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Porous nano- and microfibrous polymeric membrane material for catalytic support
chemical engineering research and design 8 9 ( 2 0 1 1 ) 621–630
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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Porous nano- and microfibrous polymeric membrane material for catalytic support R.S. Barhate a,∗ , S. Koeppl b , S. Ramakrishna a,∗ a
NUS Nanoscience and Nanotechnology Initiative, National University of Singapore, Faculty of Engineering, 2 Engineering Drive 3, Singapore 117576, Singapore b Mechanical Engineering, Technical University of Munich, Munich, Germany
a b s t r a c t High surface area is essential for attachment of functional groups, ions, moieties and nanoparticles. Surface area of fibrous membrane can be enhanced by reducing the fiber diameter or producing the porous fibers. Flow properties of the fibrous membrane can be improved by placing the fibers apart in the fibrous network. By electrospinning, it is feasible to produce the fibrous membrane of specific surface area and Darcy permeability higher than 60 m2 /g and 1 × 10−11 m2 , respectively. The interconnected irregular shape mesopores (2–50 nm) within the fibers increase the accessible surface area. On the other hand, presence of macropores (pores larger than 50 nm) largely increases the pore volume (porosity) in fibers and helps to reduce the diffusion resistances. Beaded fibers in the membrane can be used to reduce the bulk transport resistances. We developed a method for incorporating the mesopores and macropores in the nano/microfibers made of engineering plastics. To achieve ∼60 m2 /g specific surface area by reducing the fiber diameter, one needs to draw the fibers down to 50–60 nm. In present study, 60 m2 /g of specific surface area is achieved through the porous fibers of average diameter of 900 nm. A specific surface area result from the porous fiber is much higher than one can achieve by reducing the diameter of fibers. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Electrospinning; Nanofiber; Fibrous membrane; Surface area; Porous material
1.
Introduction
Nanofiber research is getting momentum to produce new nanomaterials having high surface area and unique functionality. Wide range of polymers are being electrospun from the solution and melt phase into nano/microfibers in pursuit of improved thermal, mechanical, physical (structural), chemical and biological properties. Nanofibrous membranes are finding applications in filtration and catalysis. A few reported applications of nanofibrous membranes are surface and depth filters for air purification (Barhate and Ramakrishna, 2007; Barhate et al., 2008), antimicrobial filter (Lala et al., 2007), respiratory particulate filter (Barhate et al., 2007), microfiltration membrane (Aussawasathien et al., 2008), support for interfacial polymerized ultrafiltration and nanofiltration membranes (Tang et al., 2009; Yoon et al., 2009), distillation membrane (Feng et al., 2008), affinity membrane for highly selective purifications (Ma et al., 2006), membrane for ion exchange sep-
∗
aration (An et al., 2006) and catalysis (Ramaseshan et al., 2006; Sun et al., 2007). There are seven major processes for producing nanofibers on mass scale, which include electrostatic spinning or electrospinning (Luzhansky, 2003), melt electrospinning (Lyons et al., 2004), modified melt spinning (Ward, 2001), electroblowing (Armantrout et al., 2007), splitting of jet of melt/solution (Gerking and Stobik, 2007) and centrifuge spinning (Dauner, 2006). Electrospinning is being used extensively at laboratory and industrial scale for production of nano- and microfibrous membranes. Donaldson company has used? the nanofiber production (nanofiber coating) line of 10,000 m2 per day for the past 20 years (Luzhansky, 2003). Nano- and microfibrous membranes have several unique properties such as high surface area, flexibility, superior directional strength, good interconnectivity of pores and potential to incorporate the active chemistry or functionality on nanoscale. These properties make the nano- and microfi-
Corresponding authors. Tel.: +65 65164266; fax: +65 68725563. E-mail addresses: [email protected] (R.S. Barhate), [email protected] (S. Ramakrishna). Received 3 March 2009; Received in revised form 7 September 2010; Accepted 10 September 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.09.007
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Nomenclature Acs BD C CM D M N Q (%) QD QL Qw
R S St T Vm W Wm Z d P rp rk t ˛ ˇ
cross-sectional area for heaxagonal close˚ 2) packed nitrogen monolayer at 77 K (16.2 A Darcy permeability (m2 ) BET constant (indicative of adsorbent/adsorbate interactions) constant (2.861 when P is taken in Pa) pore size distribution molecular weight of adsorbate Avagadaro’s Number (6.023 × 1023 molecules/mol) percentage filter flow (%) fluid flow through dry filter (subscript “h” and “l” denotes high and low pressure limits) (L/min) percentage filter flow of previous value (%) fluid flow through wet filter (subscript “h” and “l” denotes high and low pressure limits) (L/min) gas constant (8.314 × 107 ergs/K/mol) specific surface area (m2 /g) total surface area (m2 ) boiling point of nitrogen (77 K) molar volume of liquid nitrogen (34.7 cm3 /mol) weight of gas adsorbed at relative pressure of P/P0 (g) weight of adsorbate constituting monolayer surface coverage (g) thickness of the membrane (m) pore diameter (m) pressure drop across the filter membrane (Pa) mesopore radius (Å) Kelvin radius (radius of pore in which condensation occurs at relative pressure of P/P0 ) (Å) thickness of the adsorbed layer (Å) viscous term coefficient (m−2 ) inertial term coefficient (m−1 ) surface tension of nitrogen at its boiling point (8.85 ergs/cm2 at 77 K) density of fluid (kg/m3 ) surface tension of wetting solution (Galwick 0.0159 N/m) velocity of fluid flowing through membrane (m/s) viscosity of the fluid (kg/m s)
brous membranes suitable for catalytic filtration application. Fibers having the diameter less than 1000 nm are designated as nanofibers; whereas the fibers having the diameter more than 1000 nm are designated as microfibers in this investigation. There are a few attempts to prepare porous nano/microfibers (Bognitzki et al., 2001a,b; Han et al., 2005; MacCann et al., 2006; Moon et al., 2008; Qi et al., 2009). Many techniques have been used to prepare the porous fibers such as the introduction of dispersed phase morphologies in the spun fibers followed by a selective extraction of dispersed phase morphologies from the fibers (Bognitzki et al., 2001a; Moon et al., 2008), electrospinning a polymeric solution into cryogenic liquid bath followed by vacuum drying (MacCann et al., 2006), phase separation due to rapid evaporation of solvent during electrospinning (Bognitzki et al., 2001b; Han
et al., 2005) and preferential evaporation of solvent from a ternary system during electrospinning process (Qi et al., 2009). The most of these attempts are either centric to hydrophilic or water soluble polymeric materials, which are not suitable for catalysis and catalytic filtration applications. Furthermore, none of the above approach has shown the specific surface area more than 15 m2 /g. There exists a need to prepare porous fibers by easy means from the engineering plastics and hydrophobic water insoluble polymeric material. Engineering plastics and hydrophobic water insoluble polymeric materials [such as polysulfone (PSU), polyvinylidine difluoride (PVDF), polystyrene (PS), polyetherketone (PEK), polyetheretherketone (PEEK), polyamide (PA), polyimide (PI), polyphenylene oxide (PPO), polyethylene terepthalate (PET), polycarbonate (PC)] which are stable in acidic, basic and hot gaseous environment would be useful for catalysis and catalytic filtration applications. The main focus of this investigation is to develop a simple method for preparation of porous nano- and microfibrous membrane made of engineering plastics such as polysulfone. This study also includes the characterization (surface, structural and flow properties) of porous nano- and microfibrous membrane.
2.
Theoretical aspects
2.1. Surface properties of nano- and microfibrous membrane The interconnected irregular shape mesopores (2–50 nm) within the fibers increase the surface area of the fibrous materials. On the other hand, presence of macropores (pores larger than 50 nm) largely increases the pore volume (porosity) in the fibers. Nitrogen gas adsorption [the Brunauer–Emmett–Teller (BET) method] is a method of choice for characterization of porous materials to obtain specific surface area and pore volume distribution (Brunauer et al., 1938). Adsorption of nitrogen at its boiling point is a useful tool for characterization of materials having large micropores, mesopores and a wide pore size distribution and where the surface layering and capillary condensation are part of the adsorption process (Naguyean and Do, 2000). BET method is used to estimate the specific surface area. BET equation requires a linear plot of 1/[W(P0 /P) − 1] vs. P/P0 using nitrogen as the adsorbate. Plot is restricted to a limited region of the adsorption isotherm usually P/P0 in the range of 0.05–0.35 (Operating manual, Quantachrome Corporation). In case of microporous materials, the linear region of the plot gets shifted to a lower relative pressure side. BET equation used to calculate specific surface area is given below. 1 1 C−1 P = + Wm C Wm C P0 W[(P0 /P) − 1] Wm can be calculated from the slope and Y-intercept of the BET equation. Specific surface area is calculated from “Wm ” by using the following equation. S=
1 St = W W
W NA m cs M
Size and shape of pores determine the pore filling in the BET method. In case of micropores, the pore filling occurs in a continuous way. In case of mesopores, the pore filling occurs by condensation process (reflecting the first order gas–liquid phase transition). The Barrett, Joyner and Halenda (BJH) method (which uses the Kelvin equation) is based on
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the pore condensation phenomena (Barrett et al., 1951), and can be conveniently used for analysis of mesopores. The BJHmethod does not describe the pore filling of micropores and even narrow mesopores accurately. A convenient method for determining “t” is proposed by Halsey (1948), which is used to estimate “t”. The following equation can be used to calculate the actual radius of pores (in Å) in the fibers. rP = rk + t
rk =
−2V m RT ln(P/P0 )
5 t = 3.54 2.303 log(P/P0 )
1/3
This statistical thickness (t) can be considered as 3.54 ˚ is the thickness of one nitrogen (Vads /Vm ), in which 3.54 A molecular layer. Vads /Vm is the ratio of the volume of nitrogen adsorbed at a given relative pressure to the volume adsorbed at the completion of a monolayer for a nonporous solid of the same composition as the porous sample. If the material contains no macropores the adsorption isotherm remains nearly horizontal over a range of P/P0 approaching unity and the pore volume can be well defined. However, in the presence of macropores the adsorption isotherm rises rapidly near the value of P/P0 = 1. In the limit of large macropores, the adsorption curve exhibits an essentially vertical rise near the value of P/P0 = 1. In this case, the limiting adsorption value cannot be identified reliably with total pore volume.
2.2.
Pore size distribution in nanofibrous membrane
Liquid extrusion flow porometry (capillary flow porometry) method is used to obtain the pore size distribution data of a sheet of material that is wetted with a liquid wetting agent (Galwick having surface tension 0.0159 N/m is used in present study). When a liquid wets a fibrous sheet of material and pores of membrane material get filled due to capillary action. An equilibrium condition exists with the force of gravity. To overcome this equilibrium condition, a higher pressure than the atmospheric pressure must be applied to one end of the pores. There is a simple relationship existing between the pressure required to empty the pores and their size. The porometer increases the pressure on the sample in small increments and monitors the gradually increasing flow that occurs as the pores empty. The end of the “wet run” is defined as the point when all pores have been emptied. A second “dry run” is made to give a reference pressure. Typical flow plot is graphically shown in Fig. 1. The combined data set is used to calculate the maximum pore size (bubble Point) and the pore size distribution. The percentage of filter flow passing through a particular range of pore size (obtained from bubble point pressure) is arrived from dry and wet curves by the following equation.
Q(%) =
Qwh QDh
Qwl −
QDl
× 100
Subscripts “h” and “l” to Qw and QD denotes high and low pressure limits. The mean pore diameter can be estimated from the pressure corresponding to intersection of the wet curve and the half dry curve.
Fig. 1 – Principle of pore size determination of nanofibrous membrane. The pore size distribution (D) can be obtained from the following equation. D=
Q − QL dL − d
Subscript “L” to Q and d is referred to previous value. The pore size (diameter) is calculated by using the following equation. d = CM
P
When the P is the bubble point pressure then d becomes the maximum pore diameter. Here CM is constant; its value is dependent on the instrument design, membrane material and wetting agent characteristics. The value of CM is 2.86 when the P and is in Pa and N/m, respectively.
2.3.
Flow characterization of nanofibrous membrane:
Forchheimer equation (Dullien, 1979) is used to characterize the effect of inertia on flow through porous media, which is given below. P = ˛v + ˇv2 z The contribution of viscous and inertial flow can be analyzed by rearranging the above equation as follow and making a Forchheimer plot [P/(z)] vs. (/). P ˇ =˛+ zv In the low flow rate regime, the viscous flow is predominant and the pressure drop varies linear with the velocity. The permeability constant (BD ) is also known as the Darcy permeability, can be obtained by the following inverse relationship.
BD =
1 . ˛
3.
Materials and methods
3.1.
Material
Polysulfone (PSU, Sigma–Aldrich Product No. 374296 average Mw 56,000), Polyvinylidene difluoride (PVDF, MW 440,000, Kynar K761, Elf Atochem, USA) were procured.
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Table 1 – Composition of spinning solutions used in preparation of nano- and microfibrous membranes for determination of specific surface area and pore volume distribution (within the nano/microfibers). Composition of solution (at 25 ◦ C)
Solution
A B C
Polymer (% w/w)
Solvent-1 (% w/w)
Polysulphone (17%) Polysulphone (20%) Polysulphone (17%) + camphor (0.85%)
N,N-dimethylformamide (66.4%) N,N-dimethylacetamide (56%) N,N-dimethylformamide (65.72%)
Solvent-2 (% w/w) Chloroform (16.6%) Acetone (24%) Chloroform (16.43%)
Table 2 – Composition of spinning solutions used in preparation of nanofibrous membranes for pore size determination. Composition of solution(at 25 ◦ C)
Solution
D E
Polymer (% w/w)
Solvent-1 (% w/w)
Solvent-2 (% w/w)
Polyvinylidene difluoride (15%) Polyvinylidene difluoride (15%)
N,N-dimethylformamide (68%) N,N-dimethylformamide (72%)
Acetone (17%) Acetone (13%)
Fig. 2 – Electrospinning setup used for preparation of nanoand microfibrous membrane. N,N-dimethylformamide (DMF, 99% by GC) and N,Ndimethylacetamide (DMAC) and acetone (purity of 99.6%) were procured from Fisher Scientific, Schwerte, Germany. Chloroform (min assay of 99.8%) was obtained from Merk, Darmstadt, Germany. Acetone was obtained from Sigma–Aldrich. All solvents were dried before use with the molecular sieves. Camphor was purchased from local market.
3.2.
Preparation of nano- and microfibrous membranes
A solution for electrospinning was prepared using polymer and solvents. The compositions of polymer and solvents used in all experiments are given in Tables 1 and 2. The experimental setup used for the preparation of nano- and microfibrous membrane is shown in Fig. 2. A three milliliter plastic syringe was used to hold the electrospinning solution. The electrospinning solution was pumped at a constant rate with the
help of a metering pump (model KDS 100, K.D. Scientific Inc., Holliston, MA, USA) through the stainless steel needle whose tip was made circular by using a Buehler Ecomet polishing machine. The stainless steel drum of diameter 6.206 cm and length 13.500 cm was connected to variable speed motor and was used to deposit the nano/microfibers. The drum rotational speed was limited to within a range of 277–1385 revolutions per minutes (0.9–4.5 m/s). The high DC voltage was applied to the needle with the help of a high voltage regulated DC power supply (Model RR 50-1.25R/230/DDPM, Gamma High Voltage Research, Ormond Beach, FL, USA). The applied voltage was limited to within a range of 10,000–20,000 V while the upright distance between the tip of needle and facing surface of drum was fixed. The collecting drum was grounded so as to generate the desirable electric field strength between the tip of spinneret and collector surface. This arrangement directed the fibers path from the point of high potential (tip of spinneret) to the surface having the lowest potential, i.e. surface of drum. The resulted fibrous mat was carefully removed from the collector and the residual solvent associated with the mat was removed by keeping the mat in a vacuum oven (0.6 bar absolute pressure) for at least two days at 20 ◦ C. The dried electrsopun mats were stored in desiccator and used for measurement of pore size determination.
3.3. Determination of specific surface area and pore volume distribution within the nano- and microfibers Phase compositions and conditions used to prepare membranes for determination of specific surface area and pore volume distribution (within the nano/microfibers) is given in Tables 1 and 3, respectively. Around 50 mg of nano- and microfibrous membrane sample was weighed and subjected to degassing at 60 ◦ C for at least 6 h using inert gas. BET (Instru-
Table 3 – Conditions of electrospinning, drying and collection of fibers used in preparation of membrane for determination of specific surface area and pore volume distribution (within the nano/microfibers). Solution
Electrospinning conditions Spinneret Feed rate of inner diameter solution (ml/h) (m)
A B C
210 210 420
3 0.66 6
Applied electric field (V/m) 141666.7 100000.0 170000.0
Drying and collection conditions of fibers Temperature (◦ C)
25.0 26.8 26.0
Humidity (% RH)
63.0 44.0 50.0
Distance between spinneret and collector (m) 0.12 0.10 0.10
Fibers collection speed (m/s) 0.9 2.7 2.7
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Table 4 – Conditions of electrospinning, drying and collection of fibers used in preparation of membranes for pore size determination. Solution
Electrospinning conditions Spinneret inner diameter (m)
D E
950 950
Feed rate of solution (ml/h)
0.9 0.9
Drying and collection conditions of fibers Applied electric field (V/m) 1.25 × 105 1.25 × 105
Temperature Humidity (% RH) (◦ C)
24 22
71 87
Distance between spinneret and collector (m)
Fibers collection speed (m/s)
0.12 0.12
0.9 0.9
Fig. 3 – Schematic of setup used for determination of flow properties of nanofibrous membrane. ment NOVA-3000 Version 6.07, Quantachrome Corporation) was used to calculate the specific surface area and pore size distribution within the nanofibers. Multipoint (7 points) BET adsorption isotherm was used to calculate specific surface
area. BJH dV/dD (desorption) isotherm was used to calculate pore volume distribution within the fibers.
3.4. Determination of pore size distribution of nanofibrous membrane Phase composition and conditions used to prepare membranes for determination of pore size distribution in nanofibrous membrane is given in Tables 2 and 4, respectively. The membrane was cut into a circular shape of a diameter of 25 mm. The capillary flow porometer (CFP-1200-A, PMI–Porous Materials Inc., Ithaca, NY, USA) was used to measure the distribution of pores present in the membrane. Percentage filter flow from the wet curve (filter wetted with liquid, GalwickTM of surface tension 0.0159 N/m) and dry curve was calculated. From wet curve and dry curve, pore size distribution was calculated for a given pore aperture opening.
3.5. Transport (flow) property of nanofibrous membrane
Fig. 4 – Transmission electron microscopic image of polysulphone fiber (spun using solution A).
Experimental setup used for evaluation of transport properties of nanofibrous membrane is shown in Fig. 3. A predried fibrous mat was cut into circular disc of 47 mm diameter by indigenously fabricated membrane cutter. A circular piece of membrane was placed in a 47 mm diameter plastic holder
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3
400 Membrane A Membrane C
300
Membrane A
2.5
Pore Volume (cc/Å/g)x10e-3
Volume of gas adsorbed (cc/g)
350
250 200 150 100
Membrane C
2
1.5
1
0.5
50 0 0
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
100
200
300
400
500
600
700
800
900
1000
Pore Diameter (Å)
P/Po Fig. 5 – Nitrogen adsorption desorption isotherm of nanofibrous polysulphone materials at 77 K. (Millipore Inc., USA) and nitrogen gas was allowed to pass through the membrane. The temperature during the measurement was maintained at 20–22 ◦ C. The inlet pressure was measured by a digital pressure gauge (Meriam Merigauge 3910 model). The difference in pressure between the upstream and downstream sides of membrane was measured by a sensitive differential pressure gauge (Model JGE50DT, DCT Instruments, products of Sensotec, Inc., Columbus, OH, USA). The flow rate was measured by the flow meter (Aalborg Instruments, Orangeburg, NY, USA). The viscosity and density of nitrogen were considered as 17.85 × 10−6 kg/(m s) and 1.22 kg/m3 , respectively.
4.
Results and analysis
4.1. Surface properties of nano- and microfibrous materials It is envisaged that the surface area of fibrous materials can be considerably enhanced by introducing the micropores
Fig. 6 – Pore volume distribution in the fibers of nano- and microfibrous membrane. (less than 2 nm) and mesopores in the fibers. To introduce the micropores and mesopores in the fibers, the rupturing tendency of harden jet surface prior to drying of fibers in electrospinning process has to be improved by (a) selecting a pair of low volatile and high volatile solvents for preparation of solution for electrospinning, and (b) enhancing a proportion of low volatile solvent in the spinning solution. Therefore, 17% PSU was electrospun from a blend of DMF (66.4%) and chloroform (16.6%). Average fiber diameter and specific surface area of membrane A was found out to be 1.4 m and 57.08 m2 /g, respectively. The image of a typical fiber captured under the Transmission Electron Microscope (TEM) is shown in Fig. 4. Nanometer scale rupturing of the harden jet in electrospinning process can be also enhanced by increasing the viscosity of spinning solution, which contains a low volatile and high volatile solvent pair. Therefore, 20% PSU was electrospun from a blend of DMAC (56%) and acetone (24%). Average fiber diameter and specific surface area of membrane B was found out to be 900 nm and 60.26 m2 /g, respectively. Fibers diameter in
Table 5 – Pore volume and average pore diameter in the fibers. Membrane
A C
Average fiber diameter (nm) 1400 1400
Average pore diameter in the fibers (nm) 12.31 9.80
Total pore volume in the fibers (cm3 /g)
0.17561 cm3 /g for all pores of diameter smaller than 218.0 nm 0.12866 cm3 /g for all pores of diameter smaller than 207.6 nm
Fig. 7 – Scanning electron microscopic images of nanofibrous membrane, (a) surface image of membrane D, (b) vertical image of membrane D.
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Fig. 8 – Scanning electron microscopic images of PVDF nanofibrous membranes, (a) membrane D, (b) membrane E.
Fig. 9 – Pore size distribution in membrane D. membrane B is smaller compared to the fibers in membrane A, this is mainly due to high draw ratio during formation of fibers. Specific surface area of membrane C was found out to be 52.54 m2 /g. Compositions selected for preparation of membrane A and membrane C differ in amount of volatile additives used in the spinning solution. Spinning solution used for preparation of membrane C contains the camphor as an additional volatile additive.
Adsorption–desorption isotherm of porous polysulfone fibers (of membrane A and membrane C) is shown in Fig. 5. The two curves represented by the black and gray colour differ in amount of volatile additives in spinning solution. Both the curves show the presence of hysteresis, which indicates the presence of mesopores and larger micropores in the nano/microfibers of polysulfone material. The steady nature of adsorption curves (from P/P0 —0.1 to 0.85) indicates
Fig. 10 – Pore size distribution in membrane E.
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0
weak interaction between the adsorbate and adsorbent. The Adsorption isotherm does not remain nearly horizontal over a range of P/P0 approaching unity indicating the presence of macropores in the material. In presence of macropores, the adsorption isotherm rises rapidly when the value of P/P0 approaches to unity. The Desorption isotherm is used to calculate the pore volume distribution in the fibers and shown in Fig. 6. The total pore volume and the average pore diameter in the fibers are tabulated in Table 5. It was noted that the diameter of introduced pores in the fibers varied from 2 to 70 nm with an average pore diameter in the range of 9–13 nm.
0
0.5
1 y = 1E+10x - 3E+10
-2E+10
-ΔP/ (zν μ)
1.5
-4E+10 -6E+10 -8E+10
y = 4E+10x - 1E+11
-1E+11
4.2.
Pore size distribution of nanofibrous membrane -1.2E+11
A fibrous membrane of randomly oriented nanofibers was prepared by allowing fibers to deposit over the rotating collector. Deposition of fibers and thickness of membrane was controlled by collecting the fibers over the rotating collector. A Scanning electron microscopic picture of nanofibrous membrane is shown in Fig. 7. The nanofibrous membrane can be characterized by the fiber diameter, thickness, pore size distribution, porosity and tortuosity. The tortuosity factor is an indicator of geometry and interconnectivity of pores. It defines the effective length of the path of the hydraulic flow through the porous fibrous mat. In the case of low porous filters, the flow takes place through a
ν (m/s)
Fig. 11 – Flow characterization of PVDF nanofibrous membranes (black curve: membrane D, gray colour curve: membrane E). longer and more tortuous path resulting in a greater resistance or pressure drop. In case of highly porous filters, the flow takes place through shorter and less tortuous path resulting in a smaller resistance or pressure drop. For randomly oriented fibers, tortuosity factor is inversely related to porosity. Capillary flow porometry is a suitable method for
Table 6 – Viscous and inertial terms coefficients of nanofibrous membranes. Membrane
Viscous term coefficient (˛) (m−2 )
Membrane D Membrane E
Inertial term coefficient (ˇ) (m−1 )
10 × 1010 3 × 1010
Darcy permeability (BD ) (m2 ) 1 × 10−11 3.33 × 10−11
585245.90 146311.48
Table 7 – Porous polymeric micro- and nanofibrous materials. SN
Polymer
1
Polylactide
2
Poly-l-lactide
3
Cellulose triacetate
4
5
6
7
Polystyrene Polyacrylonitrile Poly(vinylidene fluride) Poly (e-caprolactone) Polyacrylonitrile
Method used for preparation of porous fibers Electrospinning folio wed by selective extraction of dispersed phase morphologies Phase separation during electrospinning using two component system Phase separation during electrospinning using two component system Phase separation during electrospinning using three component system Electrospining into a cryogenic liquid (thermally induced phase separation after electrospinning)
Electrospinning folio wed by selective extraction of dispersed phase morphologies Poly(l-lactic acid) Preferential evaporation of more volatile solvent from polymer jet during electrospinning process Polysulfone Preferential evaporation of more volatile solvent from polymer jet during electrospinning process
Fiber diameter (m)
Pore size in the fibers (nm)
Specific surface area (m2 /g)
Reference
1–1.2
100–200
n.d.
Bognitzki et al. (2001a)
1–1.2
100–250
n.d.
Bognitzki et al. (2001b)
1.4–1.6
50–100
14.47
Han et al. (2005)
1.4
200–500
13.65
n.d. n.d. n.d. 30–50
n.d.
0.5–2.1
20–70
n.d.
5.3–5.6
50 nm to few hundred n.d.
Qi et al. (2009)
0.9
n.d.
60.26
Present study
1.4
20–70
57.08
1114
9.50
n.d.
MacCann et al. (2006)
Moon et al. (2008)
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characterization of nanofibrous membrane amongst various other methods like mercury intrusion porosimetry because it measures the pore throat diameter without distorting pore structure (since it uses low pressure in measurement). Nanofibrous PVDF membranes (membranes D and E) were prepared by electrospinning technique. These two membranes were prepared using two different solutions. In case of membrane D, the ratio of DMF and acetone used to solubilize PVDF polymer was 80:20; while in the case of membrane E, it was 85:15. Scanning electron microscopic picture of these membranes (Fig. 8) reveals the absence of beads in membrane D and the presence of beads in membrane E. The average diameter of fibers in membrane D and E is found out to be 300 and 400 nm, respectively. Pore size distribution of membrane D and E measured by capillary flow porometry is shown in Figs. 9 and 10, respectively. Membrane D has pore size distribution between 0.3 and 1 m. Due to presence of beads, the membrane E has a larger pore size distribution (between 0.95 and 3 m).
4.3. Flow characterization (transport property) of nanofibrous membrane
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after the drying of the fibers. Introduction of mesopores in the fibers enhances the surface area and helps to make the surface area accessible to contacting fluids. On the other hand, the introduction of macropores in the fibers increases the pore volume and helps to reduce the diffusion resistances. To achieve ∼ 60 m2 /g specific surface area by reducing the fiber diameter, one needs to draw the fibers down to 50–60 nm (Gibson et al., 2001). In the present study, 60 m2 /g of specific surface area is achieved in the fibers of average diameter of 900 nm. Thus, we conclude that specific surface area results from the porous fibers are much higher than one can achieve by reducing the diameter of fibers. Surface, flow and structural characterization of nano- and microfibrous polymeric material is important for designing materials for catalytic filtration. From the results obtained, we conclude that by electrospinning technique, it is feasible to produce the fibrous membrane of specific surface area and Darcy permeability higher than 60 m2 /g and 1 × 10−11 m2 , respectively. Beaded fibers in the fibrous materials can be used to reduce the bulk transport resistances during application of these materials in catalytic filtration.
Acknowledgement Forchheimer equation was used to estimate the viscous and inertial term coefficients of PVDF nanofibrous membranes. The results are obtained from Fig. 11. The estimated viscous and inertial terms coefficients are tabulated in Table 6. Presence of beads in the membrane E reduced the values of viscous and inertial resistance terms coefficients, which is evinced from Table 6. Presence of beads in the membrane E increased the overall pore size distribution (Fig. 10); thereby increase the permeability of gas through membrane [Darcy permeability increased due to presence of beads in the membrane E (Table 6)].
5.
Discussion and conclusions
The solid fraction in the fibrous material can be as low as 0.01% and as high as 20%. The electrospinning process can be conveniently used to prepare a fibrous material of different solid fraction. In the electrospinning process, the polymer solution of suitable conductivity, viscosity, surface tension and volatility is forced through a spinneret under the influence of high electric voltage. A resultant liquid jet then passes through a controlled temperature and humidity cabinet to evaporate the solvent before the deposition of fibers on the collector. The deposition rate of nano/microfiber varies depending upon the solution properties and electrospinning conditions. The influence of electrospinning process parameters on the formation of nano/microfibers and the quality of fibers has been discussed by Ramakrishana et al. (2005). We report a method for incorporating the mesopores and macropores in the nanoand microfibrous membrane material made of polysulfone. Results are compared with reported ones from the literature and shown in Table 7. It can be noted that the specific surface area obtained in this study is nearly four times higher than that reported by Han et al. (2005). The principle of the developed method is based on preferential evaporation of higher volatile solvent from the surface of the harden jet of a polymer solution in the electrospinning process. As more volatile solvent evaporates from the polymer jet, the nanometer scale morphologies (2–70 nm) remain behind in the fibers, which get preserved
This work is supported by the Agency for Science and Technology and Research (A*Star) of Singapore under the Singapore-Poland research grant (Grant No. WBS: R-398-000041-305).
References An, H., Shin, C., Chase, G.G., 2006. Ion exchanger using electrospun polystyrene nanofibers. J. Membr. Sci. 283 (1–2), 84–87. Armantrout, J.E., Bryner, M.A., Spiers, C.B., 2007. Improved electroblowing fiber spinning process, PCT WO 2007/022390. Aussawasathien, D., Teerawattananon, C., Vongachariya, A., 2008. Separation of micron to sub-micron particles from water: electrospun nylon-6 nanofibrous membranes as pre-filters. J. Membr. Sci. 315 (1–2), 11–19. Barhate, R.S., Ramakrishna, S., 2007. Nanofibrous filtering media: filtration problems and solutions from tiny materials. J. Membr. Sci. 296 (1–2), 1–8. Barhate, R.S., Ramaseshan, R., Liu, Y., Sundarrajan, S., Lala, N.L., Ramakrishna, S., 2007. Nanotechnology for protection from chemical and biological warfare agents: separation and decontamination aspects. In: Borrelli, J.V. (Ed.), Bioterrorism: Prevention, Preparedness and Protection. Nova Sci Pub, pp. 83–123. Barhate, R.S., Sundarrajan, S., Pliszka, D., Ramakrishna, S., 2008. Fine chemical processing: the potential of nanofibres in filtration. Filtr. Sep. 45 (4), 32–35. Barrett, E.P., Joyner, L.G., Halenda, P.P., 1951. The determination of pore volume and area distribution in pure substances. J. Am. Chem. Soc. 73, 373. Bognitzki, M., Frese, T., Steinhart, M., Greiner, A., Wendorff, J.H., 2001a. Preparation of fibers with nanoscaled morphologies: electrospining of polymer blends. Polym. Eng. Sci. 41 (6), 982–989. Bognitzki, M., Czado, W., Frese, T., Schaper, A., Hellwig, M., Steinhart, M., Greiner, A., Wendorff, J.H., 2001b. Nanostructured fibers via electrospinning. Adv. Mater. 13 (1), 70–72. Brunauer, S., Emmett, P., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, 309–319. Dauner, M., 2006. Centrifuge Spinning Chemical Nanotechnology Talks VII Status and Future of Nanofibers by Electrospinning
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and CNT-From Small to Smart, DECHEMA Haus, Frankfurt, Germany, October 23–25, p. 110. Dullien, F.A.L., 1979. Porous Media-fluid Transport and Pore Structure. Academic Press, New York, USA. Feng, C., Khulbe, K.C., Matsuura, T., Gopal, R., Kaur, S., Ramakrishna, S., Khayet, M., 2008. Production of drinking water from saline water by air-gap membrane distillation using polyvinylidene fluoride nanofiber membrane. J. Membr. Sci. 311 (1–2), 1–6. Gerking, L., Stobik, M., 2007. Nanoval spiltspinning-from coarse to nano. Chem. Fibers Int. 4, 210–211. Gibson, P., Schreuder-Gibson, H., Rivin, D., 2001. Transport properties of porous membranes based on electrospun nanofibers. Colloids Surf. A 187/188, 469–481. Halsey, G.D., 1948. Physical adsorption on non-uniform surfaces. J. Chem. Phys. 16, 931–937. Han, S.O., Son, W.K., Youk, J.H., Lee, T.S., Park, W.H., 2005. Ultrafine porous fibers electrospun from cellulose triacetate. Mater. Lett. 59, 2998–3001. Lala, N.L., Ramaseshan, R., Bojun, L., Sundarrajan, S., Barhate, R.S., Liu, Y., Ramakrishna, S., 2007. Fabrication of nanofibers with antimicrobial functionality used as filters: protection against bacterial contaminants. Biotechnol. Bioeng. 97 (6), 1357–1365. Luzhansky, D.M., 2003. Quality control in manufacturing of electrospun nanofiber composites. In: Inter. Nanowoven. Tech. Conf., Baltimore, MA, USA, September 15–18, pp. 1–7. Lyons, J., Li, C., Ko, F., 2004. Melt-electrospinning part I: processing parameters and geometric properties. Polymer 45, 7597–7603. Ma, Z., Kotaki, M., Ramakrishna, S., 2006. Surface modified nonwoven polysulphone (PSU) fiber mesh by electrospinning: a novel affinity membrane. J. Membr. Sci. 272 (1–2), 179–187. MacCann, J.T., Marquez, M., Xia, Y., 2006. Highly porous fibers by electrospinning into a cryogenic liquid. J. Am. Chem. Soc. 128 (5), 1436–1437.
Moon, S.C., Choi, J., Farris, R.J., 2008. Highly porous polyacrylonitrile/polystyrene nanofibers by electrospinning. Fiber Polym. 9 (3), 276–280. Naguyean, C., Do, D.D., 2000. Simple optimization approach for the characterization of pore size distribution. Langmuir 16, 1319–1322. Quantachrome Corporation, 2007. Operating Manual, version 6.07. Quantachrome Corporation, Nova Manual, High speed gas sorption Analyser, pp. 80–83. Qi, Z., Yu, H., Chen, Y., Zhu, M., 2009. Highly porous fibers prepared by electrospinning a ternary system of nonsolvent/solvent/poly (l-lactic acid). Mater. Lett. 63, 415–418. Ramakrishana, S., Fujihara, K., Teo, W.-E., Lim, T.-C., Ma, Z., 2005. An Introduction to Electrospinning and Nanofibers. World Scientific Publishing Co. Pte. Ltd, pp. 24–86. Ramaseshan, R., Sundarrajan, S., Liu, Y.-J., Barhate, R.S., Lala, N.L., Ramarishna, S., 2006. Functionalized polymer nanofibet membranes for protection from chemical warfare stimulants. Nanotechnology 17 (12), 2947–2953. Sun, S., Xiwang, Z., Du, A., Fung, L.P., Yinjie, W., Weijiog, F., Shiping, X., Leckie, J.O., 2007. Membrane technology: removing contaminants in wastewater. Filtr. Sep. 44 (7), 14–16. Tang, Z., Wei, J., Yung, L., Ji, B., Ma, H., Qiu, C., Yoon, K., Wan, F., Fang, D., Hsiao, B.S., Chu, B., 2009. UV-cured poly (vinyl alcohol) ultrafiltration nanofibrous membrane based on electrospun nanofiber scaffolds. J. Membr. Sci. 328 (1–2), 1–5. Ward, G.F., 2001. Meltblown nanofibres for nonwoven filtration applications. Filtr. Sep. 38, 42–43. Yoon, K., Hsiao, B.S., Chu, B., 2009. High flux nanofiltration membranes based on interfacially polymerized polyamide barrier layer on polyacrylonitrile nanofibrous scaffolds. J. Membr. Sci. 326 (2), 484–492.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Porous nano- and microfibrous polymeric membrane material for catalytic support R.S. Barhate a,∗ , S. Koeppl b , S. Ramakrishna a,∗ a
NUS Nanoscience and Nanotechnology Initiative, National University of Singapore, Faculty of Engineering, 2 Engineering Drive 3, Singapore 117576, Singapore b Mechanical Engineering, Technical University of Munich, Munich, Germany
a b s t r a c t High surface area is essential for attachment of functional groups, ions, moieties and nanoparticles. Surface area of fibrous membrane can be enhanced by reducing the fiber diameter or producing the porous fibers. Flow properties of the fibrous membrane can be improved by placing the fibers apart in the fibrous network. By electrospinning, it is feasible to produce the fibrous membrane of specific surface area and Darcy permeability higher than 60 m2 /g and 1 × 10−11 m2 , respectively. The interconnected irregular shape mesopores (2–50 nm) within the fibers increase the accessible surface area. On the other hand, presence of macropores (pores larger than 50 nm) largely increases the pore volume (porosity) in fibers and helps to reduce the diffusion resistances. Beaded fibers in the membrane can be used to reduce the bulk transport resistances. We developed a method for incorporating the mesopores and macropores in the nano/microfibers made of engineering plastics. To achieve ∼60 m2 /g specific surface area by reducing the fiber diameter, one needs to draw the fibers down to 50–60 nm. In present study, 60 m2 /g of specific surface area is achieved through the porous fibers of average diameter of 900 nm. A specific surface area result from the porous fiber is much higher than one can achieve by reducing the diameter of fibers. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Electrospinning; Nanofiber; Fibrous membrane; Surface area; Porous material
1.
Introduction
Nanofiber research is getting momentum to produce new nanomaterials having high surface area and unique functionality. Wide range of polymers are being electrospun from the solution and melt phase into nano/microfibers in pursuit of improved thermal, mechanical, physical (structural), chemical and biological properties. Nanofibrous membranes are finding applications in filtration and catalysis. A few reported applications of nanofibrous membranes are surface and depth filters for air purification (Barhate and Ramakrishna, 2007; Barhate et al., 2008), antimicrobial filter (Lala et al., 2007), respiratory particulate filter (Barhate et al., 2007), microfiltration membrane (Aussawasathien et al., 2008), support for interfacial polymerized ultrafiltration and nanofiltration membranes (Tang et al., 2009; Yoon et al., 2009), distillation membrane (Feng et al., 2008), affinity membrane for highly selective purifications (Ma et al., 2006), membrane for ion exchange sep-
∗
aration (An et al., 2006) and catalysis (Ramaseshan et al., 2006; Sun et al., 2007). There are seven major processes for producing nanofibers on mass scale, which include electrostatic spinning or electrospinning (Luzhansky, 2003), melt electrospinning (Lyons et al., 2004), modified melt spinning (Ward, 2001), electroblowing (Armantrout et al., 2007), splitting of jet of melt/solution (Gerking and Stobik, 2007) and centrifuge spinning (Dauner, 2006). Electrospinning is being used extensively at laboratory and industrial scale for production of nano- and microfibrous membranes. Donaldson company has used? the nanofiber production (nanofiber coating) line of 10,000 m2 per day for the past 20 years (Luzhansky, 2003). Nano- and microfibrous membranes have several unique properties such as high surface area, flexibility, superior directional strength, good interconnectivity of pores and potential to incorporate the active chemistry or functionality on nanoscale. These properties make the nano- and microfi-
Corresponding authors. Tel.: +65 65164266; fax: +65 68725563. E-mail addresses: [email protected] (R.S. Barhate), [email protected] (S. Ramakrishna). Received 3 March 2009; Received in revised form 7 September 2010; Accepted 10 September 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.09.007
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Nomenclature Acs BD C CM D M N Q (%) QD QL Qw
R S St T Vm W Wm Z d P rp rk t ˛ ˇ
cross-sectional area for heaxagonal close˚ 2) packed nitrogen monolayer at 77 K (16.2 A Darcy permeability (m2 ) BET constant (indicative of adsorbent/adsorbate interactions) constant (2.861 when P is taken in Pa) pore size distribution molecular weight of adsorbate Avagadaro’s Number (6.023 × 1023 molecules/mol) percentage filter flow (%) fluid flow through dry filter (subscript “h” and “l” denotes high and low pressure limits) (L/min) percentage filter flow of previous value (%) fluid flow through wet filter (subscript “h” and “l” denotes high and low pressure limits) (L/min) gas constant (8.314 × 107 ergs/K/mol) specific surface area (m2 /g) total surface area (m2 ) boiling point of nitrogen (77 K) molar volume of liquid nitrogen (34.7 cm3 /mol) weight of gas adsorbed at relative pressure of P/P0 (g) weight of adsorbate constituting monolayer surface coverage (g) thickness of the membrane (m) pore diameter (m) pressure drop across the filter membrane (Pa) mesopore radius (Å) Kelvin radius (radius of pore in which condensation occurs at relative pressure of P/P0 ) (Å) thickness of the adsorbed layer (Å) viscous term coefficient (m−2 ) inertial term coefficient (m−1 ) surface tension of nitrogen at its boiling point (8.85 ergs/cm2 at 77 K) density of fluid (kg/m3 ) surface tension of wetting solution (Galwick 0.0159 N/m) velocity of fluid flowing through membrane (m/s) viscosity of the fluid (kg/m s)
brous membranes suitable for catalytic filtration application. Fibers having the diameter less than 1000 nm are designated as nanofibers; whereas the fibers having the diameter more than 1000 nm are designated as microfibers in this investigation. There are a few attempts to prepare porous nano/microfibers (Bognitzki et al., 2001a,b; Han et al., 2005; MacCann et al., 2006; Moon et al., 2008; Qi et al., 2009). Many techniques have been used to prepare the porous fibers such as the introduction of dispersed phase morphologies in the spun fibers followed by a selective extraction of dispersed phase morphologies from the fibers (Bognitzki et al., 2001a; Moon et al., 2008), electrospinning a polymeric solution into cryogenic liquid bath followed by vacuum drying (MacCann et al., 2006), phase separation due to rapid evaporation of solvent during electrospinning (Bognitzki et al., 2001b; Han
et al., 2005) and preferential evaporation of solvent from a ternary system during electrospinning process (Qi et al., 2009). The most of these attempts are either centric to hydrophilic or water soluble polymeric materials, which are not suitable for catalysis and catalytic filtration applications. Furthermore, none of the above approach has shown the specific surface area more than 15 m2 /g. There exists a need to prepare porous fibers by easy means from the engineering plastics and hydrophobic water insoluble polymeric material. Engineering plastics and hydrophobic water insoluble polymeric materials [such as polysulfone (PSU), polyvinylidine difluoride (PVDF), polystyrene (PS), polyetherketone (PEK), polyetheretherketone (PEEK), polyamide (PA), polyimide (PI), polyphenylene oxide (PPO), polyethylene terepthalate (PET), polycarbonate (PC)] which are stable in acidic, basic and hot gaseous environment would be useful for catalysis and catalytic filtration applications. The main focus of this investigation is to develop a simple method for preparation of porous nano- and microfibrous membrane made of engineering plastics such as polysulfone. This study also includes the characterization (surface, structural and flow properties) of porous nano- and microfibrous membrane.
2.
Theoretical aspects
2.1. Surface properties of nano- and microfibrous membrane The interconnected irregular shape mesopores (2–50 nm) within the fibers increase the surface area of the fibrous materials. On the other hand, presence of macropores (pores larger than 50 nm) largely increases the pore volume (porosity) in the fibers. Nitrogen gas adsorption [the Brunauer–Emmett–Teller (BET) method] is a method of choice for characterization of porous materials to obtain specific surface area and pore volume distribution (Brunauer et al., 1938). Adsorption of nitrogen at its boiling point is a useful tool for characterization of materials having large micropores, mesopores and a wide pore size distribution and where the surface layering and capillary condensation are part of the adsorption process (Naguyean and Do, 2000). BET method is used to estimate the specific surface area. BET equation requires a linear plot of 1/[W(P0 /P) − 1] vs. P/P0 using nitrogen as the adsorbate. Plot is restricted to a limited region of the adsorption isotherm usually P/P0 in the range of 0.05–0.35 (Operating manual, Quantachrome Corporation). In case of microporous materials, the linear region of the plot gets shifted to a lower relative pressure side. BET equation used to calculate specific surface area is given below. 1 1 C−1 P = + Wm C Wm C P0 W[(P0 /P) − 1] Wm can be calculated from the slope and Y-intercept of the BET equation. Specific surface area is calculated from “Wm ” by using the following equation. S=
1 St = W W
W NA m cs M
Size and shape of pores determine the pore filling in the BET method. In case of micropores, the pore filling occurs in a continuous way. In case of mesopores, the pore filling occurs by condensation process (reflecting the first order gas–liquid phase transition). The Barrett, Joyner and Halenda (BJH) method (which uses the Kelvin equation) is based on
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the pore condensation phenomena (Barrett et al., 1951), and can be conveniently used for analysis of mesopores. The BJHmethod does not describe the pore filling of micropores and even narrow mesopores accurately. A convenient method for determining “t” is proposed by Halsey (1948), which is used to estimate “t”. The following equation can be used to calculate the actual radius of pores (in Å) in the fibers. rP = rk + t
rk =
−2V m RT ln(P/P0 )
5 t = 3.54 2.303 log(P/P0 )
1/3
This statistical thickness (t) can be considered as 3.54 ˚ is the thickness of one nitrogen (Vads /Vm ), in which 3.54 A molecular layer. Vads /Vm is the ratio of the volume of nitrogen adsorbed at a given relative pressure to the volume adsorbed at the completion of a monolayer for a nonporous solid of the same composition as the porous sample. If the material contains no macropores the adsorption isotherm remains nearly horizontal over a range of P/P0 approaching unity and the pore volume can be well defined. However, in the presence of macropores the adsorption isotherm rises rapidly near the value of P/P0 = 1. In the limit of large macropores, the adsorption curve exhibits an essentially vertical rise near the value of P/P0 = 1. In this case, the limiting adsorption value cannot be identified reliably with total pore volume.
2.2.
Pore size distribution in nanofibrous membrane
Liquid extrusion flow porometry (capillary flow porometry) method is used to obtain the pore size distribution data of a sheet of material that is wetted with a liquid wetting agent (Galwick having surface tension 0.0159 N/m is used in present study). When a liquid wets a fibrous sheet of material and pores of membrane material get filled due to capillary action. An equilibrium condition exists with the force of gravity. To overcome this equilibrium condition, a higher pressure than the atmospheric pressure must be applied to one end of the pores. There is a simple relationship existing between the pressure required to empty the pores and their size. The porometer increases the pressure on the sample in small increments and monitors the gradually increasing flow that occurs as the pores empty. The end of the “wet run” is defined as the point when all pores have been emptied. A second “dry run” is made to give a reference pressure. Typical flow plot is graphically shown in Fig. 1. The combined data set is used to calculate the maximum pore size (bubble Point) and the pore size distribution. The percentage of filter flow passing through a particular range of pore size (obtained from bubble point pressure) is arrived from dry and wet curves by the following equation.
Q(%) =
Qwh QDh
Qwl −
QDl
× 100
Subscripts “h” and “l” to Qw and QD denotes high and low pressure limits. The mean pore diameter can be estimated from the pressure corresponding to intersection of the wet curve and the half dry curve.
Fig. 1 – Principle of pore size determination of nanofibrous membrane. The pore size distribution (D) can be obtained from the following equation. D=
Q − QL dL − d
Subscript “L” to Q and d is referred to previous value. The pore size (diameter) is calculated by using the following equation. d = CM
P
When the P is the bubble point pressure then d becomes the maximum pore diameter. Here CM is constant; its value is dependent on the instrument design, membrane material and wetting agent characteristics. The value of CM is 2.86 when the P and is in Pa and N/m, respectively.
2.3.
Flow characterization of nanofibrous membrane:
Forchheimer equation (Dullien, 1979) is used to characterize the effect of inertia on flow through porous media, which is given below. P = ˛v + ˇv2 z The contribution of viscous and inertial flow can be analyzed by rearranging the above equation as follow and making a Forchheimer plot [P/(z)] vs. (/). P ˇ =˛+ zv In the low flow rate regime, the viscous flow is predominant and the pressure drop varies linear with the velocity. The permeability constant (BD ) is also known as the Darcy permeability, can be obtained by the following inverse relationship.
BD =
1 . ˛
3.
Materials and methods
3.1.
Material
Polysulfone (PSU, Sigma–Aldrich Product No. 374296 average Mw 56,000), Polyvinylidene difluoride (PVDF, MW 440,000, Kynar K761, Elf Atochem, USA) were procured.
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Table 1 – Composition of spinning solutions used in preparation of nano- and microfibrous membranes for determination of specific surface area and pore volume distribution (within the nano/microfibers). Composition of solution (at 25 ◦ C)
Solution
A B C
Polymer (% w/w)
Solvent-1 (% w/w)
Polysulphone (17%) Polysulphone (20%) Polysulphone (17%) + camphor (0.85%)
N,N-dimethylformamide (66.4%) N,N-dimethylacetamide (56%) N,N-dimethylformamide (65.72%)
Solvent-2 (% w/w) Chloroform (16.6%) Acetone (24%) Chloroform (16.43%)
Table 2 – Composition of spinning solutions used in preparation of nanofibrous membranes for pore size determination. Composition of solution(at 25 ◦ C)
Solution
D E
Polymer (% w/w)
Solvent-1 (% w/w)
Solvent-2 (% w/w)
Polyvinylidene difluoride (15%) Polyvinylidene difluoride (15%)
N,N-dimethylformamide (68%) N,N-dimethylformamide (72%)
Acetone (17%) Acetone (13%)
Fig. 2 – Electrospinning setup used for preparation of nanoand microfibrous membrane. N,N-dimethylformamide (DMF, 99% by GC) and N,Ndimethylacetamide (DMAC) and acetone (purity of 99.6%) were procured from Fisher Scientific, Schwerte, Germany. Chloroform (min assay of 99.8%) was obtained from Merk, Darmstadt, Germany. Acetone was obtained from Sigma–Aldrich. All solvents were dried before use with the molecular sieves. Camphor was purchased from local market.
3.2.
Preparation of nano- and microfibrous membranes
A solution for electrospinning was prepared using polymer and solvents. The compositions of polymer and solvents used in all experiments are given in Tables 1 and 2. The experimental setup used for the preparation of nano- and microfibrous membrane is shown in Fig. 2. A three milliliter plastic syringe was used to hold the electrospinning solution. The electrospinning solution was pumped at a constant rate with the
help of a metering pump (model KDS 100, K.D. Scientific Inc., Holliston, MA, USA) through the stainless steel needle whose tip was made circular by using a Buehler Ecomet polishing machine. The stainless steel drum of diameter 6.206 cm and length 13.500 cm was connected to variable speed motor and was used to deposit the nano/microfibers. The drum rotational speed was limited to within a range of 277–1385 revolutions per minutes (0.9–4.5 m/s). The high DC voltage was applied to the needle with the help of a high voltage regulated DC power supply (Model RR 50-1.25R/230/DDPM, Gamma High Voltage Research, Ormond Beach, FL, USA). The applied voltage was limited to within a range of 10,000–20,000 V while the upright distance between the tip of needle and facing surface of drum was fixed. The collecting drum was grounded so as to generate the desirable electric field strength between the tip of spinneret and collector surface. This arrangement directed the fibers path from the point of high potential (tip of spinneret) to the surface having the lowest potential, i.e. surface of drum. The resulted fibrous mat was carefully removed from the collector and the residual solvent associated with the mat was removed by keeping the mat in a vacuum oven (0.6 bar absolute pressure) for at least two days at 20 ◦ C. The dried electrsopun mats were stored in desiccator and used for measurement of pore size determination.
3.3. Determination of specific surface area and pore volume distribution within the nano- and microfibers Phase compositions and conditions used to prepare membranes for determination of specific surface area and pore volume distribution (within the nano/microfibers) is given in Tables 1 and 3, respectively. Around 50 mg of nano- and microfibrous membrane sample was weighed and subjected to degassing at 60 ◦ C for at least 6 h using inert gas. BET (Instru-
Table 3 – Conditions of electrospinning, drying and collection of fibers used in preparation of membrane for determination of specific surface area and pore volume distribution (within the nano/microfibers). Solution
Electrospinning conditions Spinneret Feed rate of inner diameter solution (ml/h) (m)
A B C
210 210 420
3 0.66 6
Applied electric field (V/m) 141666.7 100000.0 170000.0
Drying and collection conditions of fibers Temperature (◦ C)
25.0 26.8 26.0
Humidity (% RH)
63.0 44.0 50.0
Distance between spinneret and collector (m) 0.12 0.10 0.10
Fibers collection speed (m/s) 0.9 2.7 2.7
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Table 4 – Conditions of electrospinning, drying and collection of fibers used in preparation of membranes for pore size determination. Solution
Electrospinning conditions Spinneret inner diameter (m)
D E
950 950
Feed rate of solution (ml/h)
0.9 0.9
Drying and collection conditions of fibers Applied electric field (V/m) 1.25 × 105 1.25 × 105
Temperature Humidity (% RH) (◦ C)
24 22
71 87
Distance between spinneret and collector (m)
Fibers collection speed (m/s)
0.12 0.12
0.9 0.9
Fig. 3 – Schematic of setup used for determination of flow properties of nanofibrous membrane. ment NOVA-3000 Version 6.07, Quantachrome Corporation) was used to calculate the specific surface area and pore size distribution within the nanofibers. Multipoint (7 points) BET adsorption isotherm was used to calculate specific surface
area. BJH dV/dD (desorption) isotherm was used to calculate pore volume distribution within the fibers.
3.4. Determination of pore size distribution of nanofibrous membrane Phase composition and conditions used to prepare membranes for determination of pore size distribution in nanofibrous membrane is given in Tables 2 and 4, respectively. The membrane was cut into a circular shape of a diameter of 25 mm. The capillary flow porometer (CFP-1200-A, PMI–Porous Materials Inc., Ithaca, NY, USA) was used to measure the distribution of pores present in the membrane. Percentage filter flow from the wet curve (filter wetted with liquid, GalwickTM of surface tension 0.0159 N/m) and dry curve was calculated. From wet curve and dry curve, pore size distribution was calculated for a given pore aperture opening.
3.5. Transport (flow) property of nanofibrous membrane
Fig. 4 – Transmission electron microscopic image of polysulphone fiber (spun using solution A).
Experimental setup used for evaluation of transport properties of nanofibrous membrane is shown in Fig. 3. A predried fibrous mat was cut into circular disc of 47 mm diameter by indigenously fabricated membrane cutter. A circular piece of membrane was placed in a 47 mm diameter plastic holder
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3
400 Membrane A Membrane C
300
Membrane A
2.5
Pore Volume (cc/Å/g)x10e-3
Volume of gas adsorbed (cc/g)
350
250 200 150 100
Membrane C
2
1.5
1
0.5
50 0 0
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
100
200
300
400
500
600
700
800
900
1000
Pore Diameter (Å)
P/Po Fig. 5 – Nitrogen adsorption desorption isotherm of nanofibrous polysulphone materials at 77 K. (Millipore Inc., USA) and nitrogen gas was allowed to pass through the membrane. The temperature during the measurement was maintained at 20–22 ◦ C. The inlet pressure was measured by a digital pressure gauge (Meriam Merigauge 3910 model). The difference in pressure between the upstream and downstream sides of membrane was measured by a sensitive differential pressure gauge (Model JGE50DT, DCT Instruments, products of Sensotec, Inc., Columbus, OH, USA). The flow rate was measured by the flow meter (Aalborg Instruments, Orangeburg, NY, USA). The viscosity and density of nitrogen were considered as 17.85 × 10−6 kg/(m s) and 1.22 kg/m3 , respectively.
4.
Results and analysis
4.1. Surface properties of nano- and microfibrous materials It is envisaged that the surface area of fibrous materials can be considerably enhanced by introducing the micropores
Fig. 6 – Pore volume distribution in the fibers of nano- and microfibrous membrane. (less than 2 nm) and mesopores in the fibers. To introduce the micropores and mesopores in the fibers, the rupturing tendency of harden jet surface prior to drying of fibers in electrospinning process has to be improved by (a) selecting a pair of low volatile and high volatile solvents for preparation of solution for electrospinning, and (b) enhancing a proportion of low volatile solvent in the spinning solution. Therefore, 17% PSU was electrospun from a blend of DMF (66.4%) and chloroform (16.6%). Average fiber diameter and specific surface area of membrane A was found out to be 1.4 m and 57.08 m2 /g, respectively. The image of a typical fiber captured under the Transmission Electron Microscope (TEM) is shown in Fig. 4. Nanometer scale rupturing of the harden jet in electrospinning process can be also enhanced by increasing the viscosity of spinning solution, which contains a low volatile and high volatile solvent pair. Therefore, 20% PSU was electrospun from a blend of DMAC (56%) and acetone (24%). Average fiber diameter and specific surface area of membrane B was found out to be 900 nm and 60.26 m2 /g, respectively. Fibers diameter in
Table 5 – Pore volume and average pore diameter in the fibers. Membrane
A C
Average fiber diameter (nm) 1400 1400
Average pore diameter in the fibers (nm) 12.31 9.80
Total pore volume in the fibers (cm3 /g)
0.17561 cm3 /g for all pores of diameter smaller than 218.0 nm 0.12866 cm3 /g for all pores of diameter smaller than 207.6 nm
Fig. 7 – Scanning electron microscopic images of nanofibrous membrane, (a) surface image of membrane D, (b) vertical image of membrane D.
chemical engineering research and design 8 9 ( 2 0 1 1 ) 621–630
627
Fig. 8 – Scanning electron microscopic images of PVDF nanofibrous membranes, (a) membrane D, (b) membrane E.
Fig. 9 – Pore size distribution in membrane D. membrane B is smaller compared to the fibers in membrane A, this is mainly due to high draw ratio during formation of fibers. Specific surface area of membrane C was found out to be 52.54 m2 /g. Compositions selected for preparation of membrane A and membrane C differ in amount of volatile additives used in the spinning solution. Spinning solution used for preparation of membrane C contains the camphor as an additional volatile additive.
Adsorption–desorption isotherm of porous polysulfone fibers (of membrane A and membrane C) is shown in Fig. 5. The two curves represented by the black and gray colour differ in amount of volatile additives in spinning solution. Both the curves show the presence of hysteresis, which indicates the presence of mesopores and larger micropores in the nano/microfibers of polysulfone material. The steady nature of adsorption curves (from P/P0 —0.1 to 0.85) indicates
Fig. 10 – Pore size distribution in membrane E.
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0
weak interaction between the adsorbate and adsorbent. The Adsorption isotherm does not remain nearly horizontal over a range of P/P0 approaching unity indicating the presence of macropores in the material. In presence of macropores, the adsorption isotherm rises rapidly when the value of P/P0 approaches to unity. The Desorption isotherm is used to calculate the pore volume distribution in the fibers and shown in Fig. 6. The total pore volume and the average pore diameter in the fibers are tabulated in Table 5. It was noted that the diameter of introduced pores in the fibers varied from 2 to 70 nm with an average pore diameter in the range of 9–13 nm.
0
0.5
1 y = 1E+10x - 3E+10
-2E+10
-ΔP/ (zν μ)
1.5
-4E+10 -6E+10 -8E+10
y = 4E+10x - 1E+11
-1E+11
4.2.
Pore size distribution of nanofibrous membrane -1.2E+11
A fibrous membrane of randomly oriented nanofibers was prepared by allowing fibers to deposit over the rotating collector. Deposition of fibers and thickness of membrane was controlled by collecting the fibers over the rotating collector. A Scanning electron microscopic picture of nanofibrous membrane is shown in Fig. 7. The nanofibrous membrane can be characterized by the fiber diameter, thickness, pore size distribution, porosity and tortuosity. The tortuosity factor is an indicator of geometry and interconnectivity of pores. It defines the effective length of the path of the hydraulic flow through the porous fibrous mat. In the case of low porous filters, the flow takes place through a
ν (m/s)
Fig. 11 – Flow characterization of PVDF nanofibrous membranes (black curve: membrane D, gray colour curve: membrane E). longer and more tortuous path resulting in a greater resistance or pressure drop. In case of highly porous filters, the flow takes place through shorter and less tortuous path resulting in a smaller resistance or pressure drop. For randomly oriented fibers, tortuosity factor is inversely related to porosity. Capillary flow porometry is a suitable method for
Table 6 – Viscous and inertial terms coefficients of nanofibrous membranes. Membrane
Viscous term coefficient (˛) (m−2 )
Membrane D Membrane E
Inertial term coefficient (ˇ) (m−1 )
10 × 1010 3 × 1010
Darcy permeability (BD ) (m2 ) 1 × 10−11 3.33 × 10−11
585245.90 146311.48
Table 7 – Porous polymeric micro- and nanofibrous materials. SN
Polymer
1
Polylactide
2
Poly-l-lactide
3
Cellulose triacetate
4
5
6
7
Polystyrene Polyacrylonitrile Poly(vinylidene fluride) Poly (e-caprolactone) Polyacrylonitrile
Method used for preparation of porous fibers Electrospinning folio wed by selective extraction of dispersed phase morphologies Phase separation during electrospinning using two component system Phase separation during electrospinning using two component system Phase separation during electrospinning using three component system Electrospining into a cryogenic liquid (thermally induced phase separation after electrospinning)
Electrospinning folio wed by selective extraction of dispersed phase morphologies Poly(l-lactic acid) Preferential evaporation of more volatile solvent from polymer jet during electrospinning process Polysulfone Preferential evaporation of more volatile solvent from polymer jet during electrospinning process
Fiber diameter (m)
Pore size in the fibers (nm)
Specific surface area (m2 /g)
Reference
1–1.2
100–200
n.d.
Bognitzki et al. (2001a)
1–1.2
100–250
n.d.
Bognitzki et al. (2001b)
1.4–1.6
50–100
14.47
Han et al. (2005)
1.4
200–500
13.65
n.d. n.d. n.d. 30–50
n.d.
0.5–2.1
20–70
n.d.
5.3–5.6
50 nm to few hundred n.d.
Qi et al. (2009)
0.9
n.d.
60.26
Present study
1.4
20–70
57.08
1114
9.50
n.d.
MacCann et al. (2006)
Moon et al. (2008)
chemical engineering research and design 8 9 ( 2 0 1 1 ) 621–630
characterization of nanofibrous membrane amongst various other methods like mercury intrusion porosimetry because it measures the pore throat diameter without distorting pore structure (since it uses low pressure in measurement). Nanofibrous PVDF membranes (membranes D and E) were prepared by electrospinning technique. These two membranes were prepared using two different solutions. In case of membrane D, the ratio of DMF and acetone used to solubilize PVDF polymer was 80:20; while in the case of membrane E, it was 85:15. Scanning electron microscopic picture of these membranes (Fig. 8) reveals the absence of beads in membrane D and the presence of beads in membrane E. The average diameter of fibers in membrane D and E is found out to be 300 and 400 nm, respectively. Pore size distribution of membrane D and E measured by capillary flow porometry is shown in Figs. 9 and 10, respectively. Membrane D has pore size distribution between 0.3 and 1 m. Due to presence of beads, the membrane E has a larger pore size distribution (between 0.95 and 3 m).
4.3. Flow characterization (transport property) of nanofibrous membrane
629
after the drying of the fibers. Introduction of mesopores in the fibers enhances the surface area and helps to make the surface area accessible to contacting fluids. On the other hand, the introduction of macropores in the fibers increases the pore volume and helps to reduce the diffusion resistances. To achieve ∼ 60 m2 /g specific surface area by reducing the fiber diameter, one needs to draw the fibers down to 50–60 nm (Gibson et al., 2001). In the present study, 60 m2 /g of specific surface area is achieved in the fibers of average diameter of 900 nm. Thus, we conclude that specific surface area results from the porous fibers are much higher than one can achieve by reducing the diameter of fibers. Surface, flow and structural characterization of nano- and microfibrous polymeric material is important for designing materials for catalytic filtration. From the results obtained, we conclude that by electrospinning technique, it is feasible to produce the fibrous membrane of specific surface area and Darcy permeability higher than 60 m2 /g and 1 × 10−11 m2 , respectively. Beaded fibers in the fibrous materials can be used to reduce the bulk transport resistances during application of these materials in catalytic filtration.
Acknowledgement Forchheimer equation was used to estimate the viscous and inertial term coefficients of PVDF nanofibrous membranes. The results are obtained from Fig. 11. The estimated viscous and inertial terms coefficients are tabulated in Table 6. Presence of beads in the membrane E reduced the values of viscous and inertial resistance terms coefficients, which is evinced from Table 6. Presence of beads in the membrane E increased the overall pore size distribution (Fig. 10); thereby increase the permeability of gas through membrane [Darcy permeability increased due to presence of beads in the membrane E (Table 6)].
5.
Discussion and conclusions
The solid fraction in the fibrous material can be as low as 0.01% and as high as 20%. The electrospinning process can be conveniently used to prepare a fibrous material of different solid fraction. In the electrospinning process, the polymer solution of suitable conductivity, viscosity, surface tension and volatility is forced through a spinneret under the influence of high electric voltage. A resultant liquid jet then passes through a controlled temperature and humidity cabinet to evaporate the solvent before the deposition of fibers on the collector. The deposition rate of nano/microfiber varies depending upon the solution properties and electrospinning conditions. The influence of electrospinning process parameters on the formation of nano/microfibers and the quality of fibers has been discussed by Ramakrishana et al. (2005). We report a method for incorporating the mesopores and macropores in the nanoand microfibrous membrane material made of polysulfone. Results are compared with reported ones from the literature and shown in Table 7. It can be noted that the specific surface area obtained in this study is nearly four times higher than that reported by Han et al. (2005). The principle of the developed method is based on preferential evaporation of higher volatile solvent from the surface of the harden jet of a polymer solution in the electrospinning process. As more volatile solvent evaporates from the polymer jet, the nanometer scale morphologies (2–70 nm) remain behind in the fibers, which get preserved
This work is supported by the Agency for Science and Technology and Research (A*Star) of Singapore under the Singapore-Poland research grant (Grant No. WBS: R-398-000041-305).
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