Supramolecular Hybrid Hydrogels And Macromolecular Self-Assembly Based On Electrochemical Sensitive Inclusion Complexation Interactions

Molecular self-assembly is a "bottom-up" strategy to fabricate new functional materials. It has been considered as one of the most attractive and challenging scientific fields because of the complexity of the assembly process as well as the hierarchical structures of the resultant assemblies. In the past decade, our group first proposed and developed a new "block copolymer-free strategy" leading to "NonCovalently Connected Micelles" (NCCMs), in which self-assembly behavior of pairs of complementary polymers induced by hydrogen bonds were systematically investigated. Recently, we introduced host-guest interactions of supramolecular chemistry as the driving force for macromolecular self-assembly. This work not only opened a new research aspect of NCCM, but also helped us better understand the role of non-covalent interactions in macromolecular self-assembly. Based on the previous research work in our group, this thesis focuses on the fabrication of a electrochemically responsive "supra-crosslink" and supramolecular hybrid hydrogel, as well as polymeric micelles and colloids using the inclusion complexation between cyclodextrin (CD) and ferrocene (Fc) derivatives as the driving force. The contents of this thesis are as follows:1. Supramolecular hydrogels constructed by electrochemically responsive supra-crosslink.The majority of current research on responsive polymeric hydrogels is focusing on thermal and/or pH sensitive hydrogels, while those responsive to redox stimulus are still rare. It is known that Fc undergoes a reversible one-electron oxidation at a low potential, and this reduced-state molecule can form a stable inclusion complex withβ-CD. In this part, a novel electrochemically responsive supra-crosslink (Fc-SCL) was designed and prepared by incorporating Fc monomer (Fc-M) to the surface ofβ-CD stabilized CdS quantum dots (CD@QD) through theβ-CD/Fc inclusion complexation. A supramolecular hydrogel (Fc-Gel), which retained the fluorescent properties of QD, was successfully prepared by in situ polymerization of Fc-SCL with N,N'-dimethylacrylamide (DMA). The inclusion complexation betweenβ-CD and Fc played a crucial role during the formation of this supramolecular hybrid hydrogel, i.e. Fc-Gel. In addition, according to the results of dynamic rheology, an apparent increase of elastic modulus (G') of Fc-Gel with increasing of Fc-SCL content was observed. 2. Dual responsive supramolecular hydrogel with electrochemical activity.Based on the results from last section, the CD/Fc inclusion complex pair has been further employed for connecting the surface of QD and well-designed block copolymers, then a novel hydrogel with both thermal and electrochemical responses was designed. For this purpose, a Fc-modified RAFT chain transfer agent was synthesized. From which, block copolymer of Fc-(PDMA-b-PNIPAM) with Fc at the end was obtained by living radical polymerization. The copolymer was introduced to the surface of QD via inclusion complexation to form a "star-shape" suprastructure Fc-HIC with electrochemical responses. Hydrogel formed when the aqueous solution of Fc-HIC was heated upon the LCST of PNIPAM. The hydrophobic aggregation of PNIPAM as well as the inclusion complexation on QD surface cooperatively contributed to the formation of hydrogel. When K3Fe(CN)6 was added, Fc was transformed to its oxidized state Fc+, then the inclusion complex was dissociated, and the hydrogel turned to sol. This transition can also be achieved by adding a competitive guest, i.e. a water-soluble adamantane derivative (ADA). Because the inclusion complexation between ADA and P-CD was much stronger than that of Fc, a new complex of ADA/β-CD formed instead of Fc/β-CD. The dissociation of the hydrogel can be further confirmed by rheology measurements. Meanwhile, this supramolecular hydrogel showed satisfactory thermal reversible sol-gel transition, which can be controlled by heating-cooling cycles.3. Polymeric micelles driven by simultaneous host-guest interaction and hydrogen bonding.Our group focused on the study of polymer micelles driven by a single kind of interaction, for example, hydrogen bond or host-guest inclusion complexation. In this section, we try to explore the morphology transition of macromolecular self-assembles governed by two kinds of non-covalent interactions simultaneously. Random copolymers were made of methyl methacrylate (MMA) and Fc-containing monomer (Fc-M-2) by ATRP. Then the copolymer self-assembled into stable NCCMs with cyclodextrin-grafted poly(acrylic acid) (PAA-g-CD) in aqueous solutions through both hydrogen bonding and inclusion complexation. With a high Fc content in the copolymer, spherical micelles were obtained and could be further dissociated by addition of a competitive guest. By decreasing the Fc content, i.e. to increase the relative contribution of hydrogen bond, the morphology transition from spherical micelle to network structure was observed.4. Polymer colloids stabilized by inorganic nanoparticles based on inclusion complexation.The conventional methods to stabilize polymer colloids are achieved by using low-molecular-weight surfactants or solvophilic linear polymers. In this part, a new way to stabilize polymer colloids was suggested by using inorganic nanoparticles. Typically, gold nanoparticles covered by CDs (CD@AuNPs) were employed. The Fc-containing hydrophobic polymer (PFc) collapsed in aqueous solution, but formed stabilized particles under the aid of CD@AuNPs as a result of inclusion complexation between Fc from the polymer and CD on the inorganic particle surface. The results of DLS and TEM measurements showed that the spherical colloid particles were uniform with a narrow size distribution. In addition, the hybrid colloids achieved by this method kept stable for several weeks without precipitation.