Chapter 6: CO₂-responsive Membranes

As a widespread, non-toxic and renewable gas, carbon dioxide (CO₂) has been widely used to reversibly switch a variety of functional materials. Compared to conventional stimuli, such as temperature, pH, light, or redox agents, a CO₂ stimulus is truly “green” for responsive systems owing to its distinct advantages: it is easy to remove gas from the system without any chemical contamination and dilution; the stimulating magnitude can be precisely modulated by controlling gas bubbling; it could act as a convenient physiological signal to make the materials less harmful to biological molecules or living systems. In view of the irreplaceable feature of CO₂-responsive systems, CO₂-switching functional surfaces or membranes and their potential applications in the area of molecular absorption, wetting regulation and oil/water separation have been reported. In this chapter, we first discuss the CO₂-responsive mechanism and functional groups including amidines, tertiary amines, imidazoles, and guanidines. Then, synthesis of CO₂-responsive polymers will be revealed. Subsequently, CO₂-responsive surfaces and membranes will be summarized and their applications, such as gas-controlled oil/water separation and biological agent capture and release, will be highlighted. Finally, we forecast the potential applications and challenges for CO₂-responsive surfaces and membranes.