Cubic phases domain morphology

Asymmetric block copolymers which form hexagonal or cubic-packed spherical morphologies in the bulk, form stripe or circular domain patterns in two dimensions, as illustrated in Figure 5. The stripe pattern results from cylinders lying parallel to the substrate, and a circular domain surface pattern occurs when cylinders are oriented perpendicular to the substrate, or for spheres at the surface. Bicontinuous structures cannot exist in two dimensions therefore the gy-roid phase is suppressed in thin films. More complex multiple stripe and multiple circular domain structures can be formed at the surface of ABC triblocks (83). Nanostructures in block copolymer films can be oriented using electric fields (if the difference in dielectric permittivity is sufficient), which will be important in applications where parallel stripe (84) or perpendicular cylinder configurations (85) are desired. 

Other more complex morphologies also arise for A-B mixtures. In particular, domains A and B may enclose each other, forming entangled networks, separated by a hyperbolic interface. Those cases include mesh , bicontinuous microemulsions, bicontinuous cubic phases and their disordered counterparts, sponge phases, which are discussed below. In these cases too, the sign (convex/concave) of the interfacial mean curvature sets the Type . A representation of the disordered mesostructure in a Type 2 bicontinuous microemulsion is shown in Figure 16.3. A hyperbolic interface may be equally concave and convex (a minimal surface, e.g. see Figure 16.2(c)) so that the mesophase is neither Type 1 nor Type 2. Lamellar mesophases ( smectics or neat phases) are the simplest examples. Bicontinuous balanced microemulsions, with equal polar and apolar volume fractions are further examples. 

Self-assembly and morphology of block copolymers depend on their architecture and composition [3]. Several equilibrium phases like lamellae, gyroid, hexagonal-packed cylinders, and body-centered cubic phases were observed in melts. In thin films, microphase separation resulted in formation of lamellae, stripes, and circular domains. Various types of micellar structures and arrangements were seen in dilute solutions [4], These phase behaviors were dictated by Flory-Huggins interaction parameter (/), copolymer degree of polymerization N), and composition (/) in melts and thin films. In addition to these parameters, amphiphilicity was the most important property of block copolymers enabling them to self-assemble into various stmctures in dilute solutions [3]. 

UsingTEM to identify blend morphology, two diblocks with/ps 0.8 that form cubic-packed spherical phases and cylindrical phases respectively in the pure copolymer were found not to macrophase separate in a blend with d = 2.2, but to form single domain structures (cylinders or spheres) in the blend (Koizumi et al. 1994c). Similarly, blending a diblock with fK = 0.26 with one with fK = 0.64 (d = 1.2) led to uniform microphase-separated structures, with a lamellar phase induced in the 50 50 blend. Vilesov et al. (1994) also observed that blending two PS-PB diblocks with approximately inverse compositions (i.e. 22wt% PS and 72 wt% PS) induces a lamellar phase in the 50 50 blend. These examples all correspond to case (i). 

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