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
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摘要:我们研究了较大长径比碳纳米管（CNT）非平衡态网络结构在流动场诱导下对等规聚丙烯（iPP）/碳纳米管复合材料的性能变化作用。由于引入了大长径比的CNTs，在较低CNT含量下CNT/iPP纳米复合物即可形成CNT网络。Kharchenko等人通过实验证明和理论推断得知在某些特定情况，如在满足CNT网络足够强大以致能容纳较大的旋转畸变时，大且明显的负法向应力差（△N）可能出现在CNT复合体系中。在本研究中，我们主要通过对负法向应力差的测量来探索负△N出现的特定情况。我们发现在测试过程中施加的挤压变形所引起的初始法向应力会极大地影响测量的△N值。与Kharchenko等人的研究结果一致，只有在CNT含量超过CNT凝胶化浓度时才可以在先将法向应力归零后施加剪切场的大长径比CNT/iPP纳米复合体系中观察到幅度大的负△N。 这种对于△N测试的有趣结果继而通过CNT/iPP纳米复合体系的挤出收缩以及iPP挤出中表现的不稳定形状畸变效应在CNT/iPP纳米复合体系中能够显著减弱而得到了进一步的证实。
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.