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Polymer chain properties and thermodynamic stability in oriented-platelet nanocomposites.

We present a Monte Carlo study of the conformational properties of polymer chains in platelet nanocomposites. We find that high platelet orientation leads to an increase in chain confinement and to a substantial decrease in polymer entropy. The latter may be responsible for the observed aging of these systems in which the platelets gradually lose their orientation with time. As a test of our entropy-driven hypothesis, we suggest that aging could be substantially slowed down by the addition of small spherical nanoparticles. Using a generalized Stokes-Einstein relation, a calculation of the thermal random motion of the platelets predicts a fivefold increase in shear modulus during aging, in agreement with experimental observation on compatibilized layered silicate nanocomposites.