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Photoinduced Birefringent Pattern and Photoinactivation of Liquid Crystalline Copolymer Films with Benzoic Acid and Phenylaldehyde Side Groups Nobuhiro Kawatsuki, Shogo Inada, Ryosuke Fujii, and Mizuho Kondo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04096 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 15, 2018
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Photoinduced Birefringent Pattern and Photoinactivation of Liquid Crystalline Copolymer Films with Benzoic Acid and Phenylaldehyde Side Groups Nobuhiro Kawatsuki,* Shogo Inada, Ryosuke Fujii, Mizuho Kondo Department of Applied Chemistry, Graduate School of Engineering, University of Hyogo, Shosha, Himeji 671-2280, Japan
ABSTRACT: In situ formation of N-benzylideneaniline (NBA) side groups achieved photoinduced cooperative reorientation of photoinactive copolymers with phenylaldehyde (PA) and benzoic acid (BA) side groups doped with 4-methoxyaniline (AN) molecules. Thermally stimulated molecular reorientation of the side groups was generated due to the axis-selective photoreaction of the NBA moieties. Selective coating with AN on the copolymer film formed NBA moieties in the desired region, resulting in a photoinduced birefringent pattern. Additionally, post-annealing at an elevated temperature for a long time attained photoinactivation of the reoriented film, and recoating with AN to form NBA achieved the multiple birefringent patterns and re-patterning of the reoriented structures. The slow thermal hydrolysis of NBA, which was 50 times slower than the thermally stimulated self-organization of
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the side groups due to the presence of BA side groups, contributed to the photodurability of the reoriented film and multiple birefringent patterns.
1. INTRODUCTION Regulating the molecular orientation structure in a polymeric film based on photoalignment has potential for optical and display applications.1–10 Two major techniques have been employed to attain photoaligned polymeric films. One is based on the command surface effect and the other uses photoalignable LC polymers. The former employs photoresponsive thin photoalignment layers exposed to linearly polarized (LP) light to realize homogeneous or homeotropic orientation of liquid crystalline (LC) materials on the layers.1–3,11–16 The latter uses photoalignable LC polymers to directly realize homogeneous molecularly oriented films by exposure to LP light and in some cases additional thermally stimulated self-organization.4–7,17–22 Many types of photoalignable materials have been investigated, including azobenzenecontaining polymers that exhibit photoinduced molecular reorientation due to axis-selective trans-cis-trans photoisomerization6,23–30 and photo-cross-linkable LC polymers comprised of cinnamate-derivative mesogenic side groups that attain thermally stimulated molecular reorientation.17,18,31–34
Similar
to
azobenzene
derivatives,
polymethacrylates
with
N-
benzylideneaniline (NBA) derivative side groups show a photoinduced molecular reorientation perpendicular to the polarization of LP ultraviolet (UV) light.35,36 Cooperative thermally stimulated molecular reorientation has been attained with copolymerization of NBA-containing polymers with a comonomer comprised of photoinactive mesogenic side groups. In this scenario, adjusting the copolymerization ratio controls the generated birefringence,37 but the photoresponsive moieties for the photoalignment remain in the oriented polymeric films.
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Recently, a new approach to control the orientation of LC polymeric films from the free (air) surface side was reported.38–48 Seki et al. investigated surface segregation of an azobenzenecontaining polymer in blend and diblock LC copolymer films comprised of azobenzenecontaining units and photoinactive mesogenic units where the surface-segregated azobenzene units regulated the photoinduced molecular orientation of the inner side of the photoinactive LC polymers.38–41 Furthermore, they demonstrated that photoalignment patterning of a nonphotoresponsive LC polymer film is induced by a surface coated with a thin azobenzenecontaining polymeric layer.42–45 We have reported the in situ formation of NBA moieties from a polymethacrylate comprisied of phenylaldehyde (PA) side groups (PPA100, Fig. 1) doped with phenylamine monomeric derivatives, where the photoinduced reorientation is observed similar to NBA-containing polymeric films.46 Selective formation of NBA and photoalignment patterning are attained by coating 4-methoxyaniline (AN) onto a PPA100 film using a sublimation technique.46,47 Additionally, the photoinduced reorientation of photoinactive polymethacrylate with BA side groups (P6BAM, Fig. 1) coated with cinnamic acid derivatives yields selective photoalignment of P6BAM films. The cinnamic acid derivatives formed H-bond with BA side groups, which show the photoalignability and they are eliminated via a thermally stimulated reorientation process that introduces a photoinactive oriented structure.48,49 NBA derivatives decompose in acidic conditions because hydrolysis easily occurs in solution.50–52 However, in our study on the photoalignment of methacrylate homopolymer with NBA side groups, the reoriented structure in the film is relatively stable at room temperature for several years. If hydrolysis of the aligned NBA occurs in cooperatively reoriented copolymeric
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films with NBA and photoinactive mesogenic side groups, the oriented film will be photoinactive and reorganization using the hydrolyzed PA moieties should be feasible. Herein copolymethacrylates with PA and BA side groups (PPAs, summarized in Fig. 1) are synthesized and their photoinduced reorientation behavior is investigated using composite films of PPAs and AN. These films show in situ formation of NBA moieties (Fig. 1). Significant thermally stimulated cooperative photoinduced molecular reorientation is generated based on the axis-selective photoisomerization of the NBA moieties formed in the composite films and selective-coating of AN molecules on a PPA film realize a birefringent pattern. Furthermore, post-annealing induces thermally generated hydrolysis of the NBA moieties in the oriented film due to the presence of the BA moieties, resulting in a photoinactive-oriented film. Finally, multiple reoriented birefringent patterns and re-patterning of the alignment structure of the film are demonstrated by recoating AN molecules onto the hydrolyzed films.
(Figure 1)
2. EXPERIMENTAL 2.1. Materials. All starting materials, except AN, were used as received from Tokyo Kasei Chemicals. AN was recrystallized from ethanol prior to use. (Co)polymers with BA and PA side groups (Fig. 1; PPA10, PPA20, PPA30, PPA40, PPA50, PPA100, and P6BAM) were synthesized by a free radical polymerization from the corresponding methacrylate monomers using AIBN as an initiator in tetrahydrofuran (THF).33 Table 1 summarizes the composition, molecular weight, and thermal properties of the (co)polymers.
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(Table 1)
2.2. Film preparation. Composite films of PPA/AN (thicknesses; approximately 200 nm) were prepared by spin-coating a THF solution of the polymers (1.6 wt/wt%) onto quartz or CaF2 substrates. Adjusting the molar ratio of the aldehyde groups (CHO) in PPA and AN controlled the PPA:AN ratio in the composite films. Then the films were prebaked at 100 °C for 10 min to form the NBA side groups (Fig. 1). For the coating method, AN molecules were coated on the PPA films either by sublimation at 100 °C or an inkjet coating performed using a LaboJet-1000 (MICROJET) precision printer from an ethylene glycol or dimethyl sulfoxide (DMSO) solution (10 wt/vol%) at 40 °C. After coating, the film was prebaked at 100 °C for 10 min to form the NBA side groups. 2.3. Photoreaction. Photoreactions were carried out using a high-pressure Hg lamp equipped with a glass plate placed at Brewster’s angle and a 365 nm band-pass filter (Asahi Spectra REX250), yielding a light intensity of of 30 mW/cm2 at 365 nm. After photoirradiation, the films were annealed at elevated temperatures to thermally stimulate molecular reorientation. 2.3. Characterization. 1H-NMR spectra using a Bruker DRX-500 FT-NMR and FT-IR spectra (JASCO FTIR-6600) confirmed the monomers and polymers. The molecular weights of the polymers were measured by GPC (JASCO PU-2080 and RI-2031 GPC system with a Shodex column using THF as the eluent) calibrated using polystyrene standards. The thermal properties were examined using a polarization optical microscope (POM; Olympus BX51) equipped with a Linkam TH600PM heating and cooling stage as well as differential scanning calorimetry (DSC; Seiko-I SSC5200H).
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As a measure of the photoinduced optical anisotropy, the photoinduced in-plane dichroism (D) was evaluated from the polarization absorption spectra with a Hitachi U-3010 spectrometer equipped with Glan−Taylor polarization prisms. D is estimated as
D=
A⊥ − A|| (1) A⊥ + A|| ,
where A|| and A are the absorbances parallel and perpendicular to polarization (E) of LP 365 ⊥
nm light, respectively. 3. RESULTS AND DISCUSSION 3.1. Spectroscopic Properties of PPA/AN Composite Films. The synthesized copolymers, except PPA50 and PPA100, showed nematic LC characteristics due the H-bonded dimer of BA side groups.32,53–56 Increasing the composition of methacrylate with PA side groups decreased Ti of the material. All copolymers were soluble in common organic solvents such as chloroform and THF. The PPA/AN composite films were optically transparent without scattering. Prebaking the composite film (CHO/AN=1/2 mol/mol) at 100 °C for 10 min formed photoreactive NBA side groups, as plotted in Fig. 2. An absorption band for the NBA moiety was clearly observed at 335 nm. The excess AN was sublimed upon prebaking. These spectra were similar to those of the prebaked copolymethacrylate films with similar NBA/BA compositions (PNBs; Table S1, Figs. S1a and S1b). Additionally, the AN molecules did not react with the BA side groups because prebaking the P6BAM/AN composite film resulted only in the sublimation of AN molecules, which showed the same absorption property as the P6BAM film (Fig. S2).
(Figure 2)
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3.2. Thermally Stimulated Molecular Orientation of PPA/AN Composite Films. We previously reported that thermally stimulated cooperative molecular reorientation was observed when PNB films were exposed to LP 365 nm light followed by annealing at the LC temperature range of the copolymers.37 Axis-selective trans-cis-trans photoisomerization of the NBA groups induced a slight molecular reorientation perpendicular to E of LP light. Subsequent annealing generated a significant cooperative reorientation of both the BA and NBA side groups. Similarly, the photoinduced optical anisotropy of the prebaked PPA/AN composite films was thermally amplified when the exposed films were annealed at elevated temperatures. Figure 3a shows the change in the polarized UV absorption spectra of a prebaked PPA20/AN (1/2 mol/mol) film after exposure to LP 365 nm light for 20 J/cm2 and subsequent annealing at 130 °C for 10 min. After exposure, the photoinduced anisotropy of BA was negligible (D262nm
