Thermal properties of Polyhedral oligomeric silsesquioxanes (POSS)

The presence of Si–O bonds in the central core of POSS imparts superior thermal, mechanical, and chemical stability, which makes these compounds extremely interesting from an industrial point of view. From the many valuable properties of POSS the ones that are responsible for improvement of oxidation resistance of polymers doped with POSS are of special interest. From this reason a deeper insight into thermal properties and their dependence on the POSS structure is highly needed. Already in 1997 Bolln et al. studied thermal properties of alkyl substituted T₈ POSS cages, which showed that their thermal stability increases with longer chain length (from 166 °C for octapropyl up to 355 °C for octadecyl derivatives). He also showed that the degradation processes performed in dinitrogen or in oxidative environment differ signifi­cantly. POSS degradation studies concentrating on the gas and char analysis were undertaken by Mantz et al. and gave an insight into details of the POSS degradation process. Lately, Zeng et al. reported the thermal degradation of octyl trisilanol POSS including isothermal conditions. Data showed a single step weight loss (between 250 °C and 530 °C) which led to only 8% residue. Interestingly, under isothermal conditions at 280 °C they obtained a viscous liquid due to silanol group condensation.

Many fascinating data were obtained by A. Fina et. al. They for instance studied in details the thermal degradation of octaisobutyl POSS. Using thermogravimetry they showed that under N₂ there is an evaporation of POSS leading to an almost complete weight loss independently on the heating rate. The same POSS in oxidizing atmosphere left off residue which amount was strongly related to the heating rate. Its analysis by vibrational spectroscopy and powder X-ray diffraction showed a decrease of organic moieties when the treatment temperatures were increased and the compound conversed to amorphous silica by heating up to 800 °C. Moreover they studied thermal degradation of several other substituted POSS systems including polyvinyl silsesquioxane. Interesting results on unsymmetrically substituted POSS were obtained by Blanco et. al. Apart from thermal data on well-definied POSS structures, there is various reports on the thermal degradation of silsesquioxane homopolymers and copolymers.

Thermooxidative decomposition of amide POSS 2 and 6-X (Journal of Organometallic Chemistry 2017,4 87, 173-183)

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