Photoinduced orientation of photoresponsive copolymers with N-benzylideneaniline and nonphotoreactive mesogenic side groups

Polymer 56 (2015) 318e326

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Photoinduced orientation of photoresponsive copolymers with N-benzylideneaniline and nonphotoreactive mesogenic side groups Nobuhiro Kawatsuki a, *, Teppei Washio a, Junji Kozuki a, Mizuho Kondo a, Tomoyuki Sasaki b, Hiroshi Ono b a b

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

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 October 2014 Accepted 1 November 2014 Available online 18 November 2014

The photoinduced reorientation behaviors of liquid crystalline (co)polymethacrylates (LCPs) comprised of N-benzylideneaniline (NBA) and 4-cyanobiphenyl or 4-methoxybiphenyl side groups are compared using linearly polarized (LP) 313 nm and LP 365 nm light. LCP films realize effective cooperative reorientation based on trans-cis-trans photoisomerization of the NBA moieties using LP 365 nm light and stabilization of the orientation structure due to copolymerization. In contrast, photochemical hydrolysis lowers the reorientation efficiency of LCP films when using 313 nm light. Changing the polarization direction of the LP 365 nm light controls the reorientation direction of the LCP films for several cycles, but the in-plane orientation order gradually decreases. Finally, LP 365 nm exposure and subsequent solvent annealing using a photo-cross-linked film achieve a repeatable write-erase-write of the orientation structure of the copolymer film. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Liquid crystalline polymer Photoalignment Birefringent film

1. Introduction Photoinduced reorientation of photoresponsive polymeric films has received much attention because it has potential in photoalignment of low-molecular-weight liquid crystals (LCs), fabrication of birefringent films, optical information storage, and polarization holography [1e10]. Several types of photoresponsive moieties generate a photoinduced optical anisotropy due to an axis-selective photoreaction. Of the many materials investigated for photoalignment and optical memory applications [11e19], azobenzene-containing polymeric materials have been reported to induce a photoinduced molecular reorientation perpendicular to the polarization of linearly polarized (LP) light due to axis-selective trans-cis-trans photoisomerization [11,16,17]. Additionally, controlling the polarization of LP light can alter the photoinduced reorientation direction in azobenzene-containing films because photoisomerization is reversible. Few studies have examined the photoinduced reorientation based on axis-selective photoisomerization in materials other than azobenzene-containing

* Corresponding author. E-mail address: [email protected] (N. Kawatsuki). http://dx.doi.org/10.1016/j.polymer.2014.11.007 0032-3861/© 2014 Elsevier Ltd. All rights reserved.

materials. However, several types of irreversible photoinduced molecular reorientations have achieved in photo-cross-linkable LC polymeric films comprised of cinnamates or cinnamic acid side groups [19e23]. N-benzylideneaniline (NBA) derivatives undergo a trans-cis photoisomerization similar to azobenzene derivatives, and their photoreaction mechanism and photoreacted products in solution have been investigated in detail [24e36]. Due to the transparency of several types of NBA derivatives in the visible region, polymers containing NBA derivatives can be utilized in display applications, but the photoreaction of NBA derivatives in the solid state has rarely been reported; a small photoinduced optical anisotropy in a thin poly(methyl methacrylate) (PMMA) film doped with the NBA monomer has been investigated using LP ultraviolet (UV) light [37]. Additionally, LC monomeric materials of 4,40 -alkyloxy substituted NBA derivatives have been synthesized, where the NBA moieties act as a LC mesogenic core [38,39]. Several types of polymers comprised of NBA derivative side groups or with NBA main chains have been synthesized and characterized [40e44], but few studies on the axis-selective photoreaction of NBA-containing LC polymeric films have been explored thus far [43,44]. Recently, we reported an effective photoinduced orientation of a polymethacrylate with NBA derivative side groups using LP 313 nm

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2. Experimental 2.1. Materials

Fig. 1. Chemical structure of the liquid crystalline (co)polymers used in this study.

All starting materials and solvents were used as received from Tokyo Chemical Industry. Methacrylate monomers were synthesized according to the literatures and (co)polymers (P1, P2 and P3) were synthesized by free radical copolymerization with an AIBN initiator in THF [40,46]. Table 1 summarizes the molecular weights and thermal and spectroscopic properties of the copolymers. 2.2. Photoreaction

light and its holographic surface relief (SR) formation [45,46]. Axisselective trans-cis-trans photoisomerization of the NBA side groups in the polymeric films generates molecular reorientation perpendicular to the polarization of LP 313 nm light. These polymers are expected to be new photochromic materials in photoalignment applications. However, exposure to LP 313 nm light also disturbs the photoinduced orientation structure because in addition to photoisomerization the NBA moiety undergoes sidephotoreactions [46]. Additionally, the influence of the wavelength of LP light on the photoreaction and photoinduced reorientation behavior as well as the reversibility of the photoinduced orientation have yet to be investigated. Copolymerization of comonomers comprised of nonphotoresponsive side groups with photoreactive polymers composed of photoalignable mesogenic side groups realizes a cooperative photoinduced orientation of both mesogenic groups [47e50]. Copolymerization can control the photoinduced reorientation ability and photoinduced birefringence of the aligned films by adjusting the type of the comonomer and the copolymerization ratio. Many types of comonomers comprised of nonphotoresponsive mesogenic side groups have been copolymerized with azobenzene-containing monomers; cooperative molecular reorientation of the azobenzene and the nonphotoreactive groups has been achieved [47,49]. Such an approach should effectively control the photoinduced reorientation ability of NBA-containing polymeric films. Herein, we synthesize liquid crystalline (co)polymethacrylates (LCPs) comprised of NBA derivatives and nonphotosensitive 4cyanobiphenyl or 4-methoxybiphenyl side groups (P1, P2, and P3, Fig. 1a) and explore their photoreactions and cooperative photoinduced reorientations. Specially, we investigated in detail the influence of the wavelength of irradiating UV light on the photoreaction in solution and in a thin film state using 313 nm light and 365 nm light sources as well as the difference in the photoinduced reorientation behavior of thin films. The photoreaction and photoinduced reorientation characteristics are greatly affected by the wavelength of LP light. Finally, a reversible photoinduced orientation in a copolymer films is demonstrated by irradiating with LP 365 nm light and a solvent annealing process.

Thin polymer films, which were approximately 0.15 mm thick, were prepared by spin-coating a methylene chloride solution of polymers (1 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 or 365 nm (Asahi Spectra REX-250). The light intensity was 10 (5) mW/cm2 at 313 (365) nm. 2.3. Characterization 1

H NMR spectra using a Bruker DRX-500 FT-NMR and FT-IR spectra (JASCO FTIR-410) confirmed the monomers and polymers. The molecular weights of the polymers were measured by GPC (Tosoh HLC-8020 GPC system with a Tosoh TSKgel column and 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 UVevis and FT-IR spectra were measured with a Hitachi U-3010 spectrometer equipped with GlaneTaylor polarization prisms and an FTIR-410 system with a wire-grid polarizer, respectively. The photoinduced optical dichroism (DA), which was employed as a measure of the photoinduced optical anisotropy, was evaluated using the polarization absorption spectra and was estimated as

DA ¼ A⊥  Ajj

(1)

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

S¼

A⊥  Ajj A⊥ þ 2Ajj

(2)

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

Table 1 Copolymer composition, molecular weight, and thermal and spectroscopic data of LCPs. LCP

R

Molecular weighta Mn (  10

P1 P2 P3 a b c d

e CN OCH3

1.2 4.4 1.4

4

)

Thermal propertyb

Spectroscopic data (lmax, nm)

Mw/Mn

( C)

Filmc

Solutiond (ε  104)

1.6 1.5 2.2

G 51 N 111 I G 66 SmA 147 G 67 N 133 I

283, 332 288, ~350 272, 335

283 (2.05), 332 (1.94) 292 (2.42), 345 (0.83) 276 (2.26), 333 (1.94)

Determined by GPC, polystyrene standards. Determined by DSC, 2nd heating. G: glass, N: nematic, SmA: smectic A, I: isotropic. On a quartz substrate. In THF.



320

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Fig. 2. Absorption spectra of P1, P2, and P3. (a) Films on quartz substrates and (b) in THF solutions.

(a) P2-313 nm

1 Absorbance (a.u.)

1.2

2

Exposure energy (J/cm ) initial 1.2 3 9 21 45 90 120

0.8 0.6 0.4 0.2 0

250

300 350 400 450 Wavelength (nm)

500

(b) P2-365 nm 2

Exposure energy (J/cm )

1 Absorbance (a.u.)

1.2

initial 2 5 10 20 40 100 500

0.8 0.6 0.4 0.2 0

250

300 350 400 450 Wavelength (nm)

500

Fig. 3. Changes in the absorption spectrum of the P2 film on a quartz substrate when exposed to (a) 313 nm and (b) 365 nm light.

Fig. 4. Residual film thickness of LCP films after immersing in chloroform as a function of exposure energy at (a) 313 nm and (b) 365 nm light. Change in FT-IR spectrum of a P2 film before and after exposure to (c) 313 nm and (d) 365 nm light.

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Fig. 5. Polarized UVevis absorption spectra of (a) P1, (b) P2, and (c) P3 films before and after exposure to LP 365 nm light. Exposure energies are 4 J/cm2, 45 J/cm2, and 36 J/cm2 for P1, P2, and P3, respectively.

3. Results and discussion 3.1. Thermal and spectroscopic properties of LCPs All (co)polymers were synthesized by free radical polymerization. The resulting LCPs are soluble in common organic solvents such as chloroform, toluene, and THF. Table 1 summarizes the thermal and spectroscopic data of LCPs. Homopolymer P1 and copolymer P3 exhibit a nematic LC phase, while P2 has a smectic A phase, which is induced by the 4-cyanobiphenyl side groups (Fig. S1) [51]. Fig. 2a and b plot the UVevis absorption spectra of LCP films on quartz substrates and in THF solutions, respectively. The similar absorption spectra indicate that aggregation of the mesogenic side groups does not occur in the film state. In all cases, two absorption maxima (lmax) are observed because the NBA moiety has lmax's at 283 nm and 332 nm in film and in solution. lmax of NBA at the shorter wavelength overlaps with the absorption band of the 4-cyanobiphenyl (4methoxybiphenyl) groups. However, lmax at the longer wavelength is due mainly to the NBA groups. 3.2. Photoreaction of LCPs The photoreaction behaviors of LCP in films and in THF solutions were evaluated using 313 nm and 365 nm light. Fig. 3a and b plot the changes in the absorption spectra of P2 films upon irradiating

with non-polarized (NP) 313 and 365 nm light, respectively. Exposing the film to 313 nm light decreases the absorbance at lmax but increases the absorption band around 250 nm. In contrast, irradiating with 365 nm light decreases the entire spectrum. These observations indicate that the photoreaction with 365 nm light differs from that with 313 nm light. Similar spectral changes are observed for P1 and P3 films (Fig. S2). In solution, although the absorbance greatly decreases when exposed to 313 nm light (Fig. S3a,c,e), the decrease is much slower when exposed to 365 nm light (Fig. S3b,d,f). The activation energy for the trans-cis photoisomerization of NBA derivatives in solution is lower than that for azobenzene and stilbene derivatives, and isolating the NBA cis-isomer is difficult [25e30]. Additionally, photochemical hydrolysis and photodimerization of phenylimine and NBA derivatives are initiated by a small amount of carbonyl compounds in solution [31e36]. Therefore, the spectral changes of LCPs exposed to 313 nm light suggest that side-photoreactions occur other than trans-cis photoisomerization. However, the smaller spectral change using 365 nm indicates the trans-cis photoisomerization is the main part in solution. To confirm the photoreaction of the NBA groups in the film state, solubility tests of the exposed film were performed. Fig. 4a and b plot the residual film thickness of LCP films after immersing the exposed films in THF as functions1 of exposure energy. Films

Scheme 1. Photoreactions of NBA moiety.

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Fig. 6. Changes in the FT-IR spectrum of a P2 film before and after exposure to (a) LP 313 nm and (b) LP 365 nm light. Insets are enlargements of the CeN stretching band.

exposed to 313 nm light become insoluble in the initial stage of the photoirradiation (<20 J/cm2), indicating that photo-cross-linking occurs, but irradiating with 365 nm light require more than 100 J/cm2 for P1, and 500 J/cm2 for P2 and P3 for the films to become insoluble. A larger exposure energy for the copolymers is due to the dilution effect of the NBA moieties upon cross-linking. Additionally, after irradiating with 313 nm light, the FT-IR spectra of all the LCP films exhibit a new absorption band around 3200e3600 cm1 while the band around 2900 cm1 decreases, which correspond to NeH stretching and CeH stretching bands, respectively (Fig. 4c, Fig. S4a,c). However, after irradiating with 365 nm light, the appearance of the NeH band is ambiguous and the CeH stretching increases (Fig. 4d, Fig. S4b,d). When an insoluble film exposed to 313 nm light is immersed in fluorescamine in a diethyl ether solution (5.0  103 M), the film emits blue fluorescence (Fig. 4c, inset). This result suggests the presence of primary

amine moiety, which reacts with fluorescamine [52], is due to photochemical hydrolysis. These results indicate that the 313 nm light irradiation simultaneously initiates photochemical hydrolysis and cross-linking other than the photoisomerization of the NBA moieties. These side-photoreactions are much slower, especially for the copolymers, and the photochemical hydrolysis is restricted when 365 nm light is used (Scheme 1). Additionally, decreases in the CeN stretching band at 2221 cm1 and CeO stretching at 1240 cm1 occur for P2 films when exposed to 313 nm or 365 nm light (Fig. 4c,d). Combined with the decrease in the UVevis absorption spectrum, these observations indicate that the simultaneous occurrence of the out-of-plane orientation of the 4-cyanobiphenyl side groups. The angular dependence of the absorption spectrum also suggests an out-of-plane orientation (Fig. S5a,b). Other LCPs also indicate the photoinduced out-of-plane reorientation. The

Fig. 7. Changes in the absorbances of A⊥ and Ajj of LCP films at lmax as functions of exposure energy with LP 313 nm light for (a) P1, (b) P2, and (c) P3. (d) Photoinduced order parameter of the LCP films as a function of exposure energy.

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Fig. 8. Changes in the absorbances of A⊥ and Ajj of LCP films at lmax as functions of exposure energy with LP 365 nm light for (a) P1, (b) P2, and (c) P3. (d) Photoinduced order parameter of the LCP films as a function of exposure energy.

continuous trans-cis-trans photoisomerization of the NBA groups to the actinic light propagation axis causes a cooperative out-ofplane reorientation, which has been observed in many types of azobenzene-containing copolymer films when using NP light [53e56]. However, when the exposure energy of 313 nm light is 120 J/cm2, angular dependence does not show the out-of-plane characteristics (Fig. S5a). This means that side-photoreactions caused by irradiating with 313 nm light disturbs the orientation structure at the high exposure energy. 3.3. Axis-selective photoreaction in LCP films Cooperative photoinduced orientation of both mesogenic side groups is observed when the LCP films are exposed to LPUV light. Fig. 5aec plot the changes in the polarized UVevis absorption spectra of LCP films before and after exposure to LP 365 nm light. In all cases, the absorbance perpendicular (A⊥) increases, while that parallel (Ajj) decreases, indicating the cooperative reorientation Table 2 Photoinduced optical anisotropy and three-dimensional refractive indices of LCP films exposed to LP 365 nm light.

P1 P1 P2 P2 P3 P3

Dose (J/cm2)

DAa

nxb

ny b

nz b

3 200 45 600 60 300

0.64 0.10 0.77 0.73 0.79 0.74

1.642 1.614 1.644 1.630 1.646 1.629

1.550 1.592 1.535 1.541 1.522 1.526

1.609 1.595 1.614 1.629 1.632 1.642

A⊥  Ajj at lmax (shorter wavelength). ±0.002 at 517 nm. nx (ny) is refractive index perpendicular (parallel) to E of LP 365 nm light. nz is refractive index in light incident direction. a

b

occurs perpendicular to E. Cooperative orientation is also achieved when LP 313 nm light is used (Fig. S6aec). The polarized FT-IR spectroscopy suggests a cooperative photoinduced in-plane orientation in P2 films (Fig. 6a and b). For both exposure light wavelengths, A⊥ at 2221 cm1 (CeN stretching) and at 1240 cm1 (CeO stretching) increase before exposure. These absorption bands indicate a negative dichroism, suggesting reorientation is perpendicular to E of LP light. Fig. 7aec plot the changes in the absorbances of A⊥ and Ajj of LCP films at lmax, while Fig. 7d plots the photoinduced in-plane order parameter of LCP films as functions of exposure energy of LP 313 light. For P1, the maximum DA is obtained when the exposure energy is 10 J/cm2, but further increasing the exposure energy decreases the orientation (Fig. 7a). Similar cooperative reorientation behaviors are observed for P2 and P3, but the required energies for maximum DA's are larger than that for P1 (Fig. 7b,c). As plotted in Fig. 7d, the maximum S values are less than 0.4, and rapidly decrease as the exposure doses increase. The rapid decrease in the absorbance in both directions and the decrease in S values after exhibiting maxima are due to the side-photoreactions that

Table 3 Parameters obtained by fitting Eq. (3) to the birefringence growth curves in LCP films. LP light P1 P2 P3 P1 P2 P3 a

313 313 313 365 365 365

Xa 0.66 0.57 0.41 0.23 0.36 0.33

Ya 0.34 0.43 0.59 0.77 0.64 0.67

Converted so that X þ Y ¼ 1.0.

ka (s1) 3.5 4.2 9.3 3.5 5.2 8.9

     

kb (s1) 3

10 104 105 102 103 103

4.5 4.0 9.0 7.9 1.4 2.0

     

kc (s1) 4

10 104 105 105 103 103

4.9 4.0 1.1 1.3 2.1 1.8

     

104 104 104 103 105 105

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Fig. 9. Photoinduced birefringence curves of LCP films (small dots) when exposed to (a) LP 313 nm, and (b) LP 365 nm light. Solid lines are the fits of Eq. (3) to the birefringence curves for the LCP films. Insets are photoinduced birefringence curves at the initial stage of the photo-exposure.

occur from the initial stage of photoirradiation with 313 nm light. Especially, the formation of 4-aminophenyl groups in the polymer side chain due to the photochemical hydrolysis disorders the orientation structure during the reorientation process [46]. In contrast, molecular reorientation using LP 365 nm light requires a lower exposure energy than that using LP 313 nm light (Fig. 8 aec). The excitation of the absorption band at a longer wavelength of NBA moieties effectively reorients the mesogenic side groups upon trans-cis-trans photoisomerization. Additionally, the decrease in the cooperative photoinduced orientation after reaching the maximum DA and S values is slower using LP 365 nm light irradiation (Fig. 8aed), and the maximum DA and S values (DAmax > 0.75, Smax > 0.42 at lmax) for P2 and P3 film are larger than those obtained by LP 313 nm light exposure (DAmax < 0.6). In these cases, photochemical hydrolysis is restricted, limiting the decrease in the reorientation structure. Additionally, for all cases in Fig. 8aec, (A⊥ þ Ajj)/2 after the exposure is smaller than that before exposure, indicating that the out-of-plane reorientation is simultaneously generated similar to azobenzene-containing polymeric films [55e58]. The estimated three-dimensional refractive indices of the reoriented LCP films confirm the biaxial photoinduced orientation characteristics of the reoriented LCP films (Table 2). At high exposure doses (200 J/cm2) for the P1 film, DA drastically decreases and (A⊥ þ Ajj)/2 is much smaller than that before exposure. Considering that the angular dependence of the absorption spectrum does not show out-of-plane orientation

characteristics (Fig. S7a), this angular dependence is due to the disruption of the in-plane orientation caused by photo-crosslinking. On the other hand, a much higher exposure energy is necessary to decrease in the in-plane order of copolymer P2 and P3 films because photo-cross-linking is slow, as mentioned in Section 3.2. The cooperative motion of the both mesogenic groups restricts the photo-cross-linking of the NBA side groups, resulting in smaller changes in the three-dimensional refractive indices (Table 2, Fig. S7b,c). Furthermore, the reorientation rates of P2 and P3 are slower than that of P1, similar to the case employing LP 313 nm light. The light absorption of the nonphotoreactive side groups and cooperative motion reduce the orientation rate. 3.4. Photoinduced reorientation kinetics To quantitatively estimate the photoinduced reorientation rate, the growth of the photoinduced Dn as a function of exposure time when LCP films are exposed to LP 313 or LP 365 nm light is evaluated [46]. Assuming that the side-photoreactions simultaneously disturb the photoinduced in-plane orientation and growth of Dn, the growth of Dn upon LP light exposure is described by a combination of equations as



Dn ∞ X

   ka  ka t kb  kb t e e  ekc t þ Y  ekc t ; kc  ka kc  kb

(3)

Fig. 10. Absorbances in the perpendicular and parallel directions for (a) P1 and (b) P2 films when the polarization from LP 365 light is orthogonally changed. Each exposure energy dose is 3 (30) J/cm2 for P1 (P2).

N. Kawatsuki et al. / Polymer 56 (2015) 318e326

where the first and second terms indicate the growth of Dn involving “fast” and “slow” response modes based on the fast reorientation of the side groups and slow polymer motion (ka and kb), respectively [59,60], and disturbance of the orientation structure including the side photoreaction (kc) [46]. Fig. 9a and b shows the growth of Dn with the fitting curves when exposed to LP 313 and LP 365 nm light, respectively. Table 3 summarizes the fitted parameters. When LP 313 nm light is used for P1, Dn rapidly increases in the initial stage of photoirradiation, but it quickly decreases once the irradiation time exceeds 800 s (~8 J/cm2) (kc/ka ~ 1/7). The rapid decrease of Dn after molecular orientation becomes saturated due to the photochemical hydrolysis and cross-linking during the reorientation process. For P2, the increase in Dn is slower than that of P1, and P3 shows even slower responses. This tendency is consistent with the photoinduced in-plane order vs. exposure energy curves (Fig. 7d). It should be noted that the calculated ka, kb, and kc values of copolymers P2 and P3 are comparable although the fitting curves slightly deviate from the experimental ones. This means that the cooperative motion of the mesogenic side groups has the same response to the polymer motion in the copolymers, resulting in the slow reorientation rate. Additionally, the difficulty of fitting the curves is attributed to the complicated motion of the both mesogenic side groups and side-photoreactions upon the photo-exposure. In contrast, when using LP 365 nm light, the decrease after Dn reaches a maximum is much slower than that using 313 nm light, although the growth of Dn shows similar trends in the initial stage of photoreaction. For P1, the reorientation process includes both “fast” and “slow” responses, and the decrease in Dn after the rapid increase is due to the side photoreaction from photo-cross-linking without photochemical hydrolysis, where the kc value is one order

325

of magnitude slower than ka (kc/ka ~ 1/30). For copolymers P2 and P3, the ka values are on the same order as kb (kb/ka ~ 1/5) similar to the case of LP 313 nm irradiation, but the kc values are much smaller than ka (kc/ka < 1/250). The cooperative motion of the mesogenic side groups induces the same response as the polymer motion, and the subsequent slow disorder of the in-plane structure is due to photo-cross-linking. Namely, copolymerization of comonomers with nonphotoreactive mesogenic monomer suppresses the disorder of the in-plane structure compared to the homopolymer. 3.5. Reversibility of the photoinduced reorientation Adjusting the polarization direction using LP 365 nm light controls the photoinduced reorientation directions in P1 and P2 films. Fig. 10a and b plot the change in absorbances of P1 and P2 films, respectively, both parallel and perpendicular to the polarization of LP 365 nm light when the films are exposed to LP 365 nm light while changing the polarization direction. For both films, when the polarization direction of LP 365 nm light is orthogonally changed, the absorbances in both directions and DA decrease. Further exposure while changing the polarization direction further decreases DA due to the generation of the out-of-plane orientation of the mesogenic side groups parallel to the polarization direction upon the second exposure. Because the out-of-plane reorientation simultaneously occurs upon LP 365 nm light exposure as described in Section 3.3, changing the polarization direction by 90 simultaneously induces an out-of-plane motion of the mesogenic side groups in the in-plane direction. The change in this reorientation process is being further investigated because it seems that the out-of-plane motion is enhanced upon exposing reoriented copolymer films. Finally, a write-erase-write process of the photoinduced orientation is performed using a partially cross-linked P2 film, which

Fig. 11. (a) Changes in the absorption spectrum of a P2 film before and after exposure to NP 365 nm light for 500 J/cm2 and subsequent solvent annealing in THF vapor. (b) Changes in the absorption spectrum of a solvent annealed P2 film (Fig. 11a) and after exposure to LP 365 nm light for 30 J/cm2 and subsequent solvent-annealing in THF vapor. (c) Changes in the absorbance of a photo-cross-linked P2 film when repeating the write-erase-write process.

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was prepared by exposing a film to NP 365 nm light for 500 J/cm2. Exposing to NP 365 nm light generates out-of-plane reorientation with partial photo-cross-linking, but the random orientation of the film is recovered when the exposed film is solvent-annealed using THF vapor (Fig. 11a). In this case, the degree of photo-cross-linking of the NBA side groups is less than 10%. Using this film, the writing process is carried out by irradiating with LP 365 nm light for 30 J/ cm2, which results in an in-plane order of 0.38 at 288 nm and Dn of 0.11. Solvent annealing erases the photoinduced orientation structure (Fig. 11b). This process can be repeated more than 10 times (Fig. 11c). 4. Conclusion Liquid crystalline (co)polymethacrylates comprised of NBA side groups and 4-cyano(methoxy)biphenyl side groups were synthesized and their photoreactions using 313 nm and 365 nm light were investigated. Upon exposing the (co)polymer films to NP 313 nm light, photoinduced out-of-plane reorientation based on the transcis-trans photoisomerization of the NBA side groups is induced accompanied by the side-photoreactions of photochemical hydrolysis and photo-cross-linking, which disturb the reorientation structure. In contrast, photochemical hydrolysis is restricted to irradiation with 365 nm light. When using LP light, cooperative inplane as well as the out-of-plane orientation of both mesogenic side groups is generated, and the order of the generated in-plane orientation in the copolymer films is larger than that of homopolymer using LP 365 nm light. Furthermore, cooperative reorientation of the copolymer films is slower than that of the homopolymer, and the orientation structure of the copolymer films is more stable against the prolonged light exposure. By adjusting the polarization of LP 365 nm light, the reorientation direction can be changed several times, but the out-of-plane motion reduces the in-plane orientation order. LP light exposure and the solvent annealing process using the photo-cross-linked film can attain repeatable write-erase-write of the orientation structure. A further study on the photocontrol of the reorientation direction while suppressing the out-of-plane motion is in progress. We anticipate that the NBA-containing copolymer films will realize new photoalignable materials for photoalignment and optical memory applications. Acknowledgments This work was partially supported by Grants-in-Aid for Scientific Research from JSPS (B24350121 and S23225003). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2014.11.007. References [1] Chigrinov VG, Kozenkov VM, Kwok HS. Photoalignment of liquid crystalline materials. West Sussex, England: John Wiley & Sons; 2008. [2] Shibaev VP, Kostromin SG, Ivanov SA. In: Shibaev VP, editor. Polymers as electroactive and photooptical media. Berlin: Springer; 1996. p. 37e110. [3] Seki T, Kawatsuki N, Kondo M. In: Goodby JW, Collings PJ, Kato T, Tschierske C, Gleeson HF, Raynes P, editors. Handbook of liquid crystals, vol. 8. Weinheim: Wiley-VCH; 2014. p. 539e79.

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