Hollow Silica Spheres: Synthesis and Mechanical Properties

Abstract

Core-shell polystyrene-silica spheres with diameters of 800 nm and 1.9 microm were synthesized by soap-free emulsion and dispersion polymerization of the polystyrene core, respectively. The polystyrene spheres were used as templates for the synthesis of silica shells of tunable thickness employing the Stöber method [Graf et al. Langmuir 2003, 19, 6693]. The polystyrene template was removed by thermal decomposition at 500 degrees C, resulting in smooth silica shells of well-defined thickness (15-70 nm). The elastic response of these hollow spheres was probed by atomic force microscopy (AFM). A point load was applied to the particle surface through a sharp AFM tip, and successively increased until the shell broke. In agreement with the predictions of shell theory, for small deformations the deformation increased linearly with applied force. The Young's modulus (18 +/- 6 GPa) was about 4 times smaller than that of fused silica [Adachi and Sakka J. Mater. Sci. 1990, 25, 4732] but identical to that of bulk silica spheres (800 nm) synthesized by the Stöber method, indicating that it yields silica of lower density. The minimum force needed to irreversibly deform (buckle) the shell increased quadratically with shell thickness.

High Frequency Acoustics in Colloid-Based Meso- and Nanostructures by Spontaneous Brillouin Light Scattering

“…The synthesis of the core-shell particles by soap-free emulsion polymerization of the PS core (slightly copolymerized with divinylbenzene) and subsequent coating by silica is described elsewhere. [109] Here, it is shown that a relatively thin SiO 2 shell acts as a 'nano-armor' of the polymer core even in the molten state above its glass transition temperature while the particles retaining remarkably high mechanical strength.…”

“…[106,107] Furthermore, silica shells are much stiffer compared to most organic shells. [108,109] Here the rigidity of thin glassy silica shells is utilized for the realization of shape persistent polymer cores at temperatures well above the polymer glass transition temperature T g . In this section BLS is employed to investigate the shape-persistence of spherical core-shell colloids consisting of a soft polystyrene (PS) core with d=400±12 nm and a hard silica (SiO 2 ) nanoshell with thickness L=37±3 nm against heating; this size was determined by scanning electron microscopy, averaging over about 100 spheres.…”

“…Therefore they started with the soap-free emulsion polymerization of the polymer core and used them as templates for the synthesis of the silica shell employing the Stöber method. [109,128] As the details are clearly and completely given in the mentioned literature, here I just want to introduce the main principles of the synthesis.…”

Characterisation of hollow Russian doll microspheres

“…The sacrificial core process is one of the most frequently used techniques to produce hollow inorganic spheres [1][2][3][4][5][6][7][8][9][10][11][12]. The sacrificial core approach involves the use of a solid organic or inorganic spherical particle as the template.…”

“…The core-shell structure is produced by coating the core with inorganic precursors or nanoparticles which are crosslinked to form the shell. Hollow spherical particles are subsequently generated by removing the template from the core-shell structure thermally [1][2][3][4][5][6][7][8][9][10] or chemically [1-3, 11, 12].…”