posted on 2020-03-09, 14:38authored byNazam Sakib, Yung P. Koh, Yucheng Huang, Katrina Irene S. Mongcopa, Amy N. Le, Brian C. Benicewicz, Ramanan Krishnamoorti, Sindee L. Simon
Two
matrix-free polystyrene-grafted silica nanocomposite samples
with graft chain lengths of 35 and 112 kg/mol are characterized by
calorimetry and rheometry, and results are compared to 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 that of the neat materials, whereas the absolute
heat capacity is approximately 4–7% lower in the glassy and
liquid states. 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′ versus ω curves shift
toward lower frequencies by approximately one decade due to the increase
in the glass transition temperature; second, terminal flow behavior
is absent; third, the rubbery plateau moduli (GN°) decreases by 7% for the 35 kg/mol grafted particles
and increases by approximately two and a half-fold for the 112 kg/mol
grafted particles; and fourth, the glassy modulus increases approximately
4% consistent with hydrodynamic reinforcement. On the other hand,
the magnitude of the rubbery modulus is attributed to two effects,
hydrodynamic reinforcement and a change in the effective entanglement
density, which is governed by corona interpenetration coupled with
the silica particles acting as physical entanglement points.