Impact of Sizeand Substitution Isomerism in PolycyclicAromatic-Substituted Trialkoxysilanes on the Formation of SoftenablePolysilsesquioxanes

Polyphenylsilsesquioxanes are known to form glassy materials that can reversibly soften when heated above their glass transition temperature, with irreversible curing occurring upon a further temperature increase. In this study, the effects of the size and isomerism of polycyclic aromatic groups on the synthesis and structure of polysilsesquioxanes are investigated, with a focus on their thermoplastic and thermoset properties. Polysilsesquioxanes were synthesized by acid-catalyzed polycondensation using 1-naphthyl, 2-naphthyl, and 9-phenanthrenyltrimethoxysilanes. Characterization techniques, including spectroscopy, thermal analysis, mass determination, and powder X-ray diffraction, showed that steric hindrance by the aromatic groups significantly affects the degree of condensation, the formation of OH groups, and the nature of intra- or intermolecular condensation. Bulky phenanthrenyl groups hinder chain mobility and prevent detectable flow behavior, while 1- and 2-naphthyl groups enable the formation of thermoplastic materials with reversible softening. Notably, 2-naphthylsilsesquioxane undergoes irreversible curing at 200 °C, whereas 1-naphthylsilsesquioxane resists this transition. The incorporation of both polycyclic substituents not only preserves the characteristic thermoplastic behavior of melting gels but also introduces additional properties, such as fluorescence, a high thermal stability up to 460 °C and a high refractive index of 1.61, enhancing the potential of these materials for optical applications.