Vesicle-to-Rod Transition of Polymer Aggregates upon Irradiation

Abstract

Atom transfer radical polymerization is utilized to synthesize an amphiphilic photoresponsive triblock copolymer poly(trifl uorostilbene methacrylate)- b -polystyrene- b -polyethylene oxide (PTFS- b -PSt- b -PEO). Adding water into tetrahydrofuran solution of PTFS- b -PSt- b -PEO results in the formation of spherical vesicular aggregates brought about as a result of the association of the water insoluble TFS and St blocks. UV irradiation of the colloidal solution causes trans–cis isomerization of the TFS units which result in morphological transition of the spherical vesicular aggregates to rod-like particles. This morphological transition is studied using dynamic light scattering, scanning electron microscopy, atomic force microscopy, transmission electron microscopy, and UV–vis and fl uorescence spectroscopy. The trans -stilbene units have almost zero dipole moment whereas the corresponding cis -form has a defi nite dipole moment. This polarity difference associated with structural change is expected to be the main reason for the structural transformation of the aggregates. It is also found that PTFS- b -PSt- b -PEO aggregates have the ability of trapping organic dyes inside their hydrophobic cavity thereby making them water soluble. Subsequently the encapsulated dye can be partially released upon UV irradiation.