Impact of Size and Substitution Isomerism in Polycyclic Aromatic-Substituted Trialkoxysilanes on the Formation of Softenable Polysilsesquioxanes

Abstract

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.