Phase separation in melting gels

Melting gels are a special class of sol-gel processed organically-modified silicates. For example, phenyltrimethoxysilane (PhTMS) and diphenyl-dimethoxysilane (DPhDMS) mixtures, as well as phenyltriethoxysilane (PhTES) and diphenyldiethoxysilane (DPhDES) mixtures, produce melting gels. PhTMS by itself produces a melting gel, while DPhDMS does not. The thermal behaviour of melting gels is that they are rigid at room temperature and flow around 100°C. The phenyl based melting gels exhibit a glass transition temperature around room temperature. Small additions of tetrafunctional siloxane (tetramethylorthosilicate (TMOS) or tetraethylorthosilicate (TEOS)) lead to slight increases in the glass transition temperature, but large additions result in phase separation and prevent melting gel behaviour. The role of the tetrafunctional siloxane, in some cases, is to accelerate the sol-gel reactions, and in other cases, to increase dilution. Either by addition of tetrafunctional siloxane or treatment at a higher temperature, additional crosslinking converts the melting gels into organic-inorganic hybrid glasses.