Chapter 1
Processing and Characterizations: State-of-the-Art and New Challenges

Visakh. P. M.

Research Assistant, Department of Ecology and Basic Safety, Tomsk Polytechnic University, Tomsk, Russia

Corresponding author: visagam143@gmail.com

Abstract

A brief account of various topics concerning the processing and characterization of nanostructured polymer membranes is presented in this chapter. The different topics that are discussed include membrane technology and chemical characterization of membranes; ceramic and inorganic polymer membranes preparation, characterization and applications; supramolecular membranes synthesis and characterizations; organic membranes and polymers to remove pollutants; membranes for CO2 separation; polymer nanomembranes; liquid membranes; recent progress in separation technology based on ionic liquid membranes; membrane distillation; and preparation, characterization and applications of alginate-based membranes and films.

Keywords: Nanostructured polymer membranes, membrane processing, membrane characterizations, supramolecular membranes, organic membranes, liquid membranes, separation technology, ionic liquid membranes

1.1 Membrane: Technology and Chemistry

Membranes are used in a broad range of applications such as protein fractionation, purification of drugs, separation of gaseous mixtures, sample simplification in analytical procedures, production of ultrapure water and wastewater treatment, among others [1-5]. The membrane can be defined as a selective barrier that allows some species to permeate the barrier while retaining others. Membrane can be symmetric or asymmetric membrane according to their macroscopic configuration. Thus, asymmetric membranes consist of two layers; the top one is a very thin dense layer and is commonly called the skin layer or active layer and determines the permeation properties. In particular, separation methods directed by pressure can be categorized into four major membrane processes: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [6, 7]. Liquid membrane processes are commonly identified as three main configuration types: bulk liquid membrane, surfactant liquid membrane (or emulsion liquid membrane) and supported liquid membrane. Membranes can be referred to as porous and dense, and this first classification permits defining the two main mass transport models through the membranes. In membrane processes, the retained or rejected species accumulate near the membrane surface and as a consequence concentration polarization is produced.

According to the transport mechanisms, the separation methods by liquid membrane can be divided into six basic mechanisms of transport: simple transport, simple transport with chemical reaction in strip solution, facilitated transport, coupled counter-transport, coupled co-transport and active transport. The range of materials used for nanofiltration and reverse osmosis membranes is much smaller than that used for microfiltration and ultrafiltration, and is limited to polymers. Membrane material is required to be resistant to operation conditions and suitable for specific application. In many cases, additives are added to membrane phase during the fabrication to increase the permeability or reduce the fouling. Inorganic membranes have high selectivity and high permeability as well as thermal, chemical and mechanical stability but the cost of these are very high in comparison with polymer membranes. Organic and inorganic membranes can be modified for different applications by changes in the material chemical properties or by changes of pore size [8]. The above can be accomplished using methods such as chemical oxidation, incorporation of additives into the membrane matrix, plasma treatment, classical organic reactions, polymer grafting, interpenetrating polymer network, surfactant modification, self-assembly of the nanoparticles, among others [9].

Plasma surface treatment usually refers to a plasma reaction that either results in modification of the molecular structure of the surface, or atomic substitution. For example, simple inert gas [10], nitrogen, or oxygen plasmas have been used to increase the surface hydrophilicity of membranes [11], and ammonia plasmas have successfully yielded functionalized polysulfone membranes [12]. There are several potential advantages for the use of enzymes in membrane modification. Currently, the pressure-driven membrane processes are widely used in water treatment, biotechnology, food industry, medicine, and other fields [13].

One of the main problems arising from the operation of the membrane units is membrane fouling, which seriously hampers the applications of membrane technologies [14]. New membrane modification methods have been proposed, including the modification of membrane surfaces via microswelling for fouling control in drinking water [15], hydrogel surface modification of reverse osmosis membranes [16], modification of Nafion membrane using fluorocarbon surfactant for all vanadium redox flow batteries [17], modification of ultrafiltration membranes via interpenetrating polymer networks for removal of boron from aqueous solution [18], among others.

1.2 Characterization of Membranes

Membrane morphology characterization is one of the indispensable components of the field of membrane research. Physical and chemical properties of membranes can be characterized with different laboratory techniques. Several microscopic techniques, both electronic as scanning and transmission electron microscopies, and atomic, as atomic force microscopy, have been used to analyze the pore structure and pore size distribution of the membrane. Microscopy methods, including scanning electron microscopy (SEM), transmission electron microscopy (TEM) or atomic force microscopy (AFM), are the most direct methods to characterize the membrane pore structure. SEM can be used in various pore size characterization studies to visually inspect pore sizes and shapes. The AFM has proven itself to be a useful and versatile tool in the field of surface characterization. Porometry measurements can also give information about the pore size distribution (PSD) of membrane surface area [19].

Gas adsorption is one of the most popular methods and is generally used for the surface characterization and structural properties of porous materials, allowing the determination of their surface area, pore volume, pore size distribution and adsorption energy distribution of polymer membranes. One of the most promising methods is permporometry, where a mixture of non-condensable gas and condensable vapor is fed to a porous membrane and the permeation rate of non-condensable gas is measured [20]. Fourier transform infrared (FTIR) spectroscopy is widely used in structural characterization of membrane surfaces. With recent advances in the technology, the instrument has become simplified and some of the problems are reduced [21]. Raman spectroscopy technique usually employs a laser source and the scattered light and analyzes in terms of wavelength, intensity and polarization. Raman scattering is capable of detecting elastic vibrations of an entire nanoparticle, therefore Raman scattering is good for detecting nanoparticles on the membrane surface [22].

Energy-dispersive X-ray spectroscopy (EDS) analysis can be helpful for both membrane characterization and foulant characterization. For example, Sile-Yüksel et al. [23] used EDS analysis to determine the location of silver nanoparticles in different polymer membrane matrix. Corneal et al. [24] coated tubular ceramic membranes with manganase oxide nanoparticles. They examined the coating layer using SEM-EDS. With the help of EDS analysis they observed that the manganase oxide nanoparticles were not just successfully placed on the surface but also penetrated the membrane matrix. Soffer et al. [25] used EDS analysis to show colloidal iron fouling on ultrafiltration membrane surface. Long-term fouling of a reverse osmosis membrane was examined by Melián-Martel et al. [26]. The measuring method must be adapted according to charge places whether it is on the surface or inside the pores.

1.3 Ceramic and Inorganic Polymer Membranes: Preparation, Characterization and Applications

Liquid electrolytes are liquid state electrolyte used to conduct the electricity. However, these conventional liquid electrolytes possess several disadvantages such as leakages of corrosive solvent and harmful gas, electrolytic degradation of electrolyte, formation of lithium dendrite growth, poor dimensional and mechanical stabilities, slow evaporation due to the gel state of polymer electrolyte, low safety performances, narrow potential window, poor interfacial stability and reduction in thermal, electrical and electrochemical stabilities as well [27]. Ionic liquids also offer some fascinating advantages, such as excellent chemical, thermal and electrochemical stabilities, high ionic conductivity due to high ion concentration, good oxidative stability and superior ion mobility as well as high cohesive energy density [28].

Krawiec et al. found that the particle size of filler is a vital parameter to govern the conductivity of the polymer electrolytes. They reported that the conductivity of nanosized Al2O3 added polymer electrolytes was higher about an order of magnitude that that of micrometer-sized Al2O3. High surface area to volume ratio of nanoparticles has become a driving force in the development of nanotechnology in various research fields, especially in materials science. The small particle size of the fillers can...