Direct measurements of interfacial shear strength of multi-walled carbon nanotube/PEEK composite using a nano-pullout method

This paper presents a proposal of a simple and easy method to evaluate the interfacial shear strength (IFSS) of CNT-dispersed polymer composites. An individual multi-wall carbon nanotube (MWNT) was pulled out from a MWNT-dispersed/PEEK composite using a nano-pullout testing system installed in an SEM. The tensile load was measured using the elastic deformation of an AFM cantilever. The pull-out length was controlled by making a through-thickness hole near the specimen edge using a focused ion beam (FIB) system. The IFSS of a MWNT/PEEK composite was measured as 3.5-14 MPa, which agrees with the IFSS estimated from the macroscopic stress-strain behavior of the MWNT/PEEK composites.     

Highly conducting poly(methyl methacrylate)/carbon nanotubes composites: Investigation on their thermal, dynamic-mechanical, electrical and dielectric properties

Nanocomposites of poly(methyl methacrylate) (PMMA) containing various multi-walled carbon nanotubes (MWCNT) contents were prepared using melt mixing. Several techniques were employed to study the influence of the MWCNT addition on the thermal, mechanical, electrical and dielectric properties of the PMMA matrix. The electrical percolation threshold (p_c) was found to be 0.5 vol.% by performing AC and DC conductivity measurements. Significantly high conductivity levels (σ_dc) were achieved: σ_dc exceeds 10⁻² S/cm already at 1.1 vol.%, the criterion for EMI shielding (σ_dc > 10⁻¹ S/cm) is fulfilled at 2.9 vol.%, and the highest loaded sample (5.2 vol.%) gave a maximum value of 0.5 S/cm. Dielectric relaxation spectroscopy measurements in broad frequency (10⁻¹−10⁶ Hz) and temperature ranges (−150 to 170 °C) indicated weak polymer-filler interactions, in consistency with differential scanning calorimetry and dynamic-mechanical analysis findings. Weak polymer-filler interactions and absence of crystallinity facilitate the achievement of high conductivity levels in the nanocomposites.     

Interconnected multi-walled carbon nanotubes reinforced polymer-matrix composites

A modified method for interconnecting multi-walled carbon nanotubes (MWCNTs) was put forward. And interconnected MWCNTs by reaction of acyl chloride and amino groups were obtained. Scanning electron microscopy shows that hetero-junctions of MWCNTs with different morphologies were formed. Then specimens of pristine MWCNTs, chemically functionalized MWCNTs and interconnected MWCNTs reinforced epoxy resin composites were fabricated by cast moulding. Tensile properties and fracture surfaces of the specimens were investigated. The results show that, compared with pristine MWCNTs and chemically functionalized MWCNTs, the chemically interconnected MWCNTs improved the fracture strain and therefore the toughness of the composites significantly.     

Preparation and properties of poly (p-phenylene sulfide)/multiwall carbon nanotube composites obtained by melt compounding

In this paper, electrical and mechanical properties of Poly (p-phenylene sulfide) (PPS)/multi-wall carbon nanotubes (MWNTs) nanocomposites were reported. The composites were obtained just by simply melt mixing PPS with raw MWNTs without any pre-treatment. The dispersion of MWNTs and interfacial interaction were investigated through SEM &TEM and Raman spectra. The rheological test and crystallization behavior were also investigated to study the effects of MWNTs concentration on the structure and chain mobility of the prepared composites. Though raw MWNTs without any pre-treatment were used, a good dispersion and interaction between PPS and MWNTs have been evidenced, resulting in a great improvement of electrical properties and mechanical properties of the composites. Raman spectra showed a remarkable decrease of G band intensity and a shift of D bond, demonstrating a strong filler–matrix interaction, which was considered as due to π–π stacking between PPS and MWNTs. The storage modulus (G′) versus frequency curve presented a plateau above the percolation threshold of about 2–3 wt% with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behavior. Meanwhile, a conductive percolation threshold of 5 wt% was achieved and the conductivity of nanocomposites increased sharply by several orders of magnitude. The difference between electrical and rheological percolation threshold, and the effect of critical percolation on the chain mobility, especially on crystallization behavior of PPS, were discussed. In summary, our work provides a simple and fast way to prepare PPS/MWNTs nanocomposites with good dispersion and improved properties.     

Vibration damping characteristics of carbon fiber-reinforced composites containing multi-walled carbon nanotubes

Vibration damping characteristic of nanocomposites and carbon fiber reinforced polymer composites (CFRPs) containing multiwall carbon nanotubes (CNTs) have been studied using the free and forced vibration tests. Several vibration parameters are varied to characterize the damping behavior in different amplitudes, natural frequencies and vibration modes. The damping ratio of the hybrid composites is enhanced with the addition of CNTs, which is attributed to sliding at the CNT-matrix interfaces. The damping ratio is dependent on the amplitude as a result of the random orientation of CNTs in the epoxy matrix. The natural frequency shows negligible influence on the damping properties. The forced vibration test indicates that the damping ratios of the CFRP composites increase with increasing CNT content in both the 1st and 2nd vibration modes. The CNT-epoxy nanocomposites also show similar increasing trends of damping ratio with CNT content, indicating the enhanced damping property of CFRPs arising mainly from the improved damping property of the modified matrix. The dynamic mechanical analysis further confirms that the CNTs have a strong influence on the composites damping properties. Both the dynamic loss modulus and loss factor of the nanocomposites and the corresponding CFRPs show consistent increases with the addition of CNTs, an indication of enhanced damping performance.     

Synthesis and characterization of conductive polypyrrole/multi-walled carbon nanotubes composites with improved solubility and conductivity

High conductivity and solubility of polypyrrole (PPy)/multi-walled carbon nanotubes (MWCNT) composites has been successfully synthesized by in situ chemical oxidation polymerization using various concentrations of cationic polyelectrolyte poly(styrenesulfonate) (PSS) and ammonium peroxodisulfate (APS). Raman spectroscopy, FTIR, EPR, FESEM and HRTEM were used to characterize their structure and morphology. These images of FESEM and HRTEM showed that the fabricated PPy/MWCNT composites are one-dimensional core-shell structures with the average thickness of the PPy/MWCNT composites without PSS is about 250 nm and considerably decreases to 100–150 nm by adding the PSS content. The results of Raman spectrum, FTIR and UV–Vis indicate the synthesized PPy/MWCNT composites are in the doped state. The conductivities of PPy/MWCNT composites synthesized with the weight ratio of PSS/pyrrole monomer at 0.5 are about two times of magnitude higher than that of PPy/MWCNT composites without PSS. These results are perhaps due to the part of cationic electrolyte served as a dopant can be incorporated to the PPy structure to improve the conductivity of fabricated PPy/MWCNT composites.     

Interfacial shear strength of a glass fiber/epoxy bonding in composites modified with carbon nanotubes

In recent years, carbon nanotubes (CNTs) grown on fibers have attracted a lot of interest as an additional reinforcing component in conventional fiber-reinforced composites to improve the properties of the fiber/matrix interface. Due to harsh growth conditions, the CNT-grafted fibers often exhibit degraded tensile properties. In the current study we explore an alternative approach to deliver CNTs to the fiber surface by dispersing CNTs in the fiber sizing formulation. This route takes advantage of the developed techniques for CNT dispersion in resins and introduces no damage to the fibers. We focus on unidirectional glass fiber/epoxy macro-composites where CNTs are introduced in three ways: (1) in the fiber sizing, (2) in the matrix and (3) in the fiber sizing and matrix simultaneously. Interfacial shear strength (IFSS) is investigated using single-fiber push-out microindentation. The results of the test reveal an increase of IFSS in all three cases. The maximum gain (over 90%) is achieved in the composite where CNTs are introduced solely in the fiber sizing.     

Stress-strain behavior of multi-walled carbon nanotube/PEEK composites

This study examined the effects of multi-walled carbon nanotube (CNT) dispersion on stress-strain behaviors of poly-ether-ether-ketone (PEEK) at room temperature. Tensile test specimens containing 9 wt.% and 15 wt.% of CNT were fabricated using injection molding. Results of focused ion beam (FIB) observations show that many CNTs in the CNT/PEEK composite are aligned longitudinally. Although the PEEK stress-strain behavior is almost linear up to 1.5% strain, the stress-strain curves of CNT/PEEK composites exhibit considerable nonlinear and hysteretic behaviors from extremely low strain (<0.1%) under both tensile and compressive loading. The experimental results suggest that the viscoelastic deformation effects on nonlinear and hysteresis behaviors are not strong below 1.5% strain. Presumably, the slippage at the CNT-PEEK interface occurs with increasing applied stress because of poor interfacial load-transfer capability.     

Influence of single-walled carbon nanotubes on the effective elastic constants of poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) is one of the most extensively used thermoplastic polyesters out on the market, and it has been implemented in many forms. There has been limited work in the area of PET reinforced with single-walled carbon nanotubes (SWCNT) in mechanical properties. Nanocomposites based on PET with small contents of SWCNT were prepared by in situ polymerization. Elastic constants were determined by tensile tests performed on specimens instrumented with strain gauges. Assuming random orientation distribution of nanotubes, experimental Young’s modulus and Poisson’s ratio values were compared to some micromechanical models (Cox and Krenchel, Halpin–Tsai and Mori–Tanaka) which take into account orientation and aspect ratio of the nanotubes. However, the waviness of the nanotubes is a factor that influences the reinforcing efficiency.     

Percolation threshold of carbon nanotubes filled unsaturated polyesters

This paper reports on the development of electrically conductive nanocomposites containing multi-walled carbon nanotubes in an unsaturated polyester matrix. The resistivity of the liquid suspension during processing is used to evaluate the quality of the filler dispersion, which is also studied using optical microscopy. The electrical properties of the cured composites are analysed by AC impedance spectroscopy and DC conductivity measurements. The conductivity of the cured nanocomposite follows a statistical percolation model, with percolation threshold at 0.026 wt.% loading of nanotubes. The results obtained show that unsaturated polyesters are a matrix suitable for the preparation of electrically conductive thermosetting nanocomposites at low nanotube concentrations. The effect of carbon nanotubes reaggregation on the electrical properties of the spatial structure generated is discussed.     

Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.     

Grain size effect on the strengthening behavior of aluminum-based composites containing multi-walled carbon nanotubes

Strengthening efficiency of multi-walled carbon nanotubes (MWCNTs) is investigated for aluminum-based composites with grain sizes ranging from ∼250 to ∼65 nm. The strength of composites is significantly enhanced proportional to an increase of the MWCNT volume. However, the increment differs depending on deformation mode of the matrix. The strengthening efficiency of MWCNTs in ultrafine-grained composites is comparable with that predicted by the discontinuous fiber model, whereas the efficiency becomes half of the theoretical prediction as grain size is reduced below ∼70 nm. For nano-grained aluminum, activities of forest dislocations diminish and dislocations emitted from grain boundaries are dynamically annihilated during the recovery process, providing a weak plastic strain field around MWCNTs. The observation may provide a basic understanding of the strengthening behavior of nano-grained metal matrix composites.     

Interfacial load transfer in carbon nanotube/ceramic microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance the composite delamination resistance, and possibly improve its toughness and any matrix-dominated elastic property as well. In the present work aligned CNTs were grown upon ceramic fibers (silica and alumina) by chemical vapor deposition (CVD) at temperatures of 650 °C and 750 °C. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on pristine as well as on CNT-grown fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Significant increases (up to 50%) are observed in the fiber tensile strength and in the interfacial adhesion (which was sometimes doubled) with all fiber types upon which CNTs are CVD-grown at 750 °C. We discuss the likely sources of these improvements as well as their implications.     

Electromagnetic interference shielding behavior of poly(trimethylene terephthalate)/multi-walled carbon nanotube composites

Poly(trimethylene terephthalate) [PTT]/multiwalled carbon nanotube [MWCNT] composites having varying amounts of MWCNTs were fabricated with an aim to investigate the potential of such composites as an effective light weight electromagnetic interference (EMI) shielding material in the frequency range of 12.4-18 GHz (Ku-band). PTT/MWCNT composite with shielding effectiveness (SE) of 36-42 dB was obtained at 10% (w/w) MWCNT loading. Shielding mechanism was studied by resolving the total SE into absorption (SE_A) and reflection loss (SE_R). PTT/MWCNT composite showed absorption dominated shielding; thus it can be used as microwave, radar absorbing and stealth material. The effect of MWCNT loadings on electrical conductivity (σ) and dielectric properties of PTT and the correlation among conductivity, tan δ, absorption loss and reflection loss were also studied.     

Multi-scale electrical response of silicon nitride/multi-walled carbon nanotubes composites

Dense silicon nitride (Si₃N₄) composites with various amounts (0-8.6 vol%) of multi-walled carbon nanotubes (MWCNTs) are electrically characterised by combining macroscopic dc-ac and nanoscale conductive scanning force microscopy (C-SFM) measurements. In this way, a coherent picture of the dominant charge transport mechanisms in Si₃N₄/MWCNTs composites is presented. A raise of more than 10 orders of magnitude in the electrical dc conductivity compared to the blank specimen is measured for MWCNTs contents above 0.9 vol%. Semiconductor and metallic-like behaviours are observed depending on both the temperature and the MWCNTs content. Macroscopic measurements are further supported at the nanoscale by means of C-SFM. The metallic-type conduction is associated to charge transporting along the nanotube shells, whereas the semiconductor behaviour is linked to hopping conduction across nanotube-nanotube contacts and across intrinsic defect clusters within the nanotubes.     

Liquid sensing properties of melt processed polypropylene/poly(ε-caprolactone) blends containing multiwalled carbon nanotubes

The sensing properties of polypropylene (PP)/poly(ε-caprolactone) (PCL) blends containing multiwalled carbon nanotubes (MWNT) were studied in terms of their electrical resistance change in presence of liquids (solvents). The preparation of co-continuous blends based on the double percolation concept was done by melt mixing of electrically conductive PCL composites containing 3 wt.% MWNT and neat PP in ratios of 30:70, 40:60, and 50:50. The electrical resistance change of the PCL-MWNT composites and blends was monitored in a solvent immersion/drying cycle. Various solvents, such as n-hexane, ethanol, methanol, water, toluene, chloroform, and tetrahydrofuran were successfully detected, yielding different responses and reversibility of the resistance changes.PP and PCL were tested separately for solvent sorption using ethanol and n-hexane, both showing a low sorption of n-hexane. Ethanol sorption was large for PCL and almost absent for PP. The 50/50 blend composites with 3 wt.% MWNT in the PCL phase presented larger resistance changes for n-hexane, showing larger sensing ability for this solvent compared to PCL composites with 1 and 3 wt.% loadings. The opposite response was observed for immersion in ethanol where the PCL-MWNT composites showed larger changes than the blends. As the ratio of the conductive PCL phase over PP in the blend composition (i.e. the overall MWNT content) decreased, larger resistance changes were observed. The liquid sensing properties of compression-moulded discs and melt-drawn filaments were compared indicating higher responses for the discs.     

The influence of carbon nanotubes on the PVC glass transition temperature

The suspension poly(vinyl chloride) with the K number 70 (PVC S-70) compound was applied as a matrix of the nanocomposite, and the multi-wall carbon nanotubes (MWCNT) in the concentration range between 0.01 wt.% and 0.05 wt.% as a nanofiller. The T_g determination was realized by means of DMTA, dielectric losses measurements and DSC measurements. In all cases the PVC matrix revealed lower T_g comparing with corresponding composites with CNTs, and a clear relationship between the frequency, CNT contain and the T_g was found. With an increase of the charging frequency an increase of T_g of about 3 °C by DMTA measurements, realized by frequencies f = 1 Hz and f = 10 Hz, and of 9 °C by f = 1000 Hz, was noted. Independent on the frequency a maximum of the T_g increase was observed by the CNT concentration of about 0.01–0.02 wt.%.     

Morphology and electrical properties of polyamide 6/polypropylene/multi-walled carbon nanotubes composites

Morphology, electrical properties and conductive mechanisms of polyamide 6/polypropylene/muti-walled carbon nanotubes (PA6/PP/MWNTs) composites with varied compositions and different blending sequences were investigated. The MWNTs were found to be located preferentially in the PA6 phase in the composites, whatever the PA6 was continuous or dispersed phase. While the incorporation of MWNTs changed the dispersed PA6 phase from spherical to elongated or irregular shape. The PA6/PP/MWNTs (20/80/4) composite with a dispersed PA6 phase exhibited a higher electrical conductivity in comparison with the PA6/PP/MWNTs (50/50/4) composite which has a co-continuous phase and exhibits double percolation. This was due to the formation of a conductive MWNTs networks in the PA6/PP/MWNTs (20/80/4) composite as proved by means of field emission scanning electron microscopy and rheological measurements. The morphology and electrical properties of the PA6/PP/MWNTs (20/80/4) composites were significantly influenced by blending sequences. When blending 3.9 phr MWNTs with a pre-mixed PA6/PP/MWNTs (20/80/0.1) composite, the dispersed PA6 phase formed an elongated structure, which was beneficial to the electrical properties.     

Application of continuously-monitored single fiber fragmentation tests to carbon nanotube/carbon microfiber hybrid composites

To assess the effect of carbon nanotube (CNT) grafting on interfacial stress transfer in fiber composites, CNTs were grown upon individual carbon T-300 fibers by chemical vapor deposition. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on both pristine and CNT-decorated fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Despite the fiber strength degradation resulting from the harsh CNT growth conditions, the CNT-modified fibers lead to a twofold increase in interfacial shear strength which correlates with the nearly threefold increase in apparent fiber diameter resulting from CNT grafting. These observations corroborate recently published studies with other CNT-grafted fibers. An analysis of the relative contributions to the interfacial strength of the fiber diameter and strength due to surface treatment is presented. It is concluded that the common view whereby an experimentally observed shorter average fragment length leads to a stronger interfacial adhesion is not necessarily correct, if the treatment has changed the fiber tensile strength or its diameter.     

Surface coating of multi-walled carbon nanotube nanopaper on shape-memory polymer for multifunctionalization

We are presenting a method of synthesizing three-dimensional self-assembled multi-walled carbon nanotube (MWCNT) nanopaper on hydrophilic polycarbonate membrane. The process is based on the very well-defined dispersion of nanotube and controlled pressure vacuum deposition procedure. The morphology and structure of the nanopaper are characterized with scanning electronic microscopy (SEM) over a wide range of scale sizes. A continuous and compact network observed from the microscopic images indicates that the MWCNT nanopaper could have highly conductive property. As a consequence, the sensing properties of conductive MWCNT nanopaper are characterized by functions of temperature and water content. Meanwhile, in combination with shape-memory polymer (SMP), the conductive MWCNT nanopaper facilitates the actuation in SMP nanocomposite induced by electrically resistive heating. Furthermore, the actuating capability of SMP nanocomposite is utilized to drive up a 5-gram mass from 0 to 30 mm in height.