Tailoring hydrophobicity properties of polyvinylidene fluoride infused graphene composite films

All chemicals are of analytical reagent grade and used as received. PVDF (MW ~534,000 by GPC, powder) and dimethylformamide (DMF, anhydrous 99.8%) were purchased from Sigma Aldrich, Inc. (St. Louis, USA).

The graphene dispersion was obtained in DMF solvent (1 mg·mL⁻¹) by ultra-sonicated graphene powder for 2 h at a temperature <60°C. This temperature was chosen since the flashpoint of DMF is ~60°C. Meanwhile, 2.0 g of PVDF powder was prepared separately in 30 mL of DMF solvent. The PVDF dispersion was obtained after a 2 h magnetic stirring at 30°C. A graphene-DMF dispersion with different graphene loading ranges (1.0 wt.%, 1.5 wt.%, 2.0 wt.%, and 2.5 wt.%) was slowly mixed into the PVDF/DMF dispersion. A well-dispersed PVDF incorporated graphene was obtained after 5 min of ultrasonication.

Additionally, the mixed solution was continuously stirred at 500 rpm at 60°C for 1 h to allow the interaction to occur for good dispersion of the composite. Then, the homogeneous solution was dried-cast on a glass substrate using a film casting knife technique. The composite film was dried in a vacuum oven at 70°C overnight to evaporate the solvent. Finally, the end-product was obtained as a thin film composite with an average thickness of 150 μm. Figure 1 illustrates a schematic for the synthesis of PVDF/G composite film.

Fig. 1

The PVDF/G composite morphology was observed using field emission scanning electron microscopy (SEM; Quanta 200 FESEM, FEI, Eindhoven, Netherlands) operated at 10.0 kV. The energy-dispersive X-ray spectroscopy (FEI, Eindhoven, Netherlands) was utilized to study the elemental mapping of the PVDF/G composite film. Meanwhile, the PVDF/G composite's structural properties were characterized using inVia Raman Microscope spectroscopy (Renishaw, Gloucestershire, UK) using 532 nm laser-excitation. The mechanical tensile properties of the samples were analyzed using a universal testing machine, INSTRON 3300-100kN (Norwood, USA), with film of <1 mm thickness as per ASTM D882. Specimen samples with dimensions of 20 mm in width and 80 mm in length were used for performing mechanical tensile tests at a strain rate of 100 mm·min⁻¹. Using an optical tensiometer (DYNE Technology, Staffordshire, UK) equipped with a micro-syringe, water CA measurements were carried out. A water droplet volume of 2 μL was deposited onto the film surface from a distance of 5 cm and left for 60 s before the image was captured.

A mechanical tensile test is performed to investigate the suitability of the PVDF and its composites for serving as a substrate from which to develop an ideal biomaterial. Table 1 compares the uniaxial tensile properties of the native heart valves, blood vessels, PVDF, and PVDF/G composites with different graphene loading ratios. The PVDF/G composite with an increment of graphene content shows an increase in ultimate tensile stress up to 1.5 wt.% of graphene content from the tabulated data. Later, the ultimate tensile stress obtained is decreasing when the graphene percentage is increased. The PVDF/G-1.5 wt.% displays extremely high ultimate tensile stress and Young's modulus values, amounting, respectively, to 90.24 MPa and 5720.88 MPa. These remarkable values might be contributed by the strong interaction between the graphene and PVDF [25], where the graphene acts as a nanofiller with a high specific surface area, able to withstand external forces better than pure PVDF [26]. However, with further increases in graphene percentages up to 2.0 wt.% of graphene loading, the ultimate tensile stress and Young's modulus values are decreased. One potential explanation for this phenomenon, which was put forward in the study of Francis et al. [27], is that increasing graphene loading restricts the movement of PVDF polymer chains, which leads to poor tensile properties. Thus, the outcomes indicate that 1.5 wt.% of graphene loading is the optimum amount to enhance the tensile stress. This well distributed rigid network of graphene filler can carry most of the applied load, resulting in better mechanical properties.

Figure 2 shows that the composites’ ultimate strength and Young's modulus (E) were significantly increased at 1.5 wt.% of graphene. However, the strain value of the composite was reduced to 2.3%. In contrast, pure PVDF (refer to Table 1) shows lower strength and large elongation, which indicates a significant plastic deformation before failure. The high strain is not favorable because it may lead to calcification of the heart valve. However, the physiological conditions in vivo can be characterized further by fatigue or bending stress. As seen in Table 1, the highest values of ultimate tensile strength and modulus of elasticity that can be attained while still retaining the original shape are obtained for PVDF/G-1.5 wt.%. The PVDF/G-1.5 wt.% curve exhibits a sharp rise in tensile stress with a little strain value, likely to imitate standard native heart valve leaflets’ behavior. Human leaflet thickness ranged from 177 μm to 1.76 μm in a relaxed state, while 150 μm to 1.75 μm is the range observed corresponding to a stressed state [30]. Meanwhile, the PVDF/G-1.0 wt.% exhibits lower ultimate tensile strength compared to PVDF/G-1.5 wt.%, but is higher in strain percentage, almost at 5%. However, an increase in graphene loading up to 2.0 wt.% shows significant decreases in ultimate tensile strength at an extended elongation strain of >3%. It can be explained that at a 2.0 wt.% loading of the graphene filler, the graphene aggregation in the network interaction within the PVDF matrix is significantly heightened, thus preventing the PVDF micro molecules from slipping along the graphene filler. Thus, the interpenetration of the PVDF polymer is rendered difficult, which leads to low tensile strength. Additionally, the film casting method, along with poor dispersion of filler at higher concentration, may also result in the porosity of composite with reduced tensile properties [31].

Fig. 2

Figure 3 shows FESEM images of pure PVDF and PVDF/G composite with 0.5 wt.% up to 2.0 wt.% graphene loading at high magnification. As demonstrated in Figure 3A, no flaky aggregate can be observed on the surface of the pure PVDF. Meanwhile, PVDF/G composite film with 0.5 wt.% up to 2.0 wt.% graphene loading showed the morphology of flaky aggregation, which proves the infusion of graphene into the PVDF matrices [32]. Furthermore, the flaky aggregates and surface agglomeration increase as the percentage of graphene loaded into the composite increases, as shown in Figure 3B–3E. These observations indicate that the composites are infused with graphene filler [8]. Nevertheless, energy-dispersive X-ray spectroscopy (EDX) analysis was carried out to confirm the elemental composition, as shown in Figure 3E, 3F. Figure 3E represents the actual chemical composition of pure PVDF, which is 65.23 wt.% of fluorine and 34.77 wt.% of carbon. Meanwhile, after being infused with graphene, the chemical composition is 62.4 wt.% F and 37.6 wt.% C, as represented in Figure 3F, which demonstrates a slightly higher weight percentage for carbon, with an increment of 2.83 wt.%. It indicates that graphene filler is infused within the PVDF/G composite film. Thus, the EDX data are accurate in obtaining the investigated composites’ elemental analysis and chemical composition. Furthermore, the absence of surface-level impurity elements other than the irregularities in morphology confirms that the PVDF/G composite is free from impurities.

Fig. 3

Raman mapping shows the structure of the composite, as exhibited in Figure 4. The effective ness of stress transferred from the PVDF matrix to the composite is due to well-defined graphene and PVDF matrix interactions. The graphene Raman spectrum exhibits three distinct prominent peaks at 1,350 cm⁻¹, 1,585 cm⁻¹, and 2,700 cm⁻¹. These prominent peaks appointed to the disordered graphite structure (D band), the tangential stretching mode of carbon–carbon bonds (G band), and also the crystalline graphitic material (2D band), which confirms the existence of graphene [33, 34]. The consistency of graphene peaks can be observed clearly in the PVDF/G composite. It confirms the existence of graphene in the PVDF/G composite.

Fig. 4

Wettability testing is performed to study the intermolecular associations between the fluid and the composite, confirming its hydrophobic properties [35]. The stronger the fluid is attracted to the composite, the wider the spread of the fluid drop on the composite, and vice versa. Generally, the chemical composition of the composite is a crucial element to determine the wettability properties. Aside from the mechanical strength and elasticity, the effect of graphene loading in PVDF could improve the polymer's hydrophobicity [32]. Herein, assessment of the PVDF/G composite film's wettability was performed by measuring the film's water CA. Figure 5 displays the primary outcomes of the wettability tests involving measurement of the water CA of pure PVDF and PVDF/G composite.

Fig. 5

The pure PVDF shows the lowest surface energy and substantial hydrophobic property, with the CA on the left and right amounting to 76.3° and 75.8°, respectively. Thus, the surface's film of pure PVDF is hydrophilic. In contrast, graphene incorporation has increased surface roughness and the water CA by 20° more than pure PVDF to 93.6° and 93.8°, respectively, for the CA on the left and right. These results indicate that the increase in hydrophobicity and fouling behavior that takes place pursuant to graphene incorporation is improved in the case of PVDF/G composite film in comparison with pure PVDF film. These significantly improved properties imply that PVDF/G composite film shows immense promise for use as an alternative antifouling biomaterial in medical applications, and that it demonstrates potential for reducing the chances of occurrences of blood clots and calcification problems.