Surface functionalization of carbon nanotubes by biological adhesive polymers carbopol for developing high‐permittivity polymer composites

Surface functionalization of carbon nanotubes (CNTs) by biological adhesive polymers carbopol (CP) was developed by simply mixing CNT suspension and an aqueous solution of CP without any toxic solvents. CP can be easily coated onto CNTs through hydrogen bonds O?C? OH?NH₂? C?O and electrostatic interaction between ? COO? on CP and ? NH₃⁺ on CNTs. After modification, the surface of the CNT is endowed with a large number of carboxyl groups, which can effectively prevent the reaggregation of CNT by electrostatic repulsion between the ionized carboxyl groups. Hence, highly dispersed functionally modifying CNT by CP (CP‐CNT) filler in polydimethylsiloxane (PDMS) matrix can be obtained. More important, with the help of adhesive properties of CP, the interfacial compatibility between fillers and matrix can also be improved. Thus, the CP‐CNT/PDMS composites exhibited higher dielectric permittivity comparing with CNT/PDMS composites at the same filler content. We present a potential and green approach of surface functionalization of CNT for preparing high‐permittivity polymer composites. J. VINYL ADDIT. TECHNOL., 26:165–172, 2020. © 2019 Society of Plastics Engineers     

Effect of phenol functionalization of carbon nanotubes on properties of natural rubber nanocomposites

This paper reports the results of studies on the effect of phenol functionalization of carbon nanotubes (CNTs) on the mechanical and dynamic mechanical properties of natural rubber (NR) composites. Fourier transform infrared spectrometry (FTIR) indicates characteristic peaks for ether and aromatic rings in the case of phenol functionalized CNT. Although differential scanning calorimetric (DSC) studies show no changes in the glass‐rubber transition temperature (T_g) of NR in the nanocomposites due to surface modification of CNT, dynamic mechanical studies show marginal shifting of T_g to higher temperature, the effect being pronounced in the case of functionalized CNT. Stress‐strain plots suggest an optimum loading of 5 phr CNT in NR formulations and the phenolic functionalization of CNT does not affect significantly the stress‐strain properties of the NR nanocomposites. The storage moduli register an increase in the presence of CNT and this increase is greater in the case of functionalized CNT. Loss tangent showed a decrease in the presence of CNT, and the effect is more pronounced in the case of phenol functionalized CNT. Transmission electron microscopy (TEM) reveals that phenol functionalization causes improvement in dispersion of CNT in NR matrix. This is corroborated by the increase in electrical resistivity in the case of phenol functionalized CNT/NR composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of functional groups of carbon nanotubes on the cyclization mechanism of polyacrylonitrile (PAN)

The cyclization mechanism of polyacrylonitrile (PAN) in PAN/functionalized carbon nanotube (CNT) composites was examined. The surface functionalization of CNTs was carried out by using diazonium reagents with 4-substituted aniline. The results strongly suggest that the type of functional groups on the CNTs strongly influences the cyclization mechanism of PAN during the stabilization process. The nitrile of PAN in F–Ph–CNT/PAN composite was cyclized through the free radical reaction during thermal stabilization whereas nitrile of PAN in COOH–Ph–CNT/PAN composite underwent cyclization via the ionic reaction due to the acid groups on the surfaces of the CNTs. The fluoro functional groups on the CNTs can act as effective external initiators for nitrile cyclization in homo PAN, in contrast to acid functional groups. Consequently, a lower cyclization temperature (265 °C) and enthalpy value (688 J/g) of F–Ph–CNT were shown compared to those of homo PAN.     

Relationship between electromechanical response and percolation threshold in carbon nanotube/poly(vinylidene fluoride) composites

This paper reports on the piezoresistive response of carbon nanotube/poly(vinylidene fluoride), CNT/PVDF, composites prepared with different CNT types with and without functionalization, via in situ-generated diazonium compounds. The results show that for a CNT concentration close to the percolation threshold, tunneling is the main mechanism responsible for the electrical response, leading also to a significant increase of the piezoresistance of the composites. Interestingly, this fact is independent of the CNT type or functionalization, as well as of the percolation threshold concentration. In this way, a close relationship between the percolation threshold and the piezoresistive response was demonstrated. The electromechanical response, as characterized by the gauge factor, reach values up to 3.9, being among the largest obtained for thermoplastic composites and demonstrating the suitability of these materials for sensor applications.     

Raman microscopy of residual strains in carbon nanotube/epoxy composites

Carbon nanotube (CNT) strain variations with temperature, as measured with Raman microscopy, are reported for pristine and functionalized CNT/epoxy composites. The CNT strain is derived from the difference in frequencies of the CNT vibrational G⁺-mode in the composite and that of a relaxed CNT, and shown to serve as a measure of the local residual strains in the composites. The magnitudes of these strains vary with both CNT functionalization and CNT concentration. At room temperature and with the same local concentration of CNTs in the composite, the strains of oxidized and polyamidoamine-functionalized CNTs are found to be 2.5 times higher than that of the composite containing pristine CNTs. The higher residual strain of the composites loaded with functionalized CNTs reflects their better adhesion and integration in the polymer matrix. These findings are in accordance with the improved tensile properties measured for the functionalized CNT composites.     

Characterization and thermal degradation of poly(d,l‐lactide‐co‐glycolide) composites with nanofillers

Chemical and thermal characterization of poly(d ,l ‐lactide‐co‐glycolide) (PLGA) composites filled with hydroxyapatite (HA) or carbon nanotubes (CNT) were evaluated by infrared spectroscopy, differential scanning calorimetry, thermogravimetry, and dynamic–mechanical–thermal analysis. The morphology and distribution of the nanoparticles were studied by transmission electron microscopy. The composites were prepared by solvent casting using 30% HA or 1, 3, and 5% of pristine and functionalized CNT as nanoparticles and PLGA 75:25 and PLGA 50:50 as copolymer matrix. The Coats–Redfern and E₂ function methodologies were used to calculate the reaction order and the activation energy (E_a) of the thermal degradation process. It was found that the addition of nanoparticles increased the glass transition temperature (T_g) of the composites. Also, higher degradation temperatures and E_a values were obtained for PLGA–HA composites and compared with the neat copolymer, and the opposite behavior was exhibited by PLGA–CNT composites. The thermal and mechanical properties were highly dependent on the morphology and dispersion of the filler. The functionalization process of CNT promoted, to some extent, a better distribution and dispersion of CNT into the matrix, and these composites exhibited a slight enhancement on storage modulus. On the other hand, PLGA–HA composites showed a good dispersion but no improvement on the storage modulus below T_g. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Characterization and thermal degradation kinetics of poly(l‐lactide) nanocomposites with carbon nanotubes

The purpose of this research was to study the thermal degradation kinetics of nanocomposites of poly(l ‐lactide) (PLLA) with carbon nanotubes (CNT) in order to provide further insight into their thermal stability. Nanocomposites were prepared by solvent casting with 1, 3, and 5% by weight of pristine CNT (P‐CNT) or functionalized CNT (F‐CNT), and were characterized using infrared spectroscopy, transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and dynamic‐mechanical‐thermal analysis. The kinetic parameters of thermal decomposition were determined employing Coats‐Redfern method to calculate the reaction order and E₂ function model to calculate the activation energy (Ea). We found no major changes in PLLA glass transition temperatures due to CNT presence, but melt‐crystallization temperature increased slightly in some composites. In general, composites consisting of 3% or 5% of F‐CNT had superior thermal stability than did pure polymer or P‐CNT composites. This improved thermal stability was revealed by slightly higher degradation and onset temperatures, and Ea obtained from kinetic analysis. In addition, 3% or 5% of F‐CNT in PLLA composites slightly enhanced the storage modulus above the glass transition. Therefore, functionalization promoted, in some extent, better morphology and dispersion of CNT into the matrix, which was responsible for improved thermal stability and thermomechanical performance of composites at higher temperatures relative to pure polymer. POLYM. ENG. SCI., 55:710–718, 2015. © 2014 Society of Plastics Engineers     

碳纳米管增强聚合物纳米复合材料研究进展

综述了近几年国内外在碳纳米管增强聚合物纳米复合材料力学性能方面的研究进展,主要介绍了以聚氨酯和聚酰亚胺为基体的复合材料。讨论了碳纳米管的各种改性方法及其作用原理,并对各种改性和制备方法的有效性进行了比较。最后,对碳纳米管增强聚合物纳米复合材料的发展前景进行了展望。     

PP/CNT复合材料制备及表征研究进展

碳纳米管（CNT）在聚丙烯（PP）中的分散及其与PP基体的界面结合是制备PP／CNT复合材料的关键问题，可采用机械剪切、超声处理和表面功能化等方法来解决。CNT的加入使PP的力学、电学、结晶、阻燃和耐热等性能都有不同程度的改善和提高。综述了近年来国内外PP／CNT复合材料的制备及表征研究进展。     

The effect of surface functionalization of carbon nanotubes on properties of natural rubber/carbon nanotube composites

The objective of this study was to prepare natural rubber composites filled with carbon nanotubes (CNTs) that show an electrical percolation threshold at very low CNT concentrations. Therefore, two methods of surface functionalization of CNTs were investigated to enable an improved dispersion of CNTs and chemical interaction between CNTs and rubber matrix. On one hand, the CNTs have been functionalized ex situ by acid treatment and silanization reaction with bis(triethoxysilylpropyl) tetrasulfide before mixing with the rubber and otherwise in situ functionalization was directly carried out during the processing of the composites in the internal mixer. The grafting of silane molecules onto CNT surface was established by Fourier transform infrared spectroscopy and scanning electron microscopy. Tensile tests revealed the outstanding properties of composites prepared by in situ silanization method. The in situ silanization led to a better dispersion of the CNTs and the formation of chemical linkages between CNT surface and rubber and this became manifest in higher reinforcement of the rubber, higher crosslink densities, and a lower electrical percolation threshold. It was also shown that the in situ silanization is retarding the vulcanization reaction. POLYM. COMPOS., 36:2113–2122, 2015. © 2014 Society of Plastics Engineer     

Carbon nanotube–polyaniline composites

The last decade has seen a growing interest in hybrid electrically conducting nanocomposites. This article aims to provide a detailed overview of the present status of research in carbon nanotube–polyaniline (CNT/PANI) composites, from processing to structural and property evaluations. CNT/PANI are synthesized by electrochemical and chemical processing. When chemical methods are used, the main challenge is to obtain processable CNT/PANI in the emeraldine salt (ES) form composites. Stable dispersions of ES–CNT in organic media are prepared using the post doping method, inverse emulsion polymerization, or ex situ polymerizations. On the contrary, stable water dispersions of CNT/ES are prepared using hydrophilization of a preformed CNT/ES composite, direct synthesis of micelle–CNT hybrid templates, interfacial polymerization, covalent functionalization of CNT with a water soluble polymer, or using electrostatic interactions between two oppositely charged ES and CNT aqueous colloids. Moreover, the strategies for the synthesis of ternary CNT/PANI composites incorporating noble metal nanoparticles, metal oxide, or graphene sheets are also presented and analyzed in depth. Finally, we give a review of potential applications, including chemical sensors, capacitors, fuel cells and electronic devices.     

Thermal properties of hyperbranched polyborate functionalized multiwall carbon nanotube/polybenzoxazine composites

Using a noncovalent functionalization strategy, hyperbranched polyborate (HBb) acts as a solubilizer for carbon nanotubes (CNTs), and a stable HBb‐CNT dispersion in N‐methyl‐pyrrolidone was produced. The thermal properties of the resulting HBb‐CNT/polybenzoxazine (B‐BOZ) composites and their carbonized structures were investigated. Scanning electron microscopy demonstrated that the fracture surface of HBb‐CNT/B‐BOZ composites was rather rough and plenty of plastic deformation was exhibited. Thermogravimetric analysis indicates an improvement in the thermal stability of the composite with CNTs, especially that of 2.0 wt% CNT modified composite. The increase in the thermal stability is due to the good nanotube dispersion and the effective polymer‐CNT interaction. Graphite‐like boron carbonitride ceramic compounds were found after the composites were carbonized at 1,100°C for 2 h, and there was more B‐C, B‐N, and C‐N bonds in the carbonized HBb‐CNT/B‐BOZ composite than that of HBb/B‐BOZ composite. The result implied that CNTs can promote the ceramic process of HBb/B‐BOZ composite, and the strategy of introducing ceramic precursor into polymer composites may be useful to improve their ablation properties. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Dispersion, interfacial interaction and re-agglomeration of functionalized carbon nanotubes in epoxy composites

The surface, interfacial and dispersion properties of carbon nanotubes (CNTs), and the mechanical properties of the CNT/epoxy composites affected by CNT functionalization are investigated. It is demonstrated that there exists strong correlations between amino-functionalization, dispersion, wettability, interfacial interaction and re-agglomeration behaviour of CNTs and the corresponding mechanical and thermo-mechanical properties of CNT/epoxy composites. The amino-functionalized CNTs exhibit higher surface energy and much better wettability with epoxy resin than the pristine CNTs, and the attached amine molecules arising from the functionalization effectively inhibit the re-agglomeration of CNTs during the curing of resin. These ameliorating effects along with improved interfacial adhesion between the matrix and functionalized CNTs through covalent bonds result in improved flexural and thermo-mechanical properties compared with those without functionalization.     

Physical interactions at carbon nanotube-polymer interface

Mechanical properties of carbon nanotube (CNT) reinforced polystyrene rod and CNT reinforced epoxy thin film were studied and the CNT-polymer interface in these composites was examined. Transmission and scanning electron microscopy examinations of CNT/polystyrene (PS) and CNT/epoxy composite showed that these polymers adhered well to CNT at the nanometer scale. Molecular mechanics simulations and elasticity calculations were used to quantify some of the important interfacial characteristics that critically control the performance of a composite material. In the absence of chemical bonding between CNT and the matrix, it is found that the non-bond interactions, consist of electrostatic and van der Waals forces, result in CNT-polymer interfacial shear stress (at 0 K) of about 138 and 186 MPa, respectively, for CNT/epoxy and CNT/PS. The high interfacial shear stress calculated, about an order of magnitude higher than micro fiber reinforced composites, is believed attributed to intimate contact between the two solid phases at the molecular scale. Simulations and calculations also showed that local non-uniformity of CNT and mismatch of the coefficients of thermal expansions between CNT and polymer matrix also promote the stress transfer ability between the two.     

Improved fracture toughness of carbon fiber composite functionalized with multi walled carbon nanotubes

Woven carbon fiber (CF) laminae are functionalized in situ with carbon nanotubes (CNTs) to test the hypothesis that growing CNTs on CF (i.e., carbon fiber bundles or tow) would enhance the properties of polymeric carbon composites, specifically epoxy–carbon composites that are used in aerospace applications. The CNT as-grown on the woven CF were shown to substantially improve the fracture toughness of the cured composite on the order of 50%. This was accompanied by no loss in structural stiffness of the final composite structure. In fact, the flexural modulus increased approximately 5%. The significant increase in the fracture toughness as tested under the ASTM D 5528 standard indicates that the damage tolerance of a composite structure would benefit from the CNT material applied in this way. Our approach has allowed for significantly larger samples to be uniformly functionalized with CNTs than is reported elsewhere in the open literature. In addition, this work demonstrated CNT functionalization on flexible substrates that remains flexible after functionalization, whereas most CNT growth substrates are rigid in order to withstand the high (>800 °C) growth temperatures often encountered in CNT synthesis.     

Effects of carbon fiber modification with multiwall CNT on the electrical conductivity and EMI shielding effectiveness of polycarbonate/carbon fiber/CNT composites

The effect of carbon fiber (CF) modification with multiwall carbon nanotube (CNT) on the electrical, mechanical, and rheological properties of the polycarbonate (PC)/CF/CNT composite was investigated. The CF and multiwall CNT (MWCNT) were treated with sulfuric acid and nitric acid (3:1 wt %) mixture, to modify the CF with the CNT. For the PC with acid-treated CNT (a-CNT) modified acid-treated CF (a-CF) (PC/a-CF/a-CNT) composite, the electrical conductivity, and the electromagnetic interference shielding effectiveness (EMI SE) showed the highest values, compared with those of the PC/a-CF and PC/a-CF/CNT composites. The EMI SE of the PC/a-CF (10 wt %)/a-CNT (0.5 wt %) composite was found to be 26 (dB at the frequency of 10.0 GHz, and the EMI SE was increased by 91.2%, compared to that of the PC/a-CF composite at the same amount of total filler content. Among the composites studied in this work, the PC/a-CF/a-CNT composite also showed the highest values of relative permittivity (ε_r) and dielectric loss factor. The above results suggest that the CF modification with the a-CNT significantly affected the electrical conductivity and EMI SE of the composite, and the hybrid fillers of the a-CNT and a-CF resulted in good electrical pathways in the PC/a-CF/a-CNT composite. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47302.     

Effect of acid-treatment and colloidal-processing conditions on the room temperature mechanical and electrical properties of 3YTZP/MWNT ceramic nanocomposites

Different colloidal powder processing routines have been used to prepare composites of 3 mol% Y₂O₃ -ZrO₂ (tetragonal zirconia polycrystals, 3YTZP) with 2.5 vol% multiwall carbon nanotubes (MWNT) with the aim of achieving a homogeneous distribution of the MWNTs in the ceramic, eliminating agglomerates but also minimizing carbon nanotube (CNT) damage during processing. Modifications of the acid treatment applied to the nanotubes, including subjecting them to stirring or ultrasonic agitation, and use of acid or basic pH during composite powder mixing have been approached.No MWNT damage during processing was detected by Raman spectroscopy. CNT bundles were found in all the composites forming different patterns depending on the processing route. Similar values of hardness were obtained for all the composites, while different anisotropy in fracture propagation was found when studying parallel and perpendicular directions to the sintering pressing axis on the cross sections of the composites due to the MWNT preferential alignment. The CNT bundles were found to act as fracture short paths. A similar anisotropic behavior was observed for the electrical conductivity. These results have been correlated to the different microstructures obtained in the composites prepared with different processing routines.     

Electrophoretic deposition of carbon nanotube–ceramic nanocomposites

The purpose of this paper is to present an up-to-date comprehensive overview of current research progress in the development of carbon nanotube (CNT)–ceramic nanocomposites by electrophoretic deposition (EPD). Micron-sized and nanoscale ceramic particles have been combined with CNTs, both multiwalled and single-walled, using EPD for a variety of functional, structural and biomedical applications. Systems reviewed include SiO₂/CNT, TiO₂/CNT, MnO₂/CNT, Fe₃O₄/CNT, hydroxyapatite (HA)/CNT and bioactive glass/CNT. EPD has been shown to be a very convenient method to manipulate and arrange CNTs from well dispersed suspensions onto conductive substrates. CNT–ceramic composite layers of thickness in the range <1–50 μm have been produced. Sequential EPD of layered nanocomposites as well as electrophoretic co-deposition from diphasic suspensions have been investigated. A critical step for the success of EPD is the prior functionalization of CNTs, usually by their treatment in acid solutions, in order to create functional groups on CNT surfaces so that they can be dispersed uniformly in solvents, for example water or organic media. The preparation and characterisation of stable CNT and CNT/ceramic particle suspensions as well as relevant EPD mechanisms are discussed. Key processing stages, including functionalization of CNTs, tailoring zeta potential of CNTs and ceramic particles in suspension as well as specific EPD parameters, such as deposition voltage and time, are discussed in terms of their influence on the quality of the developed CNT/ceramic nanocomposites. The analysis of the literature confirms that EPD is the technique of choice for the development of complex CNT–ceramic nanocomposite layers and coatings of high structural homogeneity and reproducible properties. Potential and realised applications of the resulting CNT–ceramic composite coatings are highlighted, including fuel cell and supercapacitor electrodes, field emission devices, bioelectrodes, photocatalytic films, sensors as well as a wide range of functional, structural and bioactive coatings.     

Functionalization of carbon nanotube yarn by acid treatment

Carbon nanotube (CNT) yarn was functionalized using sulfuric and nitric acid solutions in 3:1 volumetric ratio. Successful functionalization of CNT yarn with carboxyl and hydroxyl groups (e.g., COOH, COO–, OH, etc.) was confirmed by attenuated total reflectance spectroscopy. X-ray diffraction revealed no significant change to the atomic in-plane alignment in the CNTs; however, the coherent length along the diameter was significantly reduced during functionalization. A morphology change of wavy extensions protruding from the surface was observed after the functionalization treatment. The force required to fracture the yarn remained the same after the functionalization process; however, the linear density was increased (310%). The increase in linear density after functionalization reduced the tenacity. However, the resistivity density product of the CNT yarn was reduced significantly (234%) after functionalization.     

Mechanical and barrier properties of carbon nanotube reinforced PCL‐based composite films: Effect of gamma radiation

Carbon nanotube (CNT) reinforced (0.05–0.5% by wt) polycaprolactone (PCL)‐based composites were prepared by compression molding. Addition of 0.2% CNT caused a 131% improvement of tensile strength (TS) of PCL films. The tensile modulus (TM) and elongation at break (Eb) of PCL were also significantly improved with the addition of CNT. The water vapor permeability of PCL was 1.51 g·mm/m²·day but 0.2% CNT containing PCL films showed 1.08 g·mm/m²·day. Similarly, the oxygen transmission rate (OTR) of PCL films was found to decrease with the addition of CNT. But, carbon dioxide transmission rate (CO₂TR) of PCL film was improved due to incorporation of CNT. Effect of gamma radiation on PCL films and CNT reinforced PCL‐based composites were also studied. The TS of the irradiated (10 kGy) PCL films gained to 75% higher than control sample. The TS of the 0.2% CNT reinforced composite film was reached to 41 MPa at 15 kGy dose. The barrier properties of non‐irradiated and irradiated (10 kGy) PCL films and composites (0.2% CNT reinforced) were also measured. Both PCL films and composites showed lower values of WVP upon irradiation and indicated better water vapor barrier. The OTR and CO₂TR of the irradiated (10 kGy) PCL films and composites were decreased compared to their counterparts. Surface and interface morphologies of the composites were studied by scanning electron microscopy. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013