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The effect of uni-axial stretching on the roughness of microfiltration membranes
Journal of Membrane Science 280 (2006) 712–719
The effect of uni-axial stretching on the roughness of microfiltration membranes Jason A. Morehouse a , Dana L. Taylor a , Douglas R. Lloyd a,∗ , Desmond F. Lawler b , Benny D. Freeman a , Leah S. Worrel a b
a Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, United States Department of Civil, Architectural, and Environmental Engineering, The University of Texas at Austin, Austin, TX 78712, United States
Received 3 January 2006; received in revised form 10 February 2006; accepted 17 February 2006 Available online 31 March 2006
Abstract Atomic force microscopy (AFM) was used to characterize the surface morphology of uni-axially stretched and non-stretched microporous microfiltration (MF) membranes. The effect of stretching on the pore structure and bulk properties of MF membranes has been previously reported [J.A. Morehouse, L.S. Worrel, D.L. Taylor, D.R. Lloyd, B.D. Freeman, D.F. Lawler, The effect of uni-axial orientation on macroporous membrane structure, J. Porous Mater. 13 (2006) 63–75.]; this paper focuses solely on the use of AFM to characterize the surface of stretched and non-stretched MF membranes. A new way of representing surface roughness that may prove useful in relating roughness to performance in cross-flow applications is presented. © 2006 Published by Elsevier B.V. Keywords: Atomic force microscopy; Microporous; Phase inversion; Roughness; PVDF
1. Introduction Microfiltration (MF) and ultrafiltration (UF) are widely used in water treatment facilities to remove macromolecules, particles, and colloids from water [2]. A major concern in these applications is fouling of the membrane (that is, the irreversible blocking of pores by colloidal/particulate matter and by the macromolecules of natural organic matter). Numerous reports have been published on the deleterious effect fouling has on membrane performance (that is, flux and rejection) and the resulting additional cost of system operations [3–6]. The vast majority of these studies have focused on how the chemical and physical attributes of the foulant impact the extent of fouling. Only recently publications have appeared that shed some light on how fouling is impacted by the physical and chemical nature of the membrane [7–10]. This paper presents a new method of representing the physical attributes of membrane surfaces (specifically, surface roughness as determined by atomic force microscopy) that can influence membrane fouling.
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Corresponding author. Tel.: +1 512 471 4985; fax: +1 512 471 7060. E-mail address: [email protected] (D.R. Lloyd).
0376-7388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.memsci.2006.02.027
Atomic force microscopy (AFM) [11] coupled with appropriate image analysis software can provide a detailed and accurate representation of membrane surface characteristics. AFM has the advantage over scanning electron microscopy (SEM) that AFM measurements can be performed at atmospheric pressure and no membrane sample pretreatment is required prior to analysis [12–14]. It has been suggested that pore dimensions measured by AFM are more accurate than those obtained via SEM because SEM depends upon sample preparation techniques that may alter membrane structure [12–14]. However, Dietz et al. [15] finds that AFM may distort membrane pore sizes due to rounded corners near pore entrances. This distortion is on the order of the diameter of the tip used in the AFM measurements. AFM and SEM also differ significantly in their depth of field; where depth of field is a measure of an instrument’s ability to image beyond the surface of a membrane. AFM, because of the limitations of its design, can only image structure at or very near the surface of the membrane, whereas the depth of field of SEM is limited only by the ability for a metal coating to penetrate the membrane pore structure. With the limitations of each tool (SEM and AFM) it remains important to utilize both in order to fully characterize membrane surfaces.
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AFM has been used widely to analyze the surface of reverse osmosis (RO) membranes [16–25]. Vrijenhoek et al. [25] used AFM to characterize polyamide thin-film composite RO membranes in terms of RMS roughness, Ra roughness, surface area, peak count, and mean plane effect on membrane fouling. RMS roughness and Ra roughness are standard expressions of surface morphology [26] calculated as follows: N N 2 |Zi − ZAVE | i=1 (Zi − ZAVE ) RMS = Ra = i=1 N N In these equations, Zi corresponds to surface height with ZAVE being the average of N height measurements for the membrane. Roughness has often been found to correlate to membrane performance. Using a cross-flow filtration cell with a 0.01 M NaCl feed solution at a steady state trans-membrane pressure of 345 kPa, Vrijenhoek et al. [25] found that a decrease in roughness corresponded to a decrease in flux decline through the membrane. This finding was in close agreement with findings by Hirose et al. [24] and Kwak and Ihm [18] who also reported that membrane flux is proportional to membrane surface roughness in RO. Kwak and Ihm used 0.2% NaCl in deionized water with a trans-membrane pressure of 225 psig while Hirose et al. used 1500 mg/L NaCl with a trans-membrane pressure of 1.5 MPa as experimental conditions. AFM has proved to be a valuable tool in the analysis of NF, UF, and MF membranes in addition to RO. Very often, when making measurements on NF membranes, the AFM is employed in non-contact mode [27–29]. Non-contact mode relies on van der Waals interactions to repel the scanning tip from the membrane surface, thus, preventing the membrane surface from becoming damaged during the AFM scan. AFM has also been used to measure the roughness of NF membranes [25,27,30,31] as well as MF and UF membranes [32]. In addition, AFM has been used to measure pore diameter, pore density, and surface porosity in UF and MF membranes [33–36] and to elucidate the fouling of MF and UF membranes [37–40]. In the analysis of the surface roughness effect on UF membrane performance, Fell et al. [41] found that increased surface roughness adversely affected flux through Amicon UF membranes. Fell et al. used Amicon XM100 and XM300 UF membranes with nominal pore sizes between 15 and 25 nm and a feed solution of 0.1 wt.% bovine serum albumin (BSA) in a cross-flow stirred filtration cell with a trans-membrane pressure of 100 kPa. The authors hypothesized that surface roughness directly predicted the size of surface depressions found on the membrane. These surface depressions created stagnation points on the surface, which hindered flux. Cheung et al. [42], in a study of UF hollow fiber membranes with AFM, indicated results similar to those of Fell. Cheung et al. used poly(vinyl pyrrolidone) and BSA at a concentration of 200 mg/L and a trans-membrane pressure of 1.0 bar when doing hollow fiber membrane flux and rejection studies. Cheung et al. also found that the pure water flux of the membrane was proportional to the mean roughness and that increased membrane roughness resulted in decreased rejection of organic macromolecules. Using multivariate linear regression analysis, Knoell et al. [19] were able to relate MF
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membrane surface characteristics such as pore area, membrane surface height, and pore aspect ratio to bacterial attachment and water flux in modified polysulfone membranes. However, Knoell et al. [19] did not find a positive correlation between membrane surface roughness and flux or fouling. Building apon the work by Knoell et al. [19], this research intentionally modifies the pore dimensions of MF membranes in an effort to improve overall membrane performance. This modification is accomplished by uni-axially stretching membranes, thereby inducing significant and permenant membrane pore deformation. In a recent publication, the effect of stretching microporous PVDF membranes on membrane and pore structure was presented [1]. Work has also been completed by researchers at 3 M using microporous polyolefin [43–48] and poly(ethyleneco-vinyl alcohol) [49] membranes and researchers at Toten Corp. using polyethylene films [50,51]. However, none of these studies reported the effect of membrane stretching on surface roughness. In this article, standard AFM analysis methods such as roughness and bearing analysis are used to characterize stretched and non-stretched MF membranes, and a new way of expressing membrane surface morphology is developed. 2. Experimental 2.1. Sample preparation Poly(ether sulfone) (PES) and poly(vinylidene fluoride) (PVDF) membranes were obtained from Pall Corporation (http://www.pall.com) and PVDF membranes were obtained from Millipore Corporation (http://www.millipore.com). The PES membrane is an amorphous hydrophobic membrane, and the PVDF membrane is a partially crystalline hydrophilic membrane. The membranes were provided in large rolls 45.7 cm wide. When referring to membranes provided by Pall Corp., the nominal pore size is written with a tilde (∼) preceding the number to reflect the fact that the membrane provided by Pall Corporation is not exactly the same as the commercial membrane of the same nominal pore size. Instead, the membranes used in this research are laboratory samples made by Pall Corp. researchers to approximate the commercial product. Therefore, they may not have exactly the same pore characteristics as the membranes made in production scale facilities. Neither manufacturer revealed the proprietary membrane formation/modification methodology. From these rolls, samples of 11.9 cm × 7.6 cm were cut using a rectangular die. When stretched, membranes necked to various degrees depending on the total elongation of the membrane. As discussed in a previous paper, statistical analysis was completed to determine the degree of non-uniformity through the membrane [1]. The pores in the 10% of the membrane closest to the edges were found to be non-representative of the membrane’s bulk pore size and shape after stretching. For this reason, no samples were taken from this outer region. Each sample was uni-axially stretched in a T.M. Long biaxial stretcher (Inventure Labs, Knoxville TN; http://www.inventurelabs.com) at a desired temperature and
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crosshead speed (i.e., the stretching speed) and then annealed for 15 min at a specified temperature. The sample was then air cooled while being held under tension until the membrane reached room temperature. The sample was then removed and measured with a ruler accurate to 1.3 mm to determine the actual strain obtained. The strain, ε is defined as ε = (Lf –Li )/Li . Lf is the final membrane length after stretching, and Li is the initial membrane length. Elongation was then calculated by multiplying strain by 100%. All samples were stretched in the same direction that they were extruded or cast. Once the stretched membranes were annealed and cooled, they did not show any significant recovery of their length in the stretching direction when monitored for 4 months. All membranes presented herein were stretched at 160 ◦ C and at a head speed of 0.1 cm/s. Details of the procedure are given elsewhere [1]. 2.2. Atomic force microscopy Surface characterization of the membranes was performed with a Digital Instruments, Dimensions 3100 AFM (http://www.digitalinstruments.com). The AFM is equipped with a non-contact/contact head and a 100 m scanner that was operated in constant force mode. The membrane sample was attached to a 22 mm2 glass microscope cover glass slide (Corning # 2865-22) using double-sided tape (3 M Scotch 2002 Photo and Document Tape). The AFM detects the deflection of a laser beam reflecting from an oscillating tip as the tip scans the membrane surface. The sample moves laterally under the tip and a feedback loop keeps the vertical position of the tip constant by moving the surface up and down with a piezoelectric xyztranslator. Images were acquired utilizing silicon MPP-21100 AFM cantilevers with a 5 N/m spring constant and a 75 kHz resonant frequency (Nano Devices; http://www.nanodevices.com/). The cantilevers used have conical tips with nominally less than a 10 nm radius. Tapping mode was employed to reduce lateral forces on the tip and distortion of membrane surface. AFM images were acquired at a scan rate of 0.3 Hz with a scan resolution of 256 pixels × 256 pixels. All information about surface morphology (RMS roughness, Ra roughness, and bearing) was acquired using the data analysis software provided by Digital Instruments (Nanoscope 5.12b). The image scan sizes used for roughness and bearing analysis were 10 m × 10 m. This image size was chosen over smaller sizes to ensure that a significant number of pores were captured for analysis. When performing AFM scans, the direction of scan with respect to the stretching direction was not controlled.
3. Results RMS and Ra roughness were measured for several different PES and PVDF membranes. As shown in Table 1, RMS roughness is quantitatively larger than Ra roughness but that when comparing data they reveal similar trends, as evidenced in Fig. 1. For that reason, the discussion here focuses only on RMS. Fig. 1 shows SEM micrographs of ∼0.2 m PVDF membranes that have been stretched from 0 to 150% elongation. The pore structure of the ∼0.2 m PVDF was the same on both sides of the membrane. The effects of stretching on pore shape and size are obvious. RMS roughness data are presented in Fig. 2 for the ∼0.2 m PVDF membranes at different elongations. The roughness values obtained are similar to values obtained by other researchers working with MF membranes [39]. The 95% confidence limits shown in Fig. 2 were established by taking samples from three separate locations within the membrane and then measuring 10 different locations for each sample for a total of 30 measurements per membrane. It can be seen that RMS roughness does not change significantly with increased membrane stretching; however, Fig. 1 clearly shows that membrane structure changes with elongation of the membrane. The effect of stretching membranes to a 30% elongation level was determined for each of the available PES and PVDF membranes, and the results are presented in Fig. 3. Once again, 95% confidence limits are indicated in the figure. Only one sample, 0.2 m PVDF from Millipore Corp., showed a significant impact of stretching on surface roughness. However, the significant uncertainty associated with the roughness measurements makes drawing any conclusion about the correlation between surface roughness and membrane stretching impossible. The large uncertainty found when doing roughness measurements on MF membranes was also reported by Knoell et al. [19]. Two of the Pall membranes; ∼0.8 m and ∼1.2 m PES, do not have a stretched membrane roughness included in Fig. 3. Those two membranes were not able to be stretched. The ∼0.8 m PES has a nodular structure (Fig. 4) that had little structural integrity and was weak under tensile stress. It is believed that the cause for the significant increase in roughness and error for the ∼0.8 m membrane is due to the difference in the nodular (Fig. 4) versus the lacy structure (Fig. 1). Differences in the formation process are believed to result in the nodular versus lacy membranes; however, the specifics of these processes are proprietary. The ∼1.2 m PES membrane proved to be too brittle to stretch. Because the two sides of this membrane seemed to have vastly different surface appearances under SEM (see Fig. 5 a and b), both surfaces were characterized with AFM. However, as shown in Fig. 3, there is no significant difference
2.3. Scanning electron microscopy Scanning electron micrographs were also obtained for each of the samples. Samples were coated from a gold palladium target for 45 s with an Electron Microscope Sciences K575 sputter coater (http://www.emsdiasum.com/). Samples were then placed into a Hitachi S-4500 field emission scanning electron microscope. A voltage of 10 kV was used in imaging the samples.
Table 1 Roughness values for ∼0.2 m PVDF membranes Elongation (%)
RMS (nm)
Ra (nm)
0 50 100 150
300 261 318 272
229 220 234 226
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Fig. 1. Clockwise from top left ∼0.2 m PVDF membranes stretched to 0%, 50%, 100% and 150% elongation. White arrows denote stretching directions. (All membranes stretched at 0.1 cm/s and 160 ◦ C.)
in the membrane surface roughness on the front and back of the ∼1.2 m PES. The ∼1.2 m PES membrane could not be stretched because of the temperature limitations of the stretching machine. It was impossible to heat the membrane above its glass transition temperature, and only above the glass transition temperature would the polymer by ductile enough for the membrane to be significantly stretched. A comparison of the bearing of the stretched versus nonstretched ∼0.2 m PVDF was completed. Bearing analysis provides additional information beyond standard roughness measurements by revealing how much of a surface lies above or
below a given height. Surface roughness is generally represented in terms of statistical deviation from average height; however, this gives little indication of height distribution over the surface. Bearing analysis determines what percentage of the surface (the “bearing ratio”) lies above or below any arbitrarily chosen height. A bearing ratio of 100% corresponds to 100% of the surface lying above a given surface height. The zero point on the abscissa corresponds to the average height of each membrane, which can be verified by noting each curve passes through the 50% bearing line at zero depth. Fig. 6 shows the repeatability
Fig. 2. Surface roughness results for ∼0.2 m PVDF membranes at various levels of elongation. (Membranes stretched at 0.1 cm/s and 160 ◦ C.)
Fig. 3. Surface roughness results for various microporous membranes (membranes stretched at 0.1 cm/s and 160 ◦ C).
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Fig. 6. Repeated bearing analysis for non-stretched ∼0.2 m PVDF membrane. Fig. 4. SEM micrograph of ∼0.8 m PES membrane.
of bearing analysis for a non-stretched ∼0.2 m PVDF membrane, by plotting the bearing analysis for one membrane under a 50% elongation with the bearing measured at three locations on the membrane. Fig. 7 shows the effect of elongation on the bearing analysis for the ∼0.2 m PVDF membrane. Based on the repeatability results presented in Fig. 6, it can be concluded from Fig. 7 that elongation does not have a significant impact on the bearing analysis. Figs. 8 and 9 present AFM images of a stretched membrane’s surface. Fig. 8 represents looking along the membrane in the direction in which it was stretched; Fig. 9 represents looking across the membrane perpendicular to the direction of stretch. The two images, although depicting the same membrane, are very different. Along the direction of strain, the aligned pore shape takes on the form of long valleys, while perpendicular to this direction, the pore openings appear very closely packed. These differences could have a significant impact on membrane performance in cross-flow filtration if the orientation of the membrane is changed relative to the flow direction. While these ideas will be explored in future research, the objective of this paper is to develop a means of expressing the difference in surface morphology as seen in the two directions.
In an effort to quantify these differences, section plots were constructed of the membranes surface in the two directions. Samples of these surface plots are shown in Fig. 10. Further analysis was completed by allowing the AFM software to draw a center line through the section plot to bisect the data in such a way that half of the height data of the surface plot was above
Fig. 7. Bearing analysis for ∼0.2 m PVDF membrane tested at various levels of elongation.
Fig. 5. SEM micrographs of both sides of a ∼1.2 m PES membrane.
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Fig. 11. Results from AFM section analysis of ∼0.2 m PVDF that yields the average distance between pore walls. Fig. 8. AFM micrograph showing “along stretch direction” view of ∼0.2 m PVDF membrane. (Membrane stretched to a elongation of 150%.)
the line and half of the height data was below the center line. The number of times the section plot crossed the centerline was counted to determine the number of peaks and valleys that were on each plot. This procedure was repeated 10 times for each membrane and for each direction, along the direction of strain and perpendicular to that direction. The results from this analysis are given in Fig. 11 with the ordinate as the average center line distance between crossings of the section line. Another way to consider this value is as the average distance that must be traversed on the membrane surface before crossing a peak. This analysis clearly shows that stretching does have a quantifiable impact on surface roughness, and that the impact of stretching is different if one looks in the directions parallel to stretching and perpendicular to the stretching direction. 4. Conclusions
Fig. 9. AFM micrograph showing “across stretch direction” view of ∼0.2 m PVDF membrane. (Membrane stretched to a elongation of 150%.)
AFM was used in this research to characterize the surface of stretched and non-stretched MF membranes. While AFM and SEM clearly indicate that the surface morphology is impacted by uni-axially stretching MF membranes, it was found that the most common means of expressing surface morphology such as RMS, Ra , and bearing do not adequately reflect these changes in surface morphology. Consequently, a new method of expressing surface morphology was developed in this research and shown to reflect the changes in membrane morphology.
Fig. 10. AFM section analysis plots along stretch direction (left), across stretch direction (right)] for ∼0.2 m PVDF membrane. (Membrane stretched to a elongation of 150%.)
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Acknowledgements The authors wish to thank Gene Shipman of the 3 M Company for the donation of the stretching machine. We are also indebted to the following people for the contribution of membrane materials: Alex Chiang (Pall Corporation) and Anthony Allegrezza (Millipore Corporation). We gratefully acknowledge the following agencies for financial support of the research: National Water Research Institute, American Water Works Association Research Foundation, and the Office of Navel Research. We also wish to thank The University of Texas College of Engineering for fellowship support for the students. The ideas in this paper are those of the authors above and are not necessarily reflections of the supporting agencies. References [1] J.A. Morehouse, L.S. Worrel, D.L. Taylor, D.R. Lloyd, B.D. Freeman, D.F. Lawler, The effect of uni-axial orientation on macroporous membrane structure, J. Porous Mater. 13 (2006) 63–75. [2] K.K. Sirkar, W.S. Ho (Eds.), Membrane Handbook, 1st ed., van Nostrand Reinhold, New York, 1992. [3] H.C. Flemming, G. Schaule, R. McDonogh, How do performance parameters respond to initial microfilm formation on separation membranes? Vom Vasser 80 (1993) 177–186. [4] H.C. Flemming, G. Schaule, R. McDonogh, H.F. Ridgway, Effects and extent of biofilm accumulation in membrane systems, in: G.G. Geesey, Z. Lewandowski, H.-C. Flemming (Eds.), Biofouling and Biocorrosion in Industrial Water Systems, CRC Press, Boca Raton, 1994, pp. 63–89. [5] G.L. Leslie, R.P. Schneider, A.G. Fane, K.C. Marshall, C.J.D. Fell, Fouling of a microfiltration membrane by two gram-negative bacteria, Colloids Surf. A 73 (1993) 165–178. [6] M.R. Wiesner, P. Aptel, Mass Transport and Permeate Flux and Fouling in Pressure-Driven Processes in Water Treatment Membrane Processes, McGraw-Hill, New York, NJ, 1996, p162. [7] H.C. Flemming, G. Schaule, Biofouling on membranes–a microbiological approach, Desalination 70 (1988) 95–119. [8] H.F. Ridgway, J. Safarik, Biofouling on reverse osmosis membranes, in: H.C. Flemming, G.G. Geesey (Eds.), Biofouling and Biocorrosion in Industrial Water Systems, Springer, Berlin, 1991, pp. 81–111. [9] H.F. Ridgway, H.C. Flemming, Membrane biofouling, in: J. Mallevialle, P.E. Odendaal, M.R. Wiesner (Eds.), Water Treatment Membrane Processes, McGraw-Hill, New York, 1996, p. 6.1. [10] S.K. Hong, M. Elimelech, Chemical, physical aspects of natural organic matter (NOM) fouling of nanofiltration, J. Membr. Sci. 132 (1997) 159–181. [11] G. Binnig, C.F. Quate, C. Gerber, Atomic force microscopy, Phys. Rev. Lett. 56 (1986) 930. [12] A. Fritzsche, A. Arevalo, A. Connolly, M. Moore, V. Elings, C. Wu, The structure and morphology of the skin of polyethersulfone ultrafiltration membranes: a comparative atomic force microscope and electron microscope study, J. Polym. Sci. 45 (1992) 1945. [13] W.R. Bowen, N. Hilal, R. Lovitt, P.A. Williams, Atomic force microscope studies of membranes: surface pore structures of cylcopore and anopore membranes, J. Membr. Sci. 110 (1996) 233–238. [14] S. Singh, K.C. Khulbe, T. Matsuura, P. Ramamurthy, Membrane characterization by solute transport and atomic force microscopy, J. Membr. Sci. 142 (1998) 111–127. [15] P. Dietz, P. Hansma, I. Otto, H. Lehman, K. Herrmann, Surface pore structures of microfiltration and ultrafiltration membranes imaged with atomic force microscope, J. Membr. Sci. 65 (1992) 101–111. [16] K.C. Khulbe, T. Matsuura, Characterization of synthetic membranes by Raman spectroscopy, electron spin resonance and atomic force microscopy, Polymer 41 (2000) 1917–1935.
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The effect of uni-axial stretching on the roughness of microfiltration membranes Jason A. Morehouse a , Dana L. Taylor a , Douglas R. Lloyd a,∗ , Desmond F. Lawler b , Benny D. Freeman a , Leah S. Worrel a b
a Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, United States Department of Civil, Architectural, and Environmental Engineering, The University of Texas at Austin, Austin, TX 78712, United States
Received 3 January 2006; received in revised form 10 February 2006; accepted 17 February 2006 Available online 31 March 2006
Abstract Atomic force microscopy (AFM) was used to characterize the surface morphology of uni-axially stretched and non-stretched microporous microfiltration (MF) membranes. The effect of stretching on the pore structure and bulk properties of MF membranes has been previously reported [J.A. Morehouse, L.S. Worrel, D.L. Taylor, D.R. Lloyd, B.D. Freeman, D.F. Lawler, The effect of uni-axial orientation on macroporous membrane structure, J. Porous Mater. 13 (2006) 63–75.]; this paper focuses solely on the use of AFM to characterize the surface of stretched and non-stretched MF membranes. A new way of representing surface roughness that may prove useful in relating roughness to performance in cross-flow applications is presented. © 2006 Published by Elsevier B.V. Keywords: Atomic force microscopy; Microporous; Phase inversion; Roughness; PVDF
1. Introduction Microfiltration (MF) and ultrafiltration (UF) are widely used in water treatment facilities to remove macromolecules, particles, and colloids from water [2]. A major concern in these applications is fouling of the membrane (that is, the irreversible blocking of pores by colloidal/particulate matter and by the macromolecules of natural organic matter). Numerous reports have been published on the deleterious effect fouling has on membrane performance (that is, flux and rejection) and the resulting additional cost of system operations [3–6]. The vast majority of these studies have focused on how the chemical and physical attributes of the foulant impact the extent of fouling. Only recently publications have appeared that shed some light on how fouling is impacted by the physical and chemical nature of the membrane [7–10]. This paper presents a new method of representing the physical attributes of membrane surfaces (specifically, surface roughness as determined by atomic force microscopy) that can influence membrane fouling.
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Corresponding author. Tel.: +1 512 471 4985; fax: +1 512 471 7060. E-mail address: [email protected] (D.R. Lloyd).
0376-7388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.memsci.2006.02.027
Atomic force microscopy (AFM) [11] coupled with appropriate image analysis software can provide a detailed and accurate representation of membrane surface characteristics. AFM has the advantage over scanning electron microscopy (SEM) that AFM measurements can be performed at atmospheric pressure and no membrane sample pretreatment is required prior to analysis [12–14]. It has been suggested that pore dimensions measured by AFM are more accurate than those obtained via SEM because SEM depends upon sample preparation techniques that may alter membrane structure [12–14]. However, Dietz et al. [15] finds that AFM may distort membrane pore sizes due to rounded corners near pore entrances. This distortion is on the order of the diameter of the tip used in the AFM measurements. AFM and SEM also differ significantly in their depth of field; where depth of field is a measure of an instrument’s ability to image beyond the surface of a membrane. AFM, because of the limitations of its design, can only image structure at or very near the surface of the membrane, whereas the depth of field of SEM is limited only by the ability for a metal coating to penetrate the membrane pore structure. With the limitations of each tool (SEM and AFM) it remains important to utilize both in order to fully characterize membrane surfaces.
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AFM has been used widely to analyze the surface of reverse osmosis (RO) membranes [16–25]. Vrijenhoek et al. [25] used AFM to characterize polyamide thin-film composite RO membranes in terms of RMS roughness, Ra roughness, surface area, peak count, and mean plane effect on membrane fouling. RMS roughness and Ra roughness are standard expressions of surface morphology [26] calculated as follows: N N 2 |Zi − ZAVE | i=1 (Zi − ZAVE ) RMS = Ra = i=1 N N In these equations, Zi corresponds to surface height with ZAVE being the average of N height measurements for the membrane. Roughness has often been found to correlate to membrane performance. Using a cross-flow filtration cell with a 0.01 M NaCl feed solution at a steady state trans-membrane pressure of 345 kPa, Vrijenhoek et al. [25] found that a decrease in roughness corresponded to a decrease in flux decline through the membrane. This finding was in close agreement with findings by Hirose et al. [24] and Kwak and Ihm [18] who also reported that membrane flux is proportional to membrane surface roughness in RO. Kwak and Ihm used 0.2% NaCl in deionized water with a trans-membrane pressure of 225 psig while Hirose et al. used 1500 mg/L NaCl with a trans-membrane pressure of 1.5 MPa as experimental conditions. AFM has proved to be a valuable tool in the analysis of NF, UF, and MF membranes in addition to RO. Very often, when making measurements on NF membranes, the AFM is employed in non-contact mode [27–29]. Non-contact mode relies on van der Waals interactions to repel the scanning tip from the membrane surface, thus, preventing the membrane surface from becoming damaged during the AFM scan. AFM has also been used to measure the roughness of NF membranes [25,27,30,31] as well as MF and UF membranes [32]. In addition, AFM has been used to measure pore diameter, pore density, and surface porosity in UF and MF membranes [33–36] and to elucidate the fouling of MF and UF membranes [37–40]. In the analysis of the surface roughness effect on UF membrane performance, Fell et al. [41] found that increased surface roughness adversely affected flux through Amicon UF membranes. Fell et al. used Amicon XM100 and XM300 UF membranes with nominal pore sizes between 15 and 25 nm and a feed solution of 0.1 wt.% bovine serum albumin (BSA) in a cross-flow stirred filtration cell with a trans-membrane pressure of 100 kPa. The authors hypothesized that surface roughness directly predicted the size of surface depressions found on the membrane. These surface depressions created stagnation points on the surface, which hindered flux. Cheung et al. [42], in a study of UF hollow fiber membranes with AFM, indicated results similar to those of Fell. Cheung et al. used poly(vinyl pyrrolidone) and BSA at a concentration of 200 mg/L and a trans-membrane pressure of 1.0 bar when doing hollow fiber membrane flux and rejection studies. Cheung et al. also found that the pure water flux of the membrane was proportional to the mean roughness and that increased membrane roughness resulted in decreased rejection of organic macromolecules. Using multivariate linear regression analysis, Knoell et al. [19] were able to relate MF
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membrane surface characteristics such as pore area, membrane surface height, and pore aspect ratio to bacterial attachment and water flux in modified polysulfone membranes. However, Knoell et al. [19] did not find a positive correlation between membrane surface roughness and flux or fouling. Building apon the work by Knoell et al. [19], this research intentionally modifies the pore dimensions of MF membranes in an effort to improve overall membrane performance. This modification is accomplished by uni-axially stretching membranes, thereby inducing significant and permenant membrane pore deformation. In a recent publication, the effect of stretching microporous PVDF membranes on membrane and pore structure was presented [1]. Work has also been completed by researchers at 3 M using microporous polyolefin [43–48] and poly(ethyleneco-vinyl alcohol) [49] membranes and researchers at Toten Corp. using polyethylene films [50,51]. However, none of these studies reported the effect of membrane stretching on surface roughness. In this article, standard AFM analysis methods such as roughness and bearing analysis are used to characterize stretched and non-stretched MF membranes, and a new way of expressing membrane surface morphology is developed. 2. Experimental 2.1. Sample preparation Poly(ether sulfone) (PES) and poly(vinylidene fluoride) (PVDF) membranes were obtained from Pall Corporation (http://www.pall.com) and PVDF membranes were obtained from Millipore Corporation (http://www.millipore.com). The PES membrane is an amorphous hydrophobic membrane, and the PVDF membrane is a partially crystalline hydrophilic membrane. The membranes were provided in large rolls 45.7 cm wide. When referring to membranes provided by Pall Corp., the nominal pore size is written with a tilde (∼) preceding the number to reflect the fact that the membrane provided by Pall Corporation is not exactly the same as the commercial membrane of the same nominal pore size. Instead, the membranes used in this research are laboratory samples made by Pall Corp. researchers to approximate the commercial product. Therefore, they may not have exactly the same pore characteristics as the membranes made in production scale facilities. Neither manufacturer revealed the proprietary membrane formation/modification methodology. From these rolls, samples of 11.9 cm × 7.6 cm were cut using a rectangular die. When stretched, membranes necked to various degrees depending on the total elongation of the membrane. As discussed in a previous paper, statistical analysis was completed to determine the degree of non-uniformity through the membrane [1]. The pores in the 10% of the membrane closest to the edges were found to be non-representative of the membrane’s bulk pore size and shape after stretching. For this reason, no samples were taken from this outer region. Each sample was uni-axially stretched in a T.M. Long biaxial stretcher (Inventure Labs, Knoxville TN; http://www.inventurelabs.com) at a desired temperature and
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crosshead speed (i.e., the stretching speed) and then annealed for 15 min at a specified temperature. The sample was then air cooled while being held under tension until the membrane reached room temperature. The sample was then removed and measured with a ruler accurate to 1.3 mm to determine the actual strain obtained. The strain, ε is defined as ε = (Lf –Li )/Li . Lf is the final membrane length after stretching, and Li is the initial membrane length. Elongation was then calculated by multiplying strain by 100%. All samples were stretched in the same direction that they were extruded or cast. Once the stretched membranes were annealed and cooled, they did not show any significant recovery of their length in the stretching direction when monitored for 4 months. All membranes presented herein were stretched at 160 ◦ C and at a head speed of 0.1 cm/s. Details of the procedure are given elsewhere [1]. 2.2. Atomic force microscopy Surface characterization of the membranes was performed with a Digital Instruments, Dimensions 3100 AFM (http://www.digitalinstruments.com). The AFM is equipped with a non-contact/contact head and a 100 m scanner that was operated in constant force mode. The membrane sample was attached to a 22 mm2 glass microscope cover glass slide (Corning # 2865-22) using double-sided tape (3 M Scotch 2002 Photo and Document Tape). The AFM detects the deflection of a laser beam reflecting from an oscillating tip as the tip scans the membrane surface. The sample moves laterally under the tip and a feedback loop keeps the vertical position of the tip constant by moving the surface up and down with a piezoelectric xyztranslator. Images were acquired utilizing silicon MPP-21100 AFM cantilevers with a 5 N/m spring constant and a 75 kHz resonant frequency (Nano Devices; http://www.nanodevices.com/). The cantilevers used have conical tips with nominally less than a 10 nm radius. Tapping mode was employed to reduce lateral forces on the tip and distortion of membrane surface. AFM images were acquired at a scan rate of 0.3 Hz with a scan resolution of 256 pixels × 256 pixels. All information about surface morphology (RMS roughness, Ra roughness, and bearing) was acquired using the data analysis software provided by Digital Instruments (Nanoscope 5.12b). The image scan sizes used for roughness and bearing analysis were 10 m × 10 m. This image size was chosen over smaller sizes to ensure that a significant number of pores were captured for analysis. When performing AFM scans, the direction of scan with respect to the stretching direction was not controlled.
3. Results RMS and Ra roughness were measured for several different PES and PVDF membranes. As shown in Table 1, RMS roughness is quantitatively larger than Ra roughness but that when comparing data they reveal similar trends, as evidenced in Fig. 1. For that reason, the discussion here focuses only on RMS. Fig. 1 shows SEM micrographs of ∼0.2 m PVDF membranes that have been stretched from 0 to 150% elongation. The pore structure of the ∼0.2 m PVDF was the same on both sides of the membrane. The effects of stretching on pore shape and size are obvious. RMS roughness data are presented in Fig. 2 for the ∼0.2 m PVDF membranes at different elongations. The roughness values obtained are similar to values obtained by other researchers working with MF membranes [39]. The 95% confidence limits shown in Fig. 2 were established by taking samples from three separate locations within the membrane and then measuring 10 different locations for each sample for a total of 30 measurements per membrane. It can be seen that RMS roughness does not change significantly with increased membrane stretching; however, Fig. 1 clearly shows that membrane structure changes with elongation of the membrane. The effect of stretching membranes to a 30% elongation level was determined for each of the available PES and PVDF membranes, and the results are presented in Fig. 3. Once again, 95% confidence limits are indicated in the figure. Only one sample, 0.2 m PVDF from Millipore Corp., showed a significant impact of stretching on surface roughness. However, the significant uncertainty associated with the roughness measurements makes drawing any conclusion about the correlation between surface roughness and membrane stretching impossible. The large uncertainty found when doing roughness measurements on MF membranes was also reported by Knoell et al. [19]. Two of the Pall membranes; ∼0.8 m and ∼1.2 m PES, do not have a stretched membrane roughness included in Fig. 3. Those two membranes were not able to be stretched. The ∼0.8 m PES has a nodular structure (Fig. 4) that had little structural integrity and was weak under tensile stress. It is believed that the cause for the significant increase in roughness and error for the ∼0.8 m membrane is due to the difference in the nodular (Fig. 4) versus the lacy structure (Fig. 1). Differences in the formation process are believed to result in the nodular versus lacy membranes; however, the specifics of these processes are proprietary. The ∼1.2 m PES membrane proved to be too brittle to stretch. Because the two sides of this membrane seemed to have vastly different surface appearances under SEM (see Fig. 5 a and b), both surfaces were characterized with AFM. However, as shown in Fig. 3, there is no significant difference
2.3. Scanning electron microscopy Scanning electron micrographs were also obtained for each of the samples. Samples were coated from a gold palladium target for 45 s with an Electron Microscope Sciences K575 sputter coater (http://www.emsdiasum.com/). Samples were then placed into a Hitachi S-4500 field emission scanning electron microscope. A voltage of 10 kV was used in imaging the samples.
Table 1 Roughness values for ∼0.2 m PVDF membranes Elongation (%)
RMS (nm)
Ra (nm)
0 50 100 150
300 261 318 272
229 220 234 226
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Fig. 1. Clockwise from top left ∼0.2 m PVDF membranes stretched to 0%, 50%, 100% and 150% elongation. White arrows denote stretching directions. (All membranes stretched at 0.1 cm/s and 160 ◦ C.)
in the membrane surface roughness on the front and back of the ∼1.2 m PES. The ∼1.2 m PES membrane could not be stretched because of the temperature limitations of the stretching machine. It was impossible to heat the membrane above its glass transition temperature, and only above the glass transition temperature would the polymer by ductile enough for the membrane to be significantly stretched. A comparison of the bearing of the stretched versus nonstretched ∼0.2 m PVDF was completed. Bearing analysis provides additional information beyond standard roughness measurements by revealing how much of a surface lies above or
below a given height. Surface roughness is generally represented in terms of statistical deviation from average height; however, this gives little indication of height distribution over the surface. Bearing analysis determines what percentage of the surface (the “bearing ratio”) lies above or below any arbitrarily chosen height. A bearing ratio of 100% corresponds to 100% of the surface lying above a given surface height. The zero point on the abscissa corresponds to the average height of each membrane, which can be verified by noting each curve passes through the 50% bearing line at zero depth. Fig. 6 shows the repeatability
Fig. 2. Surface roughness results for ∼0.2 m PVDF membranes at various levels of elongation. (Membranes stretched at 0.1 cm/s and 160 ◦ C.)
Fig. 3. Surface roughness results for various microporous membranes (membranes stretched at 0.1 cm/s and 160 ◦ C).
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Fig. 6. Repeated bearing analysis for non-stretched ∼0.2 m PVDF membrane. Fig. 4. SEM micrograph of ∼0.8 m PES membrane.
of bearing analysis for a non-stretched ∼0.2 m PVDF membrane, by plotting the bearing analysis for one membrane under a 50% elongation with the bearing measured at three locations on the membrane. Fig. 7 shows the effect of elongation on the bearing analysis for the ∼0.2 m PVDF membrane. Based on the repeatability results presented in Fig. 6, it can be concluded from Fig. 7 that elongation does not have a significant impact on the bearing analysis. Figs. 8 and 9 present AFM images of a stretched membrane’s surface. Fig. 8 represents looking along the membrane in the direction in which it was stretched; Fig. 9 represents looking across the membrane perpendicular to the direction of stretch. The two images, although depicting the same membrane, are very different. Along the direction of strain, the aligned pore shape takes on the form of long valleys, while perpendicular to this direction, the pore openings appear very closely packed. These differences could have a significant impact on membrane performance in cross-flow filtration if the orientation of the membrane is changed relative to the flow direction. While these ideas will be explored in future research, the objective of this paper is to develop a means of expressing the difference in surface morphology as seen in the two directions.
In an effort to quantify these differences, section plots were constructed of the membranes surface in the two directions. Samples of these surface plots are shown in Fig. 10. Further analysis was completed by allowing the AFM software to draw a center line through the section plot to bisect the data in such a way that half of the height data of the surface plot was above
Fig. 7. Bearing analysis for ∼0.2 m PVDF membrane tested at various levels of elongation.
Fig. 5. SEM micrographs of both sides of a ∼1.2 m PES membrane.
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Fig. 11. Results from AFM section analysis of ∼0.2 m PVDF that yields the average distance between pore walls. Fig. 8. AFM micrograph showing “along stretch direction” view of ∼0.2 m PVDF membrane. (Membrane stretched to a elongation of 150%.)
the line and half of the height data was below the center line. The number of times the section plot crossed the centerline was counted to determine the number of peaks and valleys that were on each plot. This procedure was repeated 10 times for each membrane and for each direction, along the direction of strain and perpendicular to that direction. The results from this analysis are given in Fig. 11 with the ordinate as the average center line distance between crossings of the section line. Another way to consider this value is as the average distance that must be traversed on the membrane surface before crossing a peak. This analysis clearly shows that stretching does have a quantifiable impact on surface roughness, and that the impact of stretching is different if one looks in the directions parallel to stretching and perpendicular to the stretching direction. 4. Conclusions
Fig. 9. AFM micrograph showing “across stretch direction” view of ∼0.2 m PVDF membrane. (Membrane stretched to a elongation of 150%.)
AFM was used in this research to characterize the surface of stretched and non-stretched MF membranes. While AFM and SEM clearly indicate that the surface morphology is impacted by uni-axially stretching MF membranes, it was found that the most common means of expressing surface morphology such as RMS, Ra , and bearing do not adequately reflect these changes in surface morphology. Consequently, a new method of expressing surface morphology was developed in this research and shown to reflect the changes in membrane morphology.
Fig. 10. AFM section analysis plots along stretch direction (left), across stretch direction (right)] for ∼0.2 m PVDF membrane. (Membrane stretched to a elongation of 150%.)
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Acknowledgements The authors wish to thank Gene Shipman of the 3 M Company for the donation of the stretching machine. We are also indebted to the following people for the contribution of membrane materials: Alex Chiang (Pall Corporation) and Anthony Allegrezza (Millipore Corporation). We gratefully acknowledge the following agencies for financial support of the research: National Water Research Institute, American Water Works Association Research Foundation, and the Office of Navel Research. We also wish to thank The University of Texas College of Engineering for fellowship support for the students. The ideas in this paper are those of the authors above and are not necessarily reflections of the supporting agencies. References [1] J.A. Morehouse, L.S. Worrel, D.L. Taylor, D.R. Lloyd, B.D. Freeman, D.F. Lawler, The effect of uni-axial orientation on macroporous membrane structure, J. Porous Mater. 13 (2006) 63–75. [2] K.K. Sirkar, W.S. Ho (Eds.), Membrane Handbook, 1st ed., van Nostrand Reinhold, New York, 1992. [3] H.C. Flemming, G. Schaule, R. McDonogh, How do performance parameters respond to initial microfilm formation on separation membranes? Vom Vasser 80 (1993) 177–186. [4] H.C. Flemming, G. Schaule, R. McDonogh, H.F. Ridgway, Effects and extent of biofilm accumulation in membrane systems, in: G.G. Geesey, Z. Lewandowski, H.-C. Flemming (Eds.), Biofouling and Biocorrosion in Industrial Water Systems, CRC Press, Boca Raton, 1994, pp. 63–89. [5] G.L. Leslie, R.P. Schneider, A.G. Fane, K.C. Marshall, C.J.D. Fell, Fouling of a microfiltration membrane by two gram-negative bacteria, Colloids Surf. A 73 (1993) 165–178. [6] M.R. Wiesner, P. Aptel, Mass Transport and Permeate Flux and Fouling in Pressure-Driven Processes in Water Treatment Membrane Processes, McGraw-Hill, New York, NJ, 1996, p162. [7] H.C. Flemming, G. Schaule, Biofouling on membranes–a microbiological approach, Desalination 70 (1988) 95–119. [8] H.F. Ridgway, J. Safarik, Biofouling on reverse osmosis membranes, in: H.C. Flemming, G.G. Geesey (Eds.), Biofouling and Biocorrosion in Industrial Water Systems, Springer, Berlin, 1991, pp. 81–111. [9] H.F. Ridgway, H.C. Flemming, Membrane biofouling, in: J. Mallevialle, P.E. Odendaal, M.R. Wiesner (Eds.), Water Treatment Membrane Processes, McGraw-Hill, New York, 1996, p. 6.1. [10] S.K. Hong, M. Elimelech, Chemical, physical aspects of natural organic matter (NOM) fouling of nanofiltration, J. Membr. Sci. 132 (1997) 159–181. [11] G. Binnig, C.F. Quate, C. Gerber, Atomic force microscopy, Phys. Rev. Lett. 56 (1986) 930. [12] A. Fritzsche, A. Arevalo, A. Connolly, M. Moore, V. Elings, C. Wu, The structure and morphology of the skin of polyethersulfone ultrafiltration membranes: a comparative atomic force microscope and electron microscope study, J. Polym. Sci. 45 (1992) 1945. [13] W.R. Bowen, N. Hilal, R. Lovitt, P.A. Williams, Atomic force microscope studies of membranes: surface pore structures of cylcopore and anopore membranes, J. Membr. Sci. 110 (1996) 233–238. [14] S. Singh, K.C. Khulbe, T. Matsuura, P. Ramamurthy, Membrane characterization by solute transport and atomic force microscopy, J. Membr. Sci. 142 (1998) 111–127. [15] P. Dietz, P. Hansma, I. Otto, H. Lehman, K. Herrmann, Surface pore structures of microfiltration and ultrafiltration membranes imaged with atomic force microscope, J. Membr. Sci. 65 (1992) 101–111. [16] K.C. Khulbe, T. Matsuura, Characterization of synthetic membranes by Raman spectroscopy, electron spin resonance and atomic force microscopy, Polymer 41 (2000) 1917–1935.
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