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Template Method

The template method is a very effective means of making pores in CMs. It uses organic components doped in the precursor membrane matrix that decompose or dissolve at a later stage to leave pores. Pores formed by this method will be distributed very evenly within the membrane matrix. The organic fillers are usually characteristic, with thermal labile segments or moieties such as polyvinylpyrrolidone [65], polyfethylene glycol) [66], Pluronic triblock copolymer [67-69] and so on. After being thermally degraded they leave abundant porosity in the membrane matrix of CMs. In addition, inorganic dopants, such as silicon and porous carbon, can sometimes be used as templating agents. However, this kind of template needs to be removed by an additional etching step after membrane formation, to leave the pore structure.

Li et al. utilized an ethanol solution containing a phenolic resin and a Pluronic triblock copolymer to tailor commercialized polymeric hollow fiber ultrafiltration membranes. The surfactant self-assembled into the confined voids of the ultrafiltration membrane. After drying and pyrolysis, carbon hollow fiber membranes were obtained, the continuous membrane walls of which had an average thickness of 113 pm and a hierarchical pore structure: pores coming from hexagonally ordered mesoporous carbon had a pore diameter of ~4.3 nm and there were also disordered defect holes with a size of 8-50 nm randomly distributed inside the membrane matrix. The gas permeance results indicate that the membranes exhibit Knudsen diffusion behavior, confirming their good quality [70].

Strano et al. used spray deposition and pyrolysis of polyffurfuryl alcohol)/ polyethylene glycol) mixtures on macroporous stainless steel supports. Polyfethylene glycol) employed as a carbonization template creates a meso- porosity that leads to pores in the ultrafiltration range. When the molecular weight of polyethylene glycol) is below 2000 (g mol-1)/ the template effect disappears for microfiltration of a polydisperse dextran solution, membrane film cracking occurs, and reproducibility is inferior [71].

Antifouling Ability

Antifouling ability is one of the biggest concerns for membrane materials in practice. The fouling mechanisms for liquid separation membranes in applications like microfiltration, ultrafiltration and nanofiltration can mainly be summarized as pore fouling and cake filtration [72]. It is believed that the service life of membrane materials is tightly bound to the contaminant medium, such as oil droplets, inorganic salts, bacteria and some solid or gel impurities [73]. In this regard, researchers endeavor to avoid the direct contact of such pollutants with the membrane surface and pore wall, in order to shield the membranes and extend their service time so as to strengthen the practicality and market competitiveness.

Focusing on these aspects, studies have been conducted on the modification of the membrane surface and the interfacial hydrophilicity or hydropho- bicity by adjusting the surface chemical groups or growth of nano-whisker/ nano-fibers, to prevent the deposition of pollutants on the membrane surface and inside pores [48]. Another factor is the introduction of external forces such as external disturbances or pulse/periodic flow disturbances, which interfere with the formation of the cake layer on the membrane surface during the separation process [74]. Pan et al. analyzed membrane-fouling mechanisms based on the modified Hermia's model. Under the operating conditions used in the study, there is a switch of fouling mechanisms from standard pore fouling to cake filtration at a small apparent flow rate and trans-membrane pressure difference. When the values of the two parameters are raised, the intermediate pore fouling becomes dominant after an early period of cake filtration [75].

 
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