Modelling of the phase separation behaviour of semiflexible diblock copolymers

Abstract

For a realistic theoretical description of the structure factor and the phase behaviour of melts of block copolymers, the partial stiffness of the chains has to be taken into account, particularly for diblocks consisting of blocks of very different chain stiffness. In this paper, a simple theoretical description for the structure factor of semiflexible monodisperse diblock copolymers is presented. It is derived by introducing the persistence length in addition to the monomer length and is based on the random phase approximation (RPA) in combination with a cumulant expansion for the semiflexible form factor. The persistence lengths of the two blocks are in general different and arbitrary, encompassing all experimentally important cases of Gaussian-Gaussian, semiflexible-semiflexible, rod-rod, Gaussian-semiflexible. Gaussian-rod and semiflexible-rod diblocks. It is shown that the formalism is capable of predicting reasonable structure factors and spinodals for semiflexible diblock copolymers. In particular, the experimentally observed unsymmetry of the spinodals for copolymers with blocks of different stiffness is reflected in the theory, as presented in this investigation. Schematic illustration of the limiting behaviour of the block persistence lengths IA and IB as well as of their respective combinations.