Photoresponsive Porperties of Hybrid Colloids

Photoresponsive behavior of the hybrid colloids depends on the composition and distribution of the components in the colloids (Deng et ah, 2007). The hybrid colloid composed of BP-AZ-CA and PEAPE, whose preparation has been discussed in Section 5.3.4, is further discussed herein concerning its photoresponsive properties. As PEAPE and BP-AZ-CA contain HEAZ moieties and ACAZ moieties, the colloids can show photoinduced variation related to the azobenzene-type and pseudostilbene-type chromophores. 

The hybrid colloids can show photoinduced deformation, and deformation ability is dependent on the percentage of the BP-AZ-CA component in the colloids. The experimental conditions for the sample preparation and light irradiation are the same as those mentioned earlier. The colloidal spheres were exposed to the spatially filtered and collimated laser beam incident perpendicularly on the substrate surfaces. The morphologies of colloidal spheres were observed by TEM before and after the laser irradiation. For comparison, the hybrid colloids and the monocomponent colloids were studied by the same light irradiation condition. 

As discussed earlier, the colloidal spheres formed from BP-AZ-CA can exhibit significant deformation upon Ar laser irradiation, whereas PEAPE colloids do not show such deformation under the same condition. Since every hybrid colloidal sphere shows a very similar deformation upon the light irradiation, it indicates that the colloid spheres are all composed of both BP-AZ-CA and PEAPE components. The fact that the deformation degree is lower than that of the 

BP-AZ-CA colloids can also lead to the same conclusion. It has been discussed in Section 5.3.4 that the hybrid colloids are formed by gradual assembly from the cores to shell, where the more hydrophobic PEAPE chains compose the inner parts and the BP-AZ-CA chains form the shells. The photoisomerization study is consistent with this core-shell structure model. As the HEAZ moieties are confined in the cores, the isomerization cannot have the same free volume as the HEAZ moieties in the shells. Compared with the PEAPE colloids, where a fraction of HEAZ chromophores is distributed in the shells, the free volume for the photoisomerization in hybrid colloids is reduced. This factor can cause the photoisomerization rate of the hybrid colloids to become obviously lower than that of the PEAPE colloids. 

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