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Yaobang Li, Yonghong Deng, Yaning He, Xinlin Tuo, Xiaogong Wang*
Department of Chemical Engineering, Tsinghua University, Beijing, P. R. China 100084
Polymers bearing aromatic azo chromophores have shown a variety of interesting photoresponsive properties based on the photoinduced cis-trans isomerization of the azobenzene derivatives.1-3 Azo polyelectrolytes, polymers containing both azobenzene type chromophores and ionizable groups, havecharacteristics to self-assemble to various micelle-like structures in suitable mediums and to form ultilayers through electrostatic layer-by-layer dipping processes.4,5 H-aggregation of azo chromophores, haracterized by packing the chromophores in some order forms, is another important property of azo olyelectrolytes.6
Recently, several types of azo polyelectrolytes with varied polymeric architectures and different types of azo chromophores were synthesized by us. The azo polyelectrolytes with branched azo side-chains or hyperbranched architcture were examples of the newly synthesized polymers. Depending on the types of azo chromophores, the polyelectrolytes showed various photoresponsive properties, such the photochromic effect, photo-induced birefringence and surface relief grating. Self-assembled multilayers of the azo polyelectrolytes were fabricated by the electrostatic layer-by-layer adsorption method. Photo-responsive behaviors introduced by the cis-trans isomerization of the azo chromophores were observed for the self-assembled multilayer films.
The H-aggregation and self-assembled nano-structures of the azo polyelectrolytes were studied in organic/H2O mixed solvents and characterized with UV-vis spectroscopy, TEM and LLS. By adjusting thecomposition of the solvents and the polymer concentration in the solutions, the H-aggregation degree and morphology of the self-assembled structures were modified significantly. The H-aggregation exhibited an obvious effect on the photoresponsive behavior of azo chromophores and could be dissociated by UV light irradiation and released ‘isolated’ azo chromophores. By adjusting the polymeric architecture and processing conditions, micelle-like nano-structures with different inner structure could be prepared.The formation of vesicle-like aggregates was also observed for the photoresponsive azo polyelectrolytes. The vesicular structures were obtained by adding Milli-Q water dropwise into THF olutions of the azo polyelectrolytes in a slow rate. The morphology of the vesicle-like aggregates depended on the percentage of the azo chromophore bearing units and the concentration of the THF solutions. The forming process includes a complex interplay of the transport of molecules and the selfassembly of the polymeric chains. UV irradiation could induce the trans-cis isomerization of the azobenzene groups in the aggregates even the isomerization rate is slower than that in the solution. Due to the random copolymeric nature and photoresponsive character of the vesicle-forming polymers, the forming mechanism and photo-triggered transformation of the structures are worth further studying.
AcknowledgementThe financial support from NSFC under Projects 59925309 is gratefully acknowledged.
References1. Kumar, G. S. Azo Functional Polymers: Functional Group Approach in Macromolecular Design,Technomic Publishing Company Inc.: Lancaster Basel, 1993.2. Xie, S.; Natansohn A.; Rochon P. Chem. Mater. 1993, 5, 403.3. Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139-4175.4. Wu, L. F.; Tuo, X. L.; Cheng, H.; Chen, Z.; Wang, X. G. Macromolecules 2001, 34, 8005.5. Cheng, H.; Tuo, X. L.; Wang, G. J.; Wang, X. G. Maromol. Chem. Phys. 2001, 202, 3530.6. Wang, H. P.; He, Y. N.; Tuo, X. L.; Wang, X. G. Macromolecules, 2004, 37, 135.
论文来源：International Symposium on Polymer Chemistry,June,2004 |
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