Photoinduced Orientation of Photoresponsive ... - ACS Publications

Article pubs.acs.org/Macromolecules

Photoinduced Orientation of Photoresponsive Polymers with N‑Benzylideneaniline Derivative Side Groups Nobuhiro Kawatsuki,*,† Hitomi Matsushita,† Teppei Washio,† Junji Kozuki,† Mizuho Kondo,† Tomoyuki Sasaki,‡ and Hiroshi Ono‡ †

Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan ‡ Department of Electrical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan S Supporting Information *

ABSTRACT: Axis-selective photoreaction, photoinduced orientation, and formation of surface relief (SR) gratings of liquid crystalline polymethacrylates comprised of N-benzylideneaniline (NBA) derivative side groups are explored. Irradiating with linearly polarized (LP) 313 nm light generates molecular reorientation of the NBA side groups based on the axis-selective trans−cis−trans photoisomerization and photocleavage of the NBA groups. The inplane order parameter (S) and birefringence (Δn) are ∼0.53 and ∼0.16, respectively. Furthermore, thermally stimulated amplification of the photoinduced optical anisotropy occurs upon exposing films to LP 313 nm light and subsequently annealing in the liquid crystalline temperature range of the material. The amplified S and Δn values are ∼0.71 and ∼0.21, respectively. Finally, polarization holography using 325 nm He−Cd laser beams with various interferometric polarization conditions demonstrates the formation of SR gratings with a molecularly oriented structure based on the periodic photoinduced reorientation and molecular motion.

1. INTRODUCTION

for display applications because they are colored in the visible region. In contrast, a photoinduced molecular orientation in photoreactive liquid crystalline polymer (PLCP) films, which are transparent in visible region, have been investigated based on an axis-selective photoreaction combined with a thermally stimulated reorientation.14,28−30 Several types of PLCPs realize an effective thermally stimulated photoinduced molecular orientation (e.g., those comprised of cinnamate,18,28 cinnamic acid,29 and phenyl benzoate derivative side groups30). These PLCPs are applicable to birefringent optical devices14 and LC photoalignment layers with large azimuth anchoring.20 The distribution of the small amount of axis-selective photoreacted products between the parallel and perpendicular directions adjusts the thermally induced self-organization of the material. Additionally, polarization holography using PLCP films generates a SR structure with a periodic molecular orientation.17,18,31 However, PLCP films require an annealing process for molecular reorientation and SR formation. Consequently, new photosensitive materials, which are transparent in the visible region and have both a sufficient photoinduced orientation and SR formation ability without an annealing process, are needed.

An axis-selective photoreaction in photosensitive polymeric films using linearly polarized (LP) light generates optical anisotropy of the film due to anisotropic changes in the inherent refractive index of the material.1 Axis-selective photoreactions have led to many applications, such as optical information storage,1−3 polarization holography,4−6 and photoalignment of low-molecular-weight liquid crystals (LCs).7−12 If the axis-selective photoreaction is accompanied by molecular reorientation, a large optical anisotropy can be generated, and the resultant films are applicable to birefringent optical devices and LC photoalignment layers with large azimuthal anchoring.13−20 To date, a large optical anisotropy has been realized in many types of photosensitive materials, including polymeric and monomeric materials comprised of axis-selective photoreactive moieties.13−15 Among these, azobenzene-containing materials have been extensively studied due to their sufficient photoinduced reorientation ability using LP light.13,21−23 The axis-selective trans−cis−trans photoisomerization reorients the azobenzene molecules according to the polarization state of the light beams. Moreover, holographic exposure to an azobenzene-containing polymeric film forms surface reliefs (SRs) with a periodic molecular orientation structure due to the periodic photoinduced reorientation combined with molecular motion.24−27 Unfortunately, azobenzene-containing materials are unsuitable © 2013 American Chemical Society

Received: November 10, 2013 Revised: December 12, 2013 Published: December 19, 2013 324

dx.doi.org/10.1021/ma4023247 | Macromolecules 2014, 47, 324−332

Macromolecules

Article

Figure 1. (a) Chemical structures of P1 and P2. (b) Experimental setup for polarization holography.

Table 1. Molecular Weight and Thermal and Spectroscopic Properties of P1 and P2 molecular weighta P1 P2

λmax (nm)

thermal propertyb

Mn

Mw/Mn

°C (ΔH, J/g)

21 000 12 000

1.9 1.5

G 41 N 111 (1.7) I G 43 N 120 (1.5) I

solutionc (ε × 10−4) 284 (1.8) 284 (1.7)

332 (1.6) 332 (1.6)

filmd 283 283

332 332

a

Determined by GPC, polystyrene standards. bDetermined by DSC, second heating. G: glass; N: nematic; I: isotropic. cIn THF. dOn a quartz substrate.

N-Benzylideneaniline (NBA) derivatives with alkyl(oxy) substituents at the 4- and 4′-positions reveal a LC mesophase.32 Several types of LC polymers comprised of NBA-derivative side groups or with NBA main chains have been synthesized and characterized.33−35 Kosaka et al. investigated the specific features of LC copolymers derived from polymethacrylates with NBA-derivative side groups and (carbazolylmethylene)aniline side groups; they reported the appearance of a smectic phase and an enhanced thermal stability due to the interactions among the mesogenic side groups.34 Asaoka et al. synthesized diblock copolymers consisting of poly(ethylene oxide) and poly(methacrylate) with NBA-derivative side groups; although they investigated the ordered phase-segregated nanostructures,35 they did not investigate the photoreaction. Several types of NBA derivatives are transparent in the visible region and exhibit photoreactivity toward trans−cis photoisomerization.36−38 Because of the structural similarity to azobenzene derivatives, NBA derivatives should also undergo an axis-selective photoreaction. Although the photoreaction of monomeric NBA derivatives has been studied in solution, the photoreaction in the solid state has rarely been reported, and only a few studies have focused on the axis-selective photoreaction of NBA-containing materials.39−41 Ilieva et al. investigated an axis-selective photoreaction and polarization holography of the N-benzylideneaniline monomer in poly(methyl methacrylate) (PMMA) composite films using LP ultraviolet (UV) light and a 257 nm laser, but the generated optical anisotropy was very small (Δn = 3 × 10−4) and SR formation upon holographic exposure was not explored.39 Viswanathan et al. reported that unlike a similar polymer with azobenzene side groups, an epoxy-based polymeric film with NBA-derivative side groups did not form an applicable SR structure.40 In contrast, we have recently found that a polymethacrylate with NBA-derivative side groups (P1 in Figure 1a) undergoes an effective photoinduced orientation and SR formation using LPUV light and holographic exposure.41 However, the detailed axis-selective photoreaction, photoinduced orientation behavior, and influence of the connecting direction of the NBA-

derivative side groups on the axis-selective photoreaction have yet to be investigated. Herein two types of polymethacrylates with NBA-derivative side groups (P1 and P2, Figure 1a), which are transparent in the visible region, were synthesized, and their detailed axisselective photoreactions were explored using LP 313 nm light. Although both polymer films underwent a photoinduced orientation, the orientation ability depended on the connecting direction of the NBA-derivative side groups. Furthermore, thermal enhancement of the photoinduced optical anisotropy was achieved, but these molecularly reorientation phenomena were irreversible. Finally, SR formation with a molecularly oriented structure using 325 nm He−Cd laser beams with various interferometric polarization conditions was demonstrated.

2. EXPERIMENTAL SECTION 2.1. Materials. All starting materials were used as received from Tokyo Kasei Chemicals. Methacrylate monomers and polymers (P1 and P2) were synthesized according to Scheme S1. The Supporting Information provides detailed synthetic procedures for the monomers and polymers. Table 1 summarizes the molecular weights and thermal and spectroscopic properties of the polymers. 2.2. Photoreaction. Thin polymer films, which were approximately 0.1−0.3 μm thick, were prepared by spin-coating a methylene chloride solution of polymers (0.5−2% w/w) onto quartz or CaF2 substrates. The photoreactions were performed using a high-pressure Hg lamp equipped with a glass plate placed at Brewster’s angle and a band-pass filter at 313 nm (Asahi Spectra REX-250). The light intensity was 10 mW/cm2 at 313 nm. The photoinduced optical anisotropy was thermally amplified by annealing the exposed films at elevated temperatures for 10 min. 2.3. Polarization Holography. Polarization holography was performed using two beams from a 325 nm He−Cd laser (Kinmon Koha IK3501R-G-S) with various polarizations and 0.75 μm thick films on quartz substrates. Holographic exposure was carried out at RT. Figure 1b illustrates the experimental setup. The intensity of each beam was 20 mW (ϕ = 5 mm), and the angle between the two beams (2θ) was 5.9°; these conditions yielded a 3.15 μm grating period. 2.4. Characterization. 1H NMR spectra using a Bruker DRX-500 FT-NMR and FT-IR spectra (JASCO FTIR-410) confirmed the 325


Macromolecules

Article

Figure 2. Absorption spectra of P1 and P2 (a) in THF solution and (b) on quartz substrates. FT-IR spectra of (c) P1 and (d) P2 films on CaF2 substrates.

Figure 3. Change in the absorption spectrum when exposed to 313 nm light for (a) P1 in THF and (b) a P1 film on quartz substrate. (c) Change in the FT-IR spectrum of a P1 film when exposed to 313 nm light. (d) Intensity of the absorbance at 1621 cm−1 in LCP films as a function of exposure energy. monomers and polymers. The molecular weights of the polymers were measured by GPC (Tosoh HLC-8020 GPC system with a Tosoh TSKgel column using chloroform 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). The polarized absorption UV−vis and FT-IR spectra were measured with a Hitachi U-3010 spectrometer equipped with Glan−Taylor polarization prisms and an FTIR-410 system with a wire-grid polarizer, respectively.

The photoinduced optical dichroism (ΔA), which was used as a measure of the photoinduced optical anisotropy, was evaluated using the polarization absorption spectra and estimated as ΔA = A⊥ − A

(1)

where A∥ and A⊥ are the absorbances parallel and perpendicular to polarization (E) of LP 313 nm light, respectively. The in-plane order was evaluated using the in-plane order parameter (S), which is expressed as 326


Macromolecules S=

Article

A⊥ − A A⊥ + 2A

RT. In our case (photoirradiation at RT), a photoreaction other than photoisomerization should occur because the photoreaction is irreversible. For the film state, the spectral change in the initial stage of the photoirradiation is small for both films, and the films become insoluble in chloroform when the exposure energy exceeds 100 J/cm2, indicating that cross-linking occurs (Figure 3b and Figure S4b). Interestingly, the spectral changes of both films resemble those stored in humid conditions (Figures S2b,d), suggesting that photoirradiation is accompanied by cleavage of the CN bond, which generates cross-linking. Photocleavage of the CN bond was confirmed by FT-IR spectroscopy (Figure 3c and Figure S4c). For both PLC films, the absorption of CN (1621 cm−1) decreases. Additionally, a new absorption appears at 1684 cm−1 for the P1 film, indicating the formation of benzaldehyde groups, while a broad absorption around 3300−3500 cm−1 appears for the P2 film (−NH2 absorption). Figure 3d plots the intensity of the absorbance at 1621 cm−1 as a function of exposure energy. For both LCP films, approximately half the initial amount of the CN bond is decomposed when the exposure energy exceeds 100 J/cm2. Although trans−cis photoisomerization and thermal cis−trans relaxation occur in the initial stage of photoirradiation, photoirradiation at RT is simultaneously accompanied by photocleavage of the CN bond, which results in the spectrum irreversibility. 3.3. Axis-Selective Photoreaction of LCP Films. The NBA-derivative side groups axis-selectively photoreacted with LP 313 nm light. Figures 4a and 4b respectively plot the

(2)

The birefringence (Δn) of the reoriented film was evaluated using a polarimeter (Shintech OPTIPRO 11-200A) at 517 nm.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Thermal and Optical Properties of LCPs. Liquid crystalline polymethacrylates (LCPs) with NBAderivative side groups (P1 and P2) were synthesized from the corresponding methacrylate monomers by free radical polymerization in THF. Table 1 summarizes molecular weights and thermal and spectroscopic properties of P1 and P2. Both LCPs exhibit nematic LC characteristics, which according to DSC and POM observations, are between 41 and 111 °C for P1 and 43 and 120 °C for P2 (Figure S1a−c). These thermal properties are similar to previously reported results.34,41 Figures 2a and 2b show the absorption spectra of P1 and P2 in THF and on quartz substrates, respectively. Both LCP solutions have two absorption maxima (λmax) at 284 and 332 nm, and the films have similar absorption bands (λmax = 283 and 332 nm). These results indicate that the solution and the film state do not have conformational differences of the mesogenic side groups. The lowest-energy absorption band (λmax = 332 nm) is dominated by a HOMO−LUMO (π−π*) transition, while the second lowest-energy absorption (λmax = 283 nm) corresponds to a transition from the lower-lying occupied orbital to the π* (LUMO) orbital; these transitions have been theoretically and experimentally investigated in NBA derivatives that assume nonplanar conformations.38 Additionally, the FT-IR spectra of both LCPs exhibit similar absorption bands at 1725 cm−1 (C O stretching), 1621 cm−1 (CN stretching), 1606, 1568, and 1503 cm−1 (Ph), and 1248 cm−1 (Ph−O stretching) (Figures 2c,d). NBA derivatives are unstable under humid conditions because they are easily hydrolyzed. Although spin-coated P1 and P2 films are stable when stored under dry conditions (RH < 20%), the absorption intensity decreases when stored under humid conditions (RH > 95%) (Figure S2a−d). FT-IR spectroscopy confirmed the CN bond cleaves under humid conditions (Figures S3a,b). For a P1 film, the absorption of CN (1621 cm−1) decreases, while a new absorption due to CHO stretching appears at 1684 cm−1. The remaining CHO groups in the P1 film could be ascertained by the UV absorption spectra (Figure S2b); the absorption band around 280 nm slowly decreases (overlaps with the absorption band of 4-alkyloxybenzaldehyde). For P2, both absorption bands (two λmax) in the UV absorption spectra decrease, and FT-IR does not show CHO absorption because the monomeric 4methoxybenzaldehyde may vaporize, but Ph−NH2 vibrations are detected at 3420 and 3350 cm−1 (Figure S3b). To suppress the hydroxylation, the photoreaction and evaluation of LCP films in the subsequent sections are carried out within 3 h of film preparation under relatively dry conditions (RH ∼ 45%). 3.2. Photoreaction of LCP Films. Figures 3a,b show the changes in the UV absorption spectrum of a P1 solution in THF and a thin film upon exposure to 313 nm UV light at RT. In THF, the absorbency gradually decreases, and the spectrum does not recover when the UV light is turned off (Figure 3a). The P2 solution exhibits similar spectral changes (Figure S4a). Because of the instability of the cis isomer, trans−cis photoisomerization of NBA derivatives has been studied at low temperature,36,37 but the cis isomer has yet to be isolated at

Figure 4. Changes in the absorbances of A⊥ and A∥ of LCP films at 283 and 332 nm as functions of exposure energy for (a) P1 and (b) P2.

changes in absorbances of P1 and P2 films at 283 and 332 nm parallel (A∥) and perpendicular (A⊥) to E of LP 313 nm light as functions of exposure energy. For both LCP films, A⊥ increases in the initial stage of photoirradiation, but A∥ decreases faster due to the axis-selective photoreaction. The photoinduced optical anisotropy (ΔA = A⊥ − A∥) is maximized when the exposure energy is approximately 10−100 J/cm2 (ΔAmax at 283 nm = 0.91 for P1 and 0.67 for P2), but it decreases upon further increasing the exposure energy. Figures 5a and 5b show the polarization absorption spectra of films before and after exposure to LP 313 nm light for 50 J/cm2 for P1 and 10 J/cm2 for P2, respectively. A⊥ is larger than the initial absorbance, and the average absorbance after exposure [(A⊥ + A∥)/2] is smaller than the initial one. The increased absorbance in the perpendicular direction after exposure indicates that the mesogenic side groups reorient perpendicular to E of LP 313 nm light during the photoexposure process. Similar to the case of the photoinduced orientation of azobenzene-containing polymeric films,13−15 the 327


Macromolecules

Article

value is 0.45 (0.53) at 283 nm (332 nm), while Δn was 0.16 at 517 nm. These values are larger than those generated in the P2 film. The differences in the photoisomerization and photocleavage products between P1 and P2 affect the photoinduced reorientation ability. Polarized FT-IR spectroscopy also confirms the orientation ordering via the axis-selective photoreaction. Figures 6a and 6b show the polarization FT-IR spectra of the films when the exposure energy was 50 J/cm2 (10 J/cm2) for P1 (P2). The photoinduced anisotropy at 1621 cm−1 (CN), 1606 and 1503 cm−1 (Ph), and 1248 cm−1 (O−Ph) are positive, indicating that the NBA-derivative side groups undergo molecular reorientation. The photoinduced S values for the P1 film at 1621 and 1248 cm−1 are 0.34 and 0.50, respectively, which are comparable to those obtained by the polarized UV absorption spectra. Additionally, the photoinduced ΔA’s at 2950 cm−1 (C−H) and 1724 cm−1 (CO) are slightly negative for both films, suggesting that the reorientation process involves the alkylene spacer and polymer main chain. Furthermore, the reoriented film is stable under dry (RH < 20%) conditions, but hydrolysis occurs when stored at RH > 95% (Figures S6a,b), which reduces the orientation structure. 3.4. Photoinduced Orientation Mechanism and Kinetics. As described in section 3.3, the photoinduced orientation of the P1 and P2 films is due to both axis-selective trans−cis−trans photoisomerization and photocleavage of the NBA-derivative side groups. Namely, two types of axis-selective photoreactions of the NBA-derivative side groups contribute to the photoinduced orientation (Scheme 1). First, similar to the photoinduced orientation of azobenzene-containing polymeric materials,13,43,44 axis-selective trans−cis−trans photoisomerization of the NBA derivatives generates molecular reorientation perpendicular to E of LP 313 nm light. Second, axis-selective cleavage of the CN bond parallel to E upon the photoreaction acts as an impurity, which reduces the intrinsic ordering ability in the parallel direction in the initial stage of the axis-selective photoreaction and generates partial reorganization in the perpendicular direction, as observed in other types of photoreactive LCPs.14,30 However, the large amount of photocleavage disorganizes the orientation structure because it reduces the LC characteristics of the material (Figure 5c). Therefore, the reorientation kinetics of P1 and P2 differ from that of azobenzene-containing polymeric films.43−45 Figures 7a,b plot the growth of the photoinduced Δn as a function of exposure time and the photoinduced reorientation order parameters at each exposure time when LCP films are exposed to LP 313 nm light. For P1, Δn rapidly increases in the initial stage of photoirradiation, but it gradually decreases when

Figure 5. Polarized UV−vis absorption spectra of (a) P1 and (b) P2 films before and after exposure to LP 313 nm light. Exposure energy is 50 J/cm2 (10 J/cm2) for P1 (P2). (c) Photoinduced in-plane order parameter (S) value as a function of exposure energy.

axis-selective trans−cis−trans photoisomerization−relaxation cycle of the NBA-derivative side groups generates the molecular reorientation perpendicular to E. Because the degree of the photodecomposition is approximately 10−20% and the angular dependency of the polarized UV absorption (Figures S5a,b) shows the in-plane molecular orientation,42 the decrease in [(A⊥ + A∥)/2] is attributed to the axis-selective photocleavage of the side groups. However, disorganization of the orientation structure occurs due to the large amount of photocleavage of the side groups at the high exposure doses (>100 J/cm2). Section 3.4 describes the detailed photoinduced orientation mechanism. Table 2 summarizes the maximum photoinduced in-plane orientation order (S) and generated birefringence (Δn) values, Table 2. Maximum Photoinduced Optical Anisotropy of P1 and P2 Films Exposed to LP 313 nm Light P1 P2 a

dose (J/cm2)

ΔAa

S283

S332

Δnb

50 10

0.91 0.67

0.45 0.31

0.53 0.35

0.16 0.09

A⊥ − A∥ at 283 nm. bAt 517 nm.

and Figure 5c plots the photoinduced S values at 283 and 332 nm as functions of exposure energy. For P1, the maximum S

Figure 6. Polarized FT-IR absorption spectra of (a) P1 and (b) P2 films before and after exposure to LP 313 nm light. Exposure energy was 50 J/ cm2 (10 J/cm2) for P1 (P2). 328


Macromolecules

Article

Scheme 1. Photoinduced Reorientation Mechanism of P1 and P2

Figure 7. Photoinduced birefringence curves for (a) P1 and (b) P2 films (small dots). Solid red line is the fit of eq 3 to the birefringence curve for LCPs. Open circles are the independently measured order parameter at 332 nm.

the irradiation time exceeds 5000 s (>50 J/cm2). The photoinduced Δn and order parameter show a suitable relationship. Although both films show similar trends for Δn in the initial stage of photoreaction, the rate of decrease for a P2 film after Δn reaches a maximum is faster than that of P1. Assuming that the photoinduced orientation is simultaneously disturbed by photocleavage, the growth of Δn upon LP 313 nm light exposure is described by a combination of equations as

Table 3. Parameters Obtained by Fitting Eq 3 to the Birefringence Growth Curves in Figure 7 Xa P1 P2 a

0.52 0.66

Ya 0.48 0.34

ka (s−1) −3

1.6 × 10 3.5 × 10−3

kb (s−1) −4

2.7 × 10 4.5 × 10−4

kc (s−1) 7.3 × 10−5 4.9 × 10−4

Converted so that X + Y = 1.0.

process (kc/ka < 1/20). In contrast, kc of the P2 film is comparable to kb and is 1 order larger than that of P1. These results indicate that photocleavage disorders the orientation structure upon exposure, resulting in the lower Δnmax and faster disorganization of the orientation structure compared to P1. These differences are due to variations in the photodecomposed end groups in the side groups between P1 and P2, which influence the photoinduced orientation ability. The photogenerated 4-aminophenyl end groups in the P2 film disorganize the oriented structure upon the reorientation process, whereas the influence of the photogenerated 4benzaldeyde end groups on the reorientation process is small for P1. Because humidity affects the hydrolysis of a material, adjusting the humidity upon photoexposure will clarify the influence of photocleavage on the reorientation behavior. 3.5. Thermal Amplification of the Photoinduced Optical Anisotropy. Annealing axis-selective photoreacted LCP films enhances the molecular orientation structure. Both LCP films exhibit a significant thermal enhancement of the photoinduced optical anisotropy in the initial stage of photoexposure. Other types of LCPs comprised of azobenzene,45 benzoate, and cinnamate derivative side groups14,28−30 display similar thermal amplifications of the photoinduced optical anisotropy.

⎡ k ⎤ ⎡ k ⎤ a b (e−kat − e−kct )⎥ + Y ⎢ (e−kbt − e−kct )⎥ Δn ∝ X ⎢ ⎣ kc − ka ⎦ ⎣ kc − kb ⎦ (3)

where the first and second terms indicate the growth of Δn involving “fast” and “slow” response modes based on the fast reorientation of the side groups and slow polymer motion (ka and kb), respectively. These two modes are similar to the photoinduced reorientation based on trans−cis−trans photoisomerization cycle observed in azobenzene-containing polymeric films,43−46 but these two modes include reorientation due to axis-selective photocleavage of the side groups in the initial stage of photoexposure. However, because the large amount of photocleavage should lower the reorientation structure, both terms involve disordering the orientation structure (kc) due to the side reaction (photocleavage) (Scheme 1). Namely, the photocleavage simultaneously distorts the oriented structure upon the reorientation process. Figures 7a,b show the growth of Δn with fitting curves, and Table 3 summarizes the fitted parameters. For P1, after the “fast” and “slow” responses for the molecular orientation reach saturate, the small contribution of the photocleavage slowly disorganizes the orientation structure, but kc of the disorganization process is much less than ka and kb of the reorientation 329


Macromolecules

Article

the material. In contrast, in the initial stage of the photoreaction, not only the photoinduced reorientation structure but also the small amount of photocleavage parallel to E, which acts as an impurity and reduces the intrinsic ordering ability in the parallel direction, play important roles in the thermal enhancement of the reorientation structure perpendicular to E. Similar to other LCPs that exhibit a thermally enhanced photoinduced molecular reorientation,14 thermal enhancement is generated in the initial LC temperature range of the materials when the exposure energy is 1.0 J/cm2 (Figure 9b) 3.6. Polarization Holography. Polarization holographic exposure to P1 films was performed using a 325 nm He−Cd laser with various polarizations of two beams (Figure 1b). We have briefly reported that holographic exposure to a P1 film simultaneously generates both SR formation and molecular reorientation according to the interferometric polarization pattern.41 Figure 10a plots the generated SR height of P1 films for different polarization holography conditions as a function of

Figures 8a,b plot the polarization UV spectra of the LCP films before and after irradiating with LP 313 nm light and

Figure 8. Change in the polarized UV−vis absorption spectra of (a) P1 and (b) P2 films before and after irradiating with LP 313 nm light for 1.0 J/cm2 (P1) and 5.0 J/cm2 (P2) and subsequent annealing. Annealing temperature is of 100 °C (80 °C) for P1 (P2).

subsequent annealing. The exposure energy and annealing temperature are 1.0 J/cm2 (5.0 J/cm2) and 100 °C (80 °C) for P1 (P2), respectively. Because of the photoinduced orientation perpendicular to E of LP 313 nm light, the generated optical anisotropy ΔA is approximately 0.5 for both LCP films after exposure. Annealing the exposed films enhances the significant in-plane molecular reorientation perpendicular to E. The enhanced S at 332 nm and generated Δn values are 0.71 (0.64) and 0.21 (0.20) for P1 (P2), respectively. The maximum S and Δn values are summarized in Table 4, and Figure 9a plots Table 4. Thermal Enhancement of the Photoinduced Optical Anisotropy of P1 and P2 Films Exposed to LP 313 nm Light and Subsequent Annealing P1 P2 a

dose (J/cm2)

annealing tempa (°C)

S283

S332

Δnb

1 5

100 80

0.63 0.59

0.71 0.64

0.21 0.20

Annealed for 10 min. bAt 517 nm. Figure 10. (a) Inscribed SR heights of P1 films exposed to polarization holography as functions of exposure time. Inset shows the surface topology of the SRG. (b) First-order DEs of SRG films inscribed with ±45° polarization holography as functions of the polarization azimuth angle of the incident probe beam. Polarization of 633 nm probe beam is perpendicular to the grating vector when the incident polarization angle is 0°. (c, d) POM observations of P1 films inscribed with ±45° polarization holography between crossed polarizers. White arrows indicate the orientation direction of the film. Red arrows indicate polarization of the polarizer and analyzer direction. Exposure times of (c) 210 s and (d) 1200 s.

Figure 9. (a) Thermally enhanced S values of P1 and P2 films as functions of exposure energy. Annealing temperature is 100 °C (90 °C) for P1 (P2). (b) Thermally enhanced S values of P1 and P2 films as functions of annealing temperature with an exposure energy of 1 J/ cm2. Arrows indicate the LC temperature range of the LCPs.

exposure time, while the inset picture shows the surface topology of the SRG film. Polarization holography using two linearly polarized beams inclined 45° in opposite directions (±45°) and two circularly polarized beams with opposite circularities (±CP) generate a sufficient SR formation. The maximum SR height (hmax) of ±45° and ± CP beams are 211 and 235 nm, respectively. However, SR formation is insufficient (hmax = 25 nm) for s- and p-polarized (sp) polarization holography. These tendencies are similar to the SR formation in azobenzene-containing polymeric films, where the gradient force along the grating vector upon holographic exposure plays an important role in SR formation.47−49 Namely, because the intensity distribution in the polarization holography is uniform, photoisomerization of the molecules in glassy state creates

the photoinduced S values at 283 and 332 nm as functions of exposure energy. An effective thermal amplification occurs when the exposure energy is 0.5−10 J/cm2, and the degree of the decomposition of CN bond is less than 15%. Significant thermal amplification is not achieved when the photoinduced ΔA become saturated; the annealing procedure disorders the orientation structures of the exposed films with higher photoinduced ΔA’s because thermal distortion caused by large amount of photocleavage reduces the LC characteristics of 330


Macromolecules

Article

polarized absorption spectrum of reoriented P1 films stored in dry and humid conditions, and angular dependency of absorbance of reoriented films. This material is available free of charge via the Internet at http://pubs.acs.org.

mobile heterogeneity inside the film and the interferometric beam pattern must contain periodic linear polarization in a direction parallel to the grating vector for a sufficient SR structure.47,48 However, a long holographic exposure decreases in the SR height in all cases. Photocleavage of the side groups is partly responsible for both the reorientation and disorder of the P1 film. The fabricated SR gratings exhibit angular dependency of the first-order diffraction efficiencies (DEs). Figure 10b plots the first-order DEs of P1 films inscribed with ±45° polarization holography exposed for 210 s (h = 211 nm), 600 s (h = 196 nm), and 1200 s (h = 170 nm), as functions of the polarization azimuth angle of the incident probe beam. For the film exposed for 210 s, DE greatly depends on the polarization azimuth angle of the incident probe beam. This diffraction property is due to the periodical orientation, where the convex and concave regions are reoriented parallel and perpendicular to the grating vector, respectively (Figure 10c). This angular dependency is similar to the SR grating film inscribed to azobenzenecontaining polymeric film.25 In contrast, a SR film exposed for 1200 s shows a smaller polarization dependency of the first order DEs (DE = 6−7%), and the POM observation shows a lower optical anisotropy than that for a film exposed for 210 s (Figure 10d), indicating disordering of the orientation structure. This means that effective SR formation occurs based on the periodic photoinduced reorientation but is followed by disordering of the oriented structure. A similar tendency is observed for SR grating films inscribed with ±CP polarization holography (Figures S7 and S8).

■

Corresponding Author

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

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aid for Scientific Research from JSPS (B24350121 and S23225003).

■

REFERENCES

(1) Shibaev, V. P.; Kostromin, S. G.; Ivanov, S. A. Polymers as Electroactive and Photooptical Media; Shibaev, V. P., Ed.; Springer: Berlin, 1996; pp 37−110. (2) (a) Eich, M.; Wendorff, J. H. Macromol. Rapid Commun. 1987, 8, 467−471. (b) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4309−4312. (3) Hagen, R.; Bieringer, T. Adv. Mater. 2001, 13, 1805−1810. (4) Cipparrone, G.; Mazzulla, A.; Palto, S. P.; Yudin, S. G.; Blinov, L. M. Appl. Phys. Lett. 2000, 77, 2106−2108. (5) Crawford, G. P.; Eakin, J. N.; Radcliffe, M. D.; Callan-Jones, A.; Pelcovits, R. A. J. Appl. Phys. 2005, 98, 123102−1−10. (6) Pan, X.; Wang, C.; Wang, C.; Zhang, X. Appl. Opt. 2008, 47, 93− 98. (7) Chigrinov, V. G.; Kozenkov, V. M.; Kwok, H.-S. Photoalignment of Liquid Crystalline Materials; John Wiley & Sons: West Sussex, England, 2008. (8) O′Neill, M.; Kelly, S. M. J. Phys. D: Appl. Phys. 2000, 33, R67− R84. (9) Yaroshchuk, O.; Reznikov, Y. J. Mater. Chem. 2012, 22, 286−300. (10) Schadt, M.; Schmitt, K.; Kozinkov, V.; Chigrinov, V. Jpn. J. Appl. Phys. 1992, 31, 2155−2164. (11) Schadt, M.; Seiberle, H.; Schuster, A. Nature 1996, 381, 212− 215. (12) (a) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlin, B. J. Nature 1991, 351, 49−50. (b) Shannon, P. J.; Gibbons, W. M.; Sun, S. T. Nature 1994, 368, 532−533. (13) (a) Ichimura, K. Chem. Rev. 2000, 100, 1847−1874. (b) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139−4175. (c) Ikeda, T. J. Mater. Chem. 2003, 13, 2037−2057. (14) (a) Kawatsuki, N.; Ono, H. In Organic Electronics and Photonics; Nalwa, H. S., Ed.; American Science Publishers: Stevenson Ranch, CA, 2008; Vol. 2, pp 301−344. (b) Kawatsuki, N. Chem. Lett. 2011, 40, 548−554. (15) Seki, T.; Nagano, S.; Hara, M. Polymer 2013, 54, 6053−6072. (16) Kawatsuki, N.; Kawakami, T.; Yamamoto, T. Adv. Mater. 2001, 13, 1337−1339. (17) Kawatsuki, N.; Hasegawa, T.; Ono, H.; Tamoto, T. Adv. Mater. 2003, 15, 991−994. (18) Ono, H.; Emoto, A.; Kawatsuki, N.; Hasegawa, T. Opt. Express 2003, 11, 2379−2384. (19) Kawatsuki, N.; Goto, K.; Kawakami, T.; Yamamoto, T. Macromolecules 2002, 35, 706−713. (20) (a) Kawatsuki, N.; Ono, H.; Takatsuka, H.; Yamamoto, T.; Sangen, O. Macromolecules 1997, 30, 6680−6682. (b) Kawatsuki, N.; Hamano, K.; Ono, H.; Sasaki, T.; Goto, K. Jpn. J. Appl. Phys. 2007, 46, 339−341. (21) Häckel, M.; Kador, L.; Kropp, D.; Schmidt, H.-W. Adv. Mater. 2007, 19, 227−231. (22) Okano, K.; Tsutsumi, O.; Shishido, A.; Ikeda, T. J. Am. Chem. Soc. 2006, 128, 15368−15369.

4. CONCLUSION Two types of polymethacrylates with NBA-derivative side groups were synthesized, and the photoinduced orientation and polarization holography of the thin films were performed. Irradiating with 313 nm light induces photoisomerization and the photocleavage, which is irreversible in both solution and the film. The axis-selective photoreaction of thin films generates molecular orientation perpendicular to the polarization of LP 313 nm light, but the reorientation ability of P1 is superior to that of P2. Furthermore, the reoriented structure is amplified when films are annealed after the small amount of axis-selective photoreaction. The maximum photoinduced in-plane S and Δn are ∼0.53 and ∼0.16, respectively, and the amplified S and Δn for P1 films are ∼0.71 and ∼0.21, respectively. Finally, polarization holography generates a SR structure with periodical molecular orientation. Although the subscribed SR height reaches up to 235 nm, a prolonged exposure deteriorates the orientation and slightly decreases the SR height. These photoresponsive polymeric materials with NBA derivatives are applicable to birefringent optical devices, optical memories, and LC photoalignment layers, which are transparent in the visible region, under nonhumid conditions. Additionally, a subsequent chemical reaction using hydrolyzed aldehyde or amine end groups at the film surface may supply new functionalities to the oriented films and SR structures.

■

AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

Text and schemes providing the synthetic procedures of monomers and polymers, DSC chart, POM photographs, figures showing the changes in UV−vis and FT-IR spectra of LCPs stored in dry and humid conditions, change in the UV− vis spectra of P2 irradiated with 313 nm light, change in 331


Macromolecules

Article

(23) Han, D.; Tong, X.; Zhao, Y.; Galstian, T.; Zhao, Y. Macromolecules 2010, 43, 3664−3671. (24) (a) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136−138. (b) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166−1168. (c) Holme, N. C. R.; Nikolova, L.; Ramanujam, P. S.; Hvilsted, S. Appl. Phys. Lett. 1997, 70, 1518−1521. (25) Nishioka, E.; Kondo, M.; Emoto, A.; Ono, H.; Kawatsuki, N. Jpn. J. Appl. Phys. 2012, 51, 021601. (26) Walker, R.; Audorff, H.; Kadar, L.; Schmidt, H.-W. Adv. Funct. Mater. 2009, 19, 2630−2638. (27) Koskela, J. E.; Vapaavuori, J.; Hautala, J.; Priimagi, A.; Faul, C. F. J.; Kaivola, M.; Ras, R. H. A. J. Phys. Chem. C 2012, 116, 2363−2370. (28) (a) Kawatsuki, N.; Tsutsumi, R.; Takatsuka, H.; Sakai, T. Macromolecules 2007, 40, 6355−6360. (b) Kawatsuki, N.; Yamashita, A.; Fujii, Y.; Kitamura, C.; Yoneda, A. Macromolecules 2008, 41, 9715− 9721. (29) (a) Uchida, E.; Kawatsuki, N. Macromolecules 2006, 39, 9357− 9364. (b) Kawatsuki, N.; Kawanishi, T.; Uchida, E. Macromolecules 2008, 41, 4642−4650. (30) (a) Kawatsuki, N.; Neko, T.; Kurita, M.; Nishiyama, A.; Kondo, M. Macromolecules 2011, 44, 5736−5742. (b) Kawatsuki, N.; Matsushita, H.; Washio, T.; Kurita, M.; Kondo, M. Macromolecules 2012, 45, 8547−8554. (31) Kawatsuki, N.; Tashima, A.; Manabe, S.; Kondo, M.; Okada, M.; Matsui, S.; Emoto, A.; Ono, H. React. Funct. Polym. 2010, 70, 980− 985. (32) (a) Kumar, S. Liquid Crystals; Cambridge University Press: Cambridge, England, 2001. (b) He, Q.; Dai, H.; Tan, X.; Cheng, X.; Liu, F.; Tschierske, C. J. Mater. Chem. C 2013, 1, 7148−7154. (33) Frosini, V.; Levita, G.; Lupinacci, D.; Magagnini, P. L. Mol. Cryst. Liq. Cryst. 1981, 66, 21−36. (34) (a) Kosaka, Y.; Kato, T.; Uryu, T. Macromolecules 1994, 27, 2658−2663. (b) Kosaka, Y.; Kato, T.; Uryu, T. Macromolecules 1995, 28, 7005−7009. (35) Asaoka, S.; Uekusa, T.; Tokimori, H.; Komura, M.; Iyoda, T.; Yamada, T.; Yoshida, H. Macromolecules 2011, 44, 7645−7658. (36) (a) Fischer, E.; Frei, Y. J. Chem. Phys. 1957, 27, 808−809. (b) Warren, C. H.; Wettermark, G.; Weiss, K. J. Am. Chem. Soc. 1971, 93, 4658−4663. (c) Yoshida, M.; Kobayashi, M. Bull. Chem. Soc. Jpn. 1981, 54, 2395−2398. (d) Lewis, J. W.; Sandorfy, C. Can. J. Chem. 1981, 60, 1720−1726. (37) King, N. R.; Whale, E. A.; Davis, F. J.; Gilbert, A.; Mitchell, G. R. J. Mater. Chem. 1997, 7, 625−630. (38) Luo, Y.; Utecht, M.; Dokíc, J.; Korchak, S.; Vieth, H.-M.; Haag, R.; Saalfrank, P. ChemPhysChem 2011, 12, 2311−2321. (39) Ilieva, D.; Nedelchev, L. Opt. Appl. 2012, 42, 207−214. (40) Viswanathan, N. K.; Kim, D. Y.; Bian, S.; Williams, J.; Liu, W.; Li, L.; Samualson, L.; Kumar, J.; Tripathy, S. K. J. Mater. Chem. 1999, 9, 1941−1955. (41) Kawatsuki, N.; Matsushita, H.; Kondo, M.; Sasaki, T.; Ono, H. APL Mater. 2013, 1, 022103. (42) (a) Meier, J. G.; Ruhmann, R.; Stumpe, J. Macromolecules 2000, 33, 843−850. (b) Kawatsuki, N.; Hayashi, M.; Yamamoto, T. Macromol. Chem. Phys. 2001, 202, 3087−3098. (43) (a) Han, M.; Morino, S.; Ichimura, K. Macromolecules 2000, 33, 6360−6371. (b) Han, M.; Ichimura, K. Macromolecules 2001, 34, 90− 98. (44) Buffeteau, T.; Labarthet, F. L.; Sourisseau, C.; Kostromine, S.; Bieringer, T. Macromolecules 2004, 37, 2880−2889. (45) (a) Natansohn, A.; Ho, M. S.; Rochon, P. Macromolecules 1995, 28, 6124−6127. (b) Buffeteau, Th.; Natansohn, A.; Rochon, P.; Pézolet, M. Macromolecules 1996, 29, 8783−8790. (46) Koskela, J. E.; Vapaavuori, J.; Hautala, J.; Priimagi, A.; Faul, C. F. J.; Kaivola, M.; Ras, R. H. A. J. Phys. Chem. C 2012, 116, 2363−2370. (47) (a) Bian, S.; Li, L.; Kumar, J.; Kim, D. Y.; Williams, J.; Tripathy, S. K. Appl. Phys. Lett. 1998, 73, 1817−1819. (b) Kumar, J.; Li, L.; Jiang, X. L.; Kim, D. Y.; Seung, T.; Tripathy, S. K. Appl. Phys. Lett. 1998, 72, 2096−2098.

(48) Pedersen, T. G.; Johansen, P. M.; Holme, N. C. R.; Ramanujam, P. S.; Hvilsted, S. Phys. Rev. Lett. 1998, 80, 89−92. (49) Viswanathan, N. K.; Balasubramanian, S.; Li, L.; Tripathy, S. K.; Kumar, J. Jpn. J. Appl. Phys. 1999, 38, 5298−5937.

332


Photoinduced Orientation of Photoresponsive ... - ACS Publications

Recommend Documents