Block copolymers in dilute solution

The formation of micelles in a solvent that is selective for one of the blocks is one of the most important and useful properties of block copolymers. For example, micelles formed by block copolymers can solubilize otherwise insoluble substances, they can be used to microscopically mix incompatible substances and they can stabilize colloidal particles or form microemulsions. 

Since this chapter is concerned with block copolymers in dilute solution, it is useful to include a definition of the dilute regime for polymer solutions in the Introduction. This regime extends up to a volume fraction above which swollen coils overlap (de Gennes 1979)  

An early review of micellization in block copolymers was presented by Tuzar and Kratochvfl (1976), and these authors have recently provided an excellent review of the literature up to 1992 (Tuzar and Kratochvfl 1993). Micellar properties of block copolymers were also reviewed by Price (1982). A discussion of micellization was included in the general reviews on block copolymers by Riess et al. (1985) and Brown et al. (1989). Excellent reviews focussed on the solution properties of a particular class of copolymer, i.e. copolymers of polyoxyethylene) with poly(oxypropylene) have been presented by Alexandridis and Hatton (1995) and by Chu (1995) and Chu and Zhou (1996). Micellization and micellar association in related poly(oxyethylene)/poly(oxybutylene) copolymers have been summarized by Booth et al (1997). 

In this chapter, the focus is largely on experimental and theoretical studies of micellization in a range of solutions of model block copolymers prepared by anionic polymerization. A discussion of both neutral and ionic block copolymers is included, and features specific to the latter type are detailed. The adsorption of block copolymers at the liquid interface is also considered in this chapter. Recent experiments on copolymer monolayers absorbed at liquid-air and liquid-liquid interfaces are summarized, and recent observations of surface micelles outlined. Thus this chapter is concerned both with bulk micellization and the surface properties of dilute copolymer solutions. 

This chapter is organized as follows. The thermodynamics of the critical micelle concentration are considered in Section 3.2. Section 3.3 is concerned with a summary of experiments characterizing micellization in block copolymers, and tables are used to provide a summary of some of the studies from the vast literature. Theories for dilute block copolymer solutions are described in Section 3.4, including both scaling models and mean field theories. Computer simulations of block copolymer micelles are discussed in Section 3.5. Micellization of ionic block copolymers is described in Section 3.6. Several methods for the study of dynamics in block copolymer solutions are sketched in Section 3.7. Finally, Section 3.8 is concerned with adsorption of block copolymers at the liquid interface. 

---

Transmission electron micrography has, remarkably, been successfully used to image micelles formed by block copolymers in dilute solutions. Price and coworkers used two preparation methods. In the first method (Price and Woods 1973), f reeze etching, a drop of solution was rapidly frozen by quenching in liquid nitrogen. Solvent was then allowed to evaporate from a freshly microtomed surface of the droplet. Finally, a replica was made of collapsed micelles raised proud from the frozen surface. In the second method (Booth et al. 1978), a drop of micellar solution was allowed to spread and evaporate on a carbon substrate, and 0s04 was used to selectively stain one of the blocks. 

Xu et al. (1992) used light scattering to characterize micelles formed by a wide range of PS-PEO di- and tri-block copolymers in dilute solution in water. Although full analysis of the data was complicated by the tendency of the micelles to undergo secondary association, they did find that the micellar radius scaled as eqn 3.14, in agreement with the predictions of Halperin (1987). With values of p and RB from the star-like micelle model, Xu et al. (1992) were able to compute % parameters for the interactions of PEiO with water and with PS, in... 

Relatively few theoretical studies have been devoted to the conformational characteristics of nonlinear block copolymers in different solvent environments. Burchard and coworkers [284] studied theoretically the behavior of the static and dynamic structure factors for regular star-block copolymers in dilute solutions. They considered different cases where the refractive index (n)s of the solvent takes certain values with respect to the corresponding refractive indices of the inner and outer blocks. A different dependence of the ratios... 

In the following we will describe the self-organization of block copolymers in dilute solution, concentrated solution, and bulk. All these structures have been or can be used as templates for the preparation of nano particles and porous structures. 

KOT Kotaka, T., Ohnuma, H., and Inagaki, H., Thermodynamic and conformational properties of styrene-methyl methacrylate block copolymers in dilute solution. 11-Behavior in theta solvents. Polymer, 10, 517, 1969. 

DON Dondos, A., Investigations on conformations of styrene-a-methylstyrene block copolymers in dilute solution, JPo/y/w. Sci. Polym. Lett., 9, 871, 1971. 

UT1 Utiyama, H., Takenaka, H., Mizumori, M., and Fukuda, M., Light-scattering studies of a polystyrene-poly(methyl methacrylate) two-block copolymer in dilute solutions. 

Zhang L, Eisenberg A (1999) Thermodynamic vs kinetic aspects in the formation and morphological transitions of crew-cut aggregates produced by self-assembly of polysty-rene-b-poly(acrylic acid) block copolymers in dilute solution. Macromolecules 32 2239-2249... 

Figure 6.1. Schematic of a block copolymer in dilute solution, with one portion tightly coiled and the other highly extended.

---