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 investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d ≈ 2 nm] and poly(methyl methacrylate)(PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with Kopesky et al.3

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Two distinct oligomeric species of similar mass and chemical functionality (M w ≈ 2,000 g/mol), one a linear methyl methacrylate oligomer (radius of gyration R g ≈ 1.1 nm) and the other a hybrid organic-inorganic polyhedral silsesquioxane nanocage (methacryl-POSS, r ≈ 1.0 nm), were subjected to thermal and rheological tests to compare the behaviors of these geometrically dissimilar molecules over the entire composition range. The glass transition temperatures of the blends varied monotonically between the glass transition temperatures of the pure oligomer (T g = −47.3°C) and the pure POSS (T g = −61.0°C). Blends containing high POSS contents (with volume fraction φ POSS ≥ 0.90) exhibited enhanced enthalpy relaxation in DSC measurements, and the degree of enthalpy relaxation was used to calculate the kinetic fragility indices m of the oligomeric MMA (m = 59) and the POSS (m = 74). The temperature dependences of the viscosities were fitted by the free volume-based WLF-VFT framework and a dynamic scaling relation. The calculated values of the fragility from the WLF-VFT fits were similar for the POSS (m = 82) and for the oligomer (m = 76), and the dynamic scaling exponent was similar for the oligomeric MMA and the POSS. Within the range of known fragilities for glass-forming liquids, the temperature dependence of the viscosity was found to be similarly fragile for the two species. The difference in shape of the nanocages and oligomer chains is unimportant in controlling the glass-forming properties of the blends at low volume fractions (φ POSS < 0.20); however, at higher volume fractions, adjacent POSS cages begin to crowd each other, leading to an increase in the fractional free volume at the glass transition temperature and the observed enhanced enthalpy relaxation in DSC.