Surface-Relief Gratings and Photoinduced Birefringence in Layer-by

Caixa Postal 369, 13560-970, Sa˜o Carlos, SP, Brazil. Received July 3, 2003; Revised Manuscript Received August 25, 2003. Layer-by-layer (LBL) films ...
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Biomacromolecules 2003, 4, 1583-1588

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Surface-Relief Gratings and Photoinduced Birefringence in Layer-by-Layer Films of Chitosan and an Azopolymer C. S. Camilo,† D. S. dos Santos, Jr.,‡ J. J. Rodrigues, Jr.,‡ M. L. Vega,‡ S. P. Campana Filho,† O. N. Oliveira, Jr.,‡ and C. R. Mendonc¸ a*,‡ Departamento de Fı´sico-Quı´mica, Instituto de Quı´mica de Sa˜o Carlos, Caixa Postal 780, 13560-970, Sa˜o Carlos, SP, Brazil, and Departamento de Fı´sica e Cieˆ ncia dos Materiais, Instituto de Fı´sica de Sa˜o Carlos, Caixa Postal 369, 13560-970, Sa˜o Carlos, SP, Brazil. Received July 3, 2003; Revised Manuscript Received August 25, 2003

Layer-by-layer (LBL) films of chitosan alternated with an azopolymer, PS119, have been used for optical storage and fabrication of surface-relief gratings. The optical properties stem from the trans-cis-trans isomerization cycles undergone by the azochromophore, with a kinetics for writing the birefringence pattern that is much slower than in the spin-coated or cast films of azopolymers. The long writing times, of the order of 100 s, are due to the electrostatic interactions between adjacent chitosan and PS119 layers. Such interactions are also responsible for other features in the LBL films, namely the increase in the amount of adsorbed material when the pH of the preparation solution is decreased and the large residual birefringence after the writing laser is switched off. Gratings could be inscribed with s-polarized but not with p-polarized light, indicating a mass transport process associated with photodegradation. Introduction Azoaromatic polymers, in which azochromophores are covalently attached to the polymer backbone, have been widely investigated over the past few years because of their potential application in a series of optical devices. Optical processes in azopolymers include second harmonic generation, inscription of surface relief gratings (SRGs), and optically induced birefringence.1 The latter two processes are associated with the reversible trans-cis-trans photoisomerization and resulting molecular orientation/movement.2 For instance, when excited by linearly polarized laser light, azobenzene chromophores suffer trans-cis-trans isomerization cycles accompanied by molecular reorientation, which causes birefringence and dichroism in the film. This may be exploited in a writing mechanism, because even after the molecular relaxation following the switching off of the laser a considerable number of molecules remain oriented. The stable birefringence pattern corresponds to information stored, which can be erased thermally or by impinging a circularly polarized light onto the sample. Surface-relief gratings (SRG) are formed as a result from large-scale mass transport following photoisomerization cycles in azopolymer films.2 This process may be entirely light driven when moderate intensities of laser light are employed, and can be attributed to molecular movement away from the illuminated regions due to forces originating from the interaction between the molecule dipole moment and the optically induced electric field gradient.3 Most studies involving optical storage and SRGs are carried out with polymer films obtained with spin coating * To whom correspondence should be addressed. E-mail: [email protected]. † Instituto de Quı´mica de Sa ˜ o Carlos. ‡ Instituto de Fı´sica de Sa ˜ o Carlos.

or casting techniques.4 There have been, however, reports on the use of nanostructured films such as LangmuirBlodgett (LB)5 and layer-by-layer (LBL) films6 for the purposes mentioned. Using such techniques is advantageous because of the precise control of film thickness and molecular architecture. With the LBL films, in particular, it is possible to produce SRGs from a larger variety of materials as inert polymers may be alternated with azobenzene-containing compounds. The electrostatic interaction between adjacent layers makes it possible for the chains of the inert polymers to be dragged along with the azobenzenes.7 In this paper, we exploit the LBL technique to produce films containing chitosan, which display optical storage capability and are amenable to SRG inscription. Chitosan is the deacetylated form of chitin, which is a structural polysaccharide encountered in the shells of crabs and in the exoskeleton of shrimps and other arthropods. The applications of chitin are limited because of its poor solubility, but deacetylation in basic media leads to chitosan that is soluble in acidic media. Chitosan contains 2-acetamido-2deoxy-Dglucopyranose and 2-amino-2-deoxy-D-glucopyranose units joined by (1f4) glycosidic bonds. With a pKa of 6.2, chitosan behaves as a polycation in acid media and can be alternated with polyanions in the LBL technique, where adsorption is predominantly governed by ionic attraction between oppositely charged layers.8 Indeed, LBL films containing chitosan have been reported in the literature,9-16 being also applied in sensor units for organophosphorus compounds16 and in taste sensors. To the best of our knowledge, though, this is the first report of SRGs produced on chitosan films.

10.1021/bm034220r CCC: $25.00 © 2003 American Chemical Society Published on Web 10/07/2003

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Figure 2. Peak absorbance versus of number of bilayers for chitosan/ PS-119 films where the symbols b, O, 0, and 9 represent films prepared at pH 1, 2, 3, and 4, respectively. The inset shows a series of absorption spectra obtained for the sample with pH ) 2 for each deposition step.

Figure 1. Chemical structures of PS-119 and chitosan.

Experimental Details The LBL films were produced from chitosan and the azopolymer PS-119, whose chemical structures are shown in Figure 1. Chitin was extracted from gray shrimps with chitosan being synthesized according to the method described in ref 17 and purified by dissolution in acetic acid, precipitated in NH4OH and filtered using 80 µm and 45 µm membranes. Its molecular weight is Mw ) 8 × 104 g/mol determined by viscometric analysis,18 and the degree of acetylation (DA) was 14, determined with proton nuclear magnetic resonance using deuterated water.19 PS-119 was purchased from Aldrich. Chitosan and PS-119 aqueous solutions were prepared in concentrations of 1.2 g/L, using Milli-Q water supplied by a Millipore system. The pH of the solutions was adjusted within the range from 1 to 4 using hydrochloric acid. Washing solutions were the same as the aqueous solutions, but without polyelectrolyte. The LBL films were deposited on glass slides and cleaned with the RCA method,20 up to an appropriate number of layers by immersing the substrates alternately into chitosan and PS119 solutions with the same pH for 10 s. Although it may seem surprising that such a short period of time is sufficient for polymer adsorption, it has been shown that the first stage of adsorption, which may represent a considerable amount of material adsorbed, occurs within 5-10 s.21 After deposition of each layer, the substrate/film system was rinsed in the washing solution and dried with a flow of N2. The buildup of the LBL film was monitored at each deposition step by UV-vis spectroscopy using a Hitachi-U2001 spectrophotometer. Film thickness was measured with a profilometer Taylor-Hobson. The optical storage experiments were performed by inducing birefringence in the film using a diode-pumped frequency doubled, linearly polarized Nd:YAG continuous laser at 532 nm (writing beam) with a polarization angle of 45° with respect to the polarization orientation of the probe beam (reading beam). The power of the writing beam was

varied up to 60 mW for a 2 mm spot. A low-power He-Ne laser light at 632.8 nm passing through crossed polarizers was used as the reading beam to measure the induced birefringence in the sample. These measurements were performed in 30-bilayer LBL films of chitosan/PS-119. For the inscription of SRGs, 20-bilayer LBL films were employed in an experimental arrangement similar to the one described in ref 7. Basically, two laser beams from an Ar+ laser at 488 nm are made to interfere on the film surface, thus leading to a periodically modulated intensity pattern. SRG inscription was carried out using either p- or s-polarized laser beams with an intensity of 100 mW/cm2. A low power He-Ne laser operating at 632.8 nm was used as a probe beam to measure the diffraction efficiency of the first-order diffracted beam from the grating, allowing one to monitor the grating formation process. Results and Discussion Figure 2 shows that the amount of material adsorbed, taken as proportional to the optical absorbance at the peak from n-π* transitions of PS-119 (at 480 nm), increases practically linearly with the number of bilayers deposited. The only exception for the linear behavior occurs at pH 3, for which the films behave differently from the other ones, as will be commented upon later. The inset brings a series of spectra for films fabricated with solutions of pH 2. Absorption is basically due to PS-119 as chitosan does not absorb in this spectral region. The deposition rate depends on the solution pH, being higher for lower pHs, with the exception of pH 3. A similar behavior occurred for LBL films of chitosan and azodyes.14,15 The film thickness per bilayer was obtained from several measurements in samples with different numbers of layers, using a profilometer. Table 1 indicates that the average thickness per bilayer increases with decreasing pH, which points to a more efficient adsorption. Now, the data for pH 3 obey the trend, unlike the case of the absorption results from Figure 2. It is possible that the amount of PS-119 adsorbed is lower than expected at pH 3, which would lead to a lower absorbance, but the amount of chitosan (not detected with absorbance at 480 nm) is still higher than for

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Table 1. Thickness Per Bilayer for LBL Films Produced at Different pHs solution pH

thickness per bilayer (nm)

1 2 3 4

7.5 ( 0.7 5.4 ( 0.7 4.9 (0.5 3.7 ( 0.5

Figure 4. Dependence on the laser power for the amplitude of the induced birefringence for chitosan/PS-119 LBL films prepared at pH ) 3 (solid squares) and pH ) 4 (solid circles). The solid lines were drawn to guide the eye.

Figure 3. Writing and relaxation sequence in an LBL film of chitosan and PS-119. The transmitted signal is normalized between 0 and 1 for the sake of clarity of presentation.

pH 4. These differences are probably associated with a distinct coiling of chitosan at pH 3. Further experiments are required to elucidate this point, which we shall attempt with a quartz crystal microbalance to detect the mass increase for both components. Optical storage in azobenzene-containing materials results from trans-cis-trans conversions, which are accompanied by molecular reorientation. These conversions take place when a linearly polarized laser light under resonant condition is made to impinge on the azobenzene-containing film, promoting the chromophores from the lower energy trans conformation to the cis conformation. The cis-trans conversion occurs either thermally or photoinduced, thus completing the cycle. Because after each such conversion the chromophore may adopt any orientation, those oriented perpendicularly to the laser polarization direction will no longer suffer isomerization. The final result is a net population of chromophores in this particular direction, which gives rise to a birefringence in the film structure (WRITE). When the light source is switched off, molecular relaxation takes place, but a considerable number of molecules remain oriented, with the stable birefringence pattern corresponding to a STORE step. The creation of birefringence is inferred by the change in transmittance of the weak probe beam that passes through crossed polarizers (READ). The result of a typical optical storage experiment on a 30-bilayer chitosan/PS-119 LBL film, prepared at pH ) 1, is presented in Figure 3. The transmitted signal was normalized between 0 and 1. Before the writing beam is switched on, no transmission of the probe beam through the film and crossed polarizers was observed, indicating a random orientation of the chromophores. At point A, the writing beam was switched on and the transmission increased because of birefringence induced by chromophore orientation. When the writing beam was switched off at point B, the transmission decreased abruptly

to about 90% of the maximum value (point C). The same general features were observed for the samples prepared at different solution pHs, even though a significant dispersion in the induced birefringence was observed due to the nonhomogeneity of some films. The birefringence value was calculated as an average of several optical storage experiments, being 0.08 for the LBL chitosan/PS-119, regardless of the pH of the solutions. The typical writing times, obtained by fitting an exponential function to the optical storage measurements, for the films studied here vary from 15 to 70 s. This is slower than in LB or cast films but faster than that generally observed in other LBL films, probably because of the smaller electrostatic interaction between chitosan and the chromophores in the PS-119, when compared to LBL films of azodyes. The PS119 writing time is slower than that observed for LB films because the electrostatic interaction in LBL films makes the azochromophores to be more densely packed, i.e., with lower mobility. The optically induced birefringence can be determined by measuring the probe beam transmission after the second polarizer, T, according to: ∆n ) λ/πd sin-1 xT, where λ is the wavelength of the incident radiation and d is the film thickness. Figure 4 shows that the induced birefringence increases with the laser power of the writing beam up to approximately 20 mW, after which it tends to saturate. The data correspond to LBL films produced with pH 3 and 4. Essentially the same features were observed for the other pHs, which were omitted for the sake of clarify of presentation. The saturation power is smaller than that for other azodye LBL films5 but is higher than the one obtained for LB films.6 This indicates that the electrostatic interactions between chitosan and the azo chromophore are smaller than in ref 15, leading to greater chromophore mobility. Of course the saturation values are higher in the LBL than in the LB film,5 because in the latter the chromophores have higher mobility to relax, as the electrostatic interactions prevailing in the LBL film are absent. The writing time to induce the birefringence was determined by fitting the data with an exponential function, as shown in the inset in Figure 5. This time decreased drastically with increasing beam power, again

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Figure 5. Writing time to induce the birefringence versus solution pH to prepare the 30-bilayer LBL film of chitosan and PS-119. The inset shows the fitting with an exponential function for the optical storage writing sequence for pH ) 2, from which the characteristic time was determined.

up to 20 mW (results not shown here). Optically induced birefringence experiments were performed also in films with different numbers of bilayers, and there was no difference in the general behavior in terms of the maximum and residual birefringences and writing times. To study the influence from the solution pH to prepare the LBL films on the induced birefringence characteristic time, we fixed the writing beam power at 25 mW. The time evolution of the optically induced birefringence, obtained for each pH, was fitted with a single exponential function (see inset in Figure 5). Figure 5 shows that the writing times (obtained as an average of several samples) are practically independent of the solution pH, in contrast to the results obtained for chitosan/Ponceau-S LBL films.15 This is probably related to the chromophore microenvironment. For chitosan/Ponceau-S films, the free volume for the azodye (small molecule) may be strongly affected by changes in the coiling of the chitosan polymer due to changes in pH. The mobility of the chromophore could be severely hindered. On the other hand, in the chitosan/PS-119 system, both components are polymeric, and therefore, the chromophore attached to PS-119 will be less affected by changes in coiling of the two polymers. That chitosan may interact more strongly with azodyes than with an azopolymer was observed in subsidiary experiments, in which chitosan cast films became colored after being immersed into an azodye solution, but showed no color change when immersed into a PS-119 solution. The efficient photoisomerization of the azochromophores in the chitosan/PS-119 films was exploited to produce surface-relief gratings (SRGs). Gratings on azobenzenecontaining polymers were first reported by Kumar and coworkers22,23 and Nathanson and co-workers,24 which sparked a number of works (see for instance papers by Hvilsted and co-workers25-27 and a review in Oliveira et al.3). At low laser intensities, thermal effects may be neglected and the mass transport is entirely photonic. Several models have been proposed to explain the formation of SRGs in an all-light induced process,28-34 with the gradient force model35 being prominent as it can explain the polarization dependence of

Camilo et al.

Figure 6. Diffracted signal recorded during the grating inscription with s- and p-polarized light. The inset shows the decrease in absorption upon exposure to the laser light.

the inscription. According to this model, the lateral polymer chains movement is due to a force combining the change in susceptibility and the field gradient. The time average of the gradient force is given by B(r) f ) 〈[P B(r,t).∇]E B(r,t)〉

(1)

B(r,t) B P(r,t) ) 0χE

(2)

with

where B P(r,t) is the polarization, B E(r,t) is the electric (optical) field, 0 is the vacuum permittivity, and χ is the optically induced change in the susceptibility. From eqs 1 and 2, one can see that the polymer chains are subjected to a force only in the direction where there is a component of the field gradient. Thus, this force is null when the polarization is perpendicular to the gradient. The diffraction of a probe beam during the inscription of an SRG with p- and s-polarized light is shown in Figure 6. A higher intensity of the diffracted beam was observed for s polarization, whereas just a small signal was measured for p-polarized light. As mentioned above, significant surface modulation in an all-photonic process is only achieved when there is a component of the electric field gradient in the direction of the grating vector,35 which means that s-polarized light should lead to no SRG. However, our results cannot be explained by this field-gradient model,3 which means that the mass transport is not an all-photonic process. Other mechanisms predominate, the most likely being degradation of the chromophores which could cause changes in the volume occupied by the molecules, leading to mass transport. To verify the hypothesis of photodegradation, we monitored the absorbance of a chitosan/PS-119 LBL film while submitted to the laser light with the same intensity employed to inscribe the grating. The inset in Figure 6 shows a decrease in the absorbance spectrum, obtained at every 15 min, clearly indicating photodegradation. In subsidiary experiments, we observed that the same level of degradation occurred for sor p-polarized light. With degradation for both types of polarization, one may then wonder why SRGs were only formed with s polarization.36

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geneity in the AFM images. The same applies to the optical induced birefringence, as large roughness affects badly the signal detection due to scattering of the probe beam light. In the case of SRG inscription, it is known that the amplitude of deformation increases strongly with the film thickness, especially for very thin films.37 Therefore, it is easier to obtain SRGs with thicker films, and that is why we presented only the result for 20-bilayer films. Finally, the inscription of an SRG due to degradation begs the question of whether similar gratings could not be formed in polymers containing other dyes. It should be noted, however, that the chain mobility necessary for mass transport requires efficient trans-cis-trans photoisomerization cycles, which up to now has only been achieved in azobenzenes3. For example, attempts to produce SRGs on polymers functionalized with dyes that do not isomerize have failed.22 With the photoisomerization there is also the possibility of in-plane orientation of the chromophores, which would explain the polarization dependence for the SRG inscription. Conclusion

Figure 7. AFM image of an SRG inscribed on a 20-bilayer PS119/ chitosan LBL film with s-polarized light. (a) Original image and (b) Fourier treated image.

SRGs inscribed on 20-bilayer chitosan/PS-119 films had amplitudes of 13 nm, for an interference pattern of spolarized light of 100 mW/cm2. Figure 7 shows the AFM picture of the SRG with a grating spacing of approximately 2.0 µm. The image has been processed with a filter with FFT (fast Fourier transform) to eliminate effects from surface roughness. No surface modulation could be seen in the AFM image when p-polarized light was employed for the grating inscription, which is consistent with the small amplitude of the diffracted signal in Figure 6. Such low amplitudes for the SRG, in contrast to the hundreds of nm obtained in spincoated or cast films from azopolymers, are due to two factors: (i) The SRG amplitude increases considerably with film thickness,3 particularly for thin films, and therefore SRGs on ultrathin LBL films are expected to have smaller amplitudes. (ii) Mass transport in LBL films is less efficient than in spin-coated films from azopolymers as the inert polymer in the LBL film (chitosan, in our case) must be dragged by the movement of the chromophores from the other material. Indeed, such difficulties in producing SRGs on LBL films have already been discussed.3 It should be noted that in our experiments good SRGs could only be inscribed in visually uniform films with reasonable homo-

The possibility of using LBL films of biopolymer chitosan with PS119 for optical storage has been demonstrated. The amplitude and time to achieve the optically induced birefringence were determined and compared with results from the literature for polymeric systems with different deposition techniques. The formation of SRGs was demonstrated for the first time for this type of polymer. Interestingly, SRGs could be formed with s- but not with p-polarized light, in contrast to the predictions of the field-gradient model that explains mass transport in an all-photonic process. This implied that the surface modulation had other origins, the most likely being photodegradation of chromophores. The small amplitudes and effects from surface roughness indicate that further work is required to improve homogeneity of the films, which will have a positive impact on both optical storage and SRG features. Acknowledgment. This work was supported by FAPESP, CNPq, CAPES, and IMMP/MCT (Brazil). Thanks to Ma´rcia Barreto for HNMR and viscometric analysis facilities. References and Notes (1) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817-1845. (2) Oliveira, O. N., Jr.; He, J.; Zucolotto, V.; Balasubramanian, S.; Li, L.; Nalva, H.; Kumar, J.; Tripathy, S. K. In Handbook of Polyelectrolytes: New York, 2002. (3) Oliveira, O. N., Jr.; Li, L.; Kumar, J.; Tripathy, S. K. Surface relief gratings on azobenzene-containing films. In PhotoreactiVe Organic Thin Films; Sekkat, Z., Knoll, W., Eds.; Academic Press: San Diego, CA, 2002; Chapter 14, pp 429-486. (4) Fukuda, T.; Matsuda, H.; Shiraga, T.; Kimura, T.; Kato, M.; Viswanathan, N. K.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 4220-4225. (5) Mendonca, C. R.; Dhanabalan, A.; Balogh, D. T.; Misoguti, L.; dos Santos, D. S.; Pereira-da-Silva, M. A.; Giacometti, J. A.; Zilio, S. C.; Oliveira, O. N., Jr. Macromolecules 1999, 32, 1493-1499. (6) Zucolotto, V.; Mendonca, C. R.; dos Santos, D. S.; Balogh, D. T.; Zilio, S. C.; Oliveira, O. N., Jr.; Constantino, C. J. L.; Aroca, R. F. Polymer 2002, 43, 4645-4650. (7) Lee, S. H.; Balasubramanian, S.; Kim, D. Y.; Viswanathan, N. K.; Bian, S.; Kumar, J.; Tripathy, S. K. Macromolecules 2000, 33, 65346540.

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(8) Oliveira, O. N., Jr.; Raposo, M.; Dhanabalan, A. In Handbook of surface and Interface of materials: New York, 2001. (9) Lvov, Y.; Onda, M.; Ariga, K.; Kunitake, T. J. Biomater. Sci.-Polym. Ed. 1998, 9, 345-355. (10) Serizawa, T.; Goto, H.; Kishida, A.; Endo, T.; Akashi, M. J. Polym. Sci. Polym. Chem. 1999, 37, 801-804. (11) Tachaboonyakiat, W.; Serizawa, T.; Endo, T.; Akashi, M. Polym. J. 2000, 32, 481-485. (12) Serizawa, T.; Yamaguchi, M.; Matsuyama, T.; Akashi, M. Biomacromolecules 2000, 1, 306-309. (13) Serizawa, T.; Yamaguchi, M.; Akashi, M. Biomacromolecules 2002, 3, 724-731. (14) dos Santos, D. S.; Bassi, A.; Misoguti, L.; Ginani, M. F.; Oliveira, O. N., Jr.; Mendonca, C. R. Macromol. Rapid Commun. 2002, 23, 975-977. (15) dos Santos, D. S.; Bassi, A.; Misoguti, L.; Oliveira, O. N., Jr.; Mendonca, C. R. Biomacromolecules 2003, (in press). (16) Constantine, C. A.; Mello, S. V.; Dupont, A.; Cao, X. H.; Santos, D.; Oliveira, O. N., Jr.; Strixino, F. T.; Pereira, E. C.; Cheng, T. C.; Defrank, J. J.; Leblanc, R. M. J. Am. Chem. Soc. 2003, 125, 18051809. (17) Bough, W. A.; Salter, W. L.; Wu, A. C. M.; Perkins, B. E. Biotechnol. Bioeng. 1978, 20, 1931-1943. (18) Signini, R.; Campana Filho, S. P. Polı´meros: Cieˆ n. Tecnol. 1998, 8, 62-68. (19) Signini, R.; Campana, S. P. Polym. Bull. 1999, 42, 159-166. (20) Kern, W. Semiconductor Int. 1984, 94-99. (21) Raposo, M.; Pontes, R. S.; Mattoso, L. H. C.; Oliveira, O. N., Jr. Macromolecules 1997, 30, 6095-6101. (22) Kim, D. Y.; Li, L.; Jiang, X. L.; Shivshankar, V.; Kumar, J.; Tripathy, S. K. Macromolecules 1995, 28, 8835-8839. (23) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166-1168.

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