Preparation Of PH-responsive Cationic Core-shell Microgel Composites And Their Applications

Microgels are polymer colloidal particles with crosslinking network structures, and their particle size is within the scope of 10¹000 nm. Among them, p H responsive cationic microgels have important applications into many fields such as catalysis, biology, medicine and sensors for their unique stimuli-responsivity. Core-shell microgels have the advantages of better performance than that of general microgels on the regulation of properties and structures. The microgel/ noble metal nanoparticle composites prepared by the combination of cationic core-shell microgel and noble metal nanoparticles can combine with the multiple properties of organic microgel and inorganic nanoparticles. On the one hand, the three-dimensional network structure of microgels can provide stable support environment for noble metal nanoparticles, avoid the agglomeration of inorganic nanoparticles and enhance their recovery and utilization. On the other hand, the stimuli-responsivity of cationic microgels can be used to control the formation and performance of noble metal nanoparticles. These excellent properties of microgel/noble metal nanoparticles make them have more applications in catalysis, biomedicine, sensors and microreactors. However, finding a simple and effective method for the preparation of core-shell microgel/noble metal nanoparticles with high performance is still a challenge.Poly（2-（diethylamino）ethyl methacrylate）（PDEAMA） is a pH-responsive cationic polymer, and it has important applications in catalysis, drug controlled release, sensors and other fields. However, its glass transition temperature is low, which makes it easy to form film and morphology difficult to control. In consideration of this proplem, this paper illustrates the successful preparation of core-shell microgels with PDEAMA as shell by two different methods, which reduced the easy-filming properties of PDEAMA microgels to a certain extent, and their applications in Pickering emulsions were discussed. In addition, taking advantages of the reductibility of tertiary amine in PDEAMA microgels, core-shell microgel/Au nanocomposites is prepared using prepared core-shell microgels as a support and reducing agent by in-situ self-reduction method. The composites are used as a catalysis in the reduction of 4-nitrophenol（4-NP） by NaBH₄. The detailed researches and conclusions are as follows:（Ⅰ） Silicon dioxide（SiO₂） spheres were first prepared by St?ber method, and then modified with double bond, finally the core-shell SiO₂/PDEAMA microgel were prepared by distillation precipitation polymerization. The structures of SiO₂/PDEAMA microgels were characterized by Fourier transform infrared spectrometer（FT-IR）, transmission electron microscopy（TEM）, Malvern Zetasizer nano ZS90 and thermogravimetric analysis（TG）, and their pH responsibility was characterized by DLS and ultravioletvisible spectrophotometer（UV-vis）. The results show that the SiO₂/PDEAMA microgels have a core-shell structure with a uniform size. The volume and the absorbance of the microgels changed with p H, which indicates their pH responsiveness.（Ⅱ） In order to simplify the preparation process and avoid the pollution to the environment by organic solvent, methyl methacrylate（MMA） and DEAMA were used as monomers, core-shell PMMA/PDEAMA（denoted as PMD） microgels was successfully prepared by one-pot emulsion polymerization by taking advantage of the different hydrophobic/hydrophilic properties of the two monomers. In the end, their application in Pickering emulsion is discussed. The structure and thermal stability of PMD microgels were investigated via TEM, scanning electron microscope（SEM）, differential scanning calorimeter（DSC）, automatic potentiometric titration, and the p H responsibility was characterized by Malvern Zetasizer nano ZS90, DLS and UV-vis. The results show that core-shell PMD cationic microgels with uniform morphology can be simply prepared by one-pot emulsion polymerization, and the introduction of MMA improves the thermal stability of the microgel to a certain extent, the prepared PMD microgels also have a good pH responsibility. The prepared Pickering emulsions have a good chemical stability when used PMD microgels as Pickering emulsifiers.（Ⅲ） Using PMD microgels as carriers and reducing agents, HAuCl₄ as metal precursor, PMD/Au composites were prepared by self-reduction in aqueous solution without any aditional reducing agents. The morphology of the PMD/Au composites, the formation of Au nanoparticles and the catalytic rates of PMD/Au composites were analyzed by TEM, energy dispersive X-ray spectrometer（EDS）, XRD, TG, FT-IR, DLS and UV-vis. The results show that PMD/Au composites can be better prepared in the aqueous solution of pH = 6 by self-reduction method; Au nanoparticles were mainly distributed in the shell of PMD microgels; PMD/Au composites have higher catalytic efficiency compared with homopolymer PDEAMA/Au as a catalyst in the reduction of 4-NP by NaBH₄; Taking advantage of the pH responsiveness of PMD microgels, the catalytic activities of PMD/Au composites can be controlled.