Porous latex composite membranes: fabrication and properties

Porous latex composite membranes: fabrication and properties

Journal of Membrane Science 155 (1999) 79±99 Porous latex composite membranes: fabrication and properties Steve Jons1,a, Paul Ries2,b, Charles J. McD...

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Journal of Membrane Science 155 (1999) 79±99

Porous latex composite membranes: fabrication and properties Steve Jons1,a, Paul Ries2,b, Charles J. McDonaldc,* a

FilmTec Corporation, 7200 Ohms Lane, Edina, MN 55439, USA b Dow Chemical, Central Research, Midland, MI 48667, USA c Dow Chemical, Emulsion Polymers, Midland, MI 48667, USA

Received 19 June 1998; received in revised form 17 September 1998; accepted 18 September 1998

Abstract A new class of micro®ltration (MF) and ultra®ltration (UF) membranes has been developed. By placing latex particles onto the surface of a microporous substrate and stabilizing the porous array, voids are formed between the particles which provide narrowly distributed pores that serve as separation channels. The size of the interstitial voids in the array is governed by the diameter of the latex particle. This aqueous based technology has advantages relative to other membrane fabrication processes in terms of the high asymmetry of the membranes, the facile adjustment of pore sizes, and the ability to easily modify pore surfaces during the synthesis of particles. A number of approaches were examined for placement of particles and stabilization of latex composite membranes (LCMs). Filtration of particles with reactive surface groups that provide covalent linkages at the contact points in the particle array proved most effective in obtaining stable membranes. These membranes had narrow size distributions in both the UF and MF range and were capable of being cleaned and back¯ushed. The membranes were characterized in terms of gas permeabilities, pure water permeabilities and electron microscopy. The rejection properties of LCMs were also examined during ®ltration of monodispersed latex particles and a broadly dispersed dextran mixture. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Microporous and porous membranes; Membrane preparation and structure; Ultra®ltration; Micro®ltration; Composite membranes

1. Introduction Ultra®ltration (UF) and micro®ltration (MF) membranes have pore sizes that range from 0.0010 to 10.0 mm. These membranes are manufactured by a *Corresponding author. Tel.: +1-517-636-5316; fax: +1-517-638-6356; e-mail: [email protected] 1 Tel.: +1-612-897-4249; fax: +1-612-897-4268; e-mail: [email protected] 2 Tel.: +1-517-636-2341; fax: +1-517-638-7133; e-mail: [email protected]

variety of processes including phase inversion of a polymer solution stimulated by temperature or a solvent/nonsolvent combination [1]. Typically, there are a number of processing and performance limitations associated with a given membrane technology, e.g. restricted choices of surface chemistries and/or pore sizes available. A fabrication process which is aqueous based, has broad ¯exibility in surface chemistry, and an easily adjusted range of pore sizes with relatively narrow pore size distributions would provide a number of bene®ts compared to existing technology.

0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00304-4

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This paper describes a new class of composite membranes in which the discriminating layer is an array of latex particles stabilized on a porous support, such as polysulfone or nylon [2]. These latex composite membranes (LCMs) have narrow pore size distributions in the range of UF and MF applications. Our emphasis in this paper will be on their fabrication and performance properties. Gas and water permeability data as well as electron microscopy will constitute the bulk of the characterization data. The rejection of LCMs during ®ltration of monodispersed latex particles (MF) and a polydisperse mixture of dextran molecules (UF) will also be reported. 2. Theory 2.1. Array structure and pore size Latex composite membranes are formed by applying and stabilizing a water-based dispersion of uniform polymer particles onto the surface of a micro®ltration support. The particles form a thin array with the interstitial spaces serving as pores for size discrimination, and the support provides mechanical stability. The latex polymer should have a glass transition temperature suf®ciently high that the particles do not coalesce into a continuous ®lm during processing. The thickness and mean pore size of these membranes can readily be de®ned by the amount of coating and size of the particles. Fig. 1 shows atomic force micrographs of the top surface of a MF latex composite membrane typical of those developed in this study. The pore size of a latex composite membrane is primarily governed by the size and size distribution of the latex particles. It is well established that the packing of granular particles leads to the formation of porous layers suitable for use as ®lters and membranes [3±6] . The irregular interstitial spaces between particles form channels through which the ®ltrate or permeate can ¯ow. Though irregularly shaped, a pore size can be de®ned as the largest spherical particle that can pass through the interstitial spaces. Monodispersed spherical particles, such as emulsion polymerized polymers, can pack in regular crystalline arrays, and a number of packing geometries are possible: e.g. hexagonal closest packing, cubic closest packing, and body-centered cubic packing. The bulk

Fig. 1. Atomic force micrographs of the top surface of a latex composite membrane formed by application of 0.35 mm particles (Latex C) to a Nylon support using a number 4 Meyer rod. Stabilization was attained by heating at 1108C for 30 min.

porosity of the array depends on packing geometry but is independent of particle size. The hexagonal/cubic closest packed array has a porosity of 26.0%. A bodycentered cubic array has a porosity of 47.6%. The pore size, as de®ned above, however, is a function of the particle size and can be calculated from simple geometry. For hexagonal/cubic closest packed and bodycentered cubic structures, the respective pore sizes are 15.5% and 41.4% of the diameter of particles used to form the arrays. Thus, a wide range of pore sizes can be achieved simply by selecting the appropriate

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Fig. 2. The pore sizes resulting for different particle sizes in the case of hexagonal/cubic closet packing and body-centered cubic packing structures.

particle size to form the array. This is illustrated in Fig. 2. A defect-free array, as described above, can indeed be approached with careful experimentation including removal of ionic species from the continuous phase of the dispersion and slow drying rates [7]. With the facile processing described in this work, however, the particles laid down on a porous substrate have both crystalline areas as well as areas that have irregular packing, i.e. particle dislocations that disrupt the ideal crystalline structure. However, the layering of particles provides a statistical averaging of the pores such that the dislocations do not excessively broaden the pore size distribution. 2.2. Flow properties Formation of a thin discriminating array on a porous substrate results in an asymmetric structure with the potential of having very high ¯ux. The resistance of the LCM is controlled by particle size, packing geometry, and the thickness of the discriminating particle array. In theory, a single layer of well-ordered particles could establish a defect-free array. In practice, the packing structure and the rugosity of the support limits this ideal. Effective MF and UF latex composite membranes can be formed with a minimal number of particle layers, e.g. <10 layers. Assuming hexagonal closest packing, 10 layers of 0.08 and 0.64 mm diameter particles will have theoretical pore sizes of

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0.012 and 0.10 mm and corresponding thicknesses of 0.65 and 5.2 mm, respectively. Even within the thin crystalline array, porosity is relatively high because the average size of void regions is much larger than the diameter of capillary pores of equivalent rejection. Further, introducing dislocations within the array increases both the surface and bulk porosity, again tending towards producing discriminating layers having high ¯ux. One means of estimating the resistance of the discriminating array is to use the packed bed equation given by Leva [8]. This equation assumes the particles are smooth spheres, ¯ow is laminar, and end effects are minimal. Applied to LCMs, the resistance R of the latex array can be determined from its volume porosity , the diameter of latex particles Dp, the density of latex particles L, and the mass of latex applied per unit area M. R ˆ ‰200 M…1 ÿ †Š=‰L D2p 3 Š:

(1)

At a given transmembrane pressure P and viscosity , ¯ux J through the membrane can be estimated from this resistance. J ˆ P=‰ RŠ:

(2)

The number of theoretical layers in an array, L, can also be calculated from the same parameters used to calculate resistance, namely L ˆ ‰6 …1 ÿ †=Š1=3 ‰M=…L Dp …1 ÿ †Š:

(3)

Volume porosities, , of arrays fabricated using smooth particles have been experimentally determined for particles of greater than 400 mesh (38 mm) with several different morphologies (spherical, cylindrical, and granular) [8]. The porosity for an array of packed latex particles can be estimated by extrapolating the linear dependence of porosity on particle size for smooth spherical particles. This extrapolation yields a volume porosity value of ˆ0.324 for latex arrays, and dead-end ®ltration experiments have suggested the value is reasonable for larger latex particles (>0.6 mm), but is too low for the smallest particles used in this study (0.08 mm). Using the series resistance model, the resistance for a latex composite membrane would be equal to the sum of that for the discriminating array and the support. The data in Table 1 show that a latex com-

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Table 1 Predicted flow and nominal pore size for a support, a latex particle array, and the composite membrane (comparison data are also provided for the Supor 100 membrane having pore size similar to the array) Structure

Resistance (cmÿ1)

Permeability (g/min/cm2/bar)

Nominal pore sizea (mm)

Gelman Supor 450 polysulfone Array formed from 0.366 mg/cm2 of 0.65 mm particles Composite structure of latex array (0.366 mg/cm2 of 0.65 mm particles) on Supor 450 support

1.18108 3.09108 4.27108

50.8 19.4 14.1

0.45 0.1 0.1

Gelman Supor 100 polysulfone

8.27108

7.3

0.1

a

Pore size estimation assumes closest packing.

posite membrane formed from 0.366 mg/cm2 (8.13 closest packed layers) of 0.65 mm particles on a Gelman Sciences Supor 450 support should demonstrate substantially greater ¯ow than other membranes of similar pore size. Comparison to actual experiments will be provided later. 2.3. Latex particles In addition to particle size, critical variables for the performance of an LCM include structure, surface charge and reactivity of the latex particles. As illustrated in Fig. 3, each can be varied substantially by

adjustments to the polymerization process [9,10] . In particular, membrane surface properties are de®ned by the surface character of the particles. Typically, latex particles have a hydrophilic surface composed of surfactants, polymeric or oligomer species, and, in some cases, charged end groups derived from the water soluble polymerization initiator. In this work, the surface charge is anionic which would have a pH dependence due to the ionization pK of carboxylic acid containing monomers. 3. Experimental 3.1. Membrane components 3.1.1. Supports Commercial MF membranes made either of nylon or polysulfone served as supports for the latex composite membranes described in this work. The nylon substrate came from the Cuno, (Meriden, Conn., Cat. No. NM827-02-020SP). This is an isotropic microporous nylon 66 membrane with a nominal pore size of 0.2 mm and a cationic surface. Hydrophilic Supor1, poly(ethersulfone) microporous supports were obtained from Gelman Sciences (Ann Arbor, MI). Supor membranes used were of pore sizes 0.1 mm (Supor 100), 0.2 mm (Supor 200) or 0.45 mm (Supor 450).

Fig. 3. Illustration of morphologies and surface modifications readily available via synthesis techniques.

3.1.2. Latex The latex particles described in this paper were made via a semi-continuous emulsion polymerization technique. They were synthesized in a jacketed automated reactor equipped with agitator, nitrogen inlet source and multiple inlet feeds for monomer(s) and initiator. Since a variety of particles sizes were

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Table 2 Latex particles: composition, morphology and surface modification Latex

Diameter (mm)

Morphology

Description and monomer contenta

A

0.08

Ê shell Core±shell with 100 A

B C D E F

0.16 0.35 0.34 0.45 0.67

Homogeneous Homogeneous Homogeneous Homogeneous Ê shell Core±shell with 394 A

G H I

0.70 0.98 0.65

Homogeneous Homogeneous Ê shell Core±shell with 215 A

J

0.65

Ê shell Core±shell with 260 A

K

0.67

Ê shell Core±shell with 1540 A

L

0.67

Ê shell Core±shell with 1450 A

Core: MMA(99.5), AMA(0.5) Shell: BA(41), MMA(40), GMA(15), AA(3.5), AMA(0.5) MMA (100) S(99.0), DVB(1.0) MMA (100) S(100) Core: S(99.6), DVB(0.4) Shell: MMA(97.8), AMA(1.2), AA(1.0) Dow diagnostics (1B10) S(98.1), AA(1.9) Core: MMA(80.0), S(16.3), AMA(2.5), AA(0.8), DVB(0.4) Shell: MMA(59.0), BA(17.7), GMA(15.6), AMA(4.0), AA(3.8) Core: MMA(79.7), S(16.5), AMA(2.5), AA(0.8), DVB(0.4) Shell: MMA(71.7), GMA(15.6), BA(5.0), AMA(4.0), AA(3.8) Core: S(97.5), DVB(2.5) Shell: MMA(96.0), AMA(3.0), AA(1.0) Core: S(97.5), DVB(2.5) Shell: MMA(96.5), AMA(3.0), AA(0.5)

a

PS: Polystyrene, DVB: divinylbenzene, MMA: methyl methacrylate, GMA: glycidyl methacrylate, BA: butyl acrylate, AA: acrylic acid, AMA: allyl methacrylate.

required in this work, both seeded and unseeded emulsion polymerization processes were used. In the unseeded process, the initial charge in the reactor had suf®cient surfactant to nucleate the growth of particles. With knowledge of the amount of surfactant and monomer, particles of the appropriate size could be made. In the seeded process, an amount of a seed latex that served as nucleation sites was introduced into the reactor. To this was added known amounts of an initiator and monomer continuously over a given time period. A simple geometric calculation predicted the ultimate size the seed latex reached during the polymerization. Various feed streams and feed rates were used to develop particles of the appropriate morphology, composition and surface character. The approaches used to make these particles have been published in greater detail elsewhere [10,11]. The particles used in this study are listed in Table 2 along with their composition, morphology, and surface character. 3.2. Fabrication 3.2.1. Placement of particles 3.2.1.1. Coating methods. Though many approaches are possible for coating the particles onto the porous

support, the majority of the membranes examined in this study were fabricated by filtering under 0.14 bar (2 psi) pressure. Typically, 40 g of (vol ratio:1/1) methanol/water solution was passed through the support in order to ensure that all the pores were wetted, followed by 50 ml of 0.2% cationic surfactant (Arquad 2C-75, Akzo Chemie America, McCook, IL) in (volume ratio:1/1) methanol/water. This surfactant treatment increased the charge interaction between the substrate surface and the anionic particles typically used in this work. The latex was applied as a dilute water dispersion at 0.02% solids or less. For very small particles, the diluted latex was treated with ion-exchange beads to extracts the hydrated species in the serum or on the surface of the particles. Typically, between 50 and 100 ml of the diluted latex solution was ®ltered through the support. The ®ltration technique allows the amount of latex applied per unit area to be adjusted either by the dispersion concentration or the volume of dispersion ®ltered. Under the assumption of a given packing structure, these two variables allow for ®ne control of layer thickness. The equation below calculates the resulting thickness from the mass per unit area of latex solids applied M, the density of latex particles L, and the

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void fraction . For hexagonal/cubic closest packing and body-centered cubic packing, ˆ0.26 and ˆ 0.476, respectively. T ˆ M=‰L …1 ÿ †Š:

(4)

Another technique for applying latex particles to a support is the use of a wire-wrapped Meyer rod. With this approach, the coating thickness is controlled by both the gage of the wire wound around the steel rod and the concentration of the latex dispersion. The Supor support was soaked in deionized water for 1 min and then placed on a glass plate where excess water was removed using a rubber roller. A 2% solids latex dispersion was puddled in front of the Meyer rod at one end of the Supor 200 support, and the dispersion was drawn across the surface with the Meyer rod. Excess dispersion was removed by blotting it from the glass plate. The sample was then placed in a chamber at approximately 65% relative humidity and allowed to dry at ambient temperature overnight. The coating was ®xed by either thermal annealing or UV-induced polymerization, as described below. In work presented here, 0.35 mm particles (Latex C) and 0.70 mm particles (Latex G) were applied to a Supor 200 support with a #4 Meyer rod (RD Specialties, Webster, NY). Slip-casting has also been used to apply 0.35 mm particles (Latex C) to a Supor 200 support. In slipcasting, one side of a support is coated by contacting it with the surface of a latex dispersion. The support is then lifted, allowing excess liquid to drain. The dispersion concentration and the amount of time the support is allowed to drain determine the coating thickness. In this work, one side of a cationic surfactant treated Supor 200 support was contacted with the aqueous dispersion described above for 10 s before being removed and held vertically. The excess dispersion was blotted from the bottom edge after which the membrane was laid ¯at, covered, and allowed to dry at about 65% relative humidity and ambient temperature. The coating was ®xed by UV-induced polymerization of monomer previously equilibrated with the particles (10% based on weight polymer). 3.2.1.2. Role of surfactants. The placement of latex particles as a discriminating layer on the top of a porous support requires that the particles be excluded from penetrating into the support. This positioning can rely solely on size exclusion, i.e. restricting the

particle's diameter to a size greater than that of the pores in the substrate. This restriction can be lessened by prewashing the support with a surface active material which absorbs onto the support and interacts with the particles during the coating process. As mentioned, the latex particles in this study have an anionic charge on their surface. By prewashing the support with a cationic surfactant, it is possible to position particles smaller than the pores of the support on the top surface. Adsorption of the particles onto the top surface is facilitated by the electrostatic attraction of the anionic particles with the cationic support. This approach was one of the procedures used in this work. 3.2.1.3. Uniformity: ion-exchange treatment of latex particles/multilayer coating. In order to make latex composite membranes in the UF pore size range, very small particles are required. These small particle arrays have a tendency to undergo mud cracking. These mud cracks are breaks in the coatings that form from contraction that occurs with further drying after the immobilization point of the array is reached. One approach to minimizing this effect is to increase the immobilization concentration by removing salts and surfactants from the latex dispersion. These ionic components compress the electrical double layer [12], shielding the interactions between the particles, and decreasing the stability of the colloid. There are a number of techniques in the literature that enable the removal of serum species from latex dispersions [13,14] . One prominent method is the use of ion-exchange resin to adsorb the low molecular weight hydrophilic materials that are typical biproducts of an emulsion polymerization. Typically, the diluted latex was treated with an equal weight (based on solids weight) of mixed bed ion-exchange resin (Bio-Rad analytical grade mixed bed resin, AG501-x* Bio-Rad Laboratories, Hercules, CA). The mixture was shaken for 1 h, and the resin was removed by ®ltration through a 100 mesh screen. The particles were then placed on the membrane support. The second procedure that dramatically reduced defects in UF arrays is the use of multiple latex coatings. The ®rst array is placed on the support and stabilized. Mud cracks developed during the drying process are then ®lled with a second application of latex particles. This second ®ltration preferentially

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®lls in cracks in the ®rst array because of greater ¯ow through the defect areas. 3.2.2. Stabilization of the array The stabilization of the particle array may be by physical or chemical methods. 3.2.2.1. Physical. Physical stabilization is based on the interdiffusion and entanglement of polymer chains between particles. The simplest physical method is to partially anneal the latex particles at elevated temperatures. The thermal properties of the polymer in the particles determines the conditions under which limited coalescence of the particles occurs without sufficient flow to fill in the pores. The appropriate thermal conditions were determined empirically. Typically for polystyrene (glass transition temperature, 1048C), the particles were deposited by one of the methods described above and placed in an oven at 1208C for a 5 min. The presence of some residual water in the array during this treatment obscures the actual temperature it reaches. 3.2.2.2. Chemical. Chemical stabilization involves both interdiffusion of polymer chains as well as a chemical reaction at the point of contact forming a covalent linkage. The application of chemical methods proved to be most effective in stabilizing the array. Two approaches were tried: one involved placement of a reactive species on the surface of the particles, the second involved swelling the particles with a monomer/photoinitiator mixture and photopolymerizing after formation of the array. The range of surface chemistries available for developing covalent linkages between particles in LCMs is extensive [15±19]. This was summarized in Fig. 3. The surface chemistry most intensively examined in this study was the reaction of an epoxy group with a carboxylic acid to form the hydroxyl ester linkage [20±22]. Various approaches to utilize this reaction have been examined. They involved the placement of the reactive groups on the same particle, placement on different particles as in a blend, or having one reactant on the particle and the other as a difunctional additive to the serum of the latex. Examples will be presented where membranes were stabilized by the interdiffusion of reactive functional groups copolymerized in the shell of the particle.

Fig. 4. Micrograph showing a cross-section of a latex composite membrane formed with multiple coatings. In the first application, 0.37 mg/cm2 of 0.67 mm particles (Latex F) were applied to a Supor 200 support by filtration and stabilized by UV-induced polymerization of methylmethacrylate. This was followed by two successive filtrations of 0.106 mg/cm2 of 0.08 mm particles (Latex A), which were each stabilized using a room temperature reaction involving glycidylmethacrylate.

Stabilization induced by incorporation of a difunctional epoxy onto a shell containing carboxylic acid will also be demonstrated. Stability can also be induced by imbibing monomer into the particles and polymerizing after placement on the porous support. When this approach was followed, typically 8% additional monomer (based on the weight solids) was added to a dispersion of the polymer particles. The monomer contained 0.1% by weight of a photoinitiator, benzoin ethyl ether (Polysciences, Warrington, PA). Polymerization was induced by exposure to UV-light, and optimal times were empirically determined. Both polystyrene and polymethylmethacrylate arrays were stabilized with this technique. More than one method can be used to stabilize particles on a membrane. Fig. 4 shows a cross-section of an LCM formed by successive coatings on a Supor 200 support. The larger 0.67 mm particles (Latex F) were applied by ®ltration and stabilized by UVinduced polymerization of methylmethacrylate monomer. This resulted in a stabilized array of approximately eight layers. The smaller particles of Latex A (0.08 mm) were also applied by ®ltration but stabilized at elevated temperature by the reaction between gly-

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cidylmethacrylate and carboxylic acid, both polymerized into the particles' shell. Since the glass transition temperature for the shell of Latex A is approximately room temperature, interdiffusion of the surface polymer would be expected to re-enforce the stability gained by covalent bond formation. 3.3. Characterization 3.3.1. Microscopy The scanning electron micrographs of the top surfaces of the membranes were obtained from an ABT DS130 instrument on samples coated with a 0.005 mm layer of chromium. Cross-sections were prepared by freeze-fracture. Micrographs were obtained using a transmission electron microscope, JEOL 2 000FX. Atomic force microscopy (AFM) images of the latex composite membrane were obtained under distilled/deionized water using a Nanoscope II AFM (Digital Instruments, Santa Barbara, CA) operating in contact mode. A 200 mm long silicon nitride cantilever with a nominal spring constant of 0.38 N/m was used. The sample was scanned using a ``G'' scanner at a scan rate of 5.79 Hz. Gray scale height images were recorded as a 512512 point array. 3.3.2. Gas permeability data Mean ¯ow pore sizes and pore size distributions were obtained on the supports and the composite membranes in the MF pore size range by comparing the ¯ow of nitrogen through membranes before and after wetting with light mineral oil [23]. Transmembrane pressure was typically increased in either 0.34 or 0.69 bar (5 or 10 psi) increments. Flow through dry membranes was measured up to 6.9 bar (100 psi) and extrapolated to higher pressures using formulas for a compressible gas [24]. Flow through wet membranes was measured up to a maximum of 13.8 bar (200 psi), this pressure being de®ned by an instrumental limitation to measure high gas ¯ows. The maximum pressure for which wet ¯ow was examined determined the minimum pore size which could be evaluated. The mean ¯ow pore size was calculated with the Young± Laplace equation using the pressure for which ¯ow through the wet membrane was half that through the dry membrane. The distribution of ¯ow through different pore sizes was calculated from the changing ratio of wet to dry ¯ow as pressure increased. Since

laminar ¯ow through a cylindrical pore is proportional to the fourth power of the pore diameter, [25] plots of the ¯ow distribution emphasize the effect of large pores. 3.3.3. Pure water permeability Pure water permeabilities were measured in deadend mode using a stainless steel ®lter holder purchased from Fisher Scienti®c (Pittsburgh, PA, Cat No. 09753-13A). The water used for these experiments was obtained from a Barnstead E-Pure Filter system (Dubuque, IA) and was continuously cycled through a 0.0300 mm Poretics ®lter (Livormore, CA) before and during the measurement. To insure complete wetting, the membrane was soaked in a 50/50 methanol/water solution prior to placement in the apparatus. The pressure was ®rst increased to 2 bar (30 psi) for 1 min before measuring pure water permeabilities. Most commonly, this measurement was done at 0.69 bar (10 psi). The stability of latex composite membranes to back ¯ushing was also examined using this apparatus. In these experiments, the membranes were oriented such that the array was on the low pressure side, increasing the probability that particles could be blown off. For UF membranes, the pressure was increased from 0.69 (10 psi) to 4.1 bar (60 psi) in 0.69 bar (10 psi) increments. For MF membranes, the maximum pressure was increased from 0.69 (10 psi) to 3.1 bar (45 psi) in 0.34 bar (5 psi) increments. The maximum pressures attainable in each case were de®ned by the testing apparatus. The water ¯ux was measured at the new pressure and at 0.69 bar (10 psi) again after each incremental change. Stability was indicated by a linear dependence of ¯ux on pressure and a ¯ow rate at 0.69 bar (10 psi) that was independent of pressure history. 3.3.4. Filtration of cherry juice Fouling and cleaning characteristics of LCMs were investigated using cherry juice as a feedstock. Reconstituted concentrated cherry juice having approximately 68% solids was diluted to 14% for ®ltration. These experiments were carried out using a temperature controlled Minitan-S cross¯ow ®ltration cell (Millipore) operated with 0.48 bar (7 psi) transmembrane pressure. Because the pump ¯ow rate was held constant at around 300 ml/min, the percent recovery

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varied with ¯ux through the membrane. The permeate rate was measured as a function of time over several permeate-wash cycles. Two types of membranes were examined in these experiments: A Supor 100 membrane and an LCM formed by ®ltration of 0.37 mg/cm2 of 0.65 mm particles (Latex I) onto a Supor 200 support. In each case, the membranes were soaked in a 50/50 methanol/ water solution before mounting in the ®ltration cell to ensure the pores were completely wet. Methanol was removed by passing 2 l of pure water through the membranes to drain before testing. At the start of each cycle, the pure water permeability of the membrane was measured using highly puri®ed water. The feed stock was changed to cherry juice and the permeate ¯ow was measured as a function of time over 120 min. The temperature of the feed rose approximately 28C during this 2 h period. All measurements were performed at approximately 508C, but data are presented after normalizing for viscosity to 258C for consistency with other measurements. Between cycles, the membrane was rinsed with 2 l of pure water and then cleaned for 30 min by passing a 0.5% Ultrasil 10 (Ecolabs, St Paul MN) caustic solution through the membrane. The membrane was then rinsed off cleaning solution by passing 3 l of highly puri®ed RO water through the membrane.

water soluble polymer standards of various molecular weights through the membrane. This method has been described extensively in the literature [28]. The LCM was placed in an Amicon 8050 stirred ®ltration cell (Amicon, Beverly, MA) and ®ltered under gravity. The feed solution contained a mixture of various molecular weight dextrans obtained from Sigma (St. Louis, MO). Speci®cally, this dextran mixture contained 10 mg/l each of products D3759, D4133, D4626, and D9260. These are broadly dispersed standards having a nominal molecular weight of range from 9 000 to 1 00 000 amu. Permeate and retentate samples were collected at different times during the ®ltration and analyzed by size exclusion chromatography (SEC). The chromatograph was equipped with Toyosoda PW 3000 & 5000 columns (Tokyo, Japan) with a refractive index detector (Waters 410, Bedford, MA). The retention times of our SEC were calibrated using polysaccharide standards obtained from Polymer Laboratories (Amherst, MA, Cat. No. 2090-0100). Ratios of the refractive index signals from permeate and retentate sample were calculated to de®ne the retention characteristics.

3.3.5. Retention of MF latex composite membranes Rejection of micro®ltration latex composite membranes has been demonstrated using a series of latex challenge solutions. Polystyrene latex particle standards having average diameters measured by hydrodynamic chromatography [26,27] between 300 and Ê were prepared at 1.0 g/l solids. A latex com1461 A posite membrane was loaded into an Amicon 8200 dead-end separation cell, and 50 ml of the latex solution was ®ltered through the membrane at 0.34 bar (5 psi) transmembrane pressure. Passage of latex particles was qualitatively observed as turbidity in the permeate. For comparison, the same measurements were performed with the Supor 100 membrane and the Supor 450 membrane.

Fig. 5 contains electron micrographs of an LCM at three different magni®cation. This surface is an array formed by ®ltration of 0.67 mm polystyrene particles (Latex F) onto a Supor 200 support. The support has a nominal pore size of 0.2 mm, and the latex particles are retained on the surface due to size exclusion. The micrographs show both the uniformity of surface coverage and the random packing commonly resulting from ®ltration. Regions of low crystallinity are common at the surface, clearly increasing the surface porosity above that calculated from the crystalline structures possible. This suggests that the selectivity obtained from a latex composite membrane results primarily from the tortuous path below the surface. Hence, it is less accurate to consider discrimination at the top surface, and more appropriate to view these membranes as depth ®lters even though the discriminating layer usually has a thickness of much less than 10 mm.

3.3.6. Retention of UF latex composite membranes The procedure used to examine the retention characteristics of UF membranes involves passage of

4. Results and discussion 4.1. General structure of latex composite membranes

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Fig. 6. Micrograph showing the cross-section of a MF latex composite membrane. The membrane was formed by filtration of 0.37 mg/cm2 of 0.65 mm particles (Latex I) onto a Supor 450.

Fig. 6 is a cross-section of an LCM formed by ®ltration of 0.37 mg/cm2 of 0.65 mm particles (Latex I) onto a Supor 450 support. Again, ®ltration gives random packing as is seen in the interior of the array. As above, the defects in the array in¯uence the porosity of the membrane, in this case the bulk porosity. A coating thickness of between 4 and 6 mm was observed in different regions of the membrane. This corresponds reasonably to a thickness intermediate to those predicted by hexagonal/cubic closest packing (4.5 mm) and body-centered cubic packing (6.3 mm). The number of theoretical layers predicted for these two geometries would be 7.5 and 9.5, respectively. 4.2. Fabrication

Fig. 5. Micrographs at three different magnifications of the top surface of a MF latex composite membrane. The membrane was formed by filtration of 0.67 mm particles (Latex F) onto a Supor 200. The membrane was stabilized by heating at 1208C for 5 minutes.

4.2.1. Particle application The structure of the particle array in an LCM varies in crystalline order depending on the method of fabrication. In the ®ltration process, water is removed under pressure leading to rapid formation of the particle array. In contrast, the formation of the array by coating with a Meyer rod or by slip casting is slower because of its dependence on the rate of evaporation of water. These slower kinetics induce a greater degree of order among particles. The ATM array displayed in Fig. 1 was formed from the Meyer rod coating process.

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Table 3 The influence of particle coating technique on mean flow pore size of LCMs

Fig. 7. Micrograph of the top surface of a latex composite membrane formed by application of 10% solids dispersion 0.35 mm particles (Latex C) to a nylon support using a number 4 Meyer rod. Stabilization was attained by heating at 1108C for 30 min.

Fig. 7 is a scanning electron micrograph showing a larger region of the same membrane. The crystalline order of the array demonstrates both closest packing and body-centered cubic packing. Filtration dominated the membrane fabrication in this study. Despite the low degree of crystallinity, this approach was found more practical based on speed and the ease of making large scale version of these membranes. The largest LCMs made in this study have been 30 cm diameter sheets. Also, as mentioned previously this process is believed to be self-healing in that if defects in the array occur, these areas are preferentially ®lled because a greater amount of the dispersion passes through these regions due to their decreased resistance to ¯ow. Though the degree of crystalline order from the electron micrographs varies with the fabrication method, the mean ¯ow pore sizes, which range from 0.096 to 0.117 mm, are the same to within experimental error. Table 3 lists the mean ¯ow pore sizes, as determined from wet/dry gas ¯ow measurements, of the Supor 200 and three composite membranes formed by slip-casting, Meyer rod coating and ®ltration. In each case, 0.34 mm diameter particles were applied to a Supor 200 support. They were stabilized by UVinduced polymerization, as has been described in Section 3. These composites had a much reduced mean ¯ow pore size relative to the Supor 200 support. All are between the theoretically expected values for a

Membrane

Mean flow pore size (mm)

Supor 200 support Supor 200 support coated using Meyer rod with Latex C (0.35 mm) Supor 200 support coated by slip-casting with Latex C Supor 200 support coated by filtration with Latex D (0.34 mm)

0.27 0.104 0.096 0.117

hexagonal/cubic closest packed and a body-centered cubic array as shown in the curves of Fig. 2. As already mentioned in Section 3, small particle arrays are prone to mud cracking. As dehydration occurs, a point is reached where the viscosity of the array increases exponentially. This is the point at which particle±particle contacts occurs. This immobilization point is a function of particle size and the electrical double layer surrounding the particles. The smaller the particle the lower the immobilization concentration and the greater the shrinkage after immobilization as additional water of hydration is removed. This induces considerable stress within the particle array which is released by cracking. This is illustrated in the scanning electron micrograph in Fig. 8. This LCM is made from 0.16 mm diameter particles (Latex B) which was stabilized by UVinduced polymerization. There is conspicuous cracking in this array. The mean ¯ow pore size of this membrane was approximately 0.09 mm. This is greater than expected for a body-centered cubic array of 0.16 mm particles and much larger than the 0.025 mm pore size calculated for hexagonal/cubic closest packing of spheres. If the fabrication process is modi®ed to include an ion-exchange treatment of the particles prior to ®ltration, a more uniform array is formed. This treatment removes the hydrophilic species present in the serum and on the surface of a latex, expanding signi®cantly the electrical double layer of the particles. Thus, the dispersion remains stable at higher concentrations. Accordingly, after ion-exchange treatment, the immobilization point occurs at a higher volume fraction of particles, thereby reducing any subsequent shrinkage. Fig. 9 shows an electron micrograph of an LCM made

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Fig. 8. Micrograph which demonstrates mud cracking at the top surface of a UF latex composite membrane. The Supor 100 support was coated with 0.10 mg/cm2 of 0.16 mm particles (Latex B) by filtration. The array was stabilized by UV-induced polymerization.

Fig. 9. Micrograph of the top surface of a UF latex composite membrane. The membrane was formed by filtration of 0.106 mg/ cm2 of 0.08 mm core-shell particles (Latex A) onto a Supor 100. The membrane was stabilized using a room temperature cure.

by the application of 0.08 mm particles after ionexchange treatment. Though the uniformity is improved, some cracks are still formed. It was found that multiple applications of latex particles were required in order to form uniform LCMs in the UF pore size range capable of size-separating soluble polymers of less than 100 000 dalton molecular weight. The second application of particles is another self-healing process which closes the defect regions formed in the ®rst application.

4.2.2. Stabilization Three approaches were used to stabilize LCMs. These were UV-photopolymerization of particles with imbibed monomer, thermal annealing, and covalent bond formation between the particles. In the latter approach, covalent bond formation may require a high temperature, and the membrane processing would have a similar heat history to the thermal annealing process. The details of these approaches were described in Section 3. In the micrographs already present, each approach has been represented: UVphotopolymerization in Fig. 8, thermal annealing in Figs. 1, 6, 7 and 10, covalent bond formation in Figs. 6 and 9. The stabilization of LCMs were characterized with wet/dry gas ¯ow data and pure water permeabilities. The gas data were obtained with the particle array oriented either upstream or downstream relative to the nitrogen ¯ow. By feeding the gas with the array on the downstream side, the particles would be blown off if the stabilization process was ineffective. By contrast, with upstream positioning of the array, the particles would be forced into the substrate minimizing any disruption. Table 4 compares the calculated mean ¯ow pore sizes with the array oriented in these two directions. Two stabilization methods are examined: thermal annealing and UV-photopolymerization. The particles in these membranes were the 0.7 mm polystyrene/DVB (Latex G). When the arrays faced upstream, the stabilized and unstabilized membranes all had similar mean ¯ow pore sizes. When the array faced downstream, little change in the mean ¯ow pore size was observed for either of the two stabilized arrays. In contrast, when unstabilized arrays faced downstream, the mean ¯ow pore size was found to be similar to that of an uncoated Supor 200 substrate, namely 0.27 mm. These data were interpreted to mean Table 4 Stabilization of LCMs; thermal annealing and UV-induced polymerization Latex G (0.70 mm) applied to Supor 200 Stabilization method

Mean flow pore Mean flow pore size with array size with array upstream (mm) downstream (mm)

Not stabilized 0.147 Annealed at 1208C for 5 min 0.167 UV-induced polymerization 0.153

0.223 0.164 0.159

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Fig. 11. Pure water flux is shown as a function of time and pressure cycles. These data were obtained with the latex particle array downstream to the water flow and indicate that the LCM is stable to backflushing at room temperature and neutral pH. The LCM was made by filtration of 0.08 mm particles (Latex A) onto a Supor 100 support.

Fig. 10. Micrographs showing different levels of coalescence due to thermal annealing. A dispersion of 0.7 mm particles (Latex G) was filtered onto to a Supor 100 and then stabilized by heating at 1208C for either 10 or 15 min.

that both thermal annealing and UV-induced polymerization produced membranes stable to gas ¯ow up to pressure of 20.7 bar (300 psi). Back¯ushing with pure water is a more relevant test of membrane stability. In this case, the array is oriented downstream relative to water ¯ow, and water pressure is increased sequentially from 0.69 (10 psi) to 4.1 bar (60 psi) in 0.69 bar (10 psi) increments. The pressure is cycled back to 0.69 bar (10 psi) after each incremental pressure increase to measure the constancy of ¯ow. Stability was indicated by a linear dependence of the ¯ux on pressure, and a ¯ow rate at 0.69 bar (10 psi) that was independent of the pressure history. The data presented in Fig. 11 were developed on the UF membrane of Fig. 9 that

was stabilized at elevated room temperature with a reactive shell, applying the epoxy/carboxylic acid chemistry. Fig. 12(a)±(f) show similar plots for MF membranes formed by ®ltration of 0.37 g/cm2 of Latexes I or J onto a Supor 450 support. These latexes had GMA and carboxylic acid in their shell polymer which upon heating enabled covalent linkages to form. The heat treatment typically lasted 1 h at a temperature between 1108C and 1508C. Latexes I and J were both based on the same core and contained similar amounts of GMA and acrylic acid in the shell. However, the shell copolymer composition was varied to give different glass transition temperatures, 808C and 1008C, respectively. Fig. 12(a)±(c) show that curing at 1108C was insuf®cient to stabilize the particle array of Latex I, whereas curing at 1208C or 1508C yielded a stabilized array. It is also signi®cant that the ¯ux of the membrane heated at 1508C was signi®cantly less than the other two membranes of Latex I indicating that partial coalescence had occurred, which was con®rmed with SEM. By contrast, membranes formed from Latex J, with the higher Tg shell polymer, were unstable unless heated to 1508C. There was minimal coalescence of particles in this LCM, and the ¯ux was comparable to that of the Latex I membrane annealed at 1208C. One advantage of having a stabilization

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Fig. 12. Pure water flux is shown as a function of time and pressure cycles for membranes formed from two different latexes and annealed for one hour at one of the three different temperatures. Circles designate measured flux at the pressure specified. Squares show flux after returning again to 0.69 bar (10 psi). The membranes were formed by filtration of 0.37 mg/cm2 of 0.65 mm particles (Latex I and Latex J) onto a Supor 450 substrate. Latex I and Latex J were formed from the same core and had shells containing similar amounts of GMA and carboxylic acid. The respective Tg of their shells were 808C and 1008C.

process that occurs at high temperature is that the LCMs have increased tolerance to high temperature environments which is critical to many separation processes. These data are an example of the inherent ¯exibility of this membrane processing technology to tailor the viscoelastic properties of the particles and their morphology to a given separation. The stability of membranes to hot acid and hot base has also been investigated. Table 5 shows the effects of soaking two different membranes made from Latex K and Latex L for 2 h in 1 M NaOH and 1 M HCl at 958C. The GMA±carboxylic acid reaction was again used to stabilize the array. However, in this case, the shell of both latexes contained carboxylic acid but no GMA; Latex K was made with twice the acrylic acid as Latex L. A difunctional epoxy

(Dow Chemical DER-331, Freeport, TX) was added to each latex before ®ltration of 0.37 mg/cm2 of the 0.67 mm particles onto a Supor 450 substrate. Thermal curing was performed at 1408C for 1 h. Both membranes were found to be stable with the pure water permeability remaining constant after either treatment. 4.3. Characterization The characteristics of the discriminating layer of a latex composite membrane can be varied, and the subsequent in¯uence on membrane performance characterized. In this section, changes in the latex particle size and array thickness will be examined for their impact on pore size distribution, gas ¯ux, pure water

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Table 5 Pure water permeabilities of untreated and treated LCMs Membrane treatment

PWP for membrane formed with Latex K (g/cm2/min/bar)

PWP for membrane formed with Latex L (g/cm2/min/bar)

Untreated membrane NaOH (958C, 1 M, 2 h) HCl (958C, 1 M, 2 h)

20.5 18.1 18.5

19.7 18.3 17.8

permeabilities and with some LCM UF membranes, the retention of dextran molecules. 4.3.1. Coating thickness Based on the packed bed equation developed by Levy (Eq. (1)), one would expect a linear dependence of resistance on the thickness of the discriminating layer. Arrays of increasing mass were made by applying between 0.25 and 2.05 mg/cm2 of 0.45 mm particles (Latex E) to a Supor 200 substrate. The gas ¯ow at 6.2 bar (90 psi) through a dry membrane was then measured as a function of layer thickness. In each case, the LCM was oriented with the particle array on the high pressure side to ensure stability. Fig. 13 shows the relative resistances (inverse of gas ¯ow) for the Supor 200 support and four composite membranes. A linear relationship between the resistance and the mass of the coating is observed in these data, in good agreement with Eq. (1). Fig. 13 also shows that the line ®t to the relative resistance of the four composite membranes does not extrapolate to the resistance of the uncoated Supor 200 support. It is believed that this extrapolation to a lower resistance than the uncoated Supor 200 is due to the fact that the increase in the resistance at low coat weight is less than would be predicted because the coating is uneven due to the rugosity of the substrate. 4.3.2. Latex particle size The pore size of an LCM can be easily varied by changing the diameter of the latex. The effect on mean pore size has already been described in Fig. 2 for hexagonal close packed and cubic close packed arrays. This was also experimentally studied. The pore size distribution of membranes can be estimated from wet/dry gas ¯ow measurements. Fig. 14 plots the relative ¯ow through different membranes: three being commercial membranes and three latex composite membranes made from particles of

Fig. 13. The inverse of gas flow through a dry latex composite membranes is shown as a function of the mass of 0.45 mm particles (Latex E) applied to a Supor 200 support. The solid line is the best fit through the eight points which correspond to coated supports.

differing diameters. It should be noted that the relative ¯ow distribution is not the same as the pore size distribution and that this presentation of data emphasizes the effect of larger pores. Also, the method assumes cylindrical pores which is not accurate for the irregular, tortuous pores of the polyethersulfone or latex composite membranes. Fig. 14(a) shows the ¯ow distribution for a Poretics 0.1 mm track-etched polycarbonate membrane. The narrow distribution of diameters for the capillary pores produced by this method has been well documented, [1,23] and the experimental observation of a maximum near 0.1 mm re¯ects well on the accuracy of the measurements. Fig. 14(b) and (c) show the ¯ow distributions for the Supor 200 and the Supor 100 membranes. These membranes had nominal pore sizes of 0.2 and 0.1 mm, respectively. The measured distributions are quite broad with a mean ¯ow pore size greater than the nominal values. The ¯ow distribution data in Fig. 14(d) and (e) characterize the narrowing of the pore size by application of latex particles to a Supor 200 support. In

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Fig. 14(d), the membrane had 0.73 mg/cm2 of 0.7 mm particles (Latex G) on a Supor 200. It was formed by ®ltration and thermal annealing. The wet and dry gas ¯ow was measured with the array oriented toward the high pressure side. The observed mean ¯ow pore size was 0.129 mm, which is fairly close to the limiting size predicted from hexagonal/cubic closest packing. The theoretical pore sizes for body-centered cubic and hexagonal/cubic closest packing are 0.29 and 0.11 mm, respectively. Similar data were obtained with membranes made with 0.53 mg/cm2 of 0.45 mm (Latex E) and 0.98 mm (Latex H) particles applied to a Supor 200 support. The mean ¯ow pore sizes were less than 0.1 mm for the smaller particles and 0.15 mm for the larger particles, respectively. The mean ¯ow pore size for the 0.98 mm particles is again consistent with the value calculated from hexagonal/cubic closest packing, 0.15 mm. The partial curve for the LCM made from 0.45 mm particles results from the fact that the pressures required to remove liquid from the very small pores exceeded the capability of our equipment. These three examples demonstrate that narrow pore size distributions can be obtained with LCMs, and that mean ¯ow pore size can be varied by changing the diameter of the latex particles. For the 0.98 mm particle, the coat weight corresponded to less than 10 layers of particles.

Fig. 14. The relative flow through different pore sizes, as determined by wet/dry gas flow measurements, is shown for six membranes: (a) Poretics 0.1 mm polycarbonate track-etched membrane, (b) Supor 200 membrane, (c) Supor 100 membrane, (d) LCM formed by filtration of 0.73 mg/cm2 of 0.7 mm particles (Latex G) onto a Supor 200, and (e) two LCMs formed by filtration of approximately 0.53 mg/cm2 of 0.45 mm (Latex E) and 0.98 mm (Latex H) particles onto Supor 200 supports.

4.3.3. Pure water permeability The ¯ow of pure water through a membrane is an important parameter that enables one to estimate the relative ef®ciency of membranes in a separation process. In Fig. 14, the Supor 100 and the LCM derived from 0.70 mm particles (Fig. 14(c) and (d)) had nearly identical mean ¯ow pore sizes and similar ¯ow distributions. However, dry gas ¯ow through the latex composite membrane was more than twice that of the Supor 100 at similar pressures. The pure water permeabilities for Supor 100 and Supor 450 membranes have been compared to that of a latex composite membrane formed by application of 0.37 mg/cm2 of 0.67 mm particles (Latex J) to a Supor 450 support. The stabilization of this LCM was based on the GMA and acrylic acid present in the shell polymer and a curing cycle of 1508C for 1 h. This membrane had a mean ¯ow pore size as measured by gas permeability of 0.10 mm as compared to an estimated 0.20 mm for cubic close packing and 0.10 mm

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for a hexagonal closed packing. This latex composite membrane had a permeability (20.0 g/cm2/min/bar) that was 270% of that published for the Supor 100 (7.3 g/cm2/min/bar) and approximately 40% of that for the Supor 450 substrate (50.8 g/cm2/min/bar). Note that the ¯ux for the LCM composite is slightly greater than predicted in Table 1. (Eq. (1) would suggests a void volume actually closer to ˆ0.383, instead of ˆ0.324.) As pointed out earlier, the introduction of a thin latex layer serves to narrow the pore size distribution signi®cantly without undue loss in permeability. In the above comparison, the ¯ux from the LCM is signi®cantly higher than that of a Supor membrane of nominally the same pore size. 4.3.4. Fouling during filtration of cherry juice It was of interest to compare the performance of an LCM with that of a commercial membrane of similar pore size as both were challenged with a concentrated solution likely to foul the membrane. The system chosen was a 14% solution of reconstituted cherry juice. The ¯ux decline was measured during several 2 h ®ltration cycles, each separated by cleanings with a basic cleaner (Ultrasil 10) followed by a water rinse. Gelman Supor 100 membranes were compared to a latex composite membrane formed by ®ltration of 0.37 mg/cm2 of 0.65 mm particles (Latex I) onto a Supor 200 support, followed by annealing for 1 h at 1508C. No rejection properties were measured in these experiments. While the data in both Table 6 and Fig. 14 indicated that this LCM had a narrower pore size distribution than the Supor 100, this may not be re¯ected in rejection data, that is the rejection of complex feeds may differ from that of a simple particle feed. Fig. 15 shows the ¯ux measured for the two types of membranes during four ®ltration cycles, with a cleaning stage between each cycle. Both the LCM and the Supor 100 membranes showed lower permeabilities upon initial measurements with pure water than observed in previous tests using other equipment. The most likely cause of discrepancy is believed pressure measurements for this ®ltration system. The ¯ux through the LCM at the start of a cycle was usually between that measured for the two different Supor 100 membranes. That the LCM ¯ux was not greater than that for the Supor 100 membranes, as seen earlier with composites formed from similar

95

Table 6 Retention of latex particles by microfiltration LCMs, Latex J (0.65 mm) on Supor 450 Diameter of Latex in feed solution (mm)

Latex composite Gelman Gelman membrane Supor 100 Supor 450

0.0300 0.0680 0.0850 0.0910 0.1050 0.1353 0.1461

Passed Passed Passed Passed Rejected Rejected Rejected

Passed Passed Passed Passed Passed Rejected Rejected

Passed Passed Passed Passed Passed Passed Passed

Fig. 15. The time dependence of flow during filtration of concentrated cherry juice for a latex composite membrane and two Supor 100 membranes. The latex composite membranes was formed by filtering 0.37 mg/cm2 of 0.65 mm particles (Latex I) onto Supor 200 substrate and stabilized by annealing at 1508C. After each 2 h filtration at 0.48 bar (7 psi) transmembrane pressure, membranes were cleaned with water and Ultrasil 10.

sized latex particles, may be partially due to the greater resistance of the Supor 200 support used in forming this latex composite membrane, as opposed to the Supor 450 used earlier. Predicted permeabilities of LCMs formed from 0.37 mg/cm2 of 0.65 mm particles on Supor 200 and Supor 450 supports are 12.0 and 14.1 g/cm2/min/bar, respectively, as compared to a published value of 7.3 g/cm2/min/bar for a Supor 100 membrane. Variation in annealing conditions for the LCM may also be a factor. During each ®ltration cycle, the ¯ux is observed to decrease much more rapidly for the Supor 100 membranes than for the LCM. This resulted in approximately twice the permeate volume for the latter. One

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explanation for this phenomenon is that the hydrophylic latex surface is less prone to fouling than the polyethersulfone membrane. Alternatively, without simultaneous rejection information for that membrane, it might be reasonably argued that defect regions in the LCM resulted in higher ¯ux which is maintained despite fouling. In any case, all cycles demonstrated improved ¯ux after cleaning for both membrane types, and all but one cycle showed a decrease in ¯ux relative to the previous cycle. The one anomalous point at 4 h for the LCM was due to prolonged (12 h) soaking in water before continuing the experiment. There was no obvious difference between the two types of membranes in their recovery of ¯ux after cleaning. 4.3.5. Retention of latex by MF membranes The retention properties of micro®ltration latex composite membranes have been compared to those of Supor 100 and Supor 450 membranes with a series of dispersions of monodispersed polystyrene latexes (1 g/l). The passage of the particles was qualitatively observed as turbidity in the permeate. The LCM was made by ®ltration of 0.37 mg/cm2 of 0.65 mm particles (Latex J) onto a Supor 450 support. Again, stabilization with this latex involved GMA and carboxylic acid copolymerized into the particles shell with a curing step at 1508C for 1 h. The nominal pore size of this LCM membrane was expected to be 0.10 mm, and measured ¯ux was approximately 20 g/cm2/min/bar (Fig. 12(f)). Table 6 contains data that compares the particle rejection properties for this and other membranes. As is evident, all particles readily pass through the Supor 450 membrane which would be expected given that its nominal pore size is 0.45 mm and all the particles are well below this in diameter. The LCM rejected all particles above 0.1050 mm in diameter, while the Supor 100 passed the 0.1050 mm and rejected the 0.1353 mm particles. The narrower pores resulting for the LCM, as compared to the Supor 100 membrane, is again demonstrated with these data. This greater rejection is gained without the loss of permeability; the LCM has nearly three times higher ¯ux than the Supor 100 membrane. 4.3.6. Retention of dextrans by UF membranes The ability of UF latex composite membranes to separate components of different mass has been

Fig. 16. The fractional passage of a dextran mixture through an LCM, as determined by size exclusion chromatography of permeate and retentate samples, is shown as a function of time. The LCM was formed by two successive filtrations of 0.08 mm particles (Latex A) onto a Supor 100 support, resulting in a coating of 0.13 mg/cm2.

demonstrated by ®ltrations of dextran molecules. Fig. 16 shows the fractional passage of dextrans through a latex composite membrane as a function of time into the ®ltration. The membrane was formed by application of 0.13 mg/cm2 of 0.08 mm particles (Latex A), to a Supor 100 support, in two ®ltrations of 0.106 and 0.025 mg/cm2, respectively. The separation was performed in a stirred dead-end cell using a 40 mg/l mixture of dextrans ranging up to a nominal 100 000 molecular weight molecular weight. Permeate and retentate samples were analyzed by size exclusion chromatography as described in Section 3. Fig. 16 demonstrates that the 95% molecular weight cutoff is approximately 100 000 amu during the ®rst 10 min of ®ltration. As mentioned earlier, packing uniformity is substantially reduced for these small particles. The 100 000 amu roughly corresponds to about 40 nm, signi®cantly greater than that expected from geometrical arguments. Due to concentration polarization, the 95% mass cutoff increases with time, eventually reaching a steady state near 125 000 molecular weight. Although it is possible that polarization occurs within the pores of an LCM, similar changes in rejection with time were also observed using a typical skinned membrane (Millipore PTHK 100 000). Fig. 17 shows retention characteristics of a membrane formed by application of 0.08 mm particles (Latex A) to a Supor 200 support. In that case,

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Fig. 17. The fractional passage of a dextran mixture through an LCM, as determined by size exclusion chromatography of permeate and retentate samples, is shown as a function of time. The LCM was formed by application and stabilization of 0.37 mg/ cm2 of 0.67 mm particles (Latex F) on a Supor 200, followed by two successive filtrations of 0.106 mg/cm2 of 0.08 mm particles (Latex A).

Fig. 18. The fractional passage of a dextran mixture through three different LCMs, as determined by size exclusion chromatography of permeate and retentate samples. The LCMs were each formed by application and stabilization of 0.37 mg/cm2 of 0.67 mm particles (Latex F) on a Supor 200, followed by filtration and stabilization of 0.106 mg/cm2 of 0.08 mm particles (Latex A), and then application of either 0, 0.053, or 0.106 mg/cm2 of 0.08 mm particles (Latex A).

0.37 mg/cm2 of 0.67 mm particles (Latex F) was ®rst coated and stabilized on a Supor 200 support by UV-induced polymerization. This was followed by ®ltration and stabilization of 0.212 mg/cm2 of 0.08 mm particles in two ®ltrations of 0.106 mg/ cm2. Between 5 and 10 min, the 95% molecular weight cutoff for dextrans was below 45 000 amu. Between 40 and 50 min into the ®ltration, the 95% molecular weight cutoff was about 110 000 amu. In contrast to Fig. 16, it is not clear that a steady state has been achieved by 50 min in this case. One difference may be that the membrane of Fig. 17 had nearly twice the permeance during ®ltration of the membrane in Fig. 16, which exacerbates the effects of concentration polarization. The relative permeance and rejection of the membranes in Figs. 16 and 18 can be ascribed to their different structures. The greater rejection associated with the membrane of Fig. 17 is due to the greater mass of 0.08 mm latex particles deposited on the surface. This effect is demonstrated in Fig. 18 where the total mass of 0.08 mm particles applied to the composite substrate (formed of Latex F stabilized on Supor 200) was varied from 0.106 to 0.160 to 0.212 mg/cm2. That the membrane of Fig. 17 had higher permeance during ®ltration than that in Fig. 16, despite the greater thickness of the discrimi-

nating array, is due to its formation on a more porous support, the Supor 200. Table 7 compares the relative ¯ows during ®ltration for the membranes of Figs. 16 and 18, each having different total masses of 0.08 mm particles. Total resistance to ¯ow is proportional to the inverse of ¯ux. Data taken early in the ®ltrations would be expect to be less convoluted with concentration polarization effects and thus more germane to membrane structure. The inverse ¯ux values obtained at 5 min into the ®ltration for membranes formed on Supor 200 support are almost exactly linear with total mass applied to the Supor 200. However, decreased ¯ow using the Supor 100 substrate is an even more dominant effect. This example demonstrates the utility of using multiple applications of different size particles to enable formation of a discriminating layer on a less resistive support. Fig. 19 compares the latex composite membrane of Fig. 17 to a variety of commercial UF membranes made by four manufacturers: Sartorius (Hayward, CA), Millipore (Bedford, MA), Amicon (Beverly, MA), and DDSS (Nakskov, Denmark). Pure water permeance and nominal molecular weight cutoff values from commercial membranes have been obtained from the manufacturer's literature. The solid line from 45 000 to 110 000 amu represents the 95%

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Table 7 Comparison of flows during filtration of dextran mixture for different LCMs Substrate

First application (mg/cm2)

Second application (mg/cm2)

Total mass applied (mg/cm2)

Flow at 5 min (g/min/cm2/bar)

Flow at 45 min (g/min/cm2/bar)

Supor Supor Supor Supor

0.106 0.106 0.106 0.106

0.026 0 0.053 0.106

0.132 0.106 0.159 0.212

2.47 7.31 5.35 4.24

2.32 5.94 4.74 3.82

100 200 200 200

Fig. 19. The latex composite membrane of Fig. 18 is compared to commercially available UF membranes from four companies. The pure water permeance is plotted against the nominal molecular weight cutoff. The solid line from 45 000 to 110 000 amu represents the 95% cutoff observed during filtration of dextrans through the membrane of Fig. 18. Values for permeance and molecular weight cutoff have been obtained from manufacturers literature.

cutoff observed during ®ltration for the membrane of Fig. 17. It is important to recognize that molecular weight cutoffs, in particular, vary substantially depending on the operating conditions and testing method used. Still, Fig. 19 demonstrates that the latex composite membrane has relative ef®ciencies at least comparable to premier commercial UF membranes. 5. Conclusion A membrane processing technology has been developed that places an array of latex particles on a porous support and stabilizes the composite by either chemi-

cal or physical means. The interstitial spaces between the particles serve as narrowly distributed pores for both UF and MF liquid separations. The discriminating surface arrays have surface and bulk porosities that exceed expected values for crystalline arrays of hexagonal closest-packed or cubic closest-packed particles. However, these membranes maintain narrow pore size distributions based on the tortuous path through multilayers of particles. The processing and properties of these membranes have been extensively studied. Various methods have been applied to place and stabilize the particles on the porous support. Suf®cient mechanical strength and chemical resistance has been incorporated into the LCMs to enable washing in strong acid and base and back¯ushing with more than 4 bar pressure. The properties of both UF and MF LCMs are described in terms of wet and dry gas ¯ow data, pure water permeabilities and retention of dextran molecules (UF range) and monodispersed latex particles (MF range). The gas ¯uxes and water permeabilities of latex composite membranes compared to phase inverted membranes of similar mean-¯ow pore size and/or retention characteristics indicate that the narrow discriminating layer in these composites lead to highly ef®cient membranes. This aqueous based technology enables the easy adjustment of membrane surface chemistry independent of the bulk physical properties. Acknowledgements The authors are appreciative of many individuals. Scanning electron micrographs and atomic force micrographs have been obtained by Joan Marshall and Greg Meyers, respectively. Xiaoming Chen has formed and tested many of the latex composite membranes described in this work. Tim Bee and Winbin Liang measured stability and ®ltration properties of

S. Jons et al. / Journal of Membrane Science 155 (1999) 79±99

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