PPy films and particles were synthesized by plasma polymerizing Pyrrole (Aldrich, 98%) using glow discharges in a resistive arrangement with a vertical tubular glass reactor 9 cm diameter and 30 cm length at 13.56 MHz and 10− 1 mbar. A magnetic field was applied in the lower electrode. Two synthesis powers were used, 20 W and 100 W, to evaluate the hydrophobicity with a slightly different PPy chemical structure. In plasma polymers, crosslinking usually increases with the power of synthesis. The particles were collected in the region of highest magnetic field, see Fig. 1b, because the electron density and the probability of an ionizing collision with other particles is higher in this region [15]. Approximately, 27 mg of PPy particles were obtained in each synthesis, named as PPy20 and PPy100 to include the power of the synthesis in the name of the particles. Figure 1b shows the PPy fractions obtained in the synthesis: soluble and insoluble films, and insoluble particles.
The particles were added to PLA dissolved in chloroform forming homogeneous dispersions with a PPy/PLA = 0.027 mass ratio. The mixture was injected into an electrospinning machine designed and built by the team of this work with an injection speed of 1 mL/h at 12.2 kV, 12 cm distance between electrodes, internal needle diameter of 400 µm and 2500 rpm rotation collector. Sheets of fibers were obtained in this way, see Fig. 1b.
The fiber morphology was studied by scanning electron microscopy (SEM) with a Jeol IT 100 apparatus and by transmission electron microscopy (TEM) with a Jeol JEM-2010 microscope. The chemical structure was studied by infrared (IR) spectroscopy (FT-IR i5 Thermo Scientific) with attenuated total reflectance (ATR) using a diamond cell in the 550–4000 cm− 1 range.
The hydrophilicity of the polymers was evaluated measuring the contact angles in advance static mode with drops of water, KR and PBS solutions, see the salt concentration in Table 1. Consecutive drops of 2 µL were deposited on the polymers, up to 30 µL, in a Rame-Hart 250 goniometer. 15 photographs were taken observing the evolution of the angles evaluated with tangents of the drops at the coexistence point of the solid, liquid and gas phases.
Figure 2 shows photographs of a water droplet on a PPy film (a) and on electrospun hybrid PLA/PPy fibers (b) to exemplify hydrophilic and hydrophobic contact angles. The interfacial, Gas-Solid (γGS ), Solid-Liquid (γSL) and Gas-Liquid (γGL) forces are shown as arrows in Fig. 2. The equilibrium of forces has been fixed in θ = 90°, where γGS = γSL; however, if the surface forces are greater, θ < 90°, the surface becomes hydrophilic, γGS < γSL, otherwise the surface is hydrophobic, θ > 90°, where γGS > γSL [16–20].
Table 1
Concentration (mM) of solutions KR and PBS.
Solution
|
NaCl
|
Na2HPO4
|
KH2PO4
|
KCl
|
CaCl2
|
MgSO4
|
NaHCO3
|
KR
|
118
|
|
1.3
|
4.7
|
2.5
|
1.17
|
25
|
PBS
|
24
|
10
|
3
|
|
|
|
|