Manohar Group |
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Sanjeev K. ManoharTeachingTeaching PhilosophyPatentsServiceDaily Journal ClubGroup NewsScience & Democracy
UNIVERSITY OF MASSACHUSETTS LOWELL Email: sanjeev_manohar@uml.edu
Single-Walled Carbon Nanotube Image of Gandhi Plastic on Printed Using a Commercial Ink-Jet Printer
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PROJECTS The group’s research is organized along four lines of enquiry: 1. Polymer nanofibers 1. Polymer nanofibers We employed a new synthetic strategy to control the bulk morphology of conducting and conventional polymers synthesized chemically by precipitation polymerization. Called ‘nanofiber seeding’, this method allows one to synthesize bulk quantities of nanofibers of electronic polymers such as polyaniline, polypyrrole, polythiophene and PEDOT, rapidly, and in one step, by carrying out the synthesis in the presence of very small quantities (seed quantities) of nanofibers of known composition and structure. The method can also be extended to conventional, non conducting polymers, such as, poly(butylcyanoacrylate). This general method to polymer nanofibers is an improvement over existing synthetic approaches, such as, interfacial polymerization, using soft or hard templates, etc., that are system-specific.
Citations as of: 07-03-2008 A soft-template approach was also used using non-ionic surfactants as structure-directing agents to synthesize bulk quantities of nanofibers of polyaniline and polypyrrole as powders or substrate-supported films. For example, 40-60 nm diameter polyaniline nanofibers were synthesized using Triton-X100. Upon sonication, these nanofibers yielded what we believe is a single molecule nanofiber of a doped conducting polymer (~1 nm diameter polyaniline fiber). On the other hand, polypyrrole nanofibers were synthesized using cationic surfactants and it was possible to isolate highly conducting, free-standing films of nanofibrillar polypyrrole directly from the reaction mixture. Aqueous mixtures of cationic surfactant and pyrrole monomer yield an unusual solution microstructure, which we believe is responsible for the dramatic change in bulk polymer morphology from granules to fibers.
2. Polymer nanotubes When stoichiometric amounts of nanofibers are added in the above systems, polymer nanotubes are obtained in near-quantitative yield. The added nanofibers form the pores that then sheathed with polypyrrole during the polymerization. The pore can be leached out selectively, yielding highly conducting, hollow polypyrrole nanotubes having pore diameter in the 4-8 nm range. These tubes spontaneously reduce noble metal ions to the corresponding metal nanoparticles, at room temperature, without any capping or dispersing agents. These polymer/metal nanocomposites have a wide range of technological applications in fuel cells, hydrogen storage, supercapacitors, etc.
We have also used surfactants to synthe size conducting polymer nanotubes that are not accessible by existing synthetic routes. For example, a reverse emulsion polymerization method was used to chemically synthesize bulk quantities of microns long, tubes of electrically conducting PEDOT having tube diameter in the range 50-100 nm. Composites of PEDOT nanotubes with noble metals, metal oxides, etc., can also be readily synthesized using post-synthesis, and in situ polymerization methods.
3. Fundamental polymer science: The absolute molecular weights of parent polyaniline bases in the pernigraniline, emeraldine, and leucoemeraldine oxidation states have been measured by light scattering and the exact number of aniline repeat units determined. A 3-angle LS instrument equipped with a 785 nm laser has been used to measure the absolute molecular weight, a wavelength at which there is no absorbance by parent polyaniline bases. The molecular weight of the pernigraniline intermediate formed during the chemical oxidative polymerization of aniline increases by 17-20% when it is converted to emeraldine which is consistent with a two-step polymerization mechanism. These findings establish a solid experimental framework to chemically synthesize block co-polymers of polyaniline by using different monomers to intercept the reaction at the pernigraniline oxidation state.
4. Carbon nanotubes We have described an extremely simple method to obtain thin, optically transparent, strongly adherent films of single-walled carbon nanotube (SWNT) bundles on flexible plastic substrates such as poly(ethylene terephthalate) (PET: ‘overhead transparency’). These SWNT/PET films display a sheet resistance of 80 Ω/sq., >80% optical transparency, and robust flexibility, e.g., they can be bent and folded to a crease while retaining full electrical connection across the crease. Any desired circuit pattern can be obtained on paper, cloth and plastic substrates using our proprietary ink-jet printing method. These SWNT/PET films can also be used for organic vapor sensing (hexane, toluene, acetone, chloroform, acetonitrile, methanol, etc.). In addition, large, reproducible resistance changes are observed when SWNT/PET sensors are exposed to chemical warfare agent simulants (CWAs), such as, diisopropylmethylphosphonate (DIMP), a simulant for the nerve agent Sarin, or dimethylmethylphosphonate (DMMP), a simulant for the nerve agent Soman.
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