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), R₇(Si₈O₁₂)(C₆H ₄CH=CH₂), with R = isobutyl (ⁱBu). A series of styrene-styryl POSS random copolymers with 0, 6, 15, 30, and 50 wt% ⁱBuPOSS were investigated. WAXS and TEM demonstrate that the ⁱBuPOSS disperses in the polymeric matrix at a molecular level. It is observed that the ⁱBuPOSS plays a plasticizer-like effect, yielding a monotonic decrease of the glass-transition temperature with increasing ⁱBuPOSS content. Rheological measurements revealed that linear viscoelastic behavior of the copolymers is also profoundly influenced by the presence of ⁱBuPOSS. The incorporation of _iBuPOSS dramatically decreases the rubbery plateau modulus (G_N⁰), 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 ⁱBuPOSS 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 ⁱBuPOSS comonomers that act as compact volumetric branches. Conversely, intermolecular interactions between ⁱBuPOSS and PS segments do not play an essential role in determining the rheological behavior.