Photocrosslinkable Liquid Crystalline Polymers Research Articles

Fabrication and performance of efficient thin circular polarization gratings with Bragg properties using bulk photo-alignment of a liquid crystalline polymer

Thin circular polarization gratings, characterized by high diffraction efficiency and large, up to 42°, diffraction angles were created by polarization holography for the first time. The high efficiency of the gratings is the result of the specific properties of a photo-crosslinkable liquid crystalline polymer and a two-step photochemical/thermal processing procedure. A diffraction efficiency of up to 98% at 532 nm has been achieved for gratings with periods of 700 nm. In contrast to polarization gratings with larger periods these gratings exhibit Bragg properties. So one beam is either transmitted or diffracted depending on the direction of the circular polarization of the incident light, whereas the maximal diffraction efficiency is achieved only at the proper incident angle. The fabrication procedure consists of holographic exposure of the film at room temperature which provides the photo-selective cycloaddition of cinnamic ester groups. Upon subsequent thermal annealing above Tg bulk photo-alignment of the LC polymer film occurs enhancing the optical anisotropy within the grating. The holographic patterning provides high spatial resolution, the arbitrary orientation of the LC director as well as high optical quality, thermal and chemical stability of the final gratings. Highly efficient symmetric and slanted circular polarization gratings were fabricated with the proposed technique.

Evaluation of Molecular Orientation of Nanostructure of Photo-cross Linkable Liquid Crystalline Polymer Fabricated by Nanoimprint Graphoepitaxy

Evaluation of Molecular Orientation of Nanostructure of Photo-cross Linkable Liquid Crystalline Polymer Fabricated by Nanoimprint Graphoepitaxy

Formation of complex molecular orientation patterns in nanostructures via double nanoimprint graphoepitaxy

Nanoimprinting can be used to fabricate various nanostructures on materials without etching processes. We previously demonstrated that the molecular orientation of a photocrosslinkable liquid crystalline polymer (PLCP) was induced by nanoimprinting, using a process called nanoimprint graphoepitaxy. In the current study, we performed multiple nanoimprint graphoepitaxy steps on PLCP films and investigated the resulting molecular orientations. Double nanoimprint graphoepitaxy on the PLCP produced a simple 2-µm line-and-space pattern with a dot molecular orientation pattern.

Depth analysis of molecular orientation induced by nanoimprint graphoepitaxy

The nanoimprint process is a useful technique for the fabrication of various nanostructures on functional materials without the use of any etching processes. We previously confirmed that the molecular orientation of a photo-crosslinkable liquid crystalline polymer (PLCP) was induced by nanoimprinting using a process called nanoimprint graphoepitaxy. In this work, we repeat the previous O2 reactive ion etching processing and molecular orientation evaluation to examine the depth profile of the unidirectional molecular orientation that is induced by nanoimprint graphoepitaxy. The results obtained indicate that the initial PLCP film thickness affects the resulting unidirectional molecular orientation depth.

Photoalignment anchoring energy of photocrosslinkable liquid crystalline polymers doped in nematic liquid crystals

We characterized the photoalignment anchoring energy of photocrosslinkable liquid crystalline polymers (PLCPs) doped in a liquid crystal (LC). PLCP-doped LC cells with homogeneous alignment were fabricated using rubbed polyimide (PI) films. The PLCP-doped LC cells were exposed to a linearly polarized ultraviolet laser beam, and were then annealed. As a result, the PLCP-doped LCs were realigned owing to the axis-selective photocrosslink reaction. We investigated the relationship between the surface anchoring strength of the rubbed PI films and the realignment direction. The result suggested that the photoalignment anchoring strength of the PLCP in the LC is higher than 4 × 102 J/m3 at the maximum. The photoalignment realized substantial realignment in the LC cell with the surface anchoring strength of 10−4 J/m2.

Molecular reorientation behavior of photocrosslinkable liquid-crystalline polymer films containing phenylamide side chains

The cooperative molecular reorientation in methacrylate copolymer films comprising photoreactive 4-(4-methoxycinnamoyloxy)biphenyl and phenylamide in their side chains was investigated by irradiation with linearly polarized ultraviolet light and subsequent annealing. Both cinnamate and phenylamide side groups were miscible in the copolymer composition, and axis-selective photoreaction of the cinnamate groups was observed. Thermally enhanced cooperative in-plane orientation of both side groups was obtained when the irradiated films were annealed in the liquid-crystalline temperature range of the copolymers.

Tuning Thermal Stability, Photoluminescent Wavelength, and Optical Anisotropy of Oligothiophenes with Pyridyl End Groups by Using a Hydrogen-bonded Photocrosslinkable Liquid-crystalline Polymer Film

Abstract Asymmetric oligothiophenes with pyridine end groups, coated on a uniaxially aligned acid polymer film and subsequently annealed, exhibited large dichroism and linearly polarized photoluminescence parallel to the orientation direction of the aligned film. Furthermore, sublimation of the thiophenes during annealing was suppressed due to interaction with the acidic film.

Photocrosslinkable liquid crystalline polymers based on cyclohexanone and fluorescent heterocyclic ring system

New liquid crystalline polymers containing both fluorescent and photocrosslinking properties with various alkyl chain lengths (m = 2,4,6,8,10) were prepared by solution polycondensation reaction of 2,6-bis[4-(m-alkoxy)benzylidene]cyclohexanone and (4-phenyl)-2,6-bis (4-chlorocarbonyl phenyl)pyridine in THF using triethylamine as an acid acceptor. The polymers were characterized using various spectral techniques. These polymers were found to be thermally stable up to 377 °C confirmed by thermogravimetric analysis (TGA). Differential scanning Calorimetry (DSC) and polarized optical microscopy (POM) were carried out to study the phase structures and phase behaviors of this series of polymer. Some of the polymers with longer methylene chains demonstrated grainy type nematic LC phases. On irradiation with UV light at various time intervals, the prepared polymers showed different rate of crosslinking depending upon the alkyl chain lengths present in the polymer backbone. In addition, these polymers also show enhanced fluorescent property by the presence of heterocyclic unit in the polymer.

Photocrosslinkable liquid–crystalline polymers for stable photovoltaics by adjusting side-chains spacing and fullerene size to control intercalation

We report a novel copolymer system with high crystallinity and photocrosslinkable building blocks for π–π intermolecular interactions that is, an alternating copolymer with liquid–crystalline nature, and heat/solvent resistance. By copolymerization of 2,5-bis(3-bromododecylthiophen-2-yl)thieno[3,2-b]thiophene (BbTTT) monomer with thiophene and thieno[3,2-b]thiophene via Stille reaction, two novel copolymers PBbTTT-T and PBbTTT-TT have been synthesized. The balanced space between fullerene size and the side-chains of the polymer is crucial to determine the optimum polymer:fullerene blending ratios and the formation of intercalated nanostructure, in which the lamellar arrangement can be controlled by adjusting the fullerene size. This pre-optimum bimolecular crystal morphology can be frozen and preserved with long term performance after UV treatment, a clear advantage for the photo-crosslinking strategy. Furthermore, the free space in the intercalation impacts greatly on the stability of the donor–acceptor bicontinuous network, especially after photocrosslinking. The bulk-heterojunction organic photovoltaics based on PBbTTT-TT:PC71BM at 1:3.5 by weight shows an stable, well-ordered and intercalated nanostructure with an efficiency higher than 2.4% after 40h annealing at an elevated temperature of 150°C.

Thermal Nanoimprinting on Prealigned Photoreactive Polymer Liquid Crystal for Molecular Reorientation

Molecular orientation in a photo-crosslinkable liquid crystalline polymer (PLCP) film was explored by a thermal nanoinprinting technique using a SiO2/Si mold with a line-and-space (L&S) pattern. A molecular oriented structure was confirmed by polarization optical microscopy and diffraction efficiency measurements, at which the reoriented direction was parallel to the mold lines. We demonstrated thermal nanoimprinting on prealigned PLCP films by linearly polarized UV (LPUV) and evaluated the optical characteristics of imprinted PLCP films. As a result, the prealigned PLCP films were reoriented by thermal nanoimprinting.

Photoinduced reorientation and polarization holography in photo-cross-linkable liquid crystalline polymer films with large birefringence

A new photo-cross-linkable liquid crystalline polymer (PLCP) comprised 4-methoxycinnamoyloxy groups connected with a bistolane side group was synthesized to investigate thermally enhanced photoinduced molecular reorientation of a thin film with linearly polarized (LP) 365 nm light exposure. Due to the axis-selective photoreaction of the cinnamate groups followed by the thermally induced self-organization, large molecular reorientation parallel to the polarization of LPUV light ( S > 0.6) and large birefringence at the non-resonance region (Δ n = 0.34 at 632.8 nm) were obtained. The obtained Δ n value is the largest among transparent PLCPs in the visible region. The influence of the degree of photoreaction and the annealing temperature on the thermally enhanced molecular reorientation behavior was explored in detail. Finally, for an application of thin optical devices using the PLCP films, pure polarization holographic gratings with large birefringence, which showed periodic molecularly oriented structure, were fabricated using a 325 nm He–Cd laser in various polarization modes and characterized their optical properties.

Thermally Stable Binary Optical Films Based on Photocrosslinkable Liquid Crystalline Polymers Containing Azodyes

We developed thermally stable binary optical films based on azodye-containing liquid crystalline polymers (ACLCPs) through two-step exposure of linearly polarized light with the wavelength of 488 nm together with subsequent exposure for photo-crosslinking by ultraviolet light under nitrogen atmosphere. Our fabricated optical films with two in-plane optic axes showed high thermal durability and little degradation of optical efficiency less than 5% at 200°C. Due to the large birefringence acquired during the photo-alignment, the binary optical films can be used as a variety of patterned retarders for mobile display applications.

Effect of Spacer Length on Photoinduced Reorientation Behavior of Photocrosslinkable Liquid Crystalline Polymer Films Comprising 4-(4-Methoxycinnamoyloxy)biphenyl Side Groups

Methacrylate polymer liquid crystals containing 4-(4-methoxycinnamoyloxy)biphenyl (MCB) side group with different alkylene spacer length between the polymethacrylate backbone and MCB side groups are synthesized. Influence of the alkylene spacer length on the thermally enhanced molecular reorientation behavior of thin films is investigated using linearly polarized ultraviolet (LPUV) light exposure and subsequent annealing. In-plane reorientation preferentially occurs when the alkylene spacer is short, and the induced in-plane order parameter is greater than 0.6. On the other hand, when the alkylene spacer is long, thermally induced out-of-plane reorientation is observed as well as the in-plane reorientation.

Thermally Enhanced Photoinduced Reorientation in Photo-Cross-Linkable Liquid Crystalline Polymers Comprised of Cinnamate and Tolane Mesogenic Groups

Photo-cross-linkable liquid crystalline polymers (PLCPs) comprised of a 4-methoxycinnamoyloxy group connected with a tolane moiety possessing methyl substituents were synthesized in order to investigate the influence of methyl substituents on the thermally enhanced in-plane molecular reorientation of thin films. Different wavelengths from a UV light source were used to evaluate the effect of the photoreaction of the cinnamate and tolane moieties. For all PLCP films, thermally enhanced photoinduced molecular reorientation was observed, but the reorientation order and its direction greatly depended on the position and the number of the methyl substituents as well as the wavelength of light. A smaller degree of photoreaction was sufficient for effective molecular reorientation when the photoreaction of the tolane moiety was restricted.

Photoalignment and patterning of a chromonic–silica nanohybrid on photocrosslinkable polymer thin films

We recently found that the chromonic mesophase structure of an azo dye can be immobilized by a silica network to form a chromonic–silica nanohybrid. This work reports the first demonstration of the macroscopic photoalignment and photopatterning of this nanohybrid via surface-mediated orientational transfer from a photocrosslinkable liquid crystalline polymer (PPLC). Irradiation with linearly polarized light (LPL) 300 nm above the PPLC film followed by an appropriate annealing process led to strong in-plane molecular orientation in the film. The precursor sol solution containing a chromonic forming azo dye and tetraethoxysilane in the presence of an anionic surfactant and 2-(2-aminoethoxy)ethanol was placed onto this aligned film by the static deposition or dipping method. After drying, the chromonic columns hybridized with silica were macroscopically oriented orthogonal to the LPL direction, namely the orientation of the dye molecule being parallel to it. In the dip coating process, a combination of photoalignment and flow induced orientation allowed clear micropatterning of the column orientations, which could be readily discerned when observed with a polarizing film.

Synthesis and thermal properties of new chalcone-based side-chain polymers

We synthesized two types of photocrosslinkable side-chain liquid crystalline polymers (SLCPs) with a chalcone derivative in the side chain. One is a polymer having the chalconyl moiety directly connected to the benzoate component, the other is a polymer with a spacer group introduced between the cyanobiphenyl and the chalconyl moiety. The thermal properties of these polymers were investigated by polarizing optical microscopy measurements and differential scanning calorimetry measurements. Mesophase structures of these polymers were examined by X-ray measurements. A smectic phase was exhibited for the polymers.