Characterising the interfacial bonding in organic-inorganic hybrid materials from their thermal stability

Transparent hybrid materials that combine organic and inorganic components offer the possibility to obtain properties not found in conventional materials, such as simultaneous high toughness and high strength. Covalent bonding between the organic and inorganic phases is crucial to the performance and stability of sol-gel-based hybrid materials, which in turn can be achieved by using coupling agents. However, quantifying chemical coupling in hybrids is challenging due to the similarity between the reactive groups in the coupling agent and those in the organic components. Investigating the thermal stability of hybrid materials offers an alternative to assess chemical coupling, as polymers typically exhibit enhanced thermal stability when covalently bonded to stable inorganic entities such as silica. In this study, we evaluate the thermal stability of sol-gel hybrid materials based on tetraethylorthosilicate (TEOS), polyethylene glycol 200 (PEG200), and (3-Glycidyloxypropyl)-trimethoxysilane (GPTMS). The materials were analysed using thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) to determine the extent of interfacial covalent bonding between the polymer and silica networks as facilitated by the coupling agent GPTMS. The TGA results indicate a systematic increase in thermal stability with increasing GPTMS content, which is due to the covalent bonding of PEG200 to the silica network according to the FTIR results. We find that a 1:1:1 molar ratio of GPTMS, TEOS, and PEG200 yields the highest thermal stability enhancement for PEG200, where 36.8% of the organics decompose at a higher temperature compared to the native organic species. These findings demonstrate the link between the chemical structure of hybrid materials and their thermal properties as characterised using TGA.