Membrane and Desalination Technologies - TCE Moodle Website

Preparation of Polymeric Membranes 53
3. MEMBRANE MATERIALS
Most of materials such as polymer, ceramic, metal, carbon and glass can be used to make
membranes. At present, the commercial membranes are mainly fabricated from polymeric
materials. Being membrane materials, the polymers should demonstrate thermal stability over
a wide range of temperature and a chemical stability over a range of pH, and possess strong
mechanical strength. Besides, they can also be processed into flat sheet or hollow fiber
membranes easily. The polymers that are suitable for making membranes include cellulose,
cellulose acetates, cellulose acetate butyrate, cellulosic ester, cellulose nitrates, polycarbonate
esters, ethyl celluloses, polyacetylenes, polyacrylonitrile, polyamides, polyamide esters,
polyamide-hydrazide, polyamideimides, polyaryletherketone, polyetherketones, polycarbonates,
poly(phenylene oxide) polyesters, polyestercarbonate, polyethers, polyetherimides,
polyetherketones, polyethersulfones, polyethylenes, polyhydrazides, polyimides, polyphenylene
oxides, polyphenylene sulfide, poly(phthalazine ether sulfone ketone), sulfonated poly
(phthalazine ether sulfone ketone), polypropylene, polysiloxanes, polysulfones, sulfonated
polysulfones, polytetrafluoroethylene, poly(trialkylsilylacetylenes), poly(trimethylsilylpropyne),
polyureas, polyurethanes, polyvinylalcohol, polyvinylchloride, polyvinylidene fluorides,
etc. The early commercial membranes were based on cellulose acetate and polysulfone,
etc. Afterward, other polymers such as polyethersulfones, polyacrylonitrile, polyvinylidene
fluorides, polycarbonate, polyamide, polyimide, polyetherimide, etc., were used one after the
other. Some commercial and representative polymeric membrane materials are introduced
further in the following sections.
3.1. Cellulose and Cellulose Acetate
Cellulose is a polysaccharide with a regular repeating structure (Fig. 2.4), which can form
strong intermolecular hydrogen-bonds between the hydroxy groups and gives rise to a high
level of crystallinity. And the crystallinity also prevents cellulose form dissolving in water
although the polymers are highly hydrophilic. Cellulose can dissolve in N-Methylmorpholine-N-Oxide
(NMMO), N,N-dimethylacetamide (DMAC) and N-methylpyrrolidone (NMP)
with lithium chloride (LiCl) added. Regenerated cellulose has been used extensively to
fabricate dialysis membranes. Cellulose acetate, diacetate, triacetate and their blends are
widely used to form microfiltration, ultrafiltration and reverse osmosis membranes, which are
strongly hydrophilic and thus demonstrate lower fouling characteristics. The disadvantages of
cellulose and cellulose acetate membranes are their stability over a very narrow pH range
between 4 and 6.5 and their poor resistance to microbial attack.
3.2. Polysulfone
Polysulfone (PS) is an amorphous high performance polymer and demonstrates a high degree
of chemical and thermal stability, which chemical structure is shown in Fig. 2.5. Polysulfones
are mainly used to form ultrafiltration, microfiltration, and gas separation membranes, but they
are also used to form the porous support layer of many reverse osmosis, nanofiltration, and some
gas separation membranes. Compared with cellulose acetate, polysulfone membranes can