Photo-driven smart polymers and their molecular self-assembly
Date of Issue2013
School of Materials Science and Engineering
The development of responsive biomimetic self-assembled polymeric materials constitutes an exciting research field. The focus of this research is to utilize photo-sensitive molecules to trigger polymeric self-assemblies. This objective is achieved through detailed studies in the following directions: (i) the preparation and characterization of amphiphilic azobenzene derivative functionalized polymers (azobenzene containing poly(ethylene glycol), azo-PEGs) having both linear shape and star shape, (ii) the construction of their self-assemblies in both mixed solvents and aqueous solutions, and (iii) the investigation of the photo-responsive properties of these self-assemblies, (iv) the insights to mechanisms. Azobenzene is chosen as the photo-trigger in this study because it is an organic photo-chromic molecule that exhibits clear photo-isomerization mechanism. Its trans-state is hydrophobic and its cis-state is relatively hydrophilic. The synthesized azo-PEG polymers are lipid-like azobenzene-containing molecules. The simplistic and representative design of the azo-PEGs enables the in-depth studies of the self-assembly mechanism. For the water/THF mixed solvent system, investigations on the vesicle formation and its pulsation behaviour were carried out. A liquid-liquid phase separation accused owing to a partition of the solvents as the vesicles formed in the water/THF mixture. The hydrophobic and rigid azobenzene groups at chain ends aggregated in THF-rich phase, driven by the hydrophobic and π-π interactions. The hydrophilic PEG chains thus assembled in the water-rich phase forming the inner and outer layers of the membrane. The interfacial energy between the hydrophobic core-layer and the more polar solvent was one of the key factors that governed the vesicle formation. When the vesicles were exposed to UV light, the azobenzene terminal groups experienced trans-to-cis photo-isomerization. The rearrangement of the azobenzene molecules caused an increase in the surface tension. In order to lower the surface energy, the vesicle shrank by expelling the cis isomers from the parallel trans isomers and corralling of the remaining trans isomers. Polymer concentrations and volume ratios of THF and water were identified as very important factors that influence the morphological properties of self-assemblies. The different photo-isomerization behaviours of the azobenzene derivatives were identified to have contributed to regulation of the pulsation behaviour. The extraordinarily large change in vesicle size was originated from an isomerization-driven molecular assembly upon UV-visible irradiation cycles, and was associated with substantial transmembrane solvent transport. For the aqueous system, self-assembly of azo-PEGs was achieved with a modified molecular design, eventually producing a pulsating vesicle that is promising for selective ion transport. Further study was carried out on the star shaped azo-PEGs that exhibit more interesting and complicated self-assembly behaviour. Our main scientific contribution is the mechanism illustrating the extraordinarily large change in vesicle size originated from an isomerization-driven molecular assembly and the substantial transmembrane solvent transport. The photo-driven pulsation of the vesicles described in the thesis is certainly most unusual, very interesting and potentially useful. This photo-driven vesicle is a typical example of pulsating self-assemblies reported for the first time. Other examples have been reported are pH-responsive1 and thermo-responsive2 pulsating vesicles, respectively. The further studies on the vesicles self-assembled in dilute solute solutions present the photo-driven ion transport selectivity. This photo-sensitive vehicle provides promising applications in areas such as bio-sensors, controlled drug release, and water treatment.