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
志剛等在此感謝RSC Advances期刊編輯和審稿人對于我們研究工作的理解和支持！
感謝?。?/strong>
摘要:我們研究了較大長徑比碳納米管（CNT）非平衡態(tài)網(wǎng)絡(luò)結(jié)構(gòu)在流動場誘導(dǎo)下對等規(guī)聚丙烯（iPP）/碳納米管復(fù)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(fù)合物即可形成CNT網(wǎng)絡(luò)。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(wǎng)絡(luò)足夠強大以致能容納較大的旋轉(zhuǎn)畸變時，大且明顯的負法向應(yīng)力差（△N）可能出現(xiàn)在CNT復(fù)合體系中。在本研究中，我們主要通過對負法向應(yīng)力差的測量來探索負△N出現(xiàn)的特定情況。我們發(fā)現(xiàn)在測試過程中施加的擠壓變形所引起的初始法向應(yīng)力會極大地影響測量的△N值。與Kharchenko等人的研究結(jié)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(yīng)力歸零后施加剪切場的大長徑比CNT/iPP納米復(fù)合體系中觀察到幅度大的負△N。 這種對于△N測試的有趣結(jié)果繼而通過CNT/iPP納米復(fù)合體系的擠出收縮以及iPP擠出中表現(xiàn)的不穩(wěn)定形狀畸變效應(yīng)在CNT/iPP納米復(fù)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.