4794
Langmuir 2005, 21, 4794-4796
Thermally Stable Holographic Surface Relief Gratings and Switchable Optical Anisotropy in Films of an Azobenzene-Containing Polyelectrolyte Leonid M. Goldenberg,* Olga Kulikovska, and Joachim Stumpe* Fraunhofer Institute for Applied Polymer Research and Institute of Thin Film Technology and Microsensorics, Science Campus Golm, Geiselbergstrasse 69, 14476 Potsdam, Germany Received February 21, 2005 In a search for effective polymer film material for holographic surface patterning, commercially available azobenzene polyelectrolyte has been employed. Films of good optical quality in a wide range of thickness were produced. Optical dichroism up to 0.19 was induced upon irradiation with linearly polarized light. Surface relief gratings with amplitudes up to 630 nm and diffraction efficiency of 37% were inscribed by holographic exposure to the light of 488 nm. Due to the ionic nature of the material, the relief was stable at least up to the temperature of decomposition (ca. 200 °C) but could be erased and inscribed again by light.
Introduction Polymers containing azobenzene moieties are known for their ability to induce anisotropy by photoorientation upon irradiation with polarized light1 and to form surface relief gratings (SRGs) by holographic exposure.2 It is accepted that E/Z photoisomerization is a key process in both effects. The photochemically induced movement of the azobenzene moieties involves transport of whole macromolecules. Anisotropic films, orientational gratings, and SRGs have a potential for different applications in optics and information technologies. From this point of view the efficiency of the grating formation and the material properties such as film forming ability, thermal stability, etc., are of high importance. Concerning the SRG formation the presence of azobenzene moieties covalently bound to the polymer chain seems to be a requirement for the process. In this case the polymer chain ensures film building properties. SRGs have been inscribed in a large variety of different polymers such as acrylates, polyurethanes, epoxy-based polymers, etc.1b SRG formation has been also reported for some amorphous molecular azobenzenes, which are able to form films.3 Polymers in general may have good filmforming properties; however, the solubility could be a hard problem. Also some of the azobenzene polymers synthesized so far are of low molecular weights, which leads to bad mechanical properties. The utilization of organic solvents to build the films is the general disadvantage of such polymers. Another disadvantage is multistep synthesis usually required to manufacture functional polymers from commercially available chemicals.1 Consequently, they are quite expensive. The only one known to us is a commercially available azobenzene polymer that has been used for SRG writing4 (Sigma-Aldrich) and is also rather expensive. Furthermore, the purification of (1) (a) Ichimura, K. Chem. Rev. 2000, 100, 1847. (b) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139. (c) Shibaev, V.; Bobrovsky, A.; Boiko, N. Prog. Polym. Sci. 2003, 28, 729. (2) (a) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166. (b) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136. (3) Yip, W. C.; Kwok, H. S.; Kozenkov, V. M.; Chigrinov, V. G. Displays 2001, 22, 27. (b) Nakano, H.; Takahashi, T.; Kadota, T.; Shirota, Y. Adv. Mater. 2002, 14, 1157. (c) Ando, H.; Takahashi, T.; Nakano, H.; Shirota, Y. Chem. Lett. 2003, 32, 710. (d) Stracke, A.; Wendorff, J. H.; Goldmann, D.; Janietz, D. Liq. Cryst. 2000, 27, 1049. (4) Baac, H.; Lee, J.-H.; Seo, J.-M.; Park, T. H.; Chung, H.; Lee, S.-D.; Kim, S. J. Mater. Sci. Eng., C 2004, 24, 209.
the polymers is a hard problem as well. So-called “guesthost” systems were prepared by mixing azobenzene derivatives with readily available polymers such as poly(methyl methacrylate) (PMMA). It results in a rather weak ability to form SRG due to low dye concentration in homogeneous films caused by dye-polymer segregation. Such materials have been used for the generation of optical anisotropy as well.5 The so-called layer-by-layer (LbL) procedure allows use of commercially available azobenzene compounds to generate SRGs.6 In this case layered films are formed by charged azobenzene derivatives and conventional polyelectrolytes. The LbL electrostatic film assembly allows controllable build-up of thin films and is quite popular in recent years.7 However, the procedure is tedious and timeconsuming even with the use of robotic techniques. Really thick films, which are necessary for the inscription of deep SRGs, are hard to achieve. Another disadvantage of azobenzene-containing LbL films is the low concentration of photoactive azobenzene component in the film and the formation of a segregated multilayer assembly.6 It would be still a challenge to find a readily prepared azobenzene-based material for effective photoinduction of anisotropy and the formation of SRGs. Here, we report for the first time8 the film building properties, photogeneration of anisotropy, holographic SRG inscription, and thermal stability of SRGs using a commercially available azobenzene-containing polymer. SRGs with depth up to 600 nm and thermal stability of the induced relief structures up to at least 200 °C were achieved. Experimental Section Films of sodium salt of poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl} (PAZO) were cast onto glass substrates in a closed chamber at room temperature. (5) (a) Si, J.; Qiu, J.; Zhai, J.; Shen, Y.; Hirao, K. Appl. Phys. Lett. 2000, 80, 359. (b) Fiorini, C.; Prudhomme, N.; de Veyrac, G.; Maurin, I.; Raimond, P.; Nunzi, J.-M. Synth. Met. 2000, 115, 121. (6) (a) Kaneko, F.; Kato, T.; Baba, A.; Shinbo, K.; Kato, K.; Advincula, R. C. Colloids Surf., A 2002, 198, 805. (b) He, J.; Bian, S.; Li, L.; Kumar, J.; Tripathy, S. K.; Samuelson, L. A. J. Phys. Chem. B 2000, 104, 10513. (c) Zucolotto, V.; He, J.; Constantino, C. J. L.; Barbosa Neto, N. M.; Rodrigues, J. J., Jr.; Mendonc, C. R.; Zy´lioa, S. C.; Li, L.; Aroca, R. F.; Oliveira, O. N., Jr.; Kumar, J. Polymer 2003, 44, 6129. (7) Sano, M.; Lvov, Y.; Kunitake, T. In Annual Review of Material Science; AR-Inc.: Palo Alto, CA, 1996; Vol. 26, pp 153-189. (8) Goldenberg, L. M.; Kulikovska, O.; Stumpe, J.; European Patent Application 04029762.5; 040200997.5; 04029263.3.
10.1021/la050457s CCC: $30.25 © 2005 American Chemical Society Published on Web 04/19/2005
Letters
Langmuir, Vol. 21, No. 11, 2005 4795
Figure 1. Chemical formula of PAZO. The concentration was between 0.8 and 25 mg/mL in MeOH. The SRGs were imaged by atomic force microscopy (AFM) with Solver P47H Smena (NTMDT, Russia). The film thickness was estimated by measuring scratches in the film using AFM. The films were exposed to the linearly polarized light of the wavelength 488 nm (Ar+ laser Innova 90, Coherent). The induction and the relaxation of the optical anisotropy were detected in real time by means of a probe beam of a He-Ne laser operating at 633 nm. The probe light was linearly polarized at 45° to the polarization plane of the irradiating beam. The transmitted probe beam was split into two orthogonally polarized beams by means of a Wollaston prism. The intensities of both orthogonal polarization components, i.e., the component with the polarization of the incident probe beam and a new component with orthogonal polarization rising due to the induced birefringence, were measured. Saturation values of dichroism after irradiation with linearly polarized light of the wavelength 488 and 364 nm (Ar+ laser, Spectrophysics) were determined by polarized UV-vis spectroscopy (Perkin-Elmer Lamda 19 spectrometer with step-motor-controlled polarizer). The films were irradiated with the interference pattern formed by two linearly orthogonally polarized beams with polarization planes at (45° to the incidence plane. The irradiation wavelength was 488 nm, and the angle of incidence was about of 6° resulting in a period of 2.3 µm. The used experimental setup was described earlier.9 The intensities of interfering beams were equal to 250 mW/cm2. The efficiencies of zeroth, first, and second orders were monitored using 633 nm probe beam.
Results The water-soluble polyelectrolyte poly{1-[4-(3-carboxy4-hydroxyphenylazo)benzenesulfonamido]-1,2ethanediyl, sodium salt} (PAZO) is commercially available from Aldrich and contains negatively charged azobenzene moieties in its side chain (Figure 1). Recently, the material has been used to prepare films by the LbL procedure.10 However, only photoisomerization, alignment of liquid crystals, and nonlinear optical properties of such LbL films were reported.10 Tripathy and co-workers produced LbL films based on specially synthesized azobenzene containing polyelectrolytes and studied their nonlinear optical properties.11 Two of these polymers were capable to form SRGs in the form of carboxylic acid (weekly ionic form). The films were spin-coated using DMF as a solvent. However, the generation of SRGs was not effective, and these weakly ionic polymers exhibited Tg values in the range of 90-105 °C.11a (9) Kulikovska, O.; Gharagozloo-Hubmann, K.; Stumpe, J. Proc. SPIE 2002, 4802, 85. (10) (a) Lvov, Y.; Yamada, S.; Kunitake, T. Thin Solid Films 1997, 300, 107. (b) Dante, S.; Advincula, R. C.; Frank, C. W.; Stroeve, P. Langmuir 1999, 15, 193. (c) Park, M.-K.; Advincula, R. C. Langmuir 2002, 18, 4532. (d) Johal, S. M.; Ozer, B. H.; Casson, J. L.; St. John, A.; Robinson, J. M.; Wang, H.-L.; Langmuir 2004, 20, 2792. (e) Casson, J. L.; Wang, H.-L.; Roberts, J. B.; Parikh, A. N.; Robinson, J. M.; Johal, M. S. J. Phys. Chem. B 2002, 106, 1697. (11) (a) Wang, X.; Balasubramanian, S.; Kumar, J.; Tripathy, S. K.; Li, L. Chem. Mater. 1998, 10, 1546. (b) Balasubramanian, S.; Wang, X.; Wang, H. S.; Kumar, J.; Tripathy, S. K. Chem. Mater. 1998, 10, 1554.
PAZO films of good optical quality were readily prepared from MeOH solution in a wide range of thicknesses (solubility of about 25 mg/mL). Uniform films could be prepared by spin-coating or casting in the range of few tenths of nanometers up to a few micrometers. DMF (solubility of about 8 mg/mL) is also the solvent of choice. Stacked layers alternating PAZO (from MeOH solution) and usual organic solvent soluble polymers are readily obtained and as example we have stacked PAZO with azobenzene-containing acrylate using THF as solvent. When films of PAZO are exposed to the linearly polarized light E/Z isomerization cycles take place and repeated isomerization cycles result in the orientation of the long axis of azobenzene units preferably in the direction perpendicular to the electrical field vector. This orientation originates optical anisotropy of the film, i.e., birefringence and dichroism. The kinetics of the induction of anisotropy upon irradiation with linearly polarized light of 488 nm monitored by two orthogonal polarization components of the transmitted probe beam (633 nm) is exhibited in Figure 2a. From the two-exponential fit to the experimental data the induction time of 3.8 min was found for the applied intensity and wavelength. After termination of irradiation the anisotropy retains. However, it could be quickly erased by nonpolarized or circular polarized light and could be inscribed again. The direction of induced optical axis may be also changed by the change of the polarization of the incident polarized light (photoreorientation). For example, in Figure 2a the direction of the induced optical axes was changed to perpendicular by switching the polarization of the actinic light from linear vertical to linear horizontal. As it is seen from the fit to the data the induction of anisotropy in the new direction takes 3.8 min. This optical switching of anisotropy was repeated many times (Figure 2b) whereas no fatigue was noticed after at least 30 induction/erasure cycles. The saturation value of orientation was also measured by polarized spectroscopy and a dichroism of about 0.19 was found at the wavelength of absorption maximum. It was shown that the induced dichroism in this material does not depend on film thickness and irradiation wavelength. Approximately the same value was obtained for much thinner film (70 nm) irradiated with either 488 or 365 nm light. The exposure of such films to the patterned light led to the formation of relief structures. The films were irradiated with the interference pattern of two linear orthogonally polarized beams, as it was shown that such polarization configuration results in an effective formation of SRGs.9 The kinetics of the grating formation as monitored by diffraction of probe light is shown in Figure 3a. After 2 h of exposure, diffraction efficiencies (DEs) of 37.2 and 16.4% for the first and the second diffraction order, respectively, were achieved. The corresponding atomic force microscopy (AFM) image of the SRG is shown in Figure 3b. The amplitude of the inscribed SRG was approximately proportional to the time of writing, and after 2 h of exposure a depth of 630 nm was reached (Figure 3a, insert). The grating profile was approximately sinusoidal. The efficiency of the grating formation, the achieved values of DE, and the relief height are comparable to the values known for the best azobenzene-containing polymers.11,12 It may be due to the high azobenzene content in the case of PAZO. Recently, it has been shown that in the case of polymers with covalently attached azobenzene groups the efficiency of SRG formation increases with the increase (12) (a) Barrett, C. J.; Natansohn, A. L.; Rochon, P. L. J. Phys. Chem. 1996, 100, 8836. (b) Darracq, B.; Chaput, F.; Lahlil, K.; Løvy, Y.; Boilot, J.-P. Adv. Mater. 1998, 10, 1133.
4796
Langmuir, Vol. 21, No. 11, 2005
Letters
Figure 2. Induction and relaxation of the optical anisotropy: the intensity of the orthogonally polarized vertical component of the transmitted probe beam, red line shows exponential fit (a); switching between two states under alternating irradiation (eight cycles) (b).
Figure 3. Induction of a SRG: kinetics of the diffracted probe beam (a), insert shows dependence of the SRG depth on the time of writing; AFM image of the resulting SRG (b).
in azobenzene content with the maximum efficiency at about 70% content and then drops because of aggregation.13 In the case of PAZO, where azobenzene content is 100%, the effect of aggregation is probably suppressed by sterical effects due to the lateral substitution. A particular feature of gratings inscribed in PAZO is their thermal stability. Heating of the film with inscribed SRG to a temperature of ca. 200 °C, where decomposition of polymer has been visually observed (it turns first red then brown, according to TGA measurement decomposition starts at 225 °C), did not change the depth of grating. No appreciable decrease in the DEs was observed during heating at 200 °C for at least 6 h. Notice for comparison that SRGs inscribed in azobenzene-functionalized polymers are stable when kept below the glass transition temperature and may be erased thermally by heating the sample above Tg. Typically, the glass transition temperature of the functionalized polymers lies in the range of 40-150 °C restricting the thermal stability of the inscribed structures. Moreover, our experience shows that higher Tg values result in less effective SRG formation. For example, the SRGs based on an urethane-urea copolymer were thermally stable even at the Tg of 150 °C.14 However, the inscription of the grating was not effective, and a DE of only 1.5% has been achieved. Even in the case of SRGs, which were additionally photo-cross-linked after the SRG generation in order to get high thermal stability,15 DE
dropped down by half after heating to 180 °C. The extreme thermal stability of the gratings recorded in PAZO films may be assigned to the ionic nature of the material. It is of great interest that at the same time the inscribed gratings could be overwritten or erased with properly polarized light. For example, a square grating was generated in PAZO film by subsequent recording of two linear gratings with orthogonal grating vectors. In another experiment the inscribed grating was erased to about 80% by homogeneous irradiation with circularly polarized light. Afterward SRGs could be written again in the same film. To our best knowledge such combination of extraordinary thermal stability of the inscribed surface structures with the light reversibility was observed for the first time. In summary, we were able to produce films of good optical quality in a wide range of thicknesses using the commercially available azobenzene-containing polyelectrolyte PAZO. Optical dichroism up to 0.19 was induced upon irradiation with linearly polarized green or UV light. Surface relief gratings with amplitudes up to 630 nm and DEs of 37% were inscribed by holographic exposure with visible light (488 nm). Due to the ionic nature of the used material, the relief was stable at least up to the temperature of decomposition at about 200 °C but could be erased and inscribed optically again.
(13) Bo¨rger, V.; Kulikovska, O.; G-Hubmann, K.; Stumpe, J.; Huber, M.; Menzel, H. In preparation. (14) Che, Y.; Sugihara, O.; Fujimura, H.; Okamoto, N.; Egami, C.; Kawata, Y.; Tsuchimori, M.; Watanabe, O. Opt. Mater. 2002, 21, 79.
LA050457S
Acknowledgment. We thank German Ministry of Economics (BMW) for financial support.
(15) Takase, H.; Natansohn, A.; Rochon, P. Polymer 2003, 44, 7345.
