Recent advances in bio‐sourced polymeric carbohydrate/nanotube composites

Nanotubes (NTs), especially carbon nanotubes (CNTs), have attracted much attention in recent years because of their large specific surface area, and their outstanding mechanical, thermal, and electrical properties. In this review we emphasize the development of fascinating properties of polymeric carbohydrate/CNT composites, particularly in terms of their mechanical and conductivity properties and potential applications. Many methods used to modify CNTs during preparation of polymeric carbohydrate/CNT composites are presented. Moreover, we also discuss the enhanced mechanical and electrical effectiveness when hybrid CNTs or halloysite nanotubes were incorporated into different carbohydrate polymer matrices. Finally, we give a future outlook for the development of polymeric carbohydrate/CNT composites as potential alternative materials for various applications including sensors, electroactive paper, electrodes, sorbents for environmental remediation, packaging film, specialty textile, and biomedical devices. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40359.     

Polymer-functionalized carbon nanotubes in cancer therapy: a review

The increasing importance of nanotechnology in the field of biomedical applications has encouraged the development of new nanomaterials endowed with multiple functions. Novel nanoscale drug delivery systems with diagnostic, imaging and therapeutic properties hold many promises for the treatment of different types of diseases, including cancer, infection and neurodegenerative syndromes. Carbon nanotubes (CNTs) are both low-dimensional sp² carbon nanomaterials exhibiting many unique physical and chemical properties that are interesting in a wide range of areas including nanomedicine. Since 2004, CNTs have been extensively explored as drug delivery carriers for the intracellular transport of chemotherapy drugs, proteins and genes. In vivo cancer treatment with CNTs has been demonstrated in animal experiments by several different groups. Herein, the recent works on anticancer drug delivery systems based on carbon nanotubes are reviewed and some of more specific and important novel drug delivery devices are discussed in detail. This paper focuses on modifications of CNTs by polymers through covalent and non-covalent attachments: two different methods as critical steps in preparation of anticancer drug delivery systems from CNTs. In this respect the in vivo and in vitro behaviors and toxicity of the CNTs modified by polymers are summarized as well. Well-functionalized CNTs did not show any significant toxicity after injection into mice. Moreover, administration and excretion of CNT-based nanocarriers are discussed. It was concluded that future development of CNT-based nanocarriers may bring novel opportunities to cancer diagnosis and therapy.     

Development of efficient digestion procedures for quantitative determination of cobalt and molybdenum catalyst residues in carbon nanotubes

Whatever the method used for the synthesis of carbon nanotubes (CNTs), they always contain residual catalysts in variable amount. Many methods have been proposed in the literature to purify CNTs, but their efficiency strongly depends on the experimental conditions. Although the presence of residual catalysts in small amount is generally not a problem for many applications, this can become a critical issue when a high purity is required, typically for magnetic properties or for biomedical applications (because of the intrinsic toxicity of most catalysts). Quantification of the amount of residual catalysts is usually obtained by classical chemical analysis, which requires a preliminary digestion (complete mineralisation) of the CNT samples. In this work, we systematically compared 3 different digestion protocols and optimised one, reaching 100% dissolution within a very limited time (1 h) together with the requirement of only a few milligrams of sample, and safe experimental conditions. This method can be easily transferred for use in research laboratories, making accessible the quantitative analysis of CNT samples, and has been validated following ISO/IEC 17025:2005 for linearity, specificity, intermediate precision, limits of detection and quantification.     

Carbon nanotube deposits and CNT/SiO2 composite coatings by electrophoretic deposition

Abstract

Multiwalled carbon nanotube (CNT) films have been successfully fabricated by electrophoretic deposition (EPD) on stainless steel substrates. Electrophoretic deposition was performed using optimised aqueous suspensions under constant voltage conditions. Triton X-100 was used as a surfactant to disperse CNT bundles, and iodine was added as a particle charger. CNT/SiO₂ composite coatings were prepared by electrophoretic co-deposition. Experimental results show that the CNTs were efficiently mixed with SiO₂ nanoparticles to form a network structure. Layered CNT/SiO₂ porous composites were obtained by sequential EPD experiments alternating the deposition of CNT and SiO₂ nanoparticles. The structure of all films deposited was studied in detail by scanning electron microscopy. Possible applications of CNT and CNT/SiO₂ films are as porous coatings in the biomedical field, thermal management devices, biomedical sensors and other functional applications where the properties of CNTs are required.     

Fabrication of scalable,aligned and low density carbon nanotube/silicon carbide hybrid foams by polysilazane infiltration and pyrolysis

Polymer-derived ceramic (PDC) process is an attractive technique that has high ceramic yield. This versatile method allows for fabrication of porous carbon nanotube (CNT)/ silicon carbide (SiC) hybrid materials that is important high temperature structural applications. Although several forms of CNT assemblies have been used with the PDC approach, the fabricated CNT/ceramic nanocomposites were either one or two dimensional. Herein, we report, for the first time, the fabrication of a low density, three-dimensional (3D) and scalable CNT/SiC structure using PDC technique. It was synthesized by impregnating preceramic polysilazane (PSZ) into ultralow density, anisotropic, and highly aligned CNT foams, followed by thermosetting and pyrolysis processes. The ceramic phase conformally coated the CNTs. The X-ray diffraction (XRD) diffractogram confirmed the presence of β-SiC crystalline phase. The resulting hybrid foam inherited the morphology and form factor of the original CNT foam, and possessed mechanical robustness, improved electrical properties, and extraordinary thermal stability.     

From carbon nanotube coatings to high‐performance polymer nanocomposites

Since their discovery at the beginning of the 1990s, carbon nanotubes (CNTs) have been the focus of considerable research by both academia and industry due to their remarkable and unique electronic and mechanical properties. Among numerous potential applications of CNTs, their use as reinforcing materials for polymers has recently received considerable attention since their exceptional mechanical properties, combined with their low density, offer tremendous opportunities for the development of fundamentally new material systems. However, the key challenge remains to reach a high level of nanoparticle dissociation (i.e. to break down the cohesion of aggregated CNTs) as well as a fine dispersion upon melt blending within the selected matrices. Therefore, this contribution aims at reviewing the exceptional efficiency of CNT coating by a thin layer of polymer as obtained by an in situ polymerization process catalysed directly from the nanofiller surface, known as the ‘polymerization‐filling technique’. This process allows for complete destructuring of the native filler aggregates. Interestingly enough, such surface‐coated carbon nanotubes can be added as ‘masterbatch’ in commercial polymeric matrices leading to the production of polymer nanocomposites displaying much better thermomechanical, flame retardant and electrical conductive properties even at very low filler loading. Copyright © 2007 Society of Chemical Industry     

Hyperelasticity of three-dimensional carbon nanotube sponge controlled by the stiffness of covalent junctions

To expand the applications of carbon nanotubes (CNTs) at macroscale, a heteroatom doping technique has been employed to fabricate isotropic 3-D CNT architectures by inducing elbow-like covalent junctions into multiwalled CNTs. As the junctions modify the topology of each CNT by favoring the stable bends in CNTs, junction stiffness and the consequence of junction-related morphology changes in sponge's hyperelasticity remain largely elusive. In this study, two types of 3-D multiwalled CNT sponges were fabricated by inducing boron-doped or nitrogen-doped covalent junctions into CNTs. Hyperelastic properties of the sponges were experimentally quantified as the functions of CNT morphology. A novel microstructure informed continuum constitutive law was developed specifically for such isotropic CNT sponges with junctions. Analyzing the experimental data with the new theory demonstrated that, for the first time, the effective modulus of boron-doped junctions (∼100 GPa) is higher than that of nitrogen-doped junctions (∼20 GPa), and the junction stiffness is a key factor in regulating the hyperelastic compressive modulus of the material. Theoretical analysis further revealed that increased number of junctions and shorter segments on each individual CNT chain would result in stronger hyperelastic 3-D CNT networks. This study has established a fundamental knowledge base to provide guidance for the future design and fabrication of 3-D CNT macrostructures.     

Noncovalent functionalization of carbon nanotubes with maleimide polymers applicable to high-melting polymer-based composites

As novel carbon nanotube (CNT) dispersants are effective not only for obtaining stable CNT-dispersed solutions but also for high-melting polymer/CNT composites, we synthesized maleimide polymers (MIPs) using N-substituted maleimide for imparting physical adsorption on the CNT surfaces and high heat resistance. The MIPs showed strong physical adsorption on various CNT surfaces and good solubility in a wide variety of organic solvents, and acted as excellent CNT dispersants in these substances. The MIPs on the CNT surfaces were very stable at high temperatures (?∼300 °C) required for melt mixing using high-melting polymers. The addition of MIP-adsorbed CNTs (CNT/MIPs) to poly(1,4-phenylenesulfide) (PPS) as a high-melting polymer was, therefore, very effective for dispersing CNTs and improving the physical properties of the resulting PPS/CNT/MIP composites, in comparison with the PPS/CNT composites. Even at a low CNT loading (1 vol%), the storage modulus of the PPS/CNT/MIP composites increased drastically. Furthermore, thermal conductivity of the PPS/CNT/MIP composites also improved, in comparison with the PPS/CNT composites. These results are considered to be due to an increase of interactions between the CNT and PPS matrices, caused by the stable formation of MIPs on the CNT surfaces.     

Carbon nanotube induced polymer crystallization: The formation of nanohybrid shish-kebabs

Carbon nanotubes (CNTs) have attracted tremendous attention in recent years because of their superb optical, electronic and mechanical properties. In this article, we aim to discuss CNT-induced polymer crystallization with the focus on the newly discovered nanohybrid shish-kebab (NHSK) structure, wherein the CNT serves as the shish and polymer crystals are the kebabs. Polyethylene (PE) and Nylon 6,6 were successfully decorated on single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs), and vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to “size-dependent soft epitaxy”. Polymer CNT nanocomposites (PCNs) containing PE, Nylon 6,6 were prepared using a solution blending technique. Both pristine CNTs and NHSKs were used as the precursors for the PCN preparation. The impact of CNTs on the polymer crystallization behavior will be discussed. Furthermore, four different polymers were decorated on CNTs using the physical vapor deposition method, forming a two-dimensional NHSK structure. These NHSKs represent a new type of nanoscale architecture. A variety of possible applications will be discussed.     

Nanohybrid shish kebab structure and its effect on mechanical properties in poly(L‐lactide)/carbon nanotube nanocomposite fibers

In our previous work, the formation of a nanohybrid shish kebab (NHSK) structure was successfully achieved in helical polymer systems promoted by using single‐walled carbon nanotube (CNT) bundles with a unique ‘groove structure’, which is of great crystallographic interest. To further investigate the effect of surface groove structure of CNT bundles on the formation of NHSK structure in helical polymer systems, in the work reported here double‐walled carbon nanotube (DWNT) fibers with bundle structure were used as nucleating agents and orientation templates for poly(L ‐lactide) (PLLA) crystallization. A fine NHSK structure with controlled lateral size and period of kebabs was successfully obtained under various experimental conditions by using DWNT bundles. This could be due to the geometric confinement effect of the surface groove structure of the DWNT bundles, which could facilitate the orientation of PLLA chains along the DWNT axis and the lateral formation of a stable nucleus. Our work suggests an efficient method for the functionalization of CNTs with biocompatible PLLA, which may have some potential applications in biomedical areas. In addition, it is demonstrated that the formation of NHSK structure can effectively improve the physical bonding between PLLA and nanotubes, thus significantly improving the mechanical properties of PLLA/CNT nanocomposite fibers. Copyright © 2012 Society of Chemical Industry     

Fabrication and toughening behavior of carbon nanotube (CNT) scaffold reinforced SiBCN ceramic composites with high CNT loading

Uniform dispersion, high loading and three-dimensional (3D) continuous network of carbon nanotube (CNT) are desired for high-performance nanocomposites to fully utilize the superior strength and toughness of CNTs. In this work, monolithic CNTs/SiBCN composites with high CNT loading (10 wt% and 20 wt%) were prepared from 3D scaffold-like CNT cottons and a liquid polyborosilazane (PBSZ) precursor through precursor infiltration and pyrolysis process. The 3D CNT scaffold in the nanocomposite can function as passive filler and gas path to ensure formation of monolithic bulks. Moreover, direct infiltration of PBSZ into the pores among CNT cotton can hinder agglomeration of CNTs and localize CNTs at the original sites, guarantee good alignment and high CNT concentration in the final nanocomposite. This highly concentrated 3D CNT reinforcement in the nanocomposite shows unique resistance to cracking under external stress related to the complex fracture behavior of CNT bundles during the cracking formation and extension process (including CNT bridging, aligning, pulling out and then breaking), which more favors for absorbing energies and enhance toughness of the ceramic composites.     

Morphology and processing of aligned carbon nanotube carbon matrix nanocomposites

Intrinsic and scale-dependent properties of carbon nanotubes (CNTs) have led aligned CNT architectures to emerge as promising candidates for next-generation multifunctional applications. Enhanced operating regimes motivate the study of CNT-based aligned nanofiber carbon matrix nanocomposites (CNT A-CMNCs). However, in order to tailor the material properties of CNT A-CMNCs, porosity control of the carbon matrix is required. Such control is usually achieved via multiple liquid precursor infusions and pyrolyzations. Here we report a model that allows the quantitative prediction of the CNT A-CMNC density and matrix porosity as a function of number of processing steps. The experimental results indicate that the matrix porosity of A-CMNCs comprised of ∼1% aligned CNTs decreased from ∼61% to ∼55% after a second polymer infusion and pyrolyzation. The model predicts that diminishing returns for porosity reduction will occur after 4 processing steps (matrix porosity of ∼51%), and that >10 processing steps are required for matrix porosity <50%. Using this model, prediction of the processing necessary for the fabrication of liquid precursor derived A-CMNC architectures, with possible application to other nanowire/nanofiber systems, is enabled for a variety of high value applications.     

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.     

Morphology control of three-dimensional carbon nanotube macrostructures fabricated using ice-templating method

3-Dimensional (3D) carbon nanotube (CNT) macrostructures with controlled morphologies were prepared by the ice-templating method. In order to control the assembling of the CNTs into 3D macrostructures, we systematically investigated the parameters critical to the control of the morphology of the 3D CNT macrostructures formed using the ice-templating method. It was found that process parameters such as the initial characteristics of the CNT suspension and its freezing conditions significantly affected the morphologies of the resulting CNT macrostructures. By adjusting the initial characteristics of the suspension of CNTs and its freezing conditions, we could fabricate not only regular 3D CNT macrostructures that consisted of aligned lamellae but also those that had a cellular structure. This is the first instance well-aligned, cellular CNT macrostructures have been prepared using the ice-templating method. Our approach can be used to develop the ice-templating method as a technique for fabricating 3D pore- and structure-controlled CNT macrostructures.     

Experimental demonstration of meso-scale carbon nanotube self-assembled tube structures

In addition to numerous other properties of interest, carbon nanotubes (CNT) promise to form a basis for new materials of extraordinary strength owing mainly to the very high carbon–carbon bond energies and their unique tubular structure at the molecular scale. In the area of materials development, the guiding concept of bio-inspired hierarchical structures combined with controlled fabrication at multiple scales has the potential to result in substantially improved mechanical performance. Here we show examples of a multiple-scale self-assembled tube structure, which are themselves composed of multi-wall CNTs, while also demonstrating some important aspects of their nucleation and growth. These hierarchical and self-assembled objects strongly indicate the feasibility of controlled synthesis of macroscopic CNT structures and CNT-reinforced materials for use in various engineering applications. These applications could encompass the areas of structures, thermal transfer, electronics, fluid dynamics, and micro-fluidics.     

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.     

Fused deposition processing polycaprolactone of composites for biomedical applications

The polycaprolactone (PCL)-engineered scaffolds demonstrate cell viability, which is useful for bone tissue applications. The FDA-approved PCL can be engineered with various transition metals, polymers, and nanomaterials, for biomimicking of extracellular matrix via fabrication of its scaffolds. Fused deposition modeling (FDM) is an innovative technique for processing of biopolymers via biologically functionalized nanoparticles, leveraging design of stacked architecture at microlevel resolution. Present review gives progress on PCL biomaterials processed via FDM, for bioactive scaffolds and bone tissue applications. Finally, review concludes with benefits of PCL biomaterials processed via FDM and their applicability for biomedical applications.     

Chitosan: Application in tissue engineering and skin grafting

Tissue Engineering and skin grafting, an essential part of regenerative medicine is one of the fastest growing biomedical fields which could offer an important therapeutic strategy for management of hard to heal wounds. 2D and 3D polymeric scaffolds are prerequisites in this field to promote cell adhesion, proliferation and tissue regeneration. Convergence of technology and research has successfully unveiled unknown properties of Chitosan as a bioactive polymer. Natural abundance, cost effectiveness, biodegradability, biocompatibility and wound healing capabilities of chitosan and its derivatives has drawn the attention of many researchers for its use as an alternative for fabrication of a scaffold in tissue engineering and skin graft. However lower mechanical strength and solubility has limited its application in the biomedical field. It has been found that the derivatization and combination with other polymers can successfully overcome these limitations. This review focuses on the applicability of chitosan and its derivatives in combination with other polymers in tissue engineering and skin grafting along with the novel scaffold fabrication techniques. Studies so far have demonstrated the potential of chitosan and its derivative as a scaffold in the field of regenerative medicine. However, even if the promising results obtained from in-vitro and preclinical studies prove the efficacy of chitosan scaffolds it still has a long way to go to be used in clinical set ups.     

Increasing the electrical conductivity of carbon nanotube/polymer composites by using weak nanotube-polymer interactions

A weak interaction between carbon nanotubes (CNTs) and polymers was found to reduce polymer-wrapping on CNT surface, decrease the contact resistance between CNTs, and increase the electrical conductivity of their composites. Thermodynamic properties such as surface energy of components, filler-polymer interactions, and wettability of carbon/polymer systems were analyzed. It was found that the graphitized CNTs filled polyoxymethylene (POM) system exhibits the weakest CNT-polymer interaction among all the investigated systems and a poor wettability. Consequently, the graphitized CNT/POM composites possess a high electrical conductivity and a low percolation threshold of 0.5 wt.% CNT loading, which is associated with the weak CNT-polymer interaction, low contact resistance between CNTs, good connectivity of CNT networks, and high crystallinity of POM in the composites. The results obtained imply that high-performance composites with optimal CNT-network structures can be designed and fabricated by fully considering the surface properties of components and CNT-polymer interactions.