The phase behavior of block copolymers is often categorized into three regimes: weak, intermediate, and strong segregation. These approximately correspond to values of χN of order 1 (or less), 10, and 100 (or more), respectively, where N is the total degree of polymerization. In weak segregation, the effective repulsion between the blocks is a minor perturbation on the normal Gaussian behavior of a molten polymer chain; entropic effects dominate, and disordered liquids result. In strong segregation, energetic effects play a stronger role, leading to separate microdomains that are almost pure A and B, separated by a sharp interface. Most common block copolymers, such as PS–PI and PS–PBD, fall into the intermediate regime; χ is typically 0.05–0.1, and N is 100–1000. The ODT is also typically found in the intermediate regime. In the intermediate and strong segregation regimes, the chain conformation and, therefore, the domain periodicity are dictated by a balance between the interfacial energy, favoring well-separated interfaces and stretched chains, and conformational entropy, favoring coiled blocks and, therefore, more interface/unit volume. The SSSR arises when χ becomes so large that the interfacial tension completely dominates. In this case, the minor block should stretch out completely. Thus the domain size L will scale linearly with N, in contrast to L ∼ N2/3, which is characteristic of strong segregation. Figure 8 illustrates the concept of superstrong segregation for an initially spherical micelle. As the interfacial tension increases, transitions to an oblate disk, a hairy hockey puck, and eventually a flat sheet are anticipated.21 In short, new structures are anticipated, whether in solution or in the bulk. There have been no systematic explorations of this concept to date, although various studies are relevant.22, 23
The key step is to develop chemistry in which χ may be varied progressively but over a very large range. For example, χ values as large as 10 or 100 may be necessary. To achieve this, one block should be ionic or at least have an extremely high CED. For the other block, perfluorinated alkane groups offer extremely low CEDs. Nafion is an example of a commercially important material that exploits this idea, but the chain architecture and thus the material morphology are not well controlled; model block copolymers, therefore, offer an appealing possibility. An important practical issue is that of equilibration; the stronger the interactions are at the monomer level, the more likely the system will become trapped in some metastable state. By systematically varying the charge density along one block and the degree of fluorination along the other, we should be able to ramp up the degree of segregation steadily. Although this does not eliminate the problem of metastability, one expects a systematic variation in morphology with χ, and departures from such a progression should be apparent. Furthermore, by varying χ and N independently, we can characterize the crossover from strong segregation (phase behavior dependent on the product χN) to superstrong segregation (phase behavior dependent primarily on χ).