130. Influences of high aspect ratio carbon nanotube network on normal stress difference measurements and extrusion behaviors for isotactic polypropylene nanocomposite melts
writer：Jingjing Yang,Yaqiong Zhang,Zhigang Wang* and Peng Chen
keywords：聚丙烯，擠出收縮，流變，負(fù)法向應(yīng)力
source：期刊
specific source：RSC Advances, 2014, 4, 1246-1255
Issue time：2014年
摘要: 我們研究了較大長(zhǎng)徑比CNT和等規(guī)聚丙烯（iPP）納米復(fù)合材料中類(lèi)非平衡態(tài)碳納米管（CNT）網(wǎng)絡(luò)結(jié)構(gòu)在流動(dòng)誘導(dǎo)性質(zhì)變化中所起的作用。由于引入了大長(zhǎng)徑比的CNTs， CNT/iPP納米復(fù)合物在較低的CNT含量下即可形成穩(wěn)定的CNT網(wǎng)絡(luò)。Kharchenko等人通過(guò)實(shí)驗(yàn)證明和理論推斷得知在某些特定情況，如在滿(mǎn)足CNT網(wǎng)絡(luò)足夠強(qiáng)大以致能容納較大的旋轉(zhuǎn)畸變時(shí)，大且明顯的負(fù)法向應(yīng)力差（△N）會(huì)出現(xiàn)于含長(zhǎng)的CNT體系中(Nat. Mater., 2004)。在本研究中，我們主要通過(guò)對(duì)負(fù)法向應(yīng)力差的測(cè)量來(lái)探索負(fù)△N出現(xiàn)的特定情況。我們發(fā)現(xiàn)在加樣過(guò)程中擠壓形變所引起的初始法向應(yīng)力會(huì)極大影響△N值。與Kharchenko等人研究結(jié)果一致，只有在CNT含量超過(guò)CNT凝膠化濃度時(shí)才可以在先將法向應(yīng)力歸零后再施加剪切場(chǎng)的大長(zhǎng)徑比CNT/iPP納米復(fù)合體系中觀察到大的負(fù)△N。這種對(duì)于△N測(cè)試的有趣結(jié)果繼而通過(guò)擠出中表現(xiàn)的擠出收縮和iPP擠出不穩(wěn)定形狀畸變效應(yīng)在CNT/iPP納米復(fù)合體系中能夠消弱而得到了進(jìn)一步證實(shí)。
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.