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The mechanical relaxation behavior and microstructure of a series of novel norbornyl-POSS organic-inorganic copolymers have been investigated. We have examined the influence on physical properties of both the weight fraction of POSS-norbornyl monomer and the corner group composition. POSS refers to the polyhedral oligomeric silsesquioxane inorganic/organic macromer, which is composed of an inorganic Si 8O12 spherical core surrounded by seven inert organic corner groups and one reactive norbornyl moiety. It was observed that POSS copolymerization enhances the R-relaxation temperature, TR, in proportion to the weight fraction of the POSS-norbornyl comonomer. Interestingly, however, the magnitude of this dependence is larger for the POSS-norbornyl comonomer possessing cyclohexyl corner groups (CyPOSS) than for the copolymer with cyclopentyl corner groups (CpPOSS). Although POSS copolymerization yields only slight enhancement of the room temperature storage modulus, at temperatures lower than a strong mechanical relaxation, identified as a -relaxation, and near T ) -

Poly(methyl methacrylate)s (PMMA) containing both tethered and untethered polyhedral oligomeric silsesquioxanes (POSS) were examined through the use of wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and rheological characterization. The presence of tethered-POSS in entangled copolymers leads to a decrease in the plateau modulus (G N 0 ) when compared with PMMA homopolymer. Two untethered-POSS fillers, cyclohexyl-POSS and isobutyl-POSS, were blended with PMMA homopolymer. Both DSC and rheological results suggest a regime at low untethered-POSS loadings (φ e 0.05) in PMMA in which much of the POSS filler resides in the matrix in a nanoscopically dispersed state. This well-dispersed POSS decreases the zero-shear-rate viscosity (η 0). Above this regime, an apparent solubility limit is reached, and beyond this point additional untethered-POSS aggregates into crystallites in the PMMA matrix. These crystallites cause both the viscosity and the plateau modulus to increase in a way consistent with classical predictions for hard-sphere-filled suspensions. The principles of timetemperature superposition are followed by these nanocomposites; however, fits to the WLF equation show no strong trend with increasing POSS loading. Isobutyl-POSS was also blended with a POSS-PMMA copolymer containing 25 wt % tethered isobutyl-POSS distributed randomly along the chain. Blends of untethered-POSS with copolymer show a significant increase in η 0 for all loadings, greater than that expected for traditional hard-sphere fillers. This is a result of associations between untethered-POSS and tethered-POSS cages in the blend, which retard chain relaxation processes in a way not observed in either the homopolymer blends or the unfilled copolymers. Time-temperature superposition also holds for the filled copolymer system, and these blends show a strong increase in the WLF coefficients, suggesting that both free volume and viscosity increase with filler loading.

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 ( ), 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.