Article pubs.acs.org/Langmuir
Free Surface Command Layer for Photoswitchable Out-of-Plane Alignment Control in Liquid Crystalline Polymer Films Takashi Nakai,† Daisuke Tanaka,† Mitsuo Hara,† Shusaku Nagano,*,‡ 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 S Supporting Information *
ABSTRACT: To date, reversible alignment controls of liquid crystalline materials have widely been achieved by photoreactive layers on solid substrates. In contrast, this work demonstrates the reversible out-of-plane photocontrols of liquid crystalline polymer films by using a photoresponsive skin layer existing at the free surface. A polymethacrylate containing a cyanobiphenyl side-chain mesogen adopts the planar orientation. Upon blending a small amount of azobenzene-containing side-chain polymer followed by successive annealing, segregation of the azobenzene polymer at the free surface occurs and induces a planar to homeotropic orientation transition of cyanobiphenyl mesogens underneath. By irradiation with UV light, the mesogen orientation turns into the planar orientation. The orientation reverts to the homeotropic state upon visible light irradiation or thermally, and such cyclic processes can be repeated many times. On the basis of this principle, erasable optical patterning is performed by irradiating UV light through a photomask.
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INTRODUCTION
photoprocesses have great advantages that allow noncontact and higher-resolution operations.7−13 Unlike fluid low-molecular-mass LC materials, highly viscous polymer LCs are used as films cast on a solid substrate or in a self-standing state. Therefore, the films are exposed to air, and they possess free surfaces. Our recent study indicated that for the side-chain LC films the free surface plays a critical role in the mesogen orientation.14−16 Modifications of the free surface can be achieved via surface segregation by annealing or inkjet printing of a diblock copolymer consisting of a flexible amorphous block and an Az side-chain LC polymer block.14,15 The skin layer of the Az LC block copolymer on the air side alters the homeotropic to planar alignment for an LC polymer film possessing a phenyl benzoate mesogen, and effective in-plane control can be performed by linearly polarized light.15 For cyanobiphenyl (CB) side-chain polymer systems, the main-chain structure of polyacrylate or polymethacrylate critically alters the mesogen orientation.16 When a surface of
The photoalignment phenomenon of liquid crystalline (LC) materials on a photoresponsive surface was first proposed by Ichimura et al. in 1988.1 They demonstrated that the out-ofplane alignment of nematic LC molecules can be reversibly switched by the photoisomerization of azobenzene (Az) on a solid substrate surface.1,2 The E/Z (trans/cis) photoisomerization of an azobenzene monolayer on a substrate is able to alter the alignment of nematic LC molecules between the homeotropic and planar states. This active functional surface has been named a “command surface” or “command layer.” Shortly after this finding, angular selective excitation by linearly polarized light (LPL) onto an azo-dye-doped polyimide3 and photo-cross-linkable polymer films4,5 has been achieved to attain the in-plane alignment control. Approaches by the Langmuir−Blodgett (LB) technique have provided precise understandings on the molecular design of the command layer.6 A notable issue in the photoalignment processes is that they have recently been applied to practical industrial processes for the fabrication of LC display panels.7,8 As has been indicated when compared to the conventional rubbing procedure, the © 2016 American Chemical Society
Received: November 25, 2015 Revised: December 26, 2015 Published: January 6, 2016 909
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PAz to PCBMA by weight. Under these spin-cast conditions, films with ca. 150 nm thickness were obtained, as evaluated by a white light interferometric microscope (BW-S501, Nikon 129 Instruments). The films of this thickness were commonly used in this work unless stated otherwise. UV light irradiation at 365 nm was performed at 500 mW cm−2 using a Sanei supercure 203S upon slowly cooling (5 °C min−1) from an isotropic temperature (120 °C) to room temperature.
either polymer film is printed with the other one, the orientation of the whole mesogens coincide with that of the printed polymer, clearly indicating that the orientational discrimination is triggered at the free surface. Furthermore, the significance of controlling the free surface design is recognized for the orientation of microphase separation structure in the block copolymer films,14,17−19 organic semiconducting films,20,21 and discotic LC systems.22 The above previous studies deal with one-way control of mesogen orientations by the existence of a molecular or thin layer at the free surface. We have demonstrated the in-plane alternations by irradiation with LPL;15 however, no attempts have been made to realize the reversible out-of-plane control between the homeotropic and planar modes. This work reports, for the first time, that such dynamic and reversible control is actually attained by utilizing the free surface command Az layer for films of a polymethacrylate containing a CB side chain. The chemical structures of the polymers employed in this study are indicated in Figure 1. The mesogen orientation and film
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RESULTS AND DISCUSSION Figure 2 shows UV−visible absorption spectra of pure PCBMA (a) and PCBMA/PAz blended films (blending ratio 9:1 by
Figure 1. Chemical structures of PCBMA and PAz.
structures were evaluated by UV−visible absorption spectroscopy, contact angle measurements, grazing incident small-angle X-ray scattering (GI-SAXS) measurements, and polarizing optical microscopy (POM) observations.
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Figure 2. UV−visible absorption spectra of PCBMA (a) and PCBMA/ PAz blend (b) films. The black dotted and red solid lines indicate the data in the as-cast state and after annealing at 120 °C, respectively. In part b, the spectrum after UV irradiation (365 nm) is displayed by a solid blue line.
EXPERIMENTAL SECTION
The azobenzene-containing and cyanobiphenyl-containing polymers (PAz and PCBMA, Figure 1) were synthesized in tetrahydrofuran (THF) from the corresponding monomers by free radical polymerization using 2,2-azoisobutyronitrile as a radical initiator. The details of the synthesis procedures for PAz and PCBMA are described in the Supporting Information and in the previous paper.14,16 The molecular weight and thermophysical properties of the synthesized polymers were as follows. PAz: Mw = 2.7 × 104, Mw/Mn = 1.06, and glass−56 °C−smectic C−90 °C−smectic A−114 °C−isotropic. PCBMA: Mw = 1.0 × 105, Mw/Mn = 2.95, and glass−38 °C−smectic A−115 °C− isotropic. UV−visible absorption spectra were taken on an Agilent 8453 spectrometer at room temperature. X-ray scattering measurements were performed using a FR-E X-ray diffractometer (Cu Kα radiation, λ = 0.154 nm) equipped with an R-AXIS IV two-dimensional (2D) detector (Rigaku Co.). The contact angle of a water droplet was estimated with a FACE CA-XP contact angle meter (Kyowa Interface Science). The averaged values of five measurements were obtained. Birefringent characteristics of LC films were evaluated with an optical microscope under crossed polarizers using a BX51-P (Olympus Co.) equipped with a DP28 camera (OLYMPUS) operated with Cellscan controller software. Details of the experimental procedures are described in the Supporting Information. Polymer films on quartz plates were prepared by spin casting from a 2% solution by weight, and thermal annealing of the films was achieved at 120 °C for 5 min. The solution for the PCBMA/PAz blend film was prepared from a solution containing 10%
weight) (b) before and after annealing at the isotropic temperature (120 °C). Both pure and blend films gave similar absorbances of around ca. 0.9 at the absorption maximum (ca. 300 nm) of the π−π* transition of the CB moiety, suggesting that the thickness of both films was almost the same. The shoulder around 350 nm in the blend film (b) corresponds to the PAz component. In the pure PCBMA film, the absorbance at ca. 300 nm was almost unchanged by annealing (a, red line). This indicates that the mesogens of PCBMA adopt a random planar orientation (parallel to the substrate plane) after annealing.16 However, for the PCBMA/PAz blend film, the intensity of the π−π* bands showed a significant reduction for both CB and Az moieties after annealing (b, red line). The decreases in absorbance in these regions reflect that the mesogen orientation is changed to a more perpendicular direction with respect to the substrate, which will be further confirmed by X-ray data as will be mentioned later. This homeotropic alignment coincided with the orientation of a pure PAz film (Figure S1). When the UV light at 365 nm was used to irradiate this blended film, the absorbance of CB mesogens 910
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films of pure PAz and blended PCBMA/PAz indicate that PAz is preferentially segregated on the surface. This fact can also be explained by the difference in the side-chain structure. The side chain of PAz possesses a more hydrophobic (lower surface energy) and flexible tail of the pentyl group. Both factors favor the surface segregation of PAz.23,24 The orientation of the smectic layer structure in the annealed blended films was evaluated by GI-SAXS measurements. Figure 3 displays 2D GI-SAXS scattering images (upper) and 1D profiles (lower) extracted from the corresponding 2D images (lower) for an annealed pure PCBMA film (a) and a PCBMA/ PAz blend film before (b) and after (c) UV irradiation. For the pure PCBMA film, scattering spots due to the smectic lamellar structure were observed only in the in-plane direction (2θ = 2.0°, d = 4.4 nm) (a). This clearly indicates that the smectic layer is oriented perpendicularly to the substrate, with the mesogens being preferentially parallel with the substrate.16 On the other hand, in the PCBMA/PAz blended film, strong scattering spots were detected only in the out-of-plane direction in the 2D GI-SAXS image (b). The strong scattering spots were observed as a mixture of two distinct periods of smectic layers, PCBMA (2θ = 2.0°, d = 4.4 nm)16 and PAz (2θ = 2.5°, d = 3.5 nm, Figure S2).14 The existence of the two layer periods clearly shows that the two LC polymers were not mixed but phaseseparated. Together with the knowledge from the contact angle measurements, the minor component of PAz layer should form a skin layer at the free surface of PCBMA. Assuming that the densities of the two polymers are almost identical, the thickness of the surface skin layer can be calculated to be 15 nm (10% of the total thickness). Considering the strong orienting effect of the surface skin layer,14−16 we infer that the homeotropic orientation of CB mesogens was attained by the vertically aligning mesogens of PAz at the free surface. It seems somewhat strange that the scattering signals due to minor PAz were more intense than those of the major component of PCBMA. The smectic orientational order of the surface PAz layer was probably greater than that of the PCBMA film. In the blended film under UV irradiation, the scattering spots due to the PCBMA smectic layer were observed in the in-plane direction again (c) as observed for the pure PCBMA film (a).
was enhanced again (b, blue line). Therefore, the trans-to-cis photoisomerization of Az induced a homeotropic-to-planar transition of the blended film. To achieve these out-of-plane reorientations, the films needed to be heated to above the isotropization temperature of both LC polymers. Below this temperature in the LC and glassy states, reorientation did not effectively occur. We mostly adopted slow cooling (5 °C min−1), but rapid cooling within 1 min also led to orientational changes in the same manner. The cooling rate in the experimental procedure did not affect the resulting reorientation behavior. We assume that the mesogens are instantaneously oriented along with the phase transition from the isotropic to smectic A phase. The composition of the surface in the blended film was evaluated by the wettability with water. Contact angles of water droplets (θw) on pure PCBMA, pure PAz, and PCBMA/PAz blended film surfaces before and after annealing were measured, and the results are summarized in Table 1. Before annealing, Table 1. Contact Angle of Water Droplets (θw) on Polymer Surfacesa contact angle θw (deg)
a
compound
before annealing
after annealing
PCBMA PAz PCBMA/PAz
101.0 ± 0.9 103.7 ± 0.3 101.5 ± 1.3
96.5 ± 1.0 111.2 ± 1.0 110.4 ± 1.2
Annealing condition: 120 °C for 5 min.
the θw values were similar, giving ca. 101° for the three films. After annealing at 120 °C for 5 min, the θw value of the PCBMA film was reduced to 97°. In contrast, the pure PAz and PCBMA/PAz blended films after annealing exhibited significant increases in θw values to 111 and 110°, respectively. According to the θw values, PAz exhibits a lower surface free energy than does the PCBMA film. This fact can be understood by considering the polarity of the terminal substituent of mesogens, i.e., cyano and alkyl (pentyl) groups for PCBMA and PAz, respectively. The similar values of θw in the annealed
Figure 3. GI-SAXS data of the PCBMA film taken from ref 16 (a) and from the PCBMA/PAz blend film before (b) and after (c) continuous UV (365 nm) irradiation during a temperature decrease from 120 °C to room temperature, with the light intensity being 500 mW cm−2. In each part, upper and lower figures display 2D-XRD patterns and 1D intensity profiles (black, in-plane direction; red, out-of-plane direction), respectively. 911
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orientation in the whole film is induced by the segregated PAz (trans-Az) skin layer at the free surface. The right part shows the PCBMA/PAz blended film by annealing above the isotropization temperature and successive cooling under continuous UV irradiation. In this situation, the surface PAz layer adopts the cis-rich state and becomes isotropic. Accordingly, the homeotropic anchoring effect from the surface layer is lost; therefore, the PCBMA layer in the blend film adopts a random planar orientation as observed for the pure PCBMA film. When the Az unit is isomerized to the trans form photochemically with visible light or thermally, the homeotropic state is recovered. The cycles of homeotropic and planar inductions can be repeated with no deterioration (see part a). In this way, the out-of-plane commanding of CB mesogens is attained by the reversible trans−cis photoisomerization of Az existing at the free surface. We assume that changes in the molecular interaction at the interface between the segregated PAz and PCBMA base film are essential to the out-of-plane orientation switching. When the Az side chain of PAz is in the trans state, both rodlike mesogens of Az and CB interact effectively with each other most possibly via interdigitation (Figure 4c, left), which is also proven to be essential in the command layer placed on the solid substrate.6 In the course of cooling from the isotropic state to room temperature, both components of PCBMA and PAz commonly adopt the smectic A phase from ca. 114 to 90 °C. This phase matching should be important to the effective mesogen interaction. Consequently, the homeotropic aligning mesogen of the surface PAz command layer governs the orientation of the whole PCBMA film. However, when PAz becomes isotropic in the cis form of Az, the phase matching is not fulfilled. The matching/mismatching of the phase state brings about drastic compatibility changes in LC media as demonstrated in abrupt volume transitions in gel systems.28−30 The discrepancy of the phase (isotropic cis-isomerized PAz and smectic LC of PCBMA) should largely reduce the interaction between the layer interface, and the mesogens of PCBMA orient planarly as observed for the pure PCBMA film. Because the orientation of whole CB mesogens is photochemically commanded by the PAz skin layer at the free surface, the thickness dependence of PCBMA is of great concern. The induction from the planar to homeotropic transition was examined by changing the film thickness. GISAXS data were obtained for PCBMA/PAz films with various film thickness (150, 400, and 700 nm) (Figure S3). In these films, the thicknesses of the surface-segregated PAz layer were essentially unchanged, ranging from 15 to 20 nm by adjusting the blending ratio. In each 2D image of Figure S3, the scattering spots ascribed to smectic layer structures of PCBMA and PAz were observed only in the out-of-plane direction, and no scattering peaks were observed in the in-plane direction, regardless of the thickness change in the base PCBMA film. In the case of the thickest film (700 nm), the total thickness of the film reached 35 times the photoresponsive skin layer. The insensitiveness of the thickness factor gives strong evidence that the “domino effect” of the mesogen orientation is triggered from the free surface. The segregation behavior and rate (typically within 20 min) were not affected by the film thickness change as far as the blended LC polymer sample was heated above the isotropization temperature. This issue is to be noted. The molecular masses of the polymers used in this work are 2.7 × 104 and 1.0 × 105 for PAz and PCBMA, respectively, which are significantly
Thus, the GI-SAXS results obviously revealed that the photoisomerization of the PAz skin layer at the surface induces the orientational alternation of the smectic LC phase in PCBMA. Importantly, the smectic periodicity of PAz was not detected in this UV-irradiated state. The absence of the PAz scattering peaks shows that trans-to-cis photoisomerization of Az results in an LC-to-isotropic (amorphous) transition of the PAz layer.25−27 The planar orientation induced by UV irradiation can revert to the homeotropic state by irradiation with visible light at 436 nm at 120 °C or simply by gradual cooling from 120 °C without visible light irradiation (data not shown). Thus, the conversion to trans-Az regenerated the homeotropic mode again. Absorbance changes at 300 nm by the repeating cycles of UV irradiation and the thermal back process are shown in Figure 4a. As shown, these alternative homeotropic/planar LC alignment changes could be repeated at least five times. The series of processes carried out in this work are schematically summarized in Figure 4b,c. Figure 4b indicates the random planar orientation of a PCBMA film after annealing. Here, PCBMA indicated the random planar orientation.16 The left part of Figure 4c displays the PCBMA/PAz blended film after annealing. The homeotropic
Figure 4. (a) Absorbance change at 300 nm of a PCBMA/PAz blend film after repeated cycles of UV and thermal treatment. (b) Schematic illustrations of mesogen orientations of the PCBMA film.16 (c) Schematic representation of the PCBMA/PAz blended film after annealing at 120 °C (left) and after cooling from 120 °C to room temperature under continuous UV irradiation (365 nm, 500 mW cm−2) (right). Mesogens in blue and orange show CB and Az mesogens, respectively. 912
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Figure 5. POM images under crossed polarizers of the PCBMA/PAz blend film, after annealing (a), after slow cooling from 120 °C to room temperature under UV irradiation (365 nm, 500 mW cm−2) through a photomask (b), and successive annealing at 120 °C, followed by slow cooling to room temperature under visible light irradiation (436 nm, 500 mW cm−2) (c).
PCBMA film. However, this bilayer structure is difficult to confirm experimentally. The very recent development of GISAXS measurement has enabled us to obtain the depth profiles of thin polymer films by using low-energy synchrotron X-rays.34 We anticipate that this method will be of great help in obtaining more precise depth profiles and information on the layer boundary regions. Structural analysis utilizing this new technique will be involved in future investigations.
larger than the entanglement molecular mass for amorphous linear polymers such as polystyrene (1.8 × 104).31 The retained polymer mobility leading to the facile phase separation can be understood by the characteristic mechanical property of sidechain LC polymers. The entanglement in side-chain LC polymers is negligible even for reasonably high molecular mass polymers as demonstrated by viscoelasticity measurements.32,33 Patterned UV irradiation was performed through a line and space photomask. Figure 5 shows POM images under a crossed polarizer of the PCBMA/PAz blended film (150 nm) after annealing (a), under UV irradiation through a photomask (b), and after successive annealing under visible irradiation (436 nm, 500 mW cm−2) (c). In Figure 5a, the dark-field image was observed over the whole area as a result of the homeotropic orientation of PCBMA. When the blended film was irradiated with UV irradiation through a photomask during the cooling process from 120 °C to room temperature, the bright domains appeared only in the irradiated area (b). Thus, the shadowed area retained the homeotropic orientation, and the UV-exposed area was changed to the random planar orientation. After annealing to 120 °C or under visible light irradiation, the homeotropic state over the whole area was recovered again (c). In this preliminary experiment, a resolution of 10 μm was obtained.
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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/acs.langmuir.5b04325. Polymer synthesis and methods, UV−visible absorption spectra for the pure Az film, X-ray data of the PAz film and blended films with varied film thickness (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
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Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
CONCLUSIONS The orientation control for the LC polymer film by the free surface command layer has been proposed only recently; however, we believe that this strategy can be extended to widespread systems and applications. The present work has shown the reversible out-of-plane orientational control of LC polymer film, which corresponds to the out-of-plane LC command system achieved on a solid substrate reported since 1988.1,6 The successful attainment of switching should be a significant breakthrough in the free surface-mediated strategies because other systems hitherto investigated dealt with fixed one-way manipulation. The key to successful alternations of the mesogen orientation can be admitted in the reversible switching of molecular interactions between the components of the free surface layer and the polymer film underneath. As discussed, the PAz layer should be segregated on the air side of the
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (S23225003 to T.S. and B25286025 to S.N.), a Grantin-Aid for Young Scientists (B25810117 to M.H.) from The Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and the PRESTO Program of the Japan Science and Technology Agency to S.N. This work was also supported in part by a Grant-in-Aid for Scientific Research on Innovative Areas “Photosynergetics” (no. 15H01084) from MEXT, Japan. 913
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