Polymer Network Liquid Crystal Research Articles

Bidirectional Photoresponse of Surface Pretreated Azobenzene Liquid Crystal Polymer Networks

We report on the photodriven, polarization-controlled response of UV-pretreated azobenzene-based liquid crystal polymer networks (azo- LCN) of polydomain orientation to higher wavelength CW argon-ion laser light (Ar(+)) of 457-514 nm. The significant absorbance of the azo-LCN cantilever in the UV is used to form an approximately 1 microm thick cis-isomer rich skin. Subsequent exposure to the Ar(+) laser drives a bidirectional bending process that is the result of two distinguishable photochemical processes. First, 457-514 nm laser light (regardless of polarization state) drives cis-trans photoisomerization of the UV-pretreated surface, restoring the order of the azobenzene liquid crystalline moieties. Mechanically, the cis-trans process results in an expansion on the exposed surface that forces the cantilever to undergo a rapid bend away from the laser source. Once a sufficient number of trans-azobenzene moieties are regenerated, continued Ar(+) illumination promotes both the trans-cis and cis-trans processes enabling trans-cis-trans reorientation. In this particular system and conditions, trans-cis-trans reorientation enables polarization controlled mechanical bending of different angles towards the Ar(+) source. Photomechanical responses of UV-pretreated azo-LCN demonstrate the viability of photogenerated effects in UV-rich environments such as space.

Photo-controlled conformation-assisted permanent optical storage device employing a polymer network liquid crystal

A new type of optical storage device is described employing a material consisting of a host nematic liquid crystal, a photoisomerisable azobenzene component and a photopolymerizable monomer. The principle of image storing involves selectively controlling the birefringence of the medium immediately prior to photopolymerization of the monomer. We show that photoisomerisation driven nematic to isotropic transition can be employed to achieve this through proper timing of the reverse isomerization of the azobenzene compound before the nematic director fluctuations get quenched. It is also suggested that grey-shades can be created in this device using the recently discovered phenomenon of electric-field acceleration of reverse isomerisation.

Polarization-controlled, photodriven bending in monodomain liquid crystal elastomer cantilevers

We report on the fast and optically controlled angular bending of cantilevers made from liquid crystal polymer networks functionalized with azobenzene moieties (azo-LCN). For potential applications such as adaptive optics, photoresponsive cantilevers should rapidly deform to controlled angles while maintaining a modulus that can accomplish appreciable mechanical work. This work demonstrates cantilevers made of monodomain azo-LCN containing pendant (side chain) azobenzene mesogens with a storage modulus of 1.4 GPa bend 85° in less than 300 ms upon exposure to 442 nm irradiation. Moreover, the bending angle of these monodomain azo-LCN cantilevers can be controlled by the polarization angle of the source relative to the long-axis of the cantilever. The bending performance (deformation angle and speed) of this monodomain system is compared to a polydomain analogue. The impact of azobenzene concentration, laser intensity, and thickness on these parameters is also presented.

Swelling of Acrylic Interpenetrating Polymer Networks in Liquid Crystals

AbstractSummary: The swelling properties of acrylate based polymer networks in anisotropic solvents (LCs) are examined. Two Interpenetrating Polymer Networks (IPNs) consisting of crosslinked poly(ethyl hexyl‐acrylate) (PolyEHA) and poly(n butyl‐acrylate) (PolyABu) are investigated. The samples are prepared by sequential UV‐curing of the monomers in the presence of a cross‐linking agent and a photo‐initiator. Microscopy observations are used to measure the diameter of the samples in the dry and swollen states and to deduce the kinetics of polymer‐solvent swelling for different systems. The theoretical calculations based on the Fick model are presented.

Fine adjustment of network in polymer network liquid crystal film employing RAFT polymerization

Fine adjustment of network in polymer network liquid crystal film employing RAFT polymerization

Distributed Bilayer Photovoltaics Based on Nematic Liquid Crystal Polymer Networks

We discuss a liquid crystal composite approach to provide a distributed interface to vertically separate electron-donating and electron-accepting films in an organic photovoltaic device. Two different methods are used to prepare a nematic liquid crystal polymer network with a porous surface and electron-donating properties. This is infilled with an electron-accepting organic semiconductor to form a bilayer device. The interface is diffuse rather than localized so that more photogenerated excitons can reach it to generate charge before they recombine. Photoinduced absorption of a blend of the donor and acceptor materials confirms that excitons dissociate at the heterointerface. The spatial features of the diffuse interface are examined by Fourier analysis of topographic images. We find a correlation between the in-plane spatial frequencies of the interface and photovoltaic device performance. The device performance is investigated as a function of input irradiance. Any charge combination is monomolecular rather than bimolecular, and the monochromatic power conversion efficiency varies between 0.8% and 0.3% with input irradiance. Equivalent circuit analysis shows that this is limited by a high series resistance, a blocking contact, and nonoptimized spatial features of the porous interface.

Analysis of Electro-Optic Properties of a Polymer Network Liquid Crystal Display with Crossed Polarizers

An active-matrix polymer network liquid crystal display (AMPNLCD) of 1.79″ QCIF(176 × 144) was fabricated. The operating mode of normally black was reversed to normally white by using crossed polarizers. The contrast ratio, as a result, was drastically increased, which was comparable to a conventional liquid crystal display. The response time at a typical driving voltage, 2.8 V, was also fast enough to play still image smooth. The electro-optic properties of AMPNLCD with crossed polarizers were evaluated and discussed.

Molecular Alignment of Axially-Symmetric Sheared Polymer Network Liquid Crystals

An axially-symmetric sheared polymer network liquid crystal (AS-SPNLC) device is demonstrated and its performances characterized. The formation mechanism of the AS-SPNLC is investigated by observing the dynamic pattern change of the structure during the fabrication process. We also analyze the structure from the electro-optic properties and then construct a simulation model to explain the observed phenomena. Several potential applications, such as tunable-focus negative lens and spatial polarization converter, are discussed.

Driving algorithms for an endless polarization controller using two wave-plates

We present in this paper an endless polarization controller using two wave-plates with rotating axes and variable birefringence. We demonstrate the characteristics of the polymer network liquid crystal wave-plate and a polarization control method with only two degrees of freedom. Finally, some experimental attempts toward endless polarization control have been performed.

P-140: Simultaneous Phase Retardation and Optic Axis Measurements of A- and C-Plates

We demonstrate a new method for simultaneously measuring the phase retardation and optic axis of a compensation film by using an axially-symmetric sheared polymer network liquid crystal (AS-SPNLC). With overlaying a tested compensation film with a calibrated AS-SPNLC cell between crossed polarizers, the optic axis and phase retardation value of the compensation film can be determined. This method is applicable to both A- and C-plate phase compensation films.

Morphology and thermal properties of liquid crystalp-PAEB/styrene copolymer

The homopolymer of unsaturated liquid crystal (LC) monomer for p-phenylene di {4-[2-(allyloxy) ethoxy] benzoate} (p-PAEB), and copolymer poly (p-PAEB/St) of p-PAEB with styrene (St) have been synthesized. The LC behavior and thermal properties of p-PAEB and poly(p-PAEB/St) have been studied by Polarizing Optical Microscopic (POM), Differential Scanning Calorimetry, X-Ray Diffractometer (XRD), and Torsional Braid Analysis (TBA). The results demonstrate that LC phase texture and phase transition temperature of copolymers are affected by the composition of LC units in copolymers. The POM and XRD reveal that p-PAEB has a smectic phase structure; the copolymer of p-PAEB with styrene reveal deformed focal conics texture of smectic phase. The phase transition temperature range of p-PAEB is 120.5–191.5°C, but the homopolymer of p-PAEB has a broad LC temperature range from 77 to 170°C. The LC temperature range of poly(p-PAEB/St) is broadened with increased content of p-PAEB. The dynamic mechanical properties of LC polymer networks were investigated with TBA. The results indicate that the peak temperature of maximal mechanical loss is 114°C and is decreased with the addition of styrene © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5731–5736, 2006

Simultaneous measurement of phase retardation and optic axis of a phase compensation film using an axially-symmetric sheared polymer network liquid crystal

A new method for simultaneously measuring the phase retardation and optic axis of a uniaxial compensation film is demonstrated using an axially-symmetric sheared polymer network liquid crystal (SPNLC). By overlaying a tested compensation film with a calibrated SPNLC cell between crossed polarizers, two dark spots are clearly observed in a CCD image. From the orientation direction and distance of these two spots, the optic axis and phase retardation value of the compensation film can be determined. This method is particularly useful for those optical systems whose optic axis and phase retardation are dynamically changing.

Axially-symmetric sheared polymer network liquid crystals

An axially-symmetric sheared polymer network liquid crystal (SPNLC) device is demonstrated and its performances characterized. Through analyzing the structure of this axially-symmetric SPNLC, we constructed a 3-D model to explain the observed phenomena. The simulation results agree well with the experiment. Two potential applications of such an axially-symmetric SPNLC, namely tunable-focus negative lens and spatial polarization converter, are discussed.

One‐ and Two‐Dimensionally Structured Polymer Networks in Liquid Crystals for Switchable Diffractive Optical Applications

AbstractWe have created one‐ and two‐dimensionally structured polymer networks dispersed in a liquid‐crystal solvent using a holographic exposure technique. These structures have potential for electrically switchable, reverse‐mode, polarization selective and non‐selective diffractive optical elements. Using a simple phenomenological model to describe our diffraction measurements in conjunction with microscopic studies, we are able to estimate the structured polymer wall thickness as a function of monomer concentration.

58.2: Fast‐Response Polymer Network Liquid Crystals for Optical Communications

AbstractA fast‐switching and scattering‐free phase modulator using polymer network liquid crystal (PNLC) is demonstrated at λ =1.55 μm for laser beam steering application. The strong polymer network anchoring greatly reduces the visco‐elastic coefficient of the LC. As a result, the PNLC response time is improved by ∼300X, although the threshold voltage is increased by ∼25X. Computer simulation results agree with experiments well.

Fast-response and scattering-free polymer network liquid crystals for infrared light modulators

A fast-response and scattering-free homogeneously aligned polymer network liquid crystal (PNLC) light modulator is demonstrated at λ=1.55 μm wavelength. Light scattering in the near-infrared region is suppressed by optimizing the polymer concentration such that the network domain sizes are smaller than the wavelength. The strong polymer network anchoring assists LC to relax back quickly as the electric field is removed. As a result, the PNLC response time is ∼250× faster than that of the E44 LC mixture except that the threshold voltage is increased by ∼25×.

Polymer network liquid crystals for tunable microlens arrays

We demonstrate a tunable-focus microlens array using polymer network liquid crystals (PNLCs). PNLCs were prepared by ultraviolet (UV) light exposure through a patterned photomask. The UV-curable monomer in each of the exposed spots forms an inhomogeneous centro-symmetrical polymer network that functions as a lens when a homogeneous electric field is applied to the cell. The focal length of the microlens is tunable with the applied voltage.

Submillisecond response variable optical attenuator based on sheared polymer network liquid crystal

We demonstrate a variable optical attenuator (VOA) at lambda=1.55 microm using a sheared polymer network liquid crystal (SPNLC). The SPNLC exhibits a fast response time and weak wavelength dependency. Comparing with other polymer-stabilized liquid crystals, the SPNLC has lower driving voltage and negligible light scattering loss when the wavelength exceeds 700 nm. A reflection type VOA with ~0.24 ms response time and -32 dB dynamic range is demonstrated at room temperature and 35 Vrms voltage.

Liquid Crystalline Networks: Potential uses in Molecular Imprinting Technique

A novel approach to the synthesis of molecularly imprinted polymers via covalent or non-covalent linkages was studied. It relies on the use of thermotropic side-chain liquid-crystal polymer networks. The polysiloxane networks obtained after extraction of the template preserved the mesomorphic organisation set up in the presence of the guest molecule. Batch rebinding analysis were performed: this study revealed that the imprinted polymer has a much greater affinity for the template molecule than the unimprinted one, and a significant selectivity.

Tunable microlens arrays using polymer network liquid crystal

A tunable-focus microlens array based on polymer network liquid crystal (PNLC) is demonstrated. The PNLC was prepared using an ultraviolet (UV) light exposure through a patterned photomask. The photocurable monomer in each of the UV exposed spot forms an inhomogeneous centro-symmetrical polymer network which acts as a lens when a homogeneous electric field is applied to the cell. The focal length of the microlens arrays is tunable with the applied voltage.