Production of Multi-Walled Carbon Nanotube-Doped Polycaprolactone Nanofiber Membranes

Abstract

In this study, polycaprolactone (PCL) and multi-walled carbon nanotube (MWCNT) based rechargeable nanofiber membranes were produced using the nanotechnological electrospinning technique. The produced nanofiber membranes were subjected to structural (Fourier Transform Infrared Spectroscopy-FTIR), morphological (Field Emission Gun Scanning Electron Microscopy-FEGSEM) and mechanical (Tensile) analyses to determine material properties.

1. Introduction

Nanomaterials are derived from the Greek word "nanos," meaning small old man or dwarf. Today, the term "nano" is used as a technical unit of measurement. Mathematically, one nanometer is equivalent to one billionth of a meter [1-3]. The concept of nanofiber is defined as fibers with an average fiber diameter at the nanometer level, with a thickness approximately one thousandth that of a human hair. When considering the fiber concept in general, the term "nano" refers to the size of the fiber diameter. One of the most important techniques developed for creating fibers is the electrospinning method. With this production method, it is possible to obtain lightweight materials at the nano scale with high mechanical strength and biocompatibility. Nanofiber production methods are based on conventional techniques used for fibers produced from solution or melt, which involve passing the melt or solution through a die and solidifying it. However, it is not possible to produce nanofibers using these methods. This is because it is impossible to reduce the die diameter used in conventional fiber production methods to a size small enough to produce nanofibers. Today, nanofiber production can be achieved using fibrillation, meltblowing, bicomponent, spunbond and electrospinning methods [4-9]. Electrospinning is the process of transforming a viscous liquid into nano-sized fibers by applying kilovolt-level voltage to very low flow rates. In these applications, polymer solution or melt is used as the viscous liquid. At controlled flow rates, the solution reaching the tip of a medical syringe forms a spherical droplet under the influence of surface tension. Under the influence of electrical forces, it tapers (Taylor cone) and is transferred as a nanofiber to a grounded collector at a certain distance. While electrospinning systems from solution can be set up simply this way, obtaining nanofibers from melt requires a more complex system. This is because polymer granules must be melted at specific temperatures to achieve the appropriate viscosity for flow. The fundamental principle of the electrospinning process is to use electrostatic forces to overcome the viscoelastic and surface tension forces on the polymer solution, creating very fine fibril structures from the solution. These resulting structures form web-like structures of fibers with nano-scale diameters. In the electrospinning method, the polymer to be spun is dissolved in an appropriate solvent or melted by heating and placed in a pipette or syringe with a small hole at one end. An electric field is created by applying sufficient voltage between this pipette/syringe and a metal collector plate located at a specific distance, which is used for fiber collection. By gradually increasing the applied voltage, the electrostatic forces are expected to overcome the surface tension and viscoelastic forces in the polymer droplet. Once the applied voltage reaches a critical value, jet formation begins, the jet rapidly extends toward the collector plate and thins and breaks. Thus, nanofiber production begins at nanoscale. The evaporation of solvent molecules also contributes to the reduction of fiber diameter. Electrospinning is not a new technology. This process emerged in the 1600s when William Gilbert, while continuing his studies on magnetism, accidentally observed the effect of electromagnetism on liquids. In his work, he noted that a water droplet was drawn from a dry surface in a cone shape at a certain distance through electrical means. This is where the history of electro-spray and electrospinning began. With the electrospinning technique, a wide variety of materials can be produced, such as rechargeable materials, solar cell materials, biosensors, and wearable technology products. The electrospinning technique enables the production of purpose-oriented products for many sectors including healthcare, textiles, food, defense, agriculture, and filtration [1-8]. In this study, polycaprolactone (PCL) and multi-walled carbon nanotube (MWCNT) based nanofiber membrane production will be achieved using the nanotechnological electrospinning technique. Structural (FTIR), morphological (FEGSEM) and mechanical (tensile) analyses will be performed on the obtained membranes to determine nanofiber material properties.

2. Material and Method 2.1 Material

In the production of rechargeable nanofiber membranes, MWCNT (50-90 nm, over 95% carbon-based Sigma-Aldrich/Turkey) and PCL with a molecular weight of 80,000 g/mol (97% purity Sigma-Aldrich/Turkey) were selected. Organic solvents such as Dimethylformamide (DMF HCN(OH3 (CHCl3)2) (Sigma-Aldrich/Turkey) and Chloroform) (Sigma-Aldrich/Turkey) were used to dissolve the polymers. In the electrospinning method, parchment paper was used as the substrate material.

2.2 Method 2.2.1 Rechargeable Nanofiber Membrane Production

10 grams of PCL polymer by weight was dissolved in DMF/Chloroform (50-50) solvent with a heated magnetic stirrer at a stirring temperature of 60°C and a stirring time of 45 minutes. Different amounts of MWCNT (1%, 5%, 8%) were added to the created PCL solution to obtain four different composition solutions. Rechargeable nanofiber membranes were produced by applying the electrospinning working parameters listed in Table 2.1 to the obtained solutions. Table 2.1 shows the electrospinning working parameters. Figure 2.1 shows the steps for producing rechargeable nanofiber membranes. [caption id="attachment_102743" align="aligncenter"] Table 2.1. Electrospinning working parameters[/caption] [caption id="attachment_102744" align="aligncenter"] Figure 2.1. Steps for producing rechargeable nanofiber membranes[/caption]  

2.3 Characterization Method 2.3.1 Structural Characterization

FTIR analyses of the rechargeable nanofiber membranes were performed on a Jasco brand model 6600 analyzer at wavelength ranges between 400 and 4000 cm⁻¹. Based on percent transmittance (%T) values, the bonds present in the structures of the samples were determined.

2.3.2 Morphological Characterization

Rechargeable nanofiber membranes placed on holders were examined with an FEI FEG QUANTA 450 SEM microscope and photographs were taken. During the examination of the diameter and dimensions of the produced rechargeable nanofiber membranes, magnified images at x12000 magnification were examined at 5 kV potential for FEGSEM analysis. The surface morphologies of the rechargeable nanofiber membranes were determined in measurements performed on an FEI FEG QUANTA 450 device. The average diameter thicknesses of the resulting nanofibers were measured from high-resolution FEGSEM photographs using Image j (National Institutes of Health) software.

2.3.3 Mechanical Characterization

In order to conduct mechanical characterization studies of the rechargeable nanofiber membranes, samples were prepared with dimensions of 1×4 cm. A digital micrometer (795.1 MEXFL-25, Starrett, USA) device was used to measure the thickness of the rechargeable nanofiber membranes cut to appropriate size. The resulting thickness values were entered into the analysis program before mechanical analysis and used in determining the elastic modulus. The mechanical properties of the resulting nanofibers were determined using a Zwickline (Zwick/Roell Ltd. Germany) analyzer. Studies were conducted at room conditions. Samples were separated from the substrate and attached to the device clamps. The device was set to a load of 500 N, a tensile speed of 5 mm/min, and a jaw gap of 10 mm, and mechanical properties were determined.

3. Findings

3.1 Structural Characterization

Bands observed at 2945 cm⁻¹ and 2866 cm⁻¹ are CH2 stretching bands; bands observed at 1164 cm⁻¹, 1239 cm⁻¹ and 1294 cm⁻¹ frequency are symmetric C-O-C stretching bands. The intense peak observed at 1721 cm⁻¹ wavenumber is the characteristic carbonyl (C=O) stretching band of the PCL component. The MWCNT substance contains –COOH at 1488.2 cm⁻¹ frequency, C=C at 1635.6 cm⁻¹ wavenumber and OH stretching band bonded in carboxylic acid at 2735.1 cm⁻¹ wavelength. Additionally, since the MWCNT substance was not functionalized, the peaks present in the FTIR spectrum were observed as reverse absorbance [1-13]. The FTIR spectrum of the polymer and additive used in the rechargeable nanofiber membrane is shown in Figure 3.1. [caption id="attachment_102745" align="aligncenter"] Figure 3.1. FTIR spectrum of the polymer and additive used in the rechargeable nanofiber membrane[/caption]

3.2 Morphological Characterization

MWCNT FEGSEM morphological images at different magnifications are shown in Figure 3.2. As a result of FEGSEM analysis, MWCNTs with a diameter of 50-85 nm and a length of 10-15 µm were observed. Since the MWCNTs were not functionalized, carbon images were easily detected with the aid of an electron microscope. The carbon structures are located independently among themselves in a MWCNT network structure. MWCNTs are hollow, cylindrical-shaped allotropes with a high aspect ratio (length-to-diameter ratio). Their names are derived from their structures, and the walls are formed by more than one atom-thick carbon layer. MWCNTs consist of concentric graphene nanotubes in multiple layers inside other nanotubes [1]. [caption id="attachment_102746" align="aligncenter"] Figure 3.2. MWCNT FEGSEM morphological images[/caption]   FEGSEM images of the rechargeable nanofiber membranes at 6000x and 12000x magnifications are shown in Figure 3.3. Nanofiber formation was observed in all samples. The nanofibers of the PCL membrane have a uniform fiber distribution randomly distributed. As a result of MWCNT addition, certain changes and orientations occurred in the fibers. As a result of 1% MWCNT addition, clustering occurred in places on the PCL fibers. This is due to the MWCNT addition. When 5% was added, the clustering in the membrane nanofibers disappeared but widened the diameter of the PCL nanofibers. As a result of 8% MWCNT addition, it was determined that the nanofibers were oriented and the diameter of the nanofibers was reduced compared to all samples. The main reason for this is that the MWCNT additive added to the PCL solution increased the electrical conductivity of the solution. Better nanofiber production is achieved due to increased electrical conductivity [1,12-14]. [caption id="attachment_102747" align="aligncenter"] Figure 3.3. FEGSEM images of the rechargeable nanofiber membranes at 6000x and 12000x magnifications[/caption]  

3.3 Mechanical Characterization

Figure 3.4 shows the mechanical properties of the rechargeable nanofiber membranes. The produced membranes were repeated three times and mechanical properties were determined. The average tensile strength value of the PCL membrane was obtained as 16.42 MPa. As a result of MWCNT addition to PCL, linear tensile strength was achieved. When the tensile strength values of all samples were examined, the 10% PCL + 8% MWCNT membrane had the highest value with a tensile strength value of 45.03 MPa. The reason for this is that the reinforcing MWCNT additive homogeneously wraps around the PCL membranes, making the PCL nanofibers thinner and creating a more tightly packed structure [1, 12-14]. [caption id="attachment_102748" align="aligncenter"] Figure 3.4. Tensile strength values of the rechargeable nanofiber membranes[/caption]

4. Conclusions and Recommendations

In our study, PCL and MWCNT-based nanofiber membrane production was successfully accomplished using the nanotechnological electrospinning technique. According to the structural analysis results of the nanofiber membranes, due to the lack of functionalization of the MWCNT substance, a regular chemical structure could not be determined, and distinct peaks could not be detected in the nanofiber membrane structure. When the morphological images were examined, a homogeneous and regular structure of the nanofibers in the PCL membrane was observed. As the MWCNT addition rate increased, the nanofibers became increasingly thinner and the nanofibers became oriented. According to the mechanical analysis results, as the MWCNT addition increased compared to the PCL membrane, mechanical properties provided a linear increase. MWCNT addition homogeneously wrapped around PCL fibers, thinned them and enabled the formation of a more tightly packed structure. This linearly affected the increase in strength. The nanofiber membrane we obtained could be a rechargeable material. As a continuation of the study, different production methods such as dip coating can be tested in conductive composite film production. Since these conductive composite surfaces showing multifunctional properties exhibit wearable sensor characteristics, sensor behaviors can be investigated in greater detail.   References

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    Erdi Buluş Metallurgy and Materials Engineer Materials Technology Specialist Istanbul Arel University ArelPOTKAM (Polymer Technologies and Composite Application and Research Center)     Gülseren Sakarya Buluş Specialist Nurse Silivri District Health Directorate