Reinforcement Efficiency of Carbon Nanotubes - Myth and Reality

Percolation in Carbon Nanotube-Reinforced Polymers for Strain-Sensing Applications: Computational Investigation on Carbon Nanotube Distribution, Curvature, and Aggregation

The present article investigates the possibility of simulating the electrical conductivity of carbon nanotube-reinforced polymer composites by numerical methods. Periodic representative volume elements are generated by randomly distributing perfectly conductive reinforcements in an insulating matrix and are used to assemble an electrical network representative of the nanocomposite, where the nanotube-nanotube contacts are considered equivalent resistors modeled by means of Simmons' equation. A comparison of the results with experimental data from the literature supports the conclusion that a random distribution of reinforcements is not suitable for simulating this class of materials since percolation thresholds and conductivity trends are different, with experimental percolation taking place before the expectations. Including nanotube curvature does not solve the issue, since it hinders percolation even further. In agreement with experimental observations, the investigation suggests that a suitable approach requires the inclusion of aggregation during the volume element generation to reduce the volume fraction required to reach percolation. Some solutions available in the literature to generate properly representative volume elements are thus listed. Concerning strain sensing, the results suggest that representative volume elements generated with random distributions overestimate the strain sensitivity of the actual composites.

Understanding the Mechanical Reinforcement of Metal-Organic Framework-Polymer Composites: The Effect of Aspect Ratio

The aspect ratio (AR) of filler particles is one of the most critical determinants for the mechanical properties of particle-reinforced polymer composites. However, it has been challenging to solely study the effect of particle AR due to the difficulties of controlling AR without altering the physical and chemical properties of the particle. Herein, we synthesized PCN-222, a zirconium-based porphyrinic metal-organic framework (MOF) with preferential longitudinal growth as a series of particles with ARs increasing from 3.4 to 54. The synthetic MOF conditions allowed for the chemical properties of the particles to remain constant over the series. The particles were employed as reinforcers for poly(methyl methacrylate) (PMMA). MOF-polymer composite films were fabricated using doctor-blading techniques, which facilitated particle dispersion and alignment in the PMMA matrix, as revealed by optical microscopy and wide-angle X-ray diffraction. Mechanical measurements showed that both elastic and dynamic moduli increased with particle AR and particle concentrations but started to decrease as particle loading increased beyond 0.5 wt % (1.12 vol %). The data obtained at low particle loadings were fitted well with the Halpin-Tsai model. In contrast, the percolation model and the Cox model were unable to adequately fit the data, indicating the mechanical reinforcement in our system mainly originated from efficient load transfer between particles and the matrix in the particle orienting direction. Finally, we showed that the thermal stability of composite films increased with the addition of MOF particles because of the high thermal degradation temperature and restricted polymer chain mobility.

Constrained Amorphous Interphase in Poly(l-lactic acid): Estimation of the Tensile Elastic Modulus

The mechanical properties of semicrystalline PLLA containing exclusively α'- or α-crystals have been investigated. The connection between experimental elastic moduli and phase composition has been analyzed as a function of the polymorphic crystalline form. For a complete interpretation of the mechanical properties, the contribution of the crystalline regions and the constrained amorphous interphase or rigid amorphous fraction (RAF) has been quantified by a three-phase mechanical model. The mathematical approach allowed the simultaneous quantification of the elastic moduli of (i) the α'- and α-phases (11.2 and 14.8 GPa, respectively, in excellent agreement with experimental and theoretical data reported in the literature) and (ii) the rigid amorphous fractions linked to the α'- and α-forms (5.4 and 6.1 GPa, respectively). In parallel, the densities of the RAF connected with α'- and α-crystals have been measured (1.17 and 1.11 g/cm³, respectively). The slightly higher value of the elastic modulus of the RAF connected to the α-crystals and its lower density have been associated to a stronger chain coupling at the amorphous/crystal interface. Thus, the elastic moduli at T _room of the crystalline (E _C), mobile amorphous (E _MAF), and rigid amorphous (E _RAF) fractions of PLLA turned out to be quantitatively in the order of E _MAF < E _RAF < E _C, with the experimental E _MAF value equal to 3.6 GPa. These findings can allow a better tailoring of the properties of PLLA materials in relation to specific applications.

Influence of Graphene Platelet Aspect Ratio on the Mechanical Properties of HDPE Nanocomposites: Microscopic Observation and Micromechanical Modeling

A series of high-density polyethylene nanocomposites filled with different diameter sizes (5, 15, and 25 μm) of graphene nanoplatelets at various amounts (0.5-5 wt.%) are prepared by the melt-mixing method. The effect of diameter size and filler content on the mechanical properties is reported, and the results are discussed in terms of morphology and the state of dispersion within the polymer matrix. The measured stiffness and strength of the nanocomposites were found to be mainly influenced by the filler aspect ratio and the filler-matrix adhesion. Fractography was utilized to study the embrittleness of the nanocomposites, and the observations revealed that a ductile to brittle transition is caused by a micro-deformation mechanism change in the nanocomposites. Several micromechanical models for the prediction of mechanical properties of nanocomposites, taking into consideration filler aspect ratio, percolation effect, and interphase regions, are considered. The three-phase model proposed by Ji accurately predicts the stiffness of graphene nanoplatelets with a higher diameter size, while Takayanagi modified model II was found to show good agreement with the experimental results of smaller ones at low filler content. This study demonstrates that the diameter size of the filler plays a central role in determining the mechanical properties.

Fivefold enhancement of yield and toughness of copper nanowires via coating carbon nanotubes

Carbon nanotubes are outstanding reinforcements owing to their unparallel strength, while their effects on the copper nanowire are still not fully understood, hampering their broad applications. Herein, we have investigated the tensile behaviors of the nanocomposite-wire of carbon nanotube-copper using molecular dynamic simulations. For the nanocomposite, both the coated and embedded carbon nanotubes increase the Young's modulus, fracture stress and toughness of the copper nanowire. A reinforcement of over fivefold in both yield strength (5.3 times) and toughness (5.1 times) has been achieved when the carbon nanotubes are coated on the copper nanowires, as well as 1.7 times in the Young's modulus. Higher temperatures and lower loading rates reduce the reinforcement.

Percolation Model for Renewable-Carbon Doped Functional Composites in Packaging Application: A Brief Review

This review summarizes the application of percolation theory for the behavior simulation of renewable-carbon in its doped packaging composites. Such dopant-reinforced materials have sparked considerable interest due to the significant improvement on the aesthetic and mechanical properties at considerable low filler content (<1% in some cases), which would further boost their potential use in the food and pharmaceutical packaging industries. We focused mainly on the percolation behavior, which is closely related to the distribution of renewable carbon particles in the presence of polymeric matrix. The effect of geometry, alignment and surface property is of particular relevance to the percolation threshold of composites containing carbon fillers. Validity, as well as limitations of the mostly used percolation models, is further discussed. Finally, despite its recent emergence as functional filler, carbon-based nanocellulose has been extensively developed for a wide range of applications. This inspired the concept to use nanocellulose as a secondary bio-additive for packaging purposes, such as functional nanocellulose-coated film where primarily synthetic polymers are used. The microstructure and functionality of rod-like nanocellulose in its use for film composite are specifically discussed.

Amino-Functionalized Multiwalled Carbon Nanotubes Lead to Successful Ring-Opening Polymerization of Poly(ε-caprolactone): Enhanced Interfacial Bonding and Optimized Mechanical Properties

In this work, the synthesis, structural characteristics, interfacial bonding, and mechanical properties of poly(ε-caprolactone) (PCL) nanocomposites with small amounts (0.5, 1.0, and 2.5 wt %) of amino-functionalized multiwalled carbon nanotubes (f-MWCNTs) prepared by ring-opening polymerization (ROP) are reported. This method allows the creation of a covalent-bonding zone on the surface of nanotubes, which leads to efficient debundling and therefore satisfactory dispersion and effective load transfer in the nanocomposites. The high covalent grafting extent combined with the higher crystallinity provide the basis for a significant enhancement of the mechanical properties, which was detected in the composites with up to 1 wt % f-MWCNTs. Increasing filler concentration encourages intrinsic aggregation forces, which allow only minor grafting efficiency and poorer dispersion and hence inferior mechanical performance. f-MWCNTs also cause a significant improvement on the polymerization reaction of PCL. Indeed, the in situ polymerization kinetics studies reveal a significant decrease in the reaction temperature, by a factor of 30-40 °C, combined with accelerated the reaction kinetics during initiation and propagation and a drastically reduced effective activation energy.

Large-scale production of PMMA/SWCNT composites based on SWCNT modified with PMMA

In this work, a two-step method consisting of in situ polymerization of polymethyl methacrylate (PMMA) in the presence of single-walled carbon nanotubes (SWCNT), followed by the redispersion of the resulting compound in dimethylformamide (DMF), was used to fabricate SWCNT modified with PMMA (SWCNT-PMMA). Raman spectroscopy revealed that PMMA was merely wrapped around the SWCNT when raw SWCNT or purified SWCNT were used as the starting material. However, PMMA was covalently bonded to SWCNT when acid treated SWCNT (SWCNT-COOH) was used as the starting material. SWCNT-PMMA compounds were further diluted in pure PMMA by conventional melt compounding at large scale (several kilograms) to obtain transparent composites containing 0.09 wt % SWCNT. The micro- and nano-dispersion of the SWCNT in the composites were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermal and mechanical properties of the composites were determined by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile testing, and Charpy impact testing. At the the low SWCNT loading studied, the tensile properties remain unchanged, whereas the impact strength improves by 20%.