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Photoresponsive Liquid Crystalline Polymers

  • Xiao Li
  • Haifeng YuEmail author
Living reference work entry

Abstract

Light is external stimuli which is clean and highly accessible and can be precisely manipulated. Due to the fascinating photoinduced changes from molecular to nano- and macroscopic scales, photoresponsive liquid crystalline polymers (LCPs) have attracted wide interests in the fields of stimuli-responsive materials, soft robotics, biomaterials, nanotechnology and photonic devices, and so on. This entry mainly focuses on the booming researches related to photoresponsive LCPs, especially azobenzene-containing LCPs. First, the definition and photochemical properties are clarified. Then, recent advances on intriguing light-responsive behaviors of azobenzene-containing LCPs are introduced in terms of homopolymers, block copolymers, crosslinked LC systems, composite LC systems, and supramolecular liquid crystalline polymers. These photoinduced behaviors include molecular cooperative motion, nanoscale self-assembled photocontrollable structures, and macroscale photo-driven 2D and 3D mechanical movements. Finally, the researches are summarized and the possible applications are proposed.

Keywords

Liquid crystalline polymer Photoresponsive liquid crystal Liquid crystalline block copolymer Liquid crystalline elastomer Supramolecular liquid crystal 

Definition

Photoresponsive liquid crystalline polymers (LCPs) are one kind of advanced functional materials with light-controllable characteristics. Typically, photoisomerizable azobenzenes or photocrosslinkable cinnamates are often used as mesogens to prepare photoresponsive LCPs and their LC properties and self-organizing natures to be manipulated by light with suitable wavelength. In this entry, we would introduce photoresponsive properties, the ordered structures, and potential applications of several LCPs in the form of homopolymer, block copolymer (BC), crosslinked elastomer, and supramolecular matters.

Introduction

Liquid crystals (LCs) are a state of matter between solid and liquid, which has a long-range ordering with fluidity as well. Friedrich Reinitzer, an Australian botanist, first discovered the so-called the fourth state of matter. He observed two melting points when he studied a natural product, cholesteryl benzoate, opening the gate of the research on LCs. As one of the most fascinating soft matters, LC show unique properties (Ikeda 2003): (1) self-organizing nature; (2) fluidity with a long-range order; (3) cooperative motion; (4) anisotropy in all kinds of physical properties; and (5) mesogenic alignment under external fields. These distinctive natures make it hot in various fields including physics, chemistry, material science, biomedicine, and other interdisciplinary fields.

Different for three-dimensional (3D) ordered crystals, LCs possess one-dimensional (1D) or two-dimensional (2D) order. Typical phases of LCs such as nematic, smectic, and cholesteric are given in Fig. 1. In general, nematic LC molecules are centrosymmetric, and the nematic phase has only an orientational order and lack of positional order. As a result, the molecules are free to flow, and their center of mass positions are randomly distributed as in disordered liquid but still maintain their long-range directional order. The cholesteric phase not only has an orientational order similar to the nematic phase in all physical properties but also has an arrangement in a helical way. The periodic distance is often called a pitch. The smectic phases possess a positional order, and the position of the molecules is correlated in some ordered pattern. Lamellar structures with 2D ordering can be observed, as shown in Fig. 1. Due to the tilt angles (with respect to the plane normal) and packing formation, several forms of smectic phases have been discovered and defined such as smectic A (SmA) and smectic C (SmC).
Fig. 1

Schematic illustration of typical LC ordering and molecular packing

Liquid crystalline polymers (LCPs) are the combination of LCs and polymers. Polymer materials possess good mechanical properties and have entered everywhere in our life soon after their discovery, while the industry applications of LCs are restricted to liquid crystal displays (LCDs) invented in the 1960s. Most of the LC phases have been observed in LCPs till now. The integration of LCs and polymers has brought more unusual properties, thus making it possible for potential applications in diverse fields (Wang and Zhou 2004).

Recently, stimuli-responsive materials have evoked enormous interests due to their potential applications in micromachines, soft robots, biomedical systems, etc. (Li 2012). Among all these external stimulus-driven methods, such as pressure (Ilievski et al. 2011), pH (Ren et al. 2015), heat (Behl and Lendlein 2010), electric field (Daunert et al. 2006), and magnetic field (Harris et al. 2005), light is absolutely charming, as it is an abundant and clean energy which could be controlled remotely, precisely, and instantly. When photoisomerizable azobenzenes or photocrosslinkable cinnamates are used as mesogens to prepare LCPs, photoresponsive LCPs can be obtained correspondingly. This enables their LC properties and self-organizing natures to be manipulated by light. To study their photoresponsive feature and explore their potential applications, several LCP architectures such as homopolymer, block copolymer (BC), crosslinked elastomer, and supramolecular matter have been developed recently. In this entry, we would mainly focus on the photoresponsive properties, the ordered structures, and potential applications of these LCPs.

Photoreaction and Photoresponsive Properties

Photochemical reaction usually leads to both chemical and physical changes in materials, such as refractive index, solubility, and dielectric parameters, and thus can be used to fabricate various kinds of devices. So far, several light-active groups have been applied to prepare photoresponsive LCPs. For example, cinnamate and chalcone have photocrosslinking property upon irradiation of UV light. Photoinduced [2 + 2] reaction often occurs between two molecules to form cyclobutane structures, as shown in Fig. 2. In addition, chalcone and its derivative have the advantage of good hydrolysis resistance, thermal stability and very high absorption coefficient around 300–365 nm, which has been widely used in the fields of lithography printing, LCDs and nonlinear optical materials (Faghihi and Shabanian 2012).
Fig. 2

Photocrosslinking reactions of cinnamate and chalcone chromophores and the formation of cyclobutane structure upon UV irradiation

Due to its excellent properties, photoisomerizable azobenzene has become one of the most attractive chromophores in synthesis of photoresponsive LCPs. Upon UV irradiation at 365 nm, trans molecules would isomerize into their cis forms, and they come back to trans states upon thermal treatment or irradiation with visible light, as shown in Fig. 3a. This trans-cis transition occurs accompanying with the change of molecular size, which becomes the fundamental of the photoresponsive property of azobenzene molecules.
Fig. 3

Photoresponsive properties of azobenzene chromophores. (a) Photoisomerization; (b) photoalignment with the transition moment of azobenzenes perpendicular to the polarization direction; (c) reversible photoinduced LC to isotropic phase transition. (Adapted from Yu (2014b), copyright, Elsevier)

According to their photochemical behaviors, azobenzene molecules have been divided into three classes (Kumar and Neckers 1989; Rocho 1993), and they are common azobenzenes, aminoazobenzenes, and pseudostilbenes. In the first class, there is little overlap between π-π∗ and n-π∗ absorbance; thus the lifetime of cis-isomer is quite long. In the second group, there is an amino group donating electrons on one side of the azobenzene molecules and evident π-π∗ and n-π∗ overlap in UV-vis absorption spectrum. In the third class, an electron-donating group and an electron-accepting group are substituted on both sides of the azobenzene molecules, the two bands are inverted, and the lifetime of cis-molecules is the shortest, sometimes seconds. Although the photochemical transition occurs in all kinds of molecules, the third type becomes the most suitable one when it comes to photoresponsive properties.

If azobenzene molecules are incorporated into a polymer chain, a series of motions can be acquired following the interesting photoisomerization. The photoinduced motions have been classified into three levels (Natansohn and Rochon 2002). The first level is the chromophore motion, influenced by the light polarization. Upon irradiation of linearly polarized light, the molecules, whose axes are parallel to the polarization direction of the actinic light, are photoactive. Therefore, the chromophores would turn into the photo-inert direction, perpendicular to the light polarization, as shown in Fig. 3b. This effect is well-known as the Weigert effect.

The second level is the motion at nanoscale, which is in the domain level. Two requirements are needed for the second motion: (1) the chromophores are bound to the polymer matrix or become part of ordered structure, such as LCs, Langmuir-Blodgett, or monolayer films; (2) the matrix has some intrinsic sequencing, such as LC or semicrystalline ordering. Thus, the LC domain or LB monolayer would become a constraint for the motions. When the chromophores were aligned into ordered structures, this orientation would force the whole domain to transfer along the direction perpendicular to the light polarization. This is known as the cooperative motion in ordered materials.

The third level of motion happens in a larger scale, which is the macroscopic motion. This motion also requires the chromophore to be integrated to polymer chain, and it involves massive motion of the whole polymer system. Though the mechanism of the mass transition is still unclear, we can get ordered patterns through this method upon holographic recording. Gratings are the products of mass transition, which we would introduce later in this entry (Yu et al. 2005a). Furthermore, the motion in crosslinked LC elastomer system can be large enough for being visible with the naked eye (Yu et al. 2011a).

One of the most interesting properties of photoresponsive LCs is photoinduced phase transition. When azobenzene moieties possess soft alkyl substituents in their benzene rings, they often show LC phase. Then, azobenzenes may play both roles of photoresponsive groups and rodlike mesogens, as shown in Fig. 3c. Upon irradiation of UV light, the well-known trans-to-cis photoisomerization occurs, which is accompanied by the photoinduced LC to isotropic phase transition, since the trans-azobenzene can be a mesogen and the cis-azobenzene never shows any LC phase due to its bent shape. As a result, light can be easily applied to control the changes between the ordered LCs and disordered isotropic states. This is very important for their photonic applications.

Homopolymers

Homopolymers contain only one kind of photoresponsive mesogens in their macromolecular chains, incorporating LC properties with high-performance polymer materials possessing a film-forming nature, high processability, easy-fabrication characteristics, high corrosion resistance, and low manufacturing costs. More interestingly, the photoresponsive mesogens provide the designed LCP homopolymers with photocontrolled features, as shown in Fig. 4. Among them, photochemical phase transition, photo-triggered molecular cooperative motion, and photoinduced alignment are the most important ones (Yu 2014a).
Fig. 4

Photoresponsive properties of homopolymers with azobenzene (AZ) as the only mesogens in LCPs

Rodlike molecules usually show axis-selective photochemical reaction, which leads to the optical and physical anisotropy. Several materials that undergo an axis-selective photochemical reaction have been studied, which mainly includes two types: photoisomerizable azobenzenes and photocrosslinkable cinnamates. Several factors that influence the photoalignment of LCPs have been reviewed, covering (1) the photoreaction of the moieties, (2) the interaction preference between photoproducts and mesogens, (3) the azimuthal anchoring energy, and (4) pretilt formation of the LC orientation (Kawatsuki 2011).

So far, there are two ways to enhance optical anisotropy in LCPs, that is, thermal enhancement like thermal annealing and photoalignment enhancement such as exposure to UV irradiation. For photocrosslinkable LCPs, the axis-selective photoirradiation induces small optical anisotropy, while thermal annealing would largely enhance the parameters, resulting in molecular reorientation. Kawatsuki et al. prepared novel LCP films, in which the alignment could be controlled by irradiation with UV light and thermal treatment (Kawatsuki et al. 2001). Besides, the direction of molecular orientation can be adjusted by controlling the incident angle and the polarized direction of the UV light. Due to the transparency in visible region of the cinnamate derivatives, they are supposed to be used in displays.

The performance of LCP homopolymers to some degree depends on the structure of polymers main chains. For example, the glass transition temperature (Tg) of one nematic LCP of PM6ABOC2 is higher than another LCP of PA6ABOC2, as shown in Fig. 5. Even though they show similar rate in LC isomerization phase transition, the recovery process of PM6ABOC2 is slower than that of PA6ABOC2 due to the difference of main chain stiffness (Kanazawa et al. 1997). For side-chain azopolymers, the molecular reorientation depended on the length of the alkylene spacer and the amount of the axis selectively formed cis-isomers (Uchida and Kawatsuki 2006).
Fig. 5

(a) Influence of molecular structure on the property of azobenzene-containing LCP homopolymer; (b) the mechanism of application of optical storage

The application of azobenzene-containing LCPs as optical image storage materials was first reported in 1995 (Ikeda and Tsutsumi 1995). In a nematic glassy state, LCPs undergo a nematic LC-to-isotropic phase transition upon photoirradiation while they never exhibit an isotropic-to-nematic LC phase transition below their Tgs. As shown in Fig. 5b, it is obvious that the UV-irradiated area becomes isotropic since cis-azobenzene never exhibits any LC ordering due to its bent shape. By this way, an optical image can be stored in the LCP films by using a photomask. It was observed that the thermal cis-trans back-isomerization of azobenzene mesogens in polymer film occurred in 24 h at room temperature. Although the trans-form recovered completely, the isotropic glassy domains induced at the irradiated area still remained unchangeable at room temperature even after several years. These results suggest that the orientation of the mesogenic trans-azobenzenes becomes disordered below Tg through thermal cis-trans back-isomerization process. However, even after the trans-form is thermally recovered, the alignment of mesogens is difficult in the absence of segmental motions of the main chain of the LCP below Tg. The recorded images can be deleted by heating the sample films higher than its Tg, which can be used for rewritable storage of new images. Therefore, the photoresponsive LCP can be used as optical switching as well as optical image storage materials as shown in Fig. 5b.

To display a complete 3D image of an object, holograph has been developed, and this unique technique enables concomitant recording of both phases and amplitudes of light waves. As one of the most promising holographic recording materials, azobenzene-containing LCPs can be easily modulated into diverse patterns by adjusting the input light with wavelength, intensity, polarization, phase, interference, and so on. As shown in Fig. 6, holograms can be recorded by photoinduced change in orientation of LC molecules in a periodic pattern obtained from interference of two coherent laser beams. Since a large refractive index modulation can be obtained in azobenzene-containing LCPs, the recorded hologram is phase-type and often shows higher diffraction efficiency than the amplitude one (Yu et al. 2005a, 2008a, b). As shown in Fig. 6, a refractive index grating with an alternating patterned domains of LC and isotropic phase is under a surface-relief grating.
Fig. 6

Holographic gratings recorded in photoresponsive LCPs with azobenzenes as nematic mesogens. Periodical modulation of nematic LC (N) and isotropic (I) phase was obtained upon holographic recording

Photoresponsive LCPs are suitable for holographic applications, which is beneficial from their quick optical process and large modulation of refractive index. The optical birefringence can be produced by change in transition moments of chromophores by photoalignment. Among diverse of birefringent mesogens, a tolane moiety is one of the most common core structures for design of highly birefringent LC molecules because of its longer molecular conjugation length. To increase the photosensitivity and induce a larger birefringence, a concept of molecular architecture was proposed (Okano et al. 2006), in which a tolane group is directly attached onto an azobenzene molecule to prepare an azotolane mesogen, as shown in Fig. 7a. The azotolane group possesses a far longer molecular conjugation length than one single tolane or an azobenzene moiety, enabling the azotolane-containing LCPs to show a huge photoinduced change in birefringence.
Fig. 7

(a) Molecular architectures of LCPs containing azotolane mesogens for a photoinduced large change in birefringence. (b) Precise control of photoinduced birefringence by post-functionalization. (Reproduced with permission from Yu et al. (2009), copyright, John Wiley and Sons)

Recently, a post-functionalization method was elegantly explored to an azobenzene-containing LCP precursor to control the content of azotolane in the LCPs with the degree of functionalization (Yu et al. 2009). Thus, precise control of photoinduced birefringence was successfully obtained. The molecular architecture of azotolane LCPs with a giant photoinduced change in birefringence is useful for extensive optical applications in high-performance photonic devices (such as high-density optical recording, as well as holographic and multi-bit recording) and photo-switching materials.

Liquid Crystalline Block Copolymers (LCBCs)

BCs are composed of at least two kinds of polymers with different chemical characters integrated through covalent bonding. If polymer components in BCs are immiscible with each other, then the polymers would assemble into different phase-separated nanostructures according to the contents of each block. Usually, the size of the nanostructure is about 10–100 nm. The morphologies vary from spheres, cylinders, to lamellae phases with various volume ratio of polymers (Fasolka and Mayes 2001; Bates and Fredrickson 1990; Thomas et al. 1994). So far, researches on microphase separation are focused on exquisite structures, which may be applied in photonics, novel nanomaterials, and fine processing technology.

Liquid crystalline block copolymers (LCBCs) are the combination of BCs and LCs, the two soft matters with self-organization capability (Yu and Ikeda 2011b). When light-active chromophores are introduced as mesogens, LCBCs could respond to the incident actinic light and show the photoresponsive properties, for example, photoinduced phase transition, photocontrolled alignment, and photoinduced mass transition, just like that of the LCP homopolymer. These properties offer the fundamental of novel functions in this kind of materials.

To get ordered nanostructures in thin films upon microphase separation, the polymer structure must be well-defined and the molecular weight should be larger enough in LCBCs (Yu 2014b). Methods to synthesize LCBCs usually contain anionic polymerization, cationic polymerization, and conventional and controlled radical polymerizations. The earliest way to synthesize photoresponsive LCBCs is by anionic polymerization. However, the reaction condition is hard to reach: (1) the monomer must be purified, (2) the environment should be dry enough, (3) it is difficult to purify macromolecular initiator, and (4) the polymerization temperature is relatively low. Therefore, controlled radical polymerization has been widely used in synthesizing BCs, including atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, nitroxide-mediated polymerization (NMP). Among these methods, ATRP is a relative convenient and efficient way, thus popular in synthesizing LCBCs (Yu et al. 2005b, 2007a, b).

Due to the immiscibility between the two blocks, the assembled BCs would be microphase separated thermodynamically. Upon microphase separation, the major part forms the continuous matrix, and the minor part would self-assemble into ordered nanostructures. Most of the properties of LCBCs are similar with general BCs, but LCBCs are distinguished as they reserve the advantageous properties of LCs, such as self-assembly, long-range order in fluid, molecular cooperative motion, anisotropic properties, and so on. These could accelerate the phase separation of LCBCs, leading to alignment of mesogenic molecules and formation of long-range ordered nanostructures. When mesogen-containing polymers act as continuous matrix, the property of the LCBCs is similar with their LCP homopolymers. However, if the continuous matrix is composed of non-mesogenic polymers, the matrix polymer contributes more to the material characteristics. For instance, PEO provide materials with hydrophilicity, ionic electronic conductivity, and crystallinity (Yu et al. 2011c); poly(methyl methacrylate) (PMMA) improves the optical properties of the LCBCs (Yu et al. 2007a, b).

The microphase separation of LCBCs is actually a process of self-assembly from BCs and LCs. The property of the blocks may play an important role in the function of materials, but the process of microphase separation is also inevitable in the formation of nanostructures. In bulk films of LCBCs, the interplay functions between the microphase separation and the elastic deformation of LC ordering, known as supramolecular cooperative motion (Yu et al. 2011c, 2014), enables them to form hierarchically molecular structures with optical functional properties, which offers novel methods to control supramolecular self-assembled nanostructures, as shown in Fig. 8. Combining the excellent properties of LCPs with microphase separation of BCs, photoresponsive LCBCs may find diverse applications in advanced technology as well as newly promising nanotechnology.
Fig. 8

Control of microphase-separated nanostructures in LCBCs by supramolecular cooperative motion. (Reproduced with permission from Yu (2014a), copyright, Royal Society of Chemistry)

In LCBCs, mesogenic ordering may function as a bridge to transfer the external force to the microphase-separated nanostructures. Proper methods could help the formation of more regular patterns, which usually contains thermal annealing, mechanical rubbing, photocontrolled alignment, and electric field and magnetic field alignment. As we all know, light is one of the most convenient and cheap energy, whose intensity, wavelength, polarization direction, and interference patterns can be simply controlled. Totally, five kinds of alignments could be easily achieved, nonalignment or random, homeotropic, homogenous, photoinduced thermally isotropic amorphousness, and thermally isotropic liquid (Yu and Ikeda 2011b), just as shown in Fig. 9.
Fig. 9

Five kinds of alignments of LCBCs: (A) random alignment; (B) homeotropic alignment, molecules align perpendicular to the surface; (C) and (D) homogeneous alignment, molecules align parallel to the surface in different directions; (E) isotropic amorphous and the picture in the center shows the isotropic liquid phase. (Reproduced with permission from Yu (2014b), Copyright, John Wiley and Sons)

Yu et al. proposed an optical method to control a parallel alignment of PEO nanocylinders in diblock LCBCs with smectic ordering (Yu et al. 2006a). Firstly, PEO nanocylinder perpendicular to the direction of incident light was prepared by supramolecular cooperative motion; then this alignment can be easily converted into parallel alignment with the control of actinic light. This method has advantages of clean, simple, and convenient, providing a novel approach to get regular surface patterns even in curved surface, as shown Fig. 10 (Yu et al. 2006b).
Fig. 10

Photoalignment of microphase separation of azobenzene-containing diblock LCBCs. (a) Chemical structure and microphase separation scheme of the block copolymer; (b) the formation of microphase separation in the UV-irradiated and UV-unirradiated area. (Reproduced with permission from Yu et al. (2006a), Copyright, American Chemical Society)

Later, they prepared triblock LC copolymer containing both azobenzenes and cyanobiphenyl (CBs) moieties (Yu et al. 2007a). Upon microphase separation, the block of azobenzenes and CBs self-assembled into spherical domain. Due to the effect of molecular cooperative motion, the CBs in the nanospheres can be easily aligned according to the behavior of azobenzenes, which leads to a large anisotropy and stable birefringence. Novel wormlike nanostructures were prepared in bulk films of diblock copolymer with PMMA as matrix and azobenzene moieties exhibiting smectic LC phase. The azobenzene moieties may self-assemble into wormlike nanostructures, eliminating the scattering of visible light and showing outstanding transparency (Yu et al. 2007b). Later, they systematically examined a series of polymers containing azobenzene moieties including homopolymer, random copolymer, and diblock copolymer. These polymers demonstrate different behaviors in photoinduced alignment and holographic recording due to the varied contents of mesogens. For instance, LCBCs with azobenzene moieties as majority phase show similar photoalignment behavior with homopolymer. But the mass transport was partially prohibited when the mesogenic block forms the separated phase upon microphase separation, as shown in Fig. 11. LCBCs are advantageous over random copolymers since the LC phase can be easily obtained with lower mesogenic contents in BCs (Yu HF et al. 2008a).
Fig. 11

Different photoresponsive behaviors of LCBCs when their LC block form different phase upon microphase separation

It is well-known that the dipole moment of cis-azobenzene is larger than that of trans-azobenzene. Accordingly, the cis-ones are more hydrophilic than the trans-ones. By utilizing this properties, photoisomerization upon the light irradiation would change the hydrophobicity of azobenzene-containing LCBCs. Zhao et al. prepared reversible photoresponsive micelles composed of amphiphilic LCBCs based on the photochemical phase transition of azobenzene mesogenic moieties (Wang et al. 2004), as seen in Fig. 12.
Fig. 12

Photoresponsive behaviors of micelles obtained from amphiphilic LCBCs in solution

Firstly, the amphiphilic LCBC (PAzoMA-b-PAA) was synthesized by ATRP followed by hydrolysis. The micelle or vesicle aggregates formed when water was added into the dioxane solution of PAzoMA-b-PAA. Upon UV irradiation, the aggregates would break up into polymer segments, and they assemble into micelles again under visible light. Zhao’s work further proved that photoirradiation could not only result in the aggregation and cleavage of micelles (Su et al. 2007) but also reversibly change the morphologies of them. Seki reported that the photoalignment of LCBCs is efficiently influenced by the periodic film thickness upon holographic recording, as shown in Fig. 13 (Seki et al. 2013).
Fig. 13

Photoalignment of microphase separation by periodical film thickness obtained upon holographic recording of LCBC thin films

As we have discussed above, the azobenzene chromophores were aligned perpendicularly to the linearly polarized light. However, if the films were thinner than 60 nm, they would lead to a planar orientation. Thus, it is reasonable to get alternating orientations through inducing wavelike mass transition by interfered polarized beams upon holographic recording. In the thicker parts, the nanocylinders were perpendicular to the surface due to the supramolecular cooperative motion. While in the thinner ones, they aligned along the plane (Morikawa et al. 2006).

The photoalignment provides LCBC materials with large-scale ordered nanostructures without using any devices or instruments. Due to its convenience, this method has been widely used in various fields, especially in nanotemplates and optical storage. Seki et al. demonstrated a rewritable 3D photoalignment process in PS-based block copolymers (Morikawa et al. 2007). This system shows clear memory effect upon the irradiation of linearly polarized light of different directions, which would have potential application in the field of information storage. Microphase separation in LCBCs containing PEO cylinders has been used as the templates to obtain regular Ag nanoparticles. By soaking PEO cylinders into the AgNO3 solution, Ag+ is coordinated with PEO, and after the etching of PEO and reduction of Ag+, regular alignment of Ag nanoparticles would appear on the substrate (Li et al. 2007). We just summarized a part of the results in photoresponsive LCBCs, related research is still under the way, and promising applications would appear in the near future.

Crosslinked LC Systems

By crosslinking, linear polymers can be turned into network, acquiring the property of rubbery elastomers with freestanding feature. Combining the mechanical property of elastomers and the self-organized structure of LCs, the order-disorder transition in LC elastomers could occur rapidly, as shown in Fig. 14. Besides, the addition of photoresponsive chromophores such as azobenzene could easily lead to interesting photomechanical behaviors (Yu and Ikeda 2011b). Usually, the crosslinking density is one of the most important parameters in LC elastomers. As the crosslinking density increased, the mobility of mesogens was suppressed. Then, the response to external stimuli would be magnified when the linear polymer is crosslinked into 3D network. This magnification effect has been confirmed through experiments (Kurihara et al. 1998).
Fig. 14

Schematic illustration of photoresponsive feature of crosslinked LC systems containing light-active mesogens. (Reproduced with permission from Yu (2014a), copyright, Royal Society of Chemistry)

Based on the photochemical phase transition or photoinduced change in mesogenic ordering, several kinds of photomechanical behaviors have been explored in crosslinked LCP systems. Initial work mainly focused on the photoinduced 2D motions such as contraction and expansion, and later work turned into more complexed 3D motions including bending, twisting, and rotation driven by light.

In an azobenzene molecule, the distance between the 4- and 4′- carbon atoms is 9.0 Å in the trans-state, while it shortened to 5.5 Å in the cis-state (Xie et al. 1993). Such an isomerization transition demonstrates a large contraction ratio, which is suitable for the photoinduced deformable materials. When the temperature goes above the clearing point, the material contracts as the molecules are in its isotropic phase; on cooling, it returns back. Besides, the rubbery elastomer could render this transition reversible. Thus, it is easy to make thermal or photo-deformable functional materials by this way.

Eisenbach et al. first reported the photoinduced contraction of crosslinked amorphous polymer containing azobenzene moieties upon UV irradiation (Eisenbach 1980). Far from the theoretical contraction ratio, the observed contraction only restricted to 0.15–0.25%. Then Finkelmann et al. induced LC ordering into the crosslinking system by using soft polysiloxane as main chain and azobenzene as side chain (Finkelmann et al. 2001). Upon UV irradiation, the polymer film contracted as much as 20%. Keller and his coworkers synthesized a photoinitiator working at 600 nm. Through photopolymerization, they prepared several mono-domain nematic elastomers containing azobenzene moiety and observed photoinduced contraction of up to 20% (Li et al. 2003).

Since LC elastomers are one of the excellent choices in photoinduced 2D motion, it is reasonable to realize 3D mobile materials by suitable design of their structures. Ikeda’s group first discovered the bending behavior of LC elastomers and then carried out several representative works in this field (Ikeda et al. 2003). In 2003, reversible LC elastomer systems were reported to bend in the direction parallel to the incident light, and the bending direction could change according to the polarizing direction (Yu et al. 2003). As the molar extinction coefficient of azobenzene is large, the actinic light is absorbed by the surface of the film. For example, if the film is 10 um in thickness, the trans-cis photoisomerization occurs only on the surface, maybe 1 micron in thickness. Therefore, the “bilayer” structure formed and caused the bending toward incident light. After rubbing, the azobenzene mesogenic molecules aligned along the rubbing direction thus achieving the anisotropic bending behaviors, as shown in Fig. 15 (Ikeda 2003).
Fig. 15

Anisotropic photomechanical properties of LC elastomers containing azobenzenes as the only mesogens

Later, research found that homeotropically aligned LC elastomer films, in which the rodlike molecules are perpendicular to the film surface, had different photoresponsive behaviors, bending away from the incident light. Hybrid film in which one side is homeotropically aligned and the other side is homogeneous aligned was fabricated. Interestingly, two sides of the material showed different response to light. If the homogeneous surface were exposed to UV light, the film would bend toward light; otherwise, the film would bend away from the light. In addition, as both sides were influenced by actinic light, the hybrid film showed a much faster response rate compared with single homogeneous or homeotropically aligned films (Kondo et al. 2007; Yu et al. 2007). After that, various novel 3D movements except for bending were achieved by the same group (Yamada et al. 2009). Using similar molecules, they had obtained several motions including inchworm walk, a flexible robotic arm, and ring rotation by fabrication of bilayer films. The completely light-driven plastic motor was first obtained by the bilayer film combined with a simple pulley. Upon the irradiation of UV light and visible light in different site of the film, continuous rotation of the motor occurs because of the light-responsive property of the material films. These novel kinds of motions open the gate of one-step energy conversion from light energy directly into mechanical energy, which may have wide applications in auto-motors without battery or gear.

One of the applications of photoresponsive crosslinked LC systems is optical pendulum generator (OPG), which can convert light energy into electricity (Tang et al. 2015). As shown in Fig. 16, a cantilever LC actuator was first fabricated by casting azobenzene LCP on a grooved low-density polyethylene (LDPE) film. The actuator showed a fast and strong bending and unbending behaviors upon UV light exposure on and off. When attaching copper coils onto one free end of the cantilever and setting them under a magnetic field, the photomechanical movement of the cantilever concurrently drove the copper coils to cut magnetic line of force generating electricity. Electrical measurement of the OPG demonstrated that the output electricity was proportional to the changing rate of the magnetic flux through the coils. The light intensity, film thickness, and sample size exhibited strong influence on the movement rate of the coils. Continuously electrical output of OPG was also achieved by self-shielding the light exposure realizing alternatively photoinduced motions of bending and unbending. This simple and highly efficient strategy of coupling the photomechanical movement of LCP actuator with the Faraday’s law of electromagnetic induction may expand the applications of photoactive materials in capture and storage of light energy.
Fig. 16

Optical pendulum generator based on photomechanical LC elastomers. (Reproduced with permission from Tang et al. (2015), copyright, American Chemical Society)

Composite LC Systems

One of the main focuses in the rapidly developing area is the rational design and efficient fabrication of novel photoresponsive LCPs in order to provide these functional materials with potential applications in desired areas. As a result, multicomponent systems are developed based on LCPs. Although microphase-separated nanostructures can be obtained in LCBCs due to the immiscible property of component blocks, they often possess well-defined structures, and their chemical synthesis need hard work. From this point of view, LCP composite systems are more advantageous because of their easy fabrication.

One of the most popular composites is polymer-dispersed LCs (PDLCs), in which polymers act as the host in majority and the LCs in the minority. As the LC changes, the photo-inert polymer substrate can feel the response and transfer it to the whole system. Yu et al. first introduced PDLC-like structures into the hybrid film system that could bend toward UV light source, as shown in Fig. 17 (Yu et al. 2011c). In this system, LCP of PM6ABOC2 was used as the photoresponsive LC phase in the morphology of rough sphere, and poly(vinyl alcohol) (PVA) was used as the polymer matrix. The mesogenic alignment is achieved by mechanical stretching at room temperature. During the fabrication of the hybrid film, the bimetal-like structure was automatically formed because of the different density of LCP and the PVA, which explained the photomechanical behavior of the hybrid film.
Fig. 17

Photomechanical response of polymer-dispersed LCP systems. Here, PVA acted as the continuous polymer matrix, and LCP microparticles were dispersed in PVA films. Only the stretched composite films showed photomechanical response. (Reproduced with permission from Yu et al. (2011c), copyright, John Wiley and Sons)

For practical applications, the photoinduced deformation of LCPs should be accurately designed and controlled. As the photomechanical behavior of LCPs is closely related to the orientation direction of mesogens, multiple orientation directions are most urged, which can be induced by the self-assembled nanostructures. Yu and coworkers fabricated crosslinked LCP nanocomposites with excellent photomechanical behavior by utilization of aligned carbon nanotubes (CNTs) as a template to induce the alignment of LCs (Wang et al. 2012; Sun et al. 2012). The LCP nanocomposites showed distinctive photomechanical behavior when they are fabricated with different preparation processes. A rapid and reversible photoinduced deformation was achieved by alternate irradiation of UV and visible light, and the LCP nanocomposite film curled along the aligned direction of CNTs indicating mesogens oriented parallel to the align direction of CNTs. This phenomenon may be explained by that the structure of CNT sheet is just the same as the surface of rubbed polyimide film with many parallel grooves.

Nevertheless, these LCP composite films could only respond to UV light as the azobenzene group works as the LC part and such UV light often do harm to our health. What’s more, the polymer matrix moves slowly under the temperature of Tg. As a result, graphene oxide (GO) is introduced into the hybrid film, which functioned as the light absorbent and heat source for the film. Due to photothermal effect, GO could absorb visible light and transform the energy into heat. Thus, one low-molecular-weight nematic LC (5CB) was heated up to its isotropic phase, leading to the response to visible light. The photoresponsive behavior is shown in Fig. 18 (Yu et al. 2014; Yu and Yu 2015).
Fig. 18

Visible light-responsive behavior of the stretched PDLC/GO nanocomposites and the mechanism of the photothermal effect. (Reproduced with permission from Yu et al. (2014), copyright, Royal Society of Chemistry)

Except for GO, several other nanomaterials with photothermal effect has been used in light-responsive systems, including CNTs (Xie et al. 2012), graphene oxide, graphene, and gold nanoparticles (Chen et al. 2013). CNTs has been chosen in photo-deformable film as its strong absorption for visible and NIR light (Ahir and Terentjev 2005, Yang et al. 2008 and Ji et al. 2010). However, good dispersion and excellent alignment are required for CNTs, which is hard to realize and thus restricting the application of CNTs in photo-deformable materials.

Recently, upconversion nanomaterials have been used as nanocomposites to induce deformation of LCPs upon NIR light (Wu et al. 2011). In general, upconversion nanomaterials are “frequency modulation” particles that can absorb a characteristic light with the wavelength of λ1 and then emit a typical light with the wavelength of λ2 (as shown in Fig. 19). If the emission wavelength of upconversion nanomaterials λ2 is matched with that of the light inducing the trans-to-cis isomerization of azobenzene chromophores, the reversible photomechanical behavior of LCPs can be caused by light with the wavelength of λ1. The abovementioned process is how upconversion nanomaterials regulate the wavelength of stimulating light source, which has been applied in other LC systems (Wang et al. 2014).
Fig. 19

Schematic illustration of the frequency modulating function of upconversion nanomaterials and the mechanism of the photomechanical behavior of LCPs induced by upconversion nanomaterials. (Reproduced with permission from Wu et al. (2011), copyright, American Chemical Society)

Supramolecular Liquid Crystalline Polymers (SLCPs)

Self-assembly of supramolecules is a powerful way to prepare functional materials by the association of different properties of various molecules (Ikkala and Brinke 2002). Supramolecules are defined as the combination of small molecules via noncovalent interaction, which includes hydrogen-bonding, halogen-bonding, π-π interaction, hydrophobic interaction, donor-acceptor interaction, metal-ion coordination, and so on. Compared with traditional polymers, supramolecules occupy dynamic and reversible characteristic, which render them popular on stimuli-responsive materials (Xu et al. 2005). The researches on supramolecules have covered all kinds of fields in material science, chemistry, and biology, since Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen proposed and developed the concept of supramolecular chemistry (Lehn 1990). Recently, researches on supramolecular liquid crystalline polymers (SLCPs) have become a hot spot in supramolecular chemistry. Combining the photoresponsive properties of azobenzene and the self-assembled capability of pyridine, azopyridine has been widely used in photoresponsive supramolecules (Cui and Zhao 2004). Herein, we would mainly introduce the SLCPs formed by hydrogen-bonding and halogen-bonding.

So far, hydrogen-bonding is one of the most common noncovalent interactions in recognition processes (Priimagi et al. 2012a, b). Hydrogen bonds are everywhere in our life, from the microscale DNA and RNA to the water we drink daily. The first hydrogen-bonded supramolecular LC system was introduced by using the carboxylic acid and pyridyl group as the hydrogen-bond acceptor and donor (Kato and Frechet 1989). Following these examples, Zhao and his coworkers synthesized azopyridine side-chain homopolymers as well as BCs as the hydrogen bond acceptors. With several carboxylic acids as the hydrogen-bond donor, the amorphous polymers can be easily converted into SLCPs. In the example of PS-b-PAzPy BCs, three carboxylic acids were added to transform the amorphous AzPy block into the LC block, which could further enhance the photoinduced orientation of the azopyridine moieties (Cui et al. 2005).

The self-assembled morphologies of the hydrogen-bond supramolecules have also become popular, which includes fibers, spherical particles, gels, and so on. Zhou et al. has fabricated self-assembled nanofibers with different morphologies based on amphiphilic low-molecular-weight azopyridine derivatives (Zhou et al. 2011; Zhou and Yu 2012, 2013). The nanofibers are successfully obtained with different kinds of acids and salts, such as aniline hydrochloride, alkylbenzenesulfate-based anionic surfactants. These fibers are expected to have applications in conductive nanomaterials and biomaterials.

Yu et al. successfully get the self-assembled microparticles using SLCPs fabricated with one amorphous azopyridyl homopolymer and dicarboxylic acids (Liu et al. 2011; Yu et al. 2011d and Zhang et al. 2014). The SLCP microparticles show different morphologies with different alkyl chain lengths of the acids, as shown in Fig. 20. The pure homopolymer assembled into microparticles with smooth surface, and the supramolecules with long alkyl chain dicarboxylic acids show wrinkled surfaces. Besides, the SLCP microparticles show photoresponsive characteristics as the results of the photoactive azobenzene groups.
Fig. 20

Microparticles fabricated with SLCPs showed controlled morphologies and photoresponsive behaviors. (Reproduced with permission from Liu et al. (2011), copyright, American Chemical Society)

Chen et al. prepared multi-responsive reversible organogels with a carboxylic azo polymer (PM6AzCOOH) and DMSO by the interaction of H-aggregation and hydrogen-bonding (Chen et al. 2010 and Ren et al. 2015). The formed gel could be destroyed by heating, the addition of less polar solvent and the irradiation of UV light. Figure 21 demonstrates the reversible sol-gel process.
Fig. 21

Multi-responsive hydrogen-bonded organogel based on a carboxylic azo homopolymer. (Adapted with permission from Chen et al. (2010), copyright, Royal Society of Chemistry)

If the reverse process was performed, the gel could reform completely. In addition to the self-assembled morphology, the application of hydrogen bonds has extended to the field of holographic grating. As we all know, the azobenzene group is very important in the process of mass transport upon irradiation of interference pattern from two coherent later beams. However, as azobenzene group strongly absorb light, it is not preferred when using the grating. Thus, Seki proposed a method to remove the azobenzene group based on the reversibility of hydrogen bond after the grating formation (Zettsu et al. 2008).

As we have mentioned above, hydrogen-bonding could be used to introduce LC property from amorphous molecules. Parallel to hydrogen-bonding, halogen-bonding, which is the noncovalent interaction between halogen atoms and neutral or anionic Lewis bases, could achieve the same purpose (Metrangolo et al. 2005). Bruce first introduced halogen-bonded LC system. This system was composed of alkoxystilbazole and the diverse substituted phenols, and the interaction occurs between N atoms on pyridine and I atoms on the substituted phenols. The molecular structure shows the N-I distance of 2.811 Å, which is shorter than the common distance of halogen-bonding (2.762–3.127 Å). Thermal analysis and measurements of polarizing optical microscope were conducted to make sure the LC property of the mixed compound (Nguyen et al. 2004).

Later, a systematic study on almost 100 kinds of halogen-bonded LC supramolecules was performed based on Bruce’s work. By halogen-bonding, new chiral mesogens were also obtained from non-mesomorphic chiral compounds. The possible structure-function relationship was revealed in specific halogen-bonding system (Bruce et al. 2010). Then, researches on halogen-bonded LC supramolecules were preceded to the system of alkoxystilbazoles and molecular iodine (I2) (McAllister et al. 2013). Furthermore, Chen et al. first introduced the halogen-bonded LCs using azopyridine compound and molecular bromide as well as the reversible photoinduced phase transition of iodine-bonded LCs upon UV irradiation (Chen et al. 2014), as shown in Fig. 22.
Fig. 22

Fabrication of halogen-bonded LCs and their reversible photoinduced phase transition behaviors

However, there are still distinctions between the two interactions. Firstly, halogen bonds are more directional than hydrogen bonds. Secondly, the strength of halogen-bonding can be tuned by changing halogen atoms. Then, halogen-bonding is usually hydrophobic while hydrogen-bonding is hydrophilic. Lastly, the halogen atoms are much bigger than hydrogen atoms (Priimagi et al. 2013). All these characteristics render halogen-bonding a unique tool for the design of functional materials, such as halogen-bonding-triggered supramolecular gels (Meazza et al. 2012), anion transport (Jentzsch et al. 2012), and so on.

One example of photoresponsive supramolecules is surface-relief gratings based on hydrogen- and halogen-bonding. Due to the directionality and tunable properties of halogen-bonding, halogen-bonded supramolecules have shown higher diffraction efficiency that hydrogen-bonded ones in the formation of surface-relief gratings. Priimagi et al. compared the diffraction efficiency of a series of molecules that differ only in bond donor units and find that the iodoethynyl-containing azobenzene becomes the best one to produce high efficiency of producing surface patterning (Priimagi et al. 2012a; Saccone et al. 2015). Besides, photoalignment has been combined with the surface-relief grating formation in halogen-bonded supramolecular systems, which shows the remarkable potential of halogen-bonding in photoresponsive functional material (Priimagi et al. 2012b).

Conclusion and Outlooks

In summary, photoresponsive LCP systems possess both the photoactive moieties and the LC ordering, which could be easily controlled through adjusting the actinic light. Researchers mainly focus on the relationship between light and ordered structure of LCPs, especially the precise control of the structures. Through molecular cooperative motions, the photo-inert molecules are aligned following the motion of photoactive molecules. Furthermore, this photochemical transition could be magnified by the supramolecular cooperative motions into a nanoscale motion of the LCBCs. 2D and 3D photomechanical motions of LC elastomers and LCP composites are induced by light irradiation, which directly transfers light energy into mechanical energy. In SLCP systems, microparticles and nanofibers are fabricated by noncovalent interactions; besides, as an infant noncovalent interaction, halogen-bonded LCs may have quite huge space for research. All these properties of photoresponsive LCPs indicate their possible applications in energy conversion, optical devices, nanotechnology and biomaterials.

Cross-References

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Materials Science and Engineering, College of Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of EducationPeking UniversityBeijingChina

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