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J. Phys. Chem. B 2002, 106, 5378-5381
Influence of the Backbone on Photoinduced Birefringence in a Poly(malonic ester) Containing p-Cyanoazobenzene Won-Jae Joo and Cha-Hwan Oh* Department of Physics, Hanyang UniVersity, Seoul 133-791, Korea
Yang-Kyoo Han Department of Chemistry, Hanyang UniVersity, Seoul 133-791, Korea, and Center for AdVanced and Functional Polymers, KAIST, Taejeon, Korea ReceiVed: December 10, 2001; In Final Form: February 27, 2002
The influence of the backbone on the photoinduced birefringence was investigated in a poly(malonic ester) containing a p-cyanoazobenzene group as a side-chain. The photoinduced birefringence was successively measured in the fifteen cycles of the recording, relaxation, and erasing processes. The linearly polarized light, with a polarization direction of 45° relative to the analyzer, was used as the pumping beam during the first five cycles, and the same pumping beam at negative 45° was used during the next 10 cycles. When the polarization direction of the pumping beam was changed perpendicularly, a very interesting birefringence curve was observed in the recording and relaxation processes, which could be well explained by our model considering the reorientation of the backbone caused by the elastic force between the backbone and sidechain. Consequently, we confirmed the fact that the relaxation behavior of the photoinduced birefringence is closely associated with the reorientation of the backbone.
Introduction Recently, polymeric materials containing an azobenzene group have been recognized to be promising materials for holographic applications due to their large optical nonlinearity and reversibility.1-4 In general, in this type of the polymer, the optical data can be recorded by photoinduced birefringence or dichroism via the alignment of the mesogenic azobenzene groups perpendicular to the polarization direction of irradiating light.4-6 As is well-known, the alignment of mesogenic azobenzene groups occurs as a result of the trans-cis photochemical isomerization process.4-7 In the previous paper, we observed an abnormal relaxation, positive relaxation of the induced birefringence in the poly(malonic esters) containing p-cyanoazobenzene as a side-chain and we presented a novel model of the fast relaxation of the photoinduced birefringence to explain this abnormal phenomenon.8 In the model, the side-chain is first aligned by the illumination of the linearly polarized light and then the backbone is reoriented by means of the elastic force between the aligned side-chain and the backbone. Consequently, the fast relaxation is a result of the elastic force, and the remnant birefringence after the fast relaxation is closely associated with the reorientation of the backbone occurring in the recording process. Therefore, if the reorientation of the backbone can be controlled by adjusting the molecular weight, the spacer length, or other variables, it can be practically applied to improve the optical properties of polymer film, such as long-term stability and fast response. In this paper, we have investigated the reorientation of the backbone in the recording, relaxation, and erasing processes of the photoinduced birefringence in the poly(malonic esters) * Author to whom correspondence should be addressed.
Figure 1. Synthesis route for poly(malonic esters), PCN.
containing p-cyanoazobenzene. Especially, as the polarization direction of the pumping beam was changed perpendicularly, we observed very interesting recording and relaxation curves of the photoinduced birefringence, which proved our model experimentally for the reorientation of the backbone. Moreover, we could find out the concept of the influence of the reorientation of the backbone on the relaxation behavior of the induced birefringence. Experimental Section As the optical recording medium, novel liquid crystalline(LC) side-chain poly(malonic ester) was prepared according to the reported procedure shown in Figure 1.9 A new thermotropic LC malonic ester compound (MCN) was first synthesized by
10.1021/jp0144808 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/03/2002
Photoinduced Birefringence in a Poly(malonic ester)
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5379
reacting in chloroform for 48 h at 5 °C malonyl dichloride and mesogenic alcohol (mp 142 °C) with p-cyanoazobenzene. The yield and melting temperature of the MCN were 46% and 95 °C, respectively. It was observed by means of differential scanning calorimetry to show a smectic structure on both heating and cooling cycles. The phase transition temperatures to LC state appeared at 63 and 90 °C, respectively, on the heating and cooling cycles. The isotropization temperatures were 95 and 55 °C, respectively. For the phase transition temperature of the MCN monomer, the degree of supercooling between the heating and cooling cycles has a large value from 27 to 40 °C. Such a phenomenon might be due to the rapid cooling rate (average rate: 30 °C/min) of the equipment and also to the complicated structural change with temperature of the monomers. It, however, is not clear yet and needs to be examined in more detail. The LC structure of the MCN was found by means of optical polarizing microscopy to show a focal-conic texture. The MCN was then condensed with 1,6-dibromohexane in tetrahydrofuran in the presence of sodium hydride at 65 °C for 48 h to give poly(malonic ester) (PCN) with two symmetrical azo dye mesogens. The polymeric thin film (ca. 0.3 µm) was cast from the polymer solution (5 wt %) in CHCl3 onto a glass plate for 30 s using a spin coater. We employed a standard crossed polarizers setup for measurement of the photoinduced birefringence in the polymeric film.8 The polarization axes of the polarizer and the analyzer were 0° and 90° with respect to the incident plane, respectively. The linearly polarized beams from an Ar+ ion laser of 496 nm and from a He-Ne laser of 633 nm were used as the pumping and probe beams, respectively. The wavelength of the probe beam, 633 nm, is far from the absorption band of the PCN film. The polarization direction of the pumping beam was adjusted to 45° and -45° relative to the analyzer by using a λ/2 waveplate. The transmittance of the probe beam through crossed polarizers and the polymer film placed between them was measured with a photodiode. Results and Discussion In Figure 2a, repetitive recording, relaxation, and erasing curves of photoinduced birefringence in the PCN film are shown for five cycles. The polarization direction of the pumping beam was set to 45° in the recording process, the circularly polarized pumping beam was used as the erasing beam and the time interval between each repeated cycles was fixed at 3 min. As shown in Figure 2a, as the number of the repeated cycles increased, the induced birefringence in the recording process increased gradually and the initial birefringence before switching on the pumping beam also increased slightly. Since these cycles were successively carried out, the initial birefringence at each cycle was the remnant birefringence of the previous erasing process, and therefore it can be eliminated by increasing the erasing time or the erasing beam intensity. However, we can obtain information about the reorientation of the backbone from the remnant birefringence after a finite erasing process, which will be dealt with in detail later in Figure 4. In Figure 2b, the recording, relaxation, and erasing processes of the photoinduced birefringence were continued with the linearly polarized pumping beam at -45°, following the experiments of Figure 2a. As can be seen in Figure 2b, the initial birefringence remained, more or less, before the first recording process. As soon as the pumping beam was turned on at -45°, the birefringence decreased initially to zero and then increased gradually to 0.0025, which is smaller than the induced birefringence, 0.0045,
Figure 2. Photoinduced birefringence curves of PCN film in the recording, relaxation, and erasing processes, which were successively measeured fifteen times in total; (a) for the first five cycles, the polarization direction of the pumping beam was 45° relative to the analyzer, and (b) for the next 10 cycles, the polarization direction was -45°. The inset shows the first photoinduced birefringence curve after changing the polarization direction of the pumping beam.
in the first recording process of Figure 2a. However, similarly to the results of Figure 2a, as the number of the repeated cycles increased, the induced birefringence gradually increased. In the first relaxation curve of Figure 2b, it is a remarkable result that the remnant birefringence nearly became zero after the fast relaxation of the photoinduced birefringence. As the number of the repeated cycles increased, the remnant also increased gradually to the value that is shown in Figure 2a. Figure 3 shows the maximum photoinduced birefringence with the number of repeated cycles, where the pumping beam with a polarization direction of 45° was used during first five cycles, and the pumping beam at -45° was used during the following 10 cycles. The photoinduced birefringence increased gradually for the first five cycles and then decreased greatly as soon as the polarization direction was changed perpendicularly. This can be explained well by the reorientation of the backbone according to our model.8
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Figure 3. Maximum photoinduced birefringence with the number of repeated cycles.
Figure 4. Initial birefringence with the number of repeated cycles.
Figure 4 shows the variation of the initial birefringence before the recording process with the number of repeated cycles. As mentioned before, the initial birefringence is due to the remnant birefringence of the previous erasing process. In the erasing process, the reoriented side-chain can be randomized much more easily than the reoriented backbone, because of the photoresponsive property of the side-chain containing an azobenzene group. Namely, we could obtain information about the reorientation of the backbone from the remnant birefringence of an erasing process. Until the polarization direction of the pumping beam was changed, the initial birefringence increased to 4.8 × 10-4 with the number of repeated cycles. At the next cycle (7th cycle), it decreased dramatically to almost zero, but the backbone was still reoriented slightly. At the 8th cycle, the initial birefringence became zero, and the initial value then increased gradually with the number of repeated cycles. At the 6th cycle, the initial birefringence was smaller slightly than that of the 5th cycle due to taking several extra minutes to change the polarization direction of the pumping beam. Figure 5 shows the ratio of remnant birefringence after the fast and slow relaxation to the maximum induced birefringence of the recording process. The remnant birefringence was determined by fitting the relaxation curve to the biexponential function, f(t) ) Af exp(-t/τf) + As exp(- t/τs) + c, where the first and second terms are assignable to the fast and slow relaxation processes, respectively, and constant “c” denotes the
Joo et al.
Figure 5. Ratio of the remnant birefringence of the fast relaxation to the maximum photoinduced birefringence with the number of repeated cycles.
remnant birefringence after the whole relaxation. During the first five cycles, the ratio has a constant value of 0.43, but as soon as the polarization direction of the pumping beam was changed perpendicularly, the ratio dramatically decreased to 0.1. Then, as the cycle was repeated, the ratio increased gradually to 0.35. By fitting exponentially the growth, the ratio is expected to approach 0.38. From these results, we can conclude that the remnant birefringence is mainly determined by the reorientation of the backbone in the relaxation process, which is expected in our model. In this kind of polymer, the long-term stability is one of most important properties required for holographic applications such as optical data storage. Moreover, the fast response in the recording and erasing processes is also necessary for optical switching. If the reorientation of the backbone can be controlled in any way, it will be useful for improving the optical properties mentioned above in a polymer containing an azobenzene group. Conclusion The reorientation of the backbone was experimentally investigated in a poly(malonic ester) containing a p-cyanoazobenzene group as a side-chain, when the birefringence was induced by a linearly polarized pumping beam. After five cycles of recording, relaxation, and erasing processes with the linearly polarized pumping beam at 45°, the cycles are repeated 10 times with the pumping beam at -45°. In these repeated cycles, we could observe very interesting recording and relaxation curves of the photoinduced birefringence, which proved our previous model experimentally for the reorientation of the backbone. Consequently, we confirmed the fact that the relaxation behavior of the photoinduced birefringence is closely associated to the reorientation of the backbone in a polymer containing an azobenzene group. This might be practically applied for the improvement of optical properties, such as the long-term stability of the recorded optical data and the fast response time in a polymer containing an azobenzene group. Acknowledgment. This work was supported by Grant R012000-00338 from the Basic Research Program of the Korea Science & Engineering Foundation as well as the Center for Advanced Functional Polymers, KAIST in part. References and Notes (1) Rasmussen, P. H.; Ramanujam, P. S.; Hvilsted, S.; Berg, R. H. J. Am. Chem. Soc. 1999, 121, 4738.
Photoinduced Birefringence in a Poly(malonic ester) (2) Todorov, T.; Nikolova, L.; Stoyanova, K.; Tomova, N. Appl. Opt. 1985, 24, 785. (3) Anderle, K.; Birenheide, R.; Eich, M.; Wendorff, J. H. Makromol. Chem. Rapid Commun. 1989, 10, 477. (4) Ahuja, R. C.; Maack, J.; Tachibana, H. J. Phys. Chem. 1995, 99, 9221. (5) Eich, M.; Wendorff, J. J. Opt. Soc. Am. B 1990, 7, 1428.
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5381 (6) Joo, W.; Oh, C.; Song, S.; Kim, P.; Han, Y. J. Phys. Chem. B 2001, 105, 8322. (7) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2271. (8) Joo, W.; Oh, C.; Song, S.; Kim, P.; Han, Y. J. Chem. Phys. 2000, 113, 8848. (9) Han, Y.; Kim, K. M. Bull. Korean Chem. Soc. 1999, 20, 1421.
