(708a) Rapid Exfoliation and Physical Gelation of a Clay-Polymer Nanocomposite

A new exfoliation mechanism is proposed for organically modified clay (layered silicate) in a matrix of end-functionalized (“sticky”) macromolecules. The evolving clay structure gives rise to physical gelation which can be monitored rheologically. When mixing the clay particles into the polymer, sticky macromolecules anchor on the outer clay surfaces and generate an entopic pulling force that peels clay sheets away from the stack. The molecular force is due to the restricted free energy in the presence of the solid surface. The assembly of anchored macromolecules (brush) on the outside of clay particles can overcome the internal cohesion since the organically modified clay sheets are only weakly connected with each other. The entropic force increases with temperature due to increased thermal motion, but then decreases again when macromolecules begin to detach at high temperature. The experimental protocol includes time-resolved rheometry (Rheol Acta 33:385-397, 1994) and rescaling of data (Rheol Acta 45:331-338, 2006). Surprisingly, the product of equilibrium modulus and longest relaxation time, G_e(t_r)l_max(t_r), was found to remain constant throughout the late stages of physical gelation. Parameter is the duration of the isothermal ripening process (“ripening time”, t_r). Acknowledgment:  NSF support through CBET-0651888.