Liquid crystalline behavior of graphene oxide in the formation and deformation of tough nanocomposite hydrogels.

In this paper, we report the formation and transformation of graphene oxide (GO) liquid crystalline (LC) structures in the synthesis and deformation of tough GO nanocomposite hydrogels. GO aqueous dispersions form a nematic LC phase, while the addition of poly(N-vinylpyrrolidone) (PVP) and acrylamide (AAm), which are capable of forming hydrogen bonding with GO nanosheets, shifts the isotropic/nematic transition to a lower volume fraction of GO and enhances the formation of nematic droplets. During the gelation process, a phase separation of the polymers and GO nanosheets is accompanied by the directional assembly of GO nanosheets, forming large LC tactoids with a radial GO configuration. The shape of the large tactoids evolves from a sphere to a toroid as the tactoids increase in size. Interestingly, during cyclic uniaxial tensile deformation a reversible LC transition is observed in the very tough hydrogels. The isolated birefringent domains and the LC domains in the tactoids in the gels are highly oriented under a high tensile strain.

The role of unbound oligomers in the nucleation and growth of electrodeposited polypyrrole and method for preparing high strength, high conductivity films.

Polypyrrole is a material with immensely useful properties suitable for a wide range of electrochemical applications, but its development has been hindered by cumbersome manufacturing processes. Here we show that a simple modification to the standard electrochemical polymerization method produces polypyrrole films of equivalently high conductivity and superior mechanical properties in one-tenth of the polymerization time. Preparing the film as a series of electrodeposited layers with thorough solvent washing between layering was found to produce excellent quality films even when layer deposition was accelerated by high current. The washing step between the sequentially polymerized layers altered the deposition mechanism, eliminating the typical dendritic growth and generating nonporous deposits. Solvent washing was shown to reduce the concentration of oligomeric species in the near-electrode region and hinder the three-dimensional growth mechanism that occurs by deposition of secondary particles from solution. As artificial muscles, the high density sequentially polymerized films produced the highest mechanical work output yet reported for polypyrrole actuators.

Direct sub-micrometer patterning of nanostructured conducting polymer films via a low-energy infrared laser.

Despite the many attractive properties of conjugated polymers, their practical applications are often limited by the lack of a simple, scalable, and nondisruptive patterning method. Here, a direct, scalable, high-resolution patterning technique for conducting polymers is demonstrated that does not involve photoresists, masks, or postprocessing treatment. Complex, well-defined patterns down to sub-micrometer scales can be created from nanofibrous films of a wide variety of conducting polymers by photothermally welding the nanofibers using a low-energy infrared laser. The welding depth, structural robustness, and optical properties of the films are readily controlled. In addition, the electrical properties such as conductivity can be precisely tuned over a 7-order of magnitude range, while maintaining the characteristic tunable electronic properties in the nonwelded polyaniline regions.

Electroactivity and biocompatibility of polypyrrole-hyaluronic acid multi-walled carbon nanotube composite.

Electroactivity of polypyrrole hyaluronic acid, electropolymerized in the presence of oxidized carbon nanotubes (PPyHA-CNT) was studied in situ by electrochemical atomic force microscopy (EC-AFM) in physiological electrolyte solution. In situ Raman spectroscopic and quartz crystal microbalance (QCM) studies were conducted on layers of the polymer grown on AT-cut 5 MHz quartz crystals. Human adipose stem cell (ASC) attachment and viability were studied by Live/Dead staining, and the proliferation was evaluated by WST-1 Cell proliferation assay for polypyrrole samples electropolymerized on titanium. According to cyclic voltammetry, the measured specific capacitance of the material on gold is roughly 20% of the reference polypyrrole dodecylbenzene sulfonate (PPyDBS). Electrochemical-QCM (EC-QCM) analysis of a 210-nm thick film reveals that the material is very soft G' approximately 100 kPa and swells upon reduction. EC-AFM of samples polymerized on microelectrodes show that there are areas of varying electroactivity, especially for samples without a hydrophopic backing PPyDBS layer. AFM line scans show typically 20-25% thickness change during electrochemical reduction. Raman spectroscopic analysis suggests that the material supports noticeable polaron conduction. Biocompatibility study of the PPyHA-CNT on titanium with adipose stem cells showed equal or better cell attachment, viability, and proliferation compared with the reference polylactide.

Tough supersoft sponge fibers with tunable stiffness from a DNA self-assembly technique.

Tough and soft: Highly porous, spongelike materials self-assemble by calcium ion condensation of DNA-wrapped carbon nanotubes (SWNTs-DNA; see picture, IL = ionic liquid). The toughness, modulus, and swellability of the electrically conductive sponges can be tuned by controlling the density and strength of interfiber junctions. The sponges have compliances similar to the softest natural tissue, while robust interfiber junctions give high toughness.

Polymer entanglement density and its influence on interfacial friction.

The entanglement density of amorphous glassy polymers is well known to control their fracture mechanisms under tensile loading. There have been some reports indicating substantial deformation of a glassy polymer's surface region when exposed to interfacial friction. It is shown conclusively here that there is a direct correlation between the entanglement density of a glassy polymer and the deformation mechanisms that facilitate sliding friction. This correlation was shown experimentally by studying the topography of polymer surfaces following a single sliding pass by an inorganic glass sphere. Four different polymers were studied, including polystyrene cross linked to different degrees. It is also shown that permanent plastic deformation accompanies interfacial friction, and, furthermore, that the type of deformation is a direct function of the respective polymer's entanglement density. In contrast, no difference in the observed friction force could be attributed to the entanglement density of the respective polymers. The findings can be explained by the state- and rate-dependent friction model.

Free standing carbon nanotube composite bio-electrodes.

Carbon nanotubes present a new material for the construction of electrodes for electrochemical devices such as batteries, capacitors, and actuators. Such electrodes require high conductivity, strength, and surface area. The latter two requirements are often incompatible. Electrodes composed entirely of carbon nanotubes (bucky paper) have high surface areas but are typically weak, and have insufficient conductivity for practical macroscopic applications. Here we report a technique that uses naturally occurring biopolymers to produce electrodes (free standing films) that exhibit conductivities of 300 S/cm. These composites also have considerable mechanical strength (up to 145 MPa) and sufficient specific capacitance of 19-27 F/g to enable them to be used as freestanding electrodes. One potential application that deserves special attention is that of biocompatible electrodes, where the binder is a biopolymer already used in a range of implants. Preliminary studies reported here show that the new carbon nanotube biopolymer electrodes can foster prolific L929 cell growth.

Surprising shrinkage of expanding gels under an external load.

Hydrogels are fascinating and useful in that they can show large volume changes in response to various stimuli, such as temperature or chemical environment. Here we report the peculiar observation that chemically crosslinked hydrogels that normally expand owing to a change in electrolyte pH can be made to shrink in certain circumstances. Specifically, these hydrogels contract when tested at a constant compressive force and subjected to a pH change that causes expansion in the absence of the applied load. When tested under tension, the gels always expand. Although the effects of external stress on the swelling of gels is known, the concomitant change in gel mechanical properties during pH switching was found to be a more dominant effect in our studies. However, existing mechanical models used to predict dimensional changes in actuator materials could not explain both the tensile and compression results. In addition, we show that the friction between metal plates of the apparatus and the gel is a key factor in explaining the contractile actuation under compressive loads. The observations reported in this paper are important for the successful design and use of hydrogel actuators in devices such as valves for microfluidics.