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[摘要]：This article reports for the first time the observation of a series of fractal structures formed from a representative amphiphilic random azo copolymer (PPAPE). The fractal structures were obtained from colloidal spheres of PPAPE through aggregation and structural reorganization in aqueous dispersions. In the process, the colloidal spheres with hydrophobic cores were first prepared by gradual hydrophobic aggregation of PPAPE in H(2)O/THF medium with a continuous increase in the water content. The fractal structures were then obtained by gradually adding an aqueous solution of NaOH (0.2 M) into the weakly acidic dispersions of the colloidal spheres to different pH values. The fractal aggregates were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), static light scattering (SLS), and dynamic light scattering (DLS). Photoisomerization behavior of the azobenzene units was investigated to detect the local environment around the molecular probes in the aggregates. The results show that by adjusting the pH of the dispersions to different values (from 8.2 to 11.7), fractal aggregates with different morphology and fractal dimensions (d(f)) are obtained. At low pH side (pH = 8.2), the fractal structures are formed from partially dissociated colloidal spheres with d(f) of 168 +/- 0.03, which corresponds to a diffusion-limited cluster aggregation (DLCA) process. When pH is 9.4, the fractal aggregates are built up with fine needle-shaped branches with size as small as 10-20 am and df is measured to be 2.14 +/- 0.03, which is similar to a reaction-limited cluster aggregation (RLCA) case. For the high pH (pH = 11.7), the fractal dimension increases to 2.24 +/- 0.02 and fractals appear as denser aggregates. The photoisomerization study shows that the fractal structures possess a relatively loose inner structure compared to the initial colloidal spheres. The observations suggest that the well-organized fractal structures obtained in the high-pH dispersions are formed by the aggregation of partially dissociated colloidal spheres and the subsequent structural reorganization at the molecular level. The understanding of the molecular self-assembling process can be used to develop photoresponsive materials with fractal architecture for future applications. |
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