Molecules 2023 , 28 , 7984
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Taking toluene as a selective solvent, Parsonage E et al. [134] studied the polymer brush formed by the adsorption of a PS-b-PVP copolymer with different molecular weight on the substrate. PVP was affixed to the substrate surface and PS formed a polymer brush. Motschman H et al. [135] discussed the adsorption kinetics and adsorption isotherms of PS-b-PEO copolymer on the surface of silica gel in toluene solvent. The results indicated that apparent two-rate processes were showed in adsorption kinetics in a certain time range. At first, weak interactions lied in the grafting chains due to the effect of surface diffusion. Subsequently, densely polymer brushes formed by osmosis behavior among grafting chains. In the process of physisorption, desorbing of the adsorbed macromolecules happens easily due to the weak hydrogen bond or van der Waals force. 5.1.2. Covalent Attachment Covalent attachment is an irreversible process. That is, the polymer chains are con- nected by chemical bonds on substrate surface. This process can be realized by two technologies, namely “grafting to” and “grafting from”, as shown in Figure 7B. “Grafting to” means that under appropriate reaction conditions, the synthesized polymers with functional end-groups will react with modified substrate surface, and then polymers covalently bonded to substrate surface to form a polymer brush [136,137]. Auroy P et al. [138] synthesized a PSS brush on the substrate surface of silica gel by a two-step method. Firstly, PS chain with trichlorosilane end-groups was grafted on the substrate surface by using the “grafting to” technology, and then the PSS brush was obtained by in situ sulfonation reaction on substrate surface. The shortcoming of the “grafting to” technique is that only a few kinds of polymers have been grafted to the substrate surface. Once the polymer chains on substrate surface have reached a certain amount, the remaining chains could no longer be grafted due to steric hindrance. In particular, as for the grafted polyelectrolyte chains, electrostatic repulsion should be considered. Therefore, this method has a negative influence on the grafting density of polymer brushes. The “grafting from” technique is based on the surface being modified by polymeriza- tion initiators under light or heat [139–142]. The polymer brush prepared by this method is firmly bonded because of bearing initiator functionalities on substrate surface, thus leading to a dense layer of brush. Both conventional free radical polymerization and active free radical polymerization are employed to prepare polymer brush with high grafting density. Conventional free radical polymerization, by which the linear polymers are formed, is suitable for most vinyl monomers. So this method is the most widely used in the polymer industry. H and J [143] reported a poly (4-vinylpyridine) polymer brush created by free radical polymerization, and then the quaternization was commenced by adding CH 3 I to prepare a polyelectrolyte brush. When preparing brushes using acrylic acid as the monomer, it was found that increasing the reaction time and the solid content of the monomer were beneficial for increasing the length of brush. It is a simple and effective method for the synthesis of polymer brush, but low monomer utilization is a problem. It is mainly because that the two free radicals decomposed by an initiator are on the substrate surface and in the continuous phase, respectively, leading to a low monomer conversion rate (41.48%). Active radical polymerization mainly includes two strategies: atom transfer radical polymerization (ATRP) [144–147] and reversible addition–fragmentation chain transfer (RAFT) [148–151]. Zhao B et al. [152] synthesized mixed polymer brushes of PS and PMMA using ATRP technology and nitroxide-mediated radical polymerization technology. The grafting density, molecular weight, and distribution of the mixed polymer brush are controllable. Baum M et al. [153] synthesized PS, PMMA, and PAA-co PMMA polymer brushes using RAFT technology by bonding nitrogen-containing initiators onto the surface of silicon. Due to the low concentration of surface initiators, additional initiators are required to increase polymerization rate.
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