Facilitating the formation of nanohybrid shish kebab structure in helical polymer systems by using carbon nanotube bundles

Periodic patterning of carbon nanotubes (CNTs) with semi-crystalline polymers, especially the novel nanohybrid shish kebab (NHSK) superstructure, in which fibrous CNTs act as shish while polymer lamellae as kebab, is of interest both scientifically and technologically. So far the reported NHSK are mostly prepared using polymers with zigzag conformation in crystal and it seems difficult to obtain NHSK using polymer with helical conformation. In this work, we report the formation of NHSK structure by using single-walled carbon nanotube (SWNT) bundles. A promoted formation of NHSK was observed even using polymer with helical conformation, and the formation mechanism of NHSK was attributed to the unique “groove structure” formed by the stacked SWNTs in parallel arrays, which could facilitate the orientation of helical polymer chains along the SWNTs axis and the lateral formation of stable nucleus. The NHSK structure in helical polymer/SWNT bundles system could widen application of this unique superstructure, offering value in both application field and crystallography aspect as well.     

Fabrication of pH‐ or temperature‐responsive single wall carbon nanotubes via a graft from photopolymerization

An ultraviolet light initiated “graft from” polymerization method to fabricate polymer‐functionalized single wall carbon nanotubes (SWNTs) with pendant pH‐ and temperature‐responsive polymer chains is utilized. The attached polymer chains, formed from methacrylic acid and poly(ethylene glycol) methyl ether methacrylate monomers, are well established for its pH‐responsive swelling/deswelling behavior. This special property was utilized here to control the aqueous dispersibility of the carbon nanotubes. Furthermore, poly(N‐isopropylacrylamide), a temperature‐responsive polymer, was utilized in the fabrication of SWNTs whose dispersibility was dependent on solution temperature. The morphology of the polymer‐functionalized carbon nanotubes was characterized by scanning electron microscopy (SEM) before and after functionalization. Environmental SEM was used to further characterize the morphology of the functionalized SWNTs. In addition, covalent bonding of the polymer to the carbon nanotube surface was established using Raman and Fourier transform infrared spectroscopic techniques. The physical and chemical properties of the functionalized nanotubes were further characterized by energy‐dispersive X‐ray spectroscopy, thermogravimetric analysis, and differential scanning calorimetry. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2980–2986, 2012     

Effect of chemisorption on the interfacial bonding characteristics of carbon nanotube-polymer composites

The influence of chemical functionalization on the interfacial bonding characteristics of single-walled nanotubes (SWNTs) reinforced polymer composites was investigated using molecular mechanics and molecular dynamics simulations. The simulations show that functionalization of nanotubes at low densities of functionalized carbon atoms drastically increases their interfacial bonding and shear stress between the nanotubes and the polymer matrix, where chemisorption to as little as 5.0% of the nanotube carbon atoms increases the shear stress by about 1000%. This indicates that increasing the load transfer between SWNTs and a polymer matrix in a composite via chemisorption may be an effective way and chemical attachment of nanotubes during processing may be in part responsible for the enhanced stress transfer observed in some systems of the nanotube-polymer composites. Furthermore, this suggests the possibility to use functionalized nanotubes to effectively reinforce other kinds of polymer-based materials as well.     

Thermal properties of polypropylene nanocomposites: Effects of carbon nanomaterials and processing

Design of experiments was used to elucidate the complex interactions that determine nanocomposite properties and enable predictive models for optimization. The thermal properties of nanocomposites containingpolypropylene, single‐walled carbon nanotubes (SWNTs), dodecyl‐functionalized SWNTs, and vapor‐grown carbon fibers were investigated as a function of extrusion temperature, screw speed, and time. The effects of extruder processing conditions on thermal properties was dependent on the fraction of polymer chains stabilized in the interphase, the extent of polymer degradation, and the type of nanomaterial incorporated. Melting and crystallization temperatures were primarily affected by nanomaterial type. However, thermal decomposition temperature was affected significantly by processing conditions and the response wasdependent on the type of nanomaterial incorporated. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Rapid, low temperature microwave synthesis of novel carbon nanotube-silicon carbide composite

Single wall carbon nanotubes (SWNTs) incorporated into ceramic matrices are known to impart enhanced mechanical, thermal and electrical properties to the composites formed. Current procedures for their synthesis face challenges, such as, the non-uniform dispersion of the SWNTs and their damage during high temperature processing in a reactive environment. These have led to poor interfacial matrix to SWNT adhesion and the ineffective utilization of the unique properties of the nanotubes. Here we report a rapid, low temperature microwave-induced reaction to create a novel nanoscale silicon carbide (SiC)-SWNT composite. The reaction, which was completed in 10 min, involves the decomposition of chloro-trimethylsilane and the simultaneous nucleation of nanoscale SiC spheres on the SWNT bundles. The bulk composite is a branched tree-like structure comprised of three-dimensionally arrayed SiC-SWNTs. The uniqueness of this approach lies in the formation of a ceramic directly on the SWNTs, rather than physical mixing, or the growth of nanotubes in a ceramic matrix.     

Covalent functionalization of single-walled carbon nanotubes by aniline electrochemical polymerization

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178 cm⁻¹ associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one.     

Preparation and characterization of poly(styrene-co-butyl acrylate)-encapsulated single-walled carbon nanotubes under ultrasonic irradiation

Polymeric composite materials filled with single-walled carbon nanotubes (SWNTs) have attracted much attention, but successful applications of such composites require uniform dispersion of SWNTs in the polymeric matrix and the strong SWNTs-polymer interface interaction. In this paper, chemical modification combined with ultrasonically initiated in situ polymerization was successfully employed to prepare poly(styrene-co-butyl acrylate)/single-walled carbon nanotubes composites [P(St-BA)/SWNTs]. The whole procedure contained two steps: in the first step, 3-(trimethoxy)-propylmethacrylate-silane (silane-coupling agent, KH570), a kind of polymerizable vinyl monomer, was grafted onto the surface of SWNTs, forming KH570-g-SWNTs by reacting KH570 with hydroxyl groups on the surface of SWNTs, which was proved by combination of FTIR and XPS results. Due to the presence of polymerizable KH570 on the surface of SWNTs, this provides a basis for the next stage of polymerization to prepare polymer-encapsulated SWNTs composites. In the second step, an ultrasonically initiated in situ emulsion polymerization of monomer styrene (St) and n-butyl acrylate (BA) proceeded in the presence of KH570-g-SWNTs. Consequently, P(St-BA)/SWNTs composite emulsion was obtained. TEM confirmed that SWNTs were coated with the obtained polymer. FTIR and XPS further showed that even after 72 h of soxhlet extraction with boiling toluene, there were still unextracted polymers in P(St-BA)/SWNTs composite, indicating strong interaction between the polymer and carbon nanotubes. Finally, a mechanism for formation of polymer-encapsulated SWNTs through ultrasonically initiated in situ emulsion polymerization was proposed. This study could provide a new way to resolve the problems of the dispersion, stabilization, and compositing of SWNTs with polymer matrix and prepare polymer/SWNTs composites.     

Nanotubes as polymers

In this review, we show that the structure and behavior of single-walled nanotubes (SWNTs) are essentially polymeric; in fact, many have referred to SWNTs as “the ultimate polymer”. The classification of SWNTS as polymers is explored by comparing the structure, properties, phase behavior, rheology, processing, and applications of SWNTs with those of rigid-rod polymers. Special attention is given to research efforts focusing on the use of SWNTs as molecular composites (also termed nanocomposites) with SWNTs as the filler and flexible polymer chains as the host. This perspective of “SWNTs as polymers” allows the methods, applications, and theoretical framework of polymer science to be appropriated and applied to nanotubes.     

Spectroscopic study on the centrifugal fractionation of soluble single-walled carbon nanotubes

Optical absorption and resonant Raman spectra are proven to be convenient and effective to monitor the centrifugal fractionation and to evaluate the quality of soluble single-walled carbon nanotubes (SWNTs) achieved by inorganic oxidation and organic functionalization. Through a systemic study of a series of centrifuged solutions, we confirmed that heavily functionalized amorphous carbon was fractionated into the early centrifuged solutions, whereas lightly functionalized graphite fragments as well as polyhedral carbon and metal catalysts particles were fractionated into the late centrifuged solutions and centrifuged residue, and then highly pure and well dispersed SWNTs were collected from the middle centrifuged solutions. It is proposed that the purity, dispersibility, and aggregation state of SWNTs can be qualitatively estimated by the relative intensity of their absorption features, the fine structure and slope of their absorption curves. The Raman features of centrifuged SWNTs are found systematically up-shifted except the disorder-induced D band in comparison with those of as-prepared material, indicating that the SWNTs in centrifuged solutions are individual or in thin bundles. Two new features were identified at 1428 and 941 cm⁻¹ in the Raman spectrum of thermally annealed centrifuged SWNTs, which were assigned to achiral nanotubes and combined mode, respectively.     

Self organisation and photoinduced charge transfer in single-wall carbon nanotubes embedded in a thermotropic liquid crystal polymer

A hybrid material composed by a thermotropic liquid crystal (LCP) polymer (HBA–PET) and single wall carbon nanotubes has been produced in order to study the interaction at the interface matrix/filler for possible applications in electronics and optics. The nanocomposites are characterized by a mosaic-like morphology, with regions of randomly placed LC fibers intercalated with regions formed by aligned polymer fibers, that trigger in turn the alignment of carbon nanotube bundles by means of P stacking interaction. Moreover an effective electronic interaction between the nanocomposite components is demonstrated by combining use of photoluminescence and Raman spectroscopy. The photoinduced charge transfer between SWCNT and polymer could be explain on the basis of the injection of holes (generated in the polymer by light absorption), into the SWCNT valence band and followed by a radiationless decay of the excited polymer’s electron.     

Synthesis and characterization of polyaniline derivatives and related carbon nanotubes nanocomposites - Study of optical properties and band gap calculation

Poly(o-methylaniline) (POTO), poly(o-methoxyaniline) (POAS), poly(2,5-dimethylaniline) (PDMA), poly(2,5-dimethoxyaniline) (PDOA), and nanocomposite based on multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) embedded in these conducting polymers, were synthesized by oxidative polymerization. We used the Langmuir-Schaefer (LS) technique to fabricate films at the air-water interface and performed the doping process on the undoped films by dipping the substrates in 1 M hydrochloric acid (HCl) aqueous solution. We recorded UV-vis spectra for both the undoped and doped forms and calculated the related band gaps by using the Tauc equation. Experimental data showed the substituents affected the final oxidation ratio of the polymer chains and the presence of carbon nanotubes (CNTs) in the medium of reaction changed the properties in relation of the kind and number of substituents along the aromatic ring. The study of UV-vis spectra of the undoped nanocomposites and the calculated band gaps highlighted that the conducting polymer chains simply wrapped up around CNTs with no strong interaction. Both the kind and number of substituents along the aromatic rings strongly affected the protonation process, since their capability of “tuning” the formation of the polaronic state. The presence of CNTs in the polymer matrix showed no appreciable influence in the chemical properties of the doped nanocomposites with respect to the pure conducting polymers.     

Hybrid peptide–carbon nanotube dispersions and hydrogels

Scanning probe microscopy (SPM) techniques based on nano-mechanical measurements (topography, adhesion, modulus) and electric force microscopy (EFM) have been used to examine mica surfaces modified with the ionic-complementary peptide EFK8 alone and with EFK8–single-walled carbon nanotube (SWNT) dispersions in water in order to gain a deeper understanding of the interaction between nanotubes and ionic-complementary peptides. Through the use of these techniques, it has been shown for the first time that peptide fibers can be distinguished from SWNTs and peptide-wrapped SWNTs. SPM images reveal features consistent with two types of helical structures: EFK8 fibers wrapped around each other during self-assembly and EFK8 fibers wrapped around SWNTs. In this second structure, EFK8 chains should be oriented with their hydrophobic sides oriented toward the SWNTs and their hydrophilic sides toward the water, thereby enabling the dispersion of the nanotubes in aqueous media. We have also demonstrated the formation of hybrid EFK8–SWNT hydrogels that have potentially superior physical and mechanical properties over those of other hydrogels and opens up new applications for this type of material. To the best of our knowledge, this is the first work reporting the formation of a composite hydrogel made of an ionic-complementary peptide and carbon nanotubes.     

Effects of single‐walled carbon nanotubes on the crystallization behavior of polypropylene

Polypropylene matrix composites reinforced with single‐walled carbon nanotubes (SWNTs) were produced with different nanotube concentrations. The characterization of these new materials was performed by differential scanning calorimetry and Raman and Fourier transform infrared spectroscopy to obtain information on the matrix–nanotube interaction, on the crystallization kinetics of polypropylene, and especially on the macrostructure and organization of the nanotubes in the composite. On the one hand, the results confirmed the expected nucleant effect of nanotubes on the crystallization of polypropylene, but on the other hand, this effect was not linearly dependent on the SWNT content: there was a saturation of the nucleant effect at low nanotube concentrations. Raman spectroscopy was successfully applied to demonstrating that in the composite films, the crystallization kinetics were strongly affected by the distance between the nanotube bundles as a result of a different intercalation of the polymer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 708–713, 2003     

Effects of carbon nanotubes on the crystallization behavior of polypropylene

The effect of the incorporation of single‐walled carbon nanotubes (SWNTs) on the PP crystallization kinetics is investigated by thermal analysis, microscopy and Raman spectroscopy. The results of the investigation show that SWNT acts as a strong nucleation agent. A marked decrease of the half‐time of PP crystallization as well as a sensible increase of the overall crystallization rate is observed in the presence of SWNTs. Moreover, these effects are already noticeable at the lowest SWNT content in the composite with slow further effects at higher concentrations, suggesting a saturation of the nucleating action at the higher concentrations studied. The Avrami model can represent the crystallization kinetics of PP in the composite. The kinetic curves obtained under non‐isothermal conditions confirm the results obtained in isothermal tests and demonstrate the nucleation ability of SWNTs on the PP crystallization. Raman spectroscopy and scanning electron microscopy (SEM) are successfully applied to demonstrate that in the composite films, the changes in the crystallization kinetics can be explained in terms of the changes of the distance between nanotubes in bundles following a different intercalation of the polymer matrix.     

Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique

A new method was developed to disperse carbon nanotubes (CNTs) in a matrix polymer and then to prepare composites by melt processing technique. Due to high surface energy and strong adsorptive states of nano-materials, single-walled carbon nanotubes (SWNTs) were adsorbed onto the surface of polymer powders by spraying SWNT aqueous suspected solution onto fine high density polyethylene (HDPE) powders. The dried SWNTs/powders were blended in a twin-screw mixture, and the resulting composites exhibited a uniformly dispersion of SWNTs in the matrix polymer. The electrical conductivity and the rheological behavior of these composites were investigated. At low frequencies, complex viscosities become almost independent of the frequency as nanotubes loading being more than 1.5 wt%, suggesting an onset of solid-like behavior and hence a rheological percolation threshold at the loading level. However, the electrical percolation threshold is ∼4 wt% of nanotube loading. This difference in the percolation thresholds is understood in terms of the smaller nanotube-nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. The measurements of mechanical properties indicate that this processing method can obviously improve the tensile strength and the modulus of the composites.     

Modeling initial stage of phenolic pyrolysis: Graphitic precursor formation and interfacial effects

Reactive molecular dynamics simulations are used to study the initial stage of pyrolysis of phenolic polymers with carbon nanotube and carbon fiber. The products formed are characterized and water is found to be the primary product in all cases. The water formation mechanisms are analyzed and the value of the activation energy for water formation is estimated. A detailed study of graphitic precursor formation reveals the presence of two temperature zones. In the lower temperature zone (<2000 K) polymerization occurs resulting in the formation of large, stable graphitic precursors, while in the high temperature zone (>2000 K) polymer scission results in formation of short polymer chains/molecules. Simulations performed in the high temperature zone of the phenolic resin (with carbon nanotubes and carbon fibers) show that the presence of interfaces does not have a substantial effect on the chain scission rate or the activation energy value for water formation.     

Single wall carbon nanotube dispersion and exfoliation in polymers

Dispersion and exfoliation of single wall carbon nanotubes (SWNTs) have been studied in poly(acrylonitrile) (PAN), poly(p‐phenylene benzobisoxazole) (PBO) solutions, and composite fibers using transmission electron microscopy. As a result of polymer assisted dispersion and exfoliation, the average SWNT bundle diameter in SWNT/PAN (5/95) was 11 nm, while the average diameter for the pristine SWNT bundles was about 30 nm. High resolution TEM of SWNT/PBO (10/90) composite fibers did not reveal the presence of SWNT aggregates or bundles, suggesting SWNT exfoliation as individuals. On the other hand, both oriented and unoriented nanotube bundles have been observed in SWNT/PBO samples containing 15 wt % nanotubes. Carbon nanotubes are 10⁵ times more radiation resistant than flexible polymers such as polyethylene, and 10³ times more resistant than highly radiation resistant polymers such as PBO. Therefore in the high resolution TEM study of nanotube/polymer composites, nanotubes can be observed long after the polymer has been damaged by electron radiation. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 985–989, 2005     

Carbon nanotubes: dynamics of synthesis processes

Heat and mass transport are obtained in a solar reactor using ‘in situ’ measurements linked to numerical simulation and allow the interpretation of the vaporization process as well as the determination of the cooling regime. Comparison with other processes (laser ablation or electric arc) point out some common behavior like the great influence of the cooling rate of vapors on the structure and yield of nanostructured carbon material. We also investigate the growth mechanisms of single wall carbon nanotubes (SWNTs) produced by the solar method as a function of the nature of catalysts and the temperature variation in the condensing area. The Raman spectra clearly show that the change of catalyst induces differences in the diameter of SWNT whereas TEM pictures enhance the change of both length and diameter of the bundles. All these results are explained considering that the key parameter is the temperature at which the SWNTs are formed. This temperature range can be related to the sublimation temperature of the target and to the eutectic temperature of the binary phase diagram. Finally we propose a new mechanism to explain the nucleation process and segregation rate which seems to depend on the capacity of catalyst to form carbide.     

Investigation of molecular interaction between single‐walled carbon nanotubes and conjugated polymers

Single‐walled carbon nanotubes (SWNTs) have unique properties such as high electrical conductivity and high tensile strength. Their composites with polymers have a great role in new sciences such as organic solar cells and ultrastrong lightweight materials. In this article, molecular dynamic simulations with polymer consistent force field are performed to study the interaction between SWNTs and conjugated polymers including poly(2‐methoxy‐5‐(3‐7‐dimethyloctyloxy)‐1,4‐phenylenevinylene) (MDMO‐PPV), poly(3‐hexythiophene) (P3HT), and poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐bis(N,N′‐(4,butylphenyl))bis(N,N′‐phenyl‐1,4‐phenylene)diamine] (PFB). We computed the interaction energy and morphology of polymers adsorbed to the surface of SWNTs was studied by the radius of gyration (Rg). The influence of important factors such as SWNT radius, chirality, and the temperature on the interfacial adhesion of SWNT–polymer and Rg of polymers were studied. We found that the strongest interaction between the SWNTs and these polymers was, first observed for P3HT, then MDMO‐PPV, and finally PFB. Our results showed that the interaction energy is influenced by SWNT radius and the specific monomer structure of the polymers, but the effects of chirality and temperature are very weak. In addition, we found that the temperature, chirality, and radius have not any important effect on the radius of gyration. POLYM. COMPOS.,, 2012. © 2012 Society of Plastics Engineers     

Molecular dynamics simulation of dispersion and aggregation kinetics of nanorods in polymer nanocomposites

Nowadays, achieving a uniform dispersion of rod-like molecules (like carbon nanotube) in a polymer matrix is still a complicated and unsettled issue in polymer physics and chemical physics. It is very significant to fully understand the effects of deterministic factors on dispersion and aggregation processes of nanorods in the polymer matrix. Here, we adopt a coarse-grained molecular dynamics simulation to investigate the nanorod- filled polymer nanocomposites. It is found that the characteristic relaxation time of the end-to-end vector correlation exhibits an Arrhenius-like temperature-dependent behavior and both the rotational and translational diffusion coefficients have a linear relationship with temperature. By tuning the polymer–nanorod interaction in a wide range, we obtain the spatial organization of nanorods and the best dispersion state at the intermediate interfacial interaction. Meanwhile, we observe that grafting polymer chains on the nanorod surface could promote the dispersion. Moreover, a lower or higher temperature than glassy transition temperature can prevent the nanorod aggregation. The aggregation of nanorods can be significantly accelerated by nanorod–nanorod attraction, while inhibited by cross-linking of polymer chains and external shear fields. In short, by tailoring the deterministic factors above, we can effectively control the dispersion or even spatial organization of one-dimensional nanorods in polymer nanocomposites.