Preparation And Properties Of Novel Graphene Oxide/Polymer Composite Hydrogels

Graphene oxide (GO), also called functionalized-graphene, is a two-dimensional monolayer of sp2-bonded carbon atoms. GO possesses numerous unique properties, such as high specific surface area, great mechanical strength, excellent electrical and thermal conductivities, and potentially low production cost. Besides, there are a large number of hydrophilic oxygenated functional groups on the surfaces or at the edges of GO nanosheets. These groups not only supply many reaction points for their further modifications, but also dramatically improve their interfacial interactions with polymer matrix. Therefore, GO nanosheets are very suitable for fabricating polymer-based nanocomposites. Polymer hydrogels are moderately crosslinked hydrophilic polymer networks, which can absorb a certain amount of water or other aqueous fluids but cannot be dissolved into them. Owing to their good properties, polymer hydrogels have been extensively studied and widely applied in various fields, such as hygienic products, agriculture, construction engineering, and environmental protection. However, traditional hydrogels still have some drawbacks, such as heterogeneous polymer networks, weak mechanical properties, low swelling abilities, and poor intelligent sensitivities. In fact, GO is a perfect candidate for preparing polymer-based nanocomposite hydrogels because of its outstanding properties.In this thesis, GO was introduced into the polymer hydrogel networks with the monomer, initiator, and crosslinker to successfully prepare two series of novel GO/polymer composite hydrogels via in situ radical solution polymerization. One is adding various amount of GO nanosheets into a typical anionic poly (acrylic acid-co-acrylamide)(P(AA-co-AM)) polyelectrolyte hydrogel networks to synthesize novel GO/P(AA-co-AM) composite hydrogels. The introduction of GO might improve its performances and broaden its application areas, which is meaningful to be investigated profoundly. Due to its outstanding characteristics as well as its good molecular interactions with GO, the alkaline natural polysaccharide-chitosan (CS) replaced the the acrylamide monomer of the above P(AA-co-AM) hydrogel, and GO nanosheets were also filled to synthesize the other series of novel cationic/anionic GO/chitosan-g-poly (acrylic acid)(GO/CS-g-PAA) composite hydrogels. This series is very important for well understanding the molecular interactions between GO and polymer networks and further broadening the application fields of polymer hydrogels. The main research contents and conclusions of this thesis were listed as follows:(1) GO/P(AA-co-AM) composite hydrogels:The physical and chemical structures, morphologies, thermal stabilities, swelling properties, swelling kinetics, and pH-sensitivity of the hydrogels were systematically studied. The results demonstrated that relatively lower content (<0.30wt%) of GO could be dispersed well in the polymer matrix and enhanced the intermolecular interactions between the components effectively through hydrogen bondings or covalent bondings. As a result, the addition of an extremely low content (<0.30wt%) of GO could effectively improve the swelling abilities of the composite hydrogels. It was worth noting that the composite hydrogel only containing0.10wt%GO exhibited significant improvement of swelling ratios in neutral medium (1094g/g in deionized water and75g/g in0.9%NaCl solution), which were44.5%and78.6%higher than that of the pure hydrogel. Furthermore, the pseudo-second-order model was suitable to discuss the swelling kinetics of the composite hydrogels. Also, the GO/P(AA-co-AM) composite hydrogels exhibited pH-sensitive behaviors, and could retain relatively higher swelling ratios to a certain degree at acidic and basic solutions.(2) GO/CS-g-PAA composite hydrogels:The physical and chemical structures, porous microstructures, thermal stabilities, mechanical properties, and intelligent sensitivities of the hydrogels were investigated comprehensively. The results implied that a proper amount of GO could also form good interfacial interactions with the CS-g-PAA hydrogel networks by hydrogen bondings or covalent bondings and thus be dispersed well in the polymer matrix. Moreover, the addition of an extremely low content (0.05-0.30wt%) of GO nanosheets could significantly influence the microstructures of the hydrogels, leading to the formation of the interconnected and sized-controlled porous composite hydrogels. Also, the range of the pore sizes was around10-100μm. With increasing the amount of GO in the above range, the pore size became larger and more uniform gradually and there appeared parallel aligned interconnected pores. In addition, the possible microstructure models of the porous hydrogels were proposed, and the formation mechanism was also discussed briefly, which were useful for better understanding the formation process of the composite hydrogels. Different from conventional techniques for fabricating porous hydrogels, the porous structures of the as-prepared hydrogels were in situ formed after polymerization reaction. Therefore, this study might provide a novel route for preparing porous hydrogels. Meanwhile, the mechanical performances of the composite hydrogels were also reinforced significantly due to the introduction of the GO nanosheets. The composite hydrogel containing0.30wt%GO loadings acquired the highest compressive strength,339.87KPa at80%compression ratio, which is much higher than that of the pure one. Besides, the as-prepared composite hydrogels could keep the swelling abilities of the pure hydrogel and still be sensitive to salt concentration and pH value.The innovation points of this thesis were summarized as follows:GO were introduced into polymer hydrogel networks to successfully prepare two series of novel GO/polymer composite hydrogels. One series were anionic GO/P(AA-co-AM) composite hydrogels with dramatically improved swelling properties and relevant properties, and the other series were cationic/anionic GO/CS-g-PAA composite hydrogels with in situ generated porous structures and significantly reinforced mechanical properties. Additionally, the possible porous microstructure models of the GO/CS-g-PAA composite hydrogels were also proposed, and their formation mechanism was investigated. This thesis might provide a novel route for fabricating porous hydrogels. Also, this thesis was also very important for understanding the molecular interactions between GO and polymer chains. Therefore, the as-prepared composite hydrogels in this thesis were potentially promising in numerous areas, such as tissue engineering, drug delivery system, hygienic products, construction engineering, and environmental protection.