Carbon Nanotube Polymer Nanocomposites Research Articles

A two-step model for the tunneling conductivity of polymer carbon nanotube nanocomposites assuming the conduction of interphase regions

This work develops a two-step model for the conductivity of polymer carbon nanotube (CNT) nanocomposites (PCNT) assuming the properties of tunneling and interphase regions.

A simple methodology to predict the tunneling conductivity of polymer/CNT nanocomposites by the roles of tunneling distance, interphase and CNT waviness

In this work, a simple methodology is presented that describes the main tunneling conductivity of polymer/CNT nanocomposites (PCNT) assuming the tunneling distance, interphase surrounding the CNT, and CNT waviness.

Photodegradation of polymer-CNT nanocomposites: effect of CNT loading and CNT release characteristics

Single particle ICP-MS is used to characterize the evolving magnitude, rate, and form of CNT release from photodegrading polymer nanocomposites.

Piezoresistive performance characterization of strain sensitive multi-walled carbon nanotube-epoxy nanocomposites

Carbon nanotubes polymer nanocomposites have attracted a significant research, which provide interesting aspects for the realization of cost effective, highly sensitive piezoresistive strain sensors. Here, we report on an extensive piezoresistive performance investigation of multi-walled carbon nanotubes (MWCNTs)/epoxy nanocomposite under uniaxial cyclic tensile and compressive loading in low strain range. Dispersion quality of MWCNTs in the epoxy polymer matrix was monitored by scanning electron microscopy. MWCNTs/epoxy nanocomposite was deposited directly on a glass-reinforced epoxy cantilever beam on the same axis with a commercial metallic strain gauge. The strain sensitive nanocomposite presents high-performance properties, including gauge factor up to 78, good stability of ±0.26% for 10h and durability of −0.08% after cyclic loading and unloading. Furthermore, MWCNTs/epoxy nanocomposites show quite small creep up to ±0.26%, fast response and recovery times, low hysteresis under cyclic tensile and compressive loadings and temperature response make the proposed nanocomposites suitable in various potential strain sensor applications including structural health monitoring.

Multiscale metrologies for process optimization of carbon nanotube polymer composites

Carbon nanotube (CNT) polymer nanocomposites are attractive multifunctional materials with a growing range of commercial applications. With the increasing demand for these materials, it is imperative to develop and validate methods for on-line quality control and process monitoring during production. In this work, a novel combination of characterization techniques is utilized, that facilitates the non-invasive assessment of CNT dispersion in epoxy produced by the scalable process of calendering. First, the structural parameters of these nanocomposites are evaluated across multiple length scales (10−10 m to 10−3 m) using scanning gallium-ion microscopy, transmission electron microscopy and small-angle neutron scattering. Then, a non-contact resonant microwave cavity perturbation (RCP) technique is employed to accurately measure the AC electrical conductivity of the nanocomposites. Quantitative correlations between the conductivity and structural parameters find the RCP measurements to be sensitive to CNT mass fraction, spatial organization and, therefore, the processing parameters. These results, and the non-contact nature and speed of RCP measurements identify this technique as being ideally suited for quality control of CNT nanocomposites in a nanomanufacturing environment. When validated by the multiscale characterization suite, RCP may be broadly applicable in the production of hybrid functional materials, such as graphene, gold nanorod, and carbon black nanocomposites.

Study on dosimetry characteristics of polymer–CNT nanocomposites: Effect of polymer matrix

In this research work, the current density of polymer–carbon nanotube nanocomposite in different weight percentages of nanotubes, over the radiation absorbed dose under a fixed DC voltage for different polymer matrices such as high density polyethylene, polycarbonate, polyethylene terephthalate, polymethyl methacrylate, and polystyrene was investigated via finite element method. The predicted electrical percolation threshold values in different composites were validated by experimental results published by other scientists. The absorbed dose value was considered as multiplying of heat capacity and temperature rise of the composite, regarding the calorimetric approach. Results show that the polymer type having different characteristics of relative permittivity and heat capacity could affect the sensitivity and working dose range of the composite as a dosimeter.

Modeling Percolation in Polymer Nanocomposites by Stochastic Microstructuring.

A methodology was developed for the prediction of the electrical properties of carbon nanotube-polymer nanocomposites via Monte Carlo computational simulations. A two-dimensional microstructure that takes into account waviness, fiber length and diameter distributions is used as a representative volume element. Fiber interactions in the microstructure are identified and then modeled as an equivalent electrical circuit, assuming one-third metallic and two-thirds semiconductor nanotubes. Tunneling paths in the microstructure are also modeled as electrical resistors, and crossing fibers are accounted for by assuming a contact resistance associated with them. The equivalent resistor network is then converted into a set of linear equations using nodal voltage analysis, which is then solved by means of the Gauss–Jordan elimination method. Nodal voltages are obtained for the microstructure, from which the percolation probability, equivalent resistance and conductivity are calculated. Percolation probability curves and electrical conductivity values are compared to those found in the literature.

Macromol. Chem. Phys. 17/2015

Front Cover: Polymer carbon nanotube nanocomposites are prepared by melt-mixing a poly(oxymethylene)/poly(l-lactic acid) (POM/PLLA) blend and multiwalled carbon nanotubes (MWCNTs). Investigation of the crystallization behaviour shows that the MWCNTs first selectively nucleate POM to yield novel “mace-like” hybrid structure crystals, which then serve as efficient polymeric nucleating agents of PLLA. The crystallization rate and crystallinity are remarkably enhanced. Further details can be found in the article by Lijun Ye, Cuicui Ye, Xianchun Shi, Hengti Wang, Hongyan Zhao, Xiaojun Cao, Jichun You, Yongjin Li,* and Yuanyuan Liang on page 1801.

Modeling of tensile modulus in polymer/carbon nanotubes (CNT) nanocomposites

Carbon nanotubes (CNT) show significant mechanical, thermal, and electrical properties. Therefore, CNT is a good candidate for enhancement of polymer properties. Many researchers have tried to study the various characteristics of polymer/CNT nanocomposites (PCNTs) from experimental and theoretical points of view.This paper aims to predict the tensile modulus of PCNTs by a simple methodology. The experimentally measured modulus of many PCNTs from valid literature is correlated with various powers of CNT volume fractions (“ϕ”, “ϕ1/2” and “ϕ2/3”) and the best equation is chosen from analysis of calculations. It is found that the experimental moduli are well connected to “ϕ2/3” by maximum 10% disagreement between experimental and theoretical results.

Interfacial thermal transport and structural preferences in carbon nanotube-polyamide-6,6 nanocomposites: how important are chemical functionalization effects?

We employ reverse nonequilibrium molecular dynamics simulations to investigate the interfacial heat transfer in composites formed by an ungrafted or a grafted carbon nanotube which is surrounded by oligomeric polyamide-6,6 chains. The structural properties of the polymer matrix and the grafted chains are also studied. The influence of the grafting density, the length of the grafted chains as well as their chemical composition on the interfacial thermal conductivity (λi) are in the focus of our computational study. For the considered grafted polyethylene and polyamide chains we do not find a sizeable difference in the observed λi values. In contrast to this insensitivity, we predict a rather strong influence on λi by the grafting density and the length of the grafted chains. This dependence is an outcome of modifications in the structural properties of the polymer matrix as well as the grafted chains. Functionalization of the nanotube has a sizeable influence on the interfacial thermal conductivity. Its enhancement is caused by the chemical bonds between the nanotube and grafted chain atoms which reduce the number of Kapitza resistances hindering the heat transfer in polymer samples. The phonon density of states profiles confined to the bonded nanotube and the grafted chain atoms are used to emphasize the phonon support of the thermal conductivity in nanocomposites with grafted tubes. Strategies to tailor nanotube containing composites with higher thermal conductivities than that realized in the bare polymer are shortly touched.

Investigation of uniaxial stretching effects on the electrical conductivity of CNT–polymer nanocomposites

A theoretical study is carried out to investigate the effects of uniaxial stretching on the electrical conductivity of carbon nanotube (CNT)–polymer composites using a mixed micromechanics model, which incorporates two conductivity mechanisms: electron hopping and conductive networks. The uniaxial stretching induces volume expansion of the composites, re-orientation of CNTs and a change in conductive networks, which are characterized by the variation of the CNT concentration, the CNT orientation distribution function and the percolation threshold, respectively. Modelling results demonstrate that stretching decreases the electrical conductivity of the composite in both the longitudinal and transverse directions. It is also observed that stretching has more significant effects on the electrical conductivity of the composites with a lower CNT volume fraction. Furthermore, the effects of Poisson's ratio on the electrical conductivity are also investigated. Possible reasons for the observed phenomena are interpreted. This work can be claimed to provide a theoretical prediction on the trend of the stretching effects on the electrical properties of CNT–polymer composites.

Use of Fluorescence Imaging and Ionic Liquids in the Study of Carbon, Clay, and Cellulose Based-Nanomaterials

Some of the most critical issues facing the nanocomposites community include the need for multi-scale imaging of the hierarchical structure, effective methods for controlling nanoparticle surface-energy and chemistry, and new processing methods for preparing novel nanoparticles. Over the last decade we have employed ionic liquids and fluorescence microscopy to address these challenges. This presentation will discuss how the combination of ionic liquid modified materials and new measurement methods work together synergistically. Examples will include: fluorescent microscopy imaging of imidazolium- Nile blue- montmorillonite polymer nanocomposites, carbon nanotube polymer nanocomposites prepared using imidazolium ILs, Főrster resonance energy transfer (FRET) imaging of the interface in cellulose nanofibers composites, IL processing to isolate cellulose nanofibers from biomass, and surface modification of cellulose nanofibers using IL.

Computational micromechanics analysis of electron hopping and interfacial damage induced piezoresistive response in carbon nanotube-polymer nanocomposites

Carbon nanotube (CNT)-polymer nanocomposites have been observed to exhibit an effective macroscale piezoresistive response, i.e., change in macroscale resistivity when subjected to applied deformation. The macroscale piezoresistive response of CNT-polymer nanocomposites leads to deformation/strain sensing capabilities. It is believed that the nanoscale phenomenon of electron hopping is the major driving force behind the observed macroscale piezoresistivity of such nanocomposites. Additionally, CNT-polymer nanocomposites provide damage sensing capabilities because of local changes in electron hopping pathways at the nanoscale because of initiation/evolution of damage. The primary focus of the current work is to explore the effect of interfacial separation and damage at the nanoscale CNT-polymer interface on the effective macroscale piezoresistive response. Interfacial separation and damage are allowed to evolve at the CNT-polymer interface through coupled electromechanical cohesive zones, within a finite element based computational micromechanics framework, resulting in electron hopping based current density across the separated CNT-polymer interface. The macroscale effective material properties and gauge factors are evaluated using micromechanics techniques based on electrostatic energy equivalence. The impact of the electron hopping mechanism, nanoscale interface separation and damage evolution on the effective nanocomposite electrostatic and piezoresistive response is studied in comparison with the perfectly bonded interface. The effective electrostatic/piezoresistive response for the perfectly bonded interface is obtained based on a computational micromechanics model developed in the authors’ earlier work. It is observed that the macroscale effective gauge factors are highly sensitive to strain induced formation/disruption of electron hopping pathways, interface separation and the initiation/evolution of interfacial damage.

Charge carrier transport and low electrical percolation threshold in multiwalled carbon nanotube polymer nanocomposites

Electrical transport properties of multiwalled carbon nanotube (MWCNT) polymer nanocomposites based on epoxy and high density polyethylene (HDPE) as matrices have been studied. The composites have been prepared from masterbatches by dilution. Low electrical percolation thresholds of 0.05 and 0.4vol.% of carbon nanotubes in epoxy and HDPE matrices were found respectively. The analysis of the temperature dependence of dc-conductivity shows that the charge transport is fully described as the combination in series of one-dimensional (1-D) and three-dimensional (3-D) variable range hopping (VRH) regimes. The transport is limited by the 1-D VRH regime in the low temperatures limit and by the 3-D VRH regime at room temperature. The analysis of the VRH parameters when the overall volume fraction of MWCNT decreases reveals two distinct situations. In the epoxy the local characteristics of the masterbatch remain practically unchanged and a percolating network is maintained even at very low volume fractions; in the HDPE matrix, the density of the MWCNT bundles packing decreases as revealed by the strong variation of the Mott temperature parameter. Our results indicate that amorphous polymers matrices give lower percolation thresholds than semicrystalline ones when a masterbatch dilution process is used.

Computational micromechanics analysis of electron-hopping-induced conductive paths and associated macroscale piezoresistive response in carbon nanotube–polymer nanocomposites

In this study, a computational model is developed using finite-element techniques within a continuum micromechanics framework to capture the effect of electron-hopping-induced conductive paths at the nanoscale which contribute to the macroscale piezoresistive response of the nanocomposite. This is achieved by tracking the position of the nanotubes under applied deformations and modifying the conductivity of the intertube region depending on the relative proximity of individual pairs of nanotubes. The formation and disruption of the electron-hopping pathways are highly dependent on intertube distances and under deformations can result in microstructural rearrangements in terms of electrostatic properties leading to transitions in material symmetries and component magnitudes of the effective electrostatic properties. Thus, in order to capture the complexities of changing inhomogeneous nanoscale electrostatic behavior, where analytical Eshelby’s approaches cannot be used, a computational micromechanics model is needed. The effective conductivity and piezoresistive strain tensor coefficients are evaluated using volume-averaged energy equivalencies for aligned CNT–polymer nanocomposites in the transverse direction exploring different volume fractions of CNTs in the polymer and the maximum electron-hopping range. The impact of the electron-hopping mechanism on the effective piezoresistive response is studied through the macroscale effective gauge factors under different loading conditions. The effective piezoresistive strain coefficients and macroscale effective gauge factors are observed to be nonlinear with applied macroscale strain and are highly dependent on the type of boundary conditions. The effective macroscale gauge factors observed in the current study have magnitudes comparable to experimental observations reported in the literature with higher gauge factors observed closer to the percolation threshold.

Development of a conceptual framework for evaluation of nanomaterials release from nanocomposites: Environmental and toxicological implications

Despite the fact that nanomaterials are considered potentially hazardous in a freely dispersed form, they are often considered safe when encapsulated into a polymer matrix. However, systematic research to confirm the abovementioned paradigm is lacking. Our data indicates that there are possible mechanisms of nanomaterial release from nanocomposites due to exposure to environmental conditions, especially UV radiation. The degradation of the polymer matrix and potential release of nanomaterials depend on the nature of the nanofillers and the polymer matrix, as well as on the nature of environmental exposure, such as the combination of UV, moisture, mechanical stress and other factors. To the best of our knowledge there is no systematic study that addresses all these effects. We present here an initial study of the stability of nanocomposites exposed to environmental conditions, where carbon nanotube (CNT) containing polymer composites were evaluated with various spectroscopic and microscopic techniques. This work discusses various degradation mechanisms of CNT polymer nanocomposites, including such factors as UV, moisture and mechanical damage. An in vivo ingestion study with Drosophila showed reduced survivorship at each dose tested with free amine-functionalized CNTs, while there was no toxicity when these CNTs were embedded in epoxy. In addition to developing new paradigms in terms of safety of nanocomposites, the outcomes of this research can lead to recommendations on safer design strategies for the next generation of CNT-containing products.

Computational micromechanics modeling of piezoresistivity in carbon nanotube–polymer nanocomposites

The macroscale piezoresistive response, i.e. the change in electrical resistivity under the application of strain, of carbon nanotube–polymer nanocomposites has been observed to lead to gauge factors which are much larger than the gauge factors of commonly used strain gauges. Whereas most strain gauges rely on geometric effects, the gauge factors of carbon nanotube–polymer nanocomposites are the result of a combination of nanoscale mechanisms, namely electrical tunneling (electron hopping) and carbon nanotube inherent piezoresistivity, which can lead to substantial differences between the nanocomposite resistivity at zero strain and the resistivity under an applied strain. This paper focuses on modeling the piezoresistive effect of carbon nanotube–polymer nanocomposites by using computational micromechanics techniques based on finite element analysis. For nanocomposites with aligned carbon nanotubes, an electromechanically coupled code is developed for nominal well-dispersed carbon nanotube representative volume elements (RVEs) and for non well-dispersed cases in the aligned and transverse directions. The microscale mechanisms that may have a substantial influence on the overall piezoresistivity of the nanocomposites, i.e. the electrical tunneling effect, and the coupled effect of the electrical tunneling effect and the inherent piezoresistivity of the carbon nanotube, are included in microscale RVEs in order to understand their influence on macroscale piezoresistive response in terms of both the normalized change in effective resistivity and the corresponding effective gauge factor under applied strain. It is found that in the transverse directions, the electrical tunneling effect is the dominant mechanism, and in order for the inherent carbon nanotube piezoresistivity to have a noticeable coupling effect or influence, the local volume fraction of the carbon nanotube should be sufficiently high or the height of barrier of the polymer matrix should be sufficiently low. It is also found that in the axial direction, although the electrical tunneling effect is still the dominant mechanism, the inherent piezoresistivity of the carbon nanotube may have a substantial contribution to the overall axial piezoresistive response.

Effects of Functionalization on Thermal Properties of Single-Wall and Multi-Wall Carbon Nanotube–Polymer Nanocomposites

Carboxylic functionalization (-COOH groups) of carbon nanotubes is known to improve their dispersion properties and increase the electrical conductivity of carbon-nanotube-polymer nanocomposites. We have studied experimentally the effects of this type of functionalization on the thermal conductivity of the nanocomposites. It was found that while even small quantities of carbon nanotubes (~1 wt %) can increase the electrical conductivity, a larger loading fraction (~3 wt %) is required to enhance the thermal conductivity of nanocomposites. Functionalized multi-wall carbon nanotubes performed the best as filler material leading to a simultaneous improvement of the electrical and thermal properties of the composites. Functionalization of the single-wall carbon nanotubes reduced the thermal conductivity enhancement. The observed trends were explained by the fact that while surface functionalization increases the coupling between carbon nanotube and polymer matrix, it also leads to formation of defects, which impede the acoustic phonon transport in the single-wall carbon nanotubes. The obtained results are important for applications of carbon nanotubes and graphene flakes as fillers for improving thermal, electrical and mechanical properties of composites.

Atomistic Modeling of Charge Transport across a Carbon Nanotube–Polyethylene Junction

The conduction mechanism in carbon nanotube (CNT) polymer nanocomposites is complex, and there has been a considerable amount of work invested in understanding the role of the distribution of the CNTs in the composite and how it influences the conductivity. However, less interest has been devoted to the electron transport across a single CNT–polymer–CNT junction. We present a first atomistic study of the electron transmission through a CNT–polyethylene–CNT junction. The morphology of the junction is described using classical molecular dynamics simulations, and transport properties are calculated within density functional tight binding method. The electron transmission depends noticeably on the CNT–CNT separation and on the consequent polymer wrapping. At CNT–CNT distances shorter than 6 Å, the polyethylene molecules do not penetrate in the space between the CNTs. In this near contact regime, the electron transmission proceeds via direct tunneling between the two CNTs across a vacuum region without relevant contribution from the surrounding polymer. For distances larger than 6 Å, the PE molecules enter into the junction region. The frontier orbitals of the PE molecules in the junction provide localized states, which can couple to the CNT metallic states. This resonance tail increases the electron transmission probability between the CNTs across the junction by several orders of magnitude, thus lowering the effective barrier. The gradual interpenetration of the polymer is resembled in transmission fluctuations. An averaging of the transmission in energy and time along MD trajectories allows a quantitative estimation of the junction resistance and tunneling barrier.

Structure and Morphology Control in Crystalline Polymer–Carbon Nanotube Nanocomposites

Polymer nanocomposites have been an area of active research for the past 20 years. Of all potential fillers for polymer nanocomposites, carbon nanotubes (CNTs) are of particular interest due to their low mass density, high aspect ratio, and excellent mechanical, electrical, and thermal properties. In semicrystalline polymer CNT nanocomposites (PCNs), CNTs are viewed as nucleation agents that can affect polymer crystallization. However, it is challenging to quantify and compare results from different research groups, mainly due to the complexity of CNTs. Different chiralities, diameters, surface functional groups, surfactants used, and sample preparation processes can affect PCN crystallization. In this Perspective, we will focus on the structure, morphology, and related applications of semicrystalline PCNs. We will first present the introduction to semicrystalline PCNs followed by a brief discussion on transcrystallization and linear nucleation in polymers. The detailed interface structure and morphology ...