Super2- Structures in Polydisperse Multiblock-Copolymers

Microphase separation in multiblock (AB) n copolymer melts at equilibrium is considered theoretically. It is shown that any finite polydispersity of the block molecular weights results in a set of novel super 2 -structures which are characterized by two levels of microdomains, one on the top of the other. The typical size of the second-level microdomains is governed by the polydispersity degree δ, and is much larger than the size of ordinary microdomains if δ is small. The theoretical description involves the second order parameter field φ(r) (in addition to the normal order parameter ψ related to the local composition) which is proportional to the local excess of longer blocks over shorter blocks. A scaling picture including the main characteristics of the super 2 -structures is built for both weak and strong segregation regimes.