The morphology and scaling law model of polyvinylidene fluoride/carbon fiber using electrospinning technique

Electrospinning is a simple process used for polymer fibers that have diameters ranging from micro to nanometers. The purpose of this study was to investigate the morphology and scaling law model of Polyvinylidene Fluoride/Carbon (PVDF/Carbon) fiber math. Fiber is made by variations in concentration 15% w/w (FPC1), 18% w/w (FPC2), 21% w/w (FPC3) and 24% w/w (FPC4) by adding of 2% (w / w) carbon in each solution. The electrospinning process parameters used are 10 kV voltage, needle tip distance and collector 12 cm, and flowrate 0.1 ml / hour. The results showed that the morphology of FPC1-FPC2 fibers was in the form of bead and the morphology of FPC3-FPC4 are structure free bead. The avarege diameters of FPC1, FPC2, FPC3 and FPC4 are 910 nm, 1123 nm, 1349 nm and 1506 nm, respectively. The scaling law models result R Square (R2) of the experiment was 0.9992, indicating a very linear model relationship between theory and experiment. Polyvinylidene Fluoride/Carbon (PVDF/Carbon) fiber composite will be used as water filtration.


Introdution
Electrospinning is a simple process used for polymer fibers that have diameters ranging from micro to nanometers [1][2][3]. The process of making fibers is carried out by spraying a polymer solution using a high voltage electrostatic field and is supported by a solution flow pump mechanism. Fabrication via electrospinning can produce continuous fibers for large-scale, practical-efficient use and easy-to-control dimensions [1][2][3]. The nanofibers are controlled by using electrospinning to produce a fine dense fiber with a very small diameter, large surface area, and very small pore size. These properties cause nanofibers to be used for various applications, such as filtration [4,5], water filtration [6], wound dressing [6,9], drug delivery [2, 6,9] dan tissue enggenering [1,6].
Many of the polymers used to produce fibers have been successfully used by various studies using electrospinning methods for example, Poly (acrylonitrile) [9], Poly (ether sulfone) [10], Poly (vinyl alcohol) [1,6] and Poly (vinylidene Flouride) [10,11]. Polyvinylidene Fluoride (PVDF) has hydrophobic properties, and this polymer has a larger molecule than other conventional polymer membranes for example, PVP, PVA or PPL [11][12][13], so that this polymer is difficult to electrospinning. Therefore, to 2 stabilize the properties of PVDF by mixing carbon materials. The new nature of mixing the two materials can be utilized and used as an engineering material for air and water filters because they have hydrophilic functional groups such as carboxyl, epoxy, and hydroxyl [15]. Thus, the synthesis of the two materials is suitable combined.
Previous research results have reported pure PVDF electrospun with carbon [15][16][17], obtained the marphology of the fiber in the form of beadless fibers with a diameter of 80-160 [16][17], and pure PVPDF research has also produced beadless fibers in diameter 150-400 [17]. However, morphological studies and prediction of fiber diameter size models using mathematical models are still few. According to Jauhari (2020), a prediction is important in order to overcome the risk of failure of the formation of the fiber polymer mat so that a stable diameter is produced [13][14][15]. The complex system is understood through modeling the law of scale (SL) by combining process parameters (flow rate, concentration, conductivity, electricity, inertia, surface tension, and viscosity) [14,15,21]. In this study, we will discuss the morphology of PVDF / carbon fibers and the prediction of fiber size using a scaling law model.

Preparation Reduction Graphene (rGO).
Three grams of carbon and 18 grams of KmnO4 to which H2SO4: H3PO4 solution was added (360: 40 mL v / v) was stirred at 50 o C for 12 hours. Then the solution is added to 30 mL H2O2 30% then deposited for 1 day. Furthermore, the solution is centrifuged at a rotation speed of 3,000 -4,000 rpm for 15-20 minutes and decanted to separate the filtrate and the residue. The residue was washed in 200 mL of distilled water, then washed 2 times with 200 mL HCl 30%, 200 mL aceton, and 200 mL ethanol. The residue was dried, so that solid Graphene Oxide (GO) was obtained. GO was dissolved in 100 mL of distilled water at 1% (w / v) and heated to boiling. Next, stirrer the solution by adding 5% (w / v) ascorbic acid at 60 o C for 2 hours. After that, it is centrifuged at a rotation speed of 3,000-4,000 rpm for 15-20 minutes and the residue is dried at 100 o C until the final result is Graphene.

Synthesis of Fiber
Polyvinylidene Fluoride/carbon (PVDF/carbon) fibers were prepared by dissolving PVDF with various concentrations, namely 15% (w/w), 18% (w/w), 21% (w/w) and 24% (w/w) and the addition of carbon by 2% (w/w). Then dissolved using DMF solvent on hotplate-magnetic stirring (Therumo Sci., Japan) with a temperature of 80 o C, for 24 hours and a constant speed of 300 rpm and labeled FPC 1, FPC 2, FPC 3, and FPC 4 in each solution. The solution was transferred into a 10 ml syringe equipped (Terumo, Japan) and spun using electrospinning (Nanolab ES / DS 106, Malaysia). The process parameters used were a flow rate of 3.33 μl per minute, a high voltage of 13 kV, the distance between the needle tip of the collector drum of 10 cm, and the fiber rotating of 250 rpm. Also, the humidity is kept constant in the room at 45% and the temperature is 25ºC. At the time of collection of fibers can use the camera to monitor Taylor Cone that formed on the tip of the syringe.
The morphology of FPC 1, FPC 2, and FPC 3 fibers was observed using a fluorescence microscope (MiF) (Optika B-380 Material Science MET, Italy). The diameter size analysis used ImageJ 1.52a software (National Institutes of Health, USA), and the analysis results were made of normal distribution using OriginPro 2018 software (OriginLab Corporation, USA).

Scaling Law Model
Scaling Law (SL) is a law that expresses physical phenomena that occur when the size of a device or material is reduced. SL has been widely applied in solid matter physics to describe the size of polymers and particles that make up solid materials. The polymer diameter that has been synthesized using electrospinning, according to SL, will be in the form of charge voltage, solution density, molecular weight, flow rate function, electrical conductivity, polymer volume fraction, and dielectric constant. The Scaling law (SL) model can be explained in the theoretical prediction of the diameter of particles and fibers using the following equation [15,16]  (2) the prediction of polymer particle size [16][17][18][19]. The dielectric constant values were determined but predicted for the viscosity values, polymer particle size, and droplet size. Where the evaluation results are used to explain that with polymer control and the appropriate process parameters will produce polymer particles with stable sizes and controlled properties that can be produced.  (w/w). The process parameters used were the flow rate of 3.33 μl per minute, high voltage 13 kV, The distance between the needle tip of the collector drum of 10 cm, and fiber rotating of 250 rpm. The observed morphology is shaped like a continuous, regular strand. The results of MiF were successful in confirming that the lowest concentrations in this study, namely 15% and 18%, showed bead structures in FPC 1 and FPC 2 fibers. Meanwhile, at high concentrations, namely 21% and 24% showed a beadfree structure in FPC 3 and FPC 4 fibers. The formation of beads was related to viscosity and surface area. Solutions with low polymer concentration have low viscosity and few polymer chain bonds so that the elongation process when electric spinning was not perfect, this causes the formation of fiber beads [3,20]. Besides, increasing the surface tension has the effect of reducing the surface area of the mass unit of the solution [9,21]. When the free solvent molecules were in a high polymer concentration, there was a tendency for the solvent molecules to gather and form fibers with beads [3,9]. The size distribution of the diameter with the distribution of fiber diameter sizes ranging from 550 to 1800 is shown in Figure  2. The observed distribution of the diameter of FPC1, FPC2, FPC3 and FPC4 fibers had a coefficient of variance (cv) with prices of 0.16, 0.17, 0.12, and 0.09. Homogeneity occured when the ratio between the standard deviation and the mean diameter of the fibers was less than 0.3 [22,23]. The results of cv confirm that all fiber distributions were homogeneous. The mean diameters (d) of FPC1, FPC2, FPC3 and FPC 4 fiber math were found to be 910 nm, 1123 nm, 1349 nm and 1506 nm. Meanwhile, the standard deviation was 161 nm, 156 nm, 158 nm and 156 nm. It has been observed that increasing the concentration of the polymer solution results in a larger mean diameter. This is related to the increasing of polymer chains in solution when the concentration was higher [20,24]. The addition of 2% (w/w) carbon also significantly affected the total solution concentration while increasing the average fiber diameter. This was confirmed through previous studies, that pure PVDF had an average diameter ranging from 200-400 nm. In addition, the stretching of the fibers by the Coulomb force took less time because the high concentration solution dries faster.   The logarithms of fiber diameter measurements for both empirical data and theoretical calculations as a function of flow rate against electrical conductivity are shown in Figure 3. The Circle, the triangle and the hexagon are data obtained from previous studies of pure polymer [20], PVP/ETH composites [19] and experimental results.  [20,21]. The result of R Square (R 2 ) experiment approach is 0.9992, it is understood that the fiber diameter is in very good condition and more or less in accordance with the theoretical model. Thus, controlling the fiber diameter can be done by adjusting the parameters of the electrospinning process, namely electrical conductivity and flow rate.

Conclusion
The Morphology and Scaling Law Model of Polyvinylidene Fluoride/carbon (PVDF/carbon) fiber have been successfully produced and predicted using electrospinning techniques. Nanofiber was produced optimally with variations in the concentration of PVDF of 15% (w/w), 18% (w/w), 21% (w/w) and 24% (w/w) with the addition of carbon with a concentration of 2% (w/w). The process parameters of electrospinning, namely the distance between the tip of the collector drum needle were 10 cm, the flow rate was 3.33 μl per minute, the high voltage was 13 kV. MiF results have been shown a concentration of 15% and 18% were bead fiber structure and 21% and 24% were free to structure beads. The mean diameters of the FPC1, FPC2, FPC3 and FPC 4 fiber math were found to be 910 nm, 1123 nm, 1349 nm and 1506 nm, respectively. Besides, the results have been shown that the average fiber diameter was in very good condition and appropriate with the theoretical model developed in previous studies.