Phenylbornate-Diol Crosslinked Polymer Gels With Reversible Sol-Gel Transition

In recent years, the intelligent polymer gel has been the focus of considerable interest because of their appropriate stimuli-responsive actions and widely application. As one of the dynamic covalent bonds, boronate ester bond has been successfully used to prepare smart polymer gels, which have many potential applications in the fields of self-regulated insulin delivery, tissue engineering, separation, and sensor systems. At present, intelligent polymer gels can be synthesized by crosslinking copolymerization of monomers and chemical or physical crosslinking of linear polymers. The former has been the most widely used technique to produce a variety of copolymer gels, but less popular because of the limitation of involving toxic reagents like catalysts, initiators, and residual low molecular weight cross-linkers. The latter usually use polyvinylalcohol. However, it is difficult to control the polymer architecture through convention radical polymerization. Therefore, it is of great importance in synthesis of well-defined polymers, containing hydroxyls as well as phenylboronic acid side groups, for the preparation of phenylboronic-diol crosslinked polymer gels. The main results are shown as follows:(1) Monomers and polymers synthesis. The monomers (5-ethyl-2,2-dimethyl-1,3-dioxane-5-yl)methyl acrylate (EDMA) and (4-vinylbenzyl-4-(1,3,2-dioxaborinan-2-yl) benzoate (VBDB) were synthesized separately. Polymers with well-defined architecture were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization of EDMA as well as copolymerization of N,N-dimethylacrylamide (DMA) and VBDB. Both polymers were subjected to deprotection to synthesize poly(2,2-bis(hydroxymethyl)butyl acrylate) (PHBA) and 4-((4-vinylbenzyloxy)carb-onyl)phenylboronic acid-N,N-dimethylacrylamide copolymer, poly(VPB-co-DMA). The structures of monomer and polymers were characterized by NMR spectroscopy. The molecular weight and polydispersity of polymers were characterized by GPC.(2) Preparation of HBA-VPB crosslinked polymer gels. Polymer gels were synthesized by crosslinking of linear poly(VPB-co-DMA) and PHBA polymers. Analysis of the composition-property relationships of these polymer gels, specifically considering the effects of pH, molar ratio of HBA to VPB units, and gelator concentrations on dynamic rheological properties, were performed. Additionally, the polymer gels can be switched into their starting polymer solution by adjusting pH of the system. The reversible sol-gel phase transition can be performed for several cycles in a similar way of supramolecular gel. Moreover, the gel revealed interesting self-healing property, which occurred autonomously without any outside intervention. Gel behavior was quantified by subjecting the gels to dynamic rheology as a function of angular frequency. At high angular frequency, storage modulus (G') became frequency independent and plateaus. At low angular frequencies, fluid-like behavior dominates in reversible gels (i.e., G'<G"). The rheological behavior of crosslinked gels is consistent with the time-dependent viscoelastic properties of dynamic gel networks.(3) The single-walled carbon nanotube (SWNT) was dispersed in the solution of polymer to synthesize polymer gels-containing carbon nanotube. Due to more multipoint interaction with SWNT via physical gelation, the storage modulus and crosslink density of composite gels increased compared to that of polymer gels. But SWNTs had no effect on reversible sol-gel phase transition and self-healing property of gels.