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)(C6H4CHdCH2), 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 (GN o ), suggesting a strong dilation effect of isobutyl-POSS on entanglement density. Additionally, the apparent flow activation energy, obtained by fitting the Vogel-Fulcher-TammanHesse 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.