Entropy-induced smectic phases in rod-coil copolymers

TL;DR

This paper develops a self-consistent field theory for semiflexible diblock copolymers, predicting entropy-driven smectic phases solely due to differences in chain rigidity, with a detailed phase diagram including isotropic, nematic, and smectic phases.

Contribution

It introduces a novel SCFT model that predicts entropy-induced smectic phases in rod-coil copolymers based on rigidity differences, without requiring additional attractive interactions.

Findings

Smectic-A phase predicted by the model.

Second-order nematic-smectic transition.

Smectic ordering driven purely by entropic effects.

Abstract

We present a self-consistent field theory (SCFT) of semiflexible (wormlike) diblock copolymers, each consisting of a rigid and a flexible part. The segments of the polymers are otherwise identical, in particular with regard to their interactions, which are taken to be of an Onsager excluded-volume type. The theory is developed in a general three-dimensional form, as well as in a simpler one-dimensional version. Using the latter, we demonstrate that the theory predicts the formation of a partial-bilayer smectic-A phase in this system, as shown by profiles of the local density and orientational distribution functions. The phase diagram of the system, which includes the isotropic and nematic phases, is obtained in terms of the mean density and rigid-rod fraction of each molecule. The nematic-smectic transition is found to be second order. Since the smectic phase is induced solely by the difference in the rigidities, the onset of smectic ordering is shown to be an entropic effect and therefore does not have to rely on additional Flory-Huggins-type repulsive interactions between unlike chain segments. These findings are compared with other recent SCFT studies of similar copolymer models and with computer simulations of several molecular models.