**2.2. Requirements for parent polymers**

Many materials which contain functional groups can be used as supports for the IMS disregarding their form or shape: granulated form, fibrous or membranes. In all cases, when using the IMS technique it is important to take into account both the polymer properties and the final application of the nanocomposite since as both points dictate certain necessary requirements to the parent matrix (inside which the MNPs must be synthesized).

For example, when using a nanocomposite for making a sensor or a biosensor to be applied in aqueous solutions, the polymer must be insoluble in water. But, at the same time, the polymer has to provide sufficient permeability towards the analyte under study (ions or molecules). Besides, the polymer must either slightly swell in water or at least be hydrophilic to enhance both the sensor and the response rate.

Similarly, the solubility of the nanocomposite in some organic solvents allows for the preparation of homogeneous Polymer Stabilized MNPs solutions (PSMNPs "inks") that can be deposited onto the desired surfaces (e.g. electrodes) to modify their properties. This solubility would also allow the characterization via microscopic analysis, electrochemical techniques and others.[36]

The main requirements for a polymer to be used as a matrix for the IMS technique are the following:


In polymer functional matrices, ionic transport occurs in a highly amorphous, viscoelastic (solid) state. In this sense, the most intensively studied polymers are based on poly(oxa alkanes), poly(aza alkanes), or poly(thia alkanes).

In general, it is possible to state that functional groups define the chemical properties of the polymer matrix, by bearing on the surface a negative or positive charge. Due to this fact different dissociation properties of group lead to strong and weak exchangers (which are named similar to that of strong and weak electrolytes). Based on these functional groups, classification of Ion Exchange matrices involves four main groups:

	- a. strong acid exchangers (e.g., containing sulfonic acid groups or the corresponding salts)
	- b. weak acid exchangers (e.g., containing carboxylic acid groups or the corresponding salts)
	- a. strong base exchangers (e.g., containing quaternary ammonium groups)
	- b. weak base exchangers (e.g., containing amine groups)

The Ion exchange capacity (IEC) is the main feature of ion exchange materials. Taking into account that an ion exchanger can be considered as a "reservoir" containing exchangeable counterions, the counterion content in a given amount of material is defined essentially by the amount of fixed charges which must be compensated by the counterions, and thus is essentially constant.

Above all mentioned polymer matrices, this chapter is mainly focused in the use of crosslinked polymers in the form of resins and in commercial or tailored polymeric membranes.

### *2.2.1. Resin beads*

46 Ion Exchange Technologies

or bacteria.

functional groups. Consequently, ion penetration inside the matrix is balanced by the sum of two driving forces acting in opposite directions: the gradient of the ion concentration and the Donnan-effect itself. The result of these two driving forces is the formation of the MNPs

Regarding the final application of the nanocomposite, this is a really suitable distribution, since MNPs remain maximally accessible for substrates of interest such as chemical reagents

Many materials which contain functional groups can be used as supports for the IMS disregarding their form or shape: granulated form, fibrous or membranes. In all cases, when using the IMS technique it is important to take into account both the polymer properties and the final application of the nanocomposite since as both points dictate certain necessary

For example, when using a nanocomposite for making a sensor or a biosensor to be applied in aqueous solutions, the polymer must be insoluble in water. But, at the same time, the polymer has to provide sufficient permeability towards the analyte under study (ions or molecules). Besides, the polymer must either slightly swell in water or at least be

Similarly, the solubility of the nanocomposite in some organic solvents allows for the preparation of homogeneous Polymer Stabilized MNPs solutions (PSMNPs "inks") that can be deposited onto the desired surfaces (e.g. electrodes) to modify their properties. This solubility would also allow the characterization via microscopic analysis, electrochemical

The main requirements for a polymer to be used as a matrix for the IMS technique are the



In polymer functional matrices, ionic transport occurs in a highly amorphous, viscoelastic (solid) state. In this sense, the most intensively studied polymers are based on poly(oxa

In general, it is possible to state that functional groups define the chemical properties of the polymer matrix, by bearing on the surface a negative or positive charge. Due to this fact



requirements to the parent matrix (inside which the MNPs must be synthesized).

mainly near the surface of the polymer matrix (see Figure 10).

hydrophilic to enhance both the sensor and the response rate.

**2.2. Requirements for parent polymers** 

techniques and others.[36]

following:

carriers.

carriers.



alkanes), poly(aza alkanes), or poly(thia alkanes).

As IMS is based on the feasibility of the polymeric support used, which must contain ionic functional groups, one of the typical matrices that accomplish this requirement are Ionexchange resins, also known as granulated polymers.[34, 37]

Ion-exchange resins are commercial products commonly available and their shape and size allow these materials to be easily and quantitatively recovered by simple filtration or decantation. Ion exchange resins are usually used in water treatment processes (e.g. water softening) but have many other applications in chemical production. For instance, several common applications include immobilization of biological an inorganic catalysts, extraction procedures, metal recovery and separation and acid-base catalysis.[38, 39]

The first effort to obtain more stable synthetic resins for ion exchange reactions is attributed to B.A. Adams and E.L. Holmes, who in 1935 published the condensation polymerization of methanal (formaldehyde) with phenol or polysubstituted benzene compounds to give reversible exchange resins. [40] Based on the same concept Adams and Holmes quickly developed anion exchange resins, obtained by the condensation between methanal and phenylamines giving directly a copolymer matrix bearing weak basic secondary amine groups, that in presence of strong acid solutions result in acid amine salts (anion exchangers). Although nowadays the polymerization mechanism is entirely different, based on the so called addition or vinyl polymerization (first applied by D'Alelio in 1944) commercial ion exchange resins production uses the same principles as their predecessors, and the two kind of resin explained before are still the most commonly used resins.[41]

Regarding their chemical composition, most ion-exchange resins are based on cross-linked polystyrene- divinylbenzene (DVB) copolymers bearing ion-exchanging functional groups. Besides, from the morphological point of view, the following types can be considered:


Considering the feasibility of this matrix type for the synthesis of MNPs, it is noteworthy to mention that they lead to obtain a stable support for the embedded MNPs, are insoluble in water and offer different types of functionalities (e.g. sulfonic, carboxylic, quaternary ammonium), as well as different distributions of functional groups. Some common important parameters, from chemical and physical points of view, are listed in the following table (Table 1).


**Table 1.** Common important parameters in Ion Exchange Resins.

From those parameters listed in Table 1, the functional groups nature is one of the key ones since it defines the chemical properties and applicability of ion-exchange resins and, what is crucial for IMS, the sign of the matrix charge (either positive or negative). Moreover, the different dissociation properties of the functional groups leads to the distinction between strong and weak exchangers, which have to be considered separately since they have a remarkably different chemical behaviour. In this sense, in Figure 11 are shown some typical resin beads with their polymeric structure formula.

**Figure 11.** Physical and chemical features of some ion exchange resins.

Another very important parameter to be taken into account is porosity, which affects some bulk properties of the resins, and which have important consequences on their catalytic applications through direct influence on swelling capacity, equilibration rate, and selectivity. In this sense, swelling is an important to be taken account on resin behaviour, because depending on the nature of the ion-exchange resin, this interaction with the solvent may lead to a volume increase (swelling volume) up to 800% and a decrease around 90% of the cross-linking percentage, Hence, gel-type resins are generally preferred over macroporous ones due to enhanced mass transfer inside the polymer beads, resulting in good active-sites accessibility to all soluble reactants.

#### *2.2.2. Membranes*

48 Ion Exchange Technologies

velocity and diffusion.

table (Table 1).

Regarding their chemical composition, most ion-exchange resins are based on cross-linked polystyrene- divinylbenzene (DVB) copolymers bearing ion-exchanging functional groups. Besides, from the morphological point of view, the following types can be considered:




important to say that they are easily sulfonated and they are really resistant.

kinetics and allow working in high pressure conditions (i.e. chromatography).

Considering the feasibility of this matrix type for the synthesis of MNPs, it is noteworthy to mention that they lead to obtain a stable support for the embedded MNPs, are insoluble in water and offer different types of functionalities (e.g. sulfonic, carboxylic, quaternary ammonium), as well as different distributions of functional groups. Some common important parameters, from chemical and physical points of view, are listed in the following

**Chemical Parameters Physical Parameters** 

Ion Exchange Capacity Pore size and morphology

From those parameters listed in Table 1, the functional groups nature is one of the key ones since it defines the chemical properties and applicability of ion-exchange resins and, what is crucial for IMS, the sign of the matrix charge (either positive or negative). Moreover, the

Functional group type Particle Size

pH working range Form Chemical stability Density Water absorption (swelling) Shipping weight

Polymer structure, Ion conductivity Grading

**Table 1.** Common important parameters in Ion Exchange Resins.

The separation of substances by membranes has been (and still is) essential in the industry development and in the human life. Among various separation membranes, the ion exchange membranes, membranes with ionic groups permeable to electrolytes in an aqueous solution, are widely used in different fields: dialysis, solid polymer electrolyte of batteries, analytical chemistry, etc.

In its origins, the ion exchange membrane was developed from two different sources: the finding of ion exchange phenomena in soil and biological phenomena in cell membranes. In 1939 many researchers focused to the establishment of the basis of the studies on

electrochemical ion exchange membrane. For example, in 1939 K.H. Meyer, J.F. Sievers and T. Teorell obtained the first artificial charged membrane with the aim of developing a theory of membrane potential; in 1949 Sollner published a paper concerning bi-ionic potential (a measure of permselectivity between ions with the same charge through the membrane); etc. But it is not until 1950, with the work of M.R.J. Wyllie, W. Juda and M.R.C. McRae, when the first ion exchange membranes synthesis is published.

After these works, studies on ion exchange membranes, their synthetic methods, modifications, theorical explanations and applications in industry became very active. But what really made the researchers focus in the development of this kind of membrane is that the charged groups of the membrane act as a fixed carrier for various ionic materials and provide new applications of the membrane.[42, 43] (See Table 2).



Though ion exchange membranes can be used in many fields, most are used in electrochemical processes such as electrodialysis, separation of electrolytes and solid polymer electrolytes for fuel cells (which really boosted the development of these membranes).

The properties required basically depend on their final application, but generally they can be summarized as:


50 Ion Exchange Technologies

Fixed Carrier (Ion exchange groups)

**Table 2.** Principal applications of Ion Exchange membranes

first ion exchange membranes synthesis is published.

provide new applications of the membrane.[42, 43] (See Table 2).

**Characteristcs Application Example** 

Separator for electrolysis

electrochemical ion exchange membrane. For example, in 1939 K.H. Meyer, J.F. Sievers and T. Teorell obtained the first artificial charged membrane with the aim of developing a theory of membrane potential; in 1949 Sollner published a paper concerning bi-ionic potential (a measure of permselectivity between ions with the same charge through the membrane); etc. But it is not until 1950, with the work of M.R.J. Wyllie, W. Juda and M.R.C. McRae, when the

After these works, studies on ion exchange membranes, their synthetic methods, modifications, theorical explanations and applications in industry became very active. But what really made the researchers focus in the development of this kind of membrane is that the charged groups of the membrane act as a fixed carrier for various ionic materials and

Ion conductivity Electrodialysis Separation between electrolyte

and non-electrolyte

Synthesis of H2O2

Diffusion Dialysis Acid or alkali recovery from waste

Donnan Dialysis Recovery of precious metals

Up-hill transport Separation and recovery of ions

Neutralization Dialysis Desalination of water

Piezodialysis Desalination

Thermo-dialysis Desalination

Fuel cell H2-O2

Sensors Gas sensor

Hydrophilicity Pervaporation Dehydration of water miscible

Modified Electrodes -

Battery Concentration cell

Actuator Catheter for medical use

Dehumidification Dehumidification of air

Facilitated transport Separation of sugars

organic solvents

Ion Exchange membranes can be classified in various ways: by their structure and microstructure, by their functionality, materials, etc. But maybe one of the simplest classifications is the morphology which, in first instance, will determine their preparation methodology. In this sense, it is possible to classify ion exchange membranes in two main types: Heterogeneous and Homogeneous membranes.[43]

In an initial stage of membrane development, heterogeneous ion exchange membranes were actively developed by blending finely powdered ion- exchange materials and a binder. In a general procedure, cation or anion exchange resins are homogenously blended and heated with a thermoplastic polymer (i.e. polyethylene, polypropylene, etc.) and the mixture is formed as a membrane by pressing or heating.

Although these types of membranes are easily prepared and have a great mechanical strength, their electrochemical properties are lower than the ones of homogenous ones in which the fixed charged groups are evenly distributed over the entire membrane polymer matrix. This homogeneity in the homogeneous membranes structure is due to the fact that they can be produced, e.g. by polymerization or polycondensation of functional monomers such as phenylsulfonic acid with formaldehyde, or by functionalizing a polymer such as polysulfone dissolved in an appropriate solvent.

But the completely homogeneous and the macroscopically heterogeneous ion-exchange membranes are extreme structures. Most ion-exchange membranes show a certain degree of heterogeneity on the microscopic scale. Thus, other properties may be considered to classify them. In this regard, according to the distribution and species of the fixed charge (ion exchange groups) it is possible to difference between:

i. cation exchange membranes (with anionic charged groups)


On the other hand, a classification based on the constituent materials allows grouping such membranes as:


Nowadays, one of the most employed commercial ion exchange membranes is Nafion, a cation exchange homogenous perfluorinated membrane. It is an excellent proton conductor: it has excellent chemical stability, high ionic conductivity, good mechanical strength, good thermal stability, etc. ideal for performance in fuel cells. The main drawback of these membranes and of those containing Fluor in their structure (i.e. Selemion) are their high cost and, specially, the absence of pores that limits their application to the transport of ions in solution or vapour (pervaporation). Thus, the search of new homogeneous cation exchange membranes has been focussing much of efforts. In these sense, sulfonated polymers open a new window to the ion exchange membranes field. Some typical sulfonated polymers are shown in Figure 12.

**Figure 12.** Typical Sulfonated polymers.

One of the polymers that fulfils the properties to be casted as an ion exchange membrane is sulfonated poly(ether-ether ketone) (SPEEK) which is nowadays attracting great interest regarding the fabrication of membranes for fuel cells, due to its thermoplastic properties, its high chemical strength, its high stability towards oxidation and its low cost.[44, 45]

However, regarding this last option it is important to take into account that an excess of ionic groups (in this case, sulfonic groups) could cause the dissolution of the polymeric material in water since an increase of the ionic groups in the polymer directly increases the hydrophilicity of the material.

To cope with this limitation, one good option is the sulfonation of a polymer with a very hydrophobic group so as to reduce the hydrophilicity of the final polymer, the polyethersulphone with Cardo group (PES-C) [46-48] which bears a five-member lactone ring and whose sulfonation can be done in a simple way. Among all the stability properties mentioned, this polymer can be casted by wet phase inversion methodology to obtain porous membranes be applied in filtration. By controlling the ratio of sulfonic groups in the polymer, different porosity can be obtained.
