摘要: 我们研究了较大长径比CNT和等规聚丙烯（iPP）纳米复合材料中类非平衡态碳纳米管（CNT）网络结构在流动诱导性质变化中所起的作用。由于引入了大长径比的CNTs， CNT/iPP纳米复合物在较低的CNT含量下即可形成稳定的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）会出现于含长的CNT体系中(Nat. Mater., 2004)。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。我们发现在加样过程中挤压形变所引起的初始法向应力会极大影响△N值。与Kharchenko等人研究结果一致，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再施加剪切场的大长径比CNT/iPP纳米复合体系中观察到大的负△N。这种对于△N测试的有趣结果继而通过挤出中表现的挤出收缩和iPP挤出不稳定形状畸变效应在CNT/iPP纳米复合体系中能够消弱而得到了进一步证实。

Abstract: 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.