Electrospun nanofibers with both surface nanopores and internal interpenetrated nanochannels for oil absorption

Abstract

Polyoxymethylene (POM) nanofibers with both surface nanopores and internal interpenetrated channels have been fabricated by the electrospinning of POM/poly(L-lactide) (PLLA) blends and followed by the solvent etching of the PLLA component in the fibers. With the fast evaporation of the solvent, vapor-induced phase separation between the solvent and the POM/PLLA blends occurs on the surface of the nanofibers, which leads to numerous nanopores on the surface of the as-spun nanofibers. At the same time, POM and PLLA form a co-continuous structure in the fibers and the subsequent etching of the nanofibers by chloroform induces the removal of PLLA and the formation of the internal nanochannels in the fibers. The fabricated novel porous POM nanofibers with both surface pores and internal channels exhibit a much higher oil absorption capacity than both the neat POM nonwoven fabrics and the POM/PLLA blend nonwoven fabrics. The porous POM nonwoven fabrics from POM/PLLA (70/30) has a pump oil absorption capacity of 115.3 g g⁻¹, compared to 75.2 g g⁻¹ for the as-spun POM/PLLA blend and 53.2 g g⁻¹ for the neat POM nonwoven fabric. It was further found that the novel hierarchical porous structure makes the materials be an ideal alternative for both oil absorption and oil/water separation.

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1. Introduction

In recent decades, porous polymers have been received more and more attention because of their potential applications in gas storage and as separation materials, catalysts, filtration/separation membranes,^1–6 absorbents,^4,7–12 templates for structure replication and so on.^13–18 These porous polymeric materials exhibit many advantages, such as high surface area, well defined porosity, processability and diversification. Various methodologies have been developed to fabricate the porous polymers, mainly including “top-down” method (i.e. direct templating and electrospinning)^17,19–22 and “bottom-up” method (i.e. block copolymer self-assembly and polymer blending).^23–25 Polymer self-organizing provides a simple and useful process to make mesoporous or nanoporous polymers, especially for the block copolymers with gyroid morphology and the co-continuous polymer blends.^23–25 In addition, “top-down” approach offers diverse geometrical designs and excellent nanometer-level precision and accuracy. Therefore, when combining “bottom-up” method and “top-down” approach, templated-self-assembly (TSA) provides a precious opportunity for studying the self-assembly behavior in a confined environment and an innovative method for nanofabrication.²⁶ Meanwhile, such TSA strategy may also be applicable for fabricating the novel porous materials with hierarchical porous structures.

Electrospinning is a straightforward and efficient technology to prepare fibers at macro- and nano-scale and porous materials (termed nanofiber mats) can be simultaneously fabricated with the pores between the nanofibers. On the other hand, electrospinning is extremely suitable for the templates which provide confined environment for self-assembly behavior in the very thin fibers.^{9,19–22,27–36} Compared with other direct templates such as anodic aluminum oxide (AAO) template and mesoporous silicas, electrospun fibrous templates can be prepared quickly and cheaply. By choosing the specific block copolymers, the template-self-assembly (TSA) of polymers have been realized in the interior of fibers under the confined environment. For example, Harlin and Ikkala et al. have successfully controlled the internal structure of electrospun fibers by hierarchical self-assembly of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4PVP) with diameter 200–400 nm and a nominal stoichiometric amount of 3-n-pentadecylphenol (PDP).²⁵ The porous electrospun fibers with different pore sizes were obtained through removing PDP. In addition, polymeric electrospun fibers with external or internal porosity have also been reported in literatures by the mechanism of the thermally induced phase separation (TIPS) or vapor-induced phase separation (VIPS).^{9,20–22,30–31,33} For example, Ding et al. successfully fabricated the nanoporous polystyrene (PS) fibers with high absorption capacity, low cost and scalable fabrication via a one-step electrospinning process.²⁹ With controlling the electrospinning conditions (i.e., relative humidity and electrostatic field strength), and the polymer/solvent properties (i.e., copolymer content and vapor pressure of the solvent), Kongkhlang et al. had prepared surface porous poly(oxymethylene) (POM) fibers through electrospinning.²¹ However, these methodologies still have their own disadvantages. Block copolymers used in TSA experiments are often high-cost which are not suitable for scaling up production. And the surface or internal pores of fibers prepared directly through electrospun process are always not interpenetrated which limits the potential applications of such materials.^3–5,9

Recently, we have succeeded in preparing novel three dimensional interpenetrated porous polyoxymethylene (POM) membranes from the melt miscible POM/PLLA blends.^37,39 The POM crystallization induces the segregation of PLLA from the POM lamellae, and the simply solvent etching of PLLA leads to the perfect POM nanoporous materials. Especially, the POM pore structure can be adjusted by both the crystallization conditions and the component ratio of POM/PLLA. We considered that the electrospinning of the POM/PLLA blend would be accessible to fabricate the POM nonwoven fabrics with hierarchical porous structures. On the one hand, the pores with the micro-size may form naturally between the nanofibers. On the other hand, the subsequent PLLA etching process leads to the porous POM fibers following the similar mechanism of the bulk POM/PLLA blends. In this work, we have succeeded in preparing the POM nanofibers with both surface nanopores and internal interpenetrated channels by electrospinning of POM/poly(L-lactide) (PLLA) blends and subsequently the solvent etching of the PLLA component in the fibers. The nonwoven fabrics from the porous POM nanofibers have been used for both oil absorption and oil/water separation.

2. Experimental section

2.1 Materials

The PLLA materials (3001D) used was purchased from Nature Works Co. LLC (USA). The M_n and M_w/M_n were reported to be 89 300 ± 1000 g mol⁻¹ and 1.77 ± 0.02, respectively. The content of D-lactide of PLLA materials was 1.6%. The POM (MC90) samples were provided by Shenhua Co. Ltd, China. The M_n and M_w/M_n were reported to be 174 300 ± 1000 g mol⁻¹ and 2.19. Hexafluoroisopropanal (HFIP) was used as the solvent for dissolving the PLLA and POM without any purification.

2.2 Sample preparation

POM/PLLA solutions with different proportions of 100/0, 70/30, 50/50, 30/70 and 0/100 in HFIP were prepared. The resulting clear and homogeneous solutions were electrospun with voltage of 14 kV. The polymer fibers were collected onto a sheet of aluminum foil with a tip-to-collector distance of 10 cm and a volumetric flow rate of 0.05 mL h⁻¹. The spinning was carried out under a relative humidity of 80–85%. In order to prepare the porous polymeric fibers, the nonwovens were soaked in the chloroform for 12 h.

2.3 Sample characterization

Structure investigation. The surface morphology of the electrospun fibers was observed by scanning electron microscope (SEM). A Hitachi S-4800 SEM system was used for SEM measurements at an accelerating voltage of 2 kV and working distance of 2–5 mm. The phase structure of fibrous cross section was observed directly using a transmission electron microscope (TEM, Hitachi HT7700) at an accelerated voltage of 100 kV. The fibers were embedded in epoxy resin and ultramicrotomed at −120 °C to a section of thickness of about 70 nm followed by ruthenium tetroxide (RuO₄) staining. In order to analyze the external and internal structure of the fibers, the nonwoven fabrics were also stained by RuO₄ for 2 h before embedded in epoxy resin. In addition, FTIR spectra (FTIR, Bruker Tensor) were collected with a resolution of 2 cm⁻¹ by the transmittance mode. X-ray diffraction (XRD) patterns (Bruker-D8) were collected from 2θ = 5–30° at a scanning speed of 2° min⁻¹ with a step interval of 0.02°.

Thermal behavior study. The crystallization and melting behaviors were measured by differential scanning calorimeter (DSC, TA measurement DSC Q2000) at a heating rate of 5 °C min⁻¹ from 0 to 200 °C. The experiments were conducted under a continuously flowing pure nitrogen atmosphere.

Wettability behavior. A DSA 100 machine (Data-Physics, Germany) at ambient temperature was used to test the contact angle of the samples with water and oil. Droplets (about 4 μL) were dropped carefully onto the samples and the average value of 5 measurements performed at different positions on each sample was adopted as the contact angle.

Oil absorption measurement. Absorption tests were performed at ambient temperature. In order to analyze the oil absorption capacity of nonwoven fabrics in oil without water, the samples were placed in a glass beaker of 300 mL oil. After 60 min sorption, the oil was drained for 2 min and the sorbent was weighed. Three types of oils, toluene, pump oil and silicon oil, were employed to investigate the absorption capacity of samples. The oil absorption capacity of samples was determined by the following equation:

q = (m₁ − m₀)/m₀ (1)

where q was the absorption capacity (g g⁻¹), m₁ was the weight of the wet absorbent after 2 min (g), m₀ was the weight of initial absorbent (g).

Procedure of buoyancy test. The test of samples was experimentally simulated according to the published report 40 in a static system. 20 mL pump oil (stained by Tonyred III) was put into a beaker containing 30 mL water. Then 100 mg sample of POM/PLLA (70/30) nonwoven fabrics was placed into the oil/water mixture.

Oil/water separation of a mixture. The as-prepared nonwoven fabrics were used to separate a mixture of water and tetrachloromethane (CCl₄) (v/v, 1/4). And the separation flux of sample was obtained.

Mechanical properties. Tensile tests were carried out using an Instron universal material testing system (Model 5966) with a gauge length of 18 mm and crosshead speed of 2 mm min⁻¹. Dumbbell shaped samples were directly punched out from the fabric mats. Three samples were tested and averaged for each formulation.

3. Results and discussion

3.1 Electrospun POM/PLLA blend fibers

The PLLA/POM blends solution in hexafluoroisopropanal (HFIP) was homogeneous and transparent due to the miscibility between PLLA and POM. The solutions with various blend compositions at constant 8 wt% concentration were easily electrospun into fibers with diameter of less than 1 μm, as shown in Fig. 1. For the neat POM and POM/PLLA blends with less than 30% of PLLA, the diameter along each fiber was not homogeneous with some bulges. With increasing of PLLA content in the blend, the fibers became straight and smooth. On the other hand, numerous holes with the sizes of 20–30 nm were observed on the surface of the fibers for the neat POM and the blends with POM content higher than 30 wt%. Kongkhlang et al. have observed similar holes for the neat POM nanofibers and they attributed the formation of surface holes to the vapor induced phase separation which can be greatly influenced by the relative humidity of the external environment.²¹ Moisture was condensed into the nanodroplets on the surface of the fibers during the evaporation of HFIP. Obviously, the addition of PLLA suppresses such vapor-induced phase separation (VIPS) process and homogeneous solvent evaporation occurs in the blends with the PLLA as the dominant component. Therefore, the surface becomes very smooth without holes for the POM/PLLA (10/90) blend and the neat PLLA. We consider that the nanoholes on the surface will contribute the sorbent performance of the fibers with increasing surface area.^2–5 Therefore, we will focus on the blend fibers with POM ranging from 70 wt% to 30 wt% in the following investigation because the fibers from these blends not only are straight but also have nanoholes on the surface.

Fig. 1 SEM images of electrospun fibers with different POM/PLLA proportions: (A) 100/0 (B) 70/30 (C) 50/50 (D) 30/70 (E) 10/90 (F) 0/100. The scale bar in images is 5 μm.

3.2 Formation of the internal interpenetrated channels in POM nanofibers

In our previous works, we have succeeded in preparing novel three dimensional interpenetrated porous polyoxymethylene (POM) materials from the melt miscible POM/PLLA blends by simply solvent etching of PLLA.^37–39 In this work, chloroform was also used to remove the all PLLA in the nanofibers. Fig. 2 shows the TEM and SEM images of the 50/50 fibers before and after the solvent etching of PLLA. SEM experiments were also carried out to observe the fracture surface of the fiber. The POM was more readily stained by RuO₄ than PLLA, so POM was observed as the dark region in the TEM images. For the as-spun fibers, two features can be observed from the cross-section of fibers in Fig. 2(B). On the one hand, the surface of the cross-section is not even with many concaves, corresponding to the nanopores on the surface of the fibers. This is consistent with the results in Fig. 1 and 2A. On the other hand, the black dots can be seen on the surface of the fibers. It is considered that such black dots are POM rich regions formed during the vapor induced phase separation. For the POM rich blends, the evaporation of HIPF from the nanofibers induces the enrichment of POM on the surface, especially at the interface between the water droplets and the PLLA/POM matrix in Fig. 2(B) and (C). In other words, a “core–shell” structure was obtained via electrospinning where the POM is enriched on the surface of fibers as the shell and the POM/PLLA blend in the internal of fibers as the core. Moreover, it was seen from Fig. 2(A) that the as-spun fibers were solid and had no internal pores, which was further confirmed by the Fig. 2(B). The solvent etching induced the full extraction of PLLA from the fiber by the weight calculation. The PLLA was expelled out of the POM crystal lamellae and a co-continuous structure with PLLA and POM lamellae formed. The extraction of PLLA by the solvent led to the porous POM fibers. This was confirmed from both TEM and SEM images, as shown in Fig. 2(E) and (D). TEM image shows clear white channels with the size of several ten nanometers and these channels were original PLLA before the solvent etching. SEM image indicated the clearer image that the solvent etching results in an interconnected porous POM nanofiber.

Fig. 2 (A) and (D) SEM images of cross section of electrospun fibers of POM/PLLA 50/50 before and after solvent etching process; (B) and (E) TEM images of the cross section of electrospun fibers of POM/PLLA 50/50 before and after solvent etching; (C) TEM images of the surface of the blend fibers of POM/PLLA 50/50. The scale bar in images is 200 nm.

The extraction of PLLA from the fiber was further confirmed by the FTIR, XRD and DSC measurements, as shown in Fig. 3. In FTIR spectra (Fig. 3(A)), the characteristic peaks of carbonyl groups at 1757 cm⁻¹ of PLLA become very weak after etching process, indicating that the PLLA is almost completely removed (from the weight calculation) during the solvent etching. The same results were also seen in XRD curves and DSC curves, after solvent etching treatment of the fibers, neither the crystal peaks of PLLA nor the melting peak of PLLA in first heating cycle were observed (shown in Fig. 3(B) and (C)). In addition, the weight loss calculation of fibers before and after solvent etching was also tested (as shown in ESI Table S1†). It was clear that PLLA and POM are co-continuous and the etching of PLLA induces the interpenetrated channels in the fiber. In other words, the combination of the electrospinning and solvent etching process resulted in the novel nanofibers with both the surface nanopores and the internal interpenetrated nanochannels.

Fig. 3 (A) FTIR spectra of electrospun fibers before and after solvent etching process; (B) XRD curves of electrospun fibers of POM/PLLA 50/50 before and after solvent etching process; (C) DSC curves of electrospun fibers of POM/PLLA 50/50 before and after solvent etching process.

The schematic diagram for the fabrication of fibers with both surface nanopores and internal interpenetrated nanochannels was shown in Fig. 4. The nanoholes on the surface of the fibers can be attributed to the vapor induced phase separation during the evaporation of the solvent after the electrospinning. In the electrospinning process, moisture was condensed on the surface of blend fibers due to the fast evaporation of HFIP before blend fibers were deposited on the aluminum foil. HFIP and water are miscibility, so HFIP would dissolve into the water droplets at the interface between water droplets and PLLA/POM matrix in HFIP, as shown in schematic diagram. Meanwhile, with the evaporation (decreasing) of HFIP in the pre-fibers, the concentration of PLLA/POM matrix in HFIP increased quickly and the nonpolar polymer POM would dissolve out gradually at the interface between water droplets and PLLA/POM matrix because POM had a slightly better compatibility with HFIP compared with the polar polymer PLLA. Finally, POM formed a thin layer at the interface and POM rich part appears as the black dots on the surface of blend fibers and crescent-like black domains on the edge of the cross-section of fibers in TEM images in Fig. 2(B) and (C). At the same time, POM and PLLA formed co-continues structure in the internal part of fibers, as the bulk state that had been reported in our previous works.^37–39 With removing the water droplets by drying, the blend fibers with nanoholes with the sizes of 20–30 nm on the surface were obtained. In the step 4, chloroform was used for the solvent etching to extract the all PLLA in the blend fibers. And the POM nanofibers with both surface nanopores and internal interpenetrated channels were achieved.

Fig. 4 The schematic diagram of pores formation in the electrospun fibers.

3.3 Oleophilic and hydrophobic properties of the electrospun fibers

Fig. 5 shows the contact angle measurement of POM/PLLA electrospun fibers with different PLLA contents in water and oil. The water contact angle of blend fibers was larger than the neat PLLA fibers. The nanoholes structure on the surface of the blend fibers can be contributed to the changes of the water contact angle of fibers. It is seen that the all as-spun nanofiber mats had the water contact angle higher than 130°, indicating the good hydrophobic properties. In addition, the solvent etching of the PLLA led to the further increasing in the water contact angle. The POM/PLLA (70/30) sample after etching had the water contact angle of 140.5°. The more abundant hierarchical structure on the surface of POM porous fibers leads to the changes of water contact angle. At the same time, all the samples exhibit the good oleophilic properties. The oil contact angle of POM/PLLA (70/30) fibers after solvent etching was less than 10° as shown in Fig. 5D. The pores on the fibrous surface and between the fibers endued the nonwoven fabric oleophilic and hydrophobic property, which makes the nonwoven fabric have potential application as oil/water separation materials or absorbents.^2–7

Fig. 5 Water contact-angle images of the electrospun fibers before and after solvent etching: (A) neat POM, (B) and (C) POM/PLLA (70/30), oil contact-angle images of the electrospun fibers after etching: (D) POM/PLLA (70/30); (E) water contact angle curves of POM/PLLA electrospun fibers with different content of PLLA.

3.4 Oil absorption properties of the nonwoven fabrics

The oil absorption capacity of different nonwoven fabrics has been measured and the detailed results are shown in Fig. 6. With increasing the content of POM, the oil absorption capacity of nonwoven fabric increased significantly. The absorption capacity of blend nonwoven fabric increased drastically from 43.1 g g⁻¹ to 129.0 g g⁻¹ when the POM content changed from 30 wt% to 70 wt% in silicon oil. In contrast, the neat POM nonwoven fabrics show the silicon oil absorption capacity of only 76.6 g g⁻¹. The different oil absorption capacities could be ascribed to the fabric surface structures which had been discussed before. In the blend fibers, there were more nanoholes on the fibrous surface which led to bigger specific surface area of nonwoven fabric than neat POM and increased the absorption capacity observably. The nanoholes on the fibrous surface of POM/PLLA (70/30) fibers were denser than neat POM fibers and other nonwoven fabric which had the perfect absorption capacity of 22.4 g g⁻¹, 75.2 g g⁻¹ and 129.0 g g⁻¹ in toluene, pump oil and silicon oil, respectively. The oil absorption capacity of the POM/PLLA (70/30) nonwoven fabric was also tested before and after solvent etching process. In Fig. 6(B), without internal penetrated channels, the absorption capacities of neat POM was 53.2 g g⁻¹ which was bigger than neat PLLA fibers of 23.7 g g⁻¹ in pump oil. However, the absorption capacity of POM/PLLA (70/30) fibers after solvent etching process significantly increased from 75.2 g g⁻¹ to 115.3 g g⁻¹. When the all PLLA was fully removed in the blend fibers, the POM fibers with both surface nanopores and internal interpenetrated channels possessed bigger surface area to absorb the oil, which exhibited excellent oil absorption capacity compared with the materials reported.^7–12,40 In addition, the tensile properties of the nonwoven fabrics before and after solvent etching were shown in Fig. 6(C). The elongation at break of POM/PLLA (70/30) after solvent etching process still kept to be 280%, compared to the blend nonwoven fabric of 540%. Moreover, the modulus of the porous POM nonwoven fabrics was slightly higher than that of the blend nonwoven fabrics before etching. It should be noted that the mechanical properties of the nonwoven fabrics are adequate for the oil absorption or the filter.

Fig. 6 (A) and (B) The oil absorption capacities of electrospun fibers; (C) tensile stress–strain curves of electrospun mats before and after etching with chloroform.

Furthermore, the buoyancy test was also carried out to investigation the oil absorption efficiency of the porous materials. When the oleophilic and hydrophobic nonwoven fabric of POM/PLLA (70/30) was placed in a pump oil–water mixture, it floated on the liquid surface and selectively absorbed pump oil quickly and completely, as shown in Fig. 7. After 150 s, the pump oil had been totally absorbed, indicating the excellent absorption efficiency of the materials. Note that the porous materials exhibit also a good reusability when used for oil absorption (as shown in ESI Fig. S1 and 2†). The absorption capacity of the materials barely changed after 6 times oil absorption.

Fig. 7 Absorption process of pump oil by POM/PLLA (70/30) electrospun fibers (pump oil was stained with Tonyred III).

3.5 Oil/water separation performance of the nonwoven fabrics

We have shown that the POM nanofibers are highly hydrophobic and oleophilic, so one can expect the good oil/water separation performance of the materials. The oil/water separation experiment was carried out in the simple setup, as shown in Fig. 8 and Movie S1 in the ESI.† A 150 mL mixture of tetrachloromethane and water with mass ratio of 4 : 1 was used after sufficient ultrasonic treatment in the experiment. After 180 s, clear tetrachloromethane liquid was obtained from the mixture. Moreover, the nonwoven fabrics membranes exhibited very high separating efficiency with a separation flux of 2390 ± 140 L m⁻² h⁻¹. These results indicate that nanoporous POM fibers are potentially applicable to heavy oil/water separation.

Fig. 8 Oil/water separation study of the POM/PLLA (70/30) nonwoven fabrics.

4. Conclusions

In summary, a facile method was introduced to fabricate POM nanofibers with both surface nanopores and internal interpenetrated channels via electrospinning process and solvent etching of PLLA component. The pore structure and the properties can be adjusted by the variation of the PLLA/POM compositions and the preparation conditions. When used as an absorbent, the fabric mats from the nanoporous fibers exhibited much higher absorption capacity than the neat POM fabric mats and the POM/PLLA blend fabric mats. The 3D interpenetrated nanochannels contributed the extra absorption capacity to the internal porous channels in the POM fibers. This novel interesting structure in the fibers makes an ideal alternative for oil absorption, oil/water separation or catalysis in the future.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21374027, 51173036) and Program for New Century Excellent Talents in University (NCET-13-0762).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00678g

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