Confined Self-Assembly Enables Stabilization and Patterning of

4 days ago - User Resources. About Us · ACS Members · Librarians · ACS Publishing Center · Website Demos · Privacy Policy · Mobile Site ...
0 downloads 0 Views 6MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Confined Self-Assembly Enables Stabilization and Patterning of Nanostructures in Liquid-Crystalline Block Copolymers Yuxuan Chen, Shuai Huang, Tianjie Wang, Zhijiao Dong, and Haifeng Yu* Department of Material Science and Engineering, College of Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China

Macromolecules Downloaded from pubs.acs.org by TULANE UNIV on 02/16/19. For personal use only.

S Supporting Information *

ABSTRACT: Various pathways have been utilized to manipulate microphase separation (MPS) of block copolymers, but the obtained MPS nanostructures are often suffering from poor thermal stability. Here, confined self-assembly is first adopted to thermally stabilize and conveniently pattern the MPS morphologies of an amphiphilic liquidcrystalline block copolymer (LCBC) composed of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic azobenzene-containing polymethacrylate. The MPS nanostructures are thermally stabilized by top coating a layer of water-soluble polymer, sodium polystyrenesulfonate (PSSNa), as a result of coordination interaction between PEO and PSSNa. Interestingly, both thermal annealing and rubbing-treated polyimide film caused reorientation of mesogens in the PSSNa-coated LCBC film; however, the orientation of PEO nanocylinders remained unchangeable. This indicates that the PEO nanocylinders are selectively anchored by the PSSNa covering layer, regardless of the changes of mesogenic orientation. Thus, complicated hierarchical nanostructures can be fabricated effortlessly and preserved nondestructively by selected-area confined self-assembly or noncontact photopatterned technology due to the existence of photoresponsive azobenzene moieties.

■

INTRODUCTION Well-defined block copolymers (BCs) often self-assemble into diverse nanostructures ranging from 10 to 100 nm upon microphase separation (MPS).1−3 For example, a film of diblock copolymer can generate MPS morphologies of sphere, cylinder, double gyriod, and lamellae, depending on its composition and combination parameters.1,4−8 Among the formed MPS nanostructures, cylindrical morphology is of great significance for selective transport through nanocylinder channels in a BC film as a functional membrane.9 It has been reported that smectic liquid-crystalline BCs (LCBCs) show a remarkably wide nanocylinder window, resulting from the asymmetry in molecular architectures and the twodimensional (2D) lamellar structure of smectic layers.10−12 Therefore, LCBCs have been widely explored as templates or scaffolds to fabricate novel intelligent nanomaterials, which has attracted growing interests in various fields of chemistry and materials from biomedicine to electronics.13 There have been many approaches for manipulating MPS nanostructures, such as thermal or solvent annealing,14,15 mechanical rubbing,16 optical alignment,5,11,17 electric or magnetic field,18−22 chemical modification of substrate surface,7,23 templated self-assembly,24,25 and free-surface segregation,26−28 and highly ordered nanostructures have been acquired with desired structures and functionalities.29 Recently, a surface coating on MPS orientation control in amorphous BCs has been reported by Bates et al.,30 which was also effective in LCBCs.26,31−33 Seki et al. demonstrated that a homeotropic-to-parallel alignment transition of mesogens and therefore the orientation conversion of MPS nanodomains in © XXXX American Chemical Society

LCBCs could be realized by free-surface segregation with a free-surface-active polymer or exerting a command effect.27,31 Komura et al. employed one organic surface covering to control MPS nanocylinder orientation in LCBC films.33 However, it is inevitable for those systems to suffer from either sensitively etching or steeping in organic solvents to eliminate the skin layer, which could damage the MPS structures underneath it, thus limiting its further applications. Very recently, Nealey et al. adopted a water-soluble top coating of polyvinylpyrrolidone (PVP) to control the orientation of LCBC microdomains,34 but so far, the stabilization of MPS structures is still of great challenge. In this paper, we report one confined self-assembly strategy for thermal stabilization and convenient patterning of MPS nanostructures in LCBCs enabled by top coating of one watersoluble ionic polymer, poly(sodium-4-styrenesulfonate) (PSSNa). As shown in Figure 1, we adopted one amphiphilic LCBC (denoted as PEO-b-PM11AzC4), consisting of a hydrophobic polymethacrylate bearing pendant azobenzene mesogens (PM11AzC4) and a hydrophilic poly(ethylene oxide) (PEO) block. Such kinds of LCBCs have been intensively studied for easy fabrication of well-ordered MPS nanostructures in film with PEO nanocylinders dispersed in the mesogenic matrix.35 Because of the selectively anchoring effect of PSSNa on the PEO domains, effective thermal stabilization and patterning of MPS nanostructures were easily obtained. Received: November 13, 2018 Revised: February 6, 2019

A

DOI: 10.1021/acs.macromol.8b02435 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. MPS properties of the LCBC and the top coating method used in this paper. (a) Chemical structures of the amphiphilic LCBC (PEO114b-P(M11AzC4)83), the corresponding homopolymer (PM11AzC4), and the water-soluble ionic polymer (PSSNa). (b) Schematic illustration of the coordination interaction between PEO and PSSNa. AFM images (c), (d), (e), and (f) are MPS nanostructures of the LCBC films annealed at 45, 75, 95, and 125 °C, respectively.

Figure 2. PSSNa top coating-caused stabilization of MPS structure in LCBC thin films. The FESEM images of LCBC films annealed at 45 °C (a) and 125 °C (b) and the 45 °C-annealed films which were reannealed at 125 °C without (c) and with (d) a top coating. The PEO microdomains were selectively stained with RuO4 vapor prior to measurement.

42, 73, and 122 °C in its DSC curve (Figure S1a), which can be assigned to the melting peak of PEO, the transition of smectic X to smectic A, and the clearing point of the mesogenic block, respectively. On cooling, the crystallization temperature of PEO decreased to −24 °C as a result of the supercooling effect and the confinement of MPS, which is similar to other PEO-based BCs.11,16,17,35,36 Upon annealing

Moreover, it is convenient to remove the top covering layer by rinsing in water without disruption to the MPS nanostructures, ensuring their further applications in nanofabrication and nanoengineering.

■

RESULTS AND DISCUSSION Thermal Properties and MPS Nanostructures of the LCBC Film. On heating, three endothermic peaks appeared at B

DOI: 10.1021/acs.macromol.8b02435 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

125 °C, far different MPS behaviors were observed in Figures 2c,d. Without the top coating, the film showed transformed morphologies from nanodots in Figure 2a to nanolines in Figure 2c. In contrast, the PSSNa-coated film demonstrated unchangeable MPS nanostructures in Figure 2d, maintaining the MPS morphology before coating even after undergoing a high-temperature reannealing process, which further supports the stabilization effect of PSSNa. This top-coating caused stabilization of MPS morphology is different from the results reported by Seki, Komura, and Nealey et al., in which a homeotropic-to-parallel orientation transition of mesogens, and therefore the transition from out-of-plane to in-plane arrangement of dispersed phase was accomplished by being covered with a free-surface-active polymer or a top coating layer.26,33,34 Furthermore, the top coating gave rise to a threedimensional (3D) stabilization of MPS rather than limitation in 2D, as indicated by the cross-sectional AFM images. As shown in Figures 1c and 2a, the uncoated film exhibited a transformation of nanocylinder orientation from out-of-plane into in-plane after being reannealed at 125 °C (Figure S2c and Figure 2c), which was further proved by the cross-sectional AFM image in Figure S3a. However, vertical orientation of the upright PEO nanocylinders penetrating throughout the film was still preserved in the PSSNa-coated film (Figure 2d and Figure S3b) after the same thermal treatment at 125 °C, indicating the stabilization of MPS structures really occurred in 3D with the existence of the top-coating PSSNa. Influence of the PSSNa Coating on Mesogenic Orientation. Recently, a water-soluble polymer was reported as top coating to show great influence on the orientation of LCBC microdomains;34 however, the stabilization on the MPS structures has never been observed. Generally, the mesogenic alignment plays an important role in MPS nanostructures of LCBCs since the LC elastic deformation should interfere with the MPS process.16 Then, the mesogenic orientation was investigated by UV−vis absorption spectroscopy and polarizing optical microscopy (POM). Here, the orientation-independent ϕ−ϕ* transition absorption at 243 nm can be used as an internal standard to normalize the orientation-dependent π−π* transition at 337 nm for characterization of the spatial orientation of azobenzene chromophores.23,46 Almost identical declining in the ratio of these two transitions was obtained in Figures S4a,b, indicating that the existence of PSSNa top coating makes no difference to the mesogenic orientation upon thermal annealing. This is far different from the thermally induced orientation of the smectic mesogens in LCBCs, in which the homeotropic mesogens anchored by free surface were transformed into parallel arrangement upon destroying the anchoring with a command layer.31 Similarly, the same conclusions can be drawn for the 45 °C-pretreated LCBC film (Figures S4c,d): whether it was top coated with PSSNa or not had no effect on the mesogenic alignment, which is further proved by the POM observation (Figure S5).47,48 Thus, the mesogens in the continuous phase turned into out-of-plane orientation upon reannealing at 125 °C,23 which showed little influence on the dispersed PEO phase domains in the PSSNacovered LCBC film because of the thermal stabilization effect. Mechanism of Thermal Stabilization of MPS Nanostructures. Because there is little interaction between hydrophilic PSSNa and hydrophobic mesogens, the thermal stabilization of MPS structure of the PSSNa-covered LCBC film might have resulted from the selective anchoring function

the LCBC film at different temperatures, various MPS nanostructures were observed in Figures 1c−f. In the UV−vis absorption spectrum of as-cast film, two characteristic peaks were observed at 337 and 443 nm, which can be attributed to the π−π* transition of trans-azobenzene and the n−π* excitation of cis-azobenzene, respectively.37−40 After being annealed at 45 °C (just a little higher than the melting point of PEO), the maximum absorption peak at 337 nm decreased slightly in Figure S1b. Correspondingly, the sparsely disordered dotlike morphology of the as-cast film (Figure S1c) turned into a more compacted and regularly dotted arrangement (Figure 1c). When the annealing temperature was increased to 75 or 95 °C (below the clearing point), the maximum absorption peak continued declining in Figure S1b, and the MPS morphologies of stripes alternating with dots appeared in Figures 1d,e. When the annealing temperature was raised up to 125 °C (right above the clearing point), the maximum absorbance decreased to an extremely low value, suggesting the out-of-plane arrangement of chromophores. Generally, H-aggregation of chromophores leads to a blue-shift of the absorption band, whereas J-aggregation results in a redshift of the absorption band. As shown in Figure S1b, a distinct widening trend and slight shift were observed, which might be attributed to the coexistence of nonaggregated azobenzenes, H-aggregation, and J-aggregation in the annealed LCBC film.17,41 Meanwhile, an unambiguous nanostructure with parallel stripes was clearly observed, in which the PEO nanocylinders with a diameter of about 10 nm dispersed in polymer matrix with a periodicity of about 22 nm (Figure 1f).7,10,11,26−28,31,42,43 Obviously, as the annealing temperature increased, the maximum absorption peak of azobenzenes exhibited a remarkable decline, and the out-of-plane arranged cylindrical microdomains gradually transformed into in-plane. This is because the polymer chains at a high temperature are more easily movable to achieve a sufficient self-assembly upon phase segregation, making them to be in a low-energy and stable arrangement. Simultaneously, the smectic LC ordering showed great influence on the MPS nanostructures under the supramolecular cooperative motion.9 As a result, a more ordered MPS nanostructure was eventually obtained.44,45 Thermal Stabilization of MPS Nanocylinders in the LCBC Film. To separate the free surface (the interface between the LCBC film and air),26 a water-soluble ionic polymer (PSSNa) was chosen to cover the LCBC film for providing a confined self-assembly environment. For comparison, both as-cast and pretreated films with or without the top coating were annealed at 125 °C for 24 h, as shown in Figure S2. After eliminating the covering layer, both the as-prepared and the 45 °C-pretreated LCBC films exhibited unchangeable MPS nanostructures (Figures S2b,d), similarly to those in Figure S1c and Figure 1c. In contrast, films without the top coating self-assembled into unambiguous MPS nanostructures with parallel stripes over the entire region (Figures S2a,c), just like those shown in Figure 1f. These different MPS changing behaviors indicate the effect of surface-covering layer on the stabilization of LCBC nanostructures. Figure 2 shows FESEM images of LCBC films with and without top-coated PSSNa. Out-of-plane (Figure 2a) and inplane (Figure 2b) cylindrical orientations were clearly observed in the LCBC films annealed at 45 and 125 °C, respectively, which is in line with the AFM results in Figures 1c,f. When the samples in Figure 2a with and without the PSSNa coating underwent a secondary thermal treatment at C

DOI: 10.1021/acs.macromol.8b02435 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Schematic Illustration of the Thermal Stabilization of Three Kinds of MPS Nanostructures Formed in the As-Cast Film (a), the Pretreated Films Annealed at 125°C (b), and Exposed to UV Irradiation (c); Their Corresponding Stabilizations Were Obtained by the Top-Coated PSSNa (d, e, f)

During the annealing process of the PSSNa-coated LCBC films, little chemical reactions occurred between them besides the coordination interaction between PEO and PSSNa. Thus, the top coating was easily eliminated by being immersed in deionized water for 5 min, which is confirmed by UV−vis absorption spectroscopy. As shown in Figure S8a, two characteristic absorption peaks at 223 and 262 nm manifest the presence of PSSNa coating on quartz, which completely disappeared after rinsing, indicating that 5 min is sufficient to get rid of the PSSNa layer. More importantly, both the MPS nanostructure and the mesogenic orientation in film were little influenced by the dipping process (Figure S8b). Because the LCBC is insoluble in water even with prolonged dipping time, only the PSSNa layer was dissolved, as proven by Figure S8c. Here, PSSNa was chosen to provide a confined self-assembly condition for two reasons: first, it is water-soluble and immiscible with the azobenzene-containing polymers; second, its decomposition temperature of 440 °C is well above the annealing temperature for LCBC. Thus, almost no damage to the MPS nanostructures on thermal annealing was observed. Influence Factors of Thermal Stabilization of MPS Nanostructures. To avoid the contingency of the thermal stabilization and make the results repeatable and reliable, different parameters (e.g., molecular weight of LCBC, film thickness, and preprocessing method) and reannealing conditions at elevated temperatures (up to 180 °C) were systematically investigated. As shown in Scheme 1, all the results consistently indicate that the PSSNa-coated films kept their MPS nanostructures unchangeable upon reannealing treatment whatever happened prior to the top-coating process. Then the stabilization effect on the nanostructures of LCBC films at different reannealing temperatures was also studied. As shown in Figure S9, the threshold value of the efficient nanostructure stabilization should be set below 200 °C. With increasing temperature, the stabilizing effect gradually weakened because of the increased activity of PSSNa for interfering with the MPS process. Furthermore, the effect of top coating layer thickness on the thermal stabilization of the MPS structure was also studied. As shown in Figure S10, the

on the PEO nanodomains, which is possibly attributed to the coordination interaction between PEO and PSSNa (Figure 1b). Considering the incompatibility between the LCBC and PSSNa, PEO instead of the LCBC was adopted to prepare PEO−PSSNa complexes to study their coordination interaction. As shown in Figure S6b, a new characteristic peak appeared at 922 cm−1 in the FTIR spectrum of the complex, which can be assigned to the combination of symmetric CH2 rocking and C−O−C stretching vibrations. It clearly indicates the existence of sodium complex PEO−PSSNa. In other words, the typical alkali metal cation (Na+) should be coordinated to the ether oxygens of PEO as expected.49,50 Meanwhile, in addition to a weak peak at 62 °C arising from the melting of residual noncomplexed PEO, another strong endothermic peak appeared at 182 °C in the DSC curve of the PEO−PSSNa complex (Figure S6a), corresponding to its melting point.51,52 Thus, the PSSNa-caused thermal stabilization of MPS nanostructures with top coating should be attributed to the selective anchoring on the PEO microdomains, which does not influence the mesogenic orientation or the liquid crystallinity of the continuous phase. This is far different from previously reported systems,26,27,31−34 in which the orientation control of the cylindrical microdomains depends on the homeotropic-to-parallel alignment transition of mesogens rather than the interaction between nanocylinders and surface covering layer. Electrostatic force microscopy (EFM) is a subdivision imaging mode developed from atomic force microscopy (AFM) in the tapping mode, which can measure the electric field distribution of the sample surface qualitatively and therefore reveal the type of charge. When we applied a voltage of −5 V to the cantilever probe, the PSSNa-coated LCBC film (Figure S7b) showed positive phase value, as shown in Figures S7a,c, suggesting the electrostatic interaction between the negatively charged probe and the coated film is repulsive force. That is to say, the surface of the reannealed film with top coating is charged negatively indeed, which provides support for the coordination interaction between PEO and PSSNa in the current system. D

DOI: 10.1021/acs.macromol.8b02435 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 3. Thermal stabilization of photopatterned nanostructures in LCBC thin film with PSSNa coating. (a) Schematic illustration of the thermal stabilization of photopatterned nanostructure (the top view). The photoirradiation for nanopatterning was performed with UV light (365 nm, 134 mW/cm2) for 30 s. (b) AFM phase image of the photopatterned film reannealed at 125 °C with PSSNa coating. Both the UV-irradiated area with dotted structures and the nonirradiated area with linear structures were stabilized by the PSSNa coating. (c) UV−vis absorption spectra of the LCBC film upon annealing at 125 °C, UV irradiation, and reannealing treatment.

maintained the morphology even 6 months later (Figure S12b). This PSSNa-caused stabilization effect provides us a novel and versatile way for long-term nondestructive preservation of MPS nanostructures, undoubtedly extending applications of the MPS nanostructures. As shown in Figure S13, the PSSNa top coating was selectively applied in different areas of the LCBC film, thus patterning of PEO nanocylinder arrangement was obtained upon thermal annealing, as shown in Figures S13b,c. In addition to thermal stabilization, the confined self-assembly also offers an effortless method to generate patterned nanostructures in LCBC films.31,53,54 As shown in Figure S14, more complicated patterning of MPS nanostructures could be obtained facilely. Thermal Stabilization of the Photopatterned MPS Nanostructures. Because of the existence of azobenzene mesogens, vertical orientation of PEO nanocylinders has been obtained by the photoinduced phase transition of the LCBC,44 and such light-directed method was also effective for both ascast and 125 °C-annealed films with a PSSNa covering layer (Scheme 1d−f). As shown in Figure S15, the original MPS structures rapidly turned into highly ordered MPS nanostructures with hexagonal close packing of nanocylinders upon UV irradiation for 30 s, which should be resulted from the photoinduced LC-to-isotropic phase transition.44 Thus, the light-caused phase transition dominates the final MPS nanostructures in the competition of the thermal stabilization and the photocontrol. While keeping the light tunability induced by the photoresponsive azobenzene mesogens, the confined self-assembly approach also provides the current nanostructures with thermal stability, which is of critical importance for further applications.

PSSNa layer should be thicker than 3 nm to obtain remarkable thermal stabilization effect. The thermal stabilization of MPS nanostructures was still effective when one rubbing polyimide (PI) film was used as substrate for the confined self-assembly of the PSSNa-coated LCBC film. The mesogens were oriented along the rubbing direction,16 showing an order parameter (S) of 0.62 and an obvious contrast when the film was rotated by 45° under POM observation (Figures S11c,d). However, the MPS nanostructures preserved the same out-of-plane arrangement as before coating (Figures S11a,b), rather than forming in-plane nanocylinder arrangement along the rubbing direction as reported previously.16 Obviously, there exists a competition between the rubbing-induced in-plane orientation of nanocylinders and the thermal stabilization of MPS nanostructures, and the latter is obviously predominant for the present LCBC. Patterning of MPS Nanocylinder Orientation in LCBC with the PSSNa Top Coating. Until now, there still lacks efficient technology to store MPS nanostructures due to the tendency to be contaminated by defects and impurities, which greatly limits their further applications. Fortunately, the confined self-assembly may offer one convenient way for long-term preservation of nanopatterned MPS structures. Figure S12 shows the morphological changes in MPS nanostructures of PSSNa-coated and uncoated LCBC films being stored at room light. A few small defects appeared in the MPS nanostructures (Figure S12c) of the uncoated film 3 months later. What is worse, most defects gathered into larger convex or concave regions (Figure S12d), which severely disrupted the well-defined dotlike nanostructure after storage for 6 months. Contrastively, the top-coated LCBC film E

DOI: 10.1021/acs.macromol.8b02435 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Notes

After the hexagonal nanocylinder array was light-directed in the LCBC film upon the photoinduced phase transition,44 the thermal stabilization still came into effect once PSSNa was coated on such an isotropic film, and the resulted nanostructures maintained the same vertical orientation even after being reannealed at a high temperature, as shown in Figure 3a. This can be used to manipulatively pattern the MPS nanostructure by combination of the PSSNa-induced stabilization and the light tunability. By this way, stabilized nanostructures with remarkable difference between the irradiated and nonirradiated areas were observed after removal of PSSNa in Figure 3b, just like the nanostructure obtained before coating. It can be concluded that the thermal stabilization takes effect not only for as-cast and annealed MPS nanostructures but also for photopatterned ones (Scheme 1). Figure 3c shows the UV−vis absorption spectra of the LCBC film during these processes, in which the results of the annealed film coincide exactly with the reannealed one, strongly supporting that the thermal stabilization of various nanostructures is independent of the mesogenic orientation in the continuous phase, but merely decided by the anchoring effect of the PEO domains in LCBCs.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (Grants 51322301 and 51573005).

■

(1) Bates, F. S.; Fredrickson, G. H. Block CopolymersDesigner Soft Materials. Phys. Today 1999, 52, 32−38. (2) Fasolka, M. J.; Mayes, A. M. Block Copolymer Thin Films: Physics and Applications1. Annu. Rev. Mater. Res. 2001, 31, 323−355. (3) Bates, F. S.; Fredrickson, G. H. Block Copolymer Thermodynamics: Theory and Experiment. Annu. Rev. Phys. Chem. 1990, 41, 525−557. (4) Thomas, E. L.; Lescanec, R. L. Phase Morphology in BlockCopolymer Systems. Philos. Trans. R. Soc. London A 1994, 348, 149− 166. (5) Morikawa, Y.; Nagano, S.; Watanabe, K.; Kamata, K.; Iyoda, T.; Seki, T. Optical Alignment and Patterning of Nanoscale Microdomains in a Block Copolymer Thin Film. Adv. Mater. 2006, 18, 883−886. (6) Leibler, L. Theory of Microphase Separation in Block Copolymers. Macromolecules 1980, 13, 1602−1617. (7) Yu, H. Photoresponsive Liquid Crystalline Block Copolymers: From Photonics to Nanotechnology. Prog. Polym. Sci. 2014, 39, 781− 815. (8) Komura, M.; Komiyama, H.; Nagai, K.; Iyoda, T. Direct Observation of Faceted Grain Growth of Hexagonal Cylinder Domains in a Side Chain Liquid Crystalline Block Copolymer Matrix. Macromolecules 2013, 46, 9013−9020. (9) Zhou, C.; Segal-Peretz, T.; Oruc, M. E.; Suh, H. S.; Wu, G.; Nealey, P. F. Fabrication of Nanoporous Alumina Ultrafiltration Membrane with Tunable Pore Size Using Block Copolymer Templates. Adv. Funct. Mater. 2017, 27, 1701756. (10) Komiyama, H.; Sakai, R.; Hadano, S.; Asaoka, S.; Kamata, K.; Iyoda, T.; Komura, M.; Yamada, T.; Yoshida, H. Enormously Wide Range Cylinder Phase of Liquid Crystalline PEO-b-PMA(Az) Block Copolymer. Macromolecules 2014, 47, 1777−1782. (11) Yu, H.; Iyoda, T.; Ikeda, T. Photoinduced Alignment of Nanocylinders by Supramolecular Cooperative Motions. J. Am. Chem. Soc. 2006, 128, 11010−11011. (12) Xu, B.; Piñol, R.; Nono-Djamen, M.; Pensec, S.; Keller, P.; Albouy, P.-A.; Lévy, D.; Li, M.-H. Self-Assembly of Liquid Crystal Block Copolymer PEG-b-Smectic Polymer in Pure State and in Dilute Aqueous Solution. Faraday Discuss. 2009, 143, 235−250. (13) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; De Pablo, J. J.; Nealey, P. F. Epitaxial Self-Assembly of Block Copolymers on Lithographically Defined Nanopatterned Substrates. Nature 2003, 424, 411−414. (14) Komura, M.; Iyoda, T. AFM Cross-Sectional Imaging of Perpendicularly Oriented Nanocylinder Structures of MicrophaseSeparated Block Copolymer Films by Crystal-like Cleavage. Macromolecules 2007, 40, 4106−4108. (15) Peng, J.; Kim, D. H.; Knoll, W.; Xuan, Y.; Li, B.; Han, Y. Morphologies in Solvent-Annealed Thin Films of Symmetric Diblock Copolymer. J. Chem. Phys. 2006, 125, 064702. (16) Yu, H.; Li, J.; Ikeda, T.; Iyoda, T. Macroscopic Parallel Nanocylinder Array Fabrication Using a Simple Rubbing Technique. Adv. Mater. 2006, 18, 2213−2215. (17) Yu, H.; Kobayashi, T.; Hu, G. H. Photocontrolled Microphase Separation in a Nematic Liquid-Crystalline Diblock Copolymer. Polymer 2011, 52, 1554−1561. (18) Chao, C. Y.; Li, X.; Ober, C. K.; Osuji, C.; Thomas, E. L. Orientational Switching of Mesogens and Microdomains in Hydrogen-Bonded Side-Chain Liquid-Crystalline Block Copolymers Using AC Electric Fields. Adv. Funct. Mater. 2004, 14, 364−370.

■

CONCLUSION In conclusion, efficient thermal stabilization and patterning of MPS structures in an amphiphilic LCBC film via confined selfassembly were successfully achieved by top coating a watersoluble polymer of PSSNa. When performing a reannealing step at a higher temperature (below 200 °C), both the thermally annealed and UV-treated LCBC films maintained their previous nanostructures unchangeable once being topcoated with PSSNa. The thermal stabilization should be attributed to the selective anchoring of cylindrical PEO microdomains, which derives from the coordination interaction between PEO and PSSNa. The introduced top coating showed little effect on the mesogenic orientation because of the immiscibility between them. Importantly, the surface coating layer can be removed easily and nondestructively by rinsing in water. Moreover, complicated photopatterned nanostructures can be fabricated and conserved for a long time (at least half a year) without any disruption by a combination of the confined self-assembly with the photopatterned technique, promising for their further applications in nanofabrication and nanoengineering.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02435.

■

REFERENCES

Experimental details and additional data including UV− vis absorption spectra, AFM phase images and EFM images, POM images, DSC curves, and FT-IR spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shuai Huang: 0000-0001-5083-2540 Haifeng Yu: 0000-0003-0398-5055 F

DOI: 10.1021/acs.macromol.8b02435 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (19) Kamata, K.; Iyoda, T. Alignment Control and Templating Process in Amphiphilic Block Copolymer Thin Film. Res. Rep.-NIFSPROC Ser. 2007, 70, 48−50. (20) Tomikawa, N.; Lu, Z.; Itoh, T.; Imrie, C. T.; Adachi, M.; Tokita, M.; Watanabe, J. Orientation of Microphase-Segregated Cylinders in Liquid Crystalline Diblock Copolymer by Magnetic Field. Jpn. J. Appl. Phys. 2005, 44, L711−L714. (21) Osuji, C.; Ferreira, P. J.; Mao, G.; Ober, C. K.; Vander Sande, J. B.; Thomas, E. L. Alignment of Self-Assembled Hierarchical Microstructure in Liquid Crystalline Diblock Copolymers Using High Magnetic Fields. Macromolecules 2004, 37, 9903−9908. (22) Gopinadhan, M.; Majewski, P. W.; Beach, E. S.; Osuji, C. O. Magnetic Field Alignment of a Diblock Copolymer Using a Supramolecular Route. ACS Macro Lett. 2012, 1, 184−189. (23) Haque, H. A.; Hara, M.; Nagano, S.; Seki, T. Photoinduced InPlane Motions of Azobenzene Mesogens Affected by the Flexibility of Underlying Amorphous Chains. Macromolecules 2013, 46, 8275− 8283. (24) Cheng, J. Y.; Mayes, A. M.; Ross, C. A. Nanostructure Engineering by Templated Self-Assembly of Block Copolymers. Nat. Mater. 2004, 3, 823−828. (25) Aragones, J. L.; Alexander-Katz, A. Block Copolymer Templated Self-Assembly of Disk-Shaped Molecules. J. Chem. Phys. 2017, 147, 054905. (26) Fukuhara, K.; Fujii, Y.; Nagashima, Y.; Hara, M.; Nagano, S.; Seki, T. Liquid-Crystalline Polymer and Block Copolymer Domain Alignment Controlled by Free-Surface Segregation. Angew. Chem., Int. Ed. 2013, 52, 5988−5991. (27) Seki, T. A Wide Array of Photoinduced Motions in Molecular and Macromolecular Assemblies at Interfaces. Bull. Chem. Soc. Jpn. 2018, 91, 1026−1057. (28) Vapaavuori, J.; Grosrenaud, J.; Pellerin, C.; Bazuin, C. G. In Situ Photocontrol of Block Copolymer Morphology during DipCoating of Thin Films. ACS Macro Lett. 2015, 4, 1158−1162. (29) Lodge, T. P. Block Copolymers: Past Successes and Future Challenges. Macromol. Chem. Phys. 2003, 204, 265−273. (30) Bates, C. M.; Seshimo, T.; Maher, M. J.; Durand, W. J.; Cushen, J. D.; Dean, L. M.; Blachut, G.; Ellison, C. J.; Willson, C. G. Polarity-Switching Top Coats Enable Orientation of Sub-10-nm Block Copolymer Domains. Science 2012, 338, 775−779. (31) Fukuhara, K.; Nagano, S.; Hara, M.; Seki, T. Free-Surface Molecular Command Systems for Photoalignment of Liquid Crystalline Materials. Nat. Commun. 2014, 5, 3320. (32) Nagano, S. Random Planar Orientation in Liquid-Crystalline Block Copolymers with Azobenzene Side Chains by Surface Segregation. Langmuir 2018, DOI: 10.1021/acs.langmuir.8b01824. (33) Komura, M.; Yoshitake, A.; Komiyama, H.; Iyoda, T. Control of Air-Interface-Induced Perpendicular Nanocylinder Orientation in Liquid Crystal Block Copolymer Films by a Surface-Covering Method. Macromolecules 2015, 48, 672−678. (34) Xie, H.-L.; Li, X.; Suh, H. S.; Ren, J.-X.; Wan, L.-S.; Craig, G. S. W.; Arges, C. G.; Nealey, P. F. Water-Soluble Top Coats for Orientation Control of Liquid Crystal-Containing Block Copolymer Films. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1569−1574. (35) Tian, Y.; Watanabe, K.; Kong, X.; Abe, J.; Iyoda, T. Synthesis, Nanostructures, and Functionality of Amphiphilic Liquid Crystalline Block Copolymers with Azobenzene Moieties. Macromolecules 2002, 35, 3739−3747. (36) Asaoka, S.; Uekusa, T.; Tokimori, H.; Komura, M.; Iyoda, T.; Yamada, T.; Yoshida, H. Normally Oriented Cylindrical Nanostructures in Amphiphilic PEO-LC Diblock Copolymers Films. Macromolecules 2011, 44, 7645−7658. (37) Natansohn, A.; Rochon, P. Photoinduced Motions in AzoContaining Polymers. Chem. Rev. 2002, 102, 4139−4175. (38) Blair, H. S.; Pague, H. I.; Riordan, J. E. Photoresponsive Effects in Azo Polymers. Polymer 1980, 21, 1195−1198. (39) Ikeda, T. Photomodulation of Liquid Crystal Orientations for Photonic Applications. J. Mater. Chem. 2003, 13, 2037−2057.

(40) Bisoyi, H. K.; Li, Q. Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications. Chem. Rev. 2016, 116, 15089−15166. (41) Spano, F. C.; Silva, C. H- and J-Aggregate Behavior in Polymeric Semiconductors. Annu. Rev. Phys. Chem. 2014, 65, 477− 500. (42) Nagano, S. Inducing Planar Orientation in Side-Chain LiquidCrystalline Polymer Systems via Interfacial Control. Chem. Rec. 2016, 16, 378−392. (43) Seki, T. Light-Directed Alignment, Surface Morphing and Related Processes: Recent Trends. J. Mater. Chem. C 2016, 4, 7895− 7910. (44) Wang, T.; Li, X.; Dong, Z.; Huang, S.; Yu, H. Vertical Orientation of Nanocylinders in Liquid-Crystalline Block Copolymers Directed by Light. ACS Appl. Mater. Interfaces 2017, 9, 24864−24872. (45) Schacher, F. H.; Rupar, P. A.; Manners, I. Functional Block Copolymers: Nanostructured Materials with Emerging Applications. Angew. Chem., Int. Ed. 2012, 51, 7898−7921. (46) Sapich, B.; Vix, A. B. E.; Rabe, J. P.; Stumpe, J.; Wilbert, G.; Zentel, R. Ordering and Dewetting in Spin-Coated Films of a Liquid Crystalline Main Chain Polymer. Thin Solid Films 2006, 514, 165− 173. (47) Larson, R. G.; Doi, M. Mesoscopic Domain Theory for Textured Liquid Crystalline Polymers. J. Rheol. 1991, 35, 539−563. (48) Guan, L.; Zhao, Y. Self-Assembled Gels of Liquid Crystals: Hydrogen-Bonded Aggregates Formed in Various Liquid Crystalline Textures. J. Mater. Chem. 2001, 11, 1339−1344. (49) Cowie, J. M. G.; Cree, S. H. Electrolytes Dissolved in Polymers. Annu. Rev. Phys. Chem. 1989, 40, 85−113. (50) Papke, B. L.; Ratner, M. A.; Shriver, D. F. Vibrational Spectroscopy and Structure of Polymer Electrolytes, Poly(ethylene oxide) Complexes of Alkali Metal Salts. J. Phys. Chem. Solids 1981, 42, 493−500. (51) Bannister, D. J.; Davies, G. R.; Ward, I. M.; Mcintyre, J. E. Ionic Conductivities for Poly(ethylene oxide) Complexes with Lithium Salts of Monobasic and Dibasic Acids and Blends of Poly(ethylene oxide) with Lithium Salts of Anionic Polymers. Polymer 1984, 25, 1291−1296. (52) Lee, C. C.; Wright, P. V. Morphology and Ionic Conductivity of Complexes of Sodium Iodide and Sodium Thiocyanate with Poly(ethylene oxide). Polymer 1982, 23, 681−689. (53) Zhao, X.; Wang, Q.; Lee, Y. I.; Hao, J.; Liu, H. G. A New Strategy to Fabricate Composite Thin Films with Tunable Micro- and Nanostructures via Self-Assembly of Block Copolymers. Chem. Commun. 2015, 51, 16687−16690. (54) Cui, L.; Zhao, Y. Azopyridine Side Chain Polymers: An Efficient Way to Prepare Photoactive Liquid Crystalline Materials through Self-Assembly. Chem. Mater. 2004, 16, 2076−2082.

G

DOI: 10.1021/acs.macromol.8b02435 Macromolecules XXXX, XXX, XXX−XXX