Atomic force microscopy indentation to determine mechanical property for polystyrene-silica core-shell hybrid particles with controlled shell thickness

The positively charged polystyrene (PS) particles with a size of ca. 200 nm were synthesized by soap-free polymerization. The PS cores were coated with silica shells of tunable thickness employing the modified Stöber method. The PS cores were removed by thermal decomposition at 500 °C, resulting in well-defined silica hollow spheres (10-30 nm in shell thickness). The elastic response of the as-synthesized samples was probed by an atomic force microscope (AFM). A point load was applied to the particle surface through a sharp AFM tip, and the force-displacement curves were recorded. Elastic moduli (E) for the PS particles (2.01 ± 0.70 GPa) and the core-shell structured hybrid particles were determined on the basis of Hertzian contact model. The calculated E values of composites exhibited a linear dependence on the silica shell thickness. While the shell thickness increased from ca. 10 to 15 and 20 nm, the E values of composites increased from 4.42 ± 0.27 to 5.88 ± 0.48 and 9.07 ± 0.94 GPa. For core-shell structured organic/inorganic composites, the E values of the hybrid particles were much lower than those of inorganic shells, while these values were much close to those of organic cores. Moreover, the moduli of elasticity of the composites appeared to be determined by the properties of the polymer cores, the species of inorganic shells and the thickness of shells. Besides, the inorganic shells enhanced the mechanical properties of the polymer cores. This work will provide essential experimental and theoretical basis for the design and application of core-shell structured organic/inorganic composite abrasives in chemical mechanical polishing/planarization.