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摘要: 我們研究了較大長徑比CNT和等規聚丙烯（iPP）納米復合材料中類非平衡態碳納米管（CNT）網絡結構在流動誘導性質變化中所起的作用。由于引入了大長徑比的CNTs， CNT/iPP納米復合物在較低的CNT含量下即可形成穩定的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）會出現于含長的CNT體系中(Nat. Mater., 2004)。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。我們發現在加樣過程中擠壓形變所引起的初始法向應力會極大影響△N值。與Kharchenko等人研究結果一致，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再施加剪切場的大長徑比CNT/iPP納米復合體系中觀察到大的負△N。這種對于△N測試的有趣結果繼而通過擠出中表現的擠出收縮和iPP擠出不穩定形狀畸變效應在CNT/iPP納米復合體系中能夠消弱而得到了進一步證實。

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.