A new epoxy-based azo polymer (BP-AZ-CSMB), which contained chiral-substituted push-pull azo chromophores, was synthesized by the post-polymerization azo-coupling reaction. This newly synthesized polymer was characterized by 1H NMR, FTIR, GPC, UV-Vis, DSC and TGA. When exposed to interference patterns of laser beam (532 nm, 100 mW cm−2), BP-AZ-CSMB showed ability to quickly form surface-relief-gratings. Due to this property, quasi-crystal surface relief structures were feasibly fabricated on the BP-AZ-CSMB films by using the dual-beam multiple exposure technique. The quasi-crystal structures with 6-, 8-, 10-, 12-, 36-, and 72-fold rotation symmetry were successfully photofabricated.
© 2015 Optical Society of America
Quasi-crystal is a type of unconventional crystallographic structures characterized by long-range aperiodic order and rotational symmetry . Since first reported by Shechtman et al. in 1984 , investigations on quasi-crystals have aroused tremendous research enthusiasm because of their unique architecture and interesting properties [3–8]. For instance, quasi-crystal structures with meso-scale features have been exploited as photonic band-gap materials [9,10]. For the fabrication of artificial quasi-crystal patterns, methods combining holography and photo-polymerization have been developed by using materials such as photoresist , and polymer-dispersed liquid crystal (PDLC) . Using photoresist as a processing medium, two-dimension (2D) quasi-crystal patterns have been fabricated by the dual-beam multiple exposure technique . In the dual-beam multiple exposure process, reorientation of the photosensitive sample between each laser irradiation step enables the formation of the quasi-crystal pattern. It is a simple and feasible approach as accurate positioning of the sample between exposure steps is not required.
Polymers containing azobenzene and its derivatives (azo polymers for short) have been intensively investigated for various photoresponsive properties, which are triggered by the reversible trans-cis isomerization of azo chromophores [14–18]. Photoinduced surface-relief-grating (SRG) formation is one of the most fascinating properties of azo polymers [19,20]. SRGs on azo polymer films can be inscribed at temperature well below the glass transition temperatures (Tgs) of the azo polymers. The surface structures are stable below the Tgs of the polymers and erasable by heating samples to a temperature above their Tgs. Different types of azo polymers and azo molecular glasses have been used to fabricate SRGs [14,15,17,18,21–23]. The surface quasi-crystal structures can be prepared by multiple SRGs inscribed on the same film . Distinguished from the dual-beam multiple exposure technique using photoresist, the surface relief patterns are formed on the basis of the photoinduced mass transport [14,15]. However, in practical performance, this promising approach has been accomplished only for few amorphous azo molecular materials . Using this method to prepare quasi-crystal surface relief structures on polymer films has faced some challenging obstacles. Due to the high molecular weight and chain entanglement, SRG formation on azo polymer needs a relatively longer irradiation time. As a consequence, superimposed multiple exposure steps often blur the edges of the fringes and damage the patterns formed in the previous steps. To our knowledge, there is no report concerning the fabrication of quasi-crystal surface relief structures on azo polymer films in the literature.
In this study, a new epoxy-based azo polymer (BP-AZ-CSMB) was synthesized to contain chromophores with chiral substituents. BP-AZ-CSMB was synthesized from an epoxy-based precursor polymer by the post-polymerization azo-coupling reaction. It was observed that it was highly efficient for this polymer to form SRGs in a short time period. Due to this characteristic, quasi-crystal surface relief structures could be feasibly inscribed on BP-AZ-CSMB films by the dual-beam multiple exposure technique. The quasi-crystal surface relief structures were photofabricated on the BP-AZ-CSMB film to possess a rotational symmetry as high as 72-folds.
Bisphenol-A diglycidyl ether (BP), aniline (AN) and 4-aminobenzoic acid were purchased from Alfa Aesar and used as received. (S)-2-Methylbutanol was purchased from TCI and used as received. (S)-1-Bromo-2-methylbutane was synthesized from (S)-2-methylbutanol in this laboratory. Tetrahydrofuran (THF) was purified by distillation with sodium and benzophenone. Deionized water (resistivity >18 MΩ cm) was obtained from a Milli-Q water purification system. All other reagents were commercially available products and used as received without further purification.
1H NMR and 13C NMR spectra were obtained on a JEOL JNM-ECA400 or JEOL JNM-ECA600 NMR spectrometer with tetramethylsilane (TMS) as the internal standard at 25 °C in d6-DMSO. FT-IR spectra were collected on a Nicolet 560-IR spectrometer. The samples were mixed with KBr and then pressed into thin transparent disks. Thermal analyses of the polymers were carried out using TA Instruments DSC Q2000 with a heating rate of 10 °C min−1 in a nitrogen atmosphere. The UV-vis spectra of the samples in THF solutions were measured by using an Agilent 8453 UV-vis spectrophotometer. Optical microscopic (OM) observations were conducted on a Nikon LV 1000 POL microscope equipped with a Nikon DS-Fi2 CCD camera and Nikon DS-U3 digital sight. The surface images of the surface-relief-gratings and two-dimensional quasi-crystal structures were monitored using an atomic force microscope (AFM, Nanoscope V) in the tapping mode.
2.3 Synthesis of BP-AZ-CSMB
(S)-2-Methylbutyl 4-Aminobenzoate. To a stirred mixture of 4-aminobenzoic acid (3.0 g, 22 mmol) and (S)-1-bromo-2-methylbutane (2.9 g, 19 mmol) in DMF (50 mL) was added solid K2CO3 (4.2 g, 30 mmol) and KI (0.9 g, 5 mmol). The mixture was heated and stirred at 50 °C for 24 h. The reaction mixture was cooled, diluted with water (100 mL), and extracted twice with DCM (50 mL). The organic phase was washed twice with water (50 mL) and dried over Na2SO4. After evaporation of the solvent, the residue was subjected to the column chromatography on silica gel with DCM as an eluent to yield white powder (70%). 1H NMR (400 MHz, DMSO-d6, δ): 7.66 (d, J = 8.7 Hz, 2H; ArH), 6.59 (d, J = 8.7 Hz, 2H; ArH), 5.97 (br, 2H; NH2), 4.00 (m, 2H; O-CH2), 1.73 (m, 1H; CH), 1.45 (m, 1H; CH2), 1.21 (m, 1H; CH2), 0.90 (m, 6H; CH3).
BP-AZ-CSMB. The precursor polymer BP-AN was prepared by step polymerization between bisphenol-A diglycidyl ether (BP) and aniline (AN) according to the literature [25,26]. For preparing BP-AZ-CSMB, BP-AN (1.8 g, 47 μmol) was dissolved in DMF (200 mL) at 0 °C. A diazonium salt of (S)-2-methylbutyl 4-aminobenzoate was prepared by adding an aqueous solution of sodium nitrite (0.8 g, 12 mmol in 2 mL water) into a solution of (S)-2-methylbutyl 4-aminobenzoate (1.2 g, 6 mmol) in a mixture of sulfuric acid (2 mL) and glacial acetic acid (20 mL). The mixture was stirred at 0 °C for 30 min and then was added dropwise into the BP-AN solution. The solution was stirred at 0 °C for another 12 h. Then, the solution was poured into plenty of water and the precipitate was collected and dried. The product was dissolved in 60 mL THF and precipitated into 600 mL petroleum ether. The final product was vacuum dried at 50 °C for 24 h. 1H NMR (600 MHz, DMSO-d6, δ): 8.05 (br, 2H; ArH), 7.82 (d, J = 6.4 Hz, 2H; ArH), 7.73 (d, J = 6.0 Hz, 2H; ArH), 7.07 (br, 4H; ArH), 6.90 (br, 2H; ArH), 6.84 (br, 2H; ArH), 5.50 (br, 2H; OH), 4.12-3.41 (m, 12H; CH), 1.79-0.88 (m, 15H; CH). FTIR (KBr): ν = 3397 (s; ν(O-H)), 2963 (s; νas(CH3)), 2932 (m; νas(CH2)), 2875 (m; νs(CH3)), 1713 (s; νs(C = O)), 1599, 1510 (s; ν(benz. ring)), 1460 ((m; ν(C-O)), 1241 (s; ν(C-O)) cm−1. GPC: Mn = 2.4 × 104 g mol−1, Mw/Mn = 2.9. UV-Vis (THF): λmax = 426 nm. DSC: Tg = 126 °C. TGA: Td = 231 °C.
The films with smooth surfaces were prepared by spin-coating. The solution was prepared by dissolving BP-AZ-CSMB in anhydrous DMF to obtain a solution with the concentration of 10 wt %. The solution was filtered through 0.45 μm membrane and then spin-coated onto the glass slides with a speed about 1000 rpm for 30 s. The spin-coated films were dried at room temperature under vacuum for 12 h.
For inscribing surface-relief-grating, a linearly polarized beam from a diode-pumped frequency-doubled solid state laser (532 nm) was used as the light source. The p-polarized laser beam was spatially filtered, expanded, and collimated to have the intensity of 100 mW cm−2. By using the Lloyd set-up, one half of the collimated beam was incident on the film directly and the other half of the beam was reflected onto the film from a mirror. The diffraction efficiency of the first order diffracted beam from the gratings was probed with a low power He-Ne laser beam at 633 nm in transmission mode.
The two-dimensional quasi-crystal patterns were fabricated with the optical setup similar to that reported previously . A diode-pumped frequency-doubled solid state laser beam (size~9 mm) with Gaussian profile at 532 nm was split into two equal-intensity beams by using a beam splitter. Two λ/2 wave plates were used to independently rotate the plane of the polarization of the beams. Glan-Thompson prisms were applied to ensure that the electric fields of the two beams were perpendicular to each other (in x and y-axis). Two λ/4 wave plates were put in the optical paths to ensure the interfering beams with opposite circular polarizations (right circularly polarized and left circularly polarized). The intensity of each beam was in the range 30-45 mW cm−2. The spatial period of the interference pattern (Λ) was controlled by adjusting the intersection angle of the beams (θ), according to the equation λ/2sin(θ/2), where λ is the wavelength in vacuum (532 nm). The irradiation area had a size about 0.64 cm2. A piece of the BP-AZ-CSMB film mounted on a rotation stage was exposed to the interference pattern for a fixed period of time. After each exposure, the sample was rotated around the surface normal to achieve the required orientation of the grating vector relative to the previous one. To fabricate a quasi-crystal structure with 2n-fold rotation symmetry, the rotation angle was π/n and n exposure steps were required. Under typical conditions, the time period for each exposure was 5-20 s. The first-order diffraction efficiency was monitored during each writing step. The exact irradiation time for each step was adjusted by controlling the diffraction efficiency to be the same.
3. Results and discussion
3.1 Polymer synthesis
A new epoxy-based azo polymer (BP-AZ-CSMB), which contains azo chromophores with chiral substituents, was synthesized in this study. The chemical structure of BP-AZ-CSMB is shown in Fig. 1. The main difference between this polymer and other epoxy-based azo polymers reported before is that the azo chromophores bear the chiral (S)-2-methylbutyl groups. This molecular architecture was found to be important for fabricating the quasi-crystal relief structures. BP-AZ-CSMB is a linear polymer with number-average molecular weights of 2.4 × 104 and the polydispersity index of 2.9 determined by GPC. BP-AZ-CSMB shows phase behavior of a typical amorphous polymer with Tg of 126 °C. Transparent solid thin films with smooth surfaces were obtained through spin-coating by using the DMF solutions of BP-AZ-CSMB.
3.2 Surface-relief-grating formation
Photoinduced SRG formation on BP-AZ-CSMB film was investigated and compared with the epoxy-based azo polymer BP-AZ-CA. The synthesis of BP-AZ-CA has been reported in our previous article . In this study, both azo polymers were obtained from the same batch of the precursor polymer to make sure that they possessed the same degree of polymerization. The SRG was inscribed by irradiation with interfering p-polarized laser beams from a diode-pumped frequency-doubled solid state laser (532 nm). A He-Ne laser beam at 633 nm was used as the probe beam. The SRG formation was monitored by measuring the diffraction efficiencies of the probe beam in a real-time manner. The profiles and trough depths of the gratings were detected by AFM.
Figures 2(a) and 2(b) show typical AFM images of the sinusoidal surface relief structures with regular spaces formed on the BP-AZ-CSMB film. Figure 2(c) compares the first order diffraction efficiencies (DEs) as a function of the irradiation time in the process of SRG formation on the films of BP-AZ-CSMB and BP-AZ-CA. The DE of the BP-AZ-CSMB film shows an abrupt increase when irradiated by the laser beam, and then a gradual growth for further irradiation. On the contrary, BP-AZ-CA shows a gradual increase with the increasing irradiation time from the beginning. Only a small-scale DE jump at the beginning can be seen for BP-AZ-CA, which is related to the polarization grating formation [14,15]. The much higher DE of BP-AZ-CSMB, compared with that of BP-AZ-CA, can be attributed to its high efficiency to form SRG in a short time period. This quick growth of SRG in the short time period (5-20 s) for BP-AZ-CSMB is an important property for fabricating the quasi-crystal surface relief structures.
3.3 Quasi-crystal surface patterns
The quasi-crystal surface relief structures were fabricated by the dual-beam multiple exposure technique as described in experimental section. The spatially modulated light field was produced by interference of a right circularly polarized (RCP) beam and a left circularly polarized (LCP) beam. The spatial period Λ of the interference pattern was adjusted by changing the two-beam intersection angle (θ). The quasi-crystal surface relief structures were fabricated by rotating the samples around the surface normal. The 2n-fold quasi-crystals were obtained by n exposure steps, and the relative orientation of the interference fringes was changed by the π/n radians after each exposure.
Figure 3(a) and 3(b) give the AFM images of the quasi-crystal patterns with 10-fold symmetry (Penrose quasi-crystal). The quasi-crystal structures on BP-AZ-CSMB film were fabricated through 5 exposure steps by using the interference pattern with the period of 3.0 μm. The modulation depth of the pattern is 250 nm estimated from the AFM cross-section profile. Figure 3(c) gives a typical optical micrograph of the quasi-crystal structures. Figure 3(d) gives the diffraction pattern of a He-Ne laser beam with the normal incidence to the sample.
The quasi-crystal structures on BP-AZ-CSMB films with different rotation symmetry can be prepared through the same procedure. The quasi-crystal structures with 6-, 8-, 10-, 12-, 36-, and 72-fold symmetry were successfully fabricated in this work. Under typical conditions, the modulation depth in the 100-300 nm was obtained after a 5-20 s irradiation for each exposure step. Figure 4 gives the representative AFM images of the quasi-crystal patterns with 6-, 8-, 12-, and 36-fold rotation symmetry.
As the accumulated modulation is related to the total exposure time, an addition of the exposure time for each step, it is a challenging task to obtain the patterns with high fold rotation symmetry. BP-AZ-CSMB shows the ability to form the quasi-crystal structure with rotation symmetry as high as 72-fold. Figure 5 shows the AFM image, optical micrograph and the diffraction pattern of the 72-fold quasi-crystal structure. The structure (Λ = 2.0 μm) was obtained through 36 exposure steps with the sample rotation increment of π/36 radians. Figure 5(a) gives typical AFM image of the surface relief structure. The surface profile shows very complicated surface pattern with the high rotational symmetry. Figure 5(b) presents the typical optical micrograph of the 72-fold quasi-crystal structure, which further confirms the AFM observation. Figure 5(c) shows the diffraction pattern of a He-Ne laser beam incident normally on the sample, which clearly shows the 72-fold rotation symmetry of the structure.
While we used other epoxy-based azo polymers, such as BP-AZ-CA, no such quasi-crystal structure could be properly fabricated. Considering the typical conditions for fabricating the quasi-crystal surface structures, the time period for each exposure is less than 30 s but the multiple exposure steps are needed. Therefore, the fast growth of the surface pattern is necessary condition to obtain the surface structures. As shown above, the DE growth of the BP-AZ-CSMB is much faster than that of the BP-AZ-CA in the first 30 s. It is the reason why the two-dimensional quasi-crystal surface relief structures can be successfully fabricated on the BP-AZ-CSMB films. Whether the chiral structure is a necessary structural factor is currently under investigation by us.
In this work, a new epoxy-based azo polymer (BP-AZ-CSMB) was synthesized, which contains chiral-substituted push-pull azo chromophores. BP-AZ-CSMB was obtained from the precursor polymer (BP-AN) by reacting with the diazonium salts of (S)-2-methylbutyl 4-aminobenzoate. This newly synthesized polymer was carefully characterized by 1H NMR, FTIR, GPC, UV-Vis, DSC and TGA. It was observed that quasi-crystal surface relief structures could be feasibly fabricated on the BP-AZ-CSMB films by using the dual-beam multiple exposure technique. The quasi-crystal structures with 6-, 8-, 10-, 12-, 36-, and 72-fold rotation symmetry were successfully fabricated on the BP-AZ-CSMB films.
The financial support from the NSFC under Projects 51233002 is gratefully acknowledged.
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