Thermomechanical properties of poly(methyl methacrylate)s containing tethered and untethered polyhedral oligomeric silsesquioxanes

Poly(methyl methaerylate)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 (phi less than or equal to 0.05) in PMMA in which much of the POSS filler resides in the matrix in a nanoseopically dispersed state. This well-dispersed POSS decreases the zero-shear-rate viscosity (eta(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 time-temperature 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 eta(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.