Surface-initiated grafting modification

Surface graft modification includes direct graft polymerization modification and surface-initiated grafting polymerization modification. The former requires the particle surface to have active groups to copolymerize with other monomers, whereas the latter require that active groups to copolymerize with other monomers are generated from the grain surface by chemical or physical methods. 

Surfaces play crucial roles in many important properties, including optical, mechanical, thermodynamic, and chemical properties, and thus surface modification is an important issue. One of the effective surface modification methods is surface-initiated graft polymerization. Fukuda et used LRP... 

In general, in the field of materials or condensed matter, the preparation of polymer brushes on solid surfaces is of great interest for surface modification and composite material preparation [4-6]. A number of model surface grafting techniques have been used on planar surfaces and particles and have been the subject of previous reviews. While a munber of polymer brush preparation methods have been reported using physisorption or chemisorption or so-called grafting onto methods, the emphasis of this review is on surface-initiated polymerization (SIP) methods or grafting from methods. 

In the early 1970s, surface modification of most polymers was achieved using redox initiators. Ce+4-induced initiation was employed to achieve surface grafting of acrylamide onto LDPE film [117]. The film was first oxidised by chromic acid and then reduced with diborane to form a hydroxyl-rich surface which was then used to initiate graft polymerisation of acrylamide using Ce+4/HN03. The mechanism of chromic-acid-facilitated surface oxidation of LDPE surface is shown in Scheme 6a and that of free-radical generation is represented in Scheme 6b. 

Contact angles for these films were measured at initial time and after 5 minutes (Table 1). The control film (native BSA in NH4OH) presented a very hydrophilic surface, with a weak contact angle of 37°, falling to 10° after five minutes. Both modifications, either chemical grafting or modification of the solvent, resulted in an increase of these angles (up to 114°) moreover, these angles remained quite stable after 5 minutes, with values above 90°. 

In recent years, many kinds of temperature-responsive PNIPAAm and its copolymer hydrogels with other acrylic monomers have been synthesized [142]. Besides being used for hydrogels, NIPAAm monomer can be grafted on to polymer substrates by electron beam, irradiation or UV-initiated graft polymerization to achieve special modification of polymer surfaces. Thus NIPAAm has been grafted on porous polymer films such as LDPE, PP, or polyamide films in order to prepare novel films for pervaporation of liquid mixtures or separation membranes [150,151]. 

For surface modification applications, thick grafting layers are unnecessary and even undesirable because they may change bulk physical properties of the polymer, such as crystallinity and tensile modulus. A two-step method can be used to minimize the formation of the homopolymer. The polymer is preirradiated in air to produce peroxide groups on the surface. Grafting is subsequently initiated thermally in contact with a monomer. Other methods such as corona discharge, ozone treatment, and plasma treatment have also been used to generate peroxide groups on polymer surfaces. 

Currently, UV-initiated graft polymerization has been widely used for the surface modification of polymer manbranes in attempts to develop composite manbranes with enhanced resistance toward organic fouling and biofouling. As can be seen in Table 3.1, different hydrophilic monomers, such as Af-vinyl-2-pyrrolidone (NVP), A-vinylformamide (NVF), A-vinyl-caprolactam (NVC), 2-hydroxyethyl methacrylate (HEMA), acrylic acid (AA), acrylamide (AAm), 2-acrylamidoglycolic acid (AAG),... 

UV-initiated graft polymerization of AA on the surface of a PES manbrane via the immersion method using a 365 nm wavelength UV lamp was studied (Abu Seman et al. 2010). It was shown that irreversible manbrane fouling with humic acids (HA) was reduced after modification due to the increase in the hydrophilicity and the negative surface charge of the modified manbranes. 

Based on the membrane surface properties and the HA properties, various researchers have attempted to change the membrane surface characteristics by surface modification. Different techniques have been performed, such as ion beam irradiation, plasma treatment, redox-initiated graft polymerization, photochemical grafting, and interfacial polymerization (IP). In this chapter, two surface modification techniques, IP and photochemical grafting, are discussed by means of experimental examples. The surface characteristics of the unmodified membrane and the modified membranes are studied and their relationships with irreversible fouling and NF performance are reported. 

Mohanty AK, Parija S, Misra M (1996) Ce(IV)-A(-acetylglycine initiated graft copolymerization of acrylonitrile onto chemically modified pineapple leaf fibers. J Appl Polym Sci 60 931-937 Mohanty AK, Khan MA, Hinrichsen G (2000) Surface modification of jute and its influence on performance ofbiodegradable jute-fabric/Biopol composites. Compos Sci Technol 60 1115-1124 Mohanty AK, Misra M, Drzal LT, Selke SE, Harte BR, Hinrichsen G (2005) Natural fibers, biopolymers and biocomposites an introduction. In Mohanty AK, Misra M, Drzal LT (eds) Natural fibers, biopolymers and biocomposites. Taylor Francis, FL, Boca Raton Mukherjee PS, Satyanarayana KG (1986) Structure and properties of some vegetable fibres Part 2 pineapple fiber. J Mater Sci 21 51-56... 

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