Light / Temperature Response Behavior Of Amphiphilic Block Copolymer Self-assembly

Stimuli responsive polymeric materials can adjust themselves under external stimuli so as to adapt to the change of the outer environment, such as regulation of hydrophilic-hydrophobic properties of surfaces, control of ionic and molecular transportations, and transformation of chemical signals into light, electric, thermal and mechanical signals. Therefore, they have wide applications in every aspect of our lives. Stimuli responsive amphiphilic block copolymers have been a research focus recently, especially the multi-responsive block copolymers. Compared with the mono-responsive polymers, multi-responsive polymers have multiple functionalities as well as multiple control modes and thus have potential applications in controlled release, chemical catalyst, nano reactors and micro reactors. In this work, amphiphilic block copolymers with the characteristic structure, PEO-b-P(AZO-co-NIPAM), were successfully synthesized via living polymerization techniques. A variety of self-assembled aggregates including micelles, spherical vesicles, spindle-like vesicles and hollow polymer microspheres were obtained by varying the chemical structures of the block copolymers. Studies about the formation conditions, thermo and photo dual responsiveness as well as the potential functionalities of the self-assembled aggregates were also investigated.Atom transfer radical polymerization (ATRP) was utilized to synthesize the block copolymer PE048-b-P(AZO10-co-NIPAMio). Uniformly distributed polymeric micelles in aqueous solution were obtained by self-assembly method. This kind of micelles showed dual responsiveness. The micelles shrank when the temperature increased and returned to the initial size when the temperature decreased. This kind of thermo-sensitive feature could be used to realize controlled release of the encapsulated substance by repetitive heating-cooling processes. On the other hand, the micelles maintained the spherical morphology when irradiated by UV light, which was a novel feature compared with previously reported results that UV light irradiation usually destroyed the micelles. In contrast, the hydrophobicity of the micellar core could be possibly tuned by light in this work. Upon UV light irradiation, the micellar core became less hydrophobic while upon visible light irradiation, the micellar core became more hydrophobic. This feature is promising in applications such as controlled catalyst.Reversible addition-fragmentation chain transfer (RAFT) polymerization was utilized to synthesize the block copolymer PEO48-b-P(AZO5-co-NIPAM15). Giant vesicles were also obtained by self-assembly method. Studies were carried out to investigate the effect of water content, shearing force, initial concentration, solvent and water addition type on the morphologies of the giant vesicles. Spherical vesicles, spindle-like vesicles as well as rod-like vesicles were observed. On the other hand, the giant vesicles formed hollow polymer microsphere directly when the solvent was evaporated, without the traditional complicated template and chemical cross-linking processes. Florescence microscope was applied to test the load capacity of the giant vesicles and the possible mechanism of the formation of the hollow polymer microspheres was proposed. This kind of vesicles also had thermo and photo dual responsiveness. It was found that the size of the vesicles changed along with the change of the temperature. The thermo sensitiveness of the vesicles was also used to achieve the controlled release of encapsulated substance loaded in the vesicles and the release ratio reached about 36% by repetitive heating-cooling processes. On the other hand, the giant vesicles showed novel and unusual photo-responsive behavior. First, UV and visible light could tune the hydrophobicity of the vesicular walls. Second, UV light induced the transformation of irregular-shaped vesicles into regular sphere ones, indicating that the vesicles have a good self-recovery capability. A possible mechanism was proposed with reference to the theories of kinetics, thermodynamics and energy. These special and novel photo-responsive behaviors of the giant vesicles are promising in applications such as controlled release, nano reactors, controlled catalyst, et al.