Liquid Crystal-Mediated 3D Printing Process to Fabricate Nano-Ordered Layered Structures.

The emergence of three-dimensional (3D) printing promises a disruption in the design and on-demand fabrication of smart structures in applications ranging from functional devices to human organs. However, the scale at which 3D printing excels is within macro- and microlevels and principally lacks the spatial ordering of building blocks at nanolevels, which is vital for most multifunctional devices. Herein, we employ liquid crystal (LC) inks to bridge the gap between the nano- and microscales in a single-step 3D printing. The LC ink is prepared from mixtures of LCs of nanocellulose whiskers and large sheets of graphene oxide, which offers a highly ordered laminar organization not inherently present in the source materials. LC-mediated 3D printing imparts the fine-tuning required for the design freedom of architecturally layered systems at the nanoscale with intricate patterns within the 3D-printed constructs. This approach empowered the development of a high-performance humidity sensor composed of self-assembled lamellar organization of NC whiskers. We observed that the NC whiskers that are flat and parallel to each other in the laminar organization allow facile mass transport through the structure, demonstrating a significant improvement in the sensor performance. This work exemplifies how LC ink, implemented in a 3D printing process, can unlock the potential of individual constituents to allow macroscopic printing architectures with nanoscopic arrangements.

Shape memory nanocomposite fibers for untethered high-energy microengines.

Classic rotating engines are powerful and broadly used but are of complex design and difficult to miniaturize. It has long remained challenging to make large-stroke, high-speed, high-energy microengines that are simple and robust. We show that torsionally stiffened shape memory nanocomposite fibers can be transformed upon insertion of twist to store and provide fast and high-energy rotations. The twisted shape memory nanocomposite fibers combine high torque with large angles of rotation, delivering a gravimetric work capacity that is 60 times higher than that of natural skeletal muscles. The temperature that triggers fiber rotation can be tuned. This temperature memory effect provides an additional advantage over conventional engines by allowing for the tunability of the operation temperature and a stepwise release of stored energy.

Shear Rheology Control of Wrinkles and Patterns in Graphene Oxide Films.

Drying graphene oxide (GO) films are subject to extensive wrinkling, which largely affects their final properties. Wrinkles were shown to be suitable in biotechnological applications; however, they negatively affect the electronic properties of the films. Here, we report on wrinkle tuning and patterning of GO films under stress-controlled conditions during drying. GO flakes assemble at an air-solvent interface; the assembly forms a skin at the surface and may bend due to volume shrinkage while drying. We applied a modification of evaporative lithography to spatially define the evaporative stress field. Wrinkle alignment is achieved over cm2 areas. The wavelength (i.e., wrinkle spacing) is controlled in the µm range by the film thickness and GO concentration. Furthermore, we propose the use of nanoparticles to control capillary forces to suppress wrinkling. An example of a controlled pattern is given to elucidate the potential of the technique. The results are discussed in terms of classical elasticity theory. Wrinkling is the result of bending of the wet solid skin layer assembled on a highly elastic GO dispersion. Wavelength selection is the result of energy minimization between the bending of the skin and the elastic deformation of the GO supporting dispersion. The results strongly suggest the possibility to tune wrinkles and patterns by simple physicochemical routes.

Giant Electrostriction of Soft Nanocomposites Based on Liquid Crystalline Graphene.

High electromechanical coupling is critical to perform effective conversion between mechanical and electrical energy for various applications of electrostrictive polymers. Herein, a giant electrostriction effect is reported in liquid crystalline graphene-doped dielectric elastomers. The materials are formulated by a phase-transfer method which allows the solubilization of graphenic monolayers in nonpolar solvents. Dielectric spectroscopy is combined with tensile test devices to measure the true electrostriction coefficients with differentiating the Maxwell stress effect. Because of their liquid crystal structure, the resultant composites show an ultralarge electrostriction coefficient (∼10-14 m2/V2 at 0.1 Hz) coupled with good reproducibility during cycles at high deformation rates. This work offers a promising pathway to design high-performance electrostrictive polymer composites as well as to provide insights into mechanisms of true electrostriction in electrically heterogeneous systems. The use of obtained materials as a supersensitive capacitive sensor is demonstrated.

Giant Electrostrictive Response and Piezoresistivity of Emulsion Templated Nanocomposites.

Using an emulsion road and optimizing the dispersion process, we prepare polymer carbone nanotubes (CNT) and polymer reduced graphene oxide (rGO) composites. The introduction of conductive nanoparticles into polymer matrices modifies the electronic properties of the material. We show that these materials exhibit giant electrostriction coefficients in the intermediate filler concentration (below 1 wt %). This makes them very promising for applications such as capacitive sensors and actuators. In addition, the values of the piezoresistivity measured in the high filler concentration situation are at least an order of magnitude greater than the one reported in the literature. This opens the way to use these materials for stress or strain sensor applications considering their giant responses to mechanical deformations.

Superflexibility of graphene oxide.

Graphene oxide (GO), the main precursor of graphene-based materials made by solution processing, is known to be very stiff. Indeed, it has a Young's modulus comparable to steel, on the order of 300 GPa. Despite its very high stiffness, we show here that GO is superflexible. We quantitatively measure the GO bending rigidity by characterizing the flattening of thermal undulations in response to shear forces in solution. Characterizations are performed by the combination of synchrotron X-ray diffraction at small angles and in situ rheology (rheo-SAXS) experiments using the high X-ray flux of a synchrotron source. The bending modulus is found to be 1 kT, which is about two orders of magnitude lower than the bending rigidity of neat graphene. This superflexibility compares with the fluidity of self-assembled liquid bilayers. This behavior is discussed by considering the mechanisms at play in bending and stretching deformations of atomic monolayers. The superflexibility of GO is a unique feature to develop bendable electronics after reduction, films, coatings, and fibers. This unique combination of properties of GO allows for flexibility in processing and fabrication coupled with a robustness in the fabricated structure.

Graphene liquid crystal retarded percolation for new high-k materials.

Graphene flakes with giant shape anisotropy are extensively used to establish connectedness electrical percolation in various heterogeneous systems. However, the percolation behaviour of graphene flakes has been recently predicted to be far more complicated than generally anticipated on the basis of excluded volume arguments. Here we confirm experimentally that graphene flakes self-assemble into nematic liquid crystals below the onset of percolation. The competition of percolation and liquid crystal transition provides a new route towards high-k materials. Indeed, near-percolated liquid-crystalline graphene-based composites display unprecedented dielectric properties with a dielectric constant improved by 260-fold increase as compared with the polymer matrix, while maintaining the loss tangent as low as 0.4. This performance is shown to depend on the structure of monodomains of graphene liquid-crystalline phases. Insights into how the liquid crystal phase transition interferes with percolation transition and thus alters the dielectric constant are discussed.

Giant Permittivity Polymer Nanocomposites Obtained by Curing a Direct Emulsion.

Near-percolated CNT-polymer composites are promising high-permittivity materials. The main challenge in the field consists of finding compromises that allow high permittivity and low losses in frequency ranges of interest. Using an emulsion approach and optimizing the size of the droplets and the curing procedure, we obtain unprecedented performances and measure giant permittivity larger than 20,000 at 100 Hz along with a conductivity below 10(-4) S/m.

Effect of the rheological properties of carbon nanotube dispersions on the processing and properties of transparent conductive electrodes.

Transparent conductive films are made from aqueous surfactant stabilized dispersions of carbon nanotubes using an up-scalable rod coating method. The processability of the films is governed by the amount of surfactant which is shown to alter strongly the wetting and viscosity of the ink. The increase of viscosity results from surfactant mediated attractive interactions between the carbon nanotubes. Links between the formulation, ink rheological properties, and electro-optical properties of the films are determined. The provided guidelines are generalized and used to fabricate optimized electrodes using conductive polymers and carbon nanotubes. In these electrodes, the carbon nanotubes act as highly efficient viscosifiers that allow the optimized ink to be homogeneously spread using the rod coating method. From a general point of view and in contrast to previous studies, the CNTs are optimally used in the present approach as conductive additives for viscosity enhancements of electronic inks.

How polymers lose memory with age.

Uniquely in the world of materials, polymers deformed at high temperature and subsequently quenched at low temperature, memorize the temperature at which they have been processed. Polymers can even memorize multiple temperatures. This temperature memory is reflected by a maximum of residual stress restored at the temperature of initial processing. It has been speculated that this capability could arise from the presence of dynamical heterogeneities in glassy domains of polymers. Processing the material at a given temperature would result in the selection of certain heterogeneities that participate in the storage of mechanical stress. Because dynamical heterogeneities are associated with particular relaxation times, the temperature memory of polymers should depend on the time, for example, the glass transition temperature depends on the frequency. The first experimental study of temporal effects on the temperature memory of polymers is presently reported. It is found that aging at high temperature shifts the maximum of residual stress towards greater temperatures. The corresponding loss of memory is explained by the relaxation of dynamical heterogeneities with short characteristic times. The present results clarify the origin of the temperature memory and provide insights into their efficient exploitation in applications.

Thermoelectrical Memory of Polymer Nanocomposites.

The inclusion of nanoparticles improves the behavior of shape-memory polymers and allows new functionalities. It is shown in the present work that polyamide fibers loaded with carbon nanotubes (CNTs) exhibit novel memory functions associated to their electrical conductivity. Similar to classical shape memory polymers, the materials are predeformed at high temperature and then quenched down to room temperature and subsequently reheated. Their resistivity is recorded during the process and is found to decrease with temperature during the last heating stage. The rate of resistivity decrease exhibits a well-defined maximum at the temperature of predeformation. This unique response clearly shows an accurate thermoelectrical material memory. Temperature memory extended to electrical properties could serve for future sensing applications coupled to shape changes.

Carbon nanotubes induced gelation of unmodified hyaluronic acid.

This work reports an experimental study of the kinetics and mechanisms of gelation of carbon nanotubes (CNTs)-hyaluronic acid (HA) mixtures. These materials are of great interest as functional biogels for future medical applications and tissue engineering. We show that CNTs can induce the gelation of noncovalently modified HA in water. This gelation is associated with a dynamical arrest of a liquid crystal phase separation, as shown by small-angle light scattering and polarized optical microscopy. This phenomenon is reminiscent of arrested phase separations in other colloidal systems in the presence of attractive interactions. The gelation time is found to strongly vary with the concentrations of both HA and CNTs. Near-infrared photoluminescence reveals that the CNTs remain individualized both in fluid and in gel states. It is concluded that the attractive forces interplay are likely weak depletion interactions and not strong van der Waals interactions which could promote CNT rebundling, as observed in other biopolymer-CNT mixtures. The present results clarify the remarkable efficiency of CNT at inducing the gelation of HA, by considering that CNTs easily phase separate as liquid crystals because of their giant aspect ratio.

Liquid crystals of carbon nanotubes and graphene.

Liquid crystal ordering is an opportunity to develop novel materials and applications with spontaneously aligned nanotubes or graphene particles. Nevertheless, achieving high orientational order parameter and large monodomains remains a challenge. In addition, our restricted knowledge of the structure of the currently available materials is a limitation for fundamental studies and future applications. This paper presents recent methodologies that have been developed to achieve large monodomains of nematic liquid crystals. These allow quantification and increase of their order parameters. Nematic ordering provides an efficient way to prepare conductive films that exhibit anisotropic properties. In particular, it is shown how the electrical conductivity anisotropy increases with the order parameter of the nematic liquid crystal. The order parameter can be tuned by controlling the length and entanglement of the nanotubes. In the second part of the paper, recent results on graphene liquid crystals are reported. The possibility to obtain water-based liquid crystals stabilized by surfactant molecules is demonstrated. Structural and thermodynamic characterizations provide indirect but statistical information on the dimensions of the graphene flakes. From a general point of view, this work presents experimental approaches to optimize the use of nanocarbons as liquid crystals and provides new methodologies for the still challenging characterization of such materials.

Liquid Crystallinity and Dimensions of Surfactant-Stabilized Sheets of Reduced Graphene Oxide.

Graphene oxide (GO) flakes dissolved in water can spontaneously form liquid crystals. Liquid crystallinity presents an opportunity to process graphene materials into macroscopic assemblies with long-range ordering, but most graphene electronic functionalities are lost in oxidation treatments. Reduction of GO allows recovering functionalities and makes reduced graphene oxide (RGO) of greater interest. Unfortunately, chemical reduction of GO generally results in the aggregation of the flakes, with no liquid crystallinity observed. We report in the present work liquid crystals made of RGO. The addition of surfactants in appropriate conditions is used to stabilize the RGO flakes against aggregation maintaining their ability to form water-based liquid crystals. Structural and thermodynamical studies allow the dimensions of the flakes to be deduced. It is found that the thickness and diameter of RGO flakes are close to that of neat GO flakes.

Sensitivity of carbon nanotubes to the storage of stress in polymers.

Residual stress in polymers arises from the freezing of unstable molecular conformations. Residual stress is critical because its relaxation can cause shrinkage, defects, and fractures of polymer materials. The storage of stress is purposely enhanced to develop shape memory materials. Unfortunately, the storage of mechanical stress is still poorly controlled and understood. An approach to sense the storage of stress based on the spectroscopic response of carbon nanotubes is explored. The Raman response of nanotubes exhibits a variable sensitivity to strain when embedded in polymers that have experienced different thermal and mechanical treatments. This unique feature opens up new possibilities for the use of carbon nanotubes as mechanical nanosensors.

Self-assembled titanium-based hybrids with cyclopentadienyl-titanium network bonding.

Unprecedented stable hybrid materials with cyclopentadienyl-titanium bonds have been obtained from the hydrolysis of suitable precursors. Their inorganic network is not fully condensed and they show variable short-range self-organizations, the type of which depends on the shape of the ligands.

Conductivity anisotropy of assembled and oriented carbon nanotubes.

An assembly of packed and oriented rodlike particles exhibit anisotropic physical properties. We investigate in the present work the anisotropic conductivity of films made of intrinsically conducting rods. These films are obtained from more or less ordered carbon nanotube liquid crystals. Their orientational order parameter is measured by polarized Raman spectroscopy. A relationship between the anisotropy of surface conductivity and orientational order parameter is determined. The experimental results are accounted for by a model that takes into account the number of intertube contacts and density of conductive pathways in different directions, as introduced by J. Fischer et al. for magnetically aligned nanotubes. We find that a good agreement, without any fitting parameter, of the proposed model and experiments is obtained when we consider a two-dimensional (2D) Gaussian distribution of the nanotube orientation. The conductivities parallel and perpendicular to the nematic director differ by almost an order of magnitude. This anisotropy is much greater than that of conventional dielectric liquid crystals, where the behavior is governed by the mobility anisotropy of ionic current carriers. The present results do not depend on the intrinsic properties of the nanotubes and are expected to be relevant for other assemblies of conducting rodlike particles, such as metallic or semi-conducting nanowires and ribbons.

Dispersion and film-forming properties of poly(acrylic acid)-stabilized carbon nanotubes.

We present a detailed study of the influence of pH on the dispersion and film-forming properties of poly(acrylic acid)-stabilized carbon nanotubes. Poly(acrylic acid) (PAA) is a weak polyelectrolyte, with a pH-responsive behavior in aqueous solution. We obtain quantitative UV-visible measurements to show that the amount of polyelectrolyte in optimal pH conditions is weak, showing a good efficiency of the polymer as a carbon nanotube dispersing agent. The best dispersion conditions are achieved at pH 5, a value close to the pK(a) of PAA. Apart from this tenuous pH value, the PAA is not efficient at stabilizing nanotubes and atomic force microscopy allows us to explain the delicate balance between the PAA adsorption and the suspension stability. This study finally permits optimal conditions for making homogeneous and conductive composite films to be determined.

A new spacer-induced organization in highly ordered tin-based hybrid materials.

The hydrolysis of a cross-shaped bridged alpha,varpi-bis(trialkynylstannylated) compound with long alkyl side chains leads to a highly ordered hybrid material with a new type of organization consisting of striped organic-inorganic layers alternating with alkyl chain layers.

Preparation of 2-quinolones by sequential Heck reduction-cyclization (HRC) reactions by using a multitask palladium catalyst.

One-pot sequential Heck reduction-cyclization (HRC) reactions leading to the synthesis of substituted 2-quinolones have been developed by using a heterogeneous or mixed homogeneous/heterogeneous multitask palladium catalyst with charcoal as a support. The whole sequence occurs under very mild conditions without the need for additives (ligand or base) by taking advantage of the high reactivity of aryldiazonium salts as "super electrophiles". Recycling experiments showed that the reused heterogeneous Pd(0)/C catalyst was not able to promote another HRC sequence but was, however, still highly active for hydrogenation, hydrodehalogenation, as well as hydrogenolysis reactions.