D.J. Broer

Chiral nematic order in liquid crystals imposed by an engineered inorganic nanostructure

Control over the orientational order of liquid crystals (LCs) is critical to optical switching and display applications. Porous polymer networks have been used to influence the orientation of embedded chiral liquid crystals, yielding for example reflective displays. Here we show that inorganic films with a porous structure engineered on the submicrometre scale by glancing-angle deposition, can be used to control the orientation of LCs impregnated into the voids. The inorganic material contains helical columns that orient rod-like nematic LCs into a phase similar to a chiral nematic, but with direct control of the local molecular arrangement (for example, the helical pitch) imposed by the inorganic microstructure. We also show that reactive LC molecules in this composite material can be crosslinked by photopolymerization while retaining the imposed structure.

Nanomotor rotates microscale objects

Nanomachines of the future will require molecular-scale motors that can perform work and collectively induce controlled motion of much larger objects. We have designed a synthetic, light-driven molecular motor that is embedded in a liquid-crystal film and can rotate objects placed on the film that exceed the size of the motor molecule by a factor of 10,000. The changes in shape of the motor during the rotary steps cause a remarkable rotational reorganization of the liquid-crystal film and its surface relief, which ultimately causes the rotation of submillimetre-sized particles on the film.

InP-based two-dimensional photonic crystals filled with polymers

Polymer filling of the air holes of indium-phosphide-based two-dimensional photonic crystals is reported. After infiltration of the holes with a liquid monomer and solidification of the infill in situ by thermal polymerization, complete filling is proven using scanning electron microscopy. Optical transmission measurements of a filled photonic crystal structure exhibit a redshift of the air band, confirming the complete filling.

Photopatterned liquid crystalline polymers for microactuators

We have investigated the properties of thermally-responsive, patterned liquid crystalline polymers as their dimensions are scaled to a size suitable for use in microelectromechanical systems. All samples were fabricated using surface alignment and photopatterning techniques that can be used to produce integrated devices anchored on a substrate. The thermomechanical properties of free-standing macroscopic samples with varying concentration of crosslinking molecules were investigated in order to optimize the thermal response of the material. It was found that samples containing 12% crosslinker were able to expand by up 19% when heated. The thermomechanical properties of surface-anchored films were also investigated, and it was found that by employing a polymerized cholesteric structure of the liquid crystalline units a thermal expansion of up to 11% could be achieved when the sample was heated to 200 °C. Patternability was demonstrated using a simple photopatterning process that was used to fabricate samples consisting of lines of cholesteric material on a bare substrate, or alternating regions of liquid crystalline polymer in the isotropic and cholesteric phases. Actuation of these films was also demonstrated.

Physical Properties of Anisotropically Swelling Hydrogen-Bonded Liquid Crystal Polymer Actuators

Glassy polymeric actuators are described which reversibly deform in response to changes in pH and/or the presence of water. Hydrogen-bonded liquid crystalline monomers act as precursors, and these materials are photopolymerized (without mechanical drawing) into monodomain nematic networks. We discuss the influence of film composition, polymerization conditions, and chemical treatments on the properties of these anisotropic networks. We show that large-amplitude reversible motion can be generated in response to small changes in pH. Pretreatment in basic solutions also activates a response to water, and this effect is investigated in detail. Elastic moduli, which are directly related to the generation of work, have not been reported previously, and therefore particular attention is given to mechanical properties. A marked anisotropy is observed in the moduli parallel and perpendicular to the nematic director. We find that pretreatment in basic solutions reduces this anisotropy yet preserves magnitudes on the order of 2 GPa. Employing complex director profiles (such as the twisted configuration), extremely large amplitude bending-mode deformations are induced in response to small pH changes, immersion in water, or variations in the relative humidity. Since the elastic moduli are large, work output significantly greater than conventional liquid crystal elastomers is predicted

Polymer-Filled Nematics: A New Class of Light-Scattering Materials for Electro-Optical Switches

A new class of mixts. of liq. crystal materials and polymers is presented consisting of sub-micrometer sized, internally crosslinked polymeric particles dispersed in a continuous LC phase, which is referred to as polymer-filled nematics (PFNs) to distinguish them from LC mixts. contg. inorg. particles. Initial electrooptical expts. indicate that the light-scattering properties of these blends can be controlled with the application of an elec. field and that hysteresis effects are present. [on SciFinder (R)]

Alignment and switching of nematic liquid crystals embedded in porous chiral thin films

Porous thin films with engineered microstructures have been fabricated using glancing angle deposition (GLAD). GLAD films with chiral microstructures have been previously shown to exhibit unique chiral optical response. The pores of these films were embedded with (non-chiral) nematic liquid crystals (LCs) to produce a new composite optical material wherein the GLAD film induces chiral nematic-like LC orientation. We demonstrate here reversible electro-optic switching of the LC component of these hybrid films. Unaddressed, cells of GLAD/LC hybrid films show enhanced chiral optic response compared with the unfilled GLAD film. When addressed, the chiral optic response vanishes.

Liquid Crystal Alignment and Switching in Porous Chiral Thin Films

Optimization of optical properties in liquid crystal (LC) devices requires control over the long-range orientation of the LC molecules. A variety of techniques are used to produce alignment in LC devices such as the familiar liquid crystal display (LCD). One well-known technique involves surface treatment of cell substrates by coating with thin polymer films such as polyimides that are subsequently rubbed. These treatments are used to produce a known orientation of the LC molecules at the surface. For example, rubbed layers may be used to align rod-like nematic LC molecules near the substrate with their long axes parallel to the direction of rubbing. The twisted nematic LCD, as an example, has the rubbing direction of the two substrates of the display cell perpendicular to each other to generate a 90 twist in the nematic orientation aided by a small amount of chiral dopant. A second technique for controlling LC orientation is the use of obliquely deposited thin films as alignment layers which generate a certain atilto of the LC molecules at the substrates. These types of orientation layer techniques influence LC orientation near the substrate surfaces only and are reliant on the LC molecules asettlingo into some minimum potential energy arrangement which gives the desired properties. The major drawback of these surface alignment layer techniques is that they are unable to provide significant influence on LC orientation further from the substrate surfaces. Maintenance of the desired LC orientation in thicker switching cells, particularly with chiral or cholesteric LCs (CLC), becomes especially difficult. This leads for instance to irreversible switching when the LCs do not settle back into their original arrangement under influence of the alignment layer after having been addressed. For better control over LC alignment, a technique which induces LC alignment throughout the cell is required, rather than influencing the LC orientation near the substrate surfaces only. This can be accomplished, for instance, by phase separation in polymer-dispersed liquid crystals (PDLC) or photopolymerization to afreezeo in the desired LC alignment. These techniques, however, may come at the sacrifice of the ability to switch the LC. A recent innovation in LC alignment was made by Robbie, Broer, and Brett who embedded LCs into porous thin films with engineered microstructures. These porous thin films are fabricated using glancing angle deposition (GLAD), a physical vapor deposition technique which allows the fabrication of highly porous thin films (Fig. 1) with columnar microstructure controllable on the sub-micrometer scale. GLAD uses highly oblique or glancing angle deposition (typically at vapor incidence angles of a > 75 , measured relative to the substrate normal) to accentuate the atomic shadowing effects, leading to thin films with porosities tunable from 10 % to 90 %. Computer control of substrate motion during

Large-area microfabrication of three-dimensional, helical polymer structures

A technique has been developed to fabricate polymeric helices with sub-micron dimensions. These helices are made using a double-templating process, in which an inorganic thin film deposited using glancing angle deposition acts as the master. The shape, pitch, handedness and number of turns of the polymer helices can be tuned by altering the deposition parameters of the master film. The structure of this positive master is copied into a negative intermediate template of photoresist, which itself acts as a master for the templating of polymer helices. This process is demonstrated with four multifunctional acrylates. The master, intermediate template and polymer helices are characterized using scanning electron microscopy, and the polymer helices are characterized using energy dispersive x-ray spectroscopy. It is shown that a large number of polymer helical microstructures, which are anchored to both a thick substrate and a thin capping layer, can be made in parallel over areas of mm2 to cm2.

Colourful Photo-Curable Coatings for Application in the Electro-Optical Industry

The formation of mirrors reflecting a single colour or reflecting the whole visible spectrum by application of a liquid crystalline coating followed by UV polymerisation (photo-curing) is described. Also, the formation of patterned coatings obtained by a sequence of UV exposure steps is discussed. Such coatings play an important role in the improvement of the performance of liquid crystal displays (LCDs).