Microphase Separation and Ordering

In even the simplest pure diblock and triblock copolymer melts, numerous distinct microphase-separated morphologies have been observed, as depicted at the top of Fig. 13-4 (Winey et al. 1992 Forster et al. 1994 Hadjuk et al. 1994 Schulz and Bates 1996). The simplest types of domain shapes are spheres, cylinders, and lamellae. For a pure AB diblock 

The cubic packing symmetry of the spherical domains is often body-centered cubic (BCC) (Thomas et al, 1987 Almdal et al. 1993 Okamoto et al. 1994a Chu et al. 1995 Adams et al. 1996) and, when solvent is present, sometimes face-centered cubic (FCC) (McConnell et al. 1993). The BCC packing is characteristic of spheres that interact via 

A somewhat different nomenclature, Tmst, is sometimes used instead of Todt, where MST refers to microseparation transition. This nomenclature can be misleading, however, since microseparation of one type of block from another occurs gradually as the temperamre is lowered, and it is distinct from the ordering transition at which symmetry is broken and an ordered phase appears at a sharply defined temperamre. This is particularly true when the composition of one of the blocks is low ( S0.15), so that well-segregated, but disordered, spheres (or polymeric micelles ), and sometimes worms and vesicles, appear. Thus, we prefer to avoid the potentially misleading nomenclature Tmst-  

ABC-type triblock copolymers, in which A, B, and C are all chemically distinct from each other, can form a variety of additional structures, such as a cylindrical phase ordered on a square lattice, a tricontinuous diamond phase (Mogi et al. 1992 Nakazawa and Ohta 1993), and some exquisitely delicate structures, such as cylinders decorated by rings (see Fig. 13-8) or by strands winding around them in helieal fashion (Stadler et al. 1995 Krappe et al. 1995). Recent findings have greatly expanded the known types of ordered block copolymer phases. 

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