The Application Research Of Dynamic Covalent Bond In Polymer Materials

Dynamic covalent bonds are the covalent bonds which could break and reform reversibly under suitable conditions, combining the reversibility of supra-molecular non-covalent bond and the robustness of covalent bond. Dynamic covalent bonds have received extensive applications, especially in polymer materials. As one of the dynamic covalent bonds, disulfide bonds are well-known to be reduced to thiols in the presence of various reducing agents. The resulting thiols can reversibly oxidated to disulfide bonds. Due to this reversible nature and mild transformation conditions, thiol-disulfide bond reversible exchange was widely used in preparing reversible polymer micelles, organo-or hydro-gel, and so on. Meanwhile, the metathesis of special disulfide bonds could occur under suitable conditions and have been extensively used in self-healing polymer materials. In this doctoral thesis, based on the reversible oxidation-reduction transformation of thiol-disulfide bond, we produced core crosslinking micellar clusters by thermal oxidation of polythiol segments of PS-b-Polythiols in their common solvent assisted by CdS nanorods, and prepared recyclable disulfide bond reversibly chemical cross-linking high toughness, high conductivity polymer ion gels. Meanwhile, based on the metathesis of disulfide bonds, we investigated the recyclability of the above polymer ion gels based on re-shaping and restructuring in gel state. This dissertation can be further categorized into three main parts as described below:1Degradable core cross-linking (CCL) micellar clusters of diblock copolymer, PS-b-polythiols, was prepared by CdS nanorods assisted thermal oxidation of polythiol segment of PS-b-polythiol, at high concentration (2.3wt%) in a common organic solvent (toluene). The CdS nanorods acted as not only catalysts, but also templates. Because the incompatible of RAFT polymerization and thiols, the diblock copolymer, PS-b-polythiol, was obtained by aminolysis of PS-b-P1, a block copolymer synthesized via RAFT polymerization using a macroRAFT transfer agent, PS-CTA, and a methacrylate monomer that possessed protected thiol group, in the presence of n-butyl amine and tri-butyl phosphine. Comparison of1H-NMR spectra of oxidation product of PS-b-polythiol with and without CdS nanorods was performed to investigate the oxidation process. The possible mechanism of CdS nanorods catalysing cross-linking of PS-b-polythiols to form CCL micellar clusters was proposed as below:first, adsorption of polythiol segment of PS-b-polythiol on CdS nanorod via strong interaction between thiol and Cd, leaving the PS block outer side; second, oxidation of thiol to disulfide bond catalyzed by CdS nanorods and detachment of crosslinking copolymer because of the much weaker interaction between disulfide bond and CdS nanorod and high temperature to form CCL micelles. The XPS research of S2p and Cd3d of the oxidation procedure and the degradability of the CCL micelles further confirmed the proposed catalytic mechanism.2A disulfide bond reversible chemical cross-linking high toughness, high conductivity ion gels was prepared by sequential triblock copolymer self-assembly and subsequent oxidation of thiols. The triblock copolymer, SOS-SH, was synthesized by combining RAFT polymerization and click reaction.1H-NMR, FT-IR, Raman spectra, XPS, GPC, etc. strongly confirmed the successful synthesis and relative low DPI of SOS-SH. The results of dynamic shear rheological measurements confirmed the forming of gels (G’>G"). The first cross-linking ion gels showed high tensile strength (about0.1MPa), high ultimate elongation (about6), high ionic conductivity (about7mS/cm). After twice reduction-oxidation cycles, the performance loss (toughness and ionic conductivity) of the ion gels were little (less than25%) and the strength even increased by about5%, inferring the good recyclability of this ion gel. The Raman spectra gave the direct evidence of the existing of the disulfide bond, confirming the key role of it playing on the cross-linking and recyclability of the ion gels.3Another outstanding feature of the above disulfide bond reversible chemical cross-linking ion gel was showed:the ion gels could be recycled based on re-shaping and restructuring under heat and pressure in gel state, without the need of solvent or other reagents (such as reducing agent). The ion gels could undergo broken-restructuring-reshaping cycles at least six times with little performance loss (less than10%), exhibiting good recyclability. In order to illustrate the possible mechanism for restructuring of the ion gel, we synthesis two delicately different corresponding small molecular aliphatic-like disulfides as a model metathesis reaction and investigated their metathesis by ESI-MS. The results indicated the feasibility of metathesis of the two corresponding small molecular disulfides and significant exchange would occur only when both IL and copper salt exist. So we considered the process of restructuring of the ion gel as a disulfide metathesis procedure catalysed by both IL and the residual copper salt.