Normally Oriented Cylindrical Nanostructures in Amphiphilic PEO–LC Diblock Copolymers Films

Liquid Crystalline Nanoparticles via Polymerization-Induced Self-Assembly: Morphology Evolution and Function Regulation

Liquid crystalline nanoparticles (LC NPs) are a kind of polymer NPs with LC mesogens, which can form special anisotropic morphologies due to the influence of LC ordering. Owing to the stimuli-responsiveness of the LC blocks, LC NPs show abundant morphology evolution behaviors in response to external regulation. LC NPs have great application potential in nano-devices, drug delivery, special fibers and other fields. Polymerization-induced self-assembly (PISA) method can synthesize LC NPs at high solid content, reducing the harsh demand for reaction solvent of the LC polymers, being a better choice for large-scale production. In this review, we introduced recent research progress of PISA-LC NPs by dividing them into several parts according to the LC mesogen, and discussed the improvement of experimental conditions and the potential application of these polymers.

Azobenzene-Containing Liquid Crystalline Twisted Ribbons via Polymerization-Induced Hierarchical Self-Assembly

Polymerization-induced self-assembly incorporating liquid crystallization, as a polymerization-induced hierarchical self-assembly (PIHSA) method to produce polymeric particles with anisotropic morphologies facilely and efficiently, has drawn wide attention recently. However, the means of regulating the morphologies of liquid crystalline (LC) polymer assemblies still need to be explored. Herein, a route is presented to fabricate the twisted ribbons via PIHSA containing azobenzene based on poor reversible addition-fragmentation chain transfer (RAFT) control, called poorly controlled PIHSA. Cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl pentanoic acid-2-(2-pyridyldithio) ethyl ester is used as the RAFT agent with poor controllability, and the morphological evolution from ribbons to twisted ribbons can be observed in the corresponding PIHSA system. The formation mechanism of the twisted ribbons is studied systematically and the broad molecular weight distribution is considered to be the decisive factor. Moreover, the supramolecular chirality induced by symmetry breaking is also related to the twist of the ribbons. This study enriches the methods of controlling the morphologies of LC polymer particles and is helpful for further clarifying the mechanism of PIHSA.

High Mechanical Strength and Multifunctional Microphase-Separated Supramolecular Hydrogels Fabricated by Liquid-Crystalline Block Copolymer

The development of multifunctional supramolecular hydrogels with high mechanical strength and multifunction is in high demand. In this work, the diblock copolymer poly(acrylamide-co-1-benzyl-3-vinylimidazolium bromide)-block-polyAzobenzene is synthesized through reversible addition-fragmentation chain transfer polymerization. The dynamic host-guest interactions between the host molecule cucurbit[8] uril and guest units are used to fabricate a 3D network of supramolecular hydrogels. Investigations on the properties of the supramolecular hydrogels show that the tensile stress of the sample is 1.46 MPa, eight times higher than that of hydrogel without liquid-crystalline block copolymer, and the self-healing efficiency of the supramolecular hydrogels at room temperature is 88.3% (fracture stress) and 100% (fracture strain) after 24 h. Results show that microphase-separated structure plays a key role in the high-strength hydrogel, whereas the host-guest interaction endows the hydrogel with self-healing properties. The supramolecular hydrogels with high mechanical strength, photo-responsivity, injectability, and biocompatibility can be used in various potential applications.

Tunable Thermal Switch via Order-Order Transition in Liquid Crystalline Block Copolymer

Organic material-based thermal switch is drawing much attention as one of the key thermal management devices in organic electronic devices. This study aims at tuning the switching temperature (T_S) of thermal conductivity by using liquid crystalline block copolymers (BCs) with different order-order transition temperature (T_tr) related to the types of mesogens in the side chain. The BC films with low T_tr of 363 K and high T_tr of 395 K exhibit reversible thermal conductivity switching behaviors at T_S of ∼360 K and ∼390 K, respectively. The BC films also exhibit thermal conductivity variation originating from the anisotropy of the internal structures: poly(ethylene oxide) domains and liquid crystals. These results demonstrate that the switching behavior is attributed to an order-order transition between BC films with vertically arranged cylinder domains and the ones with ordered sphere domains. This highlights that BCs become a promising thermal conductivity switching material with tailored T_S.

In situ generation and evolution of polymer toroids by liquid crystallization-assisted seeded dispersion polymerization

An effective method is presented for preparing high solid content azobenzene-containing triblock copolymer toroidal assemblies by liquid crystallization-assisted seeded dispersion polymerization. Vesicles are prepared via polymerization-induced self-assembly (PISA), and used as seeds for further chain extension. By introducing smectic liquid crystalline (LC) ordering into the core-forming block, toroids are formed in situ during the polymerization. The morphological transformation from toroids to barrels is observed under ultraviolet irradiation due to the photo-isomerization of the azobenzene mesogens. This strategy expands the scope of tunable anisotropic morphologies for potential functional nanomaterials based on a LC copolymer by seeded dispersion polymerization.

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.

The self-assembly of single chain Janus nanoparticles from azobenzene-containing block copolymers and reversible photoinduced morphology transitions

Azobenzene-containing liquid crystalline single chain Janus nanoparticles (LC-SCJNPs) were employed as building blocks to construct assemblies showing a reversible photoinduced morphology transition.

A crystallization driven thermoresponsive transition in a liquid crystalline polymer

A new thermoresponsive transition in a liquid crystalline polymer is found and the reason leading to the thermoresponse is discussed.

Influence of single chain nanoparticle stabilizers on polymerization induced hierarchical self-assembly

Intramolecularly folded single chain nanoparticles (SCNPs) with steric character are used as stabilizers to construct a polymerization-induced self-assembly (PISA) formulation for the first time.

Synthesis of azobenzene-containing liquid crystalline block copolymer nanoparticles via polymerization induced hierarchical self-assembly

A facile synthesis of non-spherical photoresponsive azobenzene-containing liquid crystalline nanoparticles via polymerization-induced hierarchical self-assembly (PIHSA).

Self-template-assisted micro-phase segregation in blended liquid-crystalline block copolymers films toward three-dimensional structures

In-plane mesopatterns derived from block-copolymer (BCP) micro-phase segregation in thin films have attracted much interest in practical applications as well as fundamental research programs. However, phase segregation along the film-normal direction has been less studied. Here, we describe a strategy to concurrently, yet independently, control in-plane micro-phase and out-of-plane macro-phase segregation in multiblended films composed of liquid-crystalline BCPs (LCBCPs), affording spontaneously layered three-dimensional (3D) mesostructures. This strategy relies on sequential liquid crystallization during the cooling process in thermal annealing as follows. The constituent LCBCP with the highest isotropic-transition temperature (T _iso) first liquid-crystallizes and segregates from the other LCBCP mixture remaining in isotropic states to form a noncontaminated layer at the top surface. This preformed LCBCP layer preserves its inherent in-plane pattern and acts as a template guiding the subsequent micro-phase segregations of the other low-T _iso LCBCPs underneath. This self-template-assisted micro-phase segregation (STAMPS) readily provides 3D mesostructures, the potential toward rational material design of which is also demonstrated in water-separation applications.

Perpendicularly aligned nanodomains on versatile substrates via rapid thermal annealing assisted by liquid crystalline ordering in block copolymer films

The highly ordered perpendicularly aligned cylindrical and lamellar microdomains within block copolymer (BCP) films have important applications in diverse fields. However, the fast normal orientation of self-assembled nanostructures on arbitrary substrates without tedious pre- and postprocessing has been a challenging issue in manufacturing miniaturized devices. Here, we outline the potential for extending the hierarchical self-assembly within azobenzene-containing PS-b-PMA(Az) films to inherently assist in the formation of normally aligned domains using a rapid thermal annealing process (140 °C for 5 min). Liquid crystalline (LC) mesogens in PS-b-PMA(Az) films self-assemble to form a parallelly aligned sematic phase after thermal annealing, as confirmed by grazing-incidence small-angle X-ray scattering (GISAXS), wide-angle X-ray diffraction (WAXD) and ultraviolet-visible (UV-vis) spectra. This sub-phase contributes to broadening of the PS-cylinder-phase window (0.083 ≤ f _PS < 0.49) and ∼12 nm PS cylinder structures. Perpendicular cylinders or lamellae are observed on various substrates, such as silicon wafers, flexible polyethylene terephthalate (PET) sheets and conductive aluminum foils. Additionally, the good reactive ion etching (RIE) rate difference between the two blocks makes these BCPs more attractive for advancing the field of BCP lithographic applications for fabricating flexible microelectronic devices.

One-Pot Synthesis of Cross-linked Block Copolymer Nanowires via Polymerization-Induced Hierarchical Self-Assembly and Photodimerization

Block copolymer (BCP) nanowires are crucially important in diversified fields. However, their applications typically suffer from a tedious fabrication process, heterogeneous morphology, and degradation. Herein, we propose a facile and robust approach to synthesize stilbene-containing BCP nanowires in homogeneous morphology with high stability at high solid content (5%-20% w/w) in one pot. It is realized by the polymerization-induced hierarchical self-assembly and subsequent photodimerization cross-linking under UV irradiation of stilbene mesogens in the core-forming block in the later stage of polymerization. Because of the strong liquid crystalline ordering and photodimerization of stilbene, the cross-linked nanowires can be obtained over a broad BCP composition, which show robust morphological stability when exposed to solvent dissolution tests and water dispersion stability tests. This efficient approach to stabilized nanowires with homogeneous morphology via in situ cross-linking would be useful in various fields, such as nanomedicine and Pickering emulsifiers.

Photoinduced Reorientation in Thin Films of a Nematic Liquid Crystalline Polymer Anchored to Interfaces and Enhancement Using Small Liquid Crystalline Molecules

The photoinduced reorientation of the side-chain mesogens in nematic liquid crystalline (LC) polymer thin films triggered by the axis-selective photo-Fries rearrangements of side-chain phenyl benzoate moieties is studied to understand the regulation of the anisotropic nanostructures supported by LC polymers. The influence of the substrate surface in anchoring the side-chain mesogens near the interfaces is examined by comparing the reorientation of 30- and 120-nm-thick films. Irradiation with linearly polarized ultraviolet (UV) light and subsequent annealing causes the side-chain mesogen reorientation to align perpendicular to the electric field of the incident UV light. The inplane order in the 30-nm-thick films is lower than that in the 120-nm ones. On the other hand, the annealing period required for mesogen alignment is independent of the film thickness. It is suggested that the substrate surfaces anchor the LC mesogens to fix their orientation, rather than slowing down the reorientational motion. In addition, it is demonstrated that small LC molecules miscible with the nematic LC polymer enhance photoinduced reorientation through cooperative molecular interaction with the side-chain mesogens, remarkably accelerating the orientation and improving the inplane order of the unidirectionally aligned mesogens.

Mesostructure and orientation control of lyotropic liquid crystals in a polysiloxane matrix

As a preparation method for organic–inorganic or mesoporous inorganic materials via sol–gel condensation of a metal alkoxide, the combination of lyotropic liquid crystals (LLCs) and sol–gel chemistry is a versatile tool to fabricate various nanostructures. Despite previous investigations into such systems, no attempt has been made to utilize the dynamic switching functions of such nanostructures via the phase transition of LLCs in films. A polysiloxane containing an amine-hydrochloride group and a vinyl group was recently synthesized. By controlling the relative humidity, we achieved the phase transition of LLCs and on-demand UV-curing of LLC phases in the polysiloxane film. We further developed vertically oriented organic–inorganic nanochannels by using π−π interactions between discotic molecules and the substrate surface or the spontaneous vertical alignment of LLC containing azobenzene units.

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.

Liquid crystalline moiety-assisted perpendicular orientation of cylindrical domains within P4VP-b-PMA(Az) films with high aspect ratio

Block copolymer (BCP) films with perpendicularly aligned cylindrical domains of high aspect ratio have important applications in diverse fields. However, an aspect ratio of the cylinders as high as 200 has rarely been reported so far. Here we demonstrate an efficient route to the formation of normally aligned P4VP cylinders with high aspect ratio surrounded by a matrix of azobenzene-containing block (PMA(Az)) via hierarchical self-assembly. A crisscross structure, consisting of parallelly aligned liquid crystalline (LC) layers and normally aligned self-assembly domains, is expected to assist the formation of well-defined nanostructures. The LC layers in the cylindrical films self-assemble to form smectic phase after solvent annealing, as confirmed by WAXD and UV-vis spectra. We found that the aspect ratio of the vertical P4VP cylinders is up to 200 and the film thickness reaches 6 μm. P4VP is a functional polymer, making this P4VP-b-PMA(Az) film more suitable for advanced filters, multi-nanochannels, nanolithography, and high-density storage media, etc.

Formation of Anisotropic Liquid Crystalline Nanoparticles via Polymerization-Induced Hierarchical Self-Assembly

Polymeric nanoparticles (NPs) containing liquid crystalline (LC) mesogens with tunable anisotropic morphologies have applications in various fields, but their preparation typically suffers from tedious and low-throughput approaches. Here we present an efficient route to the preparation of anisotropic morphologies of azobenzene-containing block copolymers (BCPs) at high solids content via a polymerization-induced hierarchical self-assembly in ethanol. Various anisotropic NPs, including cuboids, short belts, lamellae, and ellipsoidal vesicles, have been obtained in a remarkably broad range of BCP compositions. The NPs exhibit a smectic phase with ordered stripes when observed under TEM. This internal LC ordering plays a significant role on the formation of these intriguing anisotropic morphologies. Morphological transitions from anisotropic to isotropic spheres can be obtained upon UV illumination due to the photoresponsive properties of the azobenzene mesogens. This work significantly expands the scope of accessible morphologies in PISA and suggests that the under explored LC BCPs may have an impactful role in the PISA field.

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.

Live Templates of a Supramolecular Block Copolymer for the Synthesis of Ordered Nanostructured TiO2 Films via Guest Exchange

In this work, we introduce a facile method based on host-guest chemistry to synthesize a range of nanostructured TiO₂ materials using supramolecular templates of a dendron-jacketed block copolymer (DJBCP). The DJBCP is composed of amphiphilic dendrons (4'-(3,4,5-tridodecyloxybenzoyloxy)benzoic acid, TDB) selectively incorporated into a P4VP block of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) via hydrogen bonding. The PS-b-P4VP host acts as a structure-directing template, while the guest molecules (TDB) assist the self-assembly nanostructures and zone-axis alignment, resulting in the nanostructured template of vertically oriented cylinders formed via successive phase transformations from Im3̅m to R3̅m to P6mm upon thermal annealing in the doctor-blade-cast film. The guest molecules subsequently direct the titania precursors into the P4VP domains of the templates via supramolecular guest exchange during immersion of the film in a designated precursor solution containing a P4VP-selective solvent. The subsequent UV irradiation step leads to the formation of PS-b-P4VP/TiO₂ hybrids. Finally, removal of the host template by calcination leaves behind mesoporous channels and makes sacrifices to be a carbon source for carbon-doping TiO₂ materials. Various TiO₂ nanoarchitectures, namely, vertical and wiggly micrometer-length channels, inverse opals, fingerprint-like channels, heterogeneous multilayers, and nanotubes, have been fabricated by highly tunable DJBCP nanostructures.

Evaluations of Mesogen Orientation in Thin Films of Polyacrylate with Cyanobiphenyl Side Chain

The orientation behavior of mesogens in a polyacrylate with cyanobiphenyl (CB) side chain in thin films was investigated in detail by UV-vis absorption spectroscopy and grazing incidence small-angle X-ray scattering (GI-SAXS) measurements using both high-energy X-rays of Cu Kα line (λ = 0.154 nm) and low-energy synchrotron X-rays (λ = 0.539 nm). By changing the film thickness ranging 7-200 nm, it is concluded that the planar orientation is predominant for thin films with thickness below 10-15 nm. This planar mesogen orientation near the substrate surface coexists with the homeotropically aligned CB mesogens in films thicker than 30 nm. For the thinnest 7 nm film, the planar orientation is unexpectedly lost, which is in consort with a disordering of smectic layer structure. Peculiar orienting characteristics of CB mesogen are suggested, which probably stem from the tendency to form an antiparallel arrangement of mesogens due to the strong dipole moment of the terminal cyano group.

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.

Linear assembly of a porphyrin–C60 complex confined in vertical nanocylinders of amphiphilic block copolymer films

Linear assemblies of a 1 : 1 porphyrin–fullerene C₆₀ complex were formed in vertical cylindrical polyether nanodomains of amphiphilic block copolymer films.