Synthesis, structure and phase behavior of liquid crystalline polyurethanes/

This dissertation describes the synthesis, structure and phase behavior of polyurethanes based on the mesogenic biphenol 4,4\sp\prime-bis(6-hydroxyhexoxy)biphenyl (BHHBP) and meta substituted tolylene/phenylene diisocyanates. The structure-property relationships were determined as a function of hydrogen-bonding, the position of the methyl group in the tolylene diisocyanate moiety (TDI) and the biphenol moiety. The liquid crystalline phase (mesophase) and crystalline phase were investigated primarily with differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS) and infrared spectroscopy. From this combination of characterization techniques, a more detailed description emerges about the thermodynamic stability and kinetic accessibility of each phase. Previous investigation of the (2,4-TDI/BHHBP) mesogenic polyurethane, 2,4-LCPU-6, has shown that this polymer is a monotropic liquid crystal. The influence of H-bonding on the structure and phase behavior of 2,4-LCPU-6 was determined by the synthesis of high molecular weight N-Methyl 2,4-LCPU-6, using a novel high temperature polymerization of a biscarbamoyl chloride with the BHHBP mesogenic diol. The comparison of the structure and physical properties of these two polymers revealed that H-bonding does not affect the mesophase morphology although its absence disrupts crystallinity and results in an enantiotropic liquid crystal. In addition, it was found that the effect of H-bonding on the mesophase-isotropic transition is enthalpic in nature. In contrast to the regular ($\alpha,\omega$-hexane diol) based polyurethanes (PUs), BHHBP derived polyurethanes (LCPUs) crystallize rapidly from their melts. This is due to the strong nucleating power of their thermodynamically unstable mesophases (monotropic L.C.) Hexafluoroisopropanol fast solvent-evaporation casting or rapid cooling from the melt resulted in thin films or bulk samples with a glassy mesophase morphology. During the subsequent heating scan, the mesophase to crystal transition takes place. Considerable amount of effort was expended to understand the nature of this transition. With the combination of vibrational spectroscopy which provides a measure of the localized structure, along with DSC and WAXS (which examine the long range order) we established the microstructural changes occurring in the different phases. Applying the results of previously mentioned analysis (kinetic control and phase perfection), highly oriented fibers were obtained for the mesogenic polyurethanes. Atomistic molecular simulations coupled with X-ray intensity refinement allowed us to determine the crystalline chain conformation and packing characteristics for the 2,6-LCPU-6 and 1,3-LCPU-6 (2,6-TDI and 1,3-Phenylene Diisocyanate (1,3-PDI) derived LCPUs). On the basis of structural similarity and well resolved WAXS powder patterns we extended the similar analysis to the regular polyurethanes as well (2,6-PU-6 and 1,3-PU-6). The good correlation between H-bonding distance and melting temperature for these four polymers suggests that melting is primarily controlled by the dissociation of H-bonds in the ordered domains

Mediated amperometric immunosensing using single walled carbon nanotube forests

A prototype amperometric immunosensor was evaluated based on the adsorption of antibodies onto perpendicularly oriented assemblies of single wall carbon nanotubes called SWNT forests. The forests were self-assembled from oxidatively shortened SWNTs onto Nafion/iron oxide coated pyrolytic graphite electrodes. The nanotube forests were characterized using atomic force microscopy and resonance Raman spectroscopy. Anti-biotin antibody strongly adsorbed to the SWNT forests. In the presence of a soluble mediator, the detection limit for horseradish peroxidase (HRP) labeled biotin was 2.5 pmol ml[-1] (2.5 nM). Unlabelled biotin was detected in a competitive approach with a detection limit of 16 nmol ml[-1] (16 μM) and a relative standard deviation of 12%. The immunosensor showed low non-specific adsorption of biotin-HRP (approx. 0.1%) when blocked with bovine serum albumin. This immunosensing approach using high surface area, patternable, conductive SWNT assemblies may eventually prove useful for nano-biosensing arrays

Electric field and tip geometry effects on dielectrophoretic growth of carbon nanotube nanofibrils on scanning probes

Single-wall carbon nanotube (SWNT) nanofibrils were assembled onto a variety of conductive scanning probes including atomic force microscope (AFM) tips and scanning tunnelling microscope (STM) needles using positive dielectrophoresis (DEP). The magnitude of the applied electric field was varied in the range of 1-20 V to investigate its effect on the dimensions of the assembled SWNT nanofibrils. Both length and diameter grew asymptotically as voltage increased from 5 to 18 V. Below 4 V, stable attachment of SWNT nanofibrils could not be achieved due to the relatively weak DEP force versus Brownian motion. At voltages of 20 V and higher, low quality nanofibrils resulted from incorporating large amounts of impurities. For intermediate voltages, optimal nanofibrils were achieved, though pivotal to this assembly is the wetting behaviour upon tip immersion in the SWNT suspension drop. This process was monitored in situ to correlate wetting angle and probe geometry (cone angles and tip height), revealing that probes with narrow cone angles and long shanks are optimal. It is proposed that this results from less wetting of the probe apex, and therefore reduces capillary forces and especially force transients during the nanofibril drawing process. Relatively rigid probes (force constant >= 2 N/m) exhibited no perceivable cantilever bending upon wetting and de-wetting, resulting in the most stable process control

Site-specific Forest-assembly of Single-Wall Carbon Nanotubes on Electron-beam Patterned SiOx/Si Substrates

Based on electron-beam direct writing on the SiOx/Si substrates, favorable absorption sites for ferric cations (Fe3+ ions) were created on the surface oxide layer. This allowed Fe3+-assisted self-assembled arrays of single-wall carbon nanotube (SWNT) probes to be produced. Auger investigation indicated that the incident energetic electrons depleted oxygen, creating more dangling bonds around Si atoms at the surface of the SiOx layer. This resulted in a distinct difference in the friction forces from unexposed regions as measured by lateral force microscopy (LFM). Atomic force microscopy (AFM) affirmed that the irradiated domains absorbed considerably more Fe3+ ions upon immersion into pH 2.2 aqueous FeCl3 solution. This rendered a greater yield of FeO(OH)/FeOCl precipitates, primarily FeO(OH), upon subsequent washing with lightly basic dimethylformamide (DMF) solution. Such selective metalfunctionalization established the basis for the subsequent patterned forest-assembly of SWNTs as demonstrated by resonance Raman spectroscopy

NMR investigation of n-alkylamine self-organization along the sidewalls of single-wall carbon nanotubes (SWNTs)

Single wall carbon nanotubes (SWNTs) have drawn considerable attention from the scientific community based on their potentially unique 1-D electronic and optical properties as well as mechanical properties. These characteristics result from the one dimensional quantum wire structure of CNTs, which have the spike-like van Hove singularities (vHs) in the electronic density of states. The detailed shape of the density of states function depends sensitively on CNT type semiconducting (sem-) versus metallic (met-)), diameter and chirality. Using the preferential affinity of amines towards sem -SWNTs, our group has been able to attain bulk separation by type based on a differential solubility of SWNTs according to the amine organization interaction on the nanotube surface. It has been argued that stable dispersions of sem -SWNTs with surfactant-amines originate from the organization of the aliphatic chain along the nanotube sidewalls, along with a small amount of zwitterions. The separation of sem-enriched SWNTs assisted by octadecylamine (ODA) had been depicted in the form of either small amount of zwitterionic interaction between carboxylic acid groups of acid-purified SWNTs or the physisorption on the SWNTs sidewall, leaving met -SWNTs in the precipitate