Optical microscopy studies of dynamics within individual polymer-dispersed liquid crystal droplets

Optical devices based on polymer-dispersed liquid crystal (PDLC) thin films derive their functional properties from the electric-field-induced reorientation of (sub)micrometer-sized polymer-dispersed liquid crystal (LC) droplets. In these materials, the LC reorientation dynamics are strongly dependent on droplet size and shape, as well as polymer/LC interfacial interactions. The dynamics also vary spatially within individual droplets. This Account describes studies of individual PDLC droplets and their field-induced dynamics by high-resolution near-field scanning optical microscopy (NSOM) and multiphoton-excited fluorescence microscopy (MPEFM). Included are studies of native ("pure") PDLCs and those doped with ionic compounds and dyes; the latter are used in sophisticated photorefractive materials.     

The kinetics and morphology of polymer dispersed liquid crystals by doped crylic acid

It is widely appreciated that electro‐optical activity in polymer dispersed liquid crystals (PDLCs) depends on phase separation of polymer and liquid crystals (LCs). The morphology of the LCs domains depends on the detail of the chemical and physical processes active during the formation of domains. This work discusses two‐phase morphology in an acrylate‐based system that developed during polymerization induced phase separation. UV/VIS spectrometer is used for monitoring the polymerization of the PDLCs by real‐time scattering. The doped crylic acid accelerated the speed of polymerization. The electro‐optical properties of PDLCs films are measured by Polarimeter (PerkinElmer Model 341). The lower threshold voltage was obtained by doped crylic acid at suitable ratio. The polarizing optical microscopy and Fourier transform infrared image system are used for depicting the morphology of LC droplets in polymer matrix. The sizes and dispersion of LC droplets were influenced by doped crylic acid which accelerated the speed of polymerization. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Optical property enhancement of dye‐PDLC using active reflector structure

The enhancement of electro‐optical properties in reflective‐type dye polymer dispersed liquid crystal (PDLC) has been achieved by applying an additional white PDLC layer along with dye PDLC layer. This newly modified structure that consists of white PDLC layer and scattering reflector acts as an active reflector. In this practice, an additional arrangement of a polymer barrier layer is made‐up over white PDLC layer, to block the absorption of any solution from dye PDLC. The contrast ratio of this new configuration is almost doubled with low driving voltage and high ON reflectance. Simultaneously, the “off” state has been observed darker than single layer dye PDLC structure. This new configuration can be potentially significant for various display applications such as E‐paper, outdoor billboard, and flexible display. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Optimization of LC droplet size and electro‐optical properties of acrylate‐based polymer‐dispersed liquid crystal by controlling photocure rate

We investigated the effects of the different content ratios of 2‐ethylhexylacrylate (2‐EHA) and 2‐ethylhexylmethacrylate (2‐EHMA) on the relationships among the photopolymerization rate, morphology of liquid crystals (LCs) droplets, and electro‐optical properties of trifunctional urethane acrylate‐based polymer‐dispersed liquid crystal (PDLC) systems. Photo‐differential scanning calorimetry (DSC) analysis and resistivity measurement revealed that increasing 2‐EHMA content gradually decreased the photocure rate of trifunctional urethane acrylate‐based PDLCs, which prolonged the phase separation between the LC molecules and the prepolymers. Morphological observations and electro‐optical measurements demonstrated that trifunctional urethane acrylate‐based PDLCs with the 2‐EHA:2‐EHMA ratios from 4:1 to 3:2 in weight percent formed the favorable microstructures of LC droplets being within the range of 1–5 µm to scatter light efficiently and showed the satisfactory off‐state opacity and on‐state transmittance and the relatively low‐driving voltage. The microstructures of LC droplets and electro‐optical properties of trifunctional urethane acrylate‐based PDLCs could be usefully optimized by controlling the photocure rate using the different 2‐EHA/2‐EHMA content ratios. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3098–3104, 2013     

Photopolymerization behavior and phase separation effects in novel polymer dispersed liquid crystal mixture based on urethane trimethacrylate monomer

Polymer dispersed liquid crystals (PDLCs) are often formed by polymer induced phase separation, based on photopolymerization of multifunctional acrylate monomers. The emerged morphology is controlled by the interplay between polymerization rate and phase separation dynamics, which depends on different parameters such as monomer structure and functionality. In this work, a new PDLC formulation containing urethane trimethacrylate (UTMA) monomer is introduced, which has different molecular weight evolution, polymer gel point, and polymerization kinetics in comparison with some common ester acrylate (such as TMPTA and DPHPA) based PDLC compositions. UTMA is synthesized and characterized by Fourier transform infrared, ¹H‐NMR, and ¹³C‐NMR spectroscopic techniques. Simultaneous examination of polymer evolution and LC phase separation by real‐time infrared spectroscopy shows that the UTMA based PDLC, which contains trifunctional urethane acrylate monomer, has greater amount of bond conversion, polymerization rate, and liquid crystal (LC) phase separation in comparison with TMPTA based PDLC. In spite of the acrylate monomers, which show gel point conversions as low as 1.83–5.72%, UTMA reaches to its maximum rate at 19.5% conversion, which causes higher phase separation and therefore greater LC domain size. The experimental results are explained more precisely by means of SEM and optical microscopy analyses. The results are confirmed by electro‐optics measurements. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fabrication of polymer‐dispersed liquid crystals with low driving voltage based on the thiol‐ene click reaction

Polymer‐dispersed liquid crystals (PDLCs ) with a well‐defined polymer matrix were successfully fabricated by the thiol‐ene click reaction based on poly(ethylene glycol) diacrylate (PEGDA ) and trimethylolpropanetris‐(3‐mercaptopropionate) (TMTP ). UV ?visible spectrophotometry, Fourier transform IR spectroscopy, SEM and polarized optical microscopy were employed to explore the PDLC films obtained. Electro‐optical properties were studied with a UV ?visible spectrophotometer. It was found that the PDLC films with optimal thiol content fabricated by the thiol‐ene click reaction showed high transmittance, low driving voltage and a low memory effect. It was concluded that the driving voltage change of PDLCs with different thiol concentrations was caused by the polymerization rate and the structure of the polymer matrix. © 2017 Society of Chemical Industry     

Effect of graft copolymer matrix prepared by reversible addition–fragmentation chain transfer and atom transfer radical polymerization on the electro‐optical properties of polymer‐dispersed liquid crystals

A type of graft macroinitiator, synthesized by reversible addition–fragmentation chain transfer (RAFT) and atom transfer radical polymerization, was employed to prepare polymer‐dispersed liquid crystals (PDLCs) with graft copolymer matrix; meanwhile, a linear macroinitiator was also synthesized via RAFT polymerization. The effect of linear and graft macroinitiators on the electro‐optical (EO) properties of the PDLCs was investigated. The results showed that the graft macroinitiator could make a large difference to the EO properties of the PDLCs. The memory effect was reduced remarkably, but the driving voltage increased and transmittance decreased. A possible mechanism is presented. © 2014 Society of Chemical Industry     

Temperature and liquid crystal concentration effect on thermal conductivity of poly(styrene) dispersed 5CB liquid crystal

In the present work, we study the thermal behavior of Polymer (Polystyrene) dispersed (4‐cyano‐4′‐pentylbiphenyl) 5CB liquid crystal film composite. A photopyroelectric device was used to study thermal conductivity at homeotropic and planar aligned of 4‐cyano‐4′‐pentylbiphenyl (5CB) liquid crystal. Thermal conductivity of polystyrene (PS) has been determined and calculated from experimental applied data reported in the literature. Thermal conductivity characteristics of the PDLC films were investigated with three prediction models as a function of both temperature and liquid crystal concentration in the polymer matrix. We particularly show the behavior of this thermal conductivity in the ON and OFF state. It was found that the difference in the film thermal conductivity ranges between 3 and 21%, depending on the ON and OFF state and the liquid crystal volume concentration. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 481–486, 2003     

Studies on electro‐optical properties of polymer matrix/LC/SiO2 nanoparticles composites

Polymer‐dispersed liquid crystal (PDLC) films were prepared by ultraviolet light‐induced polymerization of photopolymerizable monomers in nematic liquid crystal (LC)/monomers/SiO₂ nanoparticles composites, and the effect of SiO₂ nanoparticles on the electro‐optical properties of PDLC films was studied. The observed effect showed that by the adjustment of the SiO₂ nanoparticles content, the refractive index ratio of the LC and polymer could be modulated, and the electro‐optical properties of the polymer matrix/LC/SiO₂ nanoparticles composites could be optimized. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Preparation and phase behavior of blends of polysulfone‐based polymers with phosphorous‐containing smectic‐A liquid crystals

Preparation and experimental studies on the phase transitions and morphology of novel polymer‐dispersed liquid crystal (PDLC) mixtures of polysulfone UDEL‐3000 (PSU) or its derivatives (chloromethylated polysulfone or polysulfone containing lateral phosphaphenanthrene substituent) and a phosphorus‐containing liquid crystalline polymer (LCC) that exhibit isotropic/nematic/smectic‐A mesophase transition were performed. The mesomorphic properties of PDLC have been investigated by differential scanning calorimetry and polarizing optical microscopy. Mesophases structures were also investigated by X‐ray diffraction measurements, the diffraction pattern being in close dependence on the polysulfone structure and PDLC composition. The effect of the phosphorus content on the thermal properties was evaluated by thermogravimetric analysis. The results reveal that the composites were stable up to 337°C in air showing initial decomposition temperatures in the range of 337–368°C and a 10% weight loss in the range of 357–397°C. The presence of P in the structure of LCC leads to higher char yields at 700°C, for PDLCs composites, increasing with the content of phosphorus‐containing polymer. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Preparation of polymer‐dispersed liquid crystal films containing a small amount of liquid crystalline polymer and their properties

Polymer‐dispersed liquid crystal (PDLC) films were synthesized by the copolymerization of liquid crystalline polymer (LCP) precursor, urethane acrylate (UA), and mesogenic monomer (AI) at different conditions. The morphology of polymer matrix changed with the weight ratio of polymer/liquid crystal (LC) ratio and curing temperature, resulting in a large change in the droplet size of LC domains in the PDLC film. The components used in the synthesis of polymer matrix, that is, the weight ratio of LCP, AI, and UA, also strongly influenced the morphology of PDLC films. A small amount of LCP was copolymerized with UA and AI in the preparation of polymer matrix to improve the electrooptical properties such as the viewing angle. Added LCP also affected the morphology and the properties of PDLC. The hydrophobicity of LCP caused changes in the droplet size of LC domain in PDLC films and the anchoring energy between matrix polymer and LC droplets. As the hydrophobicity of the matrix increases, the droplet size of LC domain also increases; on the contrary, anchoring energy decreased, leading to the decrease of driving voltage. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3178–3188, 2000     

Studying optical and nonlinear optical properties of synthesized azo dyes doped in polymer and liquid crystal using birefringence and Z‐ scan techniques

The authors report the effect of the structure of new synthesized photo‐responsive azo dyes, based on N‐benzyl‐N‐ethyl‐aniline on their optical and nonlinear optical properties, when doped in polymer and liquid crystal (LC) hosts. Anisotropic response of dyes doped with polymer polymethyl methacrylate is studied using photo‐induced birefringence experiment and the optical nonlinearity of dyes doped in LC 5CB is measured using z‐scan technique. A correlation is established between dipole moment and optical/nonlinear optical response, which makes it possible to develop molecular design strategies to create structures with suitable anisotropic response and nonlinearity in various host systems for different applications. It is also shown how very little change in the structure of the dye may cause severe changes in the dipole moment and its nonlinear response. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

The effect of photoinitiator concentration and structures of RAFT macroinitiators on the memory effect of polymer dispersed liquid crystals

The article aimed to decrease the memory effect of polymer dispersed liquid crystals (PDLCs) films, reversible additional‐fragmental chain transfer (RAFT) polymerization was used to synthesize RAFT‐polystyrene (RAFT‐PS), which was a kind of RAFT macroinitiators and used to prepare PDLCs via photo polymerization induced phase separation (PIPS). Different RAFT agents were used to synthesize RAFT‐PS. The effect of photoinitiator concentration on the memory effect of these PDLCs was investigated, and the effect of structures of RAFT macroinitiators on the memory effect of the PDLCs was also discussed. The results showed that PDLC films with higher photoinitiator concentration and polymer matrix which had two rigid chains in two ends showed lower memory effect, which could be reduced from 10.9% to 2.5% in this experiment. POLYM. ENG. SCI., 55:8–13, 2015. © 2014 Society of Plastics Engineers     

Effect of temperature on the optical and electro-optical properties of poly(methyl methacrylate)/E7 polymer-dispersed liquid crystal composites

Experiments on the effect of temperature on the optical and electro-optical behaviors of polymer-dispersed liquid crystals (PDLCs) are considered. Composite films composed of poly(methyl methacrylate) (PMMA) and the nematic-type liquid crystal (LC) E7 were prepared by solvent casting in chloroform. The PDLC film contained droplets of E7 from 10 to 80 wt % in a PMMA matrix. Morphological studies illustrated the formation of isolated droplets of LC due to phase separation, and their homogeneous distribution increased with increasing E7 content. Thermo-optical studies showed an increase in the nematic–isotropic transition temperature of composites, which indicated preferential solvation during the phase-separation process. The electro-optical characteristics were studied under the conditions of an externally applied square wave electric field with a He–Ne laser (λ = 632.8 nm) as a light source. The responses improved as the E7 content in PMMA increased. Semipermanent memory effects were noticed in composites at higher temperatures. Changes in the transmittance due to thermal variations provided the possibility of using such a device as a temperature sensor. The results obtained indicate that under these experimental conditions, the output can be controlled to the desired level by the selection of a suitable loading of LC to prepare PDLC electro-optically active composite films with a response time on the order of only a few milliseconds. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A method for estimating interfacial tension of liquid crystal embedded in polymer matrix forming PDLC

A simple and convenient method based on sessile drop technique for measuring surface tensions of polymer and nematic liquid crystal (LC) is described. Contact angles formed by drops of probe liquids and a nematic LC on a photocurable polymer were measured. The surface energies were evaluated using the Fowkes method, Neumann's equation, and new equations developed based on Neumann's approach. The values of surface tensions were used to evaluate the interfacial interaction in term of work of adhesion between the LC and polymer. Further, the effect of dichroic dye on the extent of interaction and work of adhesion was examined by measuring contact angle in consequence of dye addition. A difference in work of adhesion between the lower and higher dye‐doped LC droplets gave an indication of affinity relationship between polymer and LC molecules. A change in work of adhesion resulted in variability of nematic director configurations inside phase separated LC droplets embedded in polymer matrix; when viewed under polarizing optical microscope. Thus, our approach of estimating surface energy of polymer and LC has found to be useful in determining interaction at polymer–LC interface. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41137.     

Influence of the multi-functional epoxy monomers structure on the electro-optical properties and morphology of polymer-dispersed liquid crystal films

Polymer dispersed liquid crystal (PDLC) films were prepared by polymerization-induced phase separation method with nematic LC content as low as 40 wt%, and the electro-optical properties were carefully investigated. To accomplish this, the structure of multi-functional curable epoxy monomers with different composition feed ratios and the weight percentages of the two groups were examined in this study. The combined effects of heat-curable monomers’ structure on the conspicuous morphology of polymer network of PDLC films formed small holes and suitably distributed coin-like networks in both groups A and B, respectively. The detailed characteristics and morphology of polymer network of PDLC films were analyzed by employing liquid crystal device parameter tester, UV-Vis-NIR spectrophotometer and scanning electron microscope. Meanwhile, the enhanced curing temperature effects on the alkyl chain length, short flexible chain length, and rigid chain segment containing epoxy monomers structure on the increasing morphology of polymer network as well as electro-optical properties of PDLC films were also studied. It was found that the LC domain size of the polymer network could be regulated by adjusting the structure and composition ratio of curable epoxy monomers, and then the electro-optics of the PDLC films could be optimized, which is beneficial for decreasing the total LC content in PDLC devices.     

Fabrication of binary metal doped CuO nanocatalyst and their application for the industrial effluents treatment

Herein, we fabricated the binary transition metal (Ce & Zn) doped CuO nanocatalyst via a single step facile co-precipitation technique by using liquid ammonia as a pH regulator and precipitating agent. The structural, morphological, and compositional studies of the fabricated samples were completed via X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and EDX techniques, respectively. The impact of binary metal-doped CuO nanocatalyst on the optical band-gap was examined via the UV-Visible spectroscopic technique. The photocatalytic aptitude of the fabricated pristine and binary metal-doped CuO nanocatalyst was examined against the 5-ppm aqueous solution of MB dye. The obtained results revealed that the doped sample removes 81.64% MB dye, via adsorption (32.65%) and degradation (48.99%) processes, while in comparison the pristine CuO sample removes just 38.77%. The superior adsorption and degradation aptitude of the binary metal-doped sample can be ascribed to its higher surface area and tuned band-gap, respectively. Moreover, the kinetic study of the degradation process also displayed that the doped sample degrades the MB dye with a higher value of the rate constant (0.0137 min^?1) than that of pristine CuO photocatalyst (0.0049 min^?1). The tuned band-gap and nanoarchitecture morphology of the doped CuO not only facilitate the excitation process but also assist in the transportation of the photo-induced species towards the surface of the photocatalyst. The observed superior photocatalytic activity of the binary metal-doped CuO photocatalyst showed its exceptional aptitude for the treatment of toxic industrial effluents.     

Structural effect of photoinitiators on electro‐optical properties of polymer‐dispersed liquid crystal composite films

Two types of photoinitiators were synthesized: (1) a α,ω‐telechelic oligomeric photoinitiator, by the reaction of poly(propylene glycol) diglycidylether (PPGDGE) and 2‐hydroxy‐2‐methyl‐1‐phenyl‐propan‐1‐one (Darocur 1173), and (2) a polymeric photoinitiator, by copolymerization of a monomer that had a liquid crystalline property, 4‐[ω‐(2‐methylpropenoyloxy)decanoxy]‐4′‐cyanobiphenyl, with a vinyl monomer that had a photosensitive group. For comparison, low‐molecular‐weight (low‐MW) photoinitiator (Darocur 1173) also was used. Attention was directed to the structural effect of the photoinitiators on the electro‐optical properties of polymer‐dispersed liquid crystal (PDLC) film in which the LC phase occupied a major volume (80 wt % of the composite film). For the preparation of PDLC films by the polymerization‐induced phase separation method, the optimum UV‐curing temperature was observed at 50°C, a temperature slightly higher than the cloud temperature (T_cloud) of the low‐MW LC/matrix‐forming material mixture. It was found that the electro‐optical performance of the PDLC cell fabricated with the oligomeric or polymeric photoinitiator was better than that of the PDLC cell made with a low‐MW photoinitiator (Darocur 1173), exhibiting lower driving voltage (V₉₀) and higher contrast ratio under identical formulation conditions. Oligomeric photoinitiators allowed premature phase separation between the LC and matrix phases, resulting in relatively pure LC‐rich phases. For the polymeric photoinitiator, incorporation of mesogenic moieties into the photoinitiator resulted in not only a well‐defined LC/matrix morphology but also in low driving voltage (V₉₀) because of reduced friction at the LC/matrix interfaces. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 162–169, 2006     

Synthesis and structural,optical, photocatalytic,and electrochemical properties of undoped and yttrium-doped tetragonal ZrO2 nanoparticles

Template-free undoped and yttrium-doped tetragonal ZrO₂ nanoparticles were successfully synthesized using a hydrothermal technique, and their structural, morphological, optical, and electrochemical properties were characterized. The photocatalytic activity for the dye degradation of the Rhodamine B (RhB) was also studied under UV light irradiation. The tetragonal crystal phase of the nanoparticles was confirmed by XRD analysis. The observed peak shift in XRD patterns confirmed the incorporation of dopant into host lattice. The Yttrium-doped ZrO₂ nanoparticles showed a higher specific surface area and smaller optical band gap (191.5?m²/g and 3.66?eV) than the pristine ZrO₂ nanoparticles (108.6?m²/g and 4.94?eV). The yttrium-doped ZrO₂ nanoparticles exhibited greater photocatalytic activity (~98%) than the pure ZrO₂ nanoparticles within 40?min, due to their high specific surface area and reduced photogenerated charge carrier recombination rate. The Nyquist plot of yttrium-doped sample exhibited lower charge transfer resistance than that of the undoped sample. The photocurrent of the doped sample (28.6?mA/cm²) was ~9 times higher than that of pristine ZrO₂ sample (3.2?mA/cm²). The doped sample showed stable and higher photocurrent density up to 520?s under light illumination.     

Interface morphology and phase separation in polymer-dispersed liquid crystal composites

It is widely appreciated that electro-optic activity in polymer-dispersed liquid crystals (PDLCs) depends on separation of the polymer and liquid crystal (LC) phases. Since the phase structure develops in a non-equilibrium system, the morphology of the LC domains depends on the details of the chemical and physical processes active during domain formation. The nature of the interface between the polymer and liquid crystal phases is of particular interest. This work discusses the two-phase morphology in an acrylate-based system that develops during polymerization-induced phase separation (PIPS). Using small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS), we find that interfaces in PDLCs developed from an acrylate-based recipe are more disordered than generally appreciated. Information gained from SAXS and USAXS is compared to data from scanning electron microscopy (SEM) and transmission electron microscopy (TEM). To elucidate the apparent discrepancies between imaging and scattering, we investigated the effects of SEM sample preparation. We observe significant alteration of the interface morphology due to the leaching of the LC phase.