Mixed-solvent behavior, polymer brushes

Figure 5.8 Schematic diagram showing switching behavior of mixed polymer brushes upon treatment with different selective solvents.

Another interesting design of mixed polymer brushes was obtained by sequential grafting of asymmetric nonsticky/sticky diblock copolymers of poly(styrene-b-3-(trimethoxysilyl)propylmethacrylate) and monomethoxypoly(ethylene glycol)-trimethoxysilane onto silicon wafers (Han et al., 2013). The nanoscopic morphology of these mixed polymer brushes exhibited stimuli responsive behavior to various temperatures and solvents. 

There is a limited number of studies of adsorption of diblock copolymers from nonselective solvents. One practical problem is that it is not enough to find a nonselective solvent, but that a brush-like configuration also requires selective adsorption of the A block. Because polymers are notorious for adsorbing onto many substrates [3,41], the best way is to use a mixed solvent with a composition such that A adsorbs and not B. Some early data were obtained by Wu et al. [19] for diblocks of dimethyl-aminoethyl-methacrylate (A) and n-butylmethacrylate (B) from propanol onto silica. For chains of 200 and 700 monomers, Wu et al. found a clear maximum at around 10% of A monomers, where the adsorbed amormt reaches a value of 13 mg/m. This behavior was thus very much in line with the theoretical predictions, as shown in Figure 7.2. The adsorbed amount of 13 mg/m corresponds with a grafting density of about 1 chain per 10 nm, which is reasonable but not very dense. Later experiments on diblocks with a cationic block (dimethyl-aminoethyl-methacrylate (AMA)) and a neutral block (dihydroxy-propyl-methacrylate (HM A)) on silica and titania were carried out by Floogeveen et al. [42] (Figure 7.3). 

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