We consider the impact of varying the aspect ratio A of carbon nanotubes (CNT) on the rheological properties of isotactic polypropylene/CNT nano- composites. Specifically, we focus on multi-wall CNT having a relatively low aspect ratio A (in the range from 28 to 56), since previous work (Kharchenko et al., Nature Materials, 2004) has emphasized the rhelogical properties of relatively high aspect ratio multi-wall CNT nanocomposites (A from 300 to 400). The proposed mechanism for the large apparent negative normal stress differences N described by Kharchenko et al. relies on the capacity of the struts of the CNT network to rotate about their impingement junctions, much like the links of a deformed chain link fence.This model implies the absence of an appreciable negative N at low CNT concentrations where the network does not yet exist and also for short CNT and at high concentrations of CNT where the mesh size of the network becomes too small to accommodate appreciable rotational distortion. In conformity with this simple mechanical model, we observe only a positive apparent N in our CNT/iPP nanocomposites, even well above the CNT gelation concentration. This striking change in the rheology of CNT nanocomposites with a change in carbon naotube aspect ratio has been further confirmed in die swell measurements, where a large die swell has been seen in the short CNT nanocomposites, rather than the die shrinkage found before for the large A or the highly entangled network counterpart.
Pub Type: Journals
aspect ratio network formation, carbon nanotubes, die swell, entanglement, fluid flow, negative normal stresses, non-equilibrium material