Colloidal sphere formation, H-aggregation, and photoresponsive properties of an amphiphilic random copolymer bearing branched azo side chains

This work studied the colloidal sphere formation, H-aggregation, and photoresponsive properties of an amphiphilic random copolymer functionalized with branched azo side chains (POAPB6P-AC). The colloidal spheres were prepared through gradual hydrophobic association of the polymer chains in THF-H2O media, which was induced by a continuous increase in the water content. The forming process and morphology of the colloidal spheres were characterized by DLS, SLS, and TEM. UV-vis spectroscopy was used to study the azo chromophore H-aggregation and structure variation in the colloid formation process by measuring the parameters such as λmax, the trans-to-cis photoisomerization rate, and the isomerization degree at the photostationary state. The results indicate that the colloidal sphere formation undergoes several different stages as the water content increases. The polymeric chains start to associate at the critical water content (CWC). CWC is related with the initial concentration of the polymer in THF and is estimated to be 23 vol % when the initial concentration is 1.0 mg/mL. When the water content increases beyond the CWC, more and more polymer chains change from isolated single chains to clusters of associated chains. When the water content reaches 37%, almost all polymer chains are involved in the clusters. As the water content further increases, the clusters experience a collapse caused by the strong hydrophobic interaction, which is indicated by a dramatic size contraction detected from the DLS measurements. When the water content reaches about 60 vol %, the azo chromophores of POAPB6P-AC start to form H-aggregates, which is indicated by a significant blue shift (from 360 to 339 nm) of the UV-vis maximum absorption band. The structure variation can also be detected from the photoisomerization behavior of the systems. When the water content changes from 50 to 60 vol %, both the trans-to-cis photoisomerization rate and the isomerization degree at the photostationary state decrease dramatically. Finally, when the water content increases from 95 to 100 vol %, the isomerization rate and degree significantly decrease again. In this stage, THF in the microphase is almost completely replaced by H2O, and colloidal structure is frozen because of the free volume decrease and chain entanglement. The above observations give insight into the occurrence of H-aggregation in colloids of amphiphilic azo polymers for the first time. The understanding can lead to further development in expanding functionalities of the photoresponsive colloidal spheres through manipulation of the chromophore aggregation.