Rheological Behavior of Entangled Polystyrene−Polyhedral Oligosilsesquioxane (POSS) Copolymers

We report on the linear viscoelastic properties of a family of entangled linear thermoplastic nonpolar hybrid inorganic−organic polymers: random copolymers of polystyrene (PS) and styryl-based polyhedral oligosilsesquioxane (POSS), R 7 (Si 8 O 12 )(C 6 H 4 CH CH 2 ), with R = isobutyl ( i Bu). A series of styrene−styryl POSS random copolymers with 0, 6, 15, 30, and 50 wt % i BuPOSS were investigated. WAXS and TEM demonstrate that the i BuPOSS disperses in the polymeric matrix at a molecular level. It is observed that the i BuPOSS plays a plasticizer-like effect, yielding a monotonic decrease of the glass-transition temperature with increasing i BuPOSS content. Rheological measurements revealed that linear viscoelastic behavior of the copolymers is also profoundly influenced by the presence of i BuPOSS. The incorporation of i BuPOSS dramatically decreases the rubbery plateau modulus ( ), suggesting a strong dilation effect of isobutyl−POSS on entanglement density. Additionally, the apparent flow activation energy, obtained by fitting the Vogel−Fulcher−Tamman−Hesse equation, monotonically increases with increasing i BuPOSS content, indicating a lower sensitivity of POSS copolymers to changes of temperature. We attribute our observations to the microscopic topology of constituent polymer chains to be altered by i BuPOSS comonomers that act as compact volumetric branches. Conversely, intermolecular interactions between i BuPOSS and PS segments do not play an essential role in determining the rheological behavior.