Mechanical properties and chemical resistance of electrospun polyterafluoroethylene fibres

Abstract

Porous fibrous polyterafluoroethylene (PTFE) membranes are widely used as high-temperature filters. However, high quality PTFE membranes with excellent mechanical properties and chemical resistance by electrospinning are still required. In this work, pure PTFE fibrous membranes were prepared by electrospinning PTFE emulsion with addition of a very small amount of polyethylene oxide (PEO) followed by a sintering process. The characterization of SEM and TEM indicated that after sintering, the PTFE particles in the fibres melted together and formed smooth fibres with uniform density. Tensile test showed that the membranes sintered at 380 °C possessed the best mechanical properties, much better than those electrospun from a blend of polyvinyl alcohol (PVA) and PTFE emulsion in previous reports. Chemical resistance test indicated that the PTFE membrane had excellent chemical resistance even on immersing in strong base and strong acid at 100 °C for 12 h. These electrospun fibrous PTFE membranes could be promising candidates as high-temperature and chemical resistance filters.

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Introduction

Polyterafluoroethylene (PTFE) is well known for its outstanding thermal stability, chemical resistance, low surface energy and good electrical insulation.^1–5 Porous PTFE membranes have been widely used as high-temperature filters.^5–8 Electrospinning is a fast developing technology to produce porous membranes from polymer solutions, melts and emulsions.^9–11 However, due to the superior solvent resistance and the high melt viscosity,^12,13 it is difficult to directly electrospin PTFE solution or melt into fibrous membranes. Muthiah et al.¹⁴ and Scheffler et al.¹⁵ prepared PTFE fibrous composite membranes by coaxial electrospinning. The porous membranes exhibited superhydrophobicity, which might be good candidates for oil/water separation and membrane distillation.^16–25 However, both reports don't show the mechanical properties and chemical resistance of the fibrous membranes, which could be a potential problem for the applications in the future.^14,15

Another feasible way to get electrospun PTFE fibrous membrane is emulsion electrospinning. Emulsion electrospinning is considered as a green and environment-friendly technology to fabricate fibrous membranes from water based emulsions together with small amount of water soluble polymer as matrix, which could avoid using large amount of toxic organic solvents in the electrospinning process.^26–29 It is widely used to process unspinnable polymer particles into fibres.^30–32 Via simple cross-linking, water treatment or sintering process, these particles can form continuous fibres and further achieve the fibrous membranes. PTFE emulsions are famous milky white dispersions of PTFE particles in water, which have been broadly applied as coating or impregnating agent. However, till now, there are countable literatures regarding using PTFE emulsion for preparation of PTFE fibrous membranes.^8,33 Xiong et al. and Zhou et al. reported the formation of PTFE fibrous membrane by electrospinning the blend of polyvinyl alcohol (PVA) and PTFE emulsion followed with sintering process.^8,33 However, the amount of PVA in pristine fibres was up to 30%, which caused many defects in PTFE fibres during the sintering process and led to the poor tensile strength (<10 MPa) and low elongation at break (<100%) of PTFE membrane.^8,33 In addition, there is also no report regarding the mechanical properties of electrospun PTFE membrane under harsh chemical environment, like in strong base and strong acid.

Ultrahigh molecular weight polyethylene oxide (PEO) is usually used as a sacrificial material to prepare fibres from unspinnable materials by electrospinning.^34,35 It has advantages as sacrificial material like easy removal by water treatment or sintering, very small amount addition in the fibres, etc.^35–38

Therefore, in this work, we aimed to prepare high performance PTFE fibrous membranes with improved mechanical properties and firstly evaluate their mechanical performance under harsh chemical environment. Emulsion electrospinning was adopted and very small amount of ultrahigh molecular weight PEO was used as sacrificial polymer. The fibre morphologies, thermal and mechanical properties of PTFE membranes under different sintering temperatures and harsh chemical environment were detailed investigated by electron microscope, thermal analysis and tensile test.

Experimental

Materials

PTFE emulsion in water (62 wt%) was obtained from Dongguan Dongzhan plastic trade co., LTD. The PTFE particles in the emulsion have an average size of 80 nm. The PEO solution (4 wt%) was prepared by dissolving PEO powder (M_W = 5 000 000 g mol⁻¹, J&K technology co., LTD) in water. The electrospinning solutions (42 wt%) with PEO : PTFE weight ratios of 1 : 99, 1.6 : 98.4, 2 : 98 and 3 : 97 were prepared by mixing PEO solution, PTFE emulsion and water according to Table 1.

Table 1 Preparation and composition of electrospinning solutions

PEO : PTFE (wt : wt)	PEO (4 wt%, g)	PTFE (62 wt%, g)	H2O (g)
1 : 99	2.50	15.97	5.34
1.6 : 98.4	4.00	15.87	3.94
2 : 98	5.00	15.81	3.00
3 : 97	7.50	15.65	0.66

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Emulsion electrospinning and fibre treatment

The electrospinning set-up including a syringe pump, a high voltage supply and stainless steel mesh (400 mesh) as collector. The electrospinning process was performed with an applied voltage, collecting distance and flow rate of 10 kV, 20 cm and 0.5 mL h⁻¹, respectively. The nanofibre membranes were collected by a steel mesh for 8 h. The obtained fibrous membrane was first dried in a vacuum oven at 70 °C for 6 h. The sintering process was carried out in a high temperature tube furnace (Sgl-1200 Tube Type Lab Furnace, Shanghai Daheng Optics And Fine Mechanics Co., Ltd) with temperature error range of ±1 °C. The membranes were sintering at 360, 380 and 410 °C for 10 min to remove PEO and get the pure PTFE fibrous membranes. The schematic for the preparation of PTFE fibres was shown in Fig. 1. 6 mol L⁻¹ of NaOH and 7.14 mol L⁻¹ of H₂SO₄ were prepared as harsh chemical environment for the chemical resistance of the obtained PTFE membrane. The membranes were immersed in NaOH and H₂SO₄ at 100 °C for 12 h. After that, the membranes were washed with distilled water until neutral and dried at 40 °C for 12 h in vacuum oven.

Fig. 1 Schematic preparation of pure PTFE electrospun nanofibers by emulsion electrospinning and sintering.

Characterization

Differential scanning calorimetric (DSC) analysis of PTFE powder (solvent removed) was performed on a NETZSCH 200 F3 thermal analyser in nitrogen atmosphere with heating rate of 10 °C min⁻¹. The PTFE particles for DSC analysis was prepared by drying the PTFE emulsion in vacuum oven at 70 °C for 12 h. Thermal stabilities were examined by thermogravimetric analyser (TGA, Beijing Henven Scientific Instrument Factory, China) in nitrogen atmosphere with heating rate of 10 °C min⁻¹. Scanning electron microscopy (SEM, FEI Quanta 200, USA) equipped with an energy dispersive X-ray spectroscopy (EDS) detector was used to observe the fibre morphology and determine the existence of PEO. Before the scanning, all the samples were coated with 2 nm thickness of gold. Image J software was used to measure the fibre diameter and summarized the distribution of fibre diameters. Transmission electron microscope (TEM, JEM-2100) was applied to study the particle assembly in fibres. X-ray diffraction patterns (CuKα irradiation) were and recorded by Bruker D8 Advance Diffractometer with scanning 2θ range from 10 to 55° and scanning rate of 2° min⁻¹. The mechanical properties were measured by computer-controlled electromechanical universal testing machine (SANS, Shenzhen suns technology stock Co., LTD, China) at a tensile speed of 5 mm min⁻¹. The samples were prepared into rectangle specimen with size of 4.0 × 0.5 cm and the gauge length was set to 10 mm. The thickness of the samples was measured by a screw micrometer. All the samples were tested for 5 times to give the average values. E modulus was obtained from the slope of the stress–strain curves in the initial linear part.

Results and discussion

Effect of PEO : PTFE weight ratios on the fibre formation

In order to get an excellent mechanical performance of PTFE fibrous membranes, the amount of PEO should be kept to a minimum amount and the fibres should be uniform without beads. Four weight ratios (1 : 99, 1.6 : 98.4, 2 : 98 and 3 : 97) of PEO : PTFE were used and led to different fibre morphologies as shown in Fig. 2. When extremely small amount of PEO (1 : 99) was applied, many beads and needle-like short fibres were obtained (Fig. 2a). This morphology could be because the amount of PEO is too small to efficiently binder the PTFE nanoparticles each other into a continuous fibre form. Further increasing the PEO amount led to the decrease of beads and the fibres became continuous and uniform (Fig. 2b–d). When the weight ratio of PEO : PTFE was 1.6 : 98.4, the amount of beads decreased and more continuous fibres were formed (Fig. 2b). Most of the beads were disappeared when further increasing the amount of PEO (2 : 98) (Fig. 2c). With this weight ratio, the obtained PEO/PTFE composite fibres exhibited an average fibre diameter of 1130 ± 430 nm, but the fibres showed a broader fibre diameter distribution from 200 nm to 2200 nm (Fig. 2e). The optimized weight ratio of PEO : PTFE for the preparation of completely beads-free fibres was found at 3 : 97, which is much smaller than the ratio (30 : 70) used in PVA/PTFE electrospinning in the previous reports.^8,33 With 3 : 97 weight ratio of PEO : PTFE, the fibres were beads-free and uniform (Fig. 2d). Compared to the broader fibre diameter distribution with PEO : PTFE weight ratio of 2 : 98, these fibres exhibited a very narrow fibre diameter distribution with average fibre diameter of 1248 ± 127 nm (Fig. 2f). Therefore, the electrospun PEO/PTFE composite fibres and their membranes from the blend with the PEO : PTFE weight ratio of 3 : 97 were used for the following investigations.

Fig. 2 SEM images of electrospun PEO/PTFE composite fibres with different weight ratios, 1 : 99 (a), 1.6 : 98.4 (b), 2 : 98 (c) and 3 : 97 (d) of PEO : PTFE.

Choose of sintering temperatures

Sintering temperature plays an important role on the formation of pure PTFE fibres. TGA curves (Fig. 3a) showed that PEO started to decompose at around 320 °C and the residual was less than 2 wt% at 430 °C. PEO/PTFE (3 : 97) composite fibres showed two weight-loss steps. The first weight loss (4.5 wt%) occurred between 300–430 °C, which was corresponding to the decomposition of PEO (3 wt%) and the residual surfactant from PTFE emulsion. The second weight loss started at 500 °C, which was attributed to the decomposition of PTFE. No weight loss was observed in the temperature range from 430 to 500 °C, suggesting the pure PTFE was thermal stable until 500 °C. DSC analysis (Fig. 3b) indicated that PTFE had a melting temperature of 327 °C. Therefore, the sintering temperature could be in the range of 330–430 °C, in which the PEO was decomposed while the PTFE particles in the electrospun composite fibres could be melt together to form continuous fibres.

Fig. 3 TGA curves of pure PEO powder and PEO/PTFE composite fibres with weight ratio of 3 : 97 (a) and DSC analysis of pure PTFE particles.

Further investigation by EDS was performed to prove the effect of sintering temperature on the composition of the fibres (Fig. 4). Before sintering, three characterized peaks at 0.27, 0.52 and 0.68 keV were the signals from elements carbon (C), oxygen (O) and fluorine (F), respectively (Fig. 4a). After sintering at 360 °C, the signal from oxygen element was disappeared (Fig. 4b), which confirmed the complete removal of PEO by sintering. Therefore, in this study, three temperatures of 360, 380 and 410 °C were chosen as the sintering temperatures for the treatment of electrospun PEO–PTFE composite membranes.

Fig. 4 EDS curves of PEO/PTFE (3 : 97) composite fibres before (a) and after 360 °C sintering (b).

Effect of sintering temperature on the fibre morphology

The effect of sintering temperature on the fibre morphology was monitored by SEM and TEM (Fig. 5). There are no obvious changes on fibre morphology when the sintering temperatures were 360 and 380 °C (Fig. 5a and b). Further increasing the temperature to 410 °C led to the shrinkage of fibres and junction-formation between fibres (Fig. 5c). High magnification TEM images presented more details on the fibre morphologies (Fig. 5b′ and d) before and after sintering. The PEO/PTFE composite fibre without sintering showed very rough surface and inhomogeneous density due to the assembly of 97% PTFE particles (Fig. 5d). After sintering at 380 °C, the PEO was completely removed and the PTFE particles were melt together. Due to the decomposition of very small amount of PEO, the fibre morphology had not changed too much and continuous fibres with homogenous density were formed in the whole fibre (Fig. 5b′), which provided guarantees to the improvement of the mechanical performance. Previous reports by Xiong et al. and Zhou et al. produced pure PTFE fibrous membrane by using 30/70 PVA/PTFE weight ratio and sintering at 390 °C.^8,33 However, due to decomposition of the large amount of PVA during sintering process, their fibres exhibited greatly inhomogeneous in fibre diameters, discontinuous in fibre length and cracks on the fibres, which significantly influenced the mechanical performance.

Fig. 5 SEM images of electrospun PEO/PTFE fibres after sintering at 360 (a), 380 (b) and 410 °C (c), and TEM images of PEO/PTFE fibre before (d) and after 380 °C sintering (b′).

XRD patterns were used to characterize the crystallinity of the electrospun PTFE fibrous membranes before and after sintering at different temperatures. As shown in Fig. 6, all the XRD patterns exhibited four typical diffraction peaks at 2θ = 18.2, 31.7, 37.2 and 41.5°, which were attributed to the crystalline planes of (100), (110), (107) and (108) respectively.^39,40 Among this peaks, the peak at 2θ = 18.2° was considered as the crystallinity phase of PTFE while other three peaks was considered as the amorphous phase and the degree of crystallinity could be calculated by dividing the integrated area of the reflection peak at 2θ = 18.2° from the sum of the integrated areas of the whole four characterized peaks. The corresponding integrated areas of each peaks and the degree of crystallinity of the as-spun PTFE membrane and the membrane with different sintering temperatures were listed in Table 2. The as-spun PTFE membrane had the highest degree of crystallinity (91.3%), which could be due to the inheritance of the assembling of PTFE particles (Fig. 5d). After sintering process, the PTFE particles experienced first melting to form pure PTFE fibres and then cooling down to the room temperature. This process led to the degree of crystallinity decrease about 10%. However, the pure PTFE membranes with sintering process still remained a comparatively high degree of crystallinity, which is benefit to get the high mechanical performance.

Fig. 6 XRD patterns of electrospun PTFE membranes before and after sintering at different temperatures.

Table 2 Integrated area of peaks and degree of crystallinity of electrospun PTFE membranes before and after sintering at different temperatures

Samples	Integrated area of peaks (cps)				Degree of crystallinity (%)
	18.2 (100)	31.7 (110)	37.2 (107)	41.5 (108)
As-spun	299.1	13.6	10.6	4.2	91.3
360	108.9	6.3	9.0	9.2	81.6
380	167	6.1	15.1	14.2	82.5
410	188.8	8.7	17.3	16.2	81.7

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Effect of sintering temperature on the mechanical properties

Although the as-spun PEO/PTFE fibrous membrane has higher degree of crystallinity than the samples after sintering, the as-spun PEO/PTFE composite fibrous membrane without sintering showed so poor mechanical performance that it even could not be measured by tensile testing. This could be explained by the structure and composition of the composite PEO/PTFE fibres. Before sintering, the as-spun fibres contained very small amount of PEO matrix (3 wt%) between the large amounts of PTFE particles (97%) (Fig. 5d). The small amount of PEO was only served as a weak binder to connect the separated PTFE nanoparticles, not a structural material to withstand stretching. Therefore, in this work, we only presented the mechanical properties of the PTFE membranes after sintering process. As shown in Fig. 7 and Table 3, the sintering temperature had significant effect on the mechanical properties of pure PTFE fibrous membranes. When the membrane was sintering at 360 °C, the PTFE membrane showed the smallest tensile strength of 9.7 ± 0.7 MPa, E modulus of 30.7 ± 2.8 MPa and elongation at break of 403% ± 35%, which could be attributed to the incomplete melting of PTFE particles at lower sintering temperature (360 °C) for short time (10 min). The mechanical performance was greatly improved when the PTFE membrane was sintering at 380 °C, where the highest tensile strength, highest E modulus and highest elongation at break were achieved. The three values were 14.5 ± 1.2 MPa, 65.1 ± 5.9 MPa and 657% ± 54%, respectively, which were 49%, 112% and 63% higher than those of sample sintering at 360 °C. Further increasing the sintering temperature to 410 °C resulted in the dramatically decrease of tensile strength and elongation at break. The corresponding tensile strength and elongation at break were 10.4 ± 0.9 MPa and 409% ± 39%, which were 39% and 61% smaller than those sintering at 380 °C. However, the E modulus was still nearly the same as that sintering at 380 °C. This mechanical performance of the PTFE membrane sintering at 410 °C might be due to the high-temperature (410 °C) leading to partial degradation of PTFE macromolecular chain (Fig. 5c). Further explanations on the effect of sintering temperatures on mechanical properties could be explained by the evolution of the structures of the fibres during sintering process. When the PEO/PTFE fibres were sintered at 360 °C for 10 min, the PEO component was decomposed to form micro/nano cavities in the fibres, but the PTFE melts with high viscosity of 10¹¹ Pa s (ref. 41) were not flowable enough to fill the micro/nano cavities in such a short time (10 min).⁸ As the temperature increased to 380 °C, the micro/nano voids from the decomposition of PEO were filled completely by the PTFE melts, and therefore the mechanical properties were improved. Although the decomposition temperature of PTFE was more than 500 °C, the PTFE molecular chain could partially be cut short before 500 °C due to possible thermal degradation, and thus led to the decreased mechanical performance of PTFE fibrous membrane with sintering temperature of 410 °C. In our previous reports, we found that the mechanical properties of electrospun polyimide fibres decreased when curing at higher temperatures although the temperatures were lower than the decomposition temperature of polyimide, which could be attributed to the cleavage of the molecular chains.^42,43

Fig. 7 Typical stress–strain curves of pure PTFE fibrous membranes with different sintering temperatures.

Table 3 Mechanical properties of PTFE membranes with different sintering temperatures of 360, 380 and 410 °C, and chemical treatment of NaOH and H₂SO₄

Samples	Tensile strength (MPa)	E modulus (MPa)	Elongation at break (%)
360 °C	9.7 ± 0.7	30.7 ± 2.8	403 ± 35
380 °C	14.5 ± 1.2	65.1 ± 5.9	657 ± 54
410 °C	10.4 ± 0.9	66.2 ± 4.5	409 ± 39
NaOH	14.5 ± 1.1	65.1 ± 6.2	622 ± 61
H2SO4	14.5 ± 1.3	66.0 ± 5.3	643 ± 67

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The report by Xiong et al. revealed that the pure PTFE fibrous membrane electrospun from PVA/PTFE blend had a tensile strength of 10 MPa and elongation at break of 70% ³³ while similar research by Zhou et al. showed only 1.2 MPa and 40%, respectively.⁸ These values were much smaller than those in our work. The reason could be that in their work too much PVA was applied as sacrificial material, which would lead to many defects/cracks in the fibres, while in our work the amount of sacrificial PEO was only 3%, which could greatly decrease the defects in the fibres.

Chemical resistance

Because of the excellent chemical resistance of PTFE, the electrospun PTFE nanofibre porous membrane would find applications in the strong acidic or alkaline media, such as used as separators in supercapacitors that normally have a strong alkaline⁴⁴ or acidic⁴⁵ electrolyte. Therefore, it would be necessary to test the chemical resistance of the electrospun PTFE nanofibre porous membrane. In this work, the chemical resistance tests of the electrospun PTFE fibrous membranes were performed by firstly treating the membranes with NaOH (6 mol L⁻¹) and H₂SO₄ (7.14 mol L⁻¹) at 100 °C for 12 h and then evaluated their mechanical properties. As shown in Fig. 8 and Table 3, the samples exhibited nearly the same mechanical properties before and after treatment with NaOH and H₂SO₄. In addition, when comparing the morphology with the sample without chemical treatment (Fig. 5b), there was no significant difference of the fibres after NaOH and H₂SO₄ treatment (Fig. 9), which also confirmed the excellent chemical resistance of pure electrospun PTFE fibrous membranes. This excellent chemical resistance of electrospun PTFE membrane could be due to the special chemical structure of PTFE molecules. The PTFE molecular chains can be seen as the result that all of the hydrogen atoms attached on the polyethylene molecular chain backbone are all replaced by fluorine atoms. The larger volume of fluorine atom, shorter length of F–C bond, larger charge repulsion between fluorine atoms make it impossible that the PTFE molecular chain is arranged as a flat zigzag form as a polyethylene chain in space, but only in an elongated spiral (twisted zigzag) arrangement.⁴⁶ In this way, the larger fluorine atoms are tightly in a pile around the C–C chain skeleton. As a result, the helical conformation of PTFE just makes the C–C chain skeleton, susceptible to chemical attack, into a shape close completely “fluorinated” layer of protection, which makes the C–C main chain of PTFE without any invasion of external agents. In addition, the strong and stable chemical bonds of C–F on PTFE and the intermolecular strong interactions between its long and helical (CF₂)_n chains are also the reasons for the superior chemical resistance of PTFE membranes.⁵

Fig. 8 Typical stress–strain curves of pure PTFE fibrous membranes before and after chemical treatment.

Fig. 9 Corresponding SEM photos of the PTFE fibrous membranes with NaOH (a and a′) and H₂SO₄ (b and b′) treatment at 100 °C for 12 h.

Conclusions

Pure PTFE fibrous membranes were successfully fabricated by electrospinning PTFE emulsion with 3 wt% PEO addition followed with high-temperature sintering. The as-spun PTFE fibres composed of particles can form smooth fibres with homogeneous density. PTFE fibrous membrane with best mechanical properties can be obtained by sintering at 380 °C. Due to the much less defects, PTFE fibrous membrane in this work have much better mechanical performance than those in the literatures. Even with strong base and strong acid treatment, the PTFE fibrous membranes can still possess the same fibre morphologies and mechanical properties, which demonstrated the excellent chemical resistance of the membrane. These electrospun fibrous PTFE membranes could open the opportunities for application as filters in high-temperature and harsh chemical environment.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grants No.: 21174058 & No.: 21374044), the Major Special Projects of Jiangxi Provincial Department of Science and Technology (Grant No.: 20114ABF05100) and the Technology Plan Landing Project of Jiangxi Provincial Department of Education (Grant No.: KJLD1104).

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Footnote

† Present address: University of Bayreuth, 95440, Germany.

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