Organic-Inorganic Nanocomposites Based On Spherical Poly Electrolyte Brushes:Preparation And Characterization By Small Angle X-ray Scattering

Nanoscience has long been one of the top research areas.Functionalized nanoparticles with unique properties have great potential in fields such as controlled catalysis and biosensors.Spherical Polyelectrolyte Brushes(SPB)can be widely applied to the selective adsorption of metal ions,protein loading,nano-catalyst thanks to their high grafting density and the relatively constant pH and counterion concentration within the brush layer.In this paper,spherical polyelectrolyte brushes(SPB)were prepared,which consist of solid polystyrene(PS)cores densely grafted with linear polyelectrolyte chains.The polyelectrolyte chains can be poly(acrylic acid)(PAA)or poly(2-aminoethyl methacrylate hydrochloride)(PAEMH),which can be ionized with negative or positive charges,respectively.SPB are responsive to change of pH and salt concentration,and have the ability to enrich counterions within the brush layer.Based on SPB,functional materials of SPB-silica,hollow silica,and SPB-gold nanocomposites were prepared,and their unique properties were characterized by small angle X-ray scattering(SAXS).The main work is as follows:Core-shell-corona silica/polymer hybrid nanoparticles with narrow size distribution were prepared in the template of SPB.The microstructure of obtained hybrid nanoparticles was studied by SAXS and in combination with dynamic light scattering(DLS)and transmission electron microscope(TEM).The model of "Core-Shell-Corona" was established.The generation of silica shell within the brush is confirmed by the significant increase of the electron density in the shell,and the silica shell showed a unique inner-loose-outer-dense structure,whose thickness is pH sensitive while insensitive to ionic strength as revealed by fitting SAXS data.After dissolving the PS core,hollow silica-polyelectrolyte composite nanoparticles were obtained and were systematically studied by SAXS.The hollow structure formed by dissolving the PS core was confirmed by the reduction of electron density to zero in the cavity through fitting SAXS data.SAXS revealed both the inward and outward expansions of the hollow silica-polyelectrolyte composite particles upon increasing pH from 3 to 9,while further increasing pH led to the partial dissolution of silica layer and even destruction of the hollow structure.SAXS was confirmed to be a unique and powerful characterization method to observe hollow silica nanoparticles,which should be ideal candidates for controlled drug delivery.Spherical polyelectrolyte nanogels(SPN)were obtained by crosslinking the PAA chains.Hollow silica nanoparticles were prepared via generating a silica layer in SPN followed by removing the PS core via solvent dissolution.SAXS in combination with TEM were employed to study SPN,silica-polymer composite,and hollow silica nanoparticles.It was confirmed that SAXS is a powerful method to monitor the generation of silica layer in SPN.The density and thickness of generated silica layer in SPN were found to be tunable by controlling the crosslinking density of the templates.The generation mechanism of silica in PAA layer was also proposed.The interaction of spherical polymer brushes with gold nanoparticles was also studied in this paper.Beside anionic PAA-SPB and cationic PAEMH-SPB,we aslo prepared electroneutral poly(N-isopropylacrylamide)brushes(PNIPAM-SPB)and weakly anionic P(NIPAM-co-AA)-SPB.Citrate-stabilized gold nanoparticles were prepared and mixed with different polymer brushes.As revelled by SAXS,neither anionic PAA-SPB nor P(NIPAM-co-AA)-SPB showed adsorption of gold nanoparticles,whereas PNIPAM-SPB adsorbed gold nanoparticles by holding them on the surface of its brush layer.The cationic PAEMH-SPB showed obvious adsorption of gold nanoparticles.After the adsorption,the size of PAEMH-SPB decreased,while gold nanoparticles formed clusters within the brush layer.Instead,gold nanoparticles prepared by in-situ reduction in PAEMH-SPB were located in the innermost brush layer and uniformly dispersed without clustering.The difference inbetween was confirmed by cryo-TEM and UV-Vis.SPB loaded with gold nanoparticles were spin-coated onto the surface-modified silicon wafer.The catalytic activity of gold nanoparticles was found to be retained,and the composite particles showed strong attraction with the substrate due to the counterion release force.