Influence of Electrostatic and Intermolecular Interactions on the Solution Behavior and Electrospinning of Functional Nanofibers
Hunley, Matthew T.
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The solution rheological and electrospinning behavior of a series of charge-containing polymers, surface-active agents, and carbon nanotube composites was studied to investigate the effect of intermolecular interactions, including electrostatic interactions, hydrogen bonding, surface activity, and surface functionalization of carbon nanotubes. The synthesis of novel polyelectrolytes with varied topologies, charge content, and counterions tailored the charged macromolecules to elucidate structure-rheology and structure-processing relationships. In addition, the use of additives for electrospinning, including surfactants and nanofillers, allows us to tailor the functionality of electrospun nanofibers for high-performance applications. Novel polyelectrolytes based on poly(2-(N,N-dimethyl)aminoethyl methacrylate) (DMAEMA) were synthesized with the counteranions Cl-, NO3-, (CN)2N-, BF4-, PF6-, triflate (TfO-), and bis(trifluoromethanesulfonyl)imide (Tf2N-). The counteranion selection controlled the thermal transitions and degradation; the larger and more charge-delocalized anions typically resulted in lower Tg and higher decomposition temperature. The polyelectrolyte behavior in solution was nearly independent of anion choice, though solution conductivity depended on the electrophoretic mobility of the counterion. Charge containing copolymers of DMAEMA and di(ethylene glycol) methyl ether methacrylate (MEO2MA) were synthesized and demonstrated that polyelectrolyte behavior in solution was also nearly independent of charge content. Low ionic contents resulted in extended solution conformations and high conductivities. Controlled atom-transfer radical polymerization allowed the synthesis of star-shaped polyelectrolytes with varying arm numbers and lengths. The solution behavior of the stars deviated slightly from the linear polyelectrolytes due to significant counterion condensation within the star core and constrained polymer conformations. The linear and star-shaped polyelectrolytes were electrospun to understand the interplay between polyelectrolyte structure and electrospinnability. Similar to other strong polyelectrolytes described in the literature, PDMAEMA-based polyelectrolytes with polar anions (e.g. Cl-) experienced significant instabilities during electrospinning, requiring high concentrations and viscosities to stabilize the electrospinning jet. The use of large, more hydrophobic anions (BF4-, TfO-) led to increased electrospinnability. Unlike neutral branched polymers, which electrospin nearly identically to linear polymers of similar molecular weight, the star-shaped PDMAEMA-based polyelectrolytes required even higher viscosities than linear polyelectrolytes for stable electrospinning. The correlations between electrospinnability and solution rheological analysis are detailed. The use of surfactants facilitates the electrospinning of neutral polymers at lower concentrations. However, we have demonstrated that specific cylindrical aggregates of surfactants (wormlike micelles) can be electrospun into microfibers under the proper conditions. Ammonium and phospholipids surfactants as well as organogelators were studied using solution rheology and DLS to determine the effects of micellar structure and solution viscosity on the electrospinnability of low molar mass surfactants. In addition, the effects of charged and uncharged surfactants on the electrospinning behavior of poly(methyl methacrylate) were determined. Added surfactant facilitated uniform fiber formation at lower PMMA concentrations. XPS analysis demonstrated the formation of core-shell fibrous structures resulting from the self-migration of surfactants to the fiber surface. Hydrogen bonding also influences fiber formation through electrospinning. Star-shaped poly(D,L-lactide)s (PDLLAs) were end-functionalized with adenine (A) or thymine (T) units. The complementary hydrogen bonding between the adenine and thymine lead to thermoresponsive rheological behavior for mixtures of PDLLA-A and PDLLA-T. The mixtures could be electrospun above the hydrogen bond dissociation temperature and resulted in thicker fibers compared to unfunctionalized PDLLA stars. The hydrogen bonding allows the preparation of polymers with a combination desirable solid-state properties and very low processing viscosities. The effects of carbon nanotube incorporation on electrospinning behavior and fiber morphology were also investigated. Nonfuntionalized and carboxylic-acid functionalized carbon nanotubes were electrospun into polyurethane nanofibers. The nonfunctionalized nanotubes required high-shear melt mixing to disperse within the polyurethane, but remained well dispersed through electrospinning. The surface functionalization with acid groups produced nanotubes which dispersed more readily into the polyurethane solutions. TEM analysis revealed that nanotube dispersion and alignment within the nanofibers was similar for both nonfunctionalized and acid-functionalized nanotubes.
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