Browsing by Author "Wendy E. Krause, Committee Member"

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    Controlling the Stereoregularity of Polyacrylonitrile and Its Determination Using Small-Molecule Host Inclusion Compounds
    (2006-11-02) Yang, Hyungchol; Wendy E. Krause, Committee Member; Alan E. Tonelli, Committee Chair; Charles M. Balik, Committee Member; Saad A. Khan, Committee Member
    This research focuses on synthesizing highly stereoregular polyacrylonitrile (PAN) and determining its tacticity (predominantly isotactic or syndiotactic), utilizing guest monomer (acrylonitrile = AN) host inclusion polymerization. Highly stereoregular PAN, with a meso or racemic diad content ~ 80%, was prepared by γ-ray irradiation of an AN urea canal complex at a low temperature (–78° C). Several essential experimental factors for ensuring the highly stereoregular PAN production were considered. After γ-ray irradiation polymerization, the tacticity of PAN was determined from triad peak intensities of the methine (CH) and nitrile (–C≡N) carbons in the 13C-NMR spectra, assuming Bernoullian statistics. When the AN guest forms an inclusion compound (IC) with urea host, it was expected that there is a structural transformation of urea into the hexagonal crystal lattice structure with a narrow channel diameter (5.25-5.5Å). However, in our FTIR observations run at room temperature, a different type of transformation was detected. AN urea IC before and the PAN urea IC after low temperature γ-ray irradiation polymerization are both large tetragonal structures, which have a larger channel diameter (> 5.5Å) at room temperature. Because these infrared observations were not carried out below -20.8° C, known as the decomposition temperature of the hexagonal IC structure, the fact that the structures of both AN urea IC and PAN urea IC are the large tetragonal does not necessarily prove that during polymerization below –20.8°C the AN urea IC was also the large tetragonal structure. If PAN was polymerized in the hexagonal (or pseudo-hexagonal) urea canal lattice (5.5Å), which provides a more confined environment for its conformation and configuration, it would likely be syndiotactic and adopt the all trans conformer. Because of the flexible nature of urea when it forms inclusion compounds with guest molecules, if AN was polymerized in the large tetragonal (> 5.5.Å) lattice structure of urea, which gives more freedom to PAN during its inclusion polymerization or inclusion compound formation, it could have either an isotactic or a syndiotactic configuration. Because no definitive evidence has been previously reported in the determination of γ-ray irradiated PAN by NMR spectroscopy, an effort to prepare PAN in another molecular host crystalline lattice, α-cyclodextrin (CD), was made. Synthesis of PAN in the columnar structure of AN α-CD–IC is a very promising method to reveal the original tacticity of highly stereoregular (~80%) PAN due to the fact that α-CD has a rigid small diameter (4.9Å) channel cavity, and only syndiotactic PAN in the all trans conformation is likely to be produced. However, γ-ray irradiation of a channel structure AN α-CD IC did not produce any PAN, implying that the AN urea IC that produced stereoregular PAN upon γ-irradiation was likely in a large tetragonal structure. Alternatively, because of the disparity in AN:α-CD and PAN:α-CD stoichiometries [1:1 (experimental) versus 3:1 (expected)], after γ-irradiation initiation of AN α-CD IC, a shortage of AN would result in the α-CD IC channels, possibly interrupting polymerization. By analogy to polypropylene (PP) polymerized in host perhydrotriphenylene (PHTP) IC (d ~ 5Å) and polyvinylchloride (PVC) polymerized in urea canals, which are both found to be syndiotactic, we suggest that stereoregular PAN polymerized in urea canals is also predominantly syndiotactic.
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    Fabrication and characterization of novel single and bicomponent electrospun nanofibrous mats
    (2008-08-08) Ojha, Satyajeet Sooryakant; Russell E. Gorga, Committee Chair; Saad A. Khan, Committee Co-Chair; Wendy E. Krause, Committee Member
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    Morphological and Mechanical Properties of Carbon Nanotube/Polymer Composites via Melt Compounding
    (2005-07-12) Dondero, William Edward; Russell E. Gorga, Committee Chair; Richard J. Spontak, Committee Member; Wendy E. Krause, Committee Member
    The mechanical properties and morphology of multi-wall carbon nanotube (MWNT)/polypropylene (PP) nanocomposites were studied as a function of nanotube orientation and concentration. Through melt mixing followed by melt drawing, using a twin screw mini-extruder with a specially designed winding apparatus, the dispersion and orientation of multi-wall carbon nanotubes was optimized in polypropylene. Tensile tests showed a 32% increase in toughness for a 0.25 wt % MWNT in PP (over pure PP). Moreover, modulus increased by 138% with 0.25 wt % MWNTs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) all demonstrated qualitative nanotube orientation. Wide angle X-ray diffraction was used to calculate the Herman's Orientation Factor for the composites as function of nanotube loading and orientation. No significant changes in PP crystal orientation were found indicating that the alignment of the nanotubes did not significantly affect the orientation of the PP crystals. In addition, differential scanning caloriometry (DSC) qualitatively revealed little change in overall crystallinity. In conclusion this work has shown that melt mixing coupled with melt drawing has yielded MWNT/PP composites with a unique combination of strength and toughness suitable for advanced fiber applications, such as smart fibers and high performance fabrics.