Gelation in block copolymer solutions

Studying this phase behaviour has been the subject of intense effort since the early 1990s. The literature reflects the relatively recent interest in this aspect of block copolymers and much work continues in this area, and will no doubt lead to an expanded version of this chapter in future editions of this book. 

Several recent reviews have included a discussion of the ordered micellar phase phase behaviour of poly(oxyethylene)/poly(oxypropylene) copolymers (Alexandridis and Hatton 1995 Chu and Zhou 1996) and poly(oxyethylene)/ poly(oxybutylene) copolymers (Booth et al, 1997), 

This chapter is concerned with experiments and theory for semidilute and concentrated block copolymer solutions.The focus is on the thermodynamics, i.e. the phase behaviour of both micellar solutions and non-micellar (e.g. swollen lamellar) phases. The chapter is organized very simply Section 4.2 contains a general account of gelation in block copolymer solutions. Section 4.3 is concerned with the solution phase behaviour of poly(oxyethylene)-containing diblocks and tri-blocks. The phase behaviour of styrenic block copolymers in selective solvents is discussed in Section 4.4. Section 4.5 is then concerned with theories for ordered block copolymer solutions, including both non-micellar phases in semidilute solutions and micellar gels. There has been little work on the dynamics of semidilute and concentrated block copolymer solutions, and this is reflected by the limited discussion of this subject in this chapter. 

It is now well established that formation of hard or stiff gels is the result of association of micelles into cubic phases. The notation hard gel follows Hvidt and co-workers (Almgren et al 1995 Hvidt et al. 1994) and refers to a micellar solution with a dynamic elastic shear modulus G 103Pa. The correlation between the formation of a cubic phase and the onset of plastic flow (i.e. formation of a gel with a finite yield stress) was first made for PS-PI solutions in 

Face-centred cubic (FCC) structures have also been observed in Pluronic copolymers, using SAXS (Berret et al. 1996). In an aqueous poly(oxyethylene)-poly(oxybutylene) (PEO-PBO) diblock solution, both BCC and FCC phases 

---

Gast and co-workers (Gast 1996 McConnell et al. 1993,1995) have used SAXS to probe ordered micellar structures in PS-PI block copolymers dissolved in decane, which is a preferential solvent for PI. They determined the form factor of a range of diblocks by performing SAXS in dilute solution (McConnell et al. 1993). The same diblocks in more concentrated solutions were found to form cubic micellar structures, and the gelation (micellar disorder-order) transition on increasing concentration was determined. Remarkably, both BCC and FCC struc-... 

Jeong et al. studied the gelation behavior of poly(ethylene glycol)-h-poly(L-alanine) (PEG-h-L-PA) block copolymers in water [117, 118]. They suggested that the P-sheet structure of l-PA plays a critical role in developing a fibrous nanostructure as well as in the sol-to-gel transition of the copolymer solutions (Fig. 14). The conformation of l-PA in water is random coil at low concentrations. As the concentration increases, block copolymers start to aggregate and the conformation... 

---