Photo-Durable Molecularly Oriented Liquid Crystalline Copolymer Film based on Photoalignment of N-benzylideneaniline

Copolymer films of photoalignable liquid crystalline (LC) copolymethacrylates comprised of a phenyl benzoate mesogen connected with N-benzylideneaniline end moiety (NBA2) and benzoic acid (BA) side groups exhibit a photoinduced reorientation behavior. Significant thermally stimulated molecular reorientation attains a dichroism (D) greater than 0.7 for all copolymer films and a birefringence of 0.113–0.181. In situ thermal hydrolysis of the oriented NBA2 groups decreases the birefringence to 0.111–0.128. However, the oriented structures of the film are maintained, demonstrating a photo-durability, even though the NBA2 side groups photo-react. The hydrolyzed oriented films show higher photo-durability without changing their optical properties.

We have systematically studied the photoinduced reorientation of PLCPs with NBA derivatives side groups (Figure 1a) [20,26]. Although the axis-selective trans-cis photoisomerization combined with thermal stimulation produces significant molecularly oriented Recently, we investigated photoinduced reorientation and thermal hydrolysis of PLC copolymers with NBA1 ( Figure 1a) and benzoic acid (BA) side groups [26]. When the exposed film is annealed at LC temperature range of the material, significant cooperative molecular orientation both side groups is achieved based on the photoinduced reorientation of NBA1 moieties. Additionally, thermally generated hydrolysis of NBA1 occurs to form non-photoreactive phenyl aldehyde end moieties under high humidity as the BA side groups act as an acid catalyst. The hydrolyzed film shows photo-durability due to the lack of photosensitive parts. However, the composition of NBA1 side groups must be limited to maintain the oriented structure because the hydrolyzed NBA1 side groups do not exhibit LC characteristics. Namely, the oriented structure collapses after the hydrolysis when the content of NBA1 is large. To avoid this disadvantage, photoalignable NBAcontaining mesogen (NBA2 in Figure 1a) is designed, which exhibits LC characteristics after the hydrolysis, and a PLCP with NBA2 side groups has been synthesized to explore the LC characteristics of oriented films after the hydrolysis [28]. Although the hydrolysis requires immersing the reoriented film in an acetic acid solution due to lack of acidic moieties, the LC characteristics remain in the hydrolyzed NBA2 side groups. Therefore, a copolymer comprised of NBA2 and BA side groups should exhibit photoinduced molecular reorientation, and subsequent thermal hydrolysis of NBA2 may be realized without a solution-immersing process.
Herein, we synthesize PLC copolymers with NBA2 and BA side groups (PNx, Figure  1b), and we investigated their thermally stimulated photoinduced cooperative reorientation. Similar to the homopolymer with NBA2 side groups and the copolymer with NBA1 and BA side groups [26,28], significant cooperative molecular reorientation is achieved. However, the annealing conditions for the thermally stimulated cooperative molecular reorientation after the photo-exposure depend on the content of the BA side groups. For all copolymer compositions, annealing under high humidity conditions generates in situ hydrolysis of the reoriented films while maintaining the molecular oriented structure at a proper temperature. Furthermore, the molecularly oriented structure is preserved upon re-exposing to 365-nm light, which induces a photoreaction of the NBA side groups. Consequently, a higher photo-durability is realized without changing the optical properties of the hydrolyzed film. Recently, we investigated photoinduced reorientation and thermal hydrolysis of PLC copolymers with NBA1 ( Figure 1a) and benzoic acid (BA) side groups [26]. When the exposed film is annealed at LC temperature range of the material, significant cooperative molecular orientation both side groups is achieved based on the photoinduced reorientation of NBA1 moieties. Additionally, thermally generated hydrolysis of NBA1 occurs to form non-photoreactive phenyl aldehyde end moieties under high humidity as the BA side groups act as an acid catalyst. The hydrolyzed film shows photo-durability due to the lack of photosensitive parts. However, the composition of NBA1 side groups must be limited to maintain the oriented structure because the hydrolyzed NBA1 side groups do not exhibit LC characteristics. Namely, the oriented structure collapses after the hydrolysis when the content of NBA1 is large. To avoid this disadvantage, photoalignable NBAcontaining mesogen (NBA2 in Figure 1a) is designed, which exhibits LC characteristics after the hydrolysis, and a PLCP with NBA2 side groups has been synthesized to explore the LC characteristics of oriented films after the hydrolysis [28]. Although the hydrolysis requires immersing the reoriented film in an acetic acid solution due to lack of acidic moieties, the LC characteristics remain in the hydrolyzed NBA2 side groups. Therefore, a copolymer comprised of NBA2 and BA side groups should exhibit photoinduced molecular reorientation, and subsequent thermal hydrolysis of NBA2 may be realized without a solution-immersing process.
Herein, we synthesize PLC copolymers with NBA2 and BA side groups (PNx, Figure 1b), and we investigated their thermally stimulated photoinduced cooperative reorientation. Similar to the homopolymer with NBA2 side groups and the copolymer with NBA1 and BA side groups [26,28], significant cooperative molecular reorientation is achieved. However, the annealing conditions for the thermally stimulated cooperative molecular reorientation after the photo-exposure depend on the content of the BA side groups. For all copolymer compositions, annealing under high humidity conditions generates in situ hydrolysis of the reoriented films while maintaining the molecular oriented structure at a proper temperature. Furthermore, the molecularly oriented structure is preserved upon re-exposing to 365-nm light, which induces a photoreaction of the NBA side groups. Consequently, a higher photo-durability is realized without changing the optical properties of the hydrolyzed film.

Photoreaction
Spin-coating onto quartz or CaF 2 substrates from a THF solution prepared thin copolymer films (100-180-nm thick). The films were photo-irradiated with a light intensity of 30 mW/cm 2 at 365-nm by a high-pressure Hg lamp equipped with a glass plate placed at Brewster's angle and a bandpass filter (Asahi Spectra, Tokyo, Japan; REX-250) at room temperature. The films were subsequently annealed at elevated temperatures under a dry N 2 atmosphere to thermally stimulate molecular reorientation.

Hydrolysis of the Oriented Films
Annealing at elevated temperatures under high humidity (RH > 70%) induced in situ hydrolysis of the oriented film. To attain high humidity at high environmental temperature, an oriented film was placed on a temperature-controlled stage placed in a box with hightemperature water. The annealing time and its temperature-controlled degree of hydrolysis (DH) were adjusted, and DH was evaluated by the change in the absorption band of NBA2 at 335 nm.

Characterization
1 H-NMR spectra acquired by a Bruker DRX-500 FT-NMR and FT-IR spectra (JASCO, Tokyo, Japan; FTIR-6600) confirmed the copolymers. The molecular weight was measured Polymers 2023, 15, 1408 4 of 13 by a gel permeation chromatography GPC; JASCO, Tokyo, Japan; PU-2080 with a Shodex column using THF as an eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarized optical microscope (POM; Olympus, Tokyo, Japan; BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as a differential scanning calorimetry (DSC; Hitachi High Technologies, Tokyo, Japan; DSC-7200). As a measure of the photoinduced optical anisotropy, polarization absorption spectra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polarization prisms. The absorption spectrum in both parallel and perpendicular direction to the polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichroism (D) is estimated as D = (A column using THF as an eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarized optical microscope (POM; Olympus, Tokyo, Japan; BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as a differential scanning calorimetry (DSC; Hitachi High Technologies, Tokyo, Japan; DSC-7200). As a measure of the photoinduced optical anisotropy, polarization absorption spectra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polarization prisms. The absorption spectrum in both parallel and perpendicular direction to the polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichroism (D) is estimated as where A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respectively.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed by the POM observation ( Figure 2a). Because the H-bonded dimer of BA is responsible for the LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA has a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (Ti) of 179 °C. H-bond formation induces LC characteristics in several types of composite materials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a small transition enthalpy and high Ti, at which simultaneous decomposition of the material is seen in POM observation ( Figure 2b, Table 1). For copolymers PN10 and PN20, Ti appears at a temperature similar to PBA with a large transition enthalpy, suggesting that the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 shows the first transition with small enthalpy at 163 °C and a second one at 205 °C. In this case, the LC characteristics are maintained even after cleaving the H-bonded dimer in the BA side groups (N2 temperature range). A large amount of NBA2 side groups can maintain the nematic LC characteristics even though the existence of free BA moieties. column using THF as an eluent) calibrated using polystyrene standards. The therma properties were examined using a polarized optical microscope (POM; Olympus, Tokyo Japan; BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as differential scanning calorimetry (DSC; Hitachi High Technologies, Tokyo, Japan; DSC 7200). As a measure of the photoinduced optical anisotropy, polarization absorption spec tra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polar ization prisms. The absorption spectrum in both parallel and perpendicular direction t the polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichro ism (D) is estimated as where A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respec tively.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed by the POM observation ( Figure 2a). Because the H-bonded dimer of BA is responsible fo the LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA has a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (Ti) o 179 °C. H-bond formation induces LC characteristics in several types of composite mate rials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays small transition enthalpy and high Ti, at which simultaneous decomposition of the mate rial is seen in POM observation ( Figure 2b, Table 1). For copolymers PN10 and PN20, T appears at a temperature similar to PBA with a large transition enthalpy, suggesting tha the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN5 shows the first transition with small enthalpy at 163 °C and a second one at 205 °C. In thi case, the LC characteristics are maintained even after cleaving the H-bonded dimer in th BA side groups (N2 temperature range). A large amount of NBA2 side groups can main tain the nematic LC characteristics even though the existence of free BA moieties.
where A || and A 5, x FOR PEER REVIEW 4 of 14 column using THF as an eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarized optical microscope (POM; Olympus, Tokyo, Japan; BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as a differential scanning calorimetry (DSC; Hitachi High Technologies, Tokyo, Japan; DSC-7200). As a measure of the photoinduced optical anisotropy, polarization absorption spectra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polarization prisms. The absorption spectrum in both parallel and perpendicular direction to the polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichroism (D) is estimated as where A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respectively.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed by the POM observation ( Figure 2a). Because the H-bonded dimer of BA is responsible for the LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA has a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (Ti) of 179 °C. H-bond formation induces LC characteristics in several types of composite materials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a small transition enthalpy and high Ti, at which simultaneous decomposition of the material is seen in POM observation ( Figure 2b, Table 1). For copolymers PN10 and PN20, Ti appears at a temperature similar to PBA with a large transition enthalpy, suggesting that the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 shows the first transition with small enthalpy at 163 °C and a second one at 205 °C. In this case, the LC characteristics are maintained even after cleaving the H-bonded dimer in the BA side groups (N2 temperature range). A large amount of NBA2 side groups can maintain the nematic LC characteristics even though the existence of free BA moieties.
are the absorbances parallel and perpendicular to E of LP light, respectively.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed by the POM observation ( Figure 2a). Because the H-bonded dimer of BA is responsible for the LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA has a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (T i ) of 179 • C. H-bond formation induces LC characteristics in several types of composite materials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a small transition enthalpy and high T i , at which simultaneous decomposition of the material is seen in POM observation ( Figure 2b, Table 1). For copolymers PN10 and PN20, T i appears at a temperature similar to PBA with a large transition enthalpy, suggesting that the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 shows the first transition with small enthalpy at 163 • C and a second one at 205 • C. In this case, the LC characteristics are maintained even after cleaving the H-bonded dimer in the BA side groups (N 2 temperature range). A large amount of NBA2 side groups can maintain the nematic LC characteristics even though the existence of free BA moieties.
Polymers 2023, 15, x FOR PEER REVIEW 4 of 13 eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarized optical microscope (POM; Olympus BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as a differential scanning calorimetry (DSC; Hitachi High Technologies, DSC-7200). As a measure of the photoinduced optical anisotropy, polarization absorption spectra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polarization prisms. The absorption spectrum in both parallel and perpendicular direction to the polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichroism (D) is estimated as where A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respectively.
Birefringence of the 120-150 nm-thick oriented films was measured by a multi-channel polarization analyzer using Optipro-compact11MQ (Shintech Co. Ltd.) at 607 nm.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed by the POM observation ( Figure 2a). Because the H-bonded dimer of BA is responsible for the LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA has a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (Ti) of 179 °C. H-bond formation induces LC characteristics in several types of composite materials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a small transition enthalpy and high Ti, at which simultaneous decomposition of the material is seen in POM observation ( Figure 2b, Table 1). For copolymers PN10 and PN20, Ti appears at a temperature similar to PBA with a large transition enthalpy, suggesting that the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 shows the first transition with small enthalpy at 163 °C and a second one at 205 °C. In this case, the LC characteristics are maintained even after cleaving the H-bonded dimer in the BA side groups (N2 temperature range). A large amount of NBA2 side groups can maintain the nematic LC characteristics even though the existence of free BA moieties.  UV-vis spectra of the copolymer depend on the copolymer composition. They show the absorption band of BA at 262 nm and that of NBA2 at 335 nm ( Figure 2c). Because the absorption band at 262 nm overlaps with the NBA2 absorption, absorption maxima slightly shift to the longer wavelength region as the NBA2 composition increases, while the absorption maxima at 335 nm rarely change due to no absorption of BA at 335 nm. FT-IR spectra show the vibration intensities of C=N (1624 cm -1 ), COOH (1683 cm -1 ), and Hbonded dimer (2681 and 2530 cm -1 ) also depend on the copolymer composition ( Figure 2d). These results indicate the random distribution of mesogenic side groups in the copolymers.

Axis-Selective Photoreaction of the Copolymer Films
Exposing the copolymer films to LP 365-nm light generates axis-selective trans-cistrans photoisomerization of the NBA2 side groups [27,28]. olumn using THF as an eluent) calibrated using polystyrene standards. The thermal roperties were examined using a polarized optical microscope (POM; Olympus, Tokyo, apan; BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as a ifferential scanning calorimetry (DSC; Hitachi High Technologies, Tokyo, Japan; DSC-200). As a measure of the photoinduced optical anisotropy, polarization absorption specra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polarzation prisms. The absorption spectrum in both parallel and perpendicular direction to he polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichrosm (D) is estimated as here A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respecively.
. Results and Discussion

.1. Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed y the POM observation (Figure 2a). Because the H-bonded dimer of BA is responsible for he LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA as a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (Ti) of 79 °C. H-bond formation induces LC characteristics in several types of composite mateials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a mall transition enthalpy and high Ti, at which simultaneous decomposition of the mateial is seen in POM observation ( Figure 2b, Table 1). For copolymers PN10 and PN20, Ti ppears at a temperature similar to PBA with a large transition enthalpy, suggesting that he LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA ide groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 hows the first transition with small enthalpy at 163 °C and a second one at 205 °C. In this ase, the LC characteristics are maintained even after cleaving the H-bonded dimer in the A side groups (N2 temperature range). A large amount of NBA2 side groups can mainain the nematic LC characteristics even though the existence of free BA moieties.
where A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respectively.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed  [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a small transition enthalpy and high Ti, at which simultaneous decomposition of the material is seen in POM observation ( Figure 2b, Table 1). For copolymers PN10 and PN20, Ti appears at a temperature similar to PBA with a large transition enthalpy, suggesting that the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 shows the first transition with small enthalpy at 163 °C and a second one at 205 °C. In this case, the LC characteristics are maintained even after cleaving the H-bonded dimer in the BA side groups (N2 temperature range). A large amount of NBA2 side groups can maintain the nematic LC characteristics even though the existence of free BA moieties.
< 0) appears due to the photoinduced reorientation of NBA2. The photoinduced optical anisotropy depends on the composition of NBA2 in the copolymer. Negative optical anisotropy is clearly seen in PN20 and PN50 films due to large content of NBA2, but it is very small for PN10.

Thermal Amplification of the Photoinduced Optical Anisotropy
In case of the photoinduced molecular reorientation of LC polymeric films, thermal stimulation of the exposed films often amplifies the photoinduced optical anisotropy, where the amplification direction depends on the type of the material [1,15]. When the photoinduced reorientation is caused by the axis-selective photoisomerization, thermal amplification often occurs in the same direction of the photoinduced optical anisotropy, but in some cases, out-of-plane direction is sometimes amplified [26,[49][50][51]. Significant inplane thermal stimulation of the photoinduced optical anisotropy was observed in the PLCP with NBA1 and BA side groups [29]. Similarly, annealing the exposed copolymer film in its LC temperature range produces a significant cooperative molecular orientation in a direction perpendicular to the polarization of LP 365-nm light. Figures 4a-c show the change in the polarized absorption spectra of PN10, PN20, and PN50 films before and after exposure to LP 365-nm light for 5 J/cm 2 and subsequent annealing at elevated temperatures. Photoinduced small anisotropy (D355 < 0.1) is significantly amplified after the annealing, where the cooperative molecular reorientation is generated for all copolymer films. D262 (D335) values of the reoriented films are 0.71 (0.71) for PN10, 0.80 (0.79) for PN20, and 0.80 (0.86) for PN50. The birefringence values of these films are 0.113 (PN10), 0.158 (PN20), and 0.181 (PN50) at 607 nm. The birefringent value increases as the amount of NBA2 groups increases due to the higher inherent birefringence of NBA2, where the birefringence of the reoriented PN100 film is 0.24 [27,28].
A similar thermal stimulation is observed when the exposure energy is 100 J/cm 2 (Figure 4d-f). In these cases, photoinduced optical anisotropy of NBA2 is larger than those for 5 J/cm 2   Additionally, absorbances for both directions at 335 nm decrease when the exposure energy exceeds 50 J/cm 2 , indicating a side photoreaction of the NBA2 [20]. Although the photoinduced optical anisotropy increases as the exposure energy increases, side photoreaction, such as photodegradation and photo-crosslinking of NBA moieties, inhibits the reorientation [46][47][48].

Thermal Amplification of the Photoinduced Optical Anisotropy
In case of the photoinduced molecular reorientation of LC polymeric films, thermal stimulation of the exposed films often amplifies the photoinduced optical anisotropy, where the amplification direction depends on the type of the material [1,15]. When the photoinduced reorientation is caused by the axis-selective photoisomerization, thermal amplification often occurs in the same direction of the photoinduced optical anisotropy, but in some cases, out-of-plane direction is sometimes amplified [26,[49][50][51]. Significant in-plane thermal stimulation of the photoinduced optical anisotropy was observed in the PLCP with NBA1 and BA side groups [29]. Similarly, annealing the exposed copolymer film in its LC temperature range produces a significant cooperative molecular orientation in a direction perpendicular to the polarization of LP 365-nm light. Figure 4a-c show the change in the polarized absorption spectra of PN10, PN20, and PN50 films before and after exposure to LP 365-nm light for 5 J/cm 2 and subsequent annealing at elevated temperatures. Photoinduced small anisotropy (D 355 < 0.1) is significantly amplified after the annealing, where the cooperative molecular reorientation is generated for all copolymer films. D 262 The birefringent value increases as the amount of NBA2 groups increases due to the higher inherent birefringence of NBA2, where the birefringence of the reoriented PN100 film is 0.24 [27,28].

Influence of the Annelaing Temperature
The annealing temperature influences the cooperative molecular reorientation. Figure 5a,b plots the change in the thermally stimulated D values of the exposed copolymer films as a function of the annealing temperature when the exposure energy is 5 J/cm 2 (100 J/cm 2 ). When the exposure energy is 5 J/cm 2 , the highest temperature for the significant D values of PN10 (PN20) is 165 °C (160 °C), which is close to Ti of the copolymers. By contrast, P50 exhibits a siginifcant thermal amplification in the N1-LC temperature range, but it does not exhibit coopertative reorientation when the annealing temperature is in the N2-LC temperature range. Cooperative reorientation occurs when the annealing temperature is 120 °C (N1 temperature range) because BA side groups reveal H-bonded dimer showing LC characteristics (Figure 6a). However, axis-selectively oriented NBA2 side groups cannot align free-BA side groups at 170 °C (Figure 6b), where the BA side groups do not form H-bonding at the N2 temperature range. A similar thermal stimulation is observed when the exposure energy is 100 J/cm 2 (Figure 4d-f). In these cases, photoinduced optical anisotropy of NBA2 is larger than those for 5 J/cm 2 -dose films-while D 262 (D 335 ) values of the thermally amplified reoriented films are 0.69 (0.63) for PN10, 0.66 (0.75) for PN20, and 0.77 (0.81) for PN50, which are similar to those obtained from the 5 J/cm 2 -dose films. However, copolymers reveal different suitable range of the exposure energy for a sufficient cooperative molecular reorientation (Figure 4a-c, inset). For P50, thermal stimulation (D > 0.7) is generated when the exposure energy is between 1 and 200 J/cm 2 . P20 films exhibit smaller exposure range between 2 and 100 J/cm 2 , but P10 has a much smaller range of 5 and 30 J/cm 2 for D > 0.6. When the composition of the NBA2 is low, the power for the cooperative thermal stimulation is small in the total mesogenic side groups.

Influence of the Annelaing Temperature
The annealing temperature influences the cooperative molecular reorientation. Figure 5a,b plots the change in the thermally stimulated D values of the exposed copolymer films as a function of the annealing temperature when the exposure energy is 5 J/cm 2 (100 J/cm 2 ). When the exposure energy is 5 J/cm 2 , the highest temperature for the significant D values of PN10 (PN20) is 165 • C (160 • C), which is close to T i of the copolymers. By contrast, P50 exhibits a siginifcant thermal amplification in the N 1 -LC temperature range, but it does not exhibit coopertative reorientation when the annealing temperature is in the N 2 -LC temperature range. Cooperative reorientation occurs when the annealing temperature is 120 • C (N 1 temperature range) because BA side groups reveal H-bonded  (Figure 6a). However, axis-selectively oriented NBA2 side groups cannot align free-BA side groups at 170 • C (Figure 6b), where the BA side groups do not form H-bonding at the N 2 temperature range.
films as a function of the annealing temperature when the exposure energy is 5 J/cm 2 (100 J/cm 2 ). When the exposure energy is 5 J/cm 2 , the highest temperature for the significant D values of PN10 (PN20) is 165 °C (160 °C), which is close to Ti of the copolymers. By contrast, P50 exhibits a siginifcant thermal amplification in the N1-LC temperature range, but it does not exhibit coopertative reorientation when the annealing temperature is in the N2-LC temperature range. Cooperative reorientation occurs when the annealing temperature is 120 °C (N1 temperature range) because BA side groups reveal H-bonded dimer showing LC characteristics (Figure 6a). However, axis-selectively oriented NBA2 side groups cannot align free-BA side groups at 170 °C (Figure 6b), where the BA side groups do not form H-bonding at the N2 temperature range. Furthermore, Figure 5 indicates that the highest temperatures for the reorientation in all films with a 100 J/cm 2 dose are lower than those with a 5 J/cm 2 dose. For PN10, a significant thermal stimulation occurs when annealing at 165 °C for an exposed film for 5-30 J/cm 2 (Figures 4a and 7a), but the orientated structure is not amplified at 165 °C for the film with a 100 J/cm 2 dose (Figure 7b). Similarly, a significant thermal stimulation appears in PN20 when annealing at 160 °C for an exposed film for 5 J/cm 2 , but reorientatiom does not occur for the film with a 100 J/cm 2 dose (Figure 7c,d). These results indicate that the side-photoreaction of the NBA2 side groups at the high doses decreases Ti of the film after the photoreaction, which is confirmed by the POM observation. Furthermore, Figure 5 indicates that the highest temperatures for the reorientation in all films with a 100 J/cm 2 dose are lower than those with a 5 J/cm 2 dose. For PN10, a significant thermal stimulation occurs when annealing at 165 • C for an exposed film for 5-30 J/cm 2 (Figures 4a and 7a), but the orientated structure is not amplified at 165 • C for the film with a 100 J/cm 2 dose (Figure 7b). Similarly, a significant thermal stimulation appears in PN20 when annealing at 160 • C for an exposed film for 5 J/cm 2 , but reorientatiom does not occur for the film with a 100 J/cm 2 dose (Figure 7c,d). These results indicate that the side-photoreaction of the NBA2 side groups at the high doses decreases T i of the film after the photoreaction, which is confirmed by the POM observation. significant thermal stimulation occurs when annealing at 165 °C for an exposed film for 5-30 J/cm 2 (Figures 4a and 7a), but the orientated structure is not amplified at 165 °C for the film with a 100 J/cm 2 dose (Figure 7b). Similarly, a significant thermal stimulation appears in PN20 when annealing at 160 °C for an exposed film for 5 J/cm 2 , but reorientatiom does not occur for the film with a 100 J/cm 2 dose (Figure 7c,d). These results indicate that the side-photoreaction of the NBA2 side groups at the high doses decreases Ti of the film after the photoreaction, which is confirmed by the POM observation.

Hydrolysis of the Reoriented Film
The oriented C=N bond in the copolymer film is hydrolyzed under humid conditions at elevated temperatures. Schiff base derivatives are hydrolyzed under the acid condition [52,53]. In case of the copolymers, BA side groups serve as the acid for the hydrolysis [26], whereas hydrolysis of reoriented PN100 requires a treatment with an acetic acid solution due to lack of the acidic moieties [28]. For the thermal hydrolysis of NBA2 in the copolymer, 4-methoxy-phenylamine formed upon the hydrolysis sublimes simultaneously upon the annealing. column using THF as an eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarized optical microscope (POM; Olympus, Tokyo, Japan; BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as a differential scanning calorimetry (DSC; Hitachi High Technologies, Tokyo, Japan; DSC-7200). As a measure of the photoinduced optical anisotropy, polarization absorption spectra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polarization prisms. The absorption spectrum in both parallel and perpendicular direction to the polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichroism (D) is estimated as where A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respectively.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed by the POM observation (Figure 2a). Because the H-bonded dimer of BA is responsible for the LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA has a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (Ti) of 179 °C. H-bond formation induces LC characteristics in several types of composite materials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a small transition enthalpy and high Ti, at which simultaneous decomposition of the material is seen in POM observation (Figure 2b, Table 1). For copolymers PN10 and PN20, Ti appears at a temperature similar to PBA with a large transition enthalpy, suggesting that the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 at 262 and 335 nm) as a function of the hydrolysis time for various annealing temperatures. All films display a diminished absorbance at 335 nm when the annealing time exceeds 60 min (degree of the hydrolysis >95%). However, the absorbance at 262 nm rarely changes, indicating that the reoriented structure is maintained after the hydrolysis (D 262 = 0.80, 0.67, and 0.78 for PN10, PN20, and PN50, respectively). This is because the LC characteristics of the oriented film are maintained, indicating that the isotropic transition temperature (T i ) hydrolyzed film is higher than the annealing temperature. To confirm this prediction, copolymers of methacrylate monomers with a 4-formylphenylbenzoate and BA side groups (PAx) are synthesized (Figure 9a). These copolymers show nematic LC characteristics, and their T i is plotted in Figure 9b. Although T i of PAx is lower than that of PNx due to the lack of NBA moiety, thermal hydrolysis temperature is low enough to hold the LC characteristics. Additionally, the hydrolyzedoriented films display decreased birefringent values of 0.111, 0.141, and 0.128 for PN10, PN20, and PN50 at 607 nm, respectively, as the C=N decomposition in the NBA2 side groups to form phenyl aldehyde end moieties decreases the inherent birefringence.
ture. To confirm this prediction, copolymers of methacrylate monomers with a 4-formylphenylbenzoate and BA side groups (PAx) are synthesized (Figure 9a). These copolymers show nematic LC characteristics, and their Ti is plotted in Figure 9b. Although Ti of PAx is lower than that of PNx due to the lack of NBA moiety, thermal hydrolysis temperature is low enough to hold the LC characteristics. Additionally, the hydrolyzedoriented films display decreased birefringent values of 0.111, 0.141, and 0.128 for PN10, PN20, and PN50 at 607 nm, respectively, as the C=N decomposition in the NBA2 side groups to form phenyl aldehyde end moieties decreases the inherent birefringence. column using THF as an eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarized optical microscope (POM; Olympus, Tokyo, Japan; BX51) equipped with a Linkam TH600PM heating and cooling stage, as well as a differential scanning calorimetry (DSC; Hitachi High Technologies, Tokyo, Japan; DSC-7200). As a measure of the photoinduced optical anisotropy, polarization absorption spectra were measure using a Hitachi U-3010 spectrometer equipped with Glan-Taylor polarization prisms. The absorption spectrum in both parallel and perpendicular direction to the polarization (E) of the LP 365-nm was measured. The photoinduced in-plane dichroism (D) is estimated as where A|| and A⏊ are the absorbances parallel and perpendicular to E of LP light, respectively.

Thermal and Spectroscopic Properties of the Copolymers
The synthesized copolymers exhibit nematic LC characteristics, which are confirmed by the POM observation (Figure 2a). Because the H-bonded dimer of BA is responsible for the LC characteristics of a polymethacrylate with BA side groups (PBA) [19,[39][40][41][42], PBA has a large transition enthalpy (29.3 J/g) and a LC-isotropic transition temperature (Ti) of 179 °C. H-bond formation induces LC characteristics in several types of composite materials [43][44][45]. By contrast, the homopolymer with NBA2 side groups (PN100) displays a small transition enthalpy and high Ti, at which simultaneous decomposition of the material is seen in POM observation (Figure 2b, Table 1). For copolymers PN10 and PN20, Ti appears at a temperature similar to PBA with a large transition enthalpy, suggesting that the LC characteristics disappear due to H-bond cleavage of the BA side groups. Free BA side groups disorder the nematic orientation of NBA2 side groups. By contrast, PN50 shows the first transition with small enthalpy at 163 °C and a second one at 205 °C. In this case, the LC characteristics are maintained even after cleaving the H-bonded dimer in the BA side groups (N2 temperature range). A large amount of NBA2 side groups can maintain the nematic LC characteristics even though the existence of free BA moieties.  The hydrolysis rate and stability of the orientated structure depend on the hydrolysis temperature. Although the hydrolysis occurs faster as operating at the higher annealing temperature (Figure 8a-c, insets), oriented structure for P10 maintains even when the annealing at 160 °C while gradually collapses for P20 at 150 °C. However, the oriented structure of P50 collapses within 30 min when annealed at 150 °C (Figure 10a-c). This is because Ti of the oriented film gradually decreases upon the hydrolysis, while Ti of the completehydrolyzed film becomes lower than the annealing temperature (Figure 9b). Assuming hydrolysis is a first order reaction, and the Arrhenius plot of the hydrolysis of the copolymer films (Figure 8d) indicates that the activation energies of the PN10, PN20, and PN50 are 87.6, 78.1, and 64.0 kJ/mol, respectively. These values are comparable as compared to the hydrolysis of phenyl ester derivatives [54]. The hydrolysis rate and stability of the orientated structure depend on the hydrolysis temperature. Although the hydrolysis occurs faster as operating at the higher annealing temperature (Figure 8a-c, insets), oriented structure for P10 maintains even when the annealing at 160 • C while gradually collapses for P20 at 150 • C. However, the oriented structure of P50 collapses within 30 min when annealed at 150 • C (Figure 10a-c). This is because T i of the oriented film gradually decreases upon the hydrolysis, while T i of the complete-hydrolyzed film becomes lower than the annealing temperature (Figure 9b). Assuming hydrolysis is a first order reaction, and the Arrhenius plot of the hydrolysis of the copolymer films (Figure 8d) indicates that the activation energies of the PN10, PN20, and PN50 are 87.6, 78.1, and 64.0 kJ/mol, respectively. These values are comparable as compared to the hydrolysis of phenyl ester derivatives [54]. ture of P50 collapses within 30 min when annealed at 150 °C (Figure 10a-c). This is because Ti of the oriented film gradually decreases upon the hydrolysis, while Ti of the completehydrolyzed film becomes lower than the annealing temperature (Figure 9b). Assuming hydrolysis is a first order reaction, and the Arrhenius plot of the hydrolysis of the copolymer films (Figure 8d) indicates that the activation energies of the PN10, PN20, and PN50 are 87.6, 78.1, and 64.0 kJ/mol, respectively. These values are comparable as compared to the hydrolysis of phenyl ester derivatives [54].

Photo-Durability of Reoriented Films
Re-exposing an oriented PN100 film to LP 365-nm light in a direction parallel to the orientation direction reverses the molecular orientation direction [27]. This is due to the axis-selective photoreaction of NBA2 side groups, which is similar to the behavior of azobenzene-containing polymeric films [29][30][31][32][33][34][35][36]. By contrast, a different photo-durability behavior is observed for the oriented copolymer films. Figure 11a-c, respectively, show the changes in the polarized absorption spectra of oriented PN10, PN20, and PN50 films before and upon the exposure to LP 365-nm light with E parallel to the orientation direction. The insets plot the D262 and D335 values as a function of the exposure energy. For PN10 and PN20, the absorption bands at 262 nm in both directions rarely change even when the re-exposure energy exceeds 100 J/cm 2 ,

Photo-Durability of Reoriented Films
Re-exposing an oriented PN100 film to LP 365-nm light in a direction parallel to the orientation direction reverses the molecular orientation direction [27]. This is due to the axis-selective photoreaction of NBA2 side groups, which is similar to the behavior of azobenzene-containing polymeric films [29][30][31][32][33][34][35][36]. By contrast, a different photo-durability behavior is observed for the oriented copolymer films. Figure 11a-c, respectively, show the changes in the polarized absorption spectra of oriented PN10, PN20, and PN50 films before and upon the exposure to LP 365-nm light with E parallel to the orientation direction. The insets plot the D 262 and D 335 values as a function of the exposure energy. For PN10 and PN20, the absorption bands at 262 nm in both directions rarely change even when the re-exposure energy exceeds 100 J/cm 2 , whereas the band at 335 nm gradually decreases. Thus, the molecularly orientated structure does not collapse even though the photoreaction of the NBA2 side groups proceeds in the parallel direction. Namely, photoreaction of NBA2 does not generate the cooperative molecular reorientation of mesogenic side groups. In this case, the birefringence of the film decreases slightly to 0.112 (PN10) and 0.157 (PN20). By contrast, the molecularly orientated structure of PN50 collapses slightly (Figure 11c). The absorption band at 262 nm shows a slight decrease in the reoriented structure. After the 100 J/cm 2 dose, D 262 and the birefringence values decrease to 0.61 and 0.132, respectively. These results indicate that a larger composition of the oriented BA side groups restricts the photoinduced molecular reorientation upon the re-exposure at room temperature. Namely, the cooperative motion of the mesogenic side groups is negligible once the reoriented structure is formed, whereas photodegradation of NBA2 partly occurs at high exposure doses. whereas the band at 335 nm gradually decreases. Thus, the molecularly orientated structure does not collapse even though the photoreaction of the NBA2 side groups proceeds in the parallel direction. Namely, photoreaction of NBA2 does not generate the cooperative molecular reorientation of mesogenic side groups. In this case, the birefringence of the film decreases slightly to 0.112 (PN10) and 0.157 (PN20). By contrast, the molecularly orientated structure of PN50 collapses slightly (Figure 11c). The absorption band at 262 nm shows a slight decrease in the reoriented structure. After the 100 J/cm 2 dose, D262 and the birefringence values decrease to 0.61 and 0.132, respectively. These results indicate that a larger composition of the oriented BA side groups restricts the photoinduced molecular reorientation upon the re-exposure at room temperature. Namely, the cooperative motion of the mesogenic side groups is negligible once the reoriented structure is formed, whereas photodegradation of NBA2 partly occurs at high exposure doses. Figure 11. Changes in the polarized absorption spectra when oriented (a) PN10, (b) PN20, and (c) PN50 films are exposed to LP 365-nm light with E parallel to the orientation direction. Insets plot the D values as a function of exposure energy.

Photo-Durability of Hydrolyzed Oriented Films
Hydrolyzed oriented films exhibit a higher photo-durability. Figure 12a-c shows the changes in the polarized absorption spectra of a hydrolyzed oriented PN10, PN20, and PN50 films (DH > 95 %) upon the exposure to LP 365-nm light with E parallel to the orientation direction, respectively. The spectra of the oriented films do not change even after the exposure dose of 100 J/cm 2 , regardless of the composition. This is due to the lack of

Photo-Durability of Hydrolyzed Oriented Films
Hydrolyzed oriented films exhibit a higher photo-durability. Figure 12a-c shows the changes in the polarized absorption spectra of a hydrolyzed oriented PN10, PN20, and PN50 films (DH > 95%) upon the exposure to LP 365-nm light with E parallel to the orientation direction, respectively. The spectra of the oriented films do not change even after the exposure dose of 100 J/cm 2 , regardless of the composition. This is due to the lack of photo-sensitive moieties in the oriented films. These films are applicable to birefringent film for display application because there is no absorption in the visible light region. Figure 11. Changes in the polarized absorption spectra when oriented (a) PN10, (b) PN20, and (c) PN50 films are exposed to LP 365-nm light with E parallel to the orientation direction. Insets plot the D values as a function of exposure energy.

Photo-Durability of Hydrolyzed Oriented Films
Hydrolyzed oriented films exhibit a higher photo-durability. Figure 12a-c shows the changes in the polarized absorption spectra of a hydrolyzed oriented PN10, PN20, and PN50 films (DH > 95 %) upon the exposure to LP 365-nm light with E parallel to the orientation direction, respectively. The spectra of the oriented films do not change even after the exposure dose of 100 J/cm 2 , regardless of the composition. This is due to the lack of photo-sensitive moieties in the oriented films. These films are applicable to birefringent film for display application because there is no absorption in the visible light region. Figure 12. Changes in the polarized absorption spectra when hyrolyazed (a) PN10, (b) PN20, and (c) PN50 films are exposed to LP 365-nm light with E parallel to the orientation direction.

Conclusions
Photoalignable liquid crystalline copolymers comprised of NBA2 and BA side groups are synthesized. Exposing these copolymers films to LP 365-nm light generates photoinduced reorientation of NBA2 side groups, and subsequent annealing in the LC temperature range of the material induces significant cooperative molecular orientation of both mesogenic side groups. However, thermal amplification of the cooperative orientation does not occur when the H-bond of the BA dimer collapses, even though the copolymer exhibits LC characteristics. The oriented films exhibit birefringent values of 0.113-0.181 at 607 nm, and the oriented structure is maintained under LP 365-nm light re-exposure when the NBA2 content is less than 20%. Because in situ hydrolysis of the NBA2 moiety does not alter the oriented structure, the resultant films display a higher photo-durability without affecting the optical properties due to the lack of photosensitive moieties. Currently, studies to apply these copolymers to birefringent devices are underway.