Optical Storage in Mixed Langmuir−Blodgett (LB) Films of Disperse

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Optical Storage in Mixed Langmuir-Blodgett (LB) Films of Disperse Red-19 Isophorone Polyurethane and Cadmium Stearate A. Dhanabalan, D. S. Dos Santos, Jr., C. R. Mendonc¸ a, L. Misoguti, D. T. Balogh, J. A. Giacometti, S. C. Zilio, and O. N. Oliveira, Jr.* Instituto de Fı´sica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, CP 369, CEP 13560-970, Sa˜ o Carlos, SP, Brazil Received August 25, 1998. In Final Form: March 4, 1999 The optical storage capability of mixed Langmuir-Blodgett (LB) films of disperse red-19 isophorone polyurethane (DR19-IPPU) and cadmium stearate has been demonstrated by measuring their optically induced birefringence that originates from the trans-cis isomerization of the azobenzene chromophore. Isomerization was achieved because the azobenzene chromophore could be introduced in the polyurethane main chain in a way that would allow for the free volume necessary for the molecular reorientation inherent in the isomerization process. The amplitude of induced birefringence decreased with the number of layers in the composite LB films, but the film thickness did not affect the time required to achieve maximum birefringence. Stored information could be optically erased by overwriting with a circularly polarized light. The identification of optimized conditions for producing LB films with the required optical properties was based on an extensive characterization process for the monolayers, using surface pressure and surface potential measurements, and for the deposited LB films. The mixed-film approach had to be used because pure monolayers of DR19-IPPU could not be transferred uniformly onto solid substrates. UV-vis and FTIR spectroscopy studies indicated the presence of both cadmium stearate and DR19-IPPU in the films, with a possible J-type aggregation of azobenzene chromophores, whereas X-ray diffraction patterns revealed that the cadmium stearate molecules form separate domains in a DR19-IPPU matrix.

Introduction Functionalized polymers with azobenzene chromophores covalently attached to the polymer main chain are versatile materials for constructing a variety of molecularly based optical devices. The nonlinear optical properties and photoisomerization characteristics of these polymers can be exploited in optical storage,1 optical switching,2 optical modulators,3 optical holography,4-6 and in constructing command surfaces for controlling the alignment of liquid crystals.7 For optical memory devices specifically, the free volume around the azobenzene chromophore must be controlled precisely to allow for the molecular rearrangement inherent in the reversible trans-cis photoisomerization.8 This is possible because the chromophore may be attached at different positions in either the main chain or a side chain. Polymers such as polyesters, polyamides, polyureas, polysilicones, polysilanes, polyacrylates, polymethacrylates, polystyrenes, and polyepoxys have been used as the main backbones,9 as have methacrylic copolymers with suitable smaller spacer groups.10 * Telephone: 55 16 271 5365. Fax: 55 16 271 3616. E-mail: [email protected]. (1) Meng, X.; Natansohn, A.; Rochon, P. Polymer 1997, 38, 2677 and references therein. (2) Maack, J.; Ahuja, R. C.; Mobius, D.; Tachibana, H.; Matsumoto, M. Thin Solid Films 1994, 242, 122. (3) Loucif-Saibi, R.; Nakatani, K.; Delaire, J. A.; Dumont, M.; Sekkat, Z. Chem. Mater. 1993, 5, 229. (4) Pham, V. P.; Galstyan, T.; Granger, A.; Lessard, R. A. Jpn. J. Appl. Phys. 1997, 36, 429. (5) Itoh, M.; Harada, K.; Matsuda, H.; Ohnishi, S.; Parfenov, A.; Tamaoki, N.; Yatagai, T. J. Phys. D: Appl. Phys. 1998, 31, 463. (6) Tripathy, S. K.; Kim, D.; Li, L.; Kumar, J. Chemtech 1998, 28, 34. (7) Tsukruk, V. V.; Bliznyuk, V. N. Prog. Polym. Sci. 1997, 22, 1089. (8) Sudesh Kumar, G.; Neckers, D. C. Chem. Rev. 1989, 89, 1915. Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479.

Diisocyanate-terminated polyurethanes are of special interest because they can easily be cross-linked by either thermal treatment or chemical reaction with -OH functionalities, thereby freezing the molecular dipole orientation induced by corona discharge, which generates stable and enhanced nonlinear optical characteristics.11-13 We are currently interested in the trans-cis photoisomerization properties of azobenzene functionalized polyurethanes because of their optical memory properties and because different spacer groups may be introduced in the main chain by the reaction of different diisocyanates with a diol or a polyol. The structure of the polymer, disperse red-19 isophorone polyurethane (DR19-IPPU), employed in the present study is shown in Figure 1. The azobenzene group is hooked as a side chain through spacer ethoxy groups to the main polyurethane chain that also contains a spacer isophorone group. The spacer in the main chain and two covalent linkages of the azobenzene group to the polymer backbone greatly reduce aggregation of azobenzene chromophores in the polymer chain,14 although azobenzene groups from distinct polymer chains may still aggregate. The optical storage characteristics of cast films from azobenzene-functionalized acrylic polymers have been studied by Natansohn et al.9 Here, we employ the Langmuir-Blodgett (LB) technique to produce ultrathin (9) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5, 403. (10) Brown, D.; Natansohn, A.; Rochon, P. Macromolecules 1995, 28, 6116. (11) Tsutsumi, N.; Yoshizaki, S.; Sakai, W.; Kiyotsukuri, T. Macromolecules 1995, 28, 6437. (12) Chen, M.; Dalton, L. R.; Yu, L. P.; Shi, Y. Q.; Steier, W. H. Macromolecules 1992, 25, 4032. (13) Mao, S. S. H.; Ra, Y. S.; Guo, L.; Zhang, C.; Dalton, L. R.; Chen, A.; Garner S.; Steier, W. H. Chem. Mater. 1998, 10, 146. (14) Meng, X.; Natansohn, A.; Rochon, P. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 1461.

10.1021/la9811012 CCC: $18.00 © 1999 American Chemical Society Published on Web 05/14/1999

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Figure 1. Structure of DR19-IPPU.

films from the azobenzene-containing polyurethane because this method offers the possibility of precise control over the film thickness and molecular packing,15 in addition to allowing the photoisomerization of azobenzene chromophores to be investigated in the solid state. Before addressing the optical storage characteristics of the LB films of DR19-IPPU, we report on the monolayer characteristics and LB film preparation. Owing to the difficulties in obtaining pure DR19-IPPU LB films, the composite LB film approach is employed in which DR19IPPU is deposited together with cadmium stearate. Experimental Section

Figure 2. (Curve a) surface pressure- and (curve b) surface potential-area per monomer isotherm of a DR19-IPPU monolayer. 4 × 10-4 M cadmium chloride and 5 × 10-5 M sodium bicarbonate whose pH was approximately 6.0. Monolayer experiments and LB depositions were carried out with a KSV-5000 LB system housed in a class 10 000 clean room. All experiments were carried out at a subphase temperature of 22 °C and in the dark. During the isotherm experiments, monolayers were compressed at a barrier speed of 10 mm/min. In the case of pure DR19-IPPU, the monomer repeat unit (552 g/mol) was considered for mean molecular area (mma) calculations. For the composite monolayer, mma was calculated based on the number of stearic acid molecules. BK7 glass, gold evaporated glass, and zinc selenide (ZnSe) plates were used as substrates that were cleaned thoroughly prior to use. UV-vis and FTIR measurements were carried out with a Hitachi-U2001 spectrophotometer and a BOMEM-MB102 Michelson series instrument, respectively. X-ray diffraction measurements were made with a Rigaku Rotaflex (Model RU200B) X-ray diffractometer in the 2θ range of 3-20° using a Cu target. Surface potential measurements of the LB film deposited on gold/glass substrates were carried out with a Trek 320B electrostatic voltmeter. The optical birefringence was induced in the LB film using a polarized Nd/YAG continuous laser at 532 nm with a polarization angle of 45° with respect to the polarization orientation of the probe beam. The power of the writing laser beam was 6.8 mW for a 2 mm spot. A low power He-Ne laser at 632.8 nm passing through crossed polarizers was used as the probe beam (reading light) to measure the induced birefringence in the sample.

The DR19-IPPU polymer was synthesized by reacting disperse red-19 (DR19) with isophorone diisocyanate in cyclohexanone at 120 °C for 2 days, as reported in Wong et al.16 After being cooled to room temperature, the reaction mixture was poured into toluene under stirring. The precipitated polymer was collected, redissolved in tetrahydrofuran, and precipitated in toluene. The polymer was washed thoroughly with 2-propanol to remove unreacted DR19. The yield was about 60%. The UV-vis spectrum (Hitachi-U2001 spectrophotometer) of a chloroform solution of DR19-IPPU displayed an absorption maximum at 460 nm, which is close to that reported for DR19-IPPU in the literature.16 A redshift of ca. 10 nm was observed for the DR-19 absorption maximum when the solvent was changed from a pure dimethyl sulfoxide (DMSO) to a chloroform-DMSO mixture, but when the same experiment was performed with DR19-IPPU, the absorption pattern was not altered. The chromophore content in the polymer was estimated from the calibration curve obtained from the chloroform-DMSO solution of DR19 dye with different concentrations. The result was 47 mol % in comparison to the theoretical prediction of 57 mol %. The FTIR spectra (BOMEMMB102 Michelson series instrument) of both the DR19 dye and the DR19-IPPU polymer were obtained in the form of KBr pellets. The FTIR spectra of DR19-IPPU exhibited an additional vibrational absorption peak at 1720 cm-1, but the strong, broad OH absorption at about 3430 cm-1 disappeared, owing to the formation of the DR19-IPPU polymer. The number-average molecular weight and glass-transition temperature (Tg) of the DR19-IPPU polymer synthesized using a similar method were about 5380 g/mol and 137 °C, respectively.16 Ultrapure water produced by a Milli-RO coupled to a Milli-Q purification system was used to prepare subphase solutions. Monolayers of DR19-IPPU were obtained by spreading a solution of DR19-IPPU in a mixture of chloroform-DMSO (8:2) on a pure water surface. Initial attempts to use chloroform as the spreading solvent resulted in unstable solutions. For the composite monolayer of DR19-IPPU and cadmium stearate, a solution with 50/ 50 wt % of DR19-IPPU and stearic acid in a chloroform-dimethyl sulfoxide mixture was spread on an aqueous subphase containing

Monolayer Studies. The surface pressure-area isotherm of a DR19-IPPU monolayer is shown in Figure 2 as curve a. The onset for surface pressure occurs at ca. 90 Å2, with the pressure increasing up to about 8 mN/m at an area of about 55 Å2, which can be considered the liquidexpanded region. A similar liquid-expanded region has been reported for the monolayer from polyurethane derivatized with long alkyl chains.17 After this region, the pressure increases steeply until the monolayer collapses at about 28 mN/m. Analogously to most polymeric monolayers, collapse is denoted by a slight change of slope in the isotherm.18 The limiting mean molecular area obtained from the extrapolation of the liquid-condensed region to zero surface pressure is ca. 60 Å2. In our earlier study, we obtained a limiting mean molecular area close to 20 Å2 for monolayers of the disperse red derivatized methacrylic

(15) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (16) Wang, N. P.; Leslie, T. M.; Wang, S.; Kowel, S. Chem. Mater. 1995, 7, 185.

(17) Nerger, D.; Ohst, H.; Shopper, H. C.; Wehrman, R. Thin Solid Films 1989, 178, 253. (18) Dhanabalan, A.; Balogh, D. T.; Riul, A., Jr.; Giacometti J. A.; Oliveira, O. N., Jr. Thin Solid Films 1998, 323, 257.

Results and Discussion

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Figure 3. (Curve a) surface pressure- and (curve b ) surface potential-area per molecule isotherm of a composite monolayer of DR19-IPPU and cadmium stearate (50/50 %). The area is per stearic acid molecule.

polymer which contained no main chain spacer group.18 It can be inferred therefore that the polyurethane chain with the spacer isophorone group is possibly anchored at the air-water interface with the azobenzene chromophore pointing toward the air. Because the estimated area required for a facile reversible trans-cis isomerization of a azobenzene group19 is comparable to the measured limiting mean molecular area, there is enough room for a facile reversible trans-cis photoisomerization of the azobenzene chromophores. The surface potential-area isotherm of the DR19-IPPU monolayer is shown in Figure 2 as curve b. The near-zero surface potential for areas above 150 Å2 indicates no considerable aggregation at this stage. The onset for the surface potential occurs at 150 Å2, which is almost twice the onset area for the pressure. We should stress that no attempts can be made here to interpret the maximum surface potential of ca. 300 mV quantitatively, because it is not straightforward to ascribe group dipole moments for polymeric materials.20 Subsequent stability experiments revealed that the monolayer is not very stable, and a decrease of mean molecular area could be noticed while holding the monolayer in the compressed state. The poor stability of the monolayer was also inferred from a significant hysteresis between compression and decompression cycles. Furthermore, the initial attempts to transfer the DR19-IPPU monolayer onto substrates at a constant surface pressure of 18 mN/m resulted in visually nonuniform LB films. To obtain uniform LB films, we codeposited DR19IPPU with cadmium stearate, as has been done with similar polymers.21 Figure 3 shows (curve a) the surface pressure- and (curve b) the surface potential-area isotherms of a composite monolayer containing DR19IPPU and cadmium stearate (50/50 wt %). Similarly to the pure DR19-IPPU monolayer, a liquid-expanded region was observed, starting at ca. 55 Å2 with the pressure increasing up to 10 mN/m (curve a). This was followed by a steep rise in pressure until the monolayer collapses at 65 mN/m, which is close to the collapse pressure of pure (19) Nishiyama, K.; Kurihara, M. A.; Fujihira, M. Thin Solid Films 1989, 179, 477. (20) Dhanabalan, A.; Riul, A., Jr.; Mattoso, L. H. C.; Oliveira, O. N., Jr. Langmuir 1997, 13, 4882. (21) Dhanabalan, A.; Balogh, D. T.; Mendonc¸ a, C. R.; Riul, A., Jr.; Constantino, C. J. L.; Giacometti, J. A.; Zilio, S. C.; Oliveira, O. N., Jr. Langmuir 1998, 14, 3614.

Dhanabalan et al.

Figure 4. Plot of absorbance at 470 nm vs number of layers of composite LB films of DR19-IPPU and cadmium stearate.

cadmium stearate. The liquid-expanded region cannot be attributed to the presence of nonionized stearic acid molecules, since such a region is not present in pure cadmium stearate monolayers obtained under similar conditions. Also, the FTIR spectra of the transferred monolayer indicated the absence of ionized stearic acid (see below). Because the calculated molecular area was based on the number of stearic acid molecules, the extrapolated area of ca. 42 Å2 (in comparison to the 20 Å2 for pure cadmium stearate) demonstrates that DR19IPPU molecules are at the air-water interface even in the compressed monolayer. The composite monolayer is reasonably stable, despite a small decrease of area while the monolayer is held in the compressed state at the initial stages. This stability indicates that there is no significant squeezing out of polymer molecules from the cadmium stearate matrix. The surface potential-area isotherm of a composite monolayer of DR19-IPPU and cadmium stearate is shown as curve b in Figure 3, with the onset occurring at ca. 80 Å2 which is almost twice the limiting mean molecular area. The maximum surface potential of ca. 200 mV is between the surface potentials of pure DR19-IPPU (300 mV) and of pure cadmium stearate (120 mV). LB Film Deposition and Characterization. Composite monolayers were transferred onto solid substrates as Y-type LB films with near-unity transfer ratios, using the vertical dipping method at a constant surface pressure of 31 mN/m. The LB films are visually uniform, which means that the builder material has enhanced the transferability of DR19-IPPU. The UV-vis spectra of an as-deposited composite LB film onto glass and of a DR19IPPU solution in the chloroform-DMSO mixture (the spreading solution used in the LB experiment) were recorded for comparison. The absorption maximum is redshifted by 10 nm in the LB film (470 nm) when compared to the solution (460 nm), which is probably due to J-type aggregation of azobenzene chromophores in the LB film. The absorption increases linearly with the number of LB layers up to 70 layers or so, as shown in Figure 4 for the absorbance at 470 nm, thus indicating that an equal amount of DR19-IPPU was transferred during each transfer step. Figure 5 shows the FTIR spectra of a 31 layer LB film of DR19-IPPU and cadmium stearate on a ZnSe substrate, from which the following main peaks can be observed: at ca. 1710 cm-1, a peak due to the CdO stretching vibrations of the urethane group in the polymer

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Langmuir, Vol. 15, No. 13, 1999 4563 Table 1. Surface Potentials of Composite LB Films of DR19-IPPU and Cadmium Stearate (55/50 wt %) with Different Numbers of Layers

Figure 5. FTIR spectrum of a composite LB film (31 layers) of DR19-IPPU and cadmium stearate.

Figure 6. X-ray diffraction pattern of a composite LB film (31 layers) of DR19-IPPU and cadmium stearate.

chain; at 1543 cm-1, a peak due to the CdO stretching vibrations of the carboxylate group from cadmium stearate; and at 2916 and 2849 cm-1, strong absorption peaks due to the CH stretching vibrations of CH2 from aliphatic chains of cadmium stearate and spacer methylene groups in DR19-IPPU. The absorption at about 1710 cm-1 cannot be attributed to nonionized headgroups of stearic acid, because pure cadmium stearate LB films prepared under similar conditions did not exhibit any absorption peak at this wavenumber. The absorption pattern in the region of 1500-1000 cm-1 is similar to that observed for pure DR19-IPPU in the form of a KBr pellet. Hence, the spectra of the composite LB film can be considered as the superimposition of the FTIR spectra of pure DR19-IPPU and pure cadmium stearate. A set of diffraction peaks with a corresponding bilayer distance of 50 ( 0.5 Å are seen in the X-ray diffraction pattern in Figure 6 for a composite LB film of DR19IPPU and cadmium stearate on glass. The pattern is similar to that of pure cadmium stearate LB films,15 indicating that the cadmium stearate molecules are present in separate domains. However, there is both peak broadening and lowering of peak intensity, which means that DR19-IPPU molecules affect the stacking order of the cadmium stearate domains. The surface potentials, ∆VLB, of DR19-IPPU and cadmium stearate composite LB films with different

no. of layers

VLB (mV)

1 3 5 7 11 15

220 420 420 430 440 460

numbers of layers (1, 3, 5, 7, 11, and 15) on gold/glass substrates are shown in Table 1. When scanning the surface potential probe over the LB film surface, the maximum dispersion detected was about 10 mV in all cases, which is within the accuracy limit of the instrument, indicating the uniformity of the film at least at the macroscopic level. A quantitative analysis of the data is certainly complicated because one cannot assign contributions from the molecular dipole moments to the different components in the composite. Nevertheless, one may recall from Figure 3 that the maximum surface potential for the composite Langmuir monolayer (∆VL) was ca. 200 mV, which is close to ∆VLB for a single monolayer (220 mV). For thicker LB films, ∆VLB lies in the 420-460 mV range. If we consider that ∆VLB is generally lower than ∆VL, owing to the negative contribution from the substratefilm interface, which is approximately -150 to -200 mV,22 the high value of ∆VLB implies that the dipole moment contribution in the LB film is considerably larger than that in the monolayer. This could be caused by the alignment of molecules during the transfer process, especially of azobenzene chromophores in the composite LB film. Optical Storage Experiments. The optically induced stable birefringence in a material can be exploited for optical information storage. When a linearly polarized light (writing beam) impinges on a sample, the chromophore absorbs light and undergoes a trans-cis isomerization, unless it is oriented perpendicularly to the polarization direction. The cis form relaxes thermally and photochemically to the more stable trans form, which is also accompanied by molecular reorientations. The final result is an excess of molecules oriented perpendicular to the laser polarization direction, thus resulting in birefringence and dichroism.9 This induced birefringence can be read with a probe beam in the nonresonance absorption region. The results of a typical optical storage experiment performed with a composite LB film of DR19-IPPU and cadmium stearate (50/50 wt %, 61 layers) are presented in Figure 7. No transmission of the probe beam was observed for the film between two crossed polarizers and with the writing beam switched off, indicating the random orientation of chromophores in the film. However, when the writing beam was switched on at point A, transmission increased and reached 60% and 90% of the saturation value in approximately 0.8 and 1.8 s, respectively. The increase in transmission is related to the induced birefringence in the film. The overall change in the transmitted signal is normalized between 0 and 1 for a clear presentation. This light-induced birefringence results from the orientation of some chromophores that are perpendicular to the laser polarization. When the writing beam was switched off at point B, transmission decreased sharply, and after a time span of 6 s, it was reduced to about 70% of the saturation value (point C). If the sample was left (22) Dhanabalan, A.; Mello, S. V.; Oliveira, O. N., Jr. Macromolecules 1998, 31, 1827.

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Figure 7. Writing and erasing sequence on a composite LB film (61 layers) of DR19-IPPU and cadmium stearate. The transmitted signal is normalized between 0 and 1 for a clear presentation.

Dhanabalan et al.

Figure 9. Plot of amplitude of induced birefringence vs the number of layers of composite LB films of DR19-IPPU and cadmium stearate.

up to 3 mW. The amplitude of induced birefringence decreases with the number of layers in the LB film, as shown in Figure 9, which may be simply attributed to the decrease in the order in the LB film when a larger number of layers are transferred.23 The time required to achieve a 50% change in birefringence did not change with the number of layers. Either the light-induced trans-cistrans and accompanying molecular reorientation processes are intensive properties of the azobenzene chromophore of DR19-IPPU or the energy supplied is more than sufficient to achieve maximum birefringence, irrespective of the film thickness in the range studied here. Conclusions

Figure 8. Plot of amplitude of induced birefringence in the composite LB film of DR19-IPPU and cadmium stearate (50/ 50 wt %, 61 layers) vs the laser power.

to relax, transmission after point C would remain practically the same for several days; i.e., a significant number of molecules still remain orientated, and the induced birefringence is maintained even after the writing beam is switched off. The birefringence could nevertheless be optically erased by overwriting the test spot with a circularly polarized light, as it has been done at point C in Figure 7. The incidence of a circularly polarized light randomizes the chromophores orientation, thus eliminating the macroscopic orientation of the dipoles in the LB films. Several cycles of writing and erasing could be repeated without significant change in the induced birefringence. The laser power of the incident beam (writing beam) was varied, and Figure 8 shows that for a 61-layer LB film of DR19-IPPU and cadmium stearate (50/50 wt %) the amplitude of birefringence (defined as the saturation value reached by birefringence) increases with the laser power up to 3 mW or so, after which saturation takes place. That is to say, 3 mW is sufficient to induce the maximum birefringence in a 61 layer composite LB film. In addition, the time required for achieving 50% of the total birefringence decreased with the laser power, again

The monolayer characteristics of pure polyurethane derivatized disperse red-19 dye and a composite of DR19IPPU and cadmium stearate as inferred through surface pressure and surface potential isotherms are presented. Good quality LB films could not be obtained from pure DR19-IPPU monolayers, because of the poor stability associated with these monolayers. However, uniform LB films were fabricated with the aid of cadmium stearate. The UV-vis results indicated a possible J-type aggregation of the azo chromophores, whereas the XRD patterns revealed separate domains of cadmium stearate in the composite LB films. The azo chromophores appear to be reoriented during transfer, contributing positively to the surface potential of the LB films. The optical storage capability of the composite DR19-IPPU LB films was demonstrated by measuring their optically induced birefringence. This birefringence decreased with the number of layers in the LB film, which is probably associated with a less ordered LB film for larger number of layers transferred.23 The incidence of a circularly polarized light erased the stored information, and several cycles of writing and erasing could be repeated without significant change in the induced birefringence. Acknowledgment. The authors thank FAPESP, CAPES, and CNPq for financial support. LA9811012 (23) Mendonc¸ a, C. R.; Dhanabalan, A.; Balogh, D. T.; Misoguti, L.; dos Santos, D. S., Jr.; Pereira-da-Silva, M. A.; Giacometti, J. A.; Zilio, S. C.; Oliveira, O. N., Jr. Macromolecules 1999, 32, 1493.