Effect of platelet size in a soft nanocomposite: Physical gelation and yielding

We investigated the role of the platelet size in the viscoelasticity of a soft solid nanocomposite, which consisted of exfoliated clay in a liquid polymer matrix. The internal connectivity of the nanocomposite reduced significantly when made up of smaller platelets. This expresses itself in a lowering of the modulus and an increase in relaxation times. Sample preparation played a critical role in these experimental findings. Two liquid polybutadienes served as matrix fluids, a nonfunctionalized polybutadiene (PB), which is a noninteracting liquid, and a carboxyl terminated polybutadiene (sPB), which caused the clay to exfoliate. Unexfoliated clay particles were suspended in PB and treated to high-intensity chaotic flow in a planetary mixer for size reduction. The size-reduced clay particles were exfoliated through the addition of sPB followed by annealing at an elevated temperature. The exfoliated clay in the 50/50 PB/sPB blend formed a percolating network, a physical gel. The resulting soft solid was characterized before and after having been sheared above its yield strain. Yielding caused the already soft physical gel to soften even further by irreversibly reducing its internal connectivity. The samples comprised of large platelets softened but still remained a solid after yielding, while nanocomposites made up of smaller platelets underwent an irreversible solid to liquid transition during yielding.