Photoinduced Reorientation and Surface Relief Formation in Diblock

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Photoinduced Reorientation and Surface Relief Formation in Diblock and Random Copolymers with Benzoic Acid and Alkyloxy Side Groups Hiromi Ikoma, Mizuho Kondo, and Nobuhiro Kawatsuki* Department of Applied Chemistry, Graduate School of Engineering, University of Hyogo, Shosha, Himeji 671-2280, Japan

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S Supporting Information *

ABSTRACT: Diblock and random copolymers are synthesized from a methacrylate with benzoic acid (BA) side groups and a nbutyl methacrylate, and the photoinduced reorientation of thin films doped with a N-benzylideneaniline carboxylic acid (NBA), which governs the H-bond with BA, is investigated using linearly polarized (LP) 365 and 313 nm light. A sufficient thermally stimulated photoinduced orientation of the BA side groups and simultaneous sublimation of NBA are attained for the diblock copolymer films, whereas a thermal treatment results in the random orientation of the random copolymers. Furthermore, surface relief (SR) formation of the film is explored when the NBA-doped films are exposed to LP light using a photomask. The liquid crystalline characteristics, thermal properties, axis-selective photoisomerization, and the photo-cross-linking of the film play important roles in the molecular reorientation and SR formations.

1. INTRODUCTION Photoinduced orientation in photosensitive polymeric films has the potential to realize molecularly oriented films, optical memories, and polarization-sensitive optical devises.1−5 Many types of photoalignable materials exhibiting photoinduced orientation based on axis-selective trans−cis−trans photoisomerization and/or photo-cross-linking have been explored using linearly polarized (LP) light and in some cases subsequent thermally stimulated self-organization. 1−10 Among them, azobenzene-containing polymers have been intensively investigated due to their sufficient photoinduced orientation ability and rather clear photochemistry.1−7,11−13 Moreover, surface relief (SR) formation without a wet process has often been observed in photoalignable materials when using patterned exposure and intensity/polarization holography.14−19 The molecular motion of the azobenzene moieties is generated due to a change in the thermal property from the liquid crystalline (LC) state to the isotropic state at the exposed region as well as the periodical change in the polarization state upon the photoinduced reorientation motion of the azobenzene moieties. Additionally, the periodical change in the stiffness of the photo-cross-linkable films generates a thermally induced molecular motion to form SR.20−23 Similar to azobenzene derivatives, N-benzylideneaniline derivatives show trans−cis photoisomerization.24−26 Our research has focused on the photoinduced molecular reorientation of polymethacrylates with N-benzylideneaniline derivative side groups, which show reorientation perpendicular to the polarization of LP ultraviolet (LPUV) light due to the axis-selective trans−cis−trans photoisomerization.27−29 Thermal stimulation of the photoinduced molecular reorientation © XXXX American Chemical Society

and the SR structure formation by means of holographic exposure have been observed.28−30 We also developed a composite material of a photoinactive LC polymethacrylate with a benzoic acid (BA) side group (P6BAM) doped with Nbenzylideneaniline carboxylic acid (NBA) derivatives.31 A significant thermally stimulated molecular reorientation is achieved due to the axis-selective photoreaction of H-bonded NBA with the BA side groups, where the photodurability of the resultant oriented films is provided due to the simultaneous sublimation of the doped NBA derivatives upon thermal selforganization of the BA side groups.31 Many studies have introduced functionality to nonphotosensitive polymers doped with low-molecular-weight photosensitive materials via Hbonds.16,17,32−39 In some cases, detaching the photosensitive moieties after the photoreaction confers the photoinactivity of the films.31,39−43 Copolymerization of a comomoner containing nonphotosensitive mesogenic side groups with a photoalignable LC polymer varies the thermal and optical properties as well as the photofunction.44−49 Controlling the photoalignment and SR formation ability via copolymerization is important to fabricate conventional birefringent and diffraction optical elements. Many kinds of copolymers with azobenzene and nonphotosensitive mesogenic side groups show cooperative photoinduced reorientation.47−49 Copolymerization of nonmesogenic side groups such as a long alkyl chain exhibits a significant change in the thermal and LC characteristics of the Received: May 22, 2018 Revised: June 20, 2018

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DOI: 10.1021/acs.macromol.8b01071 Macromolecules XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

photoresponsive LC polymers. Seki et al. investigated the SR formation of random copolymers with azobenzene and n-hexyl side groups; an effective molecular motion occurs upon the light triggered phase transition from the smectic A to the isotropic state.50 Additionally, several types of block copolymers with photoalignable and nonmesogenic side groups have been synthesized and their microphase separation structures preciously investigated.51−58 A microphase separation of both parts is expected to result in photoresponsive functionality, which should improve the optical and mechanical properties of the films.58 Herein diblock and random copolymethacylates from a methacrylate with BA side groups and an n-butyl methacrylate are synthesized (Figure 1). To elucidate the photoinduced

2.1. Materials. A methacrylate with benzoic acid side groups via a hexamethylene spacer (6BAM) and NBA were synthesized according to the literature.59 n-Butyl methacrylate (BM) was purchased from Tokyo Kasei Chemicals and was poured into a silica column prior to use to remove inhibitors. 2-Cyano-2-propyldodecyl trithiocarbonate (CPDT) and α,α′-azobis(isobutyronitrile) (AIBN) were purchased from Sigma-Aldrich. All other solvents and materials were used as received. Figure 1 depicts the chemical structures of the synthesized (co)polymers and NBA. 2.2. Copolymer Synthesis. Scheme 1 outlines the synthetic routes of the copolymers. Diblock copolymers B1 and B2 were synthesized by a reversible addition−fragmentation chain transfer (RAFT) polymerization using CPDT and AIBN as initiators in 1,4dioxane,60,61 while random (co)polymers R1, R2, P6BAM, and PBM were synthesized by a free radical copolymerization using an AIBN initiator. Detailed synthetic procedures are described in the Supporting Information. Table 1 summarizes the composition, molecular weight, and thermal properties of the copolymers.

Table 1. Composition, Molecular Weight, and Thermal Properties of the (Co)polymers polymer B1

Figure 1. Chemical structures of the copolymers (B1, B2, R1, and R2) and the N-benzylideneaniline derivative (NBA).

B2 R1 R2 P6BAM PBM

reorientation and SR formation behavior of the thin film, copolymer films doped with NBA, which forms H-bonds with BA, are exposed to LP 365 or 313 nm light. The axis-selective photoreaction of NBA generates a small photoinduced reorientation of the NBA and BA side groups. The difference in the photoinduced reorientation and thermal stimulation of the molecular reorientation behaviors between the diblock and random copolymer films and between LP 365 and 313 nm light is investigated. Furthermore, SR formation of thin composite films is investigated using a photomask, where a significant SR is formed when random copolymer/NBA composite films are exposed to 313 nm light. The thermal properties of the copolymer film and the type of the photoreaction play important roles in the molecular reorientation and SR formations.

x/ya (DP/DP)b 1/2 (120/253) 1/1 (120/113) 1/2 1/1 1/0 0/1

Mnb

Mw/Mnb

thermal propertyc

76000

1.3

G1 28 G2 110 N 179 I

51000

1.3

G1 35 G2 120 N 177 I

44000 84000 106000 35000

2.1 1.9 2.9 2.2

G G G G

45 I 57 I 143 N 177 I 22 I

a Determined by 1H NMR. bDetermined by GPC with polystyrene standards, THF as eluent. cDetermined by DSC and POM observation. G: glass transition; N: nematic; I: isotropic.

2.3. Film Preparation and Photoreaction. Composite films of the copolymer/NBA (thicknesses; approximately 200 nm) were prepared by spin-coating a THF solution of the polymers (1.6% w/w) onto quartz substrates. Adjusting the molar ratio of the BA side groups in the copolymer and NBA controlled the BA/NBA molar ratio in the composite films. Photoreactions were carried out using a high-pressure Hg lamp equipped with a glass plate placed at Brewster’s angle and a 365 (313) nm band-pass filter (Asahi Spectra REX-250), yielding a light intensity

Scheme 1. Synthetic Route of the Diblock and Random Copolymers

B


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Figure 2. Changes in the polarized absorption spectra upon exposure to LP 365 nm light and changes in the absorbance and the photoinduced Dexposed values as functions of exposure energy. (a, b) B1/NBA (1/0.7 = mol/mol) film. (c, d) R1/NBA (1/0.7 = mol/mol) film. of 30 (10) mW/cm2 at 365 (313) nm. After photoirradiation, the films were annealed at elevated temperatures. 2.4. Characterization. 1H NMR spectra using a Bruker DRX-500 FT-NMR and FT-IR spectra (JASCO FTIR-6600) confirmed the copolymers. The molecular weights of the polymers were measured by GPC (JASCO PU-2080 and RI-2031 GPC system with a Shodex column using THF as the eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarization optical microscope (POM; Olympus BX51) equipped with a Linkam TH600PM heating and cooling stage as well as differential scanning calorimetry (DSC; Seiko-I SSC5200H). As a measure of the optical anisotropy, the photoinduced and thermally stimulated in-plane dichroism (D) was evaluated from the polarization absorption spectra with a Hitachi U-3010 spectrometer equipped with Glan−Taylor polarization prisms. D is estimated as D=

characteristics. Instead, the Tg depends on the copolymer composition. 3.2. Axis-Selective Photoreaction. The copolymers do not have an absorption band longer than 310 nm wavelength (Figure S2). The photosensitivity at 365 and 313 nm light is given by doping NBA, which forms H-bonds with the BA side groups. Figures 2a and 2b (Figures 2c and 2d) respectively show the changes in the polarized absorption spectra of a B1/ NBA (BA/NBA = 1/0.7 mol/mol) [R1/NBA (BA/NBA = 1/ 0.7 mol/mol)] film, the changes in the absorbances at 360 nm (NBA) and 262 nm (NBA and BA), and the photoinduced D values (Dexposed) of the films as functions of exposure energy. For the B1/NBA film exposed to LP 365 nm light, the absorbance parallel (perpendicular) to E at 262 nm decreases (increases) as the exposure energy increases. The parallel absorbance at 360 nm has a larger decrease due to the axisselective trans−cis−trans photoisomerization of NBA, which initiates a small photoinduced molecular reorientation of NBA and the H-bonded BA side groups perpendicular to E.31 The B2/NBA (BA/NBA = 1/0.7) film shows similar spectral changes (Figure S3a,b). The sufficient H-bond between BA and NBA generates the photoinduced reorientation of the P6BAM block. These phenomena are similar to those of the P6BAM/NBA composite film.31 Additionally, the Dexposed value is maximized when the exposure energy is around 70 J/cm2 but decreases in the higher exposure range due to the side photoreaction of NBA other than photoisomerization.29,62 For a R1(R2)/NBA (BA/NBA = 1/0.7) film, the absorption spectrum of the as-coated film is broader and shows partial scattering due to a lower miscibility (Figure 2c,d and Figure S3c,d), suggesting insufficient H-bonds between BA and NBA compared to that of B1(B2)/NBA composite film, which shows a high miscibility of NBA with the P6BAM block to form H-bonds sufficiently. In addition, the photoinduced

A⊥ − A A⊥ + A

(1)

where A|| and A⊥ are the absorbances parallel and perpendicular to polarization (E) of LPUV light, respectively. The surface topology was evaluated using an optical surface profiler (VertScan2.0 R3300, Ryoka Systems Inc., 235 μm × 176 μm area).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of the Copolymers. Diblock copolymers B1 and B2 were synthesized by RAFT polymerization using a macromolecular initiator of P6BAM-CPDT (DP = 120) and BM (Scheme 1). After polymerization, the copolymers-CPDT were heated at 200 °C to decompose trithioester to the thiol end groups. The DSC curves and POM observations of B1 and B2 show a glass transition according to the PBM block around 40 °C and nematic LC characteristics similar to P6BAM (Table 1 and Figure S1). In contrast, random copolymers (R1 and R2) synthesized by free radical copolymerization do not show LC C


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Figure 3. Changes in the polarized absorption spectra upon exposure to LP 313 nm light and changes in the absorbance and the photoinduced Dexposed values as functions of exposure energy. (a, b) B1/NBA (1/0.7 = mol/mol) film. (c, d) R1/NBA (1/0.7 = mol/mol) film.

Figure 4. Changes in the polarized absorption spectra of (a) B1/NBA(BA/NBA = 1/0.7), (b) B2/NBA (BA/NBA = 1/0.7), and (c) R1/NBA (BA/NBA = 1/0.7) films before and after exposure to LP 365 nm light (exposure energy; (a) 60 J/cm2, (b, c) 40 J/cm2) and subsequent annealing at 170 °C for 10 min. Insets plot the generated D values at 262 nm before and after annealing as functions of exposure energy.

anisotropy in a P6BAM/NBA composite film is thermally amplified.31 Similarly, the B1(B2)/NBA film exhibits thermally stimulated amplification of the photoinduced optical anisotropy. Figure 4a (4b) shows the change in the polarized absorption spectra of a B1/NBA(BA/NBA = 1/0.7) [B2/ NBA(BA/NBA = 1/0.7)] film when exposed to LP 365 nm light for 60 (40) J/cm2 and subsequently annealed at 170 °C for 10 min. The initial Dexposed at 262 nm [Dexposed:262 = 0.19 (0.13)] is amplified to Dannealed:262 = 0.52 (0.56) for B1 (B2), and annealing diminishes the absorption band of NBA at 360 nm. The thermally amplified D value (Dannealed:262) is related to the initial photoinduced reorientation of NBA and Dexposed:262, as plotted in Figures 4a and 4b (insets), where Dannealed:262 is maximized at the maximum Dexposed:262. Additionally, doping a smaller amount of the NBA [B1/NBA (BA/NBA = 1/0.4)] film shows similar thermally stimulated molecular reorientation behaviors, where Dexposed:262 = 0.13 for B1/NBA is amplified to Dannealed:262 = 0.53 (Figure S5). The LC characteristics of the P6BAM block in the copolymer film generate thermally

anisotropy is somewhat smaller than that of B1(B2)/NBA, indicating that the axis-selectively photoreacted NBA molecules in the random copolymers do not effectively generate the photoinduced cooperative molecular reorientation of the BA side groups. The use of a LP 313 nm light shows different spectral changes (Figure 3a−d). The increases in the perpendicular absorbance and the photoinduced optical anisotropy are much smaller than those using LP 365 nm light. The 313 nm light exposure causes a side photoreaction other than photoisomerization at the initial stage of the photoreaction, which disturbs the photoinduced reorientation. Solubility tests of the composite films suggest that photo-cross-linking is initiated by the hydrogen abstraction of NBA,29,62 where the film becomes insoluble in the lower exposure region when using 313 nm light, while much higher exposure energy is required to become insoluble when using 365 nm light (Figure S4). 3.3. Thermally Stimulated Molecular Reorientation. We previously reported that the photoinduced optical D


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Figure 5. (a) D values at 262 and 360 nm of a B1/NBA (BA/NBA = 1/0.7) film before and after annealing at 170 °C for 10 min as functions of exposure energy of LP 313 nm light. (b, c) Changes in the polarized absorption spectra of B1/NBA (BA/NBA = 1/0.7) films before and after exposure to LP 313 nm light (exposure energy; (b) 10 J/cm2, (c) 40 J/cm2) and subsequent annealing at 170 °C for 10 min.

Figure 6. (a) Thermally amplified dichroism at 262 nm and absorbance at 360 nm of B1/NBA (BA/NBA = 1/0.7) films (exposed to LP 365 nm light for 40 J/cm2) annealed at different temperatures. (b) Changes in the thermally stimulated dichroism at 262 nm and the absorbance at 360 nm of a B1/NBA (BA/NBA = 1/0.7) film (exposed to LC 365 nm light for 40 J/cm2) as functions of annealing time. Annealing temperature is 170 °C. Insets are enlargements of the absorbance at 360 nm. Solid and dotted lines denote experimental and theoretical values, respectively. (c) Changes in the absorbance at 360 nm of a R1/NBA (BA/NBA = 1/0.7) film (exposed to LP 365 nm light for 40 J/cm2) as a function of annealing time. Annealing temperature was 170 °C. (d) Changes in the absorbance at 360 nm of a B1/NBA (BA/NBA = 1/0.7) films exposed to LP 313 nm light for 10 J/cm2 (blue dots) and for 40 J/cm2 (red circles) as functions of annealing time. Annealing temperature is 170 °C.

reorientation is amplified Dannealed:262 = 0.28 when the exposure energy is 10 J/cm2 (Figure 5b). However, when Dexposed is maximized (exposure energy >40 J/cm2), annealing randomizes the orientation (Figure 5c). A high photo-cross-linked structure inhibits the thermal stimulation even though the photoinduced reorientation of NBA is highly generated. 3.4. Influence of Annealing Temperature and Kinetics of Molecular Reorientation. The self-organization of the BA side groups accompanied by the simultaneous sublimation of the NBA molecules generates thermal stimulation of the reoriented structure in the diblock copolymer film. Figure 6a plots Dannealed:262 and the absorbance at 360 nm (NBA absorption) of B1/NBA films exposed to LP 365 nm light for 40 J/cm2 annealed at various temperatures for

stimulated self-organization of the BA-dimer side groups, while the NBA molecules sublimed in the thermal process, resulting in the UV durability. However, annealing the R1/NBA(BA/ NBA = 1/0.7) film randomizes the photoinduced molecular orientation and simultaneous sublimation of the NBA molecules (Figure 4c). Because random copolymers do not reveal LC characteristics, annealing at elevated temperatures disorders the photoinduced orientation structure. In contrast, for the use of LP 313 nm light, thermal stimulation of the BA orientation of a B1/NBA film is unrelated to the photoinduced D (Figure 5a). In this case, the photoinduced reorientation structure is thermally amplified at the lower exposure energy region, but the BA side groups are randomized when Dexposed is maximized. The photoinduced E


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Figure 7. 3D surface structure and surface profile curves of a B1/NBA(BA/NBA = 1/0.7) film. (a) As coated, (b) after exposure to LP 365 nm light for 60 J/cm2 through a 5 μm/25 μm (L/S) photomask, and (c) after subsequent annealing at 170 °C for 10 min. Insets display POM photographs of the film between crossed polarizers.

Figure 8. 3D surface structure and surface profile curves of a B1/NBA(BA/NBA = 1/0.7) film. (a) As coated, (b) after exposure to LP 313 nm light for 40 J/cm2 through a 5 μm/25 μm (L/S) photomask, and (c) after subsequent annealing at 170 °C for 10 min. Insets display POM photographs of the film between crossed polarizers.

10 min. A sufficient thermal amplification of the D values and effective elimination of NBA are generated when the annealing temperature is close to the isotropic temperature (170 °C) of P6BAM. However, the NBA molecules are not completely sublimed and Dannealed:262 is less than 0.4 when the annealing temperature is below 150 °C. To further elucidate the thermal stimulation behavior of the B1/NBA (BA/NBA = 1/0.7) film, time dependences of the self-organization of the BA side groups and the sublimation of NBA after exposing to LP 365 nm light were evaluated in detail. Figure 6b plots the absorbance of NBA (360 nm) and Dannealed:262 of a B1/NBA (BA/NBA = 1/0.7) film, which was exposed to LP 365 nm light for 40 J/cm2 and subsequently annealed at 170 °C with various annealing times. The thermal stimulation is maximized within 2 min, while complete sublimation of NBA requires more than 10 min. It should be noted that the absorbance at 360 nm initially increases but then decreases (inset of Figure 6b), indicating the in-plane motion of NBA at the initial stage of annealing. Assuming that both the thermally stimulated molecular reorientation and sublimation of NBA occur in a first-order manner,40,63,64 the self-organization rate of the BA side groups (kBA) and the sublimation rate of NBA (kNBA) after 2 min are estimated to be 8 × 10−2 s−1 and 5.5 × 10−3 s−1, respectively. In contrast, sublimation of NBA in a R1/NBA (BA/NBA = 1/0.7) film exposed to LP 365 nm light occurs faster (kNBA = 1.1 × 10−2 s−1) without increasing the absorbance (Figure 6c). For the B1/NBA (BA/NBA = 1/0.7) films exposed to LP 313 nm light, once the absorbance at 360 nm increases at the initial time of annealing, the NBA molecules slowly sublime (kNBA = 4.8 × 10−3 s−1) as thermally stimulated reorientation occurs (exposure energy = 10 J/cm2). However, NBA quickly sublimes (kNBA = 1.0 × 10−2 s−1) in the absence of thermal stimulation (Figure 6d). These results suggest that the

reoriented NBA molecules initially stimulate the thermal selforganization of the BA side groups, which is accompanied by a slower sublimation. The faster sublimation rate of NBA in the nonreoriented film is because the reorientation of the BA side groups does not contribute to sublimation. 3.5. Patterned Exposure and SR Formation. Patterned exposure of the diblock copolymer/NBA composite films to LP 365 nm light attains a UV-durable birefringent pattern. Figure 7a−c displays the changes in the 3D surface structure, POM photograph, and surface profile of a B1/NBA (BA/NBA = 1/0.7) film before and after exposure to LP 365 nm for 60 J/ cm2 via a 5 μm/25 μm (L/S) photomask and subsequent annealing at 170 °C for 10 min, respectively. After exposure, POM shows a bright pattern at the exposure region due to photoinduced reorientation, and the thickness of the exposed region decreases by 6 nm and the surface roughness (Ra) becomes 1.0 nm (initial Ra = 0.65 nm) (Figure 7a,b). The partial sublimation of NBA upon exposure is responsible for the small uneven pattern after exposure. This is confirmed by the UV absorption spectra of a quartz substrate placed on the film upon exposure as a slight absorption is detected after the exposure, but it diminishes after immersing in diethyl ether (Figure S6). After annealing, the brightness of the exposed region in POM increases and the surface unevenness of the exposed/unexposed regions almost diminishes (Figure 7c, Δd < 1 nm). This is due to the thermal stimulation of the orientation of the BA side groups and sublimation of NBA, which contribute to UV durability. The P6BAM/NBA film exhibits a similar phenomenon (Figure S7). In contrast, SR formation with 13 nm height is observed when using LP 313 nm light (Figure 8a−c). Exposure of a B1/ NBA (BA/NBA = 1/0.7) film for 40 J/cm2 induces a slight birefringent pattern, which disappears after annealing. Because photo-cross-linking occurs at the exposed region, molecular F


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Figure 9. 3D surface structure and surface profile curves of a R1/NBA (BA/NBA = 1/0.7) film. (a) As coated, (b) after exposure to LP 313 nm light for 40 J/cm2 through a 5 μm/25 μm (L/S) photomask, and (c) after subsequent annealing at 170 °C for 10 min. Insets display POM photographs of the film between crossed polarizers.

out using LP 365 nm and LP 313 nm light. LP 365 nm light effectively achieves photoinduced reorientation. The significant thermal stimulation of the reorientation in diblock copolymers is due to the LC characteristics of the material, which is accompanied by simultaneous sublimation of NBA. However, thermal stimulation does not occur for the random copolymers. A kinetic study suggests that once the axisselective photoreacted NBA stimulates the molecular reorientation of the BA side groups in the diblock copolymer film, sublimation is slow as the reorientation rate of the BA side groups is 10 times faster than that of the sublimation of NBA. In contrast, thermal treatment of the photoreacted random copolymer/NBA films results in a random orientation with fast sublimation of NBA due to the non-LC characteristics. Furthermore, patterned exposure of the random copolymer/ NBA to LP 313 nm light results in a significant SR formation, which is generated by the molecular motion due to periodical photo-cross-linking of the material. These material systems are applicable to birefringent optical devices and facile fabrication of UV and thermally durable gratings.

motion is thermally generated from softer to harder regions. A similar molecular motion has been observed in the patterned exposure to cinnamate-containing polymeric films, which causes photo-cross-linking.23 The difference in the stiffness of the film due to preferential photo-cross-linking causes the molecular motion from the nonexposed to exposed region.22,23 Moreover, a much higher SR is formed for a R1/NBA (1/ 0.7) film using LP 313 nm light (Figure 9a−c). In this case, the surface roughness of the initial film is 8.1 nm, which is larger than that of the B1/NBA film due to a poor compatibility between R1 and NBA as described in section 3.2. After exposure, a slight birefringent pattern is produced, but annealing generates the significant molecular motion and the birefringence disappears. The SR height reaches 500 nm, and the concave region partly shows the substrate’s surface, indicating that almost all the material migrates to the exposed region (Figure 9c). The significant molecular motion is due to the great difference in the stiffness of the film between the photo-cross-linking region (difficult to mobilize) and the nonexposed region (easily mobilized at 170 °C). This significant SR formation occurs when annealing at 170−250 °C (Figure 10), indicating the high thermal stability of SR. The cross-linked structure can stabilize the shape of SR even after molecular motion from the nonphotoreacted region.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01071. Synthetic procedures for the copolymers, figure summarizing the DSC curves of (co)polymers, figures showing the UV−vis absorption spectra of copolymer films, changes in the polarized UV−vis absorption spectra and absorbances at 262 and 360 nm of B2/ NBA(BA/NBA = 1/0.7) and R1/NBA(BA/NBA = 1/ 0.7) films, residual film thickness of B1/NBA(1/0.7) films exposed to 365 and 313 nm light for the solubility test, changes in the polarized UV−vis absorption spectra of a B1/NBA(BA/NBA = 1/0.4) film before and after exposure to LP 365 nm light and subsequent annealing, the D values of the exposed and subsequently annealed B1/NBA(BA/NBA = 1/0.4) films at 262 and 335 nm as functions of exposure energy, UV absorption spectra of a quartz substrate on a B1/NBA(BA/NBA = 1/0.7) film before and after exposing to LP 365 nm light for 40 J/ cm2, and after immersing in diethyl ether, 3D surface structure and surface profile curves of a P6BAM/NBA film before and after exposure to LP 365 nm light for 40 J/cm2 though a 25 μm/5 μm (L/S) photomask, and after subsequent annealing at 170 °C for 10 min (PDF)

4. CONCLUSION Comparison studies on the photoinduced reorientation, thermal stimulation of the photoinduced optical anisotropy, and SR formation of diblock and random co-polymethacrylates with BA and n-butyl side groups doped with NBA were carried

Figure 10. Inscribed SR height of R1/NBA (BA/NBA = 1/0.7) films exposed for 40 J/cm2 through a 5 μm/25 μm (L/S) photomask through a photomask followed by annealing at various temperatures for 10 min. G


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AUTHOR INFORMATION

Corresponding Author

*(N.K.) E-mail: [email protected]. ORCID

Nobuhiro Kawatsuki: 0000-0003-3276-1552 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research from JSPS (B15H03869 and S16H06355).

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REFERENCES

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Photoinduced Reorientation and Surface Relief Formation in Diblock

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