Multifunctional poly(2,5‐benzimidazole)/carbon nanotube composite films

The AB‐monomer, 3,4‐diaminobenzoic acid dihydrochloride, was recrystallized from an aqueous hydrochloric acid solution and used to synthesize high‐molecular‐weight poly(2,5‐benzimidazole) (ABPBI). ABPBI/carbon nanotube (CNT) composites were prepared via in situ polymerization of the AB‐monomer in the presence of single‐walled carbon nanotube (SWCNT) or multiwalled carbon nanotube (MWCNT) in a mildly acidic polyphosphoric acid. The ABPBI/SWCNT and ABPBI/MWCNT composites displayed good solubility in methanesulfonic acid and thus, uniform films could be cast. The morphology of these composite films was studied by X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. The results showed that both types of CNTs were uniformly dispersed into the ABPBI matrix. Tensile properties of the composite films were significantly improved when compared with ABPBI, and their toughness (～200 MPa) was close to the nature's toughest spider silk (～215 MPa). The electrical conductivities of ABPBI/SWCNT and ABPBI/MWCNT composite films were 9.10 × 10^?5 and 2.53 × 10^?1 S/cm, respectively, whereas that of ABPBI film was 4.81 × 10^?6 S/cm. These values are ～19 and 52,700 times enhanced by the presence of SWCNT and MWCNT, respectively. Finally, without acid impregnation, the ABPBI film was nonconducting while the SWCNT‐ and MWCNT‐based composites were proton conducting with maximum conductivities of 0.018 and 0.017 S/cm, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1067–1078, 2010     

Multiscale carbon nanotube-carbon fiber reinforcement for advanced epoxy composites

We report an approach to the development of advanced structural composites based on engineered multiscale carbon nanotube-carbon fiber reinforcement. Electrophoresis was utilized for the selective deposition of multi- and single-walled carbon nanotubes (CNTs) on woven carbon fabric. The CNT-coated carbon fabric panels were subsequently infiltrated with epoxy resin using vacuum-assisted resin transfer molding (VARTM) to fabricate multiscale hybrid composites in which the nanotubes were completely integrated into the fiber bundles and reinforced the matrix-rich regions. The carbon nanotube/carbon fabric/epoxy composites showed approximately 30% enhancement of the interlaminar shear strength as compared to that of carbon fiber/epoxy composites without carbon nanotubes and demonstrate significantly improved out-of-plane electrical conductivity.     

Poly(2,5-benzoxazole)/carbon nanotube composites via in situ polymerization of 3-amino-4-hydroxybenzoic acid hydrochloride in a mild poly(phosphoric acid)

Poly(2,5-benzoxzole) (ABPBO)/carbon nanotube (CNT) composites were prepared via in situ polycondensation of “protonated” AB monomer, 3-amino-4-hydroxybenzoic acid hydrochloride, in a mildly acidic poly(phosphoric acid) medium. In situ generated hydrochloric acid during the dehydrochlorination process provided additional acidity to the reaction medium. The enhanced acidity was advantageous for both the purification and dispersion of CNTs. Specifically, it was evident for the purification of as-received single-walled carbon nanotube (SWCNT), which was contained a large portion of impurity (60-70 wt%). On the basis of the data obtained from elemental analysis (EA), thermogravimetric analysis (TGA), infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) as well as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the resultant composites implicated that individual tube of multi-walled carbon nanotube (MWCNT) and bundles of SWCNT were homogeneously dispersed into the ABPBO matrix. After in situ polymerization in harsh temperature at 175 °C and subsequent work-up processes, CNTs were remained structurally intact in a mild reaction medium. Thus, the PPA medium is indeed viable for the preparation of composite.     

Responses and thermal conductivity measurements of multi-wall carbon nanotube (MWNT)/epoxy composites

In the present study, the MWNT/epoxy composites are prepared with three weight percentages (0.0, 0.3, and 0.5%) of multiwall carbon nanotube (MWNT). The temporal response of multi-wall carbon nanotube (MWNT)/epoxy composite with different wt% of multi-wall carbon nanotube (MWNT) is measured by experiment. Also, a cavity-type measuring system is designed to experimentally measure the surface temperatures and obtain the thermal conductivity of these composites at different heating rates. It is found that the responses of the 0.3 and 0.5% weight percentage of multi-wall carbon nanotube (MWNT)/epoxy composites are found to be about 25 and 47.8%, respectively, faster than that of the pure epoxy resin. Both the responding characteristics and the variation trends of the measured surface temperatures of these composites can be well predicted by the lumped-heat capacity model. Besides, the higher the weight percentage (wt%) of multi-wall carbon nanotube (MWNT) in the composite, the larger is the thermal conductivity. Relative to the pure epoxy, the thermal conductivities for the composites with 0.3 and 0.5% of multi-wall carbon nanotube (MWNT) increase by 15.9 and 44.9%, respectively. For the weight percentages studied, the thermal conductivity of these composites is found to increase mildly at low heating rates; however, it remains nearly constant at high heating rates.     

Molecular motion,morphology, and thermal properties of multiwall carbon nanotube/polysilsesquioxane composite

Diglycidyl ether of bisphenol A (DGEBA)‐bridged polyorganosiloxane precursors have been prepared successfully by reacting diglycidyl ether of bisphenol A epoxy resin with 3‐aminopropyltriethoxysilane. Acid‐modified and unmodified multiwalled carbon nanotube (MWCNT) were dispersed in the diglycidyl ether of bisphenol A‐bridged polyorganosiloxane precursors and cured to prepare the carbon nanotube/diglycidyl ether of bisphenol A‐bridged polysilsesquioxane (MWCNT/DGEBA‐PSSQ) composites. The molecular motion of MWCNT/DGEBA‐PSSQ nanocomposites was studied by high‐resolution solid‐state ¹³C NMR. Acid‐modification can improve the affinity between MWCNT and the polymer matrix. The molecular motion of the DGEBA‐PSSQ decreased with acid‐modified MWCNT content. However, when unmodified MWCNT was used, the molecular motion of the DGEBA‐PSSQ was increased. SEM and TEM microphotographs confirm that acid‐modified MWCNT exhibits better dispersion than unmodified MWCNT in DGBEA‐PSSQ. The dynamic mechanical properties of acid‐modified MWCNT/DGBEA‐PSSQ composites are more favorable than those of unmodified MWCNT. T_g of the DGEBA‐PSSQ decreased from 174.0 °C (neat DGEBA‐PSSQ) to 159.0 °C (1 wt % unmodified MWCNT) and 156.0 °C (1 wt % acid‐modified MWCNT). The storage modulus (at 30 °C) of the DGEBA‐PSSQ increased from 1.23 × 10⁹ Pa (neat DGEBA‐PSSQ) to 1.65 × 10⁹ Pa (1 wt % acid‐modified MWCNT). However, when unmodified MWCNT was used, the storage modulus of the DGEBA‐PSSQ decreased to 6.88 × 10⁸ Pa (1 wt % unmodified MWCNT). At high temperature, above 150 °C, storage modulus of nanocomposites was higher than that of neat polymer system. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 472–482, 2008     

A review on flammability of epoxy polymer,cellulosic and non‐cellulosic fiber reinforced epoxy composites

Natural fiber is well‐known reinforcement filler in polymer‐matrix composites. Composite components like organic polymers and natural fibers are natural fire conductors as the natural fiber consists of cellulose, hemicellulose, and lignin, and hence are as highly flammable as wood. Natural fiber reinforced composite materials are progressively being used in a variety of applications where their fire response is a hazardous consideration, for example, in the automotive (transportation) and building‐construction industries. As a result, an awareness of their performance or response during a fire and the use of conventional fire retardants are of great importance, as they are subject to thermal decomposition when exposed to intensive high heat or fire sources. In this review paper, fire flammability is the main concern for cellulosic and non‐cellulosic fiber‐reinforced polymer composites, especially epoxy composites. This paper reviews the literature on the recent developments in flammability studies concerning polymers, epoxy polymers, cellulosic‐fibers, and non‐cellulosic fiber‐reinforced epoxy bio‐composites. The prime objective of this review is to expand the reach of “fire retardants for polymer materials and composites” to the science community, including physicists, chemists, and engineers in order to broaden the range of their applications. Copyright © 2015 John Wiley & Sons, Ltd.     

Electrocatalytic properties of platinum nanoparticles supported on fluorine tin dioxide/multi-walled carbon nanotube composites for methanol electrooxidation in acidic medium

Fluorine tin oxide (FTO) and multi-walled carbon nanotube (MWCNT) composites synthesized by a sol-gel process followed by a hydrothermal treatment process have been explored as a support for Pt nanoparticles (Pt-FTO/MWCNTs). X-ray diffraction analysis and high resolution transmission electron microscopy show that the Pt and FTO nanoparticles with crystallite size of around 4-8 nm are highly dispersed on the surface of MWCNTs. Pt-FTO/MWCNT catalyst is evaluated in terms of the electrochemical catalytic activity for methanol electrooxidation using cyclic voltammetry, steady state polarization experiments, and electrochemical impedance spectroscopy technique in acidic medium. The Pt-FTO/MWCNT catalyst exhibits a higher intrinsic catalytic activity for methanol electrooxidation with high stability during potential cycling than Pt nanoparticles supported on tin dioxide/multi-walled carbon nanotube composites. The results suggest that FTO/MWCNT composites could be considered as an alternative support for Pt-based electrocatalysts in direct alcohol fuel cells.     

Thermal behavior and flammability of epoxy/glass fiber composites containing clay and decabromodiphenyl oxide

Epoxy/glass fiber hybrid composites with organo-montmorillonite (OMMT) and decabromodiphenyl oxide (DBDPO) flame retardants were prepared by vacuum-assisted resin infusion technique. The effects of OMMT and DBDPO on the flammability properties of epoxy/glass fiber hybrid composites were evaluated through UL-94 vertical flammability test and limiting oxygen index (LOI). Thermal decomposition was studied by means of thermogravimetric analyzer (TG). Field emission scanning electron microscopy (FESEM) was used to study the char morphology of the epoxy hybrid composites after being subjected to UL-94 vertical flammability test. Epoxy/glass fiber/OMMT hybrid composites with DBDPO loading of 40 wt% showed V-1 rating, whereas an increase to 50 wt% loading showed V-0 rating. The LOI values increased from 22.7 to 39.9 % as the loading of DBDPO increased. The obtained TG results showed that the thermal stability of epoxy hybrid composites decreased as the DBDPO loading increased. DBDPO decomposed at a lower temperature to form bromine radicals, which reacted with the combustible gases to form hydrogen bromide to inhibit the flame spread in the gas phase. The condensed phase activity was shown in FESEM, in which a layer of compact and continuous char was formed in epoxy/glass fiber/OMMT/DBDPO hybrid composites.     

Comparison of static and fatigue interlaminar testing methods for continuous fiber reinforced polymer composites

Two different interlaminar fatigue testing methods have been compared by testing a carbon fiber reinforced epoxy (CF/EP) composite and a carbon fiber/multiwalled carbon nanotube reinforced epoxy (CF/MWCNT/EP) hybrid nanocomposite. The first, conventional fatigue testing method was the end-notched flexure (ENF) test, which was used as a reference. The second, novel technique was the fatigue interpretation of the double-notch shear (DNS) test. Both tests have been performed with static and cyclic loading to compare the evaluated properties of the different systems, the effect of transition from cyclic to fatigue loading and to demonstrate if the complex ENF test can be replaced by the simpler DNS test.The test results showed the slight beneficial effect of the nanoreinforcement in both static and cyclic load conditions, and the possibility to use the DNS test for fatigue testing of continuous fiber reinforced composites. The SEM micrographs taken of the fracture surfaces of the composites after the different interlaminar tests provide valuable data on the interlaminar failure phenomena of hybrid nanocomposites in both static and fatigue loading conditions.     

The improvement in interfacial shear strength of wood fiber/epoxy composite by sizing with epoxy sizing agent containing MWCNTs

This paper discloses a feasible and high efficient strategy for wood fiber treatment to introducing multi‐wall carbon nanotubes (MWCNTs) to the surface of wood fibers for the aim of improving the interfacial shear strength of wood fiber/epoxy composite. Briefly, a layer of MWCNT was deposited on wood fibers through sizing wood fibers with epoxy sizing agent containing amine‐treated MWCNTs (MWCNT‐PEI). The surface functional groups, morphology, wettability, and interphase properties of MWCNTs on the surface of wood fiber were studied. The remarkable enhancements were achieved in interfacial shear strength of reinforced composites by dipping wood fiber in MWCNTCOOH suspension and wood fiber sizing containing MWCNT‐PEI.     

Enhanced conductivity composites for aircraft applications: carbon nanotube inclusion both in epoxy matrix and in carbonized electrospun nanofibers

The potential of carbonized electrospun nanofiber mats to render epoxy resin composites for aircraft applications electrically and thermally more conductive was investigated. The effect of carbon nanotube inclusion both inside the carbon nanofiber and in the epoxy resin matrix material was studied, in order to reveal any synergistic effects of multilevel presence of nanosized reinforcements on the conductivity and mechanical properties. The carbon nanotube inclusion into the carbonized nanofibers increased the electrical conductivity of the samples by 20–50% and the thermal conductivity by approximately three times leading to a higher value than that of the conventional composites. The preparation of layered composites with a conductive upper layer containing nonwoven carbon nanofabric and a load bearing lower layer with conventional unidirectional carbon fiber reinforcement can offer a cost‐effective and weight‐saving solution for the replacement of metal meshes in structural aircraft composites. Copyright © 2014 John Wiley & Sons, Ltd.     

Carbon nanotube detectors for microchip CE: comparative study of single-wall and multiwall carbon nanotube, and graphite powder films on glassy carbon, gold, and platinum electrode surfaces

The performance of microchip electrophoresis/electrochemistry system with carbon nanotube (CNT) film electrodes was studied. Electrocatalytic activities of different carbon materials (single-wall CNT (SWCNT), multiwall CNT (MWCNT), carbon powder) cast on different electrode substrates (glassy carbon (GC), gold, and platinum) were compared in a microfluidic setup and their performance as microchip electrochemical detectors was assessed. An MWCNT film on a GC electrode shows electrocatalytic effect toward oxidation of dopamine (E(1/2) shift of 0.09 V) and catechol (E(1/2) shift of 0.19 V) when compared to a bare GC electrode, while other CNT/carbon powder films on the GC electrode display negligible effects. Modification of a gold electrode by graphite powder results in a strong electrocatalytic effect toward oxidation of dopamine and catechol (E(1/2) shift of 0.14 and 0.11 V, respectively). A significant shift of the half-wave potentials to lower values also provide the MWCNT film (E(1/2) shift of 0.08 and 0.08 V for dopamine and catechol, respectively) and the SWCNT film (E(1/2) shift of 0.10 V for catechol) when compared to a bare gold electrode. A microfluidic device with a CNT film-modified detection electrode displays greatly improved separation resolution (R(s)) by a factor of two compared to a bare electrode, reflecting the electrocatalytic activity of CNT.     

High strength toughened epoxy nanocomposite based on poly(ether sulfone)‐grafted multi‐walled carbon nanotube

Hydroxyl terminated poly(ether sulfone) (PES) has been grafted on multi‐walled carbon nanotube (MWCNT). The grafting reaction was confirmed by different characterization techniques such as Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The extent of the grafting was found to be around 58 wt%. Hybrid nanocomposite of epoxy with the modified MWCNT was also prepared. Effect of grafting on the mechanical, thermal, and viscoelastic properties was studied. Dynamic mechanical studies show an increase in the storage modulus for the nanocomposite prepared using PES‐grafted MWCNT compared with neat epoxy system. PES‐grafted MWCNT–epoxy nanocomposite induces a significant increase in both tensile strength (26%) and fracture toughness (125%) of the epoxy matrix. Field emission scanning electron micrographs of fractured surfaces were examined to understand the toughening mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Annealing induced electrical conductivity jump of multi‐walled carbon nanotube/polypropylene composites and influence of molecular weight of polypropylene

Multi‐walled carbon nanotube (MWCNT)/polypropylene (PP) composites were prepared by a micro melt mixing process. As‐prepared composites had relatively low electrical conductivity due to the disruption of MWCNT network by strong shear. The electrical conductivity jumped to high values throughout an annealing process above the melting temperature of PP. The significant enhancement of electrical conductivity was influenced by annealing time, temperature, and content of MWCNTs. In particular, molecular weight of PP played an important role in affecting the conductivity enhancement. The molecular weight of PP was varied from 190,000 to 340,000 to examine its effect on the electrical conductivity. By comparing the conductivity enhancement behavior of composites with different molecular weight PPs and observing the morphology evolution during annealing, it was found that reaggregation of MWCNTs and the subsequent formation of MWCNT network during annealing are the main reasons for the jump of electrical conductivity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Experimental investigation of thermal properties and fire behavior of carbon/epoxy laminate and its foam core sandwich composite

According to structural characteristics, composites are classified as laminated structure and sandwich structure. Carbon/epoxy laminate and foam core sandwich composite are the most commonly used laminate and sandwich structure material in the aircraft industry. The flammability of epoxy resins and foam core material is an inherent hazard. Many previous studies focused primarily on their mechanical properties, while the studies on the thermal and fire properties of carbon/epoxy laminate and its foam core sandwich composite have rarely conducted. Therefore, to characterize their thermal and fire properties, a comprehensive experimental investigation and theoretical analysis were carried out in this work using thermogravimetric analysis, cone calorimeter, vertical/horizontal burning tests, limiting oxygen index and scanning electron microscope tests. Several typical characteristic parameters were obtained and analyzed, such as pyrolysis temperature, heat release rate, mass loss, flaming spread rate and limiting oxygen index. These experimental data coupled with theoretical analysis can provide support for fire risk assessment and fire protection design in aircrafts. The carbon/epoxy laminate and foam core sandwich composite are both characterized as the thermally thick materials. The ignition models and mass loss rate models were obtained. Foam core material negatively affects most of the thermal and fire properties of sandwich composite, but the foam core sandwich composite has self-extinguishing behavior during horizontal burning tests, whose LOI is higher than that of carbon/epoxy laminate. Thus, an important conclusion was reached that the ignition position and flame spread direction have critical effect on the fire behavior of foam core material.

     

Effect of carbon fiber surface functionality on the moisture absorption behavior of carbon fiber/epoxy resin composites

Polyacrylonitrile (PAN)‐based carbon fibers were electrochemically oxidized in aqueous ammonium bicarbonate with increasing current density. The electrochemical treatment led to significant changes of surface physical properties and chemical structures. The oxidized fibers showed much cleaner surfaces and increased levels of oxygen functionalities. However, it was found that there was no correlation between surface roughness and the fiber/resin bond strength, i.e. mechanical interlocking did not play a major role in fiber/resin adhesion. Increases in surface chemical functionality resulted in improved fiber/resin bonding and increased interlaminar shear strength (ILSS) of carbon fiber reinforced epoxy composites. The relationship between fiber surface functionality and the hydrothermal aging behavior of carbon fiber/epoxy composites was investigated. The existence of free volume resulted from poor wetting of carbon fibers by the epoxy matrix and the interfacial chemical structure were the governing factors in the moisture absorption process of carbon fiber/epoxy composites. Copyright © 2016 John Wiley & Sons, Ltd.     

Study of the fire resistant behavior of unfilled and carbon nanofibers reinforced polybenzimidazole coating for structural applications

With increasing interest in epoxy‐based carbon fiber composites for structural applications, it is important to improve the fire resistant properties of these materials. The fire resistant performance of these materials can be improved either by using high performance epoxy resin for manufacturing carbon fiber composite or by protecting the previously used epoxy‐based composite with some fire resistant coating. In this context, work is carried out to evaluate the fire resistance performance of recently emerged high performance polybenzimidazole (PBI) when used as a coating material. Furthermore, the effect of carbon nanofibers (CNFs) on fire resistant properties of inherently flame retardant PBI coating was studied. Thermogravimetric analysis of carbon/epoxy composite, unfilled PBI and nano‐filled PBI shows that the carbon/epoxy composite maintained its thermal stability up to a temperature of 400°C and afterwards showed a large decrease in mass, while both unfilled PBI and nano‐filled PBI have shown thermal stability up to a temperature of 575°C corresponding to only 11% weight loss. Cone calorimeter test results show that unfilled PBI coating did not improve the fire retardant performance of carbon/epoxy composite. Conversely, nano‐filled PBI coating has shown a significant improvement in fire retardant performance of the carbon/epoxy composite in terms of increased ignition time, reduced average and peak heat release rate and reduced smoke and carbon monoxide emission. These results indicate that addition of carbon nanofibers to inherently flame retardant coating can significantly be helpful for improving the fire resistance performance of composite materials even with low coating thickness. Copyright © 2013 John Wiley & Sons, Ltd.     

玻璃纤维对碳纳米管填充聚合物/热塑性聚氨酯共混体系导电性能的影响

研究了表面带有环氧基团的玻璃纤维(GF)对聚丙烯(PP)/热塑性聚氨酯(TPU)/多壁碳纳米管(MWCNT)、 聚甲基丙烯酸甲酯(PMMA)/TPU/MWCNT和聚乳酸(PLA)/TPU/MWCNT体系导电性能的影响. 研究结果表明, 未添加GF时, 由于MWCNTs选择性地分布在TPU分散相中, PP/TPU/MWCNT, PMMA/TPU/MWCNT和PLA/TPU/MWCNT材料的导电性能很差; 加入20%的GF使3个体系的电阻率均大幅度下降, 最高下降约13个数量级, 表明填充GF是一种具有普适性的改善以TPU为分散相的共混体系导电性能的有效方法. GF使体系电阻率降低的机理主要是形成了TPU包覆GF结构, 该结构可以看作长径比较高的导电棒, 可以有效协助导电通路的构建; 同时GF还起到了体积占位的作用, 提高了体系中导电组分在基体中的有效浓度.     

Comparison of four methods for determining fiber content of carbon fiber/epoxy composites

The most fundamental parameter that controls the properties of fiber/epoxy composites is fiber content. Thus, exact calculation of carbon fiber content is important for product quality control and process optimization. In this study, four methods for calculating fiber content of continuous carbon fiber/epoxy composites were investigated. These four methods are processing statistical (PS), optical microscopy (OM), thermogravimetric analysis (TGA), and carbonization-in-nitrogen (CIN). The results show that the CIN and PS methods have high repeatability with no more than 4.7 wt.% deviation, and a relatively exact estimation of fiber content can be obtained by using both of them. A larger difference was generated when the TGA and OM methods were used to determine the fiber content. This is because the small amount of samples required by these methods is not sufficiently representative of the whole carbon fiber composite structure. The PS and CIN methods can be used as approved ways to calculate fiber content of carbon fiber/epoxy composites effectively.