Entropy-induced smectic phases in rod–coil copolymers

Abstract

We present a self-consistent field theory (SCFT) for dilute solutions of semiflexible(wormlike) diblock copolymers, each consisting of a rigid and a flexible part. Thesegments of the polymers are otherwise identical, in particular with regard totheir interactions, which are taken to be of an Onsager excluded-volume type. Thetheory is developed in a general three-dimensional form, as well as in asimpler one-dimensional version. Using the latter, we demonstrate thatthe theory predicts the formation of a partial-bilayer smectic-A phase inthis system, as shown by profiles of the local density and orientationaldistribution functions. The phase diagram of the system, which includes theisotropic and nematic phases, is obtained in terms of the mean density andrigid-rod fraction of each molecule. The nematic–smectic transition isfound to be second order. Since the smectic phase is induced solely bythe difference in the rigidities, the onset of smectic ordering is shown tobe an entropic effect and therefore does not have to rely on additionalFlory–Huggins-type repulsive interactions between unlike chain segments.These findings are compared with other recent SCFT studies of similarcopolymer models and with computer simulations of several molecular models.