Synthesis and comparison of the rheological and thermal properties of acyclic and polycyclic forms of polyphenylsilsesquioxane

Abstract In this work, we present a synthesis of a non-functional acyclic form of polyphenylsilsesquioxane (a-PPSQ) by the Piers–Rubinsztajn reaction of hyperbranched polyphenylethoxysiloxane (PPEOS) with dimethylphenylsilane. The polycyclic form of PPSQ (pc-PPSQ) was obtained by hydrolytic polycondensation of PPEOS in acetic acid followed by addition of dimethylphenylethoxysilane as the blocking agent. The rheological and thermal properties of acyclic and polycyclic forms of PPSQ were compared. All the PPSQs have high heat resistance (T5% weight loss) ranging from 405 to 480 °C in air and up to 500 °C in an inert environment. The glass transition temperature (Tg) of a-PPSQ is significantly lower (−34 °C) than in pc-PPSQs where Tg = 29–104 °C for pc-PPSQs with MW = 1400–5050. It was demonstrated that both a-PPSQ and pc-PPSQ offer Newtonian flow behavior. a-PPSQ with a molecular weight (MW) of 2150 has the lowest dynamic viscosity of ~0.38 Pa*s, whereas that of pc-PPSQ with MW = 1400 is ~1.8 Pa*s (at 150 °C in both cases). With an increase in pc-PPSQs molecular weight, the viscosity increases stepwise from ~1.05 * 104 to ~2.9 * 104 Pa*s (both at 150 °C) for pc-PPSQs with MW = 2860 and 3850, respectively. The viscous flow activation energies of pc-PPSQs with MW = 2860 and 3850 are 186 and 195 kJ/mol, respectively.