Freestanding polyelectrolyte films as sensors for osmotic pressure.

Freestanding ultrathin polyelectrolyte-multilayer membranes, transferred to topographically structured polydimethylsiloxane (PDMS), are used as mechanical sensors. Due to the membranes' semipermeability, high-molecular-weight molecules can be either entrapped inside them or excluded, thus generating an osmotic pressure. This leads to a deformation. We investigate the deformation as a function of the osmotic pressure and present an analytical theory that fully describes the data. Thus, osmotic pressures can be determined quantitatively. The individual osmotic-sensitive elements have only microscopic dimensions, and arrays can be easily produced.     

Nanoporous piezo- and ferroelectric thin films

Nanoporous barium titanate and lead titanate thin films (～100 nm calculated from ellipsometric data) are prepared starting from sol-gel solutions modified with a commercially available block-copolymer and evaporation-induced self-assembly methodology. The tuning of the thermal treatment followed by in situ ellipsometry allows the decomposition of the organic components and of the structuring agent leading to the formation of porous tetragonal crystalline perovskite structures as observed by XRD, HRTEM, SEM, and ellipsoporosimetry. Both nanoporous barium titanate and lead titanate thin films present local piezoelectric and ferroelectric behavior measured by piezoresponse force microscopy (PFM), being promising platforms for the preparation of the generation of new multifunctional systems.     

Photochemical formation of silver nanoparticles in elastomer films

Photochemical deposition of silver nanoparticles in poly(butyl acrylate) and poly(butadiene-styrene) under the action of monochromatic UV light (254 nm) was studied. Changes in the polymer structure in the course of photolysis were revealed and analyzed. A mechanism of formation of silver nanoparticles was suggested. Electron microscopic examination showed that the efficiency of the particle formation is determined by the residual moisture content of the polymer films. Physicomechanical properties (tensile strength and relative elongation) of the elastomer films containing silver nanoparticles were studied.     

Study of smectic elastomer films under uniaxial stress

We study the mechanical properties of free-standing films of smectic liquid crystalline elastomers. Macroscopically ordered elastomer films of submicrometer thickness are prepared from freely suspended smectic A polymer films by photo crosslinking. The deformation characteristics depend criticically on the sample composition, in particular on the density of mesogenic side chains at the siloxane backbone. In materials where the siloxane backbone is only partially substituted (dilute systems), a uniaxial stretching of the films in the layer plane is accompanied by a shrinkage of the smectic layers. This layer shrinkage is to only a minor extent achieved by the induction of a molecular tilt. We conclude that the layer compression modulus (enthalpic contribution to elasticity) in such materials is very weak. In materials with a fully substituted backbone (homopolymers), the smectic layer thickness is preserved under uniaxial stress in the layer planes.     

Study of smectic elastomer films under uniaxial stress

We study the mechanical properties of free-standing films of smectic liquid crystalline elastomers. Macroscopically ordered elastomer films of submicrometer thickness are prepared from freely suspended smectic A polymer films by photo crosslinking. The deformation characteristics depend criticically on the sample composition, in particular on the density of mesogenic side chains at the siloxane backbone. In materials where the siloxane backbone is only partially substituted (dilute systems), a uniaxial stretching of the films in the layer plane is accompanied by a shrinkage of the smectic layers. This layer shrinkage is to only a minor extent achieved by the induction of a molecular tilt. We conclude that the layer compression modulus (enthalpic contribution to elasticity) in such materials is very weak. In materials with a fully substituted backbone (homopolymers), the smectic layer thickness is preserved under uniaxial stress in the layer planes.     

High-resolution inspections of ferroelectric thin PZT films

Lead zirconate titanate (PZT) materials have been widely investigated for applications in dynamic random access memories and non-volatile random access memories. The developments of such applications are conditionned by the fabrication of thin films. The main advantages of the sol-gel method include low cost and high quality control of the deposited thin films. Electric force microscopy and near-field optical microscopy are proposed as high-resolution inspection methods at the nanometric scale to extend the classical macroscopic characterizations.     

FTIR spectroscopy of smectic elastomer films under lateral strain

Polarized Fourier transform infrared (FTIR) spectroscopy is used to study the strain-induced compression of molecular layers in oriented smectic liquid crystal elastomer films. A reversible change of the smectic layer thickness in SmA and SmC* films in response to external strain was revealed earlier by optical reflectometry and X-ray measurements. However, these methods cannot probe the mechanism of layer compression on a molecular level. Polarized FTIR spectra show that the induced mesogenic tilt, one of the possible mechanisms, is too small to provide the dominating contribution to the layer shrinkage. The FTIR absorbance spectra of stretched samples are also evidence that there are no significant changes of the order parameter. Apparently, layer compression is achieved by a certain interpenetration of neighbouring layers, and/or compression of the interstitial backbone and spacer layers.     

Liquid crystalline cellulose derivative elastomer films under uniaxial strain

Mesogenic cellulose derivative chains cross-linked into free-standing thin films were prepared by a shear-casting technique from anisotropic precursor solutions of thermotropic (acetoxypropyl)cellulose. After shear cessation a macroscopically oriented serpentine structure with the director in average along the shear direction is locked resulting in anisotropic optical and mechanical properties of the material. These films were submitted to an external uniaxial mechanical field perpendicular and parallel to the shear direction. Stretching perpendicular to the shear direction produced significant director rotations and a reset of order of the director order parameter for a deformation in the range 2–3 as detected by X-rays and optical microscopy. The different response found for strains imposed parallel and perpendicular to the initial average director orientation indicates that even though our system shows a serpentine director modulation that is either attenuated or reinforced by deformations parallel or perpendicular to the shear direction, its behaviour is similar to theoretical predictions for monodomain nematic elastomers described in the literature.     

Characterization of Langmuir-Blodgett films of a ferroelectric liquid crystal

Molecular orientation, structure, and phase transition behaviors in Langmuir-Blodgett (LB) and cast films of a ferroelectric liquid crystal of sec-butyl 6-(4-(nonyloxy)benzoyloxy)-2-naphthoate (FLC-1) are determined by ultraviolet (UV) spectroscopy, X-ray diffraction, and infrared (IR) spectroscopy. It is found that the orientation angle of chromophores theta in LB films is 41 degrees from the surface normal. The tilt angle of the chromophore changes at 56, 70, and 88 degrees C, respectively, which denotes the presence of phase transitions. Two kinds of layered or isomeric crystal structures of the LB films with layer spacings of 3 and 3.5 nm at room temperature have been found while the latter disappears above 45 degrees C, as confirmed by measurement of temperature-dependent IR spectra.     

Structures and electrical properties of ferroelectric copolymer ultrathin films

Ultrathin films of ferroelectric copolymer vinylidenefluoride and trifluoroethylene, P(VDF-TrFE), were successfully obtained by spin-coating and their nanoscale structures and electrical properties were studied utilizing atomic force microscopy (AFM). We succeeded in obtaining ultrathin copolymer films on graphite whose thickness ranged from 1 nm to several tens of nanometers by controlling concentration of copolymer solutions in methylethylketone. We found that ultrathin films thinner than 4 nm showed layered structures whose layer thickness was about 0.5 nm. On the other hand, films thicker than 4 nm formed typical edge-on lamellar crystal structures. Furthermore, we investigated surface potential distribution and piezoelectric property by AFM-based techniques and discussed interaction between electrical dipoles in the molecular chains and graphite substrate.     

Freestanding reduced graphene oxide/sodium vanadate composite films for flexible aqueous zinc-ion batteries

With the booming development of portable and wearable electronic devices, flexible energy storage devices have attracted great attention. Among various energy storage devices, aqueous zinc ion batteries(ZIBs) are one of the promising candidates due to their low cost, good safety, high energy and power densities. However, the conventional cathodes of aqueous ZIBs were often prepared by mixing active materials with binders and conductive additives and then coating them onto current collectors. The resultant cathodes often suffer from unsatisfied flexibility. Herein, we fabricated freestanding reduced graphene oxide/NaV_3O_8·1.5H_2O(RGO/NVO) composite films with interlinked multilayered architecture by a vacuum filtrating process. Such composite films exhibit excellent mechanical property and high electronic conductivity. Owing to unique architecture, they display a high capacity of 410 mA h g~(-1) and excellent cycling performance up to 2000 cycles with a high capacity retention of 94%. Moreover, RGO/NVO composite films can directly serve as the cathodes of flexible aqueous ZIBs. As a proof of concept,flexible ZIBs were assembled based on the composite films. Impressively, they exhibit stable performance at different bending states, demonstrating great potential application in flexible energy storage devices.     

Electric field induced structure formation in free standing ferroelectric films

Free standing ferroelectric films in a rotating electric field have been investigated. Depending upon the applied voltage and the period of the field rotation different structure formations could be observed. At high fields, a homogeneous orientation is obtained. In the case of very low fields, a schlieren texture results. Under special conditions, a stable ring system appears. The dynamics of these rings have been studied. The dependence of the structure formation on the relation between the impulse length of the applied field and the reorientation time is discussed.     

Electric field induced structure formation in free standing ferroelectric films

Abstract

Free standing ferroelectric films in a rotating electric field have been investigated. Depending upon the applied voltage and the period of the field rotation different structure formations could be observed. At high fields, a homogeneous orientation is obtained. In the case of very low fields, a schlieren texture results. Under special conditions, a stable ring system appears. The dynamics of these rings have been studied. The dependence of the structure formation on the relation between the impulse length of the applied field and the reorientation time is discussed.     

Anisotropic hydroxypropylcellulose films as alignment layers of a bistable ferroelectric device

Hydroxypropylcellulose films are used as the alignment substrate in a liquid crystal bistable electro-optical device. These alignment films were characterized by atomic force microscopy. The electro-optical behaviour of this device, which operates in the Clark-Lagerwall mode, is identical to that observed for typical SSFLC cells. In particular the bistability of the cell was clearly observed, the threshold voltage was measured and an estimation of the anchoring energy of the LC mixture used, in this type of system, was made.     

Glycol-based sol-gel process for the fabrication of ferroelectric PZT thin films

A new sol-gel system using ethylene glycol was developed for the fabrication of PZT thin films with compositions near the morphotropic phase boundary Pb(Zr_0.52Ti_0.48)O₃. Ethylene glycol was used as both a chelating agent and a solvent to replace the highly toxic methoxyethanol used in previous formulations. Thin films were deposited by spin coating the solutions onto platinized silicon substrates. Films were completely crystallized by about 600°C and contained the ferroelectric perovskite phase. A dielectric constant of about 750–800 at 1 KHz was obtained for thin films of 0.3 µm thickness. The hysteresis measurements revealed a remanent polarization of 15 mC/cm² with a coercive field of 60 kV/cm.     

Measurement of the viscosity of ferroelectric liquid crystals in free-standing films

A new implementation of the technique for the measurement of the Goldstone mode rotational viscosity in the S_c phase is presented. An electric field is applied parallel to the surface of a free-standing liquid crystal film. The optical transmission change of the film is recorded. The viscosity can be calculated from the reorientation time between optically separated positions of the director. A comparison with the viscosity values measured in thin cells is given. The values determined in cells are always higher than the results obtained from free-standing films. This indicates the great influence of the forces of interaction between the liquid crystal molecules and the surface layer in the cells.     

Anisotropic hydroxypropylcellulose films as alignment layers of a bistable ferroelectric device

Hydroxypropylcellulose films are used as the alignment substrate in a liquid crystal bistable electro-optical device. These alignment films were characterized by atomic force microscopy. The electro-optical behaviour of this device, which operates in the Clark-Lagerwall mode, is identical to that observed for typical SSFLC cells. In particular the bistability of the cell was clearly observed, the threshold voltage was measured and an estimation of the anchoring energy of the LC mixture used, in this type of system, was made.     

Selective reaction and chemical anisotropy in epitaxial bismuth layer-structured ferroelectric thin films

Bismuth layer-structured ferroelectric thin films consisting of a stacking of pseudoperovskite and bismuth oxide blocks along the c-axis were epitaxially grown on single-crystal substrates by metalorganic chemical vapor deposition (MOCVD), and a selective reaction of the bismuth oxide layer with HCl was demonstrated. Epitaxial films with contrasting crystal orientations were used for the acid treatment in order to probe chemical anisotropy. For a/b-axis-oriented SrBi₂Ta₂O₉, Bi₄Ti₃O₁₂, and SrBi₄Ti₄O₁₅ films with sequential stacking of the two vertical blocks, notable structural selectivity of the reaction was observed only for SrBi₂Ta₂O₉. For this film, the pseudoperovskite block remained and the bismuth oxide block was removed, while both blocks of Bi₄Ti₃O₁₂ and SrBi₄Ti₄O₁₅ dissolved into the acid. In addition to the bismuth, the other cations in the pseudoperovskite blocks, strontium and titanium, also decreased for Bi₄Ti₃O₁₂ and SrBi₄Ti₄O₁₅. The selective reaction observed for a/b-axis-oriented SrBi₂Ta₂O₉, however, was not observed for c-axis-oriented SrBi₂Ta₂O₉ films in which the pseudoperovskite and bismuth oxide blocks were stacked horizontally. The results clearly show that SrBi₂Ta₂O₉ has sub-nano-order structural selectivity and chemical anisotropy in the unit cell in the reaction with HCl.     

Polymer-dispersed liquid crystals From the nematic curvilinear aligned phase to ferroelectric films

Polymer-dispersed liquid crystals (PDLC) are widely used for electro-optic applications such as flexible displays, privacy windows or projection displays. Besides these applications, the confinement of a liquid crystal to small cavities is of fundamental interest. The present paper contains a review of the work on nematic and cholesteric PDLCs. Moreover, some very recent developments are summarized such as the use of ferroelectric liquid crystals for PDLC applications.