*2.8. Thermal Analysis*

Thermal transitions of the electrospun EVOH nanofibers were evaluated by differential scanning calorimetry (DSC) using a DSC-7 analyzer from PerkinElmer, Inc. (Waltham, MA, USA) equipped with the refrigerating cooling accessory Intracooler 2. For this, ca. 2 mg samples were placed in 40-μL hermetic aluminum sealed pans, previously calibrated with an indium standard. The scanning rate was 10 ◦C/min. Samples were subjected to a first heating step from −25 to 200 ◦C, followed by a cooling step down to −25 ◦C, and a second heating step to 200 ◦C. An empty aluminum pan was used as a reference and all tests were carried out, at least, in triplicate.

#### *2.9. Dielectrical Performance and Electrical Conductivity*

The dielectric spectra of the samples were obtained using an alpha (α) mainframe frequency analyzer, in conjunction with an active cell Concept 40, from Novocontrol Technologies BmgH & Co. Kc (Hundsangen, Germany). The sample electrode assembly consisted of two stainless steel electrodes filled with the polymer. The diameters of the electrodes were 20 mm and the thickness values of each sample were determined for each measurement. A single-sweep experiment was performed. The spectra were measured in the frequency (*f*) range of 10−2–107 Hz, under isothermal conditions at a temperature of 25 ◦C in nitrogen atmosphere. The analysis was conducted through the complex dielectric permeability ε\* = ε − *i*·<sup>ε</sup>", taking into account the real (ε) and imaginary (ε") parts as well as the loss tangent (tan δ = ε"/ε). In order to discriminate polarization and conductive effects [21,22], the complex electric modulus (*M*\*) was obtained as follows:

$$M^\* = \frac{1}{\varepsilon^\*} = \frac{1}{\varepsilon' - i \cdot \varepsilon''} = \frac{\varepsilon'}{\varepsilon'^2 + \varepsilon''^2} + i \cdot \frac{\varepsilon''}{\varepsilon'^2 + \varepsilon''^2} = M' + i \cdot M''$$

where

$$M' = \frac{\varepsilon'}{\varepsilon'^2 + \varepsilon''^2}$$

$$M'' = \frac{\varepsilon''}{\varepsilon'^2 + \varepsilon''^2}$$

The complex electrical conductivity σ\* = <sup>ε</sup>\*·ε0·<sup>ω</sup> was analyzed by means of the same experimental set. The values of the direct current electrical conductivity (σdc) were calculated by extrapolating the conductivity plateau to *f*→0 [23].

#### **3. Results and Discussion**

#### *3.1. Characterization of GNPs*

To confirm the detailed microstructure of the processed-graphite particles, the crystal phases of graphite, GO, and GNPs were analyzed by WAXS. From Figure 1 it can be observed that the XRD patterns of pristine graphite exhibits a strong peak centered at ~26.6◦ (2θ), which can be attributed to the typical graphite diffraction plane (002). By applying Bragg's law, this resulted in a *d* value of 3.35 Å, which is the characteristic interlayer distance between graphite layers [24]. Instead, a weak and broad basal diffraction peak was observed in the X-ray diffraction pattern of GO at ~11.4◦ (2θ), which corresponds to a *d* value between planes of 7.71 Å. This significant spacing increase reveals that the oxygen-containing functional groups were intercalated in the interlayer of graphite, then confirming the oxidation of graphite to GO. Furthermore, the lower intensity of this characteristic peak indicates that the degree of crystallinity in the GO structure was reduced. Indeed, GO is known to consist of heavily oxidized graphene sheets, which are loosely attached to each other [11]. Finally, the absence of peaks in the WAXS spectrum of the GNPs suggests that the graphene tactoids were randomly stacked.

Figure 2 shows a representative image, obtained by TEM, of a single layer of graphene. This provides direct evidence for the existence of a flat-like structure with a large surface and a diameter in the nanometric range, the so-called nanoplatelet. Previous studies have well described the typical morphology of a graphene particle, describing that it forms a 2D structure based on a cluster composed by several monolayers in which typical sizes of individual particles can reach several microns [25].

**Figure 1.** Wide angle X-ray scattering (WAXS) diffractograms of, from top to bottom, pristine graphite, graphite oxide (GO), and graphene nanoplatelets (GNPs). The interlayer distance is represented by *d*.

**Figure 2.** Transmission electron microscope (TEM) image of a single graphene nanoplatelet (GNP). Scale marker is 200 nm.

#### *3.2. Morphology of EVOH/GNPs Fibers*

Figure 3 shows the SEM images of the electrospun EVOH and EVOH/GNPs fibers. From Figure 3a it can be seen that the neat EVOH fibers presented a fibrilar morphology, completely free of beaded regions, with a mean diameter of approximately 675 nm. The resultant morphology differs from that previously reported in the study performed by Martínez-Sanz et al. [26], in which EVOH fibers presented beaded regions, this being mainly related to the lower polymer concentration used in the solution for electrospinning. As shown in Figure 3b–e, increasing the GNPs content led to a significant decrease in the electrospun fibers diameter. In particular, the average diameter was reduced from approximately 425 nm, for the fibers containing 0.1 wt.-% GNPs, to a value below 100 nm, for those fibers containing 2 wt.-% GNPs. This diameter decrease of the electrospun EVOH fibers can be related to an increase in the solution conductivity when the GNPs were added. In addition to a decrease in the fibers diameter, the GNPs incorporation gave rise to the formation of certain beaded fibers. This effect was particularly notable at the highest GNPs content, i.e., 2 wt.-%, where the bead morphology was the most prevalent. This observation preliminary suggests that GNPs agglomeration could occur at high contents inside the EVOH fibers during electrospinning.

**Figure 3.** Scanning electron microscopy (SEM) images of electrospun fibers of: (**a**) Neat poly(ethylene-*co*vinyl alcohol) (EVOH); (**b**) EVOH/graphene nanoplatelets (GNPs) at 0.1 wt.-%; (**c**) EVOH/GNPs at 0.5 wt.-%; (**d**) EVOH/GNPs at 1 wt.-%; (**e**) EVOH/GNPs at 2 w.-t%. Scale markers are 10 μm.

The presence and distribution of the GNPs in the electrospun EVOH fibers was also analyzed by TEM. Figure 4 displays a micrograph of the submicron EVOH fibers containing 0.5 wt.-% GNPs. In this image it can be observed that the GNPs were embedded and rolled up in the form of a continuous layer along the EVOH fiber axis. A good dispersion of the GNPs seems to be attained as a result of the favorable interfacial interaction between graphene and the EVOH matrix. Rolling and alignment of the GNPs can be ascribed to the inherent high resilience of the graphene layers combined with the extensional forces provided by the electrospinning process [4]. As a result, the GNPs were successfully incorporated by electrospinning into continuous submicron fibers of EVOH. The resultant roll-like morphology can be also advantageous to prevent restacking of the individual GNPs.

**Figure 4.** Transmission electron microscope (TEM) image of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs). Image corresponds to EVOH/GNP fibers at 0.5 wt.-%. Scale marker is 500 nm.

To further elucidate the GNPs dispersion in the submicron EVOH fibers, Raman analysis was carried out. Figure 5 shows typical Raman confocal images of the electrospun EVOH fiber mats reinforced with the different GNPs contents (Figure 5a–d) and also a typical non confocal spectrum of graphene taken in the electrospun mats (Figure 5e). The contour plots represent the added band areas of the two strong graphene bands across a defined sample area for the four nanocomposites. Thus, the more the yellow through to red color in the mapping, the stronger the presence of the GNPs. From the images, it seems that the dispersion was higher at lower nanofiller contents while the lowest dispersion, i.e., higher heterogeneity across the image, seemed to be for the sample with 2 wt.-% GNPs content, due to likely partial nanofiller agglomeration.

**Figure 5.** Raman spectroscopy contour plots of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs): (**a**) 0.1 wt.-%; (**b**) 0.5 wt.-%; (**c**) 1 wt.-%; (**d**) 2 wt.-%; (**e**) Typical non-confocal Raman spectrum of the sample EVOH/GNPs fibers at 2 wt.-%. As an example, Figure 5a shows as an inset the individual spectra that corresponds with the maximum signal of graphene in the image.

In relation to the individual spectra, the D-band at ~1316 cm<sup>−</sup><sup>1</sup> originates from the disordered carbon while the G-band, centered at ~1587 cm<sup>−</sup>1, associates with the ordered graphitic carbon [27]. In particular, the D-peak presence indicates the breakdown of translational symmetry in the lattice, which is attributed to either bulk defects in the basal plane or edge defects [28]. Table 1 includes the relative intensity ratio of D-band to G-band, i.e., ID/IG, which is habitually referred to as the "R-value", for each electrospun fiber mat taken using a 40× objective in a non-confocal mode and, hence, averaging the graphene signal in the samples. This value can be used to quantitatively characterize the amount of structurally ordered graphite crystallites in carbonaceous materials. One can observe that the ID/IG ratio was kept constant at 0.98–0.99 for the electrospun EVOH fiber mats containing the GNPs in the 0.1–1 wt.-% range. This observation suggests that the GNPs were effectively dispersed as carbon nanofillers with ordered layers in the electrospun fibers. This is in good agreemen<sup>t</sup> with the recent findings by Li et al. [16], who additionally proposed that the embedded graphene particles played an important role in promoting graphitic crystallinity for PAN nanofibers. However, this value decreased to ~0.96 for the electrospun EVOH fibers filled with 2 wt.-% GNPs, which confirms that graphene was more agglomerated at the highest content. In addition, its higher separation in wavenumbers between the D-band and G-band (dIG-ID), also shown in Table 1, may also indicate a reduction in the number of layers present in the graphene tactoids, i.e., a lower degree of exfoliation [27].

**Table 1.** Distance in wavenumbers (dIG-ID) and relative intensity ratio of D-band to G-band (ID/IG) of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers as a function of the graphene nanoplatelets (GNPs) content.


#### *3.3. Thermal Properties of EVOH/GNPs Fibers*

Figure 6 shows the DSC curves of the electrospun EVOH mats with the different GNPs contents. In Figure 6a it can be seen that the neat electrospun EVOH mat presented a relatively low glass transition temperature (Tg), of around 40 ◦C, probably related to the presence of some remaining humidity in the fibers [29]. One can also observe that the Tg value increased with the GNPs content. Therefore, the presence of the carbonaceous nanoplatelets restricted the polymer chains motion. However, the highest Tg was observed at 54 ◦C, for the electrospun EVOH fibers containing 1 wt.-% GNPs, while the EVOH fibers with 2 wt.-% GNPs presented an intermediate Tg value of ~51 ◦C. This confirms that the GNPs agglomerated more strongly at their highest content, as supported by the above-described morphological and also chemical analyses. In addition, all samples presented a slight endothermic peak during glass transition, which can most likely be related to stress-relaxation mechanisms occurring in the vicinity of the Tg. Other researchers have ascribed this effect to a physical-aging process. With aging of the material after processing, the polymer chains exhibit slow thermodynamic changes in the amorphous region to attain a lower-free energy state. As a result of this segmental mobility, the aged sample has smaller free volume and lower potential energy than the just processed one [30]. Therefore, when the materials are reheated, more energy is required to surpass the glass transition, which results in a small endothermic peak [31]. This molecular rearrangemen<sup>t</sup> has been previously described for EVOH films after pressure-assisted thermal processing [32].

In addition to the thermal variations during glass transition, all the submicron EVOH fibers did not present any melting peak during the first heating scan. This observation confirms that the electrospinning process led to a fully amorphous structure. A similar effect of chain mobility restriction in the EVOH fibers due to the presence of the GNPs can be observed in the cooling scan during crystallization from the melt, shown in Figure 6b. For instance, the electrospun EVOH fibers with 0.5 wt.-% GNPs presented a crystallization temperature (Tc) of ~128 ◦C while the neat EVOH fibers showed a value of ~134 ◦C. This delay in crystallization supports the above-described statement that the embedded GNPs acted as an anti-nucleant agent, particularly at low contents due to their improved dispersion.

**Figure 6.** Differential scanning calorimetry (DSC) curves of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs): (**a**) First heating scan; (**b**) Cooling scan.

#### *3.4. Dielectrical Performance and Electrical Conductivity of EVOH/GNPs Fibers*

The dielectric analysis of polymer materials and composites permits understanding the macromolecular relaxations [33–36] and is also a valuable approach for evaluating their conductivity and electric response. In this sense, Figures 7–10 show the dielectric spectra of the EVOH and EVOH/GNP fibers in terms of ε, ε", tan δ, and M", respectively, in all the frequency range at 25 ◦C. In Figures 7 and 8 one can observe that, at low frequencies, tan δ and ε" attained higher values though these values diminished rapidly with frequency. These results can be explained due to the fact that the alternation of the electric field was slow in the low frequency region, providing sufficient time for the permanent and induced dipoles to align themselves according to the applied field and, thus, leading to enhanced polarization. Broader dipolar polarization/relaxation processes were observed at higher frequencies, labelled as β-relaxation. This relaxation may be attributed to the local-mode relaxation in the crystalline regions of the copolymer and/or to the motion of their hydroxyl groups that could interact with each other. The neat EVOH fibers and EVOH/GNPs fibers exhibited almost identical values, with a slight increase when the GNPs were added. The maximum values were observed for a GNPs content of 0.5 wt.-%.

**Figure 7.** (**a**) Isothermal dielectric curves of the loss tangent (tan δ) of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs); (**b**) Influence of the GNPs content on tan δ for all frequency decades.

**Figure 8.** (**a**) Isothermal dielectric curves of the imaginary dielectric permeability (ε") of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs); (**b**) Influence of the GNPs content on ε" for all frequency decades.

Figure 9 shows the frequency response of the isotherms of the electrospun submicron neat EVOH fibers and EVOH/GNPs composite fibers in terms of <sup>ε</sup>. Three stages can be observed with the frequency increase. In brief, these involve a first decrease at low frequencies, followed by a plateau related to the copolymer relaxation processes, and a second decrease at high frequencies. Although all samples presented the same behavior, a non-linear slight increase was observed when the GNPs content was increased. Regardless of the magnitude of the frequency, the influence of the GNPs content showed similar profiles, with maximum values of dielectric permittivity around 0.5 wt.-% GNPs, which may indicate the most relevant polarization enhancement.

**Figure 9.** (**a**) Isothermal dielectric curves of the real dielectric permeability (ε) of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs); (**b**) Influence of the GNPs content on ε for all frequency decades.

To ascertain the molecular origin of this effect, Figure 10 plots the variation of M" in the neat EVOH fibers and EVOH/GNPs nanocomposite fibers. When the conductive effects were minimized, two peaks could be recorded, that is, one prominent peak at low frequencies and a broader one at high frequencies. These peaks have been described by other authors as the α- and β-relaxations, respectively [37]. On the one hand, the α-relaxation is ascribed to the main chain segmental motion and reflects a transition from the glassy to the rubbery state, related to the so-called Tg. This is a large-scale cooperative process determined mainly by intermolecular interactions. One can observe that the presence of the GNPs shifted the α-relaxation peak to higher frequencies, showing a maximum of frequency for a content of 0.5 wt.-% GNPs in EVOH. It means that, at this particular composition, the segmental molecular mobility of the main chain was reduced and, therefore, the Tg value increased. However, when the GNPs composition was higher than 0.5 wt.-%, the frequency of the peak decreased. This further confirms that the nanoparticles may begin to agglomerate at higher contents, as supported by the above-described information during the morphological and thermal analyses. This fact highly influences the performance and especially the electrical properties of polymer nanocomposites [38]. On the other hand, the β-relaxation was attributed to the local-mode relaxation in the crystalline regions of the copolymer and/or to the motion of the hydroxyl groups. The β-relaxation peak also showed a maximum frequency value at 0.5 wt.-% GNPs.

**Figure 10.** (**a**) Isothermal dielectric curves of the imaginary electric modulus (M") of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs); (**b**) Influence of the GNPs content on the frequency (*f*).

Figure 11a shows the evolution of σ in the 10−2–107 Hz frequency range. This property is typical of electrode polarization, associated with the accumulation of charges at the interfaces between the electrodes and the polymer sample, which increases with increasing frequency. This motion of charged carriers is spatially limited within their potential wells [39–43]. At lower frequencies, plateau regions occur, which can be associated with the free-charge transfer in the rubbery state and they are related to <sup>σ</sup>dc. However, at higher frequencies, the alternating current electrical conductivity (σac) occurs. The transition from linear non-frequency dependent <sup>σ</sup>dc to the frequency dependent range of the σac regions corresponds to the change in the mechanism of electrical conduction, which can be described by the movement of charges at long distances [40,44]. In Figure 11b one can observe that the dependence of <sup>σ</sup>dc of the nanocomposite fibers on the GNPs content was not linear. Table 2 shows the values of <sup>σ</sup>dc as a function of the GNPs content. The incorporation of low nanofiller contents sharply increased the conductivity of the electrospun EVOH fibers, up to a content of 0.5 wt.-% GNPs, with a percolation threshold relatively close to 0.1 wt.-% GNPs. The conductivity behavior of the nanocomposite fibers then changed, at very low GNPs contents, from an electrical insulator to a semiconductor material. However, at the highest GNPs contents tested, the <sup>σ</sup>dc values significantly decreased. As commented above, this phenomenon can be related to a low dispersion of the carbonaceous nanoparticles, for which no conducting clusters or bridges were formed inside the electrospun submicron EVOH fibers.

**Figure 11.** (**a**) Isothermal dielectric curves of the electrical conductivity (σ) of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers containing graphene nanoplatelets (GNPs); (**b**) Influence of the GNPs content on the direct current electrical conductivity (σdc).

**Table 2.** Direct current electrical conductivity (σdc) of the electrospun poly(ethylene-*co*-vinyl alcohol) (EVOH) fibers as a function of the graphene nanoplatelets (GNPs) content.


#### *3.5. Electrospun EVOH/GNPs Films*

Finally, Figure 12 shows the optical appearance of resultant EVOH/GNPs films obtained after thermal post-treatment at 158 ◦C, below the polymer's Tm, the so-called annealing, carried out on the electrospun fiber mats. Annealing applied on electrospun fibers mats results in continuous films that have significant potential for use in food packaging applications [45]. Simple naked eye examination of this image indicated that annealing produced continuous transparent films. This process has been recently ascribed to a compact packing rearrangemen<sup>t</sup> of the electrospun fibers by a phenomenon of fiber coalescence [46,47]. Another relevant observation is that, as the GNPs content increased, the resulting films became more opaque and developed a grey-like color, albeit contact transparency was preserved. This preliminary result indicates that the electrospun fibers mats can be turned into actual films, which will be the subject of further studies, since it may be advantageous for the application of developing labels or tags to have materials in a film format.


**Figure 12.** Electrospun films of poly(ethylene-*co*-vinyl alcohol) (EVOH) containing graphene nanoplatelets (GNPs) obtained by annealing.
