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     

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     

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     

Enhancement of electro‐optical properties in holographic polymer‐dispersed liquid crystal films by incorporation of multiwalled carbon nanotubes into a polyurethane acrylate matrix

Holographic polymer‐dispersed liquid crystal (HPDLC) films were fabricated with varying amounts of multiwalled carbon nanotubes (MWCNTs) to optimize the electro‐optical performance of the HPDLC films. The MWCNTs were well dispersed in the prepolymer mixture up to 0.5 wt%, implying that polyurethane acrylate (PUA) oligomer chains wrap the MWCNTs along their length, resulting in high diffraction efficiency and good phase separation. The hardness and elastic modulus of the polymer matrix were enhanced with increasing amounts of MWCNTs because of the reinforcement effect of the MWCNTs with intrinsically good mechanical properties. The increased elasticity of the PUA matrix and the immiscibility between the matrix and the liquid crystals (LCs) gradually increased the diffraction efficiency of the HPDLC films. However, the diffraction efficiency of HPDLC films with more than 0.05 wt% MWCNTs was reduced, caused by poor phase separation between the matrix and LCs because of the high viscosity of the reactive mixture. HPDLC films showing a low driving voltage (<3 V µm^?1), a fast response time (<10 ms) and a high diffraction efficiency (>75%) could be obtained with 0.05 wt% MWCNTs at 40 wt% LCs. Copyright © 2010 Society of Chemical Industry     

Electro‐optical properties of polymer‐dispersed liquid crystal prepared by controlled graft living radical polymerization

Living graft macromolecule has been prepared through reversible addition‐fragmentation chain transfer (RAFT) living radical polymerization in one step. Then, it was used to make polymer‐dispersed liquid crystal (PDLC) by controlling the mole ratio of styrene (St) to 1,6‐hexanediol diacrylate (HDDA) and adjusting the content of prepared graft macromolecule. The results showed that electro‐optical properties of PDLC have been optimized. Different concentration of living graft macromolecule and different mole ratio of St/HDDA led to substantial improvement of driving voltage (threshold voltage and saturation voltage) and memory effect of PDLC simultaneously. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Comparative study on the electrooptical properties of polymer‐dispersed liquid crystal films with different mixtures of monomers and liquid crystals

This article deals with the study of polymer‐dispersed liquid crystal (PDLC) films that consisted of microdroplets of liquid crystals (LCs) dispersed in a polymer matrix. The PDLC films were fabricated by the photoinduced phase separation method under room‐temperature conditions. To determine the extent of the effects of the molecular structures and their physical properties of different mixtures of monomers and LCs on the morphology and electrooptical properties of the PDLC films, various mixtures were used. A detailed discussion of the obtained results is given. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Optical switching behavior of polymer‐dispersed liquid crystal composite films with various novel azobenzene derivatives

Polymer‐dispersed liquid crystal (PDLC) composite films were fabricated by thermal polymerization with E7 liquid crystal, monomers, and novel azobenzene derivatives synthesized in this study. To investigate the effects of azocompounds on the optical switching of PDLC films, a series of novel azobenzene derivatives of 4‐alkyloxy‐4′‐methoxyazobenzene with carbon numbers of 3–6, chiral compounds of amyl‐4‐(4‐hexyloxyphenylazo)benzoate, and bornyl‐4‐(4‐hexyloxyphenylazo)benzoate were synthesized. The compounds synthesized in this investigation were identified using FTIR, NMR, and elemental analysis. The optical texture of the composite films was analyzed under crossed nicols with a polarizing microscope. The morphological observation of the solid polymer in the composite films was performed with a scanning electron microscope (SEM). The optical behavior of the composite films on UV irradiation and the effects of the curing time on the thermal stability of the composite films were investigated. Isomerization of the azobenzene derivatives due to UV irradiation was confirmed by a texture study and image recording method. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 789–799, 2004     

Synthesis and mesomorphic properties of a new side‐chain,chiral smectic,liquid‐crystalline elastomer

The synthesis of the new chiral monomer 4‐(10‐undecylen‐1‐yloxy)biphenyl‐4′‐[(S)‐2‐methyl‐1‐bu‐ toxy]benzoate (M₁), the nematic crosslinking agent biphenyl 4,4′‐bis(10‐undecylen‐1‐yloxybenzoate) (M₂), and the corresponding liquid‐crystalline elastomer is described. The chemical structures of the chiral monomer and crosslinking agent have been characterized with Fourier transform infrared, elemental analyses, and proton and carbon‐13 nuclear magnetic resonance spectra. The mesomorphic properties have been investigated with differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction. Monomer M₁ shows different smectic phases (smectic A, chiral smectic C, and smectic B) and a cholesteric phase, and M₂ exhibits a nematic phase. The liquid‐crystalline elastomer shows smectic A and chiral smectic C phases. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4234–4239, 2006     

Study of optical losses in poly(3‐octylthiophene) planar and inverted RIB waveguides

Poly(3‐octylthiophene), (P3OT) in addition to its electronics properties exhibits a high Kerr coefficient, n₂, due to its third order nonlinear dielectric susceptibility. At the wavelength of 1550 nm, this coefficient n₂ is one of the highest. So, this material should be suitable to build integrated all optical switching devices. To construct this device, it is necessary to make a single‐mode optical waveguide. For the time being, such a P3OT waveguide has never been obtained due to excessive optical losses. In view to produce single‐mode waveguide with P3OT as a core, we investigated the different causes of these optical losses in the material and in the guiding structure. We characterized the optical transmission at key steps in its development. First, we demonstrated that the intrinsic polymer absorption is not a limiting factor at 1550 nm, and then we studied the transmission properties of planar (1‐D confined light) and channel waveguides (2‐D confined light). The results revealed that better transmission properties can be achieved using planar waveguides rather that confined channel waveguides. This article describes the development and the characterization of the guiding structures that enabled us to identify the main origins of optical losses. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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     

Synthesis of photoisomeric azobenzene monomers and model compound effect on electric–optical properties in PDLC films

A series of azobenzene monomers and related model compounds with various side‐chain lengths were synthesized. The electrooptical properties of a polymer‐dispersed liquid crystal (PDLC) were verified by side‐chain methoxy azobenzene in various chain lengths (n = 3, 6, 11). The properties under various voltages were measured and the effect of extra voltage on the transmittance of PDLC was researched as well. The experiment demonstrated the validity of employing these side chain methoxy azobenzene materials in electrooptical devices. The azobenzene model compound showed better electric–optical and thermal–optical properties, having a higher contrast ratio (CR = 689) and a lower saturation voltage (4.7 V/μm). All the azobenzene molecules can be photoisomerized through UV light irradiation, following the mechanism of isomerization. The reversible photo and heat isomerization property was studied. The cis‐azobenzene that was transformed from the trans‐azobenzene irradiated by UV light can decrease the clearing point of the liquid crystal phase. We used this unique characteristic to record image patterns and it worked successfully. We synthesized the azobenzene monomers can stabilize the PDLC and their relative model compounds with various alkyl chain lengths even got better electric–optical effects. We found that azobenzene monomer shows different behaviors in the electric–optical property from its relative model compound. The difference between the systems were explained using a proposed model. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 721–732, 2005     

Electrooptic studies on polymer‐dispersed liquid‐crystal composite films. III. Poly(methyl methacrylate‐co‐butyl acrylate)/E7 and poly(methyl methacrylate‐co‐butyl acrylate)/E8 composites

Composite films composed of poly(methyl methacrylate‐co‐butyl acrylate) (PMMABA) and nematic‐type liquid crystals E7 and E8 (commercial products from E. Merck, Darmstadt, Germany) were prepared through solvent casting in chloroform. The morphology and electrooptic responses were studied. Scanning electron microscopy observations showed that the liquid‐crystal phase (E7 or E8), as larger, elongated, interconnected cavities, was continuously embedded in a spongelike PMMABA matrix. At a specific level of the liquid‐crystal (E7 or E8) loading (30/70 wt %), the effects of the voltage, temperature, and frequency of an applied alternating‐current electric field on the transmittance of the composite films were measured with a He–Ne laser (wavelength = 632.8 nm). The results were interpreted in terms of the aggregation structure, interfacial interaction, and solubility of the liquid crystal in the matrix polymer. The results indicated that, under these experimental conditions, the output could be controlled to a desired level by the selection of suitable liquid crystals to prepare polymer‐dispersed liquid‐crystal, electrooptic, active composite films with a response time of the order of only milliseconds or less. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

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     

Recent advances on polymer‐stabilized blue phase liquid crystal materials and devices

Polymer‐stabilized blue phase liquid crystals (PS‐BPLC) have become an increasingly important technology trend for information display and photonics applications. BPLCs exhibit several attractive features, such as reasonably wide temperature range, submillisecond gray‐to‐gray response time, no need for alignment layer, optically isotropic voltage‐off state, and large cell gap tolerance. However, some bottlenecks such as high operation voltage, hysteresis, residual birefringence, charging issues due to the large capacitance, and relatively low transmittance remain to be overcome before their widespread applications can be realized. Recent progress on BPLC materials and devices has shown great promise. To realize the electro‐optic effect of PS‐BPLC, two driving modes: in‐plane switching and vertical field switching mode, have been developed. The material system of PS‐BPLC, including nematic LC host, chiral dopant, and polymer network, are discussed. Each component plays an essential role affecting the stability and electro‐optic properties of PS‐BPLC. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40556.     

Macro reversible addition–fragmentation chain transfer agent mixture as a means to enhance the electro‐optical performance of polymer‐dispersed liquid crystals

Macro reversible addition–fragmentation chain transfer (RAFT) agents, i.e. RAFT polystyrene (RAFT‐PS) and RAFT poly(n‐butyl acrylate) (RAFT‐PBA), were mixed. Polymer‐dispersed liquid crystals (PDLCs) were prepared using the mixture together with methyl acrylate and liquid crystal E7. The electro‐optical properties of the PDLCs obtained were investigated. The results showed that the advantages of the electro‐optical properties of RAFT‐PS‐ and RAFT‐PBA‐dependent PDLCs could be combined in RAFT agent mixture‐dependent PDLCs. Copyright © 2011 Society of Chemical Industry     

Photografting polymerization of polyacrylamide on poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) films. II. Wettability and crystallization behaviors of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)‐graft‐polyacrylamide films

The wettability and crystallization behaviors of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV)‐graft‐polyacrylamide (PAM) films were studied. X‐ray photoelectron spectroscopy analyses illustrated that about 62 atom % of the total polar functionalities on the grafted film with 17% grafting percentage (GP) was amide groups. Wide‐angle X‐ray diffraction results suggest that grafted PAM induced defects in PHBV crystals and influenced their crystal structure. Differential scanning calorimetry (DSC) spectra showed the two melting regions, 60–90 and 145–170°C, of the imperfect PHBV crystals of the grafted films. Grafted PAM could suppress the recrystallization of PHBV, which was consistent with the polarizing optical microscopy results, in which the maximum PHBV spherulite diameter decreased from 350 μm for the PHBV film to 50 μm for the film with 53% GP. In addition, DSC studies revealed that the crystallinity of the grafted films decreased with increasing GP, which facilitated the diffusion of water into the films. The water contact angle of grafted films decreased and the water‐swelling percentage increased as GP went up. These results demonstrate the potential of PHBV‐g‐PAM for wettable surface constructs in tissue engineering applications. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Light scattering materials with tunable optical properties by controlling refractive index of the dispersed phase

Traditionally, the morphologies of the dispersed phase in Polycarbonate (PC)/poly(styrene‐co‐acrylonitrile) (SAN) blends were strongly influenced if AN content in SAN changed significantly even under constant processing conditions. This would hinder the pure research intended to study the effect of the refractive index difference between the polymer host and the polymeric dispersed particles on the optical properties tough. Therefore, we respectively prepared different PC/SAN light diffusion sheets with four types of SAN containing different AN content ranging just from 20 to 25 wt %, a narrow range that sufficiently ensures the relatively stable morphology of different SAN in PC matrix. The results suggest that the refractive index of SAN increases with an decreasing AN content, thus narrowing the refractive index difference between PC and SAN and producing PC/SAN(70/30) light diffusion sheets with an increasing transmittance and decreasing haze. The interesting phenomenon is further analyzed using Mie scattering theory. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44156.     

Tunable near‐infrared optical properties based on poly(methyl methacrylate)–oxide waveguide materials

In this study, two classes of low‐loss optical planar waveguides were prepared from trialkoxysilane‐capped poly(methyl methacrylate) (PMMA)–silica and PMMA–titania hybrid materials, respectively. The prepared hybrid films had very uniform structure and surface planarity. The incorporation of the silica or titania segments into the acrylic polymer matrix reduced the intermolecular interaction and thus induced an increase in anharmonicity of the C‐H bond in the acrylic segment. Therefore, the third harmonic stretching vibration absorption of the C‐H bond was red‐shifted and resulted in a tuning of near‐infrared (NIR) optical absorption. The optical loss of the studied waveguides was reduced from 0.65 dB/cm of the PMMA waveguide to 0.26 and 0.28 dB/cm with increasing the silica and titania content in the hybrid materials, respectively. The reduction of the C‐H number density and shifting of the NIR absorption spectra accounted for the relationship between the optical loss and the inorganic oxide content. The increased anharmonicity through the incorporation of the inorganic moiety in the hybrid materials provides another approach for tuning the NIR optical properties. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1224–1228, 2005     

Investigation of plasma polymerized benzene and furan thin films for application in opto‐electronic devices

Conjugated polymers are among the most promising organic materials for opto‐electronic devices. In such applications, the main fabrication problem is to get uniform, defect‐free, and reproducible thin films of these materials. In this investigation, an RF plasma reactor was used to produce cross‐linked organic thin films from benzene and furan precursors. Uniform thin films of desired thickness were fabricated using this plasma polymerization technique. The composition of the plasma‐polymerized films was determined with X‐ray photoelectron spectroscopy. Fourier transform infrared spectra of the monomers and plasma‐polymerized thin films prepared were compared to analyze the chemical structure of the films. Ultraviolet–visible absorption spectroscopy shows a red shift of 45 nm in λ_max for the case of plasma‐polymerized benzene films and 52 nm in the case of plasma‐polymerized furan films when compared to their respective monomer spectra. Photo luminescence spectra of these films show a blue emission with a broad peak at 460 nm for the plasma‐polymerized benzene films and 445 nm for the plasma‐polymerized furan films. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 523–528, 2003     

Optical properties of ethyl‐cyanoethyl cellulose/poly(acrylic acid) cholesteric liquid crystalline composite films

Ethyl‐cyanoethyl cellulose [(E–CE)C]/poly(acrylic acid) (PAA) composite films were prepared by photopolymerizing acrylic acid (AA) in (E–CE)C/AA cholesteric liquid crystalline solutions. With the selection of suitable concentrations, (E–CE)C/PAA composite films showed vivid colors due to the selective reflection property of the cholesteric phase. It was found that the wavelength of reflection was a function of the concentration of (E‐CE)C, and the reflectivity was increased with increasing thickness of the film. The selective reflection of the composite holds well upon heating at temperatures below 160°C. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 213–217, 2004