Photoinduced In-Plane Motions of Azobenzene Mesogens Affected by

Sep 30, 2013 - Photoinduced In-Plane Motions of Azobenzene Mesogens Affected by ... Nagoya University Venture Business Laboratory, Furo-cho, Chikusa, ...
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Photoinduced In-Plane Motions of Azobenzene Mesogens Affected by the Flexibility of Underlying Amorphous Chains Hafiz Ashraful Haque,†,‡ Mitsuo Hara,† Shusaku Nagano,†,‡,* and Takahiro Seki†,* †

Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan ‡ Nagoya University Venture Business Laboratory, Furo-cho, Chikusa, Nagoya 464-8603, Japan S Supporting Information *

ABSTRACT: Photoinduced in-plane motions are explored for a series of surface-grafted diblock copolymers comprising of an amorphous chain block and a liquid crystalline (LC) azobenzene (Az) side chain polymer block. In this architecture, amorphous polymers with different glass transition temperature (Tg) and the LC Az polymer are stepwisely grafted on substrate surfaces by the surface initiated atom transfer radical polymerization. Amorphous polymers employed are poly(hexyl methacrylate) (PHMA), poly(butyl methacrylate) (PBMA), and poly(methyl methacrylate) (PMMA) with Tg of −20, +27, and +110 °C, respectively. The extent of photoinduced in-plane anisotropy of the LC Az layer by linearly polarized light irradiation at optimal temperatures are in the order of PHMA > PBMA > PMMA which coincides with that of lowering Tg. Thus, the photoinduced motions of the LC Az blocks are coupled with the dynamic nature of the underlying amorphous chain anchored to the substrate. The structural information obtained by the grazing incidence angle X-ray diffraction measurements further shows the important role of the amorphous polymer layer in the ordering and photoalignment of LC mesogens on the top.

1. INTRODUCTION The decoration of substrate surfaces with polymer brushes is a versatile approach to modulate surface properties and exert various functions.1−8 Liquid crystalline (LC) polymer brushes in which one end is tethered to substrate surfaces have also been fascinating targets from both theoretical9,10 and practical viewpoints.11−19 If the grafting density of the tethered chains is high enough, no overlapping has been observed among the neighboring chains resulting in the chain conformation stretched away from the surface to provide a brush state.20 Rühe’s group first reported side chain LC polymer brushes via grafting-f rom method adopting surface-initiated (SI) free radical polymerization to prepare high-tilt alignment layer for low molecular mass LCs.11,12 In their system, rubbing the substrate surface with a velvet cloth before attaching initiator for the grafting-from procedure exhibited a homogeneous alignment.12 Later, Hamelinck and Huck synthesized side chain LC polymer brushes by SI atom transfer radical polymerization (SI-ATRP) that provides a homogeneous alignment for a nematic LC.13 Our group has been investigating photoresponsive brushes consisting of azobenzene (Az) side chain LC polymer synthesized either by SI-ATRP14−17 or SI ringopening metathesis polymerization.18 At sufficient high densities of grafting polymers, polymer backbone stretches away from the surface, and Az side chain mesogenic groups and smectic layers are oriented almost parallel and vertical to the © XXXX American Chemical Society

substrate surface, respectively. This characteristic alignment leads to efficient light absorption of the Az unit, and thus a large in-plane alignment is attained when the linearly polarized light (LPL) is irradiated with normal incidence.14−18 We have recently proposed a strategic approach to gain inplane rotational motions of Az side mesogenic groups in diblock coplymer brushes (DCBs) synthesized by two step ATRP, in which a flexible polymer chain poly(hexyl methacrylate) (PHMA, Tg = −20 °C) is introduced between the solid substrate and Az LC layer (Scheme 1).16,17 In this approach, even larger in-plane optical anisotropy is induced after the irradiation with LPL, and the reorientation process for aligning mesogens to the orthogonal direction through LPL irradiation became more facilitated. Therefore, the flexible chain base in the rubbery state plays a significant role as a lubricant layer for molecular motions of the Az LC layer.16 With regard to the polymer design of the amorphous polymer part, further questions arise as to what extent and how the photoalignment behavior will be affected by the nature of this layer. In this context, we extended our study by systematically synthesizing homologous block copolymers exhibiting different glass transition temperatures (Tgs). The Received: July 22, 2013 Revised: September 6, 2013

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2. EXPERIMENTAL SECTION

Scheme 1. Schematic Illustration for Preparation of SurfaceTethered Diblock Copolymer Chains by Two Step SI-ATRP

2.1. Materials. Materials used for synthesis were of reagent grade and were purchased from TCI, Aldrich, and Kanto Kagaku. Copper chloride and copper bromide were washed with acetic acid and diethyl ether for several times and dried under vacuum.28,29 Tetrahydrofuran (THF) and toluene were dehydrated by distillation over sodium in the presence of benzophenone. Hexyl methacrylate (HMA), butyl methacrylate (BMA), methyl methacrylate (MMA) and diphenyl ether were purified by distillation under reduced pressure. The Az chromophore connecting with a methacrylate monomer, 4-(10methacryloydecyloxy)-4′-pentylazobenzene (5Az10MA), was synthesized according to the method previously reported.30,31 The tethered blocks of PHMA, PBMA, PMMA were estimated from the data of free polymers conducted by ATRP method. PHMA, PBMA, and PMMA free polymers were collected during the polymerization of corresponding brushes showed the thermal transitions as glass (Tg) at −20, +27, and +110 °C respectively (Supporting Information) and P5Az10MA free polymer exhibited the following thermophysical properties: glassTg53 °C−smectic A−116 °C−isotropic obtained by differential scanning calorimetry (Supporting Information, Figure S2) and polarized optical microscopic analysis.14,15 2.2. Synthesis of Surface-Attached Diblock Copolymer Chains. The overview of synthetic routes for the surface-attached block copolymers are indicated in Scheme 2. Detailed procedures are described below. 2.2.1. Immobilization of ATRP Initiator on the Substrate. A silane coupling reagent for the ATRP initiator, [11-(2-bromo-2-methyl)propionyloxy]undecyldimethyl- chlorosilane (BUS), was synthesized according to a method reported earlier.32 The BUS modified substrate (quartz) was prepared by chemical vapor adsorption (CVA) method.33−35 Quartz plates (1 cm × 1.5 cm) were washed with saturated potassium hydroxide ethanol solution, followed by washing with pure water under sonication. The static contact angle of water was 8 ± 2° (hydrophilic quartz plates). Cleaned quartz substrates were placed in a Teflon container. The container was sealed and placed in an oven maintained at 60 °C for 1 h. A glass reservoir filled with BUS liquid (0.10 vol %) in dry toluene was then placed together and maintained at 60 °C for 72 h. BUS in the vessel was evaporated and reacted with the hydroxyl groups on the sample surface. The resultant

glass transition behavior in thin polymer films is an important recent issue21,22 and is known to influence the dynamic processes of other contacting materials. Actually, surfacemediated processes such as wetting parameters,23 dynamic wetting behavior of liquid droplets,24 adhesion/friction properties,25 and metal depositions26,27 are significantly affected by the glass transition of the polymer surface. It is thus anticipated that the difference in Tg of the underlying polymer will substantially affect the photoalignment behavior of the side chain Az LC polymer in the upper layer. In this work, we introduce poly(butyl methacrylate) (PBMA) and poly(methyl methacrylate) (PMMA) in addition to the PHMA flexible chain in diblock copolymer brush.16,17 Tg values of PHMA, PBMA, and PMMA in the bulk states are −20, +27, and +110 °C, respectively (Supporting Information, Figure S2). These polymer chains can be regarded as very flexible (rubbery), moderately flexible (glass transition region or rubbery), and rigid (glassy), respectively, in the temperature range under study.

Scheme 2. Synthetic Procedures To Prepare Diblock Copolymer Brush Containing an Amorphous Polymer Block (1st Step) and a LC Az-Containing Polymer Block (2nd step) Starting from a Surface Initiator Layer of BUS-SAM

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substrates were dipped into dehydrated toluene and acetone one after another successively and washed in an ultrasonic bath for 5 min. Finally, the samples were dried under vacuum and stored in a nitrogen atmosphere. 2.2.2. Stepwise Surface-Initiated ATRP. Typical procedures for the polymer synthesis are as follows: 6.63 mg (0.0672 mmol) of Cu(I)Cl, 0.75 mdm−3 (6.72 mmol) of BMA monomer, 1 μmol dm−3(0.00672 mmol) of ethyl-2-bromoisobutyrate (EBB) as a free initiator, and 54.9 mg (0.134 mmol) 4,4′-dionyl-2,2′dipyridyl (bpy9) as a ligand were dissolved in 4.00 mdm−3 of distilled diphenylether (DPE) and purged with nitrogen gas. The mixture was then degassed by freeze−pump− thaw cycle three times. The initiator-modified quartz or silicon substrate was added in to polymerization solution of BMA and sealed in a glovebox (first step). The flask was placed in an oil bath at 70 °C for 24 h. After the polymerization, quartz substrate was washed with chloroform/THF several times to remove unreacted monomers and free polymers and then dried under vacuum at room temperature. A subsequent ATRP (second step) was carried out following the same procedure for synthesizing diblock copolymer brush using 660 mg (1.34 mmol) of the 5Az10MA monomer in 1.34 mdm−3 DPE with identical amount of initiator, ligand and catalyst. Similar procedure was carried out for methyl methacrylate (MMA) monomer with 1.43 mol dm−3 (6.72 mmol) MMA monomer in another run. 2.2.3. Detaching Procedure of Diblock Brush. An ammonium bifluoride (NH4HF2) solution (Stella Chemical) was used for the detachment of diblock polymer chains from quartz substrates. An ammonium bifluoride solution was poured onto a polymer that is tethered from quartz plate for 10−15 min with the help of a Pasteur pipet. In this procedure, tethered polymer chains have been detached and dissolved into the solution. Then, this solution was transferred to a small sample tube, add a small amount of chloroform and pure water into the tube and shaken vigorously for several times. The bottom part of the solution after still standing was an organic part dissolving the polymers, while the upper part was an aqueous solution containing ions. After removal of the aqueous part carefully with a Pasteur pipet and a little amount of water was added and the procedure is repeated twice. At the end, polymer containing organic solution was placed and dried under vacuum at 40 °C for several hours. When the solvent was completely removed, the sample tube contained only untethered polymers. The obtained sample was subjected to gel permeation chromatography (GPC) measurements using THF as an eluent. 2.3. Measurements. 1H NMR spectra were recorded on a JEOL 400GXS instrument spectrometer using tetramethylsilane as the internal standard for deuterated chloroform. Melting points (uncorrected) were measured with a Yanaco MP-S3 melting point apparatus. Molecular weight and polydispersity index (PDI) of the free polymers were evaluated by GPC using a Shodex liquid chromatography system calibrated with standard poly(methyl methacrylate)s for PHMA, PBMA, PMMA, and standard polystyrenes for P5Az10MA. THF was used as an eluent at a flow rate of 1.0 mL min−1. UV−visible absorption spectra for the diblock grafted films at room temperature were recorded on an Agilent Technology 8453 spectrometer. To obtain polarized spectra, a polarizer was mounted in a rotating holder in front of the sample. Layer thickness was measured by X-ray reflectivity (XRR) with a Rigaku ATX-G operating with Cu Kα radiation (0.154 nm) and a Nanopics 2100 (Seiko Instruments). Grazing angle incidence X-ray diffraction (GI-XRD) measurements were performed with a Rigaku Nano Viewer operating with Cu Kα radiation (0.154 nm). The scattering profile was recorded on an imaging plate. Differential scanning calorimetry (DSC) (TA DSC Q200) has been used as a measuring tool of calorimeter. DSC scans were performed within the temperature range −50 to +200 °C at a heating rate of 2 °C min−1 under nitrogen. About 5.0 mg of mass was used for DSC measurements for all samples. An empty aluminum pan was used as a reference. In the Supporting Information, more detailed explanations are provided. 2.4. Photoirradiation. The UV or visible light irradiation to the samples was performed using an Hg−Xe lamp (SAN-EI Electric UVF203S) equipped with appropriate combinations of color filters (Toshiba glass: UV-35/UV-D36A and V-44/Y-43 for 365 and 436

nm light, respectively). LPL was obtained by passing through a polarizer. Light intensity was measured by an optical power meter (Advantest TQ8210), and all LPL irradiations were performed at 1 mW cm−2. Temperature of the sample plate was controlled with a Mettler FP82HT or a Heidolph MR Hei-Tec hot stage.

3. RESULTS AND DISCUSSION 3.1. Synthesis of Surface-Grafted Diblock Copolymer Films. For attaching the initiator on a substrate, formation of a self-assembled monolayer (SAM) is a versatile tool for synthesizing high density grafted films. As the surface morphology and roughness may influence the property and orientation of the LC brush polymer film, the CVA method was employed using a monofunctional organosilane compound (BUS) as the initiator to prepare a molecularly smooth monolayered film.34,35 The surface-initiated polymerization was carried out in the presence of free initiator (EBB) in the polymerization solution. From the kinetic aspect of ATRP, it is well-known that free polymerization in solution and the grafting polymerization at substrate are occurred simultaneously.36 The coincidence of molecular weight of the free polymer and grafted one was actually confirmed as follows. One of the surface grafted chains, PMMA-b-P5Az10MA, was separately prepared and detached from quartz substrate and observed GPC data was compared with the corresponding free PMMA (first step) and P5Az10MA (second step) homopolymers. Mn of PMMA and P5Az10MA free polymers grown from the free initiator in the solutions were 9.6 × 104 and 3.0 × 104 with PDI of 1.65 and 1.16, respectively (Figure S1). From this knowledge, Mn of the surface-attached diblock copolymer chain of the brush should be the sum to give approximately 1.3 × 105. The surfacetethered diblock copolymer obtained in the above procedures was detached from the substrate (see Experimental Section) and subjected to the GPC measurement. Mn of the detached polymer was found to be 1.4 × 105 with DPI of 1.22 (Figure S1, Supporting Information). Thus, the molecular weight and PDI of surface initiated polymer agreed well upon with those for free polymers grown in the polymerization solution. Thus, the evaluation of molecular weight data with free polymer should precisely reflect those of the surface-attached polymers. We were able to control the chain lengths of PHMA, PBMA, PMMA, and P5Az10MA blocks by changing the feeding ratio and concentration of monomers. We adopted the polymer chains having comparable chain length for the photoalignment procedures, which are summarized in Table 1. In this table, data of number-average molecular weight obtained by 1H NMR and GPC measurements, polydispersity indexes (PDI, Mw/Mn) by GPC, and degree of polymerization (DP) for the three diblock copolymers are given. All Mw/Mn values ranged from 1.0 to 1.3 except for PBMA (PDI ∼ 1.50), indicating that reasonably narrow dispersions of polymer masses were obtained by the ATRP method. Hereafter, the diblock copolymer brushes, PHMA-b-P5Az10MA, PBMA-b-P5Az10MA, and PMMA-bP5Az10MA, used in this work are dubbed as Brush-h, Brushb, and Brush-m, respectively, for simplicity. 3.2. Structural Characterizations of Surface Brush Films. 3.2.1. Film Thickness. Figure 1 shows the XRR curves for all the brushes. As shown, the simulated plots are satisfactorily fitted to the experimental fringe profiles. From the simulated fringes of Figure 1, film density, thickness, and roughness were estimated. The results for Brush-h, Brush-b, and Brush-m are summarized in Table 2. It is already confirmed C

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Table 1. Synthesis Parameters for Three Different Grafted Diblock Copolymer Brushes Derived from Free Polymer Analysisa

Table 2. Simulated Parameters Obtained from XRR Experiments of BUS-SAM and Three Diblock Copolymer Brushesa

a Key: (1) Calculated from 1H NMR data. (2) Calculated from Mn of GPC. (3) Data taken from ref 16.

a

Key: (1) Data taken from ref 16. (2) Corresponding to the total (first + second) brush film. (3) Assuming the all-trans-zigzag conformation.

Table 3. Summary of Layer Thickness and Number of Repeating Units of Three Brushes and UV−Visible Absorption Spectral Dataa

a

Key: (1) Data taken from ref 16.

Brush-b (b), and Brush-m (c) on quartz substrates taken with normal incidence for the as-prepared states (solid lines) and after annealing at 130 °C for 5 min (dashed lines). The LC Az mesogenic groups showed major two peaks around 240 and 340 nm, which can be assigned to the ϕ−ϕ* transition of the aromatic ring and π−π* long-axis transitions of the Az unit, respectively.37 A slight hypsochromic peak shifts were observed for the π−π* absorption band in all the brushes compared with that in solution (352 nm, see Table 3), showing partial formation of H-aggregates. A careful observation of the band peak of the π−π* band (λmax) reveals that the extent of hypsochromic shift became larger in the order of Brush-h (336 nm), Brush-b (337 nm), and Brush-m (339 nm). This fact suggests that the more flexible amorphous chain promotes the H-aggregates. The relative absorption intensity for the peak of π−π* band (340 nm) to that of ϕ−ϕ* one (240 nm) can be a measure of Az mesogen orientations with respect to the substrate surface, because the π−π* transition is direction dependent while the ϕ−ϕ* transition is insensitive to the chromophore orientation.37 For the P5Az10MA homopolymer brush reported earlier,14,15 the Aϕ−ϕ*/Aπ−π* value is almost unchanged by the annealing above the isotropization temperature, which means that there is no marked difference in the orientation of Az mesogens. In contrast, for the diblock copolymer brushes under

Figure 1. XRR profiles of PHMA layer (a, first step of Brush-h) and PHMA-b-P5Az10MA (b, Brush-h), PBMA layer (c, first step of Brushb) PBMA-b-P5Az10MA (d, Brush-b), PMMA layer (e, first step of Brush-m), and PMMA-b-P5Az10MA (f, Brush-m) on quartz substrates. Solid curves are experimental data and different symbol plots are drawn based on the simulations for all respective fringes.

that the thickness estimated by XRR measurements and that obtained by a direct surface height profile of AFM coincides well with each other.16 Data for Brush-h are taken from reference16. The thicknesses of the first step brushes of PHMA, PBMA, and PMMA were 51, 56, and 35 nm, respectively, and those of the second step of P5Az10MA, were 7.4, 12.3, and 10 nm. In this table, the length values of the fully stretched chain (all-trans-zigzag conformation) calculated based on the DP data are also included. For all polymer chains, the observed thicknesses were nearly half of the fully stretched states. Thus, the obtained chains are reasonably recognized as the brush states.8 In the second column of Table 3, thickness data of each constituting block and the number of their repeating unit are summarized. 3.2.2. Packing State and Orientation of Az Mesogens. Figure 2 shows UV−visible absorption spectra of Brush-h (a), D

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Figure 3. 2D GI-XRD imaging plate patterns of the annealed Brush-h (a), Brush-b (b), and Brush-m (c) observed at room temperature. 1D profiles in the in-plane direction extracted from the 2D XRD patterns are shown in the insets. In the inset of part c, enlarged plots are also shown.

Figure 2. UV−visible absorption spectra of Brush-h (a), Brush-b (b), and Brush-m (c) on quartz plates. λmax values are shown in the figures. Solid and dashed lines indicate spectra before and after annealing at 130 °C for 5 min, respectively.

structures with a layer spacing of 3.4 nm. The diffraction peaks of Brush-h and Brush-b were clearly observed, while that of Brush-m was very weak. These findings indicate that Az mesogens are in a more regular layered arrangement for Brushh and Brush-b compared to Brush-m. The layer spacings (d = 3.4 nm) in all brushes coincided well with the layer spacing of free polymer (d = 3.3 nm)14,15 or homopolymer grafted films (d = 3.6 nm)15,16 of P5Az10MA. 3.3. Photoalignment by LPL Irradiation. 3.3.1. Photoinduced Dichroism. Figure 4 shows polarized UV−visible absorption spectra after a photoalignment procedure by the irradiation with LPL at 436 nm for Brush-h (a), Brush-b (b), and Brush-m (c). Before irradiation, no in-plane dichroic nature was recognized at the macroscopic level of spectroscopic measurements (figure not shown). In a similar way as the P5Az10MA homopolymer brush,14,15 the mesogens of the diblock copolymer brushes formed a polydomain structure before the photoorientation.16,17 Upon irradiation with 436 nm LPL at 500 mJ cm−2 and 70−80 °C, the absorbance of the π−π* transition band observed with the polarized light

investigation, Brush-h, Brush-b, and Brush-m, Aϕ−ϕ*/Aπ−π* values were reduced as 0.77 → 0.64, 0.60 → 0.51, and 0.80 → 0.75, respectively (Table 3). Besides, the extent of the corresponding spectral shift for these brushes changed in this order as 6, 3, and 1 nm, respectively. Thus, for all the brushes, the existence of the amorphous base layers is effective for the Az mesogens to relax the orientation to more parallel orientations with respect to the substrate plane. Probably, wiggled motions of amorphous blocks in the annealing process promote the planar orientation. 3.2.3. Smectic Layer Structure and Its Orientation. GIXRD measurements were performed for all the three diblock copolymer brushes. The brush films were annealed at 130 °C for 5 min followed by a rapid cooling, and then the samples were subjected to in situ GI-XRD measurements at room temperature (Figure 3). All brush films exhibited diffraction peaks only in the in-plane direction commonly at 2θ = 2.6°, which are attributed to vertically oriented smectic layer E

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plane order parameter was obtained for the highly (Brush-h) and moderately (Brush-b) flexible amorphous chains, but in contrast, for the glassy rigid chain (Brush-m), the induction of photoinduced anisotropy became substantially small. Thus, the level of induced optical anisotropy decreased with increasing Tg of the amorphous base chain. The level of in-plane anisotropy became larger for brushes for thicker P5Az10MA layers (Supporting Information, Figure S3 and Table S1). The thickness of P5AzMA layer of Brush-b is larger (12.3 nm) than that of Brush-h (7.4 nm), nevertheless, the degree of anisotropy is smaller for Brush-b. This fact implies that the difference in S will become more pronounced when brushes with exactly the same P5AzMA layer thickness are compared. With regard to the chain length of the base amorphous polymer, the elongation of length resulted in smaller photoinduced in-plane anisotropy (Supporting Information, Figure S3 and Table S1). Increased deviations in the chain length for longer amorphous brushes seem to increase involvement of the Az mesogens oriented in the out-of-plane direction, which can lead to reductions of the in-plane order of P5Az10MA layer. 3.3.2. Temperature Dependence. The induction of optical anisotropy for the three brushes were explored in more details changing temperature ranging from 20 to 110 °C, and the results are displayed in Figure 5. S values were obtained at a

Figure 5. Orientational order parameters (S) obtained after 436 nm LPL irradiation at 500 mJ cm−2 and various temperatures for Brush-h (triangles, solid line), Brush-b (circles, dashed line), and Brush-c (squares, dotted line) brushes. Figure 4. Polarized UV−visible absorption spectra of Brush-h (a), Brush-b (b), and Brush-m (c) observed after LPL irradiation at 500 mJ cm−2 and 70 °C for Brush-h and 80 °C for Brush-b and for Brush-m. For all cases, solid and dotted lines denote spectra at taken with probing beams perpendicular and parallel with the actinic LPL directions, respectively.

fixed exposure dose of LPL at 500 mJ cm−2. Starting from 20 °C, S value enhanced with increased temperature and reached to a maximum value at 70 °C for Brush-h and at 80 °C for Brush-b and Brush-m. Beyond this temperature, S sharply decreased. The optimum conditions correspond to temperatures higher than Tg of P5Az10MA (=53 °C) by ca. 20−30 °C. The rubbery or glassy state of the amorphous polymer layer crucially influenced the level of photoinduced optical anisotropy at all temperatures examined. S value of Brush-m with the glassy PMMA base layer was hardly enhanced, while those of Brush-h and Brush-b with rubbery base layers were readily increased to high levels. These facts indicate that the LC ordering of P5Az10MA layer is promoted when sufficient molecular mobility of the underlying layer is provided, which is also related with the contrasting smectic layer ordering behavior as revealed from the X-ray data in Figure 3. The relatively sharp decrease in S observed beyond 70 or 80 °C should be the consequence of increased thermal molecular

orthogonal to the electric vector (E) of the actinic LPL (A⊥) became significantly larger than that in the parallel direction (A||) for all three brushes. The level of photoinduced in-plane optical anisotropy of films was evaluated by the order parameter (S) of the Az mesogens by the equation S = (A⊥- A||)/(A⊥+2A||), where A⊥ and A|| denote absorbance at the λmax (around 340 nm) obtained by measurements using polarized light with E perpendicular and parallel to that of actinic polarized light, respectively. S values for the representing Brush-h, Brush-b, and Brush-m, S values reached to 0.53, 0.46 and 0.13, respectively, at corresponding optimum alignment conditions. The large inF

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motions at higher temperatures. Actually, the in-plane anisotropy was almost diminished around the isotropidization temperature of P5Az10MA (108 °C). Therefore, the optimum temperature for photoalignment will be determined by a balance between the gained mobility required for LC ordering and a thermal disordering factor at higher temperatures. When S values for Brush-h and Brush-b are compared at 80−100 °C, the dichroic property is retained to higher levels for Brush-b. This may be ascribed to a lower degree of segmental chain motion of PBMA (Tg = 27 °C) than that of PHMA (Tg = −20 °C). In case of Brush-b, we unexpectedly observed some fluctuations in S around a temperature range from 30 and 60 °C. These temperatures are slightly above Tg of PBMA and around Tg of P5Az10MA. The actual origins for such data fluctuations are unclear, but we obtained good reproducibility for this phenomenon. We presume that some interplay between the two polymer chains with similar Tg causes such effect. This can be related with X-ray data as will be described later. All arguments hitherto described are based on Tg obtained for “bulk” polymers. The glass transition temperature of these polymers may be shifted to a slightly higher temperature region for a high density polymer brush38 and also because of the mesoscopically constrained environment.21 Within the temperature range examined for the photoalignment, the shift in Tg of PHMA (Brush-h) and PMMA (Brush-m), if any, will not affect the data because Tgs are out of the exploration range. However, the glass transition region of PBMA (Brush-b) is within this region. If Tg of PBMA is shifted to a higher temperature, the fluctuating region would correspond to the glass transition region of PBMA. Of course, this interpretation requires proper justifications in the future. To our surprise, the orientatinal order parameter for photoaligned Brush-m at the optimized temperature (80 °C) is unexpectedly small (S = 0.13), which is even smaler than that of a P5Az10MA homopolymer brush of comparable thickness (S = 0.35).16 The inorganic hard surface of quartz is anticipated to constrain the molecular motion to a larger extent, however, the resulting motions of P5Az10MA are actually more restricted on the PMMA layer. One possible explanation could be the difference in adhesion strength between the two layers. If the P5Az10MA layer is adhered to the PMMA layer to a more extent than to the quartz surface, the rotational motions of Az mesogens are expected to be more impeded. The photoinduction of in-plane anisotropy could be repeated many times (at least 15 times) at the optimum temperatures without deteriorations. Once the photoinduced anisotropy is attained at elevated temperatures, the three brushes retained the in-plane anisotropy even for a month at room temperature. This fact suggests that the orientations of P5Az10MA are firmly retained at temperatures below Tg of this polymer regardless of the segmental mobility of the underlying amorphous chains. 3.3.3. XRD Measurements at Various Temperatures. GIXRD measurements were conducted for the photoaligned Brush-b by changing temperature from 30 to 110 °C (Figure 6). At temperatures above 70 °C and below the isotropization temperature, the diffraction peak at 2θ = 2.6° was clearly observed, giving a layer spacing of 3.4 nm. However, the appearance of the peak fluctuated around 30 to 60 °C. This fact observed would be consistent with the optical anisotropy data shown in Figure 5, exhibiting some fluctuations in S as mentioned above. From both spectroscopic and X-ray data, it is

Figure 6. 1D in-plane diffraction profiles in the GI-XRD measurements for Brush-b taken on the heating processes.

concluded that not only the molecular orientation but also the layer structure formation is fluctuated in the 30−60 °C region. The above X-ray data can be compared with those obtained for Brush-h (see Figure 8 in ref 16 with which the smectic layer structure is stably maintained at all the temperatures below the isotropidization point of 5Az10MA. In case of Brush-h, Tg of PHMA (= −20 °C) is positioned at the far lower temperature from the experimental temperatures. It is likely that the intrinsic assembly of the Az mesogenic groups readily occur on a sufficiently soft PHMA layer (Brush-h), whereas it is somewhat hindered and fluctuated on a PBMA layer with Tg comparable to the measurement temperatures (Brush-b).

4. CONCLUSION Surface-grafted diblock copolymer ultrathin films of photoresponsive Az LC polymer (5Az10MA) possessing sufficiently flexible (PHMA), moderately flexible (PBMA) and glassy rigid (PMMA) amorphous chains as the base layer have been successfully synthesized by the two step SI-ATRP method. In this architecture, Az side chain mesogenic groups orient nearly parallel to the substrate surface. The in-plane photoalignment behavior of Az mesogenic groups and smectic layers are largely influenced by the extent of flexibility of the underlying amorphous chains. The role and effect of the amorphous chain layer can be summarized in three aspects. (i) With lowering Tg of the underlying amorphous polymer layer, the resulting LC photoalignment ordering in the 5Az10MA layer becomes higher. This situation is schematically summarized in Figure 7. Here, the state of rubbery and glassy leads to the substantial difference. (ii) With lowering Tg of the underlying amorphous polymer, the optimum temperature range for inducing in-plane anisotropy becomes slightly lowered due to facilitated ordering motions of Az mesogens. (iii) When the photoalignment procedure is investigated near Tg of underlying polymer (PBMA, Brush-b), the LC layer ordering and alignment behavior become unstable and fluctuated. G

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(4) Peng, S.; Bhushan, B. RSC Adv. 2012, 2, 8557−8578. (5) Azzaroni, O. J. Polym. Sci.: Part A Polym. Chem. 2012, 50, 3225− 3258. (6) O’Driscoll, B. M. D.; Griffiths, G. H.; Matsen, M. W.; Hamley, I. W. Macromolecules 2011, 44, 8527−8536. (7) Kumar, S.; Dory, Y. L.; Lepage, M.; Zhao, Y. Macromolecules 2011, 44, 7385−7393. (8) Tsujii, Y.; Ohno, K.; Yamamoto, S. Adv. Polym. Sci. 2006, 197, 1− 45. (9) Helperin, A.; Williams, D. R. M. Europhys. Lett. 1993, 21, 575− 580. (10) Helperin, A.; Williams, D. R. M. J. Phys. Condens. Matter 1994, 6, A297−A300. (11) Peng, B.; Johannsmann, D.; Rühe, J. Macromolecules 1999, 32, 6759−6766. (12) Peng, B.; Rühe, J.; Johannsmann, D. Adv. Mater. 2000, 12, 821− 824. (13) Hamelinck, P. J.; Huck, T. S. J. Mater. Chem. 2005, 15, 381− 385. (14) Uekusa, T.; Nagano, S.; Seki, T. Langmuir 2007, 23, 4642− 4245. (15) Uekusa, T.; Nagano, S.; Seki, T. Macromolecules 2009, 42, 312− 318. (16) Haque, H. A.; Nagano, S.; Seki, T. Macromolecules 2012, 45, 6095−6103. (17) Haque, H. A.; Nagano, S.; Seki, T. Mol. Cryst. Liq. Cryst. 2012, in press. (18) Haque, H. A..; Kakahi, S.; Hara, M..; Nagano, S.; Seki, T. Langmuir 2013, 29, 7571−7575. (19) Camorani, P.; Cristofolini, L.; Fontana, M. P.; Angiolini, L.; Giorgini, L.; Paris, F. Mol. Cryst. Liq. Cryst. 2009, 502, 56−64. (20) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1991, 100, 31−71. (21) Jones, R. A. L. Curr. Opin. Colloid Interface Sci. 1999, 4, 153− 158. (22) Forrest, J. A.; Dalnoki-Veress, K. Adv. Colloid Interface Sci. 2001, 94, 167−195. (23) Correia, N. T.; Moura, R.; Joaquim, J.; Adao, M. H. C.V.; Saramago, B. J. V. Mol. Cryst. Liq. Cryst. 1997, 300, 45−64. (24) Zuo, B.; Qian, C.; Yan, D.; Liu, Y.; Liu, W.; Fan, H.; Tian, H.; Wang, X. Macromolecules 2013, 46, 1875−1882. (25) Zeng, H.; Maeda, N.; Chen, N.; Tirrell, M.; Israelachivili, J. Macromolecules 2006, 39, 2350−2363. (26) Tsujioka, T.; Yamaguchi, K. Jpn. J. Appl. Phys. 2013, 52, 078002. (27) Tsujioka, T.; Matsui, A. Appl. Phys. Lett. 2009, 94, 013302. (28) Borner, H. G.; Beers, K.; Matyjaszewski, K.; Sheiko, S. S.; Möller, M. Macromolecules 2001, 34, 4375−4383. (29) Keller, R. N.; Wycoff, W. M. Inorg. Synth. 1946, 2, 1−4. (30) Li, W.; Nagano, S.; Seki, T. New J. Chem. 2009, 33, 1343−1348. (31) Kadota, S.; Aoki, K.; Nagano, S.; Seki, T. Colloids Surf., A 2006, 284−285, 535−541. (32) Ohno, K.; Morinaga, T.; Koh, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2005, 38, 2137−2142. (33) Sugimura, H.; Hozumi, A.; Kameyama, T.; Takai, O. Surf. Interface Anal. 2002, 34, 550−554. (34) Hayashi, K.; Saito, N.; Sugimura, H.; Takai, O.; Nagagiri, N. Langmuir 2002, 18, 7469−74872. (35) Hozumi, A.; Yokogawa, Y.; Kameyama, Y.; Sugimura, H.; Hayashi, K.; Shinohara, H.; Takai, O. J. Vac. Sci. Technol. A 2001, 19, 1812−1816. (36) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, S.; Fukuda, T. Macromolecules 1998, 31, 5934−5936. (37) The ϕ−ϕ* transition around 250 nm corresponds to the absorption of aromatic phenyl groups and does not show polarization dependence. This transition is not influenced by both aggregation and photoisomerization state. For references, see: Fabian, J.; Hartmann, H. Light Absorption of Organic Colarants; Springer-Verlag: Berlin, 1980; p 32−79. Sapich, B.; Vix, A. B. E.; Rabe, J. P.; Stumpe, J.; Wilbert, G.; Zentel, R. Thin Solid Films 2006, 514, 165−173.

Figure 7. Schematic comparative illustration of LPL induced optical anisotropy between diblock copolymer brushes having different amorphous chains.

As shown in this work, the molecular motions and LC ordering are strongly coupled with the physical state (chain flexibility) of the connecting amorphous polymer. Strong cooperative behavior between LC polymer block and amorphous polymers is also observed in the photo(re)alignment process of microphase separation patterns in relevant block copolymer films.39,40 In the characterizations and applications of block copolymers and polymer blends, understandings of the interface between two polymer phases are of particular importance. We believe that the strategy in this work can provide valuable plane model systems to understand interplays between two polymer phases of more complicated morphologies. Another important aspect on the polymer architecture can be pointed out as follows. In the molecular design of side chain LC polymer, the existence of a flexible spacer between the side chain mesogenic group and the polymer backbone is of particular importance to decouple the motions of the two parts.41 Our approach proposes a guideline of “mesoscopic” spacer which decouples the motions of the LC layer from the solid substrate. This work has shown that the physical state of the connecting mesoscopic spacer substantially affects the LC assembly process and orientation order.

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ASSOCIATED CONTENT

S Supporting Information *

Materials and synthesis, measurements, and preparation of brushes. 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: (T.S.) [email protected]. *E-mail: (S.N.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. T. Hikage for his sincere support in conducting high intensity X-ray analysis. The research was supported by the Grant-in-Aid for Basic Research S (23225003) to T.S., B (25286025) to S.N., and Grant-in-Aid for Young Scientists B (25810117) to M.H. from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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REFERENCES

(1) Advincula, R. C.; Brittain, W. J.; Caster, K. C.; Rühe, J., Eds.; Polymer Brushes; Wiley-VCH: New York, 2004. (2) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677−710. (3) Milner, S. T. Science 1991, 251, 905−914. H

dx.doi.org/10.1021/ma401536r | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

(38) Yamamoto, S.; Tsujii, Y.; Fukuda, T. Macromolecules 2002, 35, 6077−6079. (39) Morikawa, Y.; Kondo, T.; Nagano, S.; Seki, T. Chem. Mater. 2007, 19, 1540−1542. (40) Nagano, S.; Koizuka, Y.; Murase, T.; Sano, M.; Shinohara, Y.; Amemiya, Y.; Seki, T. Angew. Chem., Int. Ed. 2012, 51, 5884−5888. (41) Finkelmann, H.; Ringsdorf, H.; Wendorff, J. H. Macromol. Chem. 1978, 179, 273−276.

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dx.doi.org/10.1021/ma401536r | Macromolecules XXXX, XXX, XXX−XXX