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摘要:我們研究了較大長徑比碳納米管（CNT）非平衡態網絡結構在流動場誘導下對等規聚丙烯（iPP）/碳納米管復合材料的性能變化作用。由于引入了大長徑比的CNTs，在較低CNT含量下CNT/iPP納米復合物即可形成CNT網絡。Kharchenko等人通過實驗證明和理論推斷得知在某些特定情況，如在滿足CNT網絡足夠強大以致能容納較大的旋轉畸變時，大且明顯的負法向應力差（△N）可能出現在CNT復合體系中。在本研究中，我們主要通過對負法向應力差的測量來探索負△N出現的特定情況。我們發現在測試過程中施加的擠壓變形所引起的初始法向應力會極大地影響測量的△N值。與Kharchenko等人的研究結果一致，只有在CNT含量超過CNT凝膠化濃度時才可以在先將法向應力歸零后施加剪切場的大長徑比CNT/iPP納米復合體系中觀察到幅度大的負△N。 這種對于△N測試的有趣結果繼而通過CNT/iPP納米復合體系的擠出收縮以及iPP擠出中表現的不穩定形狀畸變效應在CNT/iPP納米復合體系中能夠顯著減弱而得到了進一步的證實。

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.