Browsing by Author "Allen, Michael H. Jr."

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    Imidazole-Containing Polymerized Ionic Liquids for Emerging Applications: From Gene Delivery to Thermoplastic Elastomers
    Allen, Michael H. Jr. (Virginia Tech, 2013-01-07)
    Novel imidazole-containing polyelectrolytes based on poly(1-vinylimidazole) (poly(1VIM)) were functionalized with various hydroxyalkyl-substituents to investigate the influence of charge density and hydrogen bonding on nonviral DNA delivery.  Copolymers with higher charge densities exhibited increased cytotoxicity, whereas increased hydroxyl concentrations remained nontoxic.  DNA binding affinity increased with increased charge densities and increased hydroxyl content.  Dynamic light scattering determined the copolymers which delivered DNA most effectively maintained an intermediate binding affinity between copolymer and DNA.  Copolymers containing higher charge densities or hydroxyl concentrations bound DNA too tightly, preventing its release inside the cell.  Copolymers with lower charge densities failed to protect the DNA from enzymatic degradation.  Tuning hydrogen bonding concentration allowed for a less toxic and more effective alternative to conventional, highly charged polymers for the development of nonviral DNA delivery vehicles.  The synthesis of amine-containing imidazolium copolymers functionalized with low concentrations of folic acid enabled the investigation of additional polymer modifications on nonviral gene delivery.   Functionalization of 1VIM with various hydroxyalkyl and alkyl groups and subsequent conventional free radical polymerization afforded a series of imidazolium-containing polyelectrolytes.  Hydroxyl-containing homopolymers exhibited higher thermal stabilities and lower Tg's compared to the respective alkyl-analog.  X-ray scattering demonstrated the polarity of the hydroxyl group facilitated solvation of the electrostatic interactions disrupting the nanophase-separated morphology observed in the alkylated systems.  Impedance spectroscopy determined hydroxyl-containing imidazolium homopolymers displayed higher ionic conductivities compared to the alkyl-containing analogs which was attributed to increased solvation of electrostatic interactions in the hydroxyl analogs. Beyond functionalizing 1VIM monomers and homopolymers to tailor various properties, the synthesis of novel architectures in a controlled fashion remains difficult due to the radically unstable N-vinyl propagating radical.  The regioisomer 4-vinylimidazole (4VIM) contains two resonance structures affording increased radical stability of the propagating radical.  Nitroxide-mediated polymerization (NMP) and atom transfer radical polymerization (ATRP) failed to control 4VIM homopolymerizations; however, reversible addition-fragmentation chain transfer (RAFT) demonstrated unprecedented control.  Linear pseudo-first order kinetics were observed and successful chain extension with additional 4VIM suggested preservation of the trithiocarbonate functionality. Effectively controlling the polymerization of 4VIM enabled the design of amphoteric block copolymers for emerging applications.  The design of ABA triblock copolymers with 4VIM as a high Tg supporting outer block and di(ethylene glycol) methyl ether methacrylate (DEGMEMA) as a low Tg, inner block, required the development of a new difunctional RAFT chain transfer agent (CTA).  The difunctional CTA successfully mediated the synthesis of the ABA triblock copolymer, poly(4VIM-b-DEGMEMA-b-4VIM), which exhibited microphase separated morphologies.  The amphoteric nature of the imidazole ring required substantially lower concentrations of outer block incorporation compared to traditional triblock copolymers to achieve similar mechanical properties and microphase separated morphologies.
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    Solution Properties and Electrospinning of Phosphonium Gemini Surfactants
    Hemp, Sean T.; Hudson, Amanda G.; Allen, Michael H. Jr.; Pole, Sandeep S.; Moore, Robert Bowen; Long, Timothy E. (The Royal Society of Chemistry, 2014-04-14)
    Bis(diphenylphosphino)alkanes quantitatively react with excess 1-bromododecane to prepare novel phosphonium gemini surfactants with spacer lengths ranging from 2 to 4 methylenes (12-2/3/4-12P). Dodecyltriphenylphosphonium bromide (DTPP), a monomeric surfactant analog, was readily water soluble, however, in sharp contrast, phosphonium gemini surfactants were poorly soluble in water due to two hydrophobic tails and relatively hydrophobic cationic head groups containing phenyl substituents. Isothermal titration calorimetry did not reveal a measurable critical micelle concentration for the 12-2-12P phosphonium gemini surfactant in water at 25C. Subsequent studies in 50/50 v/v water_methanol at 25C showed a CMC of 1.0 mM for 12-2-12P. All phosphonium gemini surfactants effectively complexed nucleic acids, but failed to deliver nucleic acids in vitro to HeLa cells. The solution behavior of phosphonium gemini surfactants was investigated in chloroform, which is an organic solvent where reverse micellar structures are favored. Solution rheology in chloroform explored the solution behavior of the phosphonium gemini surfactants compared to DTPP. The 12-2-12P and 12-3-12P gemini surfactants were successfully electrospun from chloroform to generate uniform fibers while 12-4-12P gemini surfactant and DTPP only electrosprayed to form droplets.
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    Water-Dispersible Cationic Polyurethanes Containing Pendant Trialkylphosphoniums
    Zhang, Musan; Hemp, Sean T.; Zhang, Mingqiang; Allen, Michael H. Jr.; Carmean, Richard N.; Moore, Robert Bowen; Long, Timothy E. (The Royal Society of Chemistry, 2014-03-06)
    Novel trialkylphosphonium ionic liquids chain extenders enabled the successful synthesis of poly(ethylene glycol)-based, cationic polyurethanes with pendant phosphoniums in the hard segments (HS). Aqueous size exclusion chromatography (SEC) confirmed the charged polyurethanes, which varied the phosphonium alkyl substituent length (ethyl and butyl) and cationic HS content (25, 50, 75 mol%), achieved high absolute molecular weights. Dynamic mechanical analysis (DMA) demonstrated the triethylphosphonium (TEP) and tributylphosphonium (TBP) polyurethanes displayed similar thermomechanical properties, including increased rubbery plateau moduli and flow temperatures. Fourier transform infrared spectroscopy (FTIR) emphasized the significance of ion_dipole interaction on hydrogen bonding. Atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) supported microphase separated morphologies in the trialkylphosphonium polyurethanes, despite the presence of ionic interactions. Sorption isotherm experiments revealed the TEP polyurethane exhibited the highest water vapor sorption profile compared to the TBP, which displayed similar water sorption profiles to the noncharged analogue. The phosphonium polyurethanes displayed significantly improved tensile strain; however, lower tensile stress of the TEP polyurethane was presumably due to absorbed water. In addition to physical characterizations, we also explored the trialkylphosphonium polyurethanes as nucleic acid delivery vectors. The phosphonium polyurethanes bound DNA at low charge ratios, and the polyplexes exhibited enhanced colloidal stability under physiological salt conditions.