Vertical Orientation of Nanocylinders in Liquid-Crystalline Block

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Vertical Orientation of Nanocylinders in Liquid-Crystalline Block Copolymers Directed by Light Tianjie Wang, Xiao Li, Zhijiao Dong, Shuai Huang, and Haifeng Yu* Department of Materials Science and Engineering, College of Engineering and Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, P. R. China S Supporting Information *

ABSTRACT: 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 lightdirected 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. KEYWORDS: light-directed nanostructures, block copolymer, microphase separation, photoresponsive polymer, azobenzene, photoisomerization, liquid crystalline polymer, photoinduced phase transition

1. INTRODUCTION In the past few decades, microphase separation (MPS) of block copolymers has been intensively studied because it offers various self-assembled nanostructures for potential applications in multidisciplinary areas.1−7 The unique properties of block copolymers basically originate from their capability as regularly patterned MPS with different morphologies. Lots of methods have been explored to control MPS nanostructures, including thermal annealing, solvent treatment, templated self-assembly, volume fraction, and so forth.1−5,8−13 Recently, liquidcrystalline (LC) ordering has been elegantly incorporated into block copolymers,8−19 providing the designed LC block copolymers (LCBCs) with supramolecular cooperative motion (SMCM).17−19 This affords one effective approach to manipulate MPS nanostructures by way of mesogenic alignment, such as electric or magnetic fields, shearing or rubbing,18 molecular command,11 and light.18−30 Among them, the lightdirected method is fascinating because light energy can be remotely, instantly, and precisely controlled in one noncontact way.8−13,17−23 Generally, azobenzene is one of the most favorite chromophores for design of light-responsive materials.8−32 When it acts as one mesogen, interesting properties such as photoalignment, phototriggered molecular cooperative motion, photoinduced phase transition, and the corresponding volume change can be brought about.8−23 These light-activated processes have greatly influenced the MPS nanostructures of LCBCs. As shown in Scheme 1b, Seki and Nagano et al. © XXXX American Chemical Society

investigated in-plane regulation of MPS nanostructures by realignment of azobenzene mesogens upon irradiation of linearly polarized light (LPL) based on the well-known Weigert effect.20−22 Another photoinduced process is photoalignment of azobenzene LCs from multidomain to monodomain orientation, as given in Scheme 1a. By utilizing this method, Zhao et al. has explored MPS orientation of azobenzenecontaining LCBCs comprising pi-conjugated crystalline P3HT or photoinert LC polymers by photoalignment or photoinduced molecular cooperative motion.27,31 Very recently, controlled MPS morphologies upon UV irradiation to change the composition of block copolymers were observed in hydrogen-bonded dip-coating film or two-dimensional LB film.33,34 Iyoda and Ikeda groups also studied macroscopically in-plane alignment of MPS by combination of photoalignment with the time-consuming way of thermal annealing (Scheme 1a).18,19 However, a rapid, reversible, and precise tuning method of MPS nanostructures at room temperature that adapting to popularize in various fields is still remaining challengeable. Although the photoinduced phase transition of azobenzene LCs are very fast in most cases,23 it has not yet been explored in photoresponsive LCBCs following the ways of Scheme 1c, d. In this paper, we report one light-directed method of fast control of poly(ethylene oxide) (PEO) Received: May 1, 2017 Accepted: July 3, 2017 Published: July 3, 2017 A

DOI: 10.1021/acsami.7b06086 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

concentration of the solution (2−5 wt %) and the spin-coating speed (1000 rpm for 10 s and 3000 rpm for 30 s). The LCBC film was then annealed at a certain temperature (e.g., 95 or 145 °C) in a vacuum oven for 24 h. Characterization. 1H NMR spectra were measured by Bruker AV500M. The molecular weights of polymers were determined by gel permeation chromatography (GPC, Waters) with standard polystyrenes in THF as eluent. Thermodynamic properties of the monomers and polymers were analyzed by a differential scanning calorimeter (DSC, PerkinElmer DSC 8000) under a N2 atmosphere at a heating and cooling rate on demand. SAXS experiments were carried out on a Bruker Nanostar SAXS instrument, and the d-spacing (d) was given by 2π/q. The UV−vis absorption spectra were recorded with a PerkinElmer Lambda 750. To characterize the LC phase properties of the LCBCs, a polarizing optical microscope (POM, Zeiss Scoper A1) equipped with a hot stage and in situ UV lamp was used. The microscopic topography of LCBCs films was evaluated with an atomic force microscope (AFM, Bruker Multimode 8 in a tapping mode). Light-Directed Regulation of Nanostructures in Thin Films. One unpolarized beam of LED UV light at 360 nm (Height-LED, HTLD-4 II) was used to carry out the light direction of nanostructures in LCBC film. The intensity of the light was adjusted by the distance from the sample film (10−100 mW/cm2). Upon irradiation for a certain time, the optical birefringence change was observed under POM at room temperature, and the light-directed nanostructures in LCBC films were measured (nearly in situ) with AFM, as shown in Scheme 2.

Scheme 1. Photocontrolled Processes of LCs Used for Manipulation of Nanostructures in LCBCsa

a

Process (a) is photoalignment of mesogens from multi-domain to mono-domain orientation upon irradiation of linearly polarized light (LPL). Process (b) is photoinduced re-orientation of mono-domain LC to another ordered alignment state perpendicular to the LPL polarization direction. Process (c) is photoinduced phase transitions from multi-domain LC to isotropic phase. Process (d) is photoinduced phase transitions from mono-domain LC to isotropic phase.

Scheme 2. Setup for AFM Measurement of Nanostructures in LCBC Thin Films upon UV Irradiation

nanocylinders in thin films of a series of amphiphilic LCBCs consisting of hydrophilic PEO and hydrophobic azobenzenecontaining poly(methyl alkyl acrylate). Regularly patterned nanocylinders were quickly light-directed from random or inplane orientation to out-of-plane arrangement at room temperature on a time scale of seconds upon photoirradiation.

2. MATERIALS AND METHODS Materials. Figure 1 shows the synthetic route of a series of amphiphilic LCBCs used in this paper, which was obtained with a

3. RESULTS AND DISCUSSION MPS Nanostructures. In the heating thermogram of DSC curve (Figure 2a), the precooled LCBC sample (m = 114, n = 83) showed three peaks at 41.9, 73.3, and 122.3 °C on heating, corresponding to PEO melting point−smectic X − smectic A − isotropic phase,15,16 respectively. Because of the supercooling effect and the MPS confinement in nano space, the PEO crystallized at −23.7 °C on cooling.15−19 Upon being annealed at 145 °C (in isotropic phase of the LCBC) for 24 h in vacuum, the LCBC thin film showed unambiguous MPS morphologies in its AFM phase images (Figure 3a&b), consisting of PEO nanocylinders with about 10 nm diameter dispersed in the continuous phase of azobenzenecontaining polymeric matrix with a periodicity of about 24 nm. A hexagonal packing was clearly observed in the inset image of fast Fourier transformation (FFT) in Figure 3a, which is similar to other kinds of LCBCs with similar stuctures.8−12 Both outof-plane and in-plane arrangement of the PEO nanocylinders

Figure 1. Synthetic route of the azobenzene-containing LCBCs (PEOm-b-PM11Azn). The table below the scheme summarizes the calculated repeated units and the molecular-weight distribution of the obtained LCBCs. previously reported method.19 All the block polymers show welldefined structures and low distribution of molecular weight, which were denoted as PEOm-b-PM11AzC4n (m = 114, n = 26, 37, 67, 83, 140). The detailed synthetic step and structural characterization are given in the Supporting Information. Sample Preparation. The thin films of LCBCs were prepared by spin coating the solutions of LCBCs in toluene on substrates (silicon wafer, quartz or glass slide). The film thickness was controlled by the B


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Figure 3. Supramolecular cooperative motion in the present LCBCs. Upon microphase separation, the azobenzene mesogenic block forms the continuous phase, and the separated phase of PEO nanocylinders are dispersed in the polymer matrix. (a, b) are AFM phase images of the LCBC thin film (1 μm × 1 μm) with top right pictures of FFT images. (c, d) are schematic illustration of MPS nanostructures with PEO nanocylinders in out-of-plane or in-plane arrangement upon thermal annealing (m = 114, n = 37). The orientation of mesogens coincides with the nanocylinders.

Figure 2. Thermal properties of PEOm-b-PM11AzC4n (m = 114, n = 83). (a) Second scan of DSC cure with heating and cooling rate of 10 °C/min. (b) Thermal properties of the LCBC samples with different thermal history. (1) As-prepared sample without annealing at a heating rate of 10 °C/min; (2) sample was annealed at 145 °C and then cooled down to room temperature, the DSC cure was obtained at a heating rate of 10 °C/min; (3) sample was annealed at 145 °C and then cooled down to −40 °C at a cooling rate of 0.5 °C/min, then the DSC curve was recorded at a heating rate of 10 °C/min and the crystallized PEO melted at 41.8 °C.

were achieved by control over the film thickness or surfacetreated substrates.9−12,16−18 In the annealing process of the LCBC thin film, the additional LC elastic deformation undoubtedly interfered with the MPS process because of the immiscible feature of the amphiphilic two blocks. This has been reported as SMCM, which enables the thin film of LCBCs to show hierarchical orientation with molecular and supramolecular ordering upon self-organization.12,13,16−19 Both the MPS nanostructures and the mesogenic orientation can be depicted in Figure 3c, d. Photoresponsive Behaviors. The photoresponsive behaviors of the LCBC film was measured by UV−vis absorption spectroscopy and POM observation. As shown in Figure 4a, two characteristic peaks appear in UV−vis absorption spectrum of the as-cast LCBC film. One stronger peak at 337 nm is owing to the π−π* transition of azobenzene, the other peak at about 460 nm is attributed to the n- π* transition of azobenzene. Being annealed at the LC temperature (95 °C), the maximum absorption peak at 337 nm decreased greatly because of the reorientation of the mesogens. When the film was annealed at the isotropic phase (145 °C), more striking changes were brought about. First, the maximum absorption decreased greatly and the corresponding peak became broad.

Figure 4. Photoinduced phase transition of one LCBC (PEO114-bPM11AzC483) upon UV irradiation. (a) Change in UV−vis absorption spectra upon annealing at different temperatures. (b) Photoresponsive feature of the annealed LCBC film. (c) Typical smectic LC texture of annealed LCBC film observed with POM at room temperature. (d) Birefringence disappeared after UV irradiation, which cannot be restored at room light. (e) Possible scheme of photoinduced phase transition of the LCBC.

Second, the maximum absorption peak bathochromically shifted to 345 nm because of the formation of chromophore C


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Figure 5. Light-direction regulation of MPS nanostructures of annealed film of LCBC (m = 114, n = 83) with a thickness of 120 nm. (a−e) AFM images upon UV irradiation with an intensity of 82 mW/cm2 for 0, 2, 5, 10, and 30 s, sequentially. (f) AFM image of sample e after undergoing thermal annealing at 145 °C. (g) Possible scheme of the photoinduced change in the LCBC film.

Figure 6. Fabrication and characterization of the photoregulated LCBC film for observation of the bottom side. (a) Fabrication of the photoregulated LCBC film with PSSNa as sacrificial polymer layer and transference of the film to the TEM cupper grid. (b, c) are TEM images of the back side the photoregulated LCBC film with different magnifications. The FFT picture in the below left of panel c is obtained from the local area of the TEM image.

aggregation after this thermal treatment.19 Moreover, one typical smectic LC texture was obtained under POM observation for such annealed film, as shown in Figure 4c. Even though the annealed LCBC film possess mesogenic aggregation, it still showed quick light-response upon irradiation of UV light at 360 nm at room temperature. As shown in Figure 4b, the stronger peak at 345 nm quickly decreased, and the other peak at about 460 nm increased simultaneously, indicating the occurrence of photoisomerization of azobenzenes. The photostationary state was obtained within 5 s, and a longer irradiation did not cause obvious change in the absorption spectra (Figure 4b). Because of the rodlike structure, trans-azobenzene generally stabilizes LC phase, whereas cis-azobenzene often destabilizes LC ordering (Figure 4e) because of its bend configuration.25,26 As a result, the photoisomerization of azobenzenes led to the quick disappearance of LC texture under POM observation (Figure 4c, d and Movie S1), indicating that the photochemical phase transition from LC to isotropic phase was caused at room temperature. Light-Directed Regulation of MPS. Just as expected, the photoinduced phase transition brought about significant effect on the MPS nanostructures in the LCBC thin film, as shown in Figure 5. Upon thermal annealing, in-plane arrangement of nanocylinders in the LCBC thin film was first observed in Figure 5a.12,17−19 Then the light-directed transformation of MPS nanostructures was investigated with AFM upon exposure to UV light using a setup shown in Scheme 2. After 2 s of irradiation, obvious changes were observed in Figure 5b. A small part of in-plane nanocylinders began to fracture into dotlike nanostructures. With increasing irradiation time, the number of dotlike nanostructures gradually increased, and

Figure 7. Photoinduced change in MPS nanostructure of the LCBC (PEO114-b-PM11AzC483) film with a thickness of 200 nm upon UV irradiation with different light intensity. One AFM phase picture was completely scanned within 90 s. The right inset picture is the corresponding AFM height image upon photoirradiation of UV light with a pulse of one second. (a) UV light intensity was 86 mW/cm2, and exposure time was 5 s. (b) UV light intensity was 26 mW/cm2, and exposure time was 12 s.

almost all the in-plane nanocylinders broke into dotlike nanostructures within 5 s (Figure 5c). Under this circumstance, the birefringent texture was nearly invisible under POM observation (Figure 4c). But all the dotlike nanostructures were still in line with their previous position of in-plane arranged nanocylinders. The nanodots became progressively more ordered with continuing photoirradiation. After exposure to UV light for 10 s, almost all the in-plane nanocylinders were changed into the dotlike nanostructures trending to a hexagonal packing (Figure 5d). The light-directed regulation of nanocylinder orientation from in-plane to out-of-plane was almost D


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Figure 8. Photocontrol of MPS nanostructure in LCBCs with different molecular weights (PEOm-b-PM11AzC4n, m = 114).

LCBC sample of at 145 °C, the initial state was obtained as shown in Figure 5f. The recovered MPS nanostructures showed the same periodicity and regularity as that of the original sample, indicating the good reversibility of this light-directed manipulation process, which can be schematically illustrated in Figure 5g. Interestingly, when the same photoirradiated film was annealing at 95 °C (lower than the clearing point of the mesogenic block), the light-directed nanocylinder arrays in out-of-plane regulation remained unchangeable, showing a “memory” effect.35 Light-Directed Nanocylinder Array. All of the AFM images of LCBC film upon the light-directed regulation showed dot-like nanostructures. This is different from the thermally induced transition of MPS nanostructures from nanocylinders to nanospheres reported by Komura et al.36 They reported that the this nanostructural change occurred only in the isotropic phase of the LCBC and the formed nanospheres were BCC patterned. In the present study, the light-directed nanostructures on surface displayed a hexagonal pattern because the

accomplished in 30 s (Figure 5e), and no detectable changes were observed with extending irradiation time. Because of interference between the LC elastic deformation upon self-organization and the MPS process of the block copolymer, the mesogenic alignment was parallel to the PEO nanocylinders following the principle of SMCM (Figure 3c, d), which has been reported by many groups.12,13,17−19 Generally, azobenzene molecules with their transition moments parallel to the polarization direction of the actinic light often show much more active than those perpendicular to the polarization direction of light.12,13 As a result, the MPS nanostructures with in-plane arrangement should be more easily light-activated than those with out-of-plane arrangement when the incident light is vertical to the film.20 This was completely consistent with the results shown in Figure S3, in which the light-directed regulation of nanocylinders in-plane arrangement exhibited much more striking effect those in the vertical orientation. The reversible process was acquired by annealing the lightdirected LCBC film. Upon annealing the photoirradiated E


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Figure 9. Photocontrol of MPS nanostructure in LCBC (PEO114-b-PM11AzC483) with different film thicknesses: (a) 40, (b) 80, (c) 120, and (d) 160 nm.

of the nanocylinders was achieved in 3D and the obtained lightdirected LCBCs can be used as nanotemplates. Role of PEO in the Light Direction. To light-manipulate the orientation of the nanostructures in the LCBCs, it is necessary for both of the two phases to be easily changeable under the illumination conditions. The continuous phase of the LCBCs is smectic LC phase, which can be fast photoinduced into isotropic phase even at room temperature. Here, the azobenzene moiety acts as both a photoresponsive group and one mesogen, which enables this photoinduced phase transition often to occur even in picoseconds.23,24 As shown in Figure 4d and Movie S1, the present LCBC immediately exhibited isotropic phase once the UV light was irradiated. Recently, azobenzene-containing LC polymers with a similar structure to the present LCBCs was light-triggered directly from solid into liquid phase with acquired fluidity and self-healing properties.38 However, the manipulation of MPS nanostructures generally needs much longer time because of the relaxation of polymeric system.8−19

light-directed regulation only change the orientation of MPS nanostructures. It must be mentioned here that the light-directed regulation of nanocylinders in LCBCs not only occurred in two-dimensional (2D) plane, but also obtained in a three-dimensional way (3D). However, the structures extending through the depth of the film is still unclear only from the AFM measurement. To obtain the information on the sample film of the bottom side, one watersoluble polymer, sodium polystyrenesulfonate (PSSNa), was explored as sacrificial layer, as shown in Figure 6a. The dotlike nanostructures were clearly observed in the TEM image (Figure 6b, c), indicating the photoregulation really occurred in 3D. This was also confirmed by the cross-sectional AFM images before and after the light-directed regulation of nanostructures, as shown in Figure S4.37 Besides, the MPS nanostructures of LCBC films with and without light-regulation were used as nanotemplates to fabricate CaCO3 nanoparticle, and different morphologies were observed in Figure S5, further indicating that the light-directed manipulation F


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ACS Applied Materials & Interfaces Furthermore, it is more difficult to transform the photoinduced changes from molecular to supramolecular level because most of polymers inherently have a glass-transition temperature higher than room temperature. Thanks to the unique features of the present LCBCs, the separated phase of PEO self-assembled into nanocylinders and the MPS process impeded the crystallization process by confining them in nano space, leading to amorphous phase at room temperature. As shown in Figure 2b, the PEO block crystallized at −23.7 °C on cooling from the annealing temperature of 145 °C. This makes it thermodynamically possible to transfer the photoinduced change from the continuous phase to the low-viscously separated phase. In contrast, when the LCBC films were respectively cooled down to −40 °C and −70 °C, the PEO block partly crystallized and the photoirradiation only caused little effect on the MPS nanostructures (Figures S6 and S7). Effect of UV Light Intensity. Because the light-directed control of nanostructures in the LCBCs was realized by the photoinduced phase transition of the azobenzene mesogenic block, it should be greatly dependent on the intensity of the actinic light. As shown in Figure 7b, the LCBC film was observed with AFM in tapping mode upon UV irradiation with 1 s interval. Obviously, this photodriven process was greatly accelerated by increasing the light intensity. Using one beam of UV light of 26 mW/cm2, the light-regulation process of nanocylinders was completed within 12 s. When the intensity was increased to 86 mW/cm2, only 5 s of irradiation was enough to obtain vertically patterned nanocylinders. However, photothermal effect was observed when the light intensity was higher than 100 mW/cm2.39 Effect of Molecular Weight on Light Direction. Similarly, the molecular weight of the LCBCs also played an important role in the light-directed way. The higher of molecular weight, the higher viscosity of the LCBCs, which should lead to retardation of the photoinduced phase transition. This undoubtedly reduced the procedure of photoregulation of the nanostructures (Figure 8). Effect of Film Thickness on Light Direction. It is wellknown that azobenzene materials often show large molar extinction efficiency (about 1 × 104 cm2 L−1), and light can only penetrate less than one-micron thickness from the incident surface.23,24 As a result, the film thickness of the LCBCs should have significance effect on the light-activated approach. Obviously, the photoinduced effect started from the film surface (front side) and gradually diffused deep into the film (back side). As shown in Figure 9, quick changes in the PEO nanocylinders was observed for a thinner film, and 2 s of photoirradiation was enough for the LCBC film with a thickness less than 80 nm. In addition, the effect of relative humidity was also studied considering the hydrophilicity of the PEO block in the present LCBCs. As shown in Figure S8, the light-directed regulation was accelerated to some extent when the LCBC film was put in a highly humid surrounding, which is very similar to the block copolymer self-assembly treated by solvent annealing.40 Application for Photopattern and Nanotemplates. This present light-directed regulation method possesses fast photocontrolled feature, enabling us to conveniently pattern the MPS nanostructures of LCBC films. Being annealed at 145 °C in vacuum, the LCBC film with MPS nanostructures of nanocylinders in-plane arrangement were first obtained. Upon photoirradiation of UV light through a photomask for only 10 s, the PEO nanocylinders were quickly regulated into out-of-plane orientation in the photoirradiated area, whereas homogeneously in-plane oriented states remained unchangeable

Figure 10. Pattern of light-directed manipulation of MPS nanocylinders through a photomask. Before photo patterning, the film was first undergone annealing treatment to induce in-plane orientated nanocylinders. (a) UV-irradiated part; (b) nonirradiated part; (c, d) photopatterned edge with different magnifications.

in the nonirradiated part. As shown in Figure 10, the nanocylinders were clearly light-directed in different area of the photomask, interestingly forming the boundary region in the patterned edge area. This photopattern is also applicable for the as-cast LCBC film without thermal annealing treatment (Figure S9).

4. CONCLUSION In summary, light-directed regulation of MPS nanostructures in LCBC thin film was successfully obtained in a simple and convenient way. By the photoinduced phase transition of the continuous LC phase, in-plane orientation of the phaseseparated nanocylinders was quickly manipulated into out-ofplane. This light-directed regulation of nanocylinders can be completed on a time scale of seconds at room temperature by optimizing the factors like molecular weight of polymer, film thickness, light intensity, and relative humidity. Benefitting from this light direction of fast photocontrol, optical patterning of PEO nanocylinders was successfully achieved. Because diverse applications in nanotechnology require precise and rapid control of functional nanostructures at room temperature, the present method contributes to furthering applications in fast control of nanostructures for nanofabrication and nanoengineering.41 Recently, various metal nanostructures have been fabricated by using these photocontrolled nanostructures in the present LCBCs as nanotemplates (Figure S10).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06086. Experimental details, characterization data, moisture effect on light direction, and applications of this lightdirected regulation (PDF) Movie S1, photoinduced phase transition (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G


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(16) Asaoka, S.; Uekusa, T.; Tokimori, H.; Komura, M.; Iyoda, T.; Yamada, T.; Yoshida, H. Normally Oriented Cylindrical Nanostructures in Amphiphilic PEO-LC Diblock Copolymers Films. Macromolecules 2011, 44 (19), 7645−7658. (17) Yu, H.; Iyoda, T.; Ikeda, T. Photoinduced Alignment of Nanocylinders by Supramolecular Cooperative Motions. J. Am. Chem. Soc. 2006, 128 (34), 11010−11011. (18) Yu, H.; Li, J.; Ikeda, T.; Iyoda, T. Macroscopic Parallel Nanocylinder Array Fabrication Using a Simple Rubbing Technique. Adv. Mater. 2006, 18 (17), 2213−2215. (19) Yu, H.; Kobayashi, T.; Hu, G. Macroscopic Control of Microphase Separation in an Amphiphilic Nematic Liquid-Crystalline Diblock Copolymer. Polymer 2011, 52 (7), 1554−1561. (20) Nagano, S.; Koizuka, Y.; Murase, T.; Sano, M.; Shinohara, Y.; Amemiya, Y.; Seki, T. Synergy Effect on Morphology Switching: RealTime Observation of Photo-Orientation of Microphase Separation in a Block Copolymer. Angew. Chem., Int. Ed. 2012, 51 (24), 5884−5888. (21) Sano, M.; Hara, M.; Nagano, S.; Shinohara, Y.; Amemiya, Y.; Seki, T. New Aspects for the Hierarchical Cooperative Motions in Photoalignment Process of Liquid Crystalline Block Copolymer Films. Macromolecules 2015, 48 (7), 2217−2223. (22) Sano, M.; Shan, F.; Hara, M.; Nagano, S.; Shinohara, Y.; Amemiya, Y.; Seki, T. Dynamic Photoinduced Realignment Processes in Photoresponsive Block Copolymer Films: Effects of the Chain Length and Block Copolymer Architecture. Soft Matter 2015, 11 (29), 5918−5925. (23) Yu, H.; Ikeda, T. Photocontrollable Liquid-Crystalline Actuators. Adv. Mater. 2011, 23 (19), 2149−2180. (24) Ikeda, T. Photomodulation of Liquid Crystal Orientations for Photonic Applications. J. Mater. Chem. 2003, 13 (9), 2037−2057. (25) Yu, H.; Kobayashi, T.; Yang, H. Liquid-Crystalline Ordering Helps Block Copolymer Self-Assembly. Adv. Mater. 2011, 23, 3337− 3344. (26) Barrett, C. J.; Yager, K. G.; Mamiya, J.; Ikeda, T. PhotoMechanical Effects in Azobenzene-Containing Soft Materials. Soft Matter 2007, 3 (10), 1249−1261. (27) Han, D.; Tong, X.; Zhao, Y.; Zhao, Y. Block Copolymers Comprising P-conjugated and Liquid Crystalline Subunits: Induction of Macroscopic Nanodomain Orientation. Angew. Chem., Int. Ed. 2010, 49 (48), 9162−9165. (28) Li, Q., Ed. Intelligent Stimuli Responsive Materials: From WellDefined Nanostructures to Applications; John Wiley & Sons: Hoboken, NJ, 2013. (29) Zheng, Z.; Li, Y.; Bisoyi, H.; Wang, L.; Bunning, T.; Li, Q. Three-Dimensional Control of the Helical Axis of a Chiral Nematic Liquid Crystal by Light. Nature 2016, 531 (7594), 352−356. (30) Bisoyi, H.; Li, Q. Light-Directed Dynamic Chirality Inversion in Functional Self-Organized Helical Superstructures. Angew. Chem., Int. Ed. 2016, 55 (9), 2994−3010. (31) Zhao, Y.; Tong, X.; Zhao, Y. Photoinduced Microphase Separation in Block Copolymers: Exploring Shape Incompatibility of Mesogenic Side Groups. Macromol. Rapid Commun. 2010, 31 (11), 986−990. (32) Bisoyi, H.; Li, Q. Light-Driven Liquid Crystalline Materials: from Photo-Induced Phase Transitions and Property Modulations to Applications. Chem. Rev. 2016, 116, 15089−15166. (33) Vapaavuori, J.; Grosrenaud, J.; Pellerin, C.; Bazuin, C. G. In Situ Photocontrol of Block Copolymer Morphology During Dip-coating of Thin Films. ACS Macro Lett. 2015, 4 (10), 1158−1162. (34) Kadota, S.; Aoki, K.; Nagano, S.; Seki, T. Photocontrolled Microphase Separation of Block Copolymer in Two Dimensions. J. Am. Chem. Soc. 2005, 127 (23), 8266−8267. (35) Fu, S.; Zhao, Y. Orientation of Azobenzene Mesogens in SideChain Liquid Crystalline Polymers: Interplay between Effects of Mechanical Stretching, Photoisomerization and Thermal Annealing. Macromolecules 2015, 48, 5088−5098. (36) Komura, M.; Komiyama, H.; Nagai, K.; Iyoda, T. Direct Observation of Faceted Grain Growth of Hexagonal Cylinder

Haifeng Yu: 0000-0003-0398-5055 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The National Natural Science Foundation of China (Grants 51322301 and 51573005). Notes

The authors declare no competing financial interest.

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