Mechanical Characterization of Membranes

C H A P T E R 13

Mechanical Characterization of Membranes K. Wang1, A.A. Abdala1, N. Hilal2, M.K. Khraisheh1 1

Hamad Bin Khalifa University, Doha, Qatar; 2Swansea University, Swansea, United Kingdom

Chapter Outline 1. Introduction 259 2. Mechanical Characterization Techniques 261 2.1 2.2 2.3 2.4 2.5

Uniaxial Tensile Test 261 Bending Test 267 Dynamic Mechanical Analysis Nanoindentation 275 Bursting Test 278

272

3. Mechanical Degradation of Polymeric Membranes

282

3.1 Fouling Induced Mechanical Degradation 283 3.2 Chemical Cleaning Induced Mechanical Degradation 3.3 Membranes Delamination 286

284

4. Stress-State of Polymeric Membrane Under Actual Condition

287

4.1 Flat Sheet Membranes 288 4.2 Hollow Fiber Membranes 290

5. Advanced Techniques for Mechanical Properties Testing

293

5.1 Environmental Effects on the Mechanical Properties of Membranes 5.2 Membrane Fatigue Behavior 293 5.3 Real-Time Micromechanical Investigations 294

293

6. Conclusions 296 List of Abbreviations 297 References 298

1. Introduction Most of the reported research dealing with membranes focuses on membrane processing [1,2], surface modification [3,4], and antifouling properties [5,6]. More recently, molecular dynamics simulations of the physical and mechanical properties of nanoporous graphene membrane were conducted [7e11]. However, experimental investigations of the Membrane Characterization. http://dx.doi.org/10.1016/B978-0-444-63776-5.00013-9 Copyright © 2017 Elsevier B.V. All rights reserved.

259

260 Chapter 13

Figure 13.1 Annual publications on “desalination and water treatment membranes” and “mechanical properties of desalination and water treatment membranes”. Based on Scopus Database.

mechanical behaviors under complex loading modes with environmental effects for porous membranes are rarely reported. According to the Scopus database, the annual publications on desalination and water treatment membranes has grown from 1050 publications in 2004 to nearly 2500 publications in 2014 as shown in Fig. 13.1. On the other hand, the number of annual publications that investigate the mechanical properties of polymeric membranes has increased from 20 publications in 2004 to about 100 publications in 2014. However, these publications still represent less than 5% of the annual publication that deals with polymeric membranes. In addition to high permeate flux and high rejection of contaminant, polymeric membranes require good mechanical durability and good chemical and fouling resistances. Thus, analysis of membranes’ real stress loading conditions and examining their mechanical properties under conditions similar to actual operation conditions are very important. Furthermore, understanding the mechanical behaviors of polymeric membranes with underlying deformation mechanisms is critical not only for membrane structure design but also for their reliability and lifetime prediction.

Mechanical Characterization of Membranes 261 The purpose of this chapter is to review the most used experimental techniques for characterizing the mechanical behaviors of polymeric membranes. Moreover, the mechanisms responsible for mechanical degradations of these membranes are also discussed. To study the mechanical responses of membranes under conditions similar to their operating conditions, the stress state at both lab and commercial scale is analyzed. Finally, advanced mechanical testing methods are proposed with potential real-time monitoring techniques, to study the microstructure properties relationship for polymeric membranes.

2. Mechanical Characterization Techniques Mechanical characterization of a material requires that measuring material responds to applied force. One common way to accomplish that is through straining the material at a controllable strain rate and accurately measuring the corresponding required load. Consequently, the material stressestrain behavior is developed from the measured forces and displacements and the sample initial dimension [12]. Conventional tests are difficult to directly apply to study the mechanical properties of membranes for water treatment because of their small thickness. For bulk materials, mechanical testing provides average properties over a large specimen cross-section and the specimen dimensions usually do not interfere with fundamental length scale [13]. Whereas the thickness of membranes is usually quite small, it is therefore expected that the mechanical responses of the membranes may exhibit a sample size dependence [14]. The strain-induced microstructural changes for membranes are often significant, invalidating many assumptions, such as in-plane elastic isotropy made about plastic deformation of bulk materials. Therefore, the mechanical properties of membranes need to be accurately evaluated with the underlying mechanisms, in order to optimize the membrane design and processing, to be used with long service time. Currently, the most used methods to study the mechanical behaviors of polymeric membranes include uniaxial tensile testing, bending test, dynamic mechanical analysis, depth-sensing nanoindentation, and bursting tests. Table 13.1 provides a list of the most used approaches to study the mechanical properties of polymeric membranes. These methods determine the elastic, plastic, viscoelastic, hardness, as well as fracture and toughness properties. In the following sections, the major aspects of each technique are highlighted.

2.1 Uniaxial Tensile Test Uniaxial tensile testing is one of the most fundamental and the most used testing methods to evaluate the mechanical behaviors of materials. The test is usually carried by gripping opposite ends of the test specimen within the load frame of a universal testing machine and the specimen is stretched at a constant controlled tensile speed until a predetermined

262 Chapter 13 Table 13.1: Tests used for mechanical characterization of polymeric membranes. Approaches Uniaxial tensile test

Bending test

Dynamic mechanical analysis

Nanoindentation

Bursting test

Based Materials PVDF PAN CA PES PSF PAI PVA PP Chitosan Al2O3 Kaolin Nickel ZrO2 PES PVA PVDF PET SPES PU Clay PES PS PAN Carbon AQPz PVDF PES CMS PLA PSF PEI

References [15e28] [29,30] [31] [32e36] [37e42] [43] [44] [45e47] [48,49] [50e54] [55e59] [60] [61] [33,36, 62e64] [65,66] [67] [68] [69] [70] [71] [64,72] [73] [74] [75] [76] [77e80] [81] [82] [83] [84] [85]

Reported Mechanical Properties Stressestrain curve, Young’s modulus, yield stress, ultimate stress, elongation at break, fracture toughness

Bending strength, fracture toughness

Storage modulus, Loss modulus

Young’s modulus, hardness

Bursting strength

stress/strain or failure. The choice of grips, grip serrated surfaces, loading cells, and strain sensors depends on the nature of the studied materials and the sample geometry and dimensions as well as on the expected results. From the measured load (F), displacement (Dl), the original specimen cross-sectional area (S0) and initial length (l0), the engineering/ nominal stress (seng ¼ F/S0) and strain (εeng ¼ DI I0 ) can be determined. Other important mechanical parameters such as Young’s modulus (E), yield stress (sy) and yield strain (εy), postyield strain-softening and strain-hardening characteristics, stress at break (sR) and strain at break/elongation at break (εR), maximum stress (sB), and strain at maximum stress (εB) of materials can be obtained from the stressestrain curves as shown

Mechanical Characterization of Membranes 263

Figure 13.2 Typical stressestrain curves for (a) brittle materials, (b) ductile materials, and (c) rubber-like materials.

in Fig. 13.2. In addition, the area under the stressestrain curve determines the material toughness, which relates to the material energy absorption capacity [86]. Uniaxial tensile test is the most widely used mechanical testing method for polymeric membranes. For flat sheet membranes, the nominal/engineering stress can be determined using the original cross-sectional area, but for hollow fiber membranes, the engineering (seng (hollow fiber)) should be calculated according to their sectional surface area [80]: F  seng ðhollow fiberÞ ¼
p do2  di2

(13.1)

4 where do and di are the outer and inner diameters of the hollow fiber, respectively. Uniaxial tensile test is used to study the mechanical properties of flat sheet composite membranes. Ahmed et al. [87] studied the mechanical strength of cellulose/electrospum polyvinylidenefluoride-co-hexafluoropropylene (PVDF-HFP) composite flat sheet membranes for efficient oilewater separation using standard dog-bone specimens. The tensile tests were conducted at a speed of 1 mm/min. The results showed that the addition of up to 15 wt% cellulose led to increase in the elastic modulus and tensile strength of the electrospun PVDF-HFP membranes (Fig. 13.3(a)). However, the elongation at break decreased with increasing the cellulose content (Fig. 13.3(b)). The change in the mechanical properties was due to the entrapment of the PVDF-HFP fibers in the cellulose matrix and their inability of their chains to easily reorient. Therefore, the elastic modulus and tensile strength increased. At cellulose content above 15 wt%, the membranes suffered

264 Chapter 13

Figure 13.3 Effect of cellulose content on the (B) elastic modulus, (C) tensile strength, and (-) elongation at break of cellulose/electrospum polyvinylidenefluoride-co-hexafluoropropylene composite membranes. From Ejaz Ahmed F, Lalia BS, Hilal N, Hashaikeh R. Underwater superoleophobic cellulose/ electrospun PVDFeHFP membranes for efficient oil/water separation. Desalination 2014;344:48e54.

process-induced defects such as cracks during the drying step, which led to a decrease of membrane stiffness. Wang et al. [88] investigated the mechanical properties of a novel graphene oxide (GO)-blended PVDF ultrafiltration (UF) flat sheet membrane at room temperature using uniaxial tensile testing at a speed of 2 mm/min. As shown in Fig. 13.4, the tensile strength of PVDF/GO membrane increased with increasing the contents of GO up to 0.2 wt%, and then it gradually decreased. The increase in tensile strength was attributed to the excellent mechanical properties of GO and the very high surface area and aspect ratio. Below GO content of 0.2 wt%, GO was dispersed very well in PVDF matrix, improving the mechanical properties of the membrane. With further increase in GO content, GO aggregation and possibly weak interface led to a decrease in the tensile strength. Han et al. [89] studied the mechanical behavior of a polyamide (PA), thin film composite (TFC), pressure retarded osmosis (PRO) membrane for renewable salinity-gradient energy generation using uniaxial tension experimental technique. The flat sheet of PA TFC membranes with 5 mm width and with 30 mm initial gauge length were tested under a tensile rate of 10 mm/min. Test temperature was not mentioned. The developed membranes exhibited very high toughness, which was an important factor for the membranes outstanding compressive resistance properties. A slightly higher tensile cross-head speed, 20 mm/min, was used to test mechanical properties of polysulfone (PSF) UF flat sheet membranes with polyethylene glycol (PEG) additive [90] at the room temperature. Increasing the dosage of PEG-400 (average

Mechanical Characterization of Membranes 265

Figure 13.4 Effects of the graphene oxide content on the mechanical properties of the blended membranes (C) tensile strength, (-) elongation at break. From Wang Z, Yu H, Xia J, Zhang F, Li F, Xia Y, Li Y. Novel GO-blended PVDF ultrafiltration membranes. Desalination 2012;299:50e4.

molecular weight 400 Da) led to increased membrane porosity consequently decreasing the tensile stress at break. On the other hand, the elongation at break initially increased and then decreased with increasing PEG-400 content. Increasing the PEG molecular weight, increased the elongation at break, whereas the tensile strength increased till to the PEG molecular weight of 1500 Da and then decreased. This was due to the appropriate increase in molecular weight of PEG that could suppress the formation of macrovoids and enhance the mechanical strength. However, when the molecular weight of PEG was high, the rapid increase of porosity arising from the increase in molecular weight of PEG addition might decrease the mechanical strength. Universal testing machine with a cross-head speed of 50 mm/min and with a 1 kN load cell was used to evaluate the mechanical strength of a self-supporting electrospun polyacrylonitrile (PAN) nanofibrous flat sheet membrane for water purification [91]. The obtained stressestrain curve exhibited a nonlinear relationship between the applied stress and deformation. The ultimate tensile strength of 4.87 MPa verified that the nanofibrous membrane possessed high level of consistency with minimum number of structural defects. The measured mechanical properties indicated that this membrane could serve as a self-supporting membrane for fluid streams. Simone et al. [25] studied the mechanical behaviors of a PVDF/poly(vinyl pyrrolidone) (PVP) hollow fiber membrane for vacuum membrane distillation (VMD) application using a uniaxial tensile testing. The specimen with a gauge length of 50 mm was stretched at a constant speed of 5 mm/min and the corresponding force was measured. At low PVP

266 Chapter 13 content, the increase in PVP content induced formation of large cavities and macrovoids of the membranes, reducing the strength of the membranes. At higher PVP content, the increase in PVP content promotes formation of sponge-like structures, improving the mechanical properties of the hollow fiber membranes. A homogeneous-reinforced PVDF hollow fiber membrane was prepared via a dryewet spinning method by the PVDF surface by PVDF polymer solutions [15]. The mechanical characterization of the membrane was carried out using a tensile tester at room temperature at a cross-head speed of 10 mm/min. As shown in Fig. 13.5, the PVDF content had no significant effect on the tensile strength of the PVDF membranes. However, the elongation at break of the membranes increased with increasing PVDF content. The increased elongation at break was attributed to the stronger bonding between the membrane matrix and the coating layer leading to a higher interfacial stress transfer coefficient. Yu et al. [21] studied the mechanical response of an organiceinorganic PVDF/SiO2 composite hollow fiber UF membrane using a materials testing machine at a speed of 50 mm/ min and they found that the mechanical strength of the membranes increases with SiO2 content. The breaking strength and Young’s modulus increased with SiO2 at low SiO2 and then declined at higher loading, i.e., above 10%. This was attributed to the formation of SiO2 network at high SiO2 content increasing the rigidity of the membrane and confining the crystallization of PVDF, leading to a decrease of the elongation at break of the membranes. The uniaxial tensile testing with an initial gauge length of 100 mm and a tensile speed of 400 mm/min was used to investigate the mechanical properties of a PVDF/nanoclay

Figure 13.5 Effects of polyvinylidenefluoride concentration on the mechanical properties of the membranes. From Zhang X, Xiao C, Hu X, Bai Q. Preparation and properties of homogeneous-reinforced polyvinylidene fluoride hollow fiber membrane. Appl Surf Sci 2013;264:801e10.

Mechanical Characterization of Membranes 267 hollow fiber membrane for water treatment [19]. The addition of nanoclay improves the mechanical properties of the hollow fiber membranes. Higher nanoclay loading could suppress the formation of the macrovoids in the membranes. In addition, nanoclay also promoted a change of the PVDF crystalline phase, from a- to b-phase, which increased polarity and brought about a more efficient energy-dissipation mechanism in the PVDF/ nanoclay nanocomposite membrane. Uniaxial tensile testing is widely used to identify mechanical behaviors of ductile polymer membranes and polymer matrixebased membranes [15,17e19,23,27,29e31,36,41,45,78,92]. However, the focus of most of these studies was to correlate the mechanical properties with the processing conditions or the composite composition or loading. No significant attempts were made to correlate the mechanical properties of the membrane to its operational performance, reliability, and lifetime. In addition, majority of the studies did not follow any standards with regard to the test specimen and the cross-head speed and they did not use extensometer to accurately measure the sample deformation. For example, the grip distance was varied between 20 mm [18] and 100 mm [19,92] and the cross-head speed was varied between 2 mm/min [27] and 500 mm/min [92] depending on the initial gauge length and initial sample length. Moreover, most of the uniaxial tensile tests were carried out at room temperature and not the temperature at which the membrane will operate. It is worth mentioning that currently there is no standard test method for testing the mechanical properties of polymeric membranes. Nevertheless, ASTM D882-12 and ISO 527-3 for testing thin plastic sheets (thickness less than 1 mm) [93,94] are the closest possible standards. Nevertheless, few studies have followed standard test methods designed for thicker polymer specimens (ISO 6239 and ASTM D638) [95,96] or metallic materials (GB/T228-2002) [97]. Although uniaxial tensile is the most used mechanical testing method for thin films [81,98e102], due to the delicacy and very thin thickness of the membrane materials, the following must be considered when testing membranes using uniaxial tensile test: (1) Suitable grip and grip serrated surface should be used to avoid tearing/fracturing the specimen around the gripped area rather than at the middle of the specimen. (2) The vertical alignment of the membrane samples and the grips should be adjusted to avoid side loading or bending. (3) Clamping force should be adaptable to avoid slippage or damage of the gripped membrane during operations.

2.2 Bending Test Bending or flexure testing is an important mechanical testing method to define the ability of brittle materials to resist deformation when subjected to simple beam loading and the

268 Chapter 13

Figure 13.6 (a) 3-Point and (b) 4-point flexure tests for membranes.

results are presented in a loadedisplacement curve or in a stress/strengthestrain curve. The flexural strength represents the highest stress before the material ruptures. According to the loading point, bending test can be divided into 3-point and 4-point flexure tests as shown in Fig. 13.6. In a 3-point bending test, the specimen is placed on two parallel supporting pins and a third loading pin is lowered at the specimen midpoint at a constant speed. For a rectangular cross-section specimen, the bending strength (sf3r) is calculated from Eq. (13.2). sf 3r ¼

3FL 2bd2

(13.2)

For a hollow fiber, the 3-point bending strength (sf3h) is given by Eq. (13.3) [103]: 8FLdo  sf 3h ¼
p do4  di4

(13.3)

where F is the measured force at a given point on the deflection curve, L is the support span. b and d are the rectangular beam width and depth, respectively. do and di are the outside and inside diameters of hollow fiber, respectively.

Mechanical Characterization of Membranes 269 In contrast to 3-point bending test, 4-point bending test requires the sample to be placed on two supporting pins. Two loading pins are placed at an equal distance around the center. For a rectangular cross-sectional specimen, the flexure strength is calculated by 3Fa (13.4) 2bd2 Here, a is the distance between the supporting and loading pins. For a hollow fiber, the 4-point bending strength can be described by Eq. (13.5) [104]: sf 4r ¼

16FKD  sf 4h ¼
p do4  di4

(13.5)

Here, K indicates a half of the difference between the outer and inner span. Furthermore, bending test can be used to determine the facture toughness of materials, details about the theoretical background are reported elsewhere [105]. Both 3-point and 4-point bending tests are typically used for rigid porous ceramic and metallic polymeric membranes. It is worth noting that the thickness of some inorganic materials is not as thin as polymeric membranes. Sarkar et al. [53] investigated the impacts of clay content and sintering temperature on the bending strength of a clayealumina porous capillary support for filtration application. The 3-point bending test was performed using a span of 30 mm and a loading speed of 10 mm/min. As shown in Fig. 13.7, the flexural strength of the clayealumina porous composite membranes increased with increasing sintering temperature and with increasing clay content. The increased clay content favored the mullite formation, which had a significant consolidation effect on the material at high temperature.

Figure 13.7 Effect of clay loading and sintering temperature on the flexural strength of clayealumina porous capillary tubes. From Sarkar S, Bandyopadhyay S, Larbot A, Cerneaux S. New clayealumina porous capillary supports for filtration application. J Membr Sci 2012;392e393:130e6.

270 Chapter 13 Kritikaki and Tsetsekou [50] studied the effect of adding small amounts of fine solegel derived nanoalumina on the bending strength of a micron-sized alumina powder for the production of highly porous ceramic. The bending test was carried out with 30-mm span and with 0.5 mm/min cross-head speed. The results showed that the membrane bending strength depended on g-alumina nanopowder content, materials mixing methods, and the sintering temperature (Fig. 13.8). More specifically, the bending strength of the membrane increased for low nanopowder content (4e6 wt%) then decreased for median nanopowder content and at last increased at high nanopowder content (20 wt%), whatever materials mixing method and sintering temperature. The evolution of the membrane bending strength as a function of the nanopowder content is related to the materials’ interface, homogeneity, as well as the membrane porosity.

Figure 13.8 Open porosity and bending strength as a function of the weight percentage of g-alumina nanopowder in the powder mixture; (a) and (b) ceramics prepared by the simple mixing in the shear mixer sintered at 1500 C and 1600 C, respectively; (c) and (d) ceramics prepared by the addition of the ball milling step prior to shear mixing sintered at 1500 C and 1600 C, respectively. (-) Porosity; (B) bending strength. From Kritikaki A, Tsetsekou A. Fabrication of porous alumina ceramics from powder mixtures with solegel derived nanometer alumina: effect of mixing method. J Eur Ceram Soc 2009;29:1603e11.

Mechanical Characterization of Membranes 271 The mechanical behaviors of porous Al2O3 ceramics prepared from pure Al2O3 powder and mixtures with Al(OH)3 particles were studied using the 3-point bending test by Deng et al. [52] using 3 mm  4 mm  40 mm specimens, with a span of 30 mm and cross-head speed of 0.5 mm/min. The fracture toughness was measured using the single-edge notched-beam test, with a notch depth of 2.0 mm, a notch width of 0.1 mm, and a span of 16 mm. Due to the strong grain bonding, the bending strength of the ceramics increased with the addition of Al(OH)3. For highly porous Al2O3, the fracture toughness increased with the addition of Al(OH)3. However, the addition of Al(OH)3 did not improve the fracture toughness of low porosity ceramics. This is attributed to the fracture-mode transition from the intergranular, at high porosity, to transgranular, at low porosity. Lee et al. [60] prepared a nickel hollow fiber membrane and used the 3-point bending test to study the effect of the sintering temperature on the bending strength of the membrane. Higher sintering temperature in the investigated temperature range from 1100 C to 1300 C and significantly improved the bending strength of the hollow fiber membrane. Similar finding was reported by Nandi et al. [55] on flexural strength of low-cost ceramic membranes based on kaolin for microfiltration (MF) applications. The flexural strength of the ceramic membranes at different sintering temperatures is shown in Fig. 13.9. The bending strength of the membrane increased from 3 MPa for sintering at 850 C to 8 MPa for sintering at 1000 C. The mechanical strength improvement with sintering temperature was due to the densification of the materials at high sintering temperature [51,56,59,61].

Figure 13.9 Flexural strength of ceramic membranes as a function of kaolin sintering temperatures. From Nandi BK, Uppaluri R, Purkait MK. Preparation and characterization of low cost ceramic membranes for micro-filtration applications. Appl Clay Sci 2008;42:102e10.

272 Chapter 13

2.3 Dynamic Mechanical Analysis Dynamic mechanical analysis (DMA) is a very useful tool to investigate the viscoelastic behavior of viscoelastic materials by decoupling their elastic and viscous components. DMA can measure several material properties as a function of temperature, time, frequency, atmosphere, or a combination of these parameters. DMA involves applying a small cyclic deformation on a specimen of known geometry and measuring the corresponded stress, or alternatively applying a cyclic stress on the specimen and measuring the resulting strain. In most cases, deformation is the controlled input and stress is the measured output [106]. Using DMA, the storage modulus (E0 ), loss modulus (E00 ),  00 and the damping factor tan d ¼ EE0 of materials can be measured. The storage modulus measures the ability of the material to store energy and it correlates with the elastic nature of the material. On the other hand, the loss modulus is a measure of the ability of the material to dissipate energy. The damping factor characterizes the mechanical damping or internal friction of a viscoelastic system. A high damping factor is indicative a material that has a high nonelastic strain component, whereas a low value indicates that the material is more elastic. This approach can be also used to determine the transition temperatures corresponding to molecular motions or thermal transition. Although DMA allows various loading moduli such as cyclic tension, compression, shearing, dual and single cantilever, cyclic tensile loading is one of the most used modes to study the dynamic mechanical properties of polymeric membranes. Chung et al. [62] studied the effect of the shear rate within the spinneret during hollow fiber spinning on the mechanical properties of UF polyethersulfone (PES)ebased hollow fiber membranes using DMA. The hollow fiber samples were heated from room temperature to 250 C at a heating rate of 3 C/min and a frequency of 10 Hz. The storage and loss moduli of the obtained PES hollow fiber membrane increased with increasing spinning shearing rate as shown in Fig. 13.10. This was attributed to the highly oriented polymer chains at higher processing shear rate, resulting in a higher potential energy and inherent stiffness. To remove Pb2þ from industrial wastewater, poly(vinyl alcohol) (PVA) nanocomposite membranes were prepared with different loading of zirconium diethylene triamine pentamethylene phosphonic acid (ZrDETPMP). DMA was used to study the dynamic mechanical properties of this membrane between 30 C and 300 C with 10 C/min heating rate. The membrane stiffness was found to increase with ZrDETPMP content. This increased membrane stiffness with ZrDETPMP content is due to the hydrogen bonding of the matrix eOH groups, the high degree of cross-linking, and attuned polymer network formation (Fig. 13.11) [65]. Zhang et al. [67] measured the storage modulus of a PVDF hollow fiber membrane with PVDF or PAN polymer coating layer using DMA operating at 1 Hz in the temperature

Mechanical Characterization of Membranes 273

Figure 13.10 Storage modulus and loss modulus as a function of temperature for polyethersulfone hollow fiber membranes with different spinning shear rate. From Chung T-S, Qin J-J, Gu J. Effect of shear rate within the spinneret on morphology, separation performance and mechanical properties of ultrafiltration polyethersulfone hollow fiber membranes. Chem Eng Sci 2000;55:1077e91.

range 80e50 C at a heating rate of 5 C/min. The room temperature storage modulus of the reinforced membranes was lower than the untreated PVDF membrane probably due to the erosion effect of the polymer solution on the matrix surface during the coating processing. Singh et al. [66] also used DMA to study the dynamic performance of nanofiltration (NF) membranes containing silica monomer precursor and PVA in the temperature range 30e320 C at a heating rate of 10 C/min under N2 atmosphere. The stiffness of the

274 Chapter 13

Figure 13.11 Dynamic mechanical analysis curves for different ZrD nanocomposite membrane. ZrD-45, ZrD-55, and ZrD-65 denote 45, 55, and 65 wt% of ZrD in the membranes, respectively. From Singh S, Patel P, Shahi VK, Chudasama U. Pb2þ selective and highly cross-linked zirconium phosphonate membrane by solegel in aqueous media for electrochemical applications. Desalination 2011;276:175e83.

membrane increased with silica content because the silica monomer makes the membrane less porous and induces cross-linking of PVA. Membranes based on surface modified electrospun polyester (PET) nanofibers with cyclodextrin polymer (CDP) were prepared by Kayaci et al. [68] for removal of phenanthrene from aqueous solution. The dynamic thermomechanical properties of the materials were investigated using DMA operating in tension mode at a constant frequency of 1 Hz and strain of 20 mm. Sample sizes of 10 mm  3 mm  0.12 mm were tested in temperature range of 50e150 C at a heating rate of 3 C/min. The storage modulus of PET/CDP membrane was much higher than that of unmodified PET membrane due to the stiffening effect of cross-linked CDP coating (Fig. 13.12). For CDP-modified membranes, the peak of damping factor, which correspond to the glass transition temperature, was shifted to higher temperature indicating that the glass transition temperature for PET/CDP membranes was higher than that of unmodified PET indicating that the chains mobility of PET was affected and the segmental motion of PET chains was hindered by CDP modification. DMA can be used to obtain accurate dynamic mechanical responses of polymeric membranes as a function of time, temperature, and frequency at a small strain or with a small force. However, the test should be carefully carried out by selecting proper initial serrated force applied by DMA grips to avoid membrane slippage or damaging in the

Mechanical Characterization of Membranes 275

Figure 13.12 (a) Storage modulus and (b) loss tangent of polyester/cyclodextrin polymer membranes. a, b, and g denote three types of cyclodextrines using for materials preparation. From Kayaci F, Aytac Z, Uyar T. Surface modification of electrospun polyester nanofibers with cyclodextrin polymer for the removal of phenanthrene from aqueous solution. J Hazard Mater 2013;261:286e94.

gripped section. Moreover, as polymer membranes stretch and become softer at high temperature, a proper strain rate should be selected for different temperature ranges.

2.4 Nanoindentation Nanoindentation is a technique developed in the mid-70s of the last century to measure the elastic modulus, hardness, and fracture toughness of solid materials. Because of its high-resolution of the loadedisplacement data acquisition ability, it is now used for the mechanical characterization of small size materials [107,108]. For a typical indentation test, a hard pyramidal or spherical indenter is pressed into a studied material where the applied force and the corresponded indenter displacement are recorded [12]. From the forceedisplacement curves obtained by instrumented indentation, the measured contact stiffness, S, is determined [108e111]: qffiffiffiffiffiffiffiffiffiffiffiffiffi dF 2 (13.6) S¼ ¼ pffiffiffi Er Ap ðhc Þ dh p where F is the indentation load, h is the indentation depth, and dF/dh is the slope of the tangent to the unloading curve at the maximum loading point (Fig. 13.13), and Ap(hc) is the projected contact area of the hardness impression at the contact depth hc, which is calculated as follows: 1

1

1

1

Ap ðhc Þ ¼ C0 h2c þ C1 hc þ C2 h2c þ C3 h4c þ C4 h8c þ C5 hc16

(13.7)

276 Chapter 13

Figure 13.13 Typical indentation loadedisplacement curve.

where the constants Ci (i ¼ 1, 2, 3, 4, 5) can be determined by a series of indentations at different depths on a standard materials and employing curve-fitting procedure. The reduced modulus, Er, which accounts for the elastic deformation occurs in both the specimen and the indenter, and is given by     1  y2s 1  y2i 1 ¼ þ (13.8) Es Ei Er where Es and ys are the Young’s modulus and Poisson’s ratio for the sample, and Ei and yi are the Young’s modulus and Poisson’s ratio for the indenter. The hardness, H, is defined as the maximum load, Fmax is divided by the residual indentation area Ar: H¼

Fmax Ar

(13.9)

Yakub [71] studied the modulus and hardness of a porous ceramic membrane for removal of contaminants from water using nanoindentation. In this, loading and unloading rates of 5 mm/s were applied through a spherical indenter with a peak load of 1000 mN. Both the membrane modulus and hardness increased with decreasing the average pore size but did not have any clear relationship with the porosity. It revealed that the elastic modulus and hardness of the membrane were strongly influenced by the average membrane size rather than the porosity. To evaluate the effect of the molecular structure on the mechanical properties of a cross-linked sulfonated block copolymer for water purification applications,

Mechanical Characterization of Membranes 277 nanoindentation testing was carried out by Yeo et al. [73]. The measurements were conducted under ambient atmosphere and the results indicated that the chemical crosslinking had a significant effect on the mechanical properties of the block copolymer due to the existence of chemical cross-linking in loop/bridge conformations. At a given stress, the chemical cross-linking made the materials more resistant. The effect of GO on the strength of a nanohybrid NF membrane was analyzed by Wang et al. [74]. The results demonstrated that Young’s modulus and hardness of the PAN substrate membrane have shown insignificant change after hydrolysis (Fig. 13.14). However, a more pronounced change was observed with poly(ethyleneimine) (PEI)/ polyacrylic acid (PAA) as Young’s modulus and hardness significantly increased when PEI-modified GO/PAA was deposited on the membrane. These results also indicated that the mechanical behaviors of the composite membrane were improved by the addition of GO. In recent years, atomic force microscopy (AFM) technique has shown a versatility in the characterization of surface properties of materials [112]. It allows high-resolution topography investigations as well as mapping of the elastic and viscoelastic behaviors of materials. In this technique, an AFM cantilever serves as a soft nanoindenter allowing local investigation of small and inhomogeneous specimen at the nanometric scale [113e115]. Nevertheless, the use of this technique to study the mechanical properties of water treatment and desalination membranes is still limited [116]. Tai et al. [75] prepared a free-standing and flexible electrospun carbon-silica nanofibrous membrane for ultrafast gravity-driven oilewater separation. The Young’s modulus of the single SiO2ecarbon composite nanofiber and the carbon nanofiber (CNF) was evaluated by contact mode AFM utilizing a silicon cantilever with a pyramidal tip. The force mapping of the fiber surface was conducted on a random area of 5  5 mm2. The average Young’s modulus for CNF was three times higher than that of SiO2ecarbon composite nanofiber suggesting that SiO2ecarbon composite nanofiber is more flexible than CNF. Wang et al. [76] molecularly designed a pore-suspending biomimetic membrane embedded with Aquaporin Z (AQPz) based on different lipid protein ratios for water purification. The mechanical behavior of the membranes was studied by force indentation using AFM in the center of the unsupported membrane-covered pores with a constant piezoelectric velocity of 0.5 m/s at a temperature of 20  1 C in water. It was reported that the mechanical properties of the membrane were improved by increasing the AQPz content, suggesting that AQPz increases the energy barrier required for a normal force to punch through the biomimetic membrane. Nanoindentation is an effective technique to determine the mechanical properties of thin films and coatings. However, this method is usually used to study the local surface mechanical properties of the materials. Accuracy of determination of mechanical properties by nanoindentation can be effected by various factors, such as inadequate model

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Figure 13.14 (a) Young’s modulus as a function of displacement into surface with assembly effects. (b) Hardness as a function of displacement into surface. From Wang N, Ji S, Zhang G, Li J, Wang L. Self-assembly of graphene oxide and polyelectrolyte complex nanohybrid membranes for nanofiltration and pervaporation. Chem Eng J 2012;213:318e29.

for data processing or omission of some specific properties or factors, such as pileup, surface forces, or temperature changes [117].

2.5 Bursting Test Bursting test can present a realistic reflection of the mechanical strength of flat sheet and hollow fiber polymeric membranes. It is a suitable mechanical testing method to determine the pressure/force required to rupture the membranes under a planar stress state. In this

Mechanical Characterization of Membranes 279 test, the membrane is subjected to increased applied pressure till failure occurs. At this point, the pressure is noted as the bursting pressure or bursting strength [81,82,118e120]. To investigate the mechanical properties of a PVDF-HFP flat sheet membrane for direct contact membrane distillation, Lalia et al. [79] used a capillary flow porometer to perform the Mullen burst test with the setup as shown in Fig. 13.15(a). The membrane (50 mm  50 mm) is clamped over a rubber diaphragm to keep the water from escaping the system or touching the membrane. With increasing the applied air pressure, the diaphragm is steadily pressurized with water. At the rupture of the membrane, the rubber diaphragm suddenly expands to the failure point of the membrane and the corresponding bursting strength is recorded by the software [83,84,121]. The results obtained from the bursting test demonstrated that the burst pressure increased with increasing the concentration of PVDF-HFP used for electrospinning. This is attributed to the larger fiber diameter obtained at higher polymer solution concentration. The increase in fiber thickness ultimately increased the mechanical strength of the spun mats and hence of the membranes [79].

Figure 13.15 Illustration of burst pressure measurement setup for (a) flat sheet membrane and (b) hollow fiber membrane. Reproduced from Wang P, Chung T-S. A new-generation asymmetric multi-bore hollow fiber membrane for sustainable water production via vacuum membrane distillation. Environ Sci Technol 2013;47:6272e8; Lalia BS, Guillen-Burrieza E, Arafat HA, Hashaikeh R. Fabrication and characterization of polyvinylidenefluoride-co-hexafluoropropylene (PVDF-HFP) electrospun membranes for direct contact membrane distillation. J Membr Sci 2013;428:104e15.

280 Chapter 13 Bursting test can also be used to investigate the mechanical properties of hollow fiber membranes [77,80,85]. Wang and Chung [77] carried out burst test using a laboratory fabricated setup to investigate the mechanical properties of PVDF multibore hollow fiber (MBF) membrane developed for VMD. The laboratory setup for hollow fiber burst pressure measurement is shown in Fig. 13.15(b). A water reservoir with 3.5 wt% NaCl solutions was used to provide the testing solution. Hydraulic pressure was provided by compressed N2. The hollow fiber module was prepared by sealing one end of the fiber and leaving the other end open. The outlet of the tube was connected to the lumen side of the hollow fiber module. The whole module was put in a beaker of deionized water. During the test, hydraulic pressure was increased with a step of 0.1 bars. The conductivity of the water in the beaker was monitored by a conductivity meter to measure the liquid entry pressure. The burst pressure of the MBF membrane was much higher than that of the single-bore hollow fiber membrane, because of the larger overall cross-section area of the MBF membrane with its lotus root like structure which could maximize the mechanical stability at the radical direction. In addition, comparing the burst pressure of two MBF membranes with different spun conditions, they concluded that the burst pressure depends on the tightness of the inner-surface pore size. The tighter surface due to the smaller pore size exhibited higher burst pressure. Zhang et al. [122] used a lab-scale reverse osmosis (RO) setup to measure water permeability and burst strength of PES hollow fiber membrane. A deionized water was pumped into lumen side of the hollow fibers at a flow rate of 0.15 mL/min and pressurized to 2 bars for 20 min before permeate was collected from the shell side. After measurement of the water permeability, the applied pressure was gradually increased at intervals of 3 bars below 14 bars and 1 bar above 14 bars. The water permeability was measured at each pressure. A decrease of water permeability versus pressure was observed due to the membrane densification until the membrane failed indicated by the sudden increase in the water permeability with the corresponding pressure noted as the burst pressure [122,123]. In addition, Zhang et al. related the burst pressure, P, to the tensile stress of hollow fiber membrane by using Barlow’s formula for isotropic tubes [122]: P ¼ 2ts=do DSf

(13.10)

where t is the hollow fiber wall thickness, do is the outer diameter, and Sf is the safety factor usually taking Sf ¼ 1. s refers to the membrane tensile stress. Thus, the burst strength of hollow fiber membrane strongly depends on the wall thickness, outer diameter, and tensile stress of the membrane. By using this relationship between the burst pressure and tensile stress, Zhang et al. [122] estimated the burst pressure by using tensile stress comparing to the measured ones. It was found that the estimated burst pressure was slightly different from the measured burst pressure for a given PES hollow fiber membrane. The reason was attributed to the anisotropy of the hollow fiber membranes.

Mechanical Characterization of Membranes 281 Because the hydraulic pressure was directly exerted upon the inner layer of the hollow fiber support during the burst pressure test, the porosity and thickness of the inner skin layer became especially important at high pressures. They therefore concluded that the mechanical strength of hollow fiber membranes was highly dependent not only on outer diameter and wall thickness Eq. (13.10), but also on porosity and the microstructure of the membranes. Liu et al. [80] studied the effect of reinforcing PVDF with PET threads on the mechanical strength of PVDF hollow fiber membrane using burst test. They reported that the addition of PET threads had little effect on the membranes burst pressure (Fig. 13.16). However, the burst pressure of the membrane increased significantly with increasing PVDF concentration in the dope solution for both the unreinforced and reinforced membranes due to the tighter stack of molecule chains in the membrane at a higher PVDF concentration. The addition of PET threads led to their distribution over the membrane along the axial direction. The diameter of PET threads was very small compared to the membrane girth and PET threads did not affect the microstructure of the membrane and consequently having a very small effect on the bursting pressure. Li et al. [78] investigated the mechanical properties of PVDF/PVA hollow fiber membrane using bursting test. As shown in Fig. 13.17, the bursting pressure of the membrane decreases with PVA content because of the increase in the membrane porosity with PVA content.

Figure 13.16 Effect of polyester threads on burst pressure of polyvinylidenefluoride hollow fiber membrane with different concentrations. From Liu J, Li P, Li Y, Xie L, Wang S, Wang Z. Preparation of PET threads reinforced PVDF hollow fiber membrane. Desalination 2009;249:453e7.

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Figure 13.17 Bursting pressure and porosity of polyvinylidenefluoride/poly(vinyl alcohol) (PVA) hollow fiber membranes as a function of PVA content. From Li N, Xiao C, An S, Hu X. Preparation and properties of PVDF/PVA hollow fiber membranes. Desalination 2010;250:530e7.

Bursting test is a useful mechanical testing approach to investigate the failure strength of polymeric membranes under planar stress state. However, during the bursting test, only the failure pressure of the membranes is recorded. Therefore, the relationship between the applied force/pressure and membrane deflection, which is important to analyze the microstructure evolution under an increasing applied pressure, cannot be established. In addition, the measured bursting pressure is highly dependent on the experimental protocol such as testing devices, data acquisition methods, and step pressure. Similar to the effect of strain rate on the mechanical properties of polymer membranes using uniaxial tensile test, it is expected that a higher step pressure for the bursting test can yield a higher bursting strength of membranes.

3. Mechanical Degradation of Polymeric Membranes Mechanical strength plays an important role in reliability and durability of polymeric membranes. Mechanical degradation of membranes due to fouling, physical damage, and chemical aging induced by harsh source water, biological growth, physical pore blocking, backwashing, chemical cleaning could lead to a decrease of membranes’ stiffness, and strength [102,124e129]. More specifically, mechanical degradation induced failure of membranes can cause a variation of applied pressure, flux, particle rejection, and permeability, consequently, a loss of quality for permeate. Uniaxial tensile testing is the most used experimental technique to investigate the mechanical degradation of the membranes.

Mechanical Characterization of Membranes 283

3.1 Fouling Induced Mechanical Degradation Membrane fouling is initiated by the accumulation of inorganic, organic, colloidal, and biological species on the membrane surface and/or within its pores. It results in a reduced flux, a rapidly increasing transmembrane pressure, and possibly deterioration of mechanical properties [130e133]. Fig. 13.18 shows scanning electron microscope (SEM) micrograph of membrane cross-section with CaSO4 deposit formed on the membrane surface during membrane distillation (MD) process of brackish water. As shown by the SEM image, the deposit is formed not only onto the membrane surface, but also it penetrates its pores interior leading to destruction of the membrane structure [133]. Nghiem and Scha¨fer [134] investigated the effect of fouling on the mechanical behaviors of hollow fiber MF membrane element taken from a water recycling plant by using uniaxial tensile test. A 200-mm-long sample was characterized under a loading rate of 50 mm/min. Fig. 13.19 shows the elongation at break for five fouled samples in comparison with the virgin unfouled membranes. Fouled membranes are more brittle than the unfouled hollow fibers due to the normal wear and tear during the filtration process and backflush operations although the major foulant included colloidal particles and organic matters. Zondervan et al. systematically analyzed the parameters that influenced membrane lifetime by analysis of variance [126]. Mechanical investigation was performed using uniaxial tensile test with a 10 N loading cell. They found that the elongation at break of fouled modules was nearly half of that of the clean modules indicating that fouling significantly influenced the membrane integrity.

Figure 13.18 Scanning electron microscope image of membrane cross-section with CaSO4 deposit formed on the membrane surface during membrane distillation process of brackish water. From Gryta M. Desalination of industrial effluents using integrated membrane processes. INTECH Open Access Publisher; 2012.

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Figure 13.19 Elongation at break of five fouled hollow fiber membranes compared to virgin ones. From Nghiem LD, Scha¨fer AI. Fouling autopsy of hollow-fibre MF membranes in wastewater reclamation. Desalination 2006;188:113e21.

3.2 Chemical Cleaning Induced Mechanical Degradation To deal with membrane fouling, chemical cleaning has been proved as an effective method toward recovery of the original membrane permeability. Chemical cleaning has been accepted for foulant removal from membrane cell in large water treatment plants [130,135e139]. However, cleaning with chemical agents may not only remove foulant but also attack the membrane materials causing its mechanical degradation [140,141]. Arkhangelsky et al. studied the hypochlorite cleaning effect on the mechanical responses of a commercially available cellulose acetate (CA) UF membrane using uniaxial tensile tests [142,143]. Wet membrane samples (76 mm  5 mm  0.2 mm) were clamped between two aluminum holders. A prestretching step (force 3 N for 5 s) was applied to stabilize the investigated samples. The CA membranes were then drawn at a constant cross-head speed of 0.001 m/s till break. The exposure of CA membranes to hypochlorite was found to decrease the ultimate tensile strength, ultimate elongation, and elasticity modulus (Fig. 13.20). This mechanical degradation was more significant at higher hypochlorite dosage [144]. The decreased mechanical properties were attributed to polymer chain breakage during the hypochlorite cleaning process. Causserand et al. [145] and Rouaix et al. [146] simulated the chemical aging of a PSF membranes by soaking them in chlorine solutions and then investigated their mechanical behaviors via uniaxial tensile test. Wet samples with a length of 200 mm were stretched at a speed of 200 mm/min and the results are shown in Fig. 13.21. The behavior of elongation at break for membranes immersed in hypochlorite is very sensitive to the solution temperature, to the solution pH value, and to the soaking time. More specifically,

Mechanical Characterization of Membranes 285

Figure 13.20 Uniaxial tensile curves for the pristine cellular acetate (CA) membrane (1) and CA membranes cleaned with hypochlorite 5 g h/L (2) and 18 g h/L (3). From Arkhangelsky E, Kuzmenko D, Gitis NV, Vinogradov M, Kuiry S, Gitis V. Hypochlorite cleaning causes degradation of polymer membranes. Tribol Lett 2007;28:109e16.

Figure 13.21 Relative elongation at break of polysulfone membranes immersed in hypochlorite solutions at 100 ppm HClO, pH 7, and various temperature, El0 ¼ 37%. From Rouaix S, Causserand C, Aimar P. Experimental study of the effects of hypochlorite on polysulfone membrane properties. J Membr Sci 2006;277:137e47.

the elongation at break of these PSF membranes decreased with increasing temperature, and increasing soaking time. The reduced mechanical properties of PSF membranes were attributed to the polymer chain breakage during the exposure to sodium hypochlorite. The effect of hypochlorite cleaning on the mechanical properties of MF flat sheet membrane made from PVDF was studied by Wang et al. [147]. A pilot-scale membrane bioreactor

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Figure 13.22 Stressestrain curves of polyvinylidenefluoride membranes with different times of hypochlorite cleaning, A0 is the original membrane, A1, A2, A3, A4, and A5 are the membranes with fourth, fifth, sixth, 10th, and 12th times chemical cleaning. From Wang P, Wang Z, Wu Z, Zhou Q, Yang D. Effect of hypochlorite cleaning on the physiochemical characteristics of polyvinylidene fluoride membranes. Chem Eng J 2010;162:1050e6.

(MBR) at an existing wastewater treatment plant was operated for 63 days to investigate the impacts of frequent and high-dose sodium hypochlorite cleaning on the membrane properties. The mechanical properties of the original and cleaned membranes were measured using uniaxial tensile machine at room temperature. Samples, a gauge length of 20 cm, were drawn at 50 mm/min. Young’s modulus and yield stress of the membrane materials decreased with increasing hypochlorite cleaning time. But the ultimate elongation tended to increase as the cleaning time prolonged due to the plasticization effect of the hypochlorite cleaning process on the PVDF membrane (Fig. 13.22).

3.3 Membranes Delamination Membrane delamination and formation of blisters between the layers are common problems in multilayer membranes caused by the lack of proper adhesion between the active and supporting layers of flat sheet membranes [148]. For hollow fiber membranes, delamination possibly appears during membrane fabrication and it is caused by uneven shrinkages between inner and outer layers due to their different thermal properties, material incompatibility, and different phase inversion rates [137,139,149]. Delamination during the membrane fabrication phase will result in macrovoids between different layers affecting the permeate quality. Under this condition, most studies have focused on improving the interface adhesion and preparing multilayer membranes with initial

Mechanical Characterization of Membranes 287

Figure 13.23 Digital photographs of wet-laid thin film composite membrane after test for response to shear stress induced by moderate cross-flow velocity. Delamination at the polyesterepolysulfone interface is seen from (a) the top surface and (b) the bottom surface. The delamination shown occurred in a cell with countercurrent flow of deionized water at cross-flow velocities that were gradually increased up to 26 cm/s. From Hoover LA, Schiffman JD, Elimelech M. Nanofibers in thin-film composite membrane support layers: enabling expanded application of forward and pressure retarded osmosis. Desalination 2013;308:73e81.

delamination-free structures [150e152]. To evaluate membrane resistance to failure, Hoover et al. [153] removed the spacer in their bench-scale laboratory equipment such that the failure points could be easily achieved with controlled pressure. With a water cross-flow velocity of 26 cm/s, Hoover et al. found that their wet-laid TFC membrane delaminated internally at the PETePSF interface (Fig. 13.23). This interlayer delamination was attributed to the cross-flow induced shearing force applied on the membrane. The location of the delamination revealed that the membrane cell geometry affected the stress state on the membranes [153].

4. Stress-State of Polymeric Membrane Under Actual Condition Inorganic membranes are typically designed as MF and UF to remove relatively large particles for water treatment purpose. Due to their inherent mechanical properties, inorganic membranes have good mechanical integrity during normal water treatment process. In contrast, polymer membranes with smaller pore size and higher efficiency for particle removal are widely used in water desalination with a high applied pressure. However, polymer membranes are mechanically weaker and have lower thermal and chemical stability compared to inorganic membranes. For next generation membranes, the

288 Chapter 13 materials should combine high permeability and high selectivity with sufficient mechanical stability [154]. Therefore, in the next sections, attention will be mainly focused on polymeric membranes operated under high pressure.

4.1 Flat Sheet Membranes The mechanical performance of membranes depends not only on their materials and fabrication process, but also on the use of proper membrane module configurations. At laboratory-scale, cross-flow or tangential-flow filtration is commonly employed for flat sheet membranes. Fig. 13.24 shows the six cells (cell reference: CF042), two-line bench-scale laboratory membrane system (Sterlitech) at Qatar Environment and Energy Research Institute (QEERI). With a single membrane cell, the solution to be filtered is pumped from the tank to the inlet of membrane cell with a controlled pressure and a controlled speed. Flow then goes through the membrane cavity. The solution that passes across the membrane is referred to as permeate. The flow that does not pass through the membrane is referred to as retentate and goes back to the tank to be filtered again. During this process, the effect of shearing induced by the cross-flow on the membrane can effectively remove the particles avoiding membrane fouling and then maintaining a stable flux. As shown in Fig. 13.24(b), the membrane is positioned by two bulges between a

Figure 13.24 (a) STERLITECH membrane system at Qatar Environment and Energy Research Institute with zoom-in view of a single membrane cell and (b) membrane cell assembly order.

Mechanical Characterization of Membranes 289 feed-side spacer (figure not shown here) and a permeate-side spacer. The cell bottom and top are then closed by four bolts. The two rubber O-rings are used for leak-proof-seal and the boundary of the membrane is fixed (membrane must fit over the posts and between the two O-rings). The central cavity between cell bottom and cell top of the membrane depends on the thicknesses of spacers. If the central cavity is too much, the membrane may wrinkle. Kim et al. [155] used a laboratory membrane cell to investigate the effect of feed channel spacer on the performance of PRO membranes. The unsupported membrane areas over the open space spacer had to withstand the high pressure applied on the draw solution side. The bulging of the membrane under pressure of the draw solution subjected the membrane to a tensile stress-state. With increasing tension induced by applied pressure, the active layer of the membrane would begin to crack (Fig. 13.25(b)) and the support layer would be stretched and finally leading to membrane rupture. She et al. [156] studied the effect of spacer geometry on the performance of flat sheet membrane for PRO using a lab-scale cross-flow setup. The spacer-dependent membrane deformation is schematically illustrated in Fig. 13.26. Both spacer geometry and applied pressure had a significant effect on the membrane deformation. The spacer with large mesh size induces more severe deformation at high pressure and affecting the flow properties. Corte´s-Juan et al. [157] simulated the laminar fluid flow in a lab-scale flat circular filtration cell using computational fluid dynamics (CFD). The simulation results indicated that the flow direction, the velocity, and the wall shear stress were strongly influenced by the membrane cell configuration. The addition of grooves on the top of the cell permitted to obtain intense velocity and wall shear stress fluctuations. Flat sheet membranes for spiral-wound configuration are sandwiched between feed channel spacers and permeate collection materials and rolled around a central collection permeate tube. The influence of spacer thickness on the permeate flux for seawater RO systems was experimentally investigated by Sablani et al. [158]. They found that the different geometry of the spacer might affect the turbulence at the membrane surface and a small thickness of spacer had less turbulence. The effect of spacer geometry on the flow properties for spiral-wound membrane modules was also theoretically investigated by CFD simulations. The predictions revealed that the spacer filament configurations, mesh size, filament diameter, and Reynolds number had a complex effect on the flow direction, velocity, and wall shear-stress [159,160]. The membranes are not only under simple shearing-stress condition induced by laminar flow but also under complex stress-state induced by turbulence flow [161,162]. The distribution of the shear stress within spiralwound membrane modules is fairly nonuniform because the spacer filaments create constrictions to the flow path as shown in Fig. 13.27 [163].

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(b)

Figure 13.25 Scanning electron microscopy images of the deformed membrane after a pressure retarded osmosis experiment at 12.5 bar: (a) at 200 and (b) at 60,000. From Kim YC, Elimelech M. Adverse impact of feed channel spacers on the performance of pressure retarded osmosis. Environ Sci Technol 2012;46:4673e81.

4.2 Hollow Fiber Membranes Hollow fiber membranes used for RO applications are usually housed into tubes and tightly bundled together with high packing density and therefore with less degree of freedom. Feed solution can be introduced on the center of fiber side or on the tube shell side. Permeate is withdrawn in a cocurrent or countercurrent manner depending on the configuration of membrane module.

Mechanical Characterization of Membranes 291

Figure 13.26 Schematic representation of feed spacer induced membrane deformation under pressure retarded osmosis operation (a) undeformed state and (b) deformed state. From She Q, Hou D, Liu J, Tan KH, Tang CY. Effect of feed spacer induced membrane deformation on the performance of pressure retarded osmosis (PRO): implications for PRO process operation. J Membr Sci 2013;445:170e82.

Figure 13.27 Numerical simulation of flow in spacer-filled channels with the effect of spacer geometry (a), geometric characterization of a spacer (b), steady-state fluid particle path lines at a low Reynolds number (c), dimensionless time averaged wall shear stresses for Reynolds number approximately 80, in the normalized chromatic scale the values of 0 and 1 correspond to the minimum and maximum of the plotted variable, respectively, direction of mean flow from top left corner to bottom right. From Koutsou C, Yiantsios S, Karabelas A. Direct numerical simulation of flow in spacer-filled channels: effect of spacer geometrical characteristics. J Membr Sci 2007;291:53e69.

Kaya et al. [164] analyzed the wall shear stress of an outside-in hollow fiber membrane modules by CFD simulation. The modeling results showed that the inlet and outlet of the membrane module could cause fluctuations in local velocity and on wall shear stress. The tangential inlet and outlet enabled rotational flow, providing slightly better wall shear

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Figure 13.28 Computational fluid dynamics simulation of wall-shear distribution in a hollow fiber membrane module with tangential inlet and outlet, (a) full range display and (b) limited range display. From Kaya R, Deveci G, Turken T, Sengur R, Guclu S, Koseoglu-Imer DY, Koyuncu I. Analysis of wall shear stress on the outside-in type hollow fiber membrane modules by CFD simulation. Desalination 2014;351:109e19.

stress on the fiber surfaces. The distribution of the shear stress within hollow fiber membrane modules is inhomogeneous and the two ends have more complex stress state induced by the multidirection flow (Fig. 13.28). Therefore, flat sheet and hollow fiber membranes for water treatment operate under complex stress state that depends not only on applied pressure and velocity distribution but also on the membrane cell configurations. The conventional methods to study the mechanical properties of the membranes do not represent the actual loading conditions. Therefore, advanced mechanical testing techniques should be developed to investigate the membranes under similar stress state as that experienced in water treatment applications. A good start is to design and build a membrane mechanical testing cells that allow subjecting the membrane to loading conditions similar to actual loading conditions at different scales and different cell configurations. The next challenge that requires to be overcome is to find an adaptable technique to continuously record the three-dimensional shape of the membranes under pressure and to correctly analyze the obtained load-deflection behaviors.

Mechanical Characterization of Membranes 293 To fabricate spiral-wound modules, multilayer flat sheet membranes are rolled and the rolling radius could be small (the central permeate tubes may have a diameter of 2 cm). This small rolling radius may lead active polymer or functional coating layer to crack and bringing an earlier membrane failure. Bending test may be suitable to simulate the rolling process. Moreover, the study of the adhesion force between hollow fiber ends and resin by using pull out testing is also important to the mechanical integrity of hollow fiber membranes with tube configuration [102,139].

5. Advanced Techniques for Mechanical Properties Testing 5.1 Environmental Effects on the Mechanical Properties of Membranes The mechanical behavior of polymers is very sensitive to testing temperature [86,165e167]. More specifically, the yield stress and Young’s modulus decrease dramatically with temperature. Temperature effect on particle rejection [168,169], scaling [170], fouling [171], and permeate flux [172] of the polymeric membranes have been studied. However, investigation of the temperature effect on the mechanical properties of membranes is less reported. The Middle East, seawater temperature is higher than that of other countries [173]. Under these circumstances, the mechanical investigations of the membranes for seawater water RO application with temperature effects are especially important. The pressure rate that depends on the duration required for the pressure to increase from zero to the maximum working pressure is also an important factor to study the mechanical responses of polymeric membranes. This effect is similar to the strain rate effect on the mechanical behavior of polymer where increased strain rate leads to material hardening due to the lower molecular chain mobility at large strain rates. More specifically, an increased pressure rate may influence the yield stress and the elastic modulus of the membranes resulting in an earlier material failure. Mechanical testing with high pressure rate can also be used to simulate the pressure shock on the mechanical responses of the membrane induced by operation or by system disturbances [174].

5.2 Membrane Fatigue Behavior Membrane backwashing is an essential stage to remove foulant from the remaining membranes’ permeation and selectivity. During backwashing, the filtration process is reversed in which permeate is flushed through the membrane to the concentrate side. Backwashing can be done either by reducing the operation pressure below the osmotic pressure of the feed solution or by increasing the permeate pressure [175]. For the second case, the feed solution pressure should reduce to zero and then the permeate pressure is applied. Consequently, the water flux direction will be reversed, resulting in an opposite

294 Chapter 13 pressure loading and shearing applied to the membrane induced by tangential flow. The frequency of backwashing, which could be high, depends on the type and rate of foulant build up on the membranes. To simulate the backwashing process, mechanical fatigue analyses should be conducted. To the best of our knowledge, no investigation about the fatigue behavior of polymeric membranes under multiaxial loading conditions has been reported. Therefore, it is recommended to study the fatigue properties of the membranes under uniaxial loading state as well as biaxial loading state with controlled environmental conditions and at various frequencies. Moreover, because some fouling particles need additional chemically enhanced backwash to be removed, the fatigue tests can be conducted at different temperatures, whereas the membrane sample is immersed in a cleaning solution medium. By using this approach, the mechanical fatigue and chemical aging of the membrane could be collectively accessed. In addition, the home design membrane testing cells can also be used to conduct the fatigue test of polymeric membranes by changing the flow direction at low frequencies. Using the continuous recorded pressure (strength)edeflection behaviors, the microstructure evolution of the membranes can be studied. Moreover, environmental effects can also be coupled with the home designed membrane testing cells to study the fatigue properties of the membrane.

5.3 Real-Time Micromechanical Investigations As discussed in Section 3, the mechanical behaviors of polymeric membranes are highly dependent on pore morphology, pore size distribution, and microstructure. To study the mechanical integrity of nanoporous graphene as desalination membrane, Cohen-Tanugi and Grossman [7] conducted a molecular dynamic simulation. Their simulation results indicated that the pore size increased with increasing biaxial strain engendered by a stress concentration around the nanopores. For high strain levels, the failure of nanoporous graphene occurred in a brittle manner and was induced by nucleated defects at the nanopores. Although the simulation was well conducted to predict the physical and mechanical properties of the desalination membranes, laboratory in situ experiments (as for example tensile testing in a scanning electron microscope) would provide new insights about the intrinsic deformation and damage mechanisms of these membranes. This could improve the simulation and help researchers to develop next generation desalination membranes with improved performance. In addition, not only the mechanical deformation can result in membrane microstructure evolution, but also cleaning and aging can lead to change of membrane microstructure affecting both permeate flux and rejection [177,178]. In situ characterization would be again of high importance to reveal the influence of external factors on the intrinsic membrane microstructure evolution. Concerning mechanical strain as external factor, some relevant in situ methods to study the strain-induced microstructure evolution of the

Mechanical Characterization of Membranes 295 membranes can be found in literature. For example, at micrometer scale, strain field measurement can be conducted using 2D/3D digital image correlation (DIC) technique. Strain-induced failure initiation and failure propagation can be recorded in real time and correlated to the membrane macroscopic stressestrain response. In addition, 3D imagingbased techniques such as X-ray tomography using synchrotron or laboratory sources are also suitable for studying pore shape evolution, pore orientation, pore size distribution, and neighboring pore interaction during the deformation of the porous membranes [179,180]. X-ray tomography was used for imaging and reconstructing of the porous structure of undeformed polymeric membranes (Fig. 13.29) [176,181,182]. Recently, Viguie´ et al. [183] used this technique with a resolution of 1 mm to reconstruct 3D structure of their hollow fiber membranes. However, the acquisition time of their setup is in hours, which does not allow real-time investigation. On the contrary, synchrotron setup significantly reduces the acquisition time to minutes [184]. Furthermore, synchrotron tomography can attain resolution of about 60 nm [184]. To investigate the strain-induced (uniaxial and biaxial) microstructure evolution of the nanoporous membranes, it is therefore expected to

Figure 13.29 Quantitative three-dimensional reconstruction of a microfiltration membrane using an ultramicrotome mounted in the specimen chamber of an environmental scanning electron microscope. From Reingruber H, Zankel A, Mayrhofer C, Poelt P. Quantitative characterization of microfiltration membranes by 3D reconstruction. J Membr Sci 2011;372:66e74.

296 Chapter 13 use the synchrotron source for X-ray tomography. In addition, time-resolved X-ray diffraction, including wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS), is also an effective technique for in situ mechanical testing of membrane materials. WAXS detects scattering objects in the typical range 0.1e1 nm as crystalline planes enabling to study their formation, destruction, or orientation as was shown for polymer materials [185] On the other hand, SAXS provides information of typical size scale of 1e100 nm as crystals or structural defect and voids [186,187]. Similarly to 3D imaging techniques, the final SAXS/WAXS results and their relevancy strongly depend on the acquisition time, resolution, and data treatment that appeared as fundamental aspects for in situ investigations.

6. Conclusions Uniaxial tensile test, bending test, dynamic mechanical analysis, nanoindentation, and bursting tests are the most widely used approaches to investigate the mechanical behaviors of polymeric membranes. In this, most of the studies are conducted at room temperature, the sample geometry and the loading rate are quite different. Attention has focused on the mechanical properties of the membranes with the effects of processing conditions, the composite composition or filler content. Normal wear, tear, and polymer chain breakage induced by fouling and chemical aging are the main causes of the membrane mechanical degradation. There are very few studies that investigate the underlying deformation mechanisms of polymeric membranes and there are even fewer studies that analyze the difference in the mechanical properties of polymeric membranes when assessed by different testing methods. The stress distribution of the membranes is nonuniform as membranes are not only under simple applied pressure-induced compression and laminar floweinduced shearing-stress conditions, but also under complex stress state induced by turbulence and rotational flow depending on the flow pressure and velocity as well as membrane cell configurations. Therefore, the conventional methods for studying the mechanical properties of the membranes do not represent their actual loading conditions. Advanced mechanical testing techniques should be developed to investigate the membranes under similar stress state as that experienced in water treatment applications. Home-designed mechanical testing cells are expected to be developed to investigate the mechanical behavior of the membranes under similar loading states. In addition, it is important to evaluate temperature and pressure effects on the mechanical responses of the membranes. Moreover, the cyclic pressure condition of the membranes could be simulated by fatigue testing at different frequencies under various environmental conditions. To probe the continuous three-dimensional structural changes induced by strain and to correctly analyze the obtained load-deflection behavior, it is required to develop an

Mechanical Characterization of Membranes 297 advanced measurement system. Furthermore, real-time 3D imaging-based techniques are potential experimental tools to reveal the relationship of the microstructure evolutiondmacromechanical property of the membranes. Using rapid, high-resolution, real-time imaging technique, the mechanical behavior of the membranes can be studied with the underlying mechanisms at different scales.

List of Abbreviations AFM AQPz CA CFD CNF CDP DIC DMA GO HFP MBF MBR MD MF NF PA PAA PAN PEG PEI PES PET PRO PSF PVA PVDF PVP RO SAXS SBF

Atomic force microscopy Aquaporin Z Cellulose acetate Computational fluid dynamics Carbon nanofiber Cyclodextrin polymer Digital image correlation Dynamic mechanical analysis Graphene oxide Hexafluoropropylene Multibore hollow fiber Membrane bioreactor Membrane distillation Microfiltration Nanofiltration Polyamide Polyacrylic acid Polyacrylonitrile Polyethylene glycol Poly(ethyleneimine) Polyethersulfone Polyethylene terephthalate Pressure retarded osmosis Polysulfone Poly(vinyl alcohol) Polyvinylidenefluoride Poly(vinyl pyrrolidone) Reverse osmosis Small-angle X-ray scattering Single-bore hollow fiber membrane

298 Chapter 13 SEM SPES TFC UF VMD WAXS ZrDETPMP

Scanning electron microscope Sulfonated polyethersulfone Thin film composite Ultrafiltration Vacuum membrane distillation Wide-angle X-ray scattering Zirconium diethylene triamine pentamethylene phosphonic acid

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