Silica-Coated Core–Shell Structured Polystyrene Nanospheres and Their Size-Dependent Mechanical Properties

The core–shell structured PS/SiO₂ composite nanospheres were synthesized on the basis of a modified Stöber method. The mechanical properties of monodisperse nanospheres were characterized with nanoindentation on the basis of the atomic force microscopy (AFM). The surface morphologies of PS/SiO₂ composite nanospheres was scanned with the tapping mode of AFM, and the force–distance curves were measured with the contact mode of AFM. Different contact models were compared for the analyses of experimental data. The elastic moduli of PS/SiO₂ composite nanosphere (4–40 GPa) and PS nanosphere (∼3.4 GPa) were obtained with the Hertz and Johnson–Kendall–Roberts (JKR) models, respectively, and the JKR model was proven to be more appropriate for calculating the elastic modulus of PS/SiO₂ nanospheres. The elastic modulus of SiO₂ shell gradually approached a constant value (∼46 GPa) with the increase of SiO₂ shell thickness. A core–shell model was proposed for describing the relationship between PS/SiO₂ composite nanosphere’s elastic modulus and shell thickness. The mechanical properties of the composite nanospheres were reasonably explained on the basis of the growth mechanism of PS/SiO₂ composite nanospheres, in particular the SiO₂ shell’s formation process. Available research data of PS/SiO₂ composite nanospheres in this work can provide valuable guidance for their effective application in surface engineering, micro/nanomanufacturing, lubrication, and so on