Thermomechanical Properties of Poly(methyl methacrylates) Containing Tethered and Untethered Polyhedral Oligomeric Silsesquioxanes (POSS)

摘要

Poly(methyl methacrylates) (PMMA) containing both tethered and untethered polyhedral oligomeric silsesquioxanes (POSS) were investigated using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and rheological characterization. Unfilled, entangled polymers were synthesized and tested in small amplitude oscillatory shear. The addition of tethered-POSS to the PMMA chain leads to a decrease in the plateau modulus (GN0) as expected from previous results on POSS-polymer rheology. Cyclohexyl-POSS and isobutyl- POSS were blended with PMMA homopolymer, and isobutyl-POSS was also blended with a POSS-PMMA copolymer containing 25 wt% tethered isobutyl-POSS distributed randomly along the chain. Both DSC and rheological results suggest a regime at low untethered-POSS loadings ( 5 vol%) in PMMA in which much of the POSS filler resides in the matrix in a nanoscopically-dispersed state. This well-dispersed POSS acts as a plasticizer and leads to a decrease in the zero-shear-rate viscosity ( 0) at low loadings. Above this regime, an apparent solubility limit is reached at which point additional untethered-POSS aggregates into crystallites in the PMMA matrix and both the viscosity and the plateau modulus increase in a way consistent with classical predicitions 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 and therefore could not explain the decrease in viscosity in light of an increase in free volume. Blends of untethered-POSS with copolymer show a significant increase in 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 i.