersed liquid crystals (HPDLCs), various types and contents of silicone monomer viz. vinyloxytrimethylsilane (VOTMS), 1,3-divinyltetramethyldisiloxane (DVTMDS) and 3-dicyclohexyl-1,1,3,3-tetrakis (dimethylvinylsilyloxy) disiloxane (DCHTDS) were added to the conventional formulations for HPDLC in a broad range of content. It was found that grating formation becomes fast in proportion to its content regardless of the type of silicon monomer due to the enhanced polymer-LC phase separation driven by the increased immiscibility. SEM and AFM showed that DCHTDS gave by far the greatest extent of phase separation. VOTMS and DVTMDS preferentially migrated toward the surface or polymer-LC interface, which effectively lowered the surface anchoring and reduced driving voltage. Due to the droplet coalescence and poor migration of DCHTDS toward the interfaces, DCHTDS gave low diffraction efficiency and high driving voltage in spite of its great and fast phase separation with LC molecules.     

Effect of graphene doping of holographic polymer‐dispersed liquid crystals

Doping a polymer matrix with a minute amount of graphene (0.05–0.25%) had significant effects on the grating formation kinetics and electro‐optical performance of a holographic polymer‐dispersed liquid crystal. Low graphene contents (≤0.1%) reduced the viscosity and induced rapid diffusion, curing, grating formation, and high diffraction efficiency with a diffraction overshoot of 0.05%. Conversely, high graphene contents increased the compound viscosity, and the entire process proceeded slowly. Graphene increased the polymer conductivity and local electric field, reduced the operating voltage from 65 to 25–50 V, and increased the contrast ratio from 7 to 8–23 with a concomitant decrease in rise time. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Effects of multiwalled carbon nanotube on holographic polymer dispersed liquid crystal

The morphology and electro‐optical performance of holographic polymer dispersed liquid crystal (HPDLC) were investigated with the addition of multiwalled carbon nanotube (CNT), both pristine and chemically modified one (CNT‐C?C). With the addition of CNT, the diffraction efficiency increased and showed a maximum at 0.6% while nucleation was delayed due to the increased mixture viscosity. Film was driven only with CNT due to the induced local electric field of polymer to overcome the threshold resistance. Among the two types of CNT, chemically modified one gave finer CNT dispersion, lower mixture viscosity, larger liquid crystal (LC) droplet, higher diffraction efficiency, and shorter response time while the pristine CNT decreased the driving voltage. Copyright © 2010 John Wiley & Sons, Ltd.     

Coalescence of droplets in viscoelastic matrix with diffuse interface under simple shear flow

The coalescence process of two droplets in simple shear flow was modeled and simulated by the diffuse interface method. The collision between two droplets was investigated. The systems with small Peclet number, which denotes highly diffuse ability of concentration, were found to coalesce faster and easier due to the overlap of interfacial layers. The effect of matrix elasticity on droplet coalescence was studied thoroughly. The matrix elasticity was found to decrease the hydrodynamic interactions between droplets, and delay the coalescence process. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1856–1869, 2007     

Study on the electro-optical properties of polyimide-based polymer-dispersed liquid crystal films

The reflectivity control device, initially developed for attitude control, is utilised to control the solar sail orbit by switching the states between absorption and specular reflection. Actually, the major parts of the device are the polymer-dispersed liquid crystal (PDLC) films. Here, PDLC films based on polyimide (PI) as polymer matrix and a low molecular weight LC can be prepared by the thermally induced phase separation (TIPS) method. The influences of cooling rate and the content of LC on the size and uniformity of LC droplets dispersed in a polymer matrix by a TIPS process were investigated. It was found that a fast cooling rate gave smaller droplet sizes and hence a more uniform distribution as compared to the ones produced under a slow cooling rate. If the LC content was increased, the droplet size would be increased. Furthermore, the effect of LC droplet size on the electro-optical properties of the PI-based PDLC films was discussed, such as transmittance, threshold voltage, driving voltage and contrast ratio (CR).     

Influence of fullerene photodimerization on the PCBM crystallization in polymer: Fullerene bulk heterojunctions under thermal stress

For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo‐stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light‐induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1209–1214     

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     

Dye-dependent studies on droplet pattern and electro-optic behaviour of polymer dispersed liquid crystal

In order to study the droplet pattern and electro-optic (EO) behaviour of polymer dispersed liquid crystal (PDLC) with the addition of dye, dichroic polymer dispersed liquid crystal (DPDLC) films were prepared using a nematic liquid crystal (NLC), photo-curable polymer (NOA 65) and anthraquinone blue dichroic dye (B2), in equal ratio (1:1) of polymer and liquid crystal (LC) by polymerisation induced phase separation (PIPS) technique. Dichroic dye was taken in different concentration (wt./wt. ratio) as 0.0625%, 0.125%, 0.25%, 0.5% and 1% of the LC mixture in DPDLC films. Initially, in an open circuit when there is no proviso for external electric field (0 V), LC droplets in polymer matrix exhibited bipolar pattern, though on closing the circuit with the increase of electric field pattern of droplets starts changing, LC molecules align along the direction of applied electric field and aligned completely relatively at higher field (30 V), which illustrate vertical radial pattern. Further, results show that the DPDLC film containing 0.0625% dye concentration with consistent average droplet size ~4.30 μm, exhibits the best transmission at lower operating voltage.     

Sterically stabilized emulsion polymerization of styrene: Pseudo‐semicontinuous approach

The sterically stabilized emulsion polymerization of styrene initiated by a water‐soluble initiator at different temperatures has been investigated. The rate of polymerization (R_p) versus conversion curve shows the two non‐stationary‐rate intervals typical for the polymerization proceeding under non‐stationary‐state conditions. The shape of the R_p versus conversion curve results from two opposite effects—the increased number of particles and the decreased monomer concentration at reaction loci as the polymerization advances. At elevated temperatures the monomer emulsion equilibrates to a two‐phase or three‐phase system. The upper phase is transparent (monomer), and the lower one is blue colored, typical for microemulsion. After stirring such a multiphase system and initiation of polymerization, the initial coarse polymer emulsion was formed. The average size of monomer/polymer particles strongly decreased up to about 40% conversion and then leveled off. The initial large particles are assumed to be highly monomer‐swollen particles formed by the heteroagglomeration of unstable polymer particles and monomer droplets. The size of the “highly monomer” swollen particles continuously decreases with conversion, and they merge with the growing particles at about 40–50% conversion. The monomer droplets and/or large highly monomer‐swollen polymer particles also serve as a reservoir of monomer and emulsifier. The continuous release of nonionic (hydrophobic) emulsifier from the monomer phase increases the colloidal stability of primary particles and the number of polymer particles, that is, the particle nucleation is shifted to the higher conversion region. Variations of the square and cube of the mean droplet radius with aging time indicate that neither the coalescence nor the Ostwald ripening is the main driving force for the droplet instability. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 804–820, 2003     

Effects of surfactant and molecular weight of polyol on grating formation and switching of holographic PDLC

The interposition of surfactants between polymer and liquid crystal (LC) droplets was theoretically predicted by the positive spreading coefficient (0 < λ₃₁) and utilized to interpret the morphology, grating formation kinetics, diffraction efficiency, and switching of the holographic polymer dispersed liquid crystal (HPDLC), prepared from various types (octanoic acid, poly oxyethylene octyl phenyl ether, and perfluoro‐1‐butanesulfonyl fluoride) and amounts (0–9 wt%) of surfactant and molecular weights of polyol (PPG). Regardless of the surfactant type, diffraction efficiency increased with the addition and increasing amount of surfactant, a tendency consistent with increasing value of spreading coefficient, which is determined by the formulations of grating formation. In contrast, diffraction efficiency showed a maximum with the polypropylene glycol (PPG) molecular weight. Surfactant effectively reduced the anchoring energy and electrically drove the film which otherwise was not driven. Overall, surfactant with greater λ₃₁ gave smaller droplet, greater diffraction efficiency, driving voltage, contrast ratio, and smaller response time. Copyright © 2008 John Wiley & Sons, Ltd.     

Phase ripening in particulate binary polymer blends

Particulate polymer‐in‐polymer mezodispersions show a pronounced increase in the size of the dispersed particles during melt‐phase annealing. Three ripening mechanisms have been proposed: Brownian coalescence, Ostwald ripening, and hydrodynamic coarsening. The modified Cahn–Hilliard equation predicts growth by Ostwald ripening and diffusion‐induced coalescence. Simulations of this mechanism show a self‐similar particle size distribution, but the distribution broadens with the increasing volume fraction of the minor phase. Hydrodynamic coarsening caused by concentration gradients and random Brownian forces has been simulated according to the hydrodynamic model. The simulations show that concentration‐driven hydrodynamics have little effect on the particle size distribution. Experiments have been performed to investigate the relative importance of these ripening mechanisms for polybutadiene in a polystyrene system. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 603–612, 2004     

In situ current voltage measurements for optimization of a novel fullerene acceptor in bulk heterojunction photovoltaics

The evaluation of the power conversion efficiency (PCE) of new materials for organic bulk heterojunction (BHJ) photovoltaics is difficult due to the large number of processing parameters possible. An efficient procedure to determine the optimum conditions for thermal treatment of polymer‐based bulk heterojunction photovoltaic devices using in situ current‐voltage measurements is presented. The performance of a new fullerene derivative, 1,9‐dihydro‐64,65‐dihexyloxy‐1,9‐(methano[1,2] benzomethano)fullerene[60], in BHJ photovolatics with poly(3‐hexylthiophene) (P3HT) was evaluated using this methodology. The device characteristics of BHJs obtained from the in situ method were found to be in good agreement with those from BHJs annealed using a conventional process. This fullerene has similar performance to 1‐(3‐methoxycarbonyl)propyl‐1‐phenyl‐[6,6]‐methano fullerene in BHJs with P3HT after thermal annealing. For devices with thickness of 70 nm, the short circuit current was 6.24 mA/cm² with a fill factor of 0.53 and open circuit voltage of 0.65 V. The changes in the current‐voltage measurements during thermal annealing suggest that the ordering process in P3HT dominates the improvement in power conversion efficiency. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Optical order of the polymer phase within polymer/fullerene blend films

This study reports on how the degree of polymer order within a polymer/fullerene blend can be investigated by spectroscopic methods. Non‐annealed blend compositions with 0–80 wt % fullerene content were analyzed using temperature dependent photoluminescence (PL) and room temperature spectroscopic ellipsometry (SE) measurements. To evaluate the SE data with respect to the optical order, an optical model was developed, including a lower and higher ordered polymer phase within a fullerene matrix. This was done using an effective medium approach describing the polymer by combining lower and higher ordered polymer properties (polymer‐EMA). The polymer/fullerene blend was then evaluated using another EMA consisting of the polymer‐EMA and the dielectric function of the disordered fullerene. The degree of optical order obtained by SE, was confirmed using another independent measurement, photoluminescence spectroscopy, according to the method of Francis C. Spano (2005). The volume fraction of the ordered polymer within the polymer‐EMA was found to be between 70 and 60 vol % for fullerene contents lower than 20 wt % in the polymer/fullerene blend. Above 20 wt % fullerene, the optical order of the polymer strongly decreases all the way down to 0 vol %. In contrast to the complementary performed X‐ray diffraction measurements, which address only the long‐range structural order of the blends, we give quantitative information on the optical order, including information on the composition, that is, volume fractions of the higher and lower ordered polymer. The gained information on the tilt of the polymer molecules with respect to the substrate is discussed comparing XRD results from the literature with those obtained by our SE model. Finally, the developed model is used to describe the influence of the P3HT molecular weight on the optical order. Results obtained with our model were compared to the structural data and mobility data in the literature. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Size control of phase‐separated liquid crystal droplets in a polymer matrix based on the phase diagram

When a mixture of liquid crystal (LC) and photo reactive monomer is irradiated by UV light, polymerization occurs and LC droplets form through phase separation, producing polymer dispersed LCs (PDLCs). Although size control of LC droplets and reduced amounts of LC in PDLC films are important in applications, precise size control of LC droplets at a low LC fraction has not yet been accomplished. In this study, the phase diagrams of the LC/initial monomer and the LC/polymer during polymerization were used to control LC droplet size at various LC fractions. Both the relative position of the sample in the initial phase diagram and the shift of the phase separation line during polymerization were shown to be important in determining the size of LC droplets. Our results are expected to provide a new strategy for precise size control of LC droplets especially at a low LC fraction range, which would be a great help for PDLC applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Migration of nanosilica particles in polymer blends

Hydrophilic pyrogenic silica melt mixed in immiscible polypropylene/poly (ethylene‐co‐vinyl acetate) (PP/EVA) blend was found to migrate from the PP matrix to the EVA dispersed domains and remained confined inside them. Surprisingly, it was shown than silica was also able to migrate from a dispersed PP phase to an EVA matrix but this transfer was slower and not complete. The same silica with a hydrophobic surface treatment moved and accumulated to the blend interface and in PP. The mechanisms from which this migration proceeds are discussed. Whereas self diffusion of the particles was shown to have almost no effect, shear induced movements and collisions with dispersed drops is believed to be the most efficient mechanism. The possible trapping of silica aggregates during droplet–droplet coalescence was impossible to observe but is thought to be a possible additional mechanism. No quantification on the relative importance of the latter phenomenon can be drawn at the moment. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1976–1983, 2008     

Effect of droplet size on the dielectric properties of PDLC films

Polymer dispersed liquid crystal (PDLC) films (5CB/PMMA, 60/40) of different droplet size were prepared by a solvent-induced phase separation method under different N₂ flow speeds. The effects of droplet size on the thermal transitions of the LC and various dielectric properties such as dielectric constant, conductance, dielectric loss, and the electric field induced in a droplet were examined. The configuration of the LC in the film with smaller droplets can be identified by comparing the dielectric constant of the film with the one predicted by Boettcher's mixture formula. In addition, the effect of droplet size on the electro-optical response of the PDLC film was investigated. Variations of the conductance and the dielectric constant of the film were analyzed under various AC frequencies, with the purpose of elucidating the polarization mechanism of the LC molecules in the droplet. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1373–1381, 1997