Polymer-Based Complex Hybrid Self-assembly System

In recent years, our group developed "block-copolymer-free" strategy as a new concept in macromolecular self-assembly. Taking cyclodextrin-based host-guest inclusion complexation as new driving force in addition to hydrogen bonding, novel non-covalently connected micelles (NCCM) and vesicles (NCCV) were successfully produced. Afterwards, inorganic nanoparticles and stimuli-responsive polymers were used as new building blocks to form hybrid inclusion complex (HIC), from which stimuli-responsive supramolecular hydrogel were successfully produced. The resultant success has proved the potentiality of the combination of supramolecular chemistry and macromolecular self-assembly. Based on this background, the research in this thesis was carried out, with contents as follows: 1. Self-assembly and morphological switching of thermo-responsive polymersIt was reported that amphiphilic block copolymers with poly(N-isoproylacrylamide) (PNIPAM) as middle block self-assembled into spherical micelles below the lower critical solution temperature (LCST) of PNIPAM, then underwent morphology transition from micelles to vesicles caused by the amphiphilicity change above LCST, but the transition took as long as several weeks. Moreover, the mechanism proposed in literatures is not convincing. To achieve rapid and reversible transition, asymmetrically modified PNIPAM (C12-PNIPAM-CA) with a hydrophobic alkyl chain -C12H25 at one end and a hydrophilic carboxyl group-COOH at the other was prepared and found to form micelles at room temperature due to hydrophobic association of alkyl chains. Successful morphology transition from micelles to vesicles can be realized within 30 minutes above LCST of PNIPAM. Based on full monitoring of the transition process by light scattering, it was found that the morphology transition was fully reversible and the vesicle size can be precisely tuned by temperature. Hence new mechanism was proposed to explain this type of morphology transition. To achieve rapid and reversible micelle-to-vesicle transition, pre-formed loose micelles induced by hydrophobic association is of key importance. Direct self-assembly of these micelles resulted in vesicles. The transition was rapid since it was free from slow re-organization of polymer chains. Based on this mechanism, we could use gold nanoparticles as new building blocks to replace the loose hydrophobic cores, for construction of new functional self-assembled structures, which led to the following research.2. Self-assembly of gold nanoparticles directed by thermo-responsive block copolymersA new type of HIC was constructed from a-cyclodextrin stabilized gold nanoparticles (α-CD@AuNP) and thermo-responsive block copolymer poly(N-isopropylacylamide)-block-poly(N,N-dimethylacrylamide) with functional azobenzene groups (Azo-PNIPAM-b-PDMA) based on host-guest complexation between a-CD and Azo. Due to the light responsiveness and reversible nature of the host-guest pair, formation of the HIC was also controlled by light reversibly. Above LCST of PNIPAM, hybrid vesicles formed via self-assembly of HIC induced by the coil-to-globule transition of PNIPAM. The vesicle wall was made of a single layer of tightly connected AuNPs. This process was fully reversible that vesicles dissociated into nanoparticles as soon as temperature was decreased below LCST of PNIPAM. This is also a proof of the proposed mechanism about micelle-to-vesicle transition, producing novel hybrid vesicles with potential applications.3. Fabrication and self-assembly of protein-polymer supra-conjugates based on multiple molecular recognition interactionsIn our group, we have been focusing on development of novel macromolecular self-assembled structures based on host-guest inclusion complexation. Herein, we used molecular recognition between protein and saccharide as a new driving force, combined with host-guest inclusion complexation, to achieve hierarchical control over a complex self-assembly system. Specifically, molecular recognition between concanavalin A (ConA) and mannoside was used as a new interaction in the system. Supramolecular conjugation between native ConA and end-functionalized polymer PEG (polyethylene glycol) was achieved by dual molecular recognition interactions, via a linker βCD-Man. Between the three components, β-cyclodextrin (βCD) and ConA recognized adamantane (Ada) end of PEG and α-mannopyranoside (Man) orthogonally. Different supramolecular structures in this three-component system were discussed at different concentration. Further self-assembly of the resultant supra-conjugates of ConA-PEG was induced by addition of aCD, which was selectively threaded by PEG chains, leading to micro-crystallization of formed pseudo-polyrotaxanes (PPR) supramolecular structures. Thus hexagonal shaped particles in dilute solution or supramolecular hydrogel at a higher concentration were produced. Moreover, moduli of the obtained hydrogel were 3 magnitudes higher than that of native PPR hydrogel without ConA at the same condition.