Nanofiller Aspect Ratio Research Articles

Mechanistic insight into the role of the aspect ratio of nanofillers in the gas barrier properties of polymer nanocomposite thin films

The introduction and processing parameters of 2D graphene flakes into a polymer matrix were altered in order to understand the impact of dispersed flake geometry on the physical properties of nanocomposites.

Influence of graphene nanoplatelets geometrical characteristics on the properties of polylactic acid composites

The influence of graphene nanoplatelets (GNP) surface area and size on the dispersion, electrical percolation behavior, electrical conductivity, thermal conductivity, electromagnetic interference (EMI) attenuation, and EMI attenuation mechanisms of GNP/polylactic acid (PLA) were investigated. The GNP/PLA composite's electrical percolation threshold decreased with the increase in GNP surface area and decrease in GNP size. The higher number of GNP particles associated with the decrease in GNP size and increase in GNP surface area facilitated the creation of a conductive network at a lower GNP content. However, at a GNP loading of 30 wt%, the GNP/PLA composites' electrical, EMI SE, and thermal conductivity decreased with the increase in GNP surface area and decrease in GNP size. At high nanofiller loading, the contact resistance between the nanofiller particles is more important than the nanofiller aspect ratio or size. The smaller filler size means more particles and more filler/filler resistance points. The most exciting finding is the dependence of EMI attenuation mechanisms on the GNP surface area. EMI attenuation by absorption was found to increase with the increase in GNP surface area.

Mechanical Properties of a Biocomposite Based on Carbon Nanotube and Graphene Nanoplatelet Reinforced Polymers: Analytical and Numerical Study

Biocomposites based on thermoplastic polymers and natural fibers have recently been used in wind turbine blades, to replace non-biodegradable materials. In addition, carbon nanofillers, including carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs), are being implemented to enhance the mechanical performance of composites. In this work, the Mori–Tanaka approach is used for homogenization of a polymer matrix reinforced by CNT and GNP nanofillers for the first homogenization, and then, for the second homogenization, the effective matrix was used with alfa and E-glass isotropic fibers. The objective is to study the influence of the volume fraction Vf and aspect ratio AR of nanofillers on the elastic properties of the composite. The inclusions are considered in a unidirectional and random orientation by using a computational method by Digimat-MF/FE and analytical approaches by Chamis, Hashin–Rosen and Halpin–Tsai. The results show that CNT- and GNP-reinforced nanocomposites have better performance than those without reinforcement. Additionally, by increasing the volume fraction and aspect ratio of nanofillers, Young’s modulus E increases and Poisson’s ratio ν decreases. In addition, the composites have enhanced mechanical characteristics in the longitudinal orientation for CNT- reinforced polymer and in the transversal orientation for GNP-reinforced polymer.

Multi-Scale Numerical Modelling for Predicting Thermo-Physical Properties of Phase-Change Nanocomposites for Cooling Energy Storage

One major limitation of phase-change materials (PCM) for thermal energy storage comes from their poor thermal conductivity hindering heat transfer process and power density. Nanocomposites PCMs, where highly conductive nanofillers are dispersed into PCM matrices, have been exploited in the past decades as novel latent heat storage materials with enhanced thermal conductivity. A computational model based on continuum simulations capable to link microscopic characteristics of nanofillers and the bulk PCM with the macroscopic effective thermal conductivity of the resulting nanocomposite is the aim of this work. After preliminary mean-field simulations investigating the impact of the nanofiller aspect ratio on the thermal conductivity of the nanocomposite, finite element simulations at reduced aspect ratios have been performed with corrected thermal conductivity values of the filler, to take into account the thermal interface resistances between fillers and matrix. Finally, the thermal conductivity at the actual aspect ratios has been extrapolated by the results obtained at reduced aspect ratios thus saving computational time and meshing efforts. This method has been validated through comparison against previous literature evidence and new experimental characterizations of nanocomposite PCMs.

Mechanical and magnetic response of magneto‐rheological elastomers with different types of fillers and their hybrids

AbstractThe use of secondary reinforcing fillers such as graphene or multi‐wall carbon nanotube (CNT) for improving mechanical properties is an effective strategy to obtain high performance magneto‐rheological elastomers (MREs). Therefore, MREs were prepared via solution mixing of room‐temperature‐vulcanized silicone rubber (RTV‐SR) and iron oxide (Fe3O4), and hybridized with iron wire, Few‐layer graphene (FLG), and CNT as secondary nanofillers. The isotropic mechanical properties were studied through compressive tests, and their compressive moduli and reinforcing factors were determined. From these studies, it was found that CNT‐based hybrid MREs show the highest compressive mechanical properties. For example, at 80 per hundred parts of rubber (phr), the compressive modulus was 3.5 MPa (RTV‐SR/Fe3O4), 4.1 MPa (RTV‐SR/Fe3O4‐FLG), 5.5 MPa (RTV‐SR/Fe3O4‐CNT), and 3.7 MPa (RTV‐SR/Fe3O4‐iron wires). A nanofiller with higher aspect ratio (CNT), has a strong effect on the isotropic mechanical properties, whereas a lower aspect ratio nanofiller (FLG) has a greater effect on magnetic sensitivity. Although iron wires with lower specific surface areas can only improve the mechanical properties slightly, they can improve the magnetic sensitivity of MREs because of its stronger magnetic behavior.

A dual-role theory of the aspect ratio of the nanofillers for the thermal conductivity of graphene-polymer nanocomposites

The extremely low thickness-to-length ratio (aspect ratio) of graphene nanofillers has long been considered beneficial to the overall thermal conductivity of graphene-polymer nanocomposites as it creates large surface area and extra channels for phonon transport. From time to time, however, a lower aspect ratio of the nanofillers did not yield a higher thermal conductivity as expected. To explain such phenomena, we explored the dual role played by the aspect ratio of graphene nanofillers in the effective thermal conductivity via an effective-medium approximation based on Maxwell's far-field matching with consideration of imperfect graphene interfaces. On one hand, we found that the initiation of the graphene thermal networks relies solely on the aspect ratio. A lower aspect ratio tends to boost the establishment of the thermal channels. On the other, we discovered a persistent exponential law that relates the graphene interfacial thermal coefficient to the aspect ratio regardless of the matrix material. A lower aspect ratio causes a higher interfacial thermal resistance, thus a possible lower effective thermal conductivity. These two effects are competing against each other in the framework of this theory. The origin of the second effect was discussed and the effect cannot be neglected with an aspect ratio below 0.01. This dual-role theory was confirmed by a large number of experimental observations.

Nano-mesh superstructure in single-walled carbon nanotube/polyethylene nanocomposites, and its impact on rheological, thermal and mechanical properties

The mesh formation of single-walled carbon nanotube (SWCNT) superstructure, i.e. nano-mesh, in low- and high-density polyethylene (LDPE and HDPE) matrices was observed by using plasma etching technique and SEM analysis. Nano-mesh was established by two mechanisms: i) alignment of SWCNT bundles (forming network backbone) and ii) shish-kebab induced crystallization (forming network net). The network was formed at low SWCNT content due to the high aspect ratio of nano-filler, which additionally contributed to alignment. Established nano-mesh superstructure caused confinement effect disrupting crystallization and melting kinetics, which directly affected mechanical properties. Although mechanical properties of LDPE and HDPE nanocomposites profoundly improved, i.e. Young’s modulus increased for 230% and 30%, respectively, at only 1 wt% of SWCNTs, HDPE nanocomposites showed visible confinement that reduced effective reinforcement. It seems that HDPE nanocomposites cannot achieve full reinforcing potential since confinement effect restricts the crystal growth, which gradually reduces effective reinforcement in semi-dilute regime.

Raman imaging of surface and sub-surface graphene oxide in fiber reinforced polymer nanocomposites

Visualization of dispersion is a challenging and unresolved issue for high aspect ratio nanofillers, such as surface-modified graphene oxide (mGO) and carbon nanotubes, in fiber reinforced polymer (FRP) composites. Conventional metrologies to study dispersion (location, spacing, size, etc.) have proven difficult and impractical when the fibers are present, particularly at the industrially-relevant, low mass fractions of nanofillers. Here, Raman spectroscopy and imaging are exploited to investigate the dispersion of mGO in unsaturated polyester (UP) resin composites both with and without glass microfiber reinforcement. Through comparative TEM and Raman maps on the same sample area, we validate a detailed Raman methodology to detect mGO agglomerate positions, even at low loadings of < 0.1 % by mass, where the sensitivity of the mGO Raman signal is dependent on incident laser wavelength. More importantly, we expand this to FRP composites, where by a proper choice of incident wavelength to avoid glass fiber fluorescence, Raman imaging is able to identify surface (2D) and sub-surface (3D) mGO microstructures near microfibers in the inter- and intralaminar regions. This measurement technique will find significant use in the FRP composite community as a readily available method to qualitatively correlate dispersion properties to processing techniques and/or mechanical performance.

The effects of nano-sized carbon fillers on the physico-chemical, mechanical, and biological properties of polyester nanocomposites

Nanocomposites based on poly(ethylene terephthalate-ethylene dilinoleate) (PET-DLA) copolymers of different hard to soft segment ratios (40:60 and 60:40) and three different carbon nanofillers of different aspect ratios (dimensions), as 0D carbon black, 1D multiwalled carbon nanotubes, and 2D graphene, have been prepared in situ during two-stage polymerization. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy were used to characterize the chemical structures of the obtained nanocomposites. Scanning electron microscopy (SEM) indicated very good dispersions of all carbon nanofillers in both polymer matrices. Differential scanning calorimetry (DSC) results revealed that the addition of nano-sized fillers eliminated cold crystallization of materials containing 40% hard segments in polymer matrix. We found that the high aspect ratio, 1D nano-filler (multiwalled carbon nanotubes) strongly nucleated crystallization of materials containing 60% of hard segments. This nanofiller also yielded the greatest improvement in the Young’s modulus as assessed by tensile tests, both at 24 °C and 37 °C. We did not observe reduced bacterial adhesion to nanocomposites, likely due to increased roughness. Importantly, in vitro cytocompatibility tests with L929 murine fibroblasts demonstrated cell viability and growth on all materials except those containing carbon nanotubes. Finally, both high aspect ratio nanofillers markedly improved the barrier properties of obtained nanocomposites. New materials were successfully used for manufacturing of prototype of heart assist device, with pneumatic membrane made of graphene nanocomposite.

Electrically and thermally conductive thin elastic polymer foils containing SiC nanofibers

Design and experiment of polymeric nanocomposites (NCs) for photovoltaic applications with outstanding electrical and thermal properties has been investigated with the introduction of SiC nanofibers (NFs) into the poly(trimethylene terephthalate)-block-poly(tetramethylene oxide) (PTT-PTMO) copolymers. In order to enhance the electrical and thermal conductivity, different concentrations of SiC NFs, ranging from 0.1 to 3.0 wt %, have been selected to mix with PTT-PTMO via in situ polymerization method. This reaction method is an excellent choice for incorporation of high amount of SiC NFs (3 wt %) into the polymer that was confirmed by morphological studies. From dielectric spectroscopy studies a percolating behavior was confirmed at low percolation threshold (less than 2% wt %). Furthermore, the 15% increment for thermal conductivity appeared with combination of 0.5 wt % SiC NFs with PTT-PTMO copolymers, which can be affected by manufacturing process of NCs, state of nanofillers dispersion and aspect ratio of nanofillers.

Measurable and Influential Parameters That Influence Corrosion Performance Differences between Multiwall Carbon Nanotube Coating Material Combinations and Model Parent Material Combinations Derived from Epoxy-Amine Matrix Materials.

Protective coatings are often erroneously thought of as perfect environmental barriers for metal substrates; however, a host of corrosion inducing environmental contaminants permeate through defect-free coatings. Carbon nanotubes are high aspect ratio nanofillers with unique mechanical, electrical, and polymer interaction properties with well-established yet, for practical reasons, often unrealized potential. The research objective was to quantify and understand the influential effects and relationships between low concentration levels of multiwall carbon nanotubes (MWCNT) dispersed into epoxy-amine matrix materials and the different water hydrogen bonding interactions on corrosion rates of steel substrates. We hypothesize that when water directly hydrogen bonds with polymer, substrate and/or MWCNTS, the localized water's capacity to transfer environmental contaminants through the coating, i.e., to and from the substrate, diminishes due to a reduced potential to contribute to the formation of water hydration shells and therefore aid in diminishing the corrosion rate. We measured the absolute pre-exposure water content, and monitored to delineate between the ratio and shifting ratio of in situ free versus bound water hydrogen bonding interactions at the coating/air interface using ATR-FTIR spectroscopy in a 5% NaCl fog environment in an attempt to correlate these differences with experimental corrosion rates. Free water content was reduced from ∼20% to <1% of the total water concentration when 1.0 wt % MWCNTs was dispersed into the parent polymer network. Concurrently, the bound water content was measured to shift from ∼2% to >80% with the same MWCNT concentration. The MWCNT bound water resulted in 25% less corrosion for the same steel substrates albeit the measured water vapor diffusivity was the same for each material combination evaluated. Interestingly, the measured pre-exposure bound water content was predictive of which material would corrode slowest and fastest, i.e., the ratio of starting water states seems to be mechanistically related to the corrosion process and the values have potential to predict corrosion rates for a variety of samples evaluated.

Significantly enhanced energy storage density for poly(vinylidene fluoride) composites by induced PDA-coated 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 nanofibers

The significantly enhanced energy storage characteristics of 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 nanofibers/PVDF dielectric composites with low doping ratios and large aspect ratio nanofillers.

Structured Polyethylene Nanocomposites: Effects of Crystal Orientation and Nanofiller Alignment on High Field Dielectric Properties

ABSTRACTIn previous work we have shown that aligned high aspect-ratio (pseudo-2D) nanofillers can yield large dielectric breakdown strength (EBD) improvements for a nanocomposite with a low-crystallinity polyethylene matrix. Here, we report a systematic study which delineates the contributions of the aligned inorganic fillers and of the aligned polymer crystallites in the overall EBD improvement achieved in the nanocomposites. Specifically, extrusion blown-molded polyethylene/montmorillonite nanocomposite films were cold-stretched to various strains, to further align the nanoparticles parallel to the film surface; this filler alignment is accompanied by a commensurate alignment of the polymer crystallites, especially those heterogeneously nucleated by the fillers. A systematic series of films are studied, with increased extent of alignment of the fillers and of the crystalline lamellae (quantified through Hermans orientation order parameters from 2D X-ray diffraction studies) and the aligned structure is correlated to the electric field breakdown strength (quantified through Weibull failure studies). It is shown that aligned pseudo-2D inorganic nanofillers provide additional strong improvements in EBD, improvements that are beyond, and added in excess of, any EBD increases due to polymer-crystal orientation.

A Micromechanical Model for Effective Thermo-elastic Properties of Nanocomposites with Graded Properties of Interphase

In this study, a micromechanics-based analytical model is proposed to evaluate the effective thermo-elastic properties of polymer matrix nanocomposite materials. Accuracy, simplicity and efficiency are the main features of this micromechanical model. The constituents of representative volume element of nanocomposites are treated as three distinct phases, consisting of nanofiller, polymer matrix and interphase around the nanofiller. Young’s modulus and coefficient of thermal expansion of the interphase are continuously graded from those of the nanofiller to those of the polymer matrix. The effects of nanoparticle volume fraction, nanoparticle size, interphase thickness, nanofiller aspect ratio and number of layers in the interphase on the thermo-elastic properties of nanocomposites are studied. The comparison of results of the presented model with experimental data and other available micromechanical analysis demonstrates the validity of the proposed micromechanical model in the case of response of nanocomposites with graded properties of interphase.

An updated micromechanical model based on morphological characterization of carbon nanotube nanocomposites

Abstract By leveraging on extensive morphological analysis of carbon nanotube nanocomposites, an update of the Eshelby-Mori-Tanaka method is proposed for a more accurate estimation of the nanocomposites effective elastic response. The experimental results are employed to overcome the main modeling limitations inherent in most common micromechanical theories, such as the perfect dispersion of the nanofiller and the uniformity of the nanofiller's aspect ratio within the nanocomposite. The actual variability of the CNTs aspect ratio and the CNTs degree of dispersion are experimentally measured and introduced in the proposed model by averaging the Eshelby tensor over the actual CNT lengths distribution and by accounting for the effective CNT volume fraction. The effects of the nanofiller morphology on the mechanical response of three different thermoplastic nanocomposites with low- and high-aspect ratio CNTs are explored, and monotonic tensile tests are performed to validate the predictions of the proposed model. A good agreement is found between the predicted nanocomposites elastic moduli and the experimental data.

Effect of multi-scale diffusion on the permeability behavior of intercalated nanocomposites

This paper is aimed to study the morphology of intercalated nanocomposites, by coupling experimental permeability data with different analytical models. X-Ray diffraction provided the reference morphological features of the nanocomposite, including gallery thickness and aspect ratio of the lamellar stacks. Afterward, the water permeability of two intercalated nanocomposites was used for the calculation of the nanofiller aspect ratio, following different approaches. The obtained results indicate that an assumption of impermeable stacks involves a significant over-estimation of the nanofiller aspect ratio. Further, when the morphological features determined in the assumption of impermeable particles are used for estimation of the nanocomposite diffusivity by the use of the Fricke model, results do not show a satisfactory agreement with experimental data. On the other hand, fitting experimental permeability data with an analytical model accounting for intra-stack diffusion provided an estimation of the nanofiller aspect ratio in excellent agreement with that obtained by XRD. Further, applying the Fricke equation with the morphological features determined by the permeable stack model, an excellent agreement to the experimental data was obtained. The results indicate the relevance of intra-stack diffusion in intercalated nanocomposite, and the need to account for it when modeling mass transfer in nanocomposites.

Finite element simulation and analytical modeling of 3D multi scale diffusion in nanocomposites with permeable stacks

This work is aimed to study the mass transport in 3D nanocomposites, characterized by the presence of permeable lamellar stacks, by means of finite element (FE) analysis. To this purpose, a geometric model was developed, based on a random distribution of non-interpenetrating stacks, each one made of regularly spaced platelets, which are considered representative of an intercalated nanocomposite. The morphological features of the stacks are the number of lamellae and the thickness of lamellar galleries, which determine the thickness, and therefore the aspect ratio. FE simulation results showed the relevance of diffusion within stack, and therefore the unsuitableness of the assumption of stack impermeability. The diffusion behavior of nanocomposites made of permeable stacks was modeled by considering the probability of collision of diffusing particles on the stack surface. For a random orientation of stacks, the developed analytical model showed an excellent agreement with the FE simulation results. It was shown that other analytical models found in literature are not able to capture the dependence of diffusivity on the morphology of intercalated nanocomposites. The developed analytical model allowed estimating the error arising from the assumption of impermeable stacks in the estimation of nanofiller aspect ratio from experimental diffusivity data.

Electrical and mechanical properties of graphene/carbon nanotube hybrid nanocomposites

Polymer nanocomposites with improved electrical and mechanical properties can be formulated by proper dispersion of conductive high aspect ratio nanofiller within the host polymer matrix. In this work, the microstructure, processing behavior, electrical and mechanical properties of melt-mixed graphene nanoplatelets (GNP)/polypropylene (PP) nanocomposites were investigated. In addition, the influence of processing conditions on the electrical properties of GNP/PP nanocomposites was evaluated. Moreover, GNP: carbon nanotube (CNT)/PP hybrids were prepared aiming at formulating nanostructured material with synergistic properties. The electrical percolation threshold of GNP/PP nanocomposite was found to be ∼4vol%. However, upon the introduction of only 1wt% (0.52vol%) CNT, significant improvement in the electrical properties was obtained. The CNT was found to bridge the distance between the GNP particles. At constant overall nanofiller concentration, GNP:CNT/PP hybrids exhibited improved properties with the increase in CNT volume fraction due to the better dispersion of nanotubes and strong adhesion at the nanotube/PP interface.

Influence of conductive network structure on the EMI shielding and electrical percolation of carbon nanotube/polymer nanocomposites

Conductive polymer nanocomposites (CPNC) are promising materials for electromagnetic interference shielding (EMI) applications. However, the relatively high cost of high aspect ratio nanofillers hinders the wide commercial use of these materials. Promoting the competitiveness of CPNCs requires formulation nanocomposites with the desired EMI capabilities at the lowest possible nanofiller loading. This requires better understanding for relation between the microstructure and EMI attenuation mechanisms, which is the objective of this work. Herein, CNPCs with segregated conductive network were prepared by placing carbon nanotube (CNT) particles at the external surface of ultrahigh molecular weight polyethylene (UHMWPE) powder by wet mixing. The microstructure, electrical and EMI shielding properties of the nanocomposites after compression molding were investigated. The EMI SE was found to increase with CNT content. An EMI SE of 50dB was reported for a 1.0mm thick plate made of 10wt% CNT/UHMWPE nanocomposite. This nanostructured material is suitable for many applications in the computer and electronics industries. Compared to CNT/polymer nanocomposite of fine and well-dispersed CNT microstructure, the unique structure of the CNT/UHMWPE characterized by thick and segregated CNT network was found to enhance the EMI shielding by absorption and reduce the reflection of the EMI.

Processing and characterization of amorphous polyethylene terephthalate fibers for the alignment of carbon nanofillers in thermosetting resins

The engineering aspects, which are associated with the development of nanocomposites, involve in either final properties or processability of nanocomposites. Both are affected by the distribution of nanofiller in the matrix and by the aspect ratio of the nanofiller. A nanofilled thermosetting resin can be exploited as a matrix for continuous fibers when the alignment of a high aspect ratio nanofiller is achieved, and in this case, a hierarchical composite is obtained. In this study, a new procedure for the alignment of nanofillers in a thermosetting matrix is proposed. The two‐step approach is based on (i) the alignment of nanofillers (carbon nanotubes, graphene, etc.) in thermoplastic fibers by a fiber spinning process and (ii) the use of these nanocomposite fibers as a carrier to bring aligned nanofillers into a reactive thermosetting resin. These fibers, soluble in the thermosetting resin, release the nanofillers orientated according to the direction in which fibers are positioned, even after the curing of the matrix. The proof of concept is demonstrated by producing melt spun polyethylene terephtalate (PETg) fibers filled with graphene nanoplatelets (GNP) and multiwall carbon nanotubes (MWCNT) with a very high filler content (up to 10 wt%) in view of producing a hierarchical composite. POLYM. COMPOS., 36:1096–1103, 2015. © 2015 Society of Plastics Engineers