Thermal Stability and Degradation of POSS Cages
Polyhedral oligomeric silsesquioxanes (POSS) possess a unique inorganic–organic hybrid architecture in which a rigid Si–O–Si cage is covalently bonded to peripheral organic substituents. The presence of Si–O bonds in the central core imparts exceptional thermal, mechanical, and chemical stability, making these compounds highly attractive from an industrial standpoint. Among the many technologically relevant properties of POSS, their ability to improve the oxidation resistance of POSS-doped polymer matrices has received particular attention. For this reason, a thorough understanding of the thermal behavior of POSS and its dependence on T8 and T10 silsesquioxane cage structure and organic functionalization has become an important research objective in the field of organosilicon chemistry.
Topics covered in this post: thermal stability of POSS cage compounds and the influence of organic substituents on decomposition temperature; thermogravimetric analysis under inert and oxidative atmospheres; char and residue analysis; thermal degradation of alkyl-, isobutyl-, and unsymmetrically substituted POSS derivatives; conversion of POSS to amorphous silica at elevated temperatures; thermal behavior of silsesquioxane homopolymers and copolymers.
Thermal Stability and Substituent Effects
The presence of Si–O bonds in the central core of POSS imparts superior thermal, mechanical, and chemical stability, making these compounds exceptionally interesting from an industrial perspective. Among the many valuable properties of POSS, those responsible for the improvement of oxidation resistance in POSS-doped polymers are of particular relevance. For this reason, detailed insight into the thermal properties of POSS and their dependence on cage structure is highly warranted. As early as 1997, Bolln et al. studied the thermal properties of alkyl-substituted T8 POSS cages and demonstrated that thermal stability increases with longer chain length, ranging from 166 °C for octapropyl derivatives up to 355 °C for octadecyl analogs.18 They also showed that degradation processes performed in dinitrogen or in an oxidative environment differ significantly. Subsequent POSS degradation studies focused on gas and char analysis were undertaken by Mantz et al., who provided detailed insight into the mechanistic aspects of the POSS degradation process.19 More recently, Zeng et al. reported the thermal degradation of octyl trisilanol POSS under both dynamic and isothermal conditions.20 The data revealed a single-step weight loss occurring between 250 °C and 530 °C, leaving only 8% residue. Notably, under isothermal conditions at 280 °C, a viscous liquid was obtained as a consequence of silanol group condensation.
Thermogravimetric Studies and Degradation Mechanisms
Char Formation and Residue Analysis under Oxidative and Inert Atmospheres
Significant contributions to this area were made by A. Fina et al., who investigated in detail the thermal degradation of octaisobutyl POSS.21,22 Using thermogravimetry, they demonstrated that under N2, evaporation of POSS occurs, leading to an almost complete weight loss independently of the heating rate. The same compound in an oxidizing atmosphere yielded a residue whose amount was strongly dependent on the heating rate. Analysis by vibrational spectroscopy and powder X-ray diffraction revealed a progressive decrease of organic moieties as treatment temperatures were increased, and the compound was ultimately converted to amorphous silica upon heating to 800 °C. Furthermore, they examined the thermal degradation of several other substituted POSS systems, including polyvinyl silsesquioxane. Notable results on unsymmetrically substituted POSS were reported by Blanco et al.23,24 In addition to data on well-defined discrete POSS structures, a substantial body of literature exists on the thermal degradation of silsesquioxane homopolymers and copolymers. Methods for deriving kinetic parameters from thermal analysis curves are discussed separately in an overview of DTA reaction kinetics.
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DOI: 10.1021/cm970090f
Full text: Chemistry of Materials → ACS Publications
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Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry. (2018). Thermal Stability and Degradation of POSS Cages. https://doi.org/10.59350/34er6-ady20
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