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Preparation and characterization of nanofibrous filtering media
Journal of Membrane Science 283 (2006) 209–218
Preparation and characterization of nanofibrous filtering media R.S. Barhate, Chong Kian Loong, Seeram Ramakrishna ∗ NUS Nanoscience and Nanotechnology Initiative, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore Received 24 March 2006; received in revised form 17 June 2006; accepted 19 June 2006 Available online 23 June 2006
Abstract Nanofibrous membranes offer unique properties for filtration- and adsorption-based separations including high specific surface area, good interconnectivity of pores and the potential to incorporate active chemistry on a nanoscale. The most versatile process for producing a nanofibrous membrane is electrospinning. Electrospinning process parameters such as the applied electric field (drawing rate), the rotational speed of collector (collection rate) and the tip-to-target distance strongly influence the extent of fiber crossing that occurs while collecting the nanofibers and affect the fiber arrangements. In turn, they have an effect on structural and transport properties of the electrospun mat. We investigated the structural and transport properties of electrospun membrane in relation to the processing parameters in order to understand the distribution, deposition and orientation of nanofibers in the nanofibrous filtering media. Our results demonstrate that control over the pore size distribution can be achieved by coordinating the drawing and collection rates. © 2006 Elsevier B.V. All rights reserved. Keywords: Microfilter; Nanofibers; Electrospinning; Microfiltration media; Membrane characterization
1. Introduction Microfiltration is an important operation for biopharmaceutical processes, membrane pre-treatment, water and air purification, and food and beverage applications. The majority of commercial microfiltering media are inherently inhomogeneous (non-uniform in mass and thickness) at all locations. This affects the operational performance of the filtering media. Often the developmental objectives during fabrication of efficient microfiltration membranes are permeability, filtration performance and attaining uniformity in structure. The physical and operational properties of fibrous media can be significantly improved by preparing them from cylindrical ultra-thin fibers [1]. Recently, there have been a few attempts to prepare microfilter from nanofibers [2–5]. Nanofiber mats offer unique properties such as high specific surface area (ranging from 1–35 m2 /g depending on the diameter of the fibers), good interconnectivity of the pores and the potential to incorporate active chemistry or functionality on a nanoscale [6]. The most versatile process for producing nanofibers (of diameter 100–500 nm) is electrospinning [6–9]. The nanofiber generation by the elec-
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Corresponding author. Tel.: +65 65162142; fax: +65 67730339. E-mail address: [email protected] (S. Ramakrishna).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.06.030
trospinning process has been extensively studied worldwide [6–9]. The challenges confronted during fabrication of nanofiber mat by electrospinning are attaining: (1) homogeneity in the size (diameter) distribution of fibers in the mat, (2) uniformity in the deposition and orientation of fibers in the mat (thickness and structural indexes) and (3) durability of the fiber layers in the nanofibrous mat. The understanding the distribution, deposition and orientation of nanofibers would be extremely useful for preparation of uniform nanofibrous microfilters. Hence, this investigation is aimed at studying the orientation of nanofibers in the electrospun mat and correlating the conditions of collection of the nanofibers with the structural and transport properties of the mat. 2. Theoretical aspects 2.1. Permeability A typical plot of face velocity and driving force reveals the following relationship [10]: P = αηµ + βρµ2 , z
(1)
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where P/z is the differential pressure (P) per unit thickness (z) across the filter due to fluid flow (Pa/m), α the viscous term coefficient (m−2 ), η the viscosity of fluid (kg/ms), µ the approach velocity (m/s) [which can be obtained by dividing the volumetric flow rate of the fluid (Q) by the face area of the filter (A)], β the inertia-term (m−1 ) and ρ is the density of the fluid (kg/m3 ). The inverse of α is equal to the reference permeability measured under Darcy flow conditions, which is also known as the Darcy permeability BD (m2 ). The constants α and β, which can be determined from Eq. (1), vary with the porosity of the media [11]. Eq. (1) can be used without any modification when the absolute pressure [gauge pressure + atmospheric pressure (101 325 Pa)] on the upstream side of the filter is not greater than 1.1 times the absolute pressure on the downstream side of the filter and the rated pore diameter of the filter is more than 1 m [12]. The contribution of viscous flow (accounted by αηµ) or inertial flow (accounted by βρµ2 ) to the total flow can be found by rearranging Eq. (1) as follows [11]: f =
1 + 1, Re
(2)
where f = P/(zβρµ2 ) and Re = βρµ/αη. The relationship between the Forchheimer variables [friction factor (f) and modified Reynolds number (Re )] can be obtained for various porous materials. A transition from linear (Darcy’s law) to nonlinear behavior can be observed in the range 100 < (1/Re ) < 10 [11]. The value of β approaches zero when the flow of an incompressible fluid is very low (often less than 8 L/min). In this case Eq. (1) reduces to the following classical Darcy equation: Q BD P µ= = . (3) A η z The flow of nitrogen in the present study was considered to be incompressible because the approach velocity was always less than 0.5% of the velocity of sound (343 m/s). Darcy’s law holds for flows at low Reynolds numbers where the driving forces are small and balanced only by the viscous forces. The linearity of Darcy’s law breaks down when the flow becomes too slow (so that the interactions between the fluid and the pore wall become prominent). However, this may not be the situation in the case of nanofibers. This effect can be realized for nanofibrous media by analyzing the Knudsen flow (also known as diffusion flow). The Knudsen number (Kn) describes the molecular movements of the fluid at the fiber surface, which can be calculated via the following equation: Kn =
λ , Rf
(4)
where λ is the mean free path of air (λ = 0.066 × 10−6 m at standard conditions) and Rf is the mean value of the radius of the fibers. When Kn > 0.25, which corresponds to a fiber diameter of <528 nm, the slip flow needs to be considered. This is the unique situation that occurs in the case of nanofibrous filtering media [3]. Due to the slip flow at the nanofiber surface, the drag force acting on the fibers becomes smaller than is the case of
non-slip flow, and this translates into a lower pressure drop. On the other hand, the slip flow makes the significant portion of the fluid flowing near the fiber surface larger than in the case of non-slip flow, which results in more particles traveling near the fibers and translates to higher diffusion, interception and inertial impaction efficiencies [3]. As a result, the value of the permeability becomes larger than that calculated for a continuum non-slip flow. 2.2. Structural properties The porosity of the mat (ε) can be estimated by measuring the weight and volume of a specimen via the following equation: (w/(vo + vs )) ρmat Vo = 1− = 1− ε= Vo + V s ρfibers ρfibers w/Az = 1− , (5) ρfibers where Vo and Vs are the volumes of void and the fibers in the nanofibrous mat, respectively. ρmat and ρfibers the densities of the nanofibrous mat and fibers, respectively. w the weight of nanofiber mat, and A and z are the area and thickness of nanofiber mat, respectively. For rigid non-porous fibers, ρfibers can be considered equal to the density polymer. The size and distribution of the pores in the fibrous mat can be determined from the capillary flow porometry measurements. The diameter of pores (dp ) in this measurement is related to the pressure gradient needed to displace a wetting fluid from a capillary of diameter (dp ) by the following equation: dp =
4σ cos θ , P
(6)
where σ is the surface tension of the wetting liquid (N/m), θ the contact angle (the wetting liquid makes with the filter), and P is the differential pressure (Pa) across the filter. 2.3. Structure and permeability relations In case of pore diameters much larger than the mean free path of air, the average flow diameter of tubular pores (dav ) in the membrane can be related to the Darcy permeability using the Hagen–Poiseuille equation [12]: 2 dav µη = , 32 P/zp
(7)
where zp is the pore length. Eq. (7) can be extended to nanofibrous media by considering the realistic pore geometry and fluid flow pattern. The tortuosity factor (τ), which defines the effective length of the path of the hydraulic flow through the porous nanofiber mat, can be used to approximate the geometry and interconnectivity of the pores present in nanofibrous network. τ can be considered to be inversely proportional to ε as the fibers are randomly arranged in the mat. Eq. (7) can be modified by replacing the approach velocity (µ) by the interstitial velocity
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(µ/ε) and pore length (zp ) by average pore length (zτ or z/ε) as follows: 32(µ/ε)η 32µηz 32BD (dav ) = = 2 = . (P/(z/ε)) ε P ε2 2
(8)
3. Materials and methods 3.1. Material The solution for electrospinning was prepared by dissolving the pre-determined quantity of polyacrylonitrile (Sigma–Aldrich Product no. 181315, MW 150 000) in N,Ndimethylformamide so that the concentration was equal to 8% (w/w).
Fig. 1. A typical trajectory of moving jet in electrospinning process.
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3.2. Methods 3.2.1. Electrospinning The experimental set-up used for the preparation of nanofiber mat is shown in Fig. 1. A 3 mL plastic syringe was used to hold the electrospinning solution. The polyacrylonitrile solution was pumped at a constant rate of 2 mL/h with the help of a metering pump (model KDS 100, K.D. Scientific Inc., Holliston, MA, USA) through a stainless steel needle of inner diameter 950 m (18G 1 1/2 TW® , BD Product no. 301631, Becton-Dickinson, Franklin Lakes, NJ, USA), whose tip was made circular by using a Buehler Ecomet polishing machine. A drum of diameter of 6.206 cm and length of 13.5 cm, connected to a variable speed motor, was used to collect the nanofibers. The speed of the drum was carefully chosen so as to prevent fiber breakage during the collection. The drum rotational speed was limited to within a range of 277–1385 rev/min (0.9–4.5 m/s). A high DC voltage was applied to the needle with the help of 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 12.5–22.5 kV while the upright distance between the tip of the needle and the facing surface of the drum was varied from 10 to 16 cm. The collecting drum was ground so as to generate the desirable electric field strength between the tip of the spinneret and the collector surface. The nanofibrous mat was carefully removed from the collector, and the residual solvent associated with nanofibers mat was removed by keeping the mat in a vacuum oven (0.6 bar absolute pressure) for at least 2 days at 20 ◦ C. The dried electrospun mats were stored in desiccators. 3.2.2. Characterization of nanofiber mat The following methods were used to measure the transport and structural properties of the nanofibrous mat. The set-up used for measuring the permeability of the membrane is shown in Fig. 2. The dried anofibrous mat was cut into
Fig. 2. Schematic representation of set-up used for determination of permeability and pore size of electrospun membrane.
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a circular disc of diameter of 47 mm using an indigenously fabricated membrane cutter. The circular piece of membrane was placed in a plastic holder of 47 mm diameter (Millipore Inc., USA), and nitrogen gas was allowed to pass through the membrane. Temperature during the measurement was maintained at 20–22 ◦ C. The inlet pressure was measured by a digital pressure gauge (Merriam Merigauge 3910 model). The difference in pressure between the upstream and downstream sides of the membrane was measured by a sensitive differential pressure gauge (Model JGE50DT, DCT Instruments, products of Sensotec Inc., Columbus, Ohio, USA). The flow rate was measured by a flow meter (Aalborg Instruments, Orangeburg, NY, USA). The Darcy permeability of electrospun mat (BD ) was calculated from the value of α using Eq. (1). The viscosity and density of nitrogen were taken as 17.85 × 10−6 kg/ms and 1.22 kg/m3 , respectively. The morphological parameters of nanofibrous mat such as fiber diameter, fiber orientation and thickness were measured by the scanning electron microscope (SEM) (quanta 200F, FEI make). To determine the diameter and orientation of fibers, samples of the completely dried electrospun membrane (approximately 5 mm × 5 mm in size approximately) were fixed on the flat sample holder (stubs). The samples were cut from the electrospun web using a sharp surgical blade. The sample for the thickness measurement was prepared by cutting very thin pieces (approximately 0.5 mm × 10 mm) from the electrospun web and mounting them vertically in a metallic holder. Samples were coated with platinum in the auto fine coater (JFC-1600, JEOL, Japan) for 100 s at a current of 10 mA and vacuum of 8–10 Pa. If the sample were not coated enough, they were recoated. The distance between the electron gun and the sample, the accelerating voltage, spot size and vacuum used for capturing SEM images were 10 mm, 10 kV, 3 and 0.9 mbar, respectively. The planer images were analyzed for the fiber diameter and orientations of the fibers, while vertical or cross-sectional images were used to determine the thickness of sample. The Java image processing software [Image J 1.29 × (222 commands)] was used for calculating the diameter of the fibers and the thickness of mat from stored SEM images. At least four pictures were used to calculate the mean values of the diameter of the fibers and the thickness of the web. The differences between the diameters of the fibers in a given specimen were significant; hence the results were reported with a S.D. The porosity of nanofibrous mat was calculated by using Eq. (5). The experimental set-up for the measurement of the bubble point, mean pore size and pore size distribution was the same as that for the determination of the permeability. The nylon fiber mesh was used to support the membrane. The nanofibrous mat was placed on the supporting mesh and mounted in the filter holder (Pall Inc., USA). The pores of the nanofibrous mat were filled with wetting liquid (water). The contact angle of water droplet on the polyacrylonitrile electrospun membrane was measured using the contact angle surface analysis system (Model: VCA Optima, AST Products Inc., MA, USA), and was found out to be zero. The wetting solvent was displaced from the pores of specimen by pressurized nitrogen gas. The difference in pressure on either side of the filter was measured using a differential pres-
sure gauge (model JGE 50DT, DCT Instruments/Sensotec Inc., USA). The bubble point pressure indicated the displacement of the wetting liquid through the largest pores. The wetting liquid was displaced through the smaller sized pores when the pressure increased to higher than the bubble point pressure. This method measures constricted pore diameter [13]. The largest and smallest pore diameters were calculated using Eq. (6) from the bubble point and the largest flow pressures, respectively. The flow average pore diameter was calculated by substituting the value of the Darcy permeability in Eq. (8). 4. Results and discussion Reneker et al. [14], studied the trajectory of a moving jet in the electrospinning process, which is schematically represented in Fig. 1. The jet can be considered as partially dried and moving with a particular velocity to estimate the forces acting on it during the electrospinning process. The product of the density of air (ρa ) and the square of the velocity of the jet [v2 , where v was taken to be 4.5 m/s on the maximum side, as inferred indirectly from the SEM pictures obtained at various collection speeds] is much smaller than the product of the surface tension coefficient {0.1 kg/s2 [14]} and the inverse of the cross-sectional radius of the jet (R is assumed to be 10−5 m, as calculated by taking the draw ratio of 10 000 from a capillary of diameter of 950 m). This confirms the absence of aerodynamically driven bending instability under the experimental conditions of our investigation. The drag force and the gravity force acting on the jet are very small in magnitudes, and can be easily neglected [14]. This analysis suggests that the electrically driven bending instability is the only mechanism that influences the way in which the jet is getting oriented before depositing over the collector. The jet undergoes bending, winding, spiraling and looping path due to the electrically driven bending instability as shown in Fig. 1 [14]. The bending, winding, spiraling and looping path of the jet strongly influences the extent of fiber crossing during collection of the nanofibers, which affects the structural (pore size distribution, pore interconnectivity and porosity) and transport (permeability) properties of the electrospun filtering media. In view of this, we studied the structural and transport properties
Fig. 3. Fiber size distribution due to varying electric field strength (right portion).
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Table 1 Effect of applied electric field strength on the fiber diameter, deposition and porosity Electric field strength (V/m)
Fiber diameter (m)
Deposition (thickness) (m)
Porosity, ε (%)
L
M
R
L
M
R
L
M
R
93000 111000 130000 148000 167000
0.2 (0.041) 0.2 (0.026) 0.2 (0.059) 0.2 (0.033) 0.2 (0.038)
0.2 (0.04) 0.2 (0.04) 0.2 (0.043) 0.2 (0.03) 0.2 (0.047)
0.2 (0.053) 0.2 (0.036) 0.2 (0.040) 0.2 (0.031) 0.2 (0.054)
141 (10.66) 121 (4.52) 82 (9.94) 179 (20.08) 85 (6.16)
146 (4.41) 82 (4.86) 104 (4.47) 145 (9.02) 157 (14.4)
116 (7.18) 91 (11.42) 69 (2.07) 81 (7.85) 146 (19.58)
97.10 96.34 91.63 95.70 94.04
96.83 94.24 93.68 94.66 96.16
97.15 96.84 95.49 94.53 96.97
Values in parenthesis are S.D.; L, M and R, the left, middle and right portion of membrane, respectively.
of the electrospun mat in relation to electrospinning parameters such as the applied electric field (influencing the drawing rate), rotational speed of the collector (influencing the collection rate) and tip-to-target distance (influencing the crossing of fibers). 4.1. Effect of the electric field strength To study this effect, the electric field strength was increased from 97 000 to 167 000 V/m while keeping a fixed feed rate (2 mL/h), rotational speed of collector (1.8 m/s) and tip-tocollector distance (0.135 m). The average fiber diameter (calculated from 50 to 60 measurements using at least four SEM pictures of 120 000× magnification) remained unchanged
(Table 1). A shifting trend in the fiber distribution towards a lower fiber size was noted (Fig. 3), and this indicates that the fiber diameter slightly decreases as the electric field strength increases. The effect of electric field strength on fiber diameter is expected to be prominent in the case of highly viscous feed solutions. There was no noticeable change in thickness and porosity when the electric field strength was increased. The morphological changes in the membranes due to variation of the electric field strength are shown in Fig. 4. A reduced number of beaded fibers and smaller sized beads were observed when the electric field strength was 148 000 V/m (Fig. 4). This finding is consistent with reports in the literature, that a high electric field favors the whipping instability and suppresses the axisymmetric
Fig. 4. Morphological changes in electrospun mat due to varying electric field strength.
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Table 2 Effect of applied electric field on permeability and pore size Electric field strength (V/m)
93000 111000 130000 148000 167000
Darcy permeability (m2 )
Maximum pore diameter (m)
Minimum detectable pore diameter (m)
Average flow pore diameter (m)
L
M
R
L
M
R
L
M
R
L
M
R
5.00 × 10−13
5.00 × 10−13
5.00 × 10−13
291.20 58.24 18.20 2.09 4.48
58.24 24.27 11.65 3.94 6.62
5.94 11.65 3.51 58.24 13.24
2.17 1.90 1.53 1.40 2.21
2.04 1.76 1.57 1.37 2.14
2.24 1.97 1.89 1.57 2.31
4.12 4.15 2.33 2.96 3.47
4.13 3.00 2.70 2.67 4.16
4.12 4.13 2.96 2.68 4.13
5.00 × 10−13 1.43 × 10−13 2.50 × 10−13 3.33 × 10−13
2.50 × 10−13 2.00 × 10−13 2.00 × 10−13 5.00 × 10−13
5.00 × 10−13 2.50 × 10−13 2.00 × 10−13 5.00 × 10−13
L, M and R: the left, middle and right portion of membrane, respectively. Table 3 Effect of rotational speed of collector on the fiber diameter, deposition and porosity Speed of collector (m/s)
Fiber diameter (m)
Deposition (thickness) (m)
Porosity (ε) (%)
L
M
R
L
M
R
L
M
R
0.9 1.8 2.7 3.6 4.5
0.2 (0.045) 0.2 (0.049) 0.2 (0.062) 0.3 (0.022) 0.2 (0.035)
0.2 (0.043) 0.2 (0.036) 0.2 (0.037) 0.3 (0.046) 0.2 (0.084)
0.2 (0.098) 0.2 (0.042) 0.2 (0.036) 0.3 (0.037) 0.2 (0.064)
147 (11.82) 118 (9.41) 166 (6.91) 211 (8.12) 38 (2.44)
138 (6.89) 236 (9.51) 120 (8.30) 149 (8.15) 155 (11.43)
131 (13.46) 69 (2.25) 110 (5.91) 148 (6.57) 128 (20.34)
94.37 96.41 96.42 96.89 88.73
95.03 95.19 94.48 95.79 94.82
97.66 91.04 96.06 97.86 92.62
Values in parenthesis are S.D.; L, M and R: the left, middle and right portion of membrane, respectively.
instabilities, thereby suppressing the formation of beaded fibers [15]. The drawing rate of the nanofibers can be enhanced by increasing the electric field strength. It was hypothesized that an enhanced drawing rate would increase the number of fiber crossings and that the high extent of fiber crossing would reduce the pore size and improve the interconnectivity of pores. The pore size was measured to examine this hypothesis. Table 2 shows the effect of electric field strength on permeability and pore size. It was observed that the permeability and pore size decreased as the electric field strength increased, except at very high electric field strength like 167 000 V/m (Table 2). Charge accumulation on the fibers becomes substantial at very high electric field, and this enhances repulsion between the fibers and changes the trend in fiber arrangement as evinced from the increased permeability and the pore size values (Table 2). A considerable increase in accumulation of charge on electrospun polyacrylonitrile fibers has been noted when the electric voltage was increased [16]. The Forchheimer variables (f and Re ) from the results of electric field strength variation experiments were calculated by substituting the values of α and β (which were arrived from Eq. (1), by plotting P/(zηµ) versus ρµ/η) into Eq. (2). The plot of the inverse of the modified Reynolds number and the friction factor revealed that the Forchheimer plot in the region of 0.06 < Re < 0.5 overestimates the value of the friction factor (Fig. 5), a similar finding has been noted by Andrade et al. [11]
ing the rotational speed of the collector from 0.9 to 4.5 m/s. The average fiber diameter (calculated from 50 to 60 readings using at least four SEM pictures of 120 000× magnification) remained unchanged (Table 3) except that corresponding to a collection speed of 3.6 m/s (this variation may be attributed to
4.2. Effect of the rotational speed of collector The drawing rate (corresponding to an electric field strength of 130 000 V/m) and the tip-to-collector distance (0.135 m) were kept constant while studying the effect of rotational speed of the collector. The nanofiber collection rate was altered by vary-
Fig. 5. A plot of modified Reynolds number and friction factor (as defined in Eq. (2)).
R.S. Barhate et al. / Journal of Membrane Science 283 (2006) 209–218
215
Fig. 6. Orientation of fibers with increasing rational speed of collector (samples from right portion).
Fig. 7. Vertical section (thickness) of nanofiber mat with increasing rational speed of collector (Samples from right portion).
details of the fibrous mat. The difference between the largest pore diameter and the average flow pore diameter was found to be small in case of a membrane collected at a rotational speed of 2.7 m/s (Table 4), which indicates that for a given drawing rate the best arrangement of fibers occurs at a rotational speed of 2.7 m/s. The difference between the largest and minimum detectable pore size is high (which is indicative of a broad distribution in pore sizes) at collection speeds that are either higher or lower than 2.7 m/s. Below the rotation speed of 2.7 m/s, the deposition rate of the nanofibers was sufficiently higher than the speed of the collector, which resulted in random orientations of fibers in the mat (Fig. 6). These results demonstrate that control over the pore size distribution can be achieved by collecting the nanofibers at the optimal rotational speed of the target. The differential pressure versus air flow curve of the wet filter (which is indicative of pore size distribution) for the right portion of the membranes is shown in Fig. 8. It may be noted that the
a slight increase in the viscosity of the feed solution upon storage). Nanofibers in the electrospun mat were observed to become increasingly aligned in the direction of rotation of the collector when the rotational speed of the collector was increased (Fig. 6). The packing density and alignment of fibers increased significantly when the collection speed was increased from 3.6 to 4.5 m/s, which can be clearly seen from the vertical sections of the SEM pictures (Fig. 7). As a result, the porosity of the electrospun mat decreased considerably when the collection speed was increased from 3.6 to 4.5 m/s (Table 3). The permeability data was not adequate to define the threedimensional arrangement of the fibers. For instance, the permeability values of membranes collected at rotational speeds of 2.7 and 4.5 m/s (right portions) are the same (Table 4) but their fiber arrangements and pore sizes are different (Fig. 6 and Table 4). The permeability value, in conjunction with pore size data, would be a more meaningful way to express the structural Table 4 Effect of rotational speed of collector on permeability and pore size Speed of collector (m/s)
0.9 1.8 2.7 3.6 4.5
Darcy permeability (m2 )
Maximum pore diameter (m)
Min detectable pore diameter (m)
Average flow pore diameter (m)
L
M
R
L
M
R
L
M
R
L
M
R
10.00 × 10−13 5.00 × 10−13 5.00 × 10−13 5.00 × 10−13 1.43 × 10−13
5.00 × 10−13 5.00 × 10−13 3.33 × 10−13 5.00 × 10−13 5.00 × 10−13
5.00 × 10−13 2.50 × 10−13 3.33 × 10−13 5.00 × 10−13 3.33 × 10−13
41.60 97.07 41.01 145.60 –
105.89 24.47 7.92 66.48 4.89
78.70 – 17.13 582.40 61.96
2.62 2.57 5.39 2.22 –
4.66 2.47 3.22 2.51 2.20
4.41 – 1.82 3.00 2.29
5.99 4.15 4.15 4.13 2.41
4.21 4.20 3.45 4.18 4.22
4.09 3.11 3.40 4.09 3.53
(−) not detected due to rupture of membrane during measurements; L, M and R: the left, middle and right portion of membrane, respectively.
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Fig. 8. Effect of collection speed on the wet curve of nanofiber mat (samples from right portion). Table 5 Effect of tip-to-collector distance on the fiber diameter, deposition and porosity Tip-to-collector distance (m)
Fiber diameter (m)
Deposition (thickness) (m)
Porosity (ε) (%)
L
M
R
L
M
R
L
M
R
0.10 0.12 0.13 0.14 0.16
0.2 (0.029) 0.2 (0.027) 0.2 (0.027) 0.2 (0.033) 0.2 (0.052)
0.2 (0.035) 0.2 (0.037) 0.2 (0.04) 0.2 (0.034) 0.2 (0.084)
0.2 (0.058) 0.2 (0.029) 0.2 (0.044) 0.2 (0.041) 0.2 (0.033)
81 (16.11) 114 (8.01) 60 (6.61) 90 (14.54) 104 (13.6)
94 (11.89) 96 (6.78) 104 (15.26) 109 (7.76) 115 (4.28)
61 (8.45) 113 (9.23) 80 (7.38) 132 (12.37) 185 (16.16)
93.15 97.52 95.70 95.02 93.40
93.79 94.68 95.23 94.95 94.75
95.13 95.48 94.52 96.98 98.29
Values in parenthesis are S.D.; L, M and R: the left, middle and right portion of membrane, respectively.
wet curves of membranes collected at rotational speeds of 1.8 and 2.7 m/s do not show any difference (Fig. 8a), however, a difference may be noted when the pressure drop per thickness of the mat is considered instead of the pressure drop (Fig. 8b). In spite of having a lower membrane thickness, the wet curve of the membrane derived from a collection rate of 2.7 m/s (Fig. 8b), indicating better pore interconnectivity or networking of fibers in the vertical direction. This effect can be visualized from Fig. 7 where the membrane collected at a speed of 1.8 m/s shows the better pore interconnectivity than those collected at various other collection speeds. 4.3. Effect of the tip-to-collector distance The drawing rate (corresponding to electric field strength of 140 000 V/m) and collecting rate (1.8 m/s) were kept constant and the tip-to-target distance was altered from 0.10 to
0.16 m to study this effect. The extent of drying, deposition and orientation of fibers can be affected by increasing the tip-tocollector distance in electrospinning process. A densely packed membrane resulted instead of a porous nanofibrous membrane, when the distance between the spinneret and the collector was less than 0.08 m indicating inadequate drying of the jet. In this case the drying time was not long enough to evaporate the solvent before the deposition of nanofibers and as a result partially dried fibers were fused and spread on the collector and formed a densely packed membrane. A tip-to-collector distance of 0.10 m has improved the drying of the jet and collection of the nanofibers. An increase in the tip-to-collector distance from 0.10 to 0.16 m did not influence the average diameter of the nanofibers (Table 5); this revealed that the fibers were sufficiently dried in transit between the spinneret and the collector. The deposition of the fibers was mostly confined to the middle portion of the mandrel when the deposition distance was 0.10 m,
Table 6 Effect of tip-to-collector distance on permeability and pore size Tip-to-collector distance (m)
0.10 0.12 0.13 0.14 0.16
Darcy permeability (m2 )
Maximum pore diameter (m)
Min detectable pore diameter (m)
Average flow pore diameter (m)
L
M
R
L
M
R
L
M
R
L
M
R
2.00 × 10−13 10.00 × 10−13 2.00 × 10−13 2.00 × 10−13 1.67 × 10−13
2.00 × 10−13 5.00 × 10−13 2.00 × 10−13 2.00 × 10−13 2.00 × 10−13
2.00 × 10−13 5.00 × 10−13 2.00 × 10−13 3.33 × 10−13 5.00 × 10−13
2.45 194.13 145.60 145.60 3.10
48.53 194.13 145.60 19.41 291.20
97.07 194.13 4.94 291.20 145.60
1.66 3.94 1.85 1.54 1.46
1.51 3.07 1.47 1.47 1.48
1.66 3.39 1.55 1.79 1.65
2.72 5.80 2.64 2.66 2.48
2.70 4.22 2.66 2.66 2.67
2.66 4.19 2.68 3.37 4.07
L, M and R: the left, middle and right portion of membrane, respectively
R.S. Barhate et al. / Journal of Membrane Science 283 (2006) 209–218
Fig. 9. Effect of tip-to-collector distance on the wet curve of nanofiber mat.
which indicates that the area of the envelope cone created by the electrically driven bending jet was not adequate to cover entire length of the drum (0.135 m). The thickness of a membrane collected at a tip-to-target distance of 0.12 was measured at three-different locations. This revealed that the fiber deposi-
217
tion was fairly uniform over the entire potion of the collector and that the area of the envelope cone was sufficient to cover the length of the drum. Yarin et al. [17] have observed that the radius of the envelope cone does not extend beyond a certain distance especially when the jet solidifies and becomes much more rigid, i.e. unstretchable. In our study, we observed that by further increasing the deposition distance (0.13, 0.14 and 0.16 m), there was a tendency for the fibers to deposit unevenly on the collector (the fibers were being deposited more at the right portion of the collector). The extent of uneven deposition of the fibers increased as the deposition distance was increased from 0.13 to 0.16 m. This may be attributed to different rates of drying and charge decay from the nanofibers before depositing. The exact mechanism is still unclear. The permeability constants of the right side of the membranes progressively increased when the tip-to-collector distance was increased from 0.13 m (Table 6). The effect of tip-to-collector distance on the wet curves of the samples is apparent in Fig. 9, which prominently shows that an increase in the tip-to-collector distance from 0.10 to 0.12 m resulted in an interesting trend in the wet curves (Fig. 9), porosity values (Table 5) and pore sizes (Table 6). Fiber fusing in the direction of rotation of the collector was quite intense in the case of a tip-to-collector distance of 0.10 m, which did not yield desirable three-dimensional inter-pore connectivity and, in turn, better permeability values. This effect can be also seen in the vertical sections of nanofibrous mat (Fig. 10). Nanofibrous mat collected at a deposition distance of 0.12 m has not only yielded an even deposition on both end of the collector but also higher permeability constants (ranging from 5.0 to 10.0 × 10−13 m2 , Table 6). These results show that 0.12 m is the optimum tip-to-collector distance for collection of the nanofibers spun from 8% polyacrylonitrile in DMF at a field strength of 140 000 V/m.
Fig. 10. Vertical section (thickness) of nanofiber mat at 0.10 and 0.12 m tip-to-collector distance.
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5. Conclusion In the absence of an aerodynamically driven bending instability, electrical bending instability is the only mechanism that influences the way in which the jet is oriented before its deposition over the collector. The fiber crossing and pore size can be optimized to attain improved structural (pore size distribution, pore interconnectivity and porosity) and transport (permeability) properties of the electrospun filtering media. This can be achieved by coordination of the drawing and collection rates. Since the jet follows a bending, winding, spiraling and looping path in three-dimensions due to electrical bending instability before its deposition over the collector, the tip-to-collector distance is another parameter that may alter the degree of the fiber crossing while collecting the nanofibers. The area of the envelope cone (resulting from the electrically driven bending instability) is an important parameter to consider when targeting uniform deposition of nanofibers. Charge accumulation on the nanofibers is one of the predominately deteriorating factors when optimizing the structural and transport properties of electrospun filtering media. The results of this study demonstrate that control over the pore size distribution can be achieved by coordinating the drawing and collection rates. Acknowledgements This work is supported by the Defense Science and Technology Agency (DSTA), Government of Singapore under Grant no. WBS R-398-000-027-422. The help rendered by Mr. Ramakrishnan Ramaseshan during the SEM analysis is greatly acknowledged.
Nomenclature A BD dav f P Q Rf vo vs w z zp
face area of filter (m2 ) Darcy permeability (m2 ) average flow diameter of tubular pores (m) friction factor differential pressure across the filter (Pa) volumetric flow rate of fluid (m3 /s) mean radius of the fibers (m) volume of voids in the filtering media volume of solid (fibers) in the filtering media weight of the filtering media thickness of filter (m) pore length (m)
Greek letters α viscous term coefficient (m−2 ) β inertia-term (m−1 )
ε η θ λ µ ρ σ
differential or difference porosity of nanofiber mat viscosity of fluid (kg/ms) contact angle of wetting liquid with the filter (◦ ) mean free path of air (0.066 × 10−6 m) approach velocity (m/s) density of fluid (nitrogen gas density 1.22 kg/m3 ) surface tension of wetting liquid (N/m)
References [1] A.V. Genis, Relationship of structure and physical properties of fibre materials, Fiber Chem. 33 (1) (2001) 33. [2] T. Grafe, M. Gogins, M. Barris, J. Schaefer, R. Canepa, Nanofibers in filtration applications in transportation, in: Proceedings of the Filtration 2001 International Conference and Exposition of the INDA (Association of the Nowovens Fabric Industry), Chicago, IL, December 3–5, 2001. [3] K. Graham, M. Ouyang, T. Raether, T. Grafe, B. McDonald, P. Knauf, Polymeric nanofibers in air filtration applications, in: Proceedings of the Fifteenth Annual Technical Conference & Expo of the American Filtration & Separations Society, Galveston, Texas, April 9–12, 2002. [4] F. Montefusco, The use of nonwovens in air filtration, feature article, Filtr. Sep. 42 (2) (2005) 30. [5] Z. Ma, M. Kotaki, S. Ramakrishna, Surface modified nonwoven polysulphone fiber mesh by electrospinning: a novel affinity membrane, J. Membr. Sci. 272 (1/2) (2006) 179. [6] S. Ramakrishana, K. Fujihara, W.-E. Teo, T.-C. Lim, Z. Ma, An Introduction to Electrospinning and Nanofibers, World Scientific Publishing Co., Pvt. Ltd., 2005. [7] D.H. Reneker, H. Fong, Polymeric Nanofibers, Oxford University Press, 2006. [8] P. Gibson, H.S. Gibson, R. Rivin, Transport properties of porous membranes based on electrospun nanofibers, Colloids Surf. A 187–188 (2001) 469. [9] Y. Dzenis, Spinning continuous fibers for nanotechnology, Science 304 (2004) 1917. [10] L. Green, P. Duwez, Fluid flow through porous metals, J. Appl. Mech. 18 (1951) 39. [11] J.S. Andrade, U.M.S. Costa, M.P. Almeida, H.A. Makse, H.E. Stanley, Inertial effects on fluid flow through disordered porous media, Phys. Rev. Lett. 82 (26) (1999) 5249. [12] P.R. Johnston, Fluid Sterilization by Filtration, 3rd ed., CRC Press LLC, 2004. [13] A. Jena, K. Gupta, Pore volume of nanofiber nonwovens, Int. Nonwovens J. (2005) 25. [14] D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, Bending instability of electrically charged liquid jets of polymer solutions in electrospinning, J. Appl. Phys. 87 (9) (2000) 4531. [15] W. Zuo, M. Zhu, W. Yang, H. Yu, Y. Chen, Y. Zhang, Experimental study on relationship between jet instability and formation of beaded fibers during electrospinning, Polym. Eng. Sci. 45 (5) (2005) 704. [16] V.E. Kalayci, P.K. Patra, Y.K. Kim, S.C. Ugbolue, S.B. Warner, Charge consequences in electrospun polyacrylonitrile (PAN) nanofibers, Polymer 46 (18) (2005) 7191. [17] A.L. Yarin, S. Koombhongse, D.H. Reneker, Bending instability in electrospinning of nanofibers, J. Appl. Phys. 89 (5) (2001) 3018–3026.
Preparation and characterization of nanofibrous filtering media R.S. Barhate, Chong Kian Loong, Seeram Ramakrishna ∗ NUS Nanoscience and Nanotechnology Initiative, Faculty of Engineering, National University of Singapore, Singapore 117576, Singapore Received 24 March 2006; received in revised form 17 June 2006; accepted 19 June 2006 Available online 23 June 2006
Abstract Nanofibrous membranes offer unique properties for filtration- and adsorption-based separations including high specific surface area, good interconnectivity of pores and the potential to incorporate active chemistry on a nanoscale. The most versatile process for producing a nanofibrous membrane is electrospinning. Electrospinning process parameters such as the applied electric field (drawing rate), the rotational speed of collector (collection rate) and the tip-to-target distance strongly influence the extent of fiber crossing that occurs while collecting the nanofibers and affect the fiber arrangements. In turn, they have an effect on structural and transport properties of the electrospun mat. We investigated the structural and transport properties of electrospun membrane in relation to the processing parameters in order to understand the distribution, deposition and orientation of nanofibers in the nanofibrous filtering media. Our results demonstrate that control over the pore size distribution can be achieved by coordinating the drawing and collection rates. © 2006 Elsevier B.V. All rights reserved. Keywords: Microfilter; Nanofibers; Electrospinning; Microfiltration media; Membrane characterization
1. Introduction Microfiltration is an important operation for biopharmaceutical processes, membrane pre-treatment, water and air purification, and food and beverage applications. The majority of commercial microfiltering media are inherently inhomogeneous (non-uniform in mass and thickness) at all locations. This affects the operational performance of the filtering media. Often the developmental objectives during fabrication of efficient microfiltration membranes are permeability, filtration performance and attaining uniformity in structure. The physical and operational properties of fibrous media can be significantly improved by preparing them from cylindrical ultra-thin fibers [1]. Recently, there have been a few attempts to prepare microfilter from nanofibers [2–5]. Nanofiber mats offer unique properties such as high specific surface area (ranging from 1–35 m2 /g depending on the diameter of the fibers), good interconnectivity of the pores and the potential to incorporate active chemistry or functionality on a nanoscale [6]. The most versatile process for producing nanofibers (of diameter 100–500 nm) is electrospinning [6–9]. The nanofiber generation by the elec-
∗
Corresponding author. Tel.: +65 65162142; fax: +65 67730339. E-mail address: [email protected] (S. Ramakrishna).
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.06.030
trospinning process has been extensively studied worldwide [6–9]. The challenges confronted during fabrication of nanofiber mat by electrospinning are attaining: (1) homogeneity in the size (diameter) distribution of fibers in the mat, (2) uniformity in the deposition and orientation of fibers in the mat (thickness and structural indexes) and (3) durability of the fiber layers in the nanofibrous mat. The understanding the distribution, deposition and orientation of nanofibers would be extremely useful for preparation of uniform nanofibrous microfilters. Hence, this investigation is aimed at studying the orientation of nanofibers in the electrospun mat and correlating the conditions of collection of the nanofibers with the structural and transport properties of the mat. 2. Theoretical aspects 2.1. Permeability A typical plot of face velocity and driving force reveals the following relationship [10]: P = αηµ + βρµ2 , z
(1)
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where P/z is the differential pressure (P) per unit thickness (z) across the filter due to fluid flow (Pa/m), α the viscous term coefficient (m−2 ), η the viscosity of fluid (kg/ms), µ the approach velocity (m/s) [which can be obtained by dividing the volumetric flow rate of the fluid (Q) by the face area of the filter (A)], β the inertia-term (m−1 ) and ρ is the density of the fluid (kg/m3 ). The inverse of α is equal to the reference permeability measured under Darcy flow conditions, which is also known as the Darcy permeability BD (m2 ). The constants α and β, which can be determined from Eq. (1), vary with the porosity of the media [11]. Eq. (1) can be used without any modification when the absolute pressure [gauge pressure + atmospheric pressure (101 325 Pa)] on the upstream side of the filter is not greater than 1.1 times the absolute pressure on the downstream side of the filter and the rated pore diameter of the filter is more than 1 m [12]. The contribution of viscous flow (accounted by αηµ) or inertial flow (accounted by βρµ2 ) to the total flow can be found by rearranging Eq. (1) as follows [11]: f =
1 + 1, Re
(2)
where f = P/(zβρµ2 ) and Re = βρµ/αη. The relationship between the Forchheimer variables [friction factor (f) and modified Reynolds number (Re )] can be obtained for various porous materials. A transition from linear (Darcy’s law) to nonlinear behavior can be observed in the range 100 < (1/Re ) < 10 [11]. The value of β approaches zero when the flow of an incompressible fluid is very low (often less than 8 L/min). In this case Eq. (1) reduces to the following classical Darcy equation: Q BD P µ= = . (3) A η z The flow of nitrogen in the present study was considered to be incompressible because the approach velocity was always less than 0.5% of the velocity of sound (343 m/s). Darcy’s law holds for flows at low Reynolds numbers where the driving forces are small and balanced only by the viscous forces. The linearity of Darcy’s law breaks down when the flow becomes too slow (so that the interactions between the fluid and the pore wall become prominent). However, this may not be the situation in the case of nanofibers. This effect can be realized for nanofibrous media by analyzing the Knudsen flow (also known as diffusion flow). The Knudsen number (Kn) describes the molecular movements of the fluid at the fiber surface, which can be calculated via the following equation: Kn =
λ , Rf
(4)
where λ is the mean free path of air (λ = 0.066 × 10−6 m at standard conditions) and Rf is the mean value of the radius of the fibers. When Kn > 0.25, which corresponds to a fiber diameter of <528 nm, the slip flow needs to be considered. This is the unique situation that occurs in the case of nanofibrous filtering media [3]. Due to the slip flow at the nanofiber surface, the drag force acting on the fibers becomes smaller than is the case of
non-slip flow, and this translates into a lower pressure drop. On the other hand, the slip flow makes the significant portion of the fluid flowing near the fiber surface larger than in the case of non-slip flow, which results in more particles traveling near the fibers and translates to higher diffusion, interception and inertial impaction efficiencies [3]. As a result, the value of the permeability becomes larger than that calculated for a continuum non-slip flow. 2.2. Structural properties The porosity of the mat (ε) can be estimated by measuring the weight and volume of a specimen via the following equation: (w/(vo + vs )) ρmat Vo = 1− = 1− ε= Vo + V s ρfibers ρfibers w/Az = 1− , (5) ρfibers where Vo and Vs are the volumes of void and the fibers in the nanofibrous mat, respectively. ρmat and ρfibers the densities of the nanofibrous mat and fibers, respectively. w the weight of nanofiber mat, and A and z are the area and thickness of nanofiber mat, respectively. For rigid non-porous fibers, ρfibers can be considered equal to the density polymer. The size and distribution of the pores in the fibrous mat can be determined from the capillary flow porometry measurements. The diameter of pores (dp ) in this measurement is related to the pressure gradient needed to displace a wetting fluid from a capillary of diameter (dp ) by the following equation: dp =
4σ cos θ , P
(6)
where σ is the surface tension of the wetting liquid (N/m), θ the contact angle (the wetting liquid makes with the filter), and P is the differential pressure (Pa) across the filter. 2.3. Structure and permeability relations In case of pore diameters much larger than the mean free path of air, the average flow diameter of tubular pores (dav ) in the membrane can be related to the Darcy permeability using the Hagen–Poiseuille equation [12]: 2 dav µη = , 32 P/zp
(7)
where zp is the pore length. Eq. (7) can be extended to nanofibrous media by considering the realistic pore geometry and fluid flow pattern. The tortuosity factor (τ), which defines the effective length of the path of the hydraulic flow through the porous nanofiber mat, can be used to approximate the geometry and interconnectivity of the pores present in nanofibrous network. τ can be considered to be inversely proportional to ε as the fibers are randomly arranged in the mat. Eq. (7) can be modified by replacing the approach velocity (µ) by the interstitial velocity
R.S. Barhate et al. / Journal of Membrane Science 283 (2006) 209–218
(µ/ε) and pore length (zp ) by average pore length (zτ or z/ε) as follows: 32(µ/ε)η 32µηz 32BD (dav ) = = 2 = . (P/(z/ε)) ε P ε2 2
(8)
3. Materials and methods 3.1. Material The solution for electrospinning was prepared by dissolving the pre-determined quantity of polyacrylonitrile (Sigma–Aldrich Product no. 181315, MW 150 000) in N,Ndimethylformamide so that the concentration was equal to 8% (w/w).
Fig. 1. A typical trajectory of moving jet in electrospinning process.
211
3.2. Methods 3.2.1. Electrospinning The experimental set-up used for the preparation of nanofiber mat is shown in Fig. 1. A 3 mL plastic syringe was used to hold the electrospinning solution. The polyacrylonitrile solution was pumped at a constant rate of 2 mL/h with the help of a metering pump (model KDS 100, K.D. Scientific Inc., Holliston, MA, USA) through a stainless steel needle of inner diameter 950 m (18G 1 1/2 TW® , BD Product no. 301631, Becton-Dickinson, Franklin Lakes, NJ, USA), whose tip was made circular by using a Buehler Ecomet polishing machine. A drum of diameter of 6.206 cm and length of 13.5 cm, connected to a variable speed motor, was used to collect the nanofibers. The speed of the drum was carefully chosen so as to prevent fiber breakage during the collection. The drum rotational speed was limited to within a range of 277–1385 rev/min (0.9–4.5 m/s). A high DC voltage was applied to the needle with the help of 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 12.5–22.5 kV while the upright distance between the tip of the needle and the facing surface of the drum was varied from 10 to 16 cm. The collecting drum was ground so as to generate the desirable electric field strength between the tip of the spinneret and the collector surface. The nanofibrous mat was carefully removed from the collector, and the residual solvent associated with nanofibers mat was removed by keeping the mat in a vacuum oven (0.6 bar absolute pressure) for at least 2 days at 20 ◦ C. The dried electrospun mats were stored in desiccators. 3.2.2. Characterization of nanofiber mat The following methods were used to measure the transport and structural properties of the nanofibrous mat. The set-up used for measuring the permeability of the membrane is shown in Fig. 2. The dried anofibrous mat was cut into
Fig. 2. Schematic representation of set-up used for determination of permeability and pore size of electrospun membrane.
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a circular disc of diameter of 47 mm using an indigenously fabricated membrane cutter. The circular piece of membrane was placed in a plastic holder of 47 mm diameter (Millipore Inc., USA), and nitrogen gas was allowed to pass through the membrane. Temperature during the measurement was maintained at 20–22 ◦ C. The inlet pressure was measured by a digital pressure gauge (Merriam Merigauge 3910 model). The difference in pressure between the upstream and downstream sides of the membrane was measured by a sensitive differential pressure gauge (Model JGE50DT, DCT Instruments, products of Sensotec Inc., Columbus, Ohio, USA). The flow rate was measured by a flow meter (Aalborg Instruments, Orangeburg, NY, USA). The Darcy permeability of electrospun mat (BD ) was calculated from the value of α using Eq. (1). The viscosity and density of nitrogen were taken as 17.85 × 10−6 kg/ms and 1.22 kg/m3 , respectively. The morphological parameters of nanofibrous mat such as fiber diameter, fiber orientation and thickness were measured by the scanning electron microscope (SEM) (quanta 200F, FEI make). To determine the diameter and orientation of fibers, samples of the completely dried electrospun membrane (approximately 5 mm × 5 mm in size approximately) were fixed on the flat sample holder (stubs). The samples were cut from the electrospun web using a sharp surgical blade. The sample for the thickness measurement was prepared by cutting very thin pieces (approximately 0.5 mm × 10 mm) from the electrospun web and mounting them vertically in a metallic holder. Samples were coated with platinum in the auto fine coater (JFC-1600, JEOL, Japan) for 100 s at a current of 10 mA and vacuum of 8–10 Pa. If the sample were not coated enough, they were recoated. The distance between the electron gun and the sample, the accelerating voltage, spot size and vacuum used for capturing SEM images were 10 mm, 10 kV, 3 and 0.9 mbar, respectively. The planer images were analyzed for the fiber diameter and orientations of the fibers, while vertical or cross-sectional images were used to determine the thickness of sample. The Java image processing software [Image J 1.29 × (222 commands)] was used for calculating the diameter of the fibers and the thickness of mat from stored SEM images. At least four pictures were used to calculate the mean values of the diameter of the fibers and the thickness of the web. The differences between the diameters of the fibers in a given specimen were significant; hence the results were reported with a S.D. The porosity of nanofibrous mat was calculated by using Eq. (5). The experimental set-up for the measurement of the bubble point, mean pore size and pore size distribution was the same as that for the determination of the permeability. The nylon fiber mesh was used to support the membrane. The nanofibrous mat was placed on the supporting mesh and mounted in the filter holder (Pall Inc., USA). The pores of the nanofibrous mat were filled with wetting liquid (water). The contact angle of water droplet on the polyacrylonitrile electrospun membrane was measured using the contact angle surface analysis system (Model: VCA Optima, AST Products Inc., MA, USA), and was found out to be zero. The wetting solvent was displaced from the pores of specimen by pressurized nitrogen gas. The difference in pressure on either side of the filter was measured using a differential pres-
sure gauge (model JGE 50DT, DCT Instruments/Sensotec Inc., USA). The bubble point pressure indicated the displacement of the wetting liquid through the largest pores. The wetting liquid was displaced through the smaller sized pores when the pressure increased to higher than the bubble point pressure. This method measures constricted pore diameter [13]. The largest and smallest pore diameters were calculated using Eq. (6) from the bubble point and the largest flow pressures, respectively. The flow average pore diameter was calculated by substituting the value of the Darcy permeability in Eq. (8). 4. Results and discussion Reneker et al. [14], studied the trajectory of a moving jet in the electrospinning process, which is schematically represented in Fig. 1. The jet can be considered as partially dried and moving with a particular velocity to estimate the forces acting on it during the electrospinning process. The product of the density of air (ρa ) and the square of the velocity of the jet [v2 , where v was taken to be 4.5 m/s on the maximum side, as inferred indirectly from the SEM pictures obtained at various collection speeds] is much smaller than the product of the surface tension coefficient {0.1 kg/s2 [14]} and the inverse of the cross-sectional radius of the jet (R is assumed to be 10−5 m, as calculated by taking the draw ratio of 10 000 from a capillary of diameter of 950 m). This confirms the absence of aerodynamically driven bending instability under the experimental conditions of our investigation. The drag force and the gravity force acting on the jet are very small in magnitudes, and can be easily neglected [14]. This analysis suggests that the electrically driven bending instability is the only mechanism that influences the way in which the jet is getting oriented before depositing over the collector. The jet undergoes bending, winding, spiraling and looping path due to the electrically driven bending instability as shown in Fig. 1 [14]. The bending, winding, spiraling and looping path of the jet strongly influences the extent of fiber crossing during collection of the nanofibers, which affects the structural (pore size distribution, pore interconnectivity and porosity) and transport (permeability) properties of the electrospun filtering media. In view of this, we studied the structural and transport properties
Fig. 3. Fiber size distribution due to varying electric field strength (right portion).
R.S. Barhate et al. / Journal of Membrane Science 283 (2006) 209–218
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Table 1 Effect of applied electric field strength on the fiber diameter, deposition and porosity Electric field strength (V/m)
Fiber diameter (m)
Deposition (thickness) (m)
Porosity, ε (%)
L
M
R
L
M
R
L
M
R
93000 111000 130000 148000 167000
0.2 (0.041) 0.2 (0.026) 0.2 (0.059) 0.2 (0.033) 0.2 (0.038)
0.2 (0.04) 0.2 (0.04) 0.2 (0.043) 0.2 (0.03) 0.2 (0.047)
0.2 (0.053) 0.2 (0.036) 0.2 (0.040) 0.2 (0.031) 0.2 (0.054)
141 (10.66) 121 (4.52) 82 (9.94) 179 (20.08) 85 (6.16)
146 (4.41) 82 (4.86) 104 (4.47) 145 (9.02) 157 (14.4)
116 (7.18) 91 (11.42) 69 (2.07) 81 (7.85) 146 (19.58)
97.10 96.34 91.63 95.70 94.04
96.83 94.24 93.68 94.66 96.16
97.15 96.84 95.49 94.53 96.97
Values in parenthesis are S.D.; L, M and R, the left, middle and right portion of membrane, respectively.
of the electrospun mat in relation to electrospinning parameters such as the applied electric field (influencing the drawing rate), rotational speed of the collector (influencing the collection rate) and tip-to-target distance (influencing the crossing of fibers). 4.1. Effect of the electric field strength To study this effect, the electric field strength was increased from 97 000 to 167 000 V/m while keeping a fixed feed rate (2 mL/h), rotational speed of collector (1.8 m/s) and tip-tocollector distance (0.135 m). The average fiber diameter (calculated from 50 to 60 measurements using at least four SEM pictures of 120 000× magnification) remained unchanged
(Table 1). A shifting trend in the fiber distribution towards a lower fiber size was noted (Fig. 3), and this indicates that the fiber diameter slightly decreases as the electric field strength increases. The effect of electric field strength on fiber diameter is expected to be prominent in the case of highly viscous feed solutions. There was no noticeable change in thickness and porosity when the electric field strength was increased. The morphological changes in the membranes due to variation of the electric field strength are shown in Fig. 4. A reduced number of beaded fibers and smaller sized beads were observed when the electric field strength was 148 000 V/m (Fig. 4). This finding is consistent with reports in the literature, that a high electric field favors the whipping instability and suppresses the axisymmetric
Fig. 4. Morphological changes in electrospun mat due to varying electric field strength.
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Table 2 Effect of applied electric field on permeability and pore size Electric field strength (V/m)
93000 111000 130000 148000 167000
Darcy permeability (m2 )
Maximum pore diameter (m)
Minimum detectable pore diameter (m)
Average flow pore diameter (m)
L
M
R
L
M
R
L
M
R
L
M
R
5.00 × 10−13
5.00 × 10−13
5.00 × 10−13
291.20 58.24 18.20 2.09 4.48
58.24 24.27 11.65 3.94 6.62
5.94 11.65 3.51 58.24 13.24
2.17 1.90 1.53 1.40 2.21
2.04 1.76 1.57 1.37 2.14
2.24 1.97 1.89 1.57 2.31
4.12 4.15 2.33 2.96 3.47
4.13 3.00 2.70 2.67 4.16
4.12 4.13 2.96 2.68 4.13
5.00 × 10−13 1.43 × 10−13 2.50 × 10−13 3.33 × 10−13
2.50 × 10−13 2.00 × 10−13 2.00 × 10−13 5.00 × 10−13
5.00 × 10−13 2.50 × 10−13 2.00 × 10−13 5.00 × 10−13
L, M and R: the left, middle and right portion of membrane, respectively. Table 3 Effect of rotational speed of collector on the fiber diameter, deposition and porosity Speed of collector (m/s)
Fiber diameter (m)
Deposition (thickness) (m)
Porosity (ε) (%)
L
M
R
L
M
R
L
M
R
0.9 1.8 2.7 3.6 4.5
0.2 (0.045) 0.2 (0.049) 0.2 (0.062) 0.3 (0.022) 0.2 (0.035)
0.2 (0.043) 0.2 (0.036) 0.2 (0.037) 0.3 (0.046) 0.2 (0.084)
0.2 (0.098) 0.2 (0.042) 0.2 (0.036) 0.3 (0.037) 0.2 (0.064)
147 (11.82) 118 (9.41) 166 (6.91) 211 (8.12) 38 (2.44)
138 (6.89) 236 (9.51) 120 (8.30) 149 (8.15) 155 (11.43)
131 (13.46) 69 (2.25) 110 (5.91) 148 (6.57) 128 (20.34)
94.37 96.41 96.42 96.89 88.73
95.03 95.19 94.48 95.79 94.82
97.66 91.04 96.06 97.86 92.62
Values in parenthesis are S.D.; L, M and R: the left, middle and right portion of membrane, respectively.
instabilities, thereby suppressing the formation of beaded fibers [15]. The drawing rate of the nanofibers can be enhanced by increasing the electric field strength. It was hypothesized that an enhanced drawing rate would increase the number of fiber crossings and that the high extent of fiber crossing would reduce the pore size and improve the interconnectivity of pores. The pore size was measured to examine this hypothesis. Table 2 shows the effect of electric field strength on permeability and pore size. It was observed that the permeability and pore size decreased as the electric field strength increased, except at very high electric field strength like 167 000 V/m (Table 2). Charge accumulation on the fibers becomes substantial at very high electric field, and this enhances repulsion between the fibers and changes the trend in fiber arrangement as evinced from the increased permeability and the pore size values (Table 2). A considerable increase in accumulation of charge on electrospun polyacrylonitrile fibers has been noted when the electric voltage was increased [16]. The Forchheimer variables (f and Re ) from the results of electric field strength variation experiments were calculated by substituting the values of α and β (which were arrived from Eq. (1), by plotting P/(zηµ) versus ρµ/η) into Eq. (2). The plot of the inverse of the modified Reynolds number and the friction factor revealed that the Forchheimer plot in the region of 0.06 < Re < 0.5 overestimates the value of the friction factor (Fig. 5), a similar finding has been noted by Andrade et al. [11]
ing the rotational speed of the collector from 0.9 to 4.5 m/s. The average fiber diameter (calculated from 50 to 60 readings using at least four SEM pictures of 120 000× magnification) remained unchanged (Table 3) except that corresponding to a collection speed of 3.6 m/s (this variation may be attributed to
4.2. Effect of the rotational speed of collector The drawing rate (corresponding to an electric field strength of 130 000 V/m) and the tip-to-collector distance (0.135 m) were kept constant while studying the effect of rotational speed of the collector. The nanofiber collection rate was altered by vary-
Fig. 5. A plot of modified Reynolds number and friction factor (as defined in Eq. (2)).
R.S. Barhate et al. / Journal of Membrane Science 283 (2006) 209–218
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Fig. 6. Orientation of fibers with increasing rational speed of collector (samples from right portion).
Fig. 7. Vertical section (thickness) of nanofiber mat with increasing rational speed of collector (Samples from right portion).
details of the fibrous mat. The difference between the largest pore diameter and the average flow pore diameter was found to be small in case of a membrane collected at a rotational speed of 2.7 m/s (Table 4), which indicates that for a given drawing rate the best arrangement of fibers occurs at a rotational speed of 2.7 m/s. The difference between the largest and minimum detectable pore size is high (which is indicative of a broad distribution in pore sizes) at collection speeds that are either higher or lower than 2.7 m/s. Below the rotation speed of 2.7 m/s, the deposition rate of the nanofibers was sufficiently higher than the speed of the collector, which resulted in random orientations of fibers in the mat (Fig. 6). These results demonstrate that control over the pore size distribution can be achieved by collecting the nanofibers at the optimal rotational speed of the target. The differential pressure versus air flow curve of the wet filter (which is indicative of pore size distribution) for the right portion of the membranes is shown in Fig. 8. It may be noted that the
a slight increase in the viscosity of the feed solution upon storage). Nanofibers in the electrospun mat were observed to become increasingly aligned in the direction of rotation of the collector when the rotational speed of the collector was increased (Fig. 6). The packing density and alignment of fibers increased significantly when the collection speed was increased from 3.6 to 4.5 m/s, which can be clearly seen from the vertical sections of the SEM pictures (Fig. 7). As a result, the porosity of the electrospun mat decreased considerably when the collection speed was increased from 3.6 to 4.5 m/s (Table 3). The permeability data was not adequate to define the threedimensional arrangement of the fibers. For instance, the permeability values of membranes collected at rotational speeds of 2.7 and 4.5 m/s (right portions) are the same (Table 4) but their fiber arrangements and pore sizes are different (Fig. 6 and Table 4). The permeability value, in conjunction with pore size data, would be a more meaningful way to express the structural Table 4 Effect of rotational speed of collector on permeability and pore size Speed of collector (m/s)
0.9 1.8 2.7 3.6 4.5
Darcy permeability (m2 )
Maximum pore diameter (m)
Min detectable pore diameter (m)
Average flow pore diameter (m)
L
M
R
L
M
R
L
M
R
L
M
R
10.00 × 10−13 5.00 × 10−13 5.00 × 10−13 5.00 × 10−13 1.43 × 10−13
5.00 × 10−13 5.00 × 10−13 3.33 × 10−13 5.00 × 10−13 5.00 × 10−13
5.00 × 10−13 2.50 × 10−13 3.33 × 10−13 5.00 × 10−13 3.33 × 10−13
41.60 97.07 41.01 145.60 –
105.89 24.47 7.92 66.48 4.89
78.70 – 17.13 582.40 61.96
2.62 2.57 5.39 2.22 –
4.66 2.47 3.22 2.51 2.20
4.41 – 1.82 3.00 2.29
5.99 4.15 4.15 4.13 2.41
4.21 4.20 3.45 4.18 4.22
4.09 3.11 3.40 4.09 3.53
(−) not detected due to rupture of membrane during measurements; L, M and R: the left, middle and right portion of membrane, respectively.
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Fig. 8. Effect of collection speed on the wet curve of nanofiber mat (samples from right portion). Table 5 Effect of tip-to-collector distance on the fiber diameter, deposition and porosity Tip-to-collector distance (m)
Fiber diameter (m)
Deposition (thickness) (m)
Porosity (ε) (%)
L
M
R
L
M
R
L
M
R
0.10 0.12 0.13 0.14 0.16
0.2 (0.029) 0.2 (0.027) 0.2 (0.027) 0.2 (0.033) 0.2 (0.052)
0.2 (0.035) 0.2 (0.037) 0.2 (0.04) 0.2 (0.034) 0.2 (0.084)
0.2 (0.058) 0.2 (0.029) 0.2 (0.044) 0.2 (0.041) 0.2 (0.033)
81 (16.11) 114 (8.01) 60 (6.61) 90 (14.54) 104 (13.6)
94 (11.89) 96 (6.78) 104 (15.26) 109 (7.76) 115 (4.28)
61 (8.45) 113 (9.23) 80 (7.38) 132 (12.37) 185 (16.16)
93.15 97.52 95.70 95.02 93.40
93.79 94.68 95.23 94.95 94.75
95.13 95.48 94.52 96.98 98.29
Values in parenthesis are S.D.; L, M and R: the left, middle and right portion of membrane, respectively.
wet curves of membranes collected at rotational speeds of 1.8 and 2.7 m/s do not show any difference (Fig. 8a), however, a difference may be noted when the pressure drop per thickness of the mat is considered instead of the pressure drop (Fig. 8b). In spite of having a lower membrane thickness, the wet curve of the membrane derived from a collection rate of 2.7 m/s (Fig. 8b), indicating better pore interconnectivity or networking of fibers in the vertical direction. This effect can be visualized from Fig. 7 where the membrane collected at a speed of 1.8 m/s shows the better pore interconnectivity than those collected at various other collection speeds. 4.3. Effect of the tip-to-collector distance The drawing rate (corresponding to electric field strength of 140 000 V/m) and collecting rate (1.8 m/s) were kept constant and the tip-to-target distance was altered from 0.10 to
0.16 m to study this effect. The extent of drying, deposition and orientation of fibers can be affected by increasing the tip-tocollector distance in electrospinning process. A densely packed membrane resulted instead of a porous nanofibrous membrane, when the distance between the spinneret and the collector was less than 0.08 m indicating inadequate drying of the jet. In this case the drying time was not long enough to evaporate the solvent before the deposition of nanofibers and as a result partially dried fibers were fused and spread on the collector and formed a densely packed membrane. A tip-to-collector distance of 0.10 m has improved the drying of the jet and collection of the nanofibers. An increase in the tip-to-collector distance from 0.10 to 0.16 m did not influence the average diameter of the nanofibers (Table 5); this revealed that the fibers were sufficiently dried in transit between the spinneret and the collector. The deposition of the fibers was mostly confined to the middle portion of the mandrel when the deposition distance was 0.10 m,
Table 6 Effect of tip-to-collector distance on permeability and pore size Tip-to-collector distance (m)
0.10 0.12 0.13 0.14 0.16
Darcy permeability (m2 )
Maximum pore diameter (m)
Min detectable pore diameter (m)
Average flow pore diameter (m)
L
M
R
L
M
R
L
M
R
L
M
R
2.00 × 10−13 10.00 × 10−13 2.00 × 10−13 2.00 × 10−13 1.67 × 10−13
2.00 × 10−13 5.00 × 10−13 2.00 × 10−13 2.00 × 10−13 2.00 × 10−13
2.00 × 10−13 5.00 × 10−13 2.00 × 10−13 3.33 × 10−13 5.00 × 10−13
2.45 194.13 145.60 145.60 3.10
48.53 194.13 145.60 19.41 291.20
97.07 194.13 4.94 291.20 145.60
1.66 3.94 1.85 1.54 1.46
1.51 3.07 1.47 1.47 1.48
1.66 3.39 1.55 1.79 1.65
2.72 5.80 2.64 2.66 2.48
2.70 4.22 2.66 2.66 2.67
2.66 4.19 2.68 3.37 4.07
L, M and R: the left, middle and right portion of membrane, respectively
R.S. Barhate et al. / Journal of Membrane Science 283 (2006) 209–218
Fig. 9. Effect of tip-to-collector distance on the wet curve of nanofiber mat.
which indicates that the area of the envelope cone created by the electrically driven bending jet was not adequate to cover entire length of the drum (0.135 m). The thickness of a membrane collected at a tip-to-target distance of 0.12 was measured at three-different locations. This revealed that the fiber deposi-
217
tion was fairly uniform over the entire potion of the collector and that the area of the envelope cone was sufficient to cover the length of the drum. Yarin et al. [17] have observed that the radius of the envelope cone does not extend beyond a certain distance especially when the jet solidifies and becomes much more rigid, i.e. unstretchable. In our study, we observed that by further increasing the deposition distance (0.13, 0.14 and 0.16 m), there was a tendency for the fibers to deposit unevenly on the collector (the fibers were being deposited more at the right portion of the collector). The extent of uneven deposition of the fibers increased as the deposition distance was increased from 0.13 to 0.16 m. This may be attributed to different rates of drying and charge decay from the nanofibers before depositing. The exact mechanism is still unclear. The permeability constants of the right side of the membranes progressively increased when the tip-to-collector distance was increased from 0.13 m (Table 6). The effect of tip-to-collector distance on the wet curves of the samples is apparent in Fig. 9, which prominently shows that an increase in the tip-to-collector distance from 0.10 to 0.12 m resulted in an interesting trend in the wet curves (Fig. 9), porosity values (Table 5) and pore sizes (Table 6). Fiber fusing in the direction of rotation of the collector was quite intense in the case of a tip-to-collector distance of 0.10 m, which did not yield desirable three-dimensional inter-pore connectivity and, in turn, better permeability values. This effect can be also seen in the vertical sections of nanofibrous mat (Fig. 10). Nanofibrous mat collected at a deposition distance of 0.12 m has not only yielded an even deposition on both end of the collector but also higher permeability constants (ranging from 5.0 to 10.0 × 10−13 m2 , Table 6). These results show that 0.12 m is the optimum tip-to-collector distance for collection of the nanofibers spun from 8% polyacrylonitrile in DMF at a field strength of 140 000 V/m.
Fig. 10. Vertical section (thickness) of nanofiber mat at 0.10 and 0.12 m tip-to-collector distance.
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5. Conclusion In the absence of an aerodynamically driven bending instability, electrical bending instability is the only mechanism that influences the way in which the jet is oriented before its deposition over the collector. The fiber crossing and pore size can be optimized to attain improved structural (pore size distribution, pore interconnectivity and porosity) and transport (permeability) properties of the electrospun filtering media. This can be achieved by coordination of the drawing and collection rates. Since the jet follows a bending, winding, spiraling and looping path in three-dimensions due to electrical bending instability before its deposition over the collector, the tip-to-collector distance is another parameter that may alter the degree of the fiber crossing while collecting the nanofibers. The area of the envelope cone (resulting from the electrically driven bending instability) is an important parameter to consider when targeting uniform deposition of nanofibers. Charge accumulation on the nanofibers is one of the predominately deteriorating factors when optimizing the structural and transport properties of electrospun filtering media. The results of this study demonstrate that control over the pore size distribution can be achieved by coordinating the drawing and collection rates. Acknowledgements This work is supported by the Defense Science and Technology Agency (DSTA), Government of Singapore under Grant no. WBS R-398-000-027-422. The help rendered by Mr. Ramakrishnan Ramaseshan during the SEM analysis is greatly acknowledged.
Nomenclature A BD dav f P Q Rf vo vs w z zp
face area of filter (m2 ) Darcy permeability (m2 ) average flow diameter of tubular pores (m) friction factor differential pressure across the filter (Pa) volumetric flow rate of fluid (m3 /s) mean radius of the fibers (m) volume of voids in the filtering media volume of solid (fibers) in the filtering media weight of the filtering media thickness of filter (m) pore length (m)
Greek letters α viscous term coefficient (m−2 ) β inertia-term (m−1 )
ε η θ λ µ ρ σ
differential or difference porosity of nanofiber mat viscosity of fluid (kg/ms) contact angle of wetting liquid with the filter (◦ ) mean free path of air (0.066 × 10−6 m) approach velocity (m/s) density of fluid (nitrogen gas density 1.22 kg/m3 ) surface tension of wetting liquid (N/m)
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