Phase-Equilibria and Nanostructure Formation in Charged Rigid-Rod Polymers and Carbon Nanotubes

An important element in the microstructure of high-performance fibers and film fabricated from rigid polymers is an interconnected network of oriented microfibrils, the lateral size of which is about 10 nm. This study is an attempt to elucidate the mechanism by which the microfibrils are formed, so that larger lateral dimensions can be achieved by suitable processing. Since this morphology emerges in the coagulation stage of the spinning process, we compared the microfibrillar network formed by drastically different coagulation conditions. Ribbons, spun from solution of poly(pphenylene benzobisthiazole) in polyphosphoric acid through a slit die, were coagulated either in the ordinary rapid process with water (timescale of seconds) or in a slow process with phosphoric acid (timescale of hours). The coagulated microfibrillar network was dried with with supercritical CO2 for x-ray scattering measurements, and impregnated with epoxy resin for sectioning and imaging by transmission electron microsocpy. Slow coagulation yields better-aligned microfibrils of enhanced chain packing, but the lateral size of the microfibrils formed in both cases is similar, about 10 nm. Heat-treatment increases the width of water-coagulated microfibrils but not the acid-coagulated ones. The observations do not support spinodal decomposition as the mechanism of microfibril formation during coagulation, as was previously suggested. spc004066-final: Y.Cohen 5 Part II: Evaluation of Single-Walled Carbon Nanotube Dispersion Processes by Amphiphilic Polymers, and Processing by Electrospinning. Introduction Carbon nanotubes were first discovered by Iijima in 1991 revealing a new and unique structure of carbon. They are considered as elongated fullerenes built from a rolled graphene sheet forming a seamless, usually capped tubular structure. Multi-wall carbon nanotubes (MWNT) consist of several concentric layers while single-wall nanotubes (SWNT) consist just one layer forming a very thin nanotube with diameter of 0.8-1.4 nm and length of few microns. This uncommon and extremely high aspect ratio is responsible for the unique behavior, properties and applications of these quasione dimensional molecules. The small dimensions and the lightweight of the carbon nanotubes combined with their exceptional properties opened a large window to a whole range of promising applications and... dreams. Two main challenges are to be overcome in order to realize the huge potential of the carbon nanotubes: first, a large-scale production of high quality nanotubes with controlled properties is required. Secondly, these remarkable properties are attributed to the molecular level. Thus, the separation of bundles into individual nanotubes and their dispersion individually becomes vital for achieving the expected results in many applications. Among the wide range of potential applications, a few are briefly mention herein. Based on their mechanical properties, carbon nanotubes can be used as reinforcing agents of ultra strong and lightweight nano-composite materials and also used to form a light and strong macro cable. Due to their size, strength, flexibility (ability to bend) and high conductivity nanotubes can be used as a nanoprobe in scanning microscope tips, ultrasmall pipettes for injection of molecules to cells and more. Carbon nanotubes are of best candidate to make high yeild field electron emitters due to their nano-size, high conductivity and low threshold electrical field. Field electron emitters are used for technological applications, such as flat panel displays and electron gun for electron microscopy. Recent researches use carbon nanotubes for single electron transistor (SET) which are the future alternative to conventional semiconductor transistors. Also, carbon nanotubes may be used as hydrogen storage for full cells due to their high hydrogen storage capacity. spc004066-final: Y.Cohen 6 Preparation of stable dispersion and separation of the bundles are the crucial first steps required for any application and molecular-scale study of carbon nanotubes. Two main types of dispersions are reported in the literature: chemical and physical dispersions. Chemical reactions of carbon nanotubes involve the attack on the carbon lattice by breaking the c-c double bond to form a covalent bond with the reactant. The oxidation of nanotubes by strong acids is one of the primary chemical reactions. Nitric acid, sulfuric acid or the combination of the two (at ratio of 3:1 H2SO4:HNO3) form acidic functional groups like carboxylic acid, carbonyl and lactone groups onto the surface of the nanotubes, leaving a hole. The acidic groups may be subsequently neutralized by base, introducing charges (like carboxylate groups), which contribute to the dispersion stability of the nanotubes in a polar solvents, especially water and ethanol, by electric repulsion. Derivatization of the acid-treated short SWNT with a long alkyl amine, as was reported by Chen at al., provides the ability to solubilize the nanotubes in organic solvents, such as benzene, toluene and chloroform. Another chemical manipulation is the fluorination of SWNT. In this process F2 gas is reacted at high temperature with SWNT to produce side-wall fluorinated SWNT. Chemical treatment modifies the nanotubes structure and may even cause a fatal destruction of the tubes, especially SWNTs. Monthioux at al. have shown that severe acid treatment in addition to the side-wall attack, causes a complete amorphisation of the graphene tubes. Morever, in contrary to other works, they proved that the chemical treatment does not exfoliate the bundles into the individual tubes. Partial recovering of the damages can be achieved by vacuum annealing at very high temperature (1200C) by regraphitization of the tube shell. However, annealed samples are more difficult to disperse, since during the annealing process larger and more well ordered bundles are formed. Most of the proposed purification processes of raw materials involve an oxidation step in order to remove impurities such as catalyst particles and oxidize amorphous carbon. As the purification process is prolonged the amount of impurities decreases but the cost of damaging the nanotubes increases accordingly. In order to disperse SWNTs the inter-tube van der Waals interaction must be overcome by other alternative physical interactions. This can be achieved by favorable adsorbing of potential candidates onto the surface of the nanotubes. In this way the dispersability is improved without damaging the nanotubes structure and their properties. One of the simplest methods is by sonicating SWNT with an ionic spc004066-final: Y.Cohen 7 surfactant, such as Sodium dodecyl sulfate (SDS) or nonionic one like Triton X at a concentration below the CMC. In these systems the amphiphilic character of the surfactant stabilizes a colloidal suspension in water. However, there is no evidence that this process separates the bundles into individual tubes. In fact, viscosity measurements performed on these systems revealed a low viscosity (as water), which demonstrates the colloidal/aggregated character of these suspensions. Another approach to disperse nanotubes is based on introducing nanotubes with polymers. The similarity of the sizes and chemical structure of caebon nanotubes and certain polymers, the variety of the functional groups of the polymers and the low surface tension of liquid polymers make the polymers very good potential wetting and dispersive agents of SWNT. One of the most investigated system is a conjugated polymer Poly(p-phenylene vinylene) (PPV) and its derived copolymer Poly(mphenylenevinylene-co-2,5 dioctoxy-p-phenylenevinylene (PmPV) solution in toluene. The combination of the nanotubes having unique electronic properties with the conjugated polymer having optical properties opens a wide range of applications in the electro-optical field. The conjugated copolymer tends to coil forming a helical structure. Under this condition the polymer can wet the nanotubes in a close contact, which enables π−π interactions to occur. Curran at al., while working with MWNT, suggested that the polymer wraps itself around nanotubes due to his helicity thus exofliating the bundles. Furthermore, it was claimed that the wrapping of the polymer (studied by transmission electron microscopy-TEM image) occurs in a periodic fashion, which arises from the van der Waals interaction between the phenyl group of the polymer and the hexagonal lattice of the nanotubes. This work has been extended to SWNT showing that the polymer tends to coat the nanotubes while leaving other carbon forms bare. Moreover, it was found that the polymer selectively interacts with nanotubes having specific diameters. Thus, the polymer simultaneously acts as an intercalating agent dispersing the nanotubes, purifying agent providing the separation of the nanotubes from other carboneous impurities, and selective filter of nanotubes size. This concept of using polymer as dispersive agent was recently used by Smalley and coworkers as well. In their work, SWNTs were reacted with PVP (Polyvinyl pyrrolidone) and PSS (Polystyrene sulfonate) to form a stable dispersion by replacing the self-interaction of van der Waals among the nanotubes by a strong spc004066-final: Y.Cohen 8 association between the polymer and individual SWNT. Based on atomic force microscopy (AFM) images it was once again suggested that the linear polymer is uniformly wrapped around the SWNT thus forming a uniform diameter polymerSWNT unit. After drying, these hybrid units are easily redispersed in water by minimal sonication at concentration up to 1.4 g/l. From thermodynamic considerations it is proposed that the entropy loss due to the wrapping of the polymer around the nanotube, compared to the freely coiled conformation, is excessively compensated by the loss of the hydrophobic interaction between the nanotube surface and the surrounding water. Once the solvent is replaced by non-polar ones like THF, the thermodynamic driving force is no longer present. Thus, reversible disassociation between polymer and nanotube takes place. A new, simple one-step process was recently reported using a natural polysaccharide, namely Gum Arabic, to disperse SWNT in aqueous solution. The dispersion was prepared by simple mild sonication, resulting in a stable, visually homogeneous ink-like suspension. After drying, the nanotubes may be redispersed in water at high concentration up to 15% by weight. As was concluded from wide-angle x-ray scattering (WAXS) of the dried dispersion and reinforced by TEM images, the triangular packing of the bundles is destroyed so that individual SWNTs are well separated (the length of the separated SWNT is above 1 μm). Here, in contrast to the “wrapping” model, a more “loose” adsorbance of the polymer on the nanotubes is suggested. In this configuration the polymer chains are adsorbed to the nanotubes without loosing their coiled conformation. The repulsive entropy between the polymer chains, which are immersed in a good solvent having en excluded volume, overcomes the van der Waals attraction between the embedded nanotubes. On the other hand, the exfoliation of the bundles into individual tubes is accompanied by an increase of the overall translational entropy since the number of the separated units is larger relative to the bundled state. Both contributions encourage the separation of the nanotubes and stabilize the dispersion. So far, only few reports have been presented concerning the processing of carbon nanotube dispersions to a final macroscopic product. One of the first attempts was done by Vigolo and co workers. This work introduced a way to assemble SWNTs into a macroscopic long ribbon or fiber. The SWNTs (0.4 wt%) were dispersed by sonication in an aqueous solution of the anionic surfactant SDS (1 wt%). spc004066-final: Y.Cohen 9 At intermediate concentration of SDS, a stable homogeneous dispersion was formed due to electrostatic repulsion between the adsorbed surfactant molecules. The viscosity of this dispersion was “as low as water”. This dispersion was injected through a syringe needle into a rotating solution of polyvinylalcohol (PVA). PVA adsorbed onto the nanotubes due to its amphiphilic character replacing some SDS molecules but in the lack of electrostatic repulsion immediate aggregation took place. Both shearing flow at the tip of the needle due to the relative high viscosity of the PVA solution, and elongation flow achieved by the rotating flow drawing the dispersion filament in a spiral path, induced the alignment of the nanotubes bundles and formation of a ribbon. By pumping the PVA solution from the bottom, a long ribbon without entanglements could be drawn out easily. The Young’s modulus of these fibers after rinsing and drying varied between 9-15 GPa which is much lower then the individual nanotubes but is one order of magnitude greater than the modulus of the “bucky paper”. X-ray diffraction of these fibers has shown that the nanotubes as well as PVA and residual graphite preferentially oriented along the fiber axis. Yet, the distribution of nanotubes orientation is still large as indicated by FWHM (75) of the angular distribution peak. Another method to process nanotubes dispersion was applied by Schreuder-Gibson at al. using the electrospinning technique. In this work carbon naotubes were dispersed in a solution of polyurethane in DMF (dimethylformamide) and polyaniline to produce a spinnable solution (10% polymer, 10% carbon nanotubes and 80% DMF). The solution was electrospun, resulting in a composite nanofiber enclosing oriented clusters of nanotubes along its axis (see section 2.2.5.2). A new work just published demonstrated the formation of a hybrid fiber by dry jet wet spinning technique. In this case a liquid crystalline phase of poly(para phenylene benzobisoxazole) (PBO) was prepared by in situ polymerization of the monomers in the present of SWNTs (~10 wt% of the polymer weight). The resulting hybrid fiber exhibited tensile strength greater by a factor of 50 than the PBO fiber. spc004066-final: Y.Cohen 10