spacer by chemical reduction showed improved biofouling control in seawater
desalination (Yang et al. 2009). In addition, fouling of positively charged
nanofiltration membrane can be prevented to some extent by modifying the surface
properties such as hydrophilicity, roughness, and charge by adsorbing a layer of
water-soluble polymers like dextran and poly(acrylic acid sodium salt) onto the
surface in a dynamic manner (Ba et al. 2010).
4.5.3 Surface Treatment
Surface treatments which include techniques like chemical treatment and surface
grafting are employed to attain higher desalination rate by utilizing the enhanced
properties of surface-modified membranes. Chemical treatment is a covalent binding
of phosphorylcholine to the membrane resulting in stable modified surface. The
modified membranes exhibited good mechanical property as that of the original
membranes, maintained the morphological structure, and showed higher water
permeability than the original membranes. The presence of phospholipid side chains
also showed high biocompatibility and hydrophilicity (Xu et al. 2009). Dai et al. and
Xu et al. proposed an efficient method for attaching phospholipid analogous polymers onto a polypropylene membrane. N,N-dimethylaminoethyl methacrylate
(DMAEMA) was photoinduced by graft polymerization onto the membrane surface
as ring-opening reagent followed by the ring-opening reaction of grafted poly
(DMAEMA) with 2-alkyloxy-2-oxo-1,3,2-dioxaphospholanes (Dai et al. 2004; Xu
et al. 2004). Surface grafting polymerization is a chemical tailoring of polymeric
membrane with distinct properties while maintaining the substrate properties. Compared with physical coating and adsorption, covalent attachment of phospholipid
analogous chain onto polymeric membrane can avoid desorption and maintain a
long-term chemical stability. The surface grafting methods include initiator grafting
polymerization, photoinduced grafting polymerization, corona irradiation grafting
polymerization, and ATRP grafting polymerization (Xu et al. 2009). The hydrophilic
character of polyacrylonitrile (PAN) and polysulfone (PSf) ultrafiltration
(UF) membranes was improved by graft polymerization of hydrophilic monomers
such as 2-hydroxyethyl methacrylate (HEMA) and acrylic or methacrylic acid onto
the membrane surfaces by thermal decomposition of peroxides (Ulbricht and Belfort
1996). Polypropylene microfiltration membrane (PPMM) showed improved hydrophilicity after modification by UV photo-assisted and γ-ray pre-irradiation-induced
graft polymerization of hydrophilic polymer, poly(N-vinyl-2-pyrrolidone) (Liu et al.
2004).
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
143
desalination (Yang et al. 2009). In addition, fouling of positively charged
nanofiltration membrane can be prevented to some extent by modifying the surface
properties such as hydrophilicity, roughness, and charge by adsorbing a layer of
water-soluble polymers like dextran and poly(acrylic acid sodium salt) onto the
surface in a dynamic manner (Ba et al. 2010).
4.5.3 Surface Treatment
Surface treatments which include techniques like chemical treatment and surface
grafting are employed to attain higher desalination rate by utilizing the enhanced
properties of surface-modified membranes. Chemical treatment is a covalent binding
of phosphorylcholine to the membrane resulting in stable modified surface. The
modified membranes exhibited good mechanical property as that of the original
membranes, maintained the morphological structure, and showed higher water
permeability than the original membranes. The presence of phospholipid side chains
also showed high biocompatibility and hydrophilicity (Xu et al. 2009). Dai et al. and
Xu et al. proposed an efficient method for attaching phospholipid analogous polymers onto a polypropylene membrane. N,N-dimethylaminoethyl methacrylate
(DMAEMA) was photoinduced by graft polymerization onto the membrane surface
as ring-opening reagent followed by the ring-opening reaction of grafted poly
(DMAEMA) with 2-alkyloxy-2-oxo-1,3,2-dioxaphospholanes (Dai et al. 2004; Xu
et al. 2004). Surface grafting polymerization is a chemical tailoring of polymeric
membrane with distinct properties while maintaining the substrate properties. Compared with physical coating and adsorption, covalent attachment of phospholipid
analogous chain onto polymeric membrane can avoid desorption and maintain a
long-term chemical stability. The surface grafting methods include initiator grafting
polymerization, photoinduced grafting polymerization, corona irradiation grafting
polymerization, and ATRP grafting polymerization (Xu et al. 2009). The hydrophilic
character of polyacrylonitrile (PAN) and polysulfone (PSf) ultrafiltration
(UF) membranes was improved by graft polymerization of hydrophilic monomers
such as 2-hydroxyethyl methacrylate (HEMA) and acrylic or methacrylic acid onto
the membrane surfaces by thermal decomposition of peroxides (Ulbricht and Belfort
1996). Polypropylene microfiltration membrane (PPMM) showed improved hydrophilicity after modification by UV photo-assisted and γ-ray pre-irradiation-induced
graft polymerization of hydrophilic polymer, poly(N-vinyl-2-pyrrolidone) (Liu et al.
2004).
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
143