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

摘要

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)(C6H4CH=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_N~o),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.