15.1 Types of polymeric membranes and their preparation

Polymeric membranes come in various types, each with unique structures and properties. From symmetric to asymmetric, porous to non-porous, these membranes play crucial roles in separation processes across industries.

Preparation methods like phase inversion, interfacial polymerization, and stretching shape membrane characteristics. Factors such as polymer concentration, solvent choice, and additives fine-tune properties, balancing advantages and limitations for specific applications.

Types of Polymeric Membranes

Types of polymeric membranes

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• Symmetric membranes have a homogeneous structure and composition throughout the entire membrane thickness (microporous membranes, dense membranes)
• Asymmetric membranes possess a non-uniform structure and composition across the membrane thickness consisting of a thin, dense selective layer supported by a porous substructure (anisotropic membranes, composite membranes)
• Porous membranes contain interconnected pores that allow selective transport based on size exclusion with pore sizes classified as microporous ($<2$ nm), mesoporous ($2-50$ nm), and macroporous ($>50$ nm)
• Non-porous (dense) membranes lack well-defined pores and rely on the solution-diffusion mechanism for transport, where separation is based on differences in solubility and diffusivity of permeants
• Charged membranes (ion-exchange membranes) contain charged functional groups that allow selective transport of ions, such as cation-exchange membranes with negative fixed charges and anion-exchange membranes with positive fixed charges

Membrane Preparation Methods and Factors Influencing Properties

Methods for membrane preparation

• Phase inversion transforms a polymer solution into a solid membrane through controlled phase separation using techniques like immersion precipitation, vapor-induced phase separation, and thermally-induced phase separation, widely used for preparing asymmetric membranes with a dense skin layer and porous substructure
• Interfacial polymerization involves the reaction of two monomers at the interface of two immiscible solvents forming a thin, dense selective layer on a porous support membrane, commonly used for preparing composite membranes like thin-film composite (TFC) membranes for reverse osmosis and nanofiltration
• Stretching involves the uniaxial or biaxial stretching of a partially crystalline polymer film creating a microporous structure through the formation of interconnected voids, used for preparing microporous membranes such as polytetrafluoroethylene (PTFE) and polypropylene (PP) membranes

Factors in membrane properties

• Polymer concentration influences membrane structure, with higher concentrations leading to denser and less porous membranes, while lower concentrations result in more porous and less selective membranes
• Solvent selection affects phase separation kinetics and membrane morphology, where good solvents promote delayed demixing and result in more porous membranes, while poor solvents promote instantaneous demixing and lead to less porous, more selective membranes
• Additives modify membrane properties, with pore-forming agents (polyvinylpyrrolidone, polyethylene glycol) increasing porosity and permeability, hydrophilic additives improving wettability and fouling resistance, and inorganic particles (zeolites, metal-organic frameworks) enhancing selectivity and stability

Membrane techniques: advantages vs limitations

• Phase inversion advantages include versatility, scalability, and suitability for preparing a wide range of membrane structures and morphologies, while limitations involve limited control over the selective layer thickness and pore size distribution
• Interfacial polymerization advantages produce thin, dense, and highly selective layers for high-performance separations (desalination, organic solvent nanofiltration), while limitations require careful control of reaction conditions and may suffer from defects and scalability issues
• Stretching advantages are simplicity, cost-effectiveness, and suitability for preparing microporous membranes with high porosity and narrow pore size distribution, while limitations are restricted to partially crystalline polymers and may result in mechanical weakness and low selectivity