Ionomers azobenzene crystallines liquides photoactive
Date de publication2006
Azobenzene chemistry is an intensive area of exploration because it continues to surprise us with totally unforeseen phenomena some of which are still not totally explained. If the azobenzene chromophore is attached to a polymer chain or even if it is introduced in a polymer matrix without bonding, the photoisomerization of the chromophore can initiate a series of motions. The applications of the photo-induced motions of liquid crystalline polymers can be numerous. The use of the active photo-function where the reversibility of the photo-induced alignment is achieved can be used in erasable optical recordings, image processing etc. On the other hand when the passive function is put into use, long-lasting molecular orientations can be achieved with high optical anisotropy. This can be used in liquid crystalline aligning layers that are required for LC display devices. In the present work we had synthesized a new type of liquid crystalline polymers: namely, azobenzene liquid crystalline ionomers. In order to do so, first the azobenzene monomer was co-polymerized with methacrylic acid. Then, the acid group of the random copolymers was partially neutralized by reaction with metal acetate, which introduced metal ions in the liquid crystalline azobenzene polymer. If the azobenzene chromophore can initiate a series of motions it must be due to its photoisomerization property. Irradiating them with UV light at 360nm performed a test for photoisomerization of the new materials synthesized and their UV-vis spectrum showed that the trans-cis isomerization took place. This gave us the green light for possible motion of the azobenzene chromophore on the azobenzene ionomers and we irradiated our samples with UV light and then with linearly polarized visible light to produce a desired orientation. Knowing that the chemistry of the azobenzene varies with its environment, we investigated the effects of ionic aggregates, which are characteristic of ionomers, on the photo-induced orientation of azobenzene mesogens. We then studied the effect on the photo orientation by introducing the sample into the oven at a particular temperature. This gave different results for the different ionomers. Another experiment, which was of interest, was to study the different aggregations and their relative populations at different temperatures. Finally, the photochemical phase transitions from nematic to isotropic state at different reduced temperatures were examined for the different ionomers. The hypothesis assumed was that because of the different ionic aggregates that the different ionomers can adopt there will be differences in the rate of disappearing and in the rate of recovering of the nematic phase. A special set-up was constructed for this last experiment where a sample was irradiated with UV light and the luminosity of the phase, between crossed polarizers, was monitored by an oscilloscope. We found that the presence of metal ions can either increase or decrease the thermal stability of the Liquid Crystalline phase formed by the azobenzene mesogens, which in turns can enhance or reduce the degree of photo-induced orientation of the azobenzene mesogens.
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