Synthesis and self-assembly of well-defined block copolymers with photoresponsive groups

Abstract

This thesis deals with synthesis and self-assembly of well-defined block copolymers containing photoresponsive azobenzene groups. Well-defined block copolymers were synthesized through combination of controlled polymerization methods such as chain-growth condensation polymerization (CGCP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT) polymerization. Self-assembled nanostructures of the block copolymers and their responsiveness to external stimuli in thin films as well as in solution were investigated. We investigated the reversible dramatic transmittance modulation as a result of photoinduced aggregation and segregation of self-assembled block copolymer micelles which are composed of well-defined rod-coil diblock copolymers. The block copolymers were synthesized through CGCP of AB type monomers for rod blocks incorporated azobenzene units along the polymer backbone, poly(arylene ether azobenzene) (PAEAz), and subsequent ATRP of vinyl monomers with proper macroinitiators for coil blocks. A nucleophilic aromatic substitution (SNAr) reaction was a key for the synthesis of well-controlled PAEAz by displacement of the leaving group activated by a trifluoromethyl group. In cyclohexane, uniform micellar aggregates of the diblock copolymer were formed as the spherical shape consisted of the core rod and the periphery coil. Irradiated by UV light, the transmittance of the homogenous solution decreased down to ca. 22 %, and became turbid, but returned to a clear solution when exposed to visible light. The solution obstructed incident light whenever a certain amount of UV irradiation was illuminated. By CGCP of the AB type monomer, we also synthesized well-defined PAEAz and macromonomers by modification of the chain end of the polymers. Comb-coil type block copolymers (CCBPCs) were synthesized through sequential RAFT polymerization of appropriate monomers and macromonomers. Because the designed macromonomers have extended side chains, only two step RAFT polymerizations generated the architecture of CCBPCs. And we also investigated microphase-separated nanostructure of the block copolymer thin films. The polymer thin films exhibited vertically oriented cylindrical morphologies regardless of surface energy that are driven by the architecture to overcome the interactions between the substrates and the polymer chains. Finally, well-defined amphiphilic block copolymers were synthesized through CGCP of the monomer with hydrophilic poly(ethylene oxide)s (PEOs) macroinitiator. CGCP of the hydrophobic monomer with the hydrophilic coil blocks allowed not only the sysnthesis of well-defined amphiphilic rod-coil type diblock copolymers but also modulation of their hydrophilic-hydrophobic balance (HHB) through the volume fraction of the hydrophobic block. Self-assembly behavior of the well-defined amphiphilic block copolymers was investigated. Poly(arylene ether azobenzene)-block-poly(ethylene oxide)s (PAEAz-b-PEOs) with proper HHB self-assemble into fibrilar network gel in polar solvents such as alcohols. Self-assembled organogelators of PAEAz-b-PEOs containing azobenzene units in the main chain of the hydrophobic rod block reversibly undergo reversible sol-gel transition under irradiation of UV and visible light or thermal treatment. Moreover, molecular recognition of PAEAz and PEO blocks produced the inclusion complexes with different CDs, resulting in turbidity changes of solution.