Article pubs.acs.org/IECR
Fabrication and Characterization of Conductive Nanofiber-Based Composite Membranes Chitrabala Subramanian,*,† R. A. Weiss,‡ and Montgomery T. Shaw† †
Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States Department of Polymer Engineering, University of Akron, Akron, Ohio 44325, United States
‡
ABSTRACT: Cross-linked electrospun fiber mats of highly sulfonated polystyrene were combined with a polymer matrix to fabricate nanofiber-based composite membranes and characterized. In-plane conductivity of the membrane was controlled by varying the concentration of fiber mats in polymer matrix. Unlike dispersing acidic particles in polymer matrix, the electrospun fiber mats are always percolated; hence, this morphology helps achieve high conductivities across the membrane, and the variation in conductivity with respect to fiber loading is practically linear. The cross-linked fiber mats were subjected to sonication to chop the fiber mat to short fibers that can conceivably be dispersed and aligned in a polymer matrix to achieve bulk conductivity through the membranes.
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INTRODUCTION
is a built-in percolation structure for through-plane conductivity. Electrospun nanofibers of SPEEKK (IEC = 2 meq/g),9 sulfonated poly(arylene ether sulfone) (IEC = 2.5 meq/g),3 and post sulfonated electrospun PS fibers (IEC = 0.24 meq/ g)10 have been used to fabricate nanofiber membranes. The IEC of the polymers used has been much lower than the theoretical sulfonation values for those polymers, largely because of the difficulty of electrospinning polyelectrolytes and the water solubility of noncross-linked high-IEC fibers. Thus, membranes based on such electropsun nanofibers have resulted in membranes with exceedingly low IECs. This paper focuses on fabrication and characterization of electrospun nanofiber-based composite membranes made from highly sulfonated polystyrene (SPS) nanofibers (IEC = ∼4.5). Cross-linked electro-spun fiber mats of SPS were combined with a PDMS matrix to fabricate composite membranes (concept as shown in Figure 1) that were characterized with respect to performance and properties.
Conventional proton exchange membranes (PEM) materials, for example, Nafion, are random copolymers composed of hydrophobic and acid-containing hydrophilic groups. One limitation of the random-copolymer PEM is that the transport and mechanical properties are coupled; hence optimizing one property usually comes at the expense of the other. As a result, during the past decade there has been a renewed effort to develop composite materials that allow one to manage the transport and mechanical properties independently. Some of the approaches that have been explored are block copolymers,1,2 nanofiber-based networks,3 particle-filled systems wherein the particles have very high ion-exchange capacities (IEC)4,5 and blends.6,7 Two important aspects controlling proton transport in a composite membrane comprising acidic particles dispersed in a nonconducting matrix are the concentration of the acid groups at the particle surface that can exchange easily and a path for proton transport between these particles. The preferred composite structure is one where the particles form an interconnected structure in the nonconducting matrix. Higher specific surface area of the particles will increase the number of sulfonic acid groups on the surface. Smaller particle size also means that the acid groups located in the particle are more available when swollen with water. One way to achieve these characteristics is to electrospin ionomer or polyelectrolyte nanofibers and embed the fibers in a suitable matrix.3 In the past, cross-linked sulfonated polystyrene particles have been dispersed in polymer matrixes such as PDMS and lightly sulfonated PEKK to fabricate composite membranes.4,6 If dispersed randomly, the concentration of the particles in the polymer matrix should be higher than the percolation threshold to achieve high conductivity. To achieve percolation at lower concentrations, the particles can be aligned in the polymer matrix using external electric or magnetic fields.8 The advantage of using a nanofiber network over particles is that the fiber mesh is automatically composed of continuous fibers, so there © 2013 American Chemical Society
Figure 1. Concept drawing of nanofiber-based composite membrane.
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EXPERIMENTAL SECTION Materials. Poly(ethylene oxide) (PEO) with Mw = 100,000 g/mol and the sodium salt of sulfonated polystyrene (SPS) with Mw = 500,000 g/mol (Na-SPS-500) were obtained from Polysciences, Inc. and Scientific Polymer Products, respectively. The IEC of the SPS was ∼4.5 meq/g. N,N-dimethylformamide Received: Revised: Accepted: Published: 15088
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Transmission Electron Microscopy (TEM). The morphology of the SPS-PEO fiber mats was examined using a transmission electron microscope (Tecnai G2 Spirit Biotwin). Because the fibers were extremely sensitive to humidity, they were stained using OsO4 vapor instead of the aqueous solutions typically used for staining. The staining process using OsO4 vapor involved mounting the sample onto a 100-mesh Ni folding grid in a glovebag and then placing the grid along with 0.5 g of OsO4 in a desiccator under vacuum for 40 min. The stained samples were analyzed with the TEM using an accelerating voltage of 80 kV. Thermal Analysis. The as-spun fiber mats were also analyzed by DSC. Typically a 5- to 10-mg sample was heated in an aluminum pan in N2 atmosphere after holding the sample isothermally at 30 °C for 15 min. The temperature was then ramped to 130 °C at 20 °C/min. There was no attempt to hold the fibers isothermally at higher temperatures to remove the residual water. X-ray Diffraction (XRD). X-ray diffraction patterns of the asspun and heat-treated electrospun fiber mats were collected using an Oxford diffractometer, XCalibur PX Ultra with an Onyx area detector. The diffraction patterns were obtained at room temperature using CuKα X-ray radiation (λ = 0.15418 nm). Impedance Spectroscopy. Conductivity of the electrospun fiber mats was determined by electrochemical impedance spectroscopy using a Solartron 1260 impedance analyzer over a frequency range of 20−106 Hz. The applied voltage was 50 mV. A custom-built cell, Figure 2, was used to measure the in-plane
(DMF) was obtained from Acros Organics and used as received. The sodium salt of sulfonated polystyrene was converted to the sulfonic acid form with an ion exchange column filled with Dowex Marathon C (Sigma Aldrich Co.). Gelest, Inc. supplied the vinyl-terminated poly(dimethyl siloxane) (DMS V25: M = 17.2 kDa and a vinyl concentration of 0.11−0.13 mmol/g, as reported by Gelest) and the methyl hydrogen polysiloxane cross-linker (HMS-301). A platinum catalyst was also obtained from Gelest Inc. (SIP6830.3). Osmium tetroxide (Stevens Metallurgical) was used for staining the fiber mats for TEM analysis. Membrane Preparation. Polymer nanofiber mats were fabricated by electrospinning. The electrospinning system used a 1-mL Norm-Ject syringe with the tip of the syringe needle (20 Gauge, 12.7 mm length) connected to the positive terminal of a high-voltage DC source (Gamma High Voltage Research, Inc., ES 30R/DDPM). The flow rate at which the solution was dispensed from the syringe was controlled using a syringe pump (KD Scientific, model number 780212). The electrospun web was collected onto a grounded collector and stored in a desiccator.11,12 SPS was coelectrospun with PEO using DMF as the solvent; the electrospun fiber mat was then subjected to heat treatment. At higher temperatures the two polymers react and cross-link the fibers, hence improving its stability in water. The hypothesis is that the PEO is hydrolyzed at higher temperatures to form PEO oligomers with hydroxyl end groups that react with the sulfonic acid groups to form esters. A detailed procedure of the cross-linking process was described in an earlier paper.12 A typical formulation for the PDMS matrix consisted of 0.1 g of methyl hydrogen polysiloxane cross-linker and 9 mg of the Pt catalyst for each gram of DMS-V25. The heat-treated fiber mat was dip coated with the PDMS mixture and squeezed between two glass plates to remove the excess PDMS. This procedure removed trapped air and ensured uniform coating of the fibers with the silicone matrix. Various loadings of the fiber mats were combined with the PDMS formulation. To control the proportion of fiber mats in the composite, the heat-treated fiber mat was pressed using a Carver lab press (model C). A rectangular section of the fiber mat [10 mm × 30 mm (W × L)] was sandwiched between two Kapton polyimide sheets backed by steel plates and pressed with a 70-MPa hydraulic pressure for 20 s. Compacting the fiber mats under pressure increased the number of fibers per unit area of the mat (60 to 90 fibers/100 μm2). The reactive PDMS mixture imbibed into the fiber mat was typically cured overnight at room temperature. The 70−30 w/w SPS-PEO fiber mats were optimal in terms of resistance to breakage on handling and limited swelling on exposure to water; hence, these mats were used to fabricate the composite membranes. Characterization. Water Uptake. The water uptake of electrospun fibers was determined using a TA Instruments Q5000 SA, moisture analyzer. For a typical run, a 7- to 10-mg sample was weighed and transferred into a tared semispherical metal-coated quartz crucible. A sorption−desorption cycle was performed at room temperature by varying the relative humidity from 0 to 90% in steps of 10%. The water uptake at each value of relative humidity was calculated using the formula % water uptake = 100[(Wwet − Wdry)/ Wdry ]
Figure 2. Schematic of the cell used for in-plane conductivity testing.
conductivity. The samples were mounted in the cell and stored in a desiccator. The fiber mats were humidified with water vapor by adding 50 mL of distilled water to the desiccator. The fiber samples were allowed to equilibrate overnight in the tightly sealed desiccator at 98% relative humidity, as measured with a humidity meter placed inside. To avoid drying of the fiber mats, the conductivity measurements were done with the sample and cell within the desiccator. Conductivity of the sample was measured every 5 min initially and then every 30 min until equilibrium was effectively attained. Conductivity, σ, was calculated from the equation
σ = l /RA
(2)
where, l is the distance between the electrodes, R is the resistance, and A is the cross sectional area of the membrane perpendicular to the current flow.
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RESULTS AND DISCUSSION Composite Fabrication. Early attempts to combine the asspun SPS nanofiber mat with a PDMS matrix were
where, Wwet is mass of the of the completely hydrated sample and Wdry is mass of the sample after equilibration at 0% R.H. 15089
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unsuccessful, because the fiber structure was completely lost when the mats contacted the PDMS precursors. This was thought to be the interaction of the fiber with reactive components of the formula, particularly the hydrosilane. When the composite was exposed to water, the fiber remnants leached out completely into the water. In the past, we successfully electro-spun highly sulfonated polystyrene fibers,11 and to improve the stability of the electrospun fibers in water and to fabricate composite membranes, SPS was coelectrospun with PEO and the fiber mats were cross-linked.12 As shown in Figure 3, cross-linking
Figure 4. TEM image of SPS-PEO 70−30 w/w as-spun fiber mat stained with OsO4.
Thermal Analysis. Figure 5 shows that in SPS-PEO fiber mats with lower concentrations of PEO, PEO produce a very
Figure 3. FESEM images of a cross section of a membrane comprising a cross-linked SPS fiber mat within a PDMS matrix.
rendered the nanofibers insoluble, so composite membranes could be fabricated without losing the fiber morphology. Figure 3 is an SEM image of a fractured surface of the composite membrane, showing the cylindrical morphology of the fibers within the composite. Morphology of the SPS-PEO Fiber Mats. Microscopy. The SPS-PEO as-spun fiber mat was examined by TEM to study the details of the fiber morphology in the blend. To improve the contrast between the two phases, the fibers were stained with OsO4 vapor. SPS has a tendency to be stained by OsO4, which was confirmed by EDX analysis of the stained neat SPS and PEO fiber mats. The electrospun SPS-PEO blend underwent phase separation during spinning to generate a core−sheath structure, as shown in Figure 4. SPS forms the outer shell and encapsulates the PEO. The core−sheath structure of the SPS-PEO fiber mats was resolved by staining with a 20-min exposure to OsO4, but longer staining times improved the contrast and were preferred. Core−sheath structures have been observed for electrospinning other polymer blends, for example, polyaniline (PANI)/polystyrene and PANI/polycarbonate.13 During the electrospinning process, the high surface area of the fibers achieved due to the drawing can lead to high solvent evaporation rates. The loss of solvent and possibly the lower temperature favor phase separation in the fibers.14 Sun et al.15 studied the electrospinning of peptide−polymer conjugate and showed that the segment that is more polarizable has a tendency to move to the surface under the influence of an electric field, which in our case would be the SPS. This effect is highly desirable because it promotes the sulfonic acid groups to the fiber surface, which makes them available for ion transport.
Figure 5. DSC scan of heating the as-spun SPS-PEO fiber mat from 30−130 °C @ 20 °C/min.
weak endotherm at ∼60 °C. With PEO concentrations ≥30 wt %, a distinct melting endotherm was observed. This suggests that at lower concentrations of PEO, the blend was miscible, while at higher concentrations of PEO, the fibers were phaseseparated. Although water is a good solvent for both the polymers and preferable for electrospinning, the polymer solution in water electrosprayed into beads instead of spinning into fibers. Hence, dimethyl formamide (DMF) was used as the spinning solvent. XRD Patterns of the Electrospun Fiber Mats. Typically, the working temperature in a fuel cell exceeds the melting temperature of PEO (60 °C). Thus, crystalline PEO in the SPS-PEO mats would melt during operation of the fuel cell, which could cause shrinkage of the fibers and the mats and possibly change the structure of the fibers. The WAXD patterns of as-spun and heat-treated electrospun fiber mats are shown in Figure 6 and Figure 7,12 respectively. The broad peak at ∼17−18° for the as-spun and heat-treated mats corresponds to amorphous polymer. At ≥ 30 wt % of PEO in the fiber mats, the as-spun fiber mats showed two sharp crystalline peaks at 19 and 23° due to PEO crystals, and a broad amorphous peak from SPS, as shown in Figure 6. This agrees 15090
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Figure 8. Water uptake of SPS-PEO heat-treated fiber mats with respect to relative humidity at 25 °C.
Figure 6. WAXD of as-spun SPS-PEO fiber mats.
Conductivity. An example Nyquist plot (imaginary part of the impedance against the real part) of a 70/30 SPS-PEO nanofiber mat containing 50% nanofibers imbedded in a PDMS matrix is shown in Figure 9. The semicircular response results
Figure 7. WAXD of electrospun fiber mats heat-treated at 130 °C.
with the literature for e-spun PEO fibers,16 and to the TEM and DSC data, where signs of phase separation were observed when the PEO content of the mats exceeded 30 wt %. Although the PEO in the as-spun fiber mats had crystallinity, heat-treatment of the SPS-PEO fiber mats eliminated the crystallinity, Figure 7a−d. A heat-treated, neat PEO electrospun fiber mat lost its fiber morphology completely upon heating, but the crystalline peaks in the WAXD pattern, Figure 7e, were preserved. During the heat-treatment, PEO was probably hydrolyzed by the acidic environment at the elevated temperature, which produced PEO oligomers with hydroxyl end groups. The hydroxyl groups reacted with the sulfonic acid groups to form sulfonoesters, which can suppress crystallization of the PEO chains upon cooling.12 Water Uptake. Figure 8 plots the equilibrium water uptake of heat-treated SPS-PEO electrospun fiber mats at 25 °C as a function of relative humidity. The sorption isotherm can be divided into two regions ∼0−50% and >50%. Over the lower range, the absorption follows Henry’s law, and the data in Figure 8 are linear. This is consistent with what is reported in the literature for anionic and cationic membranes.17 At higher relative humidity, water can fill micropores in the membrane and the sorption becomes nonlinear. With increasing weight fraction of PEO in the SPS-PEO fiber mats, the water uptake decreases because of dilution of the hygroscopic SPS fibers with PEO and/or increased cross-linking. Although dimension changes on swelling were not measured, one expects that the major swelling axis will be in the thickness direction.
Figure 9. Nyquist plot of SPS-PEO (70−30 w/w) fibers in PDMS matrix at room temperature.
from observations taken at high frequencies, whereas the more linear response is from the low-frequency data. The membrane resistance (R) is the Z′ intercept of the extrapolated straightline portion of the data.18 Conductivity of the membrane was calculated from eq 2, membrane thickness was ∼0.05−0.1 mm. Since the electrospun fiber mat forms the conductive phase, a higher volume fraction of fiber in the composite will help achieve higher conductivity. Figure 10 shows that conductivity of the composite increased linearly (R2 = 0.95) with increasing mass fraction of fibers. Unlike dispersed nanoparticles in a polymer matrix, the electrospun fiber mats always form a percolated structure because of the high number of contacts for each nanofiber in the mesh, which insures a conductive path between the two electrodes. Hence, ideally there should not be a percolation threshold as with particles, where a change in slope in the conductivity vs particle loading plot is expected.6 Variation in conductivity with respect to wt. fraction of fibers in the composite was practically linear, as shown in Figure 10. Figure 11 is a cross section SEM image of 25 wt % fibers in PDMS matrix tested at 98% humidity. Unlike the as-spun fiber 15091
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present work had high in-plane conductivities, the nanofibers comprising the mats could serve as a source of short rods that could be dispersed in a polymer matrix and aligned to achieve high out-of-plane conductivity. When spherical particles with a higher permittivity than the medium are subjected to an external field, they align and form chains of particles that can serve as a connector within the membrane.6 Likewise, rodshaped particles align in fields and tend to chain in the field direction.19 Short nanofibers have a higher aspect ratio than do spherical particles and approximate a nanorod. Therefore, they are expected to be ideal for achieving out-of-plane conductivity. The formation of rods, the first step in the process, was tested using the cross-linked SPS-PEO (70−30 w/w) fiber mats as a source of nanorods. The nanofiber mats were “chopped” into nanorods by sonification. A 2-mg piece of a cross-linked fiber mat was swollen in 5 mL of distilled water and sonicated for 2 min using a Misonix sonicator (model S 3000) with standard probe. A 3μL aliquot of the freshly sonicated suspension was transferred onto a plasma-treated carbon-coated Cu grid (400 mesh), dried for ∼10 min, and observed with a Tecnai TEM at an accelerating voltage of 80 kV (Figure 12). The number average length of the “chopped” fibers was ∼6 μm, and sample standard deviation based on 165 measurements was 3 μm. One possible mechanism for breakage of these long fibers is sonication induced cavitation. Hydrodynamic stresses acting on the long fibers can potentially cause them to fragment.20 The remaining steps of the process would follow along the lines of the work of Oren et al.8 and Brijmohan et al.6
Figure 10. Plot of conductivity of SPS-PEO (70−30 w/w) fiber mats in PDMS as a function of fiber composition in the composite (25 °C, 98% R.H).
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CONCLUSIONS Nanofiber based composite membranes were prepared by combining cross-linked SPS fiber mats with PDMS matrix and characterized. The conductivity of these membranes was measured by impedance spectroscopy. By varying the weight fraction of fibers in the composite, conductivity of the membrane could be controlled. Unlike particles, the membranes did not have a percolation threshold and the variation in conductivity with respect to fiber loading was practically linear. The cross-linked fiber mats had good in-plane conductivities, ∼0.1 S/cm at 25 °C and 98% R.H. In comparison to films of the same composition, high specific area of the nanofibers allowed for shorter equilibration times. The cross-linked fiber mats could be swollen in water and sonicated to generate short fibers that can potentially be aligned in a polymer matrix to achieve high bulk conductivities.
Figure 11. FESEM image of 25 wt % SPS-PEO (70−30 w/w) fiber mat in PDMS (cross-section), tested at 98% R.H.
mat, the fiber structure was retained even after the exposure to water vapor. Cross-Linked SPS-PEO Fiber Mats as a Source for Short Fibers. SXPLS particles have been previously dispersed in a PDMS matrix to fabricate composite proton-exchange membranes.4,6 Oren et al.8 aligned the particles in the polymer matrix using an external electric field to create ion-conductive pathways in the composite membrane. They showed that membranes with aligned particles had better conductivities and a lower percolation threshold than randomly dispersed particles. Because sulfonated nanofiber mats described in the
Figure 12. TEM image of cross-linked fiber mat sonicated for 2 min (left), distribution in length of short fibers after sonication; log-normal fit (right). 15092
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(18) Soboleva, T.; Xie, Z.; Shi, Z.; Tsang, E.; Navessin, T.; Holdcroft, S. Investigation of the through-plane impedance technique for evaluation of anisotropy of proton conducting polymer membranes. J. Electroanal. Chem. 2008, 622, 145−152. (19) Kanu, R. C.; Shaw, M. T. Enhanced Electrorheological Fluids using Ellipsoidal Particles. J. Rheol. 1998, 42, 657−670. (20) Huang, Y. Y.; Knowles, T. P. J.; Terentjev, E. M. Strength of nanotubes, filaments, and nanowires from sonication-induced scission. Adv. Mater. 2009, 21, 3945−3948.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the Civil, Mechanical and Manufacturing Innovation Program (Directorate of Engineering) of the National Science Foundation, CMMI Grant 1032201.
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REFERENCES
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