Applications of Polymer Nanofibers. Группа авторов

Acetone 56 20.7 23.5 Chloroform 62 4.8 26.7 Tetrahydrofuran 66 7.6 24.0 Water 100 78.5 72.0 Dimethylformamide 153 36.7 36.7 Ethanol 78 24.6 22.0 Dichloromethane 40 9.1 28.1 Formic acid 101 58.5 37.7 Acetic acid 118 6.19 26.9 Hexafluoroisopropanol (HFIP) 85 16.7 16.1

      

      Studies of electrospinning polystyrene or PEO from multiple solvents indicate that dipole moment, conductivity, and key solvent properties determine electrospinnability. Supaphol and coworkers considered 18 solvents for electrospinning polystyrene. Qualitatively, they found that solvents with high boiling point and high dipole moment that resulted in polymer solutions with high conductivity, low surface tension, and low viscosity led to fiber formation and minimized needle clogging (Jarusuwannapoom et al. 2005). Using PEO, smaller fibers were observed with solvents with higher dielectric constant (Son et al. 2004) and has been the focus of further investigation.

      The dielectric constant of the solvent is critical in electrospinning. Practically, it is how much electrical charge the solvent is capable of holding which affects the surface charge density. Generally, higher dielectric constants are preferred for achieving uniform surface charge density that results in uniform nanofibers. The fiber size is also affected by the dielectric constant of the solvent. For example, an approximate twofold increase in poly(lactic‐co‐glycolic acid) fiber size was observed when the solvent was switched from hexafluoro‐2‐propanol (ε ~ 17) to chloroform (ε ~ 5). A similar trend has been observed in electrospinning polymers from mixtures of solvents to tune the dielectric constant; introducing a solvent with high dielectric constant generally reduces fiber size, for example PCL from mixtures of chloroform ε ~ 5 and DMF (ε ~ 17). An approximate threefold decrease in fiber diameter was achieved by increasing the volume fraction of DMF from 0% to 10%. However, changing solvents affects conductivity, surface tension, polymer chain conformation, and solvent volatility in addition to affecting dielectric constant. Therefore, the observed changes cannot be solely attributed to dielectric constant.

      Luo et al. electrospun PCL from formic acid/acetic acid mixtures to systematically vary the dielectric constant at comparable molecular interactions, solubility, boiling point, viscosity, and surface tension (Luo et al. 2012). As the dielectric constant increased, decreased beading was observed and uniform nanofibers where achieved when the dielectric constant was greater than ~19. Notably, the interfiber spacing and mat porosity increased with increasing dielectric constant. The effect on porosity was attributed to increased residual charge at higher dielectric constant. Therefore, the dielectric constant may be an important consideration for tuning the fiber mat porosity.

      1.4.3 Additivities to Tune Solution Properties

      1.4.3.1 Surface Tension

      The polymer/solvent system also dictates other important solution parameters that influence the electrospinning process, specifically, surface tension and conductivity. Electrospinning requires the electrostatic force to overcome surface tension of the solution (Andrady 2008; Ramakrishna 2005). Surface tension acts to minimize the surface area per unit mass of fluid, i.e. spherical shapes which result in beaded fibers and affects the bending during the whipping instability (Ramakrishna 2005). Generally, lower surface tension is preferred for electrospinning (Andrady 2008).

      The surface tension is affected by the solvent and polymer concentration (Andrady 2008; Ramakrishna 2005). Typically, the surface tension decreases with increasing polymer concentration. To further reduce the surface tension, cosolvents such as ethanol or surfactants can be added to the polymer solution. Small amounts of surfactants (concentrations ~0.01 to 1 mM) dramatically decrease surface tension and facilitate electrospinning (Andrady 2008; Ramakrishna 2005). For example, 5–15 w/v% polystyrene in DMF/tetrahydrofuran only formed bead‐free nanofibers with the addition of surfactant (dodecyl trimethyl ammonium bromide). Triton X‐100 is a common nonionic surfactant used in aqueous systems such as polyvinyl alcohol and PEO and their blends. While nonionic surfactants change in conductivity relative to the change in surface tension is minimal, ionic surfactants such as sodium dodecyl sulfate affects surface tension as well as conductivity. The relative contribution of the conductivity and surface tension cannot be determined. Practically, ionic surfactants are effective in improving fiber quality. Zwitterionic surfactants may be especially effective at increasing the surface charge density. Nanofiber size can be affected by surfactant concentration, i.e. size decreases monotonically with surfactant concentration (Lin et al. 2004). Although, most often, cosolvents/surfactants to reduce surface tension are used to reduce beading and create uniform fibers.

      1.4.3.2 Conductivity

Solution conductivity is required for transfer of electric charge from the electrode to polymer solution (Andrady 2008; Ramakrishna 2005). Solvents commonly used in electrospinning have conductivity lower than water, but they play an important role in solution conductivity. For example, Uyar and coworker observed that the grade and supplier of the solvent affected solution conductivity. Solutions with a higher conductivity produced bead‐free fibers at lower polymer concentrations (Uyar and Besenbacher 2008). Polymer solutions generally have higher conductivity due to conducting ionic species (typically impurities or additives) from the polymer. Increasing polymer concentration can reduce its conductivity. With polyelectrolytes, i.e. polymers that have ionic functionalities, the solution conductivity is highly concentration‐dependent (Andrady 2008). The conductivity of the spinning solution can be increased using additives (Andrady 2008; Ramakrishna 2005). Typically, additives are inorganic salts such as sodium chloride. Organic compounds such as pyridinium formate or trialkylbenzyl ammonium chloride have also been used. Typically, additive concentrations below 2 wt% are used because at high salt concentrations, maintaining surface charge on the droplet becomes increasingly difficult. For example, aqueous solutions of poly(ethylene oxide) and CaCl₂ with conductivities above 5 mS/cm could not be electrospun (Andrady 2008).

      The additives increase surface charge density maintained on the jet which promotes fiber extension during whipping that can reduce beading in systems (Andrady 2008). In some cases, increased fiber extension can reduce resulting nanofiber size. The size of the conducting species is an important consideration. Ions with smaller ionic radius are thought to be more mobile and create elongational force during electrospinning. For example, solutions containing NaCl had smaller diameter than solutions with dissolved KH₂PO₄ or NaH₂PO₄ (Ramakrishna 2005). However, increasing fiber diameter with increasing solution conductivity has also been observed (Mit‐uppatham et al. 2004; Seo et al.