Phase segregated microdomains

Block copolymers comprise chemically distinct polymer chains bonded together to form a single macromolecule. The repulsion between chemically dissimilar blocks enables the formation of periodic microdomains and, as a result of the covalent bond between blocks, macroscopic phase segregation is avoided. The presence and type of equilibrium microstructure (e.g., lamella, cylinders, and spheres) depends on temperature, composition, the extent of repulsion between blocks, surface boundary conditions, and imposed fields. 

The increasing importance of block copolymers arises mainly from their unique properties in solution and in the solid state, which is a consequence of their molecular structure. In particular, sequences of different chemical composition are usually incompatible and therefore have a tendency to phase segregate. The microdomain formation process in the solid state is directly related to the specific molecular... 

Interesting information can be obtained when the melt structure is examined in detail. In particular, it is convenient to describe the nonhomogeneous melt as two categories, weakly and strongly segregated microdomains. When weakly segregated, the microdomains, or meso-phases, are destroyed by the crystallization process. Examples of this type of behavior have been found in diblocks of ethylene-ethylene a/t propylene,(60) ethylene-ethylene ethylene,(55,57) ethylene-3-methyl-1-butene,(61) caprolactone-butadiene,(58) oxyethylene-oxybutylene (62) as well as others. (63) In these cases, irrespective of the initial microdomain structure, the kinetic data are well represented by Avrami type isotherms. Typically the Avrami exponent is 3.0 for an appreciable extent of the transformation as is illustrated with ethylene-ethylene ethylene diblocks.(57) Thus, when weakly segregated, the microdomains characteristic of the initial melts are overwhelmed, and have no effect on the crystallization. Conventional lamellar-like crystallites are formed even when the mesophase in the melt is not lamellar. 

A mean field approach was applied to determine homopolymer distributions in the lamellar phase of a blend of AB diblock and A homopolymer by Shull and Winey (1992). In the strong segregation limit, complete segregation of the A homopolymer into the A microdomain was predicted. Furthermore, in this limit, the diblocks were treated as brushes , wetted by homopolymer in the A domain. Composition profiles showing the distribution of homopolymer and copolymer were determined by numerical solution of the self-consistent field equations. 

Politakos, N. Ntoukas, E. Averopoulos, A. Krikorian, V. Pate, B. D. Thomas, E. L. Hfll, R. M., Strongly Segregated Cubic Microdomain Morphology Consistent with the Double Gyroid Phase in High Molecular Weight Diblock Copolymers of Polystyrene and Poly(dimethylsiloxane). J. Polym. Sci., PartB Polym. Phys. 2009,47,2419-2427. 

---