Yehai Yan

Interface molecular engineering of single-walled carbon nanotube/epoxy composites

Dispersion of large single-walled carbon nanotube (SWCNT) bundles into individual nanotubes or small bundles and thus strengthening of the nanotube/matrix interfacial interaction are prerequisites for taking full advantage of the remarkable multifunctional properties of SWCNTs in various carbon nanotube-based composites. Noncovalent functionalization of SWCNTs is an attractive option to simultaneously achieve these conditions. Toward this end, three reactive amino-containing pyrene derivatives (AmPys) with various spacer chain lengths were synthesized. One with the longest spacer length (12 methylene units, AmPy-12) shows the highest functionalization efficiency for SWCNTs in terms of dispersibility. Systematic characterization on a SWCNT/AmPy-12 hybrid suggests that ca. 10 wt% of AmPy-12 is strongly adsorbed on SWCNTs through π–π interactions, making them steadily dispersed into individual ones and/or small bundles without noticeable change in their electronic structure. AmPy-12-functionalized SWCNTs were then used for the preparation of epoxy composites. Since the SWCNT/epoxy interface was well engineered at a molecular level by application of AmPy-12, which interacts noncovalently with SWCNT but bonds chemically to the epoxy matrix, the composite with only 0.3 wt% SWCNTs displays an increase of 54% and 27% in tensile strength and Youngs modulus, respectively, over neat resin. A low electrical percolation threshold of 0.1 wt% SWCNTs and improved thermal properties were also observed.

Aqueous dispersion of pristine single-walled carbon nanotubes prepared by using a vinylimidazole-based polymer dispersant

In most practical applications, actualizing the inherent properties of single-walled carbon nanotubes (SWNTs) depends strongly on the exfoliation and dispersion of SWNT bundles as individuals and/or small-diameter bundles in various liquid media. Aqueous dispersions are of specific interest in many ways, and polymer dispersants prove to be quite competent for such tasks. To develop a new type of polymer dispersant with accessional function, four water-soluble vinylimidazole (VI)-based polymers bearing different pendant groups were designed, synthesized, and then tested for preparation of aqueous dispersions of pristine SWNTs. The dispersing efficiency was studied as functions of molecular structure and solution nature of VI-polymer. Under assistance of VI-polymers, stable aqueous dispersions are available: the maximal dispersibility reaches 148.9 mg l−1, while initial amount of optimal VI-polymer is merely 0.06 mg ml−1 (∼6 × 10−3 wt%), far below ∼1.0 wt% of sodium dodecyl sulfate (SDS), the most commonly used surfactant, for achieving SWNT dispersion. Biocompatibility of VI-polymer–SWNT complexes obtained from pertinent aqueous dispersions was assessed by culturing L929 cells (mouse fibroblasts), and the results showed that the complex materials were non-toxic to the cells under consideration. The ready dispersions thus pave the way for a variety of applications including those with biological relevance, wherein SWNTs have to face the water-based chemical environments. Besides, it may be more important that the findings, i.e. amphiphilicity and free/sparse of polymer-micelles are two critical factors for VI-polymer to determine its dispersing efficiency, can be potentially applicable to other nanotube/dispersant systems and serve as a common model for developing aqueous dispersions of pristine nanotubes.

A novel poly[(N-vinylimidazole)-co-(1-pyrenylmethyl methacrylate)] ferric complex with fluorescence and superparamagnetism

Due to its unique physical and chemical properties, the polymer N-vinylimidazole (VI), either as a homopolymer or copolymer, has a large number of important applications. Currently, technological advances have identified further uses for it and there are practical demands for multiply functional forms of the polymer; e.g., fluorescence and magnetism in one VI-polymer material. Toward this end, a novel poly[(N-vinylimidazole)-co-(1-pyrenylmethyl methacrylate)] (VI-co-PyMMA) ferric complex prepared by complexing a VI-co-PyMMA copolymer with 1-chlorobutane and FeCl3 in succession is presented in this work. The complexed FeCl3, which predominately exists as the FeCl4− anion, endows the resultant material with superparamagnetism; while the presence of pyrene moieties allows the complex to exhibit additional fluorescence. Both the fluorescence and magnetism exhibit a strong dependence on the content of the complexed FeCl3 but in opposite manners. The preferred complex, S4, shows a balance of the properties, with a fluorescence quantum yield of 0.13 and a magnetic susceptibility of 16.1 × 10−6 emu g−1. Along with re-dispersibility, this single-component fluorescent–magnetic material may replace some corresponding multi-component nanomaterials, where the polymer component acts only as a protective coating and/or an organic fluorophore and the magnetic performance is usually provided by magnetic nanoparticles (MNPs). It may find potential applications in the booming biotechnology field.

A Versatile Platform of 2‐(3,4‐Dihydroxyphenyl) Pyrrolidine Grafted Graphene for Preparation of Various Graphene‐derived Materials

Covalent functionalization has proven an effective solution for graphene to realize its revolutionary potential in real applications, whereas the platform strategy (a reactive graphene-based material acting as the platform to undergo post-reactions for generation of various graphene-derived materials) is an attractive option to execute efficiently such a task. This contribution demonstrates that 2-(3,4-dihydroxyphenyl) pyrrolidine (DHPP) grafted graphene, G-OH, is a competent platform. Four typical but not exclusive graphene-derived materials have been prepared from G-OH by using the chemical virtue of each DHPP unit having three categories totaling six reactive sites. The controlled feature of 1,3-dipolar cycloaddition for the synthesis of G-OH ensures that the electronic structure and properties of pristine graphene are succeeded largely by G-OH and thus its derivatives. A promising alternative to graphene oxide, which has been widely used as a platform to prepare the graphene-derived materials but suffers from some intrinsic disadvantages, is thus developed.

ANNEALING EFFECTS ON ELECTRIC CONTACTS BETWEEN CARBON NANOTUBES AND ELECTRODES

Single wall carbon nanotubes suspended in isopropyl alcohol are placed between two Au electrodes by ac dielectrophoresis method. Total resistance including the contact resistance and intrinsic tube resistance is found to decrease from 105–106 Ω for as-prepared samples to 104 Ω after annealing at 300°C in ambient environment. Measured I–V curves and Schottky barrier heights suggest that the electric contacts are changed from Schottky to Ohmic characteristics after annealing. These results demonstrate that annealing in ambient environment is a simple and efficient way to decrease the contact resistance.

Functional nanoscale metal–organic particles synthesized from a new vinylimidazole-based polymeric ligand and dysprosium ions

With nanoparticles (NPs) getting more and more crucial to various key technologies, it is essential to sustainedly develop new NPs with desirable functionalities. In this regard, nanoscale metal–organic particles (NMOPs) offer a competitive choice due to their highly tunable nature, whereas seeking new organic ligands (e.g. polymeric ones) represents a preferable research direction on NMOPs. With these points in mind, a water- and ethanol-soluble vinylimidazole-based polymer, poly[1-vinylimidazole-co-(poly(ethylene glycol) methacrylate)] (VI-co-PEGMA), was designed, synthesized, and demonstrated to be exactly such a new ligand. The formation of NPs through coordination-driven self-assembly of VI-co-PEGMA and dysprosium ions (Dy3+) was studied as functions of the molecular structure of VI-co-PEGMA, Dy3+ addition rate, mixing rate and duration, and Dy3+/VI-co-PEGMA feed ratio. The as-synthesized NPs exhibit bimodal dysprosium fluorescence and magnetism with a strong dependence on the coordinated Dy3+ contents. Moreover, these NPs may also possess guest loading capacity, as indicated by their amorphous nature and the high coordination number of Dy3+. By coating the NPs with silica shells, the resultant NP@silica materials further develop an anti-disassembling ability in aqueous media, which is absent from most of the NMOPs based on water-soluble ligands. An application demonstration performed using mouse L-929 cells shows that silica-coated NPs can act as bimodal magneto-fluorescent probes for cellular bioimaging. This work thus represents a meaningful attempt for the development of new functional NPs based on the existing NMOP strategy by using an elaborately designed polymer as the organic linker and Dy3+ as the metal center.

Flexible Polydimethylsilane Nanocomposites Enhanced with a Three-Dimensional Graphene/Carbon Nanotube Bicontinuous Framework for High-Performance Electromagnetic Interference Shielding

High-performance electromagnetic interference (EMI)-shielding materials featuring lightweight, flexibility, excellent conductivity, and shielding properties, as well as superior mechanical robustness, are highly required, yet their development still remains a daunting challenge. Here, a flexible and exceptional EMI-shielding polydimethylsilane/reduced graphene oxide/single-wall carbon nanotube (PDMS/rGO/SWCNT) nanocomposite was developed by a facile backfilling approach utilizing a preformed rGO/SWCNT aerogel as the three-dimensional (3D) conducting and reinforcement skeleton. Pristine SWCNTs acting as secondary conductive fillers showed intriguing advantages, whose intrinsically high conductivity could be well preserved in the composites because of no surface acidification treatment. The robust and interconnected 3D network can not only serve as fast channels for electron transport but also effectively transfer external load. Accordingly, a prominent electrical conductivity of 1.2 S cm-1 and an outstanding EMI-shielding effectiveness of around 31 dB over the X-band frequency range were achieved for the resultant composite with an ultralow loading of 0.28 wt %, which is among the best results for currently reported conductive polymer nanocomposites. Moreover, the composite displayed excellent mechanical properties and bending stability; for example, a 233% increment in the compression strength was obtained compared with that of neat PDMS. These observations indicate the unrivalled effectiveness of 3D rGO/SWCNT aerogel as a reinforcement to endow the polymer composites with outstanding conductive and mechanical properties toward high-performance EMI-shielding application.

High-Performance Poly(vinyl alcohol) Nanocomposites Filled with Individual Montmorillonite Nanolayers

ABSTRACT Filling poly(vinyl alcohol) (PVA) with clay, typically montmorillonite (MMT), has been proven to be an attractive option to meet the high-performance requirements of PVA-based materials. In previous reports MMT or organophilic MMT (OMMT) were directly used as fillers. As a result, both exfoliated and intercalated MMT structures coexisted in the resultant nanocomposites. However, there is still a large gap between these nanocomposites and ideally designed ones where individual clay nanolayers (CNLs) are expected to be uniformly dispersed in the PVA. With this in mind, an ameliorative solution casting process is proposed here to prepare PVA nanocomposites. For this purpose the CNLs were prepared ahead of time by exfoliation of MMT in water and then used as fillers. Assessment of the dispersion state of the CNLs in PVA revealed that they (≤5.0 wt%) were randomly and uniformly dispersed (down to the level of individual silicate layers) in and formed strong interfacial interactions with the PVA. This resulted in significantly enhanced physical properties of the resultant nanocomposites relative to neat PVA. In particular, a 104.7% increment in the yield stress was achieved with 5.0 wt% CNLs, much larger than the 15–70% increments of previous PVA nanocomposites using MMT or OMMT as fillers. Additionally, excellent optical clarity of the PVA was obtained for the nanocomposites.