The influence of chain length on the thermal and rheological behavior of hairy nanoparticles is investigated using two matrix-free polystyrene-grafted silica nanocomposite samples with different graft chain lengths. Results are compared to those for neat polystyrenes of comparable molecular weights. The glass transition temperature Tg of the nanocomposites is found to be approximately 1 to 2 K higher than the neat materials, whereas the absolute heat capacity is approximately 4 - 7 % lower. The step change in heat capacity ΔCp at Tg, is 15 % lower for the nanocomposites, consistent with an immobilized glassy layer of approximately 2 nm. The linear viscoelastic behavior of the nanocomposite samples differs significantly compared to their neat analogs in several ways: first, the G' vs. ω curves shift towards lower frequencies by approximately one decade due to the increase in the glass transition temperature; second, terminal flow behavior is absent; third, the glassy modulus increases in a manner consistent with hydrodynamic reinforcement; and fourth, the rubbery plateau moduli (GNo) increases for long grafts and decreases for short unentangled grafts. The magnitude of rubbery modulus is attributed to two effects, hydrodynamic reinforcement and a change in the effective entanglement density which is governed by corona interpenetration, or lack thereof, coupled with the silica particles acting as physical entanglement points. The use of nanocomposites to address global energy challenges will also be addressed.

Characterization of grafted nanocomposites

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