Fabrication of Graphene-Polyimide Nanocomposites with Superior Electrical Conductivity.

We report on the fabrication of a novel class of lightweight materials, polyimide-graphene nanocomposites (0.01-5 vol %), with tunable electrical conductivity. The graphene-polyimide nanocomposites exhibit an ultra-low graphene percolation threshold of 0.03 vol % and maximum dc conductivity of 0.94 S/cm, which we attribute to excellent dispersion, extraordinary electron transport in the well-dispersed graphene, high number density of graphene nanosheets, and the π-π interactions between the aromatic moieties of the polyimide and the carbon rings in graphene. The dc conductivity data are shown to follow the power-law dependence on the graphene volume fraction near the percolation threshold. The ac conductivity of the nanocomposites is accurately represented by the extended pair-approximation model. The exponent s of the approximation is estimated to be 0.45-0.61, indicating anomalous diffusion of charge particles and a fractal structure for the conducting phase, lending support to the percolation model. Low-temperature dc conductivity of the nanocomposites is well-approximated by the thermal fluctuation-induced tunneling. Wide-angle X-ray scattering and transmission electron microscopy were utilized to correlate the morphology with the electrical conductivity. The lack of maxima in X-ray indicates the loss of structural registry and short-range ordering.

Trade-off between the Mechanical Strength and Microwave Electrical Properties of Functionalized and Irradiated Carbon Nanotube Sheets.

Carbon nanotube (CNT) sheets represent a novel implementation of CNTs that enable the tailoring of electrical and mechanical properties for applications in the automotive and aerospace industries. Small molecule functionalization and postprocessing techniques, such as irradiation with high-energy particles, are methods that can enhance the mechanical properties of CNTs. However, the effect that these modifications have on the electrical conduction mechanisms has not been extensively explored. By characterizing the mechanical and electrical properties of multiwalled carbon nanotube (MWCNT) sheets with different functional groups and irradiation doses, we can expand our insights into the extent of the trade-off that exists between mechanical strength and electrical conductivity for commercially available CNT sheets. Such insights allow for the optimization of design pathways for engineering applications that require a balance of material property enhancements.

Toward the Responsible Development and Commercialization of Sensor Nanotechnologies.

Nanotechnology-enabled sensors (or nanosensors) will play an important role in enabling the progression toward ubiquitous information systems as the Internet of Things (IoT) emerges. Nanosensors offer new, miniaturized solutions in physiochemical and biological sensing that enable increased sensitivity, specificity, and multiplexing capability, all with the compelling economic drivers of low cost and high-energy efficiency. In the United States, Federal agencies participating in the National Nanotechnology Initiative (NNI) "Nanotechnology for Sensors and Sensors for Nanotechnology: Improving and Protecting Health, Safety, and the Environment" Nanotechnology Signature Initiative (the Sensors NSI), address both the opportunity of using nanotechnology to advance sensor development and the challenges of developing sensors to keep pace with the increasingly widespread use of engineered nanomaterials. This perspective article will introduce and provide background on the NNI signature initiative on sensors. Recent efforts by the Sensors NSI aimed at promoting the successful development and commercialization of nanosensors will be reviewed and examples of sensor nanotechnologies will be highlighted. Future directions and critical challenges for sensor development will also be discussed.

Self-Healing of Core-Shell Magnetic Polystyrene Nanocomposites.

High heat generation is reported in core-shell magnetic nanoparticle polystyrene (PS) nanocomposites (3.5, 10 wt %) when they are placed in a high-frequency ac magnetic field. These magnetic nanoparticles with cobalt iron oxide core and manganese iron oxide shell were synthesized and characterized by wide-angle X-ray scattering (WAX), thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), and ac field gradient magnetometery. When placed in a high-frequency ac magnetic field, the thermal energy generated in the magnetic polystyrene nanocomposites resulted in a surface temperature increase. The heat generation is attributed to the contribution of Néel relaxation and hysteresis of the core-shell magnetic nanoparticles in the solid state. The maximum surface temperature increased with increasing nanoparticle content and resulted in melting of the magnetic polystyrene nanocomposite.

Carbon nanotube epoxy nanocomposites: the effects of interfacial modifications on the dynamic mechanical properties of the nanocomposites.

Surface functionalization of pretreated carbon nanotubes (CNT) using aromatic, aliphatic, and aliphatic ether diamines was performed. The pretreatment of the CNT consisted of either acid- or photo-oxidation. The acid treated CNT had a higher initial oxygen content compared to the photo-oxidized CNT and this resulted in a higher density of functionalization. X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA) were used to verify the presence of the oxygenated and amine moieties on the CNT surfaces. Epoxy/0.1 wt % CNT nanocomposites were prepared using the functionalized CNT and the bulk properties of the nanocomposites were examined. Macroscale correlations between the interfacial modification and bulk dynamic mechanical and thermal properties were observed. The amine modified epoxy/CNT nanocomposites exhibited up to a 1.9-fold improvement in storage modulus (G') below the glass transition (Tg) and up to an almost 4-fold increase above the Tg. They also exhibited a 3-10 °C increase in the glass transition temperature. The aromatic diamine surface modified epoxy/CNT nanocomposites resulted in the largest increase in shear moduli below and above the Tg and the largest increase in the Tg. Surface examination of the nanocomposites with scanning electron microscopy (SEM) revealed indications of a greater adhesion of the epoxy resin matrix to the CNT, most likely due to the covalent bonding.

Increased tensile strength of carbon nanotube yarns and sheets through chemical modification and electron beam irradiation.

The inherent strength of individual carbon nanotubes (CNTs) offers considerable opportunity for the development of advanced, lightweight composite structures. Recent work in the fabrication and application of CNT forms such as yarns and sheets has addressed early nanocomposite limitations with respect to nanotube dispersion and loading and has pushed the technology toward structural composite applications. However, the high tensile strength of an individual CNT has not directly translated into that of sheets and yarns, where the bulk material strength is limited by intertube electrostatic attractions and slippage. The focus of this work was to assess postprocessing of CNT sheets and yarns to improve the macro-scale strength of these material forms. Both small-molecule functionalization and electron-beam irradiation were evaluated as means to enhance the tensile strength and Young's modulus of the bulk CNT materials. Mechanical testing revealed a 57% increase in tensile strength of CNT sheets upon functionalization compared with unfunctionalized sheets, while an additional 48% increase in tensile strength was observed when functionalized sheets were irradiated. Similarly, small-molecule functionalization increased tensile strength of yarn by up to 25%, whereas irradiation of the functionalized yarns pushed the tensile strength to 88% beyond that of the baseline yarn.

Graphene polyimide nanocomposites; thermal, mechanical, and high-temperature shape memory effects.

Flexible graphene polyimide nanocomposites (0.1-4 wt %) with superior mechanical properties over those of neat polyimide resin have been prepared by solution blending. Imide moieties were grafted to amine-functionalized graphene using a step-by-step condensation and thermal imidization method. The imide-functionalized graphene exhibited excellent compatibility with N-methyl-2-pyrrolidone. The dynamic storage moduli of the graphene polyimide nanocomposites increased linearly with increasing graphene content for both unmodified graphene and imidized graphene. Moduli of the imidized graphene nanocomposites were 25-30% higher than those of unmodified graphene nanocomposites. Both neat polyimide and polyimide nanocomposites exhibited shape memory effects with a triggering temperature of 230 °C. where addition of graphene improved the recovery rate. Addition of graphene improved thermal stability of the polyimide nanocomposites for both graphene and modified graphene.

Transparent large-strain thermoplastic polyurethane magnetoactive nanocomposites.

Organically modified superparamagnetic MnFe(2)O(4)/thermoplastic polyurethane elastomer (TPU) nanocomposites (0.1-8 wt %) were prepared by solvent mixing followed by solution casting. Linear aliphatic alkyl chain modification of spherical MnFe(2)O(4) provided compatibility with the TPU containing a butanediol extended polyester polyol-MDI. All MnFe(2)O(4)/TPU nanocomposite films were superparamagnetic and their saturation magnetization, σ(s), increased with increasing MnFe(2)O(4) content. All nanocomposite films exhibited large deformations (>10 mm) under a magneto-static field. This is the first report of large actuation of magnetic nanoparticle nanocomposites at low-loading levels of 0.1 wt % (0.025 vol %). The maximum actuation deformation of the MnFe(2)O(4)/TPU nanocomposite films increased exponentially with increasing nanoparticle concentration. An empirical correlation between the maximum displacement, saturation magnetization, and magnetic nanoparticle loading is proposed. The cyclic deformation actuation of a 6 wt % surface modified MnFe(2)O(4)/TPU, in a low magnetic field 151 < B(y) < 303 Oe, exhibited excellent reproducibility and controllability. MnFe(2)O(4)/TPU nanocomposite films (0.1-2 wt %) were transparent and semitransparent over the wavelengths from 350 to 700 nm.

Synthesis, optical characterization, and electrochemical properties of isomeric tetraphenylbenzodifurans containing electron acceptor groups.

Isomeric tetraphenylbenzodifuran systems, benzo[1,2-b:5,4]difuran and benzo[1,2-b:4,5]difuran, containing electron acceptor groups (CF(3), CN, and NO(2)) have been synthesized and studied. Their electronic absorption, fluorescence, two-photon absorption cross sections, and electrochemical properties were investigated. The absorption and emission maxima are red-shifted for the linear-conjugated systems in comparison with the corresponding isomer. Dual fluorescence was observed and the existence of a twisted intramolecular charge transfer state was confirmed by low-temperature emission experiments. Wide HOMO-LUMO energy gaps were obtained ranging from 2.53 to 3.28 eV. HOMO levels were found in the energy range of -6.03 to -6.63 eV while LUMO are within -2.55 to -3.52 eV.

Using the dynamic bond to access macroscopically responsive structurally dynamic polymers.

New materials that have the ability to reversibly adapt to their environment and possess a wide range of responses ranging from self-healing to mechanical work are continually emerging. These adaptive systems have the potential to revolutionize technologies such as sensors and actuators, as well as numerous biomedical applications. We will describe the emergence of a new trend in the design of adaptive materials that involves the use of reversible chemistry (both non-covalent and covalent) to programme a response that originates at the most fundamental (molecular) level. Materials that make use of this approach - structurally dynamic polymers - produce macroscopic responses from a change in the material's molecular architecture (that is, the rearrangement or reorganization of the polymer components, or polymeric aggregates). This design approach requires careful selection of the reversible/dynamic bond used in the construction of the material to control its environmental responsiveness.

Reinforced thermoplastic polyimide with dispersed functionalized single wall carbon nanotubes.

Molecular pi-complexes were formed from pristine HiPCO single- wall carbon nanotubes (SWCNTs) and 1-pyrene- N-(4-N'-(5-norbornene-2,3-dicarboxyimido)phenyl butanamide, 1. Polyimide films were prepared with these complexes as well as uncomplexed SWCNTs and the effects of nanoadditive addition on mechanical, thermal, and electrical properties of these films were evaluated. Although these properties were enhanced by both nanoadditives, larger increases in tensile strength and thermal and electrical conductivities were obtained when the SWCNT/1 complexes were used. At a loading level of 5.5 wt %, the T(g) of the polyimide increased from 169 to 197 degrees C and the storage modulus increased 20-fold (from 142 to 3045 MPa). The addition of 3.5 wt % SWCNT/1 complexes increased the tensile strength of the polyimide from 61.4 to 129 MPa; higher loading levels led to embrittlement and lower tensile strengths. The electrical conductivities (DC surface) of the polyimides increased to 1 x 10(-4) Scm(-1) (SWCNT/1 complexes loading level of 9 wt %). Details of the preparation of these complexes and their effects on polyimide film properties are discussed.

Phenacenes from Diels-Alder trapping of photogenerated o-xylylenols: phenanthrenes and benzo[e]pyrene bisimide.

[structure: see text] The synthesis of phenanthrene and benzo[e]pyrene bisimides, 1 and 2, was accomplished via the Diels-Alder trapping of sterically congested o-xylylenols photochemically generated from 3,6-dibenzoyl-o-xylene and 1,4-dibenzoyl-9,10-dihydroanthracene, respectively. Absorption and emission from 2 are red-shifted from 1 and unsubstituted benzo[e]pyrene. The fluorescence quantum yield for 2 is an order of magnitude lower than that of 1 and comparable to that of the parent benzo[e]pyrene.

Twisted, Z-shaped perylene bisimide.

The first derivative of a new class of perylene bisimide chromophores, N,N'-bis(octyl)-3,9-bis(phenyl)perylene-1,2,7,8-tetracarboxyl bisimide, 1, has been synthesized and its fundamental photophysical and electrochemical properties assessed. The extended, Z-shaped structure was achieved by use of the classic photoenolization of an o-methylbenzophenone analogue, 1,5-dibenzoyl-9,10-dihydroanthracene, and in situ Diels-Alder trapping of the resulting o-xylylenol intermediates with N-octylmaleimide. Subsequent dehydration and aromatization of the resulting bisadduct afforded 1. In dichloromethane, bisimide 1 has an absorption lambdamax at 491 nm (epsilon = 29,600 M-1 cm-1), a fluorescence lambdamax at 517 nm with a high quantum yield (Phi = 0.70), and a single-exponential fluorescence decay (tau = 5.01 ns). Pure crystals of 1 have red emission, suggesting exciplex formation in the solid state. X-ray crystallographic analysis of 1 revealed significant twisting of its perylene core.