Photoresponsive Liquid Crystalline Polymers
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.
KeywordsLiquid crystalline polymer Photoresponsive liquid crystal Liquid crystalline block copolymer Liquid crystalline elastomer Supramolecular liquid crystal
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.
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.
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
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.
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 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.
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).
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
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).
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.
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.
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.
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.
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.
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).
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.
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