*3.1. Morphology of Electrospun PLA and Electrosprayed SiO2 Materials*

Table 1 includes the mean thickness values of the uncoated PET film and the multilayer films after the deposition of the PLA fibers mat and also the electrosprayed nanostructured SiO2 microparticles. One can observe that the neat PET film presented a mean thickness of 77 μm while the coatings of electrospun PLA fibers and SiO2 microparticles were approximately 4.2 and 4.6 μm, respectively. The thickness was aimed at being the lowest under the screened conditions that could be applied and that could also generate reproducible superhydrophobic and adhesion performance in the final materials.

**Table 1.** Thicknesses of the non-coated polyethylene terephthalate (PET) film and PET films coated by electrospun polylactide (PLA) fibers and electrosprayed silica (SiO2) microparticles.


Figure 2 shows the top view of the non-coated and coated PET films. In Figure 2A, one can observe the smooth and continuous surface of the neat PET film. After electrospinning, a mat composed of uniform and randomly oriented PLA fibers with a mean fiber diameter of 1.5 ± 0.7 μm was obtained (see Figure 2B). Similar morphologies and sizes have been reported in the literature for PLA fibers obtained under similar electrospinning conditions and solution properties [24]. When SiO2 microparticles were electrosprayed on the PET/PLA layers, important changes on the surface morphology were observed. Some fibers were coated with SiO2 nanoparticles (41 ± 6 nm) and also some particles agglomerates (4 ± 0.5 μm) were randomly deposited onto the electrospun PLA mat (see Figure 2C). It has been reported that ultrafine particles such as silica nanoparticles tend to agglomerate or aggregate due to the strong cohesive forces between primary particles caused by their high surface-to-volume ratio and the small distance between them [7,25,26].

**Figure 2.** (**A**) Field emission scanning electron microscopy (FESEM) images of the as-received polyethylene terephthalate (PET) film; (**B**) Electrospun polylactide (PLA) fibers deposited onto the PET film; (**C**) Electrosprayed silica (SiO2) microparticles onto the PET/PLA fibers. Scale bars of 50 μm.

#### *3.2. Characterization of the Electrospun PLA Coated PET Films*

The coated PET/PLA fibers were annealed at different temperatures and without pressure, in the range from 90 to 170 ◦C. Figure 3 shows the top view of the different PET films coated with PLA fibers and annealed at different temperatures. One can observe that, from 140 ◦C onwards, the PLA fiber morphology was lost due to a process of fibers coalescence. It has been early reported that the thermal post-processing of electrospun fibers well below their melting point leads to a packing of the material into a continuous film with virtually little or no porosity due to fibers coalescence [27,28].

**Figure 3.** Field emission scanning electron microscope (FESEM) micrographs of the polyethylene terephthalate (PET) films coated with electrospun polylactide (PLA) fibers and annealed at different temperatures: (**A**) Without annealing; (**B**) 90 ◦C; (**C**) 120 ◦C; (**D**) 140 ◦C; (**E**) 160 ◦C; (**F**) 170 ◦C. The annealing time was 15 s in all samples and scale bars are 50 μm.

Figure 4 gathers relevant information about the transparency characteristics of the uncoated and coated PET films. Figure 4A shows quantitative measurements of transparency obtained by UV-Vis spectrophotometry in the wavelength range from ca. 400 nm to 800 nm. It can be observed that the uncoated PET film presented a high transparency, with an average light transmittance of 90% in the wavelength range screened. The PLA fibers deposition resulted in a high decrease in transparency to nearly 38% of light transmittance at 600 nm. A similar absorbance was observed for the bilayer structure annealed at 90 ◦C. The higher absorbance was seen to increase with increasing wavelength. It has been reported that nanofibrous films comprise large amount of air/fiber interfaces and therefore the incident light not only reflects and refracts many times at these interfaces but is absorbed within the mat, resulting in little light being transmitted through [29,30].

An increase in the light transmittance of the bilayer materials were observed when thermal treatments above 120 ◦C were applied. The sample annealed at 120 ◦C showed a slight increase in the light transmittance with respect to the non-heated sample. Interestingly, all bilayer structures annealed at higher temperatures, that is, 140 ◦C, 160 ◦C, and 170 ◦C, presented light transmittance values close to that of the uncoated PET film. Moreover, it was observed that the film transparency was progressively increased as the annealing temperature increased. This behavior was attributed to the fact that these annealing temperatures are closer or just above the first melting feature of the electrospun PLA fibers, previously reported at 154 ◦C by DSC [22] and in agreement with other previous study [31]. Therefore, it can be stated that the light transmittance of these bilayer films can be significantly enhanced by applying annealing temperatures of at least 140 ◦C. Similar observations were reported by Cherpinski et al. [28] when carrying out the post-processing optimization of electrospun submicron poly(3-hydroxybutyrate) (PHB) fibers to obtain continuous films.

Figure 4B includes the visual aspect of the film samples to evaluate their contact transparency. It can be observed that contact transparency of the films illustrated a high transparency for PET films and a clear decrease after the PLA fibers were deposited. Interestingly, more transparent films were observed after annealing, especially at temperatures higher than 140 ◦C. These observations are in expected coherence with the results of the light transmittance studies performed by UV-Vis spectrophotometry mentioned above.

**Figure 4.** (**A**) Ultraviolet-visible (UV-Vis) transparency measures of the uncoated polyethylene terephthalate (PET) film and coated with electrospun polylactide (PLA) fibers at different annealing temperatures. (**B**) Contact transparency of the films of: A—Uncoated PET; B—PET/PLA without annealing; C—PET/PLA annealed at 90 ◦C; D—PET/PLA annealed at 120 ◦C; E—PET/PLA annealed at 140 ◦C; F—PET/PLA annealed at 170 ◦C; and G—PET/PLA annealed at 160 ◦C. The typical sample size of the films in the pictures is of ca. 2 <sup>×</sup> 1.5 cm2.

Figure 5 provides the apparent water contact angle of the PET films coated with electrospun PLA fibers without annealing and annealed at different temperatures. One can observe that the PLA electrospun fibers generated a hydrophobic microdeposition with a value of 96.17◦ ± 10.79◦ that led to a slight increase of water contact angle when compared to the uncoated PET films (82.27◦ ± 0.83◦). This higher contact angle indicates that the water droplet does not spread well on the substrate due to the intrinsic hydrophobicity of the electrospun PLA mats. Contact angles higher than 120◦ have been reported for PLA, PCL, and poly(lactic-*co*-glycolic acid) (PLGA) electrospun mats or coatings [8,32].

On the other hand, water contact angles of the PET/PLA films decreased as the temperature of the thermal post-treatment increased, obtaining values of 83.94◦ ± 3.54◦, 80.64◦ ± 3.33◦, 79.27◦ ± 9.08◦, 75.3◦ ± 3.24◦, and 73.42◦ ± 6.84◦ for the bilayer films treated at 90 ◦C, 120 ◦C, 140 ◦C, 160 ◦C, and 170 ◦C, respectively (see Figure 5). In particular, the PET/PLA samples annealed above 140 ◦C showed a reduction in the contact angles values higher than 20%, in comparison with the coated PET/PLA film without annealing. This effect can be related to the partial loss of the fibrilar roughness structure of the

PLA mat (see Figure 3) and, then, to a reduction of the overall porosity of the film surface, resulting in a decrease of the water contact angle. This behavior is in agreement with the work recently reported by Lasprilla-Botero et al. [8], where LDPE/PCL films were annealed at temperatures ranging from 55 to 90 ◦C and a loss of surface hydrophobicity was observed as a function of the annealing temperature.

**Figure 5.** (**A**) Apparent water contact angle measurements of the uncoated polyethylene terephthalate (PET) film and coated with electrospun polylactide (PLA) fibers without annealing and annealed at 90 ◦C, 120 ◦C, 140◦C, 160 ◦C, and 170 ◦C. The annealing time was 15 s in all samples. (**B**) Digital images of the contact angle performed on the PET films before (left) and after deposition (right) of the electrospun PLA fibers.
