Applications of Polymer Nanofibers. Группа авторов

Applications of Polymer Nanofibers - Группа авторов


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the higher the molecular weight, the higher the viscosity of the polymer solution. Increasing the polymer concentration also increases the viscosity of the solution. A frequent observation has been that at low viscosities/polymer concentrations, the electrospinning jet breaks up into droplets, rather than stretching to form a fiber. With increasing concentration, there is a transition to beaded fibers and a second transition to uniform fibers (Figure 1.3). The ability to form uniform fibers has been frequently attributed to polymer entanglement (Andrady 2008; Ramakrishna 2005; Li and Wang 2013).

Schematic illustration of specific viscosity as a function of polymer concentration to determine entanglement concentration for PEO of various molecular weights.

      Source: Image of beaded fibers is reprinted from Fong et al. (1999). Copyright (1999), with permission from Elsevier.

      To quantify the degree of entanglement required to achieve uniform fibers, semiempirical relationships have been used (Shenoy et al. 2005; McKee et al. 2004, 2006). The entanglement concentration can be determined by measuring the viscosity (zero‐shear) as a function of polymer concentration and examining the scaling relationship between the specific viscosity and concentration. Note that the specific viscosity (ηsp) accounts for the viscosity of the solvent

      (1.9)equation

      The entanglement concentration can also be used to predict nanofiber diameter based on polymer concentration. A master curve for fiber diameter (df) as a function of concentration φ can be constructed as follows:

      (1.10)equation

      where df,e is the diameter of the fibers electrospun at the entanglement concentration φe. This result agrees well with the theoretical scaling of 2.3 (Wang et al. 2016). Long and coworkers showed comparable results with multiple polymers including linear, randomly branched, highly branched, and star polymers (McKee et al. 2004). This approach, which considers polymer concentration, viscosity, and polymer molecular weight (because the entanglement concentration decreases as polymer molecular weight increases), is convenient (Andrady 2008). Due to the high deformation rates, the entangled polymer solutions behave like elastic swollen gels. The rapid stretching of the gel has recently been considered the main mechanism of fiber formation. These results imply that the elasticity of the entangled polymer solution rather than the viscosity influences the final fiber diameter (Wang et al. 2016).

      1.4.2 Solvent Selection


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Solvent Tb (°C) Dielectric constant Surface tension (mN/m)