New Aspects for the Hierarchical Cooperative Motions in Photoalignment Process of Liquid Crystalline Block Copolymer Films

Circular depolarization spectroscopy: A new tool to study photo-imprinting of chirality

When irradiating a molecular material containing photo-isomerizable groups with pure circularly polarized light, a particular handedness may get imprinted into the material. To study the mechanism and kinetics of this process in situ and operando, we have developed a new chiroptical tool where the circular polarization of the incident circularly polarized light is monitored after transmission through the photoactive layer. Practical limits to the resolution and sensitivity of the measurements as well as its calibration are discussed. To aid interpretation of experimental results, we present kinetic Monte Carlo simulations on a model for the active material involving photo-induced reorientation of molecules in a cholesteric organization. The simulations support the interpretation of a transient minimum in the degree of circular polarization of the transmitted light in terms of a nematic transient state during photo-inversion of a cholesteric organization in the molecular material.

Thickness-Dependent Photo-Aligned Thin-Film Morphologies of a Block Copolymer Containing an Azobenzene-Based Liquid Crystalline Polymer and a Poly(ionic liquid)

Photo-induced alignment of the thin-film morphologies of azobenzene-containing block copolymers (BCPs) is an effective method to obtain a uniaxial pattern of nanocylinders. Although film thickness is an important factor affecting the self-assembly of BCP thin films, the influence of film thickness on the photo-induced alignment of BCP thin-film morphology has never been systematically studied. Herein, we report the thickness-dependent photo-aligned film morphologies of the BCP containing an azobenzene-based liquid crystalline polymer and a poly(ionic liquid) (PIL), with a perfect uniaxial pattern of PIL nanocylinders. For films aligned with the unpolarized light (UPL), the out-of-plane PIL nanocylinders can be obtained in the film with a thickness of only 1L0 (∼30 nm, where L0 is the layer spacing of the hexagonally packed cylinder array), which is far lower than the thickness (more than 4L0) of the thermally annealed film needed to obtain the same morphology. This change is attributed to the orientation effect of UPL on azobenzene mesogens that suppresses the excluded volume effect. For the films aligned with linearly polarized light (LPL), to take advantage of the excluded volume effect to obtain the planar orientation of azobenzene mesogens, the thickness should be controlled to be no more than 3L0 to achieve an in-plane uniaxial alignment of PIL nanocylinders. The above relationship between the morphology and thickness of photo-aligned film eliminates the obstacles encountered in preparing films with well-ordered photo-aligned morphologies.

Strategies to Stabilize the Photoinduced Supramolecular Chirality in Azobenzene Liquid Crystalline Polymers

This paper describes the synthesis, thermal characterization and optical properties of liquid crystalline homopolymers and block copolymers with a repeating unit consisting of two functional units, with at least one of them being an azobenzene. Films of these polymers have been irradiated with circularly polarized light at room temperature, evaluating the intensity of the photoinduced chiral signal and its temporal stability upon storage. The paper also explores two different strategies to restrict the relaxation of the photoinduced order. Firstly, block copolymers have been prepared to confine the photoaddressable segments into nanoscopic domains where relaxation should be restricted. Secondly, an alternative homopolymer has been synthesized where the repeating unit combines two chromophores that can be separately photoaddressed, an azobenzene unit to efficiently photoinduce chirality and a cinnamate to fix the chiral signal by photocrosslinking.

Random Planar Orientation in Liquid-Crystalline Block Copolymers with Azobenzene Side Chains by Surface Segregation

Rodlike liquid-crystalline (LC) mesogens preferentially adopt a homeotropic orientation by excluded volume effects at the free surface in side-chain LC (SCLC) polymer films. The homeotropic orientation is not advantageous for in-plane LC alignment processes. Surface segregation of polymers is the phenomenon in which one component with a low surface free energy covers the surface in a mixture of two or more polymers or a block copolymer film. In SCLC block copolymer films, the surface segregation structure induces a random planar orientation due to the formation of a microphase-separated interface parallel to the substrate via the covering of one of the segregated polymer blocks. This feature article focuses on the unique, random planar orientation induced by the surface segregation of SCLC block copolymer films with the photoresponsive azobenzene (Az) mesogenic group. A transition moment of the Az mesogens is parallel to the molecular long axis, and light irradiation is conducted perpendicular to the film surface in general photoreaction processes. Therefore, the homeotropic molecular orientation in the SCLC polymer systems with Az mesogenic units inhibits efficient photoreaction reorientations in thin films. The random planar orientations by the surface segregation of a coil block in SCLC block polymers provide efficient in-plane photoreorientation and photoswitching with LC hierarchical mesostructures, such as microphase-separated structures of SCLC block copolymers and laminated LC polymer films. On the other hand, surface-segregated SCLC blocks form a high-density polymer LC brush layer with a random planar orientation by self-assembly, which exhibits efficient angular selective photoreactions. These approaches using the surface segregation of SCLC block copolymers are expected to offer new concepts for the LC photoalignment process for LC polymer devices.

Self-assembly of a silicon-containing side-chain liquid crystalline block copolymer in bulk and in thin films: kinetic pathway of a cylinder to sphere transition

The self-assembly of a high-χ silicon-containing side-chain liquid crystalline block copolymer (LC BCP) in bulk and in thin films is reported, and the structural transition process from the hexagonally packed cylinder (HEX) to the body-centered cubic structure (BCC) in thin films was examined by both reciprocal and real space experimental methods. The block copolymer, poly(dimethylsiloxane-b-11-(4'-cyanobiphenyl-4-yloxy)undecylmethacrylate) (PDMS-b-P(4CNB11C)MA) with a molecular weight of 19.5 kg mol-1 and a volume fraction of PDMS 27% self-assembled in bulk into a hierarchical nanostructure of sub-20 nm HEX cylinders of PDMS with the P(4CNB11C)MA block exhibiting a smectic LC phase with a 1.61 nm period. The structure remained HEX as the P(4CNB11C)MA block transformed to an isotropic phase at ∼120 °C. In the thin films, the PDMS cylindrical microdomains were oriented in layers parallel to the substrate surface. The LC block formed a smectic LC phase which transformed to an isotropic phase at ∼120 °C, and the microphase-separated nanostructure transformed from HEX to BCC spheres at ∼160 °C. The hierarchical structure as well as the dynamic structural transition of the thin films were characterized using in situ grazing-incidence small-angle X-ray scattering and grazing-incidence wide-angle X-ray scattering. The transient morphologies from the HEX to BCC structure in thin films were captured by scanning electron microscopy and atomic force microscopy, and the transition pathway was described.

3D Orientational Control in Self-Assembled Thin Films with Sub-5 nm Features by Light

While self-assembled molecular building blocks could lead to many next-generation functional organic nanomaterials, control over the thin-film morphologies to yield monolithic sub-5 nm patterns with 3D orientational control at macroscopic length scales remains a grand challenge. A series of photoresponsive hybrid oligo(dimethylsiloxane) liquid crystals that form periodic cylindrical nanostructures with periodicities between 3.8 and 5.1 nm is studied. The liquid crystals can be aligned in-plane by exposure to actinic linearly polarized light and out-of-plane by exposure to actinic unpolarized light. The photoalignment is most efficient when performed just under the clearing point of the liquid crystal, at which the cylindrical nanostructures are reoriented within minutes. These results allow the generation of highly ordered sub-5 nm patterns in thin films at macroscopic length scales, with control over the orientation in a noncontact fashion.

Vertical Orientation of Nanocylinders in Liquid-Crystalline Block Copolymers Directed by Light

The microphase-separated nanostructures of block copolymers are ideal nanotemplates for advanced fabrication, but they are greatly limited by the rapid and precise manipulation especially at room temperature. Here we report one method of light-directed regulation of nanostructures in thin films of liquid-crystalline diblock copolymers containing azobenzene units as photoresponsive mesogens. The in-plane orientated nanocylinders in thin film can be light-directed into out-of-plane on a time scale of seconds at room temperature. This fast regulation is beneficial from the fast process of photoinduced phase transition of the mesogenic block from liquid crystal to disordered isotropic phase. Several influence factors like the molecular weight of polymer, film thickness, light intensity, and relative humidity were studied in the light-directed processes. In addition, the photoregulated nanostructures demonstrate their capability of being photopatterned and further used as nanotemplates for fabrication of nanoparticles. The light-directed method shows noncontact, precise, and reversible features, enabling it to find further applications in fast control of nanostructures for nanofabrication and nanoengineering.

Bulk self-assembly and ionic conductivity of a block copolymer containing an azobenzene-based liquid crystalline polymer and a poly(ionic liquid)

A block copolymer containing a liquid crystalline polymer and a poly(ionic liquid) self-assembles and can be used as a solid electrolyte.

Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications

Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photochromism, liquid crystallinity, and fabrication techniques has enabled some fascinating functional materials which can be driven by ultraviolet, visible, and infrared light irradiation. Nanoscale particles have been incorporated to widen and diversify the scope of the light-driven liquid crystalline materials. The developed materials possess huge potential for applications in optics, photonics, adaptive materials, nanotechnology, etc. The challenges and opportunities in this area are discussed at the end of the Review.

Inducing Planar Orientation in Side-Chain Liquid-Crystalline Polymer Systems via Interfacial Control

For efficient photoresponses of liquid-crystal (LC) azobenzene (Az) polymer systems, planar LC orientation of the Az mesogenic group is required because the light irradiation process usually occurs with normal incidence to the film surface. However, LC molecules with a rodlike shape tend to orient perpendicularly to the film surface according to the excluded volume effect theory. This review introduces new approaches for inducing planar orientation in side-chain LC Az polymer films via interface and surface molecular designs. The planar orientation offers efficient in-plane photoalignment and photoswitching to hierarchical LC architectures from molecular LC mesogens and LC phases to mesoscopic microphase-separated structures. These approaches are expected to provide new concepts and possibilities in new LC polymer devices.

Dynamic photoinduced realignment processes in photoresponsive block copolymer films: effects of the chain length and block copolymer architecture

A series of block copolymers composed of an amorphous poly(butyl methacrylate) (PBMA) block connected with an azobenzene (Az)-containing liquid crystalline (PAz) block were synthesized by changing the chain length and polymer architecture. With these block copolymer films, the dynamic realignment process of microphase separated (MPS) cylinder arrays of PBMA in the PAz matrix induced by irradiation with linearly polarized light was studied by UV-visible absorption spectroscopy, and time-resolved grazing incidence small angle X-ray scattering (GI-SAXS) measurements using a synchrotron beam. Unexpectedly, the change in the chain length hardly affected the realignment rate. In contrast, the architecture of the AB-type diblock or the ABA-type triblock essentially altered the realignment feature. The strongly cooperative motion with an induction period before realignment was characteristic only for the diblock copolymer series, and the LPL-induced alignment change immediately started for triblock copolymers and the PAz homopolymer. Additionally, a marked acceleration in the photoinduced dynamic motions was unveiled in comparison with a thermal randomization process.