Hydrophilic Nature of Polytetrafluoroethylene through Modification with Perfluorosulfonic Acid-Based Polymers

Abstract

Polytetrafluoroethylene (PTFE), commercially known as Teflon, is a fluoropolymer with a structure containing (CF₂–CF₂)_n. It has high resistance to acids, alkalis and corrosive chemicals. PTFE is hydrophobic in nature with a water contact angle of 140°. Being hydrophobic in nature is a knotty problem, particularly in electrical applications, as it may lead to short circuits and result in reducing the lifetime of electrical equipment. Herein we describe the surface modification of PTFE from hydrophobic to hydrophilic without altering its bulk property. The surface hydrophilicity is achieved by two different techniques, viz., polymer coating (aquivion and nafion) and plasma treatment. Several characterization techniques including FTIR, Raman, XPS, WCA and SEM were used to analyze the surface of PTFE. It was found that 5% of the polymer solution and N₂ plasma treatment for 2 min can produce huge differences in the surface property, as evidenced by the reduction in water contact angle from 140° (neat Teflon) to 80° (surface-modified Teflon). The surface morphology of neat PTFE is completely changed and collapsed as evidenced by the SEM images. The FTIR, Raman and XPS analyses confirm the presence of additional hydrophilic functional groups after the polymer coating and plasma treatment. Hence, this method represents a unique approach to modifying the surface property of Teflon, while maintaining its bulk property.

1. Introduction

The interaction between liquid and solid surfaces creates hydrophobic and hydrophilic surfaces. Two different phenomenon, i.e., wettability and adhesion, govern this surface property [1]. Therefore, controlling the interaction between liquid and solid surfaces effectively is of great importance, as both hydrophobic and hydrophilic materials are used in our day-to-day life activities [2,3]. ‘Supherhydrophobic surfaces’ with lotus effect, repel water droplets and are used as self-cleaning, anti-icing and antibacterial surfaces [4,5,6,7], whereas ‘superhydrophilic surfaces’, with excellent water wettability are used as biocompatible and antifogging materials. In addition to these two surfaces, ‘gecko surfaces’, also called as ‘adhesive hydrophobic surfaces’ with a high contact angle that can pin water droplets to the surface are used to transfer small volumes of liquid from one place to another [8,9,10].

Polytetrafluoroethylene, commercially known as ‘Teflon’, is a fluoro-carbon polymer possessing the superhydrophobic property with a water contact angle >140°. It is used as a low-cost superhydrophobic layer for electro-wetting on dielectric applications. Other commercial applications of PTFE include textiles, household items, sealing, electronic applications, environmental protection, etc. This is attributed to its remarkable property of withstanding harsh environments, as it shows high resistance towards acid, alkali and corrosion. Nevertheless, in electronics, the porosity of the PTFE membrane can cause electrical breakdown, leading to short circuits and can end-up shortening the lifetime of the material. So, the ability to tune the surface of PTFE while retaining its bulk property is desirable to extend the lifetime of electronic materials [11,12,13]. There are several methods available to tune the surface of PTFE from hydrophobic to hydrophilic that include both chemical and physical methods. Chemical methods involve treatment with hydrophilic materials through dip coating, spin coating or spray coating, whereas physical methods involve plasma treatment, e-beam irradiation and so on. It should be noted that when adopting any of these fabrication techniques, there should be precise control over the shape and size of the micro and nano particles on the surface, as these are the governing factors for producing hydrophobic/hydrophilic surfaces [14,15,16,17].

Among the physical methods, plasma treatment is an effective and reproducible method for modifying the surface property of PTFE without altering its bulk property. By selecting suitable gases, such as oxygen, nitrogen and hydrogen, in plasma treatment, hydrophilic surface functional groups can be incorporated effectively, which brings about the wettability of the surface. Moreover, plasma treatment can bring about these surface changes within a very short duration of time (i.e., <2 min) [15]. In the case of chemical treatment methods, the choice of hydrophilic materials used for coating the PTFE surface plays an important role. Generally, perfluorosulphonic acid (PFSA)-based polymers can effectively induce the hydrophilic property when coated on PTFE, as they contain fluoro groups as a hydrophobic backbone and sulphonic acid groups as hydrophilic sidechains. It is believed that the hydrophobic groups get attached to the PTFE surface, leaving the hydrophilic sulphonic acid groups protruding outside, resulting in hydrophilicity [18,19,20,21,22,23,24,25,26,27].

Hence, in this work, we adopted both the physical (plasma treatment) and chemical (dip coating) methods to modify the surface of PTFE from hydrophobic to hydrophilic. This work is an extension of our previous work [28], where we used only plasma treatment to bring about hydrophilicity in PTFE. We used different gases (N₂, O₂ and Ar+H₂) as active gases with different plasma durations and found that using N₂ gas for 2 min is effective in producing hydrophilic surfaces. We were successful in reducing the hydrophobicity of PTFE by decreasing the water contact angle from 140 to 80°. However, we found that the produced hydrophilic surface was not stable for a longer period of time. Therefore, in this work, two different PFSA-based polymers, Nafion and Aquivion, were coated separately onto PTFE through dip coating and the plasma treatment was carried out on these surfaces, to further reduce the contact angle and to maintain the hydrophilic property for a longer duration. All characterizations pertaining to the work are well studied and explained in detail.

2. Materials

The PTFE sheet roll (thickness = 0.08 mm and length = 20 cm) was purchased from Flontec Co., Ltd., Incheon, South Korea, and the different gases, including Ar and N₂, were supplied from Korea Standard Gas (KSG), Gyeongsan, Republic of Korea. Aquivion D72-25BS (PFSA eq. wt. 720 g/mole SO₃H, liquid, dispersion, 25% in water, stabilized CF₃ polymer chain ends) was purchased from Sigma Aldrich, Seoul, Republic of Korea and Nafion D-520 dispersion (5% w/w in water and 1-propanol, ≥1.00 meq/g exchange capacity) was purchased from Alfa Aesar, Incheon, Republic of Korea. Iso-propanol was purchased from Daesung Chemicals, Hwaseong-si, Republic of Korea. All the chemicals and solvents were used without further purification.

3. Methods

3.1. Preparation of Coating Solutions

Two different coating solutions were prepared using aquivion and nafion. The as-received aquivion and nafion solutions were used as the stock solutions. A 5% aquivion solution was prepared in a mixture of solvents, iso-propanol and DI water in an 8:2 ratio. In a similar way, a 5% nafion solution was prepared using iso-propanol and DI water. These 5% solutions were used as the coating solutions.

3.2. Modification of PTFE

Modification of PTFE was carried out by two different processes. The modification of PTFE by aquivion and plasma treatment process was as follows: At first, the PTFE sheet with dimensions of 10 × 10 cm was taken and dipped into the 5% aquivion solution in a large Petri dish. Proper precautions were taken to place the PTFE into the dish without any folding and to make sure that the entirety of the PTFE was wetted with the aquivion solution. When it was fully wetted, the PTFE looked transparent. After 10 min of dipping in the aquivion solution, the PTFE was taken out carefully and was spread over a flat base and dried in the open air. As it dried, the transparent PTFE regained its previous texture and color. After complete drying, the PTFE underwent the second treatment, i.e., N₂ plasma treatment.

For the N₂ plasma treatment [4], two different gases were used: Ar gas was used as the carrier gas and N₂ gas was used as the active gas. From our previous work, we found that using N₂ as the active gas for 2 min of plasma treatment was successful in creating a hydrophilic surface. So we followed a similar plasma treatment procedure here. The dried PTFE was placed carefully on the sample holder of the plasma equipment. Pressure was applied so that the PTFE stuck properly to the sample holder. The flow rate of the carrier gas and active gas was maintained at 4.5 L/min and 20 mL/min, respectively. Four different plasma duration times were followed (30, 60, 90 and 120 s) (Scheme 1). Soon after the plasma treatment, the modified PTFE was packed into a zipper bag to avoid any atmospheric contact. A similar procedure was followed for PTFE modified with nafion and plasma treatment. The samples were packed properly and analyzed for several characterizations.

3.3. Instrumentation Methods

An attenuated total reflectance fourier transform infrared spectrometer ATR FT-IR, MB3000, from Perkin Elmer was employed to acquire the FT-IR spectra. The infrared spectra was obtained in the range 400–4000 cm⁻¹. The neat PTFE and plasma treated PTFE were placed on the sample disc and the spectra were recorded. For each sample, the spectrum was recorded at three different spots on the PTFE to observe the uniformity of the sample. The Raman spectra were recorded on an XploRA Micro-Raman spectrophotometer (Horiba, Palaiseau, France) with a range of 500–4000 cm⁻¹. In order to obtain high-resolution XPS spectra, K-Alpha (Thermo Scientific, Waltham, MA, USA) was utilized. CasaXPS software (2.3.22PR1.0) was used for deconvolution of high-resolution XPS spectra. As the field emission scanning electron microscopy (FESEM, Urbana, IL, USA) with energy-dispersive X-ray spectroscopy (EDS), high-resolution transmittance electron microscopy (HRTEM, Tokyo, Japan) (FESEM, Hitachi S-4800), at an accelerating voltage of 10 kV, was employed to examine the morphology of the neat PTFE and aquivion and nafion-modified PTFE. The working distance was about 8–9 mm and the SEM images were taken at different magnifications from 1 to 10 k. A Dataphysics Instrument OCA 20 model (Filderstadt, Germany) was used to determine the water contact angle of the neat PTFE and the modified PTFE. The PTFE membrane was placed horizontally on the glass slide and around 2 µL of DI water was dropped onto the membrane using a micro-syringe. The measuring range of the instrument is 0 to 180°. For each sample, the contact angle measurement was taken in five different areas and the average value was noted.

4. Results and Discussion

4.1. FT-IR Analysis

During the treatment process of PTFE with aquivion and nafion, these groups get adsorbed on the surface of the PTFE and further, upon N₂ plasma treatment, fluorination occurs with the formation of C-N bonds making the surface hydrophilic. The changes that occurred on the PTFE when treated with aquivion and nafion, along with N₂ plasma treatment, were identified with FTIR analysis. Figure 1 depicts the FTIR spectra of neat PTFE and PTFE modified with aquivion and N₂ plasma treatment. As can be seen from the spectra, the neat PTFE shows the asymmetric and symmetric vibrations of C-F at 1205 and 1149 cm⁻¹, respectively. In addition, the bending vibrations of CF₂ were found at 638 cm⁻¹ due to rocking; at 525 cm⁻¹ due to deformation; and at 501 cm⁻¹ due to wagging. The modified PTFE shows many additional bands due to aquivion coating and plasma treatment. In addition to the stretching and bending vibrations of CF₂, the spectrum shows a broad band at 3600 cm⁻¹, denoting the -OH stretching vibrations. The asymmetric and symmetric stretching vibrations of C-H were found at 2923 and 2852 cm⁻¹, respectively. Moreover, the moisture adsorbed on the surface of the PTFE shows a broad band with less intensity around 1600 cm⁻¹. The vibrations of the sulfonyl group (S=O and S-O) and carbonyl group (C-O-C) from the aquivion gave sharp peaks at 1200 and 1150 cm⁻¹ and at 970 and 780 cm⁻¹, respectively [3,7]. In a similar way, modification of PTFE with nafion and N₂ plasma treatment gave certain changes in the FTIR spectra and are represented in Figure 2. When looking into the structure of aquivion and nafion, both have a similar structure, in which nafion has additional -O-CF₂-CF(CF₃)- units, when compared with aquivion. Therefore, similar changes in vibrations were observed for nafion-treated PTFE, where only a high-intensity sharp peak was observed for C-O-C vibrations around 970 cm⁻¹, indicating an increased number of C-O-C bonds. It is evident from both of these coatings (with aquivion and nafion) that the N₂ plasma treatment duration of 2 min was effective, as a shorter duration time results in fewer intensity peaks.

4.2. Raman Spectroscopy

The stretching vibrational modes of neat PTFE and aquivion- and nafion-treated PTFE were analyzed by Raman spectroscopy and are represented in Figure 3. The PTFE shows four different vibrational modes. The asymmetric and symmetric stretching modes of CF₂ were found at 1372 and 726 cm⁻¹, respectively. The C-C stretching mode shows a broad band centered at 1638 cm⁻¹. The band at 3494 cm⁻¹ is due to the absorbed-moisture peak of PTFE due to storage. All of these vibrational modes indicate that C-F and C-C are the predominant bonds present in PTFE. When the PTFE was modified with PFSA-based polymers and N₂ plasma treatment, several changes in the vibrational modes could be observed. The vibrational modes associated with the backbone structure were observed at 744 and 1390 cm⁻¹ for aquivion-treated PTFE, and at 746 and 1419 cm⁻¹ for nafion-treated PTFE, corresponding to the symmetric and asymmetric stretching modes of CF₂ bonds. Moreover, the sidechain vibrations due to the aquivion coating, nafion coating and N₂ plasma treatment were found at 1801 and 1813 cm⁻¹, corresponding to C-O-C and S-O stretching modes; at 2644 and 2643 cm⁻¹, corresponding to C-H stretching modes; and at 3389 and 3393 cm⁻¹, corresponding to S-OH stretching modes [29,30]. All these vibrational modes prove that the surface of the PTFE was modified to a greater extent by the PFSA polymer coating along with N₂ plasma treatment. Moreover, these vibrations are in good agreement with FTIR stretching vibrations.

4.3. Water Contact Angle Analysis

The hydrophobic or hydrophilic nature of the material can be determined by analysis of its water contact angle. In general terms, the material’s surface is categorized into four different forms, based on the angle formed by the water on the surface of the material. A material is said to be either superhydrophilic (θ = 0°); hydrophilic (θ < 90°); hydrophobic (θ > 90°); or superhydrophobic (θ = 180°). Figure 4 shows the contact angle values, their images and the atomic ratio of each of the elements. As can be seen from Figure 4A,B, the neat PTFE is hydrophobic in nature and has a water contact angle of 141°. The PTFE modified with aquivion and N₂ plasma treatment shows reduced water contact angle values of 112.2, 107.1, 85.6 and 80.5° for different N₂ plasma durations of 30, 60, 90 and 120 s, respectively. Similarly, the PTFE modified with nafion and N₂ plasma treatment also shows reduced water contact angle values of 119.5, 108.2, 93.1 and 83.5°, respectively, for the 30, 60, 90 and 120 s plasma durations. The initial contact angle of the PTFE was 141°, exhibiting a hydrophobic property, whereas the PTFE treated with a polymer coating and plasma treatment showed a lower water contact angle of 80.5° or 83.5°. This behavior could be attributed to the fact that PTFE, which is originally hydrophobic in nature, becomes hydrophilic due to the polymer coating and a very short N₂ plasma treatment, accompanied by the incorporation of hydrophilic groups like –SO₃H, C-O-C and –OH on the surface of the PTFE. It was observed that a N₂ plasma duration of 2 min (120 s) for both aquivion- and nafion-coated PTFE was effective in modifying the surface of PTFE from hydrophobic (θ = 141°) to hydrophilic (θ = 80.5° for the aquivion coating and θ = 83.5° for the nafion coating) [31,32,33,34,35,36,37]. Moreover, N₂ plasma duration of more than 2 min was not effective, as it resulted in damaged PTFE and a change in color that could be observed with the naked eye. Our previous work also shows that the N₂ plasma treatment for 2 min was effective in reducing the hydrophobic nature of the PTFE. The coating with aquivion and nafion introduces the sulfonic acid group, which is hydrophilic in nature, onto the surface of PTFE. This coating, along with the N₂ plasma treatment results in terms of defluorination and hydrogen absorption, results in reduced water contact angle values.

The atomic ratios of carbon, fluorine and oxygen are given in Figure 4B. The bar diagram shows that, in neat PTFE, the atomic ratio of fluorine is twice that of carbon with a very minimal amount of oxygen. With respect to the formula, -(CF₂-CF₂)_n-, one carbon atom should be accompanied by two fluorine atoms. The oxygen atom results from atmospheric moisture. In the case of the modified PTFE, both with an aquivion and nafion coating, a decrease in the fluorine content and increases in carbon and oxygen contents were observed. This confirms that defluorination occurs with N₂ plasma treatment, and that coating with aquivion and nafion results in increased carbon and oxygen content.

4.4. XPS Analysis

The composition of elements in each material is determined by XPS analysis and represented in Figure 5 and Figure 6. Figure 5 shows the survey spectrum of neat PTFE and modified PTFE. As can be observed from the spectrum, carbon and fluorine are the predominant elements present in neat PTFE, whereas the PTFE modified with both aquivion and nafion shows visible peaks for oxygen in addition to carbon and fluorine atoms [38,39,40]. To arrive at a clearer idea about their composition, the XPS spectrum was deconvoluted and represented in Figure 5. For the neat PTFE, the C 1s spectrum is deconvoluted into four different peaks at 292.18, 292.72, 291.08 and 284.88 eV, corresponding to CF₂, CF, CF₃ and C-C/surface adsorbed C-O, respectively (Figure 6a), whereas both of the modified PTFEs show additional peaks between 286 and 284 eV, corresponding to the binding of C-S, C-O-C and C-C/C-H groups (Figure 6b,c). The O 1s spectrum is deconvoluted into two different peaks for the neat PTFE, at 532.08 and 533.38 eV, corresponding to the surface adsorbed oxygen atoms (Figure 6d). The modified PTFE shows additional deconvoluted peaks at 534 and 532 eV, corresponding to the binding energy of S=O/S-O and O-H groups, respectively (Figure 6e,f). Similarly, the F 1s spectrum for the neat PTFE is deconvoluted into three different peaks at 689.48, 689.08 and 689.78 eV, corresponding to the binding energies of the F₂-C, F-C and F₃-C groups [41,42,43,44,45,46], respectively (Figure 6g). For the modified PTFE (Figure 6h,i), there are no additional deconvoluted peaks, but the intensity of the F₂-C peak increased indicating the incorporation of the aquivion and nafion structures.

4.5. SEM Analysis

The morphology of neat PTFE and modified PTFE was investigated by FE-SEM measurements. Figure 7 and Figure 8 display the SEM images of neat PTFE and PTFE modified with aquivion and nafion. The neat PTFE membrane (Figure 7a) shows several reticular fiber-like nodule structures, in which the fibers are aligned in a uniform order and connected to another fiber through this nodule. It can also be observed that, from each nodule, several fibers arise that are connected together. This reticular fiber–nodule structure is a result of the melt-stretching membrane fabrication technology adopted to fabricate the PTFE, and the hydrophobic nature of PTFE is expected to be from this structural contribution in addition to –CF₂ groups. The modified PTFE showed drastic changes to the surface morphology. At first, the alignment of the fiber structure was completely collapsed, and at several places the fibers were torn and broken fibers are clearly visible. Therefore, we can confirm that coating PTFE with aquivion/nafion and plasma treatment brings drastic changes with respect to the PTFE’s surface morphology. The EDX spectrum shows the composition of each element present in the PTFE. It can be observed that C and F are the predominant elements present in all samples. The modified PTFE shows the presence of oxygen species in addition to carbon and fluorine atoms [47,48,49,50,51].

4.6. Stability of the Surface Treatment Process

The stability or sustainability of the process (aquivion or nafion coating and the N₂ plasma treatment process) on the PTFE surface was analyzed. The PTFE modified with aquivion and nafion showed the lowest contact angle of 80.5 and 83.5° with a N₂ plasma treatment duration of 2 min. So, both of these modified PTFE membranes were preserved for 1 week and the water contact angle was checked again. Both of the membranes showed a contact angle of 82.5 and 85.4°, indicating a slight increase in the contact angle values as evident in Figure 9. It was observed that even after several days, the hydrophilic nature is maintained, indicating a stable hydrophilic coating on the surface of the PTFE.

4.7. Bulk Property of Modified PTFE

To analyze whether the plasma treatment and chemical treatment processes made any changes to the bulk property of the materials, the untreated PTFE, PTFE-Aq-120s and PTFE-Naf-120s samples were subjected to through chemical treatment procedures. Two different environments were created, an acidic environment (2N HCl) and a basic environment (2N NaOH). The three samples were immersed in both the solutions separately for 12 h and then completely dried at 50 °C. The images of the samples during immersion and after drying are displayed in Figure 10. Generally, PTFE shows high resistivity towards chemical environments (both acidic and basic). As is evident from the figures, it can clearly be seen that the treated PTFE samples also show high resistance towards acidic and basic environments (Figure 10c–f,i–l). Therefore, it is clear that both of the treatment processes bring about changes in the surface property, while maintaining the bulk property of the material.

5. Summary and Conclusions

Tuning the surface of PTFE from hydrophobic to hydrophilic in order to broaden its application has been brought about by combining both chemical and physical processes. The chemical method includes coating PTFE with aquivion/nafion and the physical method includes the N₂ plasma treatment process. Two different modified PTFEs was fabricated: one with aquivion and plasma treatment and the other with nafion and plasma treatment. It was found that a shorter plasma duration of 2 min was effective in producing hydrophilic surfaces. The water contact angle of neat PTFE was drastically reduced upon modification [from 140 to 80.5° (for the aquivion coating) and to 83.5° (for nafion coating)]. The coating and plasma treatment process confirms the presence of additional functional groups, such as sulfonic acid groups and C-H and O-H groups, as evidenced by the FTIR results. XPS analysis also shows additional peaks, due to the binding energies of the C-O-C, C-H, S=O/S-O and O-H groups. The surface morphology also gets disturbed by these processes. The uniform fiber like structure of neat PTFE is completely changed upon modification, as evidenced by the SEM images. All these results indicate that the combination of both of these methods paves the way for PTFE surface tuning. This strategy could be adopted for binding PTFE with other polymeric, inorganic or biomaterials, so as to widen their application in several fields.