Effects of Reduced Graphene Oxide on Structural, Morphological, and Contact Angle Measurements of Poly (Vinyl Alcohol)/Polyvinyl Pyrrolidone Nanofibers

Polymer blend nanofibers are attracting considerable attention due to their wide applications in energy storage, water purification, drug delivery, and wettability studies. To date, research on poly (vinyl alcohol)/polyvinyl pyrrolidone (PVA/PVP) embedded with graphene-related materials like graphene oxide or reduced graphene oxide(rGO) are synthesized from solution mixing and casting into film, with limited efforts on developing polymer blend nanofibrous films. The present study focuses on developing the PVA/PVP blend nanofibrous film embedded with rGO using the electrospinning technique. The X-ray Diffraction (XRD), Field Emission scanning electron microscopy (FESEM), and contact angle measurements are used to investigate the structural, morphological, and wettability of the synthesized nanofibrous film. The rGO is synthesized by the improved Hummer’s method. The study provides insights into the tuning of the contact angles by modifying the morphology of nanofibers. The as-synthesized nanofibers have a diameter of about 150-250 nm and with the increase in rGO weight percentage show a drastic change in contact angle from 64° to 35.2°. These results highlight the modification of the wettability properties of blend nanofiber films by embedding rGO.


Introduction
Graphene is an sp2 carbon atom bonded in a hexagonal honeycomb-like lattice.Reduced graphene oxide (rGO) is a chemically derived form of graphene and is mostly used because of its high mechanical, and electrical conductivity, hydrophobic in nature, and properties similar to those of graphene.rGO can be produced in bulk via a chemical synthesis route and are mostly applicable in the field of energy storage devices (1), biosensors, electronic devices, and polymer composite (2).Recently polymers are attracting much attention due to their thermal stability, ease of processing, and wide range of applications.Polyvinyl alcohol (PVA) is a semicrystalline material obtained by hydrolysis of polyvinyl acetate.These are widely used as a binder and electrolytes in energy storage devices (3) Polyvinyl pyrrolidone (PVP) is a polymer with environmental stability and moderate electrical conductivity also ease of processing (Egypt and M. Zidan, 2016).PVA and PVP are miscible water-soluble materials.The area of PVA/PVP polymer blend nanofibers is attracting growing attention due to their excellent property that can be modified accordingly to achieve enhanced flexibility.
In this paper, we synthesize PVA/PVP/rGO nanofiber prepared via the electrospinning technique.While most studies focus on composite film prepared through solution mixing and casting into films, very few studies have explored electrospun PVA/PVP nanofiber embedded with reduced graphene oxide.The result will help in understanding the effect of rGO concentration on PVA/PVP polymer blend and the variation in its structural, morphological, and wettability of as-synthesized nanofiber film.
Procedure-Graphene oxide is prepared via the improved Hummer's method.First, graphite powder is added to the solution of sulfuric acid and phosphoric acid prepared in a 9: 1 ratio.The solution was kept in an ice bath, and then slowly potassium permanganate was added to this mixture, which acts as an oxidizing agent.The solution is then left overnight for stirring at room temperature and this results in a change in color to brown representing the formation of graphite oxide.Then hydrogen peroxide is added for the termination of the above solution.then the final powder is obtained by collecting the sample through the centrifuge and multiple washing with ethanol, Dil HCl, and distilled water and finally drying in a hot air oven at 80 ° C. The obtained graphite oxide is dispersed in distilled water at a 1mg/ml concentration and then performed ultrasonication to separate the layers of graphite oxide to finally get graphene oxide (GO).The obtained graphene oxide is then reduced using hydrazine hydrate via a sonication process, resulting in reduced graphene oxide (rGO).
The obtained rGO is used in the polymer blend solution to synthesize nanofibers via electrospinning.The PVA (5%, 15%) and PVP 20% solutions are prepared in distilled water under continuous stirring for 2 hours at 80 ° C and at room temperature respectively.The blend solutions are prepared by mixing PVA 5% and PVP 20% in equal ratios at room temperature until a homogenous viscous solution is formed.To this polymer blend, rGO is added in 0.5 and 1% and stirred until the rGO is uniformly dispersed in the solution.The prepared solutions are used to perform electrospinning to obtain nanofiber film on a cleaned glass slide.
Similarly, PVA 15% and PVP 20% solution is prepared.It was observed that for blend solutions of PVA 15% -PVP 20%, with each component solution of high concentrations and high molecular weight causes formation of thick viscous solution causing difficulty to stir properly.The electrospinning of PVA15%-PVP 20% was not possible due to the clogging of solution in the needle, when high voltage was applied.Thus, blend PVA 5% -PVP 20% nanofibers are studied in this work.
The PVA 15%, PVA 5%-PVP20% (hereafter mentioned as PVA/PVP), PVA/PVP/rGO-0.5% and PVA/PVP/rGO-1% solutions are used for synthesis of nanofibers.The Parameters such as distance between needle and collector, flow rate, and voltage are optimized during the electrospinning and are listed in Table 1.

Serial
No. The prepared nanofiber film was characterized using X-ray Diffraction (GI-XRD PANalytical), Field Emission Scanning Electron Microscopy (Model Ultra55 of Carl Zeiss), Contact angle meter (Kyowa Interface Science CO.LTD, Japan, DMs-401)

X-Ray Diffraction Analysis
Figure 1 shows the XRD of the prepared nanofibers, having a peak at 20° indexed as (101) plane confirming the presence of PVA in all the films.( 5).The broad peak in the range 15-35° indicates the presence of amorphous phases.The PVA/PVP/rGO-0.5% films have a small hump around 42 ° corresponding to reduced graphene oxide.

Field Emission Scanning Electron Microscopy (FESEM)
Figure 2 depicts the FESEM images of the PVA 15%, PVA/PVP, PVA/PVP/rGO-0.5%, and PVA/PVP/rGO-1% blend nanofibers along with the histogram of nanofiber diameter.The PVA 15% nanofibers are continuous, uniform, and have smooth morphology with an average diameter of 0.33 µm as shown in figure 2(a).For the PVA/PVP blend, nanofibers are continuous and were non-uniform with an average diameter of 0.27µm which shows the decrease in diameter with the addition of PVP as shown in figure 2(b).The addition of reduced graphene oxide to the PVA/PVP polymer blend results in non-uniform fibers distribution with surface modifications.The average diameter of the fiber diameter decreases from 0.27µm to 0.1 µm as the concentration of rGO increases from 0.5% to 1 % as shown in figure 2(c and d) histograms respectively.The contact angle measurements are made via the sessile drop method by depositing a 2µl of drop on the nanofiber film, and the image of the water drop shape is taken at an interval of 100ms.Table 2 shows the list of samples and the corresponding contact angles recorded.In Figure 3, PVA 15% shows the least contact angle formation of 34.1°, whereas the PVA/PVP blend indicates an improvement of contact angle to 64°.However, the further addition of rGO causes a decrease in the contact angle from 64° to 35.2°.It is clear that the addition of rGO has introduced the enhanced surface roughness as confirmed by FESEM micrographs, achieving higher wettability (6).

Conclusion
The nanofibers of PVA 15%, PVA/PVP, PVA/PVP/rGO-0.5%, and PVA/PVP/rGO-1% polymer blends on Borosilicate glass (BSG) are synthesized by electrospinning technique with a potential range 3.3.Contact angle measurements of 15-20 kV.It is found that these nanofibers have good adhesion nature on the glass substrate and stability for a long duration.The microstructural investigation shows that the PVA 15% nanofibers have an average diameter of 300-350 nm with smooth nanofiber surface, those of PVA/PVP blends are in the range of 250-300 nm, while with the addition of reduced-graphene oxide gives nanofibers diameter of 100-400 nm.The addition of rGO causes non-uniformity of the fiber diameter along the length and increased nanofiber surface roughness.The contact angle measurements indicate that the PVA 15% nanofiber has the least contact angle (34°) made with the water drop.However, the blend of PVA/PVP clearly shows that the contact angle is approximately doubled (~ 64°).With addition and an increase in the concentration of rGO, it is found that the contact angle decreases.At 0.5% rGO, the contact angle is about 59°, however, it falls drastically to 35° at 1% of rGO.These fibers can also be studied for their energy storage applications.

Table 2 .
Contact angle values of water on different prepared Nanofibers.