BACKROUND: One of the most important properties of the azobenzene chromophores is the photochemical trans-cis isomerization induced by UV or visible light. In azopolymers, the photoisomerization induces conformational changes in the polymer chains, which in turn lead to macroscopic variations in the chemical and physical properties of the surroundings and media.RESULTS: This work reports the photochromic behaviour and surface structuring capacity of azopolymers having rigid polyimide and flexible polysiloxane structures, respectively. These polymers have good thermostabilities, with the degradation process starting above 315 • C. The glass transition temperature of the azopolyimide is 228 • C, while that of the azopolysiloxane modified with thymine is 34 • C.
CONCLUSION:The experiments show that the azobenzene groups can isomerize even in the case of the rigid polyimide, but the maximum degree of conversion to the cis isomer is less than in the case of the flexible polysiloxane. This behaviour is reversed in solution, probably due to both the thymine and azo group interactions and for conformational reasons. The azopolymers show a good surface structuring capacity. The polysiloxane is more sensitive in the case of low irradiation energy and irradiation time. By increasing the irradiation time for both polymers, the modulation depth increases and has comparable values.
We report the measurements of picometer vibration amplitudes produced by a piezo mirror in an interferometric setup, using a GaAs:Cr photoconductive device. The moving light fringes (gratings) induce in this new detector periodic photocurrents characterized by efficient suppression of the low-frequency drifts of the working point and low sensitivity to the amplitude laser noise. The optimum operation conditions of this system are also shown.
Azobenzene polymers have been the subject of intensive research due to their unique and unexpected properties that allow various applications triggered by light. One of their attractive features is the possibility of changing the orientation of azobenzene chain through trans-cis-trans photoisomerization cycles to photoinduce birefringerence and linear dichroism in thin polymer films. The photochromic behavior and surface structuring capacity of rigid or flexible polymers having side azobenzene units have been investigated. The polyimides with side azobenzene units have rigid structure, while polysiloxane chains modified with azobenzene groups and nucleobases are considered materials with flexible structure. During the photochromic studies of the investigated polymers, a mechanism concerning the possibility to generate a fluid phase under UV/VIS irradiation was proposed. The photoisomerisation kinetic by UV-Vis irradiation in the solid state compared with the solution and the cis-trans azobenzene relaxation phenomena were presented. The surface structuring capacity of these polymers was studied using a Nd:YAG laser at 355 nm, at different incident fluencies and pulse numbers.
A systematic electron microscopy analysis is performed of the defects on metallic surfaces, either pre-existing or introduced by processing or as a result of powerful laser irradiation. Metallic and nonmetallic defects were revealed with a vaporisation threshold lower than that of an ideal plane metallic surface. This lower threshold might lead to a lowering of the breakdown threshold of a gas in the neighbourhood of the metallic target.
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