Dispersal of a relatively small concentration [O(1%) volume fraction] of multiwall carbon nanotubes (MWNT) into polypropylene (PP) is found to cause large and complex changes in nanocomposite transport properties. Specifically, both the shear viscosity h(g) and electrical conductivity s (g) of the MWNT nanocomposites decrease strongly with increasing shear rate and, moreover, these dispersions exhibit impressively large and negative normal stress differences. The observation of negative normal stresses is a rarely reported phenomenon in soft condensed matter and can be expected to lead to many dramatic and practically important effects under processing conditions (e.g., absence of die swell in extrusion, modification of sharkskin and shear banding instabilities, droplet distortion and thread break-up processes under flow, etc.). In the present letter, we report on the shrinkage rather than swelling of the MWNT nanocomposite extrudates under low Reynolds number flow conditions, evidencing unusual flow properties in these materials. We associate these large flow-induced property changes with the formation of non-equilibrium nanotube network structure, which leads to viscolelastic properties characteristic of a ‘jammed solid’.

Sam Kharchenko, PhD

Research Scientist

Processing Characterization Group

Polymers Division

Stop 8544

Ph.: (301) 975-4441 Fax: (301) 975-4924

E-mail: semen.kharchenko@nist.gov

URL: www.nist.gov/polymers