Ms. Jingjing Yang et al.published No. 130 paper of our group in RSC Advances (2014, 4, 1246-1255)
Influences of high aspect ratio
carbon nanotube network on normal stress difference measurements and
extrusion behaviors for isotactic polypropylene nanocomposite melts
RSC Advances, 2014, 4, 1246-1255
We have investigated the role that representative nonequilibrium carbon
nanotube (CNT) network structures play in the flow-induced property
changes of relatively high aspect ratio CNT/isotactic polypropylene
(iPP) nanocomposites. Stable CNT network can form in CNT/iPP
nanocomposites at relatively low CNT contents due to the employment of
high aspect ratio CNTs. As experimentally demonstrated and theoretically
predicted by Kharchenko et al. (Nat. Mater., 2004), large apparent negative normal stress differences (ΔN)
might occur in long CNT systems under some particular conditions, for
example, which satisfy the condition that the CNT network is strong
enough to accommodate appreciable rotational distortion. In this study,
we focused on the measurements of normal stress differences to explore
the particular conditions for appearance of negative ΔN. We find that the values of ΔN are immensely influenced by initial normal stresses induced by
squeezing deformation during the sample loading. Large negative ΔN can be observed under the subsequent shear flow with zeroing the
initial normal stresses in our high aspect ratio CNT/iPP nanocomposites
only if the CNT content is above the CNT gelation concentration, which
is consistent with the previous studies of Kharchenko et al. This interesting result about the measurements of ΔN is further confirmed by a die-shrinkage extrusion behavior and at the
same time the effect of shape-distortion instability in the extrusion of
neat iPP can be diminished in the CNT/iPP nanocomposites.