Modeling and simulations of carbon nanotube (CNT) dispersion in water/surfactant/polymer systems

Molecular dynamics simulations of carbon nanotube dispersions in water: Effects of nanotube length, diameter, chirality and surfactant structures

Development of a Novel Composite Material with Carbon Nanotubes Assisted by Self-Assembled Peptides Designed in Conjunction with β-Sheet Formation

Molecular dynamics simulations of the interactions and dispersion of carbon nanotubes in polyethylene oxide/water systems

Although ordinary Portland cement (OPC) is one of the most widely used construction materials in the world, its relatively weak tensile strength and fracture resistance limit wider structure applications. Carbon nanotubes (CNTs), having been identified as one of the strongest and stiffest materials on earth, are attractive candidates as nano-filaments in reinforcing OPC. The investigation of CNT reinforced OPC composites (CNT-OPC) is at a relatively early stage, and very limited research regarding the effectiveness of CNTs in enhancing the tensile strength or fracture toughness of OPC are available in open literature. The published results on testing of the CNT reinforcing effect often show large variations and sometimes contradict each other. Therefore, there is a significant need for further studies in this area to improve understanding of the reinforcing behaviour of CNTs in cement matrix, including dispersion of CNTs and the effect of CNT on OPC paste in terms of hydration, microstructure, tensile strength and fracture properties. This study builds on the earlier research on CNT reinforcement of the mechanical properties of OPC paste in Duan’s group in the Department of Civil Engineering at Monash University. Specifically, the objectives of this study are (1) development of high mechanical performance CNT reinforced OPC paste by considering the effect of dispersion and concentration of CNTs within OPC matrix, and (2) investigation of the reinforcing mechanisms of CNTs in cement matrix by estimating the post-peak softening behaviour of CNT-OPC paste and the interfacial bond strength between CNTs and cement matrix. In order to develop high performance CNT reinforced OPC paste, the dispersion and concentration of CNTs were studied. The combinations of chemical functionalisation (COOH functional groups on CNT surface), a polycarboxylate-based cement compatible superplasticiser (PC), and sufficient ultrasonication energy have been found essential to achieving homogeneous dispersion of CNTs in cement paste. An effective PC to CNT mass ratio of 8 is recommended to ensure effective dispersion of CNTs, at the same time maintaining satisfactory workability of CNT-OPC paste. Moreover, a CNT dosage-independent optimal ultrasonication energy for achieving mechanically superior CNT-OPC paste was found to be 50 J/ml per unit CNTs-to-suspensions weight ratio. The incorporation of CNTs (of 0.075 wt.% of cement) substantially enhanced the mechanical properties of plain cement. For example, Young’s modulus was improved by 31.5%, flexural strength by 49.9%, and fracture energy by 62.6%. Regarding the reinforcing mechanisms of CNTs within cement paste, the post-peak softening characteristics of CNT-OPC paste were investigated. It was found that the linear post-peak softening characteristics, including initial fracture energy and cohesive tensile strength of plain OPC paste (estimated using size effect tests with the assistance of cohesive crack-based finite element simulation), can be significantly improved by incorporating CNTs. Particularly, the enhancement of cohesive tensile strength may be attributed to the filling of CNTs in the nano-sized colloidal pores and bridging micro-sized capillary pores. On the other hand, the comparable brittleness numbers obtained for CNT-OPC and OPC pastes indicate the minor contribution of CNTs to the ductility of plain OPC paste. Based on cohesive tensile strength, the range of effective interfacial bond strength between CNTs and cement matrix was estimated to be from 9.5 to 24.5 MPa based on a micromechanics-based crack bridging stress-crack separation model for CNT reinforced composites developed in Duan’s group. This result suggests that the introduction of COOH functional group may enhance the interfacial bond property between CNTs and cement matrix, thereby resulting in improved mechanical performances. This project will provide key information to assist other researchers and practitioners to better understand and apply the novel CNT-cement composite with improved mechanical properties to the design of structures and codification.

In this study, the sequential dispersion of multi-walled carbon nanotubes (CNTs) in PDMS/PB (polydimethylsiloxane/polybutene) blends and the change of blend morphology by the dispersion of CNTs were investigated by rheological and morphological observations. The dispersion of CNTs into PDMS/PB blend was accomplished by the dilution of the CNT master (2 wt.% CNT in PDMS) in PDMS/PB blend using an extensional mixer. The morphological study shows that under the extensional flow, CNTs in the dispersed CNT master phase are mainly broken up by tip-streaming and the continuous pinching-off of PDMS drops during morphology evolution enhances the dispersion of CNT. It has been shown that CNTs can be disentangled as in the case of dispersing CNTs in a Boger fluid. Rheological data and TEM observations show that it is not simply a mixing of two phases and the CNTs in the master phase can be dispersed in the single CNT level.

Carbon nanotubes (CNTs) are the strongest fibres that have been made, and have been used as reinforcing additives in ordinary Portland cement (OPC) since 2005. One major aim of incorporating CNTs in OPC paste has been to improve the mechanical properties of cement, thereby reducing the consumption of cement in construction and limiting its environmental impact. However, the reported reinforcing effect of CNTs has so far shown large discrepancies due to the limited understanding of the dispersion of CNTs and reinforcing mechanism in OPC paste. This PhD study aims to improve theoretical understanding of the composite and to develop applicable fabrication methods that can produce stable and significant increases in the mechanical properties of CNT-OPC paste. To achieve this aim, three main tasks are defined as: (1) investigating the dispersion and agglomeration of CNTs in water and alkaline environment; (2) studying the reinforcing mechanism of CNTs; (3) optimizing the fabrication of CNT-OPC paste composite. Experimental, theoretical and numerical approaches are adopted to accomplish these tasks. Experimental techniques such as the use of scanning electron, transmission electron and optical microscopy, UV-vis spectrometry, centrifugation and compressive, flexural and fractural tests are employed to investigate the dispersion and reinforcing effects of CNTs as well as to optimize the fabrication protocol. In the theoretical part of this study, molecular mechanics/dynamics (MM/MD) simulation and theoretical development are conducted to develop models to study CNT dispersion and predict the reinforcing effect of CNTs. Regarding task (1), MM simulation suggests that variation in the molecular characteristics of surfactants generates different interaction energies with the CNT surface, thereby altering their packing morphologies on the CNT surface. Surfactants with long chain-like and planar like structures interact more effectively with CNTs compared with short and linear molecules. Surfactant-dispersed CNTs are found to be in a semi-stable state in a calcium based alkaline environment. A theoretical model is developed to simulate the agglomeration of the CNTs in such an environment. This model suggests that CNTs prefer first to form parallel bundles with a few tubes, before growth into large 3D agglomerates occurs. In terms of task (2), the reinforcing mechanism of CNTs is investigated by developing a micro-mechanical crack bridging model. This model is based on length distribution of the CNTs, which is found to follow a log-normal distribution. The length distribution of the CNTs is heavily affected by the ultrasonication process used in the fabrication of the CNT-OPC paste which promotes the dispersion of CNTs while scissoring them into shorter ones. This crack bridging model is used to predict the optimum ultrasonication energy for reinforcing purposes. In task (3), the prediction is further verified by mechanical testing results, where the optimum ultrasonication energy, 75 J/ml, observed in the tests matches the prediction of the model. By using this optimal energy, the increment in fracture energy and flexural strength (notched beam) is doubled compared with low (25J/ml) or very high (400J/ml) ultrasonication energy. The mechanical test results also show that the reinforcing effect is almost proportional to the concentration of the dispersed CNTs, which again matches the model’s assumption. Further experimental investigation of the maximum concentration of dispersed CNTs suggests a limit of 0.264 wt % (in water with cement-compatible surfactants), above which significant agglomeration will occur. After incorporation into fresh OPC paste, ~60 wt % of the CNTs stays dispersed for 4-16 hours and ~35 % is adsorbed by cement grains within 5 minutes. This PhD thesis enhances understanding of CNT-OPC paste and addresses some future research direction of nanoscale particle reinforced cementitious materials.

Investigation of magnetic properties of nanostructured ferromagnets, such as oriented arrays of carbon nanotubes (CNTs) containing ferromagnetic nanoparticles (FNPs), is still relevant. In addition to attractive applications in magnetoelectronics, CNTs with embedded FNPs are also a very useful model object for studying magnetic interaction of the latter through a conducting medium. For this, it is important to establish a relation between macroscopic and microscopic parameters of the system. In nanostructured ferromagnets, this dependence is described within the random magnetic anisotropy model (RAM), in which the spin system and, consequently, the main macroscopic characteristics (coercivity, susceptibility, saturation magnetization) are determined by such microscopic parameters as the exchange interaction constant, FNP magnetization, local magnetic anisotropy constant and grain size 2Rc [1].Recently, magnetic parameters like the exchange and anisotropy fields, effective magnetic anisotropy constant, Bloch and exchange constants in aligned CNT arrays containing FNPs were determined within the RAM by analyzing law of the approach to saturation magnetization and corresponding modeling of the correlation functions of the magnetic anisotropy axes of the FNPs [2,3]. Presence of the interplay between the exchange interaction and magnetic anisotropy, as well as the contribution of not only random, but also coherent anisotropy, was established [3]. In addition, the important role of magnetoelastic anisotropy in the case when a single FNP is localized inside a CNT has been revealed. [4].It was shown also that aligned CNT arrays with a low concentration of FNPs embedded only inside CNT have relatively high values of exchange fields, random and coherent anisotropy. They are manifested in CNT arrays, in which the average distance between the FNPs significantly exceeds the size of the latter, reaching hundreds of nanometers. These effects, which do not currently have a convincing explanation, are mainly associated with the presence of the exchange interaction between FNPs through the CNT matrix. However, there is still no reasonable mechanism of the long-range exchange interaction in CNT arrays. In our work [5], it was assumed that these effects are related to the indirect exchange coupling of the RKKY type via the conducting electrons of CNTs. The obtained preliminary estimations have shown that the RKKY exchange interaction is enhanced by the presence of spin-orbit coupling (SOC) and could propagate up to a micrometer scale [5].In this contribution a multiwall CNT (MWCNT) with embedded FNPs is considered. We present the results of modeling of the RKKY interaction in MWCNT depending on its diameter, chirality, chemical potential and SOC constant within the Klinovaya-Loss model [6]. The influence of the external longitudinal magnetic field is also studied. It is assumed that the main contribution to the RKKY interaction is caused by the conduction electrons of the inner wall of CNTs which is in contact with the FNP. In addition, spin-orbit interaction (SOI) in CNT is also considered. The SOI in a nanotube can occur due to the curvature effect, which significantly increases its contribution in comparison with a flat graphene. It can also be enhanced by FNP, CNT defects or impurity states. The spin susceptibility of CNT conduction electrons χ/χ0 is evaluated (χ0=a2kG/hυF, where υF is the Fermi velocity, kG is the circumferential direction, a is the lattice constant). Fast oscillations are excluded and only slowly changing envelopes of spin susceptibility are considered. The chemical potential is tuned inside the gap opened by SOI. It is shown that the decay of the amplitude of the χ/χ0 oscillations depends strongly on the chemical potential: the higher the Fermi energy of the CNT (εF), the more significant is the decay. The frequency of spin susceptibility oscillations increases with the increase of the εF (Fig. 1). This is due to the fact that with the increase of the Fermi energy of CNTs, the discrepancy between the gap opened by SOI and the value of chemical potential increases. Finally, the proposed approach allows evaluating the energy of the exchange interaction between FNPs belonging to the same CNT.The work is supported by the COST Action CA19118. **

Effects of carbon nanotube (CNT) geometries on the dispersion characterizations and adhesion properties of CNT reinforced epoxy composites

An atomic-level understanding of the strengthening mechanism of aluminum matrix composites reinforced by aligned carbon nanotubes

The efficient dispersion of carbon nanotubes (CNTs) is a challenging task in reaching the usable nanocomposites. In this study, a comparative analysis on dispersion of multiwalled CNTs multiwalled carbon nanotubes (MWNTs) in styrene-butadiene rubber (SBR) latex was carried out by using two anionic surfactants, sodium dodecyl benzene sulfonate and sodium lauryl sulfate. The MWNTs were first predispersed in distilled water using two surfactants individually followed by gentle mixing the MWNT predispersion into SBR latex. By using the technique of ultraviolet-visible spectroscopy, the study on MWNT dispersion in aqueous media was focused on surfactant concentration, MWNT functionality, and ultrasonication time. The ultraviolet-visible absorptions showed the positive effect of MWNT functionality in addition to surfactant concentration with no great effect of ultrasonication time over 15 min. In comparison with sodium lauryl sulfate, the existing benzene ring in the sodium dodecyl benzene sulfonate structure seems to result in higher adsorption of surfactant onto the MWNTs surface and, hence, better MWNT dispersion. The MWNT dispersion was further improved by using hydroxyl functionalized MWNTs mainly because of the formation of hydrogen bonding between the hydrophilic head of surfactant and the existing hydroxyl group of the functionalized MWNTs. After mixing the MWNT predispersion into SBR latex, the dispersion of MWNTs was further characterized by using electrical volume conductivity, microscopy technique, and rheological measurements. In rheometry tests of the lattices, the storage modulus at terminal zone was utilized for tracking the degree of MWNT dispersion in the nanocomposite. The pictures of scanning electron microscopy showed the efficiency of MWNT functionality in enhancing the degree of dispersion. In conductivity tests, the percolation threshold was obtained at about 1 part by weight per hundred parts of resin of the functionalized MWNT in dried film. J. VINYL ADDIT. TECHNOL., 2015. © 2015 Society of Plastics Engineers

Carbon nanotubes (CNT) with exceptional mechanical and physical properties are the prime candidate materials for metal matrix composites. But the dispersion of CNTs in the metal matrix is affected by attractive van der Waals forces. In this study, we have adopted ultrasonication and mechanical alloying to achieve homogenous dispersion of multiple reinforcements namely B4C and CNTs within the commercial purity aluminium powder. The ultrsonication of all the starting materials was followed mechanical alloying process which was carried out in a planetary ball mill up to 480 min. The milling process was systematically studied by taking a small amount of milled powder at different milling time of 60, 120, 240 and 480 min respectively. The effect of ultrasonication and mechanical alloying on both morphology of the powders and dispersion of B4C and CNTs was studied using scanning electron microscopy. The results showed uniform dispersion of both B4C and CNTs within the pure aluminium powders without any agglomeration. Both the pure aluminium and hybrid nanocomposite powder showed decrease in particle size from 20.89 µm to 5.67 µm and 2.72 µm respectively after 480 minutes of ball milling.

In initial experiments the effects of aromatic model substances on carbon nanotube (CNT) dispersions in dimethylformamide (DMF) were investigated. Electron‐deficient aromatics interact strongly with CNTs, causing increased agglomeration and sedimentation. Conversely, electron‐donating aromatics stabilize CNT dispersions in DMF. Polymers with electron‐deficient aromatics, such as polydinitrostyrene (PDNS), exhibit a concentration‐dependent effect: low concentrations lead to stabilization of dispersions, while higher concentrations lead to sedimentation. This suggests that such polymers can enhance attraction between the matrix of CNT‐reinforced polymers as well as stabilize the dispersed CNTs. Polycarbonate, modified with polydinitrocarbonate (PDNC) and reinforced with CNTs showed improved mechanical properties. The addition of 6 wt.% CNTs and 6 wt.% PDNC resulted in a notable improvement with a 22% increase in tensile strength, a 29% increase in flexural strength, a 39% increase in Young's modulus and a 47% increase in flexural modulus. This enhancement resulted in an overall mechanical performance comparable to the high‐performance polymer polyetherimide. However, there must be noted, that the addition of PDNC increases the CNT particle size, which can negatively affect mechanical properties. The results highlight the additive's dual role in enhancing adhesive interactions while potentially increasing CNT agglomerate sizes.Highlights Interactions of CNTs dispersed in DMF and various aromatics were investigated. Polydinitrocarbonate (PDNC) was synthesized as a new additive for CNT‐composites. Polycarbonate/CNT‐composites were obtained using extrusion. Test specimens with CNT contents up to 6 wt.% were obtained. Mechanical properties of polycarbonate reached the level of polyetherimide.

A poly(vinyl alcohol) (PVA) based nanocomposite using fully exfoliated graphene oxide (GO) sheets and multi-walled carbon nanotubes (CNTs) were prepared via a simple procedure. It is confirmed from optical imaging that dispersion of CNTs in the PVA matrix can be significantly improved by adding GO sheets. Molecular dynamics (MD) simulations suggest that the GO–CNT interaction is strong and the complex is thermodynamically favorable over agglomerates of CNTs. The GO–CNT scroll-like structure formed with the hydrophilic outer surface of GO can be well dispersed in water. More important, a synergistic effect arises from the combination of CNT and GO, the GO–CNT/PVA composite films show superior mechanical properties compared to PVA composite films enhanced by GO or CNT alone, not only the tensile strength and Young's modulus of the composites are significantly improved, but most of the ductility is also retained. The enhanced mechanical properties of the GO–CNT/PVA composite film can be attributed to the fully exploited reinforcement effect from GO and CNT via good dispersion.

Biomimetic modification of large diameter carbon nanotubes and the desalination behavior of its reverse osmosis membrane

Aqueous carbon nanotube (CNT) dispersions stabilized by surfactants are emerging as safe and eco-friendly approaches for fabricating CNT assemblies. However, obtaining liquid-crystalline (LC) CNT dispersions in processable quantities remains challenging and is critical for the full utilization of their exceptional properties. In this paper, we present a scalable and facile method for concentrating surfactant-assisted aqueous CNT dispersions using superabsorbent polymer (SAP) beads, inspired by the hydration behavior of chia seeds. SAP beads selectively absorb water and surfactants while excluding CNTs, enabling SAP-induced dialysis (SPID). The CNT dispersions were concentrated 12-fold (from 0.4 to 4.79 wt %), accompanied by spontaneous transitions from the isotropic to the LC phase and from the sol to the gel phase. The SPID process enables the demonstration of potential applications, such as electrically conducting liquids for flexible sensors, wet-spinning LC dope, and CNT aerogel precursors, broadening the possibilities for the solution processing of surfactant-assisted CNT dispersions.