Stimuli-Responsive Membranes for Separations

  • Raja GhoshEmail author
Living reference work entry
Part of the Polymers and Polymeric Composites: A Reference Series book series (POPOC)


Synthetic membranes are increasingly being used for a varied range of applications, in industry, medicine, agriculture, environmental sciences, geoengineering, and medical diagnostics, to name just a few. In the chemical industry, membranes are more commonly used in separation processes. Such separations are usually carried out based on one of two basic working principles, i.e., relative permeability and relative sorption. Between the two, permeability-based separations are more common. The physical form and functionality of conventional membranes are not expected to change appreciably during a separation process. This is to ensure reliable performance during separation and thereby ensure consistent product quality. However, the ability to alter membrane permeability in a controllable and reproducible manner increases its scope and range of application. This includes membranes for sequential multiple component separation, controlled release membranes, antifouling membranes, self-cleaning membranes, and membranes for biomedical devices. Such unconventional membranes are generally referred to as “stimuli-responsive” or in some case “environment-responsive” or “smart” membranes. This chapter reviews different kinds of stimuli-responsive membranes, including both permeability-based and sorption-based membranes.

1 Introduction

Synthetic membranes are used for a wide range of applications in industry and medicine [1, 2, 3, 4]. A membrane, broadly defined, is a thin barrier which separates two phases and allows selective passage of material through it. In some sense, a membrane could be thought of as a highly specialized filter. On one hand, a membrane could be used to purify extremely large volumes of material as in applications such as water treatment [5], while on the other, they could be used for regulating the release of miniscule amounts of drugs for the treatment of serious ailments such as cancer from delivery systems such as microcapsules [6]. Figure 1 shows the working principle of a reverse osmosis membrane used for desalinating seawater [7]. This figure summarizes the different forces responsible for transport of different species through a membrane. In a desalinating membrane, the principal driving force is a hydrostatic pressure drop which drives the flow of water from a high-pressure zone to a low-pressure zone through microscopic openings (or pores) present in the membrane. The salt electrolytes being bulkier cannot go through the membrane and tend to accumulate near the surface of the membrane. There is usually a fine balance between the amount of electrolyte accumulating near the membrane surface and that diffusing back into the bulk feed, this being dependent on the system hydrodynamics, i.e., how well the feed side is mixed. If significant amounts of electrolytes accumulate, their presence results in the so-called osmotic back pressure, which acts against the applied hydrostatic pressure. In a typical desalination process, the magnitude of osmotic back pressure could be very significant. Other factors and mechanisms typically at play in most membrane separation processes include Fickian diffusion, electrostatic attraction or repulsion, Donnan effect hindered diffusion and convection. Figure 2 shows a membrane-based osmotic pump [8] used for controlled release of drugs. Here, osmosis and liquid absorption are the main mechanism involved. For most applications, membranes are expected to demonstrate consistent permeability for a specific permeating species, i.e., for a given driving force, such as pressure or concentration difference, and operating condition such as temperature, the same amount of the species should go through the membrane per unit time. While the permeability of most membranes does change to some extent during operation due to factors such as concentration polarization [9] and membrane fouling [10], it is expected that this change is not very big. This is to enable the design and operation of reliable and reproducible processes with low run-to-run variability. However, there are situations where the ability to alter membrane permeability in a controllable manner significantly increases the scope and range of application. Such membranes are referred to as “stimuli-responsive” or in some case “environment-responsive” or “smart” membranes [11, 12, 13, 14, 15, 16, 17, 18].
Fig. 1

Desalination using reverse osmosis membrane

Fig. 2

Controlled release of drug from osmotic pump

The concept of a stimuli-responsive membrane is by no means a novel creation of the human mind. Almost all natural membranes such as the cell membranes that surround the innumerable cells in our body are stimuli responsive to varying extents. They precisely regulate what goes in or comes out of a cell based on very specific triggers and control mechanisms. For instance, they regulate the concentration of ionic species within our cells by controlled uptake and elimination through the utilization of different passive and active transport processes. Our cell membranes are dynamic in terms of their morphology and permeability and have, within them, very specific solute and solvent transport routes such as ion channels and aquaporins. Figure 3 summarizes some of the more important transport routes and mechanisms present in a typical cell. Controlled elimination and uptake of species are important in biological processes ranging from hemostasis and osmoregulation in individual cells to glomerular ultrafiltration and tubular reabsorption in our kidney. Therefore, stimuli-responsive membranes currently used in medicine and industry are at best poor imitations of biological membranes, both in terms of complexity of form and function.
Fig. 3

Transport of material across cell membrane

Stimuli-responsive membranes have been successfully used for many applications in the fields of separation science and technology [11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21] and drug delivery systems [5, 22, 23, 24, 25, 26, 27]. The main motivation behind the development of stimuli-responsive membranes (see Fig. 4) is to be able to modulate the permeability of a species through the membrane by providing an appropriate stimulus such as a change in temperature [21, 28, 29], pH [30], ionic strength [28], light [31], voltage [32], current [33], or charge density [34] or by the addition or removal of specific chemical entities [19] to/from the immediate environment of the membrane. This chapter focuses on stimuli-responsive membranes used in separation processes. For the reader interested in stimuli-responsive membranes for drug delivery and similar biomedical applications, there are some excellent articles and reviews [6, 22, 23, 24, 25, 26, 27]. The term “stimuli-responsive membranes” could also be broadly used to include pressure- or shear-sensitive material used in touchscreens and thin layers coated around sensors and actuators [35]. Unfortunately, any discussion on these “membranes” is beyond the scope of this chapter. Also not included are geomembranes [36] used in construction and geotechnical engineering and stimuli-responsive textile material [37] which are better discussed in other more relevant forums.
Fig. 4

Stimuli-responsive membrane

Stimuli-responsive membranes used in separation processes could be broadly categorized into two groups. The first group includes membranes that respond to a stimulus by altering the solute or solvent permeability, i.e., true stimuli-responsive membranes or stimuli-responsive permselective membrane [17, 19, 38, 39, 40]. The second group includes membranes, the selective passage of solutes through which is not governed by “permeability” but by other factors such as physical adsorption or chemical bond formation [13, 29, 41, 42, 43, 44]. Membranes of this group could be called apparent stimuli-responsive membranes or simply stimuli-responsive adsorptive membranes. These are basically adsorbents prepared in a membrane-like format, i.e., in the form of thin microporous sheets.

Stimuli-responsive membranes are generally prepared using two methods. The first method (see Fig. 5) involves combining a stimuli-responsive polymer with a standard membrane scaffold polymers to produce a copolymer blend [11, 45, 46, 47, 48, 49, 50] which is then cast into a membrane using standard fabrication techniques such as deposition, coating, and phase inversion. With such membranes, the entire structure is stimuli responsive, i.e., the functional properties are directly built into the structure of the membrane. The second method involves the attachment of a stimuli-responsive polymer component to a pre-existing porous membrane scaffold [51, 52, 53]. Three different types of membranes can be prepared using the second method. The first type includes “grafted membranes” (see Fig. 6) which are made by attachment or growth of a stimuli-responsive polymer on/from the surface of the pre-existing scaffold membrane [47, 51, 53, 54, 55, 56, 57, 58, 59, 60]. Members of the second type are referred to as “filled membranes” (see Fig. 7) which are prepared by filling the pre-existing porous membrane with a polymer solution followed by cross-linking to immobilize a stimuli-responsive polymer network domain within the pores [17, 61, 62, 63, 64, 65, 66]. Members of the third type are frequently referred to as “coated membrane,” and these are prepared by interpenetrating network formation (see Fig. 8) which effectively results in the formation of a thin stimuli-responsive layer over a pre-existing supporting polymer network [13, 29, 41, 42, 43, 67]. The immobilized polymer or polymer network described above could be tweaked using an appropriate stimulus to alter its swelling properties in the case of permselective membranes or its binding properties in the case of adsorptive membranes.
Fig. 5

Copolymer blend stimuli-responsive membrane

Fig. 6

Grafted stimuli-responsive membrane

Fig. 7

Filled stimuli-responsive membrane

Fig. 8

Coated stimuli-responsive membrane

2 Stimuli-Responsive Permselective Membranes

Before discussing the specific methods for making stimuli-responsive permselective membranes, it is perhaps better to understand the different possible motivations behind making such membranes. In separation processes carried out using regular or non-stimuli-responsive membranes (see Fig. 9), two overall product fractions are obtained, i.e., the permeate consisting of all species that go through the membrane and the retentate consisting of species retained by the membrane. Therefore, such a process is not suitable for fractionating a multicomponent mixture. A stimuli-responsive membrane, the permeability of which could be sequentially changed using appropriate stimuli would be more suitable for such applications. A simplified scheme for multicomponent separation using stimuli-responsive membrane is shown in Fig. 10. Using a membrane whose permeability could be changed in a sequential manner using multiple stimuli, multiple permeate fractions, each having a different composition, could be obtained using a single-membrane device. In order to carry out similar separations using fixed permeability (i.e., non-stimuli-responsive) membranes, several membrane devices arranged in the form of a cascade would be required [68].
Fig. 9

Binary separation using conventional membrane separation process

Fig. 10

Multicomponent separation using stimuli-responsive membrane

The second type of application where stimuli-responsive membranes would be useful is the control or regulation of the concentration of chemical species within a compartment such as a chemical reactor [39, 40]. This application would require a stimuli-responsive membrane that shows a higher permeability when a substance has to be removed from the compartment and a lower permeability when it has to be retained, just as a cell membrane would regulate concentration of different species within a cell. The third type of application of a stimuli-responsive membrane is as an “antifouling” membrane [69, 70, 71]. Fouling refers to the normally irreversible binding of material present in the feed solution on the membrane surface or within the pores. Fouling is widely considered the Achilles heel of membrane separation as cleaning of fouled membranes is very difficult. In most current applications, fouled membranes are simply replaced. By contrast, a severely fouled stimuli-responsive membrane could quite easily be cleaned either by forward or back flushing of the foulant (see Fig. 11).
Fig. 11

Cleanable stimuli-responsive membrane

2.1 Copolymer Blend Membranes

The main advantages of a copolymer blend membrane include the ease and reproducibility of fabrication. Precise amounts of the different polymer components, i.e., the scaffold and the stimuli-responsive polymer, could be blended using appropriate solvent systems and cast into membranes. Different casting methods such as coating followed by solvent evaporation, spray coating, coacervation-phase separation, and phase inversion could be used for preparing such membranes. Alternately, the different polymer components could be cross-linked together using appropriate linker molecules.

The main disadvantages with membranes made using the above approach include low permeability and limited stimuli-responsive properties. This is because the structural and functional properties of these membranes are intertwined. Due to this, the response of the stimuli-responsive polymer could be “quenched” to some degree by the scaffold polymer. Another significant disadvantage of using this approach is the limited range of stimuli that could be used with such membranes. Some of these disadvantages could be overcome by decoupling the fabrication of the scaffold membrane and the stimuli-responsive membrane using the different grafting, filling, and coating methods described below.

2.2 Grafting to Membranes

This is the easiest strategy for making a grafted stimuli-responsive membrane. An appropriate pre-existing porous scaffold membrane is selected, and preformed stimuli-responsive polymers are chemically bonded or physically attached to it [47, 51, 54, 55]. However, physical attachment is less commonly utilized as the resultant membranes tend to be relatively more unstable than membranes prepared by chemical attachment, thereby limiting the operating stimuli windows such as temperature, pH, and ionic strength ranges. The surface of the scaffold membrane is first activated followed by the addition of a polymer possessing an active functional group. The location of the active functional group determines both the density of polymer attachment and the orientation of the responsive polymer. The main advantage of using this approach is the preciseness of the identity and behavior of the stimuli-responsive polymer. After synthesis, a polymer could be purified to a high degree of homogeneity before being attached to the surface of the scaffold. By doing so, the permeability and responsiveness of the resultant membrane could be very precisely controlled. The stimuli-responsive polymer could be selectively attached in a brushlike configuration close to the pore entrance to create a surface (or lid) valve or could be attached in a brushlike configuration within the pores to create an internal valve. If required, the grafted polymer could be further cross-linked to create mesh-like valves. The main advantages of the brushlike configuration include high membrane permeability and a large range for the stimuli-responsive permeability. The main disadvantage with the brushlike configuration is low stability, i.e., drastic change in permeability in response to small change in the stimulus. When the polymer brushes are cross-linked, the magnitude of response is low but better controlled.

The main disadvantage of the grafting to approach is the low grafting density. When attaching preformed polymer chains to surfaces, steric hindrance limits the number of chains attached per unit surface area, particularly when grafting on concave surfaces. This problem can be solved by using the grafting from approach described next.

2.3 Grafting from Membranes

The grafting from approach involves growing polymer chains on the surface of the scaffold membrane [53, 56, 57, 58, 59, 60]. The first step typically involves the attachment of an initiator molecule on the surface. The polymer chain is then grown by controlled monomer addition and reaction. Different chemistries such as atom transfer radical polymerization [53, 56, 57, 58] and reversible addition-fragmentation chain transfer [59, 60] are used for the grafting from approach. Depending on the method used, different approaches are available for obtaining specific monomer composition and chain length of the responsive polymer. In addition to the potential for increasing the graft density, the grafting from approach is generally cheaper than the grafting to approach as the polymer purification step is not necessary.

The main disadvantage of the grafting from approach is the relatively greater heterogeneity of the attached responsive polymer when compared with the grafting to approach. On account of this, the magnitude and responsiveness of permeability are less controllable. Both the grafting to and the grafting from approaches rely on the covalent attachment of the responsive polymer on the membrane. On account of this, the range of material that could be used for preparing stimuli-responsive membranes gets limited. This problem can be overcome using the filled and coated membrane approaches described next where covalent attachment of the responsive polymer to the scaffold membrane is not required.

2.4 Filled Membranes

A filled membrane [17, 61, 62, 63, 64, 65, 66] could be prepared by filling the pores of a macroporous scaffold membrane with a reaction mixture consisting of appropriate monomer/s, initiator, and cross-linker followed by thermal or photochemical polymerization and cross-linking. The resultant cross-linked bulk polymer domains are physically retained within the pores of the scaffold membrane due to their bulk, shape, and continuity (see Fig. 12). Better retention of the polymer can be ensured by using a scaffold with high degree of pore connectivity. A simpler but perhaps a bit more expensive variant of the above approach is to use a reaction mixture consisting of preformed polymers and cross-linker. By using this approach, better control over the magnitude and responsiveness of the membrane permeability could be ensured.
Fig. 12

Micrograph of filled stimuli-responsive membrane

The main disadvantage of using the pore-filling approach is the relatively low permeability of the resulting membrane. Also, the selectivity of such membranes tends to be low. The problem of low permeability can be solved to some extent using the coated membrane approach discussed next.

2.5 Coated Membranes

The method for making coated membranes [13, 29, 41, 42, 43, 67] is indistinguishable from that used for making filled membranes. The main differences are in the nature of the supporting material used and the quantity of responsive polymer in the resultant membrane. With a filled membrane, the scaffold material is usually macroporous in nature, while that used for making a coated membrane is microporous. With the filled membranes, the responsive polymer remains in the form of bulk domains, while with the coated membranes, the responsive polymer intermeshes with the scaffold polymer, effectively forming a thin stimuli-responsive coating (see Fig. 13). Therefore the coated membrane is somewhat structurally similar to an interpenetrating polymer network. Figure 14 shows the change in appearance of the microporous scaffold polyvinylidene fluoride (PVDF) membrane due to the addition of a polyvinyl caprolactam coating layer. A comparison of Figs. 12 and 14 shows that the effective porosity and pore size of a coated membrane is greater than that of a filled membrane. Therefore, the permeability of a coated membrane is expected to be greater. Also due to the greater room for volume change of the responsive polymer, the range of permeability is also expected to be greater.
Fig. 13

Coated stimuli-responsive membrane

Fig. 14

Micrograph of microporous scaffold membrane (a) and coated stimuli-responsive membrane (b)

3 Stimuli-Responsive Adsorptive Membranes

Membrane adsorption or membrane chromatography involves the use of a stack of porous membranes as chromatographic media [72]. The main advantage of membrane chromatography over column chromatography is the high speed of separation due to the predominance of convective mass transport. Due to convective transport of solutes, the time taken by solute molecules to travel to and away from their binding sites is significantly lower than in resin-based columns. Membrane chromatography is typically faster than column chromatography by more than one order of magnitude. A shorter process implies higher productivity and reduced product degradation. Despite the obvious advantages of membrane chromatography, it is not that widely used, primarily due to lower solute-binding capacity compared to resin-based media. The binding capacity of conventional membranes used for membrane chromatography is low as solute binding is limited to the pore walls and external surfaces only (see Fig. 15). The use of coated and filled membranes discussed in the previous section offers the opportunity for creating three-dimensional binding domains within the membrane pores [13, 29, 41, 42, 43, 65, 67]. As shown in Fig. 15, a filled membrane offers a substantially magnification of available binding sites. With conventional membranes, solute binding takes places on a predominantly concave surface. With large solutes such as proteins and nucleic acids, this may result in significant steric hindrance and consequently low solute binding. With a filled membrane, the binding surface is predominantly convex which implies lower steric restrictions and thereby greater binding [65].
Fig. 15

Comparison of solute binding on conventional and stimuli-responsive adsorptive membranes

The use of a stimuli-responsive membrane allows greater flexibility in fine-tuning the adsorption/desorption process and thereby leads to the potential for greater resolution in multiple solute separation. For instance, conventional hydrophobic interaction membrane chromatography relies on the binding of solutes on media prepared by grafting hydrophobic ligands such as phenyl, butyl, and octyl on support material. Such media is always hydrophobic, and therefore the selectivity in solute-solute separation is limited. As shown in Fig. 16, the selectivity of such separation could be significantly enhanced using a stimuli-responsive membrane with tunable hydrophobicity, prepared by grafting polymers such as polyethylene glycol and polyvinyl caprolactam on appropriate scaffold membranes [13, 29, 41, 42, 43, 67]. In the presence of lyotropic salts, such polymers exist in a collapsed state, and therefore the membrane surface is effectively hydrophobic and thereby suitable for binding hydrophobic solutes. In the absence of lyotropic salts, these polymers exist in an extended hydrophilic state. Therefore, hydrophobic solutes are released from the surface of the membrane. The recovery of solutes from such stimuli-responsive surfaces is far greater than that from conventional hydrophobic interaction media. Moreover, the change in the hydrophobic-hydrophilic property of the stimuli-responsive membranes is generally graded; thereby high-resolution separation can be carried out.
Fig. 16

Hydrophobic interaction-based solute binding on stimuli-responsive membrane

4 Conclusion

Conventional membranes are suitable for binary separation, i.e., where a feed material has to be fractionated into two components, the permeate and the retentate. On the other hand, stimuli-responsive membranes whose permeability could be changed in a sequential manner using multiple stimuli are suitable for carrying out multicomponent separations. This opens up endless possibilities about how a separation process could be designed and operated. It also increases process efficiency by replacing a multi-device process with a single device one. Stimuli-responsive membranes could potentially be used for the regulation of concentration of a chemical species within chemical reactors just as the cell membrane regulates the concentration of different species within a cell. The same principle could be applied for regulating species concentration in bioreactors, such as those used for mammalian cell culture or tissue engineering. Membrane fouling is widely considered the Achilles heel of membrane filtration. In most conventional membrane processes, a membrane needs to be frequently cleaned, and this increases the cost of production. Stimuli-responsive membrane can function as good “self-cleaning” or “antifouling” membranes as they could quite easily be cleaned either by forward or back flushing of the foulant in conjunction with an appropriate stimulus which “opens up” the membrane. Using such membranes, separation processes could easily be carried out in a sustained or semicontinuous manner by implementing some simple in-process cleaning strategies. Stimuli-responsive membranes used in separation processes could be broadly categorized into two groups: (a) true stimuli-responsive membranes or stimuli-responsive permselective membranes that respond to a stimulus by altering the solute or solvent permeability, and (b) apparent stimuli-responsive membranes or stimuli-responsive adsorptive membranes, the selective passage of solutes through which is not governed by “permeability” but by other factors such as physical adsorption or chemical bond formation. Stimuli-responsive membranes could be prepared by “copolymer blending,” i.e., by mixing a stimuli-responsive polymer with a standard membrane scaffold polymers and then casting this into a membrane using standard fabrication techniques. However, these membranes tend to have poor permeability and smaller responses to stimuli. Grafted membranes can be prepared using a “grafting to” or a “grafting on approach.” However, grafting limits the range of material that could be used as scaffold or as the stimuli-responsive polymer. This problem could be overcome by preparing filled or coated membranes. Filled and coated membranes are prepared in a similar way, but they differ in terms of the morphology of the stimuli-responsive polymer. In a filled membrane, the responsive polymer forms the bulk domain within the pores, while with coated membranes, the responsive polymer just coats the surface of the scaffold membrane. Overall, the field of stimuli-responsive membranes is a promising new area for research, development, and application.


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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of Chemical EngineeringMcMaster UniversityHamiltonCanada

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