Lubricant Effect of Flexible Chain in the ... - ACS Publications

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Lubricant Effect of Flexible Chain in the Photoinduced Motions of Surface-Grafted Liquid Crystalline Azobenzene Polymer Brush Hafiz Ashraful Haque,† Shusaku Nagano,‡,§ and Takahiro Seki*,† †

Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan ‡ Venture Business Laboratory, Nagoya University, Nagoya 464-8603, Japan § Japan Science and Technology Agency-PRESTO, Tokyo 102-0076, Japan ABSTRACT: Poly(hexyl methacrylate)-block-poly[4-(10methacryloxydecyloxy)-4′-pentylazobenzene] (PHMA-bP5Az10MA) diblock copolymer brush was stepwisely synthesized on an initiator-immobilized quartz surface by atom transfer radical polymerization. In this chain design, a flexible PHMA chain was connected between a liquid crystalline (LC) azobenzene (Az) block and a solid substrate. The orientation and structural features of the smectic layer spacing of the azobenzene (Az) block at the outer surface were essentially the same as those of P5Az10MA homopolymer brush. Upon irradiation of 436 nm linearly polarized light (LPL) at a temperature of liquid crystalline (LC) state of the Az polymer, the block copolymer brush exhibited a significant improvement in the induction of in-plane optical anisotropy compared to a corresponding homopolymer brush as revealed by UV−vis spectroscopy and grazing angle incidence X-ray diffraction measurements. The temperature dependence of the optical and structural anisotropy and photoreorientation behavior unveiled a significant lubricant role of the flexible chain block for the improvement of photoresponsive motions. This work proposes an idea to decouple the photoinduced motions of the upper photoresponsive block from the restriction by the substrate through introduction of a “buffer spacer” chain.

1. INTRODUCTION Polymer brushes consisting of densely and covalently attached polymer chains to a solid surface are promising for wide range of applications,1 which inter alia include modifications of adhesion,2 wettability,3 switching,4 stabilization, rheology, tribology,5−7 etc. In essence, the tethered polymer chains onto a substrate exhibit improved mechanical robustness and prevention of dewetting. The brush architecture is confined to a state of a 2D restricted geometry in the molecular assembly compared with a conventional free polymer film system. When the density of the tethering is sufficiently high, the chains must stretch away from the surface to adapt a brush state.1,8 Introduction of liquid crystal (LC) materials in to the restricted architecture in brush systems has also received an upsurge of interest for their potential applications.9−13 Surface-initiated atom transfer radical polymerization (ATRP) has been widely employed for the formation of polymer brushes since ATRP is tolerable for various functionalized monomers,14−21 and the living character of the ATRP process yields polymers with low polydispersities. On the other hand, numerous attempts have so far been made to exploit free block copolymer materials for fabricating nanostructures,22−26 microphase separation structures,27−29 nanowire arrays,30 nonotubes,31 nanoporous materials,32 etc. Block copolymer brushes, consisting of two or more different © 2012 American Chemical Society

polymer blocks into the confined state, are getting of special interest to exert more and specialized functionalities.33,34 The block copolymer brush architecture combining different conformations in one polymer chain will exhibit diverse functionalities in response to ambient solvent and/or temperature.35−40 In this context, the influence of surface polarity, mechanical actuation, and wettability of brushes by external stimuli has been reported.41−43 Recently, surface-grafted polymer chains have been prepared carrying photoresponsive mesogens, which exhibited surprisingly different properties from those of spin-casted films.9,10,44 We reported that the unique orientation of azobenzene (Az) groups within the thin polymer film is obtained in the poly[4(10-methacryloxydecyloxy)-4′-pentylazobenzene] (P5Az10MA) homopolymer brush. The Az mesogenic groups were oriented preferentially parallel to the substrate to show the effective absorption of light, whereas in the case of conventional simple spin-cast films, they were oriented mostly perpendicularly with poor light absorptivity.9 Later on we also show that highly ordered in-plane optical anisotropy is attainable in the Received: April 26, 2012 Revised: June 22, 2012 Published: July 25, 2012 6095

dx.doi.org/10.1021/ma300843x | Macromolecules 2012, 45, 6095−6103

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Scheme 1. Synthetic Procedures To Prepare Diblock Copolymer Brush Containing PHMA Block (First Step) and Liquid Crystalline Az-Containing Block (Second Step) Starting from a Surface Initiator Layer of BUS-SAM

P5Az10MA grafted film, and monodomain structure can be readily realized.10 In the light of fascinating dynamic nature and functions of BCP brushes as mentioned above, we newly constituted a project to explore a framework of diblock copolymer brush containing an identical Az-containing LC polymer chain. In the present approach, an amorphous chain with a low glass transition temperature (Tg) of poly(hexyl methacrylate) (PHMA, T g = −20 °C) is introduced between the P5Az10MA chain and the substrate surface. We will compare the structure and dynamic motional properties among the brushes of the P5Az10MA homopolymer and diblock copolymer with a connecting PHMA chain and elucidate the role of this flexible chain in the dynamic photoorientation and reorientation processes. The preparation strategy of the diblock copolymer brush is shown in Scheme 1. To our knowledge, this is the first attempt to elucidate structural details and properties of BCP brush composed of a coiled block and a LC block.

measured by ESCA-3300 (Shimadzu Corp.) to confirm the formation of the initiator monolayer on the substrate surface. All binding energies were referenced to Si 2p at 99.34 eV. Atomic force microscopy (AFM) was carried out on a SPA400/SPI 3800N system (Seiko Instruments) for the observation of surface morphologies of the initiator modified substrate and roughness. 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. 2.3. Immobilization of ATRP Initiator on the Substrate. A silane coupling reagent for the ATRP initiator, [11-(2-bromo-2methyl)propionyloxy]undecyldimethylchlorosilane (BUS), was synthesized according to method reported earlier.49 The BUS modified substrate (quartz) was prepared by the chemical vapor adsorption (CVA) method.50−52 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). Quartz substrates were finally subjected to exposure to UV ozone cleaning for 45 min (Nippon Laser and Electronics). 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 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. 2.4. Stepwise Surface-Initiated ATRP. Typical procedures are as follows: 6.63 mg (0.0672 mmol) of Cu(I)Cl, 1.28 mdm−3 (6.72 mmol) of HMA monomer, 1 μdm−3 (0.006 72 mmol) of ethyl 2bromoisobutyrate (EBB) as a free initiator, and 54.9 mg (0.134 mmol) of 4,4′-dionyl-2,2′-dipyridyl (bpy9) as a ligand were dissolved in 4.00 mdm−3 of distilled diphenyl ether (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 HMA and sealed in a glovebox (first step). The flask was placed in an oil bath at 70 °C for 24 h. After the polymerization, the substrate was washed with chloroform/THF several times to remove unreacted monomers and the free polymer 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. PHMA layers have been grown at the same condition (feeding ratio, monomer concentration) in both 1 and 2 brushes. In case of P5Az10MA layers at the second step, the feeding ratio is also the same, but the monomer concentration is varied

2. EXPERIMENTAL SECTION 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.45,46 Tetrahydrofuran (THF) and toluene were dehydrated by distillation over sodium in the presence of benzophenone. Hexyl methacrylate (HMA) 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.47,48 The tethered block of PHMA (Mn = 8.00 × 104, Mw/Mn = 1.23) and surface block of P5Az10MA (Mn = 2.94 × 104, Mw/Mn = 1.12) were estimated from the data of free polymers conducted by the ATRP method. PHMA free polymer showed the thermal transitions as glass (Tg) at −20 °C (data not shown), and P5Az10MA free polymer exhibited the thermophysical properties Tg−51 °C smectic A−108 °C isotropic obtained by differential scanning calorimetry and polarized optical microscopic analysis.9 2.2. Measurements. 1H NMR spectra were recorded on a JEOL 400GXS instrument spectrometer using tetramethylsilane as the internal standard for deuterated chloroform. The melting point (uncorrected) was measured with a Yanaco MP-S3 melting point apparatus. Molecular weight and polydispersity index (PDI) of the free polymers were evaluated by gel permeation chromatography (GPC) using a Shodex liquid chromatography system calibrated with standard poly(methyl methacrylate)s and polystyrenes for PHMA and P5Az10MA, respectively. THF was used as an eluent at a flow rate of 1.0 mL/min. X-ray photoelectron spectroscopy (XPS) was 6096


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Table 1. Synthesis Parameters for the Grafted Diblock Copolymer and Homopolymer Films Derived from Free Polymer Analysis Mn polymer 1. 1. 2. 2. 3. 4.

PHMA(1st) P5Az10MA(2nd) PHMA(1st) P5Az10MA(2nd) P5Az10MA P5Az10MA

conv (%) 87 44 95 100 100 100

NMR 1.48 4.30 1.61 9.87 1.97 4.92

× × × × × ×

105 104 105 104 105 104

GPC

Mw/Mn

deg of polymer (DP)

× × × × × ×

1.23 1.12 1.28 1.13 1.15 1.16

470 58 572 83 105 60

8.00 2.90 9.74 4.10 5.17 2.95

significantly, i.e., 0.33 mol dm−3 for brush-1 and 1.0 mol dm−3 for brush-2. On the other hand, for 3 and 4 homopolymer brushes, the feeding ratio of monomer to initiator was doubled in brush-4 (400:1) than that of the brush-3 (200:1). 2.5. Photoinduced Orientation. UV−vis 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. The UV or visible light irradiation to the samples was performed using an Hg−Xe lamp (SAN-EI Electric UVF-203S) equipped with appropriate combinations of color filters (Toshiba glass: UV-35/UV-D36A and V44/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.

104 104 104 104 104 104

chains at the second step. In the table, the degree of polymerization (DP) of the polymer chains calculated from GPC data was also shown. All Mw/Mn values ranging from 1.0 to 1.3 indicated that reasonably narrow dispersions of polymer mass were obtained by the ATRP method. 3.2. Structural Characterizations of Surface Brush Films. 3.2.1. Film Thickness. Figure 1a shows the XRR profiles of the BUS-SAM (surface initiator, I), PHMA homopolymer (brush-1, -II), and diblock copolymer (brush-1, -III) fringes of BCP brush. From the simulations of the fringes, film density, thickness, and roughness were obtained and are summarized in Table 2. For polymer brushes, the ideally (all-zigzag conformation) stretched chain length values calculated from

3. RESULTS AND DISCUSSION 3.1. Synthesis of Surface-Grafted Diblock Copolymer Films. Self-assembled monolayer (SAM), which immobilizes the initiator, is an important tool for preparing high-density grafted films. As the surface morphology and roughness would influence the property and orientation of the LC brush polymer film, the CVA method was used with a monofunctional organosilane compound of the initiator preparing a smooth monolayered film.51,52 XPS measurement also revealed the formation of the initiator layer. The bromide 3d and the carbon 1s peaks were observed in the narrow scan mode around 70 and 286 eV, respectively. The carbon 1s signals attributed to CO and C−O bonds were also observed. These results are indicative of formation of a BUS-SAM on the quartz, and the prepared SAM is thus welldefined for preparing high-density polymer brush. An AFM measurement revealed formation of molecularly smooth surface. The surface-initiated polymerization was undertaken in the presence of free initiator 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. Therefore, the value of free polymers can be a good measuring tool for molecular weight, degree of polymerization, and molecular weight distribution. The number-average molecular weights obtained by NMR and GPC measurements and polydispersity index (Mw/Mn) by GPC for two PHMA-b-P5Az10MA (brush-1 and brush-2) and two homopolymer (brush-3 and brush-4) brushes are summarized in Table 1. As shown, we were able to control the chain lengths of PHMA and P5Az10MA blocks by changing the concentration of the monomer as described in the Experimental Section. For diblock copolymer brushes, PHMA chains were grown first and then followed by P5Az10MA

Figure 1. (a) XRR profiles of BUS-SAM, PHMA homopolymer (brush-1, first step), and PHMA-b-P5Az10MA (brush-1, second step) brush, indicated as I, II, and III, respectively (dotted plots are observed data and solid curves are drawn based on the simulation for all three fringes). (b) Topographical atomic force microscopic image of PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush. The lower region corresponds to a scratched area for the thickness evaluation. 6097


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Table 2. Simulated Parameters Obtained from XRR Experiments of BUS-SAM, Diblock Copolymer (Brush-1 and -2) and Homopolymer (Brush-3 and -4) Brushes layer 0. 1. 1. 2. 2. 3. 4.

BUS-SAM PHMA(1st) P5Az10MA(2nd) PHMA(1st) P5Az10MA(2nd) P5Az10MA P5Az10MA

density/ g cm−2

thickness/ nm

roughness/ nm

calcd length from DP/nm

0.96 0.90 0.83 0.95 0.83 0.80 0.84

2.9 51 7.4 58 13 8 16

0.60 1.10 1.51 1.44 1.55 0.50 0.58

117 14 143 21 15 26

DP are also listed. For brush-1, the thickness of PHMA and P5Az10MA was evaluated as 51 and 7.4 nm, respectively. The total thickness of ca. 58 nm coincides well with the value (ca. 60 nm) obtained by the AFM image. In Figure 1b, the film was partly scratched and the height difference was evaluated as for film thickness. For all polymer chains, the observed thicknesses were almost half of the ideally stretched states. Thus, the obtained chains are reasonably recognized as the brush states. According to the results, brush-1 and brush-2 provided BCP brushes possessing a common thickness of PHMA chains [51 (brush-1) and 58 nm (brush-2)] and differed in thicknesses of P5Az10MA layers [7.4 nm (brush-1) and 13 (brush-2)]. The P5Az10MA part of brush-2 was ca. 2-fold thicker than that of brush-1. The homopolymer brushes of P5Az10MA possessed the thicknesses of 8 and 16 nm for brush-3 and brush-4, respectively. In regard to the thicknesses of P5Az10MA part, brush-3 and brush-4 are comparable with that of brush-1 and brush-2, respectively. For clarity, the brush polymers are hereafter denoted as PHMA(51 nm)-b-P5Az10MA(7.4 nm), PHMA(58 nm)-b-P5Az10MA(13 nm), P5Az10MA(8 nm), and P5Az10MA(16 nm), for from brush-1 to brush-4, respectively. 3.2.2. Packing State and Orientation of Az Mesogens. Figure 2 shows UV−vis absorption spectra of the PHMA(51 nm)-b-P5Az10MA(7.4 nm) diblock grafted film (a) and P5Az10MA(8 nm) homopolymer on quartz substrates taken with the normal incidence for the as-prepared states (solid lines) and after annealing at 130 °C for 5 min (dashed lines). The LC Az blocks of BCP and homopolymer brushes showed the peaks around 240 and 340 nm, which can be assigned to the ϕ−ϕ* transition of the aromatic ring53 and π−π* long-axis transitions of the Az unit, respectively. A slight hypsochromic peak shifts were observed for the π−π* absorption band in both brushes than that in solution (352 nm, Table 3), showing the partial formation of H-aggregations. The spectra did not change by repeated washing with THF, obviously indicating that the chains were chemically tethered onto the substrate. 3.2.3. Effect of Flexible Chain on the Packing State of Mesogens. The relative intensity of absorption at the peak of π−π* band (340 nm) to that of ϕ−ϕ* one (240 nm) can be a measure of Az mesogen orientations because the π−π* transition is direction dependent, while the ϕ−ϕ* transition is insensitive to the chromophore orientation.53 The molecular orientation of LC Az units in the side chains had a nearly parallel orientation to the substrate for the high-density P5Az10MA homopolymer film.9,54 In the homopolymer case, according to our previous data, Aϕ−ϕ*/Aπ−π* gradually decreased with increasing π−π* absorbance; i.e., the chain length of Az layer reached to a limiting value of ca. 0.5 at absorbance around 0.2 (Figure 3 of ref 10). Above this chain

Figure 2. UV−vis absorption spectra of grafted PHMA(51 nm)-bP5Az10MA(7.4 nm) (a) and P5Az10MA(8 nm) (b) brushes on quartz plates. λmax values are shown in the figures. Solid and dashed lines indicate spectra before and after annealing at 130 °C, respectively.

Table 3. UV Spectral Data of Diblock Copolymer and Homopolymer Brushes and in Solution system 1. PHMA(51 nm)-bP5Az10MA(7.4 nm) (m = 470, n = 58) brush 2. PHMA(58 nm)-b-P5Az10MA (13 nm) (m = 572, n = 83) brush 3. P5Az10MA(16 nm) (n = 105) brush 4. P5Az10MA(8 nm) (n = 60) brush 5. P5Az10MA in CH3Cl solution

annealing at 130 °C

λmax(π−π*)/ nm

A(ϕ−ϕ*)/ A(π−π*)

before

336

0.77

after before

342 335

0.62 0.65

after before

339 338

0.54 0.51

after before

338 340

0.58 0.41

after

342 352

0.41 0.51

length, the Az orientation remains unchanged. With respect to λmax of the π−π* band, the spectrum essentially showed no shift at any chain lengths, indicating that the aggregation state is not altered by the change in the chain lengths.10 The spectral features (λmax of the π−π* band and Aϕ−ϕ*/Aπ−π*) were unaffected by the annealing at 130 °C (b). On the contrary, the λmax and the ratio of Aϕ−ϕ*/Aπ−π* of the grafted PHMA(51 nm)-b-P5Az10MA(7.4 nm) film exhibited significant changes by annealing at 130 °C (a). The λmax exhibited a red-shift by 6 nm by annealing, and the ratio of Aϕ−ϕ*/Aπ−π* changed from 0.77 to 0.62. Thus, by annealing, the aggregation state is slightly loosened and the long axis of Az adopted more parallel direction to the substrate. Table 3 summarizes the spectral features of the four polymer brushes. The same tendencies were recognized for both block copolymer and homopolymer brushes. The packing state and molecular orientation were altered by annealing for both block copolymer brushes, while they were essentially unchanged for 6098


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grafted PHMA(51 nm)-b-P5Az10MA(7.4 nm) film before (a) and after (b) irradiation with LPL at 436 nm. Before irradiation, no in-plane dichroism was recognized at the macroscopic level of spectroscopic measurements (a). In a similar way as the P5Az10MA homopolymer brush, the mesogens of the BCP brush formed a polydomain structure before the photoorientation.9 Upon irradiation with 500 mJ cm−2 LPL at 70 °C, the absorbance of the π−π* transition band observed with the polarized light orthogonal to the electric vector (E) of the actinic LPL (A⊥) became significantly larger than that in the parallel direction (A∥) (b). The level of photoinduced in-plane optical anisotropy of films was evaluated by the order parameter (S) of the Az mesogens by S [= (A⊥ − A∥)/(A⊥ + 2A∥)], where A⊥ and A∥ denote absorbance at the λmax (336 nm) obtained by measurements using polarized light with E perpendicular and parallel to that of actinic polarized light. In the case shown in Figure 4b, S reached 0.53 at 500 mJ cm−2. On the other hand, S value of the corresponding homopolymer P5Az10MA (8 nm) brush reached only 0.35 in the same optimum irradiation conditions (see Figure 6, dashed line). According to our previous data, the maximum S depends on the film thickness (Figure 6 of ref 10), and to obtain the same level of high orientational order (S > 0.5) requires thicker films beyond 20 nm. The obvious difference in the attainment of optical anisotropy can be ascribed to higher flexibility of PHMA chain connecting the P5Az10MA block, which is able to decouple the motional constraint from the solid surface. In other words, for the homopolymer brush, the induction of molecular orientation is achieved more effectively on the free air side, and the motions of Az mesogens in the vicinity of solid surface, probably in a range of a few nanometers from the surface, are highly restricted even at a LC fluid state. 3.3.2. Effect of Temperature. The increase in S with LPL irradiation dose is displayed in Figure 5. The large optical anisotropy was obtained at 70 °C, i.e., moderately above Tg of P5Az10MA adapting the smectic A phase. S reached above 0.5 at irradiation dose above 300 mJ cm−2. The photoalignment was hardly achieved at 20 °C (below Tg of P5Az10MA) and at 100 °C (near isotropidization temperature). These results are consistent with our observations with the P5Az10MA homopolymer brush.10 The temperature dependence was explored in more detail ranging from 20 to 110 °C, and the results are displayed in Figure 6. S values are plotted for two homopolymer brushes (8 and 16 nm thickness) and the diblock copolymer brush of

homopolymer brushes. In the diblock copolymer brushes, the flexible PHMA chain plays a significant role as a buffer layer to attain a favorable packing and orientation state of Az mesogens existing away from the substrate. In contrast, the direct attachment of LC Az chains of P5Az10MA in the homopolymer system impedes such relaxations. Hereafter, data for the PHMA(51 nm)-b-P5Az10MA(7.4 nm) and the corresponding P5Az10MA(8 nm) brushes are mainly described because essentially the same results were obtained for the longer diblock brushes. 3.2.4. GI-XRD Measurements. GI-XRD measurement was performed for PHMA(51 nm)-b-P5Az10MA(7.4 nm) block copolymer brush. The brush film was annealed at 130 °C followed by a rapid cooling, and then in-situ GI-XRD measurement was conducted at room temperature (Figure 3).

Figure 3. 2D GI-XRD imaging plate pattern of the annealed grafted PHMA(51 nm)-b-P5Az10MA(7.4 nm) diblock copolymer film recorded on an imaging plate at room temperature. 1D profile in the in-plane direction extracted from the 2D XRD pattern is shown in the inset. A strong comet-tail-like line running in the perpendicular direction is a specular reflection.

This diblock copolymer grafted film exhibited diffraction peaks only in the in-plane direction at 2θ = 2.6°, which is attributed to a vertically oriented smectic layer structure. The layer spacing (d = 3.4 nm) coincided well with the layer spacing of free polymer (d = 3.3 nm)9 or homopolymer grafted films (d = 3.6 nm).9,10 3.3. Irradiation with LPL. 3.3.1. Photoinduced Dichroism. Figure 4 shows polarized UV−vis absorption spectra of the

Figure 4. Polarized UV−vis absorption spectra of a grafted PHMA(51 nm)-b-P5Az10MA(7.4 nm) film before (a) and after LPL irradiation (b) at 500 mJ cm−2 and 70 °C. In (b), solid and dotted lines denote spectra at taken with probing beams perpendicular and parallel with the actinic LPL directions, respectively. Before LPL irradiation (a), the spectrum was taken at the same configuration as for (b). 6099


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photoinduced anisotropy is significantly maintained at higher temperature regions for the homopolymer brushes, implying a more restricted situation of the molecular motion by the directly anchored substrate surface. The photoinduced dichroism could be fully erased by thermal annealing at 130 °C (above the isotropidization temperature, 108 °C) or by dipping into a good solvent. 3.3.3. Structure Evaluation by X-ray Diffraction. The orientation of smectic LC layers in the grafted PHMA-bP5Az10MA films aligned upon the exposure to 436 nm LPL was evaluated by GI-XRD measurements. The incidence of Xray beam was achieved from direction orthogonal to the mesogenic orientation (A) or parallel to it (B) as shown in Figure 7. At room temperature, the photoaligned PHMA(51

Figure 5. Change in orientational order parameters (S) with energy doses by LPL irradiation in PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush at 20 °C (dotted line), 70 °C (solid line), and 100 °C (dashed line).

Figure 6. Orientational order parameters (S) obtained after 436 nm LPL irradiation at 500 mJ cm−2 with varying temperatures for PHMA(51 nm)-b-P5Az10MA(7.4 nm) (solid line), P5Az10MA(16 nm) (dotted line), P5Az10MA(8 nm) (dashed line) brushes.

PHMA(51 nm)-b-P5Az10MA(7.4 nm) at a fixed exposure dose of 500 mJ cm−2. For the P5Az10MA(16 nm) brush (dotted line), S increased with increasing the temperature starting from the 20 °C and reached the highest optical anisotropy (S = 0.48) at 80 °C and then was almost maintained until 100 °C and suddenly decreased above 110 °C. The thinner brush of P5Az10MA(8 nm) exhibited the same tendency and gave a similar profile but with lower S at all temperatures, the maximum S being 0.35 (dashed profile). On the other hand, PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush gave a differently featured profile in two aspects. First, the in-plane optical anisotropy increased relatively sharply with increasing the temperature, reached the maximum (S = 0.53) at 70 °C, and sharply decreased beyond this temperature. In this way, the BCP brush showed a narrower optimum temperature range to obtain high S values. Second, the higher orientational order is obtained. The maximum S for PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush was 0.53, which is considerably higher than that of the homopolymer brush of the similar length (P5Az10MA(8 nm), S = 0.35). In addition, it is even higher than the 2-fold thicker homopolymer of P5Az10MA(16 nm) (S = 0.48 at 80 °C) brush. The difference can be interpreted in the same line as discussed in section 3.3.1. For the diblock copolymer brush, the cooperative orientation behavior of Az mesogens is decoupled with the confinement from the substrate by the existence of the buffer chain of PHMA. In the case of homopolymer brush, on the other hand, the direct anchoring of the one chain end to the solid substrate would disrupt the free orienting motions. The

Figure 7. 2D GI-XRD patterns of the grafted PHMA(51 nm)-bP5Az10MA(7.4 nm) polymer film recorded on an imaging plate taken with the incident directions A (a) and B (b). The beam directions A and B are displayed in a scheme below. 1D profiles in the in-plane direction extracted from the 2D XRD patterns are shown as the inset. A strong comet-tail-like line running in the perpendicular direction is a specular reflection. 6100


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nm)-b-P5Az10MA(7.4 nm) grafted film exhibited the in-plane diffraction peak at 2θ = 2.6° with incidence from A, showing the smectic layer structure spacing of 3.4 nm. No diffraction peak was observed with the incidence from B. Temperature dependence of the GI-XRD data was obtained by in-situ heating from room temperature to 120 °C (Figure 8a). The diffraction peak at 2θ = 2.6° was observed until 90 °C

temperatures around Tg (ca. 50 °C) of P5Az10MA as clearly admitted in the heating process (a). The increased molecular motion is likely to provide a more adapted packing state within the smectic layer. 3.3.4. Photoreorientation. The in-plane rotation of the P5Az10MA is further expected to be influenced by the existence of the lubricant flexible chain. Therefore, the reorientation behavior of the molecular orientation by a subsequent exposure to LPL with the orthogonal polarization direction was examined. Figure 9 shows the reorientation

Figure 9. Change in the orientational order parameters (S) during the exposure to 436 nm LPL set orthogonal to the initial alignment at 20 °C (a) and 70 °C (b). Solid and dotted lines are data for PHMA(51 nm)-b-P5Az10MA(7.4 nm) and P5Az10MA(8 nm) brushes, respectively.

behavior (change in S) along with the irradiation with orthogonal 436 nm LPL. Here, the negative sign in S indicates that the molecular orientation in the orthogonal direction from the initial is prevailed compared to the original one. At 20 °C below Tg, the initially photoaligned P5Az10MA(8 nm) homopolymer brush did not respond to LPL irradiation with another orthogonal direction even at 500 mJ cm−2 (a). Prolonged irradiation at 3000 mJ cm−2 brought about some reduction in S to 0.20. In contrast, 200 mJ cm−2 light irradiation was considered enough to reduce S from 0.4 to 0.3 for the PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush. The optical anisotropy is almost fully lost at 3000 mJ cm−2 irradiation (a). In this way, an unequivocal difference in the reorientation behavior between the two brushes is indicated. Also at the optimum temperature for photoalignment (70 °C), an obvious difference was admitted in the reorientation rate (b). For the PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush, S started decreasing immediately upon orthogonal LPL irradiation, followed by a monotonous change to the full reorientation at 400−500 mJ cm−2 (solid line). On the other hand, the corresponding homopolymer brush of P5Az10MA(8 nm) exhibited a nonlinear behavior (dashed line). The reorientation virtually started after an induction period of 200 mJ cm−2. Beyond this point, the reduction in S proceeded at

Figure 8. 1D in-plane diffraction profiles in the GI-XRD measurements for PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush taken on the heating (a) and cooling (b) processes.

and fully disappeared at 100 °C. On the other hand, the peak disappeared slightly above 100 °C for the homopolymer brush (data not shown). This fact is consistent with the optical anisotropy data shown in Figure 6. Not only the molecular orientation but also the layer structure is retained at the higher temperature for the homopolymer brush. We also carried out the XRD measurements on the cooling process staring from 110 to 20 °C for PHMA(51 nm)-b-P5Az10MA(7.4 nm) brush. The peak reappeared at 90 °C and remained until room temperature as shown in Figure 8b. The heating and cooling processes essentially gave the same temperature dependence, but the initial clear peak observed in the heating process (a) became less intense in the cooling process at lower temperatures (b). This is ascribed to the loss of uniform in-plane photoalignment by lowering the temperature from the isotopic state above 120 °C. At all the temperatures, the d spacing of the in-plane smectic layer did not change significantly; however, a minor spacing decreases by 0.1−0.2 nm in d could be admitted at higher 6101


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ymer case, on the contrary, the reorientation requires an induction at an earlier period exhibiting a nonlinear behavior. As revealed in this work, the motional functions are readily performed by the existence of a flexible block chain anchoring to the solid surface. This concept is similar to the well-known molecular design concept in the side chain LC polymers; i.e., the existence of a spacer connecting the main chain and the mesogeic group plays an important role to decouple the influence of main chain in the assembling nature of mesogenic groups.55 This work expands this molecular design concept to macromolecular levels of block copolymers. The present work dealt with a series of block copolymer possessing a block of very low Tg (−20 °C). Our next interest is to introduce moderately or sufficiently high Tg block as the anchoring chain to the substrate surface and to elucidate the role of the chain rigidity for assembling and photoalignment behavior of the LC Az block. Work along this line is now underway. Smart surfaces in response to light may find a number of applications for switching of surface alignment of liquid crystals, tribological properties, microfluidics, biocompatibility, etc. Further, the surface grafting of polymer chains is not limited to planar rigid substrate materials but would be extended to systems of flexible substrates, fibers, and particles. In such cases, photomechanical motions and light energy transductions are further anticipated. We believe that fundamental knowledge obtained in the present work will provide useful information and guidelines for the photofunctionalization of various types of materials.

the same rate as for the BCP brush. The orientational reversion is almost fully completed at ca. 600 mJ cm−2. It is likely that the constraint by the solid surface impedes the rotating motions of Az mesogenic groups in response to the LPL below 200 mJ cm−2. Once the in-plane rotation starts, the reorientation readily occurs due to the cooperative nature of the LC material. Under suitable irradiation conditions, in any case, LPL irradiation with another orthogonal direction at 60−80 °C led to a complete rewriting without an orientational memory effect for both homopolymer and diblock copolymer brushes.

4. CONCLUSION Surface-grafted block copolymer ultrathin films have been successfully synthesized by the two-step surface-initiated ATRP method where the outer block contains the photoreactive Az mesogenic units. The orientation of Az mesogeic groups and the smectic layer structure are essentially the same as those of the corresponding homopolymer brushes. In this diblock polymer design, a flexible amorphous chain of PHMA links the LC Az polymer and the substrate. The PHMA chain works as the buffer part that decouples the influence of solid substrate in the motion of the Az LC layer at the outer surface. The role and effect of the flexible block chain can be summarized in the following three aspects. (i) More efficient cooperative photoorientation of the Az LC block is attained in diblock copolymer brush as revealed. Figure 10 illustrates the differences in the

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +81-52-789-4668; Fax +81-52-789-4669. Notes

The authors declare no competing financial interest.

■ Figure 10. Comparative illustration of LPL-induced optical anisotropy between homopolymer (upper) and diblock copolymer (lower) brushes.

ACKNOWLEDGMENTS The authors thank Mr. T. Hikage, Dr. M. Hara, Mr. S. Kakehi, and Mr. A. Nishimi for their support in conducting highintensity X-ray analysis. The research was supported by the Grant-In-Aid for Basic Research S (No. 23225003) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and PRESTO program of Japan Science and Technology Agency.

induced optical anisotropy for the homopolymer and diblock copolymer of the same length of Az polymer. High optical anisotropy (S > 0.5) is obtained even for the thinner film (8 nm thickness) in contrast with the homopolymer brush case in which thicker films above 20 nm are required to attain the same high level of anisotropy.10 (ii) The optimized temperature range for the anisotropy induction becomes narrower. In higher temperature ranges, the homopolymer chain retains at relatively higher levels of optical and structural anisotropy, probably due to the motional restriction by the solid substrate. In other words, the diblock copolymer brush affords more favorable environment for the LC Az mesogens to exert an intrinsic assembling. (iii) The rate of in-plane rotational motions, i.e., photoreorientation with orthogonal LPL irradiation, is monotonously and more readily achieved by the higher wiggled cooperative movement with the aid of underlying flexible polymer chains in diblock copolymer brush. In the homopol-

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(51) Hayashi, K.; Saito, N.; Sugimura, H.; Takai, O.; Nagagiri, N. Langmuir 2002, 18, 7469. (52) Hozumi, A.; Yokogawa, Y.; Kameyama, Y.; Sugimura, H.; Hayashi, K.; Shinohara, H.; Takai, O. J. Vac. Sci. Technol. A 2001, 19, 1812. (53) 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; pp 32−79. Sapich, B.; Vix, A. B. E.; Rabe, J. P.; Stumpe, J.; Wilbert, G.; Zentel, R. Thin Solid Films 2006, 514, 165. (54) The value of Aϕ−ϕ*/Aπ−π* does not show a significant difference with that in solution, indicating at the Az units are in a more or less randomized state. Nevertheless, the GI-SAXS measurement revealed the existence of the smectic layer structure orienting perpendicular to the substrate plane.9 This fact should show that the Az units are not in a perfect random state in the film. For this reason, we use the term “nearly parallel to the substrate” to express the orientation of the Az unit. (55) Finkelmann, H.; Ringsdorf, H.; Wendorff, J. H. Macromol. Chem. 1978, 179, 273.

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