Influence of Carbon Nanotube Aspect Ratio on Normal Stress Differences in Isotactic Polypropylene Nanocomposite Melts

Abstract

We consider the impact of varying the aspect ratio A of carbon nanotubes (CNT) on the rheological properties of isotactic polypropylene/CNT nanocomposites. Specifically, we focus on multiwall CNT having a relatively low aspect ratio (A in the range from 22 to 45), since previous work (Kharchenko et al., Nature Mater. 2004) has emphasized the rheological properties of relatively high aspect ratio multiwall CNT nanocomposites (A from 300 to 400). Correspondingly, we find that the formation of a nonequilibrium CNT network structure occurs at a higher CNT concentration in our low A CNT nanocomposites, as theoretically expected. The proposed mechanism for the large apparent negative normal stress differences (∆N) described by Kharchenko et al. relies on the capacity of the "struts" of the CNT network to rotate about their impingement junctions, much like the links of a deformed chain link fence. This model implies the absence of appreciable negative ∆N at low CNT concentrations where the network does not yet exist and also for short CNT and at high concentrations of CNT where the mesh size of the network becomes too small to accommodate appreciable rotational distortion. In conformity with this simple mechanical model, we observe only a positiVe apparent ∆N in our CNT/iPP nanocomposites, even well above the CNT gelation concentration. This striking change in the rheology of CNT nanocomposites with a change in A has been further confirmed in die-swell measurements, where a large die swell has been seen in the short CNT nanocomposites, rather than the die-shrinkage found before for the large A or the highly "entangled" CNT network counterpart.

Rheological Behaviors of Poly(Vinylidene Fluoride)/“Bud-Branched” Nanotubes Nanocomposites

“…However, the rheological properties of these low aspect ratio CNT/iPP nanocomposites were contrary to the observations of Kharchenko et al for high aspect ratio CNT in iPP. [31] In this research, another interesting result is shown in Fig. 4, which shows the normal force of PVDF and its nanocomposites.…”

Preparation and properties of functionalized multiwalled carbon nanotubes/polypropylene nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

“…A number of reports indicated that the incorporation of graphite with other conductive materials, especially carbon nan- otubes, have been considered as an effective method to develop higher bulk electrical conductivity of the nanocomposite bipolar plates due to 3D conductive networks [30][31][32][33]. Fig.…”

“…However, there are associated with two critical issues when CNTs were used as reinforcements in polymer composites [30][31][32]. The major two problems including the CNTs may aggregate into bundles and only weak interfacial bonding existed between CNTs and polymer matrix limits the improvement on the electrical or mechanical properties of composites by CNTs [33]. To overcome these difficulties, this is necessary for generating strong interfacial force between fillers and the polymer.…”

“…Consequently, the efficiency of the load transfer from polymer matrix to CNTs might be higher. Therefore, a better approach reported so far is the surface functionalizations of CNTs [33][34][35][36][37][38]. These functionalizations of CNTs could contribute to enhance the interaction between CNTs and the matrix, so as to provide efficient interfacial stress transfer from the polymer to CNTs.…”

Nanocomposites of Multiphase Polymer Blend Reinforced with Carbon Nanotubes: Processing and Characterization

“…Great number of twin-screw extrusion of major engineering polymer-based nanocomposites exists. High impact polystyrene (HIPS), [63] polypropylene (PP), [64][65][66][67][68][69][70][71] acrylonitrile-butadiene-styrene terpolymer (ABS), [8,[71][72][73][74] polyamide 6 (PA6) [75][76][77][78][79][80][81][82] or polycarbonates (PC) [5,56,[83][84][85][86] and polyurethanes (PUR) [87][88][89] are already widely investigated.…”