Random Planar Orientation in Liquid-Crystalline Block Copolymers

Invited Feature Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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

Langmuir Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/09/18. For personal use only.

Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan

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

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INTRODUCTION The molecular orientations of liquid crystalline (LC) materials are strongly affected by free surfaces and interfaces in contact with other materials. Because of the excluded volume effect, rodlike mesogens of both low-molecular-weight LCs and sidechain LC (SCLC) polymers generally tend to adopt a parallel orientation to the substrate interface (random planar orientation) on a solid substrate.1−4 On the other hand, no excluded volume interactions occur at a free surface, and the mesogens have a strong tendency to normally (homeotropically) orient by reducing the excluded volume.1−4 SCLC polymers, which are often used as free-standing films, are strongly influenced by the free surface, resulting in the adoption of a homeotropic orientation.5−9 Therefore, random planar oriented films of SCLC polymers are difficult to produce by LC alignment techniques even using alignment layers and high surface tension substrates. Photofunctional polymers containing Az chromophores have been the most widely studied for photochemical, photomechanical, and photonic applications.7,8,10−15 Azobenzene chromophores undergo trans−cis isomerization by light © XXXX American Chemical Society

irradiation. In LC matrixes, the rodlike shape of the trans-Az form behaves like a typical LC mesogenic structure. On the other hand, the cis-Az form with a bent molecular shape becomes a nonmesogenic structure.10,12,13 The trans−cis photoisomerization molecular shape changes in the Az unit upon light irradiation drive the LC phase transition (i.e., photoinduced LC phase transition).3,3,3 Furthermore, the irradiation of linearly polarized light (LPL) can trigger the angularly selective photoisomerization of Az chromophores and induce an in-plane molecularly reoriented film.7,8,10−15 When polymer films containing Az chromophores were irradiated with LPL light, the Az chromophores oriented parallel to the actinic electric field (E) of the LPL preferentially photoisomerize and reorient in a position orthogonal to E (the Weigert effect).16 By performing this angularly selective photoisomerization in LC media, highly efficient molecular photoreorientation systems can be created.7,8,10−15 In the case of SCLC polymers containing Received: May 31, 2018 Revised: August 9, 2018 Published: August 10, 2018 A

DOI: 10.1021/acs.langmuir.8b01824 Langmuir XXXX, XXX, XXX−XXX

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lower surface free energy covers the film surface; therefore, the phase-separation interface (intermaterial dividing surface, IMDS) at the surface layer forms parallel to the film surface plane.23,24 The arrangements of microphase-separated (MPS) structures in the film thickness direction are determined using the surface-segregated layer as the starting point.23,24 In the case of SCLC diblock copolymers with amorphous coil blocks (SCLC-coil diblock copolymers), the side-chain mesogens orient parallel to the IMDS since the main chain direction is preferentially normal to the IMDS (Figure 1c).25−30 Therefor, at the segregated surface of SCLC diblock copolymers, a planar orientation can be spontaneously induced.7,8,31−35 In this context, this review highlights the following surface-segregated polymer systems of SCLC-coil diblock copolymers.7,8,31−35 The random planar orientation induced in SCLC polymer systems by surface segregation of the coil block (Figure 2a)31−34 and high-density polymer brush structures with a random planar orientation in surface-segregated SCLC block layers (Figure 2b)35 is discussed. Induced Planar Orientation in SCLC Polymer Systems by Surface Segregation of a Coil Block. Block copolymers can form nano-ordered structures (MPS structures) such as spheres, cylinders, and lamellar phases by self-assembly.36−41 Applying MPS structures to nanolithographic patterning and templating materials has been extensively explored by a directed self-assembly technique.36−41 Therefore, MPS structure alignment techniques are very important and have been studied by examining mechanical shear and flow, the application of electric and magnetic fields, and graphoepitaxy in nano-ordered topographical and wetting patterns.36−41 SCLC block copolymers form hierarchical LC structures containing thermotropic orders and molecular directions within lyotropic MPS structures. The thermotropic molecular orientations of the SCLC blocks determine the orientation direction for the entire MPS structure domain. For example, Iyoda and co-workers created vertically aligned cylinder morphologies over a large area with a homeotropic LC orientation of SCLC blocks in amphiphilic Az SCLC block copolymer thin films (Figure 3a).28−30 Utilizing the angularly selective photoisomerization of Az units, in-plane alignment of the MPS cylinder structure in the Az SCLC block copolymer thin films can be performed.42,43 At almost the same time, Yu et al.42 and our group43 reported the in-plane photoalignment of an amphiphilic Az SCLC block copolymer. Moreover, our group has demonstrated in-plane and out-of-plane photoalignment control (3D photoalignment) of an MPS cylinder structure in an Az SCLC block copolymer with a polystyrene (PS) block (PS-b-PAz).44 Since these Az SCLC block copolymers adopt a vertically oriented cylinder form with the homeotropically oriented SCLC block LC matrix by selfassembly (Figure 3a), the in-plane photoalignment process requires monoaxial growth of the LC phase in an energetically unstable direction from the isotropic phase or as-cast state. In the case of SCLC-coil diblock copolymers, the surface segregation of a coil block can induce a random planar orientation due to the IMDS that forms parallel to the surface plane. We introduced a poly(butyl methacrylate) (PBMA) block as a low surface free energy coil block to the Az SCLC diblock copolymer (PBMA-b-PAz, Figure 3b).31 As expected, UV−vis spectroscopy and grazing incidence small-angle X-ray measurements revealed that the PBMA-b-PAz thin film exhibited the random planar orientation of a smectic phase for the PAz block. In a PAz homopolymer film, UV−vis spectroscopy revealed that the absorption band of the π−π* transition significantly

Az mesogens, however, the LC orientation and aggregation structure strongly depend on the side-chain structures and rigidity of the main chains.11,17,18 As noted, SCLC polymers have a tendency to preferentially adopt a homeotropic orientation in free-standing thin films (Figure 1a). The

Figure 1. Homeotropic alignment of Az SCLC polymers (a), a direction for efficient photoreaction in an Az unit (b), and an illustration of the molecular orientation in an MPS interface of SCLCcoil block copolymers (c). Reproduced with permission from ref 8. Copyright 2016, John & Wiley Sons.

homeotropic orientation is not advantageous to the light absorption of Az mesogens in SCLC polymer systems since light irradiation is generally applied normal to the film and the transition moment of Az is oriented perpendicular to the E of the light irradiation for a photoreaction (Figure 1b). The in-plane monoaxial alignment from the homeotropic (normal) orientation requires two-dimensional orientation controls consisting of homeotropic to random planar orientation and in-plane azimuthal alignment control. For an efficient photoreaction and photoalignment, a random planar orientation is advantageous. Therefore, surficial and interfacial designs inducing a random planar orientation are required for the efficient photoreaction and photoreorientation of SCLC Az systems.8 This feature article reviews planar orientations in SCLC block copolymer films by surface segregation. Surface segregation in polymer films is the phenomenon in which one component with low surface tension (or free energy) covers the surface in a mixture of two or more polymers or a block copolymer film.19−24 Polymer chains with lower surface tension and higher entropic structure, such as hydrophobic chemical structures, polymer chain terminals, and highly flexible and mobile structures, preferentially migrate to the film surface.19−24 Surface segregation has been explored in amorphous polymer blended films for interesting polymer physical characteristics and lowcost, simple-operation surface modification methods.19−24 In block copolymer films, the polymer block component with a B


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Figure 2. Surface and interface molecular orientations of photoresponsive SCLC diblock copolymer systems with surface segregation (induced random planar orientation structure).

assembly (Figure 3b). The amorphous block layer segregated at the topmost surface leads to the random planar orientation of the smectic phase in SCLC polymer films.31,32 The PBMA-b-PAz film with the random planar Az mesogens exhibits an efficient angularly selective photoreaction and inplane reorientation photoswitching of the smectic phase and MPS structure by LPL irradiation (active photoalignment, Figure 5a).31,32,45−47 Active photoalignment can photocontrol the in-plane alignment of the MPS cylinder structure a number of times in any azimuthal direction in a few minutes. Interestingly, PBMA-b-PAz provided an understanding of the photoswitching hierarchical mechanisms in the SCLC polymer system with lyotropic MPS cylinders and thermotropic PAz blocks.31,32,45−47 We conducted in situ grazing incidence smallangle X-ray scattering measurements of the photoswitching of the LC phase and MPS cylinders under LPL irradiation using a synchrotron radiation source. When LPL is irradiated from an initial state in which the LC phase and cylinder structure are monoaxially oriented, the hierarchical LC structure is reoriented in the in-plane direction opposite by 90° (Figure 5a, left to right). Then, irradiation with successive LPL with polarization rotated azimuthally by 90° with respect to that of the previous LPL direction results in the orientations of the LC phase and cylinders returning to the initial positions (Figure 5a, right to left). These photoalignment and photorealignment processes were tracked in real time by the X-ray scattering intensity. Figure 5b,c shows the time-course analysis for the scattering intensities corresponding to smectic LC and MPS cylinder phases in the LPL irradiation process.31,32 The scattering intensity due to both the smectic LC and MPS cylinder phases quickly decayed from the initial state within approximately 40 s (Figure 5b). The

Figure 3. Schematic illustrations of the vertically oriented cylinder phase with homeotropic SCLC blocks (a), planar-oriented SCLC blocks and cylinders by surface segregation of coil blocks, and (b) SCLC-coil diblock copolymer thin films. PBMA blocks with low surface free energy selectively segregate at a surface. Figure 3b was reproduced with permission from ref 8. Copyright 2016, John & Wiley Sons.

decreased after annealing at an isotropic temperature (Figure 4a), and scattering corresponding to the smectic lamellar layer (q = 2.2 nm−1, d = 2.9 nm) was observed only in the out-of-plane direction by GI-SAXS measurements (Figure 4c). These results indicated that the Az mesogenic units in the homopolymer film exhibit a homeotropic orientation. On the other hand, the spectral shape of the PBMA-b-PAz film was almost unchanged by the annealing process (Figure 4b) and exhibited smectic scattering (q = 1.9 nm−1, d = 3.3 nm) in the in-plane direction in the GI-SAXS image (Figure 4d), suggesting the random planar orientation of the PAz matrix in the annealed film. These experimental results indicate that the PBMA cylinders are oriented parallel to the substrate plane and surface by selfC


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Figure 4. Induced planar orientation of PBMA-b-Az in a thin film. UV−vis absorption spectra before (dashed line) and after thermal annealing (solid blue line) of the PAz homopolymer film (a) and the PBMA-b-PAz film (b) at the isotropic temperature. 2D GI-SAXS patterns of the PAz homopolymer film (c) and the PBMA-b-PAz film (d) after annealing. The scattering 1D profiles are indicated as white curves in c and d.

orthogonally directed structure of these structures appeared slowly over 300 s (Figure 5c). The scattering intensity profiles reveal that the photoswitching motion of the LC phase is synchronized with that of the MPS cylinders. In fact, the reorientation of the LC domains with the MPS cylinder structure was observed at the photoreorientation transition state by polarized optical microscopy (POM) and transmission electron microscopy (TEM).32 These detailed results demonstrated that the photoreorientation occurs via the rotation of the domains by retaining the LC phase and cylinder structure (Figure 6).31,32,45 The active photoalignment of the LC hierarchical structures in the PBMA-b-PAz film consisted of three stages: (stage I) fluctuations in the smectic phase of the side chains in the initial state and dividing into smaller grains (LC domains) to the submicrometer level before the orientational change, (stage II) actual rotation of the Az LC domains driven by the photoinduced reorientation, and (stage III) fusion and growth of the rotated LC domains in the realigned direction. This induced planar orientation via surface segregation of the PBMA block can be utilized for other polymer SCLC polymer systems (Figure 7a).33,48 As mentioned, the PAz homopolymer film exhibits a strongly homeotropic orientation that is unfavorable for photoreactions (Figure 3a,c). In addition, PSb-PAz also adopts a homeotropic PAz matrix (Figure 7b) and vertically oriented cylinder structure in a thin film.33,44,48 By adding a small amount (5−10% by weight) of PBMA-b-PAz, both PAz/PBMA-b-PAz and PS-b-PAz/PBMA-b-PAz (Figure 7c) mixed films exhibited a random planar orientation of the PAz smectic phase after annealing. The lamellar scattering peaks (q = 1.7 nm−1, d = 3.4 nm) in the mixed films were observed only in the in-plane direction, which clearly demonstrated that the planar orientation of the smectic phase of PAz is induced by

Figure 5. Schematic images for in-plane photoreorientation of MPS cylinders in a PBMA-b-PAz film (a). Time-course analysis of the scattering intensity in the real-time in situ GI-SAXS observation measurement for the photoreorientation of PBMA-b-PAz. (b) Decay processes attributed to the fluctuations of both the LC (open circle) and MPS cylinders (full circle) with the irradiation of LPL. (c) Enhancement processes corresponding to the ordering and growth of the periodicity of the two hierarchical molecular structures. Reproduced with permission from ref 31. Copyright 2012, John & Wiley Sons.

D


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Figure 6. Schematic chart of the active photoalignment process focusing on LC hierarchical structures in the PBMA-b-PAz film. PSS indicates the photostationary state. The main motions occur in the highlighted regions. Reproduced with permission from ref 32. Copyright 2014, American Chemical Society.

the surface-segregated film was irradiated with LPL. After that, successively patterned LPL irradiation was obliquely carried out at 45° by rotating the polarizer. The crossed Nicols POM observation showed clear birefringence patterns, demonstrating that the photoreorientation of the PAz block induced the inplane alignment of internal PBz mesogens beneath the PBMA block surface layer. The crossed Nicols POM observation showed clear birefringence patterns, demonstrating that the inplane reorientation of internal PPBz was achieved by the photoreactive PAz block beneath the PBMA block surface layer. The out-of-plane orientation and in-plane alignment of the nonphotoresponsive PPBz film with a thickness of more than 10 μm can be controlled by the photoresponse of the PBMA-b-PAz surface-segregated layer with a thickness of ca. 20 nm at the free surface. To directly prove the effect of the surface-segregated PBMA-b-PAz layer on the induced random planar and in-plane LC alignments, a PBMA-b-PAz film was painted over a PPBz film with homeotropic orientation by a superfine inkjet printing technique. The PPBz film covered with the PBMA-b-PAz film was annealed above the isotropic points of both LC components and irradiated with a successive LPL process in the PAz LC state. A painted image with birefringent patterning emerged and disappeared by the rotation of polarizers with the cross Nicol condition (Figure 9c,d). The appearing and disappearing of the printed images with every 45° rotation of the crossed polarizers suggest that the PPBz mesogens were monoaxially aligned by angular selective photoreorientation of the PAz layer upon LPL irradiation. The unprinted areas were dark fields unchanged by the polarizer rotation, indicating that homeotropically orientated pure PPBz film regions were formed by annealing. LC alignment for the production of LC devices is traditionally conducted with rubbed polymer film surfaces (polymer rubbing method)49,50 or angular, selective photoreactions on polymer surfaces (command surfaces).11,15,51−55 These surface LC alignment methods have been primarily performed on solid substrates. Compared to these LC alignment methods, the planar orientation induced via surface segregation of PBMA-bPAz can be regarded as an LC alignment technique from the free surface without substrates. The surface-segregated PPBz/

mixing with PBMA-b-PAz. These random planar-oriented films showed efficient angular selective reorientation of the PAz LC phase by LPL irradiation. In the PS-b-PAz/PBMA-b-PAz film, moreover, the polystyrene cylinders were aligned with the monoaxially aligned PAz phase (Figure 7d). These induced planar-oriented films could be due to the surface segregation of the PBMA blocks at the topmost surface. Surface-active PBMAb-PAz selectively migrates to the surface of a mixed film and forms a surface-segregated PBMA layer at the topmost surface. A TEM cross-sectional profile of the photoaligned PS-b-PAz/ PBMA-b-PAz film elucidated the PBMA block surfacesegregated layer with a thickness of ca. 20 nm (Figure 7d). Note that the addition of the same amount of PBMA homopolymer could not induce a planar orientation, which indicates that the orientational alteration requires an MPS interface (IMDS) between PBMA and PAz. In the above part, we described that the surface segregation of PBMA-b-PAz induced the planar orientation of SCLC polymer systems with the same PAz composition. The surface-segregated approach is also effective in other SCLC polymer systems with different mesogens (Figure 8a).34 The surface-segregated layer of PBMA-b-PAz was applied to a nonphotoresponsive SCLC polymer system (PPBz, Figure 8a). PPBz exhibits a smectic A phase and strongly adopts a homeotropic orientation in a thin film after an annealing process, similar to the PAz film (Figure 8a left). In the annealed film, scattering corresponding to the smectic lamella spacing (q = 2.0 nm−1, d = 3.2 nm) of PPBz was detected only in the out-of-plane direction by the GI-SAXS measurement (Figure 8b). With the addition of PBMA-b-PAz (3% by weight), the mixed film exhibited scattering attributed to the PPBz smectic spacing (q = 2.1 nm−1, d = 2.9 nm) in the inplane direction after annealing (Figure 8c). The addition of a small amount of PBMA-b-PAz leads to the random planar orientation of an SCLC polymer system with a different mesogen by the surface segregation of PBMA. Moreover, by using angular selective photoisomerization of the surfacesegregated photoresponsive Az layer of PBMA-b-PAz, in-plane photoalignment and photopatterning of the PPBz/PBMA-bPAz films were achieved (Figure 9a,b). First, the whole area of E


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Figure 7. (a) Schematic images of the inducement of planar orientation and photoalignment processes for a PS-b-PAz/PBMA-b-PAz film with surface segregation of a PBMA-b-PAz layer. Two-dimensional GI-SAXS images of the pure PS-b-PAz film (b) and PS-b-PAz/PBMA-b-PAz films (c) after annealing at 130 °C. The 1D profiles of the scattering intensity are shown as curves in white. (d) Cross-sectional TEM image of the monoaxially aligned PS-b-PAz/PBMA-b-PAz mixed film. The cross-sectional film sample was cut in the direction parallel to the electric field of irradiated LPL light. Reproduced with permission from ref 33. Copyright 2013, John & Wiley Sons.

PBMA-b-PAz film can provide in-plane photoreoriented LC systems by the simple casting of a mixed solution or painting PBMA-b-PAz onto an SCLC polymer film and a subsequent annealing process. Thus, surface-segregated PBMA-b-PAz systems can be called a command surface ink.34 Conventionally, surface segregation has been used to modify the topmost surface, affecting the wettability and adsorption properties.19−24 The present approaches can control the internal orientation structure of the film from the free surface by a surface-segregation effect. In recent years, the effect of the free surface has been utilized to control the molecular orientation in free-standing films of various molecular systems such as liquid crystalline polymers,56−58 conductive polymers,59 discotic liquid crystals,60 and MPS structures.6 These approaches are anticipated to be developed as a new molecular alignment technique. Induced Planar Orientation in a Surface-Segregated SCLC Block Layer of a Block Copolymer. Because of the surface segregation of the coil blocks of the SCLC block copolymer, the inward-facing SCLC blocks trigger the random planar orientation in the SCLC polymer systems in the previous section. This section describes the LC orientation and polymer-

aggregated structure of SCLC blocks when the SCLC blocks segregate to the free surface. In the case of PS-b-PAz, the surface free energy of the PAz block is lower than that of the PS block. When PS-b-PAz is added to a PS homopolymer film, the SCLC PAz blocks of the block copolymer segregate to the free surface. In fact, the surface-segregated PAz spontaneously forms an LC polymer brush structure exhibiting a random planar orientation (Figure 2b).35 The PS-b-PAz pure film adopts the homeotropic orientation of the PAz matrix (Figure 7b) and the normally oriented PS cylinder phase, as previously described.33,44 Spin-cast films of a binary mixture of the PS homopolymer and a small amount of PS-b-PAz (5−10% by weight) with various PAz lengths were prepared from mixed solutions and annealed at a higher temperature than the isotropic point of the PAz block and the glass transition of PS. The thickness of the resulting films was approximately 500 nm. Cross-sectional TEM observations were performed on the PS/PS-b-PAz binary films (Figure 10a−c). In these images, two-layer structures consisting of a thick layer and a thin surface layer were observed. According to the results from the films being stained with OsO4 and the high mixing ratio of F


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Figure 8. (a) Molecular structure of nonphotoresponsive PPBz and schematic images of the inducement of planar orientation and photoorientation for a nonphotoresponsive SCLC polymer film with surface segregation of a PBMA-b-PAz layer. The 2D GI-SAXS images are demonstrated for the pure PPBz film (b) and PBMA-b-PAz (3%)/PPBz (c) films after the annealing process at 125 °C for 10 min. Reproduced with permission from ref 34. Copyright 2014, Nature Publishing Group.

Figure 9. (a, b) POM photographs of the in-plane-patterned film of a PPBz/PBMA-b-PAz mixed film with crossed polarizers rotated from each other (scale bar 100 μm). (c, d) POM photographs of the in-plane and out-of-plane alignment films coated by a superfine inkjet printer under crossed polarizers 45° to one another (scale bar 200 μm). Reproduced with permission from ref 34. Copyright 2014, Nature Publishing Group.

the thickness of the PAz thin surface layers increased with the polymer length of the PAz blocks. The relationship between the length (degree of polymerization, N) of the PAz block and the thickness of the surface layer is plotted in Figure 10d. The

the polystyrene homopolymer, the thick layer can be attributed to the polystyrene layer and the thin surface layer can be attributed to the PAz block layer. In PS/PS-b-PAz, the PAz blocks preferentially segregate at the free surface. Interestingly, G


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The high-density PAz brushes formed on the PS film surface by surface segregation also exhibit efficient in-plane photoalignment with LPL irradiation. The spontaneous brush formation should be promoted by three factors: (1) surface segregation of SCLC blocks with relatively low surface free energy, (2) anchoring of the coil blocks on the polymer substrate surface, and (3) the self-assembled nature of the smectic LC phase of the SCLC polymer blocks. Random planar orientation of the SCLC high-density polymer brushes was prepared with surface-grafted PAz chains by surface-initiated (SI) living polymerization on a glass and Si substrate.63−68 The surface-segregated polymer brush approach provides the following advantages. The preparation method is very simple and easy compared to the SI living polymerization method for surface-grafted polymer brushes. The thickness control of the surface-segregated brush can be simply conducted by using presynthesized diblock copolymers. The surfacesegregated polymer brush can be formed on a flexible surface with polymer processes such as spin-casting and solvent-casting deposition and molding processes. Yokoyama et al. have demonstrated high-density polymer brush structures of amphiphilic diblock copolymers with poly(ethylene oxide) blocks by polymer surface segregation systems with solvation dynamics in water via self-assembly.69 The SCLC-segregated brush is formed and is stable in air due to the surface free energy and LC nature.

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SUMMARY LC mesogens of SCLC polymers have a strong tendency to adopt a homeotropic orientation in thin film due to the volume exclusion effect at the surface. The homeotropic orientation of Az mesogens inhibits the effective photoreaction and angular selective photoreorientation because the photoprocessing light is usually irradiated with incidence normal to the substrate plane. The induction of a random planar orientation is necessary for the in-plane orientation control of the liquid crystal, in particular. This review demonstrated that the surface segregation of the SCLC block copolymer in thin films induced the unique planar orientation that is advantageous for in-plane photoalignment processes. The surface segregation of an amorphous block provides a random planar orientation with an MPS interface formed parallel to the film surface. The planar orientations result in efficient in-plane reorientation photoswitching for a hierarchical LC mesostructure consisting of smectic SCLC and MPS cylinder structures. Moreover, the surface-segregated Az block layer can photocontrol the in-plane alignment in LC polymer systems (free surface command layer). On the other hand, when the SCLC polymer block is segregated at the free surface, the SCLC block layer forms a high-density polymer brush layer with a random planar orientation by selfassembly. It is worth noting that these induced random planar orientations are self-assembled (thermodynamically stable) by the surface segregation of polymer surfaces. Orientational controls at the interface and surface are important in the LC alignment process for polymer LC systems. The photoalignment technologies of liquid crystal materials are expected to be applied to various optical elements and devices.70−74 We anticipate that the new surface and interface polymer architectures as demonstrated here will lead to new LC polymer processes and devices.

Figure 10. Induced planar orientation of the high-density polymer brush structure in the surface-segregated SCLC block. Cross-sectional TEM images of (a) PS96-b-PAz65/PS, (b) PS96-b-PAz192/PS, and (c) PS96-b-PAz331/PS films. (All polymer cross-sectional samples were stained by the vapor treatment of RuO4.) (d) Thickness of the surfacesegregated PAz block layers as a function of the polymerization degree of the PAz block monomer unit. The red line and the dotted black line indicate the observed thickness of the PAz layer in the cross-sectional TEM and the ideal all-trans conformation chain length calculated with PMMA main chains, respectively. (e) GI-SAXS 2D image of the surface-segregated PAz block layer on a PS film. Reproduced with permission from ref 35. Copyright 2016, John & Wiley Sons.

observed thickness of the surface layer linearly increased with N of the PAz block chain, and the slope of the line calculated by the least-squares method was 0.202N (nm). The calculated value from the ideal slope (dotted line) for the all-trans conformer of poly(methyl methacrylate) was 0.254N (nm). Therefore, the surface-segregated PAz block chain was calculated to be an approximately 80% extension of the ideal elongated chain, indicating that the PAz chain elongation is comparable to that of high-density polymer brushes in a solvent swollen state.61,62 These facts demonstrate that the main chain of the surfacesegregated PAz orients vertical with respect to the surface plane (and the IDMS). Thereby, the Az mesogens should inevitably adopt a random planar orientation. GI-SAXS measurements proved the random planar-oriented smectic phase in the PAz block layer at the surface (Figure 10e). As previously mentioned, pure PS-b-PAz exhibits out-of-plane scattering in thin films, suggesting the homeotropic orientation of the PAz matrix (Figure 7b). In contrast, for the surface-segregated PAz chains on the PS layer, X-ray scattering was observed only in the inplane direction (Figure 10e). The scattering in the in-plane direction indicates the random planar-oriented smectic phase. H


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shusaku Nagano: 0000-0002-3929-6377 Notes

The author declares no competing financial interest. Biography

Shusaku Nagano is an associate professor at the Venture Business Laboratory of Nagoya University. He received his B.S. (1995) and M.S. (1997) degrees in chemistry from Gakushuin University and Ph.D. (2001) degree from Tokyo Institute of Technology under the supervision of Prof. Takahiro Seki. From 1998 to 1999, he worked in the Central Research Laboratories at Nihon Parkerizing Co., Ltd. as a researcher. In 2001, he joined the central research center at Ricoh Company, Ltd. as a researcher. In 2002, he worked as an assistant professor in the Graduate School of Engineering, Nagoya University. Since 2011, he has been an associate professor in the Venture Business Laboratory of Nagoya University. He has been working on interdisciplinary research in polymer chemistry, polymer physics focusing on liquid crystalline polymers, photoresponsive polymers, polymer semiconductor devices, polymer surfaces, and polymer ultrathin films.

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ACKNOWLEDGMENTS I thank Professor Takahiro Seki at Nagoya University for encouragement, conversations, understanding, and consideration during this study. I also acknowledge the support provided by associate professor Yukikazu Takeoka and assistant professor Mitsuo Hara of Nagoya University. I am thankful for the significant experimental effort and results of Mr. Yusuke Koizuka, Mr. Tomoya Murase, Dr. Masami Sano, Dr. Kei Fukuhara, Mr. Yuki Nagashima, Mr. Koji Mukai, and many graduate students in our laboratory at Nagoya University. The real-time in situ GI-SAXS observation studies were part of collaborative work with Dr. Yuya Shinohara at the University of Tennessee and Professor Yoshiyuki Amemiya at the University of Tokyo. This work was supported by a Grant-in-Aid for Scientific Research (B: 25286025) from JSPS, the PRESTO Program from JST, and the Toyo Gosei Memorial Foundation.

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Random Planar Orientation in Liquid-Crystalline Block Copolymers

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