Preparation And Performance Of Laponite-based Nanocomposite Functionally Gradient Hydrogels

In order to meet the requirement of extraordinarily high temperature- and low temperature-tolerance for aerospace materials, functionally gradient material(FGM) firstly was put forward by Japanese scholars in 1987. Once proposed, FGM attracted widespread attention from the research community and rapidly became multi-disciplinary research focus including mechanical engineering, bionics, tissue engineering, cell engineering, diagnostics and etal. As a novel FGM, gradient hydrogels not only has a continuous or quasi-continuous characteristic in molecular structure, chemical composition and macro-performance, but also has good biocompatibility, high permeability to small molecules, optical properties and intelligent response. However, the vast majority of gradient hydrogels are chemically crosslinked, and thus lead to poor overall mechanical properties.In this thesis, organic/inorganic nanocomposite(NC) gradient hydrogels were prepared by in-situ copolymerization hydrophilic monomers in the dispersion of Laponite and direct current electric field-induced(DC-EF) anisotropic distribution of Laponite, which was utilized as a multifunctional physical crosslinker. The hydrogels showed excellent mechanical properties, intelligent response and catalytic properties. The main contents of this thesis are as follows.(1) Laponite as a physical crosslinking agent was induced by DC-EF( electric field strength E = 400 V/m) to migrate towards the anode. At the same time, potassium persulfate(KPS) initiated in situ free radical copolymerization of monomers acrylamide(AM) and N,N- dimethylaminoethyl methacrylate(DMAEMA) to form organic/inorganic nanocomposite gradient hydrogels with crosslinking density gradient and anisotropic mechanical strength. Laponite staining experiment with Rhodamine B(RB), infrared spectroscopy(FT-IR), thermal gravimetric analysis(TGA) tests confirmed the electric field-induced gradient distribution of Laponite in the hydrogels. Gradient distribution of Laponite led to gradient crosslinking density and mechanical strength. When the distance to anode of the gradient hydrogels was decreased from 3.0 cm to 0.5 cm, the tensile strength increased from 43.4 to 135.3 k Pa while the elongation at break from 600% to 1300%; and compressive strength increased from 116 k Pa to 1100 k Pa. Moreover, overall mechanical properties of gradient hydrogels were investigated. The mechanical property of the gradient hydrogel was better with the increasing Laponite content.(2) A tubular graphite electrodes(E = 400 V/m) was used to induce the formation of radial concentration gradient of Laponite. KPS initiated in-situ free-radical copolymerization of monomers(DMAEMA, N,N-diethyl-acrylamide(DEA)). As a result, temperature sensitive and bamboo-like organic/inorganic NC gradient hydrogels were prepared. In the electric field, Laponite migrated towards the anode, while DMAEMA adsorbing on the surface of Laponite also directed to follow the migration toward the anode, forming a gradient distribution. Content of the two kinds of structural units in the polymer and Laponite hydrogel continuously changed along the direction of the field. Gradient molecular structure and chemical composition resulted in bamboo-like hydrogels. The compressive strength of gradient hydrogels reached 700 k Pa, and exhibited anisotropic mechanical strength: compressive stress was 758 k Pa and 693 k Pa respectively when force direction was towards and against the direction of electric field respectively. The hydrogels showed a gradient temperature reponsive behaviors.(3) Poly(AM-co-NIPAm) was prepared by crosslinking polymerization of AM and N-isopropylacrylamide(NIPAm). Then copper phosphate(Cu P)@poly(AM-co-NIPAm) organic/inorganic hydrogel was prepared by in-situ co-precipitation method. and the variable valence of copper in Cu P(Cu2+-Cu+) activated molecular oxygen, and catalytized oxidation of N,N-diethyl-p-phenylenediamine sulfate(DPD) and o-phenylenediamine(OPD) and other color reaction. Color reaction proved that molecular oxygen acted as oxidant. Some factors, including p H value, DPD concentration, reaction time, catalyst type and concentration of oxygen molecules were discussed with respect to the color reaction of the DPD. Besides, to replace organic crosslinking agents by Laponite, Cu P@ Laponite-based poly(AM-co-NIPAm) organic / inorganic NC gradient hydrogels were prepared and used in the detection of DPD.