EFFECTS OF SHORT DIBLOCKS AT INTERFACES OF STRONGLY SEGREGATED LONG DIBLOCKS

Mixtures of small amounts of short diblocks (cosurfactants) and chemic ally identical large diblocks (surfactants) are considered, when the l arge diblocks are in the strongly segregated regime. Due to the enthal py gain, the short diblocks segregate to the interfaces and change the interfacial properties. The competition between the interfacial free energy reduction and the packing free energy change determines the pha se behavior of the system. It is shown that large changes can be expec ted in the global phase diagram when both short and long chain composi tion ratios are varied. At some places on the phase boundaries between structures of different symmetries the increase in the packing free e nergy density corresponding to the addition of nearly symmetrical shor t diblocks cancels the reduction in the interfacial free energy densit y, and varying the concentration of small chains has no effect on the stability of the phases in the vicinity of these structural fixed poin ts. Elsewhere, large changes in the stabilities of phases of different symmetries can occur depending on the composition ratios of the short and long copolymers, and whether the short molecules behave as compat ibilizers at the domain boundaries or act more like homopolymer filler s in the bulk domains. Density profiles of both large and short dibloc ks, based on mean-field calculations, are presented.