INTRODUCTION
Electrospinning has emerged as a versatile and scalable technique for fabricating nanofiber membranes with applications in filtration, sensors, biomedical scaffolds, and surface coatings 1,2. Among the various polymers employed, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) stands out due to its excellent chemical resistance, thermal stability, mechanical flexibility, and intrinsic hydrophobicity, making it ideal for advanced functional materials 3,4. The performance of electrospun nanofibers is closely linked to their morphological and surface properties, which are, in turn, governed by processing parameters such as polymer concentration, solution viscosity, and electrospinning conditions 5,6,7. Among these, polymer concentration plays a pivotal role in determining fiber diameter, uniformity, and surface roughness, parameters that significantly influence wettability, permeability, and mechanical behavior 8,9. Despite numerous studies on PVDF-based nanofibers, detailed insights into the relationship between PVDF-co-HFP concentration and both fiber morphology and surface wetting characteristics remain limited.
Understanding this correlation is critical, especially for applications where surface wettability dictates performance, such as in water-repellent membranes and biointerface engineering 10,11,12. Water contact angle (WCA) is a reliable measure of surface hydrophobicity and serves as a direct indicator of how nanostructure geometry affects wetting behavior. In this context, controlling fiber diameter through polymer concentration can provide a tunable method for tailoring surface functionality. This study aims to systematically investigate the influence of PVDF-co-HFP concentration on the morphological and wettability properties of electrospun nanofibers. We focus on analyzing the fiber diameter using SEM, verifying chemical structure via FTIR spectroscopy, and evaluating surface hydrophobicity through static WCA measurements. The findings offer a deeper understanding of how microstructural control via polymer concentration can be leveraged to engineer nanofibers with desired surface properties for specific applications.
Although various studies have reported on the electrospinning of PVDF-based polymers 13,14, most have addressed general processing effects or focused on blended systems, with limited emphasis on systematically isolating the role of PVDF-co-HFP concentration. To date, there remains a lack of comprehensive investigations that directly correlate polymer concentration with both nanofiber morphology and wettability behavior. Addressing this gap, the present study systematically examines how varying PVDF-co-HFP concentrations affect fiber diameter, structural characteristics, and surface hydrophobicity. By linking concentration-dependent morphological features to wettability outcomes, this work provides new insights into microstructural control strategies for engineering nanofiber surfaces with tunable hydrophobic properties, offering practical relevance for applications such as water-repellent membranes, coatings, and biomedical scaffolds.
MATERIALS AND METHODS
Materials
Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) with an average molecular weight (Mw) of ~400,000 and number average molecular weight (Mn) of ~130,000 in pellet form was obtained from Sigma-Aldrich. Dimethylformamide (DMF) and acetone, both of analytical grade, were purchased from Merck and used as received.
Preparation of PVDF-co-HFP nanofiber
Solutions of PVDF-co-HFP were prepared at concentrations of 12 wt%, 14 wt%, 16 wt%, and 18 wt% by dissolving the polymer in a 6:4 (v/v) mixture of DMF and acetone to a total volume of 10 mL. The mixtures were magnetically stirred at 60°C and 600 rpm until complete dissolution of the polymer was achieved. Electrospinning was performed using a high-voltage power supply with an applied voltage of 10 kV, a tip-to-collector distance of 15 cm, and a flow rate of 1 mL/h. The electrospinning process was maintained for 8 hours to ensure adequate nanofiber mat formation. The electrospinning process was conducted at ambient conditions, with a recorded temperature of approximately 25 ± 2°C and relative humidity of 60 ± 5%.
Materials characterizations
The surface morphology and fiber diameter of the electrospun nanofibers were characterized using scanning electron microscopy (SEM, JEOL JSM-6510). Chemical bonding and functional groups were analyzed via Fourier-transform infrared spectroscopy (FTIR) using a Shimadzu IRSpirit-X Compact FTIR Spectrometer. Water contact angle (WCA) measurements were carried out using a custom-built contact angle measurement setup to evaluate the wettability of the nanofiber surfaces.
RESULTS AND DISCUSSION
The morphological evolution of electrospun PVDF-co-HFP nanofibers with varying polymer concentrations (12–18 wt%) is shown in Figure 1, complemented by the corresponding fiber diameter measurements in Figure 2. At polymer concentration of 12 wt% (i.e., PVDF-co-HFP12), the SEM images reveal the formation of thin, randomly oriented nanofibers with slightly non-uniform diameters and minor bead presence, which is indicative of insufficient solution viscosity and poor chain entanglement during electrospinning process 15. This is quantitatively confirmed by the average fiber diameter of approximately 245 nm (Figure 2a). As the polymer concentration increases to 14 wt% (i.e., PVDF-co-HFP14), the nanofiber morphology becomes more homogeneous, with smoother and thicker fibers. The mean diameter rises to approximately 371 nm, suggesting improved jet stability due to enhanced viscosity and surface tension balance. At 16 wt% (PVDF-co-HFP16), a further increase in diameter to 475 nm is observed, accompanied by a more densely packed and uniform fiber network. At the highest tested concentration (18 wt%), the fibers exhibit the most uniform morphology and the largest average diameter of 590 nm. The high viscosity of this solution limits the stretching of the polymer jet, resulting in larger fiber diameters and highly aligned networks with minimal structural defects 16.
The Fourier-transform infrared (FTIR) spectrum of the electrospun PVDF-co-HFP nanofibers (Figure 2b) reveals the presence of characteristic vibrational modes confirming the chemical structure of the copolymer. Prominent absorption peaks are observed at 3021 and 2978 cm-1, which correspond to the symmetric and asymmetric stretching vibrations of –CH bonds, respectively. These peaks are commonly attributed to the aliphatic hydrocarbon chains in the polymer backbone. A strong absorption band around 1398 cm-1 is associated with the bending vibrations of CH2 groups, while the peak at 1275 cm-1 can be ascribed to CF2 stretching. The band at 1176 cm-1 is indicative of β-phase PVDF, typically assigned to C–F stretching vibrations. This is a significant feature as the β-phase is known for its piezoelectric and ferroelectric properties, which can influence the functional behavior of the nanofibers. In addition, peaks at 1072 and 878 cm-1 are attributed to C–C and C–F stretching vibrations, respectively, further confirming the integrity of the PVDF-co-HFP structure. The absorption at 749 and 686 cm-1 corresponds to the CF2 rocking mode, typically associated with the α-phase of PVDF. This indicates the presence of a semi-crystalline structure with both α- and β-phase contributions 17,18. Overall, the FTIR data confirms the successful retention of the copolymer’s molecular structure during the electrospinning process. The coexistence of α- and β-phases suggests a partially crystalline morphology, which may influence the material’s mechanical and surface properties, such as wettability and stiffness.
Figure 3 illustrates the surface wettability behavior of PVDF-co-HFP nanofiber mats, assessed via static water contact angle (WCA) measurements. Figure 3a presents representative side-view images of water droplets on nanofiber surfaces, while Figure 3b quantifies the corresponding contact angles. The sample PVDF-co-HFP12 (12 wt%) exhibits an average WCA of approximately 108°, indicative of a moderately hydrophobic surface. With increased polymer concentration, the WCA also increases: PVDF-co-HFP14 (14 wt%) reaches ~115°, PVDF-co-HFP16 (16 wt%) shows ~123°, and PVDF-co-HFP18 (18 wt%) achieves the highest angle at ~30° (Figure 3b) 19,20,21.
A strong positive correlation is observed between the diameter of electrospun PVDF-co-HFP nanofibers and their corresponding water contact angle (WCA), as demonstrated in Figures 2a and 3b. As the polymer concentration increases from 12 wt% (PVDF-co-HFP12) to 18 wt% (PVDF-co-HFP18), the average fiber diameter increases significantly—from approximately 235 nm to over 550 nm. This morphological evolution is directly accompanied by a progressive enhancement in surface hydrophobicity, with WCA values rising from ~108° to ~128°. From a physicochemical standpoint, this trend can be explained by considering the effect of fiber diameter on surface roughness and porosity, which play pivotal roles in wetting behavior. In electrospun membranes, smaller-diameter fibers typically form densely packed, highly porous mats with high surface area-to-volume ratios 22,23. While this may enhance absorption or permeation in some applications, such fine structures often lead to incomplete air pocket formation beneath a water droplet, reducing the apparent hydrophobicity. As fiber diameter increases, the surface topography becomes coarser, and the inter-fiber spacing becomes more regular and well-defined. This enables a greater fraction of the droplet to rest on air pockets between fibers, minimizing solid–liquid contact area 24. According to the Cassie–Baxter model, this condition leads to an increased apparent contact angle, as the droplet effectively sits on a composite surface of solid and air 25,26. Hence, larger nanofiber diameters contribute to enhanced surface roughness at the microscale, which amplifies the intrinsic hydrophobic nature of the PVDF-co-HFP polymer. Moreover, thicker fibers result from higher solution viscosity during electrospinning, which stabilizes the jet and reduces the incidence of bead formation. Beads, often present in low-viscosity solutions, create wettable defects on the surface, lowering the WCA. Therefore, the absence of such defects in higher-diameter fibers further contributes to the observed increase in hydrophobicity. This relationship between decreasing fiber diameter and increasing hydrophobicity is of practical significance for designing water-repellent materials, particularly in applications such as self-cleaning surfaces, protective coatings, and membrane technology.
CONCLUSION
In this study, we successfully fabricated PVDF-co-HFP nanofiber mats via electrospinning by varying polymer concentrations from 12 wt% to 18 wt%. The morphological analysis using SEM revealed that increasing the polymer concentration led to a significant increase in nanofiber diameter, from approximately 235 nm (PVDF-co-HFP12) to 560 nm (PVDF-co-HFP18), with more uniform and bead-free structures observed at higher concentrations. FTIR spectroscopy confirmed the chemical integrity of the electrospun PVDF-co-HFP nanofibers, showing characteristic vibrational bands associated with CF2, C–F, C–C, and CH groups, with evidence of both α- and β-phases of PVDF. Water contact angle measurements demonstrated a consistent increase in hydrophobicity with fiber diameter, from ~108° for PVDF-co-HFP12 to ~128° for PVDF-co-HFP18. This enhancement is attributed to increased surface roughness and air entrapment in the nanofiber network, consistent with the Cassie–Baxter wetting model. Overall, this work highlights that PVDF-co-HFP concentration is a critical parameter in controlling nanofiber morphology and surface wettability. The ability to tune these properties through simple formulation adjustment offers a promising route for optimizing electrospun membranes for specific applications such as waterproof coatings, separation membranes, and functional sensor interfaces. Future studies will explore the effects of thermal or chemical post-treatment, as well as assess the long-term stability of hydrophobic performance under environmental stressors such as UV exposure and humidity fluctuations.