Polystyrene-Grafted Silica Nanoparticles: Investigating the Molecular Weight Dependence of Glass Transition and Fragility Behavior

Polymer-tethered nanoparticles provide a strategy to improve particle dispersion in polymer nanocomposites and as materials themselves can exhibit self-healing behavior and enhanced mechanical properties. The few studies that previously characterized the glass transition temperature (T_g) behavior of neat polymer-grafted nanoparticles in the absence of a polymer matrix largely focused on average T_g response. We synthesized polystyrene-grafted silica nanoparticles (Si-PS) via ARGET ATRP, achieving the densely grafted state. Using differential scanning calorimetry, we investigated the brush molecular weight (MW) dependence of T_g, T_g breadth, heat capacity jump (ΔC_p), and fragility from 12 to 98 kg/mol. Compared with free PS chains of the same MW, brush T_g increases by 1-2 °C, brush T_g breadth remains unchanged within error down to 36 kg/mol and increases by 3-4 °C at brush MWs of 12 and 13 kg/mol, and brush ΔC_p and fragility remain unchanged within error down to 52 kg/mol and then decrease with decreasing MW. Evidence of a significant T_g gradient from near the nanoparticle graft interface to near the free chain end was obtained for the first time via fluorescence of a pyrenyl dye labeled at specific regions along the brush chain length. In relatively high MW brushes, T_g = ∼116 °C near the graft interface and T_g = ∼102 °C near the chain end. Comparisons are made to results recently reported for similar PS brushes densely grafted to a flat substrate, which indicate that a larger T_g gradient is evident in a grafting geometry involving a flat interface as compared with a spherical nanoparticle interface. Other comparisons are also made with glass transition and fragility behaviors reported in the flat substrate geometry. Results of this study and others will help to better understand nanocomposites and tailor them for optimal properties.