Interphase In Nanocomposites Research Articles

Nanomechanical characterization of carbon nanotube-based composite interfaces tailored by electrophoretic deposition

Carbon nanotube (CNT) addition to composite materials can offer both nanoscale reinforcement and a multifunctional element due to their extraordinary mechanical, thermal and electrical properties. Electrophoretic deposition (EPD) offers a scalable processing technique to incorporate CNTs into conventional fiber-reinforced polymer composites (FRPCs), facilitating the production of unique nanoscale structures in the critical interphase region. In this study, CNTs functionalized with polyethyleneimine (CNT-PEI) were deposited onto a planar substrate via EPD followed by the infusion of epoxy matrix in order to replicate the nanocomposite interphase region present in nanomodified FRPCs. The nanocomposite films have thicknesses ranging from several hundred nanometers to a few microns to represent different fiber-matrix interphase regions found in FRPCs. The morphology and mechanical performance of CNT-PEI/epoxy nanocomposites are examined using atomic force microscopy (AFM) in both tapping and nanoindentation modes. The EPD creates a homogeneously distributed porous CNT network bridged by PEI, forming the pathway of epoxy resin infusion through interconnected pores with diameters less than 100 nm. CNT-PEI/epoxy nanocomposites exhibited significant improvements in stiffness, hardness, and creep resistance compared to constituent porous CNT-PEI films and neat epoxy. The improvement was directly related to the ability of the load bearing CNTs chemically bonded with the epoxy matrix through the grafted PEI, providing an efficient load transfer mechanism. The chemical bond between the porous CNT-PEI and epoxy also produced far greater fracture surface in nanoscale scratch tests compared to unmodified epoxy, indicating the CNT-PEI/epoxy nanocomposite is capable of distributing load and absorbing more energy prior to fracture.

A developed Takayanagi model to estimate the tensile modulus and interphase characteristics of polymer nanocellulose composites

This study analyzes the tensile modulus of nanocellulose-reinforced composites by developing the Takayanagi model considering the interphase section and filler size. The model accuracy is assessed by reported experimental moduli of composites and by parametric studies. The estimates of advanced model fit to the measured moduli of numerous examples. Also, it is revealed that the treatment of nanocellulose increases the modulus and thickness of interphase in nanocomposites. The relative modulus (Er) as the ratio of nanocomposite modulus to the modulus of matrix significantly changes when nanocellulose modulus (Ef) is raised from 50 to 100 GPa; however, Er unimportantly rises at Ef > 100 GPa. Additionally, nanocellulose radius (R) of 2 nm maximizes the Er to 1.636, though Er decreases to 1.09 at R = 58 nm and the variation of Er is insignificant at R > 20 nm. Furthermore, the interphase moduli (Ei) below 10 GPa meaningfully affect the nanocomposite modulus, but Ei > 10 GPa cannot cause the considerable change in Er. Therefore, Er does not change at the high levels of Ef, R and Ei.

Novel techniques for characterising graphene nanoplatelets using Raman spectroscopy and machine learning

A significant challenge for graphene nanoplatelet (GNP) suppliers is the characterisation of platelet morphology in industrial environments. This challenge is further exacerbated to platelet surface chemistry when scalable functionalisation processes, such as plasma treatment, are used to modify the GNPs to improve the filler-matrix interphase in nanocomposites. The costly and complex suite of analytical equipment necessary for a complete material description makes quality control and process optimisation difficult. Raman spectroscopy is a facile and accessible characterisation technique, with recent advancements unlocking fast mapping for rapid data collection. In this study, we develop novel techniques to better characterise GNP morphology and changes in surface chemistry using Raman maps of bulk powders. Providing a bespoke algorithmic framework for the analysis of these advanced materials. An unsupervised peak fitting and processing algorithm was used to extract crystallinity data and correlate it with laser-diffraction-derived lateral size values for a commercial set of GNPs rapidly and accurately. Classical machine learning was used to identify the most informative Raman features for classifying the plasma-functionalised GNPs. The initial material properties were found to affect the peak features that were the most useful for classification. In low defect density and low specific surface area GNPs, the D peak full width at half maximum is found to be the most useful, whereas the ratio is the most useful in the opposite case. Finally, a convolutional neural network was trained to discern between different GNP grades with 86% accuracy. This work demonstrates how computer vision could be deployed for rapid and accurate quality control on the factory floor.

Carbon nanotube network and interphase in buckypaper nanocomposites using atomic force microscopy

The CNT network and interphase in nanocomposites have not been studied quantitatively enough in the literature. The AFM peak force quantitative nanomechanics mapping technique was applied to polymer nanocomposites (PNC) reinforced with 0.5 wt% MWCNT and CNT buckypaper (BP). The DMA, BJH, SEM, and tension stress-strain curves were studied. BP is heterogeneous due to its local porous structure and nonuniform resin impregnation. The Interphase thickness for the CNT network is 26.8 ± 7 and 17.4 ± 5.1 nm in BP PNC and 0.5 wt% MWCNT PNC. The interphase thickness between infiltrated BP and polymer is ∼40 ± 4 nm. The CNT network sizes in BP PNC (108.5 ± 21.3 and 45.4 ± 10.3 nm) are influenced by the size of inter-bundle and intra-bundle pores. The interphase thickness normalized to the CNT network size is at least ∼7 times higher in BP PNC compared to 0.5 wt% MWCNT PNC, confirming pronounced interphase effects in the former. The Kolarik, Quali, and Takayanagi models for nanocomposite moduli were evaluated.

Hybrid hierarchical homogenization theory for unidirectional CNTs-coated fuzzy fiber composites undergoing inelastic deformations

A new hybrid homogenization approach is proposed for simulating the homogenized and local response of unidirectional fuzzy fiber nanocomposites undergoing inelastic deformations. Fuzzy fiber composites are hierarchical reinforcing structures where the fibers coated with radially aligned carbon nanotubes (CNTs) are embedded in the matrix. In this spirit, the fuzzy fiber composites are modeled as a three-scale medium. At the microscale, the CNTs-reinforced matrix is homogenized as nanocomposite interphase (NCP) attached to the main fiber via the asymptotic expansion homogenization (AEH). At the mesoscale, an intermediate equivalent fiber that substitutes for the NCP and the main fiber is constructed using the composite cylinder assemblage (CCA) and the transformation field analysis (TFA) techniques. At the macroscale, homogenization of the equivalent fiber and the surrounding matrix is handled by AEH which yields the effective response of the whole fuzzy fiber composites. The new technique facilitates accurate and efficient studies of the inelastic deformation mechanisms of periodic fuzzy fiber arrays with single or multiple inclusions under biaxial and triaxial loading conditions, eliminating exhausting interphase mesh discretizations encountered in the classical full-field homogenization. An added advantage is that the proposed theory captures the fiber-fiber interaction neglected in the classical CCA-TFA, an issue that leads to the exceptionally stiff post-yielding stress-strain response common in the mean-field micromechanics approaches.

Comparative Study of Direct Compounding, Coupling Agent-Aided and Initiator-Aided Reactive Extrusion to Prepare Cellulose Nanocrystal/PHBV (CNC/PHBV) Nanocomposite

Three compounding methods were compared to determine their effects on the organization of the interphase in nanocomposites of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and cellulose nanoc...

Modeling the Compressive Buckling Strain as a Function of the Nanocomposite Interphase Thickness in a Carbon Nanotube Sheet Wrapped Carbon Fiber Composite

Polymer matrix composites have high strengths in tension. However, their compressive strengths are much lower than their tensile strengths due to their weak fiber/matrix interfacial shear strengths. We recently developed a new approach to fabricate composites by overwrapping individual carbon fibers or fiber tows with a carbon nanotube sheet and subsequently impregnate them into a matrix to enhance the interfacial shear strengths without degrading the tensile strengths of the carbon fibers. In this study, a theoretical analysis is conducted to identify the appropriate thickness of the nanocomposite interphase region formed by carbon nanotubes embedded in a matrix. Fibers are modeled as an anisotropic elastic material, and the nanocomposite interphase region and the matrix are considered as isotropic. A microbuckling problem is solved for the unidirectional composite under compression. The analytical solution is compared with finite element simulations for verification. It is determined that the critical load at the onset of buckling is lower in an anisotropic carbon fiber composite than in an isotropic fibfer composite due to lower transverse properties in the fibers. An optimal thickness for nanocomposite interphase region is determined, and this finding provides a guidance for the manufacture of composites using aligned carbon nanotubes as fillers in the nanocomposite interphase region.

Electrostatic force microscopy for the accurate characterization of interphases in nanocomposites.

The unusual properties of nanocomposites are commonly explained by the structure of their interphase. Therefore, these nanoscale interphase regions need to be precisely characterized; however, the existing high resolution experimental methods have not been reliably adapted to this purpose. Electrostatic force microscopy (EFM) represents a promising technique to fulfill this objective, although no complete and accurate interphase study has been published to date and EFM signal interpretation is not straightforward. The aim of this work was to establish accurate EFM signal analysis methods to investigate interphases in nanodielectrics using three experimental protocols. Samples with well-known, controllable properties were designed and synthesized to electrostatically model nanodielectrics with the aim of “calibrating” the EFM technique for future interphase studies. EFM was demonstrated to be able to discriminate between alumina and silicon dioxide interphase layers of 50 and 100 nm thickness deposited over polystyrene spheres and different types of matrix materials. Consistent permittivity values were also deduced by comparison of experimental data and numerical simulations, as well as the interface state of silicone dioxide layers.

Effect of grain boundaries on the interfacial behaviour of graphene-polyethylene nanocomposite

Aim of this article was to investigate the effect of grain boundaries on the interfacial properties of bi-crystalline graphene/polyethylene based nanocomposites. Molecular dynamics based atomistic simulations were performed in conjunction with the reactive force field parameters to capture atomic interactions within graphene and polyethylene atoms, whereas non-bonded interactions were considered for the interfacial properties. Atoms at the higher energy state in bi-crystalline graphene helps in improving the interaction at the nanocomposite interphase. Geometrical imperfections such as wrinkles and ripples helps the bi-crystalline graphene in increasing the number of adhesion points between the nanofiller and matrix, which eventually improves the strength and toughness of nanocomposite. These outcomes will help in opening new opportunities for defective nanofillers in the development of nanocomposites for future applications.

Thin and Flexible Carbon Nanotube-Based Pressure Sensors with Ultrawide Sensing Range.

A scalable electrophoretic deposition (EPD) approach is used to create novel thin, flexible, and lightweight carbon nanotube-based textile pressure sensors. The pressure sensors can be produced using an extensive variety of natural and synthetic fibers. These piezoresistive sensors are sensitive to pressures ranging from the tactile range (<10 kPa), the body weight range (∼500 kPa), and very high pressures (∼40 MPa). The EPD technique enables the creation of a uniform carbon nanotube-based nanocomposite coating, in the range of 250-750 nm thick, of polyethyleneimine (PEI) functionalized carbon nanotubes on nonconductive fibers. In this work, nonwoven aramid fibers are coated by EPD onto a backing electrode followed by film formation onto the fibers creating a conductive network. The electrically conductive nanocomposite coating is firmly bonded to the fiber surface and shows piezoresistive electrical/mechanical coupling. The pressure sensor displays a large in-plane change in electrical conductivity with applied out-of-plane pressure. In-plane conductivity change results from fiber/fiber contact as well as the formation of a sponge-like piezoresistive nanocomposite "interphase" between the fibers. The resilience of the nanocomposite interphase enables sensing of high pressures without permanent changes to the sensor response, showing high repeatability.

Identifying interphase properties in polymer nanocomposites using adaptive optimization

To predict the properties of nanocomposites, computational models have demonstrated that the interphase behavior can be expressed using the matrix properties with a modified (shifted) frequency. The amount of the shift necessary for a given sample data set can be determined by achieving the best fit between the predicted curve from a computation model and the experimental data through trial-and-error. However, with the complexity of experimental data and expensive computational costs, a manual process to solve this inverse problem is impractical to handle many experimental data sets. This difficulty hinders investigation of the underlying principles behind nanocomposite interphase. In this work, we present an adaptive optimization approach that accelerates the search for interphase properties in polymer nanocomposite data sets by solving the inverse problem using global optimization. The objective is to minimize the difference between the predicted bulk property of a nanocomposite with that from the experiment data. A Gaussian Process (GP) model is built as a surrogate of the objective function with quantification of prediction uncertainty. An adaptive sampling strategy is applied to effectively navigate the complex search space by iteratively selecting the next sampling point based on an expected improvement function. The surrogate model and the optimal solution evolve until the desired objective is achieved. The approach is tested on both the simulations of dielectric and viscoelastic properties in nanocomposites. Our work provides insight into identifying the interphase properties for polymer nanocomposites using adaptive optimization and demonstrates the potential of data-driven approach for achieving a deeper understanding of the interphase properties and its origins.

"Brick-and-Mortar" Nanostructured Interphase for Glass-Fiber-Reinforced Polymer Composites.

The fiber-matrix interface plays a critical role in determining composite mechanical properties. While a strong interface tends to provide high strength, a weak interface enables extensive debonding, leading to a high degree of energy absorption. Balancing these conflicting requirements by engineering composite interfaces to improve strength and toughness simultaneously still remains a great challenge. Here, a nanostructured fiber coating was realized to manifest the critical characteristics of natural nacre, at a reduced length scale, consistent with the surface curvature of fibers. The new interphase contains a high proportion (∼90 wt %) of well-aligned inorganic platelets embedded in a polymer; the window of suitable platelet dimensions is very narrow, with an optimized platelet width and thickness of about 130 and 13 nm, respectively. An anisotropic, nanostructured coating was uniformly and conformally deposited onto a large number of 9 μm diameter glass fibers, simultaneously, using self-limiting layer-by-layer assembly (LbL); this parallel approach demonstrates a promising strategy to exploit LbL methods at scale. The resulting nanocomposite interphase, primarily loaded in shear, provides new mechanisms for stress dissipation and plastic deformation. The energy released by fiber breakage in tension appear to spread and dissipate within the nanostructured interphase, accompanied by stable fiber slippage, while the interfacial strength was improved up to 30%.

Prediction of complex modulus in phase-separated poly (lactic acid)/poly (ethylene oxide)/carbon nanotubes nanocomposites

This study focuses on the modeling of complex modulus in phase-separated poly (lactic acid) (PLA)/poly (ethylene oxide) (PEO)/carbon nanotubes (CNT) nanocomposites. Palierne model for complex modulus of immiscible blends is developed assuming the significances of CNT and interphase regions. The predictions of developed model are compared to the experimental data from rheological experiment and the predictability of the developed model is studied. Furthermore, the roles of main parameters in the complex modulus of nanocomposites are explained to validate the developed model. The calculations show proper agreements with the experimental data confirming the predictability of the developed model. A higher concentration of continuous matrix and a smaller content of PEO droplets cause thicker and stronger interphase in nanocomposites. High CNT concentration and thin CNT mainly improve the complex modulus. Additionally, both thickness and complex modulus of interphase regions directly control the complex modulus of nanocomposites. This study can afford an insight for researchers to control and optimize the complex modulus in immiscible nanocomposites.

Stochastic modeling and generation of random fields of elasticity tensors: A unified information-theoretic approach

In this Note, we present a unified approach to the information-theoretic modeling and simulation of a class of elasticity random fields, for all physical symmetry classes. The new stochastic representation builds upon a Walpole tensor decomposition, which allows the maximum entropy constraints to be decoupled in accordance with the tensor (sub)algebras associated with the class under consideration. In contrast to previous works where the construction was carried out on the scalar-valued Walpole coordinates, the proposed strategy involves both matrix-valued and scalar-valued random fields. This enables, in particular, the construction of a generation algorithm based on a memoryless transformation, hence improving the computational efficiency of the framework. Two applications involving weak symmetries and sampling over spherical and cylindrical geometries are subsequently provided. These numerical experiments are relevant to the modeling of elastic interphases in nanocomposites, as well as to the simulation of spatially dependent wood properties for instance.

Influence of clay mineral structure and polyamide polarity on the structural and morphological properties of clay polypropylene/polyamide nanocomposites

The influence of the clay mineral structure and of the polyamide dispersed phase polarity on the structure and morphology of polypropylene/polyamide blends filled with clay mineral nanoparticles was investigated. Two polyamides (PA) were used: a polar PA6 and a less polar PA12. The clay mineral nanofillers used were either organically modified montmorillonite (Mt) or synthetic talc (ST), having preferential affinity towards PA dispersed phase. For all clay polymer nanocomposites (CPN), a decrease of PA nodule size was observed. However, the mechanisms governing the morphology establishment were shown to depend mainly on the clay structure, and also on the polyamide polarity. Mt nanoparticles were shown to be mostly located at the interface, forming a nanocomposite interphase. The decrease of PA nodule size induced by Mt nanoparticles was attributed to coalescence inhibition by steric repulsions, mediated by the interphase, which is more developed in the case of PA6. Besides, the interphase was shown to play a key role in the change from a nodular to a non-nodular morphology, even at low Mt fractions. ST particles were shown to be exclusively dispersed within PA nodules. In this case, nodule size reduction was attributed to the presence of some larger ST particles, exhibiting numerous structural defects, which favor the nodule break-up, especially in the case of PA12.

Estimation of interphase thickness of epoxy-based nanocomposites

Recent literature attributes the improvement often seen in the properties of nanocomposites to the interphase layer between the polymer matrix and the filler material. However, an understanding of the interphase in nanocomposites is still elusive. The present work aims at estimating the interphase thickness of epoxy based nanocomposites. For experimental studies, epoxy resin is used as the base polymer, and aluminium oxide (Al2O3) particles with an average size of 50 nm are used as fillers. The nanocomposites are characterized in terms of space charge density, conductivity, permittivity, and breakdown strength. All properties are measured for different nanofiller loadings (0.5 vol. % to 2 vol. %). The experimental findings reveal a correlation between interphase volume fraction and change in dielectric properties. It is assumed that the greatest change in property is achieved at that filler loading where the interphase volume fraction is maximized, provided adequate dispersion is achieved at all filler loadings. An approximate interphase thickness is estimated for epoxy alumina nanocomposites by using experimental results in conjunction with a simulation model. Accuracy of the estimated interphase thickness is seen to be dependent on the scatter in particle size and the uniformity of particle dispersion in the polymer matrix.

How Thick Is the Polymer Interphase in Nanocomposites? Probing It by Local Stress Anisotropy and Gas Solubility

The influence of the inclusion of a silica nanoparticle on the spatial distribution of the local stresses and the locally resolved excess Helmholtz free energy of sorption of small penetrants (He, ...

Polymer nanocomposites: structure, interaction, and functionality

This feature article discusses the main factors determining the properties of polymer nanocomposites with special attention paid to structure and interactions. Usually more complicated structure develops in nanocomposites than in traditional particulate filled polymers, and that is especially valid for composites prepared from plate-like nanofillers. Besides the usually assumed exfoliated/intercalated morphology, i.e. individual platelets and tactoids, such nanocomposites often contain large particles, and a network structure developing at large extent of exfoliation. Aggregation and orientation are the most important structural phenomena in nanotube or nanofiber reinforced composites, and ag-gregation is a major problem also in composites prepared with spherical particles. The surface characteristics of nanofillers and interactions are rarely determined or known; the related problems are discussed in the paper in detail. The surface of these reinforcements is modified practically always. The goal of the modification is to improve dispersion and/or adhesion in nanotube and spherical particle reinforced composites, and to help exfoliation in nanocomposites containing platelets. However, modification decreases surface energy often leading to decreased interaction with the matrix. Very limited information exists about interphase formation and the properties of the interphase in nanocomposites, although they must influence properties considerably. The properties of nanocomposites are usually far from the expectations, the main reason being insufficient homogeneity, undefined structure and improper adhesion. In spite of considerable difficulties nanocomposites have great potentials especially in functional applications. Several nanocomposite products are already used in industrial practice demonstrated by a few examples in the article.

Characterization of Polymer Nanocomposite Interphase and Its Impact on Mechanical Properties

The structure of the interphase, a region between nanoparticle fillers and the bulk polymer matrix in a particle reinforced composite, was investigated using two different approaches. The polymer n...