Solvent copolymer adsorption onto

Balazs and Lewandowski (1990) have performed simulations of the adsorption of triblock copolymers onto a planar surface, and examined the conformations of the adsorbed chains. Monte Carlo simulations were performed of the motion of hydrophilic-hydrophobic chains on a cubic lattice. These simulations revealed a complex structure in the interfacial region due to the self-assembly of chains, driven by the solvent-incompatible block, reducing adsorption onto the surface. The influence on the surface coverage of length of the hydrophilic segement, polymer concentration, interaction energy between hydrophilic block and the... 

Marques et al. [116] have studied the adsorption of an A - B diblock copolymer from a dilute solution onto a solid surface that attracts the A block and repels the B block in a nonselective solvent, good for both blocks. 

Adsorption of block copolymers onto a surface is another pathway for surface functionalization. Block copolymers in solution of selective solvent afford the possibility to both self-assemble and adsorb onto a surface. The adsorption behavior is governed mostly by the interaction between the polymers and the solvent, but also by the size and the conformation of the polymer chains and by the interfacial contact energy of the polymer chains with the substrate [115-119], Indeed, in a selective solvent, one of the blocks is in a good solvent it swells and does not adsorb to the surface while the other block, which is in a poor solvent, will adsorb strongly to the surface to minimize its contact with the solvent. There have been a considerable number of studies dedicated to the adsorption of block copolymers to flat or curved surfaces, including adsorption of poly(/cr/-butylstyrcnc)-ft/od -sodium poly(styrenesulfonate) onto silica surfaces [120], polystyrene-Woc -poly(acrylic acid) onto weak polyelectrolyte multilayer surfaces [121], polyethylene-Wocfc-poly(ethylene oxide) on alkanethiol-patterned gold surfaces [122], or poly(ethylene oxide)-Woc -poly(lactide) onto colloidal polystyrene particles [123],... 

For the problem of random copolymers, unlike the situation discussed in the previous section, the randomness is now not in the medium, but in the SAW itself. One of the simpler cases is when there are two types of monomer, say A and B. Typically one has a fraction p of A-type monomers, and a fraction (1 — p) of B-type monomers. One usually assumes that the monomers are randomly distributed and constrained only by the value of p. An excellent contemporary review of this topic can be found in [2]. More generally, one can consider the case with k types of monomer, denoted m, .. , ruk, where the state of the polymer, modelled by an n-step SAW, is given by an n-tuple a = ai,..., an, where a G mi,..., TTifc. The values of a are taken from some distribution, appropriately chosen to model the problem at hand. In [2] three representative situations are discussed. The first is adsorption of a copolymer onto a surface, the second is the localization of a copolymer at an interface between two immiscible liquids, and the third is the temperature- or solvent-induced coil-ball collapse of a copolymer. We discuss these three representative problems below. 

Association is particularly noticeable in heterogeneous polymerizations. If the copolymer which is formed precipitates out of the reaction mixture, then the adsorption of the two monomers onto the precipitating polymers will be unequal. This adsorption process depends on the association in solution, which in turn depends on the solvent (or the dielectric constant of the system, to which the solvent contributes). The deviations in the reactivity ratios which are often observed in heterogeneous copolymerizations are thus not really due to the heterogeneity itself but originate from monomer association. 

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). 

The adsorption of polymers typically increases with polymer molecular weight and is higher and best (e.g. thicker, denser adsorbed layers) from a solution containing a poor solvent. For these reasons, block-copolymers are excellent steric stabihzers. Block copolymers like poly(ethylene oxide) surfactants are suitable as one part of the stabilizer has a high tendency to adsorb onto the particle surface and the other has a high affinity for the solvent. 

For longer polymers, adsorption and film formation can take place in organic solvents, which are good solvents for the neutral ferrocene-based species but bad solvents for the oxidized species [153, 154, 160-165]. In this case, even diblock copolymers precipitate on the electrode, without indication of the formation of micellar structures upon electrochemical switching [166-168]. Co-precipitation with carbon nanotubes onto electrodes has been achieved [169]. As a matter of fact, common solvents for both redox states are seldom available, unless the hydrophobic ferrocene units are complexed by cyclodextrins (in this case, water is a good solvent) [170, 171]. As another example, dendritic ferrocene-based polymers are sufficiently solubilized, even after oxidation in organic media (probably, the insoluble units are shielded in the interior of the dendrimers) [172]. Star-like... 

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