Alternation of Side-Chain Mesogen Orientation Caused by the

Letter pubs.acs.org/Langmuir

Alternation of Side-Chain Mesogen Orientation Caused by the Backbone Structure in Liquid-Crystalline Polymer Thin Films Daisuke Tanaka,† Yuki Nagashima,† 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: In side-chain-type liquid-crystalline (LC) polymers, the main chain rigidity significantly affects the LC structure and properties. We show herein a relevant new effect regarding the orientation of side-chain mesogenic groups of LC polymers in a thin-film state. A subtle change in the main chain structure, i.e., polyacrylate and polymethacrylate, leads to a clear alternation of mesogens in the homeotropic and planar modes, respectively. This orientational discrimination is triggered from the free surface region (film−air interface) as revealed by surface micropatterning via inkjet printing.

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general, the more flexible main chain can offer a higher-order mesophase for polymers with an identical mesogenic group structure, and thus the isotropization temperature increases. Percec et al. discussed the influences of backbone rigidity on the isotropization temperature and its change in entropy.6 The side-chain mesogen orientation becomes perpendicular or parallel to the oriented main chain depending on the backbone structure of polyacrylate or polymethacrylate, as revealed by 2H NMR spectroscopy of aligned samples via using a magnetic field7 and X-ray analysis of lightly cross-linked stretched fibers.8 We report herein on a new finding on a discrimination of mesogen orientation in a thin film state which is clearly altered by the subtle structural change of the substituent of the backbone (Figure 1). This small structural change led to homeotropic or planar orientation for the two polymers possessing an identical cyanobiphenyl (CB) side group. This change is found to be driven at the free surface (LC−air interface) of the polymer film as confirmed by surface micropatterning via inkjet printing.

INTRODUCTION Poly(alkyl acrylate)s and poly(alkyl methacrylate)s are classes of the most widely studied synthetic linear polymers in polymer research and are widely used in industry. The chemical structure is very similar between the two series, but the existence of a hydrogen or methyl group at the α position in the main chain causes a significant difference in the main-chain rigidity. The introduction of a methyl group leads to a more rigid main chain, which is well known for the change in the glass-transition temperature (Tg). Tg of polymethacrylates is much higher than that of their homologous polyacrylates. For example, Tg of poly(methyl methacrylate) (Tg = ca. 105 °C) is nearly 100 °C higher than that of poly(methyl acrylate) (Tg = 5 °C),1,2 and Tg of poly(tert-butyl methacrylate) (Tg = 118 °C) is 80 °C higher than that of the acrylate homologue (Tg = 35 °C).3 In the Langmuir films on water, significant differences in surface viscosity properties have been found when two homologous polymers are spread in quasi-two dimensions on a water surface.3 In terms of macroscopic properties, the rigidity of the main chain further affects the dynamic mass-transfer process of amorphous molecular materials. The formation rate and feature of photoinduced surface relief gratings via the mass migration of azobenzene-containing amorphous materials differ between acrylate- and methacrylate-based polymers.4 In liquid-crystal (LC) research, the effect of the main chain rigidity has also been a subject of fundamental interest.5 In © XXXX American Chemical Society

Received: July 2, 2015 Revised: September 23, 2015

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DOI: 10.1021/acs.langmuir.5b02442 Langmuir XXXX, XXX, XXX−XXX

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seen, contrasting POM images were obtained. The PCBA film exhibited a uniform dark-field image (a), suggesting that the mesogenic side chains of PCBA are aligned homeotropically in the thin films. On the other hand, a birefringent LC texture was obtained in the PCBMA film (b). This fact suggests that the PCBMA side chains were oriented in a random way or in a random planar orientation under the same conditions. To confirm the mesogen orientation of these LC polymer films, grazing incidence small-angle X-ray scattering (GI-SAXS) measurements were performed on the annealed films. In Figure 3, 2D scattering image, 1D profiles, and scheme of mesogen orientations for the two polymer films are displayed in the upper, middle, and bottom panels, respectively, in each part. For the PCBA film, strong scattering spots ascribed to the lamella structure of the smectic phase (2θ = 1.9°, d = 4.6 nm) were observed in the out-of-plane (vertical) direction (a). This indicates that the smectic lamellar structure is oriented preferentially parallel to the substrate plane with homeotropically aligned CB mesogens in the film, which coincided with the dark-field observation in the POM observation. Minor spots due to the same lamella structure were also detected in the inplane direction with weak intensities. These minor azimuthal peaks probably resulted from mesogens in the vicinity of the solid substrate surface.10 In contrast, the PCBMA film provided scattering peaks due to smectic lamellar structure only in the in-plane direction (2θ = 2.0°, d = 4.4 nm) (b). This clearly indicates that the smectic layer structure is oriented vertical to the substrate plane. The oriented structure is consistent with the random planar orientation of PCBMA side chains observed by POM. The above mesogen orientations are displayed schematically in the bottom of each part. On the basis of the POM and X-ray data, a clear difference in the molecular orientation in the CB side chain was revealed. To our knowledge, this is the first observation that the main chain rigidity clearly alters the molecular orientation in LC thin films. In thin films, polymer main chains are generally confined parallel to a substrate surface,10,11 and side-chain mesogens adopt a vertical orientation with respect to the film plane. This can be explained by the excluded volume effect of rodlike molecules at the free surface.12 Therefore, the homeotropic alignment as observed in the PCBA film can be regarded as a normal case, and the random planar orientation in the PCBMA film is exceptional. The inability to form the homeotropic alignment in the latter case seems to stem from the rigid backbone from which the mesogenic side chains are directed in a scattered way at the free surface. Generally, the LC orientation is influenced by surface wettability.13 Therefore, contact angles for water (θw) on the surfaces before and after annealed films were evaluated (Table 1). Before annealing, both film exhibited similar θw values of ca. 101°. After annealing at 130 °C, the θw values were slightly reduced to 99 and 97° for PCBA and PCBMA films, respectively. Therefore, the surface wettability for water was essentially unchanged between the PCBA and PCBMA films, suggesting that the chemical composition of the uppermost surface was similar. The surface wettability does not explain the difference leading to the discrimination of mesogen orientations. The important role of the free surface in the mesogen orientation has also been recognized.9,14,15 To clarify the role of the free surface, the free surfaces of the films were microscopically covered by inkjet printing. Inversely, a

Figure 1. Chemical structures of PCBA and PCBMA.

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EXPERIMENTAL SECTION

The synthesis of polymers and their characterizations and experimental methods are described in the Supporting Information. The CB-containing polyacrylate (PCBA, Figure 1) and polymethacrylate (PCBMA, Figure 1) were synthesized in tetrahydrofuran (THF) from the corresponding monomers by free radical polymerization using azoisobutyronitrile as the initiator. The molecular mass data and thermophysical properties of the resulting polymers were as follows. PCBA: Mn = 1.2 × 104, Mw/Mn = 1.83, and glass−13 °C− smectic A−89 °C−isotropic. PCBMA: Mn = 4.4 × 104, Mw/Mn = 1.68, and glass−38 °C−smectic A−115 °C−isotropic. The layer spacing of the smectic A phase of both LC polymers in the bulk state was evaluated by X-ray scattering measurements using a FR-E X-ray diffractometer equipped with an R-AXIS IV two-dimensional (2D) detector (Rigaku Co.). The layer spacings (d) were estimated to be 4.6 nm (2θ = 1.9°) and 4.4 nm (2θ = 2.0°) for PCBA and PCBMA, respectively (Supporting Information, Figure S1). Thus, the lamella layer structure was essentially unchanged by the difference in the backbone structure. Details of the experimental methods are described in the Supporting Information. Polymer films on quartz plates were prepared by spin casting from a 3% solution by weight. The thickness of the films was approximately 200 nm. Before the characterizations, these films were annealed at 130 °C (above the isotropization temperature) for 10 min and slowly cooled to 80 °C, the temperature of the smectic A phase for both PCBA and PCBMA. The film thickness was evaluated using a white-light interferometric microscope (BW-S501, Nikon Instruments) for a scratched film with a spatula. Inkjet printing was achieved by using an ST050 subfemtoliter inkjet apparatus (SIJ Technology Co.).9 Onto a PCBMA film (200 nm thickness), a solution of PCBA dissolved in o-dichlorobenzene (1% by weight) was ejected to form a 20 μm width line.

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RESULTS AND DISCUSSION The optical properties of the films were first examined by polarized optical microscopic (POM) observations. Figure 2 shows POM images of the PCBA (a) and PCBMA films (b) at 80 °C (smectic A phase) under crossed polarizers. As clearly

Figure 2. POM images of PCBA (a) and PCBMA (b) films at 80 °C (smectic A phase) under crossed polarizers after annealing at 130 °C. White arrows in the images indicate the polarization direction of the analyzer and polarizer. Circular domains in a are due to the instability of the film at the elevated temperature. B

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Figure 3. GI-SAXS data of PCBA (a) and PCBMA (b) films after annealing at 130 °C. In each part, upper, middle, and lower figures display 2DXRD patterns, 1D intensity profiles (black, in-plane direction; red, out-of-plane direction), and schematic illustration indicating the mesogen orientations of PCBA (green rod) and PCBMA (blue rod), respectively. Note that the backbone orientations in the schemes are tentatively drawn and are not based on scientific data.

Table 1. Contact Angle of Water Droplets (θw) on Polymer Surfaces before and after Annealing at 130 °C contact angle θw (deg) compound

before annealing

after annealing

PCBA PCBMA

101.6 ± 1.7 101.0 ± 0.9

99.0 ± 1.7 96.5 ± 1.0

PCBMA line was printed in the same manner onto a PCBA film. The thickness of the printed lines was approximately 50 nm as evaluated by atomic force microscopic observations using a Nanopics NPX2100 (Seiko Instruments Inc.). The films obtained as above were annealed at 130 °C for 10 min and gradually cooled to 80 °C. The two polymers were not compatible even above the isotropization temperature as confirmed by POM observation of a contacting line of the two polymers. Figure 4 shows POM images under crossed polarizers of the annealed films. Contrasting results were obtained for the two films. For the PCBA film, a bright birefringent (planar mode) line with 20 μm width was observed in the region of the PCBMA line, and other regions remained dark (a). This fact indicates that, in the printed area, the mesogen orientation of PCBA changed from a homeotropic to a planar state following the inherent orientation of PCBMA at the surface. On the other hand, for the PCBMA film, the mesogens of PCBMA in the

Figure 4. POM images of inkjet-printed (20 μm width line) LC films after annealing. In part a, a PCBMA line was printed on a PCBA film. Inversely in part b, a PCBA line was printed on PCBMA films. A schematic illustration displaying the mesogen orientation is indicated in the upper part of each image.

planar state became homeotropic in the printed area with PCBA (b). These obviously represent that the change in mesogen orientation between the acrylate-based (homeotropic mode) and methacrylate-based (planar mode) polymers was C

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controlled by the molecular orientation at the top surface in both films. We have previously demonstrated that the mesogen orientation can be controlled by a photoalignable skin layer at the free surface.9 The orientation control observed in the present work is driven essentially by the same principle. The mesogen orientation at the free surface dominates the whole mesogenic groups in the film. The influence of damage of the base film by the inkjet solvent could be ruled out from an experiment adopting surface-to-surface physical contact of two separately prepared polymers films (Supporting Information, Figure S2). We also used polystyrene (Mw = 2.8 × 105, Mw/Mn = 2.54, Aldrich), an amorphous polymer, as the printing material. When the polystyrene film was printed on the PCBA and PCBMA films, the printed regions exhibited homeotropic alignment for both cases (Supporting Information, Figure S3). Therefore, the molecular (mesogen) orientation of the overcoated layer plays an important role in the orientation discrimination shown in Figure 4. Previous investigations have revealed that the main chain consisting of polyacrylate or polymethacrylate affects the thermophysical properties of LC polymers.5 In addition, this work proposes the new effect leading to the alternation of mesogen orientation in the thin-film state. What influences the orientation change is still unclear. However, it seems reasonable to assume that the folded and extended conformations of the backbone in the polyacrylate and polymethacrylate, respectively,16,17 can alter the mesogen orientation. With respect to mesogen orientation, related phenomena have also been reported for oriented polymer materials by applying a strong magnetic field7 or by stretching.8 The mesogens commonly align parallel and perpendicular to the main chain of polyacrylate and polymethacrylate LC polymers, respectively. The feature of the present case is that the molecular orientations are induced via self-assembly without any external fields. The polyacrylate backbone is flexible; therefore, the mesogens adopt a vertical orientation at the free surface as theoretically expected.12 In contrast, the rigid polymethacrylate backbone may disturb such an expected orientation. We have so far obtained information only on the mesogen orientation. Evaluations of the main-chain orientation and surface-selective detections of molecular parts by sum frequency generation spectroscopy may help us gain further understanding of the present systems. The important role of the free surface in the molecular orientation control is to be stressed from the technical viewpoints. Orientations of rodlike9 and discotic18 mesogens and further microphase separation structures of block copolymer films have been controlled by the existence of a segregated surface14 and a topcoat layer.15,19 An unusual endon (vertical) orientation of a conducting polymer chains has also been performed via surface segregation of one end of the polymer.20 We believe that new strategies utilizing the free surface will further be developed to manipulate the orientations of various types of functional films. In summary, we propose a new mesogen orientation effect brought about by the small structural change in the main chain of polyacrylate or polymethacrylate. The homeotropic or planar orientations are governed by the air−film interface. We anticipate that this knowledge is not only of phenomenological interest but will provide new possibilities for molecular alignment in LC polymer films.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02442.

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Polymer synthesis and methods, X-ray data in the bulk, and POM images (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (S23225003 to T.S. and B25286025 to S.N.) and 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 Area “Photosynergetics” (no. 15H01084) from MEXT, Japan.

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REFERENCES

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