PoSs

Polyhedral oligomeric silsesquioxane (POSS) is a class of nanoscale inorganic–organic hybrid compounds used as building blocks in materials science. POSS compounds consist of a silica-like silicon–oxygen core in the form of a cubic or polyhedral cage, typically Si8O12, with eight silicon vertices. Each vertex bears an organic substituent R, giving the general formula (RSiO1.5)8 for a fully capped cage. The external R groups can be alkyl, aryl, or functionalized moieties, enabling tuning of solubility, compatibility with polymers, and reactivity.

Most common are octa-substituted cages with eight R groups, though mono- or multi-substituted variants exist. The cage is discrete rather than part of a continuous network, which makes POSS molecules relatively uniform in size (about 0.5 to 3 nanometers) and highly versatile as nanoscale building blocks.

Synthesis of POSS is typically achieved by hydrolysis and condensation of organosilane precursors or by condensation of silsesquoxane units under controlled conditions to yield discrete cages; careful control is required to prevent gelation and unwanted networks.

Properties of POSS include a rigid inorganic core, high thermal stability, and tunable solubility and reactivity through the chosen R groups. The cage can be covalently bonded to polymers, act as a crosslinking point, or serve as a discrete nanofiller. These characteristics enable POSS to modify the processing, mechanical, thermal, and barrier properties of materials.

Applications span polymer nanocomposites, coatings, adhesives, and electronics. POSS units can improve mechanical strength, thermal stability, and weathering resistance while maintaining optical transparency in many formulations and enabling surface modification of materials.

See also silsesquioxane, organosilicon compounds, polymer nanocomposites.