Rubbing-induced molecular reorientation on an alignment surface of an aromatic polyimide containing cyanobiphenyl side chains

Programmable digital memory devices based on nanoscale thin films of a thermally dimensionally stable polyimide

We have fabricated electrically programmable memory devices with thermally and dimensionally stable poly(N-(N',N'-diphenyl-N'-1,4-phenyl)-N,N-4,4'-diphenylene hexafluoroisopropylidene-diphthalimide) (6F-2TPA PI) films and investigated their switching characteristics and reliability. 6F-2TPA PI films were found to reveal a conductivity of 1.0 x 10(-13)-1.0 x 10(-14) S cm(-1). The 6F-2TPA PI films exhibit versatile memory characteristics that depend on the film thickness. All the PI films are initially present in the OFF state. The PI films with a thickness of >15 to <100 nm exhibit excellent write-once-read-many-times (WORM) (i.e. fuse-type) memory characteristics with and without polarity depending on the thickness. The WORM memory devices are electrically stable, even in air ambient, for a very long time. The devices' ON/OFF current ratio is high, up to 10(10). Therefore, these WORM memory devices can provide an efficient, low-cost means of permanent data storage. On the other hand, the 100 nm thick PI films exhibit excellent dynamic random access memory (DRAM) characteristics with polarity. The ON/OFF current ratio of the DRAM devices is as high as 10(11). The observed electrical switching behaviors were found to be governed by trap-limited space-charge-limited conduction and local filament formation and further dependent on the differences between the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels of the PI film and the work functions of the top and bottom electrodes as well as the PI film thickness. In summary, the excellent memory properties of 6F-2TPA PI make it a promising candidate material for the low-cost mass production of high density and very stable digital nonvolatile WORM and volatile DRAM memory devices.

Self-Assembly of Chemically Linked Rod−Disc Mesogenic Liquid Crystals

A series of new molecular discs (RDn, here n is the number of carbon atoms between the rod and disc mesogens) was synthesized via the chemical attachment of six cyanobiphenyl calamitic (rod) mesogens (R) linked to the triphenyl discotic (disc) mesogen (D) with a series of six alkyl chain linkages (n = 6-12). In this study, phase structures, transitions, and liquid crystalline (LC) behavior of the RD12 compound with 12 carbon atoms in each alkyl chain linkage between the rod and disc mesogens were investigated. Differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction (WAXD), and selected area electron diffraction (SAED) allowed us to identify three ordered phases below the isotropization temperature: nematic (N) LC and K1 and K2 crystalline phases. On the basis of the structural results obtained via 2D WAXD experiments on oriented samples and SAED experiments on single crystals, the K1 crystalline unit cell was determined to be triclinic with the dimensions of a = 1.36 nm, b = 1.45 nm, c = 2.11 nm, alpha = 85 degrees, beta = 100 degrees, and gamma = 50 degrees. The K2 phase was metastable with respect to the K1 phase. It also possessed a triclinic unit cell with a = 1.40 nm, b = 1.51 nm, c = 1.92 nm, alpha = 87 degrees, beta = 117 degrees, and gamma = 62 degrees. Molecular packing models for the crystalline phases were proposed on the basis of the diffraction results. In the whole range of ordered structures, it was found that RD12 molecular discs are intercalated. Both triphenyl discotic mesogens and cyanobiphenyl calamitic mesogens are completely interdigitated.

Revisit of the Dessy−White Intramolecular Acetylene−Acetylene [2 + 2] Cycloadditions

In this experiment, a series of thermal reactions of 4,4'-disubstituted 2,2'-bis(phenylethynyl)biphenyls with 2,3,4,5-tetraphenylcyclopenta-2,4-dienone were carried out under neat conditions and in diphenyl ether at temperatures between 260 and 270 degrees C to give rise to 9,10,11,12,13,14-hexaphenylcycloocta[l]phenanthrenes as the adducts in 12-23% yields. We traced these results to the intramolecular [2 + 2] thermal cyclization of 2,2'-bis(phenylethynyl)biphenyls to form 1,2-diphenylcyclobuta[l]phenanthrenes, which were further trapped as bridged-ketone Diels-Alder adducts, followed by thermal decarbonylative ring opening, which gave rise to the products.

Rubbing-induced anisotropy of long alkyl side chains at polyimide surfaces

Molecular organization at polyimide surfaces used as alignment layers in liquid crystal displays was investigated using vibrational sum frequency generation (SFG) spectroscopy. We focus on the orientation of the long alkyl side groups at the polymer surface using polarization-selected SFG spectra of the CH(3)- and CH(2)-stretch modes of the side chain. Mechanical rubbing and baking, an accepted industrial procedure used to produce pretilt of the liquid crystal, was found to induce pronounced azimuthal anisotropy in the orientational distribution of the alkyl side chains. Orientational analysis of the SFG vibrational spectra in terms of the azimuthal and tilt angles (in and out of plane, respectively) of the alkyl side chains shows their preferential tilt along the rubbing direction, with the azimuthal distribution narrower for stronger rubbed polymer samples.

Combination of Polarized TIRF and ATR Spectroscopies for Determination of the Second and Fourth Order Parameters of Molecular Orientation in Thin Films and Construction of an Orientation Distribution Based on the Maximum Entropy Method

This article describes two mathematical formalisms for the determination of the second and fourth order parameters of molecular films using optical spectroscopy. Method A uses polarized total internal reflection fluorescence (TIRF) to calculate the second and fourth order parameters, {P2(cos theta)} and {P4(cos theta)}, using an independently determined value for the angle between the absorption and emission dipoles, gamma. Method B uses {P2(cos theta)} obtained from attenuated total reflectance (ATR) data, along with polarized TIRF measurements to calculate {P4(cos theta)} and {cos2 gamma}. The choice of a specific method should rely on experimental considerations. We also present a method to separate the contributions of substrate surface roughness and dipole orientation with respect to the molecular axis from the spectroscopically determined second and fourth order parameters. Finally, a maximum entropy approach for construction of an orientation distribution from order parameters is compared with the commonly used delta and Gaussian distributions.

LCD-based detection of enzymatic action

By incorporating an ester-containing substrate in a self-assembled alignment layer for liquid crystal cells, the presence of a lipase (CALB) can be directly detected through its enzymatic action on the alignment layer, without the need for fluorescent labelling or enzyme assays.

Multiwalled carbon nanotubes with chemically grafted polyetherimides

Covalent attachment of a non-fluorinated polyetherimide onto the surface of carboxylic acid-functionalized multiwalled carbon nanotubes (MWNTs) has been achieved via grafting reactions. This confirms for the first time that the grafting reaction occurs at the nanotube surface when the carboxylic acid-functionalized MWNTs react with the polyetherimide with amine-terminated groups, through both amide and imide linkages formed at the interface between the carbon nanotubes and the polyetherimide. Additionally, an increase in the average molecular weight is detected in gel permeation chromatography when the polyetherimide is chemically attached onto the nanotubes. More interestingly, the chemical bonding at the interface provides much better interfacial adhesion and mechanical stress transfer, evidenced by a significant improvement in mechanical properties. As a result of the chemical attachment, the carbon nanotube-reinforced polyetherimide composite films have enhanced electrical conductivity, thermal deformation temperatures, and mechanical properties.

Control of Anchoring of Nematic Fluids at Polymer Surfaces Created by in Situ Photopolymerization

In situ photopolymerization of alkyl acrylate monomers in the presence of a nematic fluid provides a cellular matrix of liquid crystalline droplets in which the chemical structure of the encapsulating polymer exerts control over the alignment (anchoring) of the liquid crystalline molecules. Control is obtained by variation of the alkyl side chains and through copolymerization of two dissimilar monofunctional acrylates. For example, among a series of poly(methylheptyl acrylate)s, the 1-methylheptyl analogue prefers planar anchoring of a nematic (TL205) over the temperature range studied. However, the polymers of other methylheptyl side chains display a homeotropic-to-planar anchoring thermal transition temperature similar to that of the n-heptyl analogue. Copolymerization of two monofunctional acrylates with opposing tendencies of aligning liquid crystal leads to tunability of anchoring behavior over a wide temperature range. The broad anchoring transitions we observed provide a way of achieving highly tilted anchoring.

Control of the Geometry of the Adsorbed Thin Layer by the Depletion Interaction

The depletion interaction was reported to drive the mixtures of rodlike colloids and polymer coils into a variety of phase transitions such as isotropic-nematic, nematic-smectic, and so forth. We describe in this Communication a convenient preparation of the self-assembled monolayer of the platelike porphyrin molecules by the depletion interaction between the absorbent and the metal substrates. After the depletant, low molecular weight poly(ethylene oxide) (PEO), was added into the porphyrin/ethanol solution, random oriented porphyrin was then regulated to lie parallel to the adjacent metal substrates, forming the self-assembled monolayer.

Second-harmonic generation from rubbed ferroelectric liquid crystal mesogenic monolayer surfaces

We have designed self-assembled monolayers of molecules containing a ferroelectric liquid crystal mesogen attached to a glass surface through an alkane chain. After mechanical rubbing these layers induce a single domain in a cell containing a high-polarization achiral liquid crystal in the smectic-C phase. We have used optical second-harmonic generation to demonstrate that this behavior is explained by rubbing-induced in-plane anisotropy of the angular distribution function that describes the ensemble of mesogenic units. The surface order parameter is 0.094, a substantial fraction of what has been observed for rubbed polymeric alignment layers.