Pathways toward Photoinduced Alignment Switching in Liquid

Article pubs.acs.org/Macromolecules

Pathways toward Photoinduced Alignment Switching in Liquid Crystalline Block Copolymer Films Masami Sano,† Shiyuko Nakamura,† Mitsuo Hara,† Shusaku Nagano,*,‡ Yuya Shinohara,§ Yoshiyuki Amemiya,§ and Takahiro Seki*,† †

Department of Molecular Design and Engineering, Graduate School of Engineering, and ‡Nagoya University Venture Business Laboratory, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan § Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 227-8561, Japan S Supporting Information *

ABSTRACT: The pathways toward linearly polarized light (LPL)-induced alignment switching in a diblock copolymer film composed of liquid crystalline (LC) azobenzene (Az) and amorphous poly(butyl methacrylate) (PBMA) blocks were studied in detail using polarized UV−vis absorption spectroscopy, grazing incidence small-angle X-ray scattering measurements, and polarized optical microscopy and transmission electron microscopy observations. The hierarchical structures of microphaseseparated cylinders of PBMA in a smectic LC Az layer matrix were prealigned by LPL and then irradiated by orthogonal LPL, which resulted in alignment switching to the orthogonal direction. In this process, the large prealigned domains were divided into substantially smaller domains at the submicrometer level, and then the structures were realigned in the orthogonal direction in a strongly cooperative manner, most likely through the domain rotation mechanism. The alignment change consisted of three stages: (i) fluctuations in the smectic layer of LC Az side chains in the initial state and breaking up of smaller grains to the submicrometer level before the orientation change (induction period), (ii) actual rotation of the divided domains driven by the photoinduced reorientation of Az mesogens (action period), and (iii) slower fusion and growth of smaller domains in the orthogonally realigned direction (postgrowth period). New aspects of dynamic self-assembly behavior in which different hierarchical structures are involved are proposed.

1. INTRODUCTION

Azobenzene (Az) is a mesogenic group that undergoes a reversible E (trans)/Z (cis) photoisomerization, and this unit is extensively employed for many phototriggered smart lightresponsive materials.9−22 When Az-containing LC films are irradiated with linearly polarized light (LPL), the mesogens are preferentially aligned perpendicular to the electric vector (E) of the LPL.10,11,23 Photoalignment of the MPS structure has recently been demonstrated by utilizing the orientational cooperativity of Az LC block copolymers.12,15,24−27 Amorphous MPS cylinders are aligned parallel with the photoaligned rodlike Az mesogens to minimize the elastic energy of the system.24,28 The various photoaligning processes for block copolymer MPS structures have distinct technological features. First, the alignment can be achieved using an inexpensive light

The alignment and realignment of block copolymer assemblies are becoming of great significance in new patterning technologies, which are expected to play important roles in top-down lithographic methods.1−4 These assemblies form microphase-separated (MPS) structures at mesoscopic scales (typically ranging from 10 to 100 nm) depending on the overall degree of polymerization, block−block interaction parameter(s), and relative volume fraction of each block. When a calamitic liquid crystalline (LC) polymer block is involved, the phase diagram of the pattern features (mostly sphere, cylinder, and lamella) is significantly changed. For amorphous-LC-type block copolymers, cylindrical MPS domains are preferentially formed at larger volume fraction ranges of the LC block.5−8 The most significant issue for applications is developing ondemand alignment methods for mesoscopic patterns during preparation of the films.1−4 © XXXX American Chemical Society

Received: August 31, 2014 Revised: September 27, 2014

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dx.doi.org/10.1021/ma501803g | Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Chemical structure of PBMA-b-PAz. (b) Photoalignment procedures for the PBMA-b-PAz film. After annealing (left), the film was aligned by the first LPL irradiation, and then it was realigned by a second LPL in the orthogonal direction.

source, and the substrate for preparing the film does not require sophisticated surface modifications, such as those used in directed self-assembly processes.29,30 Second, micropatterning of the MPS structure is readily achieved, which is difficult to achieve using macroscopic aligning methods2 via shear, flow, and electric fields. Third, and most importantly, the initial alignment can be erased and realigned to other desired directions with subsequent LPL irradiation.24,31 Morikawa et al.24 demonstrated a 3D optical switching and patterning of a MPS cylinder with a diblock copolymer composed of polystyrene and an Az-containing LC polymer. Here, the realignment process involves two steps: an initial heating step that exceeds the isotropization temperature to erase the prealigned MPS structure (initialization) followed by LPL irradiation during gradual cooling. Nagano et al.31 investigated a homologous LC diblock copolymer system that contained a poly(butyl methacrylate) (PBMA) block with a low glass transition temperature (Tg), in which the MPS alignment was switched only by LPL irradiation without the initialization step. In this system, the real-time behavior of the LPL-induced in-plane switching was monitored simultaneously for both the smectic LC layer and MPS cylinders using grazing incidence small-angle X-ray scattering (GI-SAXS) measurements with a synchrotron X-ray source. The in-plane photoalignment switching could be repeated many times without deterioration. Interestingly, conversion to a homeotropic alignment state was fully suppressed, which is unusual for side-chain calamitic LC polymer films. This result can be attributed to the in-plane orienting effect by the segregated PBMA block at the free (air) surface.32,33 Because the orientational switching in this system proceeds isothermally to maintain the LC (smectic A) phase directly without an initialization step, investigating this unexpected behavior is of great importance. Reorientation processes of MPS patterns under shear fields34−36 and electric fields37−42 have been studied in considerable detail using experimental approaches and theoretical considerations. The alternation of the MPS pattern by an electric field proceeds via the nucleation and growth of grains and/or grain rotation.37,38 In contrast, no studies have been conducted to elucidate the pathway and

mechanism for the light-driven MPS switching processes in LC block copolymer films. In this paper, we report the results of systematic explorations to understand the pathways toward the LPL-induced direct alignment switching of MPS cylinders. Figure 1 illustrates the block copolymer used in this study (PBMA-b-PAz) (a) and presents a scheme for the photodriven realignment process (b). The MPS structure is first uniformly photoaligned by initial LPL irradiation (first LPL), and then it is irradiated by LPL in the orthogonal direction (second LPL) at 95 °C, after which the PAz block adopts a smectic A phase. We attempted to investigate the dynamic behavior of the hierarchical structures by using polarized UV−vis absorption spectroscopy, polarized optical microscopy (POM), and transmission electron microscopy (TEM). On the basis of comparisons of the time-resolved profiles obtained by UV−vis spectroscopy with the grazing incidence small-angle X-ray scattering (GI-SAXS) data, we herein propose a mechanism for the LPL-induced realignment process for a LC block copolymer film. Strongly cooperative interplay between molecular and macromolecular assemblies with different hierarchical size features was elucidated.

2. EXPERIMENTAL SECTION 2.1. Polymer Material. The synthesis of the Az-containing monomer and the polymerization procedures via atom transfer radical polymerization to obtain PBMA-b-PAz have been reported previously.32 The average numbers of repeating units of PBMA-b-PAz used in this work for PBMA and PAz were 43 and 70, respectively (Mw = 5.7 × 104 and Mw/Mn = 1.08, as determined by 1H NMR spectroscopy (GSX-270, JEOL) and gel permeation chromatography (DS-4/UV-41/RI-101, Shodex). The thermophysical properties of this polymer were obtained using differential scanning calorimetry (DSC) (Q20, TA Instruments). The phase change behavior of the Az LC polymer was glass−57 °C−smectic C−90 °C−smectic A−118 °C− isotropic, and the Tg of PBMA was approximately 20 °C. 2.2. Photoalignment Procedures. PBMA-b-PAz thin films were prepared on quartz plates by spin-casting (2000 rpm) from a 2% by weight chloroform solution. Before LPL irradiation, the film was first annealed at 130 °C (isotropic state of PAz) on a thermostat stage, and then the initial alignment was performed by irradiating with LPL at 95 °C (smectic A state of PAz block). The film thickness was determined using surface profiles obtained by atomic force microscopy (Nanopics 2100, Seiko Instruments). The film was partially scratched using a B


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spatula, and the height difference between the film surface and substrate surface was measured. The typical film thickness was ca. 200 nm. LPL irradiation at 436 nm was performed with a mercury lamp source (REX-250, Asahi Spectra) at 1 mW cm−2 with a 436 nm bandpass filter. 2.3. Measurements. Polarized UV−vis absorption spectra of the thin films were recorded using an Agilent 8453 spectrometer equipped with a polarizer in front of the samples. The anisotropic optical properties of aligned Az chromophores were evaluated by measuring the angle-dependent polarized UV−vis absorption spectra using a JASCO UV-550 spectrometer equipped with a rotating sample stage and a polarizer in front of the samples. Absorbance at 340 nm in the polarized spectra was successively acquired by rotating the polarization angles from 0° to 360° in 15° steps. GI-SAXS measurements of the PBMA-b-PAz thin films were performed using a FR-E (Rigaku) X-ray source (Cu Kα, 0.154 nm) with an R-AXIS IV (Rigaku) 2D imaging plate detector. The GI sample stage was set using a goniometer and vertical stage. 2D GIimages were recorded with the X-ray incidence angles adjusted between 0.18° and 0.22°, which is between the critical angles of the PBMA-b-PAz films. The final images were the average of 300 s of exposure. The scattering images were collected by rotating the sample stage. 2D GI-SAXS images were acquired over a range of in-plane sample rotation angles from 0° to 360° in 15° steps. The polar plots were obtained by measuring the scattering intensity for the in-plane peaks at q = 1.87 nm−1, which correspond to the Az smectic layers by extracting from the 2D images. Time-resolved GI-SAXS measurements were performed on BL (beamline)-6A at the KEK Photon Factory in Tsukuba, Japan. A monochromated X-ray radiation source was used with a wavelength λ of 0.150 nm and a beam size of ca. 50 μm × 150 μm, and the scattered X-rays were detected with an X-ray CCD detector (PILATUS 100 K detector, DECTRIS AG). Details of the procedures and analyses were described in a previous paper.31 POM was used to characterize the liquid crystallinity of the polymers and to observe the LC domains using a BX51-P (Olympus) and a DP28 camera (Olympus) with Cellscan controller software. The films were heated and maintained at a high constant LC temperature during all measurements using an HP82 thermostat stage (Mettler Toledo). High-resolution TEM images were recorded on a Hitachi H-800 at an accelerating voltage of 200 kV. For the top-view measurements, the thin films were prepared on a KBr crystal substrate. The KBr crystal substrate was dissolved by soaking in water, and the resulting thin films that floated on the surface of the water were transferred onto a Cu TEM grid. The sample films for the cross-sectional views were prepared on a Kapton (polyimide) substrate with a thickness of 100 μm. For the cross-sectional images, the samples were embedded in epoxy resin (Nisshin EM Co., Quetol-812, MNA, NSA, DMP-30) and were sliced to a thickness of 80 nm using a microtome (Leica EM UC7RT, Leica microsystems). The sliced samples were transferred onto a Cu TEM grid. The samples were stained with RuO4 by exposing the samples to vapor from a sodium hypochlorite solution containing dissolved RuO4 for 30 min to increase the mass−thickness contrast.

Figure 2. Polarized UV−vis spectroscopic data for a PBMA-b-PAz film (200 nm thickness) aligned using the procedures shown in Figure 1b. Sets of polarized absorption spectra (upper) and corresponding polar plots of absorbance at 350 nm (lower) are shown for the film before irradiation (a), after the first LPL irradiation at 600 mJ cm−2 (b), after a second LPL irradiation at 80 mJ cm−2 (c), and after a second LPL irradiation at 600 mJ cm−2 (d). The red and blue lines correspond to spectra recorded using polarized probing light set parallel and perpendicular to the E of the second LPL. In the polar plots, the direction of 0−180° is defined as the E direction of the second LPL. Changes in the in-plane DR as a function of the exposure energy are indicated in (e).

3. RESULTS 3.1. Polarized UV−vis Absorption Spectroscopy. Polarized absorption spectra were recorded during the course of the procedures, as shown in Figure 1b. The thickness of the spin-cast PBMA-b-PAz film was 200 nm. The azimuthal direction and degree of Az orientation were evaluated by the dichroic ratio (DR) using the equation DR = (A⊥ − A||)/(A⊥ + A||), where A|| and A⊥ denote the absorbance at the peak of the π−π* transition band (approximately 350 nm) measured with polarized light set parallel and perpendicular to that of the second actinic LPL light, respectively. Figure 2 presents the polarized absorption spectra, and the corresponding polar plots

that show the angular dependence, along with the scheme in Figure 1b. In the annealed state before irradiation, no optical anisotropy was observed (a). After the initial LPL irradiation at C


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436 nm (first LPL) at 95 °C and 600 mJ cm−2, strong in-plane anisotropy emerged with DR = −0.85 (b, upper). Note that the energy dose numbers correspond to the time in seconds (light intensity: 1.0 mW cm−2). The polar plots (b, lower) show strong anisotropic features in the 0−180° direction, in which the direction of 0−180° was defined as the E direction of the second LPL. The alignment and its switching by LPL were strongly hindered at lower temperatures, where PAz adopts a smectic C phase (57−90 °C) due to enhanced viscosity. The alignment change was observed only at temperatures for the smectic A phase of PAz. When successive LPL with the orthogonal direction (second LPL) was irradiated at 95 °C, the DR began to increase with an induction period until 50 mJ cm−2, and it became almost zero at 80 mJ cm−2 (c, upper). At this stage, the polar plots became almost circular in shape (c, lower). Further irradiation up to 600 mJ cm−2 resulted in a strong in-plane anisotropic property in the direction orthogonal to the initial direction (d, upper) (DR = 0.80), and the polar plots showed strong anisotropy in the 90°−270° direction (d, lower). In Figure 2e, the change in DR is plotted against light exposure (time) of the second LPL. The inversion in the sign of DR indicates that the Az mesogen orientation changes to the orthogonal direction. As indicated, the change in DR exhibited a strong cooperative behavior, providing a sigmoidal curve with an abrupt change from 50 to 120 mJ cm−2. The trans-to-cis photoisomerization of Az reached the photostationary state within the induction period. 3.2. GI-SAXS Measurements. The realignment behavior of the smectic layer of Az mesogens was evaluated using GISAXS measurements. The azimuthal angular dependence of the 2D scattering images was evaluated by rotating the sample stage as shown in Figure 3a. Figure 3b presents a series of scattering intensity profiles in the in-plane direction with changing azimuthal angles after the first LPL irradiation at 600 mJ cm−2. A scattering peak was observed at 1.87 nm−1 at an in-plane scattering vector (qy = 4π(sin θ)/λ) (for definitions of θ and λ, see Figure 3a), which corresponds to a layer spacing of 3.4 nm. Polar plots for the scattering intensity monitored at q = 1.87 nm−1 are shown in Figure 3c−f. These figures present data for the films before irradiation (c), after the first LPL irradiation at 600 mJ cm−2 (d), after the second LPL at 80 mJ cm−2 (e), and after the second LPL at 600 mJ cm−2, which correspond to the conditions used for obtaining the absorption spectral data in Figure 2. The photoalignment procedures were conducted at 95 °C (smectic A phase of PAz), followed by immediate cooling to room temperature for the GI-SAXS measurements. When the sample was slowly cooled, the smectic C phase occurred, and the GI-SAXS data did not reflect the frozen state of the smectic A phase in which the photoalignment was performed (Supporting Information, Figure S1). Prior to irradiation, the film exhibited an isotropic scattering character, providing almost identical scattering intensities at all azimuthal angles (c). After sufficient LPL irradiation at 600 mJ cm−2, the films showed a strong anisotropic nature, for both the first LPL (d) and the second LPL irradiation (f). High scattering intensities were observed when the X-ray beam was set parallel to the direction of smectic layer plane, and the intensity rapidly decreased when the stage was rotated, which is due to the deviation from the Bragg diffraction conditions. Thus, the polar plots of the X-ray intensities were more sensitive to the azimuthal rotation than those of the spectroscopic measurements (Figure 2).

Figure 3. GI-SAXS data for the PBMA-b-PAz film (200 nm thickness) aligned using the procedures in Figure 1b. (a) Schematic of the setup. (b) Series of in-plane GI-SAXS profiles (logarithm of scattering intensity vs scattering vector (q)) at various rotation angles of the stage for the first LPL irradiation at 600 mJ cm−2 as an example. Polar plots of the scattering intensity at q = 1.84 nm−1 as a function of the rotation angle of the sample stage before irradiation (c), after the first LPL irradiation at 600 mJ cm−2 (d), after a second (orthogonal) LPL irradiation at 80 mJ cm−2 (e), and after a second LPL irradiation at 600 mJ cm−2 (f). In the polar plots, the direction of 0−180° is defined as the E direction of the second LPL.

After the second LPL irradiation at 80 mJ cm−2 (e), the polar profile of the film exhibited intensities comparable to those obtained for the initial one (c). The moderate intensity X-ray scattering observed for these films indicates that polydomains were formed in the films. These polydomains were oriented in a fully random manner for the initial film before irradiation (c). On the other hand, those for the transient state of the second LPL irradiation exhibited an anisotropic feature in which the intensity showed four peaks in the diagonal direction near 45°, 135°, 225°, and 315° (e). This result suggests that the realignment mechanism is domain rotation rather than the nucleation and growth process. If the nucleation and growth mechanism prevails, the peaks should be positioned at the directions of 0°, 90°, 180°, and 270°. Figure 4 shows the 1D profiles in the in-plane direction extracted from the 2D GI-SAXS images at 90° for the initially photoaligned state (a) and for the transient state during the second LPL irradiation (b), which are taken from the data in Figures 3d and 3e, respectively. The peak intensity for the transient state became less than one-tenth of that for the uniformly photoaligned state due to the deviations from the Bragg conditions. However, the scattering profile still showed a sharp peak. The widths at the half-maximum intensity of the D


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Figure 4. In-plane GI-SAXS profiles after the first LPL irradiation at 600 mJ cm−2 (a) and after a second LPL irradiation at 80 mJ cm−2 (b).

scattering profile were 0.077 and 0.081 nm−1 for the transient and uniformly aligned states, respectively. This parameter can serve as a measure of the domain size. The similarity of the scattering profiles and the essential agreement in the width at the half-maximum intensity suggest that the domain sizes were retained at considerably large levels that do not affect the peak sharpness. Additionally, the initial stage of irradiation in the induction period was investigated in more detail using synchrotron X-ray radiation. At a very early stage of LPL irradiation at 15 mJ cm−2, the scattering intensities for both the LC layer structure and the MPS cylinder distance drastically decreased. The d spacing of the smectic layer was slightly enlarged from 3.4 nm (q = 1.87 nm−1) to 3.5 nm (q = 1.82 nm−1) under this irradiation condition. Simultaneously, the MPS cylinder-to-cylinder distance decreased by 3 nm. In this way, the trans-to-cis photoisomerization of the Az unit rapidly affected the hierarchical structures of the block copolymer film before the in-plane orientational changes (Supporting Information, Figures S2 and S3). 3.3. POM Observations. Based on the GI-SAXS data, the polydomain structure appears to possess dimensions that are detectable with optical resolution; therefore, POM observations were carried out. PBMA-b-PAz films with a thickness of 250 nm were prepared for POM observations. This slight thickness increase did not affect the efficiency of the photoprocess. Figure 5 shows POM images of the as-annealed film (a), aligned by the first LPL irradiation at 600 mJ cm−2 (b), after the second LPL irradiation at 60 mJ cm−2 (c), after the second LPL irradiation at 80 mJ cm−2 (d), and after the second LPL irradiation at 600 mJ cm−2 (e) observed under cross polarizers. In the as-annealed film, we could observe a random domain structure with submicrometer sizes (a). The major regions were observed as dark fields, and the rotation of the polarizers did not effectively change this feature. This result should indicate that the most of the LC polydomains were smaller than the optical resolution of POM (Supporting Information, Figure S4). When the film was photoaligned with the first LPL irradiation, large-sized uniform areas with sizes of several micrometers (quasi-monodomains) were formed (b). When the second LPL was irradiated at 60 mJ cm−2, which corresponded to the stage just after the DR value started to change (c), breaking up of the domains to piecewise submicrometer sizes occurred. At 80 mJ cm−2, corresponding

Figure 5. POM images of the PBMA-b-PAz film (250 nm thickness) observed using the procedures in Figure 1b: images of the film before irradiation (a), after the first LPL irradiation at 600 mJ cm−2 (b), after a second LPL irradiation at 60 mJ cm−2 (c), after a second LPL irradiation at 80 mJ cm−2 (d), and after a second LPL irradiation at 600 mJ cm−2 (e).

to the intermediate stage of the DR change, the domain sizes were almost identical. In this case, each birefringence domain blinked on and off when the cross polarizers were rotated by 45° (Figure S4), indicating that the LC mesogens were uniformly oriented within each of the divided domains. The size feature of the domains at this stage (c, d) was obviously larger than that of the initial film before irradiation (a). This will be further discussed in section 3.4 for the TEM observations. With sufficient irradiation of the second LPL at 600 mJ cm−2, the uniform domains became larger again (e) to provide a quasi-monodomain state. 3.4. TEM Observations. The GI-SAXS measurements and POM observations revealed that the smectic layer structure was retained in the divided polydomains in the transient state. We next attempted to observe the MPS cylinder structure in the PBMA-b-PAz film using TEM. Figure 6 presents top-view TEM images of the ultrathin films of PBMA-b-PAz with a thickness of 80 nm before irradiation (a), after the first LPL irradiation at 600 mJ cm−2 (b), and after the second LPL irradiation at 80 mJ cm−2 (c), along with a 2D Fourier transform (FT) image for each image. In these TEM images, the dark regions correspond to the MPS domain of PAz in which ruthenium should be more abundant after the staining treatment. According to the FT analysis, the cylinder-tocylinder distances were 21−27, 21−26, and 20−27 nm for a, b, and c images, respectively, indicating that the cylinder-toE


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in section 3.3. Furthermore, the FT in image c indicates that the MPS cylinders were primarily oriented in two distinct directions, which resembles the behavior of the polar plots in the GI-SAXS analysis in Figure 3e. However, the spots in the FT image in Figure 6c deviated from the diagonal directions in Figure 3e. This difference should be attributed to the fact that TEM images provide information at very local regions, whereas X-ray data provide the overall average of macroscopic areas. Cross-sectional TEM images are shown in Figure 7. First, TEM images were recorded for the sufficiently photoaligned

Figure 6. Top-view TEM images of the PBMA-b-PAz film (150 nm thickness) observed using the procedures in Figure 1b: images before irradiation (a), after the first irradiation at 600 mJ cm−2 (b), and after a second irradiation at 80 mJ cm−2 (b). The features of the image after the second irradiation at 600 mJ cm−2 were essentially the same as those in (b). Corresponding FT images are shown on the right side.

cylinder distance was essentially unchanged during the LPLinduced realignment process. For the initial film before irradiation, the PBMA cylinders observed as brighter lines were highly curved toward random directions, and the long-range order was lost (a). The Fourier transform profile showed nearly circular images. In contrast, after the first LPL irradiation was performed at 600 mJ cm−2, the MPS cylinders were elongated perpendicular to the E of the LPL to show structures with long-range order (b). The FT image presented higher-order spots, revealing the highly ordered anisotropic array of the PBMA cylinders. In the transient state after the second LPL irradiation at 80 mJ cm−2, the domains were divided in a piecewise manner to show mosaic structures (c). The domain size were larger (0.5−1 μm) than those observed for the initial film before irradiation (a). This result is consistent with the POM observations discussed

Figure 7. Cross-sectional TEM images of the PBMA-b-PAz film (150 nm thickness) after the first irradiation at 600 mJ cm−2 observed in the directions parallel (a) and perpendicular (b) to the actinic LPL and after a second LPL irradiation at 80 mJ cm−2 (transient state) (c).

film. The film was cut in directions perpendicular and parallel to the E of the actinic LPL. In the perpendicular direction, only white dot spots arranged almost in a hexagonal array were observed (a). By contrast, striped morphologies were observed when the film was cut parallel to E (b). Some elongated dots were also observed in (b) because small angular deviations from the exact parallel direction of the cylinders resulted in the observation of dot morphologies. This set of TEM images confirms that uniform quasi-monodomain photoalignment was also obtained for the interior of the PBMA-b-PAz film. For the F


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transient state of the photoinduced realignment process (c), the cross-sectional image consisted of a mixed morphology of stripes and dots. The domain sizes were almost consistent with those observed in the top-view observation (Figure 6c). In both the top-view and cross-sectional images of the transient state, the appearance of different morphological features, such as the formation of a spherical morphology, did not occur during the course of the photoinduced realignment process. These facts reasonably rule out the mechanism of the nucleation and growth of domains.

4. DISCUSSION 4.1. Proposed Mechanism of Photoinduced MPS Alignment Switching. The UV−vis spectroscopy, GI-SAXS, POM, and TEM measurements clearly revealed that the LPLinduced realignment process proceeds through a transient (intermediate) state with divided domains that are typically at the submicrometer level. Furthermore, the polar plots of the GI-SAXS scattering intensity (Figure 3e) and the top-view TEM image (Figure 6c) strongly suggest that domain rotation is the most probable mechanism for the smectic layer and MPS pattern switching. In particular, the existence of diagonal preference in the smectic layer (GI-SAXS) and the PBMA cylinder (TEM) in the transient state is strong evidence for this interpretation. Notably, this process involves hierarchical structures of three different size features: i.e., the Az unit with a size of 0.9 nm (in the trans form), the smectic layer with a spacing of 3.4 nm, and the MPS cylinder patterns of PBMA of ca. 25 nm. All of these hierarchical structures are simultaneously realigned by LPL in strongly cooperative manners. The correlation size of the polydomains in the transient state ranged at submicrometer levels. These relatively large domain sizes should arise from the collective behavior of the elastic LC materials. Our results may be compared with the reorientation process of MPS structures driven by a strong electric field using coil−coil-type block copolymers that do not exhibit LC properties.37−41 In some cases, GI-SAXS analyses indicate an obvious broadening of the scattering intensity profile in the transient state, indicative of larger fluctuations in the MPS structure during the realignment process.41 In contrast, for the photoalignment in the present study, the fluctuations were highly suppressed, and the local order of the LC structure was virtually unchanged during the dynamic realignment process (e.g., Figure 6c). The strong electric field reorients the MPS structure by force. On the other hand, the LPL-induced realignment process is essentially driven by self-assembling structure formation of the LC block copolymer. We have previously conducted the photoinduced MPS alignment switching with a homologous polystyrene-based photoresponsive LC block copolymer. In this system, the alignment changes required an initialization process of heating above the isotropization temperature of the Az LC polymer.24 At this higher temperature, the block copolymer adopts an optically isotopic state that does not exhibit birefringence (a cubic-like phase is most likely formed). For this case, the realignment may proceed via the nucleation and growth mechanism during the gradual cooling stage with LPL irradiation. 4.2. On the Pathway of the Alignment Switching. A series of in-situ real-time GI-SAXS measurements were reexamined to evaluate the time course correlations between the spectral data shown in Figure 2 and the GI-SAXS measurements. In Figure 8, the time course profiles of in-situ

Figure 8. Changes in GI-SAXS intensity due to the smectic layer of the Az side chain of the PBMA-b-PAz film as a function of the exposure energy of the second LPL observed in parallel (a) and perpendicular (b) to the incidence of the actinic LPL (see ref 31). Data were remeasured using synchrotron X-ray radiation under conditions that were identical to those of the spectral measurements in Figure 2. The profile of the DR changes denoted in Figure 2e is compiled in the lower part of the figure.

GI-SAXS measurements with two orthogonal directions for the smectic layer (q = 1.87 nm−1, d = 3.4 nm) and the changes in the in-plane DR are compiled as a function of the exposed light dose of the second LPL irradiation. The previous investigation demonstrated that the time course changes in the GI-SAXS data obtained for the smectic layer and the MPS cylinder patterns of PBMA are fully synchronized.31 Therefore, monitoring the GI-SAXS signals for the smectic layer also reflects the motions of the MPS structure of the PBMA cylinders. According to Figure 8, the LPL-induced realignment process of PBMA-b-PAz at 95 °C can be divided into three stages. (i) Stage I (induction period, 0−50 mJ cm−2): The GI-SAXS scattering peak observed for the prealigned direction rapidly diminishes (curve a). In this period, no scattering in the GISAXS is observed in the orthogonal direction (curve b). This stage corresponds to the induction period before changing the DR of the Az mesogens. The trans-Az isomers begin to isomerize to the cis-form by 436 nm LPL irradiation and reach a photostationary state within this light dose. (ii) Stage II (action period, 50−120 mJ cm−2): GI-SAXS signals are not observed in either incidence direction (a, b). Only the DR is drastically changed from negative to positive in sign, indicating that the Az mesogens are realigned to the orthogonal direction. (iii) Stage III (postgrowth period, 120−300 mJ cm−2): At this stage, an enhancement in the GI-SAXS peak is observed in curve b in the realigned direction at a much slower rate compared with the initial intensity reduction in curve a. On the basis of the above time (energy dose) correlations and experimental data, we propose the following detailed pathway for the overall realignment process. An overview diagram of all processes is presented in Figure 9. In stage I, the cis-Az isomers begin to be involved, which leads to fluctuations in the smectic layer structure. This should be the reason for the rapid decrease in the GI-SAXS peak intensity for the initial G


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Figure 9. Overview diagram of the LPL-induced realignment process focusing on various hierarchical structures in the PBMA-b-PAz film. PSS stands for the photostationary state. The major motions occur in the highlighted regions.

LC block copolymer film. The LPL-induced realignment process involves three distinctive stages with respect to the exposed LPL dose. The actual alignment switching occurs in a strongly cooperative manner, most likely via the domain rotation mechanism. Dynamic cooperative transformations and motions beyond hierarchical structures with different size features are often observed in biological systems. However, there are few examples in synthetic polymer systems. We believe that the dynamic process of the LC block copolymer studied in this work provides valuable knowledge on the dynamic interplay between structures with different size features. The dynamic self-assembly motions should be affected by the molecular mass and the architecture (diblock or triblock) of the block copolymer. These issues need to be addressed in future investigations. Additionally, if the domain rotation mechanism is responsible, then controlling the rotational directions (rightor left-handed rotation) in the alignment switching process is of great concern. Work with respect to these issues is currently under way.

alignment. Thus, the efficient excitation of the Az chromophore occurs because the absorption transition moment is almost parallel with the E of the second LPL. The cis-Az content at the photostationary state under 436 nm irradiation was less than 20%, which was below the level that leads to a photoinduced phase transition to the isotropic state.43,44 This means that the smectic layer structure and the MPS cylinder structure are retained at all stages. Stage II involves the actual highly cooperative realignment process via the domain rotation mechanism mentioned in section 4.1. All hierarchical structures exhibited turns to the orthogonal direction in this short period. In this study, we particularly focused on the structural characterizations of the intermediate point at 80 mJ cm−2 of the second LPL irradiation. Stage III corresponds to the fusion of divided domains and growth to form a quasi-monodomain in the subsequent direction. This self-assembly process is considerably slower than the induction period observed during stage I for the structure fluctuation of the initial alignment. In this direction, Az mesogens are not effectively excited by LPL, and therefore, perturbations due to the light excitation become minor and thermal self-assembly processes are primarily involved. A question may be raised as to whether divided but relatively large domains of submicrometer areas can actually rotate within 70 s in stage II. We hypothesize that this phenomenon is closely related with the phototriggered mass migrating system in azobenzene-containing LC polymer films with homologous structures.15,44−46 In such systems, the mass displacement motions occur over distances of some micrometers to form surface-relief gratings in 10 s to minutes depending on the light intensity. The rotational motions of the divided domains around micrometer levels of PBMA-b-PAz also appear to occur in a similarly collective fashion.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (S.N.). *E-mail: [email protected] (T.S.). Notes

The authors declare no competing financial interest.

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5. CONCLUSION This work has proposed, for the first time, detailed pathways toward photoinduced alignment changes in a photoresponsive

ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (S23225003 to T.S. and B25286025 to S.N.) and for H


Macromolecules

Article

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Young Scientists (B25810117 to M.H.) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and the PRESTO program of Japan Science and Technology Agency to S.N. The synchrotron X-ray scattering experiments were performed at BL-6A of the KEK-Photon Factory, Tsukuba (Proposal No. 2012G629).

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Pathways toward Photoinduced Alignment Switching in Liquid

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