Nonselectivity solvent

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

For highly asymmetric block copolymers with a large insoluble block, the copolymer chains can t be directly solubilized in the selective solvent. However, micelles can be obtained from these copolymers by the temporary use of a nonselective solvent which is further eliminated (see Sect. 2.2 for further details on this issue). In principle, all the copolymer chains are aggregated... 

Addition of a selective solvent to molecularly dissolved chains has been used by many research teams to prepare block copolymer micelles. The initial nonselective solvent can be further eliminated by evaporation or can be gradually replaced by the selective solvent via a dialysis process. The stepwise dialysis initially introduced by Tuzar and Kratochvil is now widely used for micelle preparation [6], especially for the formation of aqueous micelles [32],... 

This technique does not, however, overcome the formation of frozen micelles due to the formation of glassy cores at a specific nonselective solvent/selective solvent composition. Polydisperse micelles can also be generated during this preparation process if the starting material is characterized by a composition or MW polydispersity. In this respect, micelles will be first formed by the chains containing the larger insoluble block during the addition of the selective solvent. 

One of the drawbacks of crew-cut micelles is that they systematically require the use of a nonselective solvent for their preparation and they definitely represent out-of-equilibrium micelles once they have been transferred... 

Fig. 13 Phase diagram of PS310-PAA52 in dioxane/water mixtures (A). Shaded regions between sphere and rod phases and between rod and vesicle phases correspond to coexistence regions. Reversibility of vesicle formation and growth process for PS300-PAA44 as function of THF/dioxane composition of nonselective solvent (B). Reprinted with permission from [239]. Copyright (2002) American Association for the Advancement of Science...

Crew-cut micelles were prepared from PS-PMMA-PAA triblock copolymers with a large PS block [270]. The morphology of these micelles was found to be dependent on the starting nonselective solvent (dioxane, THF, or DMF), as discussed previously in Sect. 6. 

Fig. 9.20 Grafted heterogeneous polymer brushes in different environments and principle of switching (a) Structure in a nonselective solvent, (b) in a selective solvent for polymer PI (e.g. PS), and (c) in a selective solvent for polymer P2 (e.g. P4VP) (modified from ref. [214]).

Recently, Dyer used the same strategy to perform a photoinitiated synthesis of a mixed brush by using an AIBN-type initiator boimd to gold [57]. Specifically, they used initiator (24) to modify gold substrates with a binary brush composed of PS and PMMA. As Fig. 11 describes, mixed brushes will respond to the polarity of the solvent. For example, immersion into a non-selective solvent like THF brings both components to the air/hquid interface since PS and PMMA are both soluble in THF. However, immersion into a polar solvent, such as isobutanol, will selectively bring PMMA to the air/hquid interface, while the nonpolar PS collapses into the interior of the film. In contrast, immersion into cyclohexane brings PS to the air/hquid interface and PMMA is driven to the interior. The cycle is completely reversible after immersion into a nonselective solvent like THF. 

Fig. 11 Switching of a PS/PMMA brush is accomplished by immersion into various solvents. A polar solvent such as isobutanol brings PMMA to the air/liquid interface, while the PS collapses into the interior. The opposite occurs when the substrate is immersed into a nonpolar solvent such as cyclohexane. Upon immersion into a nonselective solvent, like THF, both components come to the air/liquid interface...

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. 

Blend solutions. Solutions of blends comprising immiscible polymers Pj and P2 in a nonselective solvent have miscibility gaps as shown schematically in Fig. 14. When the polymer concentration increases by solvent evaporation the polymer coils start to interpenetrate above a certain concentration. As a consequence, interactions between the polymers become operative and phase separation must start above a critical polymer concentration p. The composition of the new phases will be situated on the branches of the coexistence curve. Finally, the unmixing process is arrested owing to enhanced viscosity. This simple scheme reveals the factors directing morphology evolution in blend solutions ... 

Electron Microscopy. Figure 3 shows electron micrographs of ultra-thin sections of film specimens of the three kinds of block copolymers. As can be seen in the figure, TR-41-1647 and TR-41-1648 specimens have a heterogeneous structure in which the polystyrene domains are dispersed within a polybutadiene matrix and are connected to each other to form a swirl-like structure. On the other hand, TR-41-1649 specimen is seen to consist of alternating lamellar domains of the two components. Changes of the domain structure with fractional compositions of styrene and butadiene components are consistent with predictions of the current theories of micro-phase separation (12,13,14,15) for block copolymers cast from such a nearly nonselective solvent as the mixture of THF and methylethylketon (90/10 in volume ratio). 

The microphase separation of (styrene-g-isoprene) graft copolymers with a large number of grafted chains was investigated by Price and coworkers [349]. Those films were cast from benzene, a nonselective solvent, and a relatively well-defined microphase separated morphology was observed, but the structure was less regular compared with dispersions in hexane on carbon films [350]. In the... 

Even if all blocks of a block polymer are solvated in solution, that is, by using a nonselective solvent, phase segregation of the solvated blocks may occur at high concentration. The result is a supermolecular ordering in solution comparable to that in liquid crystals. In certain cases the aggregates are sufficiently large to diffract visible light, and, as has been mentioned earlier, block polymer solutions that are irridescent above a critical concentration have been observed (81). 

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