Effect of dual-modified CNTs on strength and chloride resistance of cementitious systems

An innovative multiscale (atomistic to mesoscale) model capable of predicting carbon nanotube (CNT) interactions and dispersion in water/surfactant/polymer systems was developed. The model was verified qualitatively with available experimental data in the literature. It can be used to computationally screen potential surfactants, solvents, polymers, and CNT with appropriate diameter and length to obtain improved CNT dispersion in aqueous medium. Thus the model would facilitate the reduction of time and cost required to produce CNT dispersed homogeneous solutions and CNT reinforced materials. CNT dispersion in any water/surfactant/polymer system depends on interactions between CNTs and surrounding molecules. Central to the study was the atomistic scale model which used the atomic structure of the surfactant, solvent, polymer, and CNT. The model was capable of predicting the CNT interactions in terms of potential of mean force (PMF) between CNTs under the influence of surrounding molecules in an aqueous solution. On the atomistic scale, molecular dynamics method was used to compute the PMF as a function of CNT separation and CNT alignment. An adaptive biasing force (ABF) method was used to speed up the calculations. Correlations were developed to determine the effective interactions between CNTs as a function of their any inter-atomic distance and orientation angle in water as well as in water/surfactant by fitting the calculated PMF data. On the mesoscale, the fitted PMF correlations were used as input An innovative multiscale (atomistic to mesoscale) model capable of predicting carbon nanotube (CNT) interactions and dispersion in water/surfactant/polymer systems was developed. The model was verified qualitatively with available experimental data in the literature. It can be used to computationally screen potential surfactants, solvents, polymers, and CNT with appropriate diameter and length to obtain improved CNT dispersion in aqueous medium. Thus the model would facilitate the reduction of time and cost required to produce CNT dispersed homogeneous solutions and CNT reinforced materials. CNT dispersion in any water/surfactant/polymer system depends on interactions between CNTs and surrounding molecules. Central to the study was the atomistic scale model which used the atomic structure of the surfactant, solvent, polymer, and CNT. The model was capable of predicting the CNT interactions in terms of potential of mean force (PMF) between CNTs under the influence of surrounding molecules in an aqueous solution. On the atomistic scale, molecular dynamics method was used to compute the PMF as a function of CNT separation and CNT alignment. An adaptive biasing force (ABF) method was used to speed up the calculations. Correlations were developed to determine the effective interactions between CNTs as a function of their any inter-atomic distance and orientation angle in water as well as in water/surfactant by fitting the calculated PMF data. On the mesoscale, the fitted PMF correlations were used as input in the Monte Carlo simulations to determine the degree of dispersion of CNTs in water and water/surfactant system. The distribution of CNT cluster size was determined for the CNTs dispersed in water with and without surfactant addition. The entropic and enthalpic contributions to the CNT interactions in water were determined to understand the dispersion mechanism of CNTs in water. The effects of CNT orientation, length, diameter, chirality and surfactant concentrations and structures on CNT interactions in water were investigated at room conditions. CNT interactions in polymer solution were also investigated with polyethylene oxide (PEO) polymer and water as a solvent. In all cases, the atomic arrangement of molecules was discussed in detailed. Simulations revealed that CNT orientation, length, diameter, and addition of surfactant and its structures can significantly affect CNT interactions (i.e., PMFs varied significantly) and in-turn the degree of CNT dispersion in aqueous solution. For all simulation cases, a uniform sampling was achieved by using the ABF method to calculate the governing PMF between CNTs indicating the effectiveness and convergence of the adaptive sampling scheme. The surfactant molecules were shown to adsorb at the CNT surface and contribute to weaker interactions between CNTs which resulted less CNT aggregate size at the mesoscale. Surfactant consisting with a benzene ring contributed much weaker interactions between CNTs as compared with that of without benzene ring. The increase in CNT length contributed the stronger CNT interactions where the increase in CNT diameter caused weaker CNT interactions in water. The interfacial characteristics between the CNT, surfactant and the polymer were also predicted and discussed. The model can be expanded for more solvents, surfactants, and polymers.

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.

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.

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

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.

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

Water based carbon nanotube (CNT) dispersion was produced by wet‐jet milling method. Commercial CNT was originally agglomerated at the particle size of less than 1 mm. The wet‐jet milling process exfoliated CNTs from the agglomerates and dispersed them into water. Sedimentation of the CNTs in the dispersion fluid was not observed for more than a month. The produced CNT dispersion was characterized by the SEM and the viscometer. CNT/PTFE composite film was formed with the CNT dispersion in this study. The electrical conductivity of the composite film increased to 10 times when the CNT dispersion, which was produced by the wet‐jet milling method, was used as a constituent of the film. Moreover, the composite film was applied to bipolar plate of fuel cell and increased the output power of the fuel cell to 1.3 times.

A latex-based concept for making carbon nanotube/polymer nanocomposites

Исследованы структурные особенности нанокомпозитов, полученных при лазерном облучении водно-белковых сред с одностенными углеродными нанотрубками (ОУНТ), электродуговым (ОУНТI) и газофазным методами (ОУНТII). С помощью спектроскопии комбинационного рассеяния нанокомпозитов определен нековалентный характер взаимодействия нанотрубок с молекулами белков. Белковая составляющая в нанокомпозитах подверглась необратимой денатурации и может выступать в качестве связующего биосовместимого материала, который является источником аминокислот для биологических тканей при имплантации нанокомпозитов в организм. Образцы, изготовленные из ОУНТI, с меньшим диаметром и длиной имели наиболее однородную структуру. При увеличении концентрации от 0.01 до 0.1 % происходило увеличение среднего размерамикропор от 45 до 85 мкм и пористости образца в общем с 46 до 58 %. При этом доля открытых пор для двух типов концентраций ОУНТI составила 2 % от общего объема композита. В нанокомпозитах на основе ОУНТI показано наличие мезопор. Увеличение концентрации нанотрубок привело к уменьшению удельных значений поверхности и объема пор образца. Исследованные нанокомпозиты могут использоваться в качестве тканеинженерных матриц для восстановления объемных дефектов биологических тканей
 
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Sorption of triclosan by carbon nanotubes in dispersion: The importance of dispersing properties using different surfactants

Electrochemical supercapacitors based on MnO2 electrode materials are currently attracting significant interest due to the high specific capacitance obtained using environmentally friendly aqueous electrolytes and low cost of MnO2. A complicating factor in the application and commercialization of MnO2 electrodes for electrochemical supercapacitors is low electronic and ionic conductivity of MnO2. This problem is usually addressed by the fabrication of porous nanocomposite electrodes, containing MnO2 nanoparticles and carbon nanotubes (CNT) or other conductive additives. Despite the impressive progress achieved in the fabrication of MnO2-CNT electrodes, there is a need for simple and versatile methods for the fabrication of MnO2 nanoparticles, efficient dispersion of MnO2 and CNT, and fabrication of porous electrodes. We report new strategies for the fabrication of MnO2-CNT composites, which are based on the use of new dispersing agents and new techniques for mixing of MnO2 and CNT. The electrochemical performance of the composites, prepared by different methods was compared. The results were used for the fabrication of advanced electrodes and devices. Efficient dispersing agents are of critical importance for the fabrication of MnO2 nanoparticles and formation of stable suspensions for colloidal processing. An important property of a dispersant is its adsorption on the particle surface. Therefore, there is a need in the development of charged dispersing agents with strong interfacial adhesion. New methods have been developed for the synthesis, surface modification and dispersion of MnO2 nanoparticles using advanced dispersing agents, which provide strong bidentate, polydentate or chelating bonding to the particle surface. Various dispersants were developed and compared, such as molecules from catechol, chromotropic acid, gallic acid, salicylic acid and bicinchoninic acid families. The influence of the nature of functional groups, number of aromatic rings, length of the hydrocarbon chain on the adsorption, dispersion and electrostastic charging in the suspensions was investigated. New dispersants were used for the synthesis of non-agglomerated MnO2 particles. Various aromatic and steroid dispersants were investigated for the dispersion of CNT. It was found that steroid dispersants outperform other dispersants in the dispersion of CNT. The polyaromatic molecules, containing chelating and charging functional groups can be used as co-dispersants for MnO2 and CNT. Further progress in the application of new dispersants was achieved by the development of liquid-liquid interface synthesis and extraction method, which allowed agglomerate free synthesis of MnO2 and their mixing with CNT. In this strategy, the problems related to particle agglomeration during the drying stage were avoided. As an extension of these investigations, chelating polymers were used for dispersion. The unique feature of this strategy is that chelating aromatic ligands of the monomers provide multiple adsorption sites for attachment on MnO2 and CNT and impart electrical charges for electrosteric dispersion. In another strategy, complexes of polymers and chelating agents were used for co-dispersion of MnO2 and CNT. As a result, we achieved an outstanding efficiency in dispersion. Heterocoagulation methods have been developed for the nanotechnology of MnO2 - CNT composites. The methods are based on selective strong adsorption of dispersants with specific adsorption ligands on MnO2 or CNT and heterocoagulation of the materials using electrostatic forces, related to opposite charges of individual dispersants or click chemistry methods. Proof of concept studies in aqueous and non-aqueous suspensions showed significant improvement in component dispersion and mixing. The capacitive performance of MnO2-CNT composites, prepared by different methods was compared. Ni foams were used as current collectors for the fabrication of electrodes with active mass loading of 30-50 mg cm-2 and mass ratio of active material to current collector of 0.3-0.42. The capacitive behavior of the composite electrodes was studied in Na2SO4 electrolyte using cyclic voltammetry, chronopotentiometry and impedance spectroscopy. The use of new dispersants and mixing techniques allowed for significant improvement in electrode performance. The composites, prepared in the presence of dispersants using liquid-liquid extraction and mixing methods showed the highest specific capacitance of 8 F cm-2. The liquid-liquid extraction method for the synthesis of MnO2 particles and their mixing with CNT allowed good capacitance retention at high charge-discharge rates. Significant improvement in capacitive behavior was achieved using heterocoagulation methods for the fabrication of MnO2-CNT composites. The new strategies allowed significant reduction in electrode resistance due to the use of efficient colloidal dispersion and mixing techniques. The capacitance retention of above 70% was achieved in the scan rate range of 2-100 mV s-1. The electrodes showed cyclic stability above 90% after 5000 cycles. The composite electrodes were used for the fabrication of asymmetric capacitors with voltage window of 1.6V. We report capacitances, power-energy characteristics and cyclic behavior of electrodes, prepared using different methods.

An electrochemical method for dispersion of single-walled carbon nanotubes (SWNTs) is described. The technique is based on grafting of oxygen-containing functional groups to the nanotube surface during electrolysis in aqueous and nonaqueous potassium bromide solutions. A dependence of the degree of functionalization of nanotubes on the solvent was revealed experimentally. Nanotubes treated in DMSO have about 14 carbon atoms per oxygen atom from functional groups (cf. nearly four C atoms per oxygen atom in the nanotubes treated in aqueous solutions). The corresponding maximum specific capacities of the electrodes are nearly 10 and 60 F g−1. The samples treated in solutions of KBr in DMSO have about 300 carbon atoms per bromine atom on the nanotube surface (cf. only 30 carbon atoms in the samples treated in aqueous solution). A mechanism of electrochemical modification of SWNTs is proposed. Its key step is production of atomic oxygen that oxidizes the nanotube surface with the formation of functional groups.

Dispersion of carbon nanotubes has been heavily studied due to its importance for their technical applications, toxic effects, and environmental impacts. Common electrolytes, such as sodium chloride and potassium chloride, promote agglomeration of nanoparticles in aqueous solutions. On the contrary, we discovered that acetic electrolytes enhanced the dispersion of multi-walled carbon nanotubes (MWCNTs) with carboxyl functional group through the strong hydrogen bond, which was confirmed by UV–Vis spectrometry, dispersion observations and aerosolization-quantification method. When concentrations of acetate electrolytes such as ammonium acetate (CH3CO2NH4) and sodium acetate (CH3CO2Na) were lower than 0.03 mol per liter, MWCNT suspensions showed better dispersion and had higher mobility in porous media. The effects by the acetic environment are also applicable to other nanoparticles with the carboxyl functional group, which was demonstrated with polystyrene latex particles as an example.

The potential properties of carbon nanotube-cement based materials strongly depend on the dispersion of carbon nanotubes (CNTs) within the cement matrix and the bonding between CNTs and the hydrated cement. The homogeneous dispersion of CNTs in the cement matrix yet is one of the main challenges due to strong van der Waals forces between nanotubes. In this study, a polycarboxylic ether based superplasticizer and ultra-sonication technique was used for dispersion of multi-walled carbon nanotubes (MWCNTs). Portland cement concrete specimens with different concentrations of MWCNTs (0.04 and 0.1 % by the weight of cement), with and without the presence of superplasticizer were investigated. Compressive strength test results revealed a significant improvement in mechanical properties of sample containing 0.1 % MWCNTs and 0.2 % superplasticizer. Moreover, field emission scanning electron microscopy (FESEM) images of fractured surfaces of hardened specimens showed a good dispersion of MWCNTs within the cement matrix. This method was developed to facilitate the uniform dispersion of MWCNTs in the cementitious concrete for better reinforcement in nanoscale and mechanical properties enhancement by transfer of load between the nanotubes and matrix.