(307b) Negative Normal Stresses in Polymer Nanocomposites

Polymer nanocomposites containing multi-walled carbon nanotubes (MWNTs) have been reported to show the remarkable property of negative normal stresses in their melt rheology (Kharchenko et al., Nature Materials, 2004). In this study, we investigate in detail the occurrence of negative normal stresses in polystyrene/MWNT nanocomposites prepared by melt-compounding. Consistent with earlier findings, we observe negative normal stresses in samples with high MWNT loadings, where a jammed network of nanotubes is present. However, we find that the normal stresses are very sensitive to the flow history and show unusual trends in transient rheological experiments. Moreover, the sign and magnitude of the normal stresses are also influenced by sample annealing at elevated temperatures. To determine whether negative normal stresses are ubiquitous in jammed particle networks, we also study the melt rheology of polystyrene/organoclay nanocomposites at high organoclay loadings. Taken together, the results will be used to construct a physical picture detailing the origin of negative normal stresses in these polymeric materials.